Variable styles of rifting expressed in crustal structure across three rift segments of the Gulf of California

NASA Astrophysics Data System (ADS)

Lizarralde, D. D.; Axen, G. J.; Brown, H. E.; Fletcher, J. M.; Fernandez, A. G.; Harding, A. J.; Holbrook, W. S.; Kent, G. M.; Paramo, P.; Sutherland, F. H.; Umhoefer, P. J.

2007-05-01

We present a summary of results from a crustal-scale seismic experiment conducted in the southern Gulf of California. This experiment, the PESCADOR experiment, imaged crustal structure across three rift segments, the Alarcon, Guaymas, and San José del Cabo to Puerto Vallarta (Cabo-PV) segments, using seismic refraction/wide-angle reflection data acquired with airgun sources and recorded by closely spaced (10-15 km) ocean-bottom seismometers (OBSs). The imaged crustal structure reveals a surprisingly large variation in rifting style and magmatism between these segments: the Alarcon segment is a wide rift with apparently little syn-rift magmatism; the Guaymas segment is a narrow, magmatically robust rift; and the Cabo-PV segment is a narrow, magmatically "normal" rift. Our explanation for the observed variability is non-traditional in that we do not invoke mantle temperature, the factor commonly invoked to explain end-member volcanic and non-volcanic rifted margins, as the source of the considerable, though non-end-member variability we observe. Instead, we invoke mantle depletion related to pre-rift arc volcanism to account for observed wide, magma-poor rifting and mantle fertility and possibly the influence of sediments to account for robust rift and post-rift magmatism. These factors may commonly vary over small lateral spatial scales in regions that have transitioned from convergent to extensional tectonics, as is the case for the Gulf of California and many other rifts. Our hypothesis suggests that substantial lateral variability may exist within the uppermost mantle beneath the Gulf of California today, and it is hoped that ongoing efforts to image upper mantle structure here will provide tests for this hypothesis.

Tectono-stratigraphic evolution of normal fault zones: Thal Fault Zone, Suez Rift, Egypt

NASA Astrophysics Data System (ADS)

Leppard, Christopher William

The evolution of linkage of normal fault populations to form continuous, basin bounding normal fault zones is recognised as an important control on the stratigraphic evolution of rift-basins. This project aims to investigate the temporal and spatial evolution of normal fault populations and associated syn-rift deposits from the initiation of early-formed, isolated normal faults (rift-initiation) to the development of a through-going fault zone (rift-climax) by documenting the tectono-stratigraphic evolution of the Sarbut EI Gamal segment of the exceptionally well-exposed Thai fault zone, Suez Rift, Egypt. A number of dated stratal surfaces mapped around the syn-rift depocentre of the Sarbut El Gamal segment allow constraints to be placed on the timing and style of deformation, and the spatial variability of facies along this segment of the fault zone. Data collected indicates that during the first 3.5 My of rifting the structural style was characterised by numerous, closely spaced, short (< 3 km), low displacement (< 200 m) synthetic and antithetic normal faults within 1 - 2 km of the present-day fault segment trace, accommodating surface deformation associated with the development of a fault propagation monocline above the buried, pre-cursor strands of the Sarbut El Gamal fault segment. The progressive localisation of displacement onto the fault segment during rift-climax resulted in the development of a major, surface-breaking fault 3.5 - 5 My after the onset of rifting and is recorded by the death of early-formed synthetic and antithetic faults up-section, and thickening of syn-rift strata towards the fault segment. The influence of intrabasinal highs at the tips of the Sarbut EI Gamal fault segment on the pre-rift sub-crop level, combined with observations from the early-formed structures and coeval deposits suggest that the overall length of the fault segment was fixed from an early stage. The fault segment is interpreted to have grown through rapid lateral propagation and early linkage of the precursor fault strands at depth before the fault segment broke surface, followed by the accumulation of displacement on the linked fault segment with minimal lateral propagation. This style of fault growth contrasts conventional fault growth models by which growth occurs through incremental increases in both displacement and length through time. The evolution of normal fault populations and fault zones exerts a first- order control on basin physiography and sediment supply, and therefore, the architecture and distribution of coeval syn-rift stratigraphy. The early syn-rift continental, Abu Zenima Formation, to shallow marine, Nukhul Formation show a pronounced westward increase in thickness controlled by the series of synthetic and antithetic faults up to 3 km west of present day Thai fault. The orientation of these faults controlled the location of fluvial conglomerates, sandstones and mudstones that shifted to the topographic lows created. The progressive localisation of displacement onto the Sarbut El Gamal fault segment during rift-climax resulted in an overall change in basin geometry. Accelerated subsidence rates led to sedimentation rates being outpaced by subsidence resulting in the development of a marine, sediment-starved, underfilled hangingwall depocentre characterised by slope-to-basinal depositional environments, with a laterally continuous slope apron in the immediate hangingwall, and point-sourced submarine fans. Controls on the spatial distribution, three dimensional architecture, and facies stacking patterns of coeval syn-rift deposits are identified as: I) structural style of the evolution and linkage of normal fault populations, ii) basin physiography, iii) evolution of drainage catchments, iv) bedrock lithology, and v) variations in sea/lake level.

Control of rift asymmetry and segmentation on the thermal architecture of hyperextended rift systems: insights from Pyrenean field observations and numerical modelling

NASA Astrophysics Data System (ADS)

Lescoutre, Rodolphe; Tugend, Julie; Brune, Sascha; Manatschal, Gianreto

2017-04-01

Mid-Cretaceous rift basins are exposed in the Pyrenees providing key information on rifted domain formation that is not available at present-day rift system. Substantial paleotemperature and thermochronological data have been collected and published in numerous recent papers. These data show a strong heterogeneity in the distribution of peak temperatures within the Cretaceous rift basins. Locations that experienced relatively high or low temperatures appear to cluster in specific areas along strike. These areas have been interpreted as either reflecting hot and cold conditions during rifting, or alternatively, a change in the polarity of a strongly asymmetric rift systems. In this study, we test if the observed variability of peak temperatures can be explained by segmentation and a change in polarity of an asymmetrical upper/lower plate rift model. To this aim we restore the observed syn- to early post-rift peak temperatures to their paleo-location within sections across the evolving rift system. In the meantime, we conduct numerical models of rift migration leading to asymmetrical extension that are benchmarked with geological and geophysical observations from the Pyrenees. From the models, we extract thermal information at different stages of rifting that are finally compared to the thermal data from the Pyrenean Cretaceous rift basins. This work employs a novel approach by comparing thermal output from numerical modelling with the distribution of peak temperatures and thermal gradient from field data. As such, these results may have substantial implications to further understand the pre-orogenic thermal evolution of the Pyrenean rift system and the role of segmentation. More generally, the results of this work may unravel the role of rift asymmetry and segmentation on the thermal architecture of hyperextended rift basins and margins.

The Afar-Red Sea-Gulf of Aden volcanic margins system : early syn-rift segmentation and tectono-magmatic evolution

NASA Astrophysics Data System (ADS)

Stab, Martin; Leroy, Sylvie; Bellahsen, Nicolas; Pik, Raphaël; Ayalew, Dereje; Yirgu, Gezahegn; Khanbari, Khaled

2017-04-01

The Afro-Arabian rift system is characterized by complex interactions between magmatism and rifting, leading to long-term segmentation of the associated continental margins. However, past studies focused on specific rift segments and no attempt has yet been made to reconcile them into a single comprehensive geodynamic model. To address this, we present interpretations of seismic profiles offshore the Eritrea-Yemeni margins in the southern Red Sea and the Yemeni margin in the Gulf of Aden and reassess the regional geodynamic evolution including the new tectonic evolution of the Central Afar Magmatic margin. We point out the role of two major transform zones in structuring the volcanism and faulting of the Red Sea-Afar-Aden margins. We show that those transform zones not only control the present-day rift organization, but were also active since the onset of rifting in Oligocene times. Early syn-rift transform zones control the emplacement and the development of seaward-dipping-reflector wedges immediately after the Continental Flood basalts (30 Ma), and are closely associated with mantle plume melts in the course of the segment extension. The margins segmentation thus appears to reflect the underlying mantle dynamics and thermal anomaly, which have directly influenced the style of rifting (wide vs. narrow rift), in controlling the development of preferential lithospheric thinning and massive transfer of magmas in the crust.

Rift flank segmentation, basin initiation and propagation: a neotectonic example from Lake Baikal

USGS Publications Warehouse

Agar, Susan M.; Klitgord, Kim D.

1995-01-01

New surficial data (field, Landsat TM and topography) define morpho-tectonic domains and rift flank segmentation in the Ol'khon region of the Central Baikal rift. Deformation, drainage and depositional patterns indicate a change in the locus of active extension that may relate to a recent (

3D displacement time series in the Afar rift zone computed from SAR phase and amplitude information

NASA Astrophysics Data System (ADS)

Casu, Francesco; Manconi, Andrea

2013-04-01

Large and rapid deformations, such as those caused by earthquakes, eruptions, and landslides cannot be fully measured by using standard DInSAR applications. Indeed, the phase information often degrades and some areas of the interferograms are affected by high fringe rates, leading to difficulties in the phase unwrapping, and/or to complete loss of coherence due to significant misregistration errors. This limitation can be overcome by exploiting the SAR image amplitude information instead of the phase, and by calculating the Pixel-Offset (PO) field SAR image pairs, for both range and azimuth directions. Moreover, it is possible to combine the PO results by following the same rationale of the SBAS technique, to finally retrieve the offset-based deformation time series. Such technique, named PO-SBAS, permits to retrieve the deformation field in areas affected by very large displacements at an accuracy that, for ENVISAT data, correspond to 30 cm and 15 cm for the range and azimuth, respectively [1]. Moreover, the combination of SBAS and PO-SBAS time series can help to better study and model deformation phenomena characterized by spatial and temporal heterogeneities [2]. The Dabbahu rift segment of the Afar depression has been active since 2005 when a 2.5 km3 dyke intrusion and hundreds of earthquakes marked the onset a rifting episode which continues to date. The ENVISAT satellite has repeatedly imaged the Afar depression since 2003, generating a large SAR archive. In this work, we study the Afar rift region deformations by using both the phase and amplitude information of several sets of SAR images acquired from ascending and descending ENVISAT tracks. We combined sets of small baseline interferograms through the SBAS algorithm, and we generate both ground deformation maps and time series along the satellite Line-Of-Sight (LOS). In areas where the deformation gradient causes loss of coherence, we retrieve the displacement field through the amplitude information. Furthermore, we could also retrieve the full 3D deformation field, by considering the North-South displacement component obtained from the azimuth PO information. The combination of SBAS and PO-SBAS information permits to better retrieve and constrain the full deformation field due to repeated intrusions, fault movements, as well as the magma movements from individual magma chambers. [1] Casu, F., A. Manconi, A. Pepe and R. Lanari, 2011. Deformation time-series generation in areas characterized by large displacement dynamics: the SAR amplitude Pixel-Offset SBAS technique, IEEE Transaction on Geosciences and Remote Sensing. [2] Manconi, A. and F. Casu, 2012. Joint analysis of displacement time series retrieved from SAR phase and amplitude: impact on the estimation of volcanic source parameters, Geophysical Research Letters, doi:10.1029/2012GL052202.

Amagmatic Accretionary Segments, Ultraslow Spreading and Non-Volcanic Rifted Margins (Invited)

NASA Astrophysics Data System (ADS)

Dick, H. J.; Snow, J. E.

2009-12-01

The evolution of non-volcanic rifted margins is key to understanding continental breakup and the early evolution of some of the world’s most productive hydrocarbon basins. However, the early stages of such rifting are constrained by limited observations on ancient heavily sedimented margins such as Newfoundland and Iberia. Ultraslow spreading ridges, however, provide a modern analogue for early continental rifting. Ultraslow spreading ridges (<20 mm/yr) comprise ~30% of the global ridge system (e.g. Gakkel, Southwest Indian, Terceira, and Knipovitch Ridges). They have unique tectonics with widely spaced volcanic segments and amagmatic accretionary ridge segments. The volcanic segments, though far from hot spots, include some of the largest axial volcanoes on the global ridge system, and have, unusual magma chemistry, often showing local isotopic and incompatible element enrichment unrelated to mantle hot spots. The transition from slow to ultraslow tectonics and spreading is not uniquely defined by spreading rate, and may also be moderated by magma supply and mantle temperature. Amagmatic accretionary segments are the 4th class of plate boundary structure, and, we believe, the defining tectonic feature of early continental breakup. They form at effective spreading rates <12 mm/yr, assume any orientation to spreading, and replace transform faults and magmatic segments. At amagmatic segments the earth splits apart with the mantle emplaced directly to the seafloor, and great slabs of peridotite are uplifted to form the rift mountains. A thick conductive lid suppresses mantle melting, and magmatic segments form only at widely spaced intervals, with only scattered volcanics in between. Amagmatic segments link with the magmatic segments forming curvilinear plate boundaries, rather than the step-like morphology found at faster spreading ridges. These are all key features of non-volcanic rifted margins; explaining, for example, the presence of mantle peridotites emplaced simultaneously on both the Newfoundland and Iberian Margins in the Jurassic and Cretaceous. Miocene Lena Trough is a new mid-ocean rift plate boundary and the final event in the separation of the North American and Eurasian continents. Mapping and sampling of Lena Trough confirms that it is both oblique and amagmatic, showing that initiation of seafloor spreading at a non-volcanic rifted continental margin follows the same pattern as ultraslow spreading ridges.

Structure and kinematics of segment-scale crustal accretion processes in Iceland and implications for analogous mid-ocean ridge systems

NASA Astrophysics Data System (ADS)

Siler, Drew Lorenz

2011-12-01

The sub-surface geologic structure of the crust is controlled by the magmatic and tectonic processes that construct the crust during plate spreading. As a result, geologic structure provides constraints on the processes that occur during plate spreading. The crust of the Skagi region of northern Iceland, where this study was focused, was accreted by magmatic construction to Iceland ˜7-10 Ma and subsequently glacially eroded, exhuming ˜1-3 km of structural relief. Continuous spreading-parallel and spreading-orthogonal mountain ranges expose the crust accreted at discrete spreading segments, the fundamental intervals upon which plate spreading and crustal accretion occur. As a result, Skagi is an ideal location to employ geologic structure analysis to study magmatic rifting processes. Within spreading segments structural patterns vary significantly between segment centers and distal fissure swarms. While segment centers are characterized by focused magmatic construction and km-scale sub-volcanic subsidence, fissure swarms are characterized by limited magmatic construction, minor sub-axial subsidence and lateral dike injection. Such along-strike variation indicates that both magma in the upper crust and gabbroic material in the lower crust must be redistributed along-strike within spreading segments during plate spreading. Material flow is directed from beneath segment centers towards distal fissure swarms. At the regional scale, each spreading segment is a structurally discrete interval of Iceland's Neovolcanic Zone. As a result of west-northwestward movement of Iceland relative to the Iceland hotspot, the rift zone axis has progressively relocated to the east-southeast with time, leaving a series of abandoned rift zones throughout western Iceland. A compilation of published K/Ar and 40Ar/39Ar age data and geologic data from across northern Iceland shows that rift relocation occurs via frequent (2-3 Ma), small-scale (˜20 km) rift propagations rather than rare, 100s of km 'rift jumps' as is conventional models suggest. The structure relationships we define in the Icelandic crust are similar to that of other magmatic rift systems including Mid-Ocean Ridges, continental rifts and ancient volcanic rift margins. As such, we suggest that many of the crustal accretion processes we have inferred from Icelandic data may be important in these analogous environments as well.

Bookshelf faulting and transform motion between rift segments of the Northern Volcanic Zone, Iceland

NASA Astrophysics Data System (ADS)

Green, R. G.; White, R. S.; Greenfield, T. S.

2013-12-01

Plate spreading is segmented on length scales from 10 - 1,000 kilometres. Where spreading segments are offset, extensional motion has to transfer from one segment to another. In classical plate tectonics, mid-ocean ridge spreading centres are offset by transform faults, but smaller 'non-transform' offsets exist between slightly overlapping spreading centres which accommodate shear by a variety of geometries. In Iceland the mid-Atlantic Ridge is raised above sea level by the Iceland mantle plume, and is divided into a series of segments 20-150 km long. Using microseismicity recorded by a temporary array of 26 three-component seismometers during 2009-2012 we map bookshelf faulting between the offset Askja and Kverkfjöll rift segments in north Iceland. The micro-earthquakes delineate a series of sub-parallel strike-slip faults. Well constrained fault plane solutions show consistent left-lateral motion on fault planes aligned closely with epicentral trends. The shear couple across the transform zone causes left-lateral slip on the series of strike-slip faults sub-parallel to the rift fabric, causing clockwise rotations about a vertical axis of the intervening rigid crustal blocks. This accommodates the overall right-lateral transform motion in the relay zone between the two overlapping volcanic rift segments. The faults probably reactivated crustal weaknesses along the dyke intrusion fabric (parallel to the rift axis) and have since rotated ˜15° clockwise into their present orientation. The reactivation of pre-existing rift-parallel weaknesses is in contrast with mid-ocean ridge transform faults, and is an important illustration of a 'non-transform' offset accommodating shear between overlapping spreading segments.

Initiation and Along-Axis Segmentation of Seaward-Dipping Volcanic Sequences Captured in Afar

NASA Astrophysics Data System (ADS)

Ebinger, C.; Wolfenden, E.; Yirgu, G.; Keir, D.

2003-12-01

The Afar triple junction zone provides a unique opportunity to examine the early development of magmatic margins, as respective limbs of the triple junction capture different stages of the breakup process. Initial rifting in the southernmost Red Sea occurred concurrent with, or soon after flood basaltic magmatism at ~31 Ma in the Ethiopia-Yemen plume province, whereas the northern part of the Main Ethiopian rift initiated after 12 Ma. Both rift systems initiated with the development of high-angle border fault systems bounding broad basins, but 8-10 My after rifting we see riftward migration of strain from the western border fault to narrow zones of increasingly more basaltic magmatism. These localised zones of faulting and volcanism (magmatic segments) show a segmentation independent of the border fault segmentation. The much older, more evolved magmatic segments in the southern Red Sea, where not onlapped by Pliocene-Recent sedimentary strata, dip steeply riftward and define a regional eastward flexure into transitional oceanic crust, as indicated by gravity models constrained by seismic refraction and receiver function data. The southern Red Sea magmatic segments have been abandoned in Pliocene-Recent triple junction reorganisations, whereas the process of seaward-dipping volcanic sequence emplacement is ongoing in the seismically and volcanically active Main Ethiopian rift. Field, remote sensing, gravity, and seismicity data from the Main Ethiopian and southern Red Sea rifts indicate that seaward-dipping volcanic sequences initiate in moderately stretched continental crust above a narrow zone of dike-intrusion. Our comparison of active and ancient magmatic segments show that they are the precursors to seaward-dipping volcanic sequences analogous to those seen on passive continental margins, and provides insights into the initiation of along-axis segmentation of seafloor-spreading centers.

The South China sea margins: Implications for rifting contrasts

USGS Publications Warehouse

Hayes, D.E.; Nissen, S.S.

2005-01-01

Implications regarding spatially complex continental rifting, crustal extension, and the subsequent evolution to seafloor spreading are re-examined for the northern and southern-rifted margins of the South China Sea. Previous seismic studies have shown dramatic differences in the present-day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south China. Although the total crustal extension is presumed to be the same along the margin and adjacent ocean basin, the amount of continental crustal extension that occurred is much less along the east and central segments of the margin than along the western segment. This difference was accommodated by the early formation of oceanic crust (creating the present-day South China Sea basin) adjacent to the eastern margin segment while continued extension of continental crust was sustained to the west. Using the observed cross-sectional areas of extended continental crust derived from deep penetration seismics, two end-member models of varying rift zone widths and varying initial crustal thicknesses are qualitatively examined for three transects. Each model implies a time difference in the initiation of seafloor spreading inferred for different segments along the margin. The two models examined predict that the oceanic crust of the South China Sea basin toward the west did not begin forming until sometime between 6-12 my after its initial formation (???32 Ma) toward the east. These results are compatible with crustal age interpretations of marine magnetic anomalies. Assuming rifting symmetry with conjugate margin segments now residing along the southern portions of the South China Sea basin implies that the total width of the zone of rifting in the west was greater than in the east by about a factor of two. We suggest the most likely causes of the rifting differences were east-west variations in the rheology of the pre-rift crust and associated east-west variations in the thermal structure of the pre-rift lithosphere. The calculated widths of rifted continental crust for the northern and southern margins, when combined with the differential widths of seafloor generated during the seafloor spreading phase, indicate the total crustal extension that occurred is about 1100 km and is remarkably consistent to within ???10% for all three (eastern, central, western) segments examined. ?? 2005 Elsevier B.V. All rights reserved.

The initiation of segmented buoyancy-driven melting during continental breakup

PubMed Central

Gallacher, Ryan J.; Keir, Derek; Harmon, Nicholas; Stuart, Graham; Leroy, Sylvie; Hammond, James O. S.; Kendall, J-Michael; Ayele, Atalay; Goitom, Berhe; Ogubazghi, Ghebrebrhan; Ahmed, Abdulhakim

2016-01-01

Melting of the mantle during continental breakup leads to magmatic intrusion and volcanism, yet our understanding of the location and dominant mechanisms of melt generation in rifting environments is impeded by a paucity of direct observations of mantle melting. It is unclear when during the rifting process the segmented nature of magma supply typical of seafloor spreading initiates. Here, we use Rayleigh-wave tomography to construct a high-resolution absolute three-dimensional shear-wave velocity model of the upper 250 km beneath the Afar triple junction, imaging the mantle response during progressive continental breakup. Our model suggests melt production is highest and melting depths deepest early during continental breakup. Elevated melt production during continental rifting is likely due to localized thinning and melt focusing when the rift is narrow. In addition, we interpret segmented zones of melt supply beneath the rift, suggesting that buoyancy-driven active upwelling of the mantle initiates early during continental rifting. PMID:27752044

The Afar rift zone deformation dynamics retrieved through phase and amplitude SAR data

NASA Astrophysics Data System (ADS)

Casu, F.; Pagli, C.; Paglia, L.; Wang, H.; Wright, T. J.; Lanari, R.

2011-12-01

The Dabbahu rift segment of the Afar depression has been active since 2005 when a 2.5 km3 dyke intrusion and hundreds of earthquakes marked the onset a rifting episode which continues to date. Since 2003, the Afar depression has been repeatedly imaged by the ENVISAT satellite, generating a large SAR archive which allow us to study the ongoing deformation processes and the dynamics of magma movements. We combine sets of small baseline interferograms through the advanced DInSAR algorithm referred to as Small BAseline Subset (SBAS), and we generate both ground deformation maps and time series along the satellite Line-Of-Sight (LOS), with accuracies on the order of 5 mm. The main limitation of DInSAR applications is that large and rapid deformations, such as those caused by dyke intrusions and eruptions in Afar, cannot be fully measured. The phase information often degrades and some areas of the interferograms are affected by high fringe rates, leading to difficulties in the phase unwrapping, and/or to complete loss of coherence due to significant misregistration errors. This limitation can be overcome by exploiting the SAR image amplitude information instead of the phase, and by calculating the Pixel-Offset (PO) field of a given SAR image pair, for both range and azimuth directions. Moreover, after computing the POs for each image pair, it is possible to combine them, following the same rationale of the SBAS technique, to finally retrieve the offset-based deformation time series. Such technique, named PO-SBAS, permits to retrieve the deformation field in areas affected by very large displacements at an accuracy that, for ENVISAT data, correspond to 30cm and 15 cm for the range and azimuth, respectively. In this work, we study the Afar rift region deformations by using both the phase and amplitude information of several sets of SAR images acquired from ascending and descending ENVISAT tracks. In particular, we use the phase information to construct dense and accurate deformation maps and time series in areas not affected by large displacements. While in areas where the deformation gradient causes loss of coherence, we retrieve the displacement field through the amplitude information. This approach allows us to obtain a spatially detailed deformation map of the study area. In addition, by combining ascending and descending data we reconstruct the vertical and East-West components of deformation field. Furthermore, in areas affected by large deformations, we can also retrieve the full 3D deformation field, by using the North-South displacement component obtained from the azimuth PO information. Distinct sources of deformations interact in Afar. Fault movements and magma chamber deflation have accompanied dyke intrusions but quantifying each contribution to the total deformation has been challenging, also due to loss of coherence in the central part of the rift. Here we combined the phase and amplitude information in order to retrieve the full deformation field of repeated intrusions. This allows us to better constrain the fault movements that occur as the dyke propagates as well as the magma movements from individual magma chambers.

Strain Localisation at Rift Segment Boundaries: An Example from the Bocana Transfer Zone in Central Baja California, Mexico

NASA Astrophysics Data System (ADS)

Seiler, C.; Gleadow, A. J.; Kohn, B. P.

2012-12-01

Rifts are commonly segmented into several hundred kilometre long zones of opposing upper-plate transport direction with boundaries defined by accommodation and transfer zones. A number of such rift segments have been recognized in the northern Gulf of California, a youthful oceanic basin that is currently undergoing the rift-drift transition. However, detailed field studies have so far failed to identify suitable structures that could accommodate the obvious deformation gradients between different rift segments, and the nature of strain transfer at segment boundaries remains enigmatic. The situation is even less clear in central and southern Baja California, where a number of rift segments have been hypothesized but it is unknown whether the intervening segment boundaries facilitate true reversals in the upper-plate transport direction, or whether they simply accommodate differences in the timing, style or magnitude of deformation. The Bocana transfer zone (BTZ) in central Baja California is a linear, WNW-ESE striking structural discontinuity separating two rift segments with different magnitudes and styles of extensional deformation. North of the BTZ, the Libertad fault is part of the Main Gulf Escarpment, which represents the breakaway fault that separates the Gulf of California rift to the east from the relatively stable western portion of the Baja peninsula. The N-striking Libertad escarpment developed during the Late Miocene (~10-8Ma) and exhibits a topographic relief of ca. 1,000m along a strike-length of ca. 50km. Finite displacement decreases from ~1000m in the central fault segment to ~500m further south, where the fault bends SE and merges with the BTZ. In the hanging wall of the Libertad fault, a series of W-tilted horsts are bound along their eastern margins by two moderate-displacement E-dipping normal faults. South of the BTZ, extension was much less than further north, which explains the comparatively subdued relief and generally shallower tilt of pre-rift strata in this area. The BTZ itself is characterized by two en echelon WNW-ESE striking dextral-oblique transfer faults with a significant down-to-the-NNE extensional component. Strain is transferred from the Libertad breakaway fault onto the transfer faults over a distance of >20km through a network of interacting normal, oblique and strike-slip faults. The shape, location and orientation of the main faults were strongly influenced by pre-existing rheological heterogeneities. Major normal faults are parallel to either the Mesozoic metamorphic foliation or Cretaceous intrusive contacts, and developed where the foliation was at a high angle to the extension direction. In contrast, the oblique-slip faults of the BTZ formed parallel to the metamorphic foliation where formlines are at a small angle to the regional extension direction. Compared to other, less well-understood accommodation zones in the Gulf of California rift, the BTZ shows a distinct lack of volcanic activity, which may help explain the different exposure and structural expression of the various segment boundaries.

Chronology of volcanism and rift basin propagation - Rungwe volcanic province, East Africa

NASA Technical Reports Server (NTRS)

Ebinger, C. J.; Deino, A. L.; Drake, R. E.; Tesha, A. L.

1989-01-01

The spatial and temporal development of along-axis segmentation in youthful continental rifts was investigated using the results of field, remote sensing, and K-Ar geochronology studies conducted in four (Rukwa, Songwe, Usangu, and Karonga) rift basins within the Rungwe volcanic province in East Africa. Results indicated that the Rukwa and Karonga border fault segments formed between 7.25 and 5 m.y. ago, the Usangu border fault segment developed between 3 and 2 m.y. ago, and subsidence along the Songwe border fault segment had occurred by 0.5 Ma. It is shown that individual basins developed diachronously, each following a similar sequence: (1) initial border fault development; (2) asymmetric basin subsidence/flank uplift and the development of monoclines opposite the border faults; and (3) continued subsidence and tilting along intrabasinal faults with flexural upwarping of the rift flanks, enhancing basinal asymmetries.

Geometry of the neoproterozoic and paleozoic rift margin of western Laurentia: Implications for mineral deposit settings

USGS Publications Warehouse

Lund, K.

2008-01-01

The U.S. and Canadian Cordilleran miogeocline evolved during several phases of Cryogenian-Devonian intracontinental rifting that formed the western mangin of Laurentia. Recent field and dating studies across central Idaho and northern Nevada result in identification of two segments of the rift margin. Resulting interpretations of rift geometry in the northern U.S. Cordillera are compatible with interpretations of northwest- striking asymmetric extensional segments subdivided by northeast-striking transform and transfer segments. The new interpretation permits integration of miogeoclinal segments along the length of the western North American Cordillera. For the U.S. Cordillera, miogeoclinal segments include the St. Mary-Moyie transform, eastern Washington- eastern Idaho upper-plate margin, Snake River transfer, Nevada-Utah lower-plate margin, and Mina transfer. The rift is orthogonal to most older basement domains, but the location of the transform-transfer zones suggests control of them by basement domain boundaries. The zigzag geometry of reentrants and promontories along the rift is paralleled by salients and recesses in younger thrust belts and by segmentation of younger extensional domains. Likewise, transform transfer zones localized subsequent transcurrent structures and igneous activity. Sediment-hosted mineral deposits trace the same zigzag geometry along the margin. Sedimentary exhalative (sedex) Zn-Pb-Ag ??Au and barite mineral deposits formed in continental-slope rocks during the Late Devonian-Mississippian and to a lesser degree, during the Cambrian-Early Ordovician. Such deposits formed during episodes of renewed extension along miogeoclinal segments. Carbonate-hosted Mississippi Valley- type (MVT) Zn-Pb deposits formed in structurally reactivated continental shelf rocks during the Late Devonian-Mississippian and Mesozoic due to reactivation of preexisting structures. The distribution and abundance of sedex and MVT deposits are controlled by the polarity and kinematics of the rift segment. Locally, discrete mineral belts parallel secondary structures such as rotated crustal blocks at depth that produced sedimentary subbasins and conduits for hydrothermal fluids. Where the miogeocline was overprinted by Mesozoic and Cenozoic deformation and magmatism, igneous rock-related mineral deposits are common. ??2008 Geological Society of America.

Along-axis segmentation and isostasy in the Western rift, East Africa

NASA Astrophysics Data System (ADS)

Upcott, N. M.; Mukasa, R. K.; Ebinger, C. J.; Karner, G. D.

1996-02-01

Structural variations along the southern sectors of the Western rift, East Africa, have previously been described, but subsurface structures in the northern sector (Uganda, Zaire) are virtually unknown. Our aims are to investigate the along-axis segmentation of the northern sector, thereby adding to the structural picture of the Western rift, and to study the isostatic compensation of the varying rift morphology along the sector's length. This study describes the first gravity survey to be carried out on the shallow Lake Albert, forward models of these and existing gravity data, and the results from inverse modeling of existing aeromagnetic data designed to delimit border and transfer fault systems. Our tectonic model shows that the northern rift sector is segmented along-axis into five 25 to 65-km-wide, 80 to 100-km-long rift segments, characterized by closed-contour Bouguer anomaly lows, and bounded by steep gravity, aeromagnetic, and topographic/bathymetric gradients. Werner and Euler deconvolution results and gravity anomaly data reveal that some faulted basins are separated by structural highs and cross-rift ramps or faults and suggest sedimentary basin depths of 4-6 km. Forward modeling of structural and free-air gravity profiles across individual basins and flanks using a model that assumes flexural compensation also suggests sediment thicknesses of up to 5.5 km, similar to the estimates from magnetic data. The basin and flank morphology can be explained by 6-9 km of extension of a lithosphere with an effective elastic thickness (Te) of 25 km (equivalent to a flexural rigidity of 1.4 × 1023 N m), similar to results in other Western rift basins. Potential field data and lithospheric strength estimates in the Western rift system show small along-axis variations in lithospheric structure, regardless of the presence or absence of Cenozoic magmatism.

Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes.

PubMed

Passarelli, L; Rivalta, E; Shuler, A

2014-01-28

As continental rifts evolve towards mid-ocean ridges, strain is accommodated by repeated episodes of faulting and magmatism. Discrete rifting episodes have been observed along two subaerial divergent plate boundaries, the Krafla segment of the Northern Volcanic Rift Zone in Iceland and the Manda-Hararo segment of the Red Sea Rift in Ethiopia. In both cases, the initial and largest dike intrusion was followed by a series of smaller intrusions. By performing a statistical analysis of these rifting episodes, we demonstrate that dike intrusions obey scaling relationships similar to earthquakes. We find that the dimensions of dike intrusions obey a power law analogous to the Gutenberg-Richter relation, and the long-term release of geodetic moment is governed by a relationship consistent with the Omori law. Due to the effects of magma supply, the timing of secondary dike intrusions differs from that of the aftershocks. This work provides evidence of self-similarity in the rifting process.

Along-strike supply of volcanic rifted margins: Implications for plume-influenced rifting and sudden along-strike transitions between volcanic and non-volcanic rifted margins

NASA Astrophysics Data System (ADS)

Ranero, C. R.; Phipps Morgan, J.

2006-12-01

The existence of sudden along-strike transitions between volcanic and non-volcanic rifted margins is an important constraint for conceptual models of rifting and continental breakup. We think there is a promising indirect approach to infer the maximum width of the region of upwelling that exists beneath a rifted margin during the transition from rifting to seafloor-spreading. We infer this width of ~30km from the minimum length of the ridge-offsets that mark the limits of the `region of influence' of on-ridge plumes on the axial relief, axial morphology, and crustal thickness along the ridge and at the terminations of fossil volcanic rifted margins. We adopt Vogt's [1972] hypothesis for along-ridge asthenospheric flow in a narrow vertical slot beneath the axis of plume-influenced `macro-segments' and volcanic rifted margins. We find that: (1) There is a threshold distance to the lateral offsets that bound plume-influenced macrosegments; all such `barrier offsets' are greater than ~30km, while smaller offsets do not appear to be a barrier to along-axis flow. This pattern is seen in the often abrupt transitions between volcanic and non-volcanic rifted margins; these transitions coincide with >30km ridge offsets that mark the boundary between the smooth seafloor morphology and thick crust of a plume- influenced volcanic margin and a neighboring non-volcanic margin, as recorded in 180Ma rifting of the early N. Atlantic, the 42Ma rifting of the Kerguelen-Broken Ridge, and the 66Ma Seychelles-Indian rifting in the Indian Ocean. (2) A similar pattern is seen in the often abrupt transitions between `normal' and plume-influenced mid- ocean ridge segments, which is discussed in a companion presentation by Phipps Morgan and Ranero (this meeting). (3) The coexistance of adjacent volcanic and non-volcanic rifted margin segments is readily explained in this conceptual framework. If the volcanic margin macrosegment is plume-fed by hot asthenosphere along an axial ridge slot, while adjacent non-volcanic margin segments stretch and upwell ambient cooler subcontinental mantle, then there will be a sudden transition from volcanic to non-volcanic margins across a transform offset. (4) A 30km width for the region of ridge upwelling and melting offers a simple conceptual explanation for the apparent 30km threshold length for the existence of strike-slip transform faults and the occurrence of non-transform offsets at smaller ridge offset-distances. (5) The conceptual model leads to the interpretation of the observed characteristic ~1000km-2000km-width of plume-influenced macro- segments as a measure of the maximum potential plume supply into a subaxial slot of 5-10 cubic km per yr. (6) If asthenosphere consumption by plate-spreading is less than plume-supply into a macro-segment, then the shallow seafloor and excess gravitational spreading stresses associated with a plume-influenced ridge can lead to growth of the axial slot by ridge propagation. We think this is a promising conceptual framework with which to understand the differences between volcanic and non-volcanic rifted margins.

Fault kinematics and tectonic stress in the seismically active Manyara Dodoma Rift segment in Central Tanzania Implications for the East African Rift

NASA Astrophysics Data System (ADS)

Macheyeki, Athanas S.; Delvaux, Damien; De Batist, Marc; Mruma, Abdulkarim

2008-07-01

The Eastern Branch of the East African Rift System is well known in Ethiopia (Main Ethiopian Rift) and Kenya (Kenya or Gregory Rift) and is usually considered to fade away southwards in the North Tanzanian Divergence, where it splits into the Eyasi, Manyara and Pangani segments. Further towards the south, rift structures are more weakly expressed and this area has not attracted much attention since the mapping and exploratory works of the 1950s. In November 4, 2002, an earthquake of magnitude Mb = 5.5 struck Dodoma, the capital city of Tanzania. Analysis of modern digital relief, seismological and geological data reveals that ongoing tectonic deformation is presently affecting a broad N-S trending belt, extending southward from the North Tanzanian Divergence to the region of Dodoma, forming the proposed "Manyara-Dodoma Rift segment". North of Arusha-Ngorongoro line, the rift is confined to a narrow belt (Natron graben in Tanzania) and south of it, it broadens into a wide deformation zone which includes both the Eyasi and Manyara grabens. The two-stage rifting model proposed for Kenya and North Tanzania also applies to the Manyara-Dodoma Rift segment. In a first stage, large, well-expressed topographic and volcanogenic structures were initiated in the Natron, Eyasi and Manyara grabens during the Late Miocene to Pliocene. From the Middle Pleistocene onwards, deformations related to the second rifting stage propagated southwards to the Dodoma region. These young structures have still limited morphological expressions compared to the structures formed during the first stage. However, they appear to be tectonically active as shown by the high concentration of moderate earthquakes into earthquake swarms, the distribution of He-bearing thermal springs, the morphological freshness of the fault scarps, and the presence of open surface fractures. Fault kinematic and paleostress analysis of geological fault data in basement rocks along the active fault lines show that recent faults often reactivate older fault systems that were formed under E-W to NW-SE horizontal compression, compatible with late Pan-African tectonics. The present-day stress inverted from earthquake focal mechanisms shows that the Manyara-Dodoma Rift segment is presently subjected to an extensional stress field with a N080°E direction of horizontal principal extension. Under this stress field, the rift develops by: (1) reactivation of the pre-existing tectonic planes of weakness, and (2) progressive development of a new fault system in a more N-S trend by the linkage of existing rift faults. This process started about 1.2 Ma ago and is still ongoing.

Upper-mantle seismic structure in a region of incipient continental breakup: northern Ethiopian rift

NASA Astrophysics Data System (ADS)

Bastow, Ian D.; Stuart, Graham W.; Kendall, J.-Michael; Ebinger, Cynthia J.

2005-08-01

The northern Ethiopian rift forms the third arm of the Red Sea, Gulf of Aden triple junction, and marks the transition from continental rifting in the East African rift to incipient oceanic spreading in Afar. We determine the P- and S-wave velocity structure beneath the northern Ethiopian rift using independent tomographic inversion of P- and S-wave relative arrival-time residuals from teleseismic earthquakes recorded by the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) passive experiment using the regularised non-linear least-squares inversion method of VanDecar. Our 79 broad-band instruments covered an area 250 × 350 km centred on the Boset magmatic segment ~70 km SE of Addis Ababa in the centre of the northern Ethiopian rift. The study area encompasses several rift segments showing increasing degrees of extension and magmatic intrusion moving from south to north into the Afar depression. Analysis of relative arrival-time residuals shows that the rift flanks are asymmetric with arrivals associated with the southeastern Somalian Plate faster (~0.65 s for the P waves; ~2 s for the S waves) than the northwestern Nubian Plate. Our tomographic inversions image a 75 km wide tabular low-velocity zone (δVP~-1.5 per cent, δVS~-4 per cent) beneath the less-evolved southern part of the rift in the uppermost 200-250 km of the mantle. At depths of >100 km, north of 8.5°N, this low-velocity anomaly broadens laterally and appears to be connected to deeper low-velocity structures under the Afar depression. An off-rift low-velocity structure extending perpendicular to the rift axis correlates with the eastern limit of the E-W trending reactivated Precambrian Ambo-Guder fault zone that is delineated by Quaternary eruptive centres. Along axis, the low-velocity upwelling beneath the rift is segmented, with low-velocity material in the uppermost 100 km often offset to the side of the rift with the highest rift flank topography. Our observations from this magmatic rift zone, which is transitional between continental and oceanic rifting, do not support detachment fault models of lithospheric extension but instead point to strain accommodation via magma assisted rifting.

Structural controls on the spatial distribution and geochemical composition of volcanism in a continental rift zone; an example from Owens Valley, eastern California

NASA Astrophysics Data System (ADS)

Haproff, P. J.; Yin, A.

2014-12-01

Bimodal volcanism is common in continental rift zones. Structural controls to the emplacement and compositions of magmas, however, are not well understood. To address this issue, we examine the location, age, and geochemistry of active volcanic centers, and geometry and kinematics of rift-related faults across the active transtensional Owens Valley rift zone. Building on existing studies, we postulate that the spatial distribution and geochemical composition of volcanism are controlled by motion along rift-bounding fault systems. Along-strike variation in fault geometry and characteristics of active volcanism allow us to divide Owens Valley into three segments: southern, northern, and central. The southern segment of Owens Valley is a simple shear, asymmetric rift bounded to the west by the east-dipping Sierra Nevada frontal fault (SNFF). Active vents of Coso volcanic field are distributed along the eastern rift shoulder and characterized by the eruption of bimodal lavas. The SNFF within this segment is low-angle and penetrates through the lithosphere and into the ductile asthenosphere, allowing for mantle-derived magma to migrate across the weakest part of the fault zone beneath the eastern rift shoulder. Magma thermally weakens wall rocks and eventually stalls in the crust where the melt develops a greater felsic component prior to eruption. The northern segment of Owens Valley displays similar structural geometry, as the west-dipping White Mountains fault (WMF) is listric at depth and offsets the crust and mantle lithosphere, allowing for vertical transport of magma and reservoir emplacement within the crust. Bimodal lavas periodically erupted in the Long Valley Caldera region along the western rift shoulder. The central segment of Owens Valley is a pure shear, symmetric graben generated by motion along the SNFF and WMF. The subvertical, right-slip Owens Valley fault (OVF) strikes along the axis of the valley and penetrates through the lithosphere into the asthenosphere. Volcanic centers of Big Pine volcanic field are located along the trace of the OVF and characterized by mafic eruptions. The OVF is interpreted to provide a subvertical conduit for asthenospheric magma to migrate across the LAB and Moho and erupt on the rift surface without significant contamination with felsic crust.

Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes

PubMed Central

L., Passarelli; E., Rivalta; A., Shuler

2014-01-01

As continental rifts evolve towards mid-ocean ridges, strain is accommodated by repeated episodes of faulting and magmatism. Discrete rifting episodes have been observed along two subaerial divergent plate boundaries, the Krafla segment of the Northern Volcanic Rift Zone in Iceland and the Manda-Hararo segment of the Red Sea Rift in Ethiopia. In both cases, the initial and largest dike intrusion was followed by a series of smaller intrusions. By performing a statistical analysis of these rifting episodes, we demonstrate that dike intrusions obey scaling relationships similar to earthquakes. We find that the dimensions of dike intrusions obey a power law analogous to the Gutenberg-Richter relation, and the long-term release of geodetic moment is governed by a relationship consistent with the Omori law. Due to the effects of magma supply, the timing of secondary dike intrusions differs from that of the aftershocks. This work provides evidence of self-similarity in the rifting process. PMID:24469260

Along-axis steps in Ethiopian rift Bouguer gravity anomalies: Implications for crustal thinning and melt emplacement prior to breakup

NASA Astrophysics Data System (ADS)

Ebinger, C. J.; Tiberi, C.; Fowler, M. R.; Hunegnaw, A.

2001-12-01

The southern Afar depression, Africa, is virtually the only area worldwide where the transition from continental rifting to seafloor spreading is exposed onshore. During mid-Miocene to Pleistocene time the rift valley was segmented along its length by long normal faults; since Pleistocene time, faulting and magmatism have jumped to a narrow ca. 60 km-long volcanic mound marked by small faults. These magmatic segments are structurally similar to slow-spreading mid-ocean ridges, yet the rift is floored by continental crust. As part of the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE), we examine new and existing Bouguer gravity anomaly data from the rift to study the modification of the lithosphere by extensional and magmatic processes. New and existing Bouguer gravity anomaly data also show an along-axis segmentation of elongate relative positive anomalies that coincide with the magmatic segments. These anomalies are superposed on a regionally eastward increasing field as one approaches true seafloor spreading in the Gulf of Aden, and crustal thickness decreases. Quite remarkably, the magmatic segment boundaries, where data coverage is good, are marked by 15-25 mGal steps. The amplitude of the along-axis steps, as well as their across-axis characteristics, indicate that magmatic intrusion and ca. 2 km relief at the crust-mantle interface contribute to the steps. We use inverse and forward models of gravity data constrained by existing seismic and petrological data to evaluate models for the along-axis steps. EAGLE seismic data will be acquired across and along the magmatic segments to improve our understanding of breakup processes.

Kinematic analysis of fractures in the Great Rift, Idaho: Implications for subsurface dike geometry, crustal extension, and magma dynamics

NASA Astrophysics Data System (ADS)

Holmes, Adrian A. J.; Rodgers, David W.; Hughes, Scott S.

2008-04-01

Extension across the southern Great Rift of the Eastern Snake River Plain (ESRP), Idaho, was measured to calculate the dimensions of underlying dikes and interpret magmatic and extensional processes. Cumulative rift-perpendicular extension ranges from 0.64 to 4.50 m along the 14 km long Kings Bowl segment, from 1.33 to 4.41 m along the 14 km long New Butte segment, and from 0.74 to 1.57 m along the 4 km long Minidoka segment. Along strike of each segment, extension increases toward coeval vents. Each rift segment is interpreted to be underlain by a subsurface dike, whose dimensions are calculated using buoyancy equilibrium and boundary element models. Dikes are calculated to have tops that are 950-530 m deep, bottoms that are 23-31 km deep, and widths that taper to zero from a maximum of 2-21 m. Modeling suggests that the Kings Bowl dike has a maximum probable width of ˜8 m and a volume of ˜2 km3, about 400 times the volume of its coeval lava flow. Dike widths and ages at the southern Great Rift provide evidence for a Holocene ESRP strain rate of about 1 to 3 × 10-16 s-1, which is as much as an order of magnitude slower than strain rates in the adjacent, seismically active Basin and Range province. Eruptive fissures are present where rift width is <1650 m. This corresponds to a depth to dike top of <700 m, which we propose was the depth where vesiculation initiated, thus increasing magma pressure and inducing eruption.

Rift Valley fever virus incorporates the 78kDa glycoprotein into virions matured in C6/36 2 mosquito cells

USDA-ARS?s Scientific Manuscript database

Rift Valley fever virus (RVFV), genus Phlebovirus, family Bunyaviridae is a zoonotic arthropod-borne virus able to transition between distant host species, causing potentially severe disease in humans and ruminants. Viral proteins are encoded by three genomic segments, with the medium M segment codi...

Magma-poor vs. magma-rich continental rifting and breakup in the Labrador Sea

NASA Astrophysics Data System (ADS)

Gouiza, M.; Paton, D.

2017-12-01

Magma-poor and magma-rich rifted margins show distinct structural and stratigraphic geometries during the rift to breakup period. In magma-poor margins, crustal stretching is accommodated mainly by brittle faulting and the formation of wide rift basins shaped by numerous graben and half-graben structures. Continental breakup and oceanic crust accretion are often preceded by a localised phase of (hyper-) extension where the upper mantle is embrittled, serpentinized, and exhumed to the surface. In magma-rich margins, the rift basin is narrow and extension is accompanied by a large magmatic supply. Continental breakup and oceanic crust accretion is preceded by the emplacement of a thick volcanic crust juxtaposing and underplating a moderately thinned continental crust. Both magma-poor and magma-rich rifting occur in response to lithospheric extension but the driving forces and processes are believed to be different. In the former extension is assumed to be driven by plate boundary forces, while in the latter extension is supposed to be controlled by sublithospheric mantle dynamics. However, this view fails in explaining observations from many Atlantic conjugate margins where magma-poor and magma-rich segments alternate in a relatively abrupt fashion. This is the case of the Labrador margin where the northern segment shows major magmatic supply during most of the syn-rift phase which culminate in the emplacement of a thick volcanic crust in the transitional domain along with high density bodies underplating the thinned continental crust; while the southern segment is characterized mainly by brittle extension, mantle seprentinization and exhumation prior to continental breakup. In this work, we use seismic and potential field data to describe the crustal and structural architectures of the Labrador margin, and investigate the tectonic and mechanical processes of rifting that may have controlled the magmatic supply in the different segments of the margin.

Analysis of the Junction of the East African Rift and the Cretaceous-Paleogene Rifts in Northern Kenya and Southern Ethiopia

NASA Astrophysics Data System (ADS)

Mariita, N. O.; Tadesse, K.; Keller, G. R.

2003-12-01

The East African rift (EAR) is a Tertiary-Miocene system that extends from the Middle East, through East Africa, to Mozambique in southern Africa. Much of the present information is from the Ethiopian and Kenyan parts of the rift. Several characteristics of the EAR such as rift-related volcanism, faulting and topographic relief being exposed make it attractive for studying continental rift processes. Structural complexities reflected in the geometries of grabens and half-grabens, the existence of transverse fault zones and accommodation zones, and the influence of pre-existing geologic structures have been documented. In particular, the EAR traverses the Anza graben and related structures near the Kenya/Ethiopian border. The Anza graben is one in a series of Cretaceous-Paleogene failed rifts that trend across Central Africa from Nigeria through Chad to Sudan and Kenya with an overall northwest-southeast trend. In spite of a number of recent studies, we do not understand the interaction of these two rift systems. In both Ethiopia and Kenya, the rift segments share some broad similarities in timing and are related in a geographic sense. For example, volcanism appears to have generally preceded or in some cases have been contemporaneous with major rift faulting. Although, these segments are distinct entities, each with its own tectonic and magmatic evolution, and they do connect in the region crossed by the Anza graben and related structures. In our present study, we are using a combination of recently collected seismic, gravity and remote sensing data to increase our understanding of these two segments of the EAR. We hope that by analysing the satellite data, the variety and differences in the volume of magmatic products extruded along in southern Ethiopia and northern Kenya will be identified. The geometry of structures (in particular, those causing the gravity axial high) will be modelled to study the impact of the older Anza graben structural trends with the younger EAR. For example there is significant crustal thinning in the Lake Turkana area of the northern Kenya segment of the EAR system. In regard to the recent EAGLE experiment in Ethiopia, we are ivestigating if the transition from relatively thick crust (~40 km) to thinned, rifted crust is as abrupt in Ethiopia as it is in Kenya.

Structural inheritance versus magmatic weakening: What controls the style of deformation at rift segment boundaries in the Gulf of California, Mexico?

NASA Astrophysics Data System (ADS)

Seiler, Christian; Gleadow, Andrew; Kohn, Barry

2013-04-01

Rifts are commonly segmented into several hundred kilometre long zones of opposing upper-plate transport direction with boundaries defined by accommodation and transfer zones. A number of such rift segments have been recognized in the Gulf of California, a youthful oceanic basin that is currently undergoing the rift-drift transition. However, detailed field studies have so far failed to identify suitable structures that could accommodate the obvious deformation gradients between different rift segments, and the nature of strain transfer at segment boundaries remains enigmatic. The Bocana transfer zone (BTZ) in central Baja California is a linear, WNW striking structural discontinuity separating two rift segments with different magnitudes and styles of extensional deformation. North of the BTZ, the Libertad fault is part of the Main Gulf Escarpment, which represents the breakaway fault that separates the Gulf of California rift to the east from the relatively stable western portion of the Baja peninsula. The N-striking Libertad escarpment developed during the Late Miocene (~10-8Ma) and exhibits a topographic relief of ca. 1,000m along a strike-length of ca. 50km. Finite displacement decreases from ~1000m in the central fault segment to ~500m further south, where the fault bends SE and merges with the BTZ. In the hanging wall of the Libertad fault, a series of W-tilted horsts are bound along their eastern margins by two moderate-displacement E-dipping normal faults. South of the BTZ, extension was much less than further north, which explains the comparatively subdued relief and generally shallower tilt of pre-rift strata in this area. The BTZ itself is characterized by two en echelon WNW-ESE striking dextral-oblique transfer faults with a significant down-to-the-NNE extensional component. Strain is transferred from the Libertad breakaway fault onto the transfer faults over a distance of >20km through a network of interacting normal, oblique and strike-slip faults. The shape, location and orientation of the main faults were strongly influenced by pre-existing rheological heterogeneities. Major normal faults are parallel to either the Mesozoic metamorphic foliation or Cretaceous intrusive contacts, and developed where the foliation was at a high angle to the extension direction. In contrast, the oblique-slip faults of the BTZ formed parallel to the metamorphic foliation where formlines are at a small angle to the regional extension direction. Compared to the BTZ, deformation in other known accommodation zones of the Gulf of California rift occurred distributed across a much wider zone, and appropriate transfer faults are either lacking or minor. In these cases, however, the accommodation zones coincide with the locations of significant pre- and synrift volcanism, suggesting that thermal weakening associated with magmatic activity may have promoted the distribution of strain across a wider region instead of localising it into discrete transfer faults.

Modeling the Sedimentary Infill of Lakes in the East African Rift: A Case Study of Multiple versus Single Rift Basin Segments

NASA Astrophysics Data System (ADS)

Zhang, C.; Scholz, C. A.

2016-12-01

The sedimentary basins in the East African Rift are considered excellent modern examples for investigating sedimentary infilling and evolution of extensional systems. Some lakes in the western branch of the rift have formed within single-segment systems, and include Lake Albert and Lake Edward. The largest and oldest lakes developed within multi-segment systems, and these include Lake Tanganyika and Lake Malawi. This research aims to explore processes of erosion and sedimentary infilling of the catchment area in single-segment rift (SSR) and multi-segment rift (MSR) systems. We consider different conditions of regional precipitation and evaporation, and assess the resulting facies architecture through forward modeling, using state-of-the-art commercial basin modeling software. Dionisos is a three-dimensional numerical stratigraphic forward modeling software program, which simulates basin-scale sediment transport based on empirical water- and gravity-driven diffusion equations. It was classically used to quantify the sedimentary architecture and basin infilling of both marine siliciclastic and carbonate environments. However, we apply this approach to continental rift basin environments. In this research, two scenarios are developed, one for a MSR and the other for a SSR. The modeled systems simulate the ratio of drainage area and lake surface area observed in modern Lake Tanganyika and Lake Albert, which are examples of MSRs and SSRs, respectively. The main parameters, such as maximum subsidence rate, water- and gravity-driven diffusion coefficients, rainfall, and evaporation, are approximated using these real-world examples. The results of 5 million year model runs with 50,000 year time steps show that MSRs are characterized by a deep water lake with relatively modest sediment accumulation, while the SSRs are characterized by a nearly overfilled lake with shallow water depths and thick sediment accumulation. The preliminary modeling results conform to the features of sedimentary infills revealed by seismic reflection data acquired in Lake Tanganyika and Lake Albert. Future models will refine the parameters of rainfall and evaporation in these two scenarios to better evaluate detailed basin facies architecture.

Comparative sequence stratigraphy of low-latitude versus high-latitude lacustrine rift basins: Seismic data examples from the East African and Baikal rifts

USGS Publications Warehouse

Scholz, C.A.; Moore, T.C.; Hutchinson, D.R.; Golmshtok, A. Ja; Klitgord, Kim D.; Kurotchkin, A.G.

1998-01-01

Lakes Baikal, Malawi and Tanganyika are the world's three largest rift valley lakes and are the classic modem examples of lacustrine rift basins. All the rift lakes are segmented into half-graben basins, and seismic reflection datasets reveal how this segmentation controls the filling of the rift basins through time. In the early stages of rifting, basins are fed primarily by flexural margin and axial margin drainage systems. At the climax of syn-rift sedimentation, however, when the basins are deeply subsided, almost all the margins are walled off by rift shoulder uplifts, and sediment flux into the basins is concentrated at accommodation zone and axial margin river deltas. Flexural margin unconformities are commonplace in the tropical lakes but less so in high-latitude Lake Baikal. Lake levels are extremely dynamic in the tropical lakes and in low-latitude systems in general because of the predominance of evaporation in the hydrologic cycle in those systems. Evaporation is minimized in relation to inflow in the high-latitude Lake Baikal and in most high-latitude systems, and consequently, major sequence boundaries tend to be tectonically controlled in that type of system. The acoustic stratigraphies of the tropical lakes are dominated by high-frequency and high-amplitude lake level shifts, whereas in high-latitude Lake Baikal, stratigraphic cycles are dominated by tectonism and sediment-supply variations.

The origin and evolution of silicic magmas during continental rifting: new constraints from trace elements and oxygen isotopes from Ethiopian volcanoes

NASA Astrophysics Data System (ADS)

Hutchison, W.; Boyce, A.; Mather, T. A.; Pyle, D. M.; Yirgu, G.; Gleeson, M. L.

2017-12-01

The petrologic diversity of rift magmas is generated by two key processes: interaction with the crust via partial melting or assimilation; and closed-system fractional crystallization of the parental magma. It is not yet known whether these two petrogenetic processes vary spatially between different rift settings, and whether there are any significant secular variations during rift evolution. The Ethiopian Rift is the ideal setting to test these hypotheses because it captures the transition from continental rifting to sea-floor spreading and has witnessed the eruption of large volumes of mafic and silicic volcanic rocks since 30 Ma. We use new oxygen isotope (δ18O) and trace element data to fingerprint fractional crystallisation and partial crustal melting processes in Ethiopia and evaluate spatial variations between three active rift segments. δ18O measurements are used to examine partial crustal melting processes. We find that most δ18O data from basalts to rhyolites fall within the bounds of modelled fractional crystallization trajectories (i.e., 5.5-6.5 ‰). Few samples deviate from this trend, emphasising that fractional crystallization is the dominant petrogenetic processes and that little fusible Precambrian crustal material (δ18O of 7-18 ‰) remain to be assimilated beneath the magmatic segments. Trace element systematics (e.g., Ba, Sr, Rb, Th and Zr) further underscore the dominant role of fractional crystallization but also reveal important variations in the degree of melt evolution between the volcanic systems. We find that the most evolved silicic magmas, i.e., those with greatest peralkalinity (molar Na2O+K2O>Al2O3), are promoted in regions of lowest magma flux off-axis and along rift. Our findings provide new information on the nature of the crust beneath Ethiopia's active magmatic segments and also have relevance for understanding ancient rift zones and the geotectonic settings that promote genesis of economically-valuable mineral deposits.

Role of the Precambrian Mughese Shear Zone on Cenozoic faulting along the Rukwa-Malawi Rift segment of the East African Rift System

NASA Astrophysics Data System (ADS)

Heilman, E.; Kolawole, F.; Mayle, M.; Atekwana, E. A.; Abdelsalam, M. G.

2017-12-01

We address the longstanding question of the role of long-lived basement structures in strain accommodation within active rift systems. Studies have highlighted the influence of pre-existing zones of lithospheric weakness in modulating faulting and fault kinematics. Here, we investigate the role of the Neoproterozoic Mughese Shear Zone (MSZ) in Cenozoic rifting along the Rukwa-Malawi rift segment of the East African Rift System (EARS). Detailed analyses of Shuttle Radar Topography Mission (SRTM) DEM and filtered aeromagnetic data allowed us to determine the relationship between rift-related basement-rooted normal faults and the MSZ fabric extending along the southern boundary of the Rukwa-Malawi Rift North Basin. Our results show that the magnetic lineaments defining the MSZ coincide with the collinear Rukwa Rift border fault (Ufipa Fault), a dextral strike-slip fault (Mughese Fault), and the North Basin hinge-zone fault (Mbiri Fault). Fault-scarp and minimum fault-throw analyses reveal that within the Rukwa Rift, the Ufipa Border Fault has been accommodating significant displacement relative to the Lupa Border Fault, which represents the northeastern border fault of the Rukwa Rift. Our analysis also shows that within the North Basin half-graben, the Mbiri Fault has accommodated the most vertical displacement relative to other faults along the half-graben hinge zone. We propose that the Cenozoic reactivation along the MSZ facilitated significant normal slip displacement along the Ufipa Border Fault and the Mbiri Fault, and minor dextral strike-slip between the two faults. We suggest that the fault kinematics along the Rukwa-Malawi Rift is the result of reactivation of the MSZ through regional oblique extension.

Seismicity and magmatic processes in the Rwenzori region of the Albertine Rift.

NASA Astrophysics Data System (ADS)

Lindenfeld, Michael; Rümpker, Georg; Kasereka, Celestin M.; Batte, Arthur; Schumann, Andreas

2013-04-01

In this presentation we summarize results from two extensive seismic field studies with temporary station networks in the Rwenzori region of the Albertine rift, located at the border between Uganda and the Democratic Republic of Congo. The first network was running from February 2006 to September 2007. It consisted of 27 seismic stations which were deployed in the Ugandan part of the area. A second network of 33 stations was operated between October 2009 and October 2011. It traversed the whole rift segment from the eastern rift shoulder in Uganda to the western shoulder in the D.R. Congo, covering the whole Rwenzori region. The data analysis revealed a pronounced local earthquake activity in this area with an average rate of more than 800 events per month and proves that this segment of the Albertine Rift belongs to the seismically most active regions of the whole East African Rift System. The earthquake distribution is highly heterogeneous. The highest activity is observed in the northeastern part of the Rwenzori area. Here, the mountains are connected to the eastern rift shoulder whereas they are surrounded by rift segments elsewhere. We were able to locate seismicity bursts with more than 300 events per day. The depth extent of seismicity ranges from 20 to 39 km and correlates well with Moho depths that were derived from teleseismic receiver functions. The majority of the derived fault plane solutions exhibit normal faulting with WNW-ESE oriented T-axes, which is perpendicular to the rift axis and in good agreement with kinematic rift models. The area of highest seismic activity is also characterized by the existence of several vertical elongated earthquake clusters in the crust. From petrological considerations we presume that these events are triggered by fluids and gases which originate from a magmatic source below the crust. The existence of a magmatic source within the lithosphere is supported by the detection of mantle earthquakes at about 40 - 60 km depth below the cluster area. We interpret these observations as an indication of deep magmatic infiltration processes that play a significant role in rift formation and that may eventually lead to the complete detachment of the Rwenzori block from the surrounding rift flanks.

Continental rift evolution: From rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa

NASA Astrophysics Data System (ADS)

Corti, Giacomo

2009-09-01

The Main Ethiopian Rift is a key sector of the East African Rift System that connects the Afar depression, at Red Sea-Gulf of Aden junction, with the Turkana depression and Kenya Rift to the South. It is a magmatic rift that records all the different stages of rift evolution from rift initiation to break-up and incipient oceanic spreading: it is thus an ideal place to analyse the evolution of continental extension, the rupture of lithospheric plates and the dynamics by which distributed continental deformation is progressively focused at oceanic spreading centres. The first tectono-magmatic event related to the Tertiary rifting was the eruption of voluminous flood basalts that apparently occurred in a rather short time interval at around 30 Ma; strong plateau uplift, which resulted in the development of the Ethiopian and Somalian plateaus now surrounding the rift valley, has been suggested to have initiated contemporaneously or shortly after the extensive flood-basalt volcanism, although its exact timing remains controversial. Voluminous volcanism and uplift started prior to the main rifting phases, suggesting a mantle plume influence on the Tertiary deformation in East Africa. Different plume hypothesis have been suggested, with recent models indicating the existence of deep superplume originating at the core-mantle boundary beneath southern Africa, rising in a north-northeastward direction toward eastern Africa, and feeding multiple plume stems in the upper mantle. However, the existence of this whole-mantle feature and its possible connection with Tertiary rifting are highly debated. The main rifting phases started diachronously along the MER in the Mio-Pliocene; rift propagation was not a smooth process but rather a process with punctuated episodes of extension and relative quiescence. Rift location was most probably controlled by the reactivation of a lithospheric-scale pre-Cambrian weakness; the orientation of this weakness (roughly NE-SW) and the Late Pliocene (post 3.2 Ma)-recent extensional stress field generated by relative motion between Nubia and Somalia plates (roughly ESE-WNW) suggest that oblique rifting conditions have controlled rift evolution. However, it is still unclear if these kinematical boundary conditions have remained steady since the initial stages of rifting or the kinematics has changed during the Late Pliocene or at the Pliocene-Pleistocene boundary. Analysis of geological-geophysical data suggests that continental rifting in the MER evolved in two different phases. An early (Mio-Pliocene) continental rifting stage was characterised by displacement along large boundary faults, subsidence of rift depression with local development of deep (up to 5 km) asymmetric basins and diffuse magmatic activity. In this initial phase, magmatism encompassed the whole rift, with volcanic activity affecting the rift depression, the major boundary faults and limited portions of the rift shoulders (off-axis volcanism). Progressive extension led to the second (Pleistocene) rifting stage, characterised by a riftward narrowing of the volcano-tectonic activity. In this phase, the main boundary faults were deactivated and extensional deformation was accommodated by dense swarms of faults (Wonji segments) in the thinned rift depression. The progressive thinning of the continental lithosphere under constant, prolonged oblique rifting conditions controlled this migration of deformation, possibly in tandem with the weakening related to magmatic processes and/or a change in rift kinematics. Owing to the oblique rifting conditions, the fault swarms obliquely cut the rift floor and were characterised by a typical right-stepping arrangement. Ascending magmas were focused by the Wonji segments, with eruption of magmas at surface preferentially occurring along the oblique faults. As soon as the volcano-tectonic activity was localised within Wonji segments, a strong feedback between deformation and magmatism developed: the thinned lithosphere was strongly modified by the extensive magma intrusion and extension was facilitated and accommodated by a combination of magmatic intrusion, dyking and faulting. In these conditions, focused melt intrusion allows the rupture of the thick continental lithosphere and the magmatic segments act as incipient slow-spreading mid-ocean spreading centres sandwiched by continental lithosphere. Overall the above-described evolution of the MER (at least in its northernmost sector) documents a transition from fault-dominated rift morphology in the early stages of extension toward magma-assisted rifting during the final stages of continental break-up. A strong increase in coupling between deformation and magmatism with extension is documented, with magma intrusion and dyking playing a larger role than faulting in strain accommodation as rifting progresses to seafloor spreading.

Geochemical signals of progressive continental rupture in the Main Ethiopian Rift

NASA Astrophysics Data System (ADS)

Furman, T.; Bryce, J.; Yirgu, G.; Ayalew, D.; Cooper, L.

2003-04-01

Mafic volcanics of the Main Ethiopian Rift record the development of magmatic rift segments during continental extension. The Ethiopian Rift is one arm of a triple junction that formed above a Paleogene mantle plume, concurrent with eruption of flood basalts ca. 30 Ma across northern Ethiopian and Yemen. The geochemistry of Ethiopian Rift lavas thus provides insight into processes associated with the shift from mechanical (lithospheric) to magmatic (asthenospheric) segmentation in the transitional phase of continental rifting. Quaternary basalts from five volcanic centers representing three magmatic segments display along-axis geochemical variations that likely reflect the degree of rifting and magma supply, which increase abruptly with proximity to the highly-extended Afar region. To first order, the geochemical data indicate a decreasing degree of shallow-level fractionation and greater involvement of depleted or plume-like mantle source materials in basalts sampled closer to the Afar. These spatially controlled geochemical signatures observed in contemporaneous basalts are similar to temporal variations documented in southern Ethiopia, where Quaternary lavas indicate a greater degree of crustal extension than those erupted at the onset of plume activity. Primitive Ethiopian Rift basalts have geochemical signatures (e.g., Ce/Pb, La/Nb, Ba/Nb, Ba/Rb, U/Th) that overlap ocean island basalt compositions, suggesting involvement of sub-lithospheric source materials. The estimated depth of melting (65-75 km) is shallower than values obtained for young primitive mafic lavas from the Western Rift and southern Kenya as well as Oligocene Ethiopian flood basalts from the onset of plume-driven activity. Basalts from the Turkana region (N. Kenya) and Erta 'Ale (Danakil depression) reflect melting at shallower levels, corresponding to the greater degree of crustal extension in these provinces. Preliminary Sr and Nd isotopic data trend towards primitive earth values, consistent with values observed previously in central Ethiopia that are associated with moderately high 3He/4He values (<19 RA; Marty et al. 1996) and interpreted as reflecting involvement of a mantle plume. Taken together, these data support a model in which upwelling plume material sampled in central Ethiopia incorporates depleted mantle during ascent beneath the more highly extended portions of the African Rift.

Silicic central volcanoes as precursors to rift propagation: the Afar case

NASA Astrophysics Data System (ADS)

Lahitte, Pierre; Gillot, Pierre-Yves; Courtillot, Vincent

2003-02-01

The Afar depression is a triple junction characterised by thinned continental crust, where three rift systems meet (Red Sea, Gulf of Aden and East African Rift). About 100 recent K-Ar ages obtained on Plio-Pleistocene lavas [Lahitte et al., J. Geophys. Res. (2002) in press; Kidane et al., J. Geophys. Res. (2002) in press], complemented by new geomorphological interpretations, allow better understanding of the volcano-tectonic activity linked to rift propagation. In Central Afar, a significant spatial and temporal correlation is observed between the occurrence of silicic central volcanoes and the initiation of the successive phases of on-land propagation of the Red Sea and Aden rifts. Inside the Afar depression, at the scale of both a whole ridge and a small rift segment, silicic lavas are systematically erupted close to the location of a future rift segment and prior to the main extensive phase associated with fissural basaltic activity. Central silicic volcanoes therefore appear to be precursor features, and their locations underline the preferred direction of future rift propagation. Evolved volcanoes (and associated magma chambers) form zones of localised lithospheric weakness, which concentrate stress and guide the development of fractures in which fissural magmatism is next emplaced. Differentiated silicic lavas are erupted first. Then, as extension increases, basaltic magma directly erupts to the surface. This composite style of rifting, with volcanic and tectonic components, is a scaled-down equivalent of the continental break-up process at the largest scale.

Controls on Early-Rift Geometry: New Perspectives From the Bilila-Mtakataka Fault, Malawi

NASA Astrophysics Data System (ADS)

Hodge, M.; Fagereng, Å.; Biggs, J.; Mdala, H.

2018-05-01

We use the ˜110-km long Bilila-Mtakataka fault in the amagmatic southern East African Rift, Malawi, to investigate the controls on early-rift geometry at the scale of a major border fault. Morphological variations along the 14 ± 8-m high scarp define six 10- to 40-km long segments, which are either foliation parallel or oblique to both foliation and the current regional extension direction. As the scarp is neither consistently parallel to foliation nor well oriented for the current regional extension direction, we suggest that the segmented surface expression is related to the local reactivation of well-oriented weak shallow fabrics above a broadly continuous structure at depth. Using a geometrical model, the geometry of the best fitting subsurface structure is consistent with the local strain field from recent seismicity. In conclusion, within this early-rift, preexisting weaknesses only locally control border fault geometry at subsurface.

Plume-induced continental break-up from Red Sea to Lake Malawi: 3D numerical models of the East African Rift System

NASA Astrophysics Data System (ADS)

Koptev, Alexander; Burov, Evgueni; Calais, Eric; Leroy, Sylvie; Gerya, Taras; Cloetingh, Sierd; Guillou-Frottier, Laurent

2017-04-01

We use numerical thermo-mechanical experiments in order to analyze the role of active mantle plume, far-field tectonic stresses and pre-existing lithospheric heterogeneities in structural development of the East African Rift system (EARS). It is commonly assumed that the Cenozoic rifts have avoided the cratons and follow the mobile belts which serve as the weakest pathways within the non-uniform material structured during pre-rift stages. Structural control of the pre-existing heterogeneities within the Proterozoic belts at the scale of individual faults or rifts has been demonstrated as well. However, the results of our numerical experiments show that the formation of two rift zones on opposite sides of a thick lithosphere segment can be explained without appealing to pre-imposed heterogeneities at the crustal level. These models have provided a unified physical framework to understand the development of the Eastern branch, the Western branch and its southern prolongation by the Malawi rift around thicker lithosphere of the Tanzanian and Bangweulu cratons as a result of the interaction between pre-stressed continental lithosphere and single mantle plume anomaly corresponding to the Kenyan plume. The second series of experiments has been designed in order to investigate northern segment of the EARS where Afro-Arabian plate separation is supposed to be related with the impact of Afar mantle plume. We demonstrate that whereas relatively simple linear rift structures are preferred in case of uni-directional extension, more complex rifting patterns combining one or several ridge-ridge-ridge triple junctions can form in response to bi-directional extensional far-field stresses. In particular, our models suggest that Afar triple junction represents an end-member mode of plume-induced bi-directional rifting combining asymmetrical northward traction and symmetrical EW extension of similar magnitudes. The presence of pre-existing linear weak zones appears to be not mandatory for deformation localization ultimately leading to present configuration of the Afar triple junction. Finally, for laterally extended experiments we have used parameters of the best-fit models for the southern and northern segments of the EARS in order to define the position of Kenyan plume and the velocity boundary conditions. These models cover all rifting and spreading structures associated with both Afar and Kenyan plumes: Red Sea Rift and the Aden Ridge to the north of the Afar Triple Junction; Main Ethiopian Rift running to the south that continues as the Kenyan Rift; Western Rift and its southern prolongation corresponding to Malawi rift. We argue that all these basic features associated with Cenozoic rifting in the EARS can be reproduced in a relatively simple context of the interaction between two mantle anomalies corresponding to Afar and Kenyan plumes and pre-stressed rheologically stratified continental lithosphere containing only first-order structural heterogeneities (such as Tanzanian and Bangweulu cratons).

Geochemistry of lavas from the Australian-Antarctic Ridge, easternmost Southeast Indian Ridge

NASA Astrophysics Data System (ADS)

Park, S.; Langmuir, C. H.; Lin, J.; Kim, S.; Hahm, D.; Michael, P. J.; Baker, E. T.

2012-12-01

The intermediate spreading Australian-Antarctic Ridge (AAR), an easternmost extension of the South East Indian Ridge located in the south of Tasmania, is one of the largest unexplored regions of the global mid-ocean ridge system, owing to its remote location and a very limited workable weather window. In early and late 2011, the Korea Polar Research Institute (KOPRI) conducted two surveys of two segments at 160°E (KR1) and 152.5°E (KR2) using the icebreaker Araon, producing a multi-beam map, 48 rock core samples and a MAPR (Miniature Autonomous Plume Recorder) hydrothermal survey. The full spreading rate of the spreading center in this area is 68 mm/yr. The axial depth of KR1 is relatively shallow (~2,000m) and is a first-order segment bounded by two large offset transform faults. The axial morphology of KR1 varies substantially from an axial high plateau (Segment 1) in the west, to a small rift valley (Segment 2), to an axial high with graben (Segment 3), and to a substantial rift valley (Segment 4) in the east. These changes occur in the absence of marked offsets in the ridge, such as overlapping spreading centers. Even so, these segments can be divided still further into shorter scale segments based on small discontinuities in the linearity of the axis and variations in rock chemistry. Small offsets in bathymetry can be associated with large chemical changes, such as between Segments 2 and 3, where incompatible element abundances change by almost a factor of ten. Incompatible trace element ratios for basalts show a regular pattern that is nonetheless not a single gradient. Along Segments 1 and 2, an axial high changes to a modest rift, (La/Sm)N of basalts decreases from 0.9 to 0.5. Then there is an abrupt step in enrichment to (La/Sm)N of 1.5, associated with a shallower depths and the appearance of an off-axis seamount south of the axis. This enrichment persists eastwards and then declines progressively to values of (La/Sm)N of 0.7 in the pronounced rift valley of Segment 4. Plume signals indicating hydrothermal vents were found in the middle of KR1 where the most enriched basalts occur and the magma supply appears robust. The first- order segment KR2 can be divided into two segments -- an axial high western segment, and a rift valley eastern segment. Hydrothermal vent signals were mainly found in the western part of the segment. The KR2 samples are mostly depleted, but KR2 also contains enriched basalts, including an E-MORB with 0.65% K2O in the western segment. Enriched KR2 basalts have different ratios of alkalis to HFSE compared to KR1, suggesting they are not derived from the same enriched component. In general in this region, inflated axial morphology is associated with trace element enrichment, suggesting that magma flux is being influenced by changing mantle composition on the segment scale.

Continental break-up history of a deep magma-poor margin based on seismic reflection data (northeastern Gulf of Aden margin, offshore Oman)

NASA Astrophysics Data System (ADS)

Autin, Julia; Leroy, Sylvie; Beslier, Marie-Odile; d'Acremont, Elia; Razin, Philippe; Ribodetti, Alessandra; Bellahsen, Nicolas; Robin, Cécile; Al Toubi, Khalfan

2010-02-01

Rifting between Arabia and Somalia started around 35 Ma followed by spreading at 17.6 Ma in the eastern part of the Gulf of Aden. The first-order segment between Alula-Fartak and Socotra-Hadbeen fracture zones is divided into three second-order segments with different structure and morphology. Seismic reflection data were collected during the Encens Cruise in 2006 on the northeastern margin. In this study, we present the results of Pre-Stack Depth Migration of the multichannel seismic data from the western segment, which allows us to propose a tectono-stratigraphic model of the evolution of this segment of the margin from rifting to the present day. The chronological interpretation of the sedimentary sequences is mapped out within relation to the onshore observations and existing dating. After a major development of syn-rift grabens and horsts, the deformation localized where the crust is the thinnest. This deformation occurred in the distal margin graben (DIM) at the northern boundary of the ocean-continent transition (OCT) represented by the OCT ridge. At the onset of the OCT formation differential uplift induced a submarine landslide on top of the deepest tilted block and the crustal deformation was restricted to the southern part of the DIM graben, where the continental break-up finally occurred. Initial seafloor spreading was followed by post-rift magmatic events (flows, sills and volcano-sedimentary wedge), whose timing is constrained by the analysis of the sedimentary cover of the OCT ridge, correlated with onshore stratigraphy. The OCT ridge may represent exhumed serpentinized mantle intruded by post-rift magmatic material, which modified the OCT after its emplacement.

Integrated Numerical Model for the East African Rift System: Plume-induced Rifting and Continental Break-up from Lake Malawi to Red Sea

NASA Astrophysics Data System (ADS)

Koptev, A.; Leroy, S. D.; Calais, E.; Gerya, T.

2016-12-01

We present numerical experiments that target to reveal the role of active mantle plume, far-field tectonic forces and pre-existing lithospheric heterogeneities in structural development of the East African Rift system (EARS). Starting with models capturing the essential geophysical features of the central and southern parts of the EARS (two «cratonic» bodies (Tanzanian craton and Bangweulu block) embedded into a «normal» surrounding lithosphere) we show that development of the magmatic Eastern branch, the amagmatic Western branch and its southern prolongation (Malawi rift) can be the result of non-uniform splitting of some hot plume material that has been initially seeded underneath the southern part of Tanzanian craton. The second series of experiments has been designed in order to investigate northern segment of the EARS where Afro-Arabian plate separation is supposed to be related with the impact of Afar mantle plume. These models permit us to reproduce observed orientation and relative position of two spreading axes (Red Sea, Gulf of Aden) and rifting (Main Ethiopian rift) one. All are joining at Afar triple junction. Finally, for laterally extended experiment we have used parameters of the best-fit models for the southern and northern segments of the EARS in order to define the position of Kenyan plume and the velocity boundary conditions. This model cover all rifting and spreading structure associated with both Afar and Kenyan plumes: Red Sea Rift and the Aden Ridge to the north of the Afar Triple Junction; Main Ethiopian Rift running to the south that continues as the Kenyan Rift; Western Rift and its southern prolongation corresponding to Malawi rift.We argue that main features of the EARS can be reproduced in a relatively simple context of the interaction between two mantle anomalies corresponding to Afar and Kenyan plumes and pre-stressed rheologically stratified continental lithosphere containing only first-order structural heterogeneities (such as Tanzanian and Bangweulu cratons).

Structural architecture and tectonic evolution of the Maghara inverted basin, Northern Sinai, Egypt

NASA Astrophysics Data System (ADS)

Moustafa, Adel R.

2014-05-01

Large NE-SW oriented asymmetric inversion anticlines bounded on their southeastern sides by reverse faults affect the exposed Mesozoic and Cenozoic sedimentary rocks of the Maghara area (northern Sinai). Seismic data indicate an earlier Jurassic rifting phase and surface structures indicate Late Cretaceous-Early Tertiary inversion phase. The geometry of the early extensional fault system clearly affected the sense of slip of the inverted faults and the geometry of the inversion anticlines. Rift-parallel fault segments were reactivated by reverse slip whereas rift-oblique fault segments were reactivated as oblique-slip faults or lateral/oblique ramps. New syn-inversion faults include two short conjugate strike-slip sets dissecting the forelimbs of inversion anticlines and the inverted faults as well as a set of transverse normal faults dissecting the backlimbs. Small anticline-syncline fold pairs ornamenting the steep flanks of the inversion anticlines are located at the transfer zones between en echelon segments of the inverted faults.

Stress imparted by the great 2004 Sumatra earthquake shut down transforms and activated rifts up to 400 km away in the Andaman Sea

USGS Publications Warehouse

Sevilgen, Volkan; Stein, Ross S.; Pollitz, Fred F.

2012-01-01

The origin and prevalence of triggered seismicity and remote aftershocks are under debate. As a result, they have been excluded from probabilistic seismic hazard assessment and aftershock hazard notices. The 2004 M = 9.2 Sumatra earthquake altered seismicity in the Andaman backarc rift-transform system. Here we show that over a 300-km-long largely transform section of the backarc, M≥4.5 earthquakes stopped for five years, and over a 750-km-long backarc section, the rate of transform events dropped by two-thirds, while the rate of rift events increased eightfold. We compute the propagating dynamic stress wavefield and find the peak dynamic Coulomb stress is similar on the rifts and transforms. Long-period dynamic stress amplitudes, which are thought to promote dynamic failure, are higher on the transforms than on the rifts, opposite to the observations. In contrast to the dynamic stress, we calculate that the mainshock brought the transform segments approximately 0.2 bar (0.02 MPa) farther from static Coulomb failure and the rift segments approximately 0.2 bar closer to static failure, consistent with the seismic observations. This accord means that changes in seismicity rate are sufficiently predictable to be included in post-mainshock hazard evaluations.

Stress imparted by the great 2004 Sumatra earthquake shut down transforms and activated rifts up to 400 km away in the Andaman Sea

USGS Publications Warehouse

Sevilgen, Volkan; Stein, Ross S.; Pollitz, Fred F.

2012-01-01

The origin and prevalence of triggered seismicity and remote aftershocks are under debate. As a result, they have been excluded from probabilistic seismic hazard assessment and aftershock hazard notices. The 2004 M = 9.2 Sumatra earthquake altered seismicity in the Andaman backarc rift-transform system. Here we show that over a 300-km-long largely transform section of the backarc, M ≥ 4.5 earthquakes stopped for five years, and over a 750-km-long backarc section, the rate of transform events dropped by two-thirds, while the rate of rift events increased eightfold. We compute the propagating dynamic stress wavefield and find the peak dynamic Coulomb stress is similar on the rifts and transforms. Long-period dynamic stress amplitudes, which are thought to promote dynamic failure, are higher on the transforms than on the rifts, opposite to the observations. In contrast to the dynamic stress, we calculate that the mainshock brought the transform segments approximately 0.2 bar (0.02 MPa) farther from static Coulomb failure and the rift segments approximately 0.2 bar closer to static failure, consistent with the seismic observations. This accord means that changes in seismicity rate are sufficiently predictable to be included in post-mainshock hazard evaluations.

Structure of the central Terror Rift, western Ross Sea, Antarctica

USGS Publications Warehouse

Hall, Jerome; Wilson, Terry; Henrys, Stuart

2007-01-01

The Terror Rift is a zone of post-middle Miocene faulting and volcanism along the western margin of the West Antarctic Rift System. A new seismic data set from NSF geophysical cruise NBP04-01, integrated with the previous dataset to provide higher spatial resolution, has been interpreted in this study in order to improve understanding of the architecture and history of the Terror Rift. The Terror Rift contains two components, a structurally-controlled rollover anticlinal arch intruded by younger volcanic bodies and an associated synclinal basin. Offsets and trend changes in fault patterns have been identified, coincident with shifts in the location of depocenters that define rift sub-basins, indicating that the Terror Rift is segmented by transverse structures. Multiple phases of faulting all post-date 17 Ma, including faults cutting the seafloor surface, indicating Neogene rifting and possible modern activity.

Active intra-basin faulting in the Northern Basin of Lake Malawi from seismic reflection data

NASA Astrophysics Data System (ADS)

Shillington, D. J.; Chindandali, P. R. N.; Scholz, C. A.; Ebinger, C. J.; Onyango, E. A.; Peterson, K.; Gaherty, J. B.; Nyblade, A.; Accardo, N. J.; McCartney, T.; Oliva, S. J.; Kamihanda, G.; Ferdinand, R.; Salima, J.; Mruma, A. H.

2016-12-01

Many questions remain about the development and evolution of fault systems in weakly extended rifts, including the relative roles of border faults and intra-basin faults, and segmentation at various scales. The northern Lake Malawi (Nyasa) rift in the East African Rift System is an early stage rift exhibiting pronounced tectonic segmentation, which is defined by 100-km-long border faults. The basins also contain a series of intrabasinal faults and associated synrift sediments. The occurrence of the 2009 Karonga Earthquake Sequence on one of these intrabasinal faults indicates that some of them are active. Here we present new multichannel seismic reflection data from the Northern Basin of the Malawi Rift collected in 2015 as a part of the SEGMeNT (Study of Extension and maGmatism in Malawi aNd Tanzania) project. This rift basin is bound on its east side by the west-dipping Livingstone border fault. Over 650 km of seismic reflection profiles were acquired in the Northern Basin using a 500 to 1540 cu in air gun array and a 1200- to 1500-m seismic streamer. Dip lines image a series of north-south oriented west-dipping intra-basin faults and basement reflections up to 5 s twtt near the border fault. Cumulative offsets on intra-basin faults decrease to the west. The largest intra-basin fault has a vertical displacement of >2 s two-way travel time, indicating that it has accommodated significant total extension. Some of these intra-basin faults offset the lake bottom and the youngest sediments by up to 50 s twtt ( 37 m), demonstrating they are still active. The two largest intra-basin faults exhibit the largest offsets of young sediments and also correspond to the area of highest seismicity based on analysis of seismic data from the 89-station SEGMeNT onshore/offshore network (see Peterson et al, this session). Fault patterns in MCS profiles vary along the basin, suggesting a smaller scale of segmentation of faults within the basin; these variations in fault patterns appear to correlate with variations in the distribution of aftershocks from the 2009 and 2014 Karonga earthquakes and in background seismicity beneath the lake, providing new constraints on length-displacement scaling for predictive models and earthquake hazards.

A quantitative analysis of transtensional margin width

NASA Astrophysics Data System (ADS)

Jeanniot, Ludovic; Buiter, Susanne J. H.

2018-06-01

Continental rifted margins show variations between a few hundred to almost a thousand kilometres in their conjugated widths from the relatively undisturbed continent to the oceanic crust. Analogue and numerical modelling results suggest that the conjugated width of rifted margins may have a relationship to their obliquity of divergence, with narrower margins occurring for higher obliquity. We here test this prediction by analysing the obliquity and rift width for 26 segments of transtensional conjugate rifted margins in the Atlantic and Indian Oceans. We use the plate reconstruction software GPlates (http://www.gplates.org) for different plate rotation models to estimate the direction and magnitude of rifting from the initial phases of continental rifting until breakup. Our rift width corresponds to the distance between the onshore maximum topography and the last identified continental crust. We find a weak positive correlation between the obliquity of rifting and rift width. Highly oblique margins are narrower than orthogonal margins, as expected from analogue and numerical models. We find no relationships between rift obliquities and rift duration nor the presence or absence of Large Igneous Provinces (LIPs).

New perspectives on the geometry of the Albuquerque Basin, Rio Grande rift, New Mexico: Insights from geophysical models of rift-fill thickness

USGS Publications Warehouse

Grauch, V. J.; Connell, Sean D.

2013-01-01

Discrepancies among previous models of the geometry of the Albuquerque Basin motivated us to develop a new model using a comprehensive approach. Capitalizing on a natural separation between the densities of mainly Neogene basin fill (Santa Fe Group) and those of older rocks, we developed a three-dimensional (3D) geophysical model of syn-rift basin-fill thickness that incorporates well data, seismic-reflection data, geologic cross sections, and other geophysical data in a constrained gravity inversion. Although the resulting model does not show structures directly, it elucidates important aspects of basin geometry. The main features are three, 3–5-km-deep, interconnected structural depressions, which increase in size, complexity, and segmentation from north to south: the Santo Domingo, Calabacillas, and Belen subbasins. The increase in segmentation and complexity may reflect a transition of the Rio Grande rift from well-defined structural depressions in the north to multiple, segmented basins within a broader region of crustal extension to the south. The modeled geometry of the subbasins and their connections differs from a widely accepted structural model based primarily on seismic-reflection interpretations. Key elements of the previous model are an east-tilted half-graben block on the north separated from a west-tilted half-graben block on the south by a southwest-trending, scissor-like transfer zone. Instead, we find multiple subbasins with predominantly easterly tilts for much of the Albuquerque Basin, a restricted region of westward tilting in the southwestern part of the basin, and a northwesterly trending antiform dividing subbasins in the center of the basin instead of a major scissor-like transfer zone. The overall eastward tilt indicated by the 3D geophysical model generally conforms to stratal tilts observed for the syn-rift succession, implying a prolonged eastward tilting of the basin during Miocene time. An extensive north-south synform in the central part of the Belen subbasin suggests a possible path for the ancestral Rio Grande during late Miocene or early Pliocene time. Variations in rift-fill thickness correspond to pre-rift structures in several places, suggesting that a better understanding of pre-rift history may shed light on debates about structural inheritance within the rift.

Analogue modelling of microcontinent formation: a case study from the Danakil Block, southern Red Sea

NASA Astrophysics Data System (ADS)

Molnar, Nicolas; Cruden, Alexander; Betts, Peter

2017-04-01

The kinematic evolution of the Danakil Block is well constrained but the processes responsible for the formation of an isolated continental segment around 13 Ma ago with an independent pole of rotation are still matter of debate. We performed three-dimensional analogue experiments of rotational continental extension containing a pre-existing linear weakness zones in the lithospheric mantle to investigate the formation of the Red Sea, including the Danakil Block. We imposed a rotational extensional boundary condition that simulates the progressive anticlockwise rotation of the Arabian Plate with respect to the Nubia Plate over the last 13-15 Ma and we simulated the presence of a narrow thermal anomaly related to the northward channelling of Afar plume by varying the viscosity of the model lithospheric mantle. The results from experiments containing a linear zone of weakness oriented at low angles with respect to the rift axis show that early stages of deformation are characterised by the development of two rift sub-parallel compartments that delimit an intra-rift block in the vicinity of the weak lithosphere boundary zone, which are analogous to the two rift branches that confine the Danakil Block in the southern Red Sea. The imposed rotational boundary condition creates a displacement gradient along the intra-rift block and prevents the nucleation of the early rift compartments to the north of the block, enhancing the formation of an independently rotating intra-rift segment. Comparison with geodetic data supports our modelling results, which are also in agreement with the "crank-arm" model of Sichler (1980. La biellette Danakile: un modèle pour l'évolution géodynamique de l'Afar. Bull. la Société Géologique Fr. 22, 925-933). Additional analogue models of i) orthogonal extension with an identical lithospheric mantle weakness and, ii) rotational extension with a homogeneous lithosphere (i.e., no lithospheric mantle weakness) show no evidence of developing rotating intra-rift segments and therefore suggest that if these processes had acted diachronously, the Danakil Block would not have formed. Based on the modelling results, we hypothesize that the Danakil Block formed as a result of the interaction between northward rift propagation and a north-northeast-trending mantle weakness zone, associated with anticlockwise rotation of the Arabian Plate and simultaneous northward channelling of the Afar plume.

Stress imparted by the great 2004 Sumatra earthquake shut down transforms and activated rifts up to 400 km away in the Andaman Sea

PubMed Central

Sevilgen, Volkan; Stein, Ross S.; Pollitz, Fred F.

2012-01-01

The origin and prevalence of triggered seismicity and remote aftershocks are under debate. As a result, they have been excluded from probabilistic seismic hazard assessment and aftershock hazard notices. The 2004 M = 9.2 Sumatra earthquake altered seismicity in the Andaman backarc rift-transform system. Here we show that over a 300-km-long largely transform section of the backarc, M≥4.5 earthquakes stopped for five years, and over a 750-km-long backarc section, the rate of transform events dropped by two-thirds, while the rate of rift events increased eightfold. We compute the propagating dynamic stress wavefield and find the peak dynamic Coulomb stress is similar on the rifts and transforms. Long-period dynamic stress amplitudes, which are thought to promote dynamic failure, are higher on the transforms than on the rifts, opposite to the observations. In contrast to the dynamic stress, we calculate that the mainshock brought the transform segments approximately 0.2 bar (0.02 MPa) farther from static Coulomb failure and the rift segments approximately 0.2 bar closer to static failure, consistent with the seismic observations. This accord means that changes in seismicity rate are sufficiently predictable to be included in post-mainshock hazard evaluations. PMID:22949694

The distribution of near-axis seamounts at intermediate spreading ridges

NASA Astrophysics Data System (ADS)

Howell, J. K.; Bohnenstiehl, D. R.; White, S. M.; Supak, S. K.

2008-12-01

The ridge axes along the intermediate-spreading rate Galapagos Spreading Center (GSC, 46-56 mm/yr) and South East Indian Ridge (SEIR, 72-76 mm/yr) vary from rifted axial valleys to inflated axial highs independent of spreading rate. The delivery and storage of melt is believed to control axial morphology, with axial highs typically observed in areas underlain by a shallow melt lens and axial valleys in areas without a significant melt lens [e.g., Baran et al., 2005 G-cubed; Detrick et al. 2002 G-cubed]. To investigate a possible correlation between the style of seafloor volcanism and axial morphology, a closed contour algorithm is used to identify near axis (2.5km off axis) semi-circular seamounts of heights greater than 20m from shipboard multibeam bathymetry. In areas characterized by an axial high, more seamounts are formed at the ends of the segments than in the center. This is consistent with observations at fast-spreading ridges and suggests a tendency of lavas to erupt at lower effusion rates near second-order segment boundaries. Segments with a rift valley along the GSC show the opposite trend, with more seamounts at the center of second-order segments. Both patterns however are observed along SEIR segments with rift valleys where magma supply may be reflected in size and not their abundance.

New geodetic measurements in central Afar constraining the Arabia-Somalia-Nubia triple junction kinematics

NASA Astrophysics Data System (ADS)

Doubre, C.; Deprez, A.; Masson, F.; Socquet, A.; Lewi, E.; Grandin, R.; Calais, E.; Wright, T. J.; Bendick, R. O.; Pagli, C.; Peltzer, G.; de Chabalier, J. B.; Ibrahim Ahmed, S.

2014-12-01

The Afar Depression is an extraordinary submerged laboratory where the crustal mechanisms involved in the active rifting process can be studied. But the crustal movements at the regional scale are complicated by being the locus of the meeting of three divergent plate boundaries: the oceanic spreading ridges of the Red Sea and the Aden Ridge and the intra-continental East-African Rift (EAR). We present here the first GPS measurements conducted in a new network in Central Afar, complementing existing networks in Eritrea, around the Manda-Harraro 2005-2010 active segment, in the Northern part of the EAR and in Djibouti. Even if InSAR data were appropriate for mapping the deformation field, the results are difficult to interpret for analyzing the regional kinematics because of the atmospheric conditions, the lack of complete data catalogue, the acquisition configuration and the small velocity variations. Therefore, our measurements in the new sites are crucial to obtain an accurate velocity field over the whole depression, and focus specifically on the spatial organization of the deformation to characterize the tripe junction. These first results show that a small part of the motion of the Somalia plate with respect to the Nubia plate or the Arabia plate (2-3 mm/yr) occurs south of the Tadjura Gulf and East of the Adda-do segment in Southern Afar. The complex kinematic pattern involves a clockwise rotation of this Southeastern part of the Afar rift and can be related to the significant seismic activity regularly recorded in the region of Jigjiga (northern Somalia-Ethiopia border). The western continuation of the Aden Ridge into Afar extends West of the Asal rift segment and does not reach the young active segment of Manda-Inakir (MI). A slow gradient of velocity is observed across the Dobi Graben and across the large systems of faults between Lake Abhe and the MI rift segment. A striking change of the velocity direction occurs in the region of Assaïta, west of Lake Abhe, suggesting that this area represents the most probable location for the triple junction.

The inverted Triassic rift of the Marrakech High Atlas: A reappraisal of basin geometries and faulting histories

NASA Astrophysics Data System (ADS)

Domènech, Mireia; Teixell, Antonio; Babault, Julien; Arboleya, Maria-Luisa

2015-11-01

The High Atlas of Morocco is an aborted rift developed during the Triassic-Jurassic and moderately inverted during the Cenozoic. The Marrakech High Atlas, with large exposures of basement and Triassic early syn-rift deposits, is ideal to investigate the geometries of the deepest parts of a rift, constituting a good analogue for pre-salt domains. It allows unraveling geometries and kinematics of the extensional and compressional structures and the influence that they exert over one another. A detailed structural study of the main Triassic basins and basin-margin faults of the Marrakech High Atlas shows that only a few rift faults were reactivated during the Cenozoic compressional stage in contrast to previous interpretations, and emphasizes that fault reactivation cannot be taken for granted in inverted rift systems. Preserved extensional features demonstrate a dominant dip-slip opening kinematics with strike-slip playing a minor role, at variance to models proposing a major strike-slip component along the main basin-bounding faults, including faults belonging to the Tizi n'Test fault zone. A new Middle Triassic paleogeographic reconstruction shows that the Marrakech High Atlas was a narrow and segmented orthogonal rift (sub-perpendicular to the main regional extension direction which was ~ NW-SE), in contrast to the central and eastern segments of the Atlas rift which developed obliquely. This difference in orientation is attributed to the indented Ouzellarh Precambrian salient, part of the West African Craton, which deflected the general rift trend in the area evidencing the major role of inherited lithospheric anisotropies in rift direction and evolution. As for the Cenozoic inversion, total orogenic shortening is moderate (~ 16%) and appears accommodated by basement-involved large-scale folding, and by newly formed shortcut and by-pass thrusting, with rare left-lateral strike-slip indicators. Triassic faults commonly acted as buttresses.

Seismic hazard assessment of the Kivu rift segment based on a new seismotectonic zonation model (western branch, East African Rift system)

NASA Astrophysics Data System (ADS)

Delvaux, Damien; Mulumba, Jean-Luc; Sebagenzi, Mwene Ntabwoba Stanislas; Bondo, Silvanos Fiama; Kervyn, François; Havenith, Hans-Balder

2017-10-01

In the frame of the Belgian GeoRisCA multi-risk assessment project focusing on the Kivu and northern Tanganyika rift region in Central Africa, a new probabilistic seismic hazard assessment has been performed for the Kivu rift segment in the central part of the western branch of the East African rift system. As the geological and tectonic setting of this region is incompletely known, especially the part lying in the Democratic Republic of the Congo, we compiled homogeneous cross-border tectonic and neotectonic maps. The seismic risk assessment is based on a new earthquake catalogue based on the ISC reviewed earthquake catalogue and supplemented by other local catalogues and new macroseismic epicenter data spanning 126 years, with 1068 events. The magnitudes have been homogenized to Mw and aftershocks removed. The final catalogue used for the seismic hazard assessment spans 60 years, from 1955 to 2015, with 359 events and a magnitude of completeness of 4.4. The seismotectonic zonation into 7 seismic source areas was done on the basis of the regional geological structure, neotectonic fault systems, basin architecture and distribution of thermal springs and earthquake epicenters. The Gutenberg-Richter seismic hazard parameters were determined by the least square linear fit and the maximum likelihood method. Seismic hazard maps have been computed using existing attenuation laws with the Crisis 2012 software. We obtained higher PGA values (475 years return period) for the Kivu rift region than the previous estimates. They also vary laterally in function of the tectonic setting, with the lowest value in the volcanically active Virunga - Rutshuru zone, highest in the currently non-volcanic parts of Lake Kivu, Rusizi valley and North Tanganyika rift zone, and intermediate in the regions flanking the axial rift zone.

Strike-slip tectonics during rift linkage

NASA Astrophysics Data System (ADS)

Pagli, C.; Yun, S. H.; Ebinger, C.; Keir, D.; Wang, H.

2017-12-01

The kinematics of triple junction linkage and the initiation of transforms in magmatic rifts remain debated. Strain patterns from the Afar triple junction provide tests of current models of how rifts grow to link in area of incipient oceanic spreading. Here we present a combined analysis of seismicity, InSAR and GPS derived strain rate maps to reveal that the plate boundary deformation in Afar is accommodated primarily by extensional tectonics in the Red Sea and Gulf of Aden rifts, and does not require large rotations about vertical axes (bookshelf faulting). Additionally, models of stress changes and seismicity induced by recent dykes in one sector of the Afar triple junction provide poor fit to the observed strike-slip earthquakes. Instead we explain these patterns as rift-perpendicular shearing at the tips of spreading rifts where extensional strains terminate against less stretched lithosphere. Our results demonstrate that rift-perpendicular strike-slip faulting between rift segments achieves plate boundary linkage during incipient seafloor spreading.

Effects on Ridge Segmentation, Magmatic Plumbing and Eruption Style Caused by Weak Hot-spot to Ridge Interaction: the Central Indian Ridge and Rodrigues Hot-spot Couplet.

NASA Astrophysics Data System (ADS)

Murton, B. J.; Parson, L. M.; Sauter, D.

2001-12-01

The intermediate spreading, Central Indian Ridge (CIR) forms a couplet with a weak hot-spot of which the Rodrigues archipelago is an expression. Recently collected bathymetry shows that despite having little in the way of a significant topographic swell, the hot-spot is associated with a change in offset sense across adjacent transforms of the CIR causing the ridge to draw nearer to the Rodrigues island system. The most proximal ridge segment of the CIR is over 20km long and comprises three non-transform bounded sub-segments. The most northerly sub-segment has a shallow (<3000m), narrow (<5km) and featureless flat rift valley. TOBI sidescan sonar imagery shows that the segment is host to a 15km-long, 5km-wide single sheet flow. Elsewhere in the segment the valley floor is characterised by long (>5km), narrow (<1km) ridges that often terminate in conical seamounts. These ridges are the loci of some of the acoustically freshest volcanic facies in the rift valley. Samples recovered from these ridges have similar petrology along strike. With increasing distance south along the CIR, the ridge segments are typically 500m deeper than to the north. Here they are about 75km long and bounded by transform offsets that are 50 km long. However, even in the deepest parts of these segments, where the axial floor is over 4000m deep at the ridge-transform-intersections, there is fresh lava and other evidence for abundant volcanic activity. Within these segments, the rift valley comprises mainly seamounts and hummocky volcanic features. We believe the westward stepping trend of the CIR towards the Rodrigues islands is a function of the hot spot. The elevated temperature and volatile content to the west reduces mantle viscosity which, combined with thinner and hence weaker lithosphere, influencec the loci of initial oceanic rifting and the relative position of the ridge axis. The unusually great length of the northern segment has a similar origin with the presence of thin and weak lithosphere and less viscous mantle reducing the tectonic ``memory" of the ridge system. Despite being farther from the hot-spot, the southern segments have a similarly robust melt supply to the northern segment. The main difference is melt delivery, with the northern segments supplied via long dikes that also erupt as fissure ridges, while to the south the supply is more disseminated. The massive lava sheet and robust and narrow axial valley in the northernmost sub-segment coincides with the tip of a southward propagating system. This system appears to herald the onset of more productive melting a may represent a relocation of the hot-spot-ridge interaction.

The L, M, and S Segments of Rift Valley Fever Virus MP-12 Vaccine Independently Contribute to a Temperature-Sensitive Phenotype

PubMed Central

Nishiyama, Shoko; Lokugamage, Nandadeva

2016-01-01

ABSTRACT Rift Valley fever (RVF) is endemic to Africa, and the mosquito-borne disease is characterized by “abortion storms” in ruminants and by hemorrhagic fever, encephalitis, and blindness in humans. Rift Valley fever virus (RVFV; family Bunyaviridae, genus Phlebovirus) has a tripartite negative-stranded RNA genome (L, M, and S segments). A live-attenuated vaccine for RVF, the MP-12 vaccine, is conditionally licensed for veterinary use in the United States. MP-12 is fully attenuated by the combination of the partially attenuated L, M, and S segments. Temperature sensitivity (ts) limits viral replication at a restrictive temperature and may be involved with viral attenuation. In this study, we aimed to characterize the ts mutations for MP-12. The MP-12 vaccine showed restricted replication at 38°C and replication shutoff (100-fold or greater reduction in virus titer compared to that at 37°C) at 39°C in Vero and MRC-5 cells. Using rZH501 reassortants with either the MP-12 L, M, or S segment, we found that all three segments encode a temperature-sensitive phenotype. However, the ts phenotype of the S segment was weaker than that of the M or L segment. We identified Gn-Y259H, Gc-R1182G, L-V172A, and L-M1244I as major ts mutations for MP-12. The ts mutations in the L segment decreased viral RNA synthesis, while those in the M segment delayed progeny production from infected cells. We also found that a lack of NSs and/or 78kD/NSm protein expression minimally affected the ts phenotype. Our study revealed that MP-12 is a unique vaccine carrying ts mutations in the L, M, and S segments. IMPORTANCE Rift Valley fever (RVF) is a mosquito-borne viral disease endemic to Africa, characterized by high rates of abortion in ruminants and severe diseases in humans. Vaccination is important to prevent the spread of disease, and a live-attenuated MP-12 vaccine is currently the only vaccine with a conditional license in the United States. This study determined the temperature sensitivity (ts) of MP-12 vaccine to understand virologic characteristics. Our study revealed that MP-12 vaccine contains ts mutations independently in the L, M, and S segments and that MP-12 displays a restrictive replication at 38°C. PMID:26819307

The L, M, and S Segments of Rift Valley Fever Virus MP-12 Vaccine Independently Contribute to a Temperature-Sensitive Phenotype.

PubMed

Nishiyama, Shoko; Lokugamage, Nandadeva; Ikegami, Tetsuro

2016-01-27

Rift Valley fever (RVF) is endemic to Africa, and the mosquito-borne disease is characterized by "abortion storms" in ruminants and by hemorrhagic fever, encephalitis, and blindness in humans. Rift Valley fever virus (RVFV; family Bunyaviridae, genus Phlebovirus) has a tripartite negative-stranded RNA genome (L, M, and S segments). A live-attenuated vaccine for RVF, the MP-12 vaccine, is conditionally licensed for veterinary use in the United States. MP-12 is fully attenuated by the combination of the partially attenuated L, M, and S segments. Temperature sensitivity (ts) limits viral replication at a restrictive temperature and may be involved with viral attenuation. In this study, we aimed to characterize the ts mutations for MP-12. The MP-12 vaccine showed restricted replication at 38°C and replication shutoff (100-fold or greater reduction in virus titer compared to that at 37°C) at 39°C in Vero and MRC-5 cells. Using rZH501 reassortants with either the MP-12 L, M, or S segment, we found that all three segments encode a temperature-sensitive phenotype. However, the ts phenotype of the S segment was weaker than that of the M or L segment. We identified Gn-Y259H, Gc-R1182G, L-V172A, and L-M1244I as major ts mutations for MP-12. The ts mutations in the L segment decreased viral RNA synthesis, while those in the M segment delayed progeny production from infected cells. We also found that a lack of NSs and/or 78kD/NSm protein expression minimally affected the ts phenotype. Our study revealed that MP-12 is a unique vaccine carrying ts mutations in the L, M, and S segments. Rift Valley fever (RVF) is a mosquito-borne viral disease endemic to Africa, characterized by high rates of abortion in ruminants and severe diseases in humans. Vaccination is important to prevent the spread of disease, and a live-attenuated MP-12 vaccine is currently the only vaccine with a conditional license in the United States. This study determined the temperature sensitivity (ts) of MP-12 vaccine to understand virologic characteristics. Our study revealed that MP-12 vaccine contains ts mutations independently in the L, M, and S segments and that MP-12 displays a restrictive replication at 38°C. Copyright © 2016, American Society for Microbiology. All Rights Reserved.

GPS Velocity Field at the Western Tip of the Aden Ridge ; Implications for Rifting and the Arabia-Somalia-Nubia Triple Junction Dynamics

NASA Astrophysics Data System (ADS)

Doubre, C.; Socquet, A.; Masson, F.; Cressot, C.; Mohamed, K.; Vigny, C.; Ruegg, J.

2010-12-01

Due to the presence of magma and a complex thermal structure, the dynamics of divergent plate boundaries are complicated, with microseismicity (ML<4) contributing very little to the total moment release. For the last 35 years several geodetic campaigns have been conducted at the western tip of the Aden Ridge propagating on land into Afar (Republic of Djibouti). The first segment above water, the Asal Rift, experienced a seismo-volcanic event in 1978, which was the first rifting episode, along with the 1978-1985 Icelandic Krafla event, to be monitored by terrestrial geodetic measurements. These measurements revealed the opening of two 1-2 m-wide dykes in the rift inner floor. Since then, terrestrial and spatial geodetic monitoring shows that the rift kept opening, during the post-rifting period, at a rate largely exceeding the plates’ motions. This significant opening rate is decreasing with time to tend, three decades after the rifting event, to the far-field opening rate. We present here the results of the GPS measurements of a 45 site network covering the Tadjoura-Asal Rift System, previously made every two years from 1995 to 2003, and repeated in 2010. The calculated 1999-2010 horizontal velocity field is very homogeneous with a quasi-constant N045° direction with respect to Somalia and a regular increase from the southern to the northern margin of the Asal Rift clearly controlled by a few normal faults, and reaching a maximum of 12.5 mm/yr. A non-negligible part of the Arabia-Somalia divergent movement (1 to 2 mm/yr) is observed south of this rift, which sheds light on the role of the active normal faults bounding the asymmetrical Gaggadé Basin and therefore brings important constraints on the location of the Red Sea Ridge-Aden Ridge-East African Rift triple junction. Since the last 2003 campaign, the lack of micro-seismicity within the Asal Rift seems to be associated with a ˜2 mm/yr decrease of the opening rate deduced from the GPS time series analysis. These results confirm the importance of non-steady state behavior of the Asal volcano-tectonic rift segment, and the role of geothermal/volcanic activity on the occurrence of transients, as suggested by InSAR results.

Quasi-quantitative analysis of the lithospheric rheology across an incipient continental rift based on 3-D magnetotelluric imaging of Linfen Basin within the North China Craton

NASA Astrophysics Data System (ADS)

Yin, Y.; Jin, S.; Wei, W.; Ye, G.; Dong, H.; Zhang, L.

2017-12-01

The Shanxi Rift being located within the interior of the North China Craton and far from any plate boundaries has undergone dramatic deformation and seismicity during the Cenozoic. In this study, we build 3-D lithospheric resistivity model by MT array data, across the Linfen Basin which is the most active segment of this intraplate rift. Accordingly, combined with previous rock physics experimental results, we estimate the fluid contents of lower crustal granulites and upper mantle peridotites and thereby the rough distribution of lithospheric rheological strength. On the two sides of Linfen Basin, lithosphere beneath the Precambrian terranes are of high strength. By contrast, a high-conductivity nearly upright lithosphere weak zone occurs beneath the eastern margin of the Linfen Basin and appears to be connected to the high-conductivity and therefore weak lower crust just beneath the basin, probably indicating a structure of asthenospheric upwelling causing the lower crustal decoupling through lateral drag forces. The distribution of lithospheric weak zones, brittle faults, ductile shear zones and detachment structures determined from our resistivity model is in good agreement with the 8-My stage model of a previous numerical geodynamic simulation for continental rift evolution by reconstruction of the South Atlantic plate. Accordingly, we suggest that the lithospheric weak zone could be a preexisting Precambrian shear zone and has reactivated as an asthenospheric upwelling conduit under the far-field effects of Indo- Asian collision or Pacific Plate subduction since the late Mesozoic. This process could have caused the upper crustal extension and rifting through the stress regulation by the plastic lower crust, which could be the mechanism of rift formation. In summary, we suggest the Linfen segment of the Shanxi Rift, is a simple shear mode rift in the incipient stage of rift evolution, rather than a mature pure shear mode one as determined by precious seismic imaging.

Orientations of Pre-existing Structures along the Scarp of the Bilila-Mtakataka Fault in the Central Malawi Rift.

NASA Astrophysics Data System (ADS)

Elifritz, E. A.; Johnson, S.; Beresh, S. C. M.; Mendez, K.; Mynatt, W. G.; Mayle, M.; Laó-Dávila, D. A.; Atekwana, E. A.; Chindandali, P. R. N.; Chisenga, C.; Gondwe, S.; Mkumbwa, M.; Kalindekafe, L.; Kalaguluka, D.; Salima, J.

2017-12-01

The NW-SE Bilila-Mtakataka Fault is suggested to be 100 km in length and is located in the Malawi Rift, a portion of the magma-poor Western Branch of the East African Rift System. This fault is exposed south of Lake Malawi and occurs close to the epicenter of the 1989 6.2 magnitude Salima Earthquake. Moreover, it traverses rocks with inherited Precambrian fabrics that may control the modern rifting process. The effect of the orientation of the pre-existing fabric on the formation of this potentially seismogenic fault has not been well studied. In this project, we measured the older foliations, dikes, and joints in addition to younger faults and striations to understand how the active faulting of the Bilila-Mtakataka Fault is affected by the older fabric. The Fault is divided into 5 segments and 4 linkage zones. All four linkage zones were studied in detail and a Brunton compass was used to determine orientations of structures. The linkage zone between segments 1 and 2 occurs between a regional WNW-ESE joint and the border fault, which is identified by a zig-zag pattern in SRTM data. Precambrian gneiss is cut by oblique steeply-dipping faults in this area. Striations and layer offsets suggest both right-lateral and normal components. This segment strikes NE-SW, in contrast with the NW-SE average strike of the entire fault. The foliations, faults, dikes, and joints collected in this area strike NE-SW, therefore running parallel to the segment. The last 3 southern linkage zones all strike NW-SE and the linkage zone between segment 3 and 4 has a steep dip angle. Dip angles of structures vary from segment to segment, having a wide range of results. Nonetheless, all four linkage zones show structures striking parallel to its segment direction. The results show that pre-existing meso-scale and regional structures and faults strike parallel to the fault scarp. The parallelism of the structures suggest that they serve as planes of weakness, controlling the localization of extension expressed as the border fault. Thus, further studies of the Precambrian foliation in the subsurface are necessary to understand the characterization of the fault where it is unexposed at depth.

Spatial instability of the rift in the St. Paul multifault transform fracture system, Atlantic Ocean

NASA Astrophysics Data System (ADS)

Sokolov, S. Yu.; Zaraiskaya, Yu. A.; Mazarovich, A. O.; Efimov, V. N.; Sokolov, N. S.

2016-05-01

The structure of the acoustic basement of the eastern part of the St. Paul multifault transform fracture system hosts rift paleovalleys and a paleonodal depression that mismatch the position of the currently active zones. This displacement zone, which is composed of five fault troughs, is unstable in terms of the position of the rift segments, which jumped according to redistribution of stresses. The St. Paul system is characterized by straightening of the transform transition between two remote segments of the Mid-Atlantic Ridge (MAR). The eastern part of the system contains anomalous bright-spot-like reflectors on the flattened basement, which is a result of atypical magmatism, that forms the standard ridge relief of the acoustic basement. Deformations of the acoustic basement have a presedimentation character. The present-day deformations with lower amplitude in comparison to the basement are accompanied by acoustic brightening of the sedimentary sequence. The axial Bouguer anomalies in the east of the system continue to the north for 120 km from the active segments of the St. Paul system. Currently seismically active segments of the spreading system are characterized by increasing amplitudes of the E-W displacement along the fault troughs. Cross-correlation of the lengths of the active structural elements of the MAR zone (segments of the ridge and transform fracture zones of displacement) indicates that, statistically, the multifault transform fracture system is a specific type of oceanic strike-slip faults.

Development of a Rift Valley fever real-time RT-PCR assay that can detect all three genome segments

USDA-ARS?s Scientific Manuscript database

Outbreaks of Rift Valley fever in Kenya, Madagascar, Mauritania, and South Africa had devastating effects on livestock and human health. In addition, this disease is a food security issue for endemic countries. There is growing concern for the potential introduction of RVF into non-endemic countries...

The Iceland Plate Boundary Zone: Propagating Rifts, Migrating Transforms, and Rift-Parallel Strike-Slip Faults

NASA Astrophysics Data System (ADS)

Karson, J. A.

2017-11-01

Unlike most of the Mid-Atlantic Ridge, the North America/Eurasia plate boundary in Iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures. Accordingly, geologic processes in Iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid-ocean ridge system. Combining existing and new data from across Iceland provides an integrated view of this active, mostly subaerial plate boundary. The broad Iceland plate boundary zone includes segmented rift zones linked by transform fault zones. Rift propagation and transform fault migration away from the Iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading-related structures. Rift propagation results in block rotations that are accommodated by widespread, rift-parallel, strike-slip faulting. The geometry and kinematics of faulting in Iceland may have implications for spreading processes elsewhere on the mid-ocean ridge system where rift propagation and transform migration occur.

Setting of the Father's Day Eruption at Kilauea

NASA Astrophysics Data System (ADS)

Swanson, D. A.

2007-12-01

The Father's Day eruption and associated intrusion took place within a 10-km segment of Kilauea's east rift zone between Hi`iaka and Napau Craters--a segment that has had more numerous eruptions and intrusions than any other of comparable length during the past 200, probably the past 1000, years. Fifteen known eruptions started in this area in the past 200 years: 1840, 1922, 1923, 1962, August and October 1963, March and December 1965, August and October 1968, February and May 1969, May and November 1973, and March 1980 (only 3 cubic meters!). Three others, not previously designated as distinct eruptions despite having all the appropriate characteristics, took place during on-going eruptions: two in `Alo`i Crater in 1970 and 1972, and one in Napau Crater in 1997. Two of the largest shields on the east rift zone formed during long-lasting eruptions within this area--Kane Nui o Hamo at an unknown date, perhaps the 11-12th century, and Mauna Ulu (1969-1974). In addition, many small intrusions without eruptions are known. Seven short eruptions punctuated a prolonged eruption: four within the segment during the Mauna Ulu eruption, two at the summit and southwest rift zone during that same eruption, and one in Napau Crater in 1997 during the Pu`u `O`o eruption. Thus the Father's Day eruption is not unique by virtue of taking place during an ongoing eruption elsewhere along the rift zone. The increased frequency of activity in the segment during the 20th century is obvious, particularly after 1962. For most of the past 1,000 years, eruptions were centered at Kilauea's summit, with significant but lesser activity along the rift zones. A large summit deflation in 1924 ended the nearly continuous lava lake in Halemaumau, eventually leading to the past 5 decades of dominantly east rift zone activity. This segment of the rift zone contains most of the pit craters on Kilauea and gradually changes from a SE trend near the caldera to an ENE trend that characterizes the rest of the zone. The Koa`e fault system joins the east rift zone at the curve. The complex structural setting likely affects the frequency of magmatic activity in the segment. All of the eruptive and intrusive activity results in storage of isolated magma bodies. Not surprisingly, petrologists find evidence that summit magma mixes with stored, fractionated magma. The area near Makaopuhi Crater and Kane Nui o Hamo is a particular focus, inferred since the mid-1960s to harbor a shallow magma reservoir. All of the eruptions and intrusions are accompanied by sharp deflation and shallow seismicity at the summit and shallow seismicity and uplift along the intrusion or eruptive fissures. Most often, no seismicity occurs between the summit and the area of intrusion or eruption. Within that area, seismicity commonly migrates downrift but occasionally uprift. Similarly, crack opening generally progresses downrift, with a few exceptions. Cracks generally trend about 65 degrees and can be either left- or right-stepping. Cracks open along azimuths of 155-175, with local exceptions. There is no structural difference between eruptive cracks (fissures) and non- eruptive cracks. Single eruptive fissures rarely exceed 200 m in length, instead stepping within en echelon zones above a presumably linear dike. Since the late 1960s, widening has been measured across the active area during eruptions and intrusions, first by EDM and then by satellite-based systems. The opening is nearly symmetrical within the rift zone, but farther out the north flank barely responds whereas the south flank moves seaward. Available leveling data show uplift on either side of the dike and subsidence along the crest. Examples of deformation in the 1960s and 1970s will be presented, and bibliographic references to past activity will be available.

Constraints Imposed by Rift Inheritance on the Compressional Reactivation of a Hyperextended Margin: Mapping Rift Domains in the North Iberian Margin and in the Cantabrian Mountains

NASA Astrophysics Data System (ADS)

Cadenas, P.; Fernández-Viejo, G.; Pulgar, J. A.; Tugend, J.; Manatschal, G.; Minshull, T. A.

2018-03-01

The Alpine Pyrenean-Cantabrian orogen developed along the plate boundary between Iberia and Europe, involving the inversion of Mesozoic hyperextended basins along the southern Biscay margin. Thus, this margin represents a natural laboratory to analyze the control of structural rift inheritance on the compressional reactivation of a continental margin. With the aim to identify former rift domains and investigate their role during the subsequent compression, we performed a structural analysis of the central and western North Iberian margin, based on the interpretation of seismic reflection profiles and local constraints from drill-hole data. Seismic interpretations and published seismic velocity models enabled the development of crustal thickness maps that helped to constrain further the offshore and onshore segmentation. Based on all these constraints, we present a rift domain map across the central and western North Iberian margin, as far as the adjacent western Cantabrian Mountains. Furthermore, we provide a first-order description of the margin segmentation resulting from its polyphase tectonic evolution. The most striking result is the presence of a hyperthinned domain (e.g., Asturian Basin) along the central continental platform that is bounded to the north by the Le Danois High, interpreted as a rift-related continental block separating two distinctive hyperextended domains. From the analysis of the rift domain map and the distribution of reactivation structures, we conclude that the landward limit of the necking domain and the hyperextended domains, respectively, guide and localize the compressional overprint. The Le Danois block acted as a local buttress, conditioning the inversion of the Asturian Basin.

Estimation of age of Dali-Ganis rifting and associated volcanic activity, Venus

NASA Technical Reports Server (NTRS)

Basilevsky, A. T.

1993-01-01

This paper deals with the estimation of age for the Dali and Ganis Chasma rift zones and their associated volcanism based on photogeologic analysis of stratigraphic relations of rift-associated features with impact craters which have associated features indicative of their age. The features are radar-dark and parabolic, and they are believed to be mantles of debris derived from fallout of the craters' ejecta. They are thought to be among the youngest features on the Venusian surface, so their 'parent' craters must also be very young, evidently among the youngest 10 percent of Venus' crater population. Dali Chasma and Ganis Chasma are a part of a system of rift zones contained within eastern Aphrodite and Atla Regio which is a significant component of Venus tectonics. The rifts of this system are fracture belts which dissect typical Venusian plains with rare islands of tessera terrain. The rift zone system consists of several segments following each other (Diane, Dali, Ganis) and forming the major rift zone line, about 10,000 km long, which has junctions with several other rift zones, including Parga Chasma Rift. The junctions are usually locations of rift-associated volcanism in the form of volcanic edifices (Maat and Ozza Montes) or plain-forming flows flooding some areas within the rift zones and the adjacent plains.

Role of the offshore Pedro Banks left-lateral strike-slip fault zone in the plate tectonic evolution of the northern Caribbean

NASA Astrophysics Data System (ADS)

Ott, B.; Mann, P.; Saunders, M.

2013-12-01

Previous workers, mainly mapping onland active faults on Caribbean islands, defined the northern Caribbean plate boundary zone as a 200-km-wide bounded by two active and parallel strike-slip faults: the Oriente fault along the northern edge of the Cayman trough with a GPS rate of 14 mm/yr, and and the Enriquillo-Plaintain Garden fault zone (EPGFZ) with a rate of 5-7 mm/yr. In this study we use 5,000 km of industry and academic data from the Nicaraguan Rise south and southwest of the EPGFZ in the maritime areas of Jamaica, Honduras, and Colombia to define an offshore, 700-km-long, active, left-lateral strike-slip fault in what has previously been considered the stable interior of the Caribbean plate as determined from plate-wide GPS studies. The fault was named by previous workers as the Pedro Banks fault zone because a 100-km-long segment of the fault forms an escarpment along the Pedro carbonate bank of the Nicaraguan Rise. Two fault segments of the PBFZ are defined: the 400-km-long eastern segment that exhibits large negative flower structures 10-50 km in width, with faults segments rupturing the sea floor as defined by high resolution 2D seismic data, and a 300-km-long western segment that is defined by a narrow zone of anomalous seismicity first observed by previous workers. The western end of the PBFZ terminates on a Quaternary rift structure, the San Andres rift, associated with Plio-Pleistocene volcanism and thickening trends indicating initial rifting in the Late Miocene. The southern end of the San Andreas rift terminates on the western Hess fault which also exhibits active strands consistent with left-lateral, strike-slip faults. The total length of the PBFZ-San Andres rift-Southern Hess escarpment fault is 1,200 km and traverses the entire western end of the Caribbean plate. Our interpretation is similar to previous models that have proposed the "stable" western Caribbean plate is broken by this fault whose rate of displacement is less than the threshold recognizable from the current GPS network (~3 mm/yr). The Late Miocene age of the fault indicates it may have activated during the Late Miocene to recent Hispaniola-Bahamas oblique collision event.

Fault propagation and climatic control of sedimentation on the Ghoubbet Rift Floor: insights from the Tadjouraden cruise in the western Gulf of Aden

NASA Astrophysics Data System (ADS)

Audin, L.; Manighetti, I.; Tapponnier, P.; Métivier, F.; Jacques, E.; Huchon, P.

2001-02-01

A detailed geophysical survey of the Ghoubbet Al Kharab (Djibouti) clarifies the small-scale morphology of the last submerged rift segment of the propagating Aden ridge before it enters the Afar depression. The bathymetry reveals a system of antithetic normal faults striking N130°E, roughly aligned with those active along the Asal rift. The 3.5kHz sub-bottom profiler shows how the faults cut distinct layers within the recent, up to 60m thick, sediment cover on the floor of the basin. A large volcanic structure, in the centre of the basin, the `Ghoubbet' volcano, separates two sedimentary flats. The organization of volcanism and the planform of faulting, with en echelon subrifts along the entire Asal-Ghoubbet rift, appear to confirm the westward propagation of this segment of the plate boundary. Faults throughout the rift have been active continuously for the last 8400yr, but certain sediment layers show different offsets. The varying offsets of these layers, dated from cores previously retrieved in the southern basin, imply Holocene vertical slip rates of 0.3-1.4mmyr-1 and indicate a major decrease in sedimentation rate after about 6000yr BP, and a redistribution of sediments in the deepest troughs during the period that preceded that change.

Attenuation of pathogenic Rift Valley fever virus strain through the chimeric S-segment encoding sandfly fever phlebovirus NSs or a dominant-negative PKR

PubMed Central

Nishiyama, Shoko; Slack, Olga A. L.; Lokugamage, Nandadeva; Hill, Terence E.; Juelich, Terry L.; Zhang, Lihong; Smith, Jennifer K.; Perez, David; Gong, Bin; Freiberg, Alexander N.; Ikegami, Tetsuro

2016-01-01

ABSTRACT Rift Valley fever is a mosquito-borne zoonotic disease affecting ruminants and humans. Rift Valley fever virus (RVFV: family Bunyaviridae, genus Phlebovirus) causes abortions and fetal malformations in ruminants, and hemorrhagic fever, encephalitis, or retinitis in humans. The live-attenuated MP-12 vaccine is conditionally licensed for veterinary use in the US. However, this vaccine lacks a marker for the differentiation of vaccinated from infected animals (DIVA). NSs gene is dispensable for RVFV replication, and thus, rMP-12 strains lacking NSs gene is applicable to monitor vaccinated animals. However, the immunogenicity of MP-12 lacking NSs was not as high as parental MP-12. Thus, chimeric MP-12 strains encoding NSs from either Toscana virus (TOSV), sandfly fever Sicilian virus (SFSV) or Punta Toro virus Adames strain (PTA) were characterized previously. Although chimeric MP-12 strains are highly immunogenic, the attenuation through the S-segment remains unknown. Using pathogenic ZH501 strain, we aimed to demonstrate the attenuation of ZH501 strain through chimeric S-segment encoding either the NSs of TOSV, SFSV, PTA, or Punta Toro virus Balliet strain (PTB). In addition, we characterized rZH501 encoding a human dominant-negative PKR (PKRΔE7), which also enhances the immunogenicity of MP-12. Study done on mice revealed that attenuation of rZH501 occurred through the S-segment encoding either PKRΔE7 or SFSV NSs. However, rZH501 encoding either TOSV, PTA, or PTB NSs in the S-segment uniformly caused lethal encephalitis. Our results indicated that the S-segments encoding PKRΔE7 or SFSV NSs are attenuated and thus applicable toward next generation MP-12 vaccine candidates that encode a DIVA marker. PMID:27248570

Attenuation of pathogenic Rift Valley fever virus strain through the chimeric S-segment encoding sandfly fever phlebovirus NSs or a dominant-negative PKR.

PubMed

Nishiyama, Shoko; Slack, Olga A L; Lokugamage, Nandadeva; Hill, Terence E; Juelich, Terry L; Zhang, Lihong; Smith, Jennifer K; Perez, David; Gong, Bin; Freiberg, Alexander N; Ikegami, Tetsuro

2016-11-16

Rift Valley fever is a mosquito-borne zoonotic disease affecting ruminants and humans. Rift Valley fever virus (RVFV: family Bunyaviridae, genus Phlebovirus) causes abortions and fetal malformations in ruminants, and hemorrhagic fever, encephalitis, or retinitis in humans. The live-attenuated MP-12 vaccine is conditionally licensed for veterinary use in the US. However, this vaccine lacks a marker for the differentiation of vaccinated from infected animals (DIVA). NSs gene is dispensable for RVFV replication, and thus, rMP-12 strains lacking NSs gene is applicable to monitor vaccinated animals. However, the immunogenicity of MP-12 lacking NSs was not as high as parental MP-12. Thus, chimeric MP-12 strains encoding NSs from either Toscana virus (TOSV), sandfly fever Sicilian virus (SFSV) or Punta Toro virus Adames strain (PTA) were characterized previously. Although chimeric MP-12 strains are highly immunogenic, the attenuation through the S-segment remains unknown. Using pathogenic ZH501 strain, we aimed to demonstrate the attenuation of ZH501 strain through chimeric S-segment encoding either the NSs of TOSV, SFSV, PTA, or Punta Toro virus Balliet strain (PTB). In addition, we characterized rZH501 encoding a human dominant-negative PKR (PKRΔE7), which also enhances the immunogenicity of MP-12. Study done on mice revealed that attenuation of rZH501 occurred through the S-segment encoding either PKRΔE7 or SFSV NSs. However, rZH501 encoding either TOSV, PTA, or PTB NSs in the S-segment uniformly caused lethal encephalitis. Our results indicated that the S-segments encoding PKRΔE7 or SFSV NSs are attenuated and thus applicable toward next generation MP-12 vaccine candidates that encode a DIVA marker.

Volcanic/Tectonic Characteristics of First and Second Order Segments and Ridge Discontinuities `Under the Hot-spot Influence' - TOBI Imagery from the Central Indian Ridge (CIR) Adjacent to the Rodriguez System.

NASA Astrophysics Data System (ADS)

Parson, L.; Murton, B.; Sauter, D.; Curewitz, D.; Okino, K.; German, C.; Leven, J.

2001-12-01

Deeptow sidescan sonar data (TOBI, 30kHz) acquired over more than 200 km of the Central Indian Ridge during RRS Charles Darwin cruise CD127 reveal an abundance of neovolcanic activity throughout both spreading segments and ridge non-transform discontinuities alike. Imagery of the previously unsurveyed northern section of the CIR immediately south of the Marie Celeste Fracture Zone confirms the presence of a shallow, magmatically inflated second order segment that is only recently rifted, with a rift floor surfaced throughout by virtually untectonised planar sheet flow units. First and second order segments exhibit a significant component of sheeted extrusives, ponded or in lake form, abutting or overstepped by hummocky and mounded pillow constructs. Non-transform discontinuities are commonly cut by fresh axial volcanic ridges oblique to both axial trend and offset. The depths of segment centers range from 2600m to more than 3700m, and segment forms include robust, hour-glass and rifted/starved end-members - but their overall extrusive pattern is strikingly invariant. Fracture Zone offsets of up to 65 kilometres are tectonically dominated, but their intersections with the axis are often mantled by multiple sheet flows rather than the relatively low proportions of sediment cover. The largest offsets are marked by outcrops of multiple, subparallel displacement surfaces, actively eroding transverse ridges, and ridge transform intersections with classic propagation/recession fabrics - each suggesting some instability in regional plate kinematics. While it is tempting to speculate that the Rodrigues hotspot appears to have a regional effect, enhancing magmatic delivery to the adjacent ridge and offset system, the apparent breadth of influence from what is assumed to be a rather feeble mantle anomaly is problematic.

Hazard Models From Periodic Dike Intrusions at Kı¯lauea Volcano, Hawai`i

NASA Astrophysics Data System (ADS)

Montgomery-Brown, E. K.; Miklius, A.

2016-12-01

The persistence and regular recurrence intervals of dike intrusions in the East Rift Zone (ERZ) of Kı¯lauea Volcano lead to the possibility of constructing a time-dependent intrusion hazard model. Dike intrusions are commonly observed in Kı¯lauea Volcano's ERZ and can occur repeatedly in regions that correlate with seismic segments (sections of rift seismicity with persistent definitive lateral boundaries) proposed by Wright and Klein (USGS PP1806, 2014). Five such ERZ intrusions have occurred since 1983 with inferred locations downrift of the bend in Kı¯lauea's ERZ, with the first (1983) being the start of the ongoing ERZ eruption. The ERZ intrusions occur on one of two segments that are spatially coincident with seismic segments: Makaopuhi (1993 and 2007) and Nāpau (1983, 1997, and 2011). During each intrusion, the amount of inferred dike opening was between 2 and 3 meters. The times between ERZ intrusions for same-segment pairs are all close to 14 years: 14.07 (1983-1997), 14.09 (1997-2011), and 13.95 (1993-2007) years, with the Nāpau segment becoming active about 3.5 years after the Makaopuhi segment in each case. Four additional upper ERZ intrusions are also considered here. Dikes in the upper ERZ have much smaller opening ( 10 cm), and have shorter recurrence intervals of 8 years with more variability. The amount of modeled dike opening during each of these events roughly corresponds to the amount of seaward south flank motion and deep rift opening accumulated in the time between events. Additionally, the recurrence interval of 14 years appears to be unaffected by the magma surge of 2003-2007, suggesting that flank motion, rather than magma supply, could be a controlling factor in the timing and periodicity of intrusions. Flank control over the timing of magma intrusions runs counter to the historical research suggesting that dike intrusions at Kı¯lauea are driven by magma overpressure. This relatively free sliding may have resulted from decreased friction following the 1975 Kalapana earthquake. A hazard model can be constructed from the historical intrusion record (i.e., how long has it been since an intrusion on that segment), and augmented by monitoring the accumulation of strain across the rift and local seismicity rates.

The Terceira Rift as hyper-slow, hotspot-dominated oblique spreading axis: A comparison with other slow-spreading plate boundaries

NASA Astrophysics Data System (ADS)

Vogt, P. R.; Jung, W. Y.

2004-01-01

We suggest the 550 km long Terceira Rift (TR, Azores Plateau) is the world's slowest-spreading (hyper-slow, 4 mm/a plate separation; 2.3-3.8 mm/a perpendicular to oblique axial segments) organized accreting plate boundary. In its slightly sinuous (ca. 300 km radius of curvature) axial trace, its oblique spreading angles (ca. 40°-65°), and in frequency and first motions of earthquakes, the TR resembles better-known 'ultra-' or 'super-' slow spreading ridges (e.g. Gakkel and Southwest Indian ridges). Interpreted simply as volcanically 'unfilled' rift valley segments, the inter-island basins (e.g. the 3200 m deep Hirondelle Basin) are slightly wider (30-60 km), but not significantly deeper (1000-2200 m) than the Mid-Atlantic Ridge (MAR) median valley (20-28 mm/a; 10°N-53°N). However, along-axis segmentation wavelengths (ca. 100 km) are double those along the central MAR, but make TR comparable to the 'ultra-slow' (15-16 mm/a) Southwest Indian and Gakkel (7-13 mm/a) ridges. If this segmentation wavelength reflects Rayleigh-Taylor instabilities, the viscosity contrast between the overlying axial lithosphere and the partial melt zones is about an order of magnitude greater at ca. 4-16 mm/a than at 20-30 mm/a. The TR differs dramatically from ultra-slow ridges only in the large amplitude of along-strike topography (2000-4000 m; 4200 m total variation) owing perhaps to a copious melt flux from the Azores 'hotspot', combined with a spreading-rate-determined greater axial flexural strength and plate thickness, and slower export of volcanics from the rift axis. The probable TR youth (ca. 1 Ma?, requiring less than 4 km new oceanic crust) suggests lack of steady-state spreading conditions, which may explain the published gravity evidence against TR spreading. Absolute plate motions support the creation of the Azores Plateau by successive NE jumps of the rift axis to maintain its position over a fixed 'hotspot'.

Study of the deformation in Central Afar using InSAR NSBAS chain

NASA Astrophysics Data System (ADS)

Deprez, A.; Doubre, C.; Grandin, R.; Saad, I.; Masson, F.; Socquet, A.

2013-12-01

The Afar Depression (East Africa) connects all three continental plates of Arabia, Somalia and Nubia plates. For over 20 Ma, the divergent motion of these plates has led to the formation of large normal faults building tall scarps between the high plateaus and the depression, and the development of large basins and an incipient seafloor spreading along a series of active volcano-tectonic rift segments within the depression. The space-time evolution of the active surface deformation over the whole Afar region remains uncertain. Previous tectonic and geodetic studies confirm that a large part of the current deformation is concentrated along these segments. However, the amount of extension accommodated by other non-volcanic basins and normal faulting remains unclear, despite significant micro-seismic activity. Due to the active volcanism, large transient displacements related to dyking sequence, notably in the Manda Hararo rift (2005-2010), increase the difficulty to characterize the deformation field over simple time and space scales. In this study, we attempt to obtain a complete inventory of the deformation within the whole Afar Depression and to understand the associated phenomena, which occurred in this singular tectonic environment. We study in particular, the behavior of the structures activated during the post-dyking stage of the rift segments. For this purpose, we conduct a careful processing of a large set of SAR ENVISAT images over the 2004-2010 period, we also use previous InSAR results and GPS data from permanent stations and from campaigns conducted in 1999, 2003, 2010, 2012 within a GPS network particularly dense along the Asal-Ghoubbet segment. In one hand, in the western part of Afar, the far-field response of the 2005-2010 dyke sequence appears to be the dominant surface motion on the mean velocity field. In an other hand, more eastward across the Asal-Ghoubbet rift, strong gradients of deformation are observed. The time series analysis of both InSAR and GPS data allow us to (1) point out the role of volcano activity on the localization of the extensive deformation within these rifts, (2) describe the temporal evolution of the mostly aseismic fault slips, and eventually (3) characterize the behavior of the crust after the dyking events in relation to visco-elastic relaxation. Moreover, we analyze several interesting small patches of localized deformation revealing transient displacements by combining time series results and seismic data collected by the Arta Geophysical Observatory in Djibouti. In particular, a specific clear deformation pattern on the northern margin of the Tadjoura Bay could be associated with a seismic swarm, probably resulting from the occurrence of an offshore dyking event sequence along the immerged Tadjoura rift segment.

Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains' extreme elevation?

NASA Astrophysics Data System (ADS)

Wallner, Herbert; Schmeling, Harro

2010-10-01

With heights of 4-5 km, the topography of Rwenzori Mountains, a large horst of old crustal rocks located inside a young passive rift system, poses the question “Why are the Rwenzori Mountains so high?”. The Cenozoic Western Rift branch of the East African Rift System is situated within the Late Proterozoic mobile belts between the Archean Tanzania Craton and Congo Craton. The special geological setting of the massif at a rift node encircled by the ends of the northern Western Rift segments of Lake Albert and Lake Edward suggests that the mechanism responsible for the high elevation of the Rwenzoris is related to the rifting process. Our hypothesis is based on the propagation of the rift tips, surrounding the stiff old lithosphere at Rwenzori region, thereby triggering the delamination of the cold and dense mantle lithosphere (ML) root by reducing viscosity and strength of the undermost lower crust. As a result, this unloading induces fast isostatic pop-up of the less dense crustal Rwenzori block. We term this RID—“ rift induced delamination of Mantle Lithosphere”. The physical consistency of the RID hypothesis is tested numerically. Viscous flow of 2D models is approximated by a Finite Difference Method with markers in an Eulerian formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component system. Based on laboratory data of appropriate rock samples, a temperature-, pressure- and stress-dependent rheology is assumed. Assuming a simple starting model with a locally heated ML, the ML block between the weakened zones becomes unstable and sinks into the asthenosphere, while the overlying continental crust rises up. Thus, RID seems to be a viable mechanism to explain geodynamically the extreme uplift. Important conditions are a thermal anomaly within the ML, a ductile lower crust with visco-plastic rheology allowing significant strength reduction and lateral density variations. The special situation of a two-sided rifting or offset rift segments to decouple the ML laterally from the surrounding continental lithosphere seems to be most decisive. Further support for the RID mechanism may come from additional crustal thickness and an extensive stress field. Some parameters, such as the excess temperature and yield stress, are very sensitive, small changes determine whether delamination takes place or not.

Single-Molecule FISH Reveals Non-selective Packaging of Rift Valley Fever Virus Genome Segments

PubMed Central

Wichgers Schreur, Paul J.; Kortekaas, Jeroen

2016-01-01

The bunyavirus genome comprises a small (S), medium (M), and large (L) RNA segment of negative polarity. Although genome segmentation confers evolutionary advantages by enabling genome reassortment events with related viruses, genome segmentation also complicates genome replication and packaging. Accumulating evidence suggests that genomes of viruses with eight or more genome segments are incorporated into virions by highly selective processes. Remarkably, little is known about the genome packaging process of the tri-segmented bunyaviruses. Here, we evaluated, by single-molecule RNA fluorescence in situ hybridization (FISH), the intracellular spatio-temporal distribution and replication kinetics of the Rift Valley fever virus (RVFV) genome and determined the segment composition of mature virions. The results reveal that the RVFV genome segments start to replicate near the site of infection before spreading and replicating throughout the cytoplasm followed by translocation to the virion assembly site at the Golgi network. Despite the average intracellular S, M and L genome segments approached a 1:1:1 ratio, major differences in genome segment ratios were observed among cells. We also observed a significant amount of cells lacking evidence of M-segment replication. Analysis of two-segmented replicons and four-segmented viruses subsequently confirmed the previous notion that Golgi recruitment is mediated by the Gn glycoprotein. The absence of colocalization of the different segments in the cytoplasm and the successful rescue of a tri-segmented variant with a codon shuffled M-segment suggested that inter-segment interactions are unlikely to drive the copackaging of the different segments into a single virion. The latter was confirmed by direct visualization of RNPs inside mature virions which showed that the majority of virions lack one or more genome segments. Altogether, this study suggests that RVFV genome packaging is a non-selective process. PMID:27548280

Creation of a Recombinant Rift Valley Fever Virus with a Two-Segmented Genome ▿ †

PubMed Central

Brennan, Benjamin; Welch, Stephen R.; McLees, Angela; Elliott, Richard M.

2011-01-01

Rift Valley fever virus (RVFV; family Bunyaviridae) is a clinically important, mosquito-borne pathogen of both livestock and humans, which is found mainly in sub-Saharan Africa and the Arabian Peninsula. RVFV has a trisegmented single-stranded RNA (ssRNA) genome. The L and M segments are negative sense and encode the L protein (viral polymerase) on the L segment and the virion glycoproteins Gn and Gc as well as two other proteins, NSm and 78K, on the M segment. The S segment uses an ambisense coding strategy to express the nucleocapsid protein, N, and the nonstructural protein, NSs. Both the NSs and NSm proteins are dispensable for virus growth in tissue culture. Using reverse genetics, we generated a recombinant virus, designated r2segMP12, containing a two-segmented genome in which the NSs coding sequence was replaced with that for the Gn and Gc precursor. Thus, r2segMP12 lacks an M segment, and although it was attenuated in comparison to the three-segmented parental virus in both mammalian and insect cell cultures, it was genetically stable over multiple passages. We further show that the virus can stably maintain an M-like RNA segment encoding the enhanced green fluorescent protein gene. The implications of these findings for RVFV genome packaging and the potential to develop multivalent live-attenuated vaccines are discussed. PMID:21795328

Volcanism on the fossil Galapagos Rise spreading centre, SE Pacific

NASA Astrophysics Data System (ADS)

Haase, K. M.; Stroncik, N. A.

2002-12-01

A part of the fossil spreading centre of the Galapagos Rise at 10° S, 95° W in the SE Pacific Ocean was mapped and sampled. This spreading centre was active for about 12 Ma and was abandoned about 6.5 Ma ago when the spreading rate of the East Pacific Rise (EPR) increased. The aim of this study is to understand the tectonic and petrological implications of the ridge jump for the spreading centre and to gain insights into the processes in its melting column. Bathymetric swath mapping of a part of the Galapagos Rise revealed an elongated structure with a NNE-SSW strike direction which is bounded by a large fracture zone in the north. The mapped area can be divided into three segments, each of about 50 km length. The northernmost segment consists of an ~4400 m deep rift which shows similarities to a slow-spreading centre, e.g. the Mid-Atlantic Ridge. The southern two segments are volcanic ridges with numerous volcanic flank cones which reach water depths up to 490 m. This volcanic ridge is interpreted as the continuation of the fossil spreading axis. While the northernmost segment is magmatically starved, the volcanic ridges of the southern two segments apparently formed after cessation of spreading. The rock samples from the rift flanks in the north are incompatible element-depleted (K/Ti 0.08-0.28) and plagioclase-phyric basalts resembling typical mid-ocean ridge basalts (MORB). In contrast, the lavas from the two volcanic ridge segments in the south are highly vesicular incompatible element-enriched alkali basalts with K/Ti of 0.65-1.4. The depleted rift basalts have Sr isotope ratios below 0.7027 while the alkali basalts from the ridge range between 0.7029 and 0.7031. The rift basalts have significantly lower sodium contents than the alkali basalts and thus the southern lavas are probably derived by smaller degrees of partial melting. The relatively low Si contents of the alkali basalts also indicates formation deeper in the melting column than the northern MORB-like samples. The mantle source of the alkali basalts is similar to the enriched source of off-axis seamounts along the EPR. Our preliminary data suggest that the northernmost segment formed by tectonic processes during a final slow-spreading phase of the Galapagos Rise while the southern two segments erupted alkaline lavas probably after spreading stopped.

Magma-tectonic interactions in an area of active extension; a review of recent observations, models and interpretations from Iceland

NASA Astrophysics Data System (ADS)

Pedersen, Rikke; Sigmundsson, Freysteinn; Drouin, Vincent; Rafn Heimisson, Elías; Parks, Michelle; Dumont, Stéphanie; Árnadóttir, Þóra; Masterlark, Timothy; Ófeigsson, Benedíkt G.; Jónsdóttir, Kristín; Hooper, Andrew

2016-04-01

The geological setting of Iceland provides rich opportunities of studying magma-tectonic interactions, as it constitutes Earth's largest part of the mid-oceanic ridge system exposed above sea level. A series of volcanic and seismic zones accommodate the ~2 cm/year spreading between the North-American and Eurasian plates, and the Icelandic hot-spot conveniently provides the means of exposing this oceanic crust-forming setting above sea-level. Both extinct and active plumbing system structures can be studied in Iceland, as the deeply eroded tertiary areas provide views into the structures of extinct volcanic systems, and active processes can be inferred on in the many active volcanic systems. A variety of volcanic and tectonic processes cause the Icelandic crust to deform continuously, and the availability of contemporaneous measurements of crustal deformation and seismicity provide a powerful data set, when trying to obtain insight into the processes working at depth, such as magma migration through the uppermost lithosphere, magma induced host rock deformation and volcanic eruption locations and styles. The inferences geodetic and seismic datasets allow on the active plate spreading processes and subsurface magma movements in Iceland will be reviewed, in particular in relation to the Northern Volcanic Zone (NVZ). There the three phases of a rifting cycle (rifting, post-rifting, inter-rifting) have been observed. The NVZ is an extensional rift segment, bounded to the south by the Icelandic mantle plume, and to the north by the Tjörnes transform zone. The NVZ has typically been divided into five partly overlapping en-echelon fissure swarms, each with a central main volcanic production area. Most recently, additional insight into controlling factors during active rifting has been provided by the Bárðarbunga activity in 2014-2015 that included a major rifting event, the largest effusive eruption in Iceland since 1783, and a gradual caldera collapse. It is evident from available datasets that improved rifting-cycle models do need to incorporate realistic lithospheric properties, as well as the dynamic transport of magma, in order to reproduce the variety of observations, and provide means of forecasting large future dyking events and eruptions at active rifting segments.

Pangea break-up: from passive to active margin in the Colombian Caribbean Realm

NASA Astrophysics Data System (ADS)

Gómez, Cristhian; Kammer, Andreas

2017-04-01

The break-up of Western Pangea has lead to a back-arc type tectonic setting along the periphery of Gondwana, with the generation of syn-rift basins filled with sedimentary and volcanic sequences during the Middle to Late Triassic. The Indios and Corual formations in the Santa Marta massif of Northern Andes were deposited in this setting. In this contribution we elaborate a stratigraphic model for both the Indios and Corual formations, based on the description and classification of sedimentary facies and their architecture and a provenance analysis. Furthermore, geotectonic environments for volcanic and volcanoclastic rock of both units are postulated. The Indios Formation is a shallow-marine syn-rift basin fill and contains gravity flows deposits. This unit is divided into three segments; the lower and upper segments are related to fan-deltas, while the middle segment is associated to offshore deposits with lobe incursions of submarine fans. Volcanoclastic and volcanic rocks of the Indios and Corual formations are bimodal in composition and are associated to alkaline basalts. Volcanogenic deposits comprise debris, pyroclastic and lava flows of both effusive and explosive eruptions. These units record multiple phases of rifting and reveal together a first stage in the break-up of Pangea during Middle and Late Triassic in North Colombia.

Voluminous lava flow from Axial Seamount's south rift constrains extension rate on northern Vance Segment

NASA Astrophysics Data System (ADS)

Le Saout, M.; Clague, D. A.; Paduan, J. B.

2017-12-01

Axial Seamount is characterized by a robust magma supply resulting from the interaction between the Cobb hotspot and the Juan de Fuca Ridge. During the last two decades, magmatic activity was focused within the summit caldera and upper and middle portions of the two rift zones, with eruptions in 1998, 2011, and 2015. However, the distal ends of both rift zones have experienced numerous eruptions in the past. The most voluminous flows are located near the extreme ends, greater than 40 kilometers from the caldera. Where Axial's South Rift Zone overlaps with the Vance Segment of the Juan de Fuca Ridge, the 2015 MBARI expedition mapped 16 km2 of the seafloor with our AUV, and collected 33 rocks and 33 sediment cores during two ROV dives. The data were used to confirm the boundaries of an extensive flow tentatively identified using modern ship based bathymetry. This flow is 18 km wide and 6 km long for a total surface area of 63 km2. The flow is modified by superficial ( 5 m deep) and deep (25 to 45 m deep) subsidence pits, with the deepest pits giving an indication of the minimum thickness of the flow. The maximum thickness of 100 m is measured at the margins of the flow. We thus estimate a volume between 2.5 and 6 km3, making this flow the most voluminous known on the global mid ocean ridge system. The minimum volume is equivalent to the present volume of the summit caldera. Radiocarbon ages of foraminifera from the basal sections of sediment cores suggest that this flow is 1000 years old. This flow travelled east and partially filled the axial valley of the adjacent Vance Segment. Since emplacement, this part of the flow has experienced deformation by fissures and faults aligned with the trend of the Vance Segment. The horizontal extension across these features allows us to estimate a local deformation rate of 3 cm/yr of tectonic extension on the northern end of Vance Segment during the last 1000 years.

Geodetic Measurements and Numerical Models of Rifting in Northern Iceland for 1993-1999

NASA Astrophysics Data System (ADS)

Ali, T.; Feigl, K.; Masterlark, T.; Carr, B. B.; Sigmundsson, F.; Thurber, C. H.

2009-12-01

Rifting occurs as episodes of active deformation in individual rift segments of the Northern Volcanic Zone (NVZ) of Iceland. To measure the deformation, we use interferometric analysis of synthetic aperture radar (InSAR) data acquired between 1993 and 1999. Preliminary results suggest that a complex interplay of multiple inflating and deflating sources at depth is required to account for the observed deformation. In an effort to integrate heterogeneous constraining information (kinematic plate spreading, seismic tomography and anisotropy, and thermal and rheologic structures), we develop finite element models that simulate the underlying sources and processes associated with rifting events to quantitatively understand the magmatic plumbing system beneath Krafla central volcano and rift segment, the site of the most recent rifting episode in the NVZ. Calibration parameters include the positions, geometries, and flux rates for elements of the plumbing system, as well as material properties. The General Inversion for Phase Technique (GIPhT) [Feigl and Thurber, Geophys. J. Int., 2009] is used to model the InSAR phase data directly, without unwrapping parameters. It operates on wrapped phase values ranging from -1/2 to +1/2 cycles. By defining a cost function that quantifies the misfit between observed and modeled values in terms of wrapped phase, GIPhT can estimate parameters in a geophysical model by minimizing the cost function. Since this approach can handle noisy, wrapped phase data, it avoids the pitfalls of phase-unwrapping approaches. Consequently, GIPhT allows the analysis, interpretation and modeling of more interferometric pairs than approaches that require unwrapping. GIPhT also allows statistical testing of hypotheses because the wrapped phase residuals follow a Von Mises distribution. As a result, the model parameters estimated by GIPhT include formal uncertainties. We test the hypothesis that deformation in the rift zone occurred at a constant (secular) rate of volume change over the observed time interval. We evaluate several functional forms for the temporal evolution of the sources. The best fitting model employs a linear time function, indicative of secular deformation in the rift zone. We conclude that post-rifting deformation following the 1975-1984 Krafla fires rifting episode has dissipated on a time scale on the order of a decade.

Northeastern Brazilian margin: Regional tectonic evolution based on integrated analysis of seismic reflection and potential field data and modelling

NASA Astrophysics Data System (ADS)

Blaich, Olav A.; Tsikalas, Filippos; Faleide, Jan Inge

2008-10-01

Integration of regional seismic reflection and potential field data along the northeastern Brazilian margin, complemented by crustal-scale gravity modelling, is used to reveal and illustrate onshore-offshore crustal structure correlation, the character of the continent-ocean boundary, and the relationship of crustal structure to regional variation of potential field anomalies. The study reveals distinct along-margin structural and magmatic changes that are spatially related to a number of conjugate Brazil-West Africa transfer systems, governing the margin segmentation and evolution. Several conceptual tectonic models are invoked to explain the structural evolution of the different margin segments in a conjugate margin context. Furthermore, the constructed transects, the observed and modelled Moho relief, and the potential field anomalies indicate that the Recôncavo, Tucano and Jatobá rift system may reflect a polyphase deformation rifting-mode associated with a complex time-dependent thermal structure of the lithosphere. The constructed transects and available seismic reflection profiles, indicate that the northern part of the study area lacks major breakup-related magmatic activity, suggesting a rifted non-volcanic margin affinity. In contrast, the southern part of the study area is characterized by abrupt crustal thinning and evidence for breakup magmatic activity, suggesting that this region evolved, partially, with a rifted volcanic margin affinity and character.

Four-segmented Rift Valley fever virus-based vaccines can be applied safely in ewes during pregnancy.

PubMed

Wichgers Schreur, Paul J; van Keulen, Lucien; Kant, Jet; Kortekaas, Jeroen

2017-05-25

Rift Valley fever virus (RVFV) causes severe and recurrent outbreaks on the African continent and the Arabian Peninsula and continues to expand its habitat. This mosquito-borne virus, belonging to the genus Phlebovirus of the family Bunyaviridae contains a tri-segmented negative-strand RNA genome. Previously, we developed four-segmented RVFV (RVFV-4s) variants by splitting the M-genome segment into two M-type segments each encoding one of the structural glycoproteins; Gn or Gc. Vaccination/challenge experiments with mice and lambs subsequently showed that RVFV-4s induces protective immunity against wild-type virus infection after a single administration. To demonstrate the unprecedented safety of RVFV-4s, we here report that the virus does not cause encephalitis after intranasal inoculation of mice. A study with pregnant ewes subsequently revealed that RVFV-4s does not cause viremia and does not cross the ovine placental barrier, as evidenced by the absence of teratogenic effects and virus in the blood and organs of the fetuses. Altogether, these results show that the RVFV-4s vaccine virus can be applied safely in pregnant ewes. Copyright © 2017 Elsevier Ltd. All rights reserved.

the role of magmatism and segmentation in the structural evolution of the Afar Rift

NASA Astrophysics Data System (ADS)

Stab, Martin; Bellahsen, Nicolas; Pik, Raphaël; Quidelleur, Xavier; Ayalew, Dereje; Leroy, Sylvie

2015-04-01

A common issue at volcanic passive margins (VPM) is the lack of observation of the structures that accommodate stretching and thinning. Indeed, the most distal parts and the Ocean-Continent Transition is often masked by thick seaward-dipping reflectors (SDR) sequences. Some current challenges are then to know if the observed thinning fit the divergence (thinning vs dyking); and what is the rheological effect of magma supply that re-thickens the crust during extension? In the Central Afar magmatic rift (Ethiopia), the structures related to rifting since Oligocene are cropping out onshore and are well preserved. We present here a new structural model based on field data and lavas (U-Th/He and K/Ar) datings along a balanced cross-section of the Central Afar Western Margin. We mapped continent-ward normal fault array affecting highly tilted trapp series (29-30 Ma) unconformably overlain by tilted Oligo-Miocene (25-7 Ma) acid series. The main extensional and necking/thinning event took place during the end of this Miocene magmatic episode. The Pliocene flood basalt (Stratoid series) is erupted over an already thinned crust. The bulk extension for the Afar Western Margin is ß ~ 2.50. Our main findings are: - Oligo-Miocene deformation in Central Afar appears to be largely distributed through space and time ("magmatic wide rift"). It has been accommodated in a 200-300 km wide strip being a diffuse incipient plate boundary during the whole rifting history until the formation of present-day magmatic segments. There is a period of tectonic quiescence accompanied with few magma erupted at the surface between 25 Ma and 7 Ma. We suggest that tectonic and magmatic activity was focused at that time on the highly faulted Danakil block and Southern Red Sea, away from our study zone. - ß ~ 2.50 is higher than the thinning factor of ~1.30 observed in geophysical studies. We propose that the continental crust in Central Afar has been re-thickened during extension by the syn-rift magmatic supply. The difference in tectono-magmatic style between Central Afar (distributed extension and thick crust) and Northern Afar Erta Ale segment (narrow graben, thin crust) may be explained by the difference of magma volume (extruded & underplated) brought to the crust during extension. Magma supply in Central Afar thus allows the crust to be stretched without extreme thinning despite high degree of divergence. Thus, break-up may occur in both Central and Northern Afar, not depending on the apparent thickness of the crust but rather on the ability of the system to localize deformation. - There appears to be a link between early-rift transform zones and distribution of magmatic activity that affects in turn the structural style. We suggest that the closest feature from the SDR at mature VPM is the Stratoid series. The difference of volume between the Stratoid and the enormous volume of SDR imaged in seismic studies (e.g South Atlantic) is probably best explained by an initial low mantle potential temperature in Afar. Contrasted structural styles in Afar are the product of magma supply and segmentation, controlling thinning and extension distribution in the rift.

Morphotectonic architecture of the Transantarctic Mountains rift flank between the Royal Society Range and the Churchill Mountains based on geomorphic analysis

USGS Publications Warehouse

Demyanick, Elizabeth; Wilson, Terry J.

2007-01-01

Extensional forces within the Antarctic Plate have produced the Transantarctic Mountains rift-flank uplift along the West Antarctic rift margin. Large-scale linear morphologic features within the mountains are controlled by bedrock structure and can be recognized and mapped from satellite imagery and digital elevation models (DEMs). This study employed the Antarctic Digital Database DEM to obtain slope steepness and aspect maps of the Transantarctic Mountains (TAM) between the Royal Society Range and the Churchill Mountains, allowing definition of the position and orientation of the morphological axis of the rift-flank. The TAM axis, interpreted as a fault-controlled escarpment formed by coast-parallel retreat, provides a marker for the orientation of the faulted boundary between the TAM and the rift system. Changes in position and orientation of the TAM axis suggests the rift flank is segmented into tectonic blocks bounded by relay ramps and transverse accommodation zones. The transverse boundaries coincide with major outlet glaciers, supporting interpretation of rift structures between them. The pronounced morphological change across Byrd Glacier points to control by structures inherited from the Ross orogen.

Dynamic rupture simulations on a fault network in the Corinth Rift

NASA Astrophysics Data System (ADS)

Durand, V.; Hok, S.; Boiselet, A.; Bernard, P.; Scotti, O.

2017-03-01

The Corinth rift (Greece) is made of a complex network of fault segments, typically 10-20 km long separated by stepovers. Assessing the maximum magnitude possible in this region requires accounting for multisegment rupture. Here we apply numerical models of dynamic rupture to quantify the probability of a multisegment rupture in the rift, based on the knowledge of the fault geometry and on the magnitude of the historical and palaeoearthquakes. We restrict our application to dynamic rupture on the most recent and active fault network of the western rift, located on the southern coast. We first define several models, varying the main physical parameters that control the rupture propagation. We keep the regional stress field and stress drop constant, and we test several fault geometries, several positions of the faults in their seismic cycle, several values of the critical distance (and so several fracture energies) and two different hypocentres (thus testing two directivity hypothesis). We obtain different scenarios in terms of the number of ruptured segments and the final magnitude (between M = 5.8 for a single segment rupture to M = 6.4 for a whole network rupture), and find that the main parameter controlling the variability of the scenarios is the fracture energy. We then use a probabilistic approach to quantify the probability of each generated scenario. To do that, we implement a logical tree associating a weight to each model input hypothesis. Combining these weights, we compute the probability of occurrence of each scenario, and show that the multisegment scenarios are very likely (52 per cent), but that the whole network rupture scenario is unlikely (14 per cent).

Seafloor spreading event in western Gulf of Aden during the November 2010-March 2011 period captured by regional seismic networks: evidence for diking events and interactions with a nascent transform zone

NASA Astrophysics Data System (ADS)

Ahmed, Abdulhakim; Doubre, Cécile; Leroy, Sylvie; Kassim, Mohamed; Keir, Derek; Abayazid, Ahmadine; Julie, Perrot; Laurence, Audin; Vergne, Jérome; Alexandre, Nercessian; Jacques, Eric; Khanbari, Khaled; Sholan, Jamal; Rolandone, Frédérique; Al-Ganad, Ismael

2016-05-01

In November 2010, intense seismic activity including 29 events with a magnitude above 5.0, started in the western part of the Gulf of Aden, where the structure of the oceanic spreading ridge is characterized by a series of N115°-trending slow-spreading segments set within an EW-trending rift. Using signals recorded by permanent and temporary networks in Djibouti and Yemen, we located 1122 earthquakes, with a magnitude ranging from 2.1 to 5.6 from 2010 November 1 to 2011 March 31. By looking in detail at the space-time distribution of the overall seismicity, and both the frequency and the moment tensor of large earthquakes, we re-examine the chronology of this episode. In addition, we also interpret the origin of the activity using high-resolution bathymetric data, as well as from observations of seafloor cable damage caused by high temperatures and lava flows. The analysis allows us to identify distinct active areas. First, we interpret that this episode is mainly related to a diking event along a specific ridge segment, located at E044°. In light of previous diking episodes in nearby subaerial rift segments, for which field constraints and both seismic and geodetic data exist, we interpret the space-time evolution of the seismicity of the first few days. Migration of earthquakes suggests initial magma ascent below the segment centre. This is followed by a southeastward dike propagation below the rift immediately followed by a northwestward dike propagation below the rift ending below the northern ridge wall. The cumulative seismic moment associated with this sequence reaches 9.1 × 1017 Nm, and taking into account a very low seismic versus geodetic moment, we estimate a horizontal opening of ˜0.58-2.9 m. The seismic activity that followed occurred through several bursts of earthquakes aligned along the segment axis, which are interpreted as short dike intrusions implying fast replenishment of the crustal magma reservoir feeding the dikes. Over the whole period, the opening is estimated to be ˜1.76-8.8 m across the segment. A striking feature of this episode is that the seismicity remained confined within one individual segment, whereas the adjacent en-echelon segments were totally quiescent, suggesting that the magma supply system of one segment is disconnected from those of the neighbouring segments. Second, we identify activity induced by the first intrusion with epicentres aligned along an N035°E-trending, ˜30 km long at the northwestern end of the active opening segment. This group encompasses more than seven earthquakes with magnitude larger than 5.0, and with strike-slip focal mechanisms consistent with the faults identified in the bathymetry and the structural pattern of the area. We propose that a transform fault is currently in formation which indicates an early stage of the ridge segmentation, at the locus of the trend change of the spreading ridge, which also corresponds to the boundary between a clear oceanic lithosphere and the zone of transform between continental and oceanic crust.

GPS constraints on broad scale extension in the Ethiopian Highlands and Main Ethiopian Rift

NASA Astrophysics Data System (ADS)

Birhanu, Yelebe; Bendick, Rebecca; Fisseha, Shimeles; Lewi, Elias; Floyd, Michael; King, Robert; Reilinger, Robert

2016-07-01

Measurements from GPS sites spanning the Ethiopian Highlands, Main Ethiopian Rift, and Somali Platform in Ethiopia and Eritrea show that present-day finite strain rates throughout NE Africa can be approximated at the continent scale by opening on the MER. Most sites in the Ethiopian Highlands are consistent with the motion of the Nubian plate at the level of 1 mm/yr with 95% confidence. However, sites at least as far as 60 km west of the rift show higher velocities relative to the stable Nubian frame of 1-2 mm/yr, requiring a combination of localized and distributed deformation to accommodate the African extensional domain. Off-rift velocities are consistent with ongoing strain related to either high gravitational potential energy or intrusive magmatism away from midrift magmatic segments either on the western rift margin or within the Ethiopian Highlands, especially when combined with likely rheological differences between the Ethiopian Rift and Highlands. Velocities from the Somali Platform are less well determined with uncertainties and residuals from a Somali frame definition at the level of 2-3 mm/yr but without spatially correlated residuals.

Kinematics of the asal rift (djibouti) determined from the deformation of fieale volcano.

PubMed

De Chabalier, J B; Avouac, J P

1994-09-16

Because of its subaerial exposure the Asal rift segment provides an exceptional opportunity to quantify the deformation field of an active rift and assess the contribution of tectonics and volcanism to rifting processes. The present topography of the Asal rift results from the tectonic dismemberment during the last 100,000 years of a large central volcanic edifice that formed astride the rift zone 300,000 to 100,000 years ago. Three-dimensional deformation of this volcano has been quantified from the combined analysis of the topography and geology. The analysis indicates that spreading at 17 to 29 millimeters per year in a N40 degrees +/- 5 degrees E direction accounts for most of the separation between Arabia and Somalia. The small topographic subsidence relative to extension suggests that tectonic thinning of the crust has been balanced by injection and underplating of magmatic material of near crustal density. The methodology developed in this study could also be applied to quantify deformation in relatively inaccessible areas where the main available information is topography or bathymetry.

The Geomorphometrics of the Rio Grande Rift: The role of tectonics, climate, and erosional processes in forming the Rio Grande river

NASA Astrophysics Data System (ADS)

Berry, M. A.; van Wijk, J.; Emry, E.; Axen, G. J.; Coblentz, D. D.

2016-12-01

Geomorphometrics provides a powerful tool for quantifying the topographic fabric of a landscape and can help with correlating surface features with underlying dynamic processes. Here we use a suite of geomorphometric metrics (including the topographic power spectra, fabric orientation/organization) to compare and contrast the geomorphology of two of the world's major rifts, the Rio Grande Rift (RGR) in western US and the East Africa Rift (EAR). The motivation for this study is the observation of fundamental differences between the characteristics of the intra-rift river drainage for the two rifts. The RGR consists of a series of NS trending rift basins, connected by accommodation or transfer zones. The Rio Grande river developed in the late Neogene, and follows these rift segments from the San Luis basin in Colorado to the Gulf of Mexico. Before the river system formed, basins are thought to have formed internally draining systems, characterized by shallow playa lakes. This is in contrast with lakes in the Tanganyika and Malawi rifts of the East African Rift that are deep and have existed for >5 My. We investigate the role of climate, tectonics and erosional processes in the formation of the through-going Rio Grande river. This occurred around the time of a slowing down of rift opening ( 10 Ma), but also climatic changes in the southwestern U.S. have been described for the late Neogene. To model our hypothesis, a tectonics and surface transport code TISC (Transport, Isostasy, Surface Transport, Climate) was used to evaluate the dynamics of a series of proto-rift basins and their connecting accommodation zones. Basin infill and drainage system development are studied as a result of varying sediment budgets, climate variables, and rift opening rate.

Checking the validity of superimposing analytical deformation models and implications for numerical modelling of dikes and magma chambers

NASA Astrophysics Data System (ADS)

Pascal, K.; Neuberg, J. W.; Rivalta, E.

2011-12-01

The displacement field due to magma movements in the subsurface is commonly modelled using the solutions for a point source (Mogi, 1958), a finite spherical source (McTigue, 1987), or a dislocation source (Okada, 1992) embedded in a homogeneous elastic half-space. When the magmatic system is represented by several sources, their respective deformation fields are summed, and the assumption of homogeneity in the half-space is violated. We have investigated the effects of neglecting the interaction between sources on the surface deformation field. To do so, we calculated the vertical and horizontal displacements for models with adjacent sources and we tested them against the solutions of corresponding numerical 3D finite element models. We implemented several models combining spherical pressure sources and dislocation sources, varying the pressure or opening of the sources and their relative position. We also investigated various numerical methods to model a dike as a dislocation tensile source or as a pressurized tabular crack. In the former case, the dike opening was either defined as two boundaries displaced from a central location, or as one boundary displaced relative to the other. We finally considered two case studies based on Soufrière Hills Volcano (Montserrat, West Indies) and the Dabbahu rift segment (Afar, Ethiopia) magmatic systems. We found that the discrepancies between simple superposition of the displacement field and a fully interacting numerical solution depend mostly on the source types and on their spacing. Their magnitude may be comparable with the errors due to neglecting the topography, the inhomogeneities in crustal properties or more realistic rheologies. In the models considered, the errors induced when neglecting the source interaction can be neglected (<5%) when the sources are separated by at least 4 radii for two combined Mogi sources and by at least 3 radii for juxtaposed Mogi and Okada sources. Furthermore, this study underlines fundamental issues related to the numerical method chosen to model a dike or a magma chamber. It clearly demonstrates that, while the magma compressibility can be neglected to model the deformation due to one source or distant sources, it is necessary to take it into account in models combining close sources.

Jurassic, slow-spreading ridge in the southeast Gulf of Mexico and its along-strike morpho-volcanic expression explained by a two-phase opening model

NASA Astrophysics Data System (ADS)

Lin, P.; Mann, P.

2016-12-01

Previous workers have used extensive grids of 2D seismic reflection data to describe the width, structural character, and adjacent oceanic crust of the late Jurassic, slow-spreading ridge in the southeast Gulf of Mexico (SEGOM). Characteristics of the now-buried SEGOM slow spreading ridge include: 1) wide, axial valley segments ranging from 5-20 km; 2) alternating, deep, axial valley segments up to 2 km in depth; 3) normal faults dipping towards the axial valleys; and 4) isolated seamounts within the axial valleys projecting 1 km above regional oceanic basement depth and reflecting along-strike variations in the ridge's magmatic supply. We have used additional seismic reflection, gravity, and magnetic data to map the ridge and its environs to its southern termination, a 2.6-km-high seamount - informally named here Buffler seamount. The southernmost, 427-km long section of the SEGOM ridge from Buffler seamount northwest to the southwestern limit of the DeSoto Canyon arch can be divided into four alternating ridge segments with two distinctive morphologies: 1) wide and deep axial valleys lying below regional oceanic basement depth and characterized by gravity high and magnetic lows; and 2) elevated, linear areas of clustered, seamounts characterized by gravity low and magnetic highs. The continental margins of both Yucatan and Florida exhibit a prominent N60E magnetic fabric created by Phase 1, NW-SE Triassic-early Jurassic continental rifting of the GOM that was subsequently offset at right angles by Phase 2, NE-SW late Jurassic stretching and oceanic spreading. Removal of the V-shaped area of oceanic crust of the SEGOM shows that the wide, axial valleys of the late Jurassic spreading ridge coincide with rifted areas of thicker crust on the "arches" or horst blocks of Triassic-early Jurassic, Phase 1 rifting (Sarasota, Middle Ground) while the elevated areas of elevated and clustered seamounts coincide with thinner crust of the intervening rifts (Apalachicola, Tampa, South Florida). The later SW-NE re-rifting of crust during the late Jurassic that was rifted earlier in the Triassic and early Jurassic in a NW-SE direction is supportive of the widely accepted two-phase opening model for the SEGOM and GOM as a whole.

Creation of Rift Valley Fever Viruses with Four-Segmented Genomes Reveals Flexibility in Bunyavirus Genome Packaging

PubMed Central

Oreshkova, Nadia; Moormann, Rob J. M.; Kortekaas, Jeroen

2014-01-01

ABSTRACT Bunyavirus genomes comprise a small (S), a medium (M), and a large (L) RNA segment of negative polarity. Although the untranslated regions have been shown to comprise signals required for transcription, replication, and encapsidation, the mechanisms that drive the packaging of at least one S, M, and L segment into a single virion to generate infectious virus are largely unknown. One of the most important members of the Bunyaviridae family that causes devastating disease in ruminants and occasionally humans is the Rift Valley fever virus (RVFV). We studied the flexibility of RVFV genome packaging by splitting the glycoprotein precursor gene, encoding the (NSm)GnGc polyprotein, into two individual genes encoding either (NSm)Gn or Gc. Using reverse genetics, six viruses with a segmented glycoprotein precursor gene were rescued, varying from a virus comprising two S-type segments in the absence of an M-type segment to a virus consisting of four segments (RVFV-4s), of which three are M-type. Despite that all virus variants were able to grow in mammalian cell lines, they were unable to spread efficiently in cells of mosquito origin. Moreover, in vivo studies demonstrated that RVFV-4s is unable to cause disseminated infection and disease in mice, even in the presence of the main virulence factor NSs, but induced a protective immune response against a lethal challenge with wild-type virus. In summary, splitting bunyavirus glycoprotein precursor genes provides new opportunities to study bunyavirus genome packaging and offers new methods to develop next-generation live-attenuated bunyavirus vaccines. IMPORTANCE Rift Valley fever virus (RVFV) causes devastating disease in ruminants and occasionally humans. Virions capable of productive infection comprise at least one copy of the small (S), medium (M), and large (L) RNA genome segments. The M segment encodes a glycoprotein precursor (GPC) protein that is cotranslationally cleaved into Gn and Gc, which are required for virus entry and fusion. We studied the flexibility of RVFV genome packaging and developed experimental live-attenuated vaccines by applying a unique strategy based on the splitting of the GnGc open reading frame. Several RVFV variants, varying from viruses comprising two S-type segments to viruses consisting of four segments (RVFV-4s), of which three are M-type, could be rescued and were shown to induce a rapid protective immune response. Altogether, the segmentation of bunyavirus GPCs provides a new method for studying bunyavirus genome packaging and facilitates the development of novel live-attenuated bunyavirus vaccines. PMID:25008937

Creation of Rift Valley fever viruses with four-segmented genomes reveals flexibility in bunyavirus genome packaging.

PubMed

Wichgers Schreur, Paul J; Oreshkova, Nadia; Moormann, Rob J M; Kortekaas, Jeroen

2014-09-01

Bunyavirus genomes comprise a small (S), a medium (M), and a large (L) RNA segment of negative polarity. Although the untranslated regions have been shown to comprise signals required for transcription, replication, and encapsidation, the mechanisms that drive the packaging of at least one S, M, and L segment into a single virion to generate infectious virus are largely unknown. One of the most important members of the Bunyaviridae family that causes devastating disease in ruminants and occasionally humans is the Rift Valley fever virus (RVFV). We studied the flexibility of RVFV genome packaging by splitting the glycoprotein precursor gene, encoding the (NSm)GnGc polyprotein, into two individual genes encoding either (NSm)Gn or Gc. Using reverse genetics, six viruses with a segmented glycoprotein precursor gene were rescued, varying from a virus comprising two S-type segments in the absence of an M-type segment to a virus consisting of four segments (RVFV-4s), of which three are M-type. Despite that all virus variants were able to grow in mammalian cell lines, they were unable to spread efficiently in cells of mosquito origin. Moreover, in vivo studies demonstrated that RVFV-4s is unable to cause disseminated infection and disease in mice, even in the presence of the main virulence factor NSs, but induced a protective immune response against a lethal challenge with wild-type virus. In summary, splitting bunyavirus glycoprotein precursor genes provides new opportunities to study bunyavirus genome packaging and offers new methods to develop next-generation live-attenuated bunyavirus vaccines. Rift Valley fever virus (RVFV) causes devastating disease in ruminants and occasionally humans. Virions capable of productive infection comprise at least one copy of the small (S), medium (M), and large (L) RNA genome segments. The M segment encodes a glycoprotein precursor (GPC) protein that is cotranslationally cleaved into Gn and Gc, which are required for virus entry and fusion. We studied the flexibility of RVFV genome packaging and developed experimental live-attenuated vaccines by applying a unique strategy based on the splitting of the GnGc open reading frame. Several RVFV variants, varying from viruses comprising two S-type segments to viruses consisting of four segments (RVFV-4s), of which three are M-type, could be rescued and were shown to induce a rapid protective immune response. Altogether, the segmentation of bunyavirus GPCs provides a new method for studying bunyavirus genome packaging and facilitates the development of novel live-attenuated bunyavirus vaccines. Copyright © 2014, American Society for Microbiology. All Rights Reserved.

Left-lateral transtension along the Ethiopian Rift and constrains on the mantle-reference plate motions

NASA Astrophysics Data System (ADS)

Muluneh, Ameha A.; Cuffaro, Marco; Doglioni, Carlo

2014-09-01

We present the kinematics of the Ethiopian Rift, in the northern part of East African Rift System, derived from compilation of geodetic velocities, focal mechanism inversions, structural data analysis and geological profiles. In the central Ethiopian Rift, the GPS velocity field shows a systematic magnitude increase in ENE direction, and the incremental extensional strain axes recorded by earthquake focal mechanisms and fault slip inversion show ≈ N100°E orientation. This deviation between direction of GPS velocity vectors and orientation of incremental extensional strain is developed due to left lateral transtensional deformation along the NE-SW trending segment of the rift. This interpretation is consistent with the en-échelon pattern of tensional and transtensional faults, plus the distribution of the volcanic centers, and the asymmetry of the rift itself. We analyzed the kinematics of the Ethiopian Rift also relative to the mantle comparing the results in the deep and shallow hotspot reference frames. While the oblique orientation of the rift was controlled by the pre-existing lithospheric fabric, the two reference frames predict different kinematics of Africa and Somalia plates along the rift itself, both in magnitude and direction, and with respect to the mantle. However, the observed kinematics and tectonics along the rift are more consistent with a faster WSW-ward motion of Africa than Somalia observed in the shallow hotspot framework. The faster WSW motion of Africa with respect to Somalia plate is inferred to be due to the lower viscosity in the top asthenosphere (LVZ-low-velocity zone) beneath Africa. These findings have significant implication for the evolution of continental rifting in transtensional settings and provide evidence for the kinematics of the Ethiopian Rift in the context of the Africa-Somalia plate interaction in the mantle reference frame.

North America's Midcontinent Rift: when Rift MET Lip

NASA Astrophysics Data System (ADS)

Stein, C. A.; Stein, S. A.; Kley, J.; Keller, G. R., Jr.; Bollmann, T. A.; Wolin, E.; Zhang, H.; Frederiksen, A. W.; Ola, K.; Wysession, M. E.; Wiens, D.; Alequabi, G.; Waite, G. P.; Blavascunas, E.; Engelmann, C. A.; Flesch, L. M.; Rooney, T. O.; Moucha, R.; Brown, E.

2015-12-01

Rifts are segmented linear depressions, filled with sedimentary and igneous rocks, that form by extension and often evolve into plate boundaries. Flood basalts, a class of Large Igneous Provinces (LIPs), are broad regions of extensive volcanism due to sublithospheric processes. Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. North America's Midcontinent Rift (MCR) is an unusual combination. Its 3000-km length formed as part of the 1.1 Ga rifting of Amazonia (Precambrian NE South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established, but contains an enormous volume of igneous rocks. MCR volcanics are significantly thicker than other flood basalts, due to deposition in a narrow rift rather than a broad region, giving a rift geometry but a LIP's magma volume. Structural modeling of seismic reflection data shows an initial rift phase where flood basalts filled a fault-controlled extending basin, and a postrift phase where volcanics and sediments were deposited in a thermally subsiding basin without associated faulting. The crust thinned during rifting and rethickened during the postrift phase and later compression, yielding the present thicker crust. The coincidence of a rift and LIP yielded the world's largest deposit of native copper. This combination arose when a new rift associated with continental breakup interacted with a mantle plume or anomalously hot or fertile upper mantle. Integration of diverse data types and models will give insight into questions including how the magma source was related to the rifting, how their interaction operated over a long period of rapid plate motion, why the lithospheric mantle below the MCR differs only slightly from its surroundings, how and why extension, volcanism, and compression varied along the rift arms, and how successful seafloor spreading ended the rift phase. Papers, talks, and educational material are available at http://www.earth.northwestern.edu/people/seth/research/mcr.html

DOE Office of Scientific and Technical Information (OSTI.GOV)

Holcomb, R.T.; Moore, J.G.; Lipman, P.W.

The GLORIA long-range sonar imaging system has revealed fields of large lava flows in the Hawaiian Trough east and south of Hawaii in water as deep as 5.5 km. Flows in the most extensive field (110 km long) have erupted from the deep submarine segment of Kilauea's east rift zone. Other flows have been erupted from Loihi and Mauna Loa. This discovery confirms a suspicion, long held from subaerial studies, that voluminous submarine flows are erupted from Hawaiian volcanoes, and it supports an inference that summit calderas repeatedly collapse and fill at intervals of centuries to millenia owing to voluminousmore » eruptions. These extensive flows differ greatly in form from pillow lavas found previously along shallower segments of the rift zones; therefore, revision of concepts of volcano stratigraphy and structure may be required.« less

Oblique strike-slip motion off the Southeastern Continental Margin of India: Implication for the separation of Sri Lanka from India

NASA Astrophysics Data System (ADS)

Desa, Maria Ana; Ismaiel, Mohammad; Suresh, Yenne; Krishna, Kolluru Sree

2018-05-01

The ocean floor in the Bay of Bengal has evolved after the breakup of India from Antarctica since the Early Cretaceous. Recent geophysical investigations including updated satellite derived gravity map postulated two phases for the tectonic evolution of the Bay of Bengal, the first phase of spreading occurred in the NW-SE direction forming its Western Basin, while the second phase occurred in the N-S direction resulting in its Eastern Basin. Lack of magnetic data along the spreading direction in the Western Basin prompted us to acquire new magnetic data along four tracks (totaling ∼3000 km) to validate the previously identified magnetic anomaly picks. Comparison of the synthetic seafloor spreading model with the observed magnetic anomalies confirmed the presence of Mesozoic anomalies M12n to M0 in the Western Basin. Further, the model suggests that this spreading between India and Antarctica took place with half-spreading rates of 2.7-4.5 cm/yr. The trend of the fracture zones in the Western Basin with respect to that of the Southeastern Continental Margin of India (SCMI) suggests that SCMI is an oblique transform margin with 37° obliquity. Further, the SCMI consists of two oblique transform segments separated by a small rift segment. The strike-slip motion along the SCMI is bounded by the rift segments of the Northeastern Continental Margin of India and the southern margin of Sri Lanka. The margin configuration and fracture zones inferred in its conjugate Western Enderby Basin, East Antarctica helped in inferring three spreading corridors off the SCMI in the Western Basin of the Bay of Bengal. Detailed grid reconstruction models traced the oblique strike-slip motion off the SCMI since M12n time. The strike-slip motion along the short northern transform segment ended by M11n time. The longer transform segment, found east of Sri Lanka lost its obliquity and became a pure oceanic transform fault by M0 time. The eastward propagation of the Africa-Antarctica spreading center initiated the anticlockwise separation of Sri Lanka from India by M12n time. Seafloor spreading south of Sri Lanka due to the India-Antarctica spreading episode and the simultaneously occurring strike-slip motion east of Sri Lanka restricted this separation resulting in a failed rift. Thus Sri Lanka with strike-slip motion to its east, failed rift towards west, continental extension to its north and rifting to its south behaved as a short lived microplate during the Early Cretaceous period and remained attached to India thereafter.

Deciphering the role of fluids in early stage rifting from full moment tensor inversion of East African earthquakes

NASA Astrophysics Data System (ADS)

Oliva, S. J. C.; Ebinger, C. J.; Keir, D.; Shillington, D. J.; Chindandali, P. R. N.

2016-12-01

The East African Rift splits around the Archaean Tanzania craton into the magmatic Eastern branch and the mostly amagmatic Western branch, which continues south of the craton. Temporary seismic networks recently deployed in three rift sectors allow for comparison and insights into the early stages of rifting, including areas with lower crustal earthquakes. We analyze earthquakes with ML > 3.5 in the area recorded by CRAFTI (northern Tanzania/Kenya), TANGA (Tanganyika rift), and/or SEGMeNT (Malawi rift) networks. For events not well enclosed by these arrays, nearby permanent stations are used to improve azimuthal coverage when possible. We present source mechanisms as well as better-constrained source depth estimates from moment tensor inversion using Dreger and Ford TDMT algorithm (Dreger, 2003; Minson & Dreger, 2008). Data and synthetic waveforms are bandpass filtered between 0.02 to 0.10 Hz, or a narrower frequency band within this range, depending on lake noise, which can interfere strongly on the lower end of this frequency range. Results suggest local stress reorientations as well as significant dilatation components on some events within magmatic rift sectors. The implications of these results for crustal rheology and magmatic modification will be discussed in light of the growing complementary data sets from the three projects to inform our understanding of early rifting as a whole.

Imaging an off-axis volcanic field in the Main Ethiopian Rift using 3-D magnetotellurics

NASA Astrophysics Data System (ADS)

Huebert, J.; Whaler, K. A.; Fisseha, S.; Hogg, C.

2017-12-01

In active continental rifts, asthenospheric upwelling and crustal thinning result in the ascent of melt through the crust to the surface. In the Main Ethiopian Rift (MER), most volcanic activity is located in magmatic segments in the rift centre, but there are areas of significant off-axis magmatism as well. The Butajira volcanic field is part of the Silti Debre Zeyt Fault (SDZF) zone in the western Main Ethiopian Rift. It is characterized by densely clustered volcanic vents (mostly scoria cones) and by limited seismic activity, which is mainly located along the big border faults that form the edge of a steep escarpment. Seismic P-Wave tomography reveals a crustal low velocity anomaly in this area. We present newly collected Magnetotelluric (MT) data to image the electrical conductivity structure of the area. We deployed 12 LMT instruments and 27 broadband stations in the western flank of the rift to further investigate the along-rift and depth extent of a highly conductive region under the SDZF which was previously identified by MT data collected on the central volcano Aluto and along a cross-rift transverse. This large conductor was interpreted as potential pathways for magma and fluid in the crust. MT Stations were positioned in five NW-SE running 50 km long profiles, covering overall 100km along the rift and providing good coverage for a 3-D inversion of the data to image this enigmatic area of the MER.

Young rift kinematics in the Tadjoura rift, western Gulf of Aden, Republic of Djibouti

NASA Astrophysics Data System (ADS)

Daoud, Mohamed A.; Le Gall, Bernard; Maury, René C.; Rolet, JoëL.; Huchon, Philippe; Guillou, Hervé

2011-02-01

The Tadjoura rift forms the westernmost edge of the westerly propagating Sheba ridge, between Arabia and Somalia, as it enters into the Afar depression. From structural and remote sensing data sets, the Tadjoura rift is interpreted as an asymmetrical south facing half-graben, about 40 km wide, dominated by a large boundary fault zone to the north. It is partially filled up by the 1-3 Myr old Gulf Basalts which onlapped the older Somali Basalts along its shallower southern flexural margin. The major and trace element analysis of 78 young onshore lavas allows us to distinguish and map four distinct basaltic types, namely the Gulf, Somali, Goumarre, and Hayyabley Basalts. These results, together with radiometric age data, lead us to propose a revised volcano-stratigraphic sketch of the two exposed Tadjoura rift margins and to discriminate and date several distinct fault networks of this oblique rift. Morphological and statistical analyses of onshore extensional fault populations show marked changes in structural styles along-strike, in a direction parallel to the rift axis. These major fault disturbances are assigned to the arrest of axial fault tip propagation against preexisting discontinuities in the NS-oriented Arta transverse zone. According to our model, the sinistral jump of rifting into the Asal-Ghoubbet rift segment results from structural inheritance, in contrast with the en échelon or transform mechanism of propagation that prevailed along the entire length of the Gulf of Aden extensional system.

Rift-drift transition in the Dangerous Grounds, South China Sea

NASA Astrophysics Data System (ADS)

Peng, Xi; Shen, Chuanbo; Mei, Lianfu; Zhao, Zhigang; Xie, Xiaojun

2018-04-01

The South China Sea (SCS) has a long record of rifting before and after subsequent seafloor spreading, affecting the wide continent of the Dangerous Grounds, and its scissor-shape opening manner results in the rifting structures that vary along this margin. Some 2000 km of regional multichannel seismic data combined with borehole and dredge data are interpreted to analyze the multistage rifting process, structural architecture and dynamic evolution across the entire Dangerous Grounds. Key sequence boundaries above the Cenozoic basement are identified and classified into the breakup unconformity and the rift end unconformity, which consist of the rift-related unconformities. Reflector T70 in the east of the Dangerous Grounds represents the breakup unconformity, which is likely corresponding to the spreading of the East Subbasin. T60 formed on the top of carbonate platform is time equivalent to the spreading of the Southwest Subbasin, marking the breakup unconformity of the central Dangerous Grounds. The termination of the spreading of the SCS is manifested by the rift end unconformity of T50 in the southwest and the final rift occurring in the northwest of the Dangerous Grounds is postponed to the rift end unconformity of T40. On the basis of the stratigraphic and structural analysis, distinct segments in the structural architecture of the syn-rift units and the ages of rift-drift transition show obvious change from the proximal zone to the distal zone. Three domains, which are the Reed Bank-Palawan Rift domain, the Dangerous Grounds Central Detachment domain and Nam Con Son Exhumation domain, reflect the propagation of the margin rifting developed initially by grabens formed by high angle faults, then large half-grabens controlled by listric faults and detachments and finally rotated fault blocks in the hyper-extended upper crust associated with missing lower crust or exhumed mantle revealing a migration and stepwise rifting process in the south margin of the SCS.

EAGLE The controlled source experiment

NASA Astrophysics Data System (ADS)

Maguire, P. K. H.; Eagle Controlled Source Group

2003-04-01

In January 2003, a wide-angle reflection / refraction seismic project was carried out over the north-eastern section of the Main Ethiopian Rift as part of the international EAGLE (Ethiopia Afar Geoscientific Lithospheric Experiment) programme. EAGLE comprises a combination of passive and controlled source seismic experiments to determine the geometry and kinematics of a continental rift immediately prior to break-up, enabling the development of magmatic margin break-up models. A total of ˜900 seismic instruments were deployed along two 450km profiles, one along the axis of the Ethiopian Rift into the south-west corner of Afar; and a second across the rift, extending north and south across the uplifted, flood basalt covered, Ethiopian plateau. The two profiles intersect over the Nazret volcanic segment in the rift. This may be indicative of the transition from continental style rifting in which strain is accommodated on the rift bounding border faults, to a state where strain and magmatism have migrated to a narrow zone within the rift, a necessary pre-cursor to break-up. A further ˜300 instruments were deployed in a 100x100km^2 array around the intersection of the two profiles. A total of 16 borehole and 2 lake shots were fired into the network over a period of four days. The principal objectives of the controlled source project were to examine crustal strain, the distribution of crustal magmatic intrusions, the influence of pre-rift crustal property variations on rift development and also to provide a crustal seismic velocity distribution to improve images of the deep mantle, as well as earthquake locations derived from the EAGLE passive arrays.

Comparisons of seismic and geodetic strain across the East African rift: Implications for magmatism during rifting

NASA Astrophysics Data System (ADS)

Lindsey, N.; Ebinger, C. J.; Pritchard, M. E.; Cote, D. M.

2010-12-01

Knowledge of how the continental lithosphere accommodates strain in an active rift setting is essential to both earthquake and volcanic hazard analyses. Far-field and impinging mantle plumes drive extension within the fault-bounded rift systems of East Africa. Our study aims to evaluate models of distributed strain and localized strain between multiple rigid plates using earthquake catalogs and existing constraints, including high resolution DEMs that reveal the spatial distribution of young faults across the broad uplifts of eastern and southern Africa. We determine cumulative seismic moment release within 0.5 degree bins across the Afro-Arabian rift system using the entire NEIC earthquake catalog (1973-present), and compare these results to geodetic estimates of strain and extensional velocity. The small bin size permits comparison of strain with geological factors, including geological terrain, border fault distribution, and the presence or absence of volcanism. Our results highlight the significance of magmatism in strain accommodation across the rift system, and suggest that some strain and magmatism occur within ‘rigid blocks’, such as the Tanzania craton. Throughout the Afro-Arabian rift system, seismic moment release lags geodetic moment release by a factor of 2, consistent with aseismic creep deformation. However, our comparisons indicate that aseismic deformation accounts for a much higher percent of geodetic moment release: approximately 90% in the Main Ethiopian and Eastern rifts, and >97% in the Afar rift zone where incipient seafloor spreading occurs. The time-averaged strain distributions match the estimates from intense seismo-volcanic rifting episodes in Afar, indicating the data base is representative of longer-term patterns in Afar. We see no systematic variation in interbasinal accommodation zones or rift segment offsets, arguing against the development of transform-like structures prior to plate rupture.

Geometry of the Arabia-Somalia Plate Boundary into Afar: Preliminary Results from the Seismic Profile Across the Asal Rift (Djibouti)

NASA Astrophysics Data System (ADS)

Vergne, J.; Doubre, C.; Mohamed, K.; Tiberi, C.; Leroy, S.; Maggi, A.

2010-12-01

In the Afar Depression, the Asal-Ghoubbet Rift in Djibouti is a young segment on land at the propagating tip of the Aden Ridge. This segment represents an ideal laboratory to observe the mechanisms of extension and the structural evolutions involved, from the continental break-up to the first stage of oceanic spreading. However, we lack first order information about the crustal and upper mantle structure in this region, which for example prevent detailed numerical modeling of the deformations observed at the surface from GPS or InSAR. Moreover the current permanent network is not well suited to precisely constrain the ratio of seismic/aseismic deformation and to characterize the active deformation and the rifting dynamics. Since November 2009 we have maintained a temporary network of 25 seismic stations deployed along a 150 km-long profile. Because we expect rapid variations of the lithospheric structure across the 10 km-wide central part of the rift, we gradually decreased the inter-stations spacing to less than 1 km in the middle section of the profile. In order to obtain a continuous image of the plate boundary, from the topographic surface to the upper mantle, several techniques and methods will be applied: P and S wave receiver functions, tomographies based on body waves, surface waves and seismic noise correlation, anisotropy, and finally a gravity-seismic joint inversion. We present some preliminary results deduced from the receiver functions applied to the data acquired during the first months of the experiment. We migrate several sets of receiver functions computed in various frequency bands to resolve both mantle interfaces and fine scale structures within the thin crust in the center of the rift. These first images confirm a rapid variation of the Moho depth on both sides of the rift and a very complex lithospheric structure in the central section with several low velocity zones within the top 50km that might correspond to magma lenses.

Fault-scale controls on rift geometry: the Bilila-Mtakataka Fault, Malawi

NASA Astrophysics Data System (ADS)

Hodge, M.; Fagereng, A.; Biggs, J.; Mdala, H. S.

2017-12-01

Border faults that develop during initial stages of rifting determine the geometry of rifts and passive margins. At outcrop and regional scales, it has been suggested that border fault orientation may be controlled by reactivation of pre-existing weaknesses. Here, we perform a multi-scale investigation on the influence of anisotropic fabrics along a major developing border fault in the southern East African Rift, Malawi. The 130 km long Bilila-Mtakataka fault has been proposed to have slipped in a single MW 8 earthquake with 10 m of normal displacement. The fault is marked by an 11±7 m high scarp with an average trend that is oblique to the current plate motion. Variations in scarp height are greatest at lithological boundaries and where the scarp switches between following and cross-cutting high-grade metamorphic foliation. Based on the scarp's geometry and morphology, we define 6 geometrically distinct segments. We suggest that the segments link to at least one deeper structure that strikes parallel to the average scarp trend, an orientation consistent with the kinematics of an early phase of rift initiation. The slip required on a deep fault(s) to match the height of the current scarp suggests multiple earthquakes along the fault. We test this hypothesis by studying the scarp morphology using high-resolution satellite data. Our results suggest that during the earthquake(s) that formed the current scarp, the propagation of the fault toward the surface locally followed moderately-dipping foliation well oriented for reactivation. In conclusion, although well oriented pre-existing weaknesses locally influence shallow fault geometry, large-scale border fault geometry appears primarily controlled by the stress field at the time of fault initiation.

Crustal structure and kinematics of the TAMMAR propagating rift system on the Mid-Atlantic Ridge from seismic refraction and satellite altimetry gravity

NASA Astrophysics Data System (ADS)

Kahle, Richard L.; Tilmann, Frederik; Grevemeyer, Ingo

2016-08-01

The TAMMAR segment of the Mid-Atlantic Ridge forms a classic propagating system centred about two degrees south of the Kane Fracture Zone. The segment is propagating to the south at a rate of 14 mm yr-1, 15 per cent faster than the half-spreading rate. Here, we use seismic refraction data across the propagating rift, sheared zone and failed rift to investigate the crustal structure of the system. Inversion of the seismic data agrees remarkably well with crustal thicknesses determined from gravity modelling. We show that the crust is thickened beneath the highly magmatic propagating rift, reaching a maximum thickness of almost 8 km along the seismic line and an inferred (from gravity) thickness of about 9 km at its centre. In contrast, the crust in the sheared zone is mostly 4.5-6.5 km thick, averaging over 1 km thinner than normal oceanic crust, and reaching a minimum thickness of only 3.5 km in its NW corner. Along the seismic line, it reaches a minimum thickness of under 5 km. The PmP reflection beneath the sheared zone and failed rift is very weak or absent, suggesting serpentinisation beneath the Moho, and thus effective transport of water through the sheared zone crust. We ascribe this increased porosity in the sheared zone to extensive fracturing and faulting during deformation. We show that a bookshelf-faulting kinematic model predicts significantly more crustal thinning than is observed, suggesting that an additional mechanism of deformation is required. We therefore propose that deformation is partitioned between bookshelf faulting and simple shear, with no more than 60 per cent taken up by bookshelf faulting.

Rifting Process and Geomorphic Development of the Okinawa Tough, Southwest Japan

NASA Astrophysics Data System (ADS)

Sato, T.; Arai, K.; Inoue, T.; Matsumoto, D.

2012-12-01

The Ryukyu Island Arc extends from Kyushu to Taiwan, a distance of 1,200 km, along the Ryukyu Trench where the Philippine Sea Plate is subducting beneath the Eurasian Plate. The Okinawa Trough, a back arc basin has formed behind the Ryukyu Island Arc in late Pliocene to early Pleistocene. The research cruises of GH11 (from 14 July to 15 August, 2011) and GH12 (from 20 to 30 July, 2012) were carried out around the Okinawa Trough. More than 3,600 miles multi channel high-resolution seismic profiles were acquired during these cruises by the GI-gun (Generator 250 cu inch and Injector 105 cu. inch) systems with 16ch digital streamer cable. As a result, two unconformities and three depositional sequence divided by the unconformities are recognized in the trough. The lower and the midlle sequence are tilted and blocked by many normal faults, on the other hand the upper one is not tilted and shows the pattern of onlap fill. From this result, the upper sequence started to deposit after start of the rifting. Additionally, internal reflection of the upper sequence shows the cyclic activities of the rifting. The position of the rifting axis was revealed based on dip of the normal faults. As a result, rifting axis shows echelon arrangement and the displacement of the faults are varied with the segment of the arrangement. The location of the segment boundaries is correlated with geometrical boundary of the adjacent slope. Steep slope with incised valley is distributed in southwestern part where the displacement of the normal fault is large, on the other hand, gentle slope without incised valley is distributed in northeastern part where the displacement is small. This difference of the displacement strongly controls the geometry of the adjacent slope.

Colorado Basin Structure and Rifting, Argentine passive margin

NASA Astrophysics Data System (ADS)

Autin, Julia; Scheck-Wenderoth, Magdalena; Loegering, Markus; Anka, Zahie; Vallejo, Eduardo; Rodriguez, Jorge; Marchal, Denis; Reichert, Christian; di Primio, Rolando

2010-05-01

The Argentine margin presents a strong segmentation with considerable strike-slip movements along the fracture zones. We focus on the volcanic segment (between the Salado and Colorado transfer zones), which is characterized by seaward dipping reflectors (SDR) all along the ocean-continent transition [e.g. Franke et al., 2006; Gladczenko et al., 1997; Hinz et al., 1999]. The segment is structured by E-W trending basins, which differs from the South African margin basins and cannot be explained by classical models of rifting. Thus the study of the relationship between the basins and the Argentine margin itself will allow the understanding of their contemporary development. Moreover the comparison of the conjugate margins suggests a particular evolution of rifting and break-up. We firstly focus on the Colorado Basin, which is thought to be the conjugate of the well studied Orange Basin [Hirsch et al., 2009] at the South African margin [e.g. Franke et al., 2006]. This work presents results of a combined approach using seismic interpretation and structural, isostatic and thermal modelling highlighting the structure of the crust. The seismic interpretation shows two rift-related discordances: one intra syn-rift and the break-up unconformity. The overlying sediments of the sag phase are less deformed (no sedimentary wedges) and accumulated before the generation of oceanic crust. The axis of the Colorado Basin trends E-W in the western part, where the deepest pre-rift series are preserved. In contrast, the basin axis turns to a NW-SE direction in its eastern part, where mainly post-rift sediments accumulated. The most distal part reaches the margin slope and opens into the oceanic basin. The general basin direction is almost orthogonal to the present-day margin trend. The most frequent hypothesis explaining this geometry is that the Colorado Basin is an aborted rift resulting from a previous RRR triple junction [e.g. Franke et al., 2002]. The structural interpretation partly supports this hypothesis and shows two main directions of faulting: margin-parallel faults (~N30°) and rift-parallel faults (~N125°). A specific distribution of the two fault sets is observed: margin-parallel faults are restrained to the most distal part of the margin. Starting with a 3D structural model of the basin fill based on seismic and well data the deeper structure of the crust beneath the Colorado Basin can be evaluate using isostatic and thermal modelling. Franke, D., et al. (2002), Deep Crustal Structure Of The Argentine Continental Margin From Seismic Wide-Angle And Multichannel Reflection Seismic Data, paper presented at AAPG Hedberg Conference "Hydrocarbon Habitat of Volcanic Rifted Passive Margins", Stavanger, Norway Franke, D., et al. (2006), Crustal structure across the Colorado Basin, offshore Argentina Geophysical Journal International 165, 850-864. Gladczenko, T. P., et al. (1997), South Atlantic volcanic margins Journal of the Geological Society, London 154, 465-470. Hinz, K., et al. (1999), The Argentine continental margin north of 48°S: sedimentary successions, volcanic activity during breakup Marine and Petroleum Geology 16(1-25). Hirsch, K. K., et al. (2009), Tectonic subsidence history and thermal evolution of the Orange Basin, Marine and Petroleum Geology, in press, doi:10.1016/j.marpetgeo.2009.1006.1009

Seismically imaging the structural legacy of rifting and collision events in the central and eastern U.S. crust

NASA Astrophysics Data System (ADS)

Schmandt, B.; Lin, F. C.; Karlstrom, K. E.

2015-12-01

EarthScope's USArray now provides broadband seismic data across the contiguous U.S. and southeastern Canada. We used teleseismic receiver functions and surface wave tomography to map crustal structure beneath the entire array. Crust thickness was estimated with multi-mode Ps receiver function images using <0.5 Hz Ps and <0.25 Hz 2p1s and 2s1p reverberations between the free-surface and Moho. In areas of sedimentary basins or large impedance contrasts in the middle crust the reverberations alone often provide clearer images of the Moho than the Ps mode, because of interference from conversions at shallow interfaces is reduced at greater lag times. The new results enable large-scale comparison of the structural legacy of multiple rifting and collision events in eastern North America. Some Proterozoic rift segments defined by Bouguer gravity and surface geology maintain locally thin crust while others lack correlated Moho topography or are areas of locally thicker crust. Locally thin crust is found at southern end of the mid-continent rift (MCR) in northern Kansas and southern Nebraska, along the Reelfoot rift, and beneath inferred rifts in Michigan, Indiana, and Ohio. The Oklahoma aulacogen is not associated with a coherent change in crust thickness along its length, at least at a scale resolvable by USArray data and our imaging approach. The MCR extending northeast from Nebraska to Lake Superior has locally thicker crust, consistent with other recent results. We suggest that magmatic additions to the lower crust overwhelmed extension in the northern mid-continent rift, but not the rift segments further south and east. Collision events of the Grenville orogeny and Paleozoic orogens that created the Appalachian Mountains are still associated with ~45-55 km thick crust extending from the Grenville front eastward across the Appalachian Mountains to the fall line that marks the abrupt geomorphic transition to the coastal plains. Despite the ~45-55 km crust thickness long-wavelength elevations (>50 km) across this area rarely exceed 1 km. Along the fall line we find ~15-20 km of seaward thinning that is coherent from Alabama to Pennsylvania, with a transition width similar to or less than the ~70 km.

Oblique transfer of extensional strain between basins of the middle Rio Grande rift, New Mexico: Fault kinematic and paleostress constraints

USGS Publications Warehouse

Minor, Scott A.; Hudson, Mark R.; Caine, Jonathan S.; Thompson, Ren A.

2013-01-01

The structural geometry of transfer and accommodation zones that relay strain between extensional domains in rifted crust has been addressed in many studies over the past 30 years. However, details of the kinematics of deformation and related stress changes within these zones have received relatively little attention. In this study we conduct the first-ever systematic, multi-basin fault-slip measurement campaign within the late Cenozoic Rio Grande rift of northern New Mexico to address the mechanisms and causes of extensional strain transfer associated with a broad accommodation zone. Numerous (562) kinematic measurements were collected at fault exposures within and adjacent to the NE-trending Santo Domingo Basin accommodation zone, or relay, which structurally links the N-trending, right-stepping en echelon Albuquerque and Española rift basins. The following observations are made based on these fault measurements and paleostresses computed from them. (1) Compared to the typical northerly striking normal to normal-oblique faults in the rift basins to the north and south, normal-oblique faults are broadly distributed within two merging, NE-trending zones on the northwest and southeast sides of the Santo Domingo Basin. (2) Faults in these zones have greater dispersion of rake values and fault strikes, greater dextral strike-slip components over a wide northerly strike range, and small to moderate clockwise deflections of their tips. (3) Relative-age relations among fault surfaces and slickenlines used to compute reduced stress tensors suggest that far-field, ~E-W–trending σ3 stress trajectories were perturbed 45° to 90° clockwise into NW to N trends within the Santo Domingo zones. (4) Fault-stratigraphic age relations constrain the stress perturbations to the later stages of rifting, possibly as late as 2.7–1.1 Ma. Our fault observations and previous paleomagnetic evidence of post–2.7 Ma counterclockwise vertical-axis rotations are consistent with increased bulk sinistral-normal oblique shear along the Santo Domingo rift segment in Pliocene and later time. Regional geologic evidence suggests that the width of active rift faulting became increasingly confined to the Santo Domingo Basin and axial parts of the adjoining basins beginning in the late Miocene. We infer that the Santo Domingo clockwise stress perturbations developed coevally with the oblique rift segment mainly due to mechanical interactions of large faults propagating toward each other from the adjoining basins as the rift narrowed. Our results suggest that negligible bulk strike-slip displacement has been accommodated along the north-trending rift during much of its development, but uncertainties in the maximum ages of fault slip do not allow us to fully evaluate and discriminate between earlier models that invoked northward or southward rotation and translation of the Colorado Plateau during early (Miocene) rifting.

The Afar Depression: Was There a Triple Junction Above the Mantle Plume?

NASA Astrophysics Data System (ADS)

Ebinger, C.; Wolfenden, E.; Yirgu, G.; Ayalew, D.; Eagles, G.; Gloaguen, R.; Tiberi, C.; Rowland, J. R.; Deino, A.; Tesfaye, A.; Tesfaye, S.

2002-12-01

The Red Sea - Gulf of Aden- Main Ethiopian rift systems (Afar Depression) have served as the textbook example of a R-R-R triple junction zone which formed above a mantle plume (Ethiopia-Yemen flood basalt province, 31-28 Ma). Recent work has documented the onset and propagation of seafloor along the length of the Gulf of Aden and Red Sea arms, but the timing of continental rifting had been poorly constrained. Our aims were to constrain the timing of rift initiation in each arm of the rift near the proposed Oligocene triple junction and to re-assess models for break-up above a mantle plume. Although much of the early history of rifting is deeply buried by Pliocene-Recent lavas, erosional dissection of the rift margins provides windows into the early rift history. Along the southernmost Red Sea, faults commonly marked by eruptive centers initiated at about 26 Ma, coincident with rifting along the easternmost Gulf of Aden. New data from the rift immediately south of the southernmost Red Sea basin (ca.10N) constrain the earliest rift sequences in the northern Main Ethiopian rift (MER). Field and Ar-Ar data from sequences overlying the pre-rift flood basalts show that extension in the northern MER commenced at 12-10 Ma when the two rift systems were finally linked. The active zone of extension and magmatism in the southern Red Sea and eastern Gulf of Aden, however, had migrated east and north, respectively. Summarising, rifting in southern Ethiopia had commenced by 16-18 Ma, and had propagated northward to cut across Oligo-Miocene rift structures of the Red Sea and Gulf of Aden by 10 Ma, consistent with plate kinematic data. A triple junction could have developed only during the past 10 My, long after flood basaltic magmatism. Inverse models of gravity data predict a significant step (2-4 km) in the Moho where the youthful, less extended MER breaks into the Afar Depression. Project EAGLE (UK-US-Ethiopia) is now acquiring seismic data across and along this zone to evaluate mechanisms for rift segmentation and propagation prior to breakup.

CO2 Degassing Estimates from Rift Length Analysis since Pangea Fragmentation: A Key Component of the Deep Carbon Cycle?

NASA Astrophysics Data System (ADS)

Brune, S.; Williams, S.; Müller, D.

2017-12-01

The deep carbon cycle links the carbon content of crust and mantle to Earth's surface: extensional plate boundaries and arc volcanoes release CO2 to the ocean and atmosphere while subducted lithosphere carries carbon back into the mantle. The length of extensional and convergent plate boundaries therefore exerts first-order control on solid Earth CO2 degassing rates. Here we provide a global census of plate boundary length for the last 200 million years. Focusing on rift systems, we find that the most extensive rift phase during the fragmentation of Pangea occurred in the Jurassic/Early Cretaceous with more than 50.000 km of simultaneously active continental rifts. During the Late Cretaceous, in the aftermath of this pervasive rift episode, the global rift length dropped by 60% to 20,000 km. We further find that a second pronounced rift episode with global rift lengths of up to 30,000 km started in Eocene times. A close geological link between CO2 degassing and faulting has been documented in currently active rift systems worldwide. Regional-scale CO2 flux densities at rift segments in Africa, Europe, and New Zealand feature an annual average value of 200 t of CO2 per km2. Assuming that the release of CO2 scales with rift length, we show that rift-related CO2 degassing rates during the two major Mesozoic and Cenozoic rift episodes reached more than 300% of present-day values. Most importantly, the timing of enhanced CO2 degassing from continental rifts correlates with two well-known periods of elevated atmospheric CO2 in the Mesozoic and Cenozoic as evidenced by multiple independent proxy indicators. Compiling the length of other plate boundaries (mid-ocean ridges, subduction zones, continental arcs) through time, we do not reproduce such a correlation. Finally, we conduct numerical carbon cycle models that account for key feedback-mechanisms of the long-term carbon cycle. We find that only those models that feature a strong rift degassing component reproduce the timing and amplitude of the paleo-CO2 record. We therefore suggest that rift-related degassing constitutes a key component of the deep carbon cycle.

Magmatic architecture within a rift segment: Articulate axial magma storage at Erta Ale volcano, Ethiopia

NASA Astrophysics Data System (ADS)

Xu, Wenbin; Rivalta, Eleonora; Li, Xing

2017-10-01

Understanding the magmatic systems beneath rift volcanoes provides insights into the deeper processes associated with rift architecture and development. At the slow spreading Erta Ale segment (Afar, Ethiopia) transition from continental rifting to seafloor spreading is ongoing on land. A lava lake has been documented since the twentieth century at the summit of the Erta Ale volcano and acts as an indicator of the pressure of its magma reservoir. However, the structure of the plumbing system of the volcano feeding such persistent active lava lake and the mechanisms controlling the architecture of magma storage remain unclear. Here, we combine high-resolution satellite optical imagery and radar interferometry (InSAR) to infer the shape, location and orientation of the conduits feeding the 2017 Erta Ale eruption. We show that the lava lake was rooted in a vertical dike-shaped reservoir that had been inflating prior to the eruption. The magma was subsequently transferred into a shallower feeder dike. We also find a shallow, horizontal magma lens elongated along axis inflating beneath the volcano during the later period of the eruption. Edifice stress modeling suggests the hydraulically connected system of horizontal and vertical thin magmatic bodies able to open and close are arranged spatially according to stresses induced by loading and unloading due to topographic changes. Our combined approach may provide new constraints on the organization of magma plumbing systems beneath volcanoes in continental and marine settings.

Architecture of the Distal Piedmont-Ligurian Rifted Margin in NW Italy: Hints for a Flip of the Rift System Polarity

NASA Astrophysics Data System (ADS)

Decarlis, Alessandro; Beltrando, Marco; Manatschal, Gianreto; Ferrando, Simona; Carosi, Rodolfo

2017-11-01

The Alpine Tethys rifted margins were generated by a Mesozoic polyphase magma-poor rifting leading to the opening of the Piedmont-Ligurian "Ocean." This latter developed through different phases of rifting that terminated with the exhumation of subcontinental mantle along an extensional detachment system. At the onset of simple shear detachment faulting, two margin types were generated: an upper and a lower plate corresponding to the hanging wall and footwall of the final detachment system, respectively. The two margin architectures were markedly different and characterized by a specific asymmetry. In this study the detailed analysis of the Adriatic margin, exposed in the Serie dei Laghi, Ivrea-Verbano, and Canavese Zone, enabled to recognize the diagnostic elements of an upper plate rifted margin. This thesis contrasts with the classic interpretation of the Southalpine units, previously compared with the adjacent fossil margin preserved in the Austroalpine nappes and considered as part of a lower plate. The proposed scenario suggests the segmentation and flip of the Alpine rifting system along strike and the passage from a lower to an upper plate. Following this interpretation, the European and Southern Adria margins are coevally developed upper plate margins, respectively resting NE and SW of a major transform zone that accommodates a flip in the polarity of the rift system. This new explanation has important implications for the study of the pre-Alpine rift-related structures, for the comprehension of their role during the reactivation of the margin and for the paleogeographic evolution of the Alpine orogen.

Consequences of Rift Propagation for Spreading in Thick Oceanic Crust in Iceland

NASA Astrophysics Data System (ADS)

Karson, J. A.

2015-12-01

Iceland has long been considered a natural laboratory for processes related to seafloor spreading, including propagating rifts, migrating transforms and rotating microplates. The thick, hot, weak crust and subaerial processes of Iceland result in variations on the themes developed along more typical parts of the global MOR system. Compared to most other parts of the MOR, Icelandic rift zones and transform faults are wider and more complex. Rift zones are defined by overlapping arrays of volcanic/tectonic spreading segments as much as 50 km wide. The most active rift zones propagate N and S away from the Iceland hot spot causing migration of transform faults. A trail of crust deformed by bookshelf faulting forms in their wakes. Dead or dying transform strands are truncated along pseudofaults that define propagation rates close to the full spreading rate of ~20 mm/yr. Pseudofaults are blurred by spreading across wide rift zones and laterally extensive subaerial lava flows. Propagation, with decreasing spreading toward the propagator tips causes rotation of crustal blocks on both sides of the active rift zones. The blocks deform internally by the widespread reactivation of spreading-related faults and zones of weakness along dike margins. The sense of slip on these rift-parallel strike-slip faults is inconsistent with transform-fault deformation. These various deformation features as well as subaxial subsidence that accommodate the thickening of the volcanic upper crustal units are probably confined to the brittle, seismogenic, upper 10 km of the crust. At least beneath the active rift zones, the upper crust is probably decoupled from hot, mechanically weak middle and lower gabbroic crust resulting in a broad plate boundary zone between the diverging lithosphere plates. Similar processes may occur at other types of propagating spreading centers and magmatic rifts.

Wrench related faults and their control on the tectonics and Eocene sedimentation in the L13-L15 sub-sag area, Pearl River Mouth basin, China

NASA Astrophysics Data System (ADS)

Chen, Shuping; Xu, Shunshan; Cai, Yu; Ma, Xiaodan

2017-09-01

Recent oil discoveries in the L13-L15 sub-sag area in the Pearl River Mouth basin have inspired interest in Paleogene hydrocarbon targets. However, the structures and their control on reservoirs have not been completely studied. The aim of this paper is to address the tectonics and Eocene sedimentation based on 3D seismic data. We documented characteristics from four aspects of the faults in the study area: (a) fault arrangement; (b) fault segmentation; (c) flower structures; and (d) distribution of the depocenters along the faults. Based on the above data, we propose that the structures in the studied area were formed by a right-handed wrench. The principal shear for this model was caused by NNE- to NE-ward motion of the eastern part of the Eurasia plate due to the collision of the Indian-Australian and Eurasian plates starting approximately 49 Ma ago. The L13-L15 sub-sag area underwent early Eocene rifting, a late Eocene rifting-depression transition and an Oligocene-Quaternary thermal depression. The rift phase included three stages: the initial rifting, intensive rifting and late rifting. The deep lake mudstone deposited during the intensive rifting stage is the source rock with the most potential for oil generation. Shallow lake source rocks formed in the late rifting and transition stages are the secondary source rocks. Reservoir sweet spots were formed in the early period of the intensive rifting and late rifting stages. The junction sites between the front of the meandering river delta plain and fault steps are favorable places for good reservoirs. The sediments in the transition stage are rich in sandstone, making them perfect sites for prospecting reservoirs.

Activation of preexisting transverse structures in an evolving magmatic rift in East Africa

NASA Astrophysics Data System (ADS)

Muirhead, J. D.; Kattenhorn, S. A.

2018-01-01

Inherited crustal weaknesses have long been recognized as important factors in strain localization and basin development in the East African Rift System (EARS). However, the timing and kinematics (e.g., sense of slip) of transverse (rift-oblique) faults that exploit these weaknesses are debated, and thus the roles of inherited weaknesses at different stages of rift basin evolution are often overlooked. The mechanics of transverse faulting were addressed through an analysis of the Kordjya fault of the Magadi basin (Kenya Rift). Fault kinematics were investigated from field and remote-sensing data collected on fault and joint systems. Our analysis indicates that the Kordjya fault consists of a complex system of predominantly NNE-striking, rift-parallel fault segments that collectively form a NNW-trending array of en echelon faults. The transverse Kordjya fault therefore reactivated existing rift-parallel faults in ∼1 Ma lavas as oblique-normal faults with a component of sinistral shear. In all, these fault motions accommodate dip-slip on an underlying transverse structure that exploits the Aswa basement shear zone. This study shows that transverse faults may be activated through a complex interplay among magma-assisted strain localization, preexisting structures, and local stress rotations. Rather than forming during rift initiation, transverse structures can develop after the establishment of pervasive rift-parallel fault systems, and may exhibit dip-slip kinematics when activated from local stress rotations. The Kordjya fault is shown here to form a kinematic linkage that transfers strain to a newly developing center of concentrated magmatism and normal faulting. It is concluded that recently activated transverse faults not only reveal the effects of inherited basement weaknesses on fault development, but also provide important clues regarding developing magmatic and tectonic systems as young continental rift basins evolve.

Rifting the continental lithosphere: case studies of the lithosphere-asthenosphere system in rifted settings across the western U.S. and in the southern East African Rift

NASA Astrophysics Data System (ADS)

Hopper, E.; Gaherty, J. B.; Shillington, D. J.

2016-12-01

Continental extension comes in many guises, often described in terms of two endmembers. Narrow rifting is typified by a rift valley narrower than lithospheric thickness (50-100 km), presumed to result in steep lateral changes in crustal and lithospheric topography; wide rifting by a broad zone (<1000 km) of normal faulting associated with much smaller topographic gradients. A type example for the former is the East African Rift Valley; for the latter, the Basin and Range in the western U.S.A. An important control on rift development is the state of the lithosphere: for example, its strength and thickness. We analyse common conversion point stacked Sp converted wave images of the lithosphere beneath rift systems in the contiguous U.S., both the wide Basin and Range, and narrow rift systems such as the Rio Grande Rift and Salton Trough. We use Sp waves recorded by EarthScope's Transportable Array and other available permanent and temporary broadband stations. Beneath the Basin and Range, we observe a very strong, shallow velocity decrease (the lithosphere-asthenosphere boundary, or LAB) that is relatively uniform over 100s of km. The strength of this feature indicates melt has ponded at this transition. We have not observed a clear relationship between lithospheric thickness beneath the Basin and Range, and total degree of extension, current extension rate, or age since surface volcanism. Beneath narrow rifts in the western U.S., however, more localised thinning of the lithosphere has been observed. We also compare these observations with seismic images of the Malawi Rift, at the southern end of the Western Branch of the East African Rift System, using broadband data acquired as part of the Study of Extension and MaGmatism in Malawi aNd Tanzania (SEGMeNT) experiment. The Malawi Rift is extending slowly in a magma-poor region of relatively strong lithosphere. We constrain the pattern of plate-scale extension by observations of crustal thinning, and image complex variations in deeper lithospheric structure.

Rifting the continental lithosphere: case studies of the lithosphere-asthenosphere system in rifted settings across the western U.S. and in the southern East African Rift

NASA Astrophysics Data System (ADS)

Hopper, E.; Gaherty, J. B.; Shillington, D. J.

2017-12-01

Continental extension comes in many guises, often described in terms of two endmembers. Narrow rifting is typified by a rift valley narrower than lithospheric thickness (50-100 km), presumed to result in steep lateral changes in crustal and lithospheric topography; wide rifting by a broad zone (<1000 km) of normal faulting associated with much smaller topographic gradients. A type example for the former is the East African Rift Valley; for the latter, the Basin and Range in the western U.S.A. An important control on rift development is the state of the lithosphere: for example, its strength and thickness. We analyse common conversion point stacked Sp converted wave images of the lithosphere beneath rift systems in the contiguous U.S., both the wide Basin and Range, and narrow rift systems such as the Rio Grande Rift and Salton Trough. We use Sp waves recorded by EarthScope's Transportable Array and other available permanent and temporary broadband stations. Beneath the Basin and Range, we observe a very strong, shallow velocity decrease (the lithosphere-asthenosphere boundary, or LAB) that is relatively uniform over 100s of km. The strength of this feature indicates melt has ponded at this transition. We have not observed a clear relationship between lithospheric thickness beneath the Basin and Range, and total degree of extension, current extension rate, or age since surface volcanism. Beneath narrow rifts in the western U.S., however, more localised thinning of the lithosphere has been observed. We also compare these observations with seismic images of the Malawi Rift, at the southern end of the Western Branch of the East African Rift System, using broadband data acquired as part of the Study of Extension and MaGmatism in Malawi aNd Tanzania (SEGMeNT) experiment. The Malawi Rift is extending slowly in a magma-poor region of relatively strong lithosphere. We constrain the pattern of plate-scale extension by observations of crustal thinning, and image complex variations in deeper lithospheric structure.

Insights into Rift Initiation, Evolution, and Failure from North America's Midcontinent Rift

NASA Astrophysics Data System (ADS)

Stein, C. A.; Stein, S.; Elling, R. P.; Keller, G. R.; Kley, J.; Wysession, M. E.

2017-12-01

Recent studies of the Midcontinent Rift (MCR) near Lake Superior give insights into how some rifts start, evolve, and fail because the rift-filling volcanic and sedimentary rocks are exposed at the surface and well imaged by deep seismic reflection and gravity data. The MCR was traditionally considered to have formed by midplate extension and volcanism 1.1 Ga that ended due to compression from the Grenville orogeny, the 1.3 - 0.98 Ga assembly of Amazonia (Precambrian northeast South America), Laurentia (Precambrian North America), and other continents into the supercontinent of Rodinia. We find that a more plausible scenario is that the MCR formed as part of the rifting of Amazonia from Laurentia and became inactive once seafloor spreading was established. A cusp in Laurentia's apparent polar wander path just before the onset of MCR volcanism likely reflects the rifting. Such cusps have been observed elsewhere when continents separate and a new ocean forms between the two fragments. New analyses also find that the MCR's failure did not result from Grenville compression. This view is consistent with the observation that many intracontinental rifts form and fail as part of plate boundary reorganizations. Present-day continental extension in the East African Rift and seafloor spreading in the Red Sea and Gulf of Aden form a classic three-arm rift geometry as Africa splits into Nubia, Somalia, and Arabia. The West Central African Rift system formed during the Mesozoic breakup of Africa and South America and became inactive once full seafloor spreading was established on the Mid-Atlantic Ridge. An important feature of the MCR is that it is has aspects both of a continental rift - a segmented linear depression filled with sedimentary and igneous rocks - and a large igneous province (LIP). We view it as a LIP deposited in crust weakened by rifting, and thus first a rift and then a LIP. The MCR exhibits many key features of volcanic passive margins: seaward dipping reflectors, volcanic rocks yielding magnetic anomalies landward of the oldest spreading anomalies, and a high-velocity lower crustal body. Hence the MCR can be treated as a rift that failed just short of forming a passive margin.

Contrasting magma types and steady-state, volume-predictable, basaltic volcanism along the Great Rift, Idaho.

USGS Publications Warehouse

Kuntz, M.A.; Champion, D.E.; Spiker, E. C.; Lefebvre, R.H.

1986-01-01

The Great Rift is an 85 km-long, 2-8 km-wide volcanic rift zone in the Snake River Plain, Idaho. Three basaltic lava fields, latest Pleistocene to Holocene, are located along the Great Rift: Craters of the Moon, Kings Bowl and Wapi. Craters of the Moon is the largest, covering 1600 km2 and containing approx 30 km3 of lava flows and pyroclastics. Field, radiocarbon and palaeomagnetic data show that this lava field formed in eight eruptive periods, each lasted several hundred years with a recurrence interval of several hundred to approx 3000 yr. The first eruption began approx 15 000 yr B.P. and the last ended at approx 2100 yr B.P. The other two lava fields formed approx 2250 yr B.P. Three magma types fed flows along the Great Rift. A contaminated and a fractionated type were erupted at the Craters of the Moon lava field. The third, little-fractionated Snake River Plain magma-type was erupted at the other two lava fields. The Craters of the Moon segment of the Great Rift has experienced quasi-steady state, volume-predictable volcanism for the last 15 000 yr. Based on this, about 5-6 km3 of lava will be erupted within the next 1000 yr.-L.C.H.

Geodetic measurements and models of rifting in Northern Iceland for 1993-1998 (Invited)

NASA Astrophysics Data System (ADS)

Ali, T.; Feigl, K.; Thurber, C. H.; Masterlark, T.; Carr, B.; Sigmundsson, F.

2010-12-01

Rifting occurs as episodes of active deformation in individual rift segments of the Northern Volcanic Zone (NVZ) in Iceland. Here we simulate deformation around the Krafla central volcano and rift system in NVZ in order to explain InSAR data acquired between 1993 and 1998. The General Inversion for Phase Technique (GIPhT) is used to model the InSAR phase data directly, without unwrapping [Feigl and Thurber, Geophys. J. Int., 2009]. Using a parallel simulated annealing algorithm, GIPhT minimizes the non-linear cost function that quantifies the misfit between observed and modeled values of the phase. We test the hypothesis that the observed deformation can be explained by a combination of at least three processes including: (i) secular plate spreading, (ii) post rifting relaxation following the Krafla rifting episode (1975-1984), and (iii) deflation of a shallow magma chamber beneath the central volcano. The calibration parameters include material properties of upper/lower crust and mantle as well as flux rates for the elements of the plumbing system. The best fitting Maxwell model favors a stronger lower crust (~1.0E+20 Pa.s) and a mantle viscosity of ~1.0E+18 Pa.s as well as a shallow deflating magma chamber. The deformation appears to be linear in time over the observed interval.

The temporal and spatial distribution of upper crustal faulting and magmatism in the south Lake Turkana rift, East Africa

NASA Astrophysics Data System (ADS)

Muirhead, J.; Scholz, C. A.

2017-12-01

During continental breakup extension is accommodated in the upper crust largely through dike intrusion and normal faulting. The Eastern branch of the East African Rift arguably represents the premier example of active continental breakup in the presence magma. Constraining how faulting is distributed in both time and space in these regions is challenging, yet can elucidate how extensional strain localizes within basins as rifting progresses to sea-floor spreading. Studies of active rifts, such as the Turkana Rift, reveal important links between faulting and active magmatic processes. We utilized over 1100 km of high-resolution Compressed High Intensity Radar Pulse (CHIRP) 2D seismic reflection data, integrated with a suite of radiocarbon-dated sediment cores (3 in total), to constrain a 17,000 year history of fault activity in south Lake Turkana. Here, a set of N-S-striking intra-rift faults exhibit time-averaged slip-rates as high as 1.6 mm/yr, with the highest slip-rates occurring along faults within 3 km of the rift axis. Results show that strain has localized into a zone of intra-rift faults along the rift axis, forming an approximately 20 km-wide graben in central parts of the basin. Subsurface structural mapping and fault throw profile analyses reveal increasing basin subsidence and fault-related strain as this faulted graben approaches a volcanic island in the center of the basin (South Island). The long-axis of this island trends north-south, and it contains a number of elongate cones that support recent emplacement of N-S-striking dike intrusions, which parallel recently active intra-rift faults. Overall, these observations suggest strain localization into intra-rift faults in the rift center is likely a product of both volcanic loading and the mechanical and thermal effects of diking along the rift axis. These results support the establishment of magmatic segmentation in southern Lake Turkana, and highlight the importance of magmatism for focusing upper crustal strain as rifts evolve to sea-floor spreading.

Crustal structure in Ethiopia and Kenya from receiver function analysis: Implications for rift development in eastern Africa

NASA Astrophysics Data System (ADS)

Dugda, Mulugeta T.; Nyblade, Andrew A.; Julia, Jordi; Langston, Charles A.; Ammon, Charles J.; Simiyu, Silas

2005-01-01

Crustal structure in Kenya and Ethiopia has been investigated using receiver function analysis of broadband seismic data to determine the extent to which the Cenozoic rifting and magmatism has modified the thickness and composition of the Proterozoic crust in which the East African rift system developed. Data for this study come from broadband seismic experiments conducted in Ethiopia between 2000 and 2002 and in Kenya between 2001 and 2002. Two methods have been used to analyze the receiver functions, the H-κ method, and direct stacks of the waveforms, yielding consistent results. Crustal thickness to the east of the Kenya rift varies between 39 and 42 km, and Poisson's ratios for the crust vary between 0.24 and 0.27. To the west of the Kenya rift, Moho depths vary between 37 and 38 km, and Poisson's ratios vary between 0.24 and 0.27. These findings support previous studies showing that crust away from the Kenya rift has not been modified extensively by Cenozoic rifting and magmatism. Beneath the Ethiopian Plateau on either side of the Main Ethiopian Rift, crustal thickness ranges from 33 to 44 km, and Poisson's ratios vary from 0.23 to 0.28. Within the Main Ethiopian Rift, Moho depths vary from 27 to 38 km, and Poisson's ratios range from 0.27 to 0.35. A crustal thickness of 25 km and a Poisson's ratio of 0.36 were obtained for a single station in the Afar Depression. These results indicate that the crust beneath the Ethiopian Plateau has not been modified significantly by the Cenozoic rifting and magmatism, even though up to a few kilometers of flood basalts have been added, and that the crust beneath the rifted regions in Ethiopia has been thinned in many places and extensively modified by the addition of mafic rock. The latter finding is consistent with models for rift evolution, suggesting that magmatic segments with the Main Ethiopian Rift, characterized by dike intrusion and Quaternary volcanism, act now as the locus of extension rather than the rift border faults.

Structure and evolution of the eastern Gulf of Aden: insights from magnetic and gravity data (Encens-Sheba MD117 cruise)

NASA Astrophysics Data System (ADS)

d'Acremont, Elia; Leroy, Sylvie; Maia, Marcia; Patriat, Philippe; Beslier, Marie-Odile; Bellahsen, Nicolas; Fournier, Marc; Gente, Pascal

2006-06-01

Magnetic and gravity data gathered during the Encens-Sheba cruise (2000 June) in the eastern Gulf of Aden provide insights on the structural evolution of segmentation from rifted margins to incipient seafloor spreading. In this study, we document the conjugate margins asymmetry, confirm the location of the ocean-continent transition (OCT) previously proposed by seismic data, and describe its deep structure and segmentation. In the OCT, gravity models indicate highly thinned crust while magnetic data indicate presence of non-oceanic high-amplitude magnetic anomalies where syn-rift sediments are not observed. Thus, the OCT could be made of ultra-stretched continental crust intruded by magmatic bodies. However, locally in the north, the nature of the OCT could be either an area of ultra-slow spreading oceanic crust or exhumed serpentinized mantle. Between the Alula-Fartak and Socotra fracture zones, the non-volcanic margins and the OCT are segmented by two N027°E-trending transfer fault zones. These transfer zones define three N110°E-trending segments that evolve through time. The first evidence of oceanic spreading corresponds to the magnetic anomaly A5d and is thus dated back to 17.6 Ma at least. Reconstruction of the spreading process suggests a complex non-uniform opening by an arc-like initiation of seafloor spreading in the OCT. The early segmentation appears to be directly related to the continental margin segmentation. The spreading axis segmentation evolved from three segments (17.6 to 10.95 Ma) to two segments (10.95 Ma to present). At the onset of the spreading process, the western segment propagated eastwards, thus reducing the size of the central segment. The presence of a propagator could explain the observed spreading asymmetry with the northern flank of the Sheba ridge being wider than the southern one.

A pulse of mid-Pleistocene rift volcanism in Ethiopia at the dawn of modern humans.

PubMed

Hutchison, William; Fusillo, Raffaella; Pyle, David M; Mather, Tamsin A; Blundy, Jon D; Biggs, Juliet; Yirgu, Gezahegn; Cohen, Benjamin E; Brooker, Richard A; Barfod, Dan N; Calvert, Andrew T

2016-10-18

The Ethiopian Rift Valley hosts the longest record of human co-existence with volcanoes on Earth, however, current understanding of the magnitude and timing of large explosive eruptions in this region is poor. Detailed records of volcanism are essential for interpreting the palaeoenvironments occupied by our hominin ancestors; and also for evaluating the volcanic hazards posed to the 10 million people currently living within this active rift zone. Here we use new geochronological evidence to suggest that a 200 km-long segment of rift experienced a major pulse of explosive volcanic activity between 320 and 170 ka. During this period, at least four distinct volcanic centres underwent large-volume (>10 km 3 ) caldera-forming eruptions, and eruptive fluxes were elevated five times above the average eruption rate for the past 700 ka. We propose that such pulses of episodic silicic volcanism would have drastically remodelled landscapes and ecosystems occupied by early hominin populations.

A pulse of mid-Pleistocene rift volcanism in Ethiopia at the dawn of modern humans

PubMed Central

Hutchison, William; Fusillo, Raffaella; Pyle, David M.; Mather, Tamsin A.; Blundy, Jon D.; Biggs, Juliet; Yirgu, Gezahegn; Cohen, Benjamin E.; Brooker, Richard A.; Barfod, Dan N.; Calvert, Andrew T.

2016-01-01

The Ethiopian Rift Valley hosts the longest record of human co-existence with volcanoes on Earth, however, current understanding of the magnitude and timing of large explosive eruptions in this region is poor. Detailed records of volcanism are essential for interpreting the palaeoenvironments occupied by our hominin ancestors; and also for evaluating the volcanic hazards posed to the 10 million people currently living within this active rift zone. Here we use new geochronological evidence to suggest that a 200 km-long segment of rift experienced a major pulse of explosive volcanic activity between 320 and 170 ka. During this period, at least four distinct volcanic centres underwent large-volume (>10 km3) caldera-forming eruptions, and eruptive fluxes were elevated five times above the average eruption rate for the past 700 ka. We propose that such pulses of episodic silicic volcanism would have drastically remodelled landscapes and ecosystems occupied by early hominin populations. PMID:27754479

Rift Valley Fever Virus MP-12 Vaccine Is Fully Attenuated by a Combination of Partial Attenuations in the S, M, and L Segments

PubMed Central

Hill, Terence E.; Smith, Jennifer K.; Zhang, Lihong; Juelich, Terry L.; Gong, Bin; Slack, Olga A. L.; Ly, Hoai J.; Lokugamage, Nandadeva; Freiberg, Alexander N.

2015-01-01

ABSTRACT Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to Africa and characterized by a high rate of abortion in ruminants and hemorrhagic fever, encephalitis, or blindness in humans. RVF is caused by Rift Valley fever virus (RVFV; family Bunyaviridae, genus Phlebovirus), which has a tripartite negative-stranded RNA genome (consisting of the S, M, and L segments). Further spread of RVF into countries where the disease is not endemic may affect the economy and public health, and vaccination is an effective approach to prevent the spread of RVFV. A live-attenuated MP-12 vaccine is one of the best-characterized RVF vaccines for safety and efficacy and is currently conditionally licensed for use for veterinary purposes in the United States. Meanwhile, as of 2015, no other RVF vaccine has been conditionally or fully licensed for use in the United States. The MP-12 strain is derived from wild-type pathogenic strain ZH548, and its genome encodes 23 mutations in the three genome segments. However, the mechanism of MP-12 attenuation remains unknown. We characterized the attenuation of wild-type pathogenic strain ZH501 carrying a mutation(s) of the MP-12 S, M, or L segment in a mouse model. Our results indicated that MP-12 is attenuated by the mutations in the S, M, and L segments, while the mutations in the M and L segments confer stronger attenuation than those in the S segment. We identified a combination of 3 amino acid changes, Y259H (Gn), R1182G (Gc), and R1029K (L), that was sufficient to attenuate ZH501. However, strain MP-12 with reversion mutations at those 3 sites was still highly attenuated. Our results indicate that MP-12 attenuation is supported by a combination of multiple partial attenuation mutations and a single reversion mutation is less likely to cause a reversion to virulence of the MP-12 vaccine. IMPORTANCE Rift Valley fever (RVF) is a mosquito-transmitted viral disease that is endemic to Africa and that has the potential to spread into other countries. Vaccination is considered an effective way to prevent the disease, and the only available veterinary RVF vaccine in the United States is a live-attenuated MP-12 vaccine, which is conditionally licensed. Strain MP-12 is different from its parental pathogenic RVFV strain, strain ZH548, because of the presence of 23 mutations. This study determined the role of individual mutations in the attenuation of the MP-12 strain. We found that full attenuation of MP-12 occurs by a combination of multiple mutations. Our findings indicate that a single reversion mutation will less likely cause a major reversion to virulence of the MP-12 vaccine. PMID:25948740

Rift Valley Fever Virus MP-12 Vaccine Is Fully Attenuated by a Combination of Partial Attenuations in the S, M, and L Segments.

PubMed

Ikegami, Tetsuro; Hill, Terence E; Smith, Jennifer K; Zhang, Lihong; Juelich, Terry L; Gong, Bin; Slack, Olga A L; Ly, Hoai J; Lokugamage, Nandadeva; Freiberg, Alexander N

2015-07-01

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to Africa and characterized by a high rate of abortion in ruminants and hemorrhagic fever, encephalitis, or blindness in humans. RVF is caused by Rift Valley fever virus (RVFV; family Bunyaviridae, genus Phlebovirus), which has a tripartite negative-stranded RNA genome (consisting of the S, M, and L segments). Further spread of RVF into countries where the disease is not endemic may affect the economy and public health, and vaccination is an effective approach to prevent the spread of RVFV. A live-attenuated MP-12 vaccine is one of the best-characterized RVF vaccines for safety and efficacy and is currently conditionally licensed for use for veterinary purposes in the United States. Meanwhile, as of 2015, no other RVF vaccine has been conditionally or fully licensed for use in the United States. The MP-12 strain is derived from wild-type pathogenic strain ZH548, and its genome encodes 23 mutations in the three genome segments. However, the mechanism of MP-12 attenuation remains unknown. We characterized the attenuation of wild-type pathogenic strain ZH501 carrying a mutation(s) of the MP-12 S, M, or L segment in a mouse model. Our results indicated that MP-12 is attenuated by the mutations in the S, M, and L segments, while the mutations in the M and L segments confer stronger attenuation than those in the S segment. We identified a combination of 3 amino acid changes, Y259H (Gn), R1182G (Gc), and R1029K (L), that was sufficient to attenuate ZH501. However, strain MP-12 with reversion mutations at those 3 sites was still highly attenuated. Our results indicate that MP-12 attenuation is supported by a combination of multiple partial attenuation mutations and a single reversion mutation is less likely to cause a reversion to virulence of the MP-12 vaccine. Rift Valley fever (RVF) is a mosquito-transmitted viral disease that is endemic to Africa and that has the potential to spread into other countries. Vaccination is considered an effective way to prevent the disease, and the only available veterinary RVF vaccine in the United States is a live-attenuated MP-12 vaccine, which is conditionally licensed. Strain MP-12 is different from its parental pathogenic RVFV strain, strain ZH548, because of the presence of 23 mutations. This study determined the role of individual mutations in the attenuation of the MP-12 strain. We found that full attenuation of MP-12 occurs by a combination of multiple mutations. Our findings indicate that a single reversion mutation will less likely cause a major reversion to virulence of the MP-12 vaccine. Copyright © 2015, American Society for Microbiology. All Rights Reserved.

Rio Grande rift evolution and accommodation mechanisms as revealed through low-temperature thermochronometry

NASA Astrophysics Data System (ADS)

Abbey, A. L.; Niemi, N. A.

2017-12-01

Low-temperature thermochronometry in the Rio Grande rift (RGR) in CO and NM, USA, allows for quantification of exhumation magnitudes and rates across the rift and reveals insights into rift basin segmentation and symmetry as well as the timing of extensional fault initiation and dominant mechanisms for rift accommodation. We combine new apatite helium (AHe) and zircon helium (ZHe) thermochronologic data with previously published AHe and apatite fission track (AFT) data to compile 17 vertical transects, each consisting of at least four samples, spanning more than >800 km along the RGR axis. Inverse thermal modeling (QTQt; Gallagher, 2012) of these vertical transects and compilation of bimodal rift related volcanism highlight transfer regions that separate several asymmetric basins with opposing fault dip directions. The Tularosa, Jornada and Albuquerque basins, in the southern RGR show extension initiation ca. 15 Ma with 3-4 km of exhumation accommodated on east dipping faults. Northward, the Española basin, a transfer zone of several strike slip, oblique-slip and smaller normal faults, does not record significant exhumation since the early Cenozoic. In the north-central part of the rift data from the San Luis Basin reveals 3-5 km of exhumation on west dipping faults began 20-15 Ma. East dipping faults in the upper Arkansas and Blue River grabens represent the northern extent of the rift and accommodate 3-5 km of exhumation beginning 15-10 Ma. RGR extension and magmatism initiation is commonly cited at 28 Ma (Tweto, 1979) however, our low-temperature thermochronometry modeling indicates that the majority of upper crustal extension initiated somewhat synchronously 15 Ma along the entire length of the rift. Rift related volcanism increased significantly in volume at 15 Ma, as well, but the locus of this volcanism is the Jemez lineament rather than the rift axis. As a result rifting within the RGR appears to be accommodated primarily by extensional faulting, with the exception of the central part of the rift (Española Basin) where the rift intersects the Jemez lineament. Widespread pre-rift thermochronometric ages in the Española Basin suggest that rifting in the central RGR is accommodated by, non-tectonic processes, most-likely magmatism.

Fault Growth and Propagation and its Effect on Surficial Processes within the Incipient Okavango Rift Zone, Northwest Botswana, Africa (Invited)

NASA Astrophysics Data System (ADS)

Atekwana, E. A.

2010-12-01

The Okavango Rift Zone (ORZ) is suggested to be a zone of incipient continental rifting occuring at the distal end of the southwestern branch of the East African Rift System (EARS), therefore providing a unique opportunity to investigate neotectonic processes during the early stages of rifting. We used geophysical (aeromagnetic, magnetotelluric), Shuttle Radar Tomography Mission, Digital Elevation Model (SRTM-DEM), and sedimentological data to characterize the growth and propagation of faults associated with continental extension in the ORZ, and to elucidate the interplay between neotectonics and surficial processes. The results suggest that: (1) fault growth occurs by along axis linkage of fault segments, (2) an immature border fault is developing through the process of “Fault Piracy” by fault-linkages between major fault systems, (3) significant discrepancies exits between the height of fault scarps and the throws across the faults compared to their lengths in the basement, (4) utilization of preexisting zones of weakness allowed the development of very long faults (> 25-100 km) at a very early stage of continental rifting, explaining the apparent paradox between the fault length versus throw for this young rift, (5) active faults are characterized by conductive anomalies resulting from fluids, whereas, inactive faults show no conductivity anomaly; and 6) sedimentlogical data reveal a major perturbation in lake sedimentation between 41 ka and 27 ka. The sedimentation perturbation is attributed to faulting associated with the rifting and may have resulted in the alteration of hydrology forming the modern day Okavango delta. We infer that this time period may represent the age of the latest rift reactivation and fault growth and propagation within the ORZ.

Initiation and evolution of the Oligo-Miocene rift basins of southwestern Europe: Contribution of deep seismic reflection profiling

NASA Astrophysics Data System (ADS)

Bois, C.

1993-11-01

Southwestern European Oligo-Miocene rift basins have recently been investigated by deep seismic reflection profiling. The study of these data, together with other geophysical and geological data, shows that the rifts, which run from the Rhinegraben to the western Mediterranean, do not form a single clearcut system. The N-trending rifts (Rhinegraben, Bresse and Limagne) were developed on a cold and rigid lithosphere affected by the Alpine collision. The NE-trending rifts (southeastern France, Gulf of Lions and Valencia Trough) were formed slightly later in a backarc basin associated with an active segment of the European-Iberian plate that was heated, affected by widespread calcalkaline volcanism and probably weakened. All the southwestern European rifts and basins together may, however, be related to a common heritage represented by the boundary between the European-Iberian and African-Apulian plates that was created in the Jurassic with the initiation of the Tethys Ocean. The present features of the southwestern European Oligo-Miocène rift basins may be explained by a combination of three geodynamic mechanisms: mechanical stretching of the lithosphere, active mantle uplifting, and subordinate lithospheric flexuring. All the rifts were probably initiated by passive stretching. A systematic discrepancy between stretching derived from fault analysis and attenuation of the crust has been observed in all the rifts. This suggests that these rifts were subsequently reworked by one or several active mantle upwelling events associated with late shoulder uplift, asthenosphere upwelling and anomalous P-wave velocities in the lowermost crust and the uppermost mantle. Crustal attenuation may have been achieved by mantle intrusion, metamorphism of the deep crust and/or its delamination. Some of the rifts were affected by lithospheric flexuring. Combinations, in various proportions, of a small number of geodynamic mechanisms probably controlled many basins in the world. This explains the unique characteristics of each basin, difficulties in basin classification and the frequent failure of single-mechanism models to explain the geological observations.

Geometry and architecture of faults in a syn-rift normal fault array: The Nukhul half-graben, Suez rift, Egypt

NASA Astrophysics Data System (ADS)

Wilson, Paul; Gawthorpe, Rob L.; Hodgetts, David; Rarity, Franklin; Sharp, Ian R.

2009-08-01

The geometry and architecture of a well exposed syn-rift normal fault array in the Suez rift is examined. At pre-rift level, the Nukhul fault consists of a single zone of intense deformation up to 10 m wide, with a significant monocline in the hanging wall and much more limited folding in the footwall. At syn-rift level, the fault zone is characterised by a single discrete fault zone less than 2 m wide, with damage zone faults up to approximately 200 m into the hanging wall, and with no significant monocline developed. The evolution of the fault from a buried structure with associated fault-propagation folding, to a surface-breaking structure with associated surface faulting, has led to enhanced bedding-parallel slip at lower levels that is absent at higher levels. Strain is enhanced at breached relay ramps and bends inherited from pre-existing structures that were reactivated during rifting. Damage zone faults observed within the pre-rift show ramp-flat geometries associated with contrast in competency of the layers cut and commonly contain zones of scaly shale or clay smear. Damage zone faults within the syn-rift are commonly very straight, and may be discrete fault planes with no visible fault rock at the scale of observation, or contain relatively thin and simple zones of scaly shale or gouge. The geometric and architectural evolution of the fault array is interpreted to be the result of (i) the evolution from distributed trishear deformation during upward propagation of buried fault tips to surface faulting after faults breach the surface; (ii) differences in deformation response between lithified pre-rift units that display high competence contrasts during deformation, and unlithified syn-rift units that display low competence contrasts during deformation, and; (iii) the history of segmentation, growth and linkage of the faults that make up the fault array. This has important implications for fluid flow in fault zones.

The Relationships of Subparallel Synthetic Faults and Pre-existing Structures in the Central Malawi Rift

NASA Astrophysics Data System (ADS)

Johnson, S.; Mendez, K.; Beresh, S. C. M.; Mynatt, W. G.; Elifritz, E. A.; Laó-Dávila, D. A.; Atekwana, E. A.; Abdelsalam, M. G.; Chindandali, P. R. N.; Chisenga, C.; Gondwe, S.; Mkumbura, M.; Kalaguluka, D.; Kalindekafe, L.; Salima, J.

2017-12-01

The objective of our research is to explore the evolution of synthetic fault systems in continental rifts. It has been suggested that during the rifting process border faults may become locked and strain is then accommodated within the hanging wall. The Malawi Rift provides an opportunity to study the evolution of these faults within a young (8 Ma), active and magma-poor continental rift. Two faults in central Malawi may show the transference of strain into the hanging wall. These faults are the older Chirobwe-Ntcheu with a length of 115 km and a scarp height of 300-1000 m and the younger Bilila-Mtakataka with a length of 130 km and a scarp height of 4-320 m. We used high-resolution aeromagnetic data and 30m resolution Shuttle Radar Topography Mission (SRTM) digital elevation models (DEM) to provide a 3D spatial characterization of the fault system. Additionally 10cm resolution DEMs were created using unmanned aerial system (UAS) derived aerial photography and Structure from Motion to document the regional Precambrian foliation and joint patterns. Moreover, displacement profiles where extracted from the SRTM-DEM data to compare the segmentation and linkage of the outer and inner faults. Our preliminary results show that the strike of each fault is approximately NW-SE which follows the strike of the Precambrian fabric. The magnetic fabric has a strike of NW-SE in the south changing to NE-SW in the north suggesting that the faults are controlled in part by an inherited Precambrian fabric. The displacement profile of the inner Bilila-Mtakataka fault is asymmetric and displays five fault segments supporting the interpretation that this is a relatively young fault. The expected results of this study are information about segmentation and displacement of each fault and their relationship to one another. The results from the aeromagnetic data utilizing Source Parameter Imaging to produce an approximate depth to basement which will support the displacement profiles derived from the SRTM. Additionally the basement fabrics and faults will be delineated using a combination of aeromagnetic and SRTM data to show the relationship between the surface expression and the expression at depth of the fault scarp. Finally, all remote sensing interpretations are compared to our structural field mapping to confirm our interpretations.

Making it and breaking it in the Midwest: Continental assembly and rifting from modeling of EarthScope magnetotelluric data

USGS Publications Warehouse

Bedrosian, Paul A.

2016-01-01

A three-dimensional lithospheric-scale resistivity model of the North American mid-continent has been estimated based upon EarthScope magnetotelluric data. Details of the resistivity model are discussed in relation to lithospheric sutures, defined primarily from aeromagnetic and geochronologic data, which record the southward growth of the Laurentian margin in the Proterozoic. The resistivity signature of the 1.1 Ga Mid-continent Rift System is examined in detail, in particular as relates to rift geometry, extent, and segmentation. An unrecognized expanse of (concealed) Proterozoic deltaic deposits in Kansas is identified and speculated to result from axial drainage along the southwest rift arm akin to the Rio Grande delta which drains multiple rift basins. A prominent conductor traces out Cambrian rifting in Arkansas, Missouri, Tennessee, and Kentucky; this linear conductor has not been imaged before and suggests that the Cambrian rift system may have been more extensive than previously thought. The highest conductivity within the mid-continent is imaged in Minnesota, Michigan, and Wisconsin where it is coincident with Paleoproterozoic metasedimentary rocks. The high conductivity is attributed to metallic sulfides, and in some cases, graphite. The former is a potential source of sulfur for multiple mineral deposits types, occurrences of which are found throughout the region. Finally, the imprint left within the mantle following the 1.1 Ga rifting event is examined. Variations in lithospheric mantle conductivity are observed and are interpreted to reflect variations in water content (depleted versus metasomatized mantle) imprinted upon the mantle by the Keweenawan mantle plume.

Giant seismites and megablock uplift in the East African Rift: evidence for Late Pleistocene large magnitude earthquakes.

PubMed

Hilbert-Wolf, Hannah Louise; Roberts, Eric M

2015-01-01

In lieu of comprehensive instrumental seismic monitoring, short historical records, and limited fault trench investigations for many seismically active areas, the sedimentary record provides important archives of seismicity in the form of preserved horizons of soft-sediment deformation features, termed seismites. Here we report on extensive seismites in the Late Quaternary-Recent (≤ ~ 28,000 years BP) alluvial and lacustrine strata of the Rukwa Rift Basin, a segment of the Western Branch of the East African Rift System. We document examples of the most highly deformed sediments in shallow, subsurface strata close to the regional capital of Mbeya, Tanzania. This includes a remarkable, clastic 'megablock complex' that preserves remobilized sediment below vertically displaced blocks of intact strata (megablocks), some in excess of 20 m-wide. Documentation of these seismites expands the database of seismogenic sedimentary structures, and attests to large magnitude, Late Pleistocene-Recent earthquakes along the Western Branch of the East African Rift System. Understanding how seismicity deforms near-surface sediments is critical for predicting and preparing for modern seismic hazards, especially along the East African Rift and other tectonically active, developing regions.

Giant Seismites and Megablock Uplift in the East African Rift: Evidence for Late Pleistocene Large Magnitude Earthquakes

PubMed Central

Hilbert-Wolf, Hannah Louise; Roberts, Eric M.

2015-01-01

In lieu of comprehensive instrumental seismic monitoring, short historical records, and limited fault trench investigations for many seismically active areas, the sedimentary record provides important archives of seismicity in the form of preserved horizons of soft-sediment deformation features, termed seismites. Here we report on extensive seismites in the Late Quaternary-Recent (≤ ~ 28,000 years BP) alluvial and lacustrine strata of the Rukwa Rift Basin, a segment of the Western Branch of the East African Rift System. We document examples of the most highly deformed sediments in shallow, subsurface strata close to the regional capital of Mbeya, Tanzania. This includes a remarkable, clastic ‘megablock complex’ that preserves remobilized sediment below vertically displaced blocks of intact strata (megablocks), some in excess of 20 m-wide. Documentation of these seismites expands the database of seismogenic sedimentary structures, and attests to large magnitude, Late Pleistocene-Recent earthquakes along the Western Branch of the East African Rift System. Understanding how seismicity deforms near-surface sediments is critical for predicting and preparing for modern seismic hazards, especially along the East African Rift and other tectonically active, developing regions. PMID:26042601

The North Tanganyika hydrothermal fields, East African Rift system: Their tectonic control and relationship to volcanism and rift segmentation

NASA Astrophysics Data System (ADS)

Coussement, C.; Gente, P.; Rolet, J.; Tiercelin, J.-J.; Wafula, M.; Buku, S.

1994-10-01

The two branches of the East African Rift system include numerous hydrothermal fields, which are closely related to the present fault motion and to volcanic and seismic activity. In this study structural data from Pemba and Cape Banza hydrothermal fields (western branch, North Tanganyika, Zaire) are discussed in terms of neotectonic phenomena. Different types of records, such as fieldwork (onshore and underwater) and LANDSAT and SPOT imagery, are used to explain structural controls on active and fossil hydrothermal systems and their significance. The Pemba site is located at the intersection of 000-020°-trending normal faults belonging to the Uvira Border Fault System and a 120-130°-trending transtensional fault zone and is an area of high seismicity, with events of relatively large magnitude ( Ms < 6.5). The Cape Banza site occurs at the northern end of the Ubawari Peninsula horst. It is bounded by two fault systems trending 015° and is characterized seismically by events of small magnitude ( Ms < 4). The hydrothermal area itself is tectonically controlled by structures striking 170-180° and 080°. The analysis of both hydrothermal areas demonstrates the rejuvenation of older Proterozoic structures during Recent rift faulting and the location of the hydrothermal activity at the junctions of submeridian and transverse faults. The fault motion is compatible with a regional direction of extension of 090-110°. The Cape Banza and Pemba hydrothermal fields may testify to magma chambers existing below the junctions of the faults. They appear to form at structural nodes and may represent a future volcanic province. Together with the four surface volcanic provinces existing along the western branch, they possibly indicate an incipient rift segmentation related to 'valley-valley' or 'transverse fault-valley' junctions, contrasting with the spacing of the volcanoes measured in the eastern branch. These spacings appear to express the different elastic thicknesses between the eastern and western branches of the East African Rift system, perhaps related to a difference in stage of evolution of the two branches.

Geological evolution of the Boset-Bericha Volcanic Complex, Main Ethiopian Rift: 40Ar/39Ar evidence for episodic Pleistocene to Holocene volcanism

NASA Astrophysics Data System (ADS)

Siegburg, Melanie; Gernon, Thomas M.; Bull, Jonathan M.; Keir, Derek; Barfod, Dan N.; Taylor, Rex N.; Abebe, Bekele; Ayele, Atalay

2018-02-01

The Boset-Bericha Volcanic Complex (BBVC) is one of the largest stratovolcanoes of the northern Main Ethiopian Rift (MER). However, very little is known about its eruptive history, despite the fact that approximately 4 million people live within 100 km of the complex. Here, we combine field observations, morphometric analysis using high-resolution LiDAR data, geochemistry and 40Ar/39Ar geochronology to report the first detailed account of the geological evolution of the BBVC, with a focus on extensive young lava flows covering the two edifices, Gudda and Bericha. These lavas exhibit a bimodal composition ranging dominantly from basaltic rift floor lavas and scoria cones, to pantelleritic trachytes and rhyolite flows at Gudda, and comenditic rhyolites at Bericha. Further, several intermediate compositions are associated with fissure vents along the Boset-Kone segment that also appear to link the silicic centres. We divide the BBVC broadly into four main eruptive stages, comprising: (1) early rift floor emplacement, (2) formation of Gudda Volcano within two main cycles, separated by caldera formation, (3) formation of the Bericha Volcano, and (4) sporadic fissure eruptions. Our new 40Ar/39Ar geochronology, targeting a representative array of these flows, provides evidence for episodic activity at the BBVC from 120 ka to the present-day. We find that low-volume mafic episodes are more frequent ( 10 ka cyclicity) than felsic episodes ( 100 ka cyclicity), but the latter are more voluminous. Over the last 30 ka, mafic to intermediate fissure activity might have reinvigorated felsic activity (over the last 16 ka), manifested as peralkaline lava flows and pyroclastic deposits at Gudda and Bericha. Felsic episodes have on average a higher eruption rate (2-5/1000 years) and productivity at Gudda compared to Bericha (1-2/1000 years). The young age of lavas and current fumarolic activity along the fault system, suggest that the BBVC is still potentially active. Coincident episodic activity within the BBVC and at several rift segments in the MER is observed, and facilitates continental rifting.

Post-rift deformation of the Red Sea Arabian margin

NASA Astrophysics Data System (ADS)

Zanoni, Davide; Schettino, Antonio; Pierantoni, Pietro Paolo; Rasul, Najeeb

2017-04-01

Starting from the Oligocene, the Red Sea rift nucleated within the composite Neoproterozoic Arabian-Nubian shield. After about 30 Ma-long history of continental lithosphere thinning and magmatism, the first pulse of oceanic spreading occurred at around 4.6 Ma at the triple junction of Africa, Arabia, and Danakil plate boundaries and propagated southward separating Danakil and Arabia plates. Ocean floor spreading between Arabia and Africa started later, at about 3 Ma and propagated northward (Schettino et al., 2016). Nowadays the northern part of the Red Sea is characterised by isolated oceanic deeps or a thinned continental lithosphere. Here we investigate the deformation of thinned continental margins that develops as a consequence of the continental lithosphere break-up induced by the progressive oceanisation. This deformation consists of a system of transcurrent and reverse faults that accommodate the anelastic relaxation of the extended margins. Inversion and shortening tectonics along the rifted margins as a consequence of the formation of a new segment of ocean ridge was already documented in the Atlantic margin of North America (e.g. Schlische et al. 2003). We present preliminary structural data obtained along the north-central portion of the Arabian rifted margin of the Red Sea. We explored NE-SW trending lineaments within the Arabian margin that are the inland continuation of transform boundaries between segments of the oceanic ridge. We found brittle fault zones whose kinematics is consistent with a post-rift inversion. Along the southernmost transcurrent fault (Ad Damm fault) of the central portion of the Red Sea we found evidence of dextral movement. Along the northernmost transcurrent fault, which intersects the Harrat Lunayyir, structures indicate dextral movement. At the inland termination of this fault the evidence of dextral movement are weaker and NW-SE trending reverse faults outcrop. Between these two faults we found other dextral transcurrent systems that locally are associated with metre-thick reverse fault zones. Along the analysed faults there is evidence of tectonic reworking. Relict kinematic indicators or the sense of asymmetry of sigmoidal Miocene dykes may suggest that a former sinistral movement was locally accommodated by these faults. This evidence of inversion of strike-slip movement associated with reverse structures, mostly found at the inland endings of these lineaments, suggests an inversion tectonics that could be related to the progressive and recent oceanisation of rift segments. Schettino A., Macchiavelli C., Pierantoni P.P., Zanoni D. & Rasul N. 2016. Recent kinematics of the tectonic plates surrounding the Red Sea and Gulf of Aden. Geophysical Journal International, 207, 457-480. Schlische R.W., Withjack M.O. & Olsen P.E., 2003. Relative timing of CAMP, rifting, continental breakup, and basin inversion: tectonic significance, in The Central Atlantic Magmatic Province: Insights from Fragments of Pangea, eds Hames W., Mchone J.G., Renne P. & Ruppel C., American Geophysical Union, 33-59.

China's first intermediate resolution multi-channel seismic survey in the northern Victoria Land Basin and Terror Rift, Ross Sea, Antarctica

NASA Astrophysics Data System (ADS)

Shen, Zhongyan; Gao, Jinyao; Zhang, Tao; Wang, Wei; Ding, Weifeng; Zhang, Sheng

2017-04-01

The West Antarctic Rift System (WARS) represents one of the largest active continental rift systems on Earth and is less well known than other rift systems because it is largely covered by thick ice. The Terror Rift (TR), superimposing on the Victoria Land Basin (VLB) in the western Ross Sea, is identified as the most recent deformational zone of the WARS, thus will provide knowledge of the active deformation process of the WARS. The structure and kinematics of the TR is under debate. Originally, the TR was thought to consist of two parts: the Discovery Graben and the magmatically-intruded Lee Arch. New denser seismic grid in the middle and southern segments of the TR revealed a different structure of the Lee Arch while the northern segment of the TR is not well studied. The glacial history of the VLB/TR region is another attractive issue to the geologists since this area records the behavior information of EAIS and WAIS. In the southern part of the VLB, especially in the McMurdo Sound, the framework of the glacial history is well established after several deep cores which recovery the whole stratigraphic sequences since the onset of the glaciation. However, the glacial history of the northern part of the VLB/TR is less well studied and here we emphasize its importance because the northern part of the VLB/TR is a link between the well-studied southern VLB and the sediment-well-preserved Northern Basin. During the 32nd Chinese National Antarctic Research Expedition, on the board of the RV XueLong, we collected intermediate resolution multi-channel seismic reflection data in the northern VLB/TR. These data will establish new constraints on the timing of deformation, structure and kinematics of the TR, and the history of the EAIS and WAIS.

Genetic stability of Rift Valley fever virus MP-12 vaccine during serial passages in culture cells.

PubMed

Lokugamage, Nandadeva; Ikegami, Tetsuro

2017-01-01

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to Africa which affects both ruminants and humans. RVF causes serious damage to the livestock industry and is also a threat to public health. The Rift Valley fever virus has a segmented negative-stranded RNA genome consisting of Large (L)-, Medium (M)-, and Small (S)-segments. The live-attenuated MP-12 vaccine is immunogenic in livestock and humans, and is conditionally licensed for veterinary use in the U.S. The MP-12 strain encodes 23 mutations (nine amino acid substitutions) and is attenuated through a combination of mutations in the L-, M-, and S-segments. Among them, the M-U795C, M-A3564G, and L-G3104A mutations contribute to viral attenuation through the L- and M-segments. The M-U795C, M-A3564G, L-U533C, and L-G3750A mutations are also independently responsible for temperature-sensitive (ts) phenotype. We hypothesized that a serial passage of the MP-12 vaccine in culture cells causes reversions of the MP-12 genome. The MP-12 vaccine and recombinant rMP12-ΔNSs16/198 were serially passaged 25 times. Droplet digital PCR analysis revealed that the reversion occurred at L-G3750A during passages of MP-12 in Vero or MRC-5 cells. The reversion also occurred at M-A3564G and L-U533C of rMP12-ΔNSs16/198 in Vero cells. Reversion mutations were not found in MP-12 or the variant, rMP12-TOSNSs, in the brains of mice with encephalitis. This study characterized genetic stability of the MP-12 vaccine and the potential risk of reversion mutation at the L-G3750A ts mutation after excessive viral passages in culture cells.

Regional tectonic framework of the Pranhita Godavari basin, India

NASA Astrophysics Data System (ADS)

Biswas, S. K.

2003-03-01

The Pranhita-Godavari Gondwana rift (PGR) has a co-genetic relationship with Permo-Triassic reactivation of the Narmada-Son Geofracture (NSG). The Satpura Gondwana basin represents the terminal depocentre against the NSG, which restricted the northwestward propagation of the PGR. The NE-SW tensional stress responsible for the NW-SE trending PGR could not propagate beyond the ramp formed by uplift along the NSG and transformed kinetically into an ENE directed horizontal shear along the NSG, inducing large scale strike-slip movements. The latter dynamics were responsible for ENE extension of the Satpura rift as a pull-apart basin. The PGR extends up to the present east coast of India, where it is apparently terminated by the NE-SW trending Bapatla ridge along the Eastern Ghat Rift (EGR). The subsurface data, however, shows that the PGR extends across the Bapatla ridge and continues beneath the Cretaceous-Tertiary sediments of the Krishna-Godavari basin (KG) in the EGR. Thus, the Permo-Triassic PGR appears to have continued in the Indo-Antarctic plate before the Cretaceous break up. The EGR, during break up of the continents, cuts across the PGR and the KG basin was superimposed on it. The PGR site is located on a paleo-suture between the Dharwar and Bastar proto-cratons. The master faults developed bordering the rift, and the intra-rift higher order faults followed the pre-existing fabric. The transverse transfer zones manifested as basement ridges, divide the rift into segments of tectono-sedimentary domains. The major domains are the Chintalapudi, Godavari, and Chandrapur sub-basins, each of which subsided differentially. The central Godavari sub-basin subsided most and shows maximum structural complexity and sediment accommodation. The rifting started with initial half-graben faulting along the northeastern master fault and expanded by successive half graben faulting. This gave rise to intra-basinal horsts and grabens, which exercised control on the syn-rift sedimentation. The southeastern boundary fault developed as a strike-slip fault in response to plate rotation and the rift expansion was constrained by it.The basin fill sediments were deposited during two rifting events—Early Permian to (?) Early Jurassic Lower Gondwana rifting, and Early Cretaceous Upper Gondwana rifting. The Lower Gondwana sedimentation started with a pre-rift crustal sagging over the rift site and was filled by glaciogenic Talchir sediments. This was followed by syn-rift-fluvial sedimentation in repeating cycles during the early to late rift stages. Early Cretaceous Chikiala and Gangapur sediments were deposited during the Upper Gondwana rifting. The fluvial cycles were tectonically controlled during each rift stage. The absence of igneous intrusions indicates that the PGR is a passive rift in contrast to the rifts developed in the NSG zone.

Rift systems in the southern North Atlantic: why did some fail and others not?

NASA Astrophysics Data System (ADS)

Nirrengarten, M.; Manatschal, G.; Tugend, J.; Kusznir, N. J.; Sauter, D.

2017-12-01

Orphan, Rockall, Porcupine, Parentis and Pyrenean Basins are failed rift systems surrounding the southern North Atlantic Ocean. The failure or succeessing of a rift system is intimately linked to the question of what controls lithospheric breakup and what keeps oceanic spreading alive. Extension rates and the thermal structure are usually the main parameters invoked. However, between the rifts that succeeded and those that failed, the relative control and relative importance of these parameters is not clear. Cessation of driving forces, strain hardening or competition between concurrent rifts are hypotheses often used to explain rift failure. In this work, we aim to analyze the influence of far field forces on the abandon of rift systems in the southern North Atlantic domain using plate kinematic modeling. A new reconstruction approach that integrates the spatio-temporal evolution of rifted basins has been developed. The plate modeling is based on the definition, mapping and restoration of rift domains using 3D gravity inversions methods that provide crustal thickness maps. The kinematic description of each rift system enables us to discuss the local rift evolution relative to the far field kinematic framework. The resulting model shows a strong segmentation of the different rift systems during extreme crustal thinning that are crosscut by V-shape propagators linked to the exhumation of mantle and emplacement of first oceanic crust. The northward propagating lithospheric breakup of the southern North Atlantic may be partly triggered and channeled by extreme lithospheric thinning. However, at Aptian-Albian time, the northward propagating lithospheric breakup diverts and is partitioned along a transtensional system resulting in the abandon of the Orphan and Rockall basins. The change in the propagation direction may be related to a local strain weakening along existing/inherited transfer zones and/or, alternatively, to a more global plate reorganization. The cessation of the Bay of Biscay-Pyrenean system is related to the northward motion of Africa at Campanian/Santonian time, resulting in a competition between incipient seafloor spreading and far field forces. A concordance between far field forces, lithospheric architecture and strain weakening seems necessary to create a sustainable oceanic domain.

Constraints on the 3D Sediment and Crustal Architecture of the Weakly Extended Malawi Rift from the Onshore/Offshore Wide-Angle Refraction Experiment

NASA Astrophysics Data System (ADS)

Accardo, N. J.; Shillington, D. J.; Scholz, C. A.; McCartney, T.; Ebinger, C. J.; Gaherty, J. B.; Nyblade, A.; Eatmon, A.; Chindandali, P. R. N.; Kamihanda, G.; Ferdinand, R.; Salima, J.; Mruma, A. H.

2016-12-01

Current models of continental rifting increasingly point to the important role of weakening mechanisms like the addition of magmatic products in overcoming the disparity between the magnitude of tectonic forces available for rifting and the forces required to break strong, cold lithosphere. However, many rifts have limited volcanism. To understand the controls on rifting in magma-poor systems, we conduct 3D first arrival time tomography from active-source wide-angle refraction data collected in the Malawi Rift to constrain crustal structure along and across the rift. The Malawi Rift represents a weakly extended rift system located within the southernmost portion of the EARS. The only surface magmatism present occurs within the Rungwe Volcanic Province (RVP) located at the northern termination of the Malawi Rift. We utilize active-source data collected in Lake Malawi as a part of SEGMeNT (Study of Extension and maGmatism in Malawi aNd Tanzania). Over 86,000 unique air gun shots were recorded on an array of 33 offshore "lake" bottom seismometers and 55 onshore seismometers. The resulting ray-coverage encompasses the entire northern section of the Malawi Rift spanning the North and Central basins of Lake Malawi, portions of the surround plateaus, as well as the RVP. First arrivals are picked for all shot-receiver pairs with sufficient signal-to-noise ratio and included in a 3D first-arrival tomography model. Direct arrivals (Ps and Pg) and reflections (PmP) are observed on the majority of instruments, with clear arrivals observed to offsets >220 km. Data and preliminary models indicate variations in fault structure and overall sediment thickness between and within rift basins. The North Basin is characterized by a series of synthetic intrabasin faults and sediments thickening to the east along the Livingstone border fault. The Central Basin is characterized by sediments thickening to the west along the Nkhata border fault in the south near Usisya, Malawi and then transitioning to eastward thickening at the northern termination of the Central Basin near the Manda, Tanzania.

Polyphase Rifting and Breakup of the Central Mozambique Margin

NASA Astrophysics Data System (ADS)

Senkans, Andrew; Leroy, Sylvie; d'Acremont, Elia; Castilla, Raymi

2017-04-01

The breakup of the Gondwana supercontinent resulted in the formation of the Central Mozambique passive margin as Africa and Antarctica were separated during the mid-Jurassic period. The identification of magnetic anomalies in the Mozambique Basin and Riiser Larsen Sea means that post-oceanisation plate kinematics are well-constrained. Unresolved questions remain, however, regarding the initial fit, continental breakup process, and the first relative movements of Africa and Antarctica. This study uses high quality multi-channel seismic reflection profiles in an effort to identify the major crustal domains in the Angoche and Beira regions of the Central Mozambique margin. This work is part of the integrated pluri-disciplinary PAMELA project*. Our results show that the Central Mozambique passive margin is characterised by intense but localised magmatic activity, evidenced by the existence of seaward dipping reflectors (SDR) in the Angoche region, as well as magmatic sills and volcanoclastic material which mark the Beira High. The Angoche region is defined by a faulted upper-continental crust, with the possible exhumation of lower crustal material forming an extended ocean-continent transition (OCT). The profiles studied across the Beira high reveal an offshore continental fragment, which is overlain by a pre-rift sedimentary unit likely to belong to the Karoo Group. Faulting of the crust and overlying sedimentary unit reveals that the Beira High has recorded several phases of deformation. The combination of our seismic interpretation with existing geophysical and geological results have allowed us to propose a breakup model which supports the idea that the Central Mozambique margin was affected by polyphase rifting. The analysis of both along-dip and along-strike profiles shows that the Beira High initially experienced extension in a direction approximately parallel to the Mozambique coastline onshore of the Beira High. Our results suggest that the Beira High results from strike-slip deformation localised along a proposed crustal weakness, represented by the Lurio-Pebane shear zone. A more north-south oriented extension is recorded by the continental breakup and oceanisation. A failed rift is initially formed between the Beira High and the African continent followed by the successful rifting of its southern margin. This study proposes a segmentation of the Central Mozambique margin, with oceanisation first occurring in the Angoche segment. The formation of the first oceanic crust in the Beira segment followed, likely delayed by the formation and failure of the northern Beira High rift. *The PAMELA project (PAssive Margin Exploration Laboratories) is a scientific project led by Ifremer and TOTAL in collaboration with Université Rennes 1, Université Pierre and Marie Curie, Université de Bretagne Occidentale, CNRS and IFPEN.

Pre-breakup geology of the Gulf of Mexico-Caribbean: Its relation to Triassic and Jurassic rift systems of the region

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bartok, P.

1993-02-01

A review of the pre-breakup geology of west-central Pangea, comprised of northern South America, Gulf of Mexico and West Africa, combined with a study of the Mesozoic rift trends of the region confirms a relation between the rift systems and the underlying older grain of deformation. The pre-breakup analysis focuses attention on the Precambrian, Early Paleozoic and Late Paleozoic tectonic events affecting the region and assumes a Pindell fit. Two Late Precambrian orogenic belts are observed in the west central Pangea. Along the northern South American margin and Yucatan a paleo northeast trending Pan-African aged fold belt is documented. Amore » second system is observed along West Africa extending from the High Atlas to the Mauritanides and Rockelides. During the Late Paleozoic, renewed orogenic activity, associated with the Gondwana/Laurentia suture, affected large segments of west central Pangea. The general trend of the system is northeast-southwest and essentially parallels the Gyayana Shield, West African, and eastern North American cratons. Mesozoic rifting closely followed either the Precambrian trends or the Late Paleozoic orogenic belt. The Triassic component focuses along the western portions of the Gulf of Mexico continuing into eastern Mexico and western South America. The Jurassic rift trend followed along the separation between Yucatan and northern South America. At Lake Maracaibo the Jurassic rift system eventually overlaps the Triassic rifts. The Jurassic rift resulted in the [open quotes]Hispanic Corridor[close quotes] that permitted Tethyan and Pacific marine faunas to mix at a time when the Gulf of Mexico underwent continental sedimentation.« less

Crustal and mantle structure and anisotropy beneath the incipient segments of the East African Rift System: Preliminary results from the ongoing SAFARI

NASA Astrophysics Data System (ADS)

Yu, Y.; Reed, C. A.; Gao, S. S.; Liu, K. H.; Massinque, B.; Mdala, H. S.; moidaki, M.; Mutamina, D. M.; Atekwana, E. A.; Ingate, S. F.; Reusch, A.; Barstow, N.

2013-12-01

Despite the vast wealth of research conducted toward understanding processes associated with continental rifting, the extent of our knowledge is derived primarily from studies focused on mature rift systems, such as the well-developed portions of the East African Rift System (EARS) north of Lake Malawi. To explore the dynamics of early rift evolution, the SAFARI (Seismic Arrays for African Rift Initiation) team deployed 50 PASSCAL broadband seismic stations across the Malawi, Luangwa, and Okavango rifts of the EARS during the summer of 2012. The cumulative length of the profiles is about 2500 km and the planned recording duration is 2 years. Here we present the preliminary results of systematic analyses of data obtained from the first year of acquisition for all 50 stations. A total of 446 high-quality shear-wave splitting measurements using PKS, SKKS, and SKS phases from 84 teleseismic events were used to constrain fast polarization directions and splitting times throughout the region. The Malawi and Okavango rifts are characterized by mostly NE trending fast directions with a mean splitting time of about 1 s. The fast directions on the west side of the Luangwa Rift Zone are parallel to the rift valley, and those on the east side are more N-S oriented. Stacking of approximately 1900 radial receiver functions reveals significant spatial variations of both crustal thickness and the ratio of crustal P and S wave velocities, as well as the thickness of the mantle transition zone. Stations situated within the Malawi rift demonstrate a southward increase in observed crustal thickness, which is consistent with the hypothesis that the Malawi rift originated at the northern end of the rift system and propagated southward. Both the Okavango and Luangwa rifts are associated with thinned crust and increased Vp/Vs, although additional data is required at some stations to enhance the reliability of the observations. Teleseismic P-wave travel-time residuals show a delay of about 1 s at stations in the Okavango rift relative to the Limpopo belt. The study region is characterized by a relatively average mantle transition zone thickness of 250 km except for stations located within and to the immediate NW of the Okavango rift, where it is probably abnormally thin. Additional seismological techniques will be applied to the data set, and the preliminary results from the above initial analyses will be confirmed or modified by data from the SAFARI stations in the second year.

Morphology and tectonics of the Mid-Atlantic Ridge, 7°-12°S

NASA Astrophysics Data System (ADS)

Bruguier, N. J.; Minshull, T. A.; Brozena, J. M.

2003-02-01

We present swath bathymetric, gravity, and magnetic data from the Mid-Atlantic Ridge between the Ascension and the Bode Verde fracture zones, where significant ridge-hot spot interaction has been inferred. The ridge axis in this region may be divided into four segments. The central two segments exhibit rifted axial highs, while the northernmost and southernmost segments have deep rift valleys typical of slow-spreading mid-ocean ridges. Bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since ˜1 Ma. Mantle Bouguer anomalies (MBAs) derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes, suggesting that mantle flow has been established beneath the new spreading centers for at least ˜1 Myr. Inversion of gravity data indicates that crustal thicknesses vary by ˜4 km along axis, with the thickest crust occurring beneath a large axial volcanic edifice. Once the effects of lithospheric aging have been removed, a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis-parallel seismic line than a model that includes additional along-axis variations in mantle temperature. Both geophysical and geochemical data from the region may be explained by the melting of small (<200 km) mantle chemical heterogeneities rather than elevated temperatures. Therefore, there may be no Ascension/Circe plume.

Morphotectonic analysis and 10Be dating of the Kyngarga river terraces (southwestern flank of the Baikal rift system, South Siberia)

NASA Astrophysics Data System (ADS)

Arzhannikova, A.; Arzhannikov, S.; Braucher, R.; Jolivet, M.; Aumaître, G.; Bourlès, D.; Keddadouche, K.

2018-02-01

The formation of the Baikal rift system basins is controlled by active faults separating each basin from the adjacent horsts. The kinematics of these faults is mainly explored through investigation of complex sequences of the fault-intersecting river terraces that record both tectonic and climatic events. This study focuses on the northern margin of the major Tunka basin that develops south-west of Lake Baikal. The development of the basin is controlled by the segmented Tunka fault. We performed a detailed mapping of the Kyngarga river terraces, the best preserved terraces staircase in Baikal rift system, at their intersection with the Tunka fault. In order to decipher the chronology of seismic events and the slip rates along that segment of the fault, key terraces were dated using in situ produced cosmogenic 10Be. We demonstrate that the formation of the terrace staircase occurred entirely during MIS1-MIS2. The obtained data allowed us to estimate the rate of incision at different stages of the terrace staircase formation and the relationship between the vertical and horizontal slip rates along this sub-latitudinal segment of the Tunka fault making respectively 0.8 and 1.12 mm yr- 1 over the past 12.5 ka. Analysis of the paleoseismology and paleoclimate data together with terrace dating provided the possibility to estimate the influence of tectonic and climatic factors on the terrace formation. Our proposed model of the Kyngarga river terrace development shows that the incisions into terraces T3 and T6 were induced by the abrupt climatic warming episodes GI-1 and GI-2, respectively, whereas terraces T5, T4 and T2 were abandoned due to the vertical tectonic displacement along the Tunka fault caused by coseismic ruptures.

Structure and degree of magmatism of North and South Atlantic rifted margins

NASA Astrophysics Data System (ADS)

Faleide, Jan Inge; Breivik, Asbjørn J.; Blaich, Olav A.; Tsikalas, Filippos; Planke, Sverre; Mansour Abdelmalak, Mohamed; Mjelde, Rolf; Myklebust, Reidun

2014-05-01

The structure and evolution of conjugate rifted margins in the South and North Atlantic have been studied mainly based on seismic reflection and refraction profiles, complemented by potential field data and plate reconstructions. All margins exhibit distinct along-margin structural and magmatic changes reflecting both structural inheritance extending back to a complex pre-breakup geological history and the final breakup processes. The sedimentary basins at the conjugate margins developed as a result of multiple phases of rifting, associated with complex time-dependent thermal structure of the lithosphere. A series of conjugate crustal transects reveal tectonomagmatic asymmetry, both along-strike and across the conjugate margin systems. The continent-ocean transitional domain along the magma-dominated margin segments is characterized by a large volume of flood basalts and high-velocity/high-density lower crust emplaced during and after continental breakup. Both the volume and duration of excess magmatism varies. The extrusive and intrusive complexes make it difficult to pin down a COB to be used in plate reconstructions. The continent-ocean transition is usually well defined as a rapid increase of P-wave velocities at mid- to lower crustal levels. The transition is further constrained by comparing the mean P-wave velocity to the thickness of the crystalline crust. By this comparison we can also address the magmatic processes associated with breakup, whether they are convection dominated or temperature dominated. In the NE Atlantic there is a strong correlation between magma productivity and early plate spreading rate, suggesting a common cause. A model for the breakup-related magmatism should be able to explain this correlation, but also the magma production peak at breakup, the along-margin magmatic segmentation, and the active mantle upwelling. It is likely that mantle plumes (Iceland in the NE Atlantic, Tristan da Cunha in the South Atlantic) may have influenced the volume of magmatism but they did not necessarily alter the process of rifted margin formation, implying that parts of the margins may have much in common with more magma-poor margins. Conjugate margin segments from the North and South Atlantic will be compared and discussed with particular focus on the tectonomagmatic processes associated with continental breakup.

Four-segmented Rift Valley fever virus induces sterile immunity in sheep after a single vaccination.

PubMed

Wichgers Schreur, Paul J; Kant, Jet; van Keulen, Lucien; Moormann, Rob J M; Kortekaas, Jeroen

2015-03-17

Rift Valley fever virus (RVFV), a mosquito-borne virus in the Bunyaviridae family, causes recurrent outbreaks with severe disease in ruminants and occasionally humans. The virus comprises a segmented genome consisting of a small (S), medium (M) and large (L) RNA segment of negative polarity. The M-segment encodes a glycoprotein precursor (GPC) protein that is co-translationally cleaved into Gn and Gc, which are required for virus entry and fusion. Recently we developed a four-segmented RVFV (RVFV-4s) by splitting the M-genome segment, and used this virus to study RVFV genome packaging. Here we evaluated the potential of a RVFV-4s variant lacking the NSs gene (4s-ΔNSs) to induce protective immunity in sheep. Groups of seven lambs were either mock-vaccinated or vaccinated with 10(5) or 10(6) tissue culture infective dose (TCID50) of 4s-ΔNSs via the intramuscular (IM) or subcutaneous (SC) route. Three weeks post-vaccination all lambs were challenged with wild-type RVFV. Mock-vaccinated lambs developed high fever and high viremia within 2 days post-challenge and three animals eventually succumbed to the infection. In contrast, none of the 4s-ΔNSs vaccinated animals developed clinical signs during the course of the experiment. Vaccination with 10(5) TCID50 via the IM route provided sterile immunity, whereas a 10(6) dose was required to induce sterile immunity via SC vaccination. Protection was strongly correlated with the presence of RVFV neutralizing antibodies. This study shows that 4s-ΔNSs is able to induce sterile immunity in the natural target species after a single vaccination, preferably administrated via the IM route. Copyright © 2015 Elsevier Ltd. All rights reserved.

Lower plate serpentinite diapirism in the Calabrian Arc subduction complex.

PubMed

Polonia, A; Torelli, L; Gasperini, L; Cocchi, L; Muccini, F; Bonatti, E; Hensen, C; Schmidt, M; Romano, S; Artoni, A; Carlini, M

2017-12-19

Mantle-derived serpentinites have been detected at magma-poor rifted margins and above subduction zones, where they are usually produced by fluids released from the slab to the mantle wedge. Here we show evidence of a new class of serpentinite diapirs within the external subduction system of the Calabrian Arc, derived directly from the lower plate. Mantle serpentinites rise through lithospheric faults caused by incipient rifting and the collapse of the accretionary wedge. Mantle-derived diapirism is not linked directly to subduction processes. The serpentinites, formed probably during Mesozoic Tethyan rifting, were carried below the subduction system by plate convergence; lithospheric faults driving margin segmentation act as windows through which inherited serpentinites rise to the sub-seafloor. The discovery of deep-seated seismogenic features coupled with inherited lower plate serpentinite diapirs, provides constraints on mechanisms exposing altered products of mantle peridotite at the seafloor long time after their formation.

Breakup of Pangaea and plate kinematics of the central Atlantic and Atlas regions

NASA Astrophysics Data System (ADS)

Schettino, Antonio; Turco, Eugenio

2009-08-01

A new central Pangaea fit (type A) is proposed for the late Ladinian (230 Ma), together with a plate motions model for the subsequent phases of rifting, continental breakup and initial spreading in the central Atlantic. This model is based on: (1) a reinterpretation of the process of formation of the East Coast Magnetic Anomaly along the eastern margin of North America and the corresponding magnetic anomalies at the conjugate margins of northwest Africa and the Moroccan Meseta; (2) an analysis of major rifting events in the central Atlantic, Atlas and central Mediterranean and (3) a crustal balancing of the stretched margins of North America, Moroccan Meseta and northwest Africa. The process of fragmentation of central Pangaea can be described by three major phases spanning the time interval from the late Ladinian (230 Ma) to the Tithonian (147.7 Ma). During the first phase, from the late Ladinian (230 Ma) to the latest Rhaetian (200 Ma), rifting proceeded along the eastern margin of North America, the northwest African margin and the High, Saharan and Tunisian Atlas, determining the formation of a separate Moroccan microplate at the interface between Gondwana and Laurasia. During the second phase, from the latest Rhaetian (200 Ma) to the late Pliensbachian (185 Ma), oceanic crust started forming between the East Coast and Blake Spur magnetic anomalies, whereas the Morrocan Meseta simply continued to rift away from North America. During this time interval, the Atlas rift reached its maximum extent. Finally, the third phase, encompassing the time interval from the late Pliensbachian (185 Ma) to chron M21 (147.7 Ma), was triggered by the northward jump of the main plate boundary connecting the central Atlantic with the Tethys area. Therefore, as soon as rifting in the Atlas zone ceased, plate motion started along complex fault systems between Morocco and Iberia, whereas a rift/drift transition occurred in the northern segment of the central Atlantic, between Morocco and the conjugate margin of Nova Scotia. The inversion of the Atlas rift and the subsequent formation of the Atlas mountain belt occurred during the Oligocene-early Miocene time interval. In the central Atlantic, this event was associated with higher spreading rates of the ridge segments north of the Atlantis FZ. An estimate of 170 km of dextral offset of Morocco relative to northwest Africa, in the central Atlantic, is required by an analysis of marine magnetic anomalies. Five plate tectonic reconstructions and a computer animation are proposed to illustrate the late Triassic and Jurassic process of breakup of Pangaea in the central Atlantic and Atlas regions.

Trace element and Sr-Nd-Pb isotope geochemistry of Rungwe Volcanic Province, Tanzania: Implications for a superplume source for East Africa Rift magmatism

NASA Astrophysics Data System (ADS)

Castillo, Paterno; Hilton, David; Halldórsson, Sæmundur

2014-09-01

The recently discovered high, plume-like 3He/4He ratios at Rungwe Volcanic Province (RVP) in southern Tanzania, similar to those at the Main Ethiopian Rift in Ethiopia, strongly suggest that magmatism associated with continental rifting along the entire East African Rift System (EARS) has a deep mantle contribution (Hilton et al., 2011). New trace element and Sr-Nd-Pb isotopic data for high 3He/4He lavas and tephras from RVP can be explained by binary mixing relationships involving Early Proterozoic (+/- Archaean) lithospheric mantle, present beneath the southern EARS, and a volatile-rich carbonatitic plume with a limited range of compositions and best represented by recent Nyiragongo lavas from the Virunga Volcanic Province also in the Western Rift. Other lavas from the Western Rift and from the southern Kenya Rift can also be explained through mixing between the same endmember components. In contrast, lavas from the northern Kenya and Main Ethiopian rifts can be explained through variable mixing between the same mantle plume material and the Middle to Late Proterozoic lithospheric mantle, present beneath the northern EARS. Thus, we propose that the bulk of EARS magmatism is sourced from mixing among three endmember sources: Early Proterozoic (+/- Archaean) lithospheric mantle, Middle to Late Proterozoic lithospheric mantle and a volatile-rich carbonatitic plume with a limited range of compositions. We propose further that the African Superplume, a large, seismically anomalous feature originating in the lower mantle beneath southern Africa, influences magmatism throughout eastern Africa with magmatism at RVP and Main Ethiopian Rift representing two different heads of a single mantle plume source. This is consistent with a single mantle plume origin of the coupled He-Ne isotopic signatures of mantle-derived xenoliths and/or lavas from all segments of the EARS (Halldorsson et al., 2014).

Ambient noise tomography reveals upper crustal structure of Icelandic rifts

NASA Astrophysics Data System (ADS)

Green, Robert G.; Priestley, Keith F.; White, Robert S.

2017-05-01

The structure of oceanic spreading centres and subsurface melt distribution within newly formed crust is largely understood from marine seismic experiments. In Iceland, however, sub-aerial rift elevation allows both accurate surface mapping and the installation of large broadband seismic arrays. We present a study using ambient noise Rayleigh wave tomography to image the volcanic spreading centres across Iceland. Our high resolution model images a continuous band of low seismic velocities, parallelling all three segments of the branched rift in Iceland. The upper 10 km contains strong velocity variations, with shear wave velocities 0.5 km s-1 faster in the older non-volcanically active regions compared to the active rifts. Slow velocities correlate very closely with geological surface mapping, with contours of the anomalies parallelling the edges of the neo-volcanic zones. The low-velocity band extends to the full 50 km width of the neo-volcanic zones, demonstrating a significant contrast with the narrow (8 km wide) magmatic zone seen at fast spreading ridges, where the rate of melt supply is similarly high. Within the seismically slow rift band, the lowest velocity cores of the anomalies occur above the centre of the mantle plume under the Vatnajökull icecap, and in the Eastern Volcanic Zone under the central volcano Katla. This suggests localisation of melt accumulation at these specific volcanic centres, demonstrating variability in melt supply into the shallow crust along the rift axis. Shear velocity inversions with depth show that the strongest velocity contrasts are found in the upper 8 km, and show a slight depression in the shear velocity through the mid crust (10-20 km) in the rifts. Our model also shows less intensity to the slow rift anomaly in the Western Volcanic Zone, supporting the notion that rift activity here is decreasing as the ridge jumps to the Eastern Volcanic Zone.

Geometry and kinematics of the Triassic rift basin in Jameson Land (East Greenland)

NASA Astrophysics Data System (ADS)

Guarnieri, Pierpaolo; Brethes, Anaïs.; Rasmussen, Thorkild M.

2017-04-01

The Triassic rift basin along the east Greenland margin described in this paper is represented by NE-SW trending basins and highs segmented by NW-SE trending transfer zones. Coarse-grained sediments along the eastern side of Jameson Land are shown to be hosted in half-graben structures belonging to the Carlsberg Fjord Basin that is bounded by NW dipping normal faults mapped and described after fieldwork in the Klitdal area in Liverpool Land. New aeromagnetic and electromagnetic data together with new drill cores allow the reinterpretation of available seismic lines showing the continuation of the Triassic rift basin toward the SW where it is buried under the Upper Triassic postrift sediments and the Jurassic successions of the Jameson Land Basin. The N-S trending Liverpool Land, interpreted as the boundary block of the Triassic basin, is shown to represent a structural high inherited from the Late Carboniferous tectonics and faulted during the Triassic rifting. The Carlsberg Fjord Basin and the Klitdal Fault System described in this paper should be seen as analogues to the Helgeland Basin in the Norwegian offshore that is bounded by the Ylvingen Fault Zone and to the Papa and West of Shetlands Basins that are bounded by the Spine Fault. The Triassic rift zone and transfer faults on both conjugate margins show a straightforward correlation with the trends of the initial spreading line and fracture zones of the northeast Atlantic indicating a possible inheritance of the Triassic rifting.

Rift-Related Sediments of the Passive Continental Margin of the Paleo-Asian Ocean (Baikal Segment)

NASA Astrophysics Data System (ADS)

Mazukabzov, A. M.; Stanevich, A. M.; Gladkochub, D. P.; Donskaya, T. V.; Khubanov, V. B.; Motova, Z. L.; Kornilova, T. A.

2018-02-01

The geological position, composition, and age of detrital zircons of sedimentary deposits of the Nugan Formation of the Western Baikal region underlying the Golousta Formation of the Baikal series of Ediacaran age have been studied. The formation of both stratigraphic units due to the same sources of detrital material, located within the southern flank of the Siberian Craton, has been proved. The deposits of the Nugan Formation have been demonstrated to mark the rifting stage of the formation of the passive margin of the Paleo-Asiatic Ocean: their accumulation occurred in the Late Cryogenian during the interval 720-640 Ma.

Significant Shear Preceded Rupture in the Oblique Gulf of California Rift

NASA Astrophysics Data System (ADS)

Bennett, S. E.; Oskin, M. E.

2011-12-01

Significant shear deformation during the early history of a rift may profoundly affect the efficiency and success of lithospheric rupture and formation of a new ocean basin. The active Gulf of California (GOC) rift is well suited to study the role of rift obliquity in continental rupture. Transtensional strain in the GOC is accommodated along en-echelon pull-apart basins bounded by dip-slip and oblique-slip faults and linked by strike-slip faults and accommodation zones. Lithospheric rupture is well documented at ca. 6 Ma when >90% of Pacific-North American relative plate motion localized into the GOC. In the northern GOC, the eastern rift margin of the Upper Delfín-Upper Tiburón rift segment preserves an onshore record of the earliest phase of this localization process. Two NW-striking shear zones bound this rift segment, spaced ~37 km apart. Our geologic mapping, paleomagnetic measurements, and geochronology of pre-rift and syn-rift volcanic and sedimentary rocks provide timing and displacement constraints for these shear zones. The Coastal Sonora Fault Zone, exposed on northeast Isla Tiburón and in adjacent coastal Sonora, helped form and then truncate transtensional non-marine basins beginning ca. 7 Ma. On northeast Isla Tiburón, Tertiary units do not match across the ~10 km long Yawassag fault, providing a minimum estimate for total dextral displacement. In coastal Sonora, we document ~12 km of discrete dextral displacement, clockwise block rotations up to 53°, and up to 75% extension that together accommodated 15.7 km of transtensional strain towards azimuth 294° over a 1 Myr period. These estimates do not include tens of kilometers of dextral displacement on the Sacrificio fault that bounds the NE side of this shear zone. The southern of the two shear zones is the La Cruz fault, which transects southern Isla Tiburón. Associated dextral transpression and transtension formed the elongate Southwest Isla Tiburón-Sauzal basin. This basin transitions from non-marine in the SE to marine in the NW where fossil-rich marine sandstone and conglomerate is underlain by a 6.7 ± 0.8 Ma tuff. The base of the marine basin displays ~1 km of dextral displacement, while Early Miocene volcanic and sedimentary rocks are offset tens of kilometers. This displacement history supports significant proto-Gulf shear along the La Cruz fault. Overall, our results suggest that significant shearing along strike-slip faults initiated by ca. 7 Ma, at least 1 Myr prior and proximal to the locus of continental rupture in the GOC. Thus far, this documents the easternmost and earliest phase of rift-related shear at this latitude. We hypothesize that progressive localization of dextral shear into a broader region of extension may act as a catalyst for lithospheric rupture. Such a configuration would resemble how the dextral Walker Lane has become embedded within the extensional Basin and Range Province. We envision that normal faults kinematically linked to strike-slip faults are able to localize crustal thinning and overcome negative feedback processes that otherwise lead to formation of wide rifts. Thus, shearing on strike-slip faults may have been a critical mechanism for strain localization and efficient lithospheric thinning that preceded and eventually led to continental rupture in the Gulf of California.

The Mid-atlantic Ridge (31°S-34°30'S): Temporal and spatial variations of accretionary processes

NASA Astrophysics Data System (ADS)

Fox, P. J.; Grindlay, N. R.; MacDonald, K. C.

1991-02-01

The ridge located between 31° S and 34°30'S is spreading at a rate of 35 mm yr-1, a transitional velocity between the very slow (≤20 mm yr-1) opening rates of the North Atlantic and Southwest Indian Oceans, and the intermediate rates (60 mm yr-1) of the northern limb of the East Pacific Rise, and the Galapagos and Juan de Fuca Ridges. A synthesis of multi-narrow beam, magnetics and gravity data document that in this area the ridge represents a dynamically evolving system. Here the ridge is partitioned into an ensemble of six distinct segments of variable lengths (12 to 100 km) by two transform faults (first-order discontinuities) and three small offset (< 30 km) discontinuities (second-order discontinuities) that behave non-rigidly creating complex and heterogeneous morphotectonic patterns that are not parallel to flow lines. The offset magnitudes of both the first and second-order discontinuities change in response to differential asymmetric spreading. In addition, along the fossil trace of second-order discontinuities, the lengths of abyssal hills located to either side of a discordant zone are observed to lengthen and shorten creating a saw-toothed pattern. Although the spreading rate remains the same along the length of the ridge studied, the morphology of the spreading segments varies from a deep median valley with characteristics analogous to the rift segments of the North Atlantic to a gently rifted axial bulge that is indistinguishable from the shape and relief of the intermediate rate spreading centers of the East Pacific Rise (i.e., 21°N). Like other carefully surveyed ridge segments at slow and fast rates of accretion, the along-axis profiles of each ridge segment are distinctly convex upwards, and exhibit along-strike changes in relief of 500m to 1500 between the shallowest portion of the segment (approximate center) and the segment ends. Such spatial variations create marked along-axis changes in the morphology and relief of each segment. A relatively low mantle Bouguer anomaly is known to be associated with the ridge segment characterized by a gently rifted axial bulge and is interpreted to indicate the presence of focused mantle upwelling (Kuo and Forsyth, 1988). Moreover, the terrain at the ends of each segment are known to be highly magnetized compared to the centers of each segment (Carbotte et al, 1990). Taken together, these data clearly establish that these profound spatial variations in ridge segment properties between adjoining segments, and along and across each segment, indicate that the upper mantle processes responsible for the formation of this contrasting architecture are not solely related to passive upwelling of the asthenosphere beneath the ridge axis. Rather, there must be differences in the thermal and mechanical structure of the crust and upper mantle between and along the ridge segments to explain these spatial variations in axial topography, crustal structure and magnetization. These results are consistent with the results of investigations from other parts of the ridge and suggest that the emplacement of magma is highly focused along segments and positioned beneath the depth minimum of a given segment. The profound differences between segments indicate that the processes governing the behavior of upwelling mantle are decoupled and the variations in the patterns of axis flanking morphology and rate of accretion indicate that processes controlling upwelling and melt production vary markedly in time as well. At this spreading rate and in this area, the accretionary processes are clearly three-dimensional. In addition, the morphology of a ridge segment is not governed so much by opening rate as by the thermal structure of the mantle which underlies the segment.

Inverse models of gravity data from the Red Sea-Aden-East African rifts triple junction zone

NASA Astrophysics Data System (ADS)

Tiberi, Christel; Ebinger, Cynthia; Ballu, Valérie; Stuart, Graham; Oluma, Befekadu

2005-11-01

The combined effects of stretching and magmatism permanently modify crustal structure in continental rifts and volcanic passive margins. The Red Sea-Gulf of Aden-Ethiopian rift triple junction zone provides a unique opportunity to examine incipient volcanic margin formation above or near an asthenospheric upwelling. We use gravity inversions and forward modelling to examine lateral variations in crust and upper mantle structure across the Oligocene flood basalt province, which has subsequently been extended to form the Red Sea, Gulf of Aden and Main Ethiopian rifts. We constrain and test the obtained models with new and existing seismic estimates of crustal thickness. In particular, we predict crustal thickness across the uplifted plateaux and rift valleys, and calibrate our results with recent receiver function analyses. We discuss the results together with a 3-D distribution of density contrasts in terms of magmatic margin structure. The main conclusions are: (1) a denser (+240 kg m-3) and/or a thinner crust (23 km) in the triple junction zone of the Afar depression; (2) a shallower Moho is found along the Main Ethiopian rift axis, with crustal thickness values decreasing from 32-33 km in the south to 24 km beneath the southern Afar depression; (3) thicker crust (~40 km) is present beneath the broad uplifted Oligocene flood basalt province, suggesting that crustal underplating compensates most of the plateau uplift and (4) possible magmatic underplating or a segmentation in the rift structure is observed at ~8°N, 39°W beneath several collapsed caldera complexes. These results indicate that magmatism has profoundly changed crustal structure throughout the flood basalt province.

Crustal geometry of the northeastern Gulf of Aden passive margin: localization of the deformation inferred from receiver function analysis

NASA Astrophysics Data System (ADS)

Tiberi, C.; Leroy, S.; d'Acremont, E.; Bellahsen, N.; Ebinger, C.; Al-Lazki, A.; Pointu, A.

2007-03-01

Here we use receiver function analysis to retrieve crustal thickness and crustal composition along the 35-My-old passive margin of the eastern Gulf of Aden. Our aims are to use results from the 3-D seismic array to map crustal stretching across and along the Aden margin in southern Oman. The array recorded local and teleseismic events between 2003 March and 2004 March. Seventy-eight events were used in our joint inversions for Vp/Vs ratio and depth. The major results are: (1) Crustal thickness decreases from the uplifted rift flank of the margin towards the Sheba mid-ocean ridge. We found a crustal thickness of about 35 km beneath the northern rift flank. This value decreases sharply to 26 km beneath the post-rift subsidence zone on the Salalah coastal plain. This 10 km of crustal thinning occurs across a horizontal distance of less than 30 km showing a localization of the crustal thinning below the first known rifted block of the margin. (2) A second rift margin transect located about 50 km to the east shows no thinning from the coast to 50 km onshore. The lack of crustal thickness variation indicates that the maximum crustal stretching could be restricted to offshore regions. (3) The along-strike variations in crustal structure demonstrate the scale and longevity of the regular along-axis rift segmentation. (4) Extension is still observed north of the rifted domain, 70 km onshore from the coast, making the width of margin larger than first expected from geology. (5) The crust has a felsic to normal composition with a probably strong effect of the sedimentary layer on the Vp/Vs ratio (comprised between 1.67 and 1.91).

Contribution of Transverse Structures, Magma, and Crustal Fluids to Continental Rift Evolution: The East African Rift in Southern Kenya

NASA Astrophysics Data System (ADS)

Kattenhorn, S. A.; Muirhead, J.; Dindi, E.; Fischer, T. P.; Lee, H.; Ebinger, C. J.

2013-12-01

The Magadi rift in southern Kenya formed at ~7 Ma within Proterozoic rocks of the Mozambique orogenic belt, parallel to its contact with the Archean Tanzania craton. The rift is bounded to the west by the ~1600-m-high Nguruman border fault. The rift center is intensely dissected by normal faults, most of which offset ~1.4-0.8 Ma lavas. Current E-W extensional velocities are ~2-4 mm/yr. Published crustal tomography models from the rift center show narrow high velocity zones in the upper crust, interpreted as cooled magma intrusions. Local, surface-wave, and SKS-splitting measurements show a rift-parallel anisotropy interpreted to be the result of aligned melt zones in the lithosphere. Our field observations suggest that recent fault activity is concentrated at the rift center, consistent with the location of the 1998 seismic swarm that was associated with an inferred diking event. Fault zones are pervasively mineralized by calcite, likely from CO2-rich fluids. A system of fault-fed springs provides the sole fluid input for Lake Magadi in the deepest part of the basin. Many of these springs emanate from the Kordjya fault, a 50-km-long, NW-SE striking, transverse structure connecting a portion of the border fault system (the NW-oriented Lengitoto fault) to the current locus of strain and magmatism at the rift center. Sampled springs are warm (44.4°C) and alkaline (pH=10). Dissolved gas data (mainly N2-Ar-He) suggests two-component mixing (mantle and air), possibly indicating that fluids are delivered into the fault zone from deep sources, consistent with a dominant role of magmatism to the focusing of strain at the rift center. The Kordjya fault has developed prominent fault scarps (~150 m high) despite being oblique to the dominant ~N-S fault fabric, and has utilized an en echelon alignment of N-S faults to accommodate its motion. These N-S faults show evidence of sinistral-oblique motion and imply a bookshelf style of faulting to accommodate dextral-oblique motion along the Kordjya fault. Fault relationships imply that the NW-SE transverse structures represent recent activity in the rift, and have locally tilted Late Pleistocene sediments. Given the abundance of N-S striking faults in the rift, the tendency for fault activity along transverse features suggests a change in the rifting driving forces that are likely the result of an interplay between strain localization at the rift center, inherited crustal fabric (NW structures in the Mozambique belt), a possible counterclockwise rotation of stress related to interacting rift segments in southern Kenya, and an active hydrothermal fluid regime that facilitates faulting. By connecting the Lengitoto fault to the rift center, the Kordjya fault has effectively caused the Magadi rift to bypass the Nguruman border fault, which has been rendered inactive and thus no longer a contributor to the rifting process.

The Consequences of Reconfiguring the Ambisense S Genome Segment of Rift Valley Fever Virus on Viral Replication in Mammalian and Mosquito Cells and for Genome Packaging

PubMed Central

Elliott, Richard M.

2014-01-01

Rift Valley fever virus (RVFV, family Bunyaviridae) is a mosquito-borne pathogen of both livestock and humans, found primarily in Sub-Saharan Africa and the Arabian Peninsula. The viral genome comprises two negative-sense (L and M segments) and one ambisense (S segment) RNAs that encode seven proteins. The S segment encodes the nucleocapsid (N) protein in the negative-sense and a nonstructural (NSs) protein in the positive-sense, though NSs cannot be translated directly from the S segment but rather from a specific subgenomic mRNA. Using reverse genetics we generated a virus, designated rMP12:S-Swap, in which the N protein is expressed from the NSs locus and NSs from the N locus within the genomic S RNA. In cells infected with rMP12:S-Swap NSs is expressed at higher levels with respect to N than in cells infected with the parental rMP12 virus. Despite NSs being the main interferon antagonist and determinant of virulence, growth of rMP12:S-Swap was attenuated in mammalian cells and gave a small plaque phenotype. The increased abundance of the NSs protein did not lead to faster inhibition of host cell protein synthesis or host cell transcription in infected mammalian cells. In cultured mosquito cells, however, infection with rMP12:S-Swap resulted in cell death rather than establishment of persistence as seen with rMP12. Finally, altering the composition of the S segment led to a differential packaging ratio of genomic to antigenomic RNA into rMP12:S-Swap virions. Our results highlight the plasticity of the RVFV genome and provide a useful experimental tool to investigate further the packaging mechanism of the segmented genome. PMID:24550727

Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland.

PubMed

Sigmundsson, Freysteinn; Hooper, Andrew; Hreinsdóttir, Sigrún; Vogfjörd, Kristín S; Ófeigsson, Benedikt G; Heimisson, Elías Rafn; Dumont, Stéphanie; Parks, Michelle; Spaans, Karsten; Gudmundsson, Gunnar B; Drouin, Vincent; Árnadóttir, Thóra; Jónsdóttir, Kristín; Gudmundsson, Magnús T; Högnadóttir, Thórdís; Fridriksdóttir, Hildur María; Hensch, Martin; Einarsson, Páll; Magnússon, Eyjólfur; Samsonov, Sergey; Brandsdóttir, Bryndís; White, Robert S; Ágústsdóttir, Thorbjörg; Greenfield, Tim; Green, Robert G; Hjartardóttir, Ásta Rut; Pedersen, Rikke; Bennett, Richard A; Geirsson, Halldór; La Femina, Peter C; Björnsson, Helgi; Pálsson, Finnur; Sturkell, Erik; Bean, Christopher J; Möllhoff, Martin; Braiden, Aoife K; Eibl, Eva P S

2015-01-08

Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long. Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens, or magma flowing vertically into dykes from an underlying source, with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Bárðarbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Bárðarbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Bárðarbunga caldera, accompanied by a series of magnitude M > 5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries.

Mapping rift domains within an inverted hyperextended rift system: The role of rift inheritance in controlling the present-day structure of the North Iberian margin (Bay of Biscay)

NASA Astrophysics Data System (ADS)

Cadenas, Patricia; Fernández-Viejo, Gabriela; Álvarez-Pulgar, Javier; Tugend, Julie; Manatschal, Gianreto; Minshull, Tim

2017-04-01

This study presents a new rift domain map in the central and western North Iberian margin, in the southern Bay of Biscay. This margin was structured during polyphase Triassic to Lower Cretaceous rifting events which led to hyperextension and exhumation and the formation of oceanic crust during a short-lived seafloor spreading period. Extension was halted due to the Alpine convergence between the Iberian and the European plates which led to the formation of the Cantabrian-Pyrenean orogen during the Cenozoic. In the Bay of Biscay, while the northern Biscay margin was slightly inverted, the North Iberian margin, which is at present-day part of the western branch of the Alpine belt together with the Cantabrian Mountains, exhibits several degrees of compressional reactivation. This makes this area a natural laboratory to study the influence of rift inheritance into the inversion of a passive margin. Relying on the interpretation of geological and geophysical data and the integration of wide-angle results, we have mapped five rift domains, corresponding to the proximal, necking, hyperthinned, exhumed mantle, and oceanic domains. One of the main outcomes of this work is the identification of the Asturian Basin as part of a hyperthinned domain bounded to the north by the Le Danois basement high. We interpret Le Danois High as a rift-related crustal block inherited from the margin structure. Our results suggest that the inherited rift architecture controlled the subsequent compressional reactivation. The hyperextended domains within the abyssal plain focused most of the compression resulting in the development of an accretionary wedge and the underthrusting of part of these distal domains beneath the margin. The presence of the Le Danois continental block added complexity, conditioning the inversion undergone by the Asturian Basin. This residual block of less thinned continental crust acted as a local buttress hampering further compressional reactivation within the platform and the inner basin, which were only slightly inverted and uplifted passively due to the underthrusting of the hyperextended domains beneath Le Danois High. The new inverted rift domain map adds some constraints to support kinematic reconstructions and confine palinspatic restorations of the inverted rifted margin. Furthermore, it provides more insights to comprehend the strain partitioning within the Bay of Biscay-Pyrenean inverted hyperextended rift and the broad structural variability observed in such a reduced area, arising from the strong segmentation and the obliquity between the NW-SE and WNW-ESE trending rift structures and the E-W compressional front.

Formation of an Oceanic Transform Fault During Continental Rifting

NASA Astrophysics Data System (ADS)

Illsley-Kemp, F.; Bull, J. M.; Keir, D.; Gerya, T.; Pagli, C.; Gernon, T.; Ayele, A.; Goitom, B.; Hammond, J. O. S.; Kendall, J. M.

2017-12-01

We integrate evidence from surface faults, geodetic measurements, local seismicity, and 3D numerical modelling of the subaerial Afar continental rift to show that an oceanic-style transform fault is forming during the final stages of continental breakup. Transform faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid-ocean ridges, and are vital in palaeotectonic reconstructions of passive margins. The current consensus is that transform faults initiate after the onset of seafloor spreading. However this inference has been difficult to test given the lack of observations of transform fault formation. We present the first direct observation of transform fault initiation, and shed unprecedented light on their formation mechanisms. We demonstrate that they originate during late-stage continental rifting, earlier in the rifting cycle than previously thought. Our results have important implications for reconstructing the breakup history of the continents. Palaeotectonic reconstructions that use transform fault terminations as an indicator of the continent-ocean boundary may have placed the continent-ocean boundary landward of its true location. This will have led to an overestimation of the age of continental breakup of between 8-18 Myr. Our results therefore have significant implications for studies that rely on accurate dating of continental breakup events.

Structure and evolution of the eastern Gulf of Aden conjugate margins from seismic reflection data

NASA Astrophysics Data System (ADS)

d'Acremont, Elia; Leroy, Sylvie; Beslier, Marie-Odile; Bellahsen, Nicolas; Fournier, Marc; Robin, Cécile; Maia, Marcia; Gente, Pascal

2005-03-01

The Gulf of Aden is a young and narrow oceanic basin formed in Oligo-Miocene time between the rifted margins of the Arabian and Somalian plates. Its mean orientation, N75°E, strikes obliquely (50°) to the N25°E opening direction. The western conjugate margins are masked by Oligo-Miocene lavas from the Afar Plume. This paper concerns the eastern margins, where the 19-35 Ma breakup structures are well exposed onshore and within the sediment-starved marine shelf. Those passive margins, about 200 km distant, are non-volcanic. Offshore, during the Encens-Sheba cruise we gathered swath bathymetry, single-channel seismic reflection, gravity and magnetism data, in order to compare the structure of the two conjugate margins and to reconstruct the evolution of the thinned continental crust from rifting to the onset of oceanic spreading. Between the Alula-Fartak and Socotra major fracture zones, two accommodation zones trending N25°E separate the margins into three N110°E-trending segments. The margins are asymmetric: offshore, the northern margin is narrower and steeper than the southern one. Including the onshore domain, the southern rifted margin is about twice the breadth of the northern one. We relate this asymmetry to inherited Jurassic/Cretaceous rifts. The rifting obliquity also influenced the syn-rift structural pattern responsible for the normal faults trending from N70°E to N110°E. The N110°E fault pattern could be explained by the decrease of the influence of rift obliquity towards the central rift, and/or by structural inheritance. The transition between the thinned continental crust and the oceanic crust is characterized by a 40 km wide zone. Our data suggest that its basement is made up of thinned continental crust along the southern margin and of thinned continental crust or exhumed mantle, more or less intruded by magmatic rocks, along the northern margin.

Earthquake and Volcanic Hazard Mitigation and Capacity Building in Sub-Saharan Africa

NASA Astrophysics Data System (ADS)

Ayele, A.

2012-04-01

The East African Rift System (EARS) is a classic example of active continental rifting, and a natural laboratory setting to study initiation and early stage evolution of continental rifts. The EARS is at different stages of development that varies from relatively matured rift (16 mm/yr) in the Afar to a weakly extended Okavango Delta in the south with predicted opening velocity < 3 mm/yr. Recent studies in the region helped researchers to highlight the length and timescales of magmatism and faulting, the partitioning of strain between faulting and magmatism, and their implications for the development of along-axis segmentation. Although the human resource and instrument coverage is sparse in the continent, our understanding of rift processes and deep structure has improved in the last decade after the advent of space geodesy and broadband seismology. The recent major earthquakes, volcanic eruptions and mega dike intrusions that occurred along the EARS attracted several earth scientist teams across the globe. However, most African countries traversed by the rift do not have the full capacity to monitor and mitigate earthquake and volcanic hazards. Few monitoring facilities exist in some countries, and the data acquisition is rarely available in real-time for mitigation purpose. Many sub-Saharan Africa governments are currently focused on achieving the millennium development goals with massive infrastructure development scheme and urbanization while impending natural hazards of such nature are severely overlooked. Collaborations with overseas researchers and other joint efforts by the international community are opportunities to be used by African institutions to best utilize limited resources and to mitigate earthquake and volcano hazards.

Rift Valley Fever Virus: Molecular Biologic Studies of the M Segment RNA(Ribonucleic Acids) for Application in Disease Prevention

DTIC Science & Technology

1988-12-01

correctly expresses, processes , and transports all M segment proteins (virus 7; r-eferences 2,3) were pulse -labeled with 35S- methionine and subsequently...c-ranslationii~y processed to yield the mature proteins. The first ATG codon of tP’o _iinle OrF is required for production of the 78kd protein...employed for the expression of the 78kd and l4kd proteins serves to control glycosylation arnd proteolytic. processing of the resultant poI~peptidles

Tectonic inheritance in the development of the Kivu - north Tanganyika rift segment of the East African Rift System: role of pre-existing structures of Precambrian to early Palaeozoic origin.

NASA Astrophysics Data System (ADS)

Delvaux, Damien; Fiama Bondo, Silvanos; Ganza Bamulezi, Gloire

2017-04-01

The present architecture of the junction between the Kivu rift basin and the north Tanganyika rift basin is that of a typical accommodation zone trough the Ruzizi depression. However, this structure appeared only late in the development of the Western branch of the East African Rift System and is the result of a strong control by pre-existing structures of Precambrian to early Palaeozoic origin. In the frame of a seismic hazard assessment of the Kivu rift region, we (Delvaux et al., 2016) constructed homogeneous geological, structural and neotectonic maps cross the five countries of this region, mapped the pre-rift, early rift and Late Quaternary faults and compiled the existing knowledge on thermal springs (assumed to be diagnostic of current tectonic activity along faults). We also produced also a new catalogue of historical and instrumental seismicity and defined the seismotectonic characteristics (stress field, depth of faulting) using published focal mechanism data. Rifting in this region started at about 11 Ma by initial doming and extensive fissural basaltic volcanism along normal faults sub-parallel to the axis of the future rift valley, as a consequence of the divergence between the Nubia and the Victoria plate. In a later stage, starting around 8-7 Ma, extension localized along a series of major border faults individualizing the subsiding tectonic basins from the uplifting rift shoulders, while lava evolved towards alkali basaltic composition until 2.6 Ma. During this stage, initial Kivu rift valley was extending linearly in a SSW direction, much further than its the actual termination at Bukavu, into the Mwenga-Kamituga graben, up to Namoya. The SW extremity of this graben was linked via a long oblique transfer zone to the central part of Lake Tanganyika, itself reactivating an older ductile-brittle shear zone. In the late Quaternary-early Holocene, volcanism migrated towards the center of the basin, with the development of the Virunga volcanic massif, and the Kivu-Ruzizi accommodation zone connected the northern half of the former Kivu rift basin to the northern extremity of the Tanganyika basin. This process was influenced by the highly heterogeneous basement, formed during a long geological history with a dominantly brittle structuration during the Pan-African. The local stress field revealed by earthquake focal mechanisms appears strongly influenced by this heterogeneous structure but also by the transition towards the Congo basin on the western side of the rift and towards the Tanzanian carton on its eastern side. Delvaux, D. et al., 2016. Journal of African Earth Sciences. doi: 10.1016/j.jafrearsci.2016.10.004

Earthquake source parameters for the 2010 western Gulf of Aden rifting episode

NASA Astrophysics Data System (ADS)

Shuler, Ashley; Nettles, Meredith

2012-08-01

On 2010 November 14, an intense swarm of earthquakes began in the western Gulf of Aden. Within a 48-hr period, 82 earthquakes with magnitudes between 4.5 and 5.5 were reported along an ˜80-km-long segment of the east-west trending Aden Ridge, making this swarm one of the largest ever observed in an extensional oceanic setting. In this study, we calculate centroid-moment-tensor solutions for 110 earthquakes that occurred between 2010 November and 2011 April. Over 80 per cent of the cumulative seismic moment results from earthquakes that occurred within 1 week of the onset of the swarm. We find that this sequence has a b-value of ˜1.6 and is dominated by normal-faulting earthquakes that, early in the swarm, migrate westwards with time. These earthquakes are located in rhombic basins along a section of the ridge that was previously characterized by low levels of seismicity and a lack of recent volcanism on the seafloor. Body-wave modelling demonstrates that the events occur in the top 2-3 km of the crust. Nodal planes of the normal-faulting earthquakes are consistent with previously mapped faults in the axial valley. A small number of strike-slip earthquakes observed between two basins near 44°E, where the axial valley changes orientation, depth and width, likely indicate the presence of an incipient transform fault and the early stages of ridge-transform segmentation. The direction of extension accommodated by the earthquakes is intermediate between the rift orthogonal and the direction of relative motion between the Arabian and Somalian plates, consistent with the oblique style of rifting occurring along the slow-spreading Aden Ridge. The 2010 swarm shares many characteristics with dyke-induced rifting episodes from both oceanic and continental settings. We conclude that the 2010 swarm represents the seismic component of an undersea magmatic rifting episode along the nascent Aden Ridge, and attribute the large size of the earthquakes to the combined effects of the slow spreading rate, relatively thick crust and recent quiescence. We estimate that the rifting episode was caused by dyke intrusions that propagated laterally for 12-18 hr, accommodating ˜1-14 m of opening or ˜85-800 yr of spreading along this section of the ridge. Our findings demonstrate the westward propagation of active seafloor spreading into this section of the western Gulf of Aden and illustrate that deformation at the onset of seafloor spreading may be accommodated by discrete episodes of faulting and magmatism. A comparison with similar sequences on land suggests that the 2010 episode may be only the first of several dyke-induced rifting episodes to occur in the western Gulf of Aden.

On abrupt transpression to transtension transition in the South Baikal rift system (Tunka - South Baikal segment)

NASA Astrophysics Data System (ADS)

Sankov, Vladimir; Parfeevets, Anna; Lukhnev, Andrey; Miroshnitchenko, Andrey; Ashurkov, Sergey; Sankov, Alexey; Usynin, Leonid; Eskin, Alexander; Bryzhak, Evgeny

2013-04-01

This work addresses to relation of transpression and extension stress-strain conditions in intracontinental rift system. In our investigation we use a new structural, shallow geophysics, GPS geodetic data and paleostress reconstructions. The surroundings of southern tip of Siberian platform is the region of three Late Cenozoic structures conjugation: sublatitudinal Obruchev fault (OF) controlling the northern boundary of the South Baikal basin, NW trending Main Sayan fault (MSF) as the strike-slip boundary between Siberian platform and East Sayan block and WNW trending eastern segment of Tunka fault (TF) as part of the Tunka basins system northern boundary. A new evidences of superposition of compression and extension fault structures were revealed near the southern extremity of Baikal lake. We've find a very close vicinity of Late Pleistocene - Holocene strike-slip, thrust and normal faulting in the MSF and OF junction zone. The on-land Holocene normal faulting can be considered as secondary fault paragenesis within the main strike-slip zone (Sankov et al., 2009). Active strike-slip, thrust and reverse faulting characterize the MSF and TF junction zone. The transpression conditions are replaced very sharply by transtension and extension ones in eastern direction from zone of structures conjugation - the active normal faulting is dominated within the South Baikal basin. The Bystraya rift basin located in the west shows the tectonic inversion since Middle Pleistocene as a result of the strike-slip movements partitioning between TF and MSF and inset of edition compression stress. The active strike-slip and intrabasin extension conditions are dominated father to the west in Tunka basin. The results of our GPS measurements show the present day convergence and east movements of Khamar-Daban block and eastern Tunka basins relative to Siberian platform along MSF and TF with NE-SW shortening domination. The clear NW-SE divergence across Baikal basin is documented. Holocene and present-day left lateral relative motions of about 3 mm/yr (Sankov et al., 2004) between of Siberian platform and its mounting frame are accommodated along south-eastern segment of MSF. We consider two main factors of sharp transition between transpression and transtension to extension conditions in Tunka-South Baikal segment of Baikal rift system. The first one is the influence of geometry of southern tip of Siberian platform as a first order ancient lithosphere heterogeneity in agreement with (Petit et al., 1996). The second factor is the interaction in this region of two tectonic forces driving the Cenozoic geodynamics. The initial opening of the Tunka and South Baikal basins since Oligocene time as well as father Baikal rift system development caused by long lived asthenosphere flow along NW-SE direction (Sankov et al., 2011). The addition NE-SW compression started during Pliocene (Parfeevets, Sankov, 2006) as the result of the Hindustan and Eurasia convergence. The former caused transpression deformations and clockwise horizontal block rotations along south-western boundary of the platform with their SE movements to the "free space" opened by the divergence of Siberian platform and Transbaikal block (Sankov et al., 2002, 2005).

Structural style and tectonic evolution of the easternmost Gulf of Aden conjugate margins (Socotra - Southern Oman)

NASA Astrophysics Data System (ADS)

Nonn, Chloe; Leroy, Sylvie; Castilla, Raymi; de Clarens, Philippe; Lescanne, Marc

2016-04-01

Observations from distal rifted margins in present day magma-poor rifted margins led to the discovery of hyperextended crust and exhumed sub-continental mantle. This finding allowed to better figure out how thinning process are accommodate by tectonic structures, forming various crustal domains, as the deformation localized towards the future area of breakup. However, some of the current challenges are about clarifying how factors as oblique kinematic, pre-existing structures and volcanism can control the 3D geometry and crustal architecture of the passive margins? A key to better understand the rifting evolution in its entirety is to study conjugate margins. The gulf of Aden is a young oceanic basin (with a global trend about N75°E) oblique to the divergence (about 30°N), separating Arabia from Somalia of less than 800 km. Thanks to its immerged margins and its thin post-rift sediment cover, the gulf of Aden basin is a natural laboratory to investigate conjugate margins and strain localisation throughout the rift history. In this contribution, we focus our interest on offshore Socotra Island (Yemen) and its conjugate in Southeastern Oman. This area extends from Socotra-Hadbeen (SHFZ) and the eastern Gulf of Aden fault zones (EGAFZ). In the easternmost part of the gulf of Aden, we provide new insights into crustal deformation and emplacement of the new oceanic crust thanks to bathymetric, magnetic, gravimetric data and single-, multi-channel, high speed seismic reflection data collected during Encens-Sheba (2000), Encens (2006) and the more recent Marges-Aden (2012) cruises respectively. The results obtained after compilation of these data, previous geological (field works) and geophysical (receiver functions, Pn-tomography, magnetic anomalies, heat flow) studies on the focused area, allowed us to provide new structural mapping and stratigraphic correlation between onshore and offshore parts of Socotra and Oman margins. We precisely defined and map crustal domains, syn-tectonic structures and oblique accommodation zones to highlighted asymmetrical margins, characterized by strong lateral variability of crustal domains along and across strike. From external to internal domains of the margins and in between SHFZ and EGAFZ (first-order segment), this study details sharp necking domain and complex transition from hyperextended to oceanic crust characterized by: (i) hyperextended crust affected by volcanic extrusions; (ii) detachment faulting in the distal part of the margins allowing exhumation; (iii) volcanic constructions in the exhumation domain; (iv) a complex proto-oceanic crust. We highlight a significant second-order segmentation characterized by six N20°E trending transfer zones, limiting seven 25 - 60 km length segments and affecting necking domain as well as the ocean-continent transition. Based on interpretative cross-sections and detailed stratigraphic analysis, we discuss the complex temporal and spatial evolution of conjugate margins: (i) the margins segmentation and the relationship with structural inheritance (ii) the set-up of a long-offset detachment fault and the nature of the exhumed basement (iv) the origin and timing of magmatic events and the onset of proto-oceanic crust.

Stratigraphy, Structure and Tectonics of the Eyjafjarðaráll Rift, Abandoned Southern Segment of the Kolbeinsey Ridge, North Iceland

NASA Astrophysics Data System (ADS)

Brandsdottir, B.; Karson, J. A.; Magnúsdóttir, S.; Detrick, B.; Driscoll, N. W.

2017-12-01

The multi-branched plate boundary across Iceland is made up of divergent and oblique rifts, and transform zones, characterized by entwined extensional and transform tectonics. The Tjörnes Fracture Zone (TFZ) is a complex transform linking the northern rift zone (NVZ) on land with the offshore Kolbeinsey Ridge. The TFZ lacks a clear topographic expression typical of oceanic fracture zones. The transform zone is roughly 150 km long (E-W) by 50-75 km wide (N-S) with three N-S trending pull-apart basins bounded by a complex array of normal and oblique-slip faults. The offshore extension of the NVZ, the Grímsey Oblique Rift, is composed of several active volcanic systems with N-S trending fissure swarms, including the Skjálfandadjúp Basin (SB). The magma-starved southern extension of the KR, the 80 km NS and 15-20 EW Eyjafjarðaráll Rift (ER), is made up of dominantly normal faults merging southwards with a system of right-lateral strike-slip faults with vertical displacement up to 15 m in the Húsavík Flatey Fault Zone (HFFZ). The northern ER is a 500-700 m deep asymmetric rift, framed by normal faults with 20-25 m vertical displacement, To the south, transform movement associated with the HFFZ has created a NW- striking pull-apart basin with frequent earthquake swarms. Details of the tectonic framework of the ER are documented in a compilation of data from aerial photos, satellite images, field mapping, multibeam bathymetry, high-resolution seismic reflection surveys (Chirp) and seismicity. The TFZ rift basins contain post-glacial sediments of variable thickness. Strata in the western ER and SB basins dip steeply E along the normal faults, towards the deepest part of the rift. The eastern side of the ER and SB basins differ considerably from the western side, with near-vertical faults. Correlation of Chirp reflection data and tephrachronology from a sediment core reveal major rifting episodes between 10-12.1 kyrs BP activating both the Eyjafjarðaráll and Skjálfandadjúp rift basins, followed by smaller-scale fault movements throughout Holocene. These vertical fault movements reflect elevated tectonic activity during early postglacial time coinciding with isostatic rebound and enhanced volcanism within Iceland.

Enigmatic rift-parallel, strike-slip faults around Eyjafjörður, Northern Iceland

NASA Astrophysics Data System (ADS)

Proett, J. A.; Karson, J. A.

2014-12-01

Strike-slip faults along mid-ocean ridge spreading centers are generally thought to be restricted to transform boundaries connecting rift segments. Faults that are parallel to spreading centers are generally assumed to be normal faults associated with tectonic extension. However, clear evidence of north-south (rift-parallel), strike-slip displacements occur widely around the southern portion of Eyjafjörður, northern Iceland about 50 km west of the Northern Rift Zone. The area is south of the southernmost strand (Dalvík Lineament) of the NW-SE-trending, dextral-slip, Tjӧrnes Fracture Zone (where N-S, sinistral, strike-slip "bookshelf" faulting occurs). Faults in the Eyjafjörður area cut 8.5-10 m.y. basaltic crust and are parallel to spreading-related dikes and are commonly concentrated along dike margins. Fault rocks range from fault breccia to gouge. Riedel shears and other kinematic indicators provide unambiguous evidence of shear sense. Most faults show evidence of sinistral, strike-slip movement but smaller proportions of normal and oblique-slip faults also are present. Cross cutting relations among the different types of faults are inconsistent and appear to be related to a single deformation event. Fault slip-line kinematic analysis yields solutions indicating sinistral-normal oblique-slip overall. These results may be interpreted in terms of either previously unrecognized transform-fault bookshelf faulting or slip accommodating block rotation associated with northward propagation of the Northern Rift Zone.

Calving and rifting on McMurdo Ice Shelf, Antarctica

NASA Astrophysics Data System (ADS)

Banwell, Alison; Willis, Ian; MacAyeal, Douglas; Goodsell, Becky; Macdonald, Grant; Mayer, David; Powell, Anthony

2017-04-01

On March 2, 2016, a series of small en échelon tabular icebergs calved from the seaward front of the McMurdo Ice Shelf, and a previously inactive ice-shelf rift suddenly widened and propagated by 3km, 25% of its previous length, setting the stage for future calving of an approximately 8 km2 segment of the ice shelf. Immediately prior to these events, perhaps within 24 hours, all remaining land-fast sea ice buttressing the ice shelf broke up and drifted away. The events were witnessed by time-lapse cameras at nearby Scott Base giving a unique opportunity to document the timing of the events and conditions leading up to them. In addition, the events can be put into context using nearby seismic and automatic weather station data, satellite imagery, and ground observation made 8 months later. Although the observations cannot be used definitively to identify the exact trigger of calving and rifting, the seismic records reveal superimposed sets of long-period (>10 s) sea swell, propagating into McMurdo Sound from distant storm sources in the Pacific Ocean, at the time of, and immediately prior to, the break-up of sea ice and associated ice shelf calving and rifting. This conspicuous presence suggests that sea swell should be studied further as a proximal cause of ice-shelf calving and rifting; if proven, it suggests that ice-shelf stability is tele-connected with far-field storm conditions at lower latitudes, adding a global dimension to the physics of potential ice-shelf breakup.

Reexaming Owens Valley: Partitioning of Discrete and Distributed Transtension, Structural Controls on Magmatism, and Seismic Potential within an Active Rift Zone, Eastern California.

NASA Astrophysics Data System (ADS)

Levy, D. A.; Haproff, P. J.; Yin, A.

2016-12-01

Crustal-scale transtensional deformation is common in intracontinental extensional settings. However, along-strike variations in the geometry, kinematics, and linkages between rift-related faults, along with controls on local magmatic plumbing, remain inadequately examined. In this study, we conducted geologic mapping of active structures within central and northern Owens Valley of eastern California. C. Owens Valley features right-slip oblique deformation accommodated by three discrete north-south-trending faults: (1) the right-slip Owens Valley fault (OVF) and rift-bounding (2) Sierra Nevada Frontal fault (SNFF) and (3) the White-Inyo Mountains fault (WIMF). The OVF also serves as a lithospheric-scale, vertical conduit for asthenospheric-derived magma to migrate upwards and erupt at Big Pine Volcanic Field. Right-slip shear within C. Owens Valley is transferred to the SNFF of N. Owens Valley via the Poverty Hills restraining bend. In contrast to C. Owens Valley, the northern segment is dominated by distributed E-W to NE-SW-oriented extension, evidenced by normal fault scarps throughout Volcanic Tablelands and basin floor. Furthermore, the White Mountain fault which bounds N. Owens Valley to the east consists of a master west-dipping detachment fault that thinned the lithosphere, allowing for asthenospheric upwelling into the crust beneath the western rift shoulder. Subvertical, right-slip faults of the SNFF provide a conduit for magma to erupt on the surface throughout the Long Valley Caldera, Mono-Inyo Craters, and Mono Basin region. Our mapping demonstrates complex strain partitioning of discrete and distributed deformation within an alternating pure and simple shear, transtensional rift zone. Lastly, we present previously unknown relationships in Owens Valley between lithospheric-scale fault systems, seismic potential, and rift magmatism.

Episodic kinematics in continental rifts modulated by changes in mantle melt fraction.

PubMed

Lamb, Simon; Moore, James D P; Smith, Euan; Stern, Tim

2017-07-05

Oceanic crust is created by the extraction of molten rock from underlying mantle at the seafloor 'spreading centres' found between diverging tectonic plates. Modelling studies have suggested that mantle melting can occur through decompression as the mantle flows upwards beneath spreading centres, but direct observation of this process is difficult beneath the oceans. Continental rifts, however-which are also associated with mantle melt production-are amenable to detailed measurements of their short-term kinematics using geodetic techniques. Here we show that such data can provide evidence for an upwelling mantle flow, as well as information on the dimensions and timescale of mantle melting. For North Island, New Zealand, around ten years of campaign and continuous GPS measurements in the continental rift system known as the Taupo volcanic zone reveal that it is extending at a rate of 6-15 millimetres per year. However, a roughly 70-kilometre-long segment of the rift axis is associated with strong horizontal contraction and rapid subsidence, and is flanked by regions of extension and uplift. These features fit a simple model that involves flexure of an elastic upper crust, which is pulled downwards or pushed upwards along the rift axis by a driving force located at a depth greater than 15 kilometres. We propose that flexure is caused by melt-induced episodic changes in the vertical flow forces that are generated by upwelling mantle beneath the rift axis, triggering a transient lower-crustal flow. A drop in the melt fraction owing to melt extraction raises the mantle flow viscosity and drives subsidence, whereas melt accumulation reduces viscosity and allows uplift-processes that are also likely to occur in oceanic spreading centres.

The crustal structure of the southern Argentine margin

NASA Astrophysics Data System (ADS)

Becker, Katharina; Franke, Dieter; Schnabel, Michael; Schreckenberger, Bernd; Heyde, Ingo; Krawczyk, Charlotte M.

2012-06-01

Multichannel reflection seismic profiles, combined with gravimetric and magnetic data provide insight into the crustal structure of the southernmost Argentine margin, at the transition from a rifted to a transform margin and outline the extent of the North Falkland Graben. Based on these data, we establish a regional stratigraphic model for the post-rift sediments, comprising six marker horizons with a new formation in the Barremian/Lower Cretaceous. Our observations support that a N-S trending subsidiary branch of the North Falkland Graben continues along the continental shelf and slope to the Argentine basin. During the rift phase, a wide shelf area was affected by the E-W extension, subsequently forming the North Falkland Graben and the subsidiary branch along which finally breakup occurred. We propose the division of the margin in two segments: a N-S trending rifted margin and an E-W trending transform margin. This is further underpinned by crustal scale gravity modelling. Three different tectono-dynamic processes shaped the study area. (1) The Triassic/Early Jurassic extensional phase resulting in the formation of the North Falkland Graben and additional narrower rift grabens ended synchronously with the breakup of the South Atlantic in the early Valanginian. (2) Extensional phase related to the opening of the South Atlantic. (3) The transform margin was active in the study area from about Hauterivian times and activity lasted until late Cretaceous/early Cenozoic. Both, the rifted margin and the transform margin are magma-poor. Very limited structures may have a volcanic origin but are suggested to be post-rift. The oceanic crust was found to be unusually thin, indicating a deficit in magma supply during formation. These findings in combination with the proposed breakup age in the early Valanginian that considerably predates the formation of the Paraná-Etendeka continental flood basalt provinces in Brazil and Namibia question the influence of the Tristan da Cunha hotspot during the initial formation of the South Atlantic.

Major and micro seismo-volcanic crises in the Asal Rift, Djibouti

NASA Astrophysics Data System (ADS)

Peltzer, G.; Doubre, C.; Tomic, J.

2009-05-01

The Asal-Ghoubbet Rift is located on the eastern branch of the Afar triple junction between the Arabia, Somalia, and Nubia tectonic plates. The last major seismo-volcanic crisis on this segment occurred in November 1978, involving two earthquakes of mb=5+, a basaltic fissure eruption, the development of many open fissures across the rift and up to 80 cm of vertical slip on the bordering faults. Geodetic leveling revealed ~2 m of horizontal opening of the rift accompanied by ~70 cm of subsidence of the inner-floor, consistent with models of the elastic deformation produced by the injection of magma in a system of two dykes. InSAR data acquired at 24-day intervals during the last 12 years by the Canadian Radarsat satellite over the Asal Rift show that the two main faults activated in 1978 continue to slip with periods of steady creep at rates of 0.3-1.3 mm/yr, interrupted by sudden slip events of a few millimeters, in 2000 and 2003. Slip events are coincident with bursts of micro earthquakes distributed around and over the Fieale volcanic center in the eastern part of the Asal Rift. In both cases (the 1978 crisis and micro-slip events), the observed geodetic moment released by fault slip exceeds by a few orders of magnitude the total seismic moment released by earthquakes over the same period. Aseismic fault slip is likely to be the faults response to a changing stress field associated with a volcanic process and not due to dry friction on faults. Sustained injection of magma (1978 crisis) and/or crustal fluids (micro-slip events) in dykes and fissures is a plausible mechanism to control fluid pressure in the basal parts of faults and trigger aseismic slip. In this respect, the micro-events observed by InSAR during a 12-year period of low activity in the rift and the 1978 seismo-volcanic episode are of same nature.

Active Deformation of Malawi Rift's North Basin Hinge Zone Modulated by Reactivation of Preexisting Precambrian Shear Zone Fabric

NASA Astrophysics Data System (ADS)

Kolawole, F.; Atekwana, E. A.; Laó-Dávila, D. A.; Abdelsalam, M. G.; Chindandali, P. R.; Salima, J.; Kalindekafe, L.

2018-03-01

We integrated temporal aeromagnetic data and recent earthquake data to address the long-standing question on the role of preexisting Precambrian structures in modulating strain accommodation and subsequent ruptures leading to seismic events within the East African Rift System. We used aeromagnetic data to elucidate the relationship between the locations of the 2009 Mw 6.0 Karonga, Malawi, earthquake surface ruptures and buried basement faults along the hinge zone of the half-graben comprising the North Basin of the Malawi Rift. Through the application of derivative filters and depth-to-magnetic-source modeling, we identified and constrained the trend of the Precambrian metamorphic fabrics and correlated them to the three-dimensional structure of buried basement faults. Our results reveal an unprecedented detail of the basement fabric dominated by high-frequency WNW to NW trending magnetic lineaments associated with the Precambrian Mughese Shear Zone fabric. The high-frequency magnetic lineaments are superimposed by lower frequency NNW trending magnetic lineaments associated with possible Cenozoic faults. Surface ruptures associated with the 2009 Mw 6.0 Karonga earthquake swarm aligned with one of the NNW-trending magnetic lineaments defining a normal fault that is characterized by right-stepping segments along its northern half and coalesced segments on its southern half. Fault geometries, regional kinematics, and spatial distribution of seismicity suggest that seismogenic faults reactivated the basement fabric found along the half-graben hinge zone. We suggest that focusing of strain accommodation and seismicity along the half-graben hinge zone is facilitated and modulated by the presence of the basement fabric.

Morphology and segmentation of the western Galápagos Spreading Center, 90.5°-98°W: Plume-ridge interaction at an intermediate spreading ridge

NASA Astrophysics Data System (ADS)

Sinton, John; Detrick, Robert; Canales, J. Pablo; Ito, Garrett; Behn, Mark

2003-12-01

Complete multibeam bathymetric coverage of the western Galápagos Spreading Center (GSC) between 90.5°W and 98°W reveals the fine-scale morphology, segmentation and influence of the Galápagos hot spot on this intermediate spreading ridge. The western GSC comprises three morphologically defined provinces: A Western Province, located farthest from the Galápagos hot spot west of 95°30'W, is characterized by an axial deep, rift valley morphology with individual, overlapping, E-W striking segments separated by non-transform offsets; A Middle Province, between the propagating rift tips at 93°15'W and 95°30'W, with transitional axial morphology strikes ˜276°; An Eastern Province, closest to the Galápagos hot spot between the ˜90°50'W Galápagos Transform and 93°15'W, with an axial high morphology generally less than 1800 m deep, strikes ˜280°. At a finer scale, the axial region consists of 32 individual segments defined on the basis of smaller, mainly <2 km, offsets. These offsets mainly step left in the Western and Middle Provinces, and right in the Eastern Province. Glass compositions indicate that the GSC is segmented magmatically into 8 broad regions, with Mg # generally decreasing to the west within each region. Striking differences in bathymetric and lava fractionation patterns between the propagating rifts with tips at 93°15'W and 95°30'W reflect lower overall magma supply and larger offset distance at the latter. The structure of the Eastern Province is complicated by the intersection of a series of volcanic lineaments that appear to radiate away from a point located on the northern edge of the Galápagos platform, close to the southern limit of the Galápagos Fracture Zone. Where these lineaments intersect the GSC, the ridge axis is displaced to the south through a series of overlapping spreading centers (OSCs); abandoned OSC limbs lie even farther south. We propose that southward displacement of the axis is promoted during intermittent times of increased plume activity, when lithospheric zones of weakness become volcanically active. Following cessation of the increased plume activity, the axis straightens by decapitating southernmost OSC limbs during short-lived propagation events. This process contributes to the number of right stepping offsets in the Eastern Province.

The continent-ocean transition on the northwestern South China Sea

NASA Astrophysics Data System (ADS)

Cameselle, Alejandra L.; Ranero, César R.; Franke, Dieter; Barckhausen, Udo

2015-04-01

Rifted margins are created as a result of stretching and breakup of continental lithosphere that eventually leads to oceanic spreading and formation of a new oceanic basin. A cornerstone for understanding how rift characteristics vary along strike in the same system and what processes control the final transition to seafloor spreading is the continent-ocean transition (COT). We use four regional multichannel seismic profiles and published magnetic lineations to study the structure and variability of COT on the northwest subbasin (NWSB) of the South China Sea and to discern continental from oceanic domains. The continental domain is characterized by tilted fault blocks overlaid by thick syn-rift sedimentary units and fairly continuous Moho reflections typically at 8-10 s twtt. Thickness of the continental crust changes from ~20-25 km under the uppermost slope to ~9-6 km under the lower slope. The oceanic domain is interpreted where a highly reflective top of basement, little faulting, no syntectonic strata, and fairly constant thickness basement (4-8 km) occur. The COT is imaged as a ~5-10 km wide zone where oceanic-type features abut continental-type structures. The South China margin is deformed by abundant normal faults dissecting the continental crust, whereas the conjugate Macclesfield Bank margin displays comparatively abrupt thinning and little faulting. Seismic profiles show an along-strike variation in the tectonic structure of the continental margin. The NE-most lines display ~20-40 km wide segments of intense faulting under the slope and associated continental-crust thinning. Towards the SW, faulting and thinning of the continental crust occurs across a ~100-110 km wide segment. We interpret this 3D structural variability and the narrow COT as a consequence of the abrupt termination of continental rifting tectonics by the NE to SW propagation of a spreading center. We suggest that breakup occurred by spreading center propagation to a larger degree than by lithospheric thinning during continental rifting. Based on the sedimentary successions overlying the oceanic crust, we propose a kinematic evolution for the oceanic domain of the NWSB consisting of a southward spreading center propagation followed by a first narrow ridge jump to the north, and then a younger larger jump to the SW into the east subbasin.

Sedimentary response to halfgraben dipslope faults evolution -Billefjorden Trough, Svalbard.

NASA Astrophysics Data System (ADS)

Smyrak-Sikora, Aleksandra; Kristensen, Jakob B.; Braathen, Alvar; Johannessen, Erik P.; Olaussen, Snorre; Sandal, Geir; Stemmerik, Lars

2017-04-01

Fault growth and linkage into larger segments has profound effect on the sedimentary architecture of rift basins. The uplifted Billefjorden Through located in central Spitsbergen is an excellent example of half-graben basin development. Detailed sedimentological and structural investigations supported by helicopter and ground base lidar scans along with photogrammetry analysis have been used to improve our understanding of the sedimentary response to faulting and along strike variations in footwall uplift and hanging wall subsidence. The early syn-rift basin fill, the Serpukhovian to Bashkirian Hultberget Formation and the Bashkirian Ebbaelven Member consists of fluvial to deltaic sandstones with minor marine incursions. During this early stage tens to hundred- meters-scale syn-tectonic faults disrupted the dipslope, and created local hanging wall depocentres where sediments were arrested. Changes in fluvial drainage pattern, development of small lacustrine basins along the faults, and the sharp based boundaries of some facies associations are interpreted as response to activity along these, mostly antithetic faults. The basin fill of the late syn-rift stage is composed of shallow marine to tidal mixed evaporite -carbonate facies in the hanging wall i.e. the Bashkirian Trikolorfjellet Member and the Moscovian Minkenfjellet Formation. These sediments interfinger with thick alluvial fan deposits outpouring from relay ramps on the master fault i.e. drainage from the footwall. The carbonate-evaporite cycles deposited on the hanging wall responded to both the eustatic sea level variations and tectonic movements in the rift basin. Intra-basinal footwall uplift of the dipslope controlled development of an internal unconformity and resulted in dissolution of the gypsum to produce stratiform breccia. In contrast thick gypsum-rich subbasins are preserved locally in hanging wall positions where they were protected from the erosion. The syn rift basin fill is capped by post rift carbonate ramp deposit of the Kasimovian to Asselian Wordiekammen Formation. This unit marks the final fill (and drowning) of the rift basin and covers both the hanging wall and footwall. In this presentation our focus will be on details of the sedimentary architecture related to internal and local dipslope activity within the rift basin, particularly thickness and facies variations, and transport directions.

Crustal Strain Patterns in Magmatic and Amagmatic Early Stage Rifts: Border Faults, Magma Intrusion, and Volatiles

NASA Astrophysics Data System (ADS)

Ebinger, C. J.; Keir, D.; Roecker, S. W.; Tiberi, C.; Aman, M.; Weinstein, A.; Lambert, C.; Drooff, C.; Oliva, S. J. C.; Peterson, K.; Bourke, J. R.; Rodzianko, A.; Gallacher, R. J.; Lavayssiere, A.; Shillington, D. J.; Khalfan, M.; Mulibo, G. D.; Ferdinand-Wambura, R.; Palardy, A.; Albaric, J.; Gautier, S.; Muirhead, J.; Lee, H.

2015-12-01

Rift initiation in thick, strong continental lithosphere challenges current models of continental lithospheric deformation, in part owing to gaps in our knowledge of strain patterns in the lower crust. New geophysical, geochemical, and structural data sets from youthful magmatic (Magadi-Natron, Kivu), weakly magmatic (Malawi, Manyara), and amagmatic (Tanganyika) sectors of the cratonic East African rift system provide new insights into the distribution of brittle strain, magma intrusion and storage, and time-averaged deformation. We compare and contrast time-space relations, seismogenic layer thickness variations, and fault kinematics using earthquakes recorded on local arrays and teleseisms in sectors of the Western and Eastern rifts, including the Natron-Manyara basins that developed in Archaean lithosphere. Lower crustal seismicity occurs in both the Western and Eastern rifts, including sectors on and off craton, and those with and without central rift volcanoes. In amagmatic sectors, lower crustal strain is accommodated by slip along relatively steep border faults, with oblique-slip faults linking opposing border faults that penetrate to different crustal levels. In magmatic sectors, seismicity spans surface to lower crust beneath both border faults and eruptive centers, with earthquake swarms around magma bodies. Our focal mechanisms and Global CMTs from a 2007 fault-dike episode show a local rotation from ~E-W extension to NE-SE extension in this linkage zone, consistent with time-averaged strain recorded in vent and eruptive chain alignments. These patterns suggest that strain localization via widespread magma intrusion can occur during the first 5 My of rifting in originally thick lithosphere. Lower crustal seismicity in magmatic sectors may be caused by high gas pressures and volatile migration from active metasomatism and magma degassing, consistent with high CO2 flux along fault zones, and widespread metasomatism of xenoliths. Volatile release and migration may be critical to strength reduction of initially cold, strong cratonic lithosphere. Our comparisons suggest that large offset border faults that develop very early in rift history create fluid pathways that maintain the initial along-axis segmentation until magma (if available), reaches mid-crustal levels.

A new kinematic model for the Mesozoic evolution of the Iberia plate

NASA Astrophysics Data System (ADS)

Nirrengarten, Michael; Manatschal, Gianreto; Tugend, Julie; Kusznir, Nick; Sauter, Daniel

2017-04-01

During the Mesozoic Iberia was progressively surrounded by rift systems leading to its transient individualization as a tectonic plate. The kinematic evolution of Iberia prior to oceanic magnetic anomaly C34 ( 83 Ma) is controversial. To date, no kinematic models accounts for the Late Aptian to Albian hyper-extended rift phase observed in the Pyrenees. Consistent isochronal features, such as oceanic magnetic anomalies, representing the backbones of oceanic plate reconstructions are lacking. The only potential candidate, the J-anomaly, located offshore Iberia and Newfoundland has recently been re-interpreted as resulting from polyphased and polygenic magmatic events and does not provide a useful constraint. We use a new reconstruction approach that integrates the spatio-temporal evolution of adjacent hyper-extended rift domains systems to investigate Iberia plate motion during the separation of the super-continent Pangea. The plate modeling is based on careful mapping and restoration of the rift domains with key rift events dated within the study area. The main outcomes of this new model are as follows: 1) A full-fit of the southern North Atlantic 2) Extension on the southern and eastern boundary of Iberia related to the opening of the Central Atlantic 3) Segmentation of the Iberia-Newfoundland rift system by fracture zones prior to a V-shape propagation of mantle exhumation and seafloor spreading 4) No Aptian subduction in the Pyrenean domain and a limited rotation of the Iberia plate 5) The partitioning of deformation between different micro-blocks along the Iberian-Eurasian boundary enabling Late Aptian to Albian extension in the Pyrenees The resulting plate kinematic model for Iberia differs from previous ones on three main points: it does not make use of the J magnetic anomaly because the J anomlay is neither an isochron or a COB marker; the deformation along the Iberian-Eurasian boundary is partitioned between distinct rift systems; and it incorporates extension in the Pyrenees consistent with published geological studies. Nonetheless this model is non-unique and additional observations are needed to further constrain the plate modeling notably by adding new constraints on the rift evolution of the southern and eastern margin of the Iberia plate during the Jurassic.

Morphology of the transition from an axial high to a rift valley at the Southeast Indian Ridge and the relation to variations in mantle temperature

NASA Astrophysics Data System (ADS)

Shah, Anjana K.; SempéRé, Jean-Christophe

1998-03-01

The Southeast Indian Ridge exhibits a transition in axial morphology from an East Pacific Rise-like axial high near 100°E to a Mid-Atlantic Ridge-like rift valley near 116°E but spreads at a nearly constant rate of 74-76 mm/yr. Assuming that the source of this transition lies in variations in mantle temperature, we use shipboard gravity-derived crustal thickness and ridge flank depth to estimate the variations in temperature associated with the changes in morphological style. Within the transitional region, SeaBeam 2000 bathymetry shows scattered instances of highs, valleys, and split volcanic ridges at the axis. A comparison of axial morphology to abyssal hill shapes and symmetry properties suggests that this unorganized distribution is due to the ridge axis episodically alternating between an axial valley and a volcanic ridge. Axial morphology can then be divided into three classes, with distinct geographic borders: axial highs and rifted highs are observed west of a transform fault at 102°45'E; rift valleys are observed east of a transform fault at 114°E; and an intermediate-style morphology which alternates between a volcanic ridge and a shallow axial valley is observed between the two. One segment, between 107° and 108°30'E, forms an exception to the geographical boundaries. Gravity-derived crustal thickness and flank depth generally vary monotonically over the region, with the exception of the segment between 107°E and 108°30'E. The long-wavelength variations in these properties correlate with the above morphological classification. Gravity-derived crustal thickness varies on average ˜2 km between the axial high and rift valley regions. The application of previous models relating crustal thickness and mantle temperature places the corresponding temperature variation at 25°C-50°C, depending on the model used. The average depth of ridge flanks varies by ˜550 m over the study area. For a variation of 25°-50°C, thermal models of the mantle predict depth variations of 75-150 m. These values are consistent with observations when the combined contributions of crustal thickness and mantle density to ridge flank depth are considered, assuming Airy isostasy. Crustal thickness variations differ at the two transitions described above: A difference of 750 m in crustal thickness is observed at the rift valley/intermediate-style transition, suggesting small variations in crustal thickness and mantle temperature drive this transition. At the axial high-rifted high/intermediate-style transition, crustal thickness variations are not resolvable, suggesting that this transition is controlled by threshold values of crustal thickness and mantle temperature, and is perhaps related to the presence of a steady state magma chamber.

Revisiting the Mesozoic opening of the Southeastern Gulf of Mexico

NASA Astrophysics Data System (ADS)

Marton, G.; Pascoe, R. P.

2016-12-01

The Southeastern Gulf of Mexico (SEGOM) is defined here as the seaway between Yucatan and Florida, south of the Tampa Embayment. This area is regarded as a southward propagating rift in the Gulf of Mexico. There is an overwhelming amount of previous evidence that the Yucatan block rotated counterclockwise about 42 degrees around a pole located just north of present-day Cuba (23oN, 84oW) during the Late Jurassic to Earliest Cretaceous oceanic spreading phase. North of the pole in the SEGOM the rotational movement of Yucatan was accommodated by a uniformly increasing amount of SW-NE extension. The degree of extension north of 25oN was large enough to result in rifting and oceanic spreading. Lack of salt in the area south of the Tampa embayment indicates that the SEGOM was not affected by the large amount of NW-SE continental extension as observed in the rest of the Gulf of Mexico. Thus, the area between Yucatan and the Sarasota arch remained a land bridge between the proto- GOM and the Proto-Caribbean and formed a barrier to salt deposition. During the period of late Jurassic oceanic crust formation (and Yucatan rotation), the southern tip of the oceanic spreading center propagated south from 27oN to 25oN, or about 220 km. In the 220 km long zone from 25oN to the pole (23oN) the rotation of Yucatan was accommodated by continental rifting only. The validity of the above outlined propagating rift model in the SEGOM is also supported by the age differences in the observed post-rift unconformities along its margins. At the edge of the salt basin to the north, the post-rift unconformity in the upper crust occurs at the base of the Louann salt and thus is Callovian in age. In the southern continental rift segment of the SEGOM, a seismic to well tie at the DSDP Site 535 shows that the post-rift unconformity is no younger than Late Berriasian to Early Valanginian. This latter age bracket constrains a) the cessation of continental rifting in the SEGOM, b) the time when the Yucatan block docked in its present day location relative to North America, and c) the time when oceanic crust formation ceased in the Gulf of Mexico.

The effects of thick sediment upon continental breakup: seismic imaging and thermal modeling of the Salton Trough, southern California

NASA Astrophysics Data System (ADS)

Han, L.; Hole, J. A.; Lowell, R. P.; Stock, J. M.; Fuis, G. S.; Driscoll, N. W.; Kell, A. M.; Kent, G. M.; Harding, A. J.; Gonzalez-Fernandez, A.; Lázaro-Mancilla, O.

2015-12-01

Continental rifting ultimately creates a deep accommodation space for sediment. When a major river flows into a late-stage rift, thick deltaic sediment can change the thermal regime and alter the mechanisms of extension and continental breakup. The Salton Trough, the northernmost rift segment of the Gulf of California plate boundary, has experienced the same extension as the rest of the Gulf, but is filled to sea level by sediment from the Colorado River. Unlike the southern Gulf, seafloor spreading has not initiated. Instead, seismicity, high heat flow, and minor volcanoes attest to ongoing rifting of thin, transitional crust. Recently acquired controlled-source seismic refraction and wide-angle reflection data in the Salton Trough provide constraints upon crustal architecture and active rift processes. The crust in the central Salton Trough is only 17-18 km thick, with a strongly layered but relatively one-dimensional structure for ~100 km in the direction of plate motion. The upper crust includes 2-4 km of Colorado River sediment. Crystalline rock below the sediment is interpreted to be similar sediment metamorphosed by the high heat flow and geothermal activity. Meta-sediment extends to at least 9 km depth. A 4-5 km thick layer in the middle crust is either additional meta-sediment or stretched pre-existing continental crust. The lowermost 4-5 km of the crust is rift-related mafic magmatic intrusion or underplating from partial melting in the hot upper mantle. North American lithosphere in the Salton Trough has been almost or completely rifted apart. The gap has been filled by ~100 km of new transitional crust created by magmatism from below and sedimentation from above. These processes create strong lithologic, thermal, and rheologic layering. While heat flow in the rift is very high, rapid sedimentation cools the upper crust as compared to a linear geotherm. Brittle extension occurs within new meta-sedimentary rock. The lower crust, in comparison, is maintained hot and weak by the overlying sedimentary thermal blanket. The lower crust stretches by ductile flow and magmatism is not localized. In this passive rift driven by distant plate motions, rapid sedimentation and its thermal effects delay final breakup of the crust and the onset of seafloor spreading.

Rifting mechanisms constrained by InSAR, seismicity, GPS, and surface rupture from the Karonga earthquake sequence in northern Lake Malawi (Nyasa)

NASA Astrophysics Data System (ADS)

Zheng, W.; Pritchard, M. E.; Henderson, S. T.; Gaherty, J. B.; Shillington, D. J.; Oliva, S. J.; Ebinger, C.; Nooner, S. L.; Elliott, J.; Saria, E.; Ntambila, D.; Chindandali, P. R. N.

2017-12-01

The Malawi rift is part of the archetypal East African rift where early-stage crustal extension is dominated by faulting. In the Karonga region of northern Malawi, a sequence of earthquakes in late 2009, with 15 teleseismically detected (Mw 4.5-6.0) over 13 days, provides a uniqueopportunity to evaluate faulting processes controlling present-day extension in an early-stage rift. We describe observations of this sequence including hundreds of aftershocks located by a temporary seismic array installed in 2010, ground deformation from satellite interferograms, and surface rupture from field surveys published by others. We use all of these data to model fault geometry and slip. The aftershocks from January-May 2010 suggest the involvement of multiple faults, and we test the extent that this can be resolved by the InSAR data. The InSAR and surface rupture both suggest that the major slip occurred at shallow depth (<5 km). Our preferred aftershock locations appear to correlate with this principal slip zone, although uncertainty in the shallow velocity structure can allow for a bulk of the events to fall down-dip of the geodetically constrained slip. Subsequent deformation, including that associated with a December 2014 Mw 5.1 earthquake, can be constrained from multidisciplinary data collected during the SEGMeNT (Study of Extension and maGmatism in Malawi aNd Tanzania) project, which includes the Karonga region and spans 2013-2015. We find 3 cm of potential ground movement at the location of the earthquake as determined by the SEGMeNT seismic array from Sentinel-1. Geodetic fault slip is consistent with the focal mechanism and depth determined by the local array. The location is at the northern end of the 2009-2010 aftershock zone, and aftershocks suggest some linkage with faults that slipped in 2009. InSAR observations do not provide any evidence for large aseismic slip or fluid movements during or after the 2014 sequence, which had <200 aftershocks above the network threshold. For example, we do not observe any deformation at Rungwe volcano above the 2 cm/yr detection threshold with InSAR time series from ALOS (2007-2010) or Sentinel-1 (10/2014 - 04/2017). The time series from SEGMeNT and other continuous GPS stations do not show transients related to the earthquakes, but are not optimally located in space or time.

Seismic constraints on a large dyking event in Western Gulf of Aden

NASA Astrophysics Data System (ADS)

Ahmed, A.; Doubre, C.; Leroy, S.; Perrot, J.; Audin, L.; Rolandone, F.; Keir, D.; Al-Ganad, I.; Khanbari, K.; Mohamed, K.; Vergne, J.; Jacques, E.; Nercessian, A.

2012-04-01

In November 2010, a large number of events were recorded by the world seismic networks showing important activity occurring along the western part of the Aden Ridge. West of the Shulka El Sheik transform zone, events in this large seismic swarm (magnitudes above 5) occurred in a complex area, where the change of both the ridge direction and the bathymetry suggest the propagation of the ridge into a continental lithosphere and the influence of the thermal anomaly of the Afar Hot Spot. We combine several sets of data from permanent networks and temporary 3C broad stations installed after the beginning of the event along the southern and eastern coasts of Yemen and Djibouti respectively, we located more than 600 earthquakes with magnitudes ranging from 2.5 to 5.6 that occurred during the first months following the first event. The spatial distribution of the main seismicity reveals a very clear N115°-trending alignment, parallel to the mean direction of the en-echelon spreading segments that form the ridge at this longitude. Half of the events, which represent half of the total seismic energy released during the first months, are located in the central third section of the segment. Here several volcanic cones and recent lava flows observed from bathymetric and acoustic reflectivity data during the Tadjouraden cruise (Audin, 1999, Dauteuil et al., 2001) constitute the sea floor. In addition to this main activity, two small groups of events suggest the activiation of landslides into a large fan and the activity in a volcanic area 50 km due east from the main active zone. The time evolution of the seismicity shows several bursts of activity. Some of them are clearly related to sudden activities within the volcanic areas, when others exhibit horizontal migration of the events, with velocity around ~ 1 km/h. The time-space evolution of the seismicity clearly reveals the intrusion of dykes associated with magma propagation from the crustal magmatic centres into the rift zone.We use knowledge from spatial geodetic observations of rifting episodes in the regions: the Harrat Al-Shaqah event in 2009 (Arabia Souadia), the Tanzanian one in 2007, and the major Dabahu Manda Harraro rifting episode in Afar. Taking into account that the geodetic moment is one order of magnitude higher than the seismic moment during such events, the seismic activity of this event of the Aden ridge represents a major rifting episode certainly associated with the opening of the segment by dyking estimated to be higher than 10 m.

Along strike behavior of the Tizi n' Firest fault during the Lower Jurassic rifting (Central High Atlas Carbonate basin, Morocco)

NASA Astrophysics Data System (ADS)

Sarih, S.; Quiquerez, A.; Allemand, P.; Garcia, J. P.; El Hariri, K.

2018-03-01

The purpose of this study is to document the along-strike early syn-rift history of the Lower Jurassic Carbonate basin of the Central High Atlas (Morocco) by combining sedimentological observations and high-resolution biostratigraphy. Six sections, each from the Sinemurian to the Upper Pliensbachian, were investigated along a 75 km-long transect at the hanging wall of a major fault of the Lower Jurassic Basin (i.e. the Tizi n' Firest fault). Depositional geometries of the early syn-rift deposits were reconstructed from the correlation between eight main timelines dated by biochronological markers for a time span covering about 6 Ma. Depocentre migration was examined and accommodation rates were calculated at the sub-zone timescale to discuss the along-strike-fault behavior of the Lower Jurassic basin formation. The early stages of extension are marked by contrasted along-strike variations in depositional geometry thickness, depocentre migration and accommodation rates, leading to the growth of three independent sub-basins (i.e. western, central, and eastern), ranging in size from 30 to 50 km, and displaying three contrasted tectono-sedimentary histories. Our results suggest that, during the early rifting phase, tectonic activity was not a continuous and progressive process evolving towards a rift climax stage, but rather a series of acceleration periods that alternated with periods of much reduced activity. The length of active fault segments is estimated at about 15-20 km, with a lifespan of a few ammonite sub-zones (> 2-3 Ma).

Crustal Structure of Active Deformation Zones in Africa: Implications for Global Crustal Processes

NASA Astrophysics Data System (ADS)

Ebinger, C. J.; Keir, D.; Bastow, I. D.; Whaler, K.; Hammond, J. O. S.; Ayele, A.; Miller, M. S.; Tiberi, C.; Hautot, S.

2017-12-01

The Cenozoic East African rift (EAR), Cameroon Volcanic Line (CVL), and Atlas Mountains formed on the slow-moving African continent, which last experienced orogeny during the Pan-African. We synthesize primarily geophysical data to evaluate the role of magmatism in shaping Africa's crust. In young magmatic rift zones, melt and volatiles migrate from the asthenosphere to gas-rich magma reservoirs at the Moho, altering crustal composition and reducing strength. Within the southernmost Eastern rift, the crust comprises 20% new magmatic material ponded in the lower crust and intruded as sills and dikes at shallower depths. In the Main Ethiopian Rift, intrusions comprise 30% of the crust below axial zones of dike-dominated extension. In the incipient rupture zones of the Afar rift, magma intrusions fed from crustal magma chambers beneath segment centers create new columns of mafic crust, as along slow-spreading ridges. Our comparisons suggest that transitional crust, including seaward dipping sequences, is created as progressively smaller screens of continental crust are heated and weakened by magma intrusion into 15-20 km thick crust. In the 30 Ma Recent CVL, which lacks a hot spot age progression, extensional forces are small, inhibiting the creation and rise of magma into the crust. In the Atlas orogen, localized magmatism follows the strike of the Atlas Mountains from the Canary Islands hot spot toward the Alboran Sea. CVL and Atlas magmatism has had minimal impact on crustal structure. Our syntheses show that magma and volatiles are migrating from the asthenosphere through the plates, modifying rheology, and contributing significantly to global carbon and water fluxes.

Evolution of the Southern Guinea Plateau: Implications on Guinea-Demerara Plateau formation using insights from seismic, subsidence, and gravity data

NASA Astrophysics Data System (ADS)

Olyphant, Jared R.; Johnson, Roy A.; Hughes, Amanda N.

2017-10-01

The Guinea Plateau (offshore Guinea) and its conjugate, the Demerara Plateau (offshore French Guiana), comprise two of the most prominent passive continental margins in the Atlantic Ocean. The conjugate plateaus formed as a result of two periods of rifting, the Jurassic opening of the Central Atlantic Ocean and the northward-propagating Cretaceous opening of the Southern Atlantic Ocean. Although several studies are published on the Demerara Plateau that explain the evolution of its multi-rift history and the effect of rifting on its distinct geometry, the Guinea Plateau, and in particular its south-eastern margin, remain relatively unexplored in the literature. Here we present interpretations of the structure and evolution of the Guinea Plateau using recent 2-D and 3-D seismic-reflection data collected at the intersection of the southern and eastern margins. We substantiate our study with calculated subsidence curves at four locations along the southern margin, as well as two 2-D gravity forward models along regional seismic-reflection profiles to estimate stretching factors (β) and crustal thicknesses. We combine our results with previous studies concerning the south-western Guinea margin, and compare them to published interpretations regarding the conjugate margins of the Demerara Plateau. The resolved amounts of rift-related volcanism, listric-style normal faults, and moderate stretching factors suggest that a component of upper-crustal asymmetry (simple shear) and depth-dependent stretching may have persisted at the Demerara-Guinea conjugate margins during Cretaceous rifting of the equatorial segment of the Southern Atlantic Ocean.

Tectonic and climatic control on evolution of rift lakes in the Central Kenya Rift, East Africa

NASA Astrophysics Data System (ADS)

Bergner, A. G. N.; Strecker, M. R.; Trauth, M. H.; Deino, A.; Gasse, F.; Blisniuk, P.; Dühnforth, M.

2009-12-01

The long-term histories of the neighboring Nakuru-Elmenteita and Naivasha lake basins in the Central Kenya Rift illustrate the relative importance of tectonic versus climatic effects on rift-lake evolution and the formation of disparate sedimentary environments. Although modern climate conditions in the Central Kenya Rift are very similar for these basins, hydrology and hydrochemistry of present-day lakes Nakuru, Elmenteita and Naivasha contrast dramatically due to tectonically controlled differences in basin geometries, catchment size, and fluvial processes. In this study, we use eighteen 14C and 40Ar/ 39Ar dated fluvio-lacustrine sedimentary sections to unravel the spatiotemporal evolution of the lake basins in response to tectonic and climatic influences. We reconstruct paleoclimatic and ecological trends recorded in these basins based on fossil diatom assemblages and geologic field mapping. Our study shows a tendency towards increasing alkalinity and shrinkage of water bodies in both lake basins during the last million years. Ongoing volcano-tectonic segmentation of the lake basins, as well as reorganization of upstream drainage networks have led to contrasting hydrologic regimes with adjacent alkaline and freshwater conditions. During extreme wet periods in the past, such as during the early Holocene climate optimum, lake levels were high and all basins evolved toward freshwater systems. During drier periods some of these lakes revert back to alkaline conditions, while others maintain freshwater characteristics. Our results have important implications for the use and interpretation of lake sediment as climate archives in tectonically active regions and emphasize the need to deconvolve lacustrine records with respect to tectonics versus climatic forcing mechanisms.

Mutational Analysis of the Rift Valley Fever Virus Glycoprotein Precursor Proteins for Gn Protein Expression

PubMed Central

Phoenix, Inaia; Lokugamage, Nandadeva; Nishiyama, Shoko; Ikegami, Tetsuro

2016-01-01

The Rift Valley fever virus (RVFV) M-segment encodes the 78 kD, NSm, Gn, and Gc proteins. The 1st AUG generates the 78 kD-Gc precursor, the 2nd AUG generates the NSm-Gn-Gc precursor, and the 3rd AUG makes the NSm’-Gn-Gc precursor. To understand biological changes due to abolishment of the precursors, we quantitatively measured Gn secretion using a reporter assay, in which a Gaussia luciferase (gLuc) protein is fused to the RVFV M-segment pre-Gn region. Using the reporter assay, the relative expression of Gn/gLuc fusion proteins was analyzed among various AUG mutants. The reporter assay showed efficient secretion of Gn/gLuc protein from the precursor made from the 2nd AUG, while the removal of the untranslated region upstream of the 2nd AUG (AUG2-M) increased the secretion of the Gn/gLuc protein. Subsequently, recombinant MP-12 strains encoding mutations in the pre-Gn region were rescued, and virological phenotypes were characterized. Recombinant MP-12 encoding the AUG2-M mutation replicated slightly less efficiently than the control, indicating that viral replication is further influenced by the biological processes occurring after Gn expression, rather than the Gn abundance. This study showed that, not only the abolishment of AUG, but also the truncation of viral UTR, affects the expression of Gn protein by the RVFV M-segment. PMID:27231931

Mutational Analysis of the Rift Valley Fever Virus Glycoprotein Precursor Proteins for Gn Protein Expression.

PubMed

Phoenix, Inaia; Lokugamage, Nandadeva; Nishiyama, Shoko; Ikegami, Tetsuro

2016-05-24

The Rift Valley fever virus (RVFV) M-segment encodes the 78 kD, NSm, Gn, and Gc proteins. The 1st AUG generates the 78 kD-Gc precursor, the 2nd AUG generates the NSm-Gn-Gc precursor, and the 3rd AUG makes the NSm'-Gn-Gc precursor. To understand biological changes due to abolishment of the precursors, we quantitatively measured Gn secretion using a reporter assay, in which a Gaussia luciferase (gLuc) protein is fused to the RVFV M-segment pre-Gn region. Using the reporter assay, the relative expression of Gn/gLuc fusion proteins was analyzed among various AUG mutants. The reporter assay showed efficient secretion of Gn/gLuc protein from the precursor made from the 2nd AUG, while the removal of the untranslated region upstream of the 2nd AUG (AUG2-M) increased the secretion of the Gn/gLuc protein. Subsequently, recombinant MP-12 strains encoding mutations in the pre-Gn region were rescued, and virological phenotypes were characterized. Recombinant MP-12 encoding the AUG2-M mutation replicated slightly less efficiently than the control, indicating that viral replication is further influenced by the biological processes occurring after Gn expression, rather than the Gn abundance. This study showed that, not only the abolishment of AUG, but also the truncation of viral UTR, affects the expression of Gn protein by the RVFV M-segment.

Fractures, Faults, and Hydrothermal Systems of Puna, Hawaii, and Montserrat, Lesser Antilles

NASA Astrophysics Data System (ADS)

Kenedi, Catherine Lewis

The focus of this work is to use geologic and geophysical methods to better understand the faults and fracture systems at Puna, in southeastern Hawaii, and southern Montserrat, in the Lesser Antilles. The particular interest is understanding and locating the deep fracture networks that are necessary for fluid circulation in hydrothermal systems. The dissertation first presents a study in which identification of large scale faulting places Montserrat into a tectonic context. Then follow studies of Puna and Montserrat that focus on faults and fractures of the deep hydrothermal systems. The first chapter consists of the results of the SEA-CALIPSO experiment seismic reflection data, recorded on a 48 channel streamer with the active source as a 2600 in3 airgun. This chapter discusses volcaniclastic debris fans off the east coast of Montserrat and faults off the west coast. The work places Montserrat in a transtensional environment (influenced by oblique subduction) as well as in a complex local stress regime. One conclusion is that the stress regime is inconsistent with the larger arc due to the influence of local magmatism and stress. The second chapter is a seismic study of the Puna hydrothermal system (PHS) along the Kilauea Lower East Rift Zone. The PHS occurs at a left step in the rift, where a fracture network has been formed between fault segments. It is a productive geothermal field, extracting steam and reinjecting cooled, condensed fluids. A network of eight borehole seismometers recorded >6000 earthquakes. Most of the earthquakes are very small (< M.2), and shallow (1-3 km depth), likely the result of hydrothermal fluid reinjection. Deeper earthquakes occur along the rift as well as along the south-dipping fault plane that originates from the rift zone. Seismic methods applied to the PHS data set, after the initial recording, picking, and locating earthquakes, include a tomographic inversion of the P-wave first arrival data. This model indicates a high seismic velocity under the field that is thought to be an intrusion and the heat source of the hydrothermal system. A shear wave splitting study suggested the PHS fracture system is largely oriented rift-parallel with some orthogonal fractures. Shear wave splitting data also were used in a tomographic inversion for fracture density. The fracture density is high in the PHS, which indicates high permeability and potential for extensive fluid circulation. This has been confirmed by high fluid flow and energy generation. The high fracture density is consistent with the interpretation of a transfer zone between the rift segments where a fracture mesh would be expected. In Puna the transfer zone is a relay ramp. The results from the PHS are used as an example to examine the proposed hydrothermal system at St. George's Hill, Montserrat. In southern Montserrat, hot springs and fumaroles suggest a deep hydrothermal system heated by local magmatism. A magnetotelluric study obtained resistivity data that suggest focused alteration under southeastern Montserrat that is likely to be along fault segments. Several faults intersect under SGH, making it the probable center of the hydrothermal system. At Puna, and also Krafla, Iceland, where faults interact is an area of increased permeability, acting as a model to be applied to southern Montserrat. The conclusion is that in both Puna and Montserrat large faults interact to produce local areas of stress transfer that lead to fracturing and permeable networks; these networks allow for high-temperature hydrothermal circulation.

Detailed study of the Cobb offset of the Juan de Fuca ridge: evolution of a propagating rift

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, H.P.; Karsten, J.L.; Delaney, J.R.

The Cobb Offset on the northern portion of the Juan de Fuca Ridge has been identified as the tip of a northward propagating rift (Hey and Wilson, 1982). Map compilations of magnetic and seismic data from four new cruises define the present locus of spreading and volcanism on the two ridge segments abutting the Offset and permit detailed modeling of the recent evolution within this transform zone. The axis of recent spreading on the southern ridge segment bends from the normal rdige trend (N20/sup 0/E) to a N-S trend, north of 47/sup 0/15'N. The spreading axis on the northern ridgemore » segment generally defines a N20/sup 0/E trend, except at the southern terminus, where the spreading center is offset slightly to the east. The two spreading centers overlap by about 33 km in the Offset vicinity, and there is evidence of recent volcanism on both segments. Present ridge axis morphology exhibits a transitional sequence from a symmetrical, axial high along the more 'normal' portions of each ridge segment to a grabenlike depression as the tip is approached. The magnetic anomaly patterns observed in the Cobb Offset vicinity are not consistent with the patterns predicted by models of continuous, northward propagation. The magnetic anomaly patterns of the Brunhes Epoch require an event of rapid northward propagation about 0.7 m.y. B.P., followed by a more gradual southward propagation in the middle Brunhes Epoch; most recently, the spreading center on the southern ridge has extended northward to its present configuration. Prior to the Brunhes Epoch, modeling of the magnetic anomaly patterns does not indicate a unique solution; however, net propagation has been northward. We present alternative models for the period beginning 1.7 m.y. B.P.« less

A comparative analysis of pre-Silurian crustal building blocks of the northern and the southern Appalachian orogen

USGS Publications Warehouse

Hibbard, J.P.; van Staal, C.R.; Rankin, D.W.

2007-01-01

The New York promontory serves as the divide between the northern and southern segments of the Appalachian orogen. Antiquated subdivisions, distinct for each segment, implied that they had lithotectonic histories that were independent of each other. Using new lithotectonic subdivisions we compare first order features of the pre-Silurian orogenic 'building blocks' in order to test the validity of the implication of independent lithotectonic histories for the two segments. Three lithotectonic divisions, termed here the Laurentian, Iapetan, and the peri-Gondwanan realms, characterize the entire orogen. The Laurentian realm, composed of native North American rocks, is remarkably uniform for the length of the orogen. It records the multistage Neoproterozoic-early Paleozoic rift-drift history of the Appalachian passive margin, formation of a Taconic Seaway, and the ultimate demise of both in the Middle Ordovician. The Iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the Iapetus Ocean, between Laurentia and Gondwana. In the northern segment, the realm is divisible on the basis of stratigraphy and faunal provinciality into peri-Laurentian and peri-Gondwanan tracts that were amalgamated in the Late Ordovician. South of New York, stratigraphic and faunal controls decrease markedly; rock associations are not inconsistent with those of the northern Appalachians, although second-order differences exist. Exposed exotic crustal blocks of the peri-Gondwanan realm include Ganderia, Avalonia, and Meguma in the north, and Carolinia in the south. Carolinia most closely resembles Ganderia, both in early evolution and Late Ordovician-Silurian docking to Laurentia. Our comparison indicates that, to a first order, the pre-Silurian Appalachian orogen developed uniformly, starting with complex rifting and a subsequent drift phase to form the Appalachian margin, followed by the consolidation of Iapetan components and ending with accretion of the peri-Gonwanan Ganderia and Carolinia. This deduction implies that any first-order differences between northern and southern segments post-date Late Ordovician consolidation of a large portion of the orogen.

Numerical reconstruction of Late-Cenosoic evolution of normal-fault scarps in Baikal Rift Zone

NASA Astrophysics Data System (ADS)

Byzov, Leonid; San'kov, Vladimir

2014-05-01

Numerical landscape development modeling has recently become a popular tool in geo-logic and geomorphic investigations. We employed this technique to reconstruct Late-Cenosoic evolution of Baikal Rift Zone mountains. The objects of research were Barguzin Range and Svyatoy Nos Upland. These structures are formed under conditions of crustal extension and bounded by active normal faults. In our experiments we used instruments, engineered by Greg Tucker (University of Colo-rado) - CHILD (Channel-Hillslope Integrated Landscape Development) and 'Bedrock Fault Scarp'. First program allowed constructing the complex landscape model considering tectonic uplift, fluvial and hillslope processes; second program is used for more accurate simulating of triangular facet evolution. In general, our experiments consisted in testing of tectonic parameters, and climatic char-acteristic, erosion and diffusion properties, hydraulic geometry were practically constant except for some special runs. Numerous experiments, with various scenarios of development, showed that Barguzin range and Svyatoy Nos Upland has many common features. These structures characterized by internal differentiation, which appear in height and shape of slopes. At the same time, individual segments of these objects are very similar - this conclusion refers to most developing parts, with pronounced facets and V-shaped valleys. Accordingly modelling, these landscapes are in a steady state and are undergoing a uplift with rate 0,4 mm/yr since Early Pliocene (this solution accords with AFT-dating). Lower segments of Barguzin Range and Svyatoy Nos Upland also have some general fea-tures, but the reasons of such similarity probably are different. In particular, southern segment of Svyatoy Nos Upland, which characterized by relative high slope with very weak incision, may be formed as result very rapid fault movement or catastrophic landslide. On the other hand, a lower segment of Barguzin Range (Ulun segment, for example) probably has small height and relative weak incision over later beginning of uplift.

From continental to oceanic rifting in the Gulf of California

NASA Astrophysics Data System (ADS)

Ferrari, Luca; Bonini, Marco; Martín, Arturo

2017-11-01

The continental margin of northwestern Mexico is the youngest example of the transition from a convergent plate boundary to an oblique divergent margin that formed the Gulf of California rift. Subduction of the Farallon oceanic plate during the Cenozoic progressively brought the East Pacific Rise (EPR) toward the North America trench. In this process increasingly younger and buoyant oceanic lithosphere entered the subduction zone until subduction ended just before most of the EPR could collide with the North America continental lithosphere. The EPR segments bounding the unsubducted parts of the Farallón plate remnants (Guadalupe and Magdalena microplates) also ceased spreading (Lonsdale, 1991) and a belt of the North American plate (California and Baja California Peninsula) became coupled with the Pacific Plate and started moving northwestward forming the modern Gulf of California oblique rift (Nicholson et al., 1994; Bohannon and Parsons, 1995). The timing of the change from plate convergence to oblique divergence off western Mexico has been constrained at the middle Miocene (15-12.5 Ma) by ocean floor morphology and magnetic anomalies as well as plate tectonic reconstructions (Atwater and Severinghaus, 1989; Stock and Hodges, 1989; Lonsdale, 1991), although the onset of transtensional deformation and the amount of right lateral displacement within the Gulf region are still being studied (Oskin et al., 2001; Fletcher et al., 2007; Bennett and Oskin, 2014). Other aspects of the formation of the Gulf of California remain not well understood. At present the Gulf of California straddles the transition from continental transtension in the north to oceanic spreading in the south. Seismic reflection-refraction data indicate asymmetric continent-ocean transition across conjugate margins of rift segments (González-Fernández et al., 2005; Lizarralde et al., 2007; Miller and Lizarralde, 2013; Martín-Barajas et al., 2013). The asymmetry may be related to crustal heterogeneities and thus early evidence of extension may provide useful information about the thermal conditions of the crust over a broader region encompassing the effects of coeval subduction and crustal stretching. On the other hand, onshore and offshore geologic studies have shown that lithospheric extension associated with a wide rift mode was already ongoing during the final stage of subduction of the Farallon plate and its remnants in the early to middle Miocene times (Ferrari et al., 2013; Murray et al., 2013; Bryan et al., 2014; Duque-Trujillo et al., 2014, 2015). More broadly, the complexity in the present rift architecture and Plio-Quaternary magmatism is related to the pre-middle Miocene geodynamic history that accompanied the removal of the slab since the Eocene (Ferrari et al., 2017).

Use of controlled dynamic impacts on hierarchically structured seismically hazardous faults for seismically safe relaxation of shear stresses

NASA Astrophysics Data System (ADS)

Ruzhich, Valery V.; Psakhie, Sergey G.; Levina, Elena A.; Shilko, Evgeny V.; Grigoriev, Alexandr S.

2017-12-01

In the paper we briefly outline the experience in forecasting catastrophic earthquakes and the general problems in ensuring seismic safety. The purpose of our long-term research is the development and improvement of the methods of man-caused impacts on large-scale fault segments to safely reduce the negative effect of seismodynamic failure. Various laboratory and large-scale field experiments were carried out in the segments of tectonic faults in Baikal rift zone and in main cracks in block-structured ice cove of Lake Baikal using the developed measuring systems and special software for identification and treatment of deformation response of faulty segments to man-caused impacts. The results of the study let us to ground the necessity of development of servo-controlled technologies, which are able to provide changing the shear resistance and deformation regime of fault zone segments by applying vibrational and pulse triggering impacts. We suppose that the use of triggering impacts in highly stressed segments of active faults will promote transferring the geodynamic state of these segments from a metastable to a more stable and safe state.

Seismic evidence for arc segmentation, active magmatic intrusions and syn-rift fault system in the northern Ryukyu volcanic arc

NASA Astrophysics Data System (ADS)

Arai, Ryuta; Kodaira, Shuichi; Takahashi, Tsutomu; Miura, Seiichi; Kaneda, Yoshiyuki

2018-04-01

Tectonic and volcanic structures of the northern Ryukyu arc are investigated on the basis of multichannel seismic (MCS) reflection data. The study area forms an active volcanic front in parallel to the non-volcanic island chain in the eastern margin of the Eurasian plate and has been undergoing regional extension on its back-arc side. We carried out a MCS reflection experiment along two across-arc lines, and one of the profiles was laid out across the Tokara Channel, a linear bathymetric depression which demarcates the northern and central Ryukyu arcs. The reflection image reveals that beneath this topographic valley there exists a 3-km-deep sedimentary basin atop the arc crust, suggesting that the arc segment boundary was formed by rapid and focused subsidence of the arc crust driven by the arc-parallel extension. Around the volcanic front, magmatic conduits represented by tubular transparent bodies in the reflection images are well developed within the shallow sediments and some of them are accompanied by small fragments of dipping seismic reflectors indicating intruded sills at their bottoms. The spatial distribution of the conduits may suggest that the arc volcanism has multiple active outlets on the seafloor which bifurcate at crustal depths and/or that the location of the volcanic front has been migrating trenchward over time. Further distant from the volcanic front toward the back-arc (> 30 km away), these volcanic features vanish, and alternatively wide rift basins become predominant where rapid transitions from normal-fault-dominant regions to strike-slip-fault-dominant regions occur. This spatial variation in faulting patterns indicates complex stress regimes associated with arc/back-arc rifting in the northern Okinawa Trough.[Figure not available: see fulltext.

Galapagos hotspot-spreading center system: 1. Spatial petrological and geochemical variations (83/sup 0/W-101/sup 0/W)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schilling, J.; Kingsley, R.H.; Devine, J.D.

We report on the petrology and geochemistry of basalts dredged at 40--50 km intervals along the Galapagos Spreading Center, between 83/sup 0/W and 101/sup 0/W (40 stations). Emphasis is on spatial variations of 'whole rock' major elements, rare earths, trace metals of the first transition series, and the nature of phenocryst assemblages and their abundances. These results provide new constraints on the nature and scale of mantle source heterogeneities, melting conditions, thermal field, and dynamics of crustal formation of the region. We suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a resultmore » of passive seafloor spreading. Basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements, which appears of worldwide extent. We reject Vogt and DeBoer's (1976) model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line, because this model would not explain the gradients in REE observed about the Galapagos Platform. Our preferred model combines the mantle-plume binary mixing model of Schilling (1973) with the concept of recurring rift propagation proposed by Hey (1977a). We further propose that pulsating mantle plume flux, perhaps in the form of a chain of blobs, may initiate the development of new rifts and their propagation. The present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow. A two million year period is suggested for the last 4 m.y. from Wilson and Hey's (1979) information Rigorous testing of our preferred model is possible.« less

New Pleomagnetic Evidence for Counter Clockwise Rotation of the Dofan Magmatic Segment Linked to Variation in Fault Slip Directions Along the Different Fault Systems, Main Ethiopian Rift.

NASA Astrophysics Data System (ADS)

Birke, T. K.; Nugsse, K.

2017-12-01

Twenty-six paleomagnetic sites were sampled from basalt, trachyte and ignimbrite flows of the Dofan magmatic segment, Southern Afar Depression. The samples were then cut in to 200 standard and their Natural Remanent Magnetization (NRM) directions were measured using the JR6A Spinner Magnetometer, of Addis Ababa University. Twin specimens from same sample were subjected to stepwise alternative field (AF) and thermal (TH) demagnetizations respectively with the corresponding directional measurements done at each step. Directional analysis of individual specimens revealed either one or two components of NRM; the first is isolated below a temperature of 300°C or AF field below 20mT; the second is isolated above those steps and defined straight lines directed towards the origin, which were interpreted as the Characteristic Remanent Magnetization (ChRM) acquired during cooling. Rock magnetic experiments on representative specimens indicated that the dominant magnetic minerals are titanium poor titanomagnetite with few cases of titanohematites. The overall mean directions calculated for the 24 stable polarity sites of Dofan is Dec=351.8°, Inc=11.5° (N=24, K=21.4, α95=6.5°). When these values are compared with the 1.5 Ma mean expected geomagnetic dipole reference field directions Dec=1.0°, Inc=16.4° (N=32, K=105.6, α95=2.3°) obtained from African Apparent Polar Wander Path Curve (Besse & Courtillot, 1991, 2003); a difference in declination DD=-9.2°± 5.6° and inclination DI=4.9°±5.5° are determined. This declination difference is interpreted as counterclockwise rotation of the Dofan segment about vertical axis and it is consistent with previous paleomagnetic reports in Fentale area (Kidane et al., 2009) and also with the recent analogue models of RE-Orientation of extension directions and pure extensional faulting at the oblique rift margins of the Main Ethiopian Rift (MER) (Corti et al., 2013).

3D crustal structure beneath the Costa Rica Rift from seismic tomography: insight into magmatic activity

NASA Astrophysics Data System (ADS)

Zhang, L.; Tong, V.; Wilson, D. J.; Hobbs, R. W.; Haughton, G.; Murton, B. J.

2016-12-01

During the cruise JC114, which was carried out in the intermediate-spreading Costa Rica Rift(CRR) in 2015, we acquired seismic records from 25 ocean-bottom seismographs in a 5x5 grid with an approximate node spacing of 5 km over the rift's axis. We picked 69,000 Pg and Pn events and inverted 3D crustal Vp structure beneath the CRR by using the First-Arrival Refraction Tomography (FAST). Our results show that at the depth of 1.0 2.0km below sea floor beneath the axis, there exists a 5km-wide low-velocity zone(LVZ), which extends along the axis but breaks into two segments at 83°48'W. At a deeper depth (>2.5km below sea floor), an underlying wider LVZ extends horizontally and vertically, probably stretching through the Moho. The shallower LVZ indicates the accretion of magma in the upper crust or the presence of highly porous or cracked rocks, while the deeper LVZ is inferred to be a partially molten zone, i.e. the representative of the axial magma chamber. The deeper LVZ is connected with the shallower one by upwelling conduits, which bifurcate and provide melts for both the west and east segments of the overlying LVZ. The conduit to the east segment is more prominent, providing more robust magma supply and leading to more intense negative velocity perturbation. It may reflect that the magma supply is fluctuating and migrating in the lower crust and the upper mantle. From analysing traveltime residual to study azimuthal anisotropy, we conclude that the fast direction varies roughly around 90° in the upper crust, implying that the vertically aligned cracks are nearly parallel to the axis and favour along-axis hydrothermal circulation. By comparing the anisotropy features of the two flanks of the CRR, we propose that the magmatic activity is more vigorous in the shallow subsurface of the north flank, i.e. the Cocos Plate. This research is part of a major, interdisciplinary NERC-funded collaboration entitled: Oceanographic and Seismic Characterisation of heat dissipation and alteration by hydrothermal fluids at an Axial Ridge (OSCAR).

Satellite gravity gradient views help reveal the Antarctic lithosphere

NASA Astrophysics Data System (ADS)

Ferraccioli, F.; Ebbing, J.; Pappa, F.; Kern, M.; Forsberg, R.

2017-12-01

Here we present and analyse satellite gravity gradient signatures derived from GOCE and superimpose these on tectonic and bedrock topography elements, as well as seismically-derived estimates of crustal thickness for the Antarctic continent. The GIU satellite gravity component images the contrast between the thinner crust and lithosphere underlying the West Antarctic Rift System and the Weddell Sea Rift System and the thicker lithosphere of East Antarctica. The new images also suggest that more distributed wide-mode lithospheric and crustal extension affects both the Ross Sea Embayment and the less well known Ross Ice Shelf segment of the rift system. However, this pattern is less clear towards the Bellingshousen Embayment, indicating that the rift system narrows towards the southern edge of the Antarctic Peninsula. In East Antarctica, the satellite gravity data provides new views into the Archean to Mesoproterozoic Terre Adelie Craton, and clearly shows the contrast wrt to the crust and lithosphere underlying both the Wilkes Subglacial Basin to the east and the Sabrina Subglacial Basin to the west. This finding augments recent interpretations of aeromagnetic and airborne gravity data over the region, suggesting that the Mawson Continent is a composite lithospheric-scale entity, which was affected by several Paleoproterozoic and Mesoproterozoic orogenic events. Thick crust is imaged beneath the Transantarctic Mountains, the Terre Adelie Craton, the Gamburtsev Subglacial Mountains and also Eastern Dronning Maud Land, in particular beneath the recently proposed region of the Tonian Oceanic Arc Superterrane. The GIA and GIU components help delineate the edges of several of these lithospheric provinces. One of the most prominent lithospheric-scale features discovered in East Antarctica from satellite gravity gradient imaging is the Trans East Antarctic Shear Zone that separates the Gamburtsev Province from the Eastern Dronning Maud Land Province and appears to form the southern boundary of the Recovery Province. We propose, based on geological data in the Lutzow Holm Complex region and formerly adjacent segments of India, Madagascar and eastern Africa that it may represent a major Pan-African age suture and/or shear zone related to Gondwana assembly.

Segmentation and disruption of the East Pacific Rise in the mouth of the Gulf of California

NASA Astrophysics Data System (ADS)

Lonsdale, Peter

1995-08-01

Analysis of new multibeam bathymetry and all available magnetic data shows that the 340 km-long crest of the East Pacific Rise between Rivera and Tamayo transforms contains segments of both the Pacific-Rivera and the Pacific-North America plate boundaries. Another Pacific-North America spreading segment (“Alarcon Rise”) extends 60 km further north to the Mexican continental margin. The Pacific-North America-Rivera triple junction is now of the RRR type, located on the risecrest 60 km south of Tamayo transform. Slow North America-Rivera rifting has ruptured the young lithosphere accreted to the east flank of the rise, and extends across the adjacent turbidite plain to the vicinity of the North America-Rivera Euler pole, which is located on the plate boundary. The present absolute motion of the Rivera microplate is an anticlockwise spin at 4° m.y.-1 around a pole located near its southeast corner; its motion has recently changed as the driving forces applied to its margins have changed, especially with the evolution of the southern margin from a broad shear zone between Rivera and Mathematician microplates to a long Pacific-Rivera transform. Pleistocene rotations in spreading direction, by as much as 15° on the Pacific-Rivera boundary, have segmented the East Pacific Rise into a staircase of en echelon spreading axes, which overlap at lengthening and migrating nontransform offsets. The spreading segments vary greatly in risecrest geomorphology, including the full range of structural types found on other rises with intermediate spreading rates: axial rift valleys, split shield volcanoes, and axial ridges. Most offsets between the segments have migrated southward, but within the past 1 m.y. the largest of them (with 14 27 km of lateral displacement) have shown “dueling” behavior, with short-lived reversals in migration direction. Migration involves propagation of a spreading axis into abyssal hill terrain, which is deformed and uplifted while it occupies the broad shear zones between overlapping spreading axes. Tectonic rotation of the deformed crust occurs by bookshelf faulting, which generates teleseismically recorded strike-slip earthquakes. When reversals of migration direction occur, plateaus of rotated crust are shed onto the rise flanks.

The Eastern Sardinian Margin (Tyrrhenian Sea, Western Mediterranean) : a key area to study the rifting and post-breakup evolution of a back-arc passive continental margin

NASA Astrophysics Data System (ADS)

Gaullier, Virginie; Chanier, Frank; Vendeville, Bruno; Maillard, Agnès; Thinon, Isabelle; Graveleau, Fabien; Lofi, Johanna; Sage, Françoise

2016-04-01

The Eastern Sardinian passive continental margin formed during the opening of the Tyrrhenian Sea, which is a back-arc basin created by continental rifting and oceanic spreading related to the eastward migrating Apennine subduction system (middle Miocene to Pliocene). Up to now, rifting in this key area was considered to be pro parte coeval with the Messinian Salinity Crisis (MSC, 5.96-5.32 Ma). We use the MSC seismic markers and the deformation of viscous salt and its brittle overburden as proxies to better delineate the timing of rifting and post-rift reactivation, and especially to quantify vertical and horizontal movements. On this young, highly-segmented margin, the Messinian Erosion Surface and the Upper and Mobile Units are systematically associated, respectively, to basement highs and deeper basins, showing that a rifted deep-sea domain already existed by Messinian times, therefore a major pre-MSC rifting episode occurred across the entire domain. Data show that there are no signs of Messinian syn-rift sediments, hence no evidence for rifting after Late Tortonian times. Moreover, because salt tectonics creates fan-shaped geometries in sediments, syn-rift deposits have to be carefully re-examined to distinguish the effects of crustal tectonics (rifting) and salt tectonics. We also precise that rifting is clearly diachronous from the upper margin (East-Sardinia Basin) to the lower margin (Cornaglia Terrace) with two unconformities, attributed respectively to the necking and to the lithospheric breakup unconformities. The onshore part of the upper margin has been recently investigated in order to characterize the large crustal faults affecting the Mesozoic series (geometry, kinematics and chronology) and to decipher the role of the structural inheritance and of the early rifting. Seaward, we also try to constrain the architecture and timing of the continent-ocean transition, between the hyper-extended continental crust and the first oceanic crust. Widespread post-breakup deformation also occurred during the Pliocene. Some Pliocene vertical movements have been evidenced by discovering localized gravity gliding of the salt and its Late Messinian (UU) and Early Pliocene overburden. To the South, crustal-scale southward tilting triggered along-strike gravity gliding of salt and cover recorded by upslope extension and downslope shortening. To the North, East of the Baronie Ridge, there was some post-salt crustal activity along a narrow N-S basement trough, bounded by crustal faults. The salt geometry would suggest that nothing happened after Messinian times, but some structural features (confirmed by analogue modelling) show that basement fault slip was accommodated by lateral salt flow, which thinned upslope and thickened downslope, while the overlying sediments remained sub-horizontal. Along the inner domain of Eastern Sardinian margin, the post-rift deformation style greatly varies. Compressional structures (reverse faults and folds) are observed both onshore and offshore while post-rift extensional structures are mainly identified offshore. Such late deformation could be attributed to mechanisms acting alone or combined, such as : i. the reactivation of the margin, as already described for the Ligurian, Algerian or South-Balearic margins due to the Eurasian-African convergence ; 2. the Zanclean reflooding and the resulting water overload on the elastic lithosphere ; 3. an episodic mantle upwelling.

Rifting by continental rotation, mantle flow and hotspot volcanism in the salt-depositing South Atlantic

NASA Astrophysics Data System (ADS)

Szatmari, P.

2012-04-01

Rabinowitz & LaBrecque (1979) proposed that Africa and South America separated between 130 Ma and 107 Ma by 11.1° rigid plate rotation about an Euler pole in NE Brazil. According to those authors, the two continents remained contiguous in the north as the wedge-shaped South Atlantic opened up between them and deposited salt mostly over oceanic crust. Subsequent seismic profiling and drilling showed that salt, restricted to north of the volcanic proto-Walvis Ridge, deposited over rift sediments and stretched-thinned continental crust. Increasingly accurate restorations by Nürnberg & Müller (1991), Aslanian et al. (2009), Torsvik et al. (2009), and Moulin et al. (2010) differentiated in both continents several rigid plates separated by active deformation zones. Still, tectonic analysis of the rifted margins indicates that the main Early Cretaceous event was the clockwise rotation of South America about an Euler pole in its northeast. Both rifting and volcanism, including the Paraná-Etendeka large igneous province, where most flood basalts erupted at 134.6 ± 0.6 Ma (Thiede & Vasconcelos, 2010), were controlled by distance and orientation of rift segments relative to that pole in NE Brazil. Rifting was active from latest Jurassic to early Albian time (Magnavita et al., 2011) over inherited late Proterozoic fold-thrust belts. By Aptian time a long, dry wedge-shaped basin formed north of the volcanic barrier of the proto-Walvis Ridge, widening southward to 700 km and subsiding deep below sea level in the Santos Basin. The basin was filled with oil-rich lacustrine limestone and marine salt, each more than 2 km thick, deposited in often desiccating shallow water over the partially hyperextended continental crust of the São Paulo Plateau (Zalán et al., 2010; Magnavita et al., 2011; Szatmari, 2011). Further south marine sediments deposited over oceanic crust. Surface subsidence of the long, deep sediment-starved rift wedge was shaped, prior to the deposition of the lacustrine limestones and marine evaporites, by South America's continental rotation and by hotspot activity; asthenosphere inflow was limited by the bordering two old continents.

High resolution 2D numerical models from rift to break-up: Crustal hyper-extension, Margin asymmetry, Sequential faulting

NASA Astrophysics Data System (ADS)

Brune, Sascha; Heine, Christian; Pérez-Gussinyé, Marta; Sobolev, Stephan

2013-04-01

Numerical modelling is a powerful tool to integrate a multitude of geological and geophysical data while addressing fundamental questions of passive margin formation such as the occurrence of crustal hyper-extension, (a-)symmetries between conjugate margin pairs, and the sometimes significant structural differences between adjacent margin segments. This study utilises knowledge gathered from two key examples of non-magmatic, asymmetric, conjugate margin pairs, i.e. Iberia-New Foundland and Southern Africa-Brazil, where many published seismic lines provide solid knowledge on individual margin geometry. While both margins involve crustal hyper-extension, it is much more pronounced in the South Atlantic. We investigate the evolution of these two margin pairs by carefully constraining our models with detailed plate kinematic history, laboratory-based rheology, and melt fraction evaluation of mantle upwelling. Our experiments are consistent with observed fault patterns, crustal thickness, and basin stratigraphy. We conduct 2D thermomechanical rift models using the finite element code SLIM3D that operates with nonlinear stress- and temperature-dependent elasto-visco-plastic rheology, with parameters provided by laboratory experiments on major crustal and upper mantle rocks. In our models we also calculate the melt fraction within the upwelling asthenosphere, which allows us to control whether the model indeed corresponds to the non-magmatic margin type or not. Our modelling highlights two processes as fundamental for the formation of hyper-extension and margin asymmetry at non-magmatic margins: (1) Strain hardening in the rift center due to cooling of upwelling mantle material (2) The formation of a weak crustal domain adjacent to the rift center caused by localized viscous strain softening and heat transfer from the mantle. Simultaneous activity of both processes promotes lateral rift migration in a continuous way that generates a wide layer of hyper-extended crust on one side of the rift basin. This mechanism implies that syn-rift deformation at the distal margin postdates faulting at the proximal margin by several million years. The succession of events holds intriguing implications not only for peak heat flow migration but also for processes like serpentinization and magmatic underplating.

A multidisciplinary study in the geodynamic active western Eger rift (Central Europe): The Quaternary volcanic complex Mytina and the recent CO2-degassing zone Hartousov

NASA Astrophysics Data System (ADS)

Flechsig, C.; Heinicke, J.; Kaempf, H. W.; Nickschick, T.; Mrlina, J.

2013-12-01

The Eger rift (Central Europe) belongs to the European Cenozoic rift system and represents an approximately 50 km wide and 300 km long ENE-WSW striking continental rift that formed during the Upper Cretaceous-Tertiary transition. This rift zone is one of the most active seismic regions in Central Europe. Especially, the western part of the Eger rift area is dominated by ongoing hidden magmatic processes in the intra-continental lithospheric mantle. Besides of known quaternary volcanoes, these processes take place in absence of any presently active volcanism at the surface. However, they are expressed by a series of phenomena distributed over a relatively large area, like occurrence of repeated earthquake swarms, surface exhalation of mantle-derived and CO2-enriched fluids at mofettes and mineral springs, and enhanced heat flow. At present this is the only known intra-continental region where such deep-seated, active lithospheric processes currently occur. The aim of the project is to investigate the tectonic/geologic near surface structure and the degassing processes of the mofette field of Hartousov, where soil gas measurements (concentration and flux rate) in an area of appr. 3x2 km traced a permeable NS extended segment of a fault zone and revealed highly permeable Diffuse Degassing Structures (DDS). The second target is volcanic environment of the Quaternary volcanic complex Mytina maar and the cinder cone Zelezna hurka/Eisenbühl. The investigations are intended to clarify: a) the spatio-temporal reconstruction of the maar complex, and the palaeo volcanic scenario (geological model, tectonic settings, distribution of pyroclastica, b) the geological structure and the tectonic control of the recent degassing zone, and c) the comperative interpretation of both regions in the consideration of potential future volcanic risk assessment in sub-regions of the western Eger Rift. To investigate both regions the following methods are used: geoelectrics, geomagnetics, shallow seismics, gravity and CO2-soil gas measurements, petrographic/petrophysical and remote sensing data. The results will be serve as for better understanding of geologic, volcanic and tectonic settings of the two regions as well as for the preparation of the ICDP drilling project 'Drilling the Eger rift' with a multidisciplinary approach consisting of geophysical, geochemical and other disciplines to understand the role of crustal fluid activity for swarm earthquake generation.

3D Numerical Models of the Effect of Diking on the Faulting Pattern at Incipient Continental Rifts and Steady-State Spreading Centers

NASA Astrophysics Data System (ADS)

Tian, X.; Choi, E.; Buck, W. R.

2015-12-01

The offset of faults and related topographic relief varies hugely at both continental rifts and mid-ocean ridges (MORs). In some areas fault offset is measured in 10s of meters while in places marked by core complexes it is measured in 10s of kilometers. Variation in the magma supply is thought to control much of these differences. Magma supply is most usefully described by the ratio (M) between rates of lithospheric extension accommodated by magmatic dike intrusion and that occurring via faulting. 2D models with different values of M successfully explain much of the observed cross-sectional structure seen at rifts and ridges. However, magma supply varies along the axis of extension and the interactions between the tectonics and magmatism are inevitably three-dimensional. We investigate the consequences of this along-axis variation in diking in terms of faulting patterns and the associated structures using a 3D parallel geodynamic modeling code, SNAC. Many observed 3D structural features are reproduced: e.g., abyssal hill, oceanic core complex (OCC), inward fault jump, mass wasting, hourglass-shaped median valley, corrugation and mullion structure. An estimated average value of M = 0.65 is suggested as a boundary value for separating abyssal hills and OCCs formation. Previous inconsistency in the M range for OCC formation between 2D model results (M = 0.3˜0.5) and field observations (M < 0.3 or M > 0.5) is reconciled by the along-ridge coupling between different faulting regimes. We also propose asynchronous faulting-induced tensile failure as a new possibility for explaining corrugations seen on the surface of core complexes. For continental rifts, we will describe a suite of 2D and 3D model calculations with a range of initial lithospheric structures and values of M. In one set of the 2D models we limit the extensional tectonic force and show how this affects the maximum topographic relief produced across the rift. We are also interested in comparing models in which the value of M varies as the rift evolves with observations from real rifts and continental margins. Finally, we plan to show how the faulting pattern in 3D can depend on the distribution of dike opening rate along segments for incipient continental rifts.

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

NASA Astrophysics Data System (ADS)

Plasman, M.; Tiberi, C.; Ebinger, C.; Gautier, S.; Albaric, J.; Peyrat, S.; Déverchère, J.; Le Gall, B.; Tarits, P.; Roecker, S.; Wambura, F.; Muzuka, A.; Mulibo, G.; Mtelela, K.; Msabi, M.; Kianji, G.; Hautot, S.; Perrot, J.; Gama, R.

2017-07-01

Rifting in a cratonic lithosphere is strongly controlled by several interacting processes including crust/mantle rheology, magmatism, inherited structure and stress regime. In order to better understand how these physical parameters interact, a 2 yr long seismological experiment has been carried out in the North Tanzanian Divergence (NTD), at the southern tip of the eastern magmatic branch of the East African rift, where the southward-propagating continental rift is at its earliest stage. We analyse teleseismic data from 38 broad-band stations ca. 25 km spaced and present here results from their receiver function (RF) analysis. The crustal thickness and Vp/Vs ratio are retrieved over a ca. 200 × 200 km2 area encompassing the South Kenya magmatic rift, the NTD and the Ngorongoro-Kilimanjaro transverse volcanic chain. Cratonic nature of the lithosphere is clearly evinced through thick (up to ca. 40 km) homogeneous crust beneath the rift shoulders. Where rifting is present, Moho rises up to 27 km depth and the crust is strongly layered with clear velocity contrasts in the RF signal. The Vp/Vs ratio reaches its highest values (ca. 1.9) beneath volcanic edifices location and thinner crust, advocating for melting within the crust. We also clearly identify two major low-velocity zones (LVZs) within the NTD, one in the lower crust and the second in the upper part of the mantle. The first one starts at 15-18 km depth and correlates well with recent tomographic models. This LVZ does not always coexist with high Vp/Vs ratio, pleading for a supplementary source of velocity decrease, such as temperature or composition. At a greater depth of ca. 60 km, a mid-lithospheric discontinuity roughly mimics the step-like and symmetrically outward-dipping geometry of the Moho but with a more slanting direction (NE-SW) compared to the NS rift. By comparison with synthetic RF, we estimate the associated velocity reduction to be 8-9 per cent. We relate this interface to melt ponding, possibly favouring here deformation process such as grain-boundary sliding (EAGBS) due to lithospheric strain. Its geometry might have been controlled by inherited lithospheric fabrics and heterogeneous upper mantle structure. We evidence that crustal and mantle magmatic processes represent first order mechanisms to ease and locate the deformation during the first stage of a cratonic lithospheric breakup.

A bottom-driven mechanism for distributed faulting: Insights from the Gulf of California Rift

NASA Astrophysics Data System (ADS)

Persaud, P.; Tan, E.; Choi, E.; Contreras, J.; Lavier, L. L.

2017-12-01

The Gulf of California is a young oblique rift that displays a variation in rifting style along strike. Despite the rapid localization of strain in the Gulf at 6 Ma, the northern rift segment has the characteristics of a wide rift, with broadly distributed extensional strain and small gradients in topography and crustal thinning. Observations of active faulting in the continent-ocean transition of the Northern Gulf show multiple oblique-slip faults distributed in a 200 x 70 km2area developed some time after a westward relocation of the plate boundary at 2 Ma. In contrast, north and south of this broad pull-apart structure, major transform faults accommodate Pacific-North America plate motion. Here we propose that the mechanism for distributed brittle deformation results from the boundary conditions present in the Northern Gulf, where basal shear is distributed between the Cerro Prieto strike-slip fault (southernmost fault of the San Andreas fault system) and the Ballenas Transform fault. We hypothesize that in oblique-extensional settings whether deformation is partitioned in a few dip-slip and strike-slip faults, or in numerous oblique-slip faults may depend on (1) bottom-driven, distributed extension and shear deformation of the lower crust or upper mantle, and (2) the rift obliquity. To test this idea, we explore the effects of bottom-driven shear on the deformation of a brittle elastic-plastic layer with pseudo-three dimensional numerical models that include side forces. Strain localization results when the basal shear is a step-function while oblique-slip on numerous faults dominates when basal shear is distributed. We further investigate how the style of faulting varies with obliquity and demonstrate that the style of faulting observed in the Northern Gulf of California is reproduced in models with an obliquity of 0.7 and distributed basal shear boundary conditions, consistent with the interpreted obliquity and boundary conditions of the study area. Our findings motivate a suite of 3D models of the early plate boundary evolution in the Gulf, and highlight the importance of local stress field perturbations as a mechanism for broadening the deformation zone in other regions such as the Basin and Range, Rio Grande Rift and Malawi Rift.

Lithospheric Layering beneath Southern Africa Constrained by S-to-P Receiver Functions

NASA Astrophysics Data System (ADS)

Liu, L.; Liu, K. H.; Gao, S. S.

2016-12-01

To investigate the existence of intra-lithospheric interfaces in an area of active rifting of ancient lithosphere, we stack S-to-P receiver functions (SRFs) recorded by broadband seismic stations in the vicinity of the non-volcanic sections of the East African Rift System and the stable Kaapvaal and Zimbabwe cratons. The data set was recorded by about 200 permanent and portable seismic stations installed over the past 30 years. The SRFs are computed using frequency-domain deconvolution, and are stacked in consecutive circles with a radius of 2 degrees. They are converted to depth series after moveout corrections using the IASP91 Earth model. In the upper mantle , a robust negative arrival is found in virtually all the stacked traces in the depth range of 50-100 km. Comparison with results from seismic tomography and mantle xenolith studies suggests that this discontinuity represents a mid-lithospheric discontinuity (MLD), similar to what was observed beneath the North American continent. The absence of observable negative arrivals in the anticipated depth of 250 km or greater beneath the study area suggests a gradual instead of sharp transition from the lithosphere to the asthenosphere. No significant shallowing of the MLD is observed beneath the young rift segments, suggesting that rifting is limited in the crust, an observation that is consistent with recent results from the SAFARI (Seismic Arrays for African Rift Initiation) project. The shallowest MLD of about 65 km in the study area is found in a NW-SE trending zone across central Zimbabwe and western Zambia. The MLD may reflect a low velocity zone caused by metasomatism, a process commonly found beneath ancient cratons.

Continuous Spectrum of Crustal Structures and Spreading Processes from Volcanic Rifted Margins to Mid-Ocean Ridges

NASA Astrophysics Data System (ADS)

Karson, J. A.

2016-12-01

Structures generated by seafloor spreading in oceanic crust (and ophiolites) and thick oceanic crust of Iceland show a continuous spectrum of features that formed by similar mechanisms but at different scales. A high magma budget near the Iceland hotspot generates thick (40-25 km) mafic crust in a plate boundary zone about 50 km wide. The upper crust ( 10 km thick) is constructed by the subaxial subsidence and thickening of lavas fed by dense dike swarms over a hot, weak lower crust to produce structures analogous to seaward-dipping reflectors of volcanic rifted margins. Segmented rift zones propagate away from the hotspot creating migrating transform fault zones, microplate-like crustal blocks and rift-parallel strike-slip faults. These structures are decoupled from the underlying lower crustal gabbroic rocks that thin by along-axis flow that reduces the overall crustal thickness and smooths-out local crustal thickness variations. Spreading on mid-ocean ridges with high magma budgets have much thinner crust (10-5 km) generated at a much narrower (few km) plate boundary zone. Subaxial subsidence accommodates the thickening of the upper crust of inward-dipping lavas and outward-dipping dikes about 1-2 km thick over a hot weak lower crust. Along-axis (high-temperature ductile and magmatic) flow of lower crustal material may help account for the relatively uniform seismic thickness of oceanic crust worldwide. Spreading along even slow-spreading mid-ocean ridges near hotspots (e.g., the Reykjanes Ridge) probably have similar features that are transitional between these extremes. In all of these settings, upper crustal and lower crustal structures are decoupled near the plate boundary but eventually welded together as the crust ages and cools. Similar processes are likely to occur along volcanic rifted margins as spreading begins.

New determinations of 40Ar/39Ar isotopic ages and flow volumes for Cenozoic volcanism in the Terror Rift, Ross Sea, Antarctica

NASA Astrophysics Data System (ADS)

Rilling, S.; Mukasa, S.; Wilson, T.; Lawver, L.; Hall, C.

2009-12-01

This study provides new determinations of 40Ar/39Ar isotopic ages and flow volumes for submarine and subaerial Neogene volcanism developed within the Terror Rift, Ross Sea, Antarctica, the youngest segment of the West Antarctic Rift System. The study is based on the first dredged samples from seven seamounts north of Ross Island, as well as new data from Franklin and Beaufort Islands. The sampled foidite and basanitic lavas range in age from Quaternary (90 ± 66 ka) on a small seamount ˜10 km north of Franklin Island to 6.80 ± 0.05 Ma on Beaufort Island. These ages are consistent with ages of volcanism in both the Melbourne and Erebus Volcanic Provinces and significantly expand the documented area of Neogene magmatism in Victoria Land. There is no geographic progression of volcanism through time, but volcanism was voluminous in the Pliocene and particularly widespread during the Pleistocene. Two of the dredges sampled edifices comprised of less than 0.2 km3 of volcanic materials. The largest seamount in the study area has 58.8 km3 of volcanic material and represents growth over a period of several thousand years. Estimated minimum eruption rates range from 2 × 10-4 km3 y-1 to 2 × 10-3 km3 y-1, consistent with rates proposed for other rift systems and nearby Mt. Erebus. Recent estimates of extension magnitude for the Terror Rift correspond to minimal decompression of only 0.10 to 0.22 GPa and therefore limited melt output of a typical peridotite source.

Volcanic unrest in Kenya: geological history from a satellite perspective

NASA Astrophysics Data System (ADS)

Robertson, E.; Biggs, J.; Edmonds, M.; Vye-Brown, C.

2013-12-01

The East African Rift (EAR) system is a 5,000 km long series of fault bounded depressions that run from Djibouti to Mozambique. In the Kenyan Rift, fourteen Quaternary volcanoes lie along the central rift axis. These volcanoes are principally composed of trachyte pyroclastics and trachyte and basaltic lavas forming low-angle multi-vent edifices. Between 1997 and 2008, geodetic activity has been observed at five Kenyan volcanoes, all of which have undergone periods of caldera collapse and explosive activity. We present a remote-sensing study to investigate the temporal and spatial development of volcanic activity at Longonot volcano. High-resolution mapping using ArcGIS and an immersive 3D visualisation suite (GeovisionaryTM) has been used with imagery derived from ASTER, SPOT5 and GDEM data to identify boundaries of eruptive units and establish relative age in order to add further detail to Longonot's recent eruptive history. Mapping of the deposits at Longonot is key to understand the recent geological history and forms the basis for future volcanic hazard research to inform risk assessments and mitigation programs in Kenya. Calderas at Kenyan volcanoes are elliptical in plan view and we use high-resolution imagery to investigate the regional stresses and structural control leading to the formation of these elliptical calderas. We find that volcanoes in the central and northern segments of the Kenyan rift are elongated nearly parallel to the direction of least horizontal compressive stress, likely as a reflection of the direction of the plate motion vector at the time of caldera collapse. The southern volcanoes however are elongated at an acute angle to the plate motion vector, most likely as a result of oblique opening of the Kenyan rift in this region.

State-of-stress in magmatic rift zones: Predicting the role of surface and subsurface topography

NASA Astrophysics Data System (ADS)

Oliva, S. J. C.; Ebinger, C.; Rivalta, E.; Williams, C. A.

2017-12-01

Continental rift zones are segmented along their length by large fault systems that form in response to extensional stresses. Volcanoes and crustal magma chambers cause fundamental changes to the density structure, load the plates, and alter the state-of-stress within the crust, which then dictates fracture orientation. In this study, we develop geodynamic models scaled to a < 7 My rift sector in the Eastern rift, East Africa where geophysical imaging provides tight constraints on subsurface structure, petrologic and thermodynamic studies constrain material densities, and seismicity and structural analyses constrain active and time-averaged kinematics. This area is an ideal test area because a 60º stress rotation is observed in time-averaged fault and magma intrusion, and in local seismicity, and because this was the site of a large volume dike intrusion and seismic sequence in 2007. We use physics-based 2D and 3D models (analytical and finite elements) constrained by data from active rift zones to quantify the effects of loading on state-of-stress. By modeling varying geometric arrangements, and density contrasts of topographic and subsurface loads, and with reasonable regional extensional forces, the resulting state-of-stress reveals the favored orientation for new intrusions. Although our models are generalized, they allow us to evaluate whether a magmatic system (surface and subsurface) can explain the observed stress rotation, and enable new intrusions, new faults, or fault reactivation with orientations oblique to the main border faults. Our results will improve our understanding of the different factors at play in these extensional regimes, as well as contribute to a better assessment of the hazards in the area.

High-resolution estimates of Nubia-Somalia plate motion since 20 Ma from reconstructions of the Southwest Indian Ridge, Red Sea and Gulf of Aden

NASA Astrophysics Data System (ADS)

DeMets, C.; Merkouriev, S.

2016-10-01

Large gaps and inconsistencies remain in published estimates of Nubia-Somalia plate motion based on reconstructions of seafloor spreading data around Africa. Herein, we use newly available reconstructions of the Southwest Indian Ridge at ˜1-Myr intervals since 20 Ma to estimate Nubia-Somalia plate motion farther back in time than previously achieved and with an unprecedented degree of temporal resolution. At the northern end of the East African rift, our new estimates of Nubia-Somalia motion for six times from 0.78 Ma to 5.2 Ma differ by only 2 per cent from the rift-normal component of motion that is extrapolated from a recently estimated GPS angular velocity. The rate of rift-normal extension thus appears to have remained steady since at least 5.2 Ma. Our new rotations indicate that the two plates have moved relative to each other since at least 16 Ma and possibly longer. Motion has either been steady since at least 16 Ma or accelerated modestly between 6 and 5.2 Ma. Our Nubia-Somalia rotations predict 42.5 ± 3.8 km of rift-normal extension since 10.6 Ma across the well-studied, northern segment of the Main Ethiopian Rift, consistent with 40-50 km estimates for extension since 10.6 Myr based on seismological surveys of this narrow part of the plate boundary. Nubia-Somalia rotations are also derived by combining newly estimated Somalia-Arabia rotations that reconstruct the post-20-Ma opening of the Gulf of Aden with Nubia-Arabia rotations estimated via a probabilistic analysis of plausible opening scenarios for the Red Sea. These rotations predict Nubia-Somalia motion since 5.2 Myr that is consistent with that determined from Southwest Indian Ridge data and also predict 40 ± 3 km of rift-normal extension since 10.6 Ma across the Main Ethiopian Rift, consistent with our 42.5 ± 3.8 km Southwest Indian Ridge estimate. Our new rotations exclude at high confidence level previous estimates of 12 ± 13 and 123 ± 14 km for rift-normal extensions across the Main Ethiopian Rift since 10.6 Ma based on reconstructions of Chron 5n.2 along the Southwest Indian Ridge. Sparse coverage of magnetic reversals older than 16 Ma along the western third of the Southwest Indian Ridge precludes reliable determinations of Nubia-Somalia plate motion before 16 Ma, leaving unanswered the key question of when the motion between the two plates began.

The structures, stratigraphy and evolution of the Gulf of Corinth rift, Greece

NASA Astrophysics Data System (ADS)

Taylor, Brian; Weiss, Jonathan R.; Goodliffe, Andrew M.; Sachpazi, Maria; Laigle, Mireille; Hirn, Alfred

2011-06-01

A multichannel seismic and bathymetry survey of the central and eastern Gulf of Corinth (GoC), Greece, reveals the offshore fault geometry, seismic stratigraphy and basin evolution of one of Earths most active continental rift systems. Active, right-stepping, en-echelon, north-dipping border faults trend ESE along the southern Gulf margin, significantly overlapping along strike. The basement offsets of three (Akrata-Derveni, Sithas and Xylocastro) are linked. The faults are biplanar to listric: typically intermediate angle (˜35° in the centre and 45-48° in the east) near the surface but decreasing in dip and/or intersecting a low- or shallow-angle (15-20° in the centre and 19-30° in the east) curvi-planar reflector in the basement. Major S-dipping border faults were active along the northern margin of the central Gulf early in the rift history, and remain active in the western Gulf and in the subsidiary Gulf of Lechaio, but unlike the southern border faults, are without major footwall uplift. Much of the eastern rift has a classic half-graben architecture whereas the central rift has a more symmetric w- or u-shape. The narrower and shallower western Gulf that transects the >40-km-thick crust of the Hellenides is associated with a wider distribution of overlapping high-angle normal faults that were formerly active on the Peloponnesus Peninsula. The easternmost sector includes the subsidiary Gulfs of Lechaio and Alkyonides, with major faults and basement structures trending NE, E-W and NW. The basement faults that control the rift architecture formed early in the rift history, with little evidence (other than the Vrachonisida fault along the northern margin) in the marine data for plan view evolution by subsequent fault linkage. Several have maximum offsets near one end. Crestal collapse graben formed where the hanging wall has pulled off the steeper onto the shallower downdip segment of the Derveni Fault. The dominant strikes of the Corinth rift faults gradually rotate from 090-120° in the basement and early rift to 090-100° in the latest rift, reflecting a ˜10° rotation of the opening direction to the 005° presently measured by GPS. The sediments include a (locally >1.5-km-) thick, early-rift section, and a late-rift section (also locally >1.5-km-thick) that we subdivide into three sequences and correlate with seven 100-ka glacio-eustatic cycles. The Gulf depocentre has deepened through time (currently >700 mbsl) as subsidence has outpaced sedimentation. We measure the minimum total horizontal extension across the central and eastern Gulf as varying along strike between 4 and 10 km, and estimate full values of 6-11 km. The rift evolution is strongly influenced by the inherited basement fabric. The regional NNW structural fabric of the Hellenic nappes changes orientation to ESE in the Parnassos terrane, facilitating the focused north-south extension observed offshore there. The basement-penetrating faults lose seismic reflectivity above the 4-14-km-deep seismogenic zone. Multiple generations and dips of normal faults, some cross-cutting, accommodate extension beneath the GoC, including low-angle (15-20°) interfaces in the basement nappes. The thermally cool forearc setting and cross-orogen structures unaccompanied by magmatism make this rift a poor analogue and unlikely precursor for metamorphic core complex formation.

Rift valley fever virus nonstructural protein NSs promotes viral RNA replication and transcription in a minigenome system.

PubMed

Ikegami, Tetsuro; Peters, C J; Makino, Shinji

2005-05-01

Rift Valley fever virus (RVFV), which belongs to the genus Phlebovirus, family Bunyaviridae, has a tripartite negative-strand genome (S, M, and L segments) and is an important mosquito-borne pathogen for domestic animals and humans. We established an RVFV T7 RNA polymerase-driven minigenome system in which T7 RNA polymerase from an expression plasmid drove expression of RNA transcripts for viral proteins and minigenome RNA transcripts carrying a reporter gene between both termini of the M RNA segment in 293T cells. Like other viruses of the Bunyaviridae family, replication and transcription of the RVFV minigenome required expression of viral N and L proteins. Unexpectedly, the coexpression of an RVFV nonstructural protein, NSs, with N and L proteins resulted in a significant enhancement of minigenome RNA replication. Coexpression of NSs protein with N and L proteins also enhanced minigenome mRNA transcription in the cells expressing viral-sense minigenome RNA transcripts. NSs protein expression increased the RNA replication of minigenomes that originated from S and L RNA segments. Enhancement of minigenome RNA synthesis by NSs protein occurred in cells lacking alpha/beta interferon (IFN-alpha/beta) genes, indicating that the effect of NSs protein on minigenome RNA replication was unrelated to a putative NSs protein-induced inhibition of IFN-alpha/beta production. Our finding that RVFV NSs protein augmented minigenome RNA synthesis was in sharp contrast to reports that Bunyamwera virus (genus Bunyavirus) NSs protein inhibits viral minigenome RNA synthesis, suggesting that RVFV NSs protein and Bunyamwera virus NSs protein have distinctly different biological roles in viral RNA synthesis.

Palaeontological evidence for an Oligocene divergence between Old World monkeys and apes.

PubMed

Stevens, Nancy J; Seiffert, Erik R; O'Connor, Patrick M; Roberts, Eric M; Schmitz, Mark D; Krause, Cornelia; Gorscak, Eric; Ngasala, Sifa; Hieronymus, Tobin L; Temu, Joseph

2013-05-30

Apes and Old World monkeys are prominent components of modern African and Asian ecosystems, yet the earliest phases of their evolutionary history have remained largely undocumented. The absence of crown catarrhine fossils older than ∼20 million years (Myr) has stood in stark contrast to molecular divergence estimates of ∼25-30 Myr for the split between Cercopithecoidea (Old World monkeys) and Hominoidea (apes), implying long ghost lineages for both clades. Here we describe the oldest known fossil 'ape', represented by a partial mandible preserving dental features that place it with 'nyanzapithecine' stem hominoids. Additionally, we report the oldest stem member of the Old World monkey clade, represented by a lower third molar. Both specimens were recovered from a precisely dated 25.2-Myr-old stratum in the Rukwa Rift, a segment of the western branch of the East African Rift in Tanzania. These finds extend the fossil record of apes and Old World monkeys well into the Oligocene epoch of Africa, suggesting a possible link between diversification of crown catarrhines and changes in the African landscape brought about by previously unrecognized tectonic activity in the East African rift system.

Tectonic evolution and extension at the Møre Margin - Offshore mid-Norway

NASA Astrophysics Data System (ADS)

Theissen-Krah, S.; Zastrozhnov, D.; Abdelmalak, M. M.; Schmid, D. W.; Faleide, J. I.; Gernigon, L.

2017-11-01

Lithospheric stretching is the key process in forming extensional sedimentary basins at passive rifted margins. This study explores the stretching factors, resulting extension, and structural evolution of the Møre segment on the Mid-Norwegian continental margin. Based on the interpretation of new and reprocessed high-quality seismic, we present updated structural maps of the Møre margin that show very thick post-rift sediments in the central Møre Basin and extensive sill intrusion into the Cretaceous sediments. A major shift in subsidence and deposition occurred during mid-Cretaceous. One transect across the Møre continental margin from the Slørebotn Subbasin to the continent-ocean boundary is reconstructed using the basin modelling software TecMod. We test different initial crustal configurations and rifting events and compare our structural reconstruction results to stretching factors derived both from crustal thinning and the classical backstripping/decompaction approach. Seismic interpretation in combination with structural reconstruction modelling does not support the lower crustal bodies as exhumed and serpentinised mantle. Our extension estimate along this transect is 188 ± 28 km for initial crustal thickness varying between 30 and 40 km.

Rift Valley fever virus NSs protein functions and the similarity to other bunyavirus NSs proteins.

PubMed

Ly, Hoai J; Ikegami, Tetsuro

2016-07-02

Rift Valley fever is a mosquito-borne zoonotic disease that affects both ruminants and humans. The nonstructural (NS) protein, which is a major virulence factor for Rift Valley fever virus (RVFV), is encoded on the S-segment. Through the cullin 1-Skp1-Fbox E3 ligase complex, the NSs protein promotes the degradation of at least two host proteins, the TFIIH p62 and the PKR proteins. NSs protein bridges the Fbox protein with subsequent substrates, and facilitates the transfer of ubiquitin. The SAP30-YY1 complex also bridges the NSs protein with chromatin DNA, affecting cohesion and segregation of chromatin DNA as well as the activation of interferon-β promoter. The presence of NSs filaments in the nucleus induces DNA damage responses and causes cell-cycle arrest, p53 activation, and apoptosis. Despite the fact that NSs proteins have poor amino acid similarity among bunyaviruses, the strategy utilized to hijack host cells are similar. This review will provide and summarize an update of recent findings pertaining to the biological functions of the NSs protein of RVFV as well as the differences from those of other bunyaviruses.

Crustal architecture and deep structure of the Namibian passive continental margin around Walvis Ridge from wide-angle seismic data

NASA Astrophysics Data System (ADS)

Behrmann, Jan H.; Planert, Lars; Jokat, Wilfried; Ryberg, Trond; Bialas, Jörg; Jegen, Marion

2013-04-01

The opening of the South Atlantic ocean basin was accompanied by voluminous magmatism on the conjugate continental margins of Africa and South America, including the formation of the Parana and Entendeka large igneous provinces (LIP), the build-up of up to 100 km wide volcanic wedges characterized by seaward dipping reflector sequences (SDR), as well as the formation of paired hotspot tracks on the rifted African and South American plates, the Walvis Ridge and the Rio Grande Rise. The area is considered as type example for hotspot or plume-related continental break-up. However, SDR, and LIP-related features on land are concentrated south of the hotspot tracks. The segmentation of the margins offers a prime opportunity to study the magmatic signal in space and time, and investigate the interrelation with rift-related deformation. A globally significant question we address here is whether magmatism drives continental break-up, or whether even rifting accompanied by abundant magmatism is in response to crustal and lithospheric stretching governed by large-scale plate kinematics. In 2010/11, an amphibious set of wide-angle seismic data was acquired around the landfall of Walvis Ridge at the Namibian passive continental margin. The experiments were designed to provide crustal velocity information and to investigate the structure of the upper mantle. In particular, we aimed at identifying deep fault zones and variations in Moho depth, constrain the velocity signature of SDR sequences, as well as the extent of magmatic addition to the lower crust near the continent-ocean transition. Sediment cover down to the igneous basement was additionally constrained by reflection seismic data. Here, we present tomographic analysis of the seismic data of one long NNW oriented profile parallel to the continental margin across Walvis Ridge, and a second amphibious profile from the Angola Basin across Walvis Ridge and into the continental interior, crossing the area of the Etendeka Plateau basalts. The most striking feature is the sharp transition in crustal structure and thickness across the northern boundary of Walvis Ridge. Thin oceanic crust (6.5 km) of the Angola Basin lies next to the up to 35 km thick igneous crustal root founding the highest elevated northern portions of Walvis Ridge. Both structures are separated by a very large transform fault zone. The velocity structure of Walvis Ridge lower crust is indicative of gabbro, and, in the lowest parts, of cumulate sequences. On the southern side of Walvis Ridge there is a smooth gradation into the adjacent 25-30 km thick crust underlying the ocean-continent boundary, with a velocity structure resembling that of Walvis Ridge The second profile shows a sharp transition from oceanic to rifted continental crust. The transition zone may be underlain by hydrated uppermost mantle. Below the Etendeka Plateau, an extensive high-velocity body, likely representing gabbros and their cumulates at the base of the crust, indicates magmatic underplating. We summarize by stating that rift-related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism, dividing the margin in a magmatic-dominated segment to the south, and an amagmatic segment north of Walvis Ridge.

Spatiotemporal evolution of dike opening and décollement slip at Kīlauea Volcano, Hawai'i

USGS Publications Warehouse

Montgomery-Brown, E. K.; Sinnett, D.K.; Larson, K.M.; Poland, Michael P.; Segall, P.; Miklius, Asta

2011-01-01

Rapid changes in ground tilt and GPS positions on Kīlauea Volcano, Hawai'i, are interpreted as resulting from a shallow, two-segment dike intrusion into the east rift zone that began at 1217 UTC (0217 HST) on 17 June 2007 and lasted almost 3 days. As a result of the intrusion, a very small volume of basalt (about 1500 m3) erupted on 19 June. Northward tilt at a coastal tiltmeter, subsidence of south flank GPS sites, southeastward displacements at southwestern flank GPS sites, and a swarm of flank earthquakes suggest that a slow slip event occurred on the décollement beneath Kīlauea's south flank concurrent with the rift intrusion. We use 4 min GPS positions that include estimates of time-dependent tropospheric gradients and ground tilt data to study the spatial and temporal relationships between the two inferred shallow, steeply dipping dike segments extending from the surface to about 2 km depth and décollement slip at 8 km depth. We invert for the temporal evolution of distributed dike opening and décollement slip in independent inversions at each time step using a nonnegative least squares algorithm. On the basis of these inversions, the intrusion occurred in two stages that correspond spatially and temporally with concentrated rift zone seismicity. The dike opening began on the western of the two segments before jumping to the eastern segment, where the majority of opening accumulated. Dike opening preceded the start of décollement slip at an 84% confidence level; the latter is indicated by the onset of northward tilt of a coastal tiltmeter. Displacements at southwest flank GPS sites began about 18 h later and are interpreted as resulting from slow slip on the southwestern flank. Additional constraints on the evolution of the intrusion and décollement slip come from inversion of an Envisat interferogram that spans the intrusion until 0822 UTC on 18 June 2007, combined with GPS and tilt data. This inversion shows that up to 0822 UTC on 18 June, décollement slip is only required in a limited region offshore of Ka'ena Point. A similar inversion of the complete event, which includes GPS and tilt data up to 21 June and a second Envisat interferogram spanning the complete intrusion until 21 June, shows décollement slip spread westward across the south flank. This may suggest westward migration of the décollement slip as the event progressed.

The dynamics of continental breakup-related magmatism on the Norwegian volcanic margin

NASA Astrophysics Data System (ADS)

Breivik, A. J.; Faleide, J. I.; Mjelde, R.

2007-12-01

The Vøring margin off mid-Norway was initiated during the earliest Eocene (~54 Ma), and large volumes of magmatic rocks were emplaced during and after continental breakup. In 2003, an ocean bottom seismometer survey was acquired on the Norwegian margin to constrain continental breakup and early seafloor spreading processes. The profile P-wave model described here crosses the northern part of the Vøring Plateau. Maximum igneous crustal thickness was found to be 18 km, decreasing to ~6.5 km over ~6 M.y. after continental breakup. Both the volume and the duration of excess magmatism after breakup is about twice of what is observed off the Møre Margin south of the Jan Mayen Fracture Zone, which offsets the margin segments by ~170 km. A similar reduction in magmatism occurs to the north over an along-margin distance of ~100 km to the Lofoten margin, but without a margin offset. There is a strong correlation between magma productivity and early plate spreading rate, which are highest just after breakup, falling with time. This is seen both at the Møre and the Vøring margin segments, suggesting a common cause. A model for the breakup- related magmatism should be able to (1) explain this correlation, (2) the magma production peak at breakup, and (3) the magmatic segmentation. Proposed end-member hypotheses are elevated upper-mantle temperatures caused by a hot mantle plume, or edge-driven small-scale convection fluxing mantle rocks through the melt zone. Both the average P-wave velocity and the major-element data at the Vøring margin indicate a low degree of melting consistent with convection. However, small scale convection does not easily explain the issues listed above. An elaboration of the mantle plume model by N. Sleep, in which buoyant plume material fills the rift-topography at the base of the lithosphere, can explain these: When the continents break apart, the buoyant plume-material flows up into the rift zone, causing excess magmatism by both elevated temperature and excess flux, and magmatism dies off as this rift-restricted material is spent. The buoyancy of the plume-material also elevates the plate boundaries and enhances plate spreading forces initially. The rapid drop in magma productivity to the north correlates with the northern boundary of the wide and deep Cretaceous Vøring Basin, thus less plume material was accommodated off Lofoten. This model predicts that the magma segmentation will show little variation in the geochemical signature.

Geophysical Survey of the 1978 Seismo-volcanic Crisis in the Asal-Ghoubbet Rift (Afar Depression, Djibouti) and the Post-rifting Deformations

NASA Astrophysics Data System (ADS)

Doubre, C.; Ruegg, J.; de Chabalier, J.; Vigny, C.; Jacques, E.

2006-12-01

In November 1978, a seismo-volcanic crisis occurred in the Asal-Ghoubbet Rift, which is located at the western termination of the oceanic Aden Ridge propagating inland into the Afar Depression and accommodates a large part of the divergent motion of the Arabia and Somalia plates. This episode offered the opportunity to study the rifting process controlling the evolution of a sub-aerial opening segment at the transition from continental break-up to oceanic spreading. This major crustal spreading episode started with two major earthquakes in the subaerial part of the rift (mb=5.3 and 5.0) and was followed by the week-long, basaltic fissure eruption of the Ardukoba at the western tip of the central volcanic chain. The geophysical survey carried out for the crisis was possible by means of the Arta Observatory in Djibouti within the framework of field surveys financed by the French agency CNRS-INSU. This allowed the measurements of the surface breaks (dry open fissures up to 100 m, normal fault throws up to 80 cm), the crustal deformation by geodetic networks and leveling (up to 2m of horizontal widening, 70 cm of inner-floor subsidence), and the evolution of the seismic activity (eastward migration along the Aden Ridge) associated with this rifting event. Elastic modeling shows that both the deformation pattern and the seismic activity can be explained by the aseismic intrusion of two dykes below the rift inner-floor. Subsequently, a continuous geodetic and seismic monitoring has been maintained and shows that the post-dyke injection evolution of the rift is dominated by two distinct periods. During the six first years (1979-1986), high rates of horizontal opening and slip of creeping normal faults accommodate the subsidence of the inner-floor surrimposed to the development of a 25 km-wide uplift. Since 1986-87, the strain rates have decreased and currently reach values consistent with long-term velocities deduced from morpho-tectonic studies. The evolution of the deformation field deduced from geodetic and seismic data is discussed in terms of kinematic relation with creeping faults and dyking, continuous aseismic dyking injection/inflation at depth, and transient variations associated with a relaxation mechanism related to the sudden opening of dykes. An active process involving fluid migration is required to explain the discrepancy between the far field and near field velocities measured by GPS, the general doming identified by leveling data, the periods of slip acceleration along continuously creeping faults observed from InSAR data, and the time variations of low-magnitude seismicity. All these observations therefore illustrate that some parts of the crust in the rift during the rifting episode and throughout the whole post-rifting period are in a critical state close to failure.

Eight Years of Surface Deformation in the Asal-Ghoubbet Rift (Afar Depression) Observed With SAR Data

NASA Astrophysics Data System (ADS)

Doubre, C.; Peltzer, G.; Manighetti, I.; Jacques, E.

2005-12-01

The volcano-tectonic Asal-Ghoubbet rift (Djibouti) is the youngest spreading segment of the Aden oceanic ridge propagating inland into the Afar Depression. The deformation in the rift is characterized by magmatic inflation and dilatation (dyking), distributed extension, fissure opening, and normal faulting, contributing to a far field opening velocity of ~1.5 cm/yr. We use radar interferometry data acquired by the Canadian satellite Radarsat on 24-day repeat, descending passes to measure the surface deformation in a 100 km wide region centered on the rift. The data set defines 87 epochs of acquisitions distributed between 1997 and 2005. We combined the SAR data into 354 full-resolution interferograms and solved for incremental displacements between epochs using a least-square approach [Berardino et al., 2002]. The resulting line of sight displacement map time series shows the following features: - A 40 km-wide zone centered on the rift is uplifted as a dome at a steady rate. - The central rift is subsiding with respect to the north and south shoulders. The velocity field shows a marked asymmetry with faster rates occurring along the northern edge of the rift. The mean velocity of the relative movement of the subsiding inner floor with respect to the northern up-lifting shoulder reaches 7 mm/yr. - Subsidence is faster in the north half of the inner floor of the rift and is associated with episodic creep events on normal faults. These includes a slip of 16 mm on the north-dipping δ fault in 2003 and an episode of accelerated creep of 10 mm occurring in 2000 on the γ fault, which is creeping at a steady rate of 3.5 mm/yr. A northern-dipping normal fault is slipping with a mean rate of 1.4 mm/yr and accommodates also the subsidence of the northern part of the inner floor. Unlike other active faults, this one does not coincide with a topographic scarp but shows evidence of surface creep in the velocity field. - The southeastern part of F fault system is the only fault clearly active on the south side of the rift axis and shows a creep event of 9 mm in 2002. We investigate the spatial and temporal relationship between deformation events observed in the SAR data and the catalog of seismicity collected by the Djibouti Observatory and during field campaign in the winter 2000/2001. We observe that creep events are generally associated with bursts of micro-seismicity distributed in the vicinity of the fault, or with swarms of small events concentrated below the fault. These observations suggest that while the overall region is deforming in response to the steady inflation of a magmatic chamber below the central rift, the faults and dykes that accommodate the deformation at the surface are sensitive and respond rapidly to small stress changes occurring episodically within the rift.

Large geodetic time series constraining the spatial distribution and the time evolution of the velocity field at the western tip of the Aden Ridge in Afar

NASA Astrophysics Data System (ADS)

Doubre, C.; Deprez, A.; Masson, F.; Socquet, A.; Ulrich, P.; Ibrahim Ahmed, S.; de Chabalier, J. B.; Ahmadine Omar, A.; Vigny, C.; Ruegg, J. C.

2014-12-01

We present the results of the last GPS campaign conducted over the Djiboutian part of Eastern Afar. A large and dense geodetic network has been measured regularly since the 90's, and allows an accurate determination of the velocity field associated with the western tip of the Arabia-Somalia divergent plate boundary. Within the Tadjoura Gulf, the Aden ridge consists of a series of 3 en échelon, submerged spreading segments, except for the Asal segment, which is partly above water. The repetition of 6 to 7 measurements together with 6 permanent continuous GNSS stations allow an opportunity to study the spatial distribution of the active extension in relation to these 3 segments, but also to study time variations of the displacements, which are greatly expected to be transitory because of the occurrence of dyking events, small to intermediate seismic events, and volcanic activity. The divergent motion of the two margins of the Gulf occurs at ~15 mm/yr, which is consistent with the long-term estimates of the Arabia-Somalia motion. Across the Asal segment, this value confirms that the effect of the dyking event in 1978 has ended. The velocity gradients show that the deformation is distributed from the southern to the northern rift shoulder. As revealed by the InSAR data however, the along-axis variations of the deformation pattern, i.e. clear superficial active faults in the SE part of the rift and deep opening in the NW part, suggests the remaining influence of the previous dyke intrusions within the segment inner floor. The time series show that the velocity field was more heterogeneous before 2003, when the micro-seismic activity was significant, particularly around the volcanic center. The striking feature of the time evolution of the velocity field consists in the transition from an extension mainly localized across the Asal segment before 2003 to an extension more distributed, implying the influence of the southern Quaternary structures forming the Gaggade and Hanle Basins. This results in a decrease of the opening velocity across the Asal segment. This crucial change suggests that the activity of the volcanic/geothermal centre in the segment is a determining factor in the spatial organization of the deformation, by affecting the activity of the normal faults and thereby favoring the concentration of the extensive deformation.

Paleomagnetic evidence for counterclockwise rotation of the Dofan magmatic segment, Main Ethiopian Rift

NASA Astrophysics Data System (ADS)

Nugsse, Kahsay; Muluneh, Ameha A.; Kidane, Tesfaye

2018-04-01

Twenty-six paleomagnetic sites in basalt and trachyte flows and ignimbrite deposits sampled in the Dofan magmatic segment, Main Ethiopian Rift (MER). From each site, 6 to 8 core samples were collected. The samples were then cut into 200 standard specimens and their Natural Remanent Magnetization (NRM) directions were measured using a JR6A spinner magnetometer. Most specimens were subjected to stepwise alternating field (AF) and at least one specimen per site to thermal (TH) demagnetization. The directional analysis of these individual specimens revealed either one or two components of NRM. Where two components are present, the first is isolated below a temperature of 300 °C or AF field below 20 mT; the second is isolated above those steps and mostly defined straight lines directed towards the origin and are interpreted as the Characteristic Remanent Magnetization (ChRM) acquired during cooling. Rock magnetic experiments on representative specimens indicate that the dominant magnetic minerals are titanium poor titanomagnetite and in few cases titanohematites. The overall mean directions calculated for the 23 sites of Dofan is Dec = 354.1°, Inc. = +11.6° (N = 23, K = 35.1, α95 = 5.2°). When these values are compared with the 1.5 Ma expected mean geomagnetic dipole reference field directions Dec = 1.0°, Inc. = +16.4° (N = 32, K = 105.6, α95 = 2.3°), obtained from African Apparent Polar Wander Path Curve; a difference in declination ΔD = -6.9° ± 4.7° and inclination ΔI = +4.8° ± 5.5° are determined. The declination difference is interpreted as a very slight counterclockwise rotation about vertical axis of the Dofan magmatic segment and the result is consistent with previous paleomagnetic reports and analogue modeling in Fentale magmatic segment.

Genetic Evidence for an Interferon-Antagonistic Function of Rift Valley Fever Virus Nonstructural Protein NSs

PubMed Central

Bouloy, Michèle; Janzen, Christian; Vialat, Pierre; Khun, Huot; Pavlovic, Jovan; Huerre, Michel; Haller, Otto

2001-01-01

Rift Valley fever virus (RVFV), a phlebovirus of the family Bunyaviridae, is a major public health threat in Egypt and sub-Saharan Africa. The viral and host cellular factors that contribute to RVFV virulence and pathogenicity are still poorly understood. All pathogenic RVFV strains direct the synthesis of a nonstructural phosphoprotein (NSs) that is encoded by the smallest (S) segment of the tripartite genome and has an undefined accessory function. In this report, we show that MP12 and clone 13, two attenuated RVFV strains with mutations in the NSs gene, were highly virulent in IFNAR−/− mice lacking the alpha/beta interferon (IFN-α/β) receptor but remained attenuated in IFN-γ receptor-deficient mice. Both attenuated strains proved to be excellent inducers of early IFN-α/β production. In contrast, the virulent strain ZH548 failed to induce detectable amounts of IFN-α/β and replicated extensively in both IFN-competent and IFN-deficient mice. Clone 13 has a defective NSs gene with a large in-frame deletion. This defect in the NSs gene results in expression of a truncated protein which is rapidly degraded. To investigate whether the presence of the wild-type NSs gene correlated with inhibition of IFN-α/β production, we infected susceptible IFNAR−/− mice with S gene reassortant viruses. When the S segment of ZH548 was replaced by that of clone 13, the resulting reassortants became strong IFN inducers. When the defective S segment of clone 13 was exchanged with the wild-type S segment of ZH548, the reassortant virus lost the capacity to stimulate IFN-α/β production. These results demonstrate that the ability of RVFV to inhibit IFN-α/β production correlates with viral virulence and suggest that the accessory protein NSs is an IFN antagonist. PMID:11152510

Post-caldera faulting of the Late Quaternary Menengai caldera, Central Kenya Rift (0.20°S, 36.07°E)

NASA Astrophysics Data System (ADS)

Riedl, Simon; Melnick, Daniel; Mibei, Geoffrey K.; Njue, Lucy; Strecker, Manfred R.

2015-04-01

A structural geological analysis of young caldera volcanoes is necessary to characterize their volcanic activity, assess their geothermal potential, and decipher the spatio-temporal relationships of faults on a larger tectonic scale. Menengai caldera is one of several major Quaternary trachytic caldera volcanoes that are aligned along the volcano-tectonic axis of the Kenya Rift, the archetypal active magmatic rift and nascent plate boundary between the Nubia and Somalia plates. The caldera covers an area of approximately 80 km² and is among the youngest and also largest calderas in the East African Rift, situated close to Nakuru - a densely populated urban area. There is an increasing interest in caldera volcanoes in the Kenya Rift, because these are sites of relatively young volcanic and tectonic activity, and they are considered important sites for geothermal exploration and future use for the generation of geothermal power. Previous studies of Menengai showed that the caldera collapsed in a multi-event, multiple-block style, possibly as early as 29 ka. In an attempt to characterize the youngest tectonic activity along the volcano-tectonic axis in the transition between the Central and Northern Kenya rifts we first used a high-resolution digital surface model, which we derived by structure-from-motion from an unmanned aerial vehicle campaign. This enabled us to identify previously unrecognized normal faults, associated dyke intrusions and volcanic eruptive centers, and transfer faults with strike-slip kinematics in the caldera interior and its vicinity. In a second step we verified these structures at outcrop scale, assessed their relationship with known stratigraphic horizons and dated units, and performed detailed fault measurements, which we subsequently used for fault-kinematic analysis. The most important structures that we mapped are a series of north-northeast striking normal faults, which cross-cut both the caldera walls and early Holocene lake shorelines outside the caldera. These faults have similar strikes as Pleistocene faults that define the left-stepping, north-northeast oriented segments of the volcano-tectonic axis of the inner trough of the Central Kenya Rift. In the center of the caldera, these faults are kinematically linked with oblique-slip and strike-slip transfer faults, similar to other sectors in the Central Kenya Rift. The structural setup of Menengai and the faults to the north and south of the eruptive center is thus compatible with tectono-magmatic activity in an oblique extensional tectonic regime, which reflects the tectonic and seismic activity along a nascent plate boundary.

Stratigraphic and structural evolution of the Selenga Delta Accommodation Zone, Lake Baikal Rift, Siberia

NASA Astrophysics Data System (ADS)

Scholz, C. A.; Hutchinson, D. R.

Seismic reflection profiles from the Lake Baikal Rift reveal extensive details about the sediment thickness, structural geometry and history of extensional deformation and syn-rift sedimentation in this classic continental rift. The Selenga River is the largest single source of terrigenous input into Lake Baikal, and its large delta sits astride the major accommodation zone between the Central and South basins of the lake. Incorporating one of the world's largest lacustrine deltas, this depositional system is a classic example of the influence of rift basin structural segmentation on a major continental drainage. More than 3700km of deep basin-scale multi-channel seismic reflection (MCS) data were acquired during the 1989 Russian and the 1992 Russian-American field programs. The seismic data image most of the sedimentary section, including pre-rift basement in several localities. The MCS data reveal that the broad bathymetric saddle between these two major half-graben basins is underlain by a complex of severely deformed basement blocks, and is not simply a consequence of long-term deltaic deposition. Maximum sediment thickness is estimated to be more than 9km in some areas around the Selenga Delta. Detailed stratigraphic analyses of the Selenga area MCS data suggest that modes of deposition have shifted markedly during the history of the delta. The present mode of gravity- and mass-flow sedimentation that dominates the northern and southern parts of the modern delta, as well as the pronounced bathymetric relief in the area, are relatively recent developments in the history of the Lake Baikal Rift. Several episodes of major delta progradation, each extending far across the modern rift, can be documented in the MCS data. The stratigraphic framework defined by these prograding deltaic sequences can be used to constrain the structural as well as depositional evolution of this part of the Baikal Rift. An age model has been established for this stratigraphy, by tying the delta sequences to the site of the Baikal Drilling Project 1993 Drill Hole. Although the drill hole is only 100m deep, and the base of the cores is only 670ka in age, ages were extrapolated to deeper stratigraphic intervals using the Reflection-Seismic-Radiocarbon method of Cohen et al. (1993). The deep prograding delta sequences now observed in the MCS data probably formed in response to major fluctuations in sediment supply, rather than in response to shifts in lake level. This stratigraphic framework and age model suggest that the deep delta packages developed at intervals of approximately 400ka and may have formed as a consequence of climate changes affiliated with the northern hemisphere glaciations. The stratigraphic analysis also suggests that the Selenga Basin and Syncline developed as a distinct depocentre only during the past 2-3Ma.

Evidence for Terrane Accretion, Localized Rifting and Magmatism from the Crustal Velocity Structure of the Southeastern United States

NASA Astrophysics Data System (ADS)

Marzen, R. E.; Shillington, D. J.; Lizarralde, D.; Harder, S. H.

2017-12-01

The crustal structure in the Southeastern United States records a rich tectonic history, including multiple terrane accretion events, the formation of the supercontinent Pangea, widespread magmatism from the Central Atlantic Magmatic Province (CAMP), and crustal thinning before the breakup of Pangea. We use wide-angle refraction seismic data from Lines 1 and 2 of the SUGAR (SUwannee suture and GeorgiA Rift basin) seismic experiment to constrain crustal structure in order to better understand these tectonic events. The 320 and 420 km lines extend from the northwest to the southeast, crossing the Mesozoic rift basins that record crustal thinning prior to the breakup of Pangea and multiple potential suture zones between accreted terranes. We model crustal P-wave velocity structure with reflection/refraction tomography based on refractions through the sediments, crust and mantle and reflections from the base of the sediments, within the crust and the Moho. To the north on Line 2, we observe high Vp and Vs within the Inner Piedmont and Carolina accreted terranes underlain by a low velocity zone at 5 km depth. These observations are consistent with metamorphosed terranes accreting onto the Laurentian margin along a low velocity region that represents meta-sedimentary rocks and/or an Appalachian detachment. Additionally, differences in the basin structure, lower crustal velocities, and crustal thickness between Lines 1 and 2 reflect varying extension and magmatism between the two Mesozoic rift segments. Line 1 has thicker and more laterally extensive syn-rift sediments and a more pronounced region of crustal thinning. In contrast, syn-rift sediments along Line 2 are thinner and limited to a couple of smaller basins, and the crust of Line 2 gradually thins towards the coast. The thinned crust beneath Line 1 is characterized by high velocities of >7.0 km/s, which we interpret as mafic intrusions related to rifting or CAMP; in contrast, no evidence of elevated lower crustal velocities is observed on Line 2. Because intrusions into the lower crust increase both lower crustal velocities and crustal thickness, the correspondence of high lower crustal velocities with regions of greater crustal thinning suggests that extension and magmatism were more localized than one would infer based only on variations in crustal thickness.

Evolution of the Lake Victoria basin in the context of coeval rift initiation in East Africa: a 3D numerical model approach

NASA Astrophysics Data System (ADS)

Wichura, Henry; Quinteros, Javier; Melnick, Daniel; Brune, Sascha; Schwanghart, Wolfgang; Strecker, Manfred R.

2015-04-01

Over the last four years sedimentologic and thermochronologic studies in the western and eastern branches of the Cenozoic East African Rift System (EARS) have supported the notion of a broadly contemporaneous onset of normal faulting and rift-basin formation in both segments. These studies support previous interpretations based on geophysical investigations from which an onset of rifting during the Paleogene had been postulated. In light of these studies we explore the evolution of the Lake Victoria basin, a shallow, unfaulted sedimentary basin centered between both branches of the EARS and located in the interior of the East African Plateau (EAP). We quantify the fluvial catchment evolution of the Lake Victoria basin and assess the topographic response of African crust to the onset of rifting in both branches. Furthermore, we evaluate and localize the nature of strain and flexural rift-flank uplift in both branches. We use a 3D numerical forward model that includes nonlinear temperature- and stress-dependent elasto-visco-plastic rheology. The model is able to reproduce the flexural response of variably thick lithosphere to rift-related deformation processes such as lithospheric thinning and asthenospheric upwelling. The model domain covers the entire EAP and integrates extensional processes in a heterogeneous, yet cold and thick cratonic block (Archean Tanzania craton), which is surrounded by mechanically weaker Proterozoic mobile belts, which are characterized by thinner lithosphere ("thin spots"). The lower limits of the craton (170 km) and the mobile belts (120 km) are simulated by different depths of the 1300 °C lithosphere-asthenosphere boundary. We assume a constant extension rate of 4 mm/a throughout the entire simulation of 30 Ma and neglect the effect of dynamic topography and magmatism. Even though the model setup is very simple and the resolution is not high enough to calculate realistic rift-flank uplift, it intriguingly reveals important topographic trends. The model shows that elevation differences of 120 to 180 m between the plateau interior and bordering rift shoulders are pronounced enough to form a closed basin after 6.5 Ma of extension. By that time the catchment area is already comparable to the present-day Lake Victoria catchment. Moreover, the final modeled topography, including 1000 m of dynamic and 500 m of pre-plume topography, yields a base basin elevation of 1110 m, which is also in good agreement with the present-day elevation of Lake Victoria. The combined effects of the formation of an extensive lacustrine depositional environment in the interior of the EAP after 6.5 Ma and rift-shoulder uplift may have forced far-reaching environmental impacts. These may have included the onset of the Lake Victoria microclimate, the influence of the basin and surrounding orographic barriers on precipitation patterns in East Africa, and the establishment of a unique flora and fauna.

Rifts never die: Structure of the Upper Rhine Graben, and bearing on young and recent tectonics

NASA Astrophysics Data System (ADS)

Behrmann, J. H.

2003-04-01

The Upper Rhine Graben (URG) is a 300 km long, NNE trending, low-strain, small-displacement continental rift of mid-Tertiary age. Its structure can be adequately retrodeformed in 3D if sinistrally transtensive strain and displacement paths along the major faults and associated contact deformation in the wall rocks are restored. The overall structure of the URG is characterised by low listric curvature of the principal faults and large (16-20 km) depth to a basal detachment zone. This deformation geometry and kinematics inhibits block rotation, minimises displacement on individual faults, and apparently leads to strain dissipation into intricate fault networks and/or "en masse" fracturing of large rock volumes, and propagation of dominantly brittle deformation deep into the continental crust. A net result of such deformation may be permanent reduction of tensional and shear strength on a crustal scale, making oblique rifts like the URG particularly prone to tectonic reactivation. Continued Quaternary and recent tectonic activity of the URG is documented by the following phenomena: (1) strong local differential subsidence and sedimentary basin filling, especially in the northern and southern parts of the rift. (2) Formation of morphological scarps at the locations of some major faults and offset of Quaternary stata at depth, especially in the southern (Freiburg-Basel) segment (3) Changes in relative elevation of reference points along precise levelling traverses. (4) Considerable microearthquake activity (> 50 events since 1995 in the Freiburg area), concentrated in the middle and upper crust on or in the vicinity of depth projections of faults. One possible conclusion to be drawn from the URG data and observations is that rifts can remain in a near-critical mechanical state very long after formation, even if plate-scale principal stresses have changed orientations and/or differential magnitudes. Rates of movement and seismicity are up to one order of magnitude lower than in areas of active rifting. However, they may be large enough to define a sizeable geological risk to the human environment, especially by large earthquakes with very long recurrence time.

Structural peculiarities of the sedimentary cover of the MAR crest depressions (5°-8°N) in the Equatorial Atlantics

NASA Astrophysics Data System (ADS)

Skolotnev, S. G.; Tsukanov, N. V.; Turko, N. N.; Peyve, A. A.

2003-04-01

The analysis of the bottom relief structure investigated with multibeam SIMRAD 12S and sedimentary cover of depressions investigeted with PARASAUND in the MAR crest zone near Sierra-Leone Fault (22 Cruise of the RV "Akademik Nikolay Strachov", and 10 Cruise of the RV "Akademik Ioffe") point on the complicated character of the tectonic activity distribution in this region. The left-lateral displacements of the rift velley and the absence of transform faults are typical for this region. Two extremely deep rift depressions (up to 5000 m) are located in the rift valley: one on 5°54'N latitude (Markov depression) and the other on 5°46'N latitude. About 40 m sediments cover their bottom. On contary, in the depression located parallel to the rift valley directly to the west from the two mentioned rift depressions the sedimentary cover is absent and bottom has very dissected, apparently volcanic, relief. In the MAR crest zone in 20 miles to the south-west from the Markov and 5°46'N depressions one can see a system of depressions oriented both parallel and oblique to the rift valley. There are filled by the sediments of different thickness. The sedimentary cover of these depressions often is tired and deformed by diapir and horst uplifts. Dredging data show, that basalts, which represent, according to their petro-geo-chemical characteristics the enriched MORB basalts and alkaline basalts compose these uplifts. Irregular distribution and character of composition of the sediments in the depressions of crest zone of the MAR segment under consideration along with high volcanic activity outside the axial spreding zone show that tectonic and volcanic activity in this area are distributed all over the crest zone. The complicated character of this activity is obviosly caused by two reasons. From one hand it may be a deep mantle plum, as inferred from basalt composition and from the other hand it may be the lithosphere blocks displacements along the left-lateral strike slips.

Multimillion-Year Evolution of a Sublacustrine Fan System: Source-to-Sink History of the South Rukuru and Ruhuhu River Drainages, Lake Malawi (Nyasa) Rift, East Africa

NASA Astrophysics Data System (ADS)

Scholz, C. A.; Shillington, D. J.; McCartney, T.

2017-12-01

The development of long-lived continental rifts can be markedly influenced by surface processes, including sediment input and footwall erosion. This occurs through modifying crustal thickness and loading, as well as by influencing behaviors of individual faults. Here we report on the evolution of a long-lived system of sublacustrine fans in the Central Basin of the Lake Malawi (Nyasa) rift, East Africa. An extensive suite of crustal-scale seismic reflection data was acquired in 2015 as part of the SEGMeNT project, which resulted superb images of the syn-rift section. These data are augmented by legacy single-channel high resolution reflection data that provide detailed information on facies geometries and stacking architecture of the deep-water fan systems. The ages and lithologic character of the stratal surfaces observed in the reflection seismic data are constrained by ties to the 2005 scientific drill cores acquired during the Lake Malawi Scientific Drilling Project. The South Rukuru River is an eastward flowing regional drainage (11,900 km2) that enters Lake Malawi through an incision in the western border fault of the rift's Central Basin. The Rukuru River drainage (17,230 km2) enters the eastern side of the lake at an accommodation zone margin between the North and Central Basins. Both are antecedent drainages that prior to rifting may have delivered sediments to the Indian Ocean continental margin. Both systems now deliver sediment to a highly confined and focused depocenter in the Central Basin. The complex interplay of extension, mainly on the border fault systems, and high-frequency and high-amplitude lake levels shifts, has led to unique coarse sediment facies stacking architectures, with vertical stacking controlled by hydroclimate, and lateral positioning localized by fault behavior. Focused deep-water (700 m) deposition has resulted in overpressure within the sedimentary section in the localized depocenter, producing dramatic mud diapirs. Long-lived channel-levee systems observed in the seismic data demonstrate that both drainages systems have been operative for the past several million years.

Seismic hazard assessment of the Kivu rift segment based on a new sismo-tectonic zonation model (Western Branch of the East African Rift system)

NASA Astrophysics Data System (ADS)

Havenith, Hans-Balder; Delvaux, Damien

2015-04-01

In the frame of the Belgian GeoRisCA multi-risk assessment project focused on the Kivu and Northern Tanganyika Region, a seismic hazard map has been produced for this area. It is based on a on a recently re-compiled catalogue using various local and global earthquake catalogues. The use of macroseismic epicenters determined from felt earthquakes allowed to extend the time-range back to the beginning of the 20th century, thus spanning about 100 years. The magnitudes have been homogenized to Mw and the coherence of the catalogue has been checked and validated. The seismo-tectonic zonation includes 10 seismic source areas that have been defined on the basis of the regional geological structure, neotectonic fault systems, basin architecture and distribution of earthquake epicenters. The seismic catalogue was filtered by removing obvious aftershocks and Gutenberg-Richter Laws were determined for each zone. On the basis of this seismo-tectonic information and existing attenuation laws that had been established by Twesigomwe (1997) and Mavonga et al. (2007) for this area, seismic hazard has been computed with the Crisis 2012 (Ordaz et al., 2012) software. The outputs of this assessment clearly show higher PGA values (for 475 years return period) along the Rift than the previous estimates by Twesigomwe (1997) and Mavonga (2007) while the same attenuation laws had been used. The main reason for these higher PGA values is likely to be related to the more detailed zonation of the Rift structure marked by a strong gradient of the seismicity from outside the rift zone to the inside. Mavonga, T. (2007). An estimate of the attenuation relationship for the strong ground motion in the Kivu Province, Western Rift Valley of Africa. Physics of the Earth and Planetary Interiors 62, 13-21. Ordaz M, Martinelli F, Aguilar A, Arboleda J, Meletti C, D'Amico V. (2012). CRISIS 2012, Program for computing seismic hazard. Instituto de Ingeniería, Universidad Nacional Autónoma de México. Twesigomwe, E. (1997). Probabilistic seismic hazard assessment of Uganda, Ph.D. Thesis, Dept. of Physics, Makare University, Uganda.

The Rome trough and evolution of the Iapetean margin

DOE Office of Scientific and Technical Information (OSTI.GOV)

Walker, D.; Hamilton-Smith, T.; Drahovzal, J.A.

1991-08-01

Recent structural mapping of the Rome trough suggests a complex structure very different from the symmetrical and laterally continuous graben commonly depicted. Early and Middle Cambrian extension in the Rome trough of eastern Kentucky and adjacent areas resulted in a series of alternately facing half-grabens with variable displacement. These half-grabens are bounded by southwest-northeast-trending normal faults (e.g., Kentucky River and Warfield faults), which are laterally continuous only on the order to tens of kilometers. The Rome trough is laterally segmented by north-south-trending faults (e.g., Lexington fault) commonly expressed as flexures in younger rocks (e.g., Burning Springs anticline and Floyd Countymore » channel). Many of these north-south-trending faults have significant left-lateral displacement, and probably represent reactivated thrust faults of the Grenville tectonic front. The Rome trough and the associated Mississippi Valley, Rough Creek, and Birmingham fault systems were initiated during an Early Cambrian shift in sea-floor spreading from the Blue Ridge-Pine Mountain rift to the Ouachita rift along the Alabama-Oklahoma transform fault. These fault systems have been proposed as having originated from extensional stress propagated northward from the Ouachita rift across the transform fault. In the alternate model proposed here, faulting was brittle, extensional failure resulting form subsidence and flexure of the continental margin to the east. Following initiation of sea-floor spreading at the Blue Ridge-Pine Mountain rift in the latest Proterozoic, margin subsidence in the presence of the Alabama-Oklahoma transform boundary and the inherited Grenville tectonic front resulted in this interior cratonic fault system.« less

Variable modes of rifting in the eastern Basin and Range, USA from on-fault geological evidence

NASA Astrophysics Data System (ADS)

Stahl, T.; Niemi, N. A.

2017-12-01

Continental rifts are often divided along their axes into magmatic (or magma-assisted) and amagmatic (or magma-poor) segments. Less is known about magmatic versus non-magmatic extension across `wide' continental rift margins like the Basin and Range province of the USA. Paleoseismic trench investigations, Quaternary geochronology (10Be and 3He exposure-age, luminescence, and 40Ar/39Ar dating), and high-resolution topographic surveys (terrestrial laser scanning and UAV photogrammetry) were used to assess the timing and spatial variability of faulting at the Basin and Range-Colorado Plateau transition zone in central Utah. Results show that while the majority of strain is accommodated by a single, range- and province-bounding fault (the Wasatch fault zone, WFZ, slip rate of c. 3-4 mm yr-1), a transition to magma-assisted rifting occurs near the WFZ southern termination marked by a diffuse zone of faults associated with Pliocene to Holocene volcanism. Paleoseismic analysis of faults within and adjacent to this zone reveal recent (<18 ka) surface-ruptures on these faults. A single event displacement of 10-15 m for the Tabernacle fault at c. 15-18 ka (3He exposure-age) and large fault displacement gradients imply that slip was coeval with lava emplacement and that the faults in this region are linked, at least in part, to dike injection in the uppermost crust rather than slip at seismogenic depths. These results have implications for the controversial nature of regional seismic hazard and the structural evolution of the eastern Basin and Range.

Insights Into the Stress Field Around Bárðarbunga Volcano From the 2014/2015 Holuhraun Rifting Event

NASA Astrophysics Data System (ADS)

Spaans, Karsten; Hooper, Andrew

2018-04-01

The two weeklong rifting event at Bárðarbunga volcano in 2014 led to the Holuhraun eruption, which produced 1.5 km3 of lava and was the largest in Iceland in over 200 years. Predicting when and where an intrusion will lead to eruption requires detailed knowledge of the underlying stress field. Previous studies have explained the dike propagation path with a model that includes a tectonically induced stress field set up by a uniform amount of plate spreading across a straight rift axis. Here we test this hypothesis by modeling the tractions acting on the dike walls, constrained by data from Global Navigation Satellite System and Interferometric Synthetic Aperture Radar. Our results show that the majority of the opening and shearing in the final two dike segments is due to stresses built up by plate spreading since the last eruption at Holuhraun, as expected, but that the tectonically induced stress magnitude must be much lower to explain the movement of the dike walls further south. This result implies that most of the tectonically induced stress beneath the ice cap has been released, presumably due to intrusions associated with the Bárðarbunga volcanic system and the nearby Grímsvötn volcanic system, which have not been detected due to their subglacial nature. Modeling of the 2014 Bárðarbunga rifting event therefore not only yields insights into the event but also provides a window into undetected volcanic activity in the past.

The crustal structure of the north-eastern Gulf of Aden continental margin: insights from wide-angle seismic data

NASA Astrophysics Data System (ADS)

Watremez, L.; Leroy, S.; Rouzo, S.; D'Acremont, E.; Unternehr, P.; Ebinger, C.; Lucazeau, F.; Al-Lazki, A.

2011-02-01

The wide-angle seismic (WAS) and gravity data of the Encens survey allow us to determine the deep crustal structure of the north-eastern Gulf of Aden non-volcanic passive margin. The Gulf of Aden is a young oceanic basin that began to open at least 17.6 Ma ago. Its current geometry shows first- and second-order segmentation: our study focusses on the Ashawq-Salalah second-order segment, between Alula-Fartak and Socotra-Hadbeen fracture zones. Modelling of the WAS and gravity data (three profiles across and three along the margin) gives insights into the first- and second-order structures. (1) Continental thinning is abrupt (15-20 km thinning across 50-100 km distance). It is accommodated by several tilted blocks. (2) The ocean-continent transition (OCT) is narrow (15 km wide). The velocity modelling provides indications on its geometry: oceanic-type upper-crust (4.5 km s-1) and continental-type lower crust (>6.5 km s-1). (3) The thickness of the oceanic crust decreases from West (10 km) to the East (5.5 km). This pattern is probably linked to a variation of magma supply along the nascent slow-spreading ridge axis. (4) A 5 km thick intermediate velocity body (7.6 to 7.8 km s-1) exists at the crust-mantle interface below the thinned margin, the OCT and the oceanic crust. We interpret it as an underplated mafic body, or partly intruded mafic material emplaced during a `post-rift' event, according to the presence of a young volcano evidenced by heat-flow measurement (Encens-Flux survey) and multichannel seismic reflection (Encens survey). We propose that the non-volcanic passive margin is affected by post-rift volcanism suggesting that post-rift melting anomalies may influence the late evolution of non-volcanic passive margins.

Effective elastic thickness along the conjugate passive margins of India, Madagascar and Antarctica: A re-evaluation using the Hermite multitaper Bouguer coherence application

NASA Astrophysics Data System (ADS)

Ratheesh-Kumar, R. T.; Xiao, Wenjiao

2018-05-01

Gondwana correlation studies had rationally positioned the western continental margin of India (WCMI) against the eastern continental margin of Madagascar (ECMM), and the eastern continental margin of India (ECMI) against the eastern Antarctica continental margin (EACM). This contribution computes the effective elastic thickness (Te) of the lithospheres of these once-conjugated continental margins using the multitaper Bouguer coherence method. The results reveal significantly low strength values (Te ∼ 2 km) in the central segment of the WCMI that correlate with consistently low Te values (2-3 km) obtained throughout the entire marginal length of the ECMM. This result is consistent with the previous Te estimates of these margins, and confirms the idea that the low-Te segments in the central part of the WCMI and along the ECMM represents paleo-rift inception points of the lithospheric margins that was thermally and mechanically weakened by the combined action of the Marion hotspot and lithospheric extension during the rifting. The uniformly low-Te value (∼2 km) along the EACM indicates a mechanically weak lithospheric margin, probably due to considerable stretching of the lithosphere, considering the fact that this margin remained almost stationary throughout its rift history. In contrast, the ECMI has comparatively high-Te variations (5-11 km) that lack any correlation with the regional tectonic setting. Using gravity forward and inversion applications, we find a leading order of influence of sediment load on the flexural properties of this marginal lithosphere. The study concludes that the thick pile of the Bengal Fan sediments in the ECMI masks and has erased the signal of the original load-induced topography, and its gravity effect has biased the long-wavelength part of the observed gravity signal. The hence uncorrelated flat topography and deep lithospheric flexure together contribute a bias in the flexure modeling, which likely accounts a relatively high Te estimate.

Meso-Cenozoic tectonic evolution of the SE Brazilian continental margin: Petrographic, kinematic and dynamic analysis of the onshore Araruama Lagoon Fault System

NASA Astrophysics Data System (ADS)

Souza, Pricilla Camões Martins de; Schmitt, Renata da Silva; Stanton, Natasha

2017-09-01

The Ararauama Lagoon Fault System composes one of the most prominent set of lineaments of the SE Brazilian continental margin. It is located onshore in a key tectonic domain, where the basement inheritance rule is not followed. This fault system is characterized by ENE-WSW silicified tectonic breccias and cataclasites showing evidences of recurrent tectonic reactivations. Based on field work, microtectonic, kinematic and dynamic analysis, we reconstructed the paleostresses in the region and propose a sequence of three brittle deformational phases accountable for these reactivations: 1) NE-SW dextral transcurrence; 2) NNW-SSE dextral oblique extension that evolved to NNW-SSE "pure" extension; 3) ENE-WSW dextral oblique extension. These phases are reasonably correlated with the tectonic events responsible for the onset and evolution of the SE onshore rift basins, between the Neocretaceous and Holocene. However, based on petrographic studies and supported by regional geological correlations, we assume that the origin of this fault system is older, related to the Early Cretaceous South Atlantic rifting. This study provides significant information about one of the main structural trends of the SE Brazilian continental margin and the tectonic events that controlled its segmentation, since the Gondwana rifting, and compartmentalization of its onshore sedimentary deposits during the Cenozoic.

Phylogeography of Rift Valley Fever Virus in Africa and the Arabian Peninsula

PubMed Central

Peterson, A. Townsend; Hall, Matthew

2017-01-01

Rift Valley Fever is an acute zoonotic viral disease caused by Rift Valley Fever virus (RVFV) that affects ruminants and humans in Sub-Saharan Africa and the Arabian Peninsula. We used phylogenetic analyses to understand the demographic history of RVFV populations, using sequence data from the three minigenomic segments of the virus. We used phylogeographic approaches to infer RVFV historical movement patterns across its geographic range, and to reconstruct transitions among host species. Results revealed broad circulation of the virus in East Africa, with many lineages originating in Kenya. Arrival of RVFV in Madagascar resulted from three major waves of virus introduction: the first from Zimbabwe, and the second and third from Kenya. The two major outbreaks in Egypt since 1977 possibly resulted from a long-distance introduction from Zimbabwe during the 1970s, and a single introduction took RVFV from Kenya to Saudi Arabia. Movement of the virus between Kenya and Sudan, and CAR and Zimbabwe, was in both directions. Viral populations in West Africa appear to have resulted from a single introduction from Central African Republic. The overall picture of RVFV history is thus one of considerable mobility, and dynamic evolution and biogeography, emphasizing its invasive potential, potentially more broadly than its current distributional limits. PMID:28068340

Phylogeography of Rift Valley Fever Virus in Africa and the Arabian Peninsula.

PubMed

Samy, Abdallah M; Peterson, A Townsend; Hall, Matthew

2017-01-01

Rift Valley Fever is an acute zoonotic viral disease caused by Rift Valley Fever virus (RVFV) that affects ruminants and humans in Sub-Saharan Africa and the Arabian Peninsula. We used phylogenetic analyses to understand the demographic history of RVFV populations, using sequence data from the three minigenomic segments of the virus. We used phylogeographic approaches to infer RVFV historical movement patterns across its geographic range, and to reconstruct transitions among host species. Results revealed broad circulation of the virus in East Africa, with many lineages originating in Kenya. Arrival of RVFV in Madagascar resulted from three major waves of virus introduction: the first from Zimbabwe, and the second and third from Kenya. The two major outbreaks in Egypt since 1977 possibly resulted from a long-distance introduction from Zimbabwe during the 1970s, and a single introduction took RVFV from Kenya to Saudi Arabia. Movement of the virus between Kenya and Sudan, and CAR and Zimbabwe, was in both directions. Viral populations in West Africa appear to have resulted from a single introduction from Central African Republic. The overall picture of RVFV history is thus one of considerable mobility, and dynamic evolution and biogeography, emphasizing its invasive potential, potentially more broadly than its current distributional limits.

Development of real-time RT-PCR for the detection of low concentrations of Rift Valley fever virus.

PubMed

Maquart, Marianne; Temmam, Sarah; Héraud, Jean-Michel; Leparc-Goffart, Isabelle; Cêtre-Sossah, Catherine; Dellagi, Koussay; Cardinale, Eric; Pascalis, Hervé

2014-01-01

In recent years, Madagascar and the Comoros archipelago have been affected by epidemics of Rift Valley fever (RVF), however detection of Rift Valley fever virus (RVFV) in zebu, sheep and goats during the post epidemic periods was frequently unsuccessful. Thus, a highly sensitive real-time RT-PCR assay was developed for the detection of RVFV at low viral loads. A new RVF SYBR Green RT-PCR targeting the M segment was tested on serum from different RVF seronegative ruminant species collected from May 2010 to August 2011 in Madagascar and the Comoros archipelago and compared with a RVF specific quantitative real time RT-PCR technique, which is considered as the reference technique. The specificity was tested on a wide range of arboviruses or other viruses giving RVF similar clinical signs. A total of 38 out of 2756 serum samples tested positive with the new RT-PCR, whereas the reference technique only detected 5 out of the 2756. The described RT-PCR is an efficient diagnostic tool for the investigation of enzootic circulation of the RVF virus. It allows the detection of low viral RNA loads adapted for the investigations of reservoirs or specific epidemiological situations such as inter-epizootic periods. Copyright © 2013 Elsevier B.V. All rights reserved.

Ridge Tectonics, Magma Supply, and Ridge-Hotpot Interaction at the Eastern End of the Australian-Antarctic Ridge

NASA Astrophysics Data System (ADS)

Kim, S.; Lin, J.; Park, S.; Choi, H.; Lee, S.

2013-12-01

During 2011-2013 the Korea Polar Research Institute (KOPRI) conducted three successive expeditions to the eastern end of the Australian-Antarctic Ridge (AAR) to investigate the tectonics, geochemistry, and hydrothermal activity of this intermediate fast spreading system. On board the Korean icebreaker R/V Araon, the science party collected multiple types of data including multibeam bathymetry, gravity, magnetics, as well as rock and water column samples. In addition, Miniature Autonomous Plume Recorders (MAPRs) were deployed at each of the wax-core rock sampling sites to detect the presence of active hydrothermal vents. In this study, we present a detailed analysis of a 360-km-long super-segment at the eastern end of the AAR to quantify the spatial variations in ridge morphology and investigate its respond to changes in melt supply. The study region contains several intriguing bathymetric features including (1) abrupt changes in the axial topography, alternating between rift valleys and axial highs within relatively short ridge segments; (2) overshooting ridge tips at the ridge-transform intersections; (3) systematic migration patterns of hooked ridges; (4) a 350-km-long mega-transform fault; and (5) robust axial and off-axis volcanisms. To obtain a proxy for regional variations in magma supply, we calculated residual mantle Bouguer gravity anomalies (RMBA), gravity-derived crustal thickness, and residual topography for seven sub-segments. The results of the analyses revealed that the southern flank of the AAR is associated with a shallower seafloor, more negative RMBA, thicker crust, and/or less dense mantle than the conjugate northern flank. Furthermore, this N-S asymmetry becomes more prominent toward the super-segment of the AAR. Such regional variations in seafloor topography and RMBA are consistent with the hypothesis that ridge segments in the study area have interacted with the Balleny hotspot, currently lies southwest of the AAR. However, the influence of the Balleny hotpot is not dominant in the axial morphology of the AAR super-segment. The axial topography of this super-segment exhibits a sharp transition from axial highs at the western end to rift valleys at the eastern end, with regions of axial highs being associated with more magma supply as indicated by more negative RMBA. The eastern AAR will be further compared with other intermediate fast spreading ridges, such as the Juan de Fuca Ridge, Galápagos Spreading Center, and Southeast Indian Ridge west of the Australian-Antarctic Discordance, to better understand the influence of ridge-hotspot interaction on ridge magma supply and tectonics.

Geochemistry of Peralkaline Melts at Kone Volcanic Complex, Main Ethiopian Rift

NASA Astrophysics Data System (ADS)

Iddon, F. E.; Edmonds, M.; Jackson, C.; Hutchison, W.; Mather, T. A.; Fontijn, K.; Pyle, D. M.

2016-12-01

The East Africa rift system (EARS) is the archetypal example of continental rifting, with the Main Ethiopian rift (MER) segment offering a unique opportunity to examine the dynamics of peralkaline magmas; the development of central volcanoes; melt distribution and transport in the crust; the volatile budgets of rift magmas and their implications for the formation of ore deposits. The alkali- and halogen-rich magmas of the MER differ from their calc-alkaline counterparts in other settings due to their lower viscosities and higher volatile contents, which have important implications for magma transport, reservoir dynamics and eruptive hazards. The high halogen contents of the magmas give rise to halogen-rich vapor which has the capacity to transport and concentrate metals and REE. The Kone Volcanic complex is one of the lesser studied Quaternary peralkaline centres, located on the axial portion of the MER. It comprises two superimposed calderas, surrounded by ignimbrite deposits and unwelded felsic pyroclastic material, small basaltic vents and rhyolitic domes. Unusually for the central volcanoes of the MER, the caldera has refilled with basaltic lava, not pyroclastic material. We use whole rock and micro-analysis to characterize a range of Kone tephras, glasses, crystal phases and melt inclusions in terms of major, trace and volatile element abundances, alongside detailed textural analysis using QEMSCAN and SEM. The whole rock geochemistry reflects the clear peralkaline nature of the suite, with a distinct compositional gap between 50 wt% and 65 wt% SiO2, controlled largely by fractional crystallization. Trace element systematics illustrate that trachytes entrain alkali feldspars, with the crystal cargo of the entire suite reflecting the structure of the magma reservoir at depth, with liquid-rich lenses and regions of syenitic mush. Melt inclusion geochemistry allows reconstruction of complex, multiphase differentiation processes and the exsolution of both a vapor phase and a brine, allowing the fluid-melt partitioning behaviour of halogens and metals to be reconstructed.

Crustal structure of the Gulf of Aden southern margin: Evidence from receiver functions on Socotra Island (Yemen)

NASA Astrophysics Data System (ADS)

Ahmed, Abdulhakim; Leroy, Sylvie; Keir, Derek; Korostelev, Félicie; Khanbari, Khaled; Rolandone, Frédérique; Stuart, Graham; Obrebski, Mathias

2014-12-01

Breakup of continents in magma-poor setting occurs primarily by faulting and plate thinning. Spatial and temporal variations in these processes can be influenced by the pre-rift basement structure as well as by early syn-rift segmentation of the rift. In order to better understand crustal deformation and influence of pre-rift architecture on breakup we use receiver functions from teleseismic recordings from Socotra which is part of the subaerial Oligo-Miocene age southern margin of the Gulf of Aden. We determine variations in crustal thickness and elastic properties, from which we interpret the degree of extension related thinning and crustal composition. Our computed receiver functions show an average crustal thickness of ~ 28 km for central Socotra, which decreases westward along the margin to an average of ~ 21 km. In addition, the crust thins with proximity to the continent-ocean transition to ~ 16 km in the northwest. Assuming an initial pre-rift crustal thickness of 35 km (undeformed Arabian plate), we estimate a stretching factor in the range of ~ 2.1-2.4 beneath Socotra. Our results show considerable differences between the crustal structure of Socotra's eastern and western sides on either side of the Hadibo transfer zone; the east displays a clear intracrustal conversion phase and thick crust when compared with the western part. The majority of measurements across Socotra show Vp/Vs ratios of between 1.70 and 1.77 and are broadly consistent with the Vp/Vs values expected from the granitic and carbonate rock type exposed at the surface. Our results strongly suggest that intrusion of mafic rock is absent or minimal, providing evidence that mechanical thinning accommodated the majority of crustal extension. From our observations we interpret that the western part of Socotra corresponds to the necking zone of a classic magma-poor continental margin, while the eastern part corresponds to the proximal domain.

Syntectonic fluid flux during rift faulting: Record from the MIS core, Victoria Land Basin, Antarctica

NASA Astrophysics Data System (ADS)

Millan, C.; Wilson, T. J.; Paulsen, T. S.

2009-12-01

The McMurdo Ice Shelf project successfully recovered 1285 m of Neogene sedimentary core from the Victoria Land Basin, a large rift basin within the West Antarctic Rift System (WARS) of Antarctica. The core contains 1475 natural fractures that were logged as faults, veins and clastic dikes, associated with the southern extension of the Neogene-active? Terror Rift fault zone. Veins constitute about 625 of this population. Most veins are filled with calcite, although zeolites and minor chlorite are common towards the bottom of the core. In the lower ~300 m of the core, veins contain opening-mode fiber fills and are wavy to tightly folded due to vertical shortening. Folded, opening-mode folded veins are filled by calcite fibers that grew normal to vein walls, indicating the host sediment was cohesive enough to fracture but was not fully lithified and accommodated vein buckling during compaction. Fold hinges are fractured and wedging of vein segments is marked by overlapping tips separated by zones with strong chlorite and clay fabrics, suggesting shearing during further vertical contraction of the host rock. Calcite veins are commonly strongly twinned. Cathodoluminescence microscopy shows minor changes in color and intensity and minimal concentric or sectoral zoning, suggesting relatively rapid crystallization of fluids of similar chemistry. However, stable isotope analyses reveal large variations in values, with carbon values ranging from -21.91 to -7.15 (VPBD) and oxygen values ranging from -5.35 to -11.97 (VPBD). Further detailed investigation of the fracture fills using cathodoluminescence and electron microscopy combined with isotopic analysis of carbon and oxygen will document the generations of the filling material in more detail and will constrain the sources and evolution of the fluids. There has clearly been significant structural control on fluid pathways during lithification, compaction and diagenesis of strata deforming within the Terror Rift zone.

Broad, Undulated Rift Structure Hidden Under Thick Sediment in the Niigata region, Japan

NASA Astrophysics Data System (ADS)

Sekiguchi, S.; Enescu, B.; Takeda, T.; Asano, Y.; Obara, K.

2011-12-01

Niigata area is part of a broader region, located in the central and north-eastern part of Japan, known for its high strain rates (Sagiya et al., 2000). The reverse fault system in the Niigata area may indicate present reactivation of an ancient rift structure formed in the Miocene, at the opening of the Sea of Japan (Sato, 1994). To have a detailed understanding of seismotectonics in the Niigata area, we have installed a dense temporary network of 300 seismic stations. Enescu et al. (2011) presented preliminary tomography results, using earthquake data recorded during intense observations. The present study uses an improved dataset and refined data processing to reveal the detailed velocity structure and accurate earthquake locations. The data consists of 1805 events that have 151,780 P-picks and 169,696 S-picks, recorded at 434 temporary and permanent seismic stations. We first use a JHD algorithm (Kissling et al., 1994) to determine an optimum 1D velocity structure and more accurate hypocenters (both used later as input for the tomography inversion). As a result of relocation, the hypocenters in the on-shore Niigata basin region are shifted upwards by ~3 km, while the off-shore events become shallower by as much as 10 km, in agreement with results obtained in previous studies (e.g., Kato et al., 2009). We next use the "tomoDD" software (Zhang and Thurber, 2003) to invert for the 3D velocity structure and relocate simultaneously the hypocenters. The horizontal and vertical grid spacing were of 5 ~ 10 km and 2 ~ 4 km, respectively. The obtained velocity model shows a wide and relatively low velocity (< 5.5 km/s for the P-wave velocity and < 3 km/s for the S-wave velocity) band extending in a NE-SW direction, which widens and narrows along its extent. The thickness of the low-velocity region varies from place to place and exceeds 8 km in some parts. We have constructed iso-velocity maps to better visualize the shape and depth extent of the low-velocity region. Such maps clearly reveal the "meandered" basin structure and the fairly undulated surface of the basement rock below the low-velocity layer. The aftershocks of the 2004 and 2007 Niigata earthquakes occurred on the flanks of the lower velocity band. The high and low velocity pattern corresponds well to the Bouguer gravity anomalies mapped in the region. Moreover, the shallow velocity structure along a cross-section in the central part of the study area shows a good agreement with the structure mapped by a controlled-source seismology survey on a nearby profile. Kato et al. (2009), based on local tomography results, suggested the presence of a buried rift structure in the area of the 2004 and 2007 M6.8 Niigata earthquakes. Our study demonstrates that the hidden rift has a larger extent and a fairly undulated and segmented structure. The rift segmentation may have implications on the maximum size of earthquakes that could occur in the region and thus detailed mapping of the rift structure helps mitigating the seismic risk in this high-seismicity region.

Sedimentary record of relay zone evolution, Central Corinth Rift (Greece): Role of fault propagation and structural inheritance.

NASA Astrophysics Data System (ADS)

Hemelsdaël, Romain; Ford, Mary; Meyer, Nicolas

2013-04-01

Relay zones along rift border fault systems form topographic lows that are considered to allow the transfer of sediment from the footwall into hanging wall depocentres. Present knowledge focuses on the modifications of drainage patterns and sediment pathways across relay zones, however their vertical motion during growth and interaction of faults segments is not well documented. 3D models of fault growth and linkage are also under debate. The Corinth rift (Greece) is an ideal natural laboratory for the study of fault system evolution. Fault activity and rift depocentres migrated northward during Pliocene to Recent N-S extension. We report on the evolution of a relay zone in the currently active southern rift margin fault system from Pleistocene to present-day. The relay zone lies between the E-W East Helike (EHF) and Derveni faults (DF) that lie just offshore and around the town of Akrata. During its evolution the relay zone captured the antecedent Krathis river which continued to deposit Gilbert-type deltas across the relay zone during fault interaction, breaching and post linkage phases. Moreover our work underlines the role that pre-existing structure in the location of the transfer zone. Offshore fault geometry and kinematics, and sediment distribution were defined by interpretation and depth conversion of high resolution seismic profiles (from Maurice Ewing 2001 geophysical survey). Early lateral propagation of the EHF is recorded by synsedimentary fault propagation folds while the DF records tilted block geometries since initiation. Within the relay zone beds are gradually tilted toward the basin before breaching. These different styles of deformation highlight mechanical contrasts and upper crustal partition associated with the development of the Akrata relay zone. Onshore detailed lithostratigraphy, structure and geomorphological features record sedimentation across the subsiding relay ramp and subsequent footwall uplift after breaching. The area is characterised by the successive deposition of the northward prograding Platanos Gilbert-type delta (Middle group; deposited in hangingwall of the Pirgaki-Mamoussia fault) and the NE to E prograding Akrata Gilbert-type delta (Upper group). The Akrata Gilbert-type delta records progressive rotation and lengthening of the relay ramp as the East Helike fault and Derveni fault propagated laterally (from around 0.8 Ma) and started to overlap. The relay ramp was then breached by the Krathis fault (around 0.45 Ma) and the latter reactivated a NW-SE oriented inherited structure. Onshore-offshore correlation and profile restoration of the Upper group demonstrate the presence of this pre-existing structure (detachment fault?) below the Akrata relay zone that was responsible for significant eastward thickening in early rift sediments (Lower to Middle group). Our evolution model is consistent with the 'isolated fault' model where a fault array initially develops from growth of kinematically independent fault segments and fault displacement gradually accumulates during pre- and post-linkage stages. Despite the prominent control of pre-existing fabrics on the location of the transfer zone, lateral fault propagation and interaction can be well documented.

Mixing of M Segment DNA Vaccines to Hantaan Virus and Puumala Virus Reduces Their Immunogenicity in Hamsters

DTIC Science & Technology

2008-01-01

vaccines for Rift Valley fever virus, tick- borne encephalitis virus, Hantaan virus, and Crimean Congo hemorrhagic fever virus. Vaccine 2006;24(May 22 (21)):4657–66. ...Valley fever virus, tick-borne encephalitis virus, TNV, and Crimean Congo hemorrhagic fever virus [19]. Thus, it s clearly possible to develop certain...online 25 April 2008 eywords: a b s t r a c t To determine if DNA vaccines for two hantaviruses causing hemorrhagic

Effects of Oblique Extension and Inherited Structure Geometry on Transfer Zone Development in Continental Rifts: A 4D Analogue Modeling Approach

NASA Astrophysics Data System (ADS)

Zwaan, Frank; Schreurs, Guido

2015-04-01

INTRODUCTION Inherited structures in the crust form weak zones along which deformation will focus during rifting. Along-strike connection of rift segments may occur along transfer zones, as observed in East Africa. Previous studies have focused on numerical and analog modeling of transfer zones (e.g. Acocella et al., 1999, Allken et al., 2012). We elaborate upon those by investigating the effects of 1) oblique extension and 2) the geometry of linked and non-linked inherited structures on the development of transfer zones. A further improvement is the use of X-ray Computer Tomography (CT) for detailed internal analysis. METHODS The experimental set-up (see Schreurs & Colleta, 1998) contains two sidewalls with a base of compressed foam and plexiglass bars stacked in between. Decompressing this base results in distributed deformation of the overlying model materials. Deforming the model laterally with a mobile base plate produces the strike-slip components for oblique extension. Divergence velocities are in the order of 5 mm/h, translating to ca. 5 mm/Ma in nature, and 1 cm represents 10 km. A 2 cm thick layer of viscous silicone represents the ductile lower crust and a 2 cm quartz sand layer the brittle upper crust. Inherited structures are created with thin lines of silicon laid down on top of the basal silicone layer. Several models were run in a CT-scanner to reveal the 3D evolution of internal structures with time, hence 4D. RESULTS Localization of deformation along the pre-defined structures works well. The models show that the structural style changes with extension obliquity, from wide rift structures to narrower rifts with internal oblique-slip and finally strike-slip structures. Furthermore, rift offset is an important parameter influencing the occurrence of linkage: increasing rift offset decreases linkage as previously observed by Allken et al. (2012). However, increasing divergence obliquity promotes transfer zone formation, as does the presence of rift-connecting inherited zones, whose strike is at an angle of >15° with respect to the divergence direction. CT-analysis indicates that faulting initiated shortly after the start of the experiments, while structures become only clearly visible at the surface only after 1:30h (4% extension). Rift boundary fault angles tend to decrease from an initial 70° to ca. 55° after 4:00h (10% extension). Further CT-analysis will reveal the 3D evolution of the transform zones in more detail. REFERENCES Acocella, V., Faccenna, C., Funiciello, R., Rossetti, F., 1999. Sand-box modelling of basement-controlled transfer zones in extensional domains. Terra Nova, Vol. 11, No. 4, pp 149-156 Allken, V., Huismans, R. S., Thieulot, C., 2012. Factors controlling the mode of rift interaction in brittle-ductile coupled systems: A 3D numerical study, Geochem. Geophys. Geosyst. Vol. 13, Q05010 Schreurs, G., Colletta, B. (1998) Analogue modelling of faulting in zones of continental transpression and transtension. In: Holdsworth, R. E., Strachan R. A., Dewey, J. F., (eds.) 1998. Continental Transpressional and Transtensional Tectonics. Geological Society, London, Special Publications. No. 135, pp 59-79

Tectonic Evolution of the Terceira Rift (Azores)

NASA Astrophysics Data System (ADS)

Stratmann, Sjard; Huebscher, Christian; Terrinha, Pedro; Ornelas Marques, Fernando; Weiß, Benedik

2017-04-01

The Azores Plateau is located in the Central Atlantic at the Eurasian, Nubian and North-American plates (RRT) Azores Triple Junction. The Terceira Rift (TR) connects the Mid-Atlantic Ridge with the Gloria Fault, hence establishing a transtensional-transform present day plate boundary between the Eurasian and the Nubian plates. Three volcanic islands arose along the TR, Graciosa, Terceira and Sao Miguel. In the geological past, the plate boundary in the Azores area between the Eurasian and Nubian plates was located further south at the East Azores Fracture Zone. The timing of the plate boundary jump, which marks the onset of rifting along the TR, is heavily disputed. Published ages vary from 36 to 1 Ma. Based on bathymetric data and high-resolution marine 2D multi-channel seismic data acquired during M113 cruise of R/V Meteor in 2014/2015 we discuss the structural evolution of the TR and address the question whether the divergence between both plates is entirely accommodated by the TR. The central TR between São Miguel and Terceira, also known as Hirondelle Basin, is up to 70 km wide. Rifting created two asymmetric graben sections separated by a rift parallel horst. The north-eastern and south-western graben sections are ca. 4 km and 3 km deep, respectively, and the corresponding graben floors are tilted towards the central horst. Volcanic cones emerged on the central horst and rift shoulders. Bright spots in the basin fill deposits indicate fluid flow out of the volcanic basement. The seafloor is displaced by faults which suggest recent fault displacement. In the Eastern Graciosa Basin between Terceira and Graciosa Islands the rift narrows to ca. 40 km and shallows to ca. 3200 m water depth. The central horst is no longer detectable. Instead, a buried normal fault and a small escarpment are observed. Shallow faults and block rotation are less pronounced compared to the basins to the south-east and north-west. The Western Graciosa Basin is about 30 km wide and ca. 3050 m deep. The floor of the wider and deeper north-eastern rift valley dips to the northeast. The southwestern basin is represented by tilted fault blocks. The relatively undisturbed rift valley between Terceira and Graciosa (Eastern Graciosa Basin) is consistent with a rather low earthquake activity compared to the other TR segments. We therefore conclude that the TR west of Terceira does not accommodate the entire Nubia-Eurasia plate motion. In fact, we assume that tectonic stress is also dissipated in a seismically active area south of the TR where the lineaments of Pico and São Jorge Island are located. Consequently, the new seismic data support the assumption of a diffuse plate boundary in the western half of the TR. Estimating the age of the TR on the basis of fault geometry and present day extension rates supports all those previous studies which suggested a TR age of 1-3 Ma.

Stratigraphic and structural evolution of the Selenga Delta Accommodation Zone, Lake Baikal Rift, Siberia

USGS Publications Warehouse

Scholz, C.A.; Hutchinson, D.R.

2000-01-01

Seismic reflection profiles from the Lake Baikal Rift reveal extensive details about the sediment thickness, structural geometry and history of extensional deformation and syn-rift sedimentation in this classic continental rift. The Selenga River is the largest single source of terrigenous input into Lake Baikal, and its large delta sits astride the major accommodation zone between the Central and South basins of the lake. Incorporating one of the world's largest lacustrine deltas, this depositional system is a classic example of the influence of rift basin structural segmentation on a major continental drainage. More than 3700 km of deep basin-scale multi-channel seismic reflection (MCS) data were acquired during the 1989 Russian and the 1992 Russian–American field programs. The seismic data image most of the sedimentary section, including pre-rift basement in several localities. The MCS data reveal that the broad bathymetric saddle between these two major half-graben basins is underlain by a complex of severely deformed basement blocks, and is not simply a consequence of long-term deltaic deposition. Maximum sediment thickness is estimated to be more than 9 km in some areas around the Selenga Delta. Detailed stratigraphic analyses of the Selenga area MCS data suggest that modes of deposition have shifted markedly during the history of the delta. The present mode of gravity- and mass-flow sedimentation that dominates the northern and southern parts of the modern delta, as well as the pronounced bathymetric relief in the area, are relatively recent developments in the history of the Lake Baikal Rift. Several episodes of major delta progradation, each extending far across the modern rift, can be documented in the MCS data. The stratigraphic framework defined by these prograding deltaic sequences can be used to constrain the structural as well as depositional evolution of this part of the Baikal Rift. An age model has been established for this stratigraphy, by tying the delta sequences to the site of the Baikal Drilling Project 1993 Drill Hole. Although the drill hole is only 100 m deep, and the base of the cores is only ∼670 ka in age, ages were extrapolated to deeper stratigraphic intervals using the Reflection-Seismic-Radiocarbon method of Cohen et al. (1993). The deep prograding delta sequences now observed in the MCS data probably formed in response to major fluctuations in sediment supply, rather than in response to shifts in lake level. This stratigraphic framework and age model suggest that the deep delta packages developed at intervals of approximately 400 ka and may have formed as a consequence of climate changes affiliated with the northern hemisphere glaciations. The stratigraphic analysis also suggests that the Selenga Basin and Syncline developed as a distinct depocentre only during the past ∼2–3 Ma.

Rotation, narrowing and preferential reactivation of brittle structures during oblique rifting

NASA Astrophysics Data System (ADS)

Huismans, R. S.; Duclaux, G.; May, D.

2017-12-01

Occurrence of multiple faults populations with contrasting orientations in oblique continental rifts and passive margins has long sparked debate about relative timing of deformation events and tectonic interpretations. Here, we use high-resolution three-dimensional thermo-mechanical numerical modeling to characterize the evolution of the structural style associated with moderately oblique rifting in the continental lithosphere. Automatic analysis of the distribution of active extensional shears at the surface of the model demonstrates a characteristic deformation sequence. We show that upon localization, Phase 1 wide oblique en-échelon grabens develop, limited by extensional shears oriented orthogonal to σ3. Subsequent widening of the grabens is accompanied by a progressive rotation of the Phase 1 extensional shears that become sub-orthogonal the plate motion direction. Phase 2 is marked by narrowing of active deformation resulting from thinning of the continental lithosphere and development of a second-generation of extensional shears. During Phase 2 deformation localizes both on plate motion direction-orthogonal structures that reactivate rotated Phase 1 shears, and on new oblique structures orthogonal to σ3. Finally, Phase 3 consists in the oblique rupture of the continental lithosphere and produces an oceanic domain where oblique ridge segments are linked with highly oblique accommodation zones. We conclude that while new structures form normal to σ3 in an oblique rift, progressive rotation and long-term reactivation of Phase 1 structures promotes orthorhombic fault systems, critical to accommodate upper crustal extension and control oblique passive margin architecture. The distribution, orientation, and evolution of frictional-plastic structures observed in our models is remarkably similar to documented fault populations in the Gulf of Aden conjugate passive margins, which developed in moderately oblique extensional settings.

Relaxation of the south flank after the 7.2-magnitude Kalapana earthquake, Kilauea Volcano, Hawaii

USGS Publications Warehouse

Dvorak, John J.; Klein, Fred W.; Swanson, Donald A.

1994-01-01

An M = 7.2 earthquake on 29 November 1975 caused the south flank of Kilauea Volcano, Hawaii, to move seaward several meters: a catastrophic release of compression of the south flank caused by earlier injections of magma into the adjacent segment of a rift zone. The focal mechanisms of the mainshock, the largest foreshock, and the largest aftershock suggest seaward movement of the upper block. The rate of aftershocks decreased in a familiar hyperbolic decay, reaching the pre-1975 rate of seismicity by the mid-1980s. Repeated rift-zone intrusions and eruptions after 1975, which occurred within 25 km of the summit area, compressed the adjacent portion of the south flank, apparently masking continued seaward displacement of the south flank. This is evident along a trilateration line that continued to extend, suggesting seaward displacement, immediately after the M = 7.2 earthquake, but then was compressed during a series of intrusions and eruptions that began in September 1977. Farther to the east, trilateration measurements show that the portion of the south flank above the aftershock zone, but beyond the area of compression caused by the rift-zone intrusions and eruptions, continued to move seaward at a decreasing rate until the mid-1980s, mimicking the decay in aftershock rate. Along the same portion of the south flank, the pattern of vertical surface displacements can be explained by continued seaward movement of the south flank and development of two eruptive fissures along the east rift zone, each of which extended from a depth of ∼3 km to the surface. The aftershock rate and continued seaward movement of the south flank are reminiscent of crustal response to other large earthquakes, such as the 1966 M = 6 Parkfield earthquake and the 1983 M = 6.5 Coalinga earthquake.

Lithospheric rheological heterogeneity across an intraplate rift basin (Linfen Basin, North China) constrained from magnetotelluric data: Implications for seismicity and rift evolution

NASA Astrophysics Data System (ADS)

Yin, Yaotian; Jin, Sheng; Wei, Wenbo; Ye, Gaofeng; Jing, Jian'en; Zhang, Letian; Dong, Hao; Xie, Chengliang; Liang, Hongda

2017-10-01

We take the Linfen Basin, which is the most active segment of the Cenozoic intraplate Shanxi Rift, as an example, showing how to use magnetotelluric data to constrain lithospheric rheological heterogeneities of intraplate tectonic zones. Electrical resistivity models, combined with previous rheological numerical simulation, show a good correlation between resistivity and rheological strength, indicating the mechanisms of enhanced conductivity could also be reasons of reduced viscosity. The crust beneath the Linfen Basin shows overall stratified features in both electrical resistivity and rheology. The uppermost crustal conductive layer is dominated by friction sliding-type brittle fracturing. The high-resistivity mid-crust is inferred to be high-viscosity metamorphic basement being intersected by deep fault. The plastic lower crust show significantly high-conductivity feature. Seismicity appears to be controlled by crustal rheological heterogeneity. Micro-earthquakes mainly distribute at the brittle-ductile transition zones as indicated by high- to low-resistivity interfaces or the high pore pressure fault zones while the epicenters of two giant destructive historical earthquakes occur within the high-resistivity and therefore high-strength blocks near the inferred rheological interfaces. The lithosphere-scale lateral rheological heterogeneity along the profile can also be illustrated. The crust and upper mantle beneath the Ordos Block, Lüliang Mountains and Taihang Mountains are of high rheological strength as indicated by large-scale high-resistivity zones while a significant high-conductivity, lithosphere-scale weak zone exists beneath the eastern margin of the Linfen Basin. According to previous geodynamic modeling works, we suggest that this kind of lateral rheological heterogeneity may play an essential role for providing driving force for the formation and evolution of the Shanxi Rift, regional lithospheric deformation and earthquake activities under the far-field effects of the India-Eurasian Collision.

High-resolution estimates of Nubia-Somalia plate motion since 20 Ma from reconstructions of the Southwest Indian Ridge, Red Sea, and Gulf of Aden

NASA Astrophysics Data System (ADS)

DeMets, C.; Merkuryev, S. A.

2015-12-01

We estimate Nubia-Somalia rotations at ~1-Myr intervals for the past 20 Myr from newly available, high-resolution reconstructions of the Southwest Indian Ridge and reconstructions of the Red Sea and Gulf of Aden. The former rotations are based on many more data, extend farther back in time, and have more temporal resolution than has previously been the case. Nubia-Somalia plate motion has remained remarkably steady since 5.2 Ma. For example, at the northern end of the East Africa rift, our Nubia-Somalia plate motion estimates at six different times between 0.78 Ma and 5.2 Ma agree to within 3% with the rift-normal component of motion that is extrapolated from the recently estimated Saria et al. (2014) GPS angular velocity. Over the past 10.6 Myr, the Nubia-Somalia rotations predict 42±4 km of rift-normal extension across the northern segment of the Main Ethiopian Rift. This agrees with approximate minimum and maximum estimates of 40 km and 53 km for post-10.6-Myr extension from seismological surveys of this narrow part of the plate boundary and is also close to 55-km and 48±3 km estimates from published and our own reconstructions of the Nubia-Arabia and Somalia-Arabia seafloorspreading histories for the Red Sea and Gulf of Aden. Our new rotations exclude at high confidence level two previously published estimates of Nubia-Somalia motion based on inversions of Chron 5n.2 along the Southwest Indian Ridge, which predict rift-normal extensions of 13±14 km and 129±16 km across the Main Ethiopian Rift since 11 Ma. Constraints on Nubia-Somalia motion before ~15 Ma are weaker due to sparse coverage of pre-15-Myr magnetic reversals along the Nubia-Antarctic plate boundary, but appear to require motion before 15 Ma. Nubia-Somalia rotations that we estimate from a probabilistic analysis of geometric and age constraints from the Red Sea and Gulf of Aden are consistent with those determined from Southwest Indian Ridge data, particularly for the past 11 Myr. Nubia-Somalia rotations determined from the Red Sea/Gulf of Aden rotations and Southwest Indian Ridge rotations independently predict that motion during its oldest phase was highly oblique to the rift and a factor-of-two or more faster than at present, although large uncertainties remain in the rotation estimates for times before ~15 Ma.

Rift Valley Fever, Sudan, 2007 and 2010

PubMed Central

Aradaib, Imadeldin E.; Erickson, Bobbie R.; Elageb, Rehab M.; Khristova, Marina L.; Carroll, Serena A.; Elkhidir, Isam M.; Karsany, Mubarak E.; Karrar, AbdelRahim E.; Elbashir, Mustafa I.

2013-01-01

To elucidate whether Rift Valley fever virus (RVFV) diversity in Sudan resulted from multiple introductions or from acquired changes over time from 1 introduction event, we generated complete genome sequences from RVFV strains detected during the 2007 and 2010 outbreaks. Phylogenetic analyses of small, medium, and large RNA segment sequences indicated several genetic RVFV variants were circulating in Sudan, which all grouped into Kenya-1 or Kenya-2 sublineages from the 2006–2008 eastern Africa epizootic. Bayesian analysis of sequence differences estimated that diversity among the 2007 and 2010 Sudan RVFV variants shared a most recent common ancestor circa 1996. The data suggest multiple introductions of RVFV into Sudan as part of sweeping epizootics from eastern Africa. The sequences indicate recent movement of RVFV and support the need for surveillance to recognize when and where RVFV circulates between epidemics, which can make data from prediction tools easier to interpret and preventive measures easier to direct toward high-risk areas. PMID:23347790

Large and giant hydrocarbon accumulations in the transitional continent-ocean zone

NASA Astrophysics Data System (ADS)

Khain, V. E.; Polyakova, I. D.

2008-05-01

The petroleum resource potential is considered for the Atlantic, West Pacific, and East Pacific types of deepwater continental margins. The most considerable energy resources are concentrated at the Atlantic-type passive margins in the zone transitional to the ocean. The less studied continental slope of backarc seas of the generally active margins of the West Pacific type is currently not so rich in discoveries as the Atlantic-type margin, but is not devoid of certain expectations. In some of their parameters, the margins bounded by continental slopes may be regarded as analogs of classical passive margins. At the margins of the East Pacific type, the petroleum potential is solely confined to transform segments. In the shelf-continental-slope basins of the rift and pull-apart nature, petroleum fields occur largely in the upper fan complex, and to a lesser extent in the lower graben (rift) complex. In light of world experience, the shelf-continental-slope basins of the Arctic and Pacific margins of Russia are evaluated as highly promising.

Initial excavation and dating of Ngalue Cave: a Middle Stone Age site along the Niassa Rift, Mozambique.

PubMed

Mercader, Julio; Asmerom, Yemane; Bennett, Tim; Raja, Mussa; Skinner, Anne

2009-07-01

Direct evidence for a systematic occupation of the African tropics during the early late Pleistocene is lacking. Here, we report a record of human occupation between 105-42ka, based on results from a radiometrically-dated cave section from the Mozambican segment of the Niassa (Malawi/Nyasa) Rift called Ngalue. The sedimentary sequence from bottom to top has five units. We concentrate on the so-called "Middle Beds," which contain a Middle Stone Age industry characterized by the use of the discoidal reduction technique. A significant typological feature is the presence of formal types such as points, scrapers, awls, and microliths. Special objects consist of grinders/core-axes covered by ochre. Ngalue is one of the few directly-dated Pleistocene sites located along the biogeographical corridor for modern human dispersals that links east, central, and southern Africa, and, with further study, may shed new light on hominin cave habitats during the late Pleistocene.

Back-Arc Opening in the Western End of the Okinawa Trough Revealed From GNSS/Acoustic Measurements

NASA Astrophysics Data System (ADS)

Chen, Horng-Yue; Ikuta, Ryoya; Lin, Cheng-Horng; Hsu, Ya-Ju; Kohmi, Takeru; Wang, Chau-Chang; Yu, Shui-Beih; Tu, Yoko; Tsujii, Toshiaki; Ando, Masataka

2018-01-01

We measured seafloor movement using a Global Navigation Satellite Systems (GNSS)/Acoustic technique at the south of the rifting valley in the western end of the Okinawa Trough back-arc basin, 60 km east of northeastern corner of Taiwan. The horizontal position of the seafloor benchmark, measured eight times between July 2012 and May 2016, showed a southeastward movement suggesting a back-arc opening of the Okinawa Trough. The average velocity of the seafloor benchmark shows a block motion together with Yonaguni Island. The westernmost part of the Ryukyu Arc rotates clockwise and is pulled apart from the Taiwan Island, which should cause the expansion of the Yilan Plain, Taiwan. Comparing the motion of the seafloor benchmark with adjacent seismicity, we suggest a gentle episodic opening of the rifting valley accompanying a moderate seismic activation, which differs from the case in the segment north off-Yonaguni Island where a rapid dyke intrusion occurs with a significant seismic activity.

Contribution of the FUTUREVOLC project to the study of segmented lateral dyke growth in the 2014 rifting event at Bárðarbunga volcanic system, Iceland

NASA Astrophysics Data System (ADS)

Sigmundsson, Freysteinn; Hooper, Andrew; Hreinsdóttir, Sigrún; Vogfjörd, Kristín S.; Ófeigsson, Benedikt; Rafn Heimisson, Elías; Dumont, Stéphanie; Parks, Michelle; Spaans, Karsten; Guðmundsson, Gunnar B.; Drouin, Vincent; Árnadóttir, Thóra; Jónsdóttir, Kristín; Gudmundsson, Magnús T.; Samsonov, Sergey; Brandsdóttir, Bryndís; White, Robert S.; Ágústsdóttir, Thorbjörg; Björnsson, Helgi; Bean, Christopher J.

2015-04-01

The FUTUREVOLC project (a 26-partner project funded by FP7 Environment Programme of the European Commission, addressing topic "Long-term monitoring experiment in geologically active regions of Europe prone to natural hazards: the Supersite concept) set aims to (i) establish an innovative volcano monitoring system and strategy, (ii) develop new methods for near real-time integration of multi-parametric datasets, (iii) apply a seamless transdisciplinary approach to further scientific understanding of magmatic processes, and (iv) to improve delivery, quality and timeliness of transdisciplinary information from monitoring scientists to civil protection. The project duration is 1 October 2012 - 31 March 2016. Unrest and volcanic activity since August 2014 at one of the focus areas of the project in Iceland, at the Bárðarbunga volcanic system, near the middle of the project duration, has offered unique opportunities for this project. On 16 August 2014 an intense seismic swarm started in Bárðarbunga, the beginning of a major volcano-tectonic rifting event forming over 45 km long dyke extending from the caldera to Holuhraun lava field outside the northern margin of Vatnajökull. A large basaltic, effusive fissure eruption began in Holuhraun on 31 August which had by January formed a lava field with a volume in excess of one cubic kilometre. We document how the FUTUREVOLC project has contributed to the study and response to the subsurface dyke formation, through increased seismic and geodetic coverage and joint interpreation of the data. The dyke intrusion in the Bárðarbunga volcanic system, grew laterally for over 45 km at a variable rate, with an influence of topography on the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Bárðarbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with a magma source deflation and slow collapse at the Bárðarbunga caldera, accompanied by a series of M>5 earthquakes. Joint interpretation of seismic and geodetic data was reported daily to the civil protection of Iceland and used for effective response and mitigation of the associated hazards. The response to, and studies of, the Bárðarbunga rifting event and eruptions have thus contributed to the achievements of all the objectives of the FUTUREVOLC project.

Development, Interaction and Linkage of Normal Fault Segments along the 100-km Bilila-Mtakataka Fault, Malawi

NASA Astrophysics Data System (ADS)

Fagereng, A.; Hodge, M.; Biggs, J.; Mdala, H. S.; Goda, K.

2016-12-01

Faults grow through the interaction and linkage of isolated fault segments. Continuous fault systems are those where segments interact, link and may slip synchronously, whereas non-continuous fault systems comprise isolated faults. As seismic moment is related to fault length (Wells and Coppersmith, 1994), understanding whether a fault system is continuous or not is critical in evaluating seismic hazard. Maturity may be a control on fault continuity: immature, low displacement faults are typically assumed to be non-continuous. Here, we study two overlapping, 20 km long, normal fault segments of the N-S striking Bilila-Mtakataka fault, Malawi, in the southern section of the East African Rift System. Despite its relative immaturity, previous studies concluded the Bilila-Mtakataka fault is continuous for its entire 100 km length, with the most recent event equating to an Mw8.0 earthquake (Jackson and Blenkinsop, 1997). We explore whether segment geometry and relationship to pre-existing high-grade metamorphic foliation has influenced segment interaction and fault development. Fault geometry and scarp height is constrained by DEMs derived from SRTM, Pleiades and `Structure from Motion' photogrammetry using a UAV, alongside direct field observations. The segment strikes differ on average by 10°, but up to 55° at their adjacent tips. The southern segment is sub-parallel to the foliation, whereas the northern segment is highly oblique to the foliation. Geometrical surface discontinuities suggest two isolated faults; however, displacement-length profiles and Coulomb stress change models suggest segment interaction, with potential for linkage at depth. Further work must be undertaken on other segments to assess the continuity of the entire fault, concluding whether an earthquake greater than that of the maximum instrumentally recorded (1910 M7.4 Rukwa) is possible.

Geodiversity and Geoheritage of the Danakil and Afar region

NASA Astrophysics Data System (ADS)

van Wyk de Vries, Benjamin; Hagos, Miruts

2017-04-01

The Danakil Depression is well known for its spectacular and wild desert scenery, with volcanoes, faults and salt plains. It' northern part is thought to be a the very last stage of continental rifting, before opening into an ocean. Such an area is a unique and fully representative example of the birth of an ocean, through hot-spot related rifting that should have World Heritage status. The area is however, a remote extreme environment with cultural and political issues, and a highly active geological setting, where rifting and volcanic events have the potential to change the landscape rapidly. The area is also the focus of increasing economic activity with interest in hydrothermal and resource's such as Potash being mined. In order to best sustainably develop the area, and to take into account all aspects of its environment, the geological environment needs to be fully mapped, characterised and monitored. The geodiversity and geohertiage needs to be assessed, and geosites inventoried. We present some of the first mapping, done to this purpose, by an international team focussed around Mekel'le University. We show a first order assessment of the geoheritage value of the main identified sites. We focus on the Dallol - Erta Ale segment, as a more detailed example of this work. This can be used by the local stakeholders to discuss the various options open for the future development of the area.

Tectonic evolution of west Antarctica and its relation to east Antarctica

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dalziel, I.W.D.

1987-05-01

West Antarctica consists of five major blocks of continental crust separated by deep sub-ice basins. Marie Byrd Land appears to have been rifted off the adjacent margin of the East Antarctic craton along the line of the Transantarctic Mountains during the Mesozoic. Ellsworth-Whitmore mountains and Haag Nunataks blocks were also rifted from the margin of the craton. They appear to have moved together with the Antarctic Peninsula and Thurston Island blocks, segments of a Pacific margin Mesozoic-Cenozoic magmatic arc, during the Mesozoic opening of the Weddell Sea basin. Paleomagnetic data suggest that all four of these blocks remained attached tomore » western Gondwanaland (South America-Africa) until approximately 125 m.y. ago, and that the present geographic configuration of the Antarctic continent was essentially complete by the mid-Cretaceous, although important Cenozoic rifting has also occurred. Fragmentation of the Gondwanaland supercontinent was preceded in the Middle to Late Jurassic by an important and widespread thermal event of uncertain origin that resulted in the emplacement of an extensive bimodal igneous suite in South America, Africa, Antarctica, and Australia. This was associated with the development of the composite back-arc basin along the western margin of South America. Inversion of this basin in the mid-Cretaceous initiated Andean orogenesis. The presentation will include new data from the joint US-UK West Antarctic Tectonics Project.« less

Relief evolution of the Continental Rift of Southeast Brazil revealed by in situ-produced 10Be concentrations in river-borne sediments

NASA Astrophysics Data System (ADS)

Salgado, André Augusto Rodrigues; Rezende, Eric de Andrade; Bourlès, Didier; Braucher, Régis; da Silva, Juliana Rodrigues; Garcia, Ricardo Alexandrino

2016-04-01

This study aims to quantify the denudation dynamics of the Brazilian passive margin along a segment of the Continental Rift of Southeast Brazil. The denudation rates of 30 basins that drain both horsts of the continental rift, including the mountain ranges of the Serra do Mar (seaside horst); and the Serra da Mantiqueira (continental horst); were derived from 10Be concentrations measured in sand-sized river sediment. The mean denudation rate ranges from 9.2 m Ma-1 on the plateau of the Serra do Mar to 37.1 m Ma-1 along the oceanic escarpment of the Serra do Mar. The seaward-facing scarps of both mountain ranges exhibit mean denudation rates that are approximately 1.5 times those of the inland-facing scarps. The escarpments of the horst nearer to the ocean (Serra do Mar) exhibit higher denudation rates (mean 30.2 m Ma-1) than the escarpments of the continental horst (Serra da Mantiqueira) (mean 16.5 m Ma-1). The parameters that impact these denudation rates include the catchment relief, the slope gradient, the rock and the climate. The incongruent combination of a mountainous landscape and moderate to low 10Be-based denudation rates averaging at ∼20 m Ma-1 suggests a reduction in intraplate tectonic activity beginning in the Middle Quaternary or earlier.

Pathogenic effects of Rift Valley fever virus NSs gene are alleviated in cultured cells by expressed antiviral short hairpin RNAs.

PubMed

Scott, Tristan; Paweska, Janusz T; Arbuthnot, Patrick; Weinberg, Marc S

2012-01-01

Rift Valley fever virus (RVFV), a member of the Bunyaviridae family, may cause severe hepatitis, encephalitis and haemorrhagic fever in humans. There are currently no available licensed vaccines or therapies to treat the viral infection in humans. RNA interference (RNAi)-based viral gene silencing offers a promising approach to inhibiting replication of this highly pathogenic virus. The small (S) segment of the RVFV tripartite genome carries the genetic determinates for pathogenicity during infection. This segment encodes the non-structural S (NSs) and essential nucleocapsid (N) genes. To advance RNAi-based inhibition of RVFV replication, we designed several Pol III short hairpin RNA (shRNA) expression cassettes against the NSs and N genes, including a multimerized plasmid vector that included four shRNA expression cassettes. Effective target silencing was demonstrated using full- and partial-length target reporter assays, and confirmed by western blot analysis of exogenous N and NSs expression. Small RNA northern blots showed detectable RNAi guide strand formation from single and multimerized shRNA constructs. Using a cell culture model of RVFV replication, shRNAs targeting the N gene decreased intracellular nucleocapsid protein concentration and viral replication. The shRNAs directed against the NSs gene reduced NSs protein concentrations and alleviated NSs-mediated cytotoxicity, which may be caused by host transcription suppression. These data are the first demonstration that RNAi activators have a potential therapeutic benefit for countering RVFV infection.

Continental breakup by oblique extension: the Gulf of California

NASA Astrophysics Data System (ADS)

van Wijk, J.; Axen, G. J.

2017-12-01

We address two aspects of oblique extension: 1) the evolution of pull-apart basins, and how/when they may evolve into seafloor spreading segments; and 2) the formation of microcontinents. The Gulf of California formed by oblique extension. Breakup resulted in oceanic crust generation in the southern and central parts, while in the northern Gulf/Salton Trough a thick layer of (meta-)sediments overlies thinned continental crust. We propose a simple mechanism to explain this N-S variation. We assume that oblique rifting of the proto-Gulf province resulted in pull-apart basins, and use numerical models to show that such pull-apart basins do not develop into seafloor spreading segments when their length-to-width ratios are small, as is the case in the northern Gulf. In the central and southern Gulf the length-to-width ratios were larger, promoting continent rupture. The mechanisms behind this fate of pull-apart basins will be discussed in the presentation. In the southern Gulf, potential field models show that the Tamayo Bank in the southern Gulf is likely a microcontinent, separated from the main continent by the Tamayo trough. The thickness of the ocean crust in the Tamayo trough is anomalously small, suggesting that initial seafloor spreading was magma-starved and unsuccessful, causing the location of rifting and seafloor spreading to jump. As a consequence a sliver of continent broke off, forming the microcontinent. We suggest that worldwide this may be a common process for microcontinent formation.

Attenuation and protective efficacy of Rift Valley fever phlebovirus rMP12-GM50 strain.

PubMed

Ly, Hoai J; Nishiyama, Shoko; Lokugamage, Nandadeva; Smith, Jennifer K; Zhang, Lihong; Perez, David; Juelich, Terry L; Freiberg, Alexander N; Ikegami, Tetsuro

2017-12-04

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to Africa and the Arabian Peninsula that affects sheep, cattle, goats, camels, and humans. Effective vaccination of susceptible ruminants is important for the prevention of RVF outbreaks. Live-attenuated RVF vaccines are in general highly immunogenic in ruminants, whereas residual virulence might be a concern for vulnerable populations. It is also important for live-attenuated strains to encode unique genetic markers for the differentiation from wild-type RVFV strains. In this study, we aimed to strengthen the attenuation profile of the MP-12 vaccine strain via the introduction of 584 silent mutations. To minimize the impact on protective efficacy, codon usage and codon pair bias were not de-optimized. The resulting rMP12-GM50 strain showed 100% protective efficacy with a single intramuscular dose, raising a 1:853 mean titer of plaque reduction neutralization test. Moreover, outbred mice infected with one of three pathogenic reassortant ZH501 strains, which encoded rMP12-GM50 L-, M-, or S-segments, showed 90%, 50%, or 30% survival, respectively. These results indicate that attenuation of the rMP12-GM50 strain is significantly attenuated via the L-, M-, and S-segments. Recombinant RVFV vaccine strains encoding similar silent mutations will be also useful for the surveillance of reassortant strains derived from vaccine strains in endemic countries. Copyright © 2017 Elsevier Ltd. All rights reserved.

The distribution and composition characteristics of siliceous rocks from Qinzhou Bay-Hangzhou Bay joint belt, South China: constraint on the tectonic evolution of plates in South China.

PubMed

Li, Hongzhong; Zhai, Mingguo; Zhang, Lianchang; Zhou, Yongzhang; Yang, Zhijun; He, Junguo; Liang, Jin; Zhou, Liuyu

2013-01-01

The Qinzhou Bay-Hangzhou Bay joint belt is a significant tectonic zone between the Yangtze and Cathaysian plates, where plentiful hydrothermal siliceous rocks are generated. Here, the authors studied the distribution of the siliceous rocks in the whole tectonic zone, which indicated that the tensional setting was facilitating the development of siliceous rocks of hydrothermal genesis. According to the geochemical characteristics, the Neopalaeozoic siliceous rocks in the north segment of the Qinzhou Bay-Hangzhou Bay joint belt denoted its limited width. In comparison, the Neopalaeozoic Qinzhou Bay-Hangzhou Bay joint belt was diverse for its ocean basin in the different segments and possibly had subduction only in the south segment. The ocean basin of the north and middle segments was limited in its width without subduction and possibly existed as a rift trough that was unable to resist the terrigenous input. In the north segment of the Qinzhou Bay-Hangzhou Bay joint belt, the strata of hydrothermal siliceous rocks in Dongxiang copper-polymetallic ore deposit exhibited alternative cycles with the marine volcanic rocks, volcanic tuff, and metal sulphide. These sedimentary systems were formed in different circumstances, whose alternative cycles indicated the release of internal energy in several cycles gradually from strong to weak.

The Transition from Volcanic to Rift Dominated Crustal Breakup - From the Vøring Plateau to the Lofoten Margin, Norway

NASA Astrophysics Data System (ADS)

Breivik, A. J.; Faleide, J. I.; Mjelde, R.; Flueh, E.; Murai, Y.

2017-12-01

The Vøring Plateau was part of the Northeast Atlantic igneous province (NAIP) during early Cenozoic crustal breakup. Crustal breakup at the Vøring Plateau occurred marginal to the deep Cretaceous basins on the shelf, with less extension of the crust. Intrusive magmatism and oceanic crust up to three times normal thickness caused a period of sub-aerial magmatism around breakup time. The transition to the Lofoten Margin is rapid to a deep-water plain. Still, there is some excess magmatism north of this transition, where early oceanic crustal thickness is reduced to half of that of the Vøring Plateau 150 km away. Our estimates of the earliest seafloor spreading rates using new ship-track magnetic profiles on different margin segments offer a clue to what caused this rapid transition. While crustal breakup occurred within the magnetic polarity C24r in other parts of the NAIP, there is a delayed breakup for the Lofoten/Vesterålen margin. Modeling of the earliest seafloor spreading with geomagnetic reversals, indicate a breakup within C24n.3n (anomaly 24b), approximately 1 m.y. later. Both old wide-angle seismic models (from Ocean Bottom Seismometers) off southern Lofoten and a newly published profile farther north show a strongly extended outer margin. Applying early seafloor half-spreading rates ( 30 mm/y) from other NAIP margin segments for 1 m.y. can account for 30 km extra extension, giving a factor of three crustal thinning, and gives a high strain rate of 3.2 ·10-14. Crustal breakup at the magma-poor Iberian Margin occurred at a low strain rate of 4.4·10-15, allowing the ascending mantle to cool, favoring tectonic extension over magmatism. Similar strain rates are found within the main Ethiopian Rift, but there is much magmatism and crustal separation is dominated by dike injection. Mantle tomography models show an exceptionally low seismic velocity below the area interpreted as an unusually hot upper mantle, which will favor magmatism. The transition from the Vøring Plateau to the Lofoten Margin can therefore be explained by the presence/absence of hot mantle plume material under the different segments during rifting. Only after significant extension and close to crustal breakup time did a minor amount of plume material reach the Lofoten/Vesterålen margin to cause some elevated but short-lived excess magmatism there.

Palaeogeographic evolution of the central segment of the South Atlantic during Early Cretaceous times: palaeotopographic and geodynamic implications

NASA Astrophysics Data System (ADS)

Chaboureau, A. C.; Guillocheau, F.; Robin, C.; Rohais, S.; Moulin, M.; Aslanian, D.

2012-04-01

The tectonic and sedimentary evolution of the Early Cretaceous rift of the central segment of the South Atlantic Ocean is debated. Our objective is to better constraint the timing of its evolution by drawing palaeogeographic and deformation maps. Eight palaeogeographic and deformations maps were drawn from the Berriasian to the Middle-Late Aptian, based on a biostratigraphic (ostracodes and pollens) chart recalibrated on absolute ages (chemostratigraphy, interstratified volcanics, Re-Os dating of the organic matter). The central segment of the South Atlantic is composed of two domains that have a different history in terms of deformation and palaeogeography. The southern domain includes Namibe, Santos and Campos Basins. The northern domain extends from Espirito Santo and North Kwanza Basins, in the South, to Sergipe-Alagoas and North Gabon Basins to the North. Extension started in the northern domain during Late Berriasian (Congo-Camamu Basin to Sergipe-Alagoas-North Gabon Basins) and migrated southward. At that time, the southern domain was not a subsiding domain. This is time of emplacement of the Parana-Etendeka Trapp (Late Hauterivian-Early Barremian). Extension started in this southern domain during Early Barremian. The brittle extensional period is shorter in the South (5-6 Ma, Barremian to base Aptian) than in the North (19 to 20 Myr, Upper Berriasian to Base Aptian). From Late Berriasian to base Aptian, the northern domain evolves from a deep lake with lateral highs to a shallower one, organic-rich with no more highs. The lake migrates southward in two steps, until Valanginian at the border between the northern and southern domains, until Early Barremian, North of Walvis Ridge. The Sag phase is of Middle to Late Aptian age. In the southern domain, the transition between the brittle rift and the sag phase is continuous. In the northern domain, this transition corresponds to a hiatus of Early to Middle Aptian age, possible period of mantle exhumation. Marine influences were clearly occurring since the Early Aptian in the Northern domain and the Campos Basin. They seem sharp, brief flooding coming from the North, i.e. from the Tethys-Central Atlantic, trough a seaway crossing South America from Sao Luis, Parnaiba, Araripe and Almada basins (Arai, 1989). In the absence of data, the importance of those marine flooding during the Middle Aptian in the Santos Basin is still discussed. Keywords: South Atlantic Ocean, Early Cretaceous, Rift, Palaeogeography, Geodynamic

Lithospheric structure along wide-angle seismic profile GEORIFT 2013 in Pripyat-Dnieper-Donets Basin (Belarus and Ukraine)

NASA Astrophysics Data System (ADS)

Starostenko, V.; Janik, T.; Yegorova, T.; Czuba, W.; Środa, P.; Lysynchuk, D.; Aizberg, R.; Garetsky, R.; Karataev, G.; Gribik, Y.; Farfuliak, L.; Kolomiyets, K.; Omelchenko, V.; Komminaho, K.; Tiira, T.; Gryn, D.; Guterch, A.; Legostaeva, O.; Thybo, H.; Tolkunov, A.

2018-03-01

The GEORIFT 2013 (GR'13) WARR (wide-angle reflection and refraction) experiment was carried out in 2013 in the territory of Belarus and Ukraine with broad international co-operation. The aim of the work is to study basin architecture and deep structure of the Pripyat-Dnieper-Donets Basin (PDDB), which is the deepest and best studied Palaeozoic rift basin in Europe. The PDDB is located in the southern part of the East European Craton (EEC) and crosses Sarmatia—one of the three segments of the EEC. The PDDB was formed by Late Devonian rifting associated with domal basement uplift and magmatism. The GR'13 extends in NW-SE direction along the PDDB strike and crosses the Pripyat Trough (PT) and Dnieper Graben (DG) separated by the Bragin Uplift (BU) of the basement. The field acquisition along the GR'13 (of 670 km total length) involved 14 shots and recorders deployed every ˜2.2 km for several shot points. The good quality of the data, with first arrivals visible up to 670 km for several shot points, allowed for construction of a velocity model extending to 80 km depth using ray-tracing modelling. The thickness of the sediments (Vp < 6.0 km s-1) varies from 1-4 km in the PT, to ˜5 km in the NW part of the DG, to 10-13 km in the SE part of the profile. Below the DG, at ˜330-530 km distance, we observed an upwarping of the lower crust (with Vp of ˜7.1 km s-1) to ˜25 km depth that represents a rift pillow or mantle underplate. The Moho shallows southeastwards from ˜47 km in the PT to 40-38 km in the DG with mantle velocities of 8.35 and ˜8.25 km s-1 in the PT and DG, respectively. A near-horizontal mantle discontinuity was found beneath BU (a transition zone from the PT to the DG) at the depth of 50-47 km. It dips to the depth of ˜60 km at distances of 360-405 km, similar to the intersecting EUROBRIDGE'97 profile. The crust and upper mantle structure on the GR'13 may reflect varying intensity of rifting in the PDDB from a passive stage in the PT to active rifting in the DG. The absence of Moho uplift and relatively thick crystalline crust under the PT is explained by its tectonic position as a closing unit of the PDDB, with a gradual attenuation of rifting from the southeast to the northwest. The most active stage of rifting is evidenced in the DG by a shallower Moho and by a presence of a rift pillow caused by mafic and ultramafic intrusions during the active phase. The junction of the PT and the DG (the BU) locates just at its intersection with the NS regional tectonic zone Odessa-Gomel. Most likely, the `blocking' effect of this zone did not allow for further propagation of active rifting to the NW.

Lithospheric structure of an incipient rift basin: Results from receiver function analysis of Bransfield Strait, NW Antarctic Peninsula

NASA Astrophysics Data System (ADS)

Biryol, C. Berk; Lee, Stephen J.; Lees, Jonathan M.; Shore, Michael J.

2018-06-01

Bransfield Basin (BB), located northwest of the Antarctic Peninsula (AP) and southeast of the South Shetland Islands (SSI), is the most active section of the Antarctic continental margin. The region has long been (50 Ma) a convergent plate boundary where the Phoenix plate was subducting beneath the Antarctic Plate and is characterized by long-lived arc magmatism and accretion. However, the collision of the Antarctic-Phoenix spreading center with the subduction front near SSI (ca. 4 Ma) gave way to the opening of slab windows and dramatic decrease in the subduction rate of the Phoenix plate beneath AP and SSI. Consequently, the Phoenix slab began to rollback slowly along the South Shetland Trench (SST), giving way to slow extension in the back-arc region and rifting along the BB. Although there is consensus on the factors that control the current deformation and extension of the BB, the origin of the BB and the tectonic configuration of the basin are still unclear. Most of the controversy stems from uncertainties regarding the crustal thickness of the BB. Hence, we computed teleseismic receiver functions for 10 broadband stations in the region that belong to existing permanent and temporary deployments in order obtain robust constraints on the lithospheric structure and crustal thickness of the BB, as well as the AP and SSI. Our results indicate that the crust is thinning from 30 km to 26 km from the AP towards the South Shetland trench and Central BB showing the asymmetrical character of the rift basin. The crustal thickness and Vp/Vs variations are less pronounced along the AP but very significant across the SSB indicating the lithospheric scale segmentation of the South Shetland Block (SSB) and the incipient rift basin under the control of the opening of slab window and the roll-back of stalled Phoenix slab. High Vp/Vs ratios (∼1.9) beneath BB and SSI, agree well with the nascent rift character of BB, the presence of a steep Phoenix slab and consequently a wider mantle wedge characterized by the presence of underplating partial melts beneath SSI and BB.

How does continental lithosphere break-apart? A 3D seismic view on the transition from magma-poor rifted margin to magmatic oceanic lithosphere

NASA Astrophysics Data System (ADS)

Emmanuel, M.; Lescanne, M.; Picazo, S.; Tomasi, S.

2017-12-01

In the last decade, high-quality seismic data and drilling results drastically challenged our ideas about how continents break apart. New models address their observed variability and are presently redefining basics of rifting as well as exploration potential along deepwater rifted margins. Seafloor spreading is even more constrained by decades of scientific exploration along Mid Oceanic Ridges. By contrast, the transition between rifting and drifting remains a debated subject. This lithospheric breakup "event" is geologically recorded along Ocean-Continent Transitions (OCT) at the most distal part of margins before indubitable oceanic crust. Often lying along ultra-deepwater margin domains and buried beneath a thick sedimentary pile, high-quality images of these domains are rare but mandatory to get strong insights on the processes responsible for lithospheric break up and what are the consequences for the overlying basins. We intend to answer these questions by studying a world-class 3D seismic survey in a segment of a rifted margin exposed in the Atlantic. Through these data, we can show in details the OCT architecture between a magma-poor hyper-extended margin (with exhumed mantle) and a classical layered oceanic crust. It is characterized by 1- the development of out-of-sequence detachment systems with a landward-dipping geometry and 2- the increasing magmatic additions oceanwards (intrusives and extrusives). Geometry of these faults suggests that they may be decoupled at a mantle brittle-ductile interface what may be an indicator on thermicity. Furthermore, magmatism increases as deformation migrates to the future first indubitable oceanic crust what controls a progressive magmatic crustal thickening below, above and across a tapering rest of margin. As the magmatic budget increases oceanwards, full-rate divergence is less and less accommodated by faulting. Magmatic-sedimentary architectures of OCT is therefore changing from supra-detachment to magmatic oceanic half-grabens (low-crustal extension, high magma additions) and to ultimate layered oceanic crust (quasi-none crustal extension, full magmatic accretion). All of these elements suggest that lithospheric breakup can be addressed as a tectonic-magma competition as the brittle-ductile mantle interface is shallowing along OCT.

Rift Valley Fever Virus Lacking the NSs and NSm Genes Is Highly Attenuated, Confers Protective Immunity from Virulent Virus Challenge, and Allows for Differential Identification of Infected and Vaccinated Animals▿

PubMed Central

Bird, Brian H.; Albariño, César G.; Hartman, Amy L.; Erickson, Bobbie Rae; Ksiazek, Thomas G.; Nichol, Stuart T.

2008-01-01

Rift Valley fever (RVF) virus is a mosquito-borne human and veterinary pathogen associated with large outbreaks of severe disease throughout Africa and more recently the Arabian peninsula. Infection of livestock can result in sweeping “abortion storms” and high mortality among young animals. Human infection results in self-limiting febrile disease that in ∼1 to 2% of patients progresses to more serious complications including hepatitis, encephalitis, and retinitis or a hemorrhagic syndrome with high fatality. The virus S segment-encoded NSs and the M segment-encoded NSm proteins are important virulence factors. The development of safe, effective vaccines and tools to screen and evaluate antiviral compounds is critical for future control strategies. Here, we report the successful reverse genetics generation of multiple recombinant enhanced green fluorescent protein-tagged RVF viruses containing either the full-length, complete virus genome or precise deletions of the NSs gene alone or the NSs/NSm genes in combination, thus creating attenuating deletions on multiple virus genome segments. These viruses were highly attenuated, with no detectable viremia or clinical illness observed with high challenge dosages (1.0 × 104 PFU) in the rat lethal disease model. A single-dose immunization regimen induced robust anti-RVF virus immunoglobulin G antibodies (titer, ∼1:6,400) by day 26 postvaccination. All vaccinated animals that were subsequently challenged with a high dose of virulent RVF virus survived infection and could be serologically differentiated from naïve, experimentally infected animals by the lack of NSs antibodies. These rationally designed marker RVF vaccine viruses will be useful tools for in vitro screening of therapeutic compounds and will provide a basis for further development of RVF virus marker vaccines for use in endemic regions or following the natural or intentional introduction of the virus into previously unaffected areas. PMID:18199647

Seafloor spreading on the Amsterdam-St. Paul hotspot plateau

NASA Astrophysics Data System (ADS)

Conder, James A.; Scheirer, Daniel S.; Forsyth, Donald W.

2000-04-01

The Amsterdam-St. Paul (ASP) platform on the intermediate rate Southeast Indian Ridge (SEIR) is the only oceanic hotspot plateau outside the Atlantic Ocean containing an active, mid-ocean ridge spreading axis. Because the ASP hotspot is small and remotely located, it has been relatively unstudied, and the ridge axis location in many places near the ASP plateau was previously unknown or ambiguous. We mapped the SEIR out to 1 Ma crust (Jaramillo anomaly) both on and near the ASP platform. We located the spreading center to within a few kilometers, based on side-scan sonar reflectivity. Recent off-platform magnetic anomalies and lineated abyssal hill topography are consistent with a simple spreading history. Off-platform full spreading rates increase from ˜63 km/Myr on segment H to the north of the platform to ˜65.5 km/Myr on segment K to the south. In contrast, inversions of seafloor magnetization based on uniform and variable thickness magnetic source layers reflect a complex on-platform tectonic history with ridge jumps, off-axis volcanism, and propagating rifts. On one section of the ASP plateau the spreading location has stabilized and is beginning to rift the plateau apart, generating symmetric magnetic anomalies and lineated topography for the last several hundred thousand years. The larger, more stable, spreading segments of the ASP platform are aligned with major volcanic edifices, suggesting that along-axis magma flow away from plume-fed centers is an important influence on spreading geometry. Many complex tectonic features observed on the ASP plateau, such as ridge jumps, en echelon, oblique spreading centers, and transforms oblique to the spreading direction, are comparable to features observed on Iceland. The similarities suggest that moderate crustal thickening at an intermediate rate spreading center may have similar effects to pronounced thickening at a slow rate spreading center.

Characterization of Rift Valley Fever Virus MP-12 Strain Encoding NSs of Punta Toro Virus or Sandfly Fever Sicilian Virus

PubMed Central

Lihoradova, Olga A.; Indran, Sabarish V.; Kalveram, Birte; Lokugamage, Nandadeva; Head, Jennifer A.; Gong, Bin; Tigabu, Bersabeh; Juelich, Terry L.; Freiberg, Alexander N.; Ikegami, Tetsuro

2013-01-01

Rift Valley fever virus (RVFV; genus Phlebovirus, family Bunyaviridae) is a mosquito-borne zoonotic pathogen which can cause hemorrhagic fever, neurological disorders or blindness in humans, and a high rate of abortion in ruminants. MP-12 strain, a live-attenuated candidate vaccine, is attenuated in the M- and L-segments, but the S-segment retains the virulent phenotype. MP-12 was manufactured as an Investigational New Drug vaccine by using MRC-5 cells and encodes a functional NSs gene, the major virulence factor of RVFV which 1) induces a shutoff of the host transcription, 2) inhibits interferon (IFN)-β promoter activation, and 3) promotes the degradation of dsRNA-dependent protein kinase (PKR). MP-12 lacks a marker for differentiation of infected from vaccinated animals (DIVA). Although MP-12 lacking NSs works for DIVA, it does not replicate efficiently in type-I IFN-competent MRC-5 cells, while the use of type-I IFN-incompetent cells may negatively affect its genetic stability. To generate modified MP-12 vaccine candidates encoding a DIVA marker, while still replicating efficiently in MRC-5 cells, we generated recombinant MP-12 encoding Punta Toro virus Adames strain NSs (rMP12-PTNSs) or Sandfly fever Sicilian virus NSs (rMP12-SFSNSs) in place of MP-12 NSs. We have demonstrated that those recombinant MP-12 viruses inhibit IFN-β mRNA synthesis, yet do not promote the degradation of PKR. The rMP12-PTNSs, but not rMP12-SFSNSs, replicated more efficiently than recombinant MP-12 lacking NSs in MRC-5 cells. Mice vaccinated with rMP12-PTNSs or rMP12-SFSNSs induced neutralizing antibodies at a level equivalent to those vaccinated with MP-12, and were efficiently protected from wild-type RVFV challenge. The rMP12-PTNSs and rMP12-SFSNSs did not induce antibodies cross-reactive to anti-RVFV NSs antibody and are therefore applicable to DIVA. Thus, rMP12-PTNSs is highly efficacious, replicates efficiently in MRC-5 cells, and encodes a DIVA marker, all of which are important for vaccine development for Rift Valley fever. PMID:23638202

Characterization of Rift Valley fever virus MP-12 strain encoding NSs of Punta Toro virus or sandfly fever Sicilian virus.

PubMed

Lihoradova, Olga A; Indran, Sabarish V; Kalveram, Birte; Lokugamage, Nandadeva; Head, Jennifer A; Gong, Bin; Tigabu, Bersabeh; Juelich, Terry L; Freiberg, Alexander N; Ikegami, Tetsuro

2013-01-01

Rift Valley fever virus (RVFV; genus Phlebovirus, family Bunyaviridae) is a mosquito-borne zoonotic pathogen which can cause hemorrhagic fever, neurological disorders or blindness in humans, and a high rate of abortion in ruminants. MP-12 strain, a live-attenuated candidate vaccine, is attenuated in the M- and L-segments, but the S-segment retains the virulent phenotype. MP-12 was manufactured as an Investigational New Drug vaccine by using MRC-5 cells and encodes a functional NSs gene, the major virulence factor of RVFV which 1) induces a shutoff of the host transcription, 2) inhibits interferon (IFN)-β promoter activation, and 3) promotes the degradation of dsRNA-dependent protein kinase (PKR). MP-12 lacks a marker for differentiation of infected from vaccinated animals (DIVA). Although MP-12 lacking NSs works for DIVA, it does not replicate efficiently in type-I IFN-competent MRC-5 cells, while the use of type-I IFN-incompetent cells may negatively affect its genetic stability. To generate modified MP-12 vaccine candidates encoding a DIVA marker, while still replicating efficiently in MRC-5 cells, we generated recombinant MP-12 encoding Punta Toro virus Adames strain NSs (rMP12-PTNSs) or Sandfly fever Sicilian virus NSs (rMP12-SFSNSs) in place of MP-12 NSs. We have demonstrated that those recombinant MP-12 viruses inhibit IFN-β mRNA synthesis, yet do not promote the degradation of PKR. The rMP12-PTNSs, but not rMP12-SFSNSs, replicated more efficiently than recombinant MP-12 lacking NSs in MRC-5 cells. Mice vaccinated with rMP12-PTNSs or rMP12-SFSNSs induced neutralizing antibodies at a level equivalent to those vaccinated with MP-12, and were efficiently protected from wild-type RVFV challenge. The rMP12-PTNSs and rMP12-SFSNSs did not induce antibodies cross-reactive to anti-RVFV NSs antibody and are therefore applicable to DIVA. Thus, rMP12-PTNSs is highly efficacious, replicates efficiently in MRC-5 cells, and encodes a DIVA marker, all of which are important for vaccine development for Rift Valley fever.

The Links Between the Formation of the Gulf of Mexico and the Late Proterozoic to Mesozoic Tectonic Evolution of Southern North America

NASA Astrophysics Data System (ADS)

Keller, G. R.; Mickus, K. L.; Gurrola, H.; Harry, D. L.; Pulliam, J.

2016-12-01

A full understanding of the Gulf of Mexico's geologic history depends on understanding the tectonic framework along the southern margin of North America. The first step in establishing this framework was the breakup of Laurentia during the Early Paleozoic. At least one tectonic block rifted away from Laurentia's southern margin at this time, and is interpreted to be presently located in Argentina. Rifting resulted in a sinuous margin consisting of alternating ridge and transform segments extending from the southeastern U.S. across Texas into northern Mexico. The Paleozoic margin is associated with a clearly defined gravity high, and ends in the trend of this high are associated with intersections of ridge and transform segments along the margin. By the end of the Paleozoic, continental assembly via the Appalachian-Ouachita orogeny added new terranes to the eastern and southern margins of Laurentia and the assembly of the supercontinent Pangea was complete. Triassic through Late Jurassic opening of the Gulf of Mexico (GOM) created a complex margin, initially mobilizing several crustal blocks that were eventually left behind on the North American margin as seafloor spreading developed within the Gulf and the Yucatan block separated and rotated into its current position. Recent deep seismic reflection profiles along the northern margin of the GOM show that rifted continental crust extends offshore for 250 km before the oceanic crust of the Gulf of Mexico is encountered. Our group has worked to produce four integrated models of the lithospheric structure based upon reflection, refraction, and teleseismic data acquired across this margin integrated with gravity, magnetic, geologic and drilling data. These models define a complex zone of crustal thinning along the Gulf Coastal plain of Texas that is covered by up to 10km of primarily Cretaceous and younger sedimentary rocks. To the east along the coastal plain region, we have defined two large crustal blocks that were essentially left behind by the opening of the Gulf of Mexico.

Dyke-path formation in relation to the eruptions of Eyjafjallajökull 2010 and Bardarbunga-Holuhraun 2014

NASA Astrophysics Data System (ADS)

Gudmundsson, Agust

2015-04-01

Dykes are extension fractures and form when the magmatic overpressure is high enough to rupture (break) the host rock. Their formation is entirely analogous to that of many joints and human-made hydraulic fractures, such as are used to increase permeability in reservoirs. When generating their paths, dykes use existing weaknesses (e.g., cooling joints) in the host rock. The maximum depth of large tension fractures below the surface of a rift zone, however, is mostly less than a few hundred metres. If the fractures extend to greater depths, they must change into closed normal faults which are generally not used as magma paths. There are thus no large tension fractures or wide-open faults at great depths ready to be filled with magma to form a dyke. While magma flow in dykes, as in other fluid-driven fractures, is at any point in various directions dyke segmentation may indicate the overall large-scale flow direction. Thus, dykes composed of large-spaced disconnected segments in lateral sections are primarily formed in vertical magma flow at segmentation depth whereas those composed of large-spaced disconnected segments in vertical sections are primarily formed in lateral magma flow. The far-field displacement and stress fields of segmented dykes are similar to those generated by single, continuous dykes of similar dimensions, particularly when the distances between the nearby tips of the segments become small in comparison with segment lengths. Most dykes become arrested and never supply magma to eruptions. Feeder-dykes normally reach the surface only along parts of their lengths (strike-dimensions). The volumetric flow or effusion rate of magma through a feeder-dyke or volcanic fissure depends on the aperture (opening) of the dyke or fissure in the 3rd power. All these theoretical and observational results are here applied to the dyke emplacements associated with the eruptions of Eyjafjallajökull 2010 and Bardarbunga-Holuhraun 2014. The results make it possible to (1) explain, broadly, the propagation-paths of the associated dykes, (2) the arrest and deflection (into sills) of many dyke segments, (3) the dimensions of the dykes, in particular (4) the dyke thicknesses, (5) the volumetric flow or effusion rates of the volcanic fissures, and (6) the location of the magma sources of the dykes. Galindo, I., Gudmundsson, A., 2012. Basaltic feeder dykes in rift zones: geometry, emplacement, and effusion rates. Nat. Hazards Earth Syst. Sci., 12, 3683-3700. Becerril, L., Galindo, I., Gudmundsson, A., Morales, J.M., 2013. Depth of origin of magma in eruptions. Sci. Reports (Nature Publishing), 3, 2762, doi: 10.1038/srep02762. Gudmundsson, A., Lecoeur, N., Mohajeri, N., Thordarson, T., 2014. Dike emplacement at Bardarbunga, Iceland, induces unusual stress changes, caldera deformation, and earthquakes. Bull. Volcanol., 76, 869, doi: 10.1007/s00445-014-0869-8.

Low intensity hawaiian fountaining as exemplified by the March 2011, Kamoamoa eruption at Kilauea Volcano, Hawai`i (Invited)

NASA Astrophysics Data System (ADS)

Orr, T. R.; Houghton, B. F.; Poland, M. P.; Patrick, M. R.; Thelen, W. A.; Sutton, A. J.; Parcheta, C. E.; Thornber, C. R.

2013-12-01

The latest 'classic' hawaiian high-fountaining activity at Kilauea Volcano occurred in 1983-1986 with construction of the Pu`u `O`o pyroclastic cone. Since then, eruptions at Kilauea have been dominated by nearly continuous effusive activity. Episodes of sustained low hawaiian fountaining have occurred but are rare and restricted to short-lived fissure eruptions along Kilauea's east rift zone. The most recent of these weakly explosive fissure eruptions--the Kamoamoa eruption--occurred 5-9 March 2011. The Kamoamoa eruption was probably the consequence of a decrease in the carrying capacity of the conduit feeding the episode 58 eruptive vent down-rift from Pu`u `O`o in Kilauea's east rift zone. As output from the vent waned, Kilauea's summit magma storage and east rift zone transport system began to pressurize, as manifested by an increase in seismicity along the upper east rift zone, inflation of the summit and Pu`u `O`o, expansion of the east rift zone, and rising lava levels at both the summit and Pu`u `O`o. A dike began propagating towards the surface from beneath Makaopuhi Crater, 6 km west of Pu`u `O`o, at 1342 Hawaiian Standard Time (UTC - 10 hours) on 5 March. A fissure eruption started about 3.5 hours later near Nāpau Crater, 2 km uprift of Pu`u `O`o. Activity initially jumped between numerous en echelon fissure segments before centering on discrete vents near both ends of the 2.4-km-long fissure system for the final two days of the eruption. About 2.6 mcm of lava was erupted over the course of four days with a peak eruption rate of 11 m3/s. The petrologic characteristics of the fissure-fed lava indicate mixing between hotter mantle-derived magma and cooler rift-stored magma, with a greater proportion of the cooler component than was present in east rift zone lava erupting before March 2011. The fissure eruption was accompanied by the highest SO2 emission rates since 1986. Coincidentally, the summit and Pu`u `O`o deflated as magma drained away, causing expansion of the ERZ. The geological, geophysical, and geochemical datasets collected before, during, and after the Kamoamoa eruption provide an unprecedented account of what, at least in recent decades, is the 'normal' mode for hawaiian fountaining at Kilauea--that is, low intensity fissure-fed eruptions. This activity differs from high fountaining in that there is little physical coupling between melt and magmatic gas--for much of the Kamoamoa eruption lava ponded sluggishly over the vents and was weakly disrupted by bursts from trains of very closely spaced gas bubbles. Such eruptions enable us to examine the middle ground between strombolian and classical hawaiian behaviors at basaltic volcanoes.

High-precision mapping of seismicity in the 2014 Bárdarbunga volcanic episode

NASA Astrophysics Data System (ADS)

Vogfjörd, Kristín S.; Hensch, Martin; Gudmundsson, Gunnar B.; Jónsdóttir, Kristín

2015-04-01

The Bárdarbunga volcano and its associated fissure swarm in Iceland's Eastern volcanic zone is a highly active system with over 20 eruptions in the last 11 centuries. The location of this active volcano and much of the fissure swarm under several hundred metres thick ice gives rise to multiple hazards, including explosive, subglacial eruptions and associated subglacial floods (jökulhlaups), as well as fissure eruptions extruding large volumes of lava. After a decade of increasing seismic activity, volcanic unrest at Bárdarbunga suddenly escalated into a minor subglacial eruption on 16 August 2014. In the following weeks seismic activity soared and surface deformation of tens of cm were observed, caused by rifting and a dyke intrusion, which propagated 48 km northward from the central volcano (Sigmundsson et al., 2014). The dyke propagation stopped just outside the glacial margin and ended in a fissure eruption at Holuhraun at the end of August. At the time of writing the eruption is ongoing, having extruded a lava volume of over 1 km3 and released high rates of SO2 into the atmosphere. Over twenty thousand microearthquakes have been recorded. Initially most were in the dyke, but after the first two weeks the activity around the caldera rim increased and over 70 shallow earthquakes with MW > 5 have been located along the caldera rim accompanied by caldera subsidence. At the onset of the unrest on 16 August, the seismicity was located in the caldera and north of the caldera rim, but already in the first few hours the activity propagated out of the caldera to the SE. Still, the activity continued for a few days in the fissure swarm to the NE of the rim. High-precision earthquake locations in the propagating dyke have revealed its very detailed, planar rifting segments, with the events distributed over a 3-4 km depth range, and mostly between 6 and 9 km. These very planar event distributions are highly unusual in volcanic areas and strongly suggest rifting of unbroken crust as the magma intruded. The lateral propagation direction of the seismicity abruptly changes direction along each new segment, sometimes by up to 90 degrees and the propagation was sporadic, advancing with varying speed and sometimes stopping for periods of time. During these times of arrest, continuous low-frequency seismic tremor was sometimes recorded for several hours. A few days following these episodes, depressions appeared on the ice surface, confirming initial assumptions that the tremor was revealing temporary magma-ice interaction on the surface below the glacier. Relative locations of microearthquakes around the caldera rim are much less constrained and their distribution is more diffuse. However, along the southern caldera rim, the events follow the linear trend of the rim and extend to a few km depth. At the northern caldera margin the distribution is more diffuse and appears to dip towards north. An overview will be given of the high-precision locations in the dyke and around the caldera rim and estimation of absolute location accuracies in horizontal and vertical direction discussed. A joint interpretation of the best fitting focal mechanisms with the rifting planes, as defined by the event distribution in the dyke, will also be presented to show the lateral variation in the stress field orientation along the dyke. The high relative, lateral location accuracy in the dyke allows detailed examination of the temporal propagation so some examples of the temporal dyke advance will also be shown. Sigmundsson and 36 others. 2014. Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland. Nature. doi:10.1038/nature14111.

Tectonic evolution of Gorda Ridge inferred from sidescan sonar images

USGS Publications Warehouse

Masson, D.G.; Cacchione, D.A.; Drake, D.E.

1988-01-01

Gorda Ridge is the southern segment of the Juan de Fuca Ridge complex, in the north-east Pacific. Along-strike spreading-rate variation on Gorda Ridge and deformation of Gorda Plate are evidence for compression between the Pacific and Gorda Plates. GLORIA sidescan sonographs allow the spreading fabric associated with Gorda Ridge to be mapped in detail. Between 5 and 2 Ma, a pair of propagating rifts re-orientated the northern segment of Gorda Ridge by about 10?? clockwise, accommodating a clockwise shift in Pacific-Juan de Fuca plate motion that occurred around 5 Ma. Deformation of Gorda Plate, associated with southward decreasing spreading rates along southern Gorda Ridge, is accommodated by a combination of clockwise rotation of Gorda Plate crust, coupled with left-lateral motion on the original normal faults of the ocean crust. Segments of Gorda Plate which have rotated by different amounts are separated by narrow deformation zones across which sharp changes in ocean fabric trend are seen. Although minor lateral movement may occur on these NW to WNW structures, no major right-lateral movement, as predicted by previous models, is observed. ?? 1988 Kluwer Academic Publishers.

Refining the Formation and Early Evolution of the Eastern North American Margin: New Insights From Multiscale Magnetic Anomaly Analyses

NASA Astrophysics Data System (ADS)

Greene, John A.; Tominaga, Masako; Miller, Nathaniel C.; Hutchinson, Deborah R.; Karl, Matthew R.

2017-11-01

To investigate the oceanic lithosphere formation and early seafloor spreading history of the North Atlantic Ocean, we examine multiscale magnetic anomaly data from the Jurassic/Early Cretaceous age Eastern North American Margin (ENAM) between 31 and 40°N. We integrate newly acquired sea surface magnetic anomaly and seismic reflection data with publicly available aeromagnetic and composite magnetic anomaly grids, satellite-derived gravity anomaly, and satellite-derived and shipboard bathymetry data. We evaluate these data sets to (1) refine magnetic anomaly correlations throughout the ENAM and assign updated ages and chron numbers to M0-M25 and eight pre-M25 anomalies; (2) identify five correlatable magnetic anomalies between the East Coast Magnetic Anomaly (ECMA) and Blake Spur Magnetic Anomaly (BSMA), which may document the earliest Atlantic seafloor spreading or synrift magmatism; (3) suggest preexisting margin structure and rifting segmentation may have influenced the seafloor spreading regimes in the Atlantic Jurassic Quiet Zone (JQZ); (4) suggest that, if the BSMA source is oceanic crust, the BSMA may be M series magnetic anomaly M42 ( 168.5 Ma); (5) examine the along and across margin variation in seafloor spreading rates and spreading center orientations from the BSMA to M25, suggesting asymmetric crustal accretion accommodated the straightening of the ridge from the bend in the ECMA to the more linear M25; and (6) observe anomalously high-amplitude magnetic anomalies near the Hudson Fan, which may be related to a short-lived propagating rift segment that could have helped accommodate the crustal alignment during the early Atlantic opening.

Refining the formation and early evolution of the Eastern North American Margin: New insights from multiscale magnetic anomaly analyses

USGS Publications Warehouse

Greene, John A.; Tominaga, Masako; Miller, Nathaniel; Hutchinson, Deborah; Karl, Matthew R.

2017-01-01

To investigate the oceanic lithosphere formation and early seafloor spreading history of the North Atlantic Ocean, we examine multiscale magnetic anomaly data from the Jurassic/Early Cretaceous age Eastern North American Margin (ENAM) between 31 and 40°N. We integrate newly acquired sea surface magnetic anomaly and seismic reflection data with publicly available aeromagnetic and composite magnetic anomaly grids, satellite-derived gravity anomaly, and satellite-derived and shipboard bathymetry data. We evaluate these data sets to (1) refine magnetic anomaly correlations throughout the ENAM and assign updated ages and chron numbers to M0–M25 and eight pre-M25 anomalies; (2) identify five correlatable magnetic anomalies between the East Coast Magnetic Anomaly (ECMA) and Blake Spur Magnetic Anomaly (BSMA), which may document the earliest Atlantic seafloor spreading or synrift magmatism; (3) suggest preexisting margin structure and rifting segmentation may have influenced the seafloor spreading regimes in the Atlantic Jurassic Quiet Zone (JQZ); (4) suggest that, if the BSMA source is oceanic crust, the BSMA may be M series magnetic anomaly M42 (~168.5 Ma); (5) examine the along and across margin variation in seafloor spreading rates and spreading center orientations from the BSMA to M25, suggesting asymmetric crustal accretion accommodated the straightening of the ridge from the bend in the ECMA to the more linear M25; and (6) observe anomalously high-amplitude magnetic anomalies near the Hudson Fan, which may be related to a short-lived propagating rift segment that could have helped accommodate the crustal alignment during the early Atlantic opening.

The Distribution and Composition Characteristics of Siliceous Rocks from Qinzhou Bay-Hangzhou Bay Joint Belt, South China: Constraint on the Tectonic Evolution of Plates in South China

PubMed Central

Li, Hongzhong; Zhai, Mingguo; Zhang, Lianchang; Zhou, Yongzhang; Yang, Zhijun; He, Junguo; Liang, Jin; Zhou, Liuyu

2013-01-01

The Qinzhou Bay-Hangzhou Bay joint belt is a significant tectonic zone between the Yangtze and Cathaysian plates, where plentiful hydrothermal siliceous rocks are generated. Here, the authors studied the distribution of the siliceous rocks in the whole tectonic zone, which indicated that the tensional setting was facilitating the development of siliceous rocks of hydrothermal genesis. According to the geochemical characteristics, the Neopalaeozoic siliceous rocks in the north segment of the Qinzhou Bay-Hangzhou Bay joint belt denoted its limited width. In comparison, the Neopalaeozoic Qinzhou Bay-Hangzhou Bay joint belt was diverse for its ocean basin in the different segments and possibly had subduction only in the south segment. The ocean basin of the north and middle segments was limited in its width without subduction and possibly existed as a rift trough that was unable to resist the terrigenous input. In the north segment of the Qinzhou Bay-Hangzhou Bay joint belt, the strata of hydrothermal siliceous rocks in Dongxiang copper-polymetallic ore deposit exhibited alternative cycles with the marine volcanic rocks, volcanic tuff, and metal sulphide. These sedimentary systems were formed in different circumstances, whose alternative cycles indicated the release of internal energy in several cycles gradually from strong to weak. PMID:24302882

Age constraints for the present fault configuration in the Imperial Valley, California: Evidence for northwestward propagation of the Gulf of California rift system

NASA Technical Reports Server (NTRS)

Larsen, Shawn; Reilinger, Robert

1990-01-01

Releveling and other geophysical data for the Imperial Valley of southern California suggest the northern section of the Imperial-Brawley fault system, which includes the Mesquite Basin and Brawley Seismic Zone, is much younger than the 4 to 5 million year age of the valley itself. A minimum age of 3000 years is calculated for the northern segment of the Imperial fault from correlations between surface topography and geodetically observed seismic/interseismic vertical movements. Calculations of a maximum age of 80,000 years is based upon displacements in the crystalline basement along the Imperial fault, inferred from seismic refraction surveys. This young age supports recent interpretations of heat flow measurements, which also suggest that the current patterns of seismicity and faults in the Imperial Valley are not long lived. The current fault geometry and basement morphology suggest northwestward growth of the Imperial fault and migration of the Brawley Seismic Zone. It is suggested that this migration is a manifestation of the propagation of the Gulf of California rift system into the North American continent.

Tectonic/climatic control on sediment provenance in the Cape Roberts Project core record (southern Victoria Land, Antarctica): A pulsing late Oligocene/early Miocene signal from south revealed by detrital thermochronology

NASA Astrophysics Data System (ADS)

Olivetti, V.; Balestrieri, M. L.; Rossetti, F.; Talarico, F. M.

2012-04-01

The Mesozoic-Cenozoic West Antarctic Rift System (WARS) is one of the largest intracontinental rift on Earth. The Transantarctic Mountains (TAM) form its western shoulder, marking the boundary between the East and West Antarctica. The rifting evolution is commonly considered polyphase and involves an Early Cretaceous phase linked to the Gondwana break-up followed by a major Cenozoic one, starting at c. 50-40 Ma. This Cenozoic episode corresponds to the major uplift/denudation phase of the TAM, which occurred concurrently with transition from orthogonal to oblique rifting. The Cenozoic rift reorganization occurred concurrently with a major change in the global climate system and a global reorganization of plate motions. This area thus provide an outstanding natural laboratory for studying a range of geological problems that involve feedback relationships between tectonics and climate. A key to address the tectonic/climate feedback relations is to look on apparent synchronicity in erosion signal between different segments, and to compare these with well-dated regional and global climatic events. However, due to the paucity of Cenozoic rock sequences exposed along the TAM front, a few information is available about the neotectonics of the rift and rift-flank uplift system. The direct physical record of the tectonic/climate history of the WARS recovered by core drillings along the western margin of the Ross sea (DSDP, CIROS, Cape Roberts and ANDRILL projects) provides an invaluable tool to address this issue. Twenty-three samples distributed throughout the entire composite drill-cored stratigraphic succession of Cape Roberts were analyzed. Age probability plots of eighteen detrital samples with depositional ages between 34 Ma and the Pliocene were decomposed into statistically significant age populations or peaks using binomial peak-fitting. Moreover, three granitic pebbles, one dolerite clast and one sample of Beacon sandstones have been dated. From detrital samples, three peaks are detected reflecting different bedrock provenance areas. Two peaks older than 40 Ma (P2 and P3) are compatible with thermochronological data from TAM bedrock that underwent a stepwise denudation in Cretaceous times. A Peak younger than 40 Ma (P1) has been detected occasionally, recording the signal of a source area exhumed during late Oligocene /early Miocene with a constant denudation rate of 0.4 mm/yr (constant lag-time up-section), but absent in the onshore portion of the proximal TAM. Indeed, when compared with AFT data from ANDRILL cores, the relatively young P1 ages, suggest that part of sediments in the Cape Robert Rift basin have a provenance from source regions probably located far away in the south (i.e. Skelton-Byrd glaciers region) where bedrock experienced compatible thermal histories. This provenance would imply glacial systems with main flow patterns from south to the north, therefore orthogonal to the orientation of present-day drainage. We thus infer that the post-Eocene glacial and erosional history of the TAM front was significantly controlled by the N-S-trending transtensional regime that affected the western Ross Sea margin during transition from orthogonal to oblique rifting in the region. The appearance and disappearance of P1 along the drill-cored stratigraphic succession seems to be linked to the oscillation in the extent of the ice sheet.

From rifting to subduction: the role of inheritance in the Wilson Cycle

NASA Astrophysics Data System (ADS)

Beaussier, Stéphane; Gerya, Taras; Burg, Jean-Pierre

2017-04-01

The Wilson Cycle entails that oceans close and reopen. This cycle is a fundamental principle in plate tectonics, inferring continuity from divergence to convergence and that continental rifting takes place along former suture zones. This view questions the role of inherited structures at each stage of the Wilson Cycle. Using the 3D thermo-mechanical code, I3ELVIS (Gerya and Yuen 2007) we present a high-resolution continuous model of the Wilson cycle from continental rifting, breakup and oceanic spreading to convergence and spontaneous subduction initiation. Therefore, all lateral and longitudinal structures of the lithospheres are generated self-consistently and are consequences of the initial continental structure, tectono-magmatic inheritance and material rheology. In the models, subduction systematically initiates off-ridge and is controlled by the convergence-induced swelling of the ridge. Geometry and dynamics of the developing off-ridge subduction is controlled by four main factors: (1) the obliquity of the ridge with respect to the convergence direction; (2) fluid-induced weakening of the oceanic crust; (3) irregularity of ridge and margins inherited from rifting and spreading; (4) strain localization at transform faults formed during ocean floor spreading. Further convergence can lead to obduction of the oceanic crust and segments of ridge after the oceanic lithosphere is entrained into subduction. We show that the main parameters controlling the occurrence and geometry of obducted ophiolite are the convergence rate and the inherited structure of the passive margins and ridge. Our numerical experiments results show the essential role played by inheritance during the Wilson Cycle and are consistent with nature observations such as the tectonic history of the Oman subduction-obduction system. REFERENCES Gerya, T. V., and D. A. Yuen. 2007: "Robust Characteristics Method for Modelling Multiphase Visco-Elasto-Plastic Thermo-Mechanical Problems, Physics of the Earth and Planetary Interiors, 163 (1-4), 83-105.

The wide-angle seismic image of a complex rifted margin, offshore North Namibia: Implications for the tectonics of continental breakup

NASA Astrophysics Data System (ADS)

Planert, Lars; Behrmann, Jan; Jokat, Wilfried; Fromm, Tanja; Ryberg, Trond; Weber, Michael; Haberland, Christian

2017-10-01

Voluminous magmatism during the South Atlantic opening has been considered as a classical example for plume related continental breakup. We present a study of the crustal structure around Walvis Ridge, near the intersection with the African margin. Two wide-angle seismic profiles were acquired. One is oriented NNW-SSE, following the continent-ocean transition and crossing Walvis Ridge. A second amphibious profile runs NW-SE from the Angola Basin into continental Namibia. At the continent-ocean boundary (COB) the mafic crust beneath Walvis Ridge is up to 33 km thick, with a pronounced high-velocity lower crustal body. Towards the south there is a smooth transition to 20-25 km thick crust underlying the COB in the Walvis Basin, with a similar velocity structure, indicating a gabbroic lower crust with associated cumulates at the base. The northern boundary of Walvis Ridge towards the Angola Basin shows a sudden change to oceanic crust only 4-6 km thick, coincident with the projection of the Florianopolis Fracture Zone, one of the most prominent tectonic features of the South Atlantic ocean basin. In the amphibious profile the COB is defined by a sharp transition from oceanic to rifted continental crust, with a magmatic overprint landward of the intersection of Walvis Ridge with the Namibian margin. The continental crust beneath the Congo Craton is 40 km thick, shoaling to 35 km further SE. The velocity models show that massive high-velocity gabbroic intrusives are restricted to a narrow zone directly underneath Walvis Ridge and the COB in the south. This distribution of rift-related magmatism is not easily reconciled with models of continental breakup following the establishment of a large, axially symmetric plume in the Earth's mantle. Rift-related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism, dividing the margin in a magma-dominated southern and an essentially amagmatic northern segment.

Initial Results from the 2002 Gulf of California Conjugate Margin Seismic Experiment

NASA Astrophysics Data System (ADS)

Holbrook, S.; Lizarralde, D.; Kent, G.; Harding, A.; Fletcher, J.; Gonzalez-Fernandez, A.; Umhoefer, P.; Axen, G.

2003-04-01

The Gulf of California, which marks the ongoing separation of Baja California from mainland Mexico, is one of the few locales where active continental breakup can be studied along unambiguous flow lines that join clear conjugate margin pairs. In Fall 2002, we conducted an onshore-offshore seismic experiment across the conjugate rifted margins of the Gulf of California in several rift segments. The joint U.S.-Mexico project, sponsored principally by the MARGINS program of the U.S. National Science Foundation, aimed to image crustal structure across conjugate margins of four major basins to determine the modes of extension and the influence of sedimentation and magmatism on breakup. Here we present an overview of the experiment, which was substantially altered at sea due to concerns for marine-mammal safety, and present some preliminary findings. Three flow-line transects were acquired, in the Alarcon Basin, the Guaymas Basin, and between Cabo and Tres Marias Islands. In addition, a fourth transect across the Baja Peninsula was acquired. Data acquired included (1) multichannel seismic reflection data using the R/V Ewing’s 20-gun array and 480-channel, 6-km-long streamer, (2) wide-angle reflection/refraction data recorded on ocean-bottom seismometers, from 206 deployments conducted by the R/V New Horizon, and (3) onshore-offshore data recorded on portable seismometers deployed up to 100 km inland on all transects. Initial results from the experiment include (1) clear evidence for asymmetric basement structure on the conjugate rifted margins and across the active mid-ocean spreading center, of the Guaymas Basin, (2) the suggestion of substantial magmatism in an early failed rift of the Alarcon Basin, and (3) active subduction beneath the margin at the Tres Marias islands. In addition, we will discuss new procedures for mitigating effects on marine mammals that may have a significant impact on future U.S.-sponsored seismic reflection activities.

Galapagos Tectonics and Evolution (Invited)

NASA Astrophysics Data System (ADS)

Hey, R. N.

2010-12-01

Galapagos is now considered one of the type-examples of hotspot-ridge interaction, although in the early years of plate tectonics it was generally thought that this interpretation was demonstrably wrong, with two influential groups insisting that non-hotspot models were required for this area. The key to understanding Galapagos tectonic evolution was the recognition that small ridge axis jumps toward the hotspot had occurred, producing complicated magnetic anomalies and asymmetric lithospheric accretion. My dissertation work, guided by Jason Morgan, showed that this simple modification to plate tectonic theory could resolve the seemingly compelling geometric arguments against the Cocos and Carnegie aseismic ridges being Galapagos hotspot tracks, and further that if Galapagos were a hotspot near Fernandina, fixed with respect to the Hawaii hotspot, there should be aseismic ridges on the Cocos and Nazca plates with the observed Cocos and Carnegie ridge geometry, both aseismic ridges forming when the hotspot was ridge-centered, but only the Carnegie ridge since the plate boundary migrated north of the hotspot. A great deal of subsequent research has shown that some areas are considerably more complicated than originally thought, but the following basic model still appears to hold. The Farallon plate split apart along the Grijalva scarp, possibly a preexisting Pacific-Farallon FZ that intersected the hotspot at this time (although alternative interpretations exist), probably in response to tensional stress caused by slab pull in different directions at the Mid-America and Peru-Chile trenches. This break-up allowed more orthogonal subduction of independent Cocos and Nazca plates beginning shortly after 25 Ma. The original Cocos-Nazca ridge trended E-NE, but soon reorganized into N-S spreading segments. The subsequent evolution involved substantial northward ridge migration and ridge jumps, mostly toward the Galapagos hotspot. Recent ridge jumps have occurred in systematic patterns as a result of new rifts almost always propagating “downhill” away from the hotspot, as shown by the characteristic patterns of pseudofaults, failed rifts (sometimes grabens, sometimes abandoned ridges), and zones of transferred lithosphere, with Galapagos 95.5W the type-example propagator. These propagators are probably driven by gravity sliding stresses due to the shallow lithosphere near the hotspot (although alternative interpretations exist). The origin of many propagation sequences appears to involve discrete southward jumps forming new segments near the hotspot. The observed petrological and geochemical variations are interpreted as consistent with mantle plume and propagating rift effects. However, the speculation of Schilling et al. (1982) that, in analogy to Iceland (the other type-example of hotspot-ridge interaction), Galapagos might be a pulsing plume, and that plume pulses might drive the Galapagos propagators, hasn’t received much support. It is interesting that the Galapagos and Iceland hotspots have produced such apparently different effects along the ridge segments they so obviously strongly influence.

Post-eruptive Deformation following the 2014 Holuhraun Rift, Iceland.

NASA Astrophysics Data System (ADS)

Grapenthin, R.; Li, S.; Ofeigsson, B.; Sigmundsson, F.; Drouin, V.; Hreinsdottir, S.; Parks, M.; Friðriksdóttir, H. M.

2017-12-01

On August 16, 2014 an intense seismic swarm started below the eastern part of Bárdarbunga caldera at the NW edge of the Vatnajökull ice cap in Iceland. The seismicity migrated in 3 major segments changing direction at least twice until the advance stopped around 10 km south of Askja Volcano, more than 45 km from Bárdarbunga. The dike opening was accompanied by a 65 m collapse of the Bárdarbunga caldera floor and broad deflation due to magma removal from a 12 km deep reservoir (Gudmundsson et al., 2016). The area of the produced lava flow is 84 km2 with a volume of about 1.4 km3, which makes it the second largest eruption in Iceland since the Laki Fires in 1783 that produced an order of magnitude more lava. The caldera collapse was accompanied by over 40 M5 earthquakes; an immense seismic energy release for a volcano. The majority of seismicity in the dike clustered between 6-8 km depth. Sigmundsson et al. (2015) derive a maximum opening of 5 m shallower than 6 km from GPS and InSAR data. The co-eruptive deformation is followed by a complex juxtaposition of predominantly viscoelastic post-eruptive processes that include post-rifting relaxation and isostatic adjustment to the new lava flow, which modulate the long-term processes of plate spreading, subsidence at Askja Volcano, reinflation at Bárdarbunga, and glacial isostatic adjustment due to the melting of the nearby ice caps. Here, we present first results deciphering this deformation field using data from the continuous GPS network that was supplemented during the eruption specifically to capture these processes. We include InSAR analysis of Sentinel-1 data and analyze the observations through viscoelastic modeling approaches. GPS data show an asymmetric deformation field around the rift with 2-year GPS velocities between 0.1-1.5 cm/yr predominantly moving away from the rift. Preliminary modeling suggests several centimeters of horizontal displacement in plate spreading directions due to post-rifting relaxation and 1 cm of generally opposing motion due to the added lava load. Vertical displacements of several centimeters due to the load are expected with a maximum under the lava flow.

Late Cretaceous-Paleocene strike-slip faults along the East Greenland margin (63°N to 75°N): constraints for the North East Atlantic opening

NASA Astrophysics Data System (ADS)

Guarnieri, P.

2012-04-01

The East Greenland margin is a long stretch starting from 60°N up to 81°N in a distance of almost 3000 km. It represents the conjugate of the European margin now separated by the North East Atlantic (NEA). After a long period of E-W extension and almost N-S oriented rift basins since Early Cretaceous, separation between Greenland and Europe began at 55 Ma following a NE-SW oriented line of breakup and the emplacement of the North Atlantic Igneous Province (NAIP). Post-breakup thermal subsidence followed in the Eocene, and the Oligocene initiated a period of plate re-organization together with the initial separation of Jan Mayen microcontinent, a complex tectonic history with inversion structures and uplifts along both the East Greenland and European margins. The effect of this history is represented by exhumed sedimentary basins, dyke swarms, fault systems, intrusive centers, shield volcanoes and plateau lavas constituting highest mountain of Greenland with some peaks up to 3700 m (e.g. Watkins Bjerge). During expeditions for fieldwork in East Greenland (2009 to 2011) to collect new geological and structural data related to the North East Atlantic tectonics, four areas were visited: Skjoldungen 63°N, Kangerlussuaq 68°N, Traill Ø 72°N and Wollaston Forland 75°N. More than 1000 measurement of fault-slip data for structural analysis along major faults were collected and helicopter flights to collect oblique pictures for 3D-photogeology and 3D-mapping were taken. Kinematic analysis of brittle deformation associated with Late Cretaceous-Paleocene rift shows strike-slip movements. Palaeo-stress tensors reconstructed from fault-slip data highlight a NE-SW maximum horizontal stress in a strike-slip tectonic setting along the entire East Greenland margin (Guarnieri 2011a; Guarnieri 2011b; Guarnieri et al. 2011). Structural data show clear evidence for oblique rifting that corresponds in time to the "volcanic rift" (61-55 Ma) with in some cases the magmatic segmentation of macro-dyke complexes or the activation of major shear zones with strike-slip movements. Oblique rifting and strike-slip deformation along the East Greenland margin reflect the progressive clockwise shift, from W-E to NW-SE, of the separation trend between Greenland and Europe probably in response to the opening of the Labrador Sea.

Magmatic effects of the Cobb hot spot on the Juan de Fuca Ridge

USGS Publications Warehouse

Chadwick, John; Perfit, M.; Ridley, I.; Jonasson, I.; Kamenov, G.; Chadwick, W.; Embley, R.; le, Roux P.; Smith, M.

2005-01-01

The interaction of the Juan de Fuca Ridge with the Cobb hot spot has had a considerable influence on the magmatism of the Axial Segment of the ridge, the second-order segment that overlies the hot spot. In addition to the construction of the large volcanic edifice of Axial Seamount, the Axial Segment has shallow bathymetry and a prevalence of constructional volcanic features along its 100-km length, suggesting that hot spot-derived magmas supplement and oversupply the ridge. Lavas are generally more primitive at Axial Seamount and more evolved in the Axial Segment rift zones, suggesting that fractional crystallization is enhanced with increasing distance from the hot spot because of a reduced magma supply and more rapid cooling. Although the Cobb hot spot is not an isotopically enriched plume, it produces lavas with some distinct geochemical characteristics relative to normal mid-ocean ridge basalt, such as enrichments in alkalis and highly incompatible trace elements, that can be used as tracers to identify the presence and prevalence of the hot spot influence along the ridge. These characteristics are most prominent at Axial Seamount and decline in gradients along the Axial Segment. The physical model that can best explain the geochemical observations is a scenario in which hot spot and mid-ocean ridge basalt (MORB) magmas mix to varying degrees, with the proportions controlled by the depth to the MORB source. Modeling of two-component mixing suggests that MORB is the dominant component in most Axial Segment basalts. Copyright 2005 by the American Geophysical Union.

Multi-type Tectonic Responses to Plate Motion Changes of Mega-Offset Transform Faults at the Pacific-Antarctic Ridge

NASA Astrophysics Data System (ADS)

Zhang, F.; Lin, J.; Yang, H.; Zhou, Z.

2017-12-01

Magmatic and tectonic responses of a mid-ocean ridge system to plate motion changes can provide important constraints on the mechanisms of ridge-transform interaction and lithospheric properties. Here we present new analysis of multi-type responses of the mega-offset transform faults at the Pacific-Antarctic Ridge (PAR) system to plate motion changes in the last 12 Ma. Detailed analysis of the Heezen, Tharp, and Udintsev transform faults showed that the extensional stresses induced by plate motion changes could have been released through a combination of magmatic and tectonic processes: (1) For a number of ridge segments with abundant magma supply, plate motion changes might have caused the lateral transport of magma along the ridge axis and into the abutting transform valley, forming curved "hook" ridges at the ridge-transform intersection. (2) Plate motion changes might also have caused vertical deformation on steeply-dipping transtensional faults that were developed along the Heezen, Tharp, and Udintsev transform faults. (3) Distinct zones of intensive tectonic deformation, resembling belts of "rift zones", were found to be sub-parallel to the investigated transform faults. These rift-like deformation zones were hypothesized to have developed when the stresses required to drive the vertical deformation on the steeply-dipping transtensional faults along the transform faults becomes excessive, and thus deformation on off-transform "rift zones" became favored. (4) However, to explain the observed large offsets on the steeply-dipping transtensional faults, the transform faults must be relatively weak with low apparent friction coefficient comparing to the adjacent lithospheric plates.

The May 29 2008 earthquake aftershock sequence within the South Iceland Seismic Zone: Fault locations and source parameters of aftershocks

NASA Astrophysics Data System (ADS)

Brandsdottir, B.; Parsons, M.; White, R. S.; Gudmundsson, O.; Drew, J.

2010-12-01

The mid-Atlantic plate boundary breaks up into a series of segments across Iceland. The South Iceland Seismic Zone (SISZ) is a complex transform zone where left-lateral E-W shear between the Reykjanes Peninsula Rift Zone and the Eastern Volcanic Zone is accommodated by bookshelf faulting along N-S lateral strike-slip faults. The SISZ is also a transient feature, migrating sideways in response to the southward propagation of the Eastern Volcanic Zone. Sequences of large earthquakes (M > 6) lasting from days to years and affecting most of the seismic zone have occurred repeatedly in historical time (last 1100 years), separated by intervals of relative quiescence lasting decades to more than a century. On May 29 2008, a Mw 6.1 earthquake struck the western part of the South Iceland Seismic Zone, followed within seconds by a slightly smaller event on a second fault ~5 km further west. Aftershocks, detected by a temporal array of 11 seismometers and three permanent Icelandic Meteorological Office stations were located using an automated Coalescence Microseismic Mapping technique. The epicenters delineate two major and several smaller N-S faults as well as an E-W zone of activity stretching further west into the Reykjanes Peninsula Rift Zone. Fault plane solutions show both right lateral and oblique strike slip mechanisms along the two major N-S faults. The aftershocks deepen from 3-5 km in the north to 8-9 km in the south, suggesting that the main faults dip southwards. The faulting is interpreted to be driven by the local stress due to transform motion between two parallel segments of the divergent plate boundary crossing Iceland.

Dynamics of continental rift propagation: the end-member modes

NASA Astrophysics Data System (ADS)

Van Wijk, J. W.; Blackman, D. K.

2005-01-01

An important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation. In regions of rift propagation, specifically transition zones from continental rifting to seafloor spreading, it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones. The focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere. More specifically, this study explores how modeled rift behavior depends on the value of rheological parameters of the crust. A three-dimensional finite element model is used to simulate lithosphere deformation in an extensional regime. The chosen approach emphasizes understanding the tectonic forces involved in rift propagation. Dependent on plate strength, two end-member modes are distinguished. The stalled rift phase is characterized by absence of rift propagation for a certain amount of time. Extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone, which requires a buildup of shear stresses near the rift tip and significant intra-plate deformation. This stage represents a situation in which a rift meets a locked zone. Localized deformation changes to distributed deformation in the locked zone, and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend. In the alternative rift propagation mode, rift propagation is a continuous process when the initial crust is weak. The extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses. Model predictions address aspects of previously unexplained rift evolution in the Laptev Sea, and its contrast with the tectonic evolution of, for example, the Gulf of Aden and Woodlark Basin.

Magnetotelluric Investigations of Convergent Margins and of Incipient Rifting: Preliminary Results from the EarthScope MT Transportable Array and MT FlexArray Deployments in Cascadia and in the North American Mid-Continent Region

NASA Astrophysics Data System (ADS)

Schultz, A.; Bedrosian, P.; Key, K.; Livelybrooks, D.; Egbert, G. D.; Bowles-martinez, E.; Wannamaker, P. E.

2014-12-01

We report on preliminary analyses of data from the EarthScope MT Transportable Array, and from two high-resolution EarthScope MT studies in Cascadia. The first of these, iMUSH, is acquiring wideband MT data at 150 sites, as well as active and passive seismic data in SW Washington (including Mounts Saint Helens, Adams and Rainier). iMUSH seeks to determine details of crustal magma transport and storage, and to resolve major tectonic controls on volcanism along the arc. iMUSH may help to settle a debate over the origin of the SW Washington Crustal Conductor (SWCC), which covers ~5000 km2and that has alternately been attributed to accreted Eocene metasediments or to an extensive region of partial melt in the lower crust beneath the three volcanoes. The iMUSH array is continguous with an amphibious ~150 station MT experiment (MOCHA) onshore and offshore of the Washington and Oregon forearc. MOCHA iwill image the crust and upper mantle of the subduction system in 3D, constraining the fluid input to the system from offshore and the distribution of fluids released from the down-going slab, including along the transitional zone where Episodic Tremor and Slip occurs. Our goal is to refine our understanding of the segmentation, structure and fluid distribution along the convergent margin segments, and their relationship to the spatial pattern of ETS. In contrast to the active Cascadia margin, the Mid-Continent Rift (MCR) is the trace of a massive igneous event that nearly split North America 1.1 billion years ago. Initial results from 3D inversion of MT Transportable Array data show less fine-scale heterogeneity in the upper mantle (250 km depth) than is evident in western, tectonic North America, but a division at the base of thick lithosphere, with higher conductivities beneath and immediately south of the Great Lakes, than to the south. From the base of the lithosphere to the Moho, this high conductivity feature narrows, ultimately disappearing in the mid-crust. In the upper crust above this feature, an E-W elongated conductive feature appears that maps to surface expressions of the MCR. The significance of this deep feature, and its relationship to the failed rifting event of the mesoproterozoic era will be discussed.

Miocene extension and extensional folding in an anticlinal segment of the Black Mountains accommodation zone, Colorado River extensional corridor, southwestern United States

USGS Publications Warehouse

Varga, R.J.; Faulds, J.E.; Snee, L.W.; Harlan, S.S.; Bettison-Varga, L.

2004-01-01

Recent studies demonstrate that rifts are characterized by linked tilt domains, each containing a consistent polarity of normal faults and stratal tilt directions, and that the transition between domains is typically through formation of accommodation zones and generally not through production of throughgoing transfer faults. The mid-Miocene Black Mountains accommodation zone of southern Nevada and western Arizona is a well-exposed example of an accommodation zone linking two regionally extensive and opposing tilt domains. In the southeastern part of this zone near Kingman, Arizona, east dipping normal faults of the Whipple tilt domain and west dipping normal faults of the Lake Mead domain coalesce across a relatively narrow region characterized by a series of linked, extensional folds. The geometry of these folds in this strike-parallel portion of the accommodation zone is dictated by the geometry of the interdigitating normal faults of opposed polarity. Synclines formed where normal faults of opposite polarity face away from each other whereas anticlines formed where the opposed normal faults face each other. Opposed normal faults with small overlaps produced short folds with axial trends at significant angles to regional strike directions, whereas large fault overlaps produce elongate folds parallel to faults. Analysis of faults shows that the folds are purely extensional and result from east/northeast stretching and fault-related tilting. The structural geometry of this portion of the accommodation zone mirrors that of the Black Mountains accommodation zone more regionally, with both transverse and strike-parallel antithetic segments. Normal faults of both tilt domains lose displacement and terminate within the accommodation zone northwest of Kingman, Arizona. However, isotopic dating of growth sequences and crosscutting relationships show that the initiation of the two fault systems in this area was not entirely synchronous and that west dipping faults of the Lake Mead domain began to form between 1 m.y. to 0.2 m.y. prior to east dipping faults of the Whipple domain. The accommodation zone formed above an active and evolving magmatic center that, prior to rifting, produced intermediate-composition volcanic rocks and that, during rifting, produced voluminous rhyolite and basalt magmas. Copyright 2004 by the American Geophysical Union.

An updated global earthquake catalogue for stable continental regions: Reassessing the correlation with ancient rifts

USGS Publications Warehouse

Schulte, S.M.; Mooney, W.D.

2005-01-01

We present an updated global earthquake catalogue for stable continental regions (SCRs; i.e. intraplate earthquakes) that is available on the Internet. Our database contains information on location, magnitude, seismic moment and focal mechanisms for over 1300 M (moment magnitude) ??? 4.5 historic and instrumentally recorded crustal events. Using this updated earthquake database in combination with a recently published global catalogue of rifts, we assess the correlation of intraplate seismicity with ancient rifts on a global scale. Each tectonic event is put into one of five categories based on location: (i) interior rifts/taphrogens, (ii) rifted continental margins, (iii) non-rifted crust, (iv) possible interior rifts and (v) possible rifted margins. We find that approximately 27 per cent of all events are classified as interior rifts (i), 25 per cent are rifted continental margins (ii), 36 per cent are within non-rifted crust (iii) and 12 per cent (iv and v) remain uncertain. Thus, over half (52 per cent) of all events are associated with rifted crust, although within the continental interiors (i.e. away from continental margins), non-rifted crust has experienced more earthquakes than interior rifts. No major change in distribution is found if only large (M ??? 6.0) earthquakes are considered. The largest events (M ??? 7.0) however, have occurred predominantly within rifts (50 per cent) and continental margins (43 per cent). Intraplate seismicity is not distributed evenly. Instead several zones of concentrated seismicity seem to exist. This is especially true for interior rifts/taphrogens, where a total of only 12 regions are responsible for 74 per cent of all events and as much as 98 per cent of all seismic moment released in that category. Of the four rifts/taphrogens that have experienced the largest earthquakes, seismicity within the Kutch rift, India, and the East China rift system, may be controlled by diffuse plate boundary deformation more than by the presence of the ancient rifts themselves. The St. Lawrence depression, Canada, besides being an ancient rift, is also the site of a major collisional suture. Thus only at the Reelfoot rift (New Madrid seismic zone, NMSZ, USA), is the presence of features associated with rifting itself the sole candidate for causing seismicity. Our results suggest that on a global scale, the correlation of seismicity within SCRs and ancient rifts has been overestimated in the past. Because the majority of models used to explain intraplate seismicity have focused on seismicity within rifts, we conclude that a shift in attention more towards non-rifted as well as rifted crust is in order. ?? 2005 RAS.

Preliminary geologic map of the Big Costilla Peak area, Taos County, New Mexico, and Costilla County, Colorado

USGS Publications Warehouse

Fridrich, Christopher J.; Shroba, Ralph R.; Hudson, Adam M.

2012-01-01

This map covers the Big Costilla Peak, New Mex.&nash;Colo. quadrangle and adjacent parts of three other 7.5 minute quadrangles: Amalia, New Mex.–Colo., Latir Peak, New Mex., and Comanche Point, New Mex. The study area is in the southwesternmost part of that segment of the Sangre de Cristo Mountains known as the Culebra Range; the Taos Range segment lies to the southwest of Costilla Creek and its tributary, Comanche Creek. The map area extends over all but the northernmost part of the Big Costilla horst, a late Cenozoic uplift of Proterozoic (1.7-Ga and less than 1.4-Ga) rocks that is largely surrounded by down-faulted middle to late Cenozoic (about 40 Ma to about 1 Ma) rocks exposed at significantly lower elevations. This horst is bounded on the northwest side by the San Pedro horst and Culebra graben, on the northeast and east sides by the Devils Park graben, and on the southwest side by the (about 30 Ma to about 25 Ma) Latir volcanic field. The area of this volcanic field, at the north end of the Taos Range, has undergone significantly greater extension than the area to the north of Costilla Creek. The horsts and grabens discussed above are all peripheral structures on the eastern flank of the San Luis basin, which is the axial part of the (about 26 Ma to present) Rio Grande rift at the latitude of the map. The Raton Basin lies to the east of the Culebra segment of the Sangre de Cristo Mountains. This foreland basin formed during, and is related to, the original uplift of the Sangre de Cristo Mountains which was driven by tectonic contraction of the Laramide (about 70 Ma to about 40 Ma) orogeny. Renewed uplift and structural modification of these mountains has occurred during formation of the Rio Grande rift. Surficial deposits in the study area include alluvial, mass-movement, and glacial deposits of middle Pleistocene to Holocene age.

Controls of inherited lithospheric heterogeneity on rift linkage: Numerical and analog models of interaction between the Kenyan and Ethiopian rifts across the Turkana depression

NASA Astrophysics Data System (ADS)

Brune, Sascha; Corti, Giacomo; Ranalli, Giorgio

2017-09-01

Inherited rheological structures in the lithosphere are expected to have large impact on the architecture of continental rifts. The Turkana depression in the East African Rift connects the Main Ethiopian Rift to the north with the Kenya rift in the south. This region is characterized by a NW-SE trending band of thinned crust inherited from a Mesozoic rifting event, which is cutting the present-day N-S rift trend at high angle. In striking contrast to the narrow rifts in Ethiopia and Kenya, extension in the Turkana region is accommodated in subparallel deformation domains that are laterally distributed over several hundred kilometers. We present both analog experiments and numerical models that reproduce the along-axis transition from narrow rifting in Ethiopia and Kenya to a distributed deformation within the Turkana depression. Similarly to natural observations, our models show that the Ethiopian and Kenyan rifts bend away from each other within the Turkana region, thus forming a right-lateral step over and avoiding a direct link to form a continuous N-S depression. The models reveal five potential types of rift linkage across the preexisting basin: three types where rifts bend away from the inherited structure connecting via a (1) wide or (2) narrow rift or by (3) forming a rotating microplate, (4) a type where rifts bend towards it, and (5) straight rift linkage. The fact that linkage type 1 is realized in the Turkana region provides new insights on the rheological configuration of the Mesozoic rift system at the onset of the recent rift episode.

An equine herpesvirus type 1 (EHV-1) vector expressing Rift Valley fever virus (RVFV) Gn and Gc induces neutralizing antibodies in sheep.

PubMed

Said, Abdelrahman; Elmanzalawy, Mona; Ma, Guanggang; Damiani, Armando Mario; Osterrieder, Nikolaus

2017-08-14

Rift Valley fever virus (RVFV) is an arthropod-borne bunyavirus that can cause serious and fatal disease in humans and animals. RVFV is a negative-sense RNA virus of the Phlebovirus genus in the Bunyaviridae family. The main envelope RVFV glycoproteins, Gn and Gc, are encoded on the M segment of RVFV and known inducers of protective immunity. In an attempt to develop a safe and efficacious RVF vaccine, we constructed and tested a vectored equine herpesvirus type 1 (EHV-1) vaccine that expresses RVFV Gn and Gc. The Gn and Gc genes were custom-synthesized after codon optimization and inserted into EHV-1 strain RacH genome. The rH_Gn-Gc recombinant virus grew in cultured cells with kinetics that were comparable to those of the parental virus and stably expressed Gn and Gc. Upon immunization of sheep, the natural host, neutralizing antibodies against RVFV were elicited by rH_Gn-Gc and protective titers reached to 1:320 at day 49 post immunization but not by parental EHV-1, indicating that EHV-1 is a promising vector alternative in the development of a safe marker RVFV vaccine.

The evolution of shallow crustal structures in early rift-transform interaction: a case study in the northern Gulf of California.

NASA Astrophysics Data System (ADS)

Farangitakis, Georgios-Pavlos; van Hunen, Jeroen; Kalnins, Lara M.; Persaud, Patricia; McCaffrey, Kenneth J. W.

2017-04-01

The Gulf of California represents a young oblique rift/transtensional plate boundary in which all of the transform faults are actively shearing the crust, separated by active rift segments. Previous workers have shown that in the northern Gulf of California, the relative plate motion between the Pacific and North American plates is distributed between: a) the Cerro Prieto Fault (CPF) in the NE b) the Ballenas Transform Fault (BTF) in the SW and c) a pull-apart structure located between these two faults consisting of a number of extensional basins (the Wagner, Consag, and Upper and Lower Delfin basins). A plate boundary relocation at approximately 2 Ma, continued to separate Isla Angel de la Guarda from the Baja California peninsula and created the 200x70 km2 NE-SW pull-apart structure located northeast of the BTF. Here we use seismic stratigraphy analysis of the UL9905 high resolution reflection seismic dataset acquired by the Lamont-Doherty Earth Observatory, Caltech, and the Centro de Investigación Científica y de Educación Superior de Ensenada to build on previous structural interpretations and seek to further understand the processes that formed the structural and sedimentary architecture of the pull-apart basin in the northern Gulf of California. We examine the formation of depositional and deformation structures in relation to the regional tectonics to provide insight into the development of structural patterns and related seismic-stratigraphic features in young rift-transform interactions. Using bathymetric data, characteristic seismic-stratigraphic packages, and seismic evidence of faulting, we confirm the existence of three major structural domains in the northern Gulf of California and examine the interaction of the seismic stratigraphy and tectonic processes in each zone. The first and most distinctive is an abrupt NE-SW 28x5 km2 depression on the seabed of the Lower Delfin Basin. This is aligned orthogonally to the BTF, is situated at its northern end, and is an active rift. The second structural domain is a large, NE-SW-trending anticlinorium 60 km wide to the southeast of the rift zone, towards the Tiburon basin. One possibility is that it represents a positive flower structure and thus indicates a transpressional domain. However, individual structures within the broader zone are normal faults and negative flower structures, suggesting transtensional deformation, and the overall structure may be a roll-over antiform formed on a deep detachment structure. Finally, a strike-slip-dominated zone occurs along the northward continuation of the Ballenas Transform Fault. This is accompanied by the formation of submarine volcanic knolls. These patterns can be compared with seismic stratigraphy facies and structural patterns in mature transform margins and potentially give insight into their early history.

Thermomechanical Controls on the Success and Failure of Continental Rift Systems

NASA Astrophysics Data System (ADS)

Brune, S.

2017-12-01

Studies of long-term continental rift evolution are often biased towards rifts that succeed in breaking the continent like the North Atlantic, South China Sea, or South Atlantic rifts. However there are many prominent rift systems on Earth where activity stopped before the formation of a new ocean basin such as the North Sea, the West and Central African Rifts, or the West Antarctic Rift System. The factors controlling the success and failure of rifts can be divided in two groups: (1) Intrinsic processes - for instance frictional weakening, lithospheric thinning, shear heating or the strain-dependent growth of rift strength by replacing weak crust with strong mantle. (2) External processes - such as a change of plate divergence rate, the waning of a far-field driving force, or the arrival of a mantle plume. Here I use numerical and analytical modeling to investigate the role of these processes for the success and failure of rift systems. These models show that a change of plate divergence rate under constant force extension is controlled by the non-linearity of lithospheric materials. For successful rifts, a strong increase in divergence velocity can be expected to take place within few million years, a prediction that agrees with independent plate tectonic reconstructions of major Mesozoic and Cenozoic ocean-forming rift systems. Another model prediction is that oblique rifting is mechanically favored over orthogonal rifting, which means that simultaneous deformation within neighboring rift systems of different obliquity and otherwise identical properties will lead to success and failure of the more and less oblique rift, respectively. This can be exemplified by the Cretaceous activity within the Equatorial Atlantic and the West African Rifts that lead to the formation of a highly oblique oceanic spreading center and the failure of the West African Rift System. While in nature the circumstances of rift success or failure may be manifold, simplified numerical and analytical models allow the isolated analysis of various contributing factors and to define a characteristic time scale for each process.

Rôle de l'halocinèse dans l'évolution du bassin d'Essaouira (Sud-Ouest marocain)

NASA Astrophysics Data System (ADS)

Mehdi, Khalid; Griboulard, Roger; Bobier, Claude

2004-04-01

The seismic reflection studies carried in the Essaouira Basin, intermediate zone between the High Atlas and the Atlantic Margin, show the presence of halokinetic structures that played a significant role in the evolution of the basin. Salt dynamics was controlled by the Atlantic rifting and the Atlasic orogen. These tectonic controls and the margin segmentation are responsible for the diachronism of salt movement. Halokinesis varied in time and space in the basin and was more active offshore, where autochtonous and allochtonous salt layers are present. To cite this article: K. Mehdi et al., C. R. Geoscience 336 (2004).

Intracontinental rift comparisons: Baikal and Rio Grande Rift Systems

NASA Astrophysics Data System (ADS)

Lipman, P. W.; Logatchev, N. A.; Zorin, Y. A.; Chapman, C. E.; Kovalenko, V.; Morgan, P.

Both the Baikal rift in Siberia and the Rio Grande rift in New Mexico, Colorado and Texas are major intracontinental extensional structures of Cenozoic age that affect regions about 1500 km long and several hundred km wide (Figures 1, 2). In the summer of 1988 these rifts were visited by study groups of U.S. and Soviet geoscientists during cooperative field workshops sponsored by the Soviet Academy of Sciences, U.S. National Academy of Sciences, and U.S. Geological Survey.In the Rio Grande region, we spent 2 weeks examining rift features between El Paso, Tex., and Denver, Colo. Particular emphasis was on the sedimentary record of rift evolution, widespread volcanic activity from inception of rifting to the present, geophysical expression of rift features, and relations between rifting and the larger-scale evolution of the North American Cordillera. In the Baikal region, which presents formidable logistic problems for a workshop, we travelled by bus, truck, helicopter, and ship to examine young seismotectonic features, rift-related basalt, and bounding structures of the Siberian craton that influenced rift development (Figure 3).

Hydrothermal activity at slow-spreading ridges: variability and importance of magmatic controls

NASA Astrophysics Data System (ADS)

Escartin, Javier

2016-04-01

Hydrothermal activity along mid-ocean ridge axes is ubiquitous, associated with mass, chemical, and heat exchanges between the deep lithosphere and the overlying envelopes, and sustaining chemiosynthetic ecosystems at the seafloor. Compared with hydrothermal fields at fast-spreading ridges, those at slow spreading ones show a large variability as their location and nature is controlled or influenced by several parameters that are inter-related: a) tectonic setting, ranging from 'volcanic systems' (along the rift valley floor, volcanic ridges, seamounts), to 'tectonic' ones (rift-bounding faults, oceanic detachment faults); b) the nature of the host rock, owing to compositional heterogeneity of slow-spreading lithosphere (basalt, gabbro, peridotite); c) the type of heat source (magmatic bodies at depth, hot lithosphere, serpentinization reactions); d) and the associated temperature of outflow fluids (high- vs.- low temperature venting and their relative proportion). A systematic review of the distribution and characteristics of hydrothermal fields along the slow-spreading Mid-Atlantic Ridge suggests that long-lived hydrothermal activity is concentrated either at oceanic detachment faults, or along volcanic segments with evidence of robust magma supply to the axis. A detailed study of the magmatically robust Lucky Strike segment suggests that all present and past hydrothermal activity is found at the center of the segment. The association of these fields to central volcanos, and the absence of indicators of hydrothermal activity along the remaining of the ridge segment, suggests that long-lived hydrothermal activity in these volcanic systems is maintained by the enhanced melt supply and the associated magma chamber(s) required to build these volcanic edifices. In this setting, hydrothermal outflow zones at the seafloor are systematically controlled by faults, indicating that hydrothermal fluids in the shallow crust exploit permeable fault zones to circulate. While less studied, similar hydrothermal systems are found elsewhere associated to other central volcanoes along the ridge axis (e.g., Menez Gwenn at the Mid-Atlantic Ridge and Soria Mornia or Troll Wall at the Arctic Ridges). Long-lived hydrothermal activity plays an important role in controlling the thermal structure of the lithosphere and its accretion at and near-axis, and also determining the distribution and biogeography of vent communities. Along slow-spreading segments, long-lived hydrothermal activity can be provided both by volcanic systems (e.g., Lucky Strike) and tectonic systems (oceanic detachment faults). While magmatic and hydrothermal activity is relatively well understood now in volcanic systems (e.g., Lucky Strike), tectonic systems (oceanic detachment faults) require further integrated studies to constrain the links between long-lived localization of deformation along oceanic detachment faults, hydrothermal activity, and origin and nature of off-axis heat sources animating hydrothermal circulation.

Two-stage magmatism during the evolution of the transitional Ethiopian rift

NASA Astrophysics Data System (ADS)

Cornwell, D. G.; England, R. W.; Maguire, P. K.; Kendall, M.; Stuart, G. W.

2008-12-01

The Ethiopian rift marks the transition between continental rifting and incipient seafloor spreading. The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) included a 400 km-long cross-rift profile with 97 broadband passive seismometers with the aim to investigate the change from mechanical to magmatic extension by defining the lithospheric structure and extent of magmatism beneath the rift. Complimentary studies of P-wave receiver functions, shear-wave splitting and teleseismic earthquake arrival times show that the lithospheric structure is inherently different beneath the north-western rift flank, rift valley and south- eastern rift flank, with contrasting crustal thickness and composition, upper mantle velocity and lithospheric anisotropy. Two stages of magmatic addition are interpreted: 1) a 6--18 km-thick underplate lens at the base of the crust, which probably formed synchronous with an Oligocene flood basalt event (and therefore pre-dates the adjacent rifting by ~20 Myr); and 2) a 20--30 km-wide zone of intense dyking and partial melt, which most likely pervades the entire crust beneath the rift valley and marks the locus of current rift extension. Furthermore, Precambrian collision-related lithospheric fabric is proposed to be the main source of the strong anisotropy that is observed along the entire cross-rift profile, which may be augmented by magmatism beneath the rift. An active, followed by a passive magma-assisted rifting model that is controlled by a combination of far-field plate stresses, the pre-existing lithospheric framework and magmatism is invoked to explain the rift evolution.

Crustal Structure in the Southern Rockall Trough from Satellite Gravity Data: Evidence for Sea-floor Spreading

NASA Astrophysics Data System (ADS)

Chappell, A.; Kusznir, N. J.

2005-05-01

The southern Rockall Trough south of 57 N has previously been interpreted as either an intra-continental rift floored with highly extended continental crust, or a failed oceanic rift formed by Cretaceous sea floor spreading. Satellite gravity, bathymetry data and seismic estimates of sediment thickness are used to derive crustal basement thickness for the southern Rockall Trough and adjacent regions using a gravity inversion method incorporating a correction for the large negative thermal gravity component present in oceanic and stretched continental lithosphere. The marine Bouguer anomaly, derived from satellite free air gravity (Sandwell & Smith 1997) and Gebco 2003 bathymetry data, is inverted using the method of Oldenberg (1974), incorporating an iteratively applied thermal anomaly correction, to give Moho depth. For oceanic crust the thermal anomaly correction is calculated using isochron ages (Muller et al. 1997) and for continental crust from the beta stretching factors resulting from gravity derived crustal basement thickness and an assumed rift age. When sediment thickness and volcanic addition are assumed to be zero, the resulting upper bound of crustal thickness from the gravity inversion is as little as 10 km in the southern Rockall Trough. A segmented axial thickening of the crust at the centre of the Rockall Trough is predicted, between the Barra volcanic ridge and the Anton Dohrn seamount and is interpreted as having a volcanic origin. Inclusion of a sediment thickness correction in the gravity inversion further reduces predicted crustal thickness. A pseudo-sediment-thickness map has been constructed from the available wide-angle data and incorporated in the gravity inversion. The addition of up to 5.5 km of sediment in the gravity inversion reduces the upper bound of crustal thickness to less than 3 km in some locations. The segmented axial thickening and thin crust shown by the gravity inversion, the lack of intra-basinal faulting, and the volcanic origin for the axis shown by normal incidence seismic data, are consistent with a sea-floor spreading origin for the southern Rockall Trough and not formation by intra-continental rifting. We investigate the formation of the southern Rockall Trough using SfMargin, a new model of continental lithosphere thinning leading to continental breakup and sea-floor spreading initiation. Comparisons of the geometry of the southern Rockall Trough predicted by SfMargin with that observed are consistent with a short period (20Ma) of slow Cretaceous sea-floor spreading, followed by thermal subsidence to present day. This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Badley Geoscience & Schlumberger Cambridge Research supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, ConocoPhillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, A Chappell, J Eccles, R Fletcher, D Healy, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, V Tymms & R Spitzer.

Quantitative challenges to our understanding of the tectonostratigraphic evolution of rift basin systems

NASA Astrophysics Data System (ADS)

Olsen, P. E.; Kent, D. V.

2012-12-01

Pervasive orbitally-paced lake level cycles combined with magnetic polarity stratigraphy in central Pangean early Mesozoic rift basins provide a thus far unique and very large-scale quantitative basis for observing patterns of basin fill and comparisons with other basins. The 32 Myr accumulation rate history of the Newark basin is segmented into intervals lasting millions of years with virtually no change in the long-term accumulation rate (at the 400-kyr-scale), and the transitions between segments are abrupt and apparently basin-wide. This is startling, because the basin geometry was, and is, a half graben - triangular in cross section and dish-shaped in along-strike section. The long periods of time with virtually no change is challenging given a simple model of basin growth (1), suggesting some kind of compensation in sediment input for the increasing surface of the area of the basin through time. Perhaps even more challenging are observations based on magnetic polarity stratigraphy and the cyclicity, that basins distributed over a huge area of central Pangea (~700,000 km2) show parallel and correlative quantitative changes in accumulation rate with those of the Newark basin. The synchronous changes in the accumulation rate in these basins suggests a very large-scale linkage, the only plausible mechanism for which would seem to be at the plate-tectonic scale, perhaps involving extension rates. Together, we can speculate that some kind of balance between extension rates, basin accommodation space and perhaps regional drainage basin size might have been in operation The most dramatic accumulation rate change in the basins' histories occurred close to, and perhaps causally related to, the Triassic-Jurassic boundary and end-Triassic extinction. The Newark basin, for example exhibits a 4-to-5-fold increase in accumulation rate during the emplacement of the brief (<1 Myr) and aerially massive Central Atlantic Magmatic Province (CAMP) beginning at 201.5 Ma, the only igneous event known during this long rifting episode. Parallel and correlative accumulation rate changes are seen in several of the other northern basins within central Pangea. Surprisingly, the rate of accommodation growth apparently increased dramatically during this time, because not only did the accumulation rate dramatically increase, the lakes apparently deepened during the same time as a huge volume of CAMP igneous material entered the basins. At the same time, the more southern basins in the southeastern US, apparently ceased to subside (2). Our ability to measure time in these rift basins using the orbitally-paced cycles, coupled with the ability to correlate between the basins using magnetic polarity stratigraphy, challenges us to form new mechanistic explanations and quantitative models to test against this rich library of observations. References: 1) Schlische RW & Olsen PE, 1990, Jour. Geol. 98:135. 2) Schlische et al., 2003, in Hames WE et al. (eds), Geophys. Monogr. 136:61.

Abrupt plate acceleration during rifted margin formation: Cause and effect

NASA Astrophysics Data System (ADS)

Brune, Sascha; Williams, Simon; Butterworth, Nathaniel; Müller, Dietmar

2017-04-01

Extension rate is known to control key processes during rifted margin formation such as crust-mantle coupling, decompression melting, magmatism, and serpentinisation. Here we build on recent advances in plate tectonic reconstructions by quantifying the extension velocity history of Earth's major rifted margins during the last 240 million years. We find that many successful rifts start with a slow phase of extension followed by rapid acceleration that introduces a fast phase. The transition from slow to fast rifting takes place long before crustal break-up: approximately half of the present day rifted margin area was created during the slow, and the other half during the fast rift phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. In these models, rift velocities are not imposed but instead evolve naturally in response to the changing strength of the rift. Our results demonstrate that abrupt plate acceleration during continental rifting is controlled by a rift-intrinsic strength-velocity feedback. The abruptness of rift acceleration is thereby governed by the nonlinearity of lithospheric localization. Realistic brittle and power-law rheologies lead to a speed-up duration between two and ten million years. For successful rifts that generate a new ocean basin, the duration of rift speed-up is notably almost independent of the applied extensional force. Instead, the force controls the duration of the slow phase: higher forces shorten the slow phase while lower forces prolong it. If the force is too low, however, delocalisation processes prevent the rift from reaching the point of speed-up and produce a failed rift, even if the extensional system was active for many million years.

Two-stage rifting in the Kenya rift: implications for half-graben models

NASA Astrophysics Data System (ADS)

Mugisha, F.; Ebinger, C. J.; Strecker, M.; Pope, D.

1997-09-01

The Kerio sub-basin in the northern Kenya rift is a transitional area between the southern Kenya rift, where crustal thickness is 30 km, and the northern Kenya rift, where crustal thickness is 20 km. The lack of data on the shallow crustal structure, geometry of rift-bounding faults, and rift evolution makes it difficult to determine if the crustal thickness variations are due to pre-rift structure, or along-axis variations in crustal stretching. We reprocessed reflection seismic data acquired for the National Oil Corporation of Kenya, and integrated results with field and gravity observations to (1) delineate the sub-surface geometry of the Kerio sub-basin, (2) correlate seismic stratigraphic sequences with dated strata exposed along the basin margins, and (3) use new and existing results to propose a two-stage rifting model for the central Kenya rift. Although a classic half-graben form previously had been inferred from the attitude of uppermost strata, new seismic data show a more complex form in the deeper basin: a narrow full-graben bounded by steep faults. We suggest that the complex basin form and the northwards increase in crustal thinning are caused by the superposition of two or more rifting events. The first rifting stage may have occurred during Palaeogene time contemporaneous with sedimentation and rifting in northwestern Kenya and southern Sudan. The distribution of seismic sequences suggests that a phase of regional thermal subsidence occurred prior to renewed faulting and subsidence at about 12 Ma after the eruption of flood phonolites throughout the central Kenya rift. A new border fault developed during the second rifting stage, effectively widening the basin. Gravity and seismic data indicate sedimentary and volcanic strata filling the basin are 6 km thick, with up to 4 km deposited during the first rifting stage.

Aeromagnetic signatures over western Marie Byrd Land provide insight into magmatic arc basement, mafic magmatism and structure of the Eastern Ross Sea Rift flank

NASA Astrophysics Data System (ADS)

Ferraccioli, F.; Bozzo, E.; Damaske, D.

2002-03-01

Aeromagnetic signatures over the Edward VII Peninsula (E7) provide new insight into the largely ice-covered and unexplored eastern flank of the Ross Sea Rift (RSR). Positive anomalies, 10-40 km in wavelength and with amplitudes ranging from 50 to 500 nT could reveal buried Late Devonian(?)-Early Carboniferous Ford Granodiorite plutons. This is suggested by similar magnetic signature over exposed, coeval Admiralty Intrusives of the Transantarctic Mountains (TAM). Geochemical data from mid-Cretaceous Byrd Coast Granite, contact metamorphic effects on Swanson Formation and hornblende-bearing granitoid dredge samples strengthen this magnetic interpretation, making alternative explanations less probable. These magnetic anomalies over formerly adjacent TAM and western Marie Byrd Land (wMBL) terranes resemble signatures typically observed over magnetite-rich magmatic arc plutons. Shorter wavelength (5 km) 150 nT anomalies could speculatively mark mid-Cretaceous mafic dikes of the E7, similar to those exposed over the adjacent Ford Ranges. Anomalies with amplitudes of 100-360 nT over the Sulzberger Bay and at the margin of the Sulzberger Ice Shelf likely reveal mafic Late Cenozoic(?) volcanic rocks emplaced along linear rift fabric trends. Buried volcanic rock at the margin of the interpreted half-graben-like "Sulzberger Ice Shelf Block" is modelled in the Kizer Island area. The volcanic rock is marked by a coincident positive Bouguer gravity anomaly. Late Cenozoic volcanic rocks over the TAM, in the RSR, and beneath the West Antarctic Ice Sheet exhibit comparable magnetic anomaly signature reflecting regional West Antarctic Rift fabric. Interpreted mafic magmatism of the E7 is likely related to mid-Cretaceous and Late Cenozoic regional crustal extension and possible mantle plume activity over wMBL. Magnetic lineaments of the E7 are enhanced in maximum horizontal gradient of pseudo-gravity, vertical derivative and 3D Euler Deconvolution maps. Apparent vertical offsets in magnetic basement at the location of the lineaments and spatially associated mafic dikes and volcanic rocks result from 2.5D magnetic modelling. A rift-related fault origin for the magnetic lineaments, segmenting the E7 region into horst and graben blocks, is proposed by comparison with offshore seismic reflection, marine gravity, on-land gravity, radio-echo sounding, apatite fission track data and structural geology. The NNW magnetic lineament, which we interpret to mark the eastern RSR shoulder, forms the western margin of the "Alexandra Mountains horst". This fundamental aeromagnetic feature lies on strike with the Colbeck Trough, a prominent NNW half-graben linked to Late Cretaceous(?) and Cenozoic(?) faulting in the eastern RSR. East-west and north-north-east to NE magnetic trends are also imaged. Magnetic trends, if interpreted as reflecting the signature of rift-related normal faults, would imply N-S to NE crustal extension followed by later northwest-southeast directed extension. NW-SE extension would be compatible with Cenozoic(?) oblique RSR rifting. Previous structural data from the Ford Ranges have, however, been interpreted to indicate that both Cretaceous and Cenozoic extensions were N-S to NE-SW directed.

Cenozoic evolution of the Socotra Island: opening of the Gulf of Aden

NASA Astrophysics Data System (ADS)

Razin, P.; Robin, C.; Serra Kiel, J.; Leroy, S.; Bellahsen, N.; Khanbari, K.

2009-12-01

A complete stratigraphic and geological map revision of the Tertiary of Socotra Island is undertaken in order to better characterize the geometry and the tecto-sedimentary evolution of the southern margin of the Gulf of Aden, and compare them with those of the conjugate northern margin in Oman. An increase of the rate of subsidence is recorded during the Late Eocene and is associated with a transgressive peak within carbonate platform deposits (Aydim Fm.). At the scale of the Arabian plate, the extent of this platform is reduced to the future rift area. This evolution of the platform system shows a modification of the sedimentary profiles, controlled by the beginning of the rifting. The syn-rift deposits of the Early Oligocene correspond to sub-reef carbonate platform facies (Ashawq Fm.). First, the throw of synsedimentary faults and the movements linked with differential subsidence are widely compensated by carbonate production which manages to maintain a platform profile. These movements are recorded by thickness variations, significant lateral variations in the platform facies and by a local inversion of sedimentary polarities controlled by the tilting of faulted blocks. Like on the northern margin, an acceleration of the extension process leads, during the Late Oligocene, to a collapse of the platform and to the creation of deep sub-basins with carbonate gravity-flow sedimentation. Marginal reef platforms keep growing at this stage on the structural highs and feed gravity-flow sedimentary systems. The sedimentation rate stays then relatively low in the basin, forming a complex topography of the margin, marked by a segmentation into numerous sub-basins more or less connected and separated by submarine escarpments marked by wedges of breccia deposits along active normal faults. In different points, these faults are sealed by sedimentary deposits characterized by progressive unconformities and onlap geometries on the fault escarpments. These geometries show the relatively short length of the phase of « stretching » of the continental crust. Around the end of the Early Miocene, the progradation of conglomerate fan-delta deposits locally results in the fill of the basins and shows a major phase of uplift. It is very rapidly followed by a new phase of subsidence which allows the preservation of thick fan-delta and equivalent reef platform complex unconformably overlying different units of the syn-rift and pre-rift sequences, or even the exhumed Proterozoic basement. This tectonic-sedimentary phase is interpreted as synchronous to the continental breakup and the onset of the OCT at the foot of the margin. The analogy with the phase of development of «sag basins» on the Atlantic margins has to be analyzed. This major uplift at the transition syn-rift/post-rift seems to be expressed symmetrically on both margins. These syn-OCT deposits are then uplifted and affected by late tilting events. However, the most recent deposits, probably Late Miocene to plio-Quaternary in age, have only been affected by small uplifts, unlike those of the Dhofar on the northern margin

Evolution of the northern Main Ethiopian rift: birth of a triple junction

NASA Astrophysics Data System (ADS)

Wolfenden, Ellen; Ebinger, Cynthia; Yirgu, Gezahegn; Deino, Alan; Ayalew, Dereje

2004-07-01

Models for the formation of the archetypal rift-rift-rift triple junction in the Afar depression have assumed the synchronous development of the Red Sea-Aden-East African rift systems soon after flood basaltic magmatism at 31 Ma, but the timing of intial rifting in the northern sector of the East African rift system had been poorly constrained. The aims of our field, geochronology, and remote sensing studies were to determine the timing and kinematics of rifting in the 3rd arm, the Main Ethiopian rift (MER), near its intersection with the southern Red Sea rift. New structural data and 10 new SCLF 40Ar/39Ar dates show that extension in the northern Main Ethiopian rift commenced after 11 Ma, more than 17 My after initial rifting in the southern Red Sea and Gulf of Aden. The triple junction, therefore, could have developed only during the past 11 My, or 20 My after the flood basaltic magmatism. Thus, the flood basaltic magmatism and separation of Arabia from Africa are widely separated in time from the opening of the Main Ethiopian rift, which marks the incipient Nubia-Somalia plate boundary; triple junction formation is not a primary feature of breakup above the Afar mantle plume. The East African rift system appears to have propagated northward from the Mesozoic Anza rift system into the Afar depression to cut across Oligo-Miocene rift structures of the Red Sea and Gulf of Aden, in response to global plate reorganisations. Structural patterns reveal a change from 130°E-directed extension to 105°E-directed extension sometime in the interval 6.6 to 3 Ma, consistent with predictions from global plate kinematic studies. The along-axis propagation of rifting in each of the three arms of the triple junction has led to a NE-migration of the triple junction since 11 Ma.

Seismic Anisotropy Beneath the Eastern Flank of the Rio Grande Rift

NASA Astrophysics Data System (ADS)

Benton, N. W.; Pulliam, J.

2015-12-01

Shear wave splitting was measured across the eastern flank of the Rio Grande Rift (RGR) to investigate mechanisms of upper mantle anisotropy. Earthquakes recorded at epicentral distances of 90°-130° from EarthScope Transportable Array (TA) and SIEDCAR (SC) broadband seismic stations were examined comprehensively, via the Matlab program "Splitlab", to determine whether SKS and SKKS phases indicated anisotropic properties. Splitlab allows waveforms to be rotated, filtered, and windowed interactively and splitting measurements are made on a user-specified waveform segment via three independent methods simultaneously. To improve signal-to-noise and improve reliability, we stacked the error surfaces that resulted from grid searches in the measurements for each station location. Fast polarization directions near the Rio Grande Rift tend to be sub-parallel to the RGR but then change to angles that are consistent with North America's average plate motion, to the east. The surface erosional depression of the Pecos Valley coincides with fast polarization directions that are aligned in a more northerly direction than their neighbors, whereas the topographic high to the east coincides with an easterly change of the fast axis.The area above a mantle high velocity anomaly discovered separately via seismic tomography which may indicate thickened lithosphere, corresponds to unusually large delay times and fast polarization directions that are more closely aligned to a north-south orientation. The area of southeastern New Mexico that falls between the mantle fast anomaly and the Great Plains craton displays dramatically smaller delay times, as well as changes in fast axis directions toward the northeast. Changes in fast axis directions may indicate flow around the mantle anomaly; small delay times could indicate vertical or attenuated flow.

Latest Jurassic ammonoid provinces: Paleoecological implications using a general circulation model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ross, C.A.; Moore, G.T.; Hayashida, D.N.

1992-01-01

The Lake Permian-early Mesozoic megacontinent Pangea was progressively fragmented by two rift systems that propagated westward out of the Tethys Sea and a third more persistent rift system that connected the Boreal and Tethys seas. By the late Tithonian, these major rift systems produced interconnected oceanic seaways that divided Pangea into four continental segments: North America, Eur-Asia, and northern and southern Gondwana. Increased rates of sea-floor spreading during the Jurassic reduced the volumetric capacity of ocean basins and produced a sea level rise through the period that culminated in the Lake Jurassic. The extensive marine shelf margins and epeiric seasmore » hosted a widely distributed and diverse ammonoid fauna. By the early Tithonian, faunal communication existed between the northwestern Tethys Sea and the eastern Panthalassa Ocean through the proto-Gulf of Mexico. By the late Tithonian, faunal similarities indicate the opening of the proto-Indian Ocean so that northern and southern Gondwana had become separate continents. A region of the equatorial Tethys that includes most of the present Arabian Peninsular contains neuritic platform facies but lacks ammonoids. In high northern latitudes, cool to cold water faunas formed a Boreal Realm which extended westward across northern North America, Europe, and Siberia during middle and late Tithonian. Late Kimmeridgian and Tithonian ammonoid distributions when compared with Late Jurassic paleoclimate simulations show likely causal relationships with sea surface water temperatures and upwelling, and possibly shed light on the temperature limitations of ammonoids. Results from modeled seasonal sea surface temperature, sea ice distribution, precipitation-evaporation, and wind-driven upwelling permit the evaluation and quantification of paleoenvironmental factors favorable as well as pernicious for ammonoid distribution.« less

Molecular detection of Rift Valley fever virus in serum samples from selected areas of Tanzania.

PubMed

Chengula, Augustino Alfred; Kasanga, Christopher Jacob; Mdegela, Robinson Hammerthon; Sallu, Raphael; Yongolo, Mmeta

2014-04-01

Rift Valley fever (RVF) is an acute mosquito-borne viral zoonotic disease affecting domestic animals and humans caused by the Rift Valley fever virus (RVFV). The virus belongs to the genus Phlebovirus of the family Bunyaviridae. The main aim of this study was to detect the presence of antibodies to RVFV as well as the virus in the serum samples that were collected from livestock during the 2006/2007 RVF outbreaks in different locations in Tanzania. Analysis of selected samples was done using a RVF-specific inhibition enzyme-linked immunosorbent assay (I-ELISA) and reverse transcription polymerase chain reaction (RT-PCR). Genomic viral RNA was extracted directly from serum samples using a QIAamp Viral RNA Mini Kit (QIAGEN), and a one-step RT-PCR protocol was used to amplify the S segment of RVFV. Positive results were obtained in 39.5% (n = 200) samples using the RVF I-ELISA, and 17.6% (n = 108) of samples were positive by RT-PCR. I-ELISA detected 41 (38.7%), 32 (39.0%), and 6 (50.0%) positive results in cattle, goats, and sheep sera, respectively, whereas the RT-PCR detected 11 (0.2%), 7 (0.2%), and 1 (0.1%) positive results in cattle, goats, and sheep sera, respectively. These findings have demonstrated the presence of RVFV in Tanzania during the 2006/2007 RVF outbreaks. To our knowledge, this is the first report to detect RVFV in serum samples from domestic animals in Tanzania using PCR technique. Therefore, a detailed molecular study to characterize the virus from different geographical locations in order to establish the profile of strains circulating in the country and develop more effective and efficient control strategies should be done.

Evidence of Tectonic Rotations and Magmatic Flow Within the Sheeted Dike Complex of Super-Fast Spread Crust Exposed at the Pito Deep Rift

NASA Astrophysics Data System (ADS)

Horst, A. J.; Varga, R. J.; Gee, J. S.; Karson, J. A.

2008-12-01

Escarpments bounding the Pito Deep Rift expose cross-sections into ~3 Ma oceanic crust accreted at a super-fast spreading (>140 mm/yr) segment of the East Pacific Rise (EPR). Dikes within the sheeted dike complex persistently strike NE, parallel to local abyssal hill lineaments and magnetic anomaly stripes, and dip SE, outward and away from the EPR. During the Pito Deep 2005 Cruise, both ALVIN and JASON II used the Geocompass to fully orient a total of 69 samples [63 basaltic dikes, 6 massive gabbros] collected in situ. Paleomagnetic analyses of these oriented samples provide a quantitative constraint of kinematics of structural rotations of dikes. Magnetic remanence of dike samples indicates a dominant normal polarity with almost all directions rotated clockwise from the expected direction. The most geologically plausible model to account for these dispersions using these data coupled with the general orientation of the dikes incorporates two different structural rotations: 1) A horizontal-axis rotation that occurred near the EPR axis, related to sub-axial subsidence, and 2) A clockwise vertical-axis rotation, associated with the rotation of the Easter microplate consistent with current models. Additionally, the anisotropy of magnetic susceptibility (AMS) of dike samples indicates rock fabric and magmatic flow direction within dikes. In most samples, two of three AMS eigenvectors lie near the dike plane orientations. Generally, Kmin lies perpendicular to dike planes, while Kmax is often shallow within the dike planes, indicating dominantly subhorizontal magma flow. Steep Kmax in a few samples indicates vertical flow directions that suggest either primary flow or gravitational back-flow during waning stages of dike intrusion. These results provide the first direct evidence for primarily horizontal magma flow in sheeted dikes of super-fast spread oceanic crust. Results for Pito Deep Rift and previous results for Hess Deep Rift reveal outward dipping dikes that are interpreted as a result of subaxial spreading processes that are not evident from surface studies of spreading centers. Both areas show evidence of subaxial subsidence during accretion and lateral magmatic flow in the sheeted dike complex.

The Orosirian-Statherian banded iron formation-bearing sequences of the southern border of the Espinhaço Range, Southeast Brazil

NASA Astrophysics Data System (ADS)

Rolim, Vassily Khoury; Rosière, Carlos A.; Santos, João Orestes Schneider; McNaughton, Neal J.

2016-01-01

The Serra da Serpentina and the Serra de São José groups are two distinct banded iron formation-bearing metasedimentary sequences along the eastern border of the southern Espinhaço Range that were deposited on the boundary between the Orosirian and Statherian periods. The Serra da Serpentina Group (SSG) has an Orosirian maximum depositional age (youngest detrital zircon grain age = 1990 ± 16 Ma) and consists of fine clastic metasediments at the base and chemical sediments, including banded iron formations (BIFs), on the top, corresponding to the Meloso and Serra do Sapo formations, respectively, and correlating with the pre-Espinhaço Costa Sena Group. The SSG represents sedimentary deposition on an epicontinental-epeiric, slow downwarping sag basin with little tectonic activity. The younger Serra de São José Group (SJG) is separated from the older SSG by an erosional unconformity and was deposited in a tectonically active continental rift-basin in the early stages of the opening of the Espinhaço Trough. The Serra do São José sediments stretch along the north-south axis of the rift and comprise a complete cycle of transgressive sedimentary deposits, which were subdivided, from base to top, into the Lapão, Itapanhoacanga, Jacém and Canjica formations. The Itapanhoacanga Formation has a maximum depositional age of 1666 ± 32 Ma (Statherian), which coincides with the maximum depositional age (i.e., 1683 ± 11 Ma) of the São João da Chapada Formation, one of the Espinhaço Supergroup's basal units. The Serra de São José Rift and the Espinhaço Rift likely represent the same system, with basal units that are facies variations of the same sequence. The supracrustal rocks have undergone two stages of deformation during the west-verging Brasiliano orogeny that affected the eastern margin of the São Francisco Craton and generated a regional-scale, foreland N-S trending fold-thrust belt, which partially involved the crystalline basement. Thrust faults have segmented the terrain into a large number of tectonic blocks, where the stratigraphic sequence was nevertheless well preserved.

The Effects of Rapid Sedimentation upon Continental Breakup: Kinematic and Thermal Modeling of the Salton Trough, Southern California, Based upon Recent Seismic Images

NASA Astrophysics Data System (ADS)

Han, L.; Hole, J. A.; Lowell, R. P.; Stock, J. M.; Fuis, G. S.

2016-12-01

The Salton Seismic Imaging Project (SSIP) illuminated crustal and upper mantle structure of the Salton Trough, the northern-most rift segment of the Gulf of California plate boundary. The crust is 17-18 km thick and homogeneous for 100 km in the plate motion direction. New crust is being created by distributed rift magmatism, Colorado River sedimentation, and metamorphism of the sediment. A 5 km thick pre-existing crustal layer may still exist. The crust has not broken apart to enable initiation of seafloor spreading. A one-dimensional time-dependent kinematic and thermal model was developed to simulate these observations. We assume that all crustal layers are stretched uniformly during extension. Distributed mafic magmatism and sedimentation are added simultaneously to compensate for the crustal thinning. The ratio of magmatism to sedimentation is constrained by the seismic observations. Heat is transported by thermal conduction and by advection due to stretching of the crust. A constant temperature boundary at the Moho is used to represent partial melting in the upper mantle. Assuming a constant plate motion rate, the zone of active rifting extends linearly with time. The crustal thickness and internal structure also evolve with time. The model constraints are the observed seismic structure and heat flow. The model rapidly reaches quasi-steady state, and could continue for many millions of years. The observed seismic structure and heat flow are reproduced after 3 Myr. The yield strength profile calculated from lithology and model temperature indicates that ductile deformation in the middle and lower crust dominates the crustal rheology. Rapid sedimentation delays crustal breakup and the initiation of seafloor spreading by maintaining the thickness of the crust and keeping it predominantly ductile. This process probably occurs wherever a large river flows into an active rift driven by far-field extension. It may have built passive margins in many locations globally, such as the Gulf of Mexico. This type of passive margin consists of mostly new crust created by magmatism and metamorphism of sediment. Along such margins, metamorphosed sediment could be misinterpreted as stretched pre-existing continental crust.

Deep magmatism alters and erodes lithosphere and facilitates decoupling of Rwenzori crustal block

NASA Astrophysics Data System (ADS)

Wallner, Herbert; Schmeling, Harro

2013-04-01

The title is the answer to the initiating question "Why are the Rwenzori Mountains so high?" posed at the EGU 2008. Our motivation origins in the extreme topography of the Rwenzori Mountains. The strong, cold proterozoic crustal horst is situated between rift segments of the western branch of the East African Rift System. Ideas of rift induced delamination (RID) and melt induced weakening (MIW) have been tested with one- and two-phase flow physics. Numerical model parameter variations and new observations lead to a favoured model with simple and plausible definitions. Results coincide in the scope of their comparability with different observations or vice versa reduce ambiguity and uncertainties in model input. Principle laws of the thermo-mechanical physics are the equations of conservation of mass, momentum, energy and composition for a two-phase (matrix-melt) system with nonlinear rheology. A simple solid solution model determines melting and solidification under consideration of depletion and enrichment. The Finite Difference Method with markers is applied to visco-plastic flow using the streamfunction in an Eulerian formulation in 2D. The Compaction Boussinesq and the high Prandtl number Approximation are employed. Lateral kinematic boundary conditions provide long-wavelength asthenospheric upwelling and extensional stress conditions. Partial melts are generated in the asthenosphere, extracted above a critical fraction, and emplaced into a given intrusion level. Temperature anomalies positioned beneath the future rifts, the sole specialization to the Rwenzori situation, localize melts which are very effective in weakening the lithosphere. Convection patterns tend to generate dripping instabilities at the lithospheric base; multiple slabs detach and distort uprising asthenosphere; plumes migrate, join and split. In spite of appearing chaotic flow behaviour a characteristic recurrence time of high velocity events (drips, plumes) emerges. Chimneys of increased enrichment develop above the anomalies and evolve to narrow low viscous mechanical decoupling zones. Deep rooting dynamic forces then affect the surface, showing a vigorous topography. A geodynamic model, linking magmatism. mantle dynamics and lithospheric extension, qualitatively explains most of observed phenomena. Depending on physical model parameters we cover the whole spectrum from dripping lithospheric base instabilities to the full break off of the mantle lithosphere block below the Rwenzoris.

Structure of the active rift zone and margins of the northern Imperial Valley from Salton Seismic Imaging Project (SSIP) data

NASA Astrophysics Data System (ADS)

Livers, A.; Han, L.; Delph, J. R.; White-Gaynor, A. L.; Petit, R.; Hole, J. A.; Stock, J. M.; Fuis, G. S.

2012-12-01

First-arrival refraction data were used to create a seismic velocity model of the upper crust across the actively rifting northern Imperial Valley and its margins. The densely sampled seismic refraction data were acquired by the Salton Seismic Imaging Project (SSIP) , which is investigating rift processes in the northern-most rift segment of the Gulf of California extensional province and earthquake hazards at the southern end of the San Andreas Fault system. A 95-km long seismic line was acquired across the northern Imperial Valley, through the Salton Sea geothermal field, parallel to the five Salton Butte volcanoes and perpendicular to the Brawley Seismic Zone and major strike-slip faults. Nineteen explosive shots were recorded with 100 m seismometer spacing across the valley and with 300-500 m spacing into the adjacent ranges. First-arrival travel times were picked from shot gathers along this line and a seismic velocity model was produced using tomographic inversion. Sedimentary basement and seismic basement in the valley are interpreted to be sediment metamorphosed by the very high heat flow. The velocity model shows that this basement to the west of the Brawley Seismic Zone is at ~4-km depth. The basement shallows to ~2-km depth in the active geothermal field and Salton Buttes volcanic field which locally coincide with the Brawley Seismic Zone. At the eastern edge of the geothermal field, the basement drops off again to ~3.5-km depth. The eastern edge of the valley appears to be fault bounded by the along-strike extension of the Sand Hills Fault, an inactive strike-slip fault. The seismic velocities to the east of the fault correspond to metamorphic rock of the Chocolate Mountains, different from the metamorphosed basement in the valley. The western edge of the valley appears to be fault bounded by the active Superstition Hills Fault. To the west of the valley, >4-km deep valley basement extends to the active Superstition Hills Fault. Basement then shallows westward towards exposures of granitic basement in the Superstition Mountains. The basin between the Superstition Mountains and Coyote Mountains is ~2 km deep.

Intracontinental Rifts As Glorious Failures

NASA Astrophysics Data System (ADS)

Burke, K.

2012-12-01

Rifts: "Elongate depressions overlying places where the lithosphere has ruptured in extension" develop in many environments because rocks are weak in extension (Sengor 2nd edn. Springer Encycl. Solid Earth Geophys.). I focus on intra-continental rifts in which the Wilson Cycle failed to develop but in which that failure has led to glory because rocks and structures in those rifts throw exceptional light on how Earth's complex continental evolution can operate: The best studied record of human evolution is in the East African Rift; The Ventersdorp rifts (2.7 Ga) have yielded superb crustal-scale rift seismic reflection records; "Upside-down drainage" (Sleep 1997) has guided supra-plume-head partial melt into older continental rifts leading Deccan basalt of ~66Ma to erupt into a Late Paleozoic (~ 300Ma) rift and the CAMP basalts of ~201 Ma into Ladinian, ~230 Ma, rifts. Nepheline syenites and carbonatites, which are abundant in rifts that overlie sutures in the underlying mantle lithosphere, form by decompression melting of deformed nepheline syenites and carbonatites ornamenting those sutures (Burke et al.2003). Folding, faulting and igneous episodes involving decompression melting in old rifts can relate to collision at a remote plate margin (Guiraud and Bosworth 1997, Dewey and Burke 1974) or to passage of the rift over a plume generation zone (PGZ Burke et al.2008) on the Core Mantle Boundary (e.g.Lake Ellen MI kimberlites at ~206 Ma).

Comparative Riftology: Insights into the Evolution of Passive Continental Margins and Continental Rifts from the Failed Midcontinent Rift (MCR)

NASA Astrophysics Data System (ADS)

Elling, R. P.; Stein, C. A.; Stein, S.; Kley, J.; Keller, G. R.; Wysession, M. E.

2017-12-01

Continental rifts evolve to seafloor spreading and are preserved in passive margins, or fail and remain as fossil features in continents. Rifts at different stages give insight into these evolutionary paths. Of particular interest is the evolution of volcanic passive margins, which are characterized by seaward dipping reflectors, volcanic rocks yielding magnetic anomalies landward of the oldest spreading anomalies, and are underlain by high-velocity lower crustal bodies. How and when these features form remains unclear. Insights are given by the Midcontinent Rift (MCR), which began to form during the 1.1 Ga rifting of Amazonia from Laurentia, but failed when seafloor spreading was established elsewhere. MCR volcanics are much thicker than other continental flood basalts, due to deposition in a narrow rift rather than a broad region, giving a rift's geometry but a LIP's magma volume. The MCR provides a snapshot of the deposition of a thick and highly magnetized volcanic section during rifting. Surface exposures and reflection seismic data near Lake Superior show a rift basin filled by inward-dipping flood basalt layers. Had the rift evolved to seafloor spreading, the basin would have split into two sets of volcanics with opposite-facing SDRs, each with a magnetic anomaly. Because the rift formed as a series of alternating half-grabens, structural asymmetries between conjugate margins would have naturally occurred had it gone to completion. Hence the MCR implies that many passive margin features form prior to seafloor spreading. Massive inversion of the MCR long after it failed has provided a much clearer picture of its structure compared to failed rifts with lesser degrees of inversion. Seismic imaging as well as gravity and magnetic modeling provide important insight into the effects of inversion on failed rifts. The MCR provides an end member for the evolution of actively extending rifts, characterized by upwelling mantle and negative gravity anomalies, to failed and inverted rifts without upwelling mantle and positive gravity anomalies.

Time dependent deformation of Kilauea Volcano, Hawaii

NASA Astrophysics Data System (ADS)

Montgomery-Brown, Emily Kvietka Desmarais

In 1997 the continuous Global Positioning System (GPS) network was completed on Kilauea, providing the first network of daily position measurements during eruptions and earthquakes on Kilauea. Kilauea has been studied for many decades with continuous seismic and tilt instruments. Other geodetic data (e.g., campaign GPS, leveling, electronic distance measurements) are also available although they contain only sparse data. Data analysis methods used here include inverting multiple data sets for optimal source parameters and the spatio-temporal distribution of magma volume and fault slip, and combining GPS and seismic observations to understand flank tectonics. The field area for this study, Kilauea Volcano, was chosen because of its frequent activity and potential hazards. The 1997 East Rift Zone eruption (Episode 54) was the first major event to occur after the completion of the continuous GPS network. The event lasted 2 days, but transient deformation continued for six months. This long-duration transient allowed the first spatio-temporal study of transient dike deformation on Kilauea from daily GPS positions. Slow-slip events were discovered on Kilauea during which the southern flank of the volcano would accelerate seaward for approximately 2 days. The discovery was made possible because of the continuously operating GPS network. These slip events were also observed to correlate with small swarms of microearthquakes found to follow temporal pattern consistent with them being co- and aftershocks of the slow-slip event (Segall, 2006). Half-space models of geodetic data favor a shallow fault plane (˜ 5 km), which is much too shallow to have increased the Coulomb stress at the depths of the co- and aftershocks. However, optimizations for the slow-slip source parameters including a layered elastic structure and a topographic correction favor deeper models within the range of the co- and aftershocks. Additionally, the spatial distribution of seaward fault slip, fixed to a decollement structure 8 km under the south flank, and the locations of the microearthquakes suggest that both occur on the same structure. In 2007, Episode 56 of the Pu'u 'O'o-Kupianaha eruption occurred. This episode was exciting both because it was the largest intrusion in the last decade, and because it occurred concurrently with a flank slow-slip event. The intrusion started on Father's day (June 17th), 2007 with increased seismicity and abrupt tilts at the summit and rift zones. Quasi-static models of the total deformation determined from GPS, tilt, and InSAR indicate that the intrusion occurred on two en echelon dike segments in the upper East Rift Zone along with deformation consistent with slow-slip in the same areas of previous events. The ˜ 2 m maximum opening occurred on the eastern segment near Makaopui crater. Unlike previous intrusions in 1997, 1999, and 2000, the dike model was not sufficient to explain deformation on the western flank. Additionally, a coastal tiltmeter installed in anticipation of a slow-slip event recorded tilts consistent with those observed during the 2005 slow-slip event. These observations led to the conclusion that a concurrent slow-slip event occurred. Geodetic models indicate a similar amount of decollement slip occurred as in previous slow-slip events. Sub-daily GPS positions were used to study the spatio-temporal distribution of the dike intrusion. The time-dependent intrusion model shows that the intrusion began on the western en echelon segment before jumping to the eastern segment, which accumulated the majority of the 2 m of opening. Sub-daily GPS positions limit the number of stations available since there are very few continuous stations north of the East Rift Zone, where coverage is critical for separating the intrusion from the slow-slip. However, an ENVISAT interferogram at 08:22 on June 18, 2007 provides additional spatial coverage of deformation up to that point. Combining this image with the GPS and tilt data up to that point, we perform a quasi-static inversion for the intrusion source. The residual deformation indicates that slow-slip had not significantly progressed by the ENVISAT image. The slow-slip event occurred therefore at least 20 hours after the initiation of the dike intrusion. (Abstract shortened by UMI.)

Post-extension shortening strains preserved in calcites of the Keweenawan rift

DOE Office of Scientific and Technical Information (OSTI.GOV)

Donnelly, K.; Craddock, J.; McGovern, M.

1993-02-01

The Keweenawan rift is part of failed triple junction system that underlies Lake Superior and the Michigan Basin. The rift experienced extensional stresses dating about 1.1 Ga, which were followed by compressional stresses from about 1,060 Ma to < 350 Ma. Associated with the rift are two thrust faults: the Douglas (dipping southeast) and the Keweenawan-Lake Owen (dipping northwest). To determine the direction of rifting, calcite twins were used to calculate strain ellipsoids (Groshong method) which are indicative of the intensity and direction of the stress applied to a rocks in a region at a given time. Rock samples whichmore » contain significant calcite within the zone of rifting were collected, slabbed, and made into thin sections. Calcite appears as amygdule, vein, and cement filings, as well as limestones. Analyses show that different calcite types show different stain orientations. Two principle directions of sub-horizontal shortening are present: one parallel to rift, and one normal to the rift, indicating that rifting motion varied out the time in which different calcite types were deposited. Shortening parallel to the rift is seen predominantly on the western margin while shortening normal to the rift is seen predominantly on the eastern margin.« less

Crustal and Mantle Structure beneath the Okavango and Malawi Rifts and Its Geodynamic Implications

NASA Astrophysics Data System (ADS)

Gao, S. S.; Liu, K. H.; Yu, Y.; Reed, C. A.; Mickus, K. L.; Moidaki, M.

2017-12-01

To investigate crustal and mantle structure beneath the young and incipient sections of the East African Rift System and provide constraints on rifting models, a total of 50 broadband seismic stations were placed along three profiles across the Okavango and Malawi rifts, with a total length of about 2500 km. Results to date suggest minor crustal thinning and nearly normal seismic velocities in the upper mantle beneath both rifts. The thickness of the mantle transition zone is comparable to the global average, suggesting the lack of thermal upwelling from the lower mantle beneath the rifts. In addition, shear-wave splitting analysis found no anomalies in either the fast polarization orientation or the splitting time associated with the rifts, and thus has ruled out the existence of small-scale mantle convection or plume-related mantle flow beneath the rifts. While the Okavango rift has long been recognized to be located in a Precambrian orogenic zone between the Kalahari and Congo cratons, our results suggest that the Malawi Rift is also developing along the western edge of a lithospheric block with relatively greater thickness relative to the surrounding area. Those seismological and gravity modeling results are consistent with a passive rifting model, in which rifts develop along pre-existing zones of lithospheric weakness, where rapid variations of lithospheric thickness is observed. Lateral variations of dragging stress applied to the bottom of the lithosphere are the most likely cause for the initiation and development of both rifts.

Abrupt plate acceleration through oblique rifting: Geodynamic aspects of Gulf of California evolution

NASA Astrophysics Data System (ADS)

Brune, S.

2016-12-01

The Gulf of California formed by oblique divergence across the Pacific-North America plate boundary. This presentation combines numerical forward modeling and plate tectonic reconstructions in order to address 2 important aspects of rift dynamics: (1) Plate motions during continental rifting are decisively controlled by the non-linear decay of rift strength. This conclusion is based on a recent plate-kinematic analysis of post-Pangea rift systems (Central Atlantic, South Atlantic, Iberia/Newfoundland, Australia/Antarctica, North Atlantic, South China Sea). In all cases, continental rifting starts with a slow phase followed by an abrupt acceleration within a few My introducing a fast rift phase. Numerical forward modeling with force boundary conditions shows that the two-phase velocity behavior and the rapid speed-up during rifting are intrinsic features of continental rupture that can be robustly inferred for different crust and mantle rheologies. (2) Rift strength depends on the obliquity of the rift system: the force required to maintain a given rift velocity can be computed from simple analytical and more realistic numerical models alike, and both modeling approaches demonstrate that less force is required to perpetuate oblique extension. The reason is that plastic yielding requires a smaller plate boundary force when extension is oblique to the rift trend. Comparing strike slip and pure extension end-member scenarios, it can be shown that about 50% less force is required to deform the lithosphere under strike-slip. This result implies that rift systems involving significant obliquity are mechanically preferred. These two aspects shed new light on the underlying geodynamic causes of Gulf of California rift history. Continental extension is thought to have started in Late Eocene/Oligocene times as part of the southern Basin and Range Province and evolved in a protracted history at low extension rate (≤15 mm/yr). However, with a direction change in Baja California microplate motion 13-6 My ago, plate divergence drastically increased its obliquity, which reduced the rifts mechanical resistance to extension. This effective loss of rift strength sparked an acceleration of the Gulf of California rift and ultimately enabled today's divergence velocities of more than 45 mm/yr.

The Red Sea and Gulf of Aden Basins

NASA Astrophysics Data System (ADS)

Bosworth, William; Huchon, Philippe; McClay, Ken

2005-10-01

We here summarize the evolution of the greater Red Sea-Gulf of Aden rift system, which includes the Gulfs of Suez and Aqaba, the Red Sea and Gulf of Aden marine basins and their continental margins, and the Afar region. Plume related basaltic trap volcanism began in Ethiopia, NE Sudan (Derudeb), and SW Yemen at ˜31 Ma, followed by rhyolitic volcanism at ˜30 Ma. Volcanism thereafter spread northward to Harrats Sirat, Hadan, Ishara-Khirsat, and Ar Rahat in western Saudi Arabia. This early magmatism occurred without significant extension, and continued to ˜25 Ma. Much of the Red Sea and Gulf of Aden region was at or near sea level at this time. Starting between ˜29.9 and 28.7 Ma, marine syn-tectonic sediments were deposited on continental crust in the central Gulf of Aden. At the same time the Horn of Africa became emergent. By ˜27.5-23.8 Ma a small rift basin was forming in the Eritrean Red Sea. At approximately the same time (˜25 Ma), extension and rifting commenced within Afar itself. At ˜24 Ma, a new phase of volcanism, principally basaltic dikes but also layered gabbro and granophyre bodies, appeared nearly synchronously throughout the entire Red Sea, from Afar and Yemen to northern Egypt. This second phase of magmatism was accompanied in the Red Sea by strong rift-normal extension and deposition of syn-tectonic sediments, mostly of marine and marginal marine affinity. Sedimentary facies were laterally heterogeneous, being comprised of inter-fingering siliciclastics, evaporite, and carbonate. Throughout the Red Sea, the principal phase of rift shoulder uplift and rapid syn-rift subsidence followed shortly thereafter at ˜20 Ma. Water depths increased dramatically and sedimentation changed to predominantly Globigerina-rich marl and deepwater limestone. Within a few million years of its initiation in the mid-Oligocene the Gulf of Aden continental rift linked the Owen fracture zone (oceanic crust) with the Afar plume. The principal driving force for extension was slab-pull beneath the Urumieh-Doktar arc on the north side of the narrowing Neotethys. Drag of Arabia by the northward-moving Indian plate across the partially locked northern Owen fracture zone and the position of the Carlsberg oceanic ridge probably also influenced the geometry of the Aden rift. The trigger for the onset of rifting, though, was the impingement of the Afar plume at ˜31 Ma. The Red Sea propagated away from the plume head, perpendicular to the extensional stresses then operating in Arabia, and arrived at the bend in the African-Levant margin, which itself may have been a stress concentration ripe for rifting. The local geometry of the early Red Sea rift was strongly influenced by pre-existing basement structures, and as a consequence followed a complex path from Afar to Suez. Each segment of the rift was initially an asymmetric half graben, with well-defined accommodation zones between sub-basins. In the Gulf of Aden, the positions of accommodation zones were strongly influenced by older Mesozoic rift basins. Early rift structures can be restored to their original contiguous geometries along both the Red Sea and Gulf of Aden conjugate margins. In both basins, present-day shorelines restore to a separation of 40-60 km along most of their lengths. The initial rift basins were 60-80 km in width. Oceanic spreading initiated on the Sheba Ridge east of the Alula-Fartaq fracture zone at ˜19-18 Ma. After stalling at this fracture zone, the ridge probably propagated west into the central Gulf of Aden by ˜16 Ma. This matches the observed termination of syn-tectonic deposition along the onshore Aden margins at approximately the same time. At ˜14 Ma, a transform boundary cut through Sinai and the Levant continental margin, linking the northern Red Sea with the Bitlis-Zagros convergence zone. This corresponded with collision of Arabia and Eurasia, which resulted in a new plate geometry with different boundary forces. Red Sea extension changed from rift normal (N60°E) to highly oblique and parallel to the Aqaba-Levant transform (N15°E). North of Suez in Egypt the rift system became emergent, perhaps due to minor compression of the Sinai sub-plate, and the marine connection to the Mediterranean Sea became restricted but not terminated. Red Sea sedimentation changed from predominantly open marine to evaporitic, although deep water persisted in many regions. A third phase of magmatism commenced, locally in Ethiopia but predominantly in western Saudi Arabia and extending north to Harrat Ash Shama and Jebel Druse in Jordan, Lebanon, and Syria. At ˜10 Ma, the Sheba Ridge rapidly propagated west over 400 km from the central Gulf of Aden to the Shukra al Sheik discontinuity. Oceanic spreading followed in the south-central Red Sea at ˜5 Ma. This corresponded in time to an important unconformity throughout the Red Sea basin and along the margins of the Gulf of Aden, coeval with the Messinian unconformity of the Mediterranean basin. A major phase of pull-apart basin development also occurred along the Aqaba-Levant transform. In the early Pliocene the influx of marine waters through Bab al Mandeb increased and Red Sea sedimentation thereafter returned to predominantly open marine conditions. By ˜3-2 Ma, oceanic spreading moved west of the Shukra al Sheik discontinuity, and the entire Gulf of Aden was an oceanic rift. During the last ˜1 My, the southern Red Sea plate boundary linked to the Aden spreading center through the Gulf of Zula, Danakil Depression, and Gulf of Tadjoura. Presently, the Red Sea spreading center appears to be propagating toward the northern Red Sea to link with the Aqaba-Levant transform. Alkali basaltic volcanism continues within the Younger Harrats of western Saudi Arabia and Yemen and offshore southern Red Sea islands. Most of the Arabian plate is now experiencing N-S upper crustal compression, whereas the maximum horizontal stress is oriented E-W in NE Africa. Arabia and Africa, now on separate plates, are therefore completely decoupled in terms of regional, far-field stresses.

Structure of backarc inner rifts as a weakest zone of arc-backarc system: a case study of the Sea of Japan

NASA Astrophysics Data System (ADS)

Sato, Hiroshi; Ishiyama, Tasuya; Kato, Naoko; Abe, Susumu; Saito, Hideo; Shiraishi, Kazuya; Abe, Shiori; Iwasaki, Takaya; Inaba, Mitsuru; No, Tetsuo; Sato, Takeshi; Kodaira, Shuichi; Takeda, Tetsuya; Matsubara, Makoto; Kodaira, Chihiro

2015-04-01

A backarc inner rift is formed after a major opening of backarc basin near a volcanic front away from the spreading center of a major backarc basin. An obvious example is the inner rift along the Izu-Bonin arc. Similar inner rift zones have been developed along the Sea of Japan coast of Honshu island, Japan. NE and SW Japan arcs experienced strong shortening after the Miocene backarc rifting. The amount of shortening shows its maximum along the backarc inner rifts, forming a fold-and-thrust of thick post-rift sediments over all the structure of backarc. The rift structure has been investigated by onshore-offshore deep seismic reflection/wide-angle reflection surveys. We got continuous onshore-offshore image using ocean bottom cable and collected offshore seismic reflection data using two ships to obtain large offset data in the difficult area for towing a long streamer cable. The velocity structure beneath the rift basin was deduced by refraction tomography in the upper curst and earthquake tomography in the deeper part. It demonstrates larger P-wave velocity in upper mantle and lower crust, suggesting a large amount of mafic intrusion and thinning of upper continental crust. The deeper seismicity in the lower crust beneath the rift basin accords well to the mafic intrusive rocks. Syn-rift volcanism was bimodal, comprising a reflective unit of mafic rocks around the rift axis and a non-reflective unit of felsic rocks near the margins of the basins. Once rifting ended, thermal subsidence, and subsequently, mechanical subsidence related to the onset of the compressional regime, allowed deposition of up to 5 km of post-rift, deep marine to fluvial sedimentation. Continued compression produced fault-related folds in the post-rift sediments, characterized by thin-skin style of deformation. The syn-rift mafic intrusion in the crust forms convex shape and the boundary between pre-rift crust and mafic intrusive shows outward dipping surface. Due to the post rift compression, the boundary of rock units reactivated as reverse faults, commonly forming a large-scale wedge thrust and produced subsidence of rift basin under compressional stress regime. Large amount of convergence of overriding plate is accommodated along the inner rift, suggesting that it is a weakest zone in whole arc-backarc system. The convergence between young (15 Ma) Shikoku basin and SW Japan arc produced intense shortening along the inner failed rift along the Sea of Japan coast. After the onset of subduction along the Nankai trough, the fold-and-thrust belt was covered by Pliocene marine sediment. Before the 2011 off-Tohoku earthquake (M9), several damaging earthquakes occurred along the backarc fold-and-thrust belt. These represents that a weak backarc inner rift is very sensitive for the stress produce by the subduction interface.

Morphostructural evidence for Recent/active extension in Central Tanzania beyond the southern termination of the Kenya Rift.

NASA Astrophysics Data System (ADS)

Le Gall, B.; Rolet, J.; Gernigon, L.; Ebinger, C.; Gloaguen, R.

2003-04-01

The southern tip zone of the Kenya Rift on the eastern branch of the East African System is usually thought to occur in the so-called North Tanzanian Divergence. In this region, the narrow (50 km-wide) axial graben of southern Kenya splays southwards, via a major EW-trending volcanic lineament, into a 200 km-wide broad rifted zone with three separate arms of normal faulting and tilted fault blocks (Eyasi, Manyara and Pangani arms from W to E). Remote sensing analysis from Central Tanzania demonstrates that rift morphology exists over an area lying 400 km beyond the southern termination of the Kenya Rift. The most prominent rift structures are observed in the Kilombero region and consist of a 100 km-wide range of uplifted basement blocks fringed to the west by an E-facing half-graben inferred to reach depths of 6-8 km from aeromagnetic dataset. Physiographic features (fault scarps), and river drainage anomalies suggest that the present-day rift pattern in the Kilombero extensional province principally results from Recent/Neogene deformation. That assumption is also supported by the seismogenic character of a number of faults. The Kilombero half-graben is superimposed upon an earlier rift system, Karoo in age, which is totally overprinted and is only evidenced from its sedimentary infill. On the other hand, the nature and thickness of the inferred Neogene synrift section is still unknown. The Kilombero rifted zone is assumed to connect northwards into the central rift arm (Manyara) of the South Kenya Rift via a seismically active transverse fault zone that follows ductile fabrics within the Mozambican crystalline basement. The proposed rift model implies that incipient rifting propagates hroughout the cold and strong crust/lithosphere of Central Tanzania along Proterozoic (N140=B0E) basement weakness zones and earlier Karoo (NS)rift structures. A second belt of Recent-active linked fault/basins also extends further East from the Pangani rift arm to the offshore Zanzibar-Kerimbas graben system. The structural connection of the Kilombero rifted zone with the Lake Malawi rift further south is also envisaged and should imply the link of the eastern and western branchs of the East African Rift System south of the Tanzanian craton.

The East African rift system in the light of KRISP 90

USGS Publications Warehouse

Keller, Gordon R.; Prodehl, C.; Mechie, J.; Fuchs, K.; Khan, M.A.; Maguire, Peter K.H.; Mooney, W.D.; Achauer, U.; Davis, P.M.; Meyer, R.P.; Braile, L.W.; Nyambok, I.O.; Thompson, G.A.

1994-01-01

On the basis of a test experiment in 1985 (KRISP 85) an integrated seismic-refraction/teleseismic survey (KRISP 90) was undertaken to study the deep structure beneath the Kenya rift down to depths of 100-150 km. This paper summarizes the highlights of KRISP 90 as reported in this volume and discusses their broad implications as well as the structure of the Kenya rift in the general framework of other continental rifts. Major scientific goals of this phase of KRISP were to reveal the detailed crustal and upper mantle structure under the Kenya rift, to study the relationship between mantle updoming and the development of sedimentary basins and other shallow structures within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system and within a global perspective and to elucidate fundamental questions such as the mode and mechanism of continental rifting. The KRISP results clearly demonstrate that the Kenya rift is associated with sharply defined lithospheric thinning and very low upper mantle velocities down to depths of over 150 km. In the south-central portion of the rift, the lithospheric mantle has been thinned much more than the crust. To the north, high-velocity layers detected in the upper mantle appear to require the presence of anistropy in the form of the alignment of olivine crystals. Major axial variations in structure were also discovered, which correlate very well with variations in the amount of extension, the physiographic width of the rift valley, the regional topography and the regional gravity anomalies. Similar relationships are particularly well documented in the Rio Grande rift. To the extent that truly comparable data sets are available, the Kenya rift shares many features with other rift zones. For example, crustal structure under the Kenya, Rio Grande and Baikal rifts and the Rhine Graben is generally symmetrically centered on the rift valleys. However, the Kenya rift is distinctive, but not unique, in terms of the amount of volcanism. This volcanic activity would suggest large-scale modification of the crust by magmatism. Although there is evidence of underplating in the form of a relatively high-velocity lower crustal layer, there are no major seismic velocity anomalies in the middle and upper crust which would suggest pervasive magmatism. This apparent lack of major modification is an enigma which requires further study. ?? 1994.

Continental rifts and mineral resources

DOE Office of Scientific and Technical Information (OSTI.GOV)

Burke, K.

1992-01-01

Continental rifts are widespread and range in age from the present to 3 b.y. Individual rifts may form parts of complex systems as in E. Africa and the Basin and Range. Rifts have originated in diverse environments such as arc-crests, sites of continental collision, collapsing mountain belts and on continents at rest over the mantle circulation pattern. Continental rift resources can be classified by depth of origin: For example, in the Great Dike, Norilsk and Mwadui magma from the mantle is the host. At shallower depths continental crust partly melted above mafic magma hosts ore (Climax, Henderson). Rift volcanics aremore » linked to local hydrothermal systems and to extensive zeolite deposits (Basin and Range, East Africa). Copper (Zambia, Belt), zinc (Red Dog) and lead ores (Benue) are related to hydrothermal systems which involve hot rock and water flow through both pre-rift basement and sedimentary and volcanic rift fill. Economically significant sediments in rifts include coals (the Gondwana of Inida), marine evaporites (Lou Ann of the Gulf of Mexico) and non-marine evaporites (East Africa). Oil and gas in rifts relate to a variety of source, reservoir and trap relations (North Sea, Libya), but rift-lake sediment sources are important (Sung Liao, Bo Hai, Mina, Cabinda). Some ancient iron ores (Hammersley) may have formed in rift lakes but Algoman ores and greenstone belt mineral deposits in general are linked to oceanic and island arc environments. To the extent that continental environments are represented in such areas as the Archean of the Superior and Slave they are Andean Arc environments which today have locally rifted crests (Ecuador, N. Peru). The Pongola, on Kaapvaal craton may, on the other hand represent the world's oldest preserved, little deformed, continental rift.« less

Large-scale variation in lithospheric structure along and across the Kenya rift

USGS Publications Warehouse

Prodehl, C.; Mechie, J.; Kaminski, W.; Fuchs, K.; Grosse, C.; Hoffmann, H.; Stangl, R.; Stellrecht, R.; Khan, M.A.; Maguire, Peter K.H.; Kirk, W.; Keller, Gordon R.; Githui, A.; Baker, M.; Mooney, W.; Criley, E.; Luetgert, J.; Jacob, B.; Thybo, H.; Demartin, M.; Scarascia, S.; Hirn, A.; Bowman, J.R.; Nyambok, I.; Gaciri, S.; Patel, J.; Dindi, E.; Griffiths, D.H.; King, R.F.; Mussett, A.E.; Braile, L.W.; Thompson, G.; Olsen, K.; Harder, S.; Vees, R.; Gajewski, D.; Schulte, A.; Obel, J.; Mwango, F.; Mukinya, J.; Riaroh, D.

1991-01-01

The Kenya rift is one of the classic examples of a continental rift zone: models for its evolution range from extension of the lithosphere by pure shear1, through extension by simple shear2, to diapiric upwelling of an asthenolith3. Following a pilot study in 19854, the present work involved the shooting of three seismic refraction and wide-angle reflection profiles along the axis, across the margins, and on the northeastern flank of the rift (Fig. 1). These lines were intended to reconcile the different crustal thickness estimates for the northern and southern parts of the rift4-6 and to reveal the structure across the rift, including that beneath the flanks. The data, presented here, reveal significant lateral variations in structure both along and across the rift. The crust thins along the rift axis from 35 km in the south to 20 km in the north; there are abrupt changes in Mono depth and uppermost-mantle seismic velocity across the rift margins, and crustal thickening across the boundary between the Archaean craton and PanAfrican orogenic belt immediately west of the rift. These results suggest that thickened crust may have controlled the rift's location, that there is a decrease in extension from north to south, and that the upper mantle immediately beneath the rift may contain reservoirs of magma generated at greater depth.

Lithospheric structure of the Rio Grande rift.

PubMed

Wilson, David; Aster, Richard; West, Michael; Ni, James; Grand, Steve; Gao, Wei; Baldridge, W Scott; Semken, Steve; Patel, Paresh

2005-02-24

A high-resolution, regional passive seismic experiment in the Rio Grande rift region of the southwestern United States has produced new images of upper-mantle velocity structure and crust-mantle topography. Synthesizing these results with geochemical and other geophysical evidence reveals highly symmetric lower-crustal and upper-mantle lithosphere extensional deformation, suggesting a pure-shear rifting mechanism for the Rio Grande rift. Extension in the lower crust is distributed over a region four times the width of the rift's surface expression. Here we propose that the laterally distributed, pure shear extension is a combined effect of low strain rate and a regionally elevated geotherm, possibly abetted by pre-existing lithospheric structures, at the time of rift initiation. Distributed extension in the lower crust and mantle has induced less concentrated vertical mantle upwelling and less vigorous small-scale convection than would have arisen from more localized deformation. This lack of highly focused mantle upwelling may explain a deficit of rift-related volcanics in the Rio Grande rift compared to other major rift systems such as the Kenya rift.

Physical characteristics and evolutionary trends of continental rifts

NASA Technical Reports Server (NTRS)

Ramberg, I. B.; Morgan, P.

1984-01-01

Rifts may be defined as zones beneath which the entire lithosphere has ruptured in extension. They are widespread and occur in a variety of tectonic settings, and range up to 2,600 m.y. in age. The object of this review is to highlight characteristic features of modern and ancient rifts, to emphasize differences and similarities in order to help characterize evolutionary trends, to identify physical conditions favorable for initiation as well as termination of rifting, and to provide constraints for future modeling studies of rifting. Rifts are characterized on the basis of their structural, geomorphic, magmatic and geophysical features and the diverse character of these features and their evolutionary trends through time are discussed. Mechanisms of rifting are critically examined in terms of the physical characteristics and evolutionary trends of rifts, and it is concluded that while simple models can give valuable insight into specific processes of rifting, individual rifts can rarely, if ever, be characterized by well defined trends predicted by these models. More data are required to clearly define evolutionary trends, and the models require development to incorporate the effects of lithospheric heterogeneities and complex geologic histories.

Crustal shear wave velocity and radial anisotropy beneath the Rio Grande rift from ambient noise tomography

NASA Astrophysics Data System (ADS)

Fu, Yuanyuan V.; Li, Aibing

2015-02-01

Shear wave velocity and radial anisotropy beneath New Mexico are obtained from ambient seismic noise tomography using data from the Transportable Array. Besides the distinct seismic structure imaged across the Rio Grande rift from the Colorado Plateau to the Great Plains, both velocity and anisotropy models also reveal significant variations along the rift. The rift at Albuquerque is characterized by remarkably low velocity in the shallow crust, high velocity and strong positive anisotropy in the middle and lower crust, and low velocity in the upper mantle. These observations can be interpreted as magma accumulation in the shallow crust and significant mafic underplating in the lower crust with abundant melt supply from the hot mantle. We propose that the Albuquerque region has recently been experiencing the most vigorous extensional deformation in the rift. Positive anisotropy with Vsh > Vsv appears in the central and southern rifts with a stronger anisotropy beneath younger volcanoes, reflecting layering of magma intrusion due to past and recent rifting activities. The low velocities in the uppermost mantle are observed under high-elevation places, the Jemez Lineament, northern rift, and east rift boundary, implying that the buoyancy of hot mantle largely compensates the local high topography. Low mantle velocities appear at the boundary of the southern rift, corresponding to the large lithosphere thickness change, instead of the rift center, consistent with the prediction from the small-scale, edge-driven mantle convection model. We conclude that the edge-driven upper mantle convection is probably the dominant mechanism for the recent and current rifting and uplift in the Rio Grande rift.

Segmentation and Accretionary Processes Near the Andrew Bain Mega-Transform Fault: The Southwest Indian Ridge 25°-35°E

NASA Astrophysics Data System (ADS)

Takeuchi, C. S.; Sclater, J. G.; Grindlay, N. R.; Madsen, J. A.; Rommevaux-Jestin, C.

2008-12-01

The ultra-slow spreading Southwest Indian Ridge (SWIR) separates the Antarctic and African plates. We present results from two surveys covering the SWIR between 26° and 27°30'E and between 32° and 35°E, lying on either side of the long-offset Andrew Bain transform fault. The objectives of the surveys were to characterize the segmentation of an ultra-slow spreading ridge on either side of a long-offset transform fault and to examine the structure of the individual segments. Four transform faults, the Du Toit, Andrew Bain, Marion, and Prince Edward, and one non-transform discontinuity bound four accretionary segments in the survey areas. Two segments lie northeast of the Andrew Bain (32°-35°E). Large central axial volcanoes, deep, broad mantle Bouguer anomaly (MBA) lows, and high magnetization intensities centered on the spreading axis result from high magmatic activity. Increased magmatism on the ridge axis is likely caused by high mantle temperatures produced by the close proximity of the Marion Plume, which abuts the northern end of the Andrew Bain. Two segments lie southwest of the Andrew Bain (26°-27°30'E). Discrepancies in the locations of the axial rift valley, central magnetization high, and an irregularly-shaped MBA low suggest complex accretionary processes at the western segment (~26°-27° E). The eastern segment (~27°-27°30'E), which abuts the southwest end of the Andrew Bain, shows a deep axial valley, MBA values which increase to the east, and nearly nonexistent magnetization intensity. These features are probably the result of amagmatic accretion caused by the transform edge effect of the Andrew Bain. A transition in the character of topography at 26°45'E suggests that the current segment configuration may not be temporally stable. High-relief (~1 km) ridge-trough structures south of the spreading axis may be the result of an episodic interplay between accretion, both magmatic and amagmatic, and tectonic extension.

Evidence for triple-junction rifting focussed on local magmatic centres along Parga Chasma, Venus

NASA Astrophysics Data System (ADS)

Graff, J. R.; Ernst, R. E.; Samson, C.

2018-05-01

Parga Chasma is a discontinuous rift system marking the southern boundary of the Beta-Atla-Themis (BAT) region on Venus. Along a 1500 km section of Parga Chasma, detailed mapping of Magellan Synthetic Aperture Radar images has revealed 5 coronae, 11 local rift zones distinct from a regional extension pattern, and 47 graben-fissure systems with radiating (28), linear (12) and circumferential (7) geometries. The magmatic centres of these graben-fissure systems typically coincide with coronae or large volcanoes, although a few lack any central magmatic or tectonic feature (i.e. are cryptic). Some of the magmatic centres are interpreted as the foci of triple-junction rifting that form the 11 local rift zones. Cross-cutting relationships between graben-fissure systems and local rift faults reveal synchronous formation, implying a genetic association. Additionally, cross-cutting relationships show that local rifting events postdate the regional extension along Parga Chasma, further indicating multiple stages of rifting. Evidence for multiple centres of younger magmatism and local rifting against a background of regional extension provides an explanation for the discontinuous morphology of Parga Chasma. Examination of the Atlantic Rift System (prior to ocean opening) on Earth provides an analogue to the rift morphologies observed on Venus.

Parga Chasma: Coronae and Rifting on Venus

NASA Technical Reports Server (NTRS)

Smrekar, S. E.; Stofan, E. R.; Buck, W. R.; Martin, P.

2005-01-01

The majority of coronae (quasicircular volcano-tectonic features) are found along rifts or fracture belts, and the majority of rifts have coronae [e.g. 1,2]. However, the relationship between coronae and rifts remains unclear [3-6]. There is evidence that coronae can form before, after, or synchronously with rifts [3,4]. The extensional fractures in the rift zones have been proposed to be a result of broad scale upwelling and traction on the lower lithosphere [7]. However, not all rift systems have a significant positive geoid anomaly, as would be expected for an upwelling site [8]. This could be explained if the rifts lacking anomalies are no longer active. Coronae are generally accepted to be sites of local upwelling [e.g. 1], but the observed rifting is frequently not radial to the coronae and extends well beyond the coronae into the surrounding plains. Thus the question remains as to whether the rifts represent regional extension, perhaps driven by mantle tractions, or if the coronae themselves create local thinning and extension of the lithosphere. In the first case, a regional extension model should be consistent with the observed characteristics of the rifts. In the latter case, a model of lithospheric loading and fracturing would be more appropriate. A good analogy may be the propagation of oceanic intraplate volcanoes [9].

Contour mapping of relic structures in the Precambrian basement of the Reelfoot rift, North American midcontinent

USGS Publications Warehouse

Dart, R.L.; Swolfs, H.S.

1998-01-01

A new contour map of the basement of the Reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features. The major features are two asymmetric intrarift basins, bounded by three structural highs, and the rift margins. The basins are oriented normal to the northeast trend of the rift. Two of the highs appear to be ridges of undetermined width that extend across the rift. The third high is an isolated dome or platform located between the basins. The minor features are three linear structures of low relief oriented subparallel to the trend of the rift. Two of these, located within the rift basins, may divide the rift basins into paired subbasins. These mapped features may be the remnants of initial extensional rifting, half graben faulting, and basement subsidence. The rift basins are interpreted as having formed as opposing half graben, and the structural highs are interpreted as having formed as associated accommodation zones. Some of these features appear to be reactivated seismogenic structures within the modem midcontinent compressional stress regime. A detailed knowledge of the geometries of the Reelfoot rift's basement features, therefore, is essential when evaluating their seismic risk potential.

Exploring Crustal Structure and Mantle Seismic Anisotropy Associated with the Incipient Southern and Southwestern Branches of the East African Rift System

NASA Astrophysics Data System (ADS)

Yu, Y.; Reed, C. A.; Gao, S. S.; Liu, K. H.; Massinque, B.; Mdala, H. S.; Chindandali, P. R. N.; Moidaki, M.; Mutamina, D. M.

2014-12-01

In spite of numerous geoscientific studies, the mechanisms responsible for the initiation and development of continental rifts are still poorly understood. The key information required to constrain various geodynamic models on rift initiation can be derived from the crust/mantle structure and anisotropy beneath incipient rifts such as the Southern and Southwestern branches of the East African Rift System. As part of a National Science Foundation funded interdisciplinary project, 50 PASSCAL broadband seismic stations were deployed across the Malawi, Luangwa, and Okavango rift zones from the summer of 2012 to the summer of 2014. Preliminary results from these 50 SAFARI (Seismic Arrays for African Rift Initiation) and adjacent stations are presented utilizing shear-wave splitting (SWS) and P-S receiver function techniques. 1109 pairs of high-quality SWS measurements, consisting of fast polarization orientations and splitting times, have been obtained from a total of 361 seismic events. The results demonstrate dominantly NE-SW fast orientations throughout Botswana as well as along the northwestern flank of the Luangwa rift valley. Meanwhile, fast orientations beneath the eastern Luangwa rift flank rotate from NNW to NNE along the western border of the Malawi rift. Stations located alongside the western Malawi rift border faults yield ENE fast orientations, with stations situated in Mozambique exhibiting more E-W orientations. In the northern extent of the study region, fast orientations parallel the trend of the Rukwa and Usangu rift basins. Receiver function results reveal that, relative to the adjacent Pan-African mobile belts, the Luangwa rift zone has a thin (30 to 35 km) crust. The crustal thickness within the Okavango rift basin is highly variable. Preliminary findings indicate a northeastward thinning along the southeast Okavango border fault system congruent with decreasing extension toward the southwest. The Vp/Vs measurements in the Okavango basin are roughly 1.75 on average, suggesting an unmodified crustal composition, while those of the Luangwa and southern Malawi rift zones are relatively high, probably suggesting ancient or ongoing magmatic emplacement. The Pan-African mobile belts enveloping the rift zones are mostly characterized by more felsic and thicker crust.

Phanerozoic geological evolution of the Equatorial Atlantic domain

NASA Astrophysics Data System (ADS)

Basile, Christophe; Mascle, Jean; Guiraud, René

2005-10-01

The Phanerozoic geological evolution of the Equatorial Atlantic domain has been controlled since the end of Early Cretaceous by the Romanche and Saint Paul transform faults. These faults did not follow the PanAfrican shear zones, but were surimposed on Palæozoic basins. From Neocomian to Barremian, the Central Atlantic rift propagated southward in Cassiporé and Marajó basins, and the South Atlantic rift propagated northward in Potiguar and Benue basins. During Aptian times, the Equatorial Atlantic transform domain appeared as a transfer zone between the northward propagating tip of South Atlantic and the Central Atlantic. Between the transform faults, oceanic accretion started during Late Aptian in small divergent segments, from south to north: Benin-Mundaú, deep Ivorian basin-Barreirinhas, Liberia-Cassiporé. From Late Aptian to Late Albian, the Togo-Ghana-Ceará basins appeared along the Romanche transform fault, and Côte d'Ivoire-Parà-Maranhão basins along Saint Paul transform fault. They were rapidly subsiding in intra-continental settings. During Late Cretaceous, these basins became active transform continental margins, and passive margins since Santonian times. In the same time, the continental edge uplifted leading either to important erosion on the shelf or to marginal ridges parallel to the transform faults in deeper settings.

Late Cretaceous to Cenozoic deformation and exhumation of the Chilean Frontal Cordillera (28°-29°S), Central Andes

NASA Astrophysics Data System (ADS)

Martínez, Fernando; Parra, Mauricio; Arriagada, César; Mora, Andrés; Bascuñan, Sebastián; Peña, Matías

2017-11-01

The Frontal Cordillera in northern Chile is located over the flat-slab subduction segment of the Central Andes. This tectonic province is characterized by a thick-skinned structural style showing evidence of tectonic inversion and basement-involved compressive structures. Field data, U-Pb geochronological and apatite fission track data were used to unravel partially the tectonic history of the area. Previous U-Pb ages of synorogenic deposits exposed on the flanks of basement-core anticlines indicate that Andean deformation started probably during Late Cretaceous with the tectonic inversion of Triassic and Jurassic half-grabens. New U-Pb ages of the synorogenic Quebrada Seca Formation suggest that this deformation continued during Paleocene (66-60 Ma) with the reverse faulting of pre-rift basement blocks. The analysis of new apatite fission-track data shows that a rapid and coeval cooling related to exhumation of the pre-rift basement blocks occurred during Eocene times. This exhumation event is interpreted for first time in the Chilean Frontal Cordillera and it could have occurred simultaneously with the propagation of basement-involved structures. The age of this exhumation event coincides with the Incaic orogenic phase, which is interpreted as the most important to the Central Andes in terms of shortening, uplift and exhumation.

3-D electrical structure across the Yadong-Gulu rift revealed by magnetotelluric data: New insights on the extension of the upper crust and the geometry of the underthrusting Indian lithospheric slab in southern Tibet

NASA Astrophysics Data System (ADS)

Wang, Gang; Wei, Wenbo; Ye, Gaofeng; Jin, Sheng; Jing, Jianen; Zhang, Letian; Dong, Hao; Xie, Chengliang; Omisore, Busayo O.; Guo, Zeqiu

2017-09-01

The approximately north-south trending Cenozoic Yadong-Gulu rift (YGR) in the eastern Lhasa block is an ideal location to investigate the extensional kinematic mechanism of the upper crust and the deformation characteristics of the Indian lithospheric slab in southern Tibet. The magnetotelluric (MT) method has been widely used in probing subsurface structures at lithospheric scale and is sensitive to high electrically conductive body (conductor). A three-dimensional (3-D) inversion of MT data was conducted to derive the east-west electrical structures across the northern segment of the YGR. The result reveals that the conductors in the middle crust are not continuous in the east-west direction. The deep conductor underneath the YGR is interpreted to result from the tearing of the Indian lithospheric slab. The upper crust to the east of the YGR is significantly intruded by underlying conductors. Based on the features of the 3-D inversion result from this study and other geophysical observations, the formation of the YGR is most likely caused by tearing of the Indian lithospheric slab through the pull of mid-lower crustal conductors that have locally weak strength beneath the YGR.

Geothermal potential on Kirtland Air Force Base lands, Bernalillo County, New Mexico

NASA Astrophysics Data System (ADS)

Grant, P. R., Jr.

1981-10-01

Public policy expressed in a number of national directives in recent years stresses the conservation of conventional fuel supplies, a switch to alternative fuels, and the application of advanced energy technologies at federal installations. Natural gas currently furnishes 85 to 95 percent of the average 94 x 1,000,000 Btu/hr energy requirements for space heating and cooling at Kirtland Air Force Base. Studies of alternatives to the use of natural gas at the base include examination of the geothermal option. Four of North America's major physiographic provinces coalesce in central New Mexico on or near Kirtland AFB. Their junction is identified throughout much of this region by a tectonic depression occupied by the Rio Grande that is structurally complex, stratigraphically and hydrologically unique, and coincides with geologically recent volcanic centers. This trough, the Rio Grande rift, has been identified as a major geothermal resource area. The western part of Kirtland AFB is in the Albuquerque Basin segment of the Rio Grande rift. Extensive sampling and geochemical analysis of groundwater in and near the base disclosed no significant geothermal parameters. However, structural conditions and current hydrologics regimes strongly suggest that thermal waters would be masked by near surface, low temperature meteoric water originating as rain and snowfall in the nearby mountains.

Temporal bone dissection simulator for training pediatric otolaryngology surgeons

NASA Astrophysics Data System (ADS)

Tabrizi, Pooneh R.; Sang, Hongqiang; Talari, Hadi F.; Preciado, Diego; Monfaredi, Reza; Reilly, Brian; Arikatla, Sreekanth; Enquobahrie, Andinet; Cleary, Kevin

2017-03-01

Cochlear implantation is the standard of care for infants born with severe hearing loss. Current guidelines approve the surgical placement of implants as early as 12 months of age. Implantation at a younger age poses a greater surgical challenge since the underdeveloped mastoid tip, along with thin calvarial bone, creates less room for surgical navigation and can result in increased surgical risk. We have been developing a temporal bone dissection simulator based on actual clinical cases for training otolaryngology fellows in this delicate procedure. The simulator system is based on pre-procedure CT (Computed Tomography) images from pediatric infant cases (<12 months old) at our hospital. The simulator includes: (1) simulation engine to provide the virtual reality of the temporal bone surgery environment, (2) a newly developed haptic interface for holding the surgical drill, (3) an Oculus Rift to provide a microscopic-like view of the temporal bone surgery, and (4) user interface to interact with the simulator through the Oculus Rift and the haptic device. To evaluate the system, we have collected 10 representative CT data sets and segmented the key structures: cochlea, round window, facial nerve, and ossicles. The simulator will present these key structures to the user and warn the user if needed by continuously calculating the distances between the tip of surgical drill and the key structures.

Identifying buried segments of active faults in the northern Rio Grande Rift using aeromagnetic, LiDAR,and gravity data, south-central Colorado, USA

USGS Publications Warehouse

Grauch, V.J.S.; Ruleman, Chester A.

2013-01-01

Combined interpretation of aeromagnetic and LiDAR data builds on the strength of the aeromagnetic method to locate normal faults with significant offset under cover and the strength of LiDAR interpretation to identify the age and sense of motion of faults. Each data set helps resolve ambiguities in interpreting the other. In addition, gravity data can be used to infer the sense of motion for totally buried faults inferred solely from aeromagnetic data. Combined interpretation to identify active faults at the northern end of the San Luis Basin of the northern Rio Grande rift has confirmed general aspects of previous geologic mapping but has also provided significant improvements. The interpretation revises and extends mapped fault traces, confirms tectonic versus fluvial origins of steep stream banks, and gains additional information on the nature of active and potentially active partially and totally buried faults. Detailed morphology of surfaces mapped from the LiDAR data helps constrain ages of the faults that displace the deposits. The aeromagnetic data provide additional information about their extents in between discontinuous scarps and suggest that several totally buried, potentially active faults are present on both sides of the valley.

Influence of crystallised igneous intrusions on fault nucleation and reactivation during continental extension

NASA Astrophysics Data System (ADS)

Magee, Craig; McDermott, Kenneth G.; Stevenson, Carl T. E.; Jackson, Christopher A.-L.

2014-05-01

Continental rifting is commonly accommodated by the nucleation of normal faults, slip on pre-existing fault surfaces and/or magmatic intrusion. Because crystallised igneous intrusions are pervasive in many rift basins and are commonly more competent (i.e. higher shear strengths and Young's moduli) than the host rock, it is theoretically plausible that they locally intersect and modify the mechanical properties of pre-existing normal faults. We illustrate the influence that crystallised igneous intrusions may have on fault reactivation using a conceptual model and observations from field and subsurface datasets. Our results show that igneous rocks may initially resist failure, and promote the preferential reactivation of favourably-oriented, pre-existing faults that are not spatially-associated with solidified intrusions. Fault segments situated along strike from laterally restricted fault-intrusion intersections may similarly be reactivated. This spatial and temporal control on strain distribution may generate: (1) supra-intrusion folds in the hanging wall; (2) new dip-slip faults adjacent to the igneous body; or (3) sub-vertical, oblique-slip faults oriented parallel to the extension direction. Importantly, stress accumulation within igneous intrusions may eventually initiate failure and further localise strain. The results of our study have important implications for the structural of sedimentary basins and the subsurface migration of hydrocarbons and mineral-bearing fluids.

Impact of fault models on probabilistic seismic hazard assessment: the example of the West Corinth rift.

NASA Astrophysics Data System (ADS)

Chartier, Thomas; Scotti, Oona; Boiselet, Aurelien; Lyon-Caen, Hélène

2016-04-01

Including faults in probabilistic seismic hazard assessment tends to increase the degree of uncertainty in the results due to the intrinsically uncertain nature of the fault data. This is especially the case in the low to moderate seismicity regions of Europe, where slow slipping faults are difficult to characterize. In order to better understand the key parameters that control the uncertainty in the fault-related hazard computations, we propose to build an analytic tool that provides a clear link between the different components of the fault-related hazard computations and their impact on the results. This will allow identifying the important parameters that need to be better constrained in order to reduce the resulting uncertainty in hazard and also provide a more hazard-oriented strategy for collecting relevant fault parameters in the field. The tool will be illustrated through the example of the West Corinth rifts fault-models. Recent work performed in the gulf has shown the complexity of the normal faulting system that is accommodating the extensional deformation of the rift. A logic-tree approach is proposed to account for this complexity and the multiplicity of scientifically defendable interpretations. At the nodes of the logic tree, different options that could be considered at each step of the fault-related seismic hazard will be considered. The first nodes represent the uncertainty in the geometries of the faults and their slip rates, which can derive from different data and methodologies. The subsequent node explores, for a given geometry/slip rate of faults, different earthquake rupture scenarios that may occur in the complex network of faults. The idea is to allow the possibility of several faults segments to break together in a single rupture scenario. To build these multiple-fault-segment scenarios, two approaches are considered: one based on simple rules (i.e. minimum distance between faults) and a second one that relies on physically-based simulations. The following nodes represents for each rupture scenario different rupture forecast models (i.e; characteristic or Gutenberg-Richter) and for a given rupture forecast, two probability models commonly used in seismic hazard assessment: poissonian or time-dependent. The final node represents an exhaustive set of ground motion prediction equations chosen in order to be compatible with the region. Finally, the expected probability of exceeding a given ground motion level is computed at each sites. Results will be discussed for a few specific localities of the West Corinth Gulf.

Gravity study of the Central African Rift system: A model of continental disruption 1. The Ngaoundere and Abu Gabra Rifts

NASA Astrophysics Data System (ADS)

Browne, S. E.; Fairhead, J. D.

1983-05-01

A regional compilation of published and unpublished gravity data for Central Africa is presented and reveals the presence of a major rift system, called here, the Central African Rift System. It is proposed that the junction area between the Ngaoundere and Abu Gabra rift arms in Western Sudan forms an incipient intraplate, triple-junction with the as yet unfractured, but domally uplifted and volcanically active, Darfur swell. It is only the Darfur swell that shows any similarities to the uplift and rift history of East Africa. The other two rifts arms are considered to be structurally similar to the early stages of passive margin development and thus reflect more closely the initial processes of continental fragmentation than the structures associated with rifting in East Africa.

The Pongola structure of southeastern Africa - The world's oldest preserved rift?

NASA Technical Reports Server (NTRS)

Burke, K.; Kidd, W. S. F.; Kusky, T. M.

1985-01-01

Rocks of the Pongola Supergroup form an elongate belt in the Archean Kaapvaal Craton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, including rapid lateral variations in thickness and character of sediments, volcanic rocks that are bimodal in silica content, coarse, basement derived conglomerates and thick sequences of shallow water sedimentary facies associations, it is suggested that the Pongola Supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in earth history reveals striking similarities, suggesting that the processes that formed this rift were not significantly different from those that form continental rifts today.

Influence of margin segmentation and anomalous volcanism upon the break-up of the Hatton Bank rifted margin, west of the UK

NASA Astrophysics Data System (ADS)

Elliott, G. M.; Parson, L. M.

2007-12-01

The Hatton Bank margin, flanking the Iceland Basin is a widely cited example of a volcanic rifted margin. Prior to this study insights into the break-up history of the margin have been limited to profiles in the north and south, yet whilst valuable, the along margin tectono-magmatic variability has not been revealed. Over 5660 line km of high quality reflection seismic profiles with supplementary multibeam bathymetry were collected to support the UK's claim to Hatton region under the United Nations Convention on Law of the Sea (UNCLOS). Integration of this new data with existing profiles, allowed the margin to be divided into three segments, each of which are flanked by oceanic crust with a smooth upper surface and internal dipping reflectors. The southernmost segment is characterised by a series of inner and outer seaward dipping reflector (SDR) packages, which are separated by an outer high feature. The outer SDR are truncated by Endymion Spur, a chain of steep sided, late stage volcanic cones linked with necks. The central sector has no inner SDR package and is characterised by the presence of a highly intruded continental block, the Hatton Bank Block (HBB). The northern sector is adjacent to Lousy Bank, with a wider region of SDR recognised than to the south and a high amount of volcanic cones imaged. The variations in the distribution of the SDR's along the margin, the presence of the HBB and Endymion Spur all suggest that the break-up process was not uniform alongstrike. The division of the margin into three sectors reveals that structural segmentation played an important role in producing the variations along the margin. Break- up initiated in the south and progressed north producing the SDR packages witnessed, when the HBB was encountered the focus of break-up moved seaward of the block. The northern sector was closer to the Iceland Hotspot and hence a greater amount of volcanism is encountered. The smooth oceanic basement also indicates a high thermal flux leading to high melt production and subsidence rates forming the dipping reflectors. Shortly after break-up the eruption of Endymion Spur occurred. The nature of the magma erupted is unknown but from the steepness of the cones, it is inferred to be viscous and considering the setting, mostly likely a tholeiitic cumulate. A possible trigger for the Endymion Spur is the passage of a pulse of hotter than normal asthenospheric material along the margin, which interacted with lower crustal material to produce melt to feed the volcanic centres. Enhanced asthenospheric heat flow has been invoked to explain the V-shaped ridges along the present day Reykjanes Ridge and it is probable that the Endymion Spur represents previous such pulses along the margin/spreading axis. The location of the enhanced volcanism is itself controlled by crustal segmentation, with the Endymion Spur limited to the southern sector. The crustal thickness in this sector is approx. 2 to 3 km thinner than that found in the central segment, in which Endymion Spur is absent. The role of the segmentation along the margin has influenced the break-up style (presence or absence of SDR) and also the location and nature of post break-up volcanism.

Rift Zone Abandonment and Reconfiguration in Hawaii: Evidence from Mauna Loa’s Ninole Rift Zone

NASA Astrophysics Data System (ADS)

Morgan, J. K.; Park, J.; Zelt, C. A.

2009-12-01

Large oceanic volcanoes commonly develop elongate rift zones that disperse viscous magmas to the distal reaches of the edifice. Intrusion and dike propagation occur under tension perpendicular to the rift zone, controlled by topography, magmatic pressures, and deformation of the edifice. However, as volcanoes grow and interact, the controlling stress fields can change, potentially altering the orientations and activities of rift zones. This phenomenon is probably common, and can produce complex internal structures that influence the evolution of a volcano and its neighbors. However, little direct evidence for such rift zone reconfiguration exists, primarily due to poor preservation or recognition of earlier volcanic configurations. A new onshore-offshore 3-D seismic velocity model for the Island of Hawaii, derived from a joint tomographic inversion of an offshore airgun shot - onshore receiver geometry and earthquake sources beneath the island, demonstrates a complicated history of rift zone reconfiguration on Mauna Loa volcano, Hawaii, including wholesale rift zone abandonment. Mauna Loa’s southeast flank contains a massive high velocity intrusive complex, now buried beneath flows derived from Mauna Loa’s active southwest rift zone (SWRZ). Introduced here as the Ninole Rift Zone, this feature extends more than 60 km south of Mauna Loa’s summit, spans a depth range of ~2-14 km below sea level, and is the probable source of the 100-200 ka Ninole volcanics in several prominent erosional hills. A lack of high velocities beneath the upper SWRZ and its separate zone of high velocities on the submarine flank, indicate that the younger rift zone was built upon a pre-existing edifice that emanated from the Ninole rift zone. The ancient Ninole rift zone may stabilize Mauna Loa’s southeast flank, focusing recent volcanic activity and deformation onto the unbuttressed west flank. The upper portion of the Ninole rift zone appears to have migrated westward over time, possibly triggered by landsliding, causing its eventual abandonment in preference to Mauna Loa’s present-day SWRZ. Subsequently, the lower SWRZ broke away, tracking rift intrusions along the trace of the Kahuku detachment fault. Similar rift zone migration is thought to be underway at Kilauea volcano, and may one-day lead to the abandonment of the east rift zone. Such rift zone reconfiguration is a reflection of changing stress conditions within growing volcanoes. It is probably much more common than previously assumed, and may enable the growth of very large volcanic edifices such as Mauna Loa.

Seismic structure of the central US crust and upper mantle: Uniqueness of the Reelfoot Rift

USGS Publications Warehouse

Pollitz, Fred; Mooney, Walter D.

2014-01-01

Using seismic surface waves recorded with Earthscope's Transportable Array, we apply surface wave imaging to determine 3D seismic velocity in the crust and uppermost mantle. Our images span several Proterozoic and early Cambrian rift zones (Mid-Continent Rift, Rough Creek Graben—Rome trough, Birmingham trough, Southern Oklahoma Aulacogen, and Reelfoot Rift). While ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences, the Reelfoot Rift is unique in its association with low mantle seismic velocity. Its mantle low-velocity zone (LVZ) is exceptionally pronounced and extends down to at least 200 km depth. This LVZ is of variable width, being relatively narrow (∼50km">∼50km wide) within the northern Reelfoot Rift, which hosts the New Madrid Seismic Zone (NMSZ). We hypothesize that this mantle volume is weaker than its surroundings and that the Reelfoot Rift consequently has relatively low elastic plate thickness, which would tend to concentrate tectonic stress within this zone. No other intraplate ancient rift zone is known to be associated with such a deep mantle low-velocity anomaly, which suggests that the NMSZ is more susceptible to external stress perturbations than other ancient rift zones.

Transient cracks and triple junctions induced by Cocos-Nazca propagating rift

NASA Astrophysics Data System (ADS)

Schouten, H.; Smith, D. K.; Zhu, W.; Montesi, L. G.; Mitchell, G. A.; Cann, J. R.

2009-12-01

The Galapagos triple junction is a ridge-ridge-ridge triple junction where the Cocos, Nazca, and Pacific plates meet around the Galapagos microplate (GMP). On the Cocos plate, north of the large gore that marks the propagating Cocos-Nazca (C-N) Rift, a 250-km-long and 50-km-wide band of NW-SE-trending cracks crosscuts the N-S-trending abyssal hills of the East Pacific Rise (EPR). These appear as a succession of minor rifts, accommodating some NE-SW extension of EPR-generated seafloor. The rifts successively intersected the EPR in triple junctions at distances of 50-100 km north of the tip of the C-N Rift. We proposed a simple crack interaction model to explain the location of the transient rifts and their junction with the EPR. The model predicts that crack locations are controlled by the stress perturbation along the EPR, induced by the dominant C-N Rift, and scaled by the distance of its tip to the EPR (Schouten et al., 2008). The model also predicts that tensile stresses are symmetric about the C-N Rift and thus, similar cracks should have occurred south of the C-N Rift prior to formation of the GMP about 1 Ma. There were no data at the time to test this prediction. In early 2009 (AT 15-41), we mapped an area on the Nazca plate south of the C-N rift out to 4 Ma. The new bathymetric data confirm the existence of a distinctive pattern of cracks south of the southern C-N gore that mirrors the pattern on the Cocos plate until about 1 Ma, and lends support to the crack interaction model. The envelope of the symmetric cracking pattern indicates that the distance between the C-N Rift tip and the EPR varied between 40 and 65 km during this time (1-4 Ma). The breakdown of the symmetry at 1 Ma accurately dates the onset of a southern plate boundary of the GMP, now Dietz Deep Rift. At present, the southern rift boundary of the GMP joins the EPR with a steep-sided, 80 km long ridge. This ridge releases the stress perturbation otherwise induced along the EPR by elastic interaction with the C-N Rift and prevents the formation of minor rifts of the type in the North of the C-N Rift. However, the seafloor displays traces of rifts formed as the Dietz Deep Rift was approaching the EPR. In fact, the present day ridge appears to have originated as one of these minor rifts, probably stabilized by enhanced magma supply from a nearby volcano at the southwestern end of Dietz Deep.

Three-dimensional inversion of the magnetic field over the Easter-Nazca propagating rift near 25°S, 112°25‧W

NASA Astrophysics Data System (ADS)

Sempere, Jean-Christophe; Gee, Jeff; Naar, David F.; Hey, Richard N.

1989-12-01

The Easter microplate boundary configuration is being reorganized by rift propagation. A Sea Beam survey of the Easter-Nazca spreading center, which forms the eastern boundary of the microplate, has revealed the presence of a young propagating rift growing northward (Naar and Hey, 1986). The tip of the propagating rift is associated with a high-amplitude positive magnetic anomaly. We have performed a three-dimensional inversion of the magnetic field over the propagating rift tip area. The magnetization solution suggests that the western and eastern pseudofaults strike 014° and 338°, respectively, and converge near the rift tip. These orientations yield a propagation to spreading rate ratio of 1.5, slightly higher than the estimate of Naar and Hey (1986). Using the revised estimate of the full spreading rate along the Easter-Nazca spreading center near 25°S (80 mm/yr) (D. F. Naar and R. N. Hey, unpublished manuscript, 1989), we obtain a propagation rate of 120 mm/yr. Within 27-30 km of the rift tip, the propagating rift curves by about 15° to the east toward the failing rift, probably as a result of the interaction between the two offset spreading centers. As at the Galapagos propagating rift, rift propagation appears to be a very orderly process along the Easter-Nazca spreading center. The magnetization distribution that we obtain exhibits a high at the propagating rift tip. At other large ridge axis discontinuities, similar magnetization highs have been interpreted as being the result of the eruption of highly differentiated basalts enriched in iron. The origin of the high magnetization zone in the case of the Easter-Nazca propagating rift near 25°S may be more complex. Preliminary rock magnetic measurements of basalts recovered in the vicinity of the propagating rift confirm the presence of highly magnetized basalts but suggest that the relationship between high magnetization intensities and high Fe content is not straightforward.

Structural geology of the African rift system: Summary of new data from ERTS-1 imagery. [Precambrian influence

NASA Technical Reports Server (NTRS)

Mohr, P. A.

1974-01-01

ERTS imagery reveals for the first time the structural pattern of the African rift system as a whole. The strong influence of Precambrian structures on this pattern is clearly evident, especially along zones of cataclastic deformation, but the rift pattern is seen to be ultimately independent in origin and nature from Precambrian tectonism. Continuity of rift structures from one swell to another is noted. The widening of the Gregory rift as its northern end reflects an underlying Precambrian structural divergence, and is not a consequence of reaching the swell margin. Although the Western Rift is now proven to terminate at the Aswa Mylonite Zone, in southern Sudan, lineaments extend northeastwards from Lake Albert to the Eastern Rift at Lake Stefanie. The importance of en-echelon structures in the African rifts is seen to have been exaggerated.

The origin of strike-slip tectonics in continental rifts

NASA Astrophysics Data System (ADS)

Ebinger, C. J.; Pagli, C.; Yun, S. H.; Keir, D.; Wang, H.

2016-12-01

Although continental rifts are zones of lithospheric extension, strike-slip tectonics is also accommodated within rifts and its origin remains controversial. Here we present a combined analysis of recent seismicity, InSAR and GPS derived strain maps to reveal that the plate motion in Afar is accommodated primarily by extensional tectonics in all rift arms and lacks evidences of regional scale bookshelf tectonics. However in the rifts of central Afar we identify crustal extension and normal faulting in the central part of the rifts but strike-slip earthquakes at the rift tips. We investigate if strike-slip can be the result of Coulomb stress changes induced by recent dyking but models do not explain these earthquakes. Instead we explain strike-slips as shearing at the tips of a broad zone of spreading where extension terminates against unstretched lithosphere. Our results demonstrate that plate spreading can develop both strike-slip and extensional tectonics in the same rifts.

Inferring the width of the upwelling region at mid-ocean ridges from the throttling effect of small-offset transforms: Implications for the dynamics of `normal' and plume-influenced mid- ocean ridges

NASA Astrophysics Data System (ADS)

Phipps Morgan, J.; Ranero, C. R.

2006-12-01

The fundamental question "How wide is the upwelling and melting region beneath mid-ocean ridges (MORs)?" remains a subject of ongoing debate after 4 decades of intensive study. The basic observational difficulty is that lateral melt migration has the potential to bring melt produced within a wide subaxial region to the ~2km- wide neovolcanic zone that has been observed to be the site of almost all oceanic crustal emplacement. Here we use an indirect approach to infer this width from the minimum length of the ridge-offsets that mark the limits of the `region of influence' of on-ridge plumes on the axial relief, axial morphology, and crustal thickness along the ridge — e.g. as seen along ridge segments influenced by the Galapagos and Iceland plumes, and at the terminations of fossil volcanic rifted margins and the paleo-Azores plume-ridge interaction. We adopt Vogt's [1972] hypothesis for along-ridge asthenospheric flow in a narrow vertical slot beneath the axis of plume- influenced `macro-segments'. We find that: (1) There is a threshold distance to the lateral offsets that bound plume-influenced macrosegments; all such `barrier offsets' are greater than ~30km, while smaller offsets do not appear to be a barrier to along-axis flow. (2) Recent seismic and E-M observations along the southern EPR are consistent with a narrow westward-dipping subaxial slot. (3) A similar pattern is seen in the often abrupt transitions between volcanic and non-volcanic rifted margins, which is discussed in a companion presentation by Ranero and Phipps Morgan (this meeting). (4) A ~30km width for the region of ridge upwelling and melting offers a simple conceptual explanation for the apparent ~30km threshold length for the existence of strike-slip transform faults and the occurrence of non-transform offsets at smaller ridge offset-distances. (5) It also offers a simple conceptual explanation for the largest scale of segmentation of axial relief seen at fast-spreading ridges; these 500-1000km `long wavelength undulations of the axis' (Macdonald et al., 1989) may be macro- segments sharing a single contiguous subaxial slot that is bent but not broken beneath non-transform offsets. (6) If asthenosphere consumption by plate-spreading is less than plume-supply into a macro-segment, then the shallow seafloor and excess gravitational spreading stresses associated with a plume-influenced ridge will lead to growth of the axial slot by ridge propagation, propagation that continues until the offset of the associated migrating shear zone becomes long enough to halt it. We think this is a promising conceptual framework with which to understand the dynamic similarities and differences between plume-influenced and `normal' mid-ocean ridges.

Papers presented to the Conference on the Processes of Planetary Rifting

NASA Technical Reports Server (NTRS)

1981-01-01

The basic problems of processes of planetary rifting are addressed from the following viewpoints: (1) speculation as to the origin and development of rifts; (2) rifts on other planets; (3) tectonics; (4) geology; (5) chemistry of the lithosphere; (6) physics of the lithosphere; and (7) resources associated with rifting. The state of ignorance on the subject and its remedy is debated.

Early evolution of the southern margin of the Neuquén Basin, Argentina: Tectono-stratigraphic implications for rift evolution and exploration of hydrocarbon plays

NASA Astrophysics Data System (ADS)

D'Elia, Leandro; Bilmes, Andrés; Franzese, Juan R.; Veiga, Gonzalo D.; Hernández, Mariano; Muravchik, Martín

2015-12-01

Long-lived rift basins are characterized by a complex structural and tectonic evolution. They present significant lateral and vertical stratigraphic variations that determine diverse basin-patterns at different timing, scale and location. These issues cause difficulties to establish facies models, correlations and stratal stacking patterns of the fault-related stratigraphy, specially when exploration of hydrocarbon plays proceeds on the subsurface of a basin. The present case study corresponds to the rift-successions of the Neuquén Basin. This basin formed in response to continental extension that took place at the western margin of Gondwana during the Late Triassic-Early Jurassic. A tectono-stratigraphic analysis of the initial successions of the southern part of the Neuquén Basin was carried out. Three syn-rift sequences were determined. These syn-rift sequences were located in different extensional depocentres during the rifting phases. The specific periods of rifting show distinctly different structural and stratigraphic styles: from non-volcanic to volcanic successions and/or from continental to marine sedimentation. The results were compared with surface and subsurface interpretations performed for other depocentres of the basin, devising an integrated rifting scheme for the whole basin. The more accepted tectono-stratigraphic scheme that assumes the deposits of the first marine transgression (Cuyo Cycle) as indicative of the onset of a post-rift phase is reconsidered. In the southern part of the basin, the marine deposits (lower Cuyo Cycle) were integrated into the syn-rift phase, implying the existence of different tectonic signatures for Cuyo Cycle along the basin. The rift climax becomes younger from north to south along the basin. The post-rift initiation followed the diachronic ending of the main syn-rift phase throughout the Neuquén Basin. Thus, initiation of the post-rift stage started in the north and proceeded towards the south, constituting a diachronous post-rift event. This arrangement implies that the lower part of Cuyo Cycle, traditionally related to regional thermal subsidence, may be deposited during either mechanical subsidence or thermal subsidence according to its position within the basin.

Crustal architecture of an inverted back arc rift basin, Niigata, central Japan

NASA Astrophysics Data System (ADS)

Sato, H.; Abe, S.; Kawai, N.; Saito, H.; Kato, N.; Ishiyama, T.; Iwasaki, T.; Kurashimo, E.; Inaba, M.; Van Horne, A.

2012-04-01

A back arc rift basin, formed during the Miocene opening of the Japan Sea, now uplifted and exposed in Niigata, central Japan, provides an exceptional opportunity to study a back arc rift formed on a short time scale and in a still active setting for the present day shortening deformation. Due to stress build up before the 2011 Tohoku earthquake (M9), two damaging earthquakes (M6.8) occurred in 2004 and 2007 in this inverted rift basin. Deep seismic profiling was performed along four seismic lines between 2008 and 2011. We used onshore-offshore deep seismic reflection profiling to examine the crustal architecture of the back arc basin, in particular the geometry of the source faults. We further applied refraction tomography analysis to distinguish between previously undifferentiated syn-rift volcanics and pre-rift Mesozoic rock based on P-wave velocity. Our findings indicate that the Miocene rift structure created during the extensional phase regulates the style of deformation and the geometry of the source faults in the current compressional regime. Syn-rift volcanics with a maximum thickness of 6 km filled the fault controlled basins as rifting proceeded. The volcanism was bimodal, comprising a reflective unit of mafic rocks around the rift axis and a non-reflective unit of felsic rocks near the margins of the basins. Once rifting ended, thermal subsidence, and subsequently, mechanical subsidence related to the onset of the compressional regime, allowed deposition of up to 5 km of post-rift, deep marine to fluvial sedimentation, including the Teradomari Formation, an over-pressured mudstone in the middle of the section that later became an important shallow detachment layer. Continued compression has caused fault-related fold and wedge thrusting in the post-rift sedimentary strata which are highly deformed by thin-skin style deformation. Since the Pliocene, normal faults created during the rift phase have been reactivated as reverse faults, including a shallow detachment in the Teradomari Formation which forms a complicated shortened deformation structure. Quaternary geomorphology suggests ongoing shortening. Transform faults inherited from the rift stage control the extent of present day reverse source faults and more importantly, earthquake magnitude.

Twenty-five years of geodetic measurements along the Tadjoura-Asal rift system, Djibouti, East Africa

NASA Astrophysics Data System (ADS)

Vigny, Christophe; de Chabalier, Jean-Bernard; Ruegg, Jean-Claude; Huchon, Philippe; Feigl, Kurt L.; Cattin, Rodolphe; Asfaw, Laike; Kanbari, Khaled

2007-06-01

Since most of Tadjoura-Asal rift system sits on dry land in the Afar depression near the triple junction between the Arabia, Somalia, and Nubia plates, it is an ideal natural laboratory for studying rifting processes. We analyze these processes in light of a time series of geodetic measurements from 1978 through 2003. The surveys used triangulation (1973), trilateration (1973, 1979, and 1981-1986), leveling (1973, 1979, 1984-1985, and 2000), and the Global Positioning System (GPS, in 1991, 1993, 1995, 1997, 1999, 2001, and 2003). A network of about 30 GPS sites covers the Republic of Djibouti. Additional points were also measured in Yemen and Ethiopia. Stations lying in the Danakil block have almost the same velocity as Arabian plate, indicating that opening near the southern tip of the Red Sea is almost totally accommodated in the Afar depression. Inside Djibouti, the Asal-Ghoubbet rift system accommodates 16 ± 1 mm/yr of opening perpendicular to the rift axis and exhibits a pronounced asymmetry with essentially null deformation on its southwestern side and significant deformation on its northeastern side. This rate, slightly higher than the large-scale Arabia-Somalia motion (13 ± 1 mm/yr), suggests transient variations associated with relaxation processes following the Asal-Ghoubbet seismovolcanic sequence of 1978. Inside the rift, the deformation pattern exhibits a clear two-dimensional pattern. Along the rift axis, the rate decreases to the northwest, suggesting propagation in the same direction. Perpendicular to the rift axis, the focus of the opening is clearly shifted to the northeast, relative to the topographic rift axis, in the "Petit Rift," a rift-in-rift structure, containing most of the active faults and the seismicity. Vertical motions, measured by differential leveling, show the same asymmetric pattern with a bulge of the northeastern shoulder. Although the inner floor of the rift is subsiding with respect to the shoulders, all sites within the rift system show uplift at rates varying from 0 to 10 mm/yr with respect to a far-field reference outside the rift.

Stress perturbation associated with the Amazonas and other ancient continental rifts

USGS Publications Warehouse

Zoback, M.L.; Richardson, R.M.

1996-01-01

The state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts (specifically a dense rift pillow in the lower crust) may modify substantially the regional stress field. Both shallow (2.0-2.6 km depth) breakout data and deep (20-45 km depth) crustal earthquake focal mechanisms indicate a N to NNE maximum horizontal compression in the vicinity of the Paleozoic Amazonas rift in central Brazil. This compressive stress direction is nearly perpendicular to the rift structure and represents a ???75?? rotation relative to a regional E-W compressive stress direction in the South American plate. Elastic two-dimensional finite element models of the density structure associated with the Amazonas rift (as inferred from independent gravity modeling) indicate that elastic support of this dense feature would generate horizontal rift-normal compressional stresses between 60 and 120 MPa, with values of 80-100 MPa probably most representative of the overall structure. The observed ???75?? stress rotation constrains the ratio of the regional horizontal stress difference to the rift-normal compressive stress to be between 0.25 and 1.0, suggesting that this rift-normal stress may be from 1 to 4 times larger than the regional horizontal stress difference. A general expression for the modification of the normalized local horizontal shear stress (relative to the regional horizontal shear stress) shows that the same ratio of the rift-normal compression relative to the regional horizontal stress difference, which controls the amount of stress rotation, also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress. The potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike-slip stress regimes. In the Amazonas rift case, because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift-normal compression to be between 0.25 and 1.0, our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced (for ratios >0.5) or enhanced (for ratios <0.5). Additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust-faulting stress regime in the vicinity of the Amazonas rift. A rift-normal stress associated with the seismically active New Madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike-slip faults parallel to the axis of the rift, although this results in a 20-40% reduction in the local horizontal shear stress within the seismic zone. Sparse stress data in the vicinity of the seismically quiescent Midcontinent rift of the central United States suggest a stress state similar to that of New Madrid, with the local horizontal shear stress potentially reduced by as much as 60%. Thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows. The modeling and analysis here demonstrate that rift-normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension.

Neoproterozoic stratigraphic framework of the Tarim Craton in NW China: Implications for rift evolution

NASA Astrophysics Data System (ADS)

Wu, Lin; Guan, Shuwei; Zhang, Shuichang; Yang, Haijun; Jin, Jiuqiang; Zhang, Xiaodan; Zhang, Chunyu

2018-06-01

The Tarim Craton is overlain by thick Neoproterozoic sedimentary successions in rift tectonic setting. This study examines the latest outcrop, seismic, and drilling core data with the objective of investigating the regional stratigraphy to deeply recognize the evolution of rifting in the craton. Cryogenian to Lower Ediacaran successions are mainly composed of clastic rocks with thicknesses of 2000-3000 m, and the Upper Ediacaran successions are composed of carbonate rocks with thicknesses of 500-800 m. The rift basins and stratigraphic zones are divided into northern and southern parts by a central paleo-uplift. The northern rift basin extends through the northern Tarim Craton in an E-W direction with two depocenters (Aksu and Kuruktag). The southern rift basin is oriented NE-SW. There are three or four phases of tillites in the northern zone, while there are two in the southern zone. Given the north-south difference of the stratigraphic framework, the northern rift basin initiated at ca. 740 Ma and the southern rift basin initiated at ca. 780 Ma. During the Cryogenian and Ediacaran, the northern and southern rift basins were separated by the central paleo-uplift, finally connecting with each other in the early Cambrian. Tectonic deformation in the Late Ediacaran led to the formation of a parallel unconformity in the rift basins and an angular unconformity in the central paleo-uplift. The Neoproterozoic rift basins continued to affect the distribution of Lower Cambrian hydrocarbon source rocks. The north-south distribution and evolution of the rift basins in the Tarim Craton have implications for reconstructions of the Rodinia supercontinent.

Seismicity within a propagating ice shelf rift: the relationship between icequake locations and ice shelf structure

USGS Publications Warehouse

Heeszel, David S.; Fricker, Helen A.; Bassis, Jeremy N.; O'Neel, Shad; Walter, Fabian

2014-01-01

Iceberg calving is a dominant mass loss mechanism for Antarctic ice shelves, second only to basal melting. An important known process involved in calving is the initiation and propagation of through-penetrating fractures called rifts; however, the mechanisms controlling rift propagation remain poorly understood. To investigate the mechanics of ice-shelf rifting, we analyzed seismicity associated with a propagating rift tip on the Amery Ice Shelf, using data collected during the Austral summers of 2004-2007. We investigated seismicity associated with fracture propagation using a suite of passive seismological techniques including icequake locations, back projection, and moment tensor inversion. We confirm previous results that show that seismicity is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of one to three hours. However, even during periods of quiescence, we find significant seismic deformation around the rift tip. Moment tensors, calculated for a subset of the largest icequakes (MW > -2.0) located near the rift tip, show steeply dipping fault planes, horizontal or shallowly plunging stress orientations, and often have a significant volumetric component. They also reveal that much of the observed seismicity is limited to the upper 50 m of the ice shelf. This suggests a complex system of deformation that involves the propagating rift, the region behind the rift tip, and a system of rift-transverse crevasses. Small-scale variations in the mechanical structure of the ice shelf, especially rift-transverse crevasses and accreted marine ice, play an important role in modulating the rate and location of seismicity associated with propagating ice shelf rifts.

Geophysical Characteristics of the Australian-Antarctic Ridge

NASA Astrophysics Data System (ADS)

Kim, S. S.; Lin, J.; Park, S. H.; Choi, H.; Lee, S. M.

2014-12-01

Between 2011 and 2013, the Korea Polar Research Institute (KOPRI) conducted three consecutive geologic surveys at the little explored eastern ends of the Australian-Antarctic Ridge (AAR) to characterize the tectonics, geochemistry, and hydrothermal activity of this intermediate spreading system. Using the Korean icebreaker R/V Araon, the multi-disciplinary research team collected bathymetry, gravity, magnetics, and rock and water column samples. In addition, Miniature Autonomous Plume Recorders (MAPRs) were deployed at wax-core rock sampling sites to detect the presence of active hydrothermal vents. Here we present a detailed analysis of a 300-km-long supersegment of the AAR to quantify the spatial variations in ridge morphology and robust axial and off-axis volcanisms. The ridge axis morphology alternates between rift valleys and axial highs within relatively short ridge segments. To obtain a geological proxy for regional variations in magma supply, we calculated residual mantle Bouguer gravity anomalies (RMBA), gravity-derived crustal thickness, and residual topography for seven sub-segments. The results of the analyses revealed that the southern flank of the AAR is associated with shallower seafloor, more negative RMBA, thicker crust, and/or less dense mantle than the conjugate northern flank. Furthermore, this north-south asymmetry becomes more prominent toward the KR1 supersegment of the AAR. The axial topography of the KR1 supersegment exhibits a sharp transition from axial highs at the western end to rift valleys at the eastern end, with regions of axial highs being associated with more magma supply as indicated by more negative RMBA. We also compare and contrast the characteristics of the AAR supersegment with that of other ridges of intermediate spreading rates, including the Juan de Fuca Ridge, Galápagos Spreading Center, and Southeast Indian Ridge west of the Australian-Antarctic Discordance, to investigate the influence of ridge-hotspot interaction on ridge magma supply and tectonics.

Relationship between tectonics and magmatism on Faial island (Azores, Portugal)

NASA Astrophysics Data System (ADS)

Trippanera, D.; Salvatore, M.; Porreca, M.; Ruch, J.; Pimentel, A.; Pacheco, J.; Acocella, V.

2012-04-01

The Azores Islands are located on the triple junction involving Eurasian, Nubian and North American plates. Faial is the nearest island to the Atlantic Ridge and one of the most active, with the 1957-58 Capelinhos eruption and the 1998 earthquake. Faial consists of three main structural features: a well exposed graben structure (eastern sector), a stratovolcano with a summit caldera (central part) and a fissure zone peninsula (western part). To analyse the relationships between magmatic and tectonic activity at Faial we use a multidisciplinary approach based on: 1) remote sensing analysis (DEM and aerial photographs); 2) geological field survey and 3) paleomagnetic analysis. The age of volcanism in Faial is not well constrained. Our paleomagnetic results show that the oldest rocks of the island have a reverse polarity, implying that they are older than 780 ka (Brunhes-Matuyama polarity transition). The structural data indicate that the main fault system, including the graben structure, is WNW-ESE oriented and shows a general transtensive kinematics with a dextral component and a NE-SW oriented extension direction of the island. Most of the dikes, volcanic vent alignments and extensional fractures are sub-parallel to the main fault system (WNW-ESE). A secondary system of fractures and dikes is NNE-SSW oriented. Inside the graben, the bedding attitude is parallel to the direction of the axis of the graben and dipping outward. This attitude suggests an outward tilt of the blocks between the faults and that the graben consists of two oppositely verging-dominoes. We have estimated the stretching factor (β=1,35) and the minimum extensional rate (2,54 ± 0.08 mm/a) of the graben. The obtained direction and rate of the extension within the Faial graben are similar to those of the nearby Terceira Rift. The absence of a clear westward continuity of the latter suggests that the Faial - Pico magmatic segment could be the SW continuation of the segmented Terceira Rift, above the current hot spot.

Inherited discontinuities and fault kinematics of a multiphase, non-colinear extensional setting: Subsurface observations from the South Flank of the Golfo San Jorge basin, Patagonia

NASA Astrophysics Data System (ADS)

Paredes, José Matildo; Aguiar, Mariana; Ansa, Andrés; Giordano, Sergio; Ledesma, Mario; Tejada, Silvia

2018-01-01

We use three-dimensional (3D) seismic reflection data to analyze the structural style, fault kinematics and growth fault mechanisms of non-colinear normal fault systems in the South Flank of the Golfo San Jorge basin, central Patagonia. Pre-existing structural fabrics in the basement of the South Flank show NW-SE and NE-SW oriented faults. They control the location and geometry of wedge-shaped half grabens from the "main synrift phase" infilled with Middle Jurassic volcanic-volcaniclastic rocks and lacustrine units of Late Jurassic to Early Cretaceous age. The NE-striking, basement-involved normal faults resulted in the rapid establishment of fault lenght, followed by gradual increasing in displacement, and minor reactivation during subsequent extensional phases; NW-striking normal faults are characterized by fault segments that propagated laterally during the "main rifting phase", being subsequently reactivated during succesive extensional phases. The Aptian-Campanian Chubut Group is a continental succession up to 4 km thick associated to the "second rifting stage", characterized by propagation and linkage of W-E to WNW-ESE fault segments that increase their lenght and displacement in several extensional phases, recognized by detailed measurement of current throw distribution of selected seismic horizons along fault surfaces. Strain is distributed in an array of sub-parallel normal faults oriented normal to the extension direction. A Late Cretaceous-Paleogene (pre-late Eocene) extensional event is characterized by high-angle, NNW-SSE to NNE-SSW grabens coeval with intraplate alkali basaltic volcanism, evidencing clockwise rotation of the stress field following a ∼W-E extension direction. We demonstrate differences in growth fault mechanisms of non-colinear fault populations, and highlight the importance of follow a systematic approach to the analysis of fault geometry and throw distribution in a fault network, in order to understand temporal-spatial variations in the coeval topography, potential structural traps, and distribution of oil-bearing sandstone reservoirs.

The life cycle of continental rifting as a focus for U.S.-African scientific collaboration

NASA Astrophysics Data System (ADS)

Abdelsalam, Mohamed G.; Atekwana, Estella A.; Keller, G. Randy; Klemperer, Simon L.

2004-11-01

The East African Rift System (EARS) provides the unique opportunity found nowhere else on Earth, to investigate extensional processes from incipient rifting in the Okavango Delta, Botswana, to continental breakup and creation of proto-oceanic basins 3000 km to the north in the Afar Depression in Ethiopia, Eritrea, and Djibouti.The study of continental rifts is of great interest because they represent the initial stages of continental breakup and passive margin development, they are sites for large-scale sediment accumulation, and their geomorphology may have controlled human evolution in the past and localizes geologic hazards in the present. But there is little research that provides insights into the linkage between broad geodynamic processes and the life cycle of continental rifts: We do not know why some rifts evolve into mid-ocean ridges whereas others abort their evolution to become aulacogens. Numerous studies of the EARS and other continental rifts have significantly increased our understanding of rifting processes, but we particularly lack studies of the embryonic stages of rift creation and the last stages of extension when continental breakup occurs.

Midplate seismicity exterior to former rift-basins

USGS Publications Warehouse

Dewey, J.W.

1988-01-01

Midplate seismicity associated with some former rift-zones is distributed diffusely near, but exterior to, the rift basins. This "basin-exterior' seismicity cannot be attributed to reactivation of major basin-border faults on which uppercrustal extension was concentrated at the time of rifting, because the border faults dip beneath the basins. The seismicity may nonetheless represent reactivation of minor faults that were active at the time of rifting but that were located outside of the principal zones of upper-crustal extension; the occurrence of basin-exterior seismicity in some present-day rift-zones supports the existence of such minor basin-exterior faults. Other hypotheses for seismicity exterior to former rift-basins are that the seismicity reflects lobes of high stress due to lithospheric-bending that is centered on the axis of the rift, that the seismicity is localized on the exteriors of rift-basins by basin-interiors that are less deformable in the current epoch than the basin exteriors, and that seismicity is localized on the basin-exteriors by the concentration of tectonic stress in the highly elastic basin-exterior upper-crust. -from Author

Quantitative analysis of the tectonic subsidence in the Potiguar Basin (NE Brazil)

NASA Astrophysics Data System (ADS)

Lopes, Juliana A. G.; de Castro, David L.; Bertotti, Giovanni

2018-06-01

The Potiguar Basin, located in the Brazilian Equatorial Margin, evolved from a complex rifting process implemented during the Atlantic Ocean opening in the Jurassic/Cretaceous. Different driving mechanisms were responsible for the onset of an aborted onshore rift and an offshore rift that initiated crustal rupture and the formation of a continental transform margin. Therefore, we applied the backstripping method to quantify the tectonic subsidence during the rift and post-rift phases of Potiguar Basin formation and to analyze the spatial variation of subsidence during the two successive and distinct tectonic events responsible for the basin evolution. The parameters required to apply this methodology were extracted from 2D seismic lines and exploratory well data. The tectonic subsidence curves present periods with moderate subsidence rates (up to 300 m/My), which correspond to the evolution of the onshore Potiguar Rift (∼141 to 128 Ma). From 128-118 Ma, the tectonic subsidence curves show no subsidence in the onshore Potiguar Basin, whereas subsidence occurred at high rates (over 300 m/My) in the offshore rift. The post-rift phase began ca. 118 Ma (Aptian), when the tectonic subsidence drastically slowed to less than 35 m/My, probably related to thermal relaxation. The tectonic subsidence rates in the various sectors of the Potiguar Rift, during the different rift phases, indicate that more intense faulting occurred in the southern portion of the onshore rift, along the main border faults, and in the southeastern portion of the offshore rift. During the post-rift phase, the tectonic subsidence rates increased from the onshore portion towards the offshore portion until the continental slope. The highest rates of post-rift subsidence (up to 35 m/My) are concentrated in the central region of the offshore portion and may be related to lithospheric processes related to the continental crust rupture and oceanic seafloor spreading. The variation in subsidence rates and the pattern of tectonic subsidence curves allowed us to interpret the tectonic signature recorded by the sedimentary sequences of the Potiguar Basin during its evolution. In the onshore rift area, the tectonic subsidence curves presented subsidence rates up to 300 m/My during a long-term rift phase (13 Ma), which confirmed that this portion had an extensional tectonic regime. In the offshore rift, the curves presented high subsidence rates of over 300 m/My in a shorter period (5-10 My), typical of basins formed in a transtensional tectonic regime.

A global census of continental rift activity since 250 Ma reveals a missing element of the deep carbon cycle

NASA Astrophysics Data System (ADS)

Brune, Sascha; Williams, Simon; Müller, Dietmar

2017-04-01

The deep carbon cycle connects CO2 concentrations within the atmosphere to the vast carbon reservoir in Earth's mantle: subducted lithosphere carries carbon into the mantle, while extensional plate boundaries and arc volcanoes release it back to Earth's surface. The length of plate boundaries thereby exerts first-order control on global CO2 fluxes on geological time scales. Here we provide a global census of rift length from the Triassic to present day, combining a new plate reconstruction analysis technique with data from the geological rift record. We find that the most extensive rift phase during the fragmentation of Pangea occurred in the Jurassic/Early Cretaceous with extension along the South Atlantic (9700 km) and North Atlantic rifts (9100 km), within East Gondwana (8500 km), the failed African rift systems (4900 km), and between Australia and Antarctica (3700 km). The combined extent of these and other rift systems amounts to more than 50.000 km of simultaneously active continental rifts. During the Late Cretaceous, in the aftermath of this massive rift episode, the global rift length dropped by 60% to 20.000 km. We further show that a second pronounced rift episode starts in the Eocene with global rift lengths of up to 30.000 km. It is well-accepted that volcanoes at plate boundaries release large amounts of CO2 from the Earth's interior. Recent work, however, highlights the importance of deep-cutting faults and diffuse degassing on CO2 emissions in the East African Rift, which appear to be comparable to CO2 release rates at mid-ocean ridges worldwide. Upscaling measured CO2 fluxes from East Africa to all concurrently active global rift zones with due caution, we compute the first-order history of cumulative rift-related CO2 degassing rates for the last 250 Myr. We demonstrate that rift-related CO2 release in the Early Cretaceous may have reached 400% of present-day rates. In first-order agreement with paleo-atmospheric CO2 concentrations from proxy indicators, our degassing rates correlate with the two distinct periods of elevated atmospheric CO2 in the Mesozoic and Cenozoic. Compiling the length of other plate boundaries through time (mid-ocean ridges, subduction zones, continental arcs), we do not find such a correlation with the paleo-CO2 record, which leads us to suggest that rift-related degassing constitutes an important element of the deep carbon cycle.

Implications of new gravity data for Baikal Rift zone structure

NASA Technical Reports Server (NTRS)

Ruppel, C.; Kogan, M. G.; Mcnutt, M. K.

1993-01-01

Newly available, 2D Bouguer gravity anomaly data from the Baikal Rift zone, Siberia, indicate that this discrete, intracontinental rift system is regionally compensated by an elastic plate about 50 km thick. However, spectral and spatial domain analyses and isostatic anomaly calculations show that simple elastic plate theory does not offer an adequate explanation for compensation in the rift zone, probably because of significant lateral variations in plate strength and the presence of subsurface loads. Our results and other geophysical observations support the interpretation that the Baikal Rift zone is colder than either the East African or Rio Grande rift.

Main Ethiopian Rift Kinematic analogue modeling: Implications for Nubian-Somalian plate motion.

NASA Astrophysics Data System (ADS)

Erbello, A.; Corti, G.; Sani, F.; Agostini, A.; Buccianti, A.; Kidane, T. B.

2016-12-01

In this contribution, analogue modeling is used to provide new insights into the kinematics of the Nubia and Somalia plates responsible for development and evolution of the Main Ethiopian Rift (MER), at the northern termination of the East African Rift. In particular, we performed new crustal-scale, brittle models to analyze the along-strike variations in fault architecture in the MER and their relations with the rift trend, plate motion and the resulting Miocene-recent kinematics of rifting. The models reproduced the overall geometry of the 600km-long MER with its along-strike variation in orientation to test different hypothesis proposed to explain rift evolution. Analysis of model results in terms of statistics of fault length and orientation, as well as deformation architecture, and its comparison with the MER suggests that models of two-phase rifting (with a first phase of NW-SE extension followed by E-W rifting) or constant NW-SE extension, as well as models of constant ENE-WSW rifting are not able to reproduce the fault architecture observed in nature. Model results suggest instead that the rift has likely developed under a constant, post-11 Ma extension oriented roughly ESE-WNW (N97.5°E), consistent with recent plate kinematics models.

Modeling along-axis variations in fault architecture in the Main Ethiopian Rift: Implications for Nubia-Somalia kinematics

NASA Astrophysics Data System (ADS)

Erbello, Asfaw; Corti, Giacomo; Agostini, Andrea; Sani, Federico; Kidane, Tesfaye; Buccianti, Antonella

2016-12-01

In this contribution, analogue modeling is used to provide new insights into the Nubia-Somalia kinematics responsible for development and evolution of the Main Ethiopian Rift (MER), at the northern termination of the East African Rift system. In particular, we performed new crustal-scale, brittle models to analyze the along-strike variations in fault architecture in the MER and their relations with the rift trend, plate motion and the resulting Miocene-recent kinematics of rifting. The models reproduced the overall geometry of the ∼600 km-long MER with its along-strike variation in orientation to test different hypothesis proposed to explain rift evolution. Analysis of model results in terms of statistics of fault length and orientation, as well as deformation architecture, and its comparison with the MER suggest that models of two-phase rifting (with a first phase of NW-SE extension followed by E-W rifting) or constant NW-SE extension, as well as models of constant ENE-WSW rifting are not able to reproduce the fault architecture observed in nature. Model results suggest instead that the rift has likely developed under a constant, post-11 Ma extension oriented roughly ESE-WNW (N97.5°E), consistent with recent plate kinematics models.

The Phuket Terrane: A Late Palaeozoic rift at the margin of Sibumasu

NASA Astrophysics Data System (ADS)

Ridd, Michael F.

2009-09-01

It is widely accepted that Sibumasu rifted from Gondwana in the Late Palaeozoic. But the rifts themselves have not previously been documented in Southeast Asia. This paper identifies the pre-Middle Permian Kaeng Krachan Group of Upper Peninsular Thailand as the infill of one such rift, which is given the name Phuket Terrane. Indirect evidence suggests the rift-infill is several kilometres thick and glacially-influenced diamictites are conspicuous in the succession. There are significant similarities with the >3 km thick pre-Middle Permian rift-infill of the Carnarvon Basin of Western Australia. East of the Khlong Marui Fault belt the succession is thinner and diamictites are a minor component. A tectono-stratigraphic model is proposed involving Gondwana glaciers dropping their load at the (present) western margin of the Phuket Terrane from where it was re-sedimented in the rapidly subsiding marine rift basin. It is suggested that the Khlong Marui Fault formed part of the eastern boundary of the rift system. The Three Pagodas Fault belt similarly juxtaposes different pre-Middle Permian successions. Rifting ceased in the Early Permian and a passive margin formed as the Mesotethys ocean widened, the upper part of the Kaeng Krachan Group and the overlying Ratburi Limestone representing the post-rift sequence.

The role of long-term strain history on the generation and amplification of inherited heterogeneities in continental lithosphere extensional settings

NASA Astrophysics Data System (ADS)

Morena Salerno, V.; Capitanio, Fabio A.

2017-04-01

The Earth's lithosphere is characters by various types of heterogeneities, at different scales and located at variable depth. They can be represented at crustal level by remnants of earlier tectonics evolution, such as previous orogenetic structures, remains of passive margins and magmatic bodies intrusion, or at deeper level by mantle anisotropies. These heterogeneities can severely affect the stress and strain localization in subsequent continental lithospheric extension and rift basins evolution, hence contributing to the formation of diverse and complex rift basin types and architectures. In order to explain the difference in rift basin and passive margin types, their subsidence patterns and melt production, previous studies have exanimated the role of initial heterogeneities, rheological layering, geothermal gradients, and extension rates during a single rifting event. However, this approach does not consider the previous strain history of many basins that are characterized by multiple rifting events. In this study we use numerical models of a pristine lithosphere undergoing two rifting events separated by cooling, to show the effect of early events on later evolution. The strain histories are controlled by the variation of velocity of boundary displacement during two rifting events. We use both fast and slow first rifting events, followed by a cooling period, producing diverse mechanical heterogeneities at Moho level that represent inherited initial conditions for the second rifting event. These inherited heterogeneities range from several small perturbations distributed along the numerical domain at the end of the slowest first rifting event, to a single large perturbation at the end of first fastest rifting event. In the second rifting event, the inherited heterogeneities are amplified at different degree and time, depending on the velocity of boundary displacement used. To highlight the role of previous strain history, we parametrize the inherited heterogeneities by calculating localization indexes for all the models at the onset of the second rifting event. This calculation embeds the inherited rheology from the previous rift event. We show that the lithosphere progressively localises along the inherited heterogeneities leading to the formation of various rift basin types, ranging from narrow to wide to hyperextended and with variation degrees of symmetry. Our result show that rift basin types and structural styles are strongly affected by inherited heterogeneities generated from previous rifting events, showing cases in which the previous strain history cannot be neglected. The subsidence patterns and melt production result to be very sensitive to the strain history, the type of inherited heterogeneities and their interplay with variation of boundary displacement velocity. Our numerical simulations replicate the first-order features of rift basins and provide a general framework to assess the inherited heterogeneities' role in the interpretation of extensional basins and their evolution.

Aerosol and Cloud Microphysical Characteristics of Rifts and Gradients in Maritime Stratocumulus Clouds

NASA Technical Reports Server (NTRS)

Sharon, Tarah M.; Albrecht, Bruce A.; Jonsson, Haflidi H.; Minnis, Patrick; Khaiyer, Mandana M.; Van Reken, Timothy; Seinfeld, John; Flagan, Rick

2008-01-01

A cloud rift is characterized as a large-scale, persistent area of broken, low reflectivity stratocumulus clouds usually surrounded by a solid deck of stratocumulus. A rift observed off the coast of Monterey Bay, California on 16 July 1999 was studied to compare the aerosol and cloud microphysical properties in the rift with those of the surrounding solid stratus deck. Variables measured from an instrumented aircraft included temperature, water vapor, and cloud liquid water. These measurements characterized the thermodynamic properties of the solid deck and rift areas. Microphysical measurements made included aerosol, cloud drop and drizzle drop concentrations and cloud condensation nuclei (CCN) concentrations. The microphysical characteristics in a solid stratus deck differ substantially from those of a broken, cellular rift where cloud droplet concentrations are a factor of 2 lower than those in the solid cloud. Further, CCN concentrations were found to be about 3 times greater in the solid cloud area compared with those in the rift and aerosol concentrations showed a similar difference as well. Although drizzle was observed near cloud top in parts of the solid stratus cloud, the largest drizzle rates were associated with the broken clouds within the rift area. In addition to marked differences in particle concentrations, evidence of a mesoscale circulation near the solid cloud rift boundary is presented. This mesoscale circulation provides a mechanism for maintaining a rift, but further study is required to understand the initiation of a rift and the conditions that may cause it to fill.

Deformation and seismicity associated with continental rift zones propagating toward continental margins

NASA Astrophysics Data System (ADS)

Lyakhovsky, V.; Segev, A.; Schattner, U.; Weinberger, R.

2012-01-01

We study the propagation of a continental rift and its interaction with a continental margin utilizing a 3-D lithospheric model with a seismogenic crust governed by a damage rheology. A long-standing problem in rift-mechanics, known as thetectonic force paradox, is that the magnitude of the tectonic forces required for rifting are not large enough in the absence of basaltic magmatism. Our modeling results demonstrate that under moderate rift-driving tectonic forces the rift propagation is feasible even in the absence of magmatism. This is due to gradual weakening and "long-term memory" of fractured rocks that lead to a significantly lower yielding stress than that of the surrounding intact rocks. We show that the style, rate and the associated seismicity pattern of the rift zone formation in the continental lithosphere depend not only on the applied tectonic forces, but also on the rate of healing. Accounting for the memory effect provides a feasible solution for thetectonic force paradox. Our modeling results also demonstrate how the lithosphere structure affects the geometry of the propagating rift system toward a continental margin. Thinning of the crystalline crust leads to a decrease in the propagation rate and possibly to rift termination across the margin. In such a case, a new fault system is created perpendicular to the direction of the rift propagation. These results reveal that the local lithosphere structure is one of the key factors controlling the geometry of the evolving rift system and seismicity pattern.

The evolving contribution of border faults and intra-rift faults in early-stage East African rifts: insights from the Natron (Tanzania) and Magadi (Kenya) basins

NASA Astrophysics Data System (ADS)

Muirhead, J.; Kattenhorn, S. A.; Dindi, E.; Gama, R.

2013-12-01

In the early stages of continental rifting, East African Rift (EAR) basins are conventionally depicted as asymmetric basins bounded on one side by a ~100 km-long border fault. As rifting progresses, strain concentrates into the rift center, producing intra-rift faults. The timing and nature of the transition from border fault to intra-rift-dominated strain accommodation is unclear. Our study focuses on this transitional phase of continental rifting by exploring the spatial and temporal evolution of faulting in the Natron (border fault initiation at ~3 Ma) and Magadi (~7 Ma) basins of northern Tanzania and southern Kenya, respectively. We compare the morphologies and activity histories of faults in each basin using field observations and remote sensing in order to address the relative contributions of border faults and intra-rift faults to crustal strain accommodation as rifting progresses. The ~500 m-high border fault along the western margin of the Natron basin is steep compared to many border faults in the eastern branch of the EAR, indicating limited scarp degradation by mass wasting. Locally, the escarpment shows open fissures and young scarps 10s of meters high and a few kilometers long, implying ongoing border fault activity in this young rift. However, intra-rift faults within ~1 Ma lavas are greatly eroded and fresh scarps are typically absent, implying long recurrence intervals between slip events. Rift-normal topographic profiles across the Natron basin show the lowest elevations in the lake-filled basin adjacent to the border fault, where a number of hydrothermal springs along the border fault system expel water into the lake. In contrast to Natron, a ~1600 m high, densely vegetated, border fault escarpment along the western edge of the Magadi basin is highly degraded; we were unable to identify evidence of recent rupturing. Rift-normal elevation profiles indicate the focus of strain has migrated away from the border fault into the rift center, where faults pervasively dissect 1.2-0.8 Ma trachyte lavas. Unlike Natron, intra-rift faults in the Magadi basin exhibit primarily steep, little-degraded fault scarps, implying greater activity than Natron intra-rift faults. Numerous fault-associated springs feed water into perennial Lake Magadi, which has no surface drainage input, yet survives despite a high evaporation rate that has created economically viable evaporite deposits. Calcite vein-filled joints are common along fault zones around Lake Magadi, as well as several cm veins around columnar joints that imply isotropic expansion of the fracture network under high pressures of CO2-rich fluids. Our work indicates that the locus of strain in this portion of the EAR transfers from the border fault to the center of the rift basin some time between 3 and 7 million years after rift initiation. This transition likely reflects the evolving respective roles of crustal flexure and magma budget in focusing strain, as well as the hydrothermal fluid budget along evolving fault zones.

Crustal tomographic imaging of a transitional continental rift: the Ethiopian rift

NASA Astrophysics Data System (ADS)

Daly, E.; Keir, D.; Ebinger, C. J.; Stuart, G. W.; Bastow, I. D.; Ayele, A.

2008-03-01

In this study we image crustal structure beneath a magmatic continental rift to understand the interplay between crustal stretching and magmatism during the late stages of continental rifting: the Main Ethiopian Rift (MER). The northern sector of this region marks the transition from continental rifting in the East African Rift to incipient seafloor spreading in the southern Red Sea and western Gulf of Aden. Our local tomographic inversion exploits 172 broad-band instruments covering an area of 250 × 350 km of the rift and adjacent plateaux. The instruments recorded a total of 2139 local earthquakes over a 16-month period. Several synthetic tests show that resolution is good between 12 and 25 km depth (below sea level), but some horizontal velocity smearing is evident along the axis of the Main Ethiopian Rift below 16 km. We present a 3-D P-wave velocity model of the mid-crust and present the first 3-D Vp/Vs model of the region. Our models show high P-wave velocities (6.5 km s-1) beneath the axis of the rift at a depth of 12-25 km. The presence of high Vp/Vs ratios (1.81-1.84) at the same depth range suggest that they are cooled mafic intrusions. The high Vp/Vs values, along with other geophysical evidence, suggest that dyking is pervasive beneath the axis of the rift from the mid-crustal depths to the surface and that some portion of partial melt may exist at lower crustal depths. Although the crustal stretching factor across the Main Ethiopian Rift is ~1.7, our results indicate that magma intrusion in narrow zones accommodates a large proportion of extensional strain, with similarities to slow-spreading mid-ocean ridge processes.

Seismological Constraints on the Magmato-tectonic Behavior of the Asal-Ghoubbet Rift (Afar Depression, Republic of Djibouti) Since the Last 1978-Rifting Episode

NASA Astrophysics Data System (ADS)

Doubre, C.; Manighetti, I.; Bertil, D.; Dorbath, C.; Dorbath, L.; Jacques, E.

2004-12-01

The Asal-Ghoubbet rift was the locus of a seismic and volcanic crisis in 1978 followed by 8 years of rapid opening (60 mm/yr) before returning to its long-term opening rate of 16 mm/yr. We analyze the space-time evolution of the seismicity that occurred in the rift between 1979 and 2001. The data recorded by the Djibouti Observatory provide only hypocentral locations before 1995 and P and S-wave arrival times since 1996. Additional data acquired during a five months experiment in 2000-2001 allowed us to determine a 3D-velocity model of the rift, used to precisely relocate post 1996 events. The 2545 small-magnitude earthquakes (Md ≤ 3.2) recorded in the rift since the 1978 crisis provide a negligible contribution to the total extension across the rift, which occurs essentially aseismically. The temporal evolution of the seismicity reveals two distinct phases consistent with those observed in the geodetic data. The post-crisis period (1979-1986) is characterized by large-magnitude earthquakes exclusively located below the northern rift shoulder. These events are associated with the contraction of the side of the rift resulting from the fast opening of the central dyke system. The subsequent period (1987-2001) corresponding to normal opening rate across the rift is characterized by a micro-seismicity essentially located below the major rift caldera (Fieale). Most recorded events during this period concentrate within the rift inner floor at the top of an aseismic, low velocity zone located below the Fiale caldera, which we interpret as hot material above the magma chamber. Outside from post-crisis periods, the seismicity tends to cluster in time in response to stress changes in the brittle layer induced by episodic magmatic movements.

Paleorift structure constrained by gravity and stratigraphic data: The Statherian Araí rift case

NASA Astrophysics Data System (ADS)

Martins-Ferreira, Marco Antonio Caçador; Campos, José Eloi Guimarães; Von Huelsen, Monica Giannoccaro; Neri, Brandow Lee

2018-07-01

Gravimetric and stratigraphic data were used to investigate the Paleoproterozoic Araí Paleorift, a failed Statherian continental rift located in the western margin of the São Francisco Craton, where basement and cover were affected by the Neoproterozoic Brasiliano Orogeny. Euler deconvolution, tilt, total horizontal gradient amplitude and upward continuation technics were applied to terrestrial gravimetric data in order to investigate the rift's main faults location, direction and depth, allowing to identify its main horsts, grabens, volcanic and plutonic centers. We found that rift faults occur to a maximum depth of ca. 38 km, but major fault throw occurs from 4 to 8 km deep and attenuates from 8 to 12 km, probably the brittle-ductile transition zone at the time of rifting, practically disappearing at 20 km. Stratigraphic data and basement mapping were used in order to constraint gravimetric results. We classify the Araí Rift as a passive, three-armed failed rift, narrow to divergent type, that produced preferably anorogenic rapakivi-related magmas, most of it still lodged in the crust from surface down to ca. 19 km deep and subsidiary mafic magmatism. The results indicate the deep occurrence of low-density magmas beneath the rift's main axis, detected up to 20 km deep. Correlation to other global Statherian rifts show that the São Francisco Craton was strongly affected by taphrogenesis during the Statherian, together with Siberia, North America and North China cratons. Finally, by comparing our results to recent rifts we found that the Ethiopian rift's morphology is quite similar to the Araí. Surrounding the Tanzanian craton, the Cenozoic East Africa rift system morphology is compared to the Araí-Espinhaço rift system, which surrounds the São Francisco craton. The major contribution of this paper is the recognition of Araí Paleorift surface and subsurface morphology, up to now unknown, over an area of ca. 45.000 km2.

Seismic Investigations of an Accommodation zone in the Northern Rio Grande Rift, New Mexico, USA

NASA Astrophysics Data System (ADS)

Baldridge, W. S.; Valdes, J.; Nedorub, O.; Phrampus, B.; Braile, L. W.; Ferguson, J. F.; Benage, M. C.; Litherland, M.

2010-12-01

Seismic reflection and refraction data acquired in the Rio Grande rift near Santa Fe, New Mexico, in 2009 and 2010 by the SAGE (Summer of Applied Geophysical Experience) program imaged the La Bajada fault (LBF) and strata offset across the associated, perpendicular Budagher fault (BF). The LBF is a major basin-bounding normal fault, offset down to the west; the smaller BF is an extensional fault that breaks the hanging wall ramp of the LBF. We chose this area because it is in a structurally complex region of the rift, comprising a small sub-basin and plunging relay ramps, where north-trending, en echelon basin-bounding faults (including the LBF) transfer crustal extension laterally between the larger Española (to north) and Albuquerque rift basins. Our data help determine the precise location and geometry of the poorly exposed LBF, which, near the survey location, offsets the rift margin vertically about 3,000 m. When integrated with industry reflection data and other SAGE seismic, gravity, and magnetotelluric surveys, we are able to map differences in offset and extension laterally (especially southward) along the fault. We interpret only about 200 m of normal offset across the BF. Our continuing work helps define multiple structural elements, partly buried by syn-rift basin-filling sedimentary rocks, of a complex intra-rift accommodation zone. We are also able to discriminate pre-Eocene (Laramide) from post-Miocene (rift) structures. Our data help determine the amount of vertical offset of pre-rift strata across structural elements of the accommodation zone, and depth and geometry of basin fill. A goal is to infer the kinematic development of this margin of the rift, linkages among faults, growth history, and possible pre-rift structural controls. This information will be potentially useful for evaluation of resources, including oil and/or gas in pre-rift strata and ground water in Late Miocene to Holocene rift-filling units.

A model for Iapetan rifting of Laurentia based on Neoproterozoic dikes and related rocks

USGS Publications Warehouse

Burton, William C.; Southworth, Scott

2010-01-01

Geologic evidence of the Neoproterozoic rifting of Laurentia during breakup of Rodinia is recorded in basement massifs of the cratonic margin by dike swarms, volcanic and plutonic rocks, and rift-related clastic sedimentary sequences. The spatial and temporal distribution of these geologic features varies both within and between the massifs but preserves evidence concerning the timing and nature of rifting. The most salient features include: (1) a rift-related magmatic event recorded in the French Broad massif and the southern and central Shenandoah massif that is distinctly older than that recorded in the northern Shenandoah massif and northward; (2) felsic volcanic centers at the north ends of both French Broad and Shenandoah massifs accompanied by dike swarms; (3) differences in volume between massifs of cover-sequence volcanic rocks and rift-related clastic rocks; and (4) WNW orientation of the Grenville dike swarm in contrast to the predominately NE orientation of other Neoproterozoic dikes. Previously proposed rifting mechanisms to explain these features include rift-transform and plume–triple-junction systems. The rift-transform system best explains features 1, 2, and 3, listed here, and we propose that it represents the dominant rifting mechanism for most of the Laurentian margin. To explain feature 4, as well as magmatic ages and geochemical trends in the Northern Appalachians, we propose that a plume–triple-junction system evolved into the rift-transform system. A ca. 600 Ma mantle plume centered east of the Sutton Mountains generated the radial dike swarm of the Adirondack massif and the Grenville dike swarm, and a collocated triple junction generated the northern part of the rift-transform system. An eastern branch of this system produced the Long Range dike swarm in Newfoundland, and a subsequent western branch produced the ca. 554 Ma Tibbit Hill volcanics and the ca. 550 Ma rift-related magmatism of Newfoundland.

A refinement of the chronology of rift-related faulting in the Broadly Rifted Zone, southern Ethiopia, through apatite fission-track analysis

NASA Astrophysics Data System (ADS)

Balestrieri, Maria Laura; Bonini, Marco; Corti, Giacomo; Sani, Federico; Philippon, Melody

2016-03-01

To reconstruct the timing of rift inception in the Broadly Rifted Zone in southern Ethiopia, we applied the fission-track method to basement rocks collected along the scarp of the main normal faults bounding (i) the Amaro Horst in the southern Main Ethiopian Rift and (ii) the Beto Basin in the Gofa Province. At the Amaro Horst, a vertical traverse along the major eastern scarp yielded pre-rift ages ranging between 121.4 ± 15.3 Ma and 69.5 ± 7.2 Ma, similarly to two other samples, one from the western scarp and one at the southern termination of the horst (103.4 ± 24.5 Ma and 65.5 ± 4.2 Ma, respectively). More interestingly, a second traverse at the Amaro northeastern terminus released rift-related ages spanning between 12.3 ± 2.7 and 6.8 ± 0.7 Ma. In the Beto Basin, the ages determined along the base of the main (northwestern) fault scarp vary between 22.8 ± 3.3 Ma and 7.0 ± 0.7 Ma. We ascertain through thermal modeling that rift-related exhumation along the northwestern fault scarp of the Beto Basin started at 12 ± 2 Ma while in the eastern margin of the Amaro Horst faulting took place later than 10 Ma, possibly at about 8 Ma. These results suggest a reconsideration of previous models on timing of rift activation in the different sectors of the Ethiopian Rift. Extensional basin formation initiated more or less contemporaneously in the Gofa Province (~ 12 Ma) and Northern Main Ethiopian Rift (~ 10-12 Ma) at the time of a major reorganization of the Nubia-Somalia plate boundary (i.e., 11 ± 2 Ma). Afterwards, rift-related faulting involved the Southern MER (Amaro Horst) at ~ 8 Ma, and only later rifting seemingly affected the Central MER (after ~ 7 Ma).

Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System

USGS Publications Warehouse

Hutchinson, D.R.; White, R.S.; Cannon, W.F.; Schulz, K.J.

1990-01-01

The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which we call the Keweenaw hot spot. Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection profiles constrain end-member models of melt thickness and stretching factors, which yield an inferred mantle potential temperature of 1500°–1570°C during rifting. Combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased, and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material. The isotopic signature of Keweenawan volcanic rocks, presented in a companion paper by Nicholson and Shirey (this issue), is consistent with our model of passive rifting above an asthenospheric mantle plume.

Closing of the Midcontinent-Rift - a far-field effect on Grenvillian compression

USGS Publications Warehouse

Cannon, W.F.

1994-01-01

The Midcontinent rift formed in the Laurentian supercontinent between 1109 and 1094 Ma. Soon after rifting, stresses changed from extensional to compressional, and the central graben of the rift was partly inverted by thrusting on original extensional faults. Thrusting culminated at about 1060 Ma but may have begun as early as 1080 Ma. On the southwest-trending arm of the rift, the crust was shortened about 30km; on the southeast-trending arm, strike-slip motion was dominant. The rift developed adjacent to the tectonically active Grenville province, and its rapid evolution from an extensional to a compressional feature at c1080 Ma was coincident with renewal of northwest-directed thrusting in the Grenville, probably caused by continent-continent collision. A zone of weak lithosphere created by rifting became the locus for deformation within the otherwise strong continental lithosphere. Stresses transmitted from the Grenville province utilized this weak zone to close and invert the rift. -Author

The influence of inherited structures on magmatic and amagmatic processes in the East African Rift.

NASA Astrophysics Data System (ADS)

Biggs, J.; Lloyd, R.; Hodge, M.; Robertson, E.; Wilks, M.; Fagereng, A.; Kendall, J. M.; Mdala, H. S.; Lewi, E.; Ayele, A.

2017-12-01

The idea that crustal heterogeneities, particularly inherited structures, influence the initiation and evolution of continental rifts is not new, but now modern techniques allow us to explore these controls from a fresh perspective, over a range of lengthscales, timescales and depths. In amagmatic rifts, I will demonstrate that deep fault structure is controlled by the stress orientation during the earliest phase of rifting, while the surface expression exploits near-surface weaknesses. I will show that pre-existing structures control the storage and orientation of deeper magma reservoirs in magmatic rifts, while the tectonic stress regime controls intra-rift faulting and shallow magmatism and stresses related to surface loading and cycles of inflation and deflation dominate at volcanic edifices. Finally, I will show how cross-rift structures influence short-term processes such as deformation and seismicity. I will illustrate the talk throughout using examples from along the East African Rift, including Malawi, Tanzania, Kenya and Ethiopia.

Mantle Flow Across the Baikal Rift Constrained With Integrated Seismic Measurements

NASA Astrophysics Data System (ADS)

Lebedev, S.; Meier, T.; van der Hilst, R. D.

2005-12-01

The Baikal Rift is located at the boundary of the stable Siberian Craton and deforming central Mongolia. The origin of the late Cenozoic rifting and volcanism are debated, as is the mantle flow beneath the rift zone. Here we combine new evidence from azimuthally-anisotropic upper-mantle tomography and from a radially-anisotropic inversion of interstation surface-wave dispersion curves with previously published shear-wave-splitting measurements of azimuthal anisotropy across the rift (Gao et al. 1994). While our tomographic model maps isotropic and anisotropic shear-velocity heterogeneity globally, the inversion of interstation phase-velocity measurements produces a single, radially-anisotropic, shear-velocity profile that averages from the rift to 500 km SE of it. The precision and the broad band (8-340 s) of the Rayleigh and Love wave curves ensures high accuracy of the profile. Tomography and shear-wave splitting both give a NW-SE fast direction (perpendicular to the rift) in the vicinity of the rift, changing towards W-E a few hundred kilometers from it. Previously, this has been interpreted as evidence for mantle flow similar to that beneath mid-ocean ridges, with deeper vertical flow directly beneath the rift also proposed. Our radially anisotropic profile, however, shows that while strong anisotropy with SH waves faster than SV waves is present in the thin lithosphere and upper asthenosphere beneath and SE of the rift, no anisotropy is required below 110 km. The tomographic model shows thick cratonic lithosphere north of the rift. These observations suggest that instead of a flow diverging from the rift axis in NW and SE directions, the most likely pattern is the asthenospheric flow in SE direction from beneath the Siberian lithosphere and across the rift. Possible driving forces of the flow are large-scale lithospheric deformation in East Asia and the draining of asthenosphere at W-Pacific subduction zones; a plume beneath the Siberian craton also cannot be ruled out. As shown for the model of subcontinental asthenospheric flow by Morgan and Morgan (2005), this mantle flow pattern can explain not only the rifting but also the basaltic volcanism observed in the Lake Baikal region.

Low lower crustal velocity across Ethiopia: Is the Main Ethiopian Rift a narrow rift in a hot craton?

NASA Astrophysics Data System (ADS)

Keranen, Katie M.; Klemperer, Simon L.; Julia, Jordi; Lawrence, Jesse F.; Nyblade, Andy A.

2009-05-01

The Main Ethiopian Rift (MER) is a classic narrow rift that developed in hot, weak lithosphere, not in the initially cold, thick, and strong lithosphere that would be predicted by common models of rift mode formation. Our new 1-D seismic velocity profiles from Rayleigh wave/receiver function joint inversion across the MER and the Ethiopian Plateau indicate that hot lower crust and upper mantle are present throughout the broad region affected by Oligocene flood basalt volcanism, including both the present rift and the adjacent Ethiopian Plateau hundreds of kilometers from the rift valley. The region of hot lithosphere closely corresponds to the region of flood basalt volcanism, and we interpret that the volcanism and thermal perturbation were jointly caused by impingement of the Afar plume head. Across the affected region, Vs is 3.6-3.8 km/s in the lowermost crust and ≤4.3 km/s in the uppermost mantle, both ˜0.3 km/s lower than in the eastern and western branches of the East African Rift System to the south. We interpret the low Vs in the lower crust and upper mantle as indicative of hot lithosphere with partial melt. Our results lead to a hybrid rift mode, in which the brittle upper crust has developed as a narrow rift along the Neoproterozoic suture between East and West Gondwana, while at depth lithospheric deformation is distributed over the broad region (˜400 km wide) thermally perturbed by the broad thermal upwelling associated with the Afar plume head. Development of both the East African Rift System to the south (in cold, strong lithosphere) and the MER to the north (in hot, weak lithosphere) as narrow rifts, despite their vastly different initial thermal states and depth-integrated lithospheric strength, indicates that common models of rift mode formation that focus only on temperature, thickness, and vertical strength profiles do not apply to these classic continental rifts. Instead, inherited structure and associated lithospheric weaknesses are the primary control on the mode of extension.

Low lower crustal velocity across Ethiopia: Is the Main Ethiopian Rift a narrow rift in a hot craton?

USGS Publications Warehouse

Keranen, K.M.; Klemperer, S.L.; Julia, J.; Lawrence, J. F.; Nyblade, A.A.

2009-01-01

[1] The Main Ethiopian Rift (MER) is a classic narrow rift that developed in hot, weak lithosphere, not in the initially cold, thick, and strong lithosphere that would be predicted by common models of rift mode formation. Our new 1-D seismic velocity profiles from Rayleigh wave/receiver function joint inversion across the MER and the Ethiopian Plateau indicate that hot lower crust and upper mantle are present throughout the broad region affected by Oligocene flood basalt volcanism, including both the present rift and the adjacent Ethiopian Plateau hundreds of kilometers from the rift valley. The region of hot lithosphere closely corresponds to the region of flood basalt volcanism, and we interpret that the volcanism and thermal perturbation were jointly caused by impingement of the Afar plume head. Across the affected region, Vs is 3.6-3.8 km/s in the lowermost crust and ???4.3 km/s in the uppermost mantle, both ??0.3 km/s lower than in the eastern and western branches of the East African Rift System to the south. We interpret the low Vs in the lower crust and upper mantle as indicative of hot lithosphere with partial melt. Our results lead to a hybrid rift mode, in which the brittle upper crust has developed as a narrow rift along the Neoproterozoic suture between East and West Gondwana, while at depth lithospheric deformation is distributed over the broad region (??400 km wide) thermally perturbed by the broad thermal upwelling associated with the Afar plume head. Development of both the East African Rift System to the south (in cold, strong lithosphere) and the MER to the north (in hot, weak lithosphere) as narrow rifts, despite their vastly different initial thermal states and depth-integrated lithospheric strength, indicates that common models of rift mode formation that focus only on temperature, thickness, and vertical strength profiles do not apply to these classic continental rifts. Instead, inherited structure and associated lithospheric weaknesses are the primary control on the mode of extension. ?? 2009 by the American Geophysical Union.

Evidence for heterogeneous (and possibly transient) geothermal flux beneath the Ross-Amundsen ice divide of the West Antarctic ice sheet

NASA Astrophysics Data System (ADS)

Blankenship, D. D.; Danque, H. A.; Quartini, E.; Young, D. A.

2012-12-01

It is well established that the geological framework for the evolution of the marine-based West Antarctic ice sheet (WAIS) is the Cretaceous through Cenozoic rifting of the underlying lithosphere. The southern flank of this rift along the Whitmore Mountains underlies the upper reaches of the Ross Sea catchment of the WAIS and has been identified as a site of active subglacial volcanism. Interestingly, the northern flank of this rift represented by the upward doming of the Marie Byrd Land volcanic province has not yet been associated with active subglacial volcanism. Similarly, it is not known whether the heterogeneity of geothermal flux associated with these existing and potential rift flank volcanic provinces extends across the floor of the rift between the rift flanks. Here we present geophysical evidence for heterogeneous geothermal flux associated with active subglacial volcanism along the northern rift flank adjacent to Marie Byrd Land where it intersects the ice divide for the Ross and Amundsen Sea sectors for the WAIS. We further evaluate the evidence for the continuity of heterogeneous geothermal flux along this ice divide and across the rift floor between the two flanks of the West Antarctic rift system.

The Volcanic Myths of the Red Sea - Temporal Relationship Between Magmatism and Rifting

NASA Astrophysics Data System (ADS)

Stockli, D. F.; Bosworth, W.

2017-12-01

The Cenozoic Red Sea is one of the premier examples of continental rifting and active break-up. It has been cited as an example for both prototypical volcanic, pure shear rift systems with limited crustal stretching as well as magma-poor simple-shear rifting and highly asymmetric rift margins characterized by low-angle normal faults. In light of voluminous Oligocene continental flood basalts in the Afar/Ethiopian region, the Red Sea has often been viewed as a typical volcanic rift, despite evidence for asymmetric extension and hyperextended crust (Zabargad Island). An in-depth analysis of the timing, spatial distribution, and nature of Red Sea volcanism and its relationship to late Cenozoic extensional faulting should shed light on some of the misconceptions. The Eocene appearance of the East African super-plume was not accompanied by any recognized significant extensional faulting or rift-basin formation. The first phase of volcanism more closely associated with the Red Sea occurred in northern Ethiopia and western Yemen at 31-30 Ma and was synchronous with the onset of continental extension in the Gulf of Aden. Early Oligocene volcanism has also been documented in southern and central Saudi Arabia and southern Sudan. However, this voluminous Oligocene volcanism entirely predates Red Sea extensional faulting and rift formation. Marking the onset of Red Sea rifting, widespread, spatially synchronous intrusion of basaltic dikes occurred at 24-21 Ma along the entire Red Sea-Gulf of Suez rift and continuing into northern Egypt. While the initiation of lithospheric extension in the central and northern and central Red Sea and Gulf of Suez was accompanied by only sparse basaltic volcanism and possible underplating, the main phase of rifting in the Miocene Red Sea/Gulf of Suez completely lacks any significant rift-related volcanism, suggesting plate-boundary forces probably drove overall separation of Arabia from Africa. During progressive rifting, there is also no evidence for the formation of SDRs or the accretion of a thick proto-oceanic crust. In fact, there appears to be evidence for hyperextension and possible mantle exhumation prior to Pliocene inception of seafloor spreading, making the Red Sea overall a rather magma-poor rift - and hardly the poster child for magmatic rifting and continental break-up.

Middle Jurassic - Early Cretaceous rifting on the Chortis Block in Honduras: Implications for proto-Caribbean opening (Invited)

NASA Astrophysics Data System (ADS)

Rogers, R. D.; Emmet, P. A.

2009-12-01

Regional mapping integrated with facies analysis, age constraints and airborne geophysical data reveal WNW and NE trends of Middle Jurassic to Early Cretaceous basins which intersect in southeast Honduras that we interpret as the result of rifting associated with the breakup of the Americas and opening of the proto-Caribbean seaway. The WNW-trending rift is 250 km long by 90 km wide and defined by a basal 200 to 800 m thick sequence of Middle to Late Jurassic fluvial channel and overbank deposits overlain by transgressive clastic shelf strata. At least three sub-basins are apparent. Flanking the WNW trending rift basins are fault bounded exposures of the pre-Jurassic continental basement of the Chortis block which is the source of the conglomeratic channel facies that delineate the axes of the rifts. Cretaceous terrigenous strata mantle the exposed basement-cored rift flanks. Lower Cretaceous clastic strata and shallow marine limestone strata are dominant along this trend indicating that post-rift related subsidence continued through the Early Cretaceous. The rifts coincide with a regional high in the total magnetic intensity data. We interpret these trends to reflect NNE-WSW extension active from the Middle Jurassic through Early Cretaceous. These rifts were inverted during Late Cretaceous shortening oriented normal to the rift axes. To the east and at a 120 degree angle to the WNW trending rift is the 300 km long NE trending Guayape fault system that forms the western shoulder of the Late Jurassic Agua Fria rift basin filled by > 2 km thickness of clastic marine shelf and slope strata. This NE trending basin coincides with the eastern extent of the surface exposure of continental basement rocks and a northeast-trending fabric of the Jurassic (?) metasedimentary basement rocks. We have previously interpreted the eastern basin to be the Jurassic rifted margin of the Chortis block with the Guayape originating as a normal fault system. These two rifts basin intersect at near 120 degree angle in southeastern Honduras. We suggest that the intersection of these two trends represents part of a R-R-R triple junction during the breakup of the Americas. The WNW trending rift produced the WNW trending fabric of the central Chortis block and failed in the Early Cretaceous while the NE trending rift continued opening to form the south-facing passive margin of the northern proto-Caribbean basin.

Constraining the dynamic response of subcontinental lithospheric mantle to rifting using Re-Os model ages in the Western Ross Sea, Antarctica

NASA Astrophysics Data System (ADS)

Doherty, C.; Class, C.; Goldstein, S. L.; Shirey, S. B.; Martin, A. P.; Cooper, A. F.; Berg, J. H.; Gamble, J. A.

2012-12-01

In order to understand the dynamic response of the subcontinental lithospheric mantle (SCLM) to rifting, it is important to be able to distinguish the geochemical signatures of SCLM vs. asthenosphere. Recent work demonstrates that unradiogenic Os isotope ratios can indicate old depletion events in the convecting upper mantle (e.g. Rudnick & Walker, 2009), and allow us to make these distinctions. Thus, if SCLM can be traced across a rifted margin, its fate during rifting can be established. The Western Ross Sea provides favorable conditions to test the dynamic response of SCLM to rifting. Re-Os measurements from 8 locations extending from the rift shoulder to 200 km into the rift basin reveal 187Os/188Os ranging from 0.1056 at Foster Crater on the shoulder, to 0.1265 on Ross Island within the rift. While individual sample model ages vary widely throughout the margin, 'aluminochron' ages (Reisberg & Lorand, 1995) reveal a narrower range of lithospheric stabilization ages. Franklin Island and Sulfur Cones show a range of Re-depletion ages (603-1522 Ma and 436-1497 Ma) but aluminochrons yield Paleoproterozoic stabilization ages of 1680 Ma and 1789 Ma, respectively. These ages coincide with U-Pb zircon ages from Transantarctic Mountain (TAM) crustal rocks, in support of SCLM stabilization at the time of crust formation along the central TAM. The Paleoproterozoic stabilization age recorded at Franklin Island is especially significant, since it lies 200km off of the rift shoulder. The similar ages beneath the rift shoulder and within the rift suggests stretched SCLM reaches into the rift and thus precludes replacement by asthenospheric mantle. The persistence of thinned Paleoproterozoic SCLM into the rifted zone in WARS suggests that it represents a 'type I' margin of Huismans and Beaumont (2011), which is characterized by crustal breakup before loss of lithospheric mantle. The Archean Re-depletion age of 3.2 Ga observed on the rift shoulder suggests that cratonic lithosphere extends beneath the TAM. With further analyses we hope to determine if there is lateral flow of cratonic lithosphere into the rift. Huismans, R., Beaumount, C., 2011. Depth-dependent extension, two stage breakup and cratonic underplating at rifted margins. Nature 473, 74-78. Reisberg, L.C., Lorand, J.P., 1995. Longevity of sub-continental mantle lithosphere from osmium isotope systematics in orogenic peridotite massifs. Nature 376, 159-162. Rudnick, R.L., Walker, R.J., 2009. Interpreting ages from Re-Os isotopes in peridotites. Lithos 1125, 1083-1095.

Synthesis of proterozoic data as a prerequisite for tectonic and thermal modeling

NASA Technical Reports Server (NTRS)

Burke, K. C.

1984-01-01

Rocks of the Pongola supergroup form an elongate belt in the Archean Kaapvaal Caton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, it is suggested that the Pongola supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in Earth history reveals striking similarities. Rocks of the Ventersdorp Supergroup were deposited in a system of northeast trending grabens on the Kaapvaal Craton approximately 2.64 Ga ago. It is suggested that it was this collision which initiated the Ventersdorp rifting. The Ventersdorp rift province is related to an extension in the Kaapval Craton associated with the collision.

The role of inheritance in structuring hyperextended rift systems

NASA Astrophysics Data System (ADS)

Manatschal, Gianreto; Lavier, Luc; Chenin, Pauline

2015-04-01

A long-standing question in Earth Sciences is related to the importance of inheritance in controlling tectonic processes. In contrast to physical processes that are generally applicable, assessing the role of inheritance suffers from two major problems: firstly, it is difficult to appraise without having insights into the history of a geological system; and secondly all inherited features are not reactivated during subsequent deformation phases. Therefore, the aim of our presentation is to give some conceptual framework about how inheritance may control the architecture and evolution of hyperextended rift systems. We use the term inheritance to refer to the difference between an "ideal" layer-cake type lithosphere and a "real" lithosphere containing heterogeneities and we define 3 types of inheritance, namely structural, compositional and thermal inheritance. Moreover, we assume that the evolution of hyperextended rift systems reflects the interplay between their inheritance (innate/"genetic code") and the physical processes at play (acquired/external factors). Thus, by observing the architecture and evolution of hyperextended rift systems and integrating the physical processes, one my get hints on what may have been the original inheritance of a system. Using this approach, we focus on 3 well-studied rift systems that are the Alpine Tethys, Pyrenean-Bay of Biscay and Iberia-Newfoundland rift systems. For the studied examples we can show that: 1) strain localization on a local scale and during early stages of rifting is controlled by inherited structures and weaknesses 2) the architecture of the necking zone seems to be influenced by the distribution and importance of ductile layers during decoupled deformation and is consequently controlled by the thermal structure and/or the inherited composition of the curst 3) the location of breakup in the 3 examples is not significantly controlled by the inherited structures 4) inherited mantle composition and rift-related mantle processes may control the rheology of the mantle, the magmatic budget, the thermal structure and the localization of final rifting Conversely, the deformation in hyperextended domains is strongly controlled by weak hydrated minerals (e.g. clay, serpentinite) that result form the breakdown of feldspar and olivine due to fluid and reaction assisted deformation and is consequently not inherited but the result of rift induced processes. These key observations show that both inheritance and rift-induced processes play a significant role in the development of magma-poor rift systems and that the role of inheritance may change as the physical conditions vary during the evolving rifting and as rift-induced processes (serpentinization; magma) become more important. Thus, it is not only important to determine the "genetic code" of a rift system, but also to understand how it interacts and evolves during rifting. Understand how far these new ideas and concepts derived from the southern North Atlantic and Alpine Tethys can be translated to other less explored hyperextended rift systems will be one of the challenges of the future research in rifted margins.

Petrological Constraints on Melt Generation Beneath the Asal Rift (Djibouti)

NASA Astrophysics Data System (ADS)

Pinzuti, P.; Humler, E.; Manighetti, I.; Gaudemer, Y.; Bézos, A.

2010-12-01

The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 95 lava flows sampled along 10 km of the rift axis and 8 km off-axis (that is for the last 650 ky). The major element composition and the trace element ratios of aphyric basalts across the Asal Rift show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. FeO, Fe8.0, Sm/YbN and Zr/Y increase, whereas SiO2 and Lu/HfN decrease from the rift axis to the rift shoulders. These variations are qualitatively consistent with a shallower melting beneath the rift axis than off-axis and the data show that the melting regime is inconsistent with a passive upwelling model. In order to quantify the depth range and extent of melting, we invert Na8.0 and Fe8.0 contents of basalts based on a pure active upwelling model. Beneath the rift axis, melting paths are shallow, from 60 to 30 km. These melting paths are consistent with adiabatic melting in normal-temperature asthenosphere, beneath an extensively thinned mantle lithosphere. In contrast, melting on the rift shoulders occurred beneath a thick mantle lithosphere and required mantle solidus temperature 180°C hotter than normal (melting paths from 110 to 75 km). The calculated rate of lithospheric thinning is high (6.0 cm yr-1) and could explain the survival of a metastable garnet within the mantle at depth shallower than 90 km beneath the modern Asal Rift.

Seismicity During Continental Breakup in the Red Sea Rift of Northern Afar

NASA Astrophysics Data System (ADS)

Illsley-Kemp, Finnigan; Keir, Derek; Bull, Jonathan M.; Gernon, Thomas M.; Ebinger, Cynthia; Ayele, Atalay; Hammond, James O. S.; Kendall, J.-Michael; Goitom, Berhe; Belachew, Manahloh

2018-03-01

Continental rifting is a fundamental component of plate tectonics. Recent studies have highlighted the importance of magmatic activity in accommodating extension during late-stage rifting, yet the mechanisms by which crustal thinning occurs are less clear. The Red Sea rift in Northern Afar presents an opportunity to study the final stages of continental rifting as these active processes are exposed subaerially. Between February 2011 and February 2013 two seismic networks were installed in Ethiopia and Eritrea. We locate 4,951 earthquakes, classify them by frequency content, and calculate 31 focal mechanisms. Results show that seismicity is focused at the rift axis and the western marginal graben. Rift axis seismicity accounts for ˜64% of the seismic moment release and exhibits a swarm-like behavior. In contrast, seismicity at the marginal graben is characterized by high-frequency earthquakes that occur at a constant rate. Results suggest that the rift axis remains the primary locus of seismicity. Low-frequency earthquakes, indicative of magmatic activity, highlight the presence of a magma complex ˜12 km beneath Alu-Dalafilla at the rift axis. Seismicity at the marginal graben predominantly occurs on westward dipping, antithetic faults. Focal mechanisms show that this seismicity is accommodating E-W extension. We suggest that the seismic activity at the marginal graben is either caused by upper crustal faulting accommodating enhanced crustal thinning beneath Northern Afar or as a result of flexural faulting between the rift and plateau. This seismicity is occurring in conjunction with magmatic extension at the rift axis, which accommodates the majority of long-term extension.

Simulations of mechanical failure in ice: Implications of terrestrial fracture models as applied to they icy satellites of the outer solar system

NASA Astrophysics Data System (ADS)

Walker, C. C.; Bassis, J. N.

2011-12-01

At the South Pole of Enceladus, a small icy moon orbiting Saturn, is a heavily fractured ice plain surrounded by a nearly-circular mountain range. Remarkably, the Cassini orbiter detected jets of water emanating from the icy shell and into space, originating from 4 parallel "tiger stripe" rifts within the center of the ice plain. The tiger stripes imaged on Enceladus are morphologically similar to rifts observed to form under extensional stress regimes in terrestrial ice shelves; the putative subsurface ocean hypothesized beneath the icy shell strengthens the analogy that their formation may have similar mechanical origins. Past studies have also suggested that the tiger stripes are the result of a process similar to that of mid-ocean ridge spreading on the Earth, but it remains to be seen whether or not such motion is consistent with the mountainous features seen at the circular cliff-like boundary of the region. In an attempt to understand the formation of these tiger stripes and their relationship to the observed mountain chains, we apply a conceptual model in which the ice is considered to be less like a continuous fluid body and, instead, behaves like a granular material made up of discrete blocks of ice. The tidal forces on the small moon tug on the shell enough that it has been cracked many times over, motivating the assumption that the ice exists in a continuum between wholly intact ice and highly pre-fractured ice. We employ several experimental setups with the intention of mapping the deformation of the south polar segment of the shell, to determine the processes that may contribute to its observed morphological state. These setups range from large scale topographical models, e.g., simulating the build up of mountains and processes that lead to overall elevation differences in the region, to small-scale, and focus on the more detailed level of fracturing. We explore our ice-shelf rifting analogy by modeling both icy moon fracturing and ice shelf rifting to compare and contrast the failure modes that we observe, results that bolster both our comparative platform and, importantly, our understanding of fracture in ice shelves on the Earth as well. A similar approach could be applied to the chaos regions of Europa, where fractures are prevalent and whose underlying causes are not well understood.

Late oligocene and miocene faulting and sedimentation, and evolution of the southern Rio Grande rift, New Mexico, USA

NASA Astrophysics Data System (ADS)

Mack, Greg H.; Seager, William R.; Kieling, John

1994-08-01

The distribution of nonmarine lithofacies, paleocurrents, and provenance data are used to define the evolution of late Oligocene and Miocene basins and complementary uplifts in the southern Rio Grande rift in the vicinity of Hatch, New Mexico, USA. The late Oligocene-middle Miocene Hayner Ranch Formation, which consists of a maximum of 1000 m of alluvial-fan, alluvial-flat, and lacustrine-carbonate lithofacies, was deposited in a narrow (12 km), northwest-trending, northeast-tilted half graben, whose footwall was the Caballo Mountains block. Stratigraphic separation on the border faults of the Caballo Mountains block was approximately 1615 m. An additional 854 m of stratigraphic separation along the Caballo Mountains border faults occurred during deposition of the middle-late Miocene Rincon Valley Formation, which is composed of up to 610 m of alluvial-fan, alluvial-flat, braided-fluvial, and gypsiferous playa lithofacies. Two new, north-trending fault blocks (Sierra de las Uvas and Dona Ana Mountains) and complementary west-northwest-tilted half graben also developed during Rincon Valley time, with approximately 549 m of stratigraphic separation along the border fault of the Sierra de las Uvas block. In latest Miocene and early Pliocene time, following deposition of the Rincon Valley Formation, movement continued along the border faults of the Caballo Mountains, Dona Ana Mountains, and Sierra de las Uvas blocks, and large parts of the Hayner Ranch and Rincon Valley basins were segmented into smaller fault blocks and basins by movement along new, largely north-trending faults. Analysis of the Hayner Ranch and Rincon Valley Formations, along with previous studies of the early Oligocene Bell Top Formation and late Pliocene-early Pleistocene Camp Rice Formation, indicate that the traditional two-stage model for development of the southern Rio Grande rift should be abandoned in favor of at least four episodes of block faulting beginning 35 Ma ago. With the exception of two northwest-trending border faults of the Caballo Mountains block that may be reactivated along Eocene compressional structures, the majority of border faults and complementary basins throughout the history of the southern Rio Grande rift were north-trending, which challenges the conventional idea of a clockwise change in stress through time.

Seismicity associated with magmatism, faulting and hydrothermal circulation at Aluto Volcano, Main Ethiopian Rift

NASA Astrophysics Data System (ADS)

Wilks, Matthew; Kendall, J.-Michael; Nowacki, Andy; Biggs, Juliet; Wookey, James; Birhanu, Yelebe; Ayele, Atalay; Bedada, Tulu

2017-06-01

The silicic volcanic centres of the Main Ethiopian Rift (MER) play a central role in facilitating continental rifting. Many of these volcanoes host geothermal resources and are located in heavily populated regions. InSAR studies have shown several are deforming, but regional seismic networks have detected little seismicity. A local network of 12 seismometers was deployed at Aluto Volcano from 2012 to 2014, and detected 2142 earthquakes within a 24-month period. We locate the events using a 1D velocity model that exploits a regional model and information from geothermal boreholes and calculate local magnitudes, b-values and focal mechanisms. Event depths generally range from the near surface to 15 km with most of the seismicity clustering in the upper 2 km. A significant amount of seismicity follows the Artu Jawa Fault Zone, which trends in alignment with the Wonji Fault Belt, NNE-SSW and is consistent with previous studies of strain localisation in the MER. Focal mechanisms are mostly normal in style, with the mean T-axes congruent to the orientation of extension in the rift at this latitude. Some show relatively small left-lateral strike-slip components and are likely associated with the reactivation of NE-ENE structures at the southern tip of the Aluto-Gedemsa segment. Events range from - 0.40 to 2.98 in magnitude and we calculate an overall b-value of 1.40 ± 0.14. This relatively elevated value suggests fluid-induced seismicity that is particularly evident in the shallow hydrothermal reservoir and above it. Subdividing our observations according to depth identifies distinct regions beneath the volcanic edifice: a shallow zone (- 2-0 km) of high seismicity and high b-values that corresponds to the hydrothermal system and is influenced by a high fluid saturation and circulation; a relatively aseismic zone (0-2 km) with low b-values that is impermeable to ascending volatiles; a region of increased fluid-induced seismicity (2-9 km) that is driven by magmatic intrusion from below and a deeper zone (below 9 km) that is interpreted as a partially crystalline, magmatic mush. These observations indicate that both the magmatic and hydrothermal systems of Aluto volcano are seismically active and highlight the need for dedicated seismic monitoring at volcanoes in the MER.

Morphologie et cinématique d'une faille holocène dans les monts Péloritains (Sicile) ; implications géodynamiques

NASA Astrophysics Data System (ADS)

Hippolyte, Jean-Claude; Bouillin, Jean-Pierre

1999-11-01

The recent fault system of eastern Sicily can be identified in the Peloritan Mountains, in particular where it cross-cuts carbonate ranges in areas preserved from strong torrential erosion. The scarp of the Mount Kalfa fault results from normal sinistral slip at a mean rate of 0.9 mm·yr -1 during the Würm to Present east-west extension. This normal fault belongs to the Apenninico-Calabro-Sicilian rift zone that cross-cuts the Tyrrhenian arc. Its Sicilian and Apenninic segments enable characterization of a Middle-Late Pleistocene change of the stress regime that could have occurred during a steepening without subduction of the Ionian slab (along Calabria) and its lateral detachment.

The thermal state of the Arabian plate derived from heat flow measurements in Oman and Yemen

NASA Astrophysics Data System (ADS)

Rolandone, Frederique; Lucazeau, Francis; Leroy, Sylvie; Mareschal, Jean-Claude; Jorand, Rachel; Goutorbe, Bruno; Bouquerel, Hélène

2013-04-01

The dynamics of the Afar plume and the rifting of the Red Sea and the Gulf of Aden affect the present-day thermal regime of the Arabian plate. However, the Arabian plate is a Precambrian shield covered on its eastern part by a Phanerozoic platform and its thermal regime, before the plume and rifting activities, should be similar to that of other Precambrian shields with a thick and stable lithosphere. The first heat flow measurements in the shield, in Saudi Arabia, yielded low values (35-44 mW/m2), similar to the typical shields values. Recent heat flow measurements in Jordan indicate higher values (56-66 mW/m2). As part of the YOCMAL project (YOung Conjugate MArgins Laboratory), we have conducted heat flow measurements in southern and northern Oman to obtain 10 new heat flux values in the eastern Arabian plate. We also derived 20 heat flux values in Yemen and Oman by processing thermal data from oil exploration wells. The surface heat flux in these different locations is uniformly low (45 mW/m2). The heat production in samples from the Dhofar and Socotra Precambrian basement is also low (0.7 µW/m3). Differences in heat flow between the eastern (60 mW/m2) and the western (45 mW/m2) parts of Arabia reflect differences in crustal heat production as well as a higher mantle heat flux in the west. We have calculated a steady state geotherm for the Arabian platform that intersects the isentropic temperature profile at a depth of about 150 km, consistent with the seismic observations. Seismic tomography studies of the mantle beneath Arabia also show this east-west contrast. Seismic studies have shown that the lithosphere is rather thin, 100 km or less below the shield and 150 km below the platform. The lithospheric thickness for the Arabian plate is 150 km, and the progressive thinning near the Red Sea, caused by the thermal erosion of the plume material, is too recent to be detected at the surface. The Afar plume mostly affects the base of the Arabian lithosphere along the Red Sea and the western part of the Gulf of Aden. The extent of this effect is explained by channeling of the asthenospheric magma by the rift. The subdued penetration into the Gulf of Aden is probably due to the important segmentation of the rift. The continental domain is not affected by rifting in the Gulf of Aden. The main thermal effect of the Arabian plate is probably the channeling of the Afar plume to the North.

A new model for the development of the active Afar volcanic margin

NASA Astrophysics Data System (ADS)

Pik, Raphaël; Stab, Martin; Bellahsen, Nicolas; Leroy, Sylvie

2016-04-01

Volcanic passive margins, that represent more than the three quarters of continental margins worldwide, are privileged witnesses of the lithospheric extension processes thatform new oceanic basins. They are characterized by voluminous amounts of underplated, intruded and extruded magmas, under the form of massive lavas prisms (seaward-dipping reflectors, or SDR) during the course of thinning and stretching of the lithosphere, that eventually form the ocean-continent transition. The origin and mechanisms of formation of these objects are still largely debated today. We have focussed our attention in the last few years on the Afar volcanic province which represents an active analogue of such volcanic margins. We explored the structural and temporal relationships that exist between the development of the major thinning and stretching structures and the magmatic production in Central Afar. Conjugate precise fieldwork analysis along with lavas geochronology allowed us to revisit the timing and style of the rift formation, since the early syn-rift period of time in the W-Afar marginal area to present days. Extension is primarily accommodated over a wide area at the surface since the very initial periods of extension (~ 25 Ma) following the emplacement of Oligocene CFBs. We propose in our reconstruction of central Afar margin history that extension has been associated with important volumes of underplated mafic material that compensate crustal thinning. This has been facilitated by major crustal-scale detachments that help localize the thinning and underplating at depth. In line with this 'magmatic wide-rift' mode of extension, we demonstrate that episodic extension steps alternate with more protracted magmatic phases. The production of syn-rift massive flood basalts (~ 4 Ma) occurs after early thinning of both the crust and the lithosphere, which suggests that SDR formation, is controlled by previous tectonic event. We determined how the melting regime evolved in response to the deformation of the lithosphere, through a petrological and geochemical study of the pre- to syn-rift lavas and concluded that the lithospheric mantle experienced the combined effect of post-plume cooling, but also thinning during the Miocene. This is accompanied by the early channelization of the plume head into narrower zones, which helped focus extension at the future volcanic margins location. The anomalous mantle potential temperature increased during the very last localization phase (< 1 Ma), which leads us to argue in favor of the focussed activity of a plume stem below the volcanic margin, instead of purely passive adiabatic decompression. Our new interpretation of the regional isotopic signatures of lavas depicts a clear framework of the Afar plume and lithospheric mantle relationships to on going extension and segmentation of these margins, and allow us to propose new contrasted models for their development.

Rifting kinematics along the Arabian Margin, Red Sea

NASA Astrophysics Data System (ADS)

Pierantoni, Pietro Paolo; Schettino, Antonio; Zanoni, Davide; Rasul, Najeeb

2017-04-01

The Red Sea represents a young basin floored by oceanic, transitional, or thinned continental crust that formed between Nubia and Arabia. According to most authors, rifting between Nubia and Arabia started in the late Oligocene ( 27 Ma) and it is still in progress in the northern part of the Red Sea at latitudes greater than 24°N. Conversely, the area south of 20.3°N displays a linear spreading ridge extending as south as 14.8°N, which formed in the early Pliocene (the first pulse of sea floor spreading occurred during chron C3n.2n, 4.62 Ma). A transition zone (between 24°N and 20.3°N, present-day coordinates) exists between the northern and the southern sectors, characterized by a segmented spreading center that started forming at 2.58 Ma (chron 2A, late Pliocene) in the southernmost area and propagated northwards. Some authors suggest that the present-day NE-SW spreading directions can be extended back to the early Miocene. However, we are going to show, on the basis of geological evidence from the Arabian margin, that at least two phases of rifting, characterized by distinct extension directions, are necessary to explain the observed structural pattern of deformation in a wide area extending from 28°N to 20°N. At present, there is no magnetic evidence for the existence of a linear spreading center in the northern Red Sea at latitudes higher than 24°N. In this area, the syn-rift pattern of deformation along the Arabian margin is only partly coherent with the present day NE-SW sea floor spreading directions and with the observed trend of fracture zones in the Red Sea. In fact, an older set of rift structures was found during 3 field trips performed along the northern and central Red Sea Arabian margin (2015-2016), suggesting the existence of an earlier rifting stage characterized by N-S trending strike-slip faults and E-W normal faults. The objective of the field trips was to investigate the hypothesis that an early phase of N-S extension and formation of left-lateral pull-apart basins characterized the separation of Arabia from Nubia, as suggested by some authors and by a preliminary analysis of remote sensing data. The necessity of performing structural observations along a wide area along the eastern margin of the northern and central Red Sea led us to select 30 sites where the preliminary morpho-structural analysis of ASTER-GDEM data and geological maps suggested the possibility to study the overprint of younger NE-SW structures on pre-existing N-S strike-slip faults. For each survey site, a number of stations were established to measure kinematic indicators. Most of the mapped structures are E-W and NW-SE normal faults or N-S and NE-SW high-angle strike-slip faults. These different faults belong to the older N-S/E-W system and the younger NW-SE/NE-SW system. Field evidence shows that the second system cuts the first one.

Diverse Eruptions at Approximately 2,200 Years B.P. on the Great Rift, Idaho: Inferences for Magma Dynamics Along Volcanic Rift Zones

NASA Technical Reports Server (NTRS)

Hughes, S. S.; Nawotniak, S. E. Kobs; Borg, C.; Mallonee, H. C.; Purcell, S.; Neish, C.; Garry, W. B.; Haberle, C. W.; Lim, D. S. S.; Heldmann, J. L.

2016-01-01

Compositionally and morphologically diverse lava flows erupted on the Great Rift of Idaho approximately 2.2 ka (kilo-annum, 1000 years ago) during a volcanic "flare-up" of activity following an approximately 2 ky (kiloyear, 1000 years) hiatus in eruptions. Volcanism at Craters of the Moon (COTM), Wapi and Kings Bowl lava fields around this time included primitive and evolved compositions, separated over 75 kilometers along the approximately 85 kilometers-long rift, with striking variability in lava flow emplacement mechanisms and surface morphologies. Although the temporal associations may be coincidental, the system provides a planetary analog to better understand magma dynamics along rift systems, including that associated with lunar floor-fractured craters. This study aims to help bridge the knowledge gap between ancient rift volcanism evident on the Moon and other terrestrial planets, and active rift volcanism, e.g., at Hawai'i and Iceland.

Martian canyons and African rifts: Structural comparisons and implications

NASA Technical Reports Server (NTRS)

Frey, H. V.

1978-01-01

The resistant parts of the canyon walls of the Martian rift complex Valled Marineris were used to infer an earlier, less eroded reconstruction of the major roughs. The individual canyons were then compared with individual rifts of East Africa. When measured in units of planetary radius, Martian canyons show a distribution of lengths nearly identical to those in Africa, both for individual rifts and for compound rift systems. A common mechanism which scales with planetary radius is suggested. Martian canyons are significantly wider than African rifts. The overall pattern of the rift systems of Africa and Mars are quite different in that the African systems are composed of numerous small faults with highly variable trend. On Mars the trends are less variable; individual scarps are straighter for longer than on earth. This is probably due to the difference in tectonic histories of the two planets: the complex history of the earth and the resulting complicated basement structures influence the development of new rifts. The basement and lithosphere of Mars are inferred to be simple, reflecting a relatively inactive tectonic history prior to the formation of the canyonlands.

Potential links between continental rifting, CO2 degassing and climate change through time

NASA Astrophysics Data System (ADS)

Brune, Sascha; Williams, Simon E.; Müller, R. Dietmar

2017-12-01

The concentration of CO2 in the atmosphere is a key influence on Earth's climate. Today, significant quantities of CO2 are emitted at continental rifts, suggesting that the spatial and temporal extent of rift systems may have influenced deep carbon fluxes and thus climate change throughout geological time. Here we test this hypothesis by conducting a worldwide census of continental rift lengths over the last 200 million years. We estimate tectonic CO2 release rates through time and show that along the extensive Mesozoic and Cenozoic rift systems, rift-related CO2 degassing rates reached more than 300% of present-day values. Using a numerical carbon cycle model, we find that two prominent periods of enhanced rifting 160 to 100 million years ago and after 55 million years ago coincided with greenhouse climate episodes, during which atmospheric CO2 concentrations were more than three times higher than today. We therefore propose that continental fragmentation and long-term climate change could plausibly be linked via massive CO2 degassing in rift systems.

East African Rift

NASA Technical Reports Server (NTRS)

2008-01-01

Places where the earth's crust has formed deep fissures and the plates have begun to move apart develop rift structures in which elongate blocks have subsided relative to the blocks on either side. The East African Rift is a world-famous example of such rifting. It is characterized by 1) topographic deep valleys in the rift zone, 2) sheer escarpments along the faulted walls of the rift zone, 3) a chain of lakes within the rift, most of the lakes highly saline due to evaporation in the hot temperatures characteristic of climates near the equator, 4) voluminous amounts of volcanic rocks that have flowed from faults along the sides of the rift, and 5) volcanic cones where magma flow was most intense. This example in Kenya displays most of these features near Lake Begoria.

The image was acquired December 18, 2002, covers an area of 40.5 x 32 km, and is located at 0.1 degrees north latitude, 36.1 degrees east longitude.

The U.S. science team is located at NASA's Jet Propulsion Laboratory, Pasadena, Calif. The Terra mission is part of NASA's Science Mission Directorate.

Fault evolution in the Potiguar rift termination, equatorial margin of Brazil

NASA Astrophysics Data System (ADS)

de Castro, D. L.; Bezerra, F. H. R.

2015-02-01

The transform shearing between South American and African plates in the Cretaceous generated a series of sedimentary basins on both plate margins. In this study, we use gravity, aeromagnetic, and resistivity surveys to identify architecture of fault systems and to analyze the evolution of the eastern equatorial margin of Brazil. Our study area is the southern onshore termination of the Potiguar rift, which is an aborted NE-trending rift arm developed during the breakup of Pangea. The basin is located along the NNE margin of South America that faces the main transform zone that separates the North and the South Atlantic. The Potiguar rift is a Neocomian structure located at the intersection of the equatorial and western South Atlantic and is composed of a series of NE-trending horsts and grabens. This study reveals new grabens in the Potiguar rift and indicates that stretching in the southern rift termination created a WNW-trending, 10 km wide, and ~ 40 km long right-lateral strike-slip fault zone. This zone encompasses at least eight depocenters, which are bounded by a left-stepping, en echelon system of NW-SE- to NS-striking normal faults. These depocenters form grabens up to 1200 m deep with a rhomb-shaped geometry, which are filled with rift sedimentary units and capped by postrift sedimentary sequences. The evolution of the rift termination is consistent with the right-lateral shearing of the equatorial margin in the Cretaceous and occurs not only at the rift termination but also as isolated structures away from the main rift. This study indicates that the strike-slip shearing between two plates propagated to the interior of one of these plates, where faults with similar orientation, kinematics, geometry, and timing of the major transform are observed. These faults also influence rift geometry.

ALVIN investigation of an active propagating rift system, Galapagos 95.5° W

USGS Publications Warehouse

Hey, R.N.; Sinton, J.M.; Kleinrock, M.C.; Yonover, R.N.; MacDonald, K.C.; Miller, S.P.; Searle, R.C.; Christie, D.M.; Atwater, T.M.; Sleep, Norman H.; Johnson, H. Paul; Neal, C.A.

1992-01-01

ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5° W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.−1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seafloor spreading center, an extensively faulted and fissured volcanic ridge, develops ~ 10 km (~ 200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed.

Petrological constraints on melt generation beneath the Asal Rift (Djibouti) using quaternary basalts

NASA Astrophysics Data System (ADS)

Pinzuti, Paul; Humler, Eric; Manighetti, Isabelle; Gaudemer, Yves

2013-08-01

The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 56 new lava flows sampled along 10 km of the rift axis and 9 km off-axis (i.e., erupted within the last 620 kyr). Petrological and primary geochemical results show that most of the samples of the inner floor of the Asal Rift are affected by plagioclase accumulation. Trace element ratios and major element compositions corrected for mineral accumulation and crystallization show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. While FeO, Fe8.0, Zr/Y, and (Dy/Yb)N decrease from the rift shoulders to the rift axis, SiO2, Na/Ti, Lu/Hf increase and Na2O and Na8.0 are constant across the rift. These variations are qualitatively consistent with shallow melting beneath the rift axis and deeper melting for off-axis lava flows. Na8.0 and Fe8.0 contents show that beneath the rift axis, melting paths are shallow, from 81 ± 4 to 43 ± 5 km. These melting paths are consistent with adiabatic melting in normal-temperature fertile asthenosphere, beneath an extensively thinned mantle lithosphere. On the contrary, melting on the rift shoulders (from 107 ± 7 to 67 ± 8 km) occurred beneath thicker lithosphere, requiring a mantle solidus temperature 100 ± 40°C hotter. In this geodynamic environment, the calculated rate of lithospheric thinning appears to be 4.0 ± 2.0 cm yr-1, a value close to the mean spreading rate (2.9 ± 0.2 cm yr-1) over the last 620 kyr.

Long wavelength magnetic anomalies over continental rifts in cratonic region

NASA Astrophysics Data System (ADS)

Friedman, S. A.; Persaud, P.; Ferre, E. C.; Martín-Hernández, F.; Feinberg, J. M.

2017-12-01

New collections of unaltered mantle xenoliths shed light on potential upper mantle contributions to long wavelength magnetic anomalies (LWMA) in continental rifts in cratonic / shield areas. The new material originates from the East African Rift (Tanzania), the Rio Grande Rift (U.S.A.), the Rhine Rift (Germany), and the West Antarctic Rift (Antarctica). The xenoliths sample the uppermost (<80 km depth) lithospheric mantle in these regions in the spinel-peridotite and plagioclase-peridotite stability fields. The most common lithology by far (95% of samples) is a spinel-lherzolite indicating relatively low oxygen fugacities (FMQ -1). Chrome spinel in these peridotites is non-magnetic (Al + Mg > 0.2 or Fe < 0.3) and primary magnetite (Fe3O4) occurs only in trace amounts, typically yielding low natural remanent magnetizations (NRM < 10-2 A/m). The low Koenigsberger ratios (Qn < 1) of these materials, combined with high geotherms (>60ºC/km) that are characteristic of rifted regions preclude any contribution to LWMA at depths >10 km. Hence, only upper basalts and hypovolcanic mafic sills would constitute potential magnetic sources. In contrast, the margins of these rifted regions consist of refractory cratonic domains, often characterized by oxidized sublithospheric mantle that host significant concentrations of primary magnetite. The higher NRMs of these peridotites (up to 15 A/m, Qn > 2.5) combined with much lower geotherms (as low as 15ºC/km) allows for a 5 to 10 km layer of uppermost mantle to potentially contribute to LWMA. Assuming that Qn values in rift margins are also <1, the new data presented here suggests that relatively young rifts would display a central negative magnetic anomaly surrounded by two broad positive anomalies. The latitude of the rift is predicted to exert a primary control on the magnitude of such anomalies, while the steepness of the magnetic gradient across the rift would primarily reflect thermal equilibration over time.

Burial and thermal history simulation of the Abu Rudeis-Sidri oil field, Gulf of Suez-Egypt: A 1D basin modeling study

NASA Astrophysics Data System (ADS)

Awadalla, Ahmed; Hegab, Omar A.; Ahmed, Mohammed A.; Hassan, Saad

2018-02-01

An integrated 1D model on seven wells has been performed to simulate the multi-tectonic phases and multiple thermal regimes in the Abu Rudeis-Sidri oilfield. Concordance between measured and calculated present-day temperatures is achieved with present-day heat flows in the range of 42-55 mW/m2. Reconstruction of the thermal and burial histories provides information on the paleotemperature profiles, the timing of thermal activation as well as the effect of the Oligo-Miocene rifting phases and its associated magmatic activity. The burial histories show the pre-rift subsidence was progressive but modest, whereas the syn-rift was more rapid (contemporaneous with the main rifting phases and basin formation). Finally, the early post-rift thermal subsidence was slow to moderate in contrast to the late post-rift thermal subsidence which was moderate to rapid. The simulated paleo heat flow illustrates a steady state for the pre-rift phase and non-steady state (transient) for syn-rift and postrift phases. Three geothermal regimes are recognized, each of which is associated with a specific geological domain. 1) A lower geothermal regime reflects the impact of stable tectonics (pre-rift). 2) The higher temperature distribution reflects the syn-rift high depositional rate as well as the impact of stretching and thinning (rifting phases) of the lithosphere. 3) A local higher geothermal pulse owing to the magmatic activity during the Oligo-Miocene time (ARM-1 and Sidri-7 wells). Paleoheat flow values of 100mW/m2 (Oligo-Miocene rifting phase) increased to 120mW/m2 (Miocene rifting phase) and lesser magnitude of 80mW/m2 (Mio- Pliocene reactivation phase) have been specified. These affected the thermal regime and temperature distribution by causing perturbations in subsurface temperatures. A decline in the background value of 60mW/m2 owing to conductive cooling has been assigned. The blanketing effect caused by low thermal conductivity of the basin-fill sediments has been simulated as well.

Pre-existing normal faults have limited control on the rift geometry of the northern North Sea

NASA Astrophysics Data System (ADS)

Claringbould, Johan S.; Bell, Rebecca E.; Jackson, Christopher A.-L.; Gawthorpe, Robert L.; Odinsen, Tore

2017-10-01

Many rifts develop in response to multiphase extension with numerical and physical models suggesting that reactivation of first-phase normal faults and rift-related variations in bulk crustal rheology control the evolution and final geometry of subsequent rifts. However, many natural multiphase rifts are deeply buried and thus poorly exposed in the field and poorly imaged in seismic reflection data, making it difficult to test these models. Here we integrate recent 3D seismic reflection and borehole data across the entire East Shetland Basin, northern North Sea, to constrain the long-term, regional development of this multiphase rift. We document the following key stages of basin development: (i) pre-Triassic to earliest Triassic development of multiple sub-basins controlled by widely distributed, NNW- to NE-trending, east- and west-dipping faults; (ii) Triassic activity on a single major, NE-trending, west-dipping fault located near the basins western margin, and formation of a large half-graben; and (iii) Jurassic development of a large, E-dipping, N- to NE-trending half-graben near the eastern margin of the basin, which was associated with rift narrowing and strain focusing in the Viking Graben. In contrast to previous studies, which argue for two discrete periods of rifting during the Permian-Triassic and Late Jurassic-Early Cretaceous, we find that rifting in the East Shetland Basin was protracted from pre-Triassic to Cretaceous. We find that, during the Jurassic, most pre-Jurassic normal faults were buried and in some cases cross-cut by newly formed faults, with only a few being reactivated. Previously developed faults thus had only a limited control on the evolution and geometry of the later rift. We instead argue that strain migration and rift narrowing was linked to the evolving thermal state of the lithosphere, an interpretation supporting the predictions of lithosphere-scale numerical models. Our study indicates that additional regional studies of natural rifts are required to test and refine the predictions of physical and numerical models, more specifically, our study suggests models not explicitly recognising or including thermal or rheological effects might over emphasise the role of discrete pre-existing rift structures such as normal faults.

Plate Speed-up and Deceleration during Continental Rifting: Insights from Global 2D Mantle Convection Models.

NASA Astrophysics Data System (ADS)

Brune, S.; Ulvrova, M.; Williams, S.

2017-12-01

The surface of the Earth is divided into a jigsaw of tectonic plates, some carrrying continents that disperse and aggregate through time, forming transient supercontinents like Pangea and Rodinia. Here, we study continental rifting using large-scale numerical simulations with self-consistent evolution of plate boundaries, where continental break-up emerges spontaneously due to slab pull, basal drag and trench suction forces.We use the StagYY convection code employing a visco-plastic rheology in a spherical annulus geometry. We consider an incompressible mantle under the Boussinesq approximation that is basally and internally heated.We show that continental separation follows a characteristic evolution with three distinctive phases: (1) A pre-rift phase that typically lasts for several hundreds of millions of years with tectonic quiescence in the suture and extensional stresses that are slowly building up. (2) A rift phase that further divides into a slow rift period of several tens of millions of years where stresses continuously increase followed by a rift acceleration period featuring an abrupt stress drop within several millions of years. The speed-up takes place before lithospheric break-up and therefore affects the structural architecture of the rifted margins. (3) The drifting phase with initially high divergence rates persists over tens of millions of years until the system adjust to new conditions and the spreading typically slows down.By illustrating the geodynamic connection between subduction dynamics and rift evolution, our results allow new interpretations of plate tectonic reconstructions. Rift acceleration within the second phase of rifting is compensated by enhanced convergence rates at subduction zones. This model outcome predicts enhanced subduction velocities, e.g. between North America and the Farallon plate during Central Atlantic rifting 200 My ago, or closure of potential back-arc basins such as in the proto-Andean ranges of South America during South Atlantic opening. Post-rift deceleration occurs when the global plate system re-equilibrates after continental rupture. This phenomenon of a plate slow-down after mechanical rupture is recorded by observations from rifted margins between Australia-Antarctica and Greenland-Eurasia.

3D Numerical Rift Modeling with Application to the East African Rift System

NASA Astrophysics Data System (ADS)

Glerum, A.; Brune, S.; Naliboff, J.

2017-12-01

As key components of plate tectonics, continental rifting and the formation of passive margins have been extensively studied with both analogue models and numerical techniques. Only recently however, technical advances have enabled numerical investigations into rift evolution in three dimensions, as is actually required for including those processes that cause rift-parallel variability, such as structural inheritance and oblique extension (Brune 2016). We use the massively parallel finite element code ASPECT (Kronbichler et al. 2012; Heister et al. 2017) to investigate rift evolution. ASPECT's adaptive mesh refinement enables us to focus resolution on the regions of interest (i.e. the rift center), while leaving other areas such as the asthenospheric mantle at coarse resolution, leading to kilometer-scale local mesh resolution in 3D. Furthermore, we implemented plastic and viscous strain weakening of the nonlinear viscoplastic rheology required to develop asymmetric rift geometries (e.g. Huismans and Beaumont 2003). Additionally created plugins to ASPECT allow us to specify initial temperature and composition conditions based on geophysical data (e.g. LITHO1.0, Pasyanos et al. 2014) or to prescribe more general along-strike variation in the initial strain seeding the rift. Employing the above functionality, we construct regional models of the East African Rift System (EARS), the world's largest currently active rift. As the EARS is characterized by both orthogonal and oblique rift sections, multi-phase extension histories as well as magmatic and a-magmatic branches (e.g. Chorowicz 2005; Ebinger and Scholz 2011), it constitutes an extensive natural laboratory for our research into the 3D nature of continental rifting. References:Brune, S. (2016), in Plate boundaries and natural hazards, AGU Geophysical Monograph 219, J. C. Duarte and W. P. Schellart (Eds.). Chorowicz, J. (2005). J. Afr. Earth Sci., 43, 379-410. Ebinger, C. and Scholz, C. A. (2011), in Tectonics of Sedimentary Basins: Recent Advances, Wiley, C. Busby and A. Azor (Eds.). Heister et al. (2017). Geophys. J. Int., 210, 833-851. Huismans, R. S. and Beaumont, C. (2003). J. Geophys. Res., 108, B10, 2496. Kronbichler et al. (2012). Geophys. J. Int., 191, 12-29. Pasyanos et al. (2014). J. of Geophys. Res., 119, 3, 2153-2173.

Spatial and Temporal Strain Distribution Along the Central Red Sea Rift - A Study of the Hamd-Jizil Basin in Saudi Arabia

NASA Astrophysics Data System (ADS)

Szymanski, E.; Stockli, D.; Johnson, P.; Kattan, F. H.; Al Shamari, A.

2006-12-01

Numerous models exploring the rupturing modes and mechanisms of continental lithosphere are based on geological evidence from the Red Sea/Gulf of Suez rift system. Individually, the Red Sea basin is the prototype for many models of orthogonal continental rifting. Despite being a classic example of continental extension, many temporal and spatial strain distribution aspects, as well as the dynamic evolution of the rift architecture of the Red Sea, remain poorly constrained. Critical data come mostly from the Gulf of Suez and the Egyptian and Yemeni margins of the Red Sea; the rift flanks in Sudan and Saudi Arabia have remained largely unstudied, leaving a large information gap along the central portions of the rift system. Improving continental lithosphere rupture models requires an absolute understanding of the timing and magnitude of strain partitioning along the full rift flank. This study focuses on the development of extensional structures, syn- extensional sedimentary deposits, and rift-related Tertiary basaltic volcanism along the central flank of the rift system in Saudi Arabia. Geo- and thermochronometric techniques are used to elucidate the evolution of inboard and outboard strain markers manifested by structurally-controlled extensional basins that parallel the trend of the main Red Sea rift. Constraints on the dynamics of rift flank deformation are achieved through the collection of thermochronometric transects that traverse both the entire Arabian shield and individual normal faults that bound inland basins. Preliminary results show inland basins as asymmetric half-grabens filled by tilted Cenozoic sedimentary strata and separated by exhumed basement fault blocks. The most prominent extensional basin is the NW-trending Hamd-Jizil basin, located north of Madinah, measuring ~200 km along strike and up to 20 km in width. The Hamd-Jizil basin is structurally characterized by two half-grabens exposing a series of syn-rift siliciclastic sedimentary sections below Tertiary basalts. In certain areas, thick basalt sequences provide basin infill and appear faulted by a younger series of normal faults. Work continues on the production of further geo- and thermochronologic data for the Tertiary basalt sequences as well as the entire rift flank region.

Neogene Rift Propagation of the East African Rift System (EARS) into Central Africa and its Implications: Tectonic, Topographic and Geomorphic Impacts of the Luangwa and Luapula Rift Valleys on the Upper Congo Drainage Basin, Lake Bangweulu Wetlands and the Development of the Diffuse Southwestern Tip of the EARS.

NASA Astrophysics Data System (ADS)

Daly, M. C.; Watts, A. B.

2017-12-01

Integration of geomorphology, seismic reflection and gravity data, seismicity, DEM analysis and modelling defines a zone of NE/SW trending rifts extending into Central and SW Africa, orthogonal to the conventionally defined East African Rift System (EARS). These large-scale tectonic features have a relatively low level of seismicity and volcanism compared to the EARS, yet they generate significant topography and control the upper Congo drainage basin. They may also represent the beginning of an active but diffuse plate boundary developing to the southwest across Central Africa. The dominant feature of this broad zone is the Luangwa Rift Valley of eastern Zambia. Seismic reflection data show the Luangwa Rift developed as a thick ( 5km) Permo-Triassic basin. Inverted in the Mesozoic, it then experienced major Neogene extensional reactivation. The latter resulted in today's major border faults of varying polarity, with fault plane escarpments of up to 1000m, and associated rift flank uplifts that elevate the Central African plateau surface by 200 m. Late Miocene alluvial fans indicate a minimum age for the initiation of reactivation. Although having similar structural features to the EARS, the Luangwa Rift has a lower level of active seismicity and volcanism. 400 km northwest of the Luangwa, the north/south Luapula rift valley passes into the NE trending Mweru and Mweru Wantipa rift lakes. Pronounced border faults and fault terraces mark the NW and SE margins of these shallow lakes. Between the Luangwa and Luapula rift valleys lies the extensive upper Congo drainage basin of the Chambeshi river and the Lake Bangweulu wetlands. DEM mapping of topography from the Luangwa rift to the Luapula-Mweru Wantipa rift shows a low amplitude, large wavelength flexure of the Central African plateau surface compatible with an effective elastic thickness of 35 km. This regional warping controls the location and shape of the Chambeshi drainage basin and the Lake Bangweulu Wetlands. These results show Neogene rift valleys are active to the southwest of the EARS and are controlling the present-day continental drainage system of Central Africa. They also define a diffuse, divergent plate boundary between the Nubian Plate and an ill-defined southern African Plate that appears to exploit a zone of crustal anisotropy and thinner lithosphere.

Dykes and structures of the NE rift of Tenerife, Canary Islands: a record of stabilisation and destabilisation of ocean island rift zones

NASA Astrophysics Data System (ADS)

Delcamp, A.; Troll, V. R.; van Wyk de Vries, B.; Carracedo, J. C.; Petronis, M. S.; Pérez-Torrado, F. J.; Deegan, F. M.

2012-07-01

Many oceanic island rift zones are associated with lateral sector collapses, and several models have been proposed to explain this link. The North-East Rift Zone (NERZ) of Tenerife Island, Spain offers an opportunity to explore this relationship, as three successive collapses are located on both sides of the rift. We have carried out a systematic and detailed mapping campaign on the rift zone, including analysis of about 400 dykes. We recorded dyke morphology, thickness, composition, internal textural features and orientation to provide a catalogue of the characteristics of rift zone dykes. Dykes were intruded along the rift, but also radiate from several nodes along the rift and form en échelon sets along the walls of collapse scars. A striking characteristic of the dykes along the collapse scars is that they dip away from rift or embayment axes and are oblique to the collapse walls. This dyke pattern is consistent with the lateral spreading of the sectors long before the collapse events. The slump sides would create the necessary strike-slip movement to promote en échelon dyke patterns. The spreading flank would probably involve a basal decollement. Lateral flank spreading could have been generated by the intense intrusive activity along the rift but sectorial spreading in turn focused intrusive activity and allowed the development of deep intra-volcanic intrusive complexes. With continued magma supply, spreading caused temporary stabilisation of the rift by reducing slopes and relaxing stress. However, as magmatic intrusion persisted, a critical point was reached, beyond which further intrusion led to large-scale flank failure and sector collapse. During the early stages of growth, the rift could have been influenced by regional stress/strain fields and by pre-existing oceanic structures, but its later and mature development probably depended largely on the local volcanic and magmatic stress/strain fields that are effectively controlled by the rift zone growth, the intrusive complex development, the flank creep, the speed of flank deformation and the associated changes in topography. Using different approaches, a similar rift evolution has been proposed in volcanic oceanic islands elsewhere, showing that this model likely reflects a general and widespread process. This study, however, shows that the idea that dykes orient simply parallel to the rift or to the collapse scar walls is too simple; instead, a dynamic interplay between external factors (e.g. collapse, erosion) and internal forces (e.g. intrusions) is envisaged. This model thus provides a geological framework to understand the evolution of the NERZ and may help to predict developments in similar oceanic volcanoes elsewhere.

Metallogeny of the midcontinent rift system of North America

USGS Publications Warehouse

Nicholson, S.W.; Cannon, W.F.; Schulz, K.J.

1992-01-01

The 1.1 Ga Midcontinent rift system of North America is one of the world's major continental rifts and hosts a variety of mineral deposits. The rocks and mineral deposits of this 2000 km long rift are exposed only in the Lake Superior region. In the Lake Superior region, the rift cuts across Precambrian basement terranes ranging in age from ??? 1850 Ma to more than 3500 Ma. Where exposed, the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks. Beneath the center of Lake Superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection profiles. This region hosts two major classes of mineral deposits, magmatic and hydrothermal. All important mineral production in this region has come from hydrothermal deposits. Rift-related hydrothermal deposits include four main types: (1) native copper deposits in basalts and interflow sediments; (2) sediment-hosted copper sulfide and native copper; (3) copper sulfide veins and lodes hosted by rift-related volcanic and sedimentary rocks; and (4) polymetallic (five-element) veins in the surrounding Archean country rocks. The scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals. Where hydrothermal sulfides occur (i.e., shale-hosted copper sulfides), the source of sulfur was local sedimentary rocks. Magmatic deposits have locally supported exploration and minor production, but most are subeconomic presently. These deposits occur in intrusions exposed near the margins of the rift and include CuNiPGE and TiFe (V) in the Duluth Complex, U-REE-Nb in small carbonatites, and breccia pipes resulting from local hydrothermal activity around small felsic intrusions. Mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization. Most of the sulfide deposits in intrusions, however, contain sulfur derived from country rocks; the interaction between magma and country rocks was important in generation of the magmatic CuNi sulfide deposits. A mantle plume origin has been proposed for the formation of the Midcontinent rift. More than 1 million km3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift, providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines. These brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume. Hydrothermal deposits were emplaced for at least 30-40 m.y. after rift magmatism and extension ceased. This time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift, or may be due to the timing of post-rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep portions of the rift. ?? 1992.

Maximum Historical Seismic Intensity Map of S. Miguel Island (azores)

NASA Astrophysics Data System (ADS)

Silveira, D.; Gaspar, J. L.; Ferreira, T.; Queiroz, G.

The Azores archipelago is situated in the Atlantic Ocean where the American, African and Eurasian lithospheric plates meet. The so-called Azores Triple Junction located in the area where the Terceira Rift, a NW-SE to WNW-ESE fault system with a dextral component, intersects the Mid-Atlantic Ridge, with an approximate N-S direction, dominates its geological setting. S. Miguel Island is located in the eastern segment of the Terceira Rift, showing a high diversity of volcanic and tectonic structures. It is the largest Azorean island and includes three active trachytic central volcanoes with caldera (Sete Cidades, Fogo and Furnas) placed in the intersection of the NW-SE Ter- ceira Rift regional faults with an E-W deep fault system thought to be a relic of a Mid-Atlantic Ridge transform fault. N-S and NE-SW faults also occur in this con- text. Basaltic cinder cones emplaced along NW-SE fractures link that major volcanic structures. The easternmost part of the island comprises an inactive trachytic central volcano (Povoação) and an old basaltic volcanic complex (Nordeste). Since the settle- ment of the island, early in the XV century, several destructive earthquakes occurred in the Azores region. At least 11 events hit S. Miguel Island with high intensity, some of which caused several deaths and significant damages. The analysis of historical documents allowed reconstructing the history and the impact of all those earthquakes and new intensity maps using the 1998 European Macrosseismic Scale were produced for each event. The data was then integrated in order to obtain the maximum historical seismic intensity map of S. Miguel. This tool is regarded as an important document for hazard assessment and risk mitigation taking in account that indicates the location of dangerous seismogenic zones and provides a comprehensive set of data to be applied in land-use planning, emergency planning and building construction.

Risk analysis of inter-species reassortment through a Rift Valley fever phlebovirus MP-12 vaccine strain.

PubMed

Ly, Hoai J; Lokugamage, Nandadeva; Nishiyama, Shoko; Ikegami, Tetsuro

2017-01-01

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to Africa and the Arabian Peninsula. The causative agent, Rift Valley fever phlebovirus (RVFV), belongs to the genus Phlebovirus in the family Phenuiviridae and causes high rates of abortions in ruminants, and hemorrhagic fever, encephalitis, or blindness in humans. Viral maintenance by mosquito vectors has led to sporadic RVF outbreaks in ruminants and humans in endemic countries, and effective vaccination of animals and humans may minimize the impact of this disease. A live-attenuated MP-12 vaccine strain is one of the best characterized RVFV strains, and was conditionally approved as a veterinary vaccine in the U.S. Live-attenuated RVF vaccines including MP-12 strain may form reassortant strains with other bunyavirus species. This study thus aimed to characterize the occurrence of genetic reassortment between the MP-12 strain and bunyavirus species closely related to RVFV. The Arumowot virus (AMTV) and Gouleako goukovirus (GOLV), are transmitted by mosquitoes in Africa. The results of this study showed that GOLV does not form detectable reassortant strains with the MP-12 strain in co-infected C6/36 cells. The AMTV also did not form any reassortant strains with MP-12 strain in co-infected C6/36 cells, due to the incompatibility among N, L, and Gn/Gc proteins. A lack of reassortant formation could be due to a functional incompatibility of N and L proteins derived from heterologous species, and due to a lack of packaging via heterologous Gn/Gc proteins. The MP-12 strain did, however, randomly exchange L-, M-, and S-segments with a genetic variant strain, rMP12-GM50, in culture cells. The MP-12 strain is thus unlikely to form any reassortant strains with AMTV or GOLV in nature.

A new tectono-magmatic model for the Lofoten/Vesterålen Margin at the outer limit of the Iceland Plume influence

NASA Astrophysics Data System (ADS)

Breivik, Asbjørn Johan; Faleide, Jan Inge; Mjelde, Rolf; Flueh, Ernst R.; Murai, Yoshio

2017-10-01

The Early Eocene continental breakup was magma-rich and formed part of the North Atlantic Igneous Province. Extrusive and intrusive magmatism was abundant on the continental side, and a thick oceanic crust was produced up to a few m.y. after breakup. However, the extensive magmatism at the Vøring Plateau off mid-Norway died down rapidly northeastwards towards the Lofoten/Vesterålen Margin. In 2003 an Ocean Bottom Seismometer profile was collected from mainland Norway, across Lofoten, and into the deep ocean. Forward/inverse velocity modeling by raytracing reveals a continental margin transitional between magma-rich and magma-poor rifting. For the first time a distinct lower-crustal body typical for volcanic margins has been identified at this outer margin segment, up to 3.5 km thick and ∼50 km wide. On the other hand, expected extrusive magmatism could not be clearly identified here. Strong reflections earlier interpreted as the top of extensive lavas may at least partly represent high-velocity sediments derived from the shelf, and/or fault surfaces. Early post-breakup oceanic crust is moderately thickened (∼8 km), but is reduced to 6 km after 1 m.y. The adjacent continental crystalline crust is extended down to a minimum of 4.5 km thickness. Early plate spreading rates derived from the Norway Basin and the northern Vøring Plateau were used to calculate synthetic magnetic seafloor anomalies, and compared to our ship magnetic profile. It appears that continental breakup took place at ∼53.1 Ma, ∼1 m.y. later than on the Vøring Plateau, consistent with late strong crustal extension. The low interaction between extension and magmatism indicates that mantle plume material was not present at the Lofoten Margin during initial rifting, and that the observed excess magmatism was created by late lateral transport from a nearby pool of plume material into the lithospheric rift zone at breakup time.

Early cretaceous platform-margin configuration and evolution in the central Oman mountains, Arabian peninsula

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pratt, B.R.; Smewing, J.D.

1993-02-01

The Hajar Supergroup (Middle Permian-Lower Cretaceous) of northeastern Oman records rifting and development of a passive margin along the edge of the Arabian platform facing Neo-Tethys. The Jurassic and Lower Cretaceous part, comprising the Sahtan, Kahmah, and Wasia groups, was deposited during the maximum extent of the broad epicontinental sea landward of this margin. These limestone units reach a total of 1500 m in thickness and correlate with the hydrocarbon reservoirs of the Arabian Peninsula. The trace of the Jurassic and Cretaceous margin in northeastern Oman followed a zigzag series of rift segments, resulting in promontories and reentrants that changedmore » in position through time in response to the configuration and differential motion of underlying rift blocks. Synsedimentary normal faulting occurred locally in the Middle Jurassic, whereas in the Late Jurassic, the margin was eroded from variable uplift of up to 300 m before subsiding to below storm wave base. This uplift may have been caused by compression from oceanic crust that obducted along the southeastern side of the platform. The Lower Cretaceous succession in the central Oman Mountains and adjacent subsurface began with regional drowning around the Jurassic-Cretaceous boundary. The succession in the east (Saih Hatat) records a single regressive sequence, ending in the progradation of the shallow-water carbonate platform by the Cenomanian. However, the succession in the west (Jebel Akhdar and interior) is dominated by shallow-water carbonate facies, but punctuated by a second regional drowning in the late Aptian. A third, Late Cretaceous drowning terminated deposition of the Wasia Group in the Turonian and was caused by convergence of oceanic crust and foreland basic formation. The record of tectonic behavior of carbonate platforms has important implications for the development of hydrocarbon source rocks and porosity. 68 refs., 11 figs., 1 tab.« less

Geophysical studies of the West Antarctic Rift System

NASA Astrophysics Data System (ADS)

Behrendt, J. C.; Lemasurier, W. E.; Cooper, A. K.; Tessensohn, F.; TréHu, A.; Damaske, D.

1991-12-01

The West Antarctic rift system extends over a 3000 × 750 km, largely ice covered area from the Ross Sea to the base of the Antarctic Peninsula, comparable in area to the Basin and Range and the East African rift system. A spectacular rift shoulder scarp along which peaks reach 4-5 km maximum elevation marks one flank and extends from northern Victoria Land-Queen Maud Mountains to the Ellsworth-Whitmore-Horlick Mountains. The rift shoulder has maximum present physiographic relief of 5 km in the Ross Embayment and 7 km in the Ellsworth Mountains-Byrd Subglacial Basin area. The Transantarctic Mountains part of the rift shoulder (and probably the entire shoulder) has been interpreted as rising since about 60 Ma, at episodic rates of ˜1 km/m.y., most recently since mid-Pliocene time, rather than continuously at the mean rate of 100 m/m.y. The rift system is characterized by bimodal alkaline volcanic rocks ranging from at least Oligocene to the present. These are exposed asymmetrically along the rift flanks and at the south end of the Antarctic Peninsula. The trend of the Jurassic tholeiites (Ferrar dolerites, Kirkpatric basalts) marking the Jurassic Transantarctic rift is coincident with exposures of the late Cenozoic volcanic rocks along the section of the Transantarctic Mountains from northern Victoria Land to the Horlick Mountains. The Cenozoic rift shoulder diverges here from the Jurassic tholeiite trend, and the tholeiites are exposed continuously (including the Dufek intrusion) along the lower- elevation (1-2 km) section of Transantarctic Mountains to the Weddell Sea. Widely spaced aeromagnetic profiles in West Antarctica indicate the absence of Cenozoic volcanic rocks in the ice covered part of the Whitmore-Ellsworth-Mountain block and suggest their widespread occurrence beneath the western part of the ice sheet overlying the Byrd Subglacial Basin. A German Federal Institute for Geosciences and Natural Resources (BGR)-U.S. Geological Survey (USGS) aeromagnetic survey over the Ross Sea continental shelf indicates rift fabric and suggests numerous submarine volcanoes along discrete NNW trending zones. A Bouguer anomaly range of approximately 200 (+50 to -150) mGal having 4-7 mGal/km gradients where measured in places marks the rift shoulder from northern Victoria Land possibly to the Ellsworth Mountains (where data are too sparse to determine maximum amplitude and gradient). The steepest gravity gradients across the rift shoulder require high density (mafic or ultramafic?) rock within the crust as well as at least 12 km of thinner crust beneath the West Antarctic rift system in contrast to East Antarctica. Sparse land seismic data reported along the rift shoulder, where velocities are greater than 7 km/s, and marine data indicating velocities above 7 km/s beneath the Ross Sea continental shelf support this interpretation. The maximum Bouguer gravity range in the Pensacola Mountains area of the Transantarctic Mountains is only about 130 mGal with a maximum 2 mGal/km gradient, which can be explained solely by 8 km of crustal thickening. Large offset seismic profiles over the Ross Sea shelf collected by the German Antarctic North Victoria Land Expedition V (GANOVEX V) combined with earlier USGS and other results indicate 17-21 km thickness for the crust beneath the Ross Sea shelf which we interpret as evidence of extended rifted continental crust. A regional positive Bouguer anomaly (0 to +50 mGal), the width of the rift, extends from the Ross Sea continental shelf throughout the Ross Embayment and Byrd Subglacial Basin area of the West Antarctic rift system and indicates that the Moho is approximately 20 km deep tied to the seismic results (probably coincident with the top of the asthenosphere) rather than the 30 km reported in earlier interpretations. The interpretation of horst and graben structures in the Ross Sea, made from marine seismic reflection data, probably can be extended throughout the rift (i.e., the Ross Ice shelf and the Byrd Subglacial Basin areas). The near absence of earthquakes in the West Antarctic rift system probably results from a combination of primarily sparse seismograph coverage and, secondarily, suppression of earthquakes by the ice sheet (e.g., Johnston, 1987) and very high seismicity shortly after deglaciation in the Ross Embayment followed by abnormally low seismicity at present (e.g., Muir Wood, 1989). The evidence of high temperatures at shallow depth beneath the Ross Sea continental shelf and adjacent Transantarctic Mountains is supportive of thermal uplift of the mountains associated with lateral heat conduction from the rift and can possibly also explain the volcanism, rifting, and high elevation of the entire rift shoulder to the Ellsworth-Horlick-Whitmore Mountains. We infer that the Gondwana breakup and the West Antarctic rift are part of a continuously propagating rift that started in the Jurassic when Africa separated from East Antarctica (including the failed Jurassic Transantarctic rift). Rifting proceeded clockwise around East Antarctica to the separation of New Zealand and the Campbell Plateau about 85-95 Ma and has continued (with a spreading center jump) to its present location in the Ross Embayment and West Antarctica. The Cenozoic activity of the West Antarctic rift system appears to be continuous in time with rifting in the same area that began only in the late Mesozoic. Although the mechanism for rifting is not completely explained, we suggest a combination of the flexural rigidity model (Stem and ten Brink, 1989) proposed for the Ross Embayment and the thermal plume or hot spot concepts. The propagating rift may have been "captured" by the thermal plume.

Birth of a Large Iceberg in Pine Island Bay, Antarctica

NASA Image and Video Library

2001-11-14

A large tabular iceberg (42 kilometers x 17 kilometers) broke off Pine Island Glacier, West Antarctica (75ºS latitude, 102ºW longitude) sometime between November 4 and 12, 2001. Images of the glacier were acquired by the Multi-angle Imaging SpectroRadiometer (MISR) instrument aboard NASA's Terra spacecraft. This event was preceded by the formation of a large crack across the glacier in mid 2000. Data gathered by other imaging instruments revealed the crack to be propagating through the shelf ice at a rate averaging 15 meters per day, accompanied by a slight rotation of about one percent per year at the seaward margin of the rift. The image set shows three views of Pine Island Glacier acquired by MISR's vertical-viewing (nadir) camera. The first was captured in late 2000, early in the development of the crack. The second and third views were acquired in November 2001, just before and just after the new iceberg broke off. The existence of the crack took the glaciological community by surprise, and the rapid rate at which the crack propagated was also not anticipated. Glaciologists predicted that the rift would reach the other side of the glacier sometime in 2002. However, the iceberg detached much sooner than anticipated, and the last 10-kilometer segment that was still attached to the ice shelf snapped off in a matter of days. http://photojournal.jpl.nasa.gov/catalog/PIA03431

Segmentation and Contrasting Magma Supply Along the South-East Indian Ridge, 130°E to 140°E: Results of the STORM Cruise

NASA Astrophysics Data System (ADS)

Briais, A.; Ruellan, E.; Maia, M.; Hemond, C.; Hanan, B. B.; Ceuleneer, G.; Graham, D. W.; Park, S. H.

2017-12-01

We present observations of the South-East Indian Ridge (SEIR) between 130°E to 140°E, mostly collected during the STORM cruise (South Tasmania Ocean Ridge and Mantle) on the N/O L'Atalante. The SEIR displays large variations of axial depth despite an almost constant intermediate full spreading rate of 75 km/m.y. In the study area the analysis of multibeam bathymetry maps shows that the axis displays a rise morphology to the east away from the discontinuities, and a rifted high morphology in the west and near the OSCs, as often observed along intermediate-spreading mid-ocean ridges. The ridge axis is offset by 27 km at 131°E and 20 km at 135°E by two large-offset overlapping spreading centers (OSCs) propagating westward, and by a smaller OSC at 137°17'E. These OSCs define four second-order ridge segments (A2 to A5 from west to east). We observe a general shallowing of the ridge axis from 3100 m depth in the west to 2400 m depth in the east, and a prominent deepening of the axis near the large OSCs. The easternmost segment A5 shows a very shallow axial ridge suggesting a robust magma supply despite its proximity to the George V transform fault (140°E). Major element variations in basalt glasses are systematically related to morphotectonic segmentation of the ridge axis, showing contrasts in crystal fractionation from one segment to another that may relate to differences in replenishment of axial melt lenses by primitive melts. Along segment A5, crystallization increases with proximity to the George V transform fault, consistent with an expected cold edge effect. In contrast, along segment A3 the extent of crystallization increases progressively from east to west in the direction of ridge propagation. *STORM Cruise Scientific Party: F. Barrere, C. Boulart, A. Briais, D. Brunelli, G. Ceuleneer, N. Ferreira, D. Graham, B. Hanan, C. Hémond, S. Macleod, M. Maia, A. Maillard, S. Merkuryev, S.H. Park, S. Révillon, E. Ruellan, A. Schohn, S. Watson, and Y.S. Yang.

Decarbonation in an intracratonic setting: Insight from petrological-thermomechanical modeling

NASA Astrophysics Data System (ADS)

Gonzalez, Christopher M.; Gorczyk, Weronika

2017-08-01

Cratons form the stable core roots of the continental crust. Despite long-term stability, cratons have failed in the past. Cratonic destruction (e.g., North Atlantic Craton) due to chemical rejuvenation at the base of the lithosphere remains poorly constrained numerically. We use 2-D petrological-thermomechanical models to assess cratonic rifting characteristics and mantle CO2 degassing in the presence of a carbonated subcontinental lithospheric mantle (SCLM). We test two tectonothermal SCLM compositions: Archon (depleted) and Tecton (fertilized) using 2 CO2 wt % in the bulk composition to represent a metasomatized SCLM. We parameterize cratonic breakup via extensional duration (7-12 Ma; full breakup), tectonothermal age, TMoho (300-600°C), and crustal rheology. The two compositions with metasomatized SCLMs share similar rifting features and decarbonation trends during initial extension. However, we show long-term (>67 Ma) stability differences due to lithospheric density contrasts between SCLM compositions. The Tecton model shows convective removal and thinning of the metasomatized SCLM during failed rifting. The Archon composition remained stable, highlighting the primary role for SCLM density even when metasomatized at its base. In the short-term, three failed rifting characteristics emerge: failed rifting without decarbonation, failed rifting with decarbonation, and semifailed rifting with dry asthenospheric melting and decarbonation. Decarbonation trends were greatest in the failed rifts, reaching peak fluxes of 94 × 104 kg m-3. Increased TMoho did not alter the effects of rifting or decarbonation. Lastly, we show mantle regions where decarbonation, mantle melting in the presence of carbonate, and preservation of carbonated mantle occur during rifting.

Fluid flow and water-rock interaction in the East Rift Zone of Kilauea Volcano, Hawaii

NASA Astrophysics Data System (ADS)

Conrad, Mark E.; Thomas, Donald M.; Flexser, Steven; Vennemann, Torsten W.

1997-07-01

The East Rift Zone of Kilauea Volcano in Hawaii represents a major area of geothermal activity. Fluid inclusion and stable isotope analyses of secondary hydrothermal minerals in core samples from three scientific observation holes (SOH) drilled into the rift zone indicate that the geothermal system is dominated by meteoric waters to depths of as much as 1500 m below sea level. Calculated δ18O and δD values for fluids on the north side of the rift zone indicate that the deep meteoric fluids may be derived from precipitation on the upper slopes of Mauna Loa Volcano. In the interior of the rift zone, recharge is dominated by seawater mixed with local meteoric water. Water/rock ratios in the rift area are approximately 2, but strongly 18O-enriched fluids in the deeper parts of the SOH-2 and SOH-4 drill holes (on the north side of the rift) indicate that the fluids underwent extensive interaction with rocks prior to reaching this part of the rift zone. Marine carbonates at the subaerial to submarine transition (between 1700 and 1780 m depth) in SOH-4 have not fully equilibrated with the fluids, suggesting that the onset of hydrothermal activity in this area was relatively recent (<2000 years). This may represent increased volcanic activity along the rift after the end of the Ai La'au phase of eruptive activity at the Kilauea summit approximately 1000 years ago, or it may reflect progressive evolution of the hydrothermal system in response to southward migration of intrusive activity within the rift.

The mesoproterozoic midcontinent rift system, Lake Superior region, USA

USGS Publications Warehouse

Ojakangas, R.W.; Morey, G.B.; Green, J.C.

2001-01-01

Exposures in the Lake Superior region, and associated geophysical evidence, show that a 2000 km-long rift system developed within the North American craton ??? 1109-1087 Ma, the age span of the most of the volcanic rocks. This system is characterized by immense volumes of mafic igneous rocks, mostly subaerial plateau basalts, generated in two major pulses largely by a hot mantle plume. A new ocean basin was nearly formed before rifting ceased, perhaps due to the remote effect of the Grenville continental collision to the east. Broad sagging/subsidence, combined with a system of axial half-grabens separated along the length of the rift by accommodation zones, provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post-rift clastic sediments, both along the rift axis and in basins flanking a central, post-volcanic horst. Pre-rift mature, quartzose sandstones imply little or no uplift prior to the onset of rift volcanism. Early post-rift red-bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings; the exception is one gray to black carbon-bearing lacustrine(?) unit. This early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red-bed, fluvial sediments with an extra-basinal provenance. ?? 2001 Elsevier Science B.V. All rights reserved.

Gravity study of the Central African Rift system: a model of continental disruption 2. The Darfur domal uplift and associated Cainozoic volcanism

NASA Astrophysics Data System (ADS)

Bermingham, P. M.; Fairhead, J. D.; Stuart, G. W.

1983-05-01

Gravity studies of the Darfur uplift, Western Sudan, show it to be associated with a circular negative Bouguer anomaly, 50 mGal in amplitude and 700 km across. A three-dimensional model interpretation of the Darfur anomaly, using constraints deduced from geophysical studies of similar but more evolved Kenya and Ethiopia domes, suggests either a low-density laccolithic body at mid-lithospheric depth (~ 60 km) or a thinned lithosphere with emplacement at high level of low-density asthenospheric material. The regional setting of the Darfur uplift is described in terms of it being an integral part of the Central African Rift System which is shown to be broadly equivalent to the early to middle Miocene stage in the development of the Afro-Arabian Rift System. Comparisons between these rift systems suggest that extensional tectonics and passive rifting, resulting in the subsiding sedimentary rift basins associated with the Ngaoundere, Abu Gabra, Red Sea and Gulf of Aden rifts, are more typical of the early stage development of passive continental margins than the active domal uplift and development of rifted features associated with the Darfur, Kenya and Ethiopia domes.

The 1974 Ethiopian rift geodimeter survey

NASA Technical Reports Server (NTRS)

Mohr, P.

1977-01-01

The field techniques and methods of data reduction for five successive geodimeter surveys in the Ethiopian rift valley are enlarged upon, with the considered conclusion that there is progressive accumulation of upper crustal strain, consonant with on-going rift extension. The extension is restricted to the Quaternary volcanotectonic axis of the rift, namely the Wonji fault belt, and is occurring at rates of 3 to 6 mm/yr in the northern sector of the rift valley. Although this concurs with the predictions of platetectonic analysis of the Afar triple junction, it is considered premature to endorse such a concurrence on the basis of only 5 years of observations. This is underlined by the detection of local tectonic contractions and expansions associated with geothermal and gravity anomalies in the central sector of the rift valley. There is a hint of a component of dextral slip along some of the rift-floor fault zones, both from geological evidence and from the strain patterns detected in the present geodetic surveys.

Structural setting of the Metán Basin (NW Argentina): new insights from 2D seismic profiles

NASA Astrophysics Data System (ADS)

Conti, Alessia; Maffucci, Roberta; Bigi, Sabina; Corrado, Sveva; Giordano, Guido; Viramonte, José G.

2017-04-01

The Metán Basin is located in the sub-Andean foreland, in the southernmost portion of the Santa Barbara system structural province (NW Argentina). The upper crust in this region shows a strong segmentation due to inherited stratigraphic and structural discontinuities, related to a Palaeozoic orogenic event and to a Cretaceous to Paleogene rifting event (Kley et al., 1999; Iaffa et al., 2011). This study seeks to unravel the deep structural setting of the basin, in order to better understand the tectonic evolution of the area. Different seismic sections are analysed, located in the Metán basin and acquired by YPF (Yacimientos Petrolíferos Fiscales, former national oil company of Argentina) in different surveys during the '70s - '80s. Stratigraphic control for the seismic interpretation is provided by petroleum exploratory wells drilled in the basin; they show a stratigraphic succession of syn-rift and post-rift deposits, mainly constituted by a continental succession of red beds, with minor limestone intercalations (Salta Group), overlain by a thick continental foreland basin succession (Orán Group) (Salfity et al., 1981). From a structural point of view, the Metán basin is characterized by a variety of structural trends, with thrust faults and related folds mainly trending N-S, NE-SW and NNE-SSW. Different mechanism can be responsible for the folding of the sedimentary cover; hangingwall anticlines are represented both by high angle thrust faults produced by inversion of Cretaceous extensional faults (Maffucci et al., 2015), and by fault propagation folds formed during the Andean shortening event. The study of the interaction between the older reactivated faults and the newly generated ones could provide new insights to unravel the complex structural setting of the area. References Iaffa D. N., Sàbat, F., Muñoz, J.A., Mon, R., Gutierrez, A.A., 2011. The role of inherited structures in a foreland basin evolution. The Metán Basin in NW Argentina. Journal of Structural Geology, 33, 1816-1828. Kley, J., Monaldi, C. R. & Salfity, J. A., 1999. Along strike segmentation of the Andean foreland: causes and consequences. Tectonophysics, 301, 75-94. Maffucci, R., Bigi, S., Corrado, S., Chiodi, A., Di Paolo, L., Giordano, G., Invernizzi, C., 2015. Quality assessment of reservoirs by means of outcrop data and "discrete fracture network" models: The case history of Rosario de La Frontera (NW Argentina) geothermal system. Tectonophysics, 647, 112-131. Salfity, J.A., Marquillas, R.A., 1981. Las unidades estratigráficas cretácicas del Norte de Argentina. In: Volkheimer, W., Musacchio, E. (Eds.), Cuencas Sedimentarias del Jurásico y Cretácico de América del Sur, 1, 303-317.

Oblique rift opening revealed by reoccurring magma injection in central Iceland.

PubMed

Ruch, Joël; Wang, Teng; Xu, Wenbin; Hensch, Martin; Jónsson, Sigurjón

2016-08-05

Extension deficit builds up over centuries at divergent plate boundaries and is recurrently removed during rifting events, accompanied by magma intrusions and transient metre-scale deformation. However, information on transient near-field deformation has rarely been captured, hindering progress in understanding rifting mechanisms and evolution. Here we show new evidence of oblique rift opening during a rifting event influenced by pre-existing fractures and two centuries of extension deficit accumulation. This event originated from the Bárðarbunga caldera and led to the largest basaltic eruption in Iceland in >200 years. The results show that the opening was initially accompanied by left-lateral shear that ceased with increasing opening. Our results imply that pre-existing fractures play a key role in controlling oblique rift opening at divergent plate boundaries.

Deep crustal earthquakes associated with continental rifts

NASA Astrophysics Data System (ADS)

Doser, Diane I.; Yarwood, Dennis R.

1994-01-01

Deep (> 20 km) crustal earthquakes have occurred within or along the margins of at least four continental rift zones. The largest of these deep crustal earthquakes ( M ⩾ 5.0) have strike-slip or oblique-slip mechanisms with T-axes oriented similarly to those associated with shallow normal faulting within the rift zones. The majority of deep crustal earthquakes occur along the rift margins in regions that have cooler, thicker crust. Several deep crustal events, however, occur in regions of high heat flow. These regions also appear to be regions of high strain, a factor that could account for the observed depths. We believe the deep crustal earthquakes represent either the relative motion of rift zones with respect to adjacent stable regions or the propagation of rifting into stable regions.

Contrasting modes of rifting: The Benue Trough and Cameroon Volcanic Line, West Africa

NASA Astrophysics Data System (ADS)

Okereke, C. S.

1988-08-01

The Benue trough of west Africa is commonly believed to be a rift feature that originated in the Cretaceous at about the time that Africa and South America began to separate. Bouguer gravity and available geological data in the trough indicate that its formation was probably the result of regional horizontal stresses in the lithosphere, causing crustal extension and surface subsidence. By contrast, the data for the adjoining Cameroon volcanic line suggests that the associated tensional stresses relate to mantle upwarp causing thinning of the lithosphere and regional crustal uplift similar to that associated with the Kenya rift. Thus the association of passive and active rifts seen in the Afro-Arabia rift system is also a feature of the Cretaceous rift system in west Africa.

Antecedent rivers and early rifting: a case study from the Plio-Pleistocene Corinth rift, Greece

NASA Astrophysics Data System (ADS)

Hemelsdaël, Romain; Ford, Mary; Malartre, Fabrice

2016-04-01

Models of early rifting present syn-rift sedimentation as the direct response to the development of normal fault systems where footwall-derived drainage supplies alluvial to lacustrine sediments into hangingwall depocentres. These models often include antecedent rivers, diverted into active depocentres and with little impact on facies distributions. However, antecedent rivers can supply a high volume of sediment from the onset of rifting. What are the interactions between major antecedent rivers and a growing normal fault system? What are the implications for alluvial stratigraphy and facies distributions in early rifts? These questions are investigated by studying a Plio-Pleistocene fluvial succession on the southern margin of the Corinth rift (Greece). In the northern Peloponnese, early syn-rift deposits are preserved in a series of uplifted E-W normal fault blocks (10-15 km long, 3-7 km wide). Detailed sedimentary logging and high resolution mapping of the syn-rift succession (400 to 1300 m thick) define the architecture of the early rift alluvial system. Magnetostratigraphy and biostratigraphic markers are used to date and correlate the fluvial succession within and between fault blocks. The age of the succession is between 4.0 and 1.8 Ma. We present a new tectonostratigraphic model for early rift basins based on our reconstructions. The early rift depositional system was established across a series of narrow normal fault blocks. Palaeocurrent data show that the alluvial basin was supplied by one major sediment entry point. A low sinuosity braided river system flowed over 15 to 30 km to the NE. Facies evolved downstream from coarse conglomerates to fined-grained fluvial deposits. Other minor sediment entry points supply linked and isolated depocentres. The main river system terminated eastward where it built stacked small deltas into a shallow lake (5 to 15 m deep) that occupied the central Corinth rift. The main fluvial axis remained constant and controlled facies distribution throughout the early rift evolution. We show that the length scale of fluvial facies transitions is greater than and therefore not related to fault spacing. First order facies variations instead occur at the scale of the full antecedent fluvial system. Strike-parallel subsidence variations in individual fault blocks represent a second order controlling factor on stratigraphic architecture. As depocentres enlarged through time, sediments progressively filled palaeorelief, and formed a continuous alluvial plain above active faults. There was limited creation of footwall relief and thus no significant consequent drainage system developed. Here, instead of being diverted toward subsiding zones, the drainage system overfilled the whole rift from the onset of faulting. Moreover, the zones of maximum subsidence on individual faults are aligned across strike parallel to the persistent fluvial axis. This implies that long-term sediment loading influenced the growth of normal faults. We conclude that a major antecedent drainage system inherited from the Hellenide mountain belt supplied high volumes of coarse sediment from the onset of faulting in the western Corinth rift (around 4 Ma). These observations demonstrate that antecedent drainage systems can be important in the tectono-sedimentary evolution of rift basins.

The Evolution of the Tethysides during the Medial to Late Triassic

NASA Astrophysics Data System (ADS)

Saǧdıç, Nurbike G.; Celâl Şengör, A. M.

2016-04-01

The Triassic is a time of widespread rifting within the future Alpides of the circum-Mediterranean countries. However, this rifting had little to do with the later, Sinemurian-Hettangian rifting that penetrated the Tethyan realm from the Atlantic Ocean. The eastern part of the rifting occurred south of the Palaeo-Tethys and seems to have been related to stretching above its extensional arc. Evidence for his stretching is seen in the Karakaya-Pelagonian-Pindos- Meliata-Hallstatt zones and the Eastern Mediterranean. The Eastern Mediterranean is separated from the other extensional zones by a Mikrasian continental fragment that had begun separating from Gondwana-Land already during the Permian. The rifting propagated eastward along the Carpathians (Transylvanian Nappes) and the Eastern and the Southern Alps from where it entered the future Provençal chains and finally the Pyrenees where evaporites were laid down in extensional basins. In the south, an area of rifting went from the Eastern Mediterranean into the High Atlas thus delimiting an Iberapulian continental fragment. The Iberapulian fragment became divided into an Iberian and an Apulian parts later during the Hettangian-Sinemurian rifting that also invaded the earlier extensional areas in the Atlas. The extension directions during the medial and late Triassic are controlled by the tectonics of the eastern end of the Palaeo-Tethys. Along its northern margin, i.e., along the Scythides, right-lateral motion dominated. Along the northern margin of the Mikrasian fragment subduction was nearly head-on (slightly oblique so as to impose a slight right-lateral motion along the arc), but the stretching along the Karakaya rift zones was probably orthogonal because of the similarly orthogonal stretching in the Eastern Mediterrarean. The kinematics is dependent on what sort of motion is imposed onto the Palaeo-Tethyan plate (s) along its (their) northern margin and the direction of stretching in the Eastern Mediterranean. The rifting in areas farther west may have been a consequence of the origin of secondary shear structures along the Mikrasian and Iberapulian fragments. The Italian rifts, such as the Lagonegro and the Sclafani seem to have resulted from a similar process. Plate kinematics, as reconstructed, imposes a slight right-lateral motion onto the East Alpine/Southern Alpine areas. It is remarkable how independent the later Jurassic rifting seems to have been. It avoided in many places the former regions of stretching and opened new avenues of rifting for itself. One wonders whether the lithosphere in the older areas of rifting had recovered sufficiently to pose a hindernis to fracturing or whether the newer rifting followed older, Hercynian zones of deformation. For the time being we favour the second alternative as the time between the Triassic rifting and the Jurassic rifting seems insufficient to allow the lithosphere to recover to build sufficient strength.

Numerical experiments of volcanic dominated rifts and passive margins

NASA Astrophysics Data System (ADS)

Korchinski, Megan; Teyssier, Christian; Rey, Patrice; Whitney, Donna; Mondy, Luke

2017-04-01

Continental rifting is driven by plate tectonic forces (passive rifting), thermal thinning of the lithosphere over a hotspot (active rifting), or a combination of the two. Successful rifts develop into passive margins where pre-drift stretching is accompanied by normal faulting, clastic sedimentation, and various degrees of magmatism. The structure of volcanic passive margins (VPM) differs substantially from margins that are dominated by sedimentation. VPMs are typically narrow, with a lower continental crust that is intruded by magma and can flow as a low-viscosity layer. To investigate the role of the deep crust in the early development of VPMs, we have developed a suite of 2D thermomechanical numerical experiments (Underworld code) in which the density and viscosity of the deep crust and the density of the rift basin fill are systematically varied. Our experiments show that, for a given rifting velocity, the viscosity of the deep crust and the density of the rift basin fill exert primary controls on early VPM development. The viscosity of the deep crust controls the degree to which the shallow crust undergoes localised faulting or distributed thinning. A weak deep crust localises rifting and is efficiently exhumed to the near-surface, whereas a strong deep crust distributes shallow crust extension and remains buried. A high density rift basin fill results in gravitational loading and increased subsidence rate in cases in which the viscosity of the deep crust is sufficiently low to allow that layer to be displaced by the sinking basin fill. At the limit, a low viscosity deep crust overlain by a thick basalt-dominated fill generates a gravitational instability, with a drip of cool basalt that sinks and ponds at the Moho. Experiment results indicate that the deep crust plays a critical role in the dynamic development of volcanic dominated rifts and passive margins. During rifting, the deep continental crust is heated and readily responds to solicitations of the shallow crust (rooting of normal faults, exhumation of the deep crust in normal fault footwalls). Gravitational instabilities caused by high density rift infill similar to those observed in our numerical experiments may be present in the Mesoproterozoic ( 1100 Ma) North American Midcontinent Rift System (MRS). The MRS is a failed rift that is filled with >20 km of dominantly basaltic volcanic deposits, and therefore represents an end member VPM (high density basin fill) where the initial structure of a pre-drift VPM is preserved. Magmatism occurred in two pulses over <15 Ma involving deep mantle melting first (>150 km), then shallow melting (40-70 km). Post-rift subsidence accumulated up to 10 km of clastic sediments in the center of the basin. Evidence of cool, dense rocks sinking into a low-viscosity deep crust as predicted in our numerical experiments may be present in the western arm of the MRS, where crustal density analyses suggest the presence of dense bodies (eclogite) at the base of the crust.

Carbon isotope and abundance systematics of Icelandic geothermal gases, fluids and subglacial basalts with implications for mantle plume-related CO2 fluxes

NASA Astrophysics Data System (ADS)

Barry, P. H.; Hilton, D. R.; Füri, E.; Halldórsson, S. A.; Grönvold, K.

2014-06-01

We report new carbon dioxide (CO2) abundance and isotope data for 71 geothermal gases and fluids from both high-temperature (HT > 150 °C at 1 km depth) and low-temperature (LT < 150 °C at 1 km depth) geothermal systems located within neovolcanic zones and older segments of the Icelandic crust, respectively. These data are supplemented by CO2 data obtained by stepped heating of 47 subglacial basaltic glasses collected from the neovolcanic zones. The sample suite has been characterized previously for He-Ne (geothermal) and He-Ne-Ar (basalt) systematics (Füri et al., 2010), allowing elemental ratios to be calculated for individual samples. Geothermal fluids are characterized by a wide range in carbon isotope ratios (δ13C), from -18.8‰ to +4.6‰ (vs. VPDB), and CO2/3He values that span eight orders of magnitude, from 1 × 104 to 2 × 1012. Extreme geothermal values suggest that original source compositions have been extensively modified by hydrothermal processes such as degassing and/or calcite precipitation. Basaltic glasses are also characterized by a wide range in δ13C values, from -27.2‰ to -3.6‰, whereas CO2/3He values span a narrower range, from 1 × 108 to 1 × 1012. The combination of both low δ13C values and low CO2 contents in basalts indicates that magmas are extensively and variably degassed. Using an equilibrium degassing model, we estimate that pre-eruptive basaltic melts beneath Iceland contain ∼531 ± 64 ppm CO2 with δ13C values of -2.5 ± 1.1‰, in good agreement with estimates from olivine-hosted melt inclusions (Metrich et al., 1991) and depleted MORB mantle (DMM) CO2 source estimates (Marty, 2012). In addition, pre-eruptive CO2 compositions are estimated for individual segments of the Icelandic axial rift zones, and show a marked decrease from north to south (Northern Rift Zone = 550 ± 66 ppm; Eastern Rift Zone = 371 ± 45 ppm; Western Rift Zone = 206 ± 24 ppm). Notably, these results are model dependent, and selection of a lower δ13C fractionation factor will result in lower source estimates and larger uncertainties associated with the initial δ13C estimate. Degassing can adequately explain low CO2 contents in basalts; however, degassing alone is unlikely to generate the entire spectrum of observed δ13C variations, and we suggest that melt-crust interaction, involving a low δ13C component, may also contribute to observed signatures. Using representative samples, the CO2 flux from Iceland is estimated using three independent methods: (1) combining measured CO2/3He values (in gases and basalts) with 3He flux estimates (Hilton et al., 1990), (2) merging basaltic emplacement rates of Iceland with pre-eruptive magma source estimates of ∼531 ± 64 ppm CO2, and (3) combining fluid CO2 contents with estimated regional fluid discharge rates. These methods yield CO2 flux estimates from of 0.2-23 × 1010 mol a-1, which represent ∼0.1-10% of the estimated global ridge flux (2.2 × 1012 mol a-1; Marty and Tolstikhin, 1998).

Magma-poor and magma-rich segments along the hyperextended, pre-Caledonian passive margin of Baltica

NASA Astrophysics Data System (ADS)

Andersen, Torgeir B.; Alsaif, Manar; Corfu, Fernando; Jakob, Johannes; Planke, Sverre; Tegner, Christian

2015-04-01

The Scandinavian Caledonides constitute a more than 1850 km long 'Himalayan-type' orogen, formed by collision between Baltica-Avalonia and Laurentia. Subduction-related magmatism in the Iapetus ended at ~430 Ma and continental convergence continued for ~30 Myr until ~400 Ma. The collision produced a thick orogenic wedge comprising the stacked remnants of the rifted to hyperextended passive Baltican margin (Andersen et al. 2012), as well as suspect, composite and outboard terranes, which were successively emplaced as large-scale nappe complexes onto Baltica during the Scandian collision (see Corfu et al. 2014 for a recent review). Large parts (~800 km) of the mountain-belt in central Scandinavia, particularly in the Särv and Seve Nappes and their counterparts in Troms, are characterised by spectacular dyke complexes emplaced into continental sediments (e.g. Svenningsen 2001, Hollocher et al. 2007). These constitute a magma-rich segment formed along the margin of Baltica or within hyperextended continental slivers outboard of Baltica. The intensity of the pre-Caledonian magmatism is comparable to that of the present NE-Atlantic and other volcanic passive margins. The volumes and available U-Pb ages of 610-597 Ma (Baird et al. 2014 and refs therein) suggest that the magmatism was short lived, intense and therefore compatible with a large igneous province (LIP). By analogy with present-day margins this LIP may have been associated with continental break-up and onset of sea-floor spreading. The remnants of the passive margin both north and south of the magma-rich segment have different architectures, and are almost devoid of rift/drift related magmatic rocks. Instead, these magma-poor segments are dominated by heterogeneous sediment-filled basins characterised by the abundant presence of solitary bodies of variably altered mantle peridotites, also commonly present as detrital serpentinites. These basins are interpreted to have formed by hyperextension. We suggest that the pre-Caledonian margin of Baltica underwent hyperextension until break-up, which was associated with emplacement of a LIP at ~600 Ma in the central segment. Andersen, T.B., Labrousse, L., Corfu, F. and Osmundsen, P.T., 2012: Evidence for hyperextension along the pre-Caledonian margin of Baltica. Jl. Geol. Soc. London, 601-612 Baird, G.B., Figg, SA. and Chamberlain, K.R., 2014: Intrusive age and geochemistry of the Kebne Dyke Complex in the Seve Nappe Complex, Kebnekaise Massif, arctic Sweden Caledonides, GFF, doi: 10.1080/11035897.2014.924553 Corfu, F., Andersen, T.B. and Gasser, D., 2014: The Scandinavian Caledonides: main features, conceptual advances and critical questions. Geol. Soc. London Spec. Publ. 390 doi:10.1144/SP390.25 Hollocher. K, Robinson, P., Walsh, E. and Terry M.P., 2007:The Neoproterozoic Ottfjellet dike swarm of the Middle Allochthon, traced geochemically into the hinterland, Western Gneiss Region, Norway. Am. Jl. Sci. 307, 901-953 Svenningsen, O., 2001: Onset of seafloor spreading in the Iapetus Ocean at 608Ma: precise age of the Sarek Dyke Swarm, northern Swedish Caledonides. Precambrian Res., 110, 241-254.

Structural evolution of the Rio Grande rift: Synchronous exhumation of rift flanks from 20-10 Ma, embryonic core complexes, and fluid-enhanced Quaternary extension

NASA Astrophysics Data System (ADS)

Ricketts, Jason William

The Rio Grande rift in Colorado and New Mexico is one of the well-exposed and well-studied continental rifts in the world. Interest in the rift is driven not only by pure scientific intrigue, but also by a desire and a necessity to quantify earthquake hazards in New Mexico as well as to assess various water related issues throughout the state. These motivating topics have thus far led to the publication of two Geological Society of America Special Publication volumes in 1994 and 2013. This dissertation aims at building on the wealth of previous knowledge about the rift, and is composed of three separate chapters that focus on the structural evolution of the Rio Grande rift at several different time and spatial scales. At the largest scale, apatite (U-Th)/He thermochronologic data suggest synchronous extension along the entire length of the Rio Grande rift in Colorado and New Mexico from 20-10 Ma, which is important for understanding and evaluating possible driving mechanisms which are responsible for the rift. Previous tectonic and magmatic events in western North America were highly influential in the formation of the Rio Grande rift, and the new thermochronologic data suggest that its formation may have been closely linked to foundering and removal of the underlying Farallon Plate. A fundamental result of rift development at these scales is a concentration of strain is some regions of the rift. In these regions of maximum extension, fault networks display a geometry involving both high- and low-angle fault networks. These geometries are similar to the early stages in the development of metamorphic core complexes, and thus these regions in the rift link incipient extensional environments to highly extended terranes. At shorter time scales, heterogeneous strain accumulation may be governed in part by fluids in fault zones. As an example, along the western edge of the Albuquerque basin, travertine deposits are cut by extensional veins that record anomalously high strain rates during the Quaternary at this location. The fluids that precipitated the travertine and calcite in veins also contain a small component of deeply-derived fluids such that surface extension in this part of the rift is coupled with processes at deeper levels. Together, these studies suggest that removal of the Farallon slab beneath Colorado and New Mexico may have been a primary mechanism establishing extension in the Rio Grande rift, while continued extension is heterogeneous in time and space and provides an important link between surface processes and processes that operate at mid-crustal levels.

Do Processes of Rhyolite Genesis Change as Icelandic Rifts Drift off of the Plume?

NASA Astrophysics Data System (ADS)

Jordan, B. T.

2004-12-01

The abandoned Snaefellsnes rift zone in western Iceland was the on-land manifestation of the Mid-Atlantic Ridge between 15 and 7 Ma. The rift zone was abandoned at 7 Ma, after it had drifted westward off of the Iceland hotspot, generally interpreted as a mantle plume. The position of the abandoned rift was initially recognized as the axis of a regional syncline analogous to the syncline developed in response to active rifting. Previous paleomagnetic and geochronologic studies have confirmed the position of the abandoned rift axis. Recent seismic tomography shows that the abandoned rift is also characterized by relatively thin crust (<20 km, versus up to 46 km above the plume). In the context of supervising Keck Geology Consortium undergraduate research projects in northwestern Iceland in 2003 and 2004, I have studied several silicic centers erupted at different times along the northern Snaefellsnes rift. A compilation of preliminary geochemical data from the Skagi area near the rift reveals several interesting trends that bear on the origin of silicic magmas as activity in the rift was waning. The compositional spectrum of silicic rocks in this area is from dacite (67 wt.% SiO2) to rhyolite (75 wt.% SiO2). Positive correlation between Na2O and SiO2 is consistent with either fractionation or decreasing degrees of crustal melting to get from dacite to rhyolite. However, Zr correlates negatively with SiO2, consistent with zircon fractionation, but inconsistent with variation in the degree of melting unless zircon is present in the source, unlikely for the meta-basaltic crust of Iceland. Therefore, I suggest these rocks reflect extreme (>90%) fractionation of a basaltic parent. A similar argument was advanced by Furman et al. (1992, J. Pet., 1405-1445) for rhyolites at Austerhorn in eastern Iceland. Compelling arguments have been previously advanced that most rhyolites erupted in the modern rifts of Iceland are the products of crustal melting. The same has been argued for some older centers erupted from the Snaefellsnes rift (Kroksfjordur, 9-10 Ma?). I propose the hypothesis that as a rift drifts off of the plume, and becomes more like a normal mid-ocean ridge (thinner crust), rhyolite genesis by crustal melting becomes uncommon, and that what rhyolites are generated are the result of extreme fractionation of a basaltic parent. Ongoing studies will test this hypothesis by more detailed trace element and O-isotope studies and the systematic study of a series of silicic centers erupted at the northern Snaefellsnes rift over its history of activity.

Graben formation — the Maltese Islands — a case history

NASA Astrophysics Data System (ADS)

Illies, J. Henning

1981-03-01

The structural setting of the Maltese Islands is governed by two rift systems of different ages and trends and the interference of both. Accompanying faults are exposed at many places along cliffs and belong to the most spectacular phenomena of rift faulting of the world. Malta is part of a wide shelf bridge that connects the Ragusa platform of southern Sicily and the Tripolitanian platform of northern Libya. The archipelago is underlain by a continental crust of African provenance. The older rift generation traversing the islands strikes about 50° to 70° to create a basin-and-range structure on western Malta, Comino and eastern Gozo. This micro-province is framed by two master faults at an average distance of 14 km. Crustal extension started during the Early Miocene, as observed by growth faulting and sedimentary dikes parallel to the future rift. A syndepositional uparching of about 200 m has preceded the physiographical rifting in post-Middle Miocene times. Discrete dip-slip faulting created an external wedge block, split by internal tilt blocks of antithetic character, both compensating an average 15% crustal spreading normal to the rift axis. Shoulder upwarping of approximately 120 m has evolved synchronously with the rifting. Structures of the first generation are crosscut by still active, second-generation rift faults, which on Malta strike about 120°, but on Gozo between 80° and 90°. These faults are associated with the Pantelleria rift, whose deep trough sets immediately south of the islands. Rifting was mainly originated during Late Miocene/Early Pliocene time to continue in parts up to the Present. A set of transform faults runs through the straits on both sides of Comino to form a complicated en echelon or Riedel shear structure on easternmost Gozo and westernmost Malta. Shoulder upwarping related to the Pantelleria rift has considerably tilted the block of Malta NNE-ward and caused the inundated river valleys of the natural harbour of Valletta. The superimposition of two rift structures of different trends has been caused principally by a rotation of the controlling stress regime about 10 m.y. ago. The active Afro—Eurasian collision front is located about 200 km north and northwest of the islands. A contemporary change of plate tectonic stresses is discussed to explain the intraplate rift pattern on Malta as foreland-specific reactions to plate tectonic processes.

Geodynamic modelling of the rift-drift transition: Application to the Red Sea

NASA Astrophysics Data System (ADS)

Fierro, E.; Schettino, A.; Capitanio, F. A.; Ranalli, G.

2017-12-01

The onset of oceanic accretion after a rifting phase is generally accompanied by an initial fast pulse of spreading in the case of volcanic margins, such that the effective spreading rate exceeds the relative far-field velocity between the two plates for a short time interval. This pulse has been attributed to edge-driven convention (EDC), although our numerical modelling shows that the shear stress at the base of the lithosphere cannot exceed 1 MPa. In general, we have developed a 2D numerical model of the mantle instabilities during the rifting phase, in order to determine the geodynamic conditions at the rift-drift transition. The model was tested using Underworld II software, variable rheological parameters, and temperature and stress-dependent viscosity. Our results show an increase of strain rates at the top of the lithosphere with the lithosphere thickness as well as with the initial width of the margin up to 300 km. Beyond this value, the influence of the initial rift width can be neglected. An interesting outcome of the numerical model is the existence of an axial zone characterized by higher strain rates, which is flanked by two low-strain stripes. As a consequence, the model suggests the existence of an area of syn-rift compression within the rift valley. Regarding the post-rift phase, we propose that at the onset of a seafloor spreading, a phase of transient creep allows the release of the strain energy accumulated in the mantle lithosphere during the rifting phase, through anelastic relaxation. Then, the conjugated margins would be subject to post-rift contraction and eventually to tectonic inversion of the rift structures. To explore the tenability of this model, we introduce an anelastic component in the lithosphere rheology, assuming both the classical linear Kelvin-Voigt rheology and a non-linear Kelvin model. The non-linear model predicts viable relaxation times ( 1-2Myrs) to explain the post-rift tectonic inversion observed along the Arabian continental margin and the episodic initial fast seafloor spreading in the central Red Sea, where the role of EDC has been invoked.

Coulomb Stress Change and Seismic Hazard of Rift Zones in Southern Tibet after the 2015 Mw7.8 Nepal Earthquake and Its Mw7.3 Aftershock

NASA Astrophysics Data System (ADS)

Dai, Z.; Zha, X.; Lu, Z.

2015-12-01

In southern Tibet (30~34N, 80~95E), many north-trending rifts, such as Yadong-Gulu and Lunggar rifts, are characterized by internally drained graben or half-graben basins bounded by active normal faults. Some developed rifts have become a portion of important transportation lines in Tibet, China. Since 1976, eighty-seven >Mw5.0 earthquakes have happened in the rift regions, and fifty-five events have normal faulting focal mechanisms according to the GCMT catalog. These rifts and normal faults are associated with both the EW-trending extension of the southern Tibet and the convergence between Indian and Tibet. The 2015 Mw7.8 Nepal great earthquake and its Mw7.3 aftershock occurred at the main Himalayan Thrust zone and caused tremendous damages in Kathmandu region. Those earthquakes will lead to significant viscoelastic deformation and stress changes in the southern Tibet in the future. To evaluate the seismic hazard in the active rift regions in southern Tibet, we modeled the slip distribution of the 2015 Nepal great earthquakes using the InSAR displacement field from the ALOS-2 satellite SAR data, and calculated the Coulomb failure stress (CFS) on these active normal faults in the rift zones. Because the estimated CFS depends on the geometrical parameters of receiver faults, it is necessary to get the accurate fault parameters in the rift zones. Some historical earthquakes have been studied using the field data, teleseismic data and InSAR observations, but results are in not agreement with each other. In this study, we revaluated the geometrical parameters of seismogenic faults occurred in the rift zones using some high-quality coseismic InSAR observations and teleseismic body-wave data. Finally, we will evaluate the seismic hazard in the rift zones according to the value of the estimated CFS and aftershock distribution.

Re-appraisal of the Magma-rich versus Magma-poor Paradigm at Rifted Margins: consequences for breakup processes

NASA Astrophysics Data System (ADS)

Tugend, J.; Gillard, M.; Manatschal, G.; Nirrengarten, M.; Harkin, C. J.; Epin, M. E.; Sauter, D.; Autin, J.; Kusznir, N. J.; McDermott, K.

2017-12-01

Rifted margins are often classified based on their magmatic budget only. Magma-rich margins are commonly considered to have excess decompression melting at lithospheric breakup compared with steady state seafloor spreading while magma-poor margins have suppressed melting. New observations derived from high quality geophysical data sets and drill-hole data have revealed the diversity of rifted margin architecture and variable distribution of magmatism. Recent studies suggest, however, that rifted margins have more complex and polyphase tectono-magmatic evolutions than previously assumed and cannot be characterized based on the observed volume of magma alone. We compare the magmatic budget related to lithospheric breakup along two high-resolution long-offset deep reflection seismic profiles across the SE-Indian (magma-poor) and Uruguayan (magma-rich) rifted margins. Resolving the volume of magmatic additions is difficult. Interpretations are non-unique and several of them appear plausible for each case involving variable magmatic volumes and mechanisms to achieve lithospheric breakup. A supposedly 'magma-poor' rifted margin (SE-India) may show a 'magma-rich' lithospheric breakup whereas a 'magma-rich' rifted margin (Uruguay) does not necessarily show excess magmatism at lithospheric breakup compared with steady-state seafloor spreading. This questions the paradigm that rifted margins can be subdivided in either magma-poor or magma-rich margins. The Uruguayan and other magma-rich rifted margins appear characterized by an early onset of decompression melting relative to crustal breakup. For the converse, where the onset of decompression melting is late compared with the timing of crustal breakup, mantle exhumation can occur (e.g. SE-India). Our work highlights the difficulty in determining a magmatic budget at rifted margins based on seismic reflection data alone, showing the limitations of margin classification based solely on magmatic volumes. The timing of decompression melting onset and melting rates (magmatic processes) relative to crustal thinning (tectonic processes) appear equally, if not more important, than the magmatic budget for unravelling the evolution of rifted margins.

A missing element of the deep carbon cycle: CO2 degassing estimates from rift length analysis during Pangea fragmentation

NASA Astrophysics Data System (ADS)

Brune, S.; Williams, S.; Müller, D.

2016-12-01

The deep carbon cycle connects CO2 within the atmosphere and oceans to the vast CO2 reservoir in Earth's mantle: subducted lithosphere carries CO2 into the mantle, while extensional plate boundaries and arc volcanoes release it back to Earth's surface. The length of plate boundaries thereby exerts first-order control on global CO2 fluxes on geological time scales. Here we provide a worldwide census of extensional plate boundary length from the Triassic to present day, in one million year time intervals, using a novel analysis technique (Brune et al. 2016, Nature, doi:10.1038/nature18319). The most extensive rift phase during the fragmentation of Pangea occurred in the Cretaceous with extension along the South Atlantic (9700 km) and North Atlantic rifts (9100 km), within East Gondwana (8500 km), and the failed African rift systems (4900 km). The combined extent of these and several smaller rifts amounts to more than 30.000 km of simultaneously active continental rifting. It is well-accepted that volcanoes at plate boundaries release large amounts of CO2 from the Earth's interior. Recent work, however, revealed the importance of deep-cutting faults and diffuse degassing on CO2 emissions in the East African rift (Lee et al. 2016, Nature Geoscience, doi: 10.1038/ngeo2622). Upscaling these measured CO2 fluxes to all concurrently active global rift zones, we compute first-order estimates of total rift-related CO2 degassing rates for the last 240 Myr. Our results show that rift-related CO2 release rates may have reached 600 Mt/yr in the Early Cretaceous, while Cenozoic rates rarely exceeded 200 Mt/yr. By comparison, present-day estimates of CO2 release at mid-ocean ridges range between 53 and 97 Mt/yr. We suggest that rift-related degassing during supercontinental breakup played a major role in maintaining high atmospheric CO2 concentrations through Mesozoic times, which exceeded Quaternary values by 400%.

Distributed deformation ahead of the Cocos-Nazca Rift at the Galapagos triple junction

NASA Astrophysics Data System (ADS)

Smith, Deborah K.; Schouten, Hans; Zhu, Wen-lu; Montési, Laurent G. J.; Cann, Johnson R.

2011-11-01

The Galapagos triple junction is not a simple ridge-ridge-ridge (RRR) triple junction. The Cocos-Nazca Rift (C-N Rift) tip does not meet the East Pacific Rise (EPR). Instead, two secondary rifts form the link: Incipient Rift at 2°40‧N and Dietz Deep volcanic ridge, the southern boundary of the Galapagos microplate (GMP), at 1°10‧N. Recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the GMP (∼1.5 Ma). South of C-N Rift a band of northeast-trending cracks cuts EPR-generated abyssal hills. It is a mirror image of a band of cracks previously identified north of C-N Rift on the same age crust. In both areas, the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the EPR spreading center and cut eastward into pre-existing topography. Each crack formed a short-lived triple junction until it was abandoned and a new crack and triple junction initiated nearby. Between 2.5 and 1.5 Ma, the pattern of cracking is remarkably symmetric about C-N Rift providing support for a crack interaction model in which crack initiation at the EPR axis is controlled by stresses associated with the tip of the westward-propagating C-N Rift. The model also shows that offsets of the EPR axis may explain times when cracking is not symmetric. South of C-N Rift, cracks are observed on seafloor as old as 10.5 Ma suggesting that this triple junction has not been a simple RRR triple junction during that time.

Geophysical evidence of pre-sag rifting and post-rifting fault reactivation in the Parnaíba basin, Brazil

NASA Astrophysics Data System (ADS)

Lopes de Castro, David; Hilário Bezerra, Francisco; Adolfo Fuck, Reinhardt; Vidotti, Roberta Mary

2016-04-01

This study investigated the rifting mechanism that preceded the prolonged subsidence of the Paleozoic Parnaíba basin in Brazil and shed light on the tectonic evolution of this large cratonic basin in the South American platform. From the analysis of aeromagnetic, aerogravity, seismic reflection and borehole data, we concluded the following: (1) large pseudo-gravity and gravity lows mimic graben structures but are associated with linear supracrustal strips in the basement. (2) Seismic data indicate that 120-200 km wide and up to 300 km long rift zones occur in other parts of the basins. These rift zones mark the early stage of the 3.5 km thick sag basin. (3) The rifting phase occurred in the early Paleozoic and had a subsidence rate of 47 m Myr-1. (4) This rifting phase was followed by a long period of sag basin subsidence at a rate of 9.5 m Myr-1 between the Silurian and the late Cretaceous, during which rift faults propagated and influenced deposition. These data interpretations support the following succession of events: (1) after the Brasiliano orogeny (740-580 Ma), brittle reactivation of ductile basement shear zones led to normal and dextral oblique-slip faulting concentrated along the Transbrasiliano Lineament, a continental-scale shear zone that marks the boundary between basement crustal blocks. (2) The post-orogenic tectonic brittle reactivation of the ductile basement shear zones led to normal faulting associated with dextral oblique-slip crustal extension. In the west, pure-shear extension induced the formation of rift zones that crosscut metamorphic foliations and shear zones within the Parnaíba block. (3) The rift faults experienced multiple reactivation phases. (4) Similar processes may have occurred in coeval basins in the Laurentia and Central African blocks of Gondwana.

The eastern arm of the Midcontinent Rift: Progress and problems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hinze, W.J.

1994-04-01

The extent and nature of the Midcontinent Rift System (MCR) was initially determined by potential-field mapping and extrapolation of geologic information from the Lake Superior region. Early interpretation suggested a rift origin which is well supported by deep crustal reflection seismic data and isotopic evidence from the related volcanic rocks that became available during the past decade. A rift origin of the eastern arm of the MCR was corroborated by sub-Phanerozoic drilling into the clastic sediment and volcanic rocks in the McClure-Sparks drill hole located on a massive anticlinal feature in the Precambrian rocks mapped by seismic reflection data. Subsequentmore » seismic profiling further detailed the character of the rift. However, these studies also indicate that the eastern arm is unlike the western, e.g., adjacent clastic rock basins are absent, late-stage compressional features are present, but definite evidence for high-angle reverse faulting is missing, and volcanic basins are not continuous. The termination of this arm of the rift also remains problematic. There is no direct evidence of the rift SE of the McClure-Sparks hole in central Michigan. Geophysical anomalies and deep drilling in the Howell anticline region suggest that the 1,100 Ma old rift is covered by Grenville-age thrusts. If the rift extends farther to the SE, its nature must have been altered by the Grenville orogeny. The hypothesized extension across Ohio east of the Grenville Front is unsupported by seismic reflection profiling and anomaly modeling. Grabens identified at the basement surface in Ohio and to the south are of unknown age, but appear to be more clearly related to late-stage Grenville activity and/or continuation of Eocambrian rifts of the Mississippi Embayment.« less

Numerical modelling of quaternary deformation and post-rifting displacement in the Asal-Ghoubbet rift (Djibouti, Africa) [rapid communication

NASA Astrophysics Data System (ADS)

Cattin, Rodolphe; Doubre, Cécile; de Chabalier, Jean-Bernard; King, Geoffrey; Vigny, Christophe; Avouac, Jean-Philippe; Ruegg, Jean-Claude

2005-11-01

Over the last three decades a host of information on rifting process relating to the geological and thermal structure, long-time scale deformation (Quaternary and Holocene) and rifting cycle displacement across the Asal-Ghoubbet rift has been made available. These data are interpreted with a two-dimensional thermo-mechanical model that incorporates rheological layering of the lithosphere, dyke inflation and faulting. Active fault locations and geometry are mainly controlled by both thermal structure and magma intrusion into the crust. The distributed slip throughout the inner rift is related to the closeness of magma chamber, leading to additional stress into the upper thinned crust. Assuming a constant Arabia-Somalia motion of 11 mm/year, the variation of subsidence rate between the last 100 and 9 ka is associated with a decrease of the average injection rate from 10 to 5 mm/year. These values, about equal to the regional opening rate, suggest that both volcanism and tectonic play an equivalent role in the rifting process. Our modelled sequence of events gives one possible explanation for both vertical and horizontal displacements observed since the 1978 seismovolcanic crisis. Although part of the post-rifting deformation could be due to viscous relaxation, the high opening rate in the first years after the event and the abrupt velocity change in 1984-1986 argue for a large dyke inflation of 12 cm/year ending in 1985. The asymmetric and constant pattern of the GPS velocity since 1991 suggests that present post-rifting deformation is mainly controlled by fault creep and regional stretching. This study demonstrates the internal consistency of the data set, highlights the role of magmatism in the mechanics of crustal stretching and reveals a complex post-rifting process including magma injection, fault creep and regional stretching.

The KRISP 90 seismic experiment-a technical review

USGS Publications Warehouse

Prodehl, C.; Mechie, J.; Achauer, U.; Keller, Gordon R.; Khan, M.A.; Mooney, W.D.; Gaciri, S.J.; Obel, J.D.

1994-01-01

On the basis of a preliminary experiment in 1985 (KRISP 85), a seismic refraction/wide-angle reflection survey and a teleseismic tomography experiment were jointly undertaken to study the lithospheric structure of the Kenya rift down to depths of greater than 200 km. This report serves as an introduction to a series of subsequent papers and will focus on the technical description of the seismic surveys of the main KRISP 90 effort. The seismic refraction/wide-angle reflection survey was carried out in a 4-week period in January and February 1990. It consisted of three profiles: one extending along the rift valley from Lake Turkana to Lake Magadi, one crossing the rift at Lake Baringo, and one located on the eastern flank of the rift proper. A total of 206 mobile vertical-component seismographs, with an average station interval of about 2 km, recorded the energy of underwater and borehole explosions to distances of up to about 550 km. During the teleseismic survey an array of 65 seismographs was deployed to record teleseismic, regional and local events for a period of about 7 months from October 1989 to April 1990. The elliptical array spanned the central portion of the rift, with Nakuru at its center, and covered an area about 300 ?? 200 km, with an average station spacing of 10-30 km. Major scientific goals of the project were to reveal the detailed crustal and upper-mantle structure under the Kenya rift, to study the relationship between deep crustal and mantle structure and the development of sedimentary basins and volcanic features within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system, and to answer fundamental questions such as the mode and mechanism of continental rifting. ?? 1994.

Neoproterozoic rift basins and their control on the development of hydrocarbon source rocks in the Tarim Basin, NW China

NASA Astrophysics Data System (ADS)

Zhu, Guang-You; Ren, Rong; Chen, Fei-Ran; Li, Ting-Ting; Chen, Yong-Quan

2017-12-01

The Proterozoic is demonstrated to be an important period for global petroleum systems. Few exploration breakthroughs, however, have been obtained on the system in the Tarim Basin, NW China. Outcrop, drilling, and seismic data are integrated in this paper to focus on the Neoproterozoic rift basins and related hydrocarbon source rocks in the Tarim Basin. The basin consists of Cryogenian to Ediacaran rifts showing a distribution of N-S differentiation. Compared to the Cryogenian basins, those of the Ediacaran are characterized by deposits in small thickness and wide distribution. Thus, the rifts have a typical dual structure, namely the Cryogenian rifting and Ediacaran depression phases that reveal distinct structural and sedimentary characteristics. The Cryogenian rifting basins are dominated by a series of grabens or half grabens, which have a wedge-shaped rapid filling structure. The basins evolved into Ediacaran depression when the rifting and magmatic activities diminished, and extensive overlapping sedimentation occurred. The distributions of the source rocks are controlled by the Neoproterozoic rifts as follows. The present outcrops lie mostly at the margins of the Cryogenian rifting basins where the rapid deposition dominates and the argillaceous rocks have low total organic carbon (TOC) contents; however, the source rocks with high TOC contents should develop in the center of the basins. The Ediacaran source rocks formed in deep water environment of the stable depressions evolving from the previous rifting basins, and are thus more widespread in the Tarim Basin. The confirmation of the Cryogenian to Ediacaran source rocks would open up a new field for the deep hydrocarbon exploration in the Tarim Basin.

South Atlantic sag basins: new petroleum system components

DOE Office of Scientific and Technical Information (OSTI.GOV)

Henry, S.G.; Mello, M.R.

Newly discovered pre-salt source rocks, reservoirs and seals need to be included as components to the petroleum systems of both sides of the South Atlantic. These new components lie between the pre-salt rift strata and the Aptian salt layers, forming large, post-rift, thermal subsidence sag basins. These are differentiated from the older rift basins by the lack of syn-rift faulting and a reflector geometry that is parallel to the base salt regional unconformity rather than to the Precambrian basement. These basins are observed in deep water regions overlying areas where both the mantle and the crust have been involved inmore » the extension. This mantle involvement creates post-rift subsiding depocenters in which deposition is continuous while proximal rift-phase troughs with little or no mantle involvement are bypassed and failed to accumulate potential source rocks during anoxic times. These features have been recognized in both West African Kwanza Basin and in the East Brasil Rift systems. The pre-salt source rocks that are in the West African sag basins were deposited in lacustrine brackish to saline water environment and are geochemically distinct from the older, syn-rift fresh to brackish water lakes, as well as from younger, post-salt marine anoxic environments of the drift phase. Geochemical analyses of the source rocks and their oils have shown a developing source rock system evolving from isolated deep rift lakes to shallow saline lakes, and culminating with the infill of the sag basin by large saline lakes to a marginally marine restricted gulf. Sag basin source rocks may be important in the South Atlantic petroleum system by charging deep-water prospects where syn-rift source rocks are overmature and the post-salt sequences are immature.« less

When Did Midcontinent Rift Volcanism End and Where Was Laurentia at that Time?

NASA Astrophysics Data System (ADS)

Fairchild, L. M.; Swanson-Hysell, N.; Ramezani, J.; Sprain, C. J.; Gaastra, K. M.; Bowring, S. A.

2015-12-01

Data from the North American Midcontinent Rift provide a robust record of Laurentia's (cratonic North America's) paleogeographic position from ca. 1110 to 1080 Ma. The resulting apparent polar wander path (APWP) reveals rapid motion of the continent towards the equator throughout the rift's lifetime. Constraints on the age of the youngest volcanics within the rift and on the paleolatitude of Laurentia at that time are important for quantifying the rate of this motion and its apparent deceleration in the late stage of rift development. Furthermore, precise calibration of the APWP enhances the robustness of paleogeographic reconstructions. The three rift successions with ca. 1090 to 1085 Ma late stage volcanics are the Lake Shore Traps of Michigan, the Michipicoten Island Formation of Ontario and the Schroeder-Lutsen basalts of Minnesota. In past studies, paleomagnetic data from the Schroeder-Lutsen basalts have been grouped with results from the North Shore Volcanic Group, which it unconformably overlies. In this study, we separate these data and add newly developed results from 40 additional flows. New data from the Michipicoten Island Formation allow for a well constrained pole that now includes data from more than 25 flows. High quality paleomagnetic data are published for the Lake Shore Traps, and we complement these with a newly developed high precision U-Pb zircon date as an update to current constraints. Taken altogether, these data strengthen our understanding of the rift's demise and the rate of Laurentia's motion as rift volcanism gave way to post-rift sedimentation.

Probable existence of a Gondwana transcontinental rift system in western India: Implications in hydrocarbon exploration in Kutch and Saurashtra offshore: A GIS-based approach

NASA Astrophysics Data System (ADS)

Mazumder, S.; Tep, Blecy; Pangtey, K. K. S.; Das, K. K.; Mitra, D. S.

2017-08-01

The Gondwanaland assembly rifted dominantly during Late Carboniferous-Early Permian forming several intracratonic rift basins. These rifts were subsequently filled with a thick sequence of continental clastic sediments with minor marine intercalations in early phase. In western part of India, these sediments are recorded in enclaves of Bikaner-Nagaur and Jaisalmer basins in Rajasthan. Facies correlatives of these sediments are observed in a number of basins that were earlier thought to be associated with the western part of India. The present work is a GIS based approach to reconnect those basins to their position during rifting and reconstruct the tectono-sedimentary environment at that time range. The study indicates a rift system spanning from Arabian plate in the north and extending to southern part of Africa that passes through Indus basin, western part of India and Madagascar, and existed from Late Carboniferous to Early Jurassic. Extensions related to the opening of Neo-Tethys led to the formation of a number of cross trends in the rift systems that acted as barriers to marine transgressions from the north as well as disrupted the earlier continuous longitudinal drainage systems. The axis of this rift system is envisaged to pass through present day offshore Kutch and Saurashtra and implies a thick deposit of Late Carboniferous to Early Jurassic sediments in these areas. Based on analogy with other basins associated with this rift system, these sediments may be targeted for hydrocarbon exploration.