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Sample records for active rifting processes

  1. The Salton Seismic Imaging Project (SSIP): Active Rift Processes in the Brawley Seismic Zone

    NASA Astrophysics Data System (ADS)

    Han, L.; Hole, J. A.; Stock, J. M.; Fuis, G. S.; Rymer, M. J.; Driscoll, N. W.; Kent, G.; Harding, A. J.; Gonzalez-Fernandez, A.; Lazaro-Mancilla, O.

    2011-12-01

    The Salton Seismic Imaging Project (SSIP), funded by NSF and USGS, acquired seismic data in and across the Salton Trough in southern California and northern Mexico in March 2011. The project addresses both rifting processes at the northern end of the Gulf of California extensional province and earthquake hazards at the southern end of the San Andreas Fault system. Seven lines of onshore refraction and low-fold reflection data were acquired in the Coachella, Imperial, and Mexicali Valleys, two lines and a grid of airgun and OBS data were acquired in the Salton Sea, and onshore-offshore data were recorded. Almost 2800 land seismometers and 50 OBS's were used in almost 5000 deployments at almost 4300 sites, in spacing as dense as 100 m. These instruments received seismic signals from 126 explosive shots up to 1400 kg and over 2300 airgun shots. In the central Salton Trough, North American lithosphere appears to have been rifted completely apart. Based primarily on a 1979 seismic refraction project, the 20-22 km thick crust is apparently composed entirely of new crust added by magmatism from below and sedimentation from above. Active rifting of this new crust is manifested by shallow (<10km depth) seismicity in the oblique Brawley Seismic Zone (BSZ), small Salton Buttes volcanoes aligned perpendicular to the transform faults, very high heat flow (~140 mW/m2), and geothermal energy production. This presentation is focused on an onshore-offshore line of densely sampled refraction and low-fold reflection data that crosses the Brawley Seismic Zone and Salton Buttes in the direction of plate motion. At the time of abstract submission, data analysis was very preliminary, consisting of first-arrival tomography of the onshore half of the line for upper crustal seismic velocity. Crystalline basement (>5 km/s), comprised of late-Pliocene to Quaternary sediment metamorphosed by the high heat flow, occurs at ~2 km depth beneath the Salton Buttes and geothermal field and ~4 km

  2. Lithospheric processes that enhance melting at rifts

    NASA Astrophysics Data System (ADS)

    Elkins-Tanton, L. T.; Furman, T.

    2008-12-01

    Continental rifts are commonly sites for mantle melting, whether in the form of ridge melting to create new oceanic crust, or as the locus of flood basalt activity, or in the long initial period of rifting before lavas evolve fully into MORBs. The high topography in the lithosphere-asthenosphere boundary under a rift creates mantle upwelling and adiabatic melting even in the absence of a plume. This geometry itself, however, is conducive to lithospheric instability on the sides of the rifts. Unstable lithosphere may founder into the mantle, producing more complex aesthenospheric convective patterns and additional opportunities to produce melt. Lithospheric instabilities can produce additional adiabatic melting in convection produced as they sink, and they may also devolatilize as they sink, introducing the possibility of flux melting to the rift environment. We call this process upside-down melting, since devolatilization and melting proceed as the foundering lithosphere sinks, rather than while rising, as in the more familiar adiabatic decompression melting. Both adiabatic melting and flux melting would take place along the edges of the rift and may even move magmatism outside the rift, as has been seen in Ethiopia. In volcanism postdating the flood basalts on and adjacent to the Ethiopian Plateau there is evidence for both lithospheric thinning and volatile enrichment in the magmas, potentially consistent with the upside-down melting model. Here we present a physical model for the conjunction of adiabatic decompression melting to produce new oceanic crust in the rift, while lithospheric gravitational instabilities drive both adiabatic and flux melting at its margins.

  3. Investigation of rifting processes in the Rio Grande Rift using data from unusually large earthquake swarms

    SciTech Connect

    Sanford, A.; Balch, R.; House, L.; Hartse, H.

    1995-12-01

    San Acacia Swarm in the Rio Grande Rift. Because the Rio Grande rift is one of the best seismically instrumented rift zones in the world, studying its seismicity provides an exceptional opportunity to explore the active tectonic processes within continental rifts. We have been studying earthquake swarms recorded near Socorro in an effort to link seismicity directly to the rifting process. For FY94, our research has focused on the San Acacia swarm, which occurred 25 km north of Socorro, New Mexico, along the accommodation zone between the Albuquerque-Belen and Socorro basins of the central Rio Grande rift. The swarm commenced on 25 February 1983, had a magnitude 4.2 main shock on 2 March and ended on 17 March, 1983.

  4. Rifting process of the Izu-Ogasawara-Mariana arc-backarc system inferred from active source seismic studies

    NASA Astrophysics Data System (ADS)

    Takahashi, N.; Kodaira, S.; Miura, S.; Sato, T.; Yamashita, M.; No, T.; Takizawa, K.; Kaiho, Y.; Kaneda, Y.

    2008-12-01

    The Izu-Ogasawara-Mariana (IBM) arc-backarc system has continued the crustal growth through crustal thickening by magmatic activities and crustal thinning by backarc opening. Tatsumi et al (2008) proposed petrological crustal growth model started from basaltic magmas rising from the slab, and showed the consistency with the seismic velocity model. Although crustal growth by the crustal thickening are modeled, crustal structural change by the backarc opening are not still unknown yet. The Shikoku Basin and Parece Vela Basin were formed by the backarc opening during approximately 15-30 Ma. Since 6 Ma, the Mariana Trough has opened and the stage already moved to spreading process from rifting process. In the northern Izu-Ogasawara arc, the Sumisu rift is in the initial rifting stage. Therefore, understanding of the crustal change by the backarc opening from rifting to spreading is indispensable to know the crustal growth of whole Izu-Ogasawara-Mariana island arc. Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has carried out seismic studies using a multichannel reflection survey system and ocean bottom seismographs (OBSs) around the IBM arc since 2003 (Takahashi et al., 2007; Kodaira et al., 2007; Takahashi et al., 2008; Kodaira et al., 2008). We already obtained eight P-wave velocity models across the IBM arc and these structures record the crustal structural change during the backarc opening process from the rifting stage to the spreading stage. As the results, we identified characteristics of the crustal structural change accompanied with backarc opening as follows. (1) Beneath the initial rifting stage without normal faults, for example, in the northern tip of the Mariana Trough, crustal thickening are identified. (2) Beneath the initial rifting stage with normal faults, for example, in the Sumisu Rift, the crustal thickness is almost similar to that beneath the volcanic front. Although an existence of the crust-mantle transition layer with

  5. Active rifting processes in the central Salton Trough, California, constrained by the Salton Seismic Imaging Project (SSIP)

    NASA Astrophysics Data System (ADS)

    Han, L.; Hole, J. A.; Stock, J. M.; Fuis, G. S.; Driscoll, N. W.; Kell, A. M.; Kent, G.; Harding, A. J.

    2012-12-01

    Seismic refraction and reflection travel times from the Salton Seismic Imaging Project (SSIP) are being used to constrain crustal structure during active continental rifting in the central Salton Trough, California. SSIP, funded by NSF and USGS, acquired seismic data in and across the Salton Trough in 2011 to investigate rifting processes at the northern end of the Gulf of California extensional province and earthquake hazards at the southern end of the San Andreas Fault system. Seven lines of refraction and low-fold reflection data were acquired onshore, two lines and a grid of airgun and OBS data were acquired in the Salton Sea, and onshore-offshore data were recorded. Based on prior studies of the central Salton Trough, North American lithosphere appears to have been rifted completely apart and replaced by entirely new crust added by magmatism from below and sedimentation from above. Ongoing active rifting of this new crust is manifested by shallow (<10km depth) seismicity in the oblique Brawley Seismic Zone (BSZ; connecting the Imperial and San Andreas faults), the small Salton Buttes volcanoes (aligned perpendicular to the direction of plate motion), and very high heat flow. Analyses of the onshore-offshore seismic line that extends along the axis of the valley, parallel to the direction of plate motion, constrain crustal structure in the valley. Crystalline basement (~5 km/s) generally occurs at ~4 km depth, but is at 2-3 km depth in a localized region beneath the Salton Buttes and Salton Sea geothermal field. This crystalline rock is interpreted to be late Pliocene to Quaternary sediment metamorphosed by high heat flow. The shallower basement under the volcanic and geothermal field is due to more intense metamorphism and hydrothermal alteration in this region. The seismic velocity of basement is slower in the BSZ than to the south and north, which may be due to seismicity-related fracturing. The basement velocity beneath the Salton Buttes and geothermal

  6. Complex seismicity patterns in the Rwenzori region: insights to rifting processes at the Albertine Rift.

    NASA Astrophysics Data System (ADS)

    Lindenfeld, M.; Rümpker, G.; Wölbern, I.; Batte, A. G.; Schumann, A.

    2012-04-01

    Numerous seismological studies in East Africa have focused on the northern and eastern branches of the East African Rift System (EARS). However, the seismic activity along the western branch is much more pronounced. Here, the Rwenzori Mountains are located within the Albertine rift valley, at the border between Uganda and D.R. Congo. During a seismic monitoring campaign between February 2006 and September 2007 we have recorded more than 800 earthquakes per month in the Rwenzori area. The earthquake distribution is highly heterogeneous. The majority of located events lie within faults zones to the East and West of the Rwenzoris with the highest seismic activity observed in the northeastern area, were the mountains are in contact with the rift shoulders. The hypocentral depth distribution peaks at 16 km depth and extends down to the Moho which was found at 20 - 32 km depths by teleseismic receiver functions. Local magnitudes range from -0.5 to 5.1 with a b-value of 1.1. Fault plane solutions of 304 events were derived from P-polarities and SV/P amplitude ratios. More than 70% of the source mechanisms exhibit normal faulting. T-axis trends are highly uniform and oriented WNW-ESE, which is perpendicular to the rift axis and in good agreement with kinematic rift models. The area of highest seismic activity NE of the Rwenzoris is characterized by the occurrence of several earthquake clusters in 5 -20 km depth. They have stable positions throughout time and form elongated pipes with 1-2 km diameter and vertical extensions of 3-5 km. From petrological considerations we presume that these earthquake swarms are triggered by fluids and gasses which originate from a magmatic source below the crust. The existence of a magmatic source within the lithosphere is supported by the detection of a shear-wave velocity reduction in 55-80 km depth from receiver-function analysis and the location of mantle earthquakes at about 60 km. We interpret these observations as indication for an

  7. Investigation of rifting processes in the Rio Grande Rift using data from an unusually large earthquake swarm. Final report, October 1, 1992--September 30, 1993

    SciTech Connect

    Sanford, A.; Balch, R.; Hartse, H.; House, L.

    1995-03-01

    Because the Rio Grande Rift is one of the best seismically instrumented rift zones in the world, studying its seismicity provides an exceptional opportunity to elucidate the active tectonic processes within continental rifts. Beginning on 29 November 1989, a 15 square km region near Bernardo, NM, produced the strongest and longest lasting sequence of earthquakes in the rift in 54 years. Our research focuses on the Bernardo swarm which occurred 40 km north of Socorro, New Mexico in the axial region of the central Rio Grande rift. Important characteristics concerning hypocenters, fault mechanisms, and seismogenic zones are discussed.

  8. Groundwater fluoride enrichment in an active rift setting: Central Kenya Rift case study.

    PubMed

    Olaka, Lydia A; Wilke, Franziska D H; Olago, Daniel O; Odada, Eric O; Mulch, Andreas; Musolff, Andreas

    2016-03-01

    Groundwater is used extensively in the Central Kenya Rift for domestic and agricultural demands. In these active rift settings groundwater can exhibit high fluoride levels. In order to address water security and reduce human exposure to high fluoride in drinking water, knowledge of the source and geochemical processes of enrichment are required. A study was therefore carried out within the Naivasha catchment (Kenya) to understand the genesis, enrichment and seasonal variations of fluoride in the groundwater. Rocks, rain, surface and groundwater sources were sampled for hydrogeochemical and isotopic investigations, the data was statistically and geospatially analyzed. Water sources have variable fluoride concentrations between 0.02-75 mg/L. 73% exceed the health limit (1.5mg/L) in both dry and wet seasons. F(-) concentrations in rivers are lower (0.2-9.2mg/L) than groundwater (0.09 to 43.6 mg/L) while saline lake waters have the highest concentrations (0.27-75 mg/L). The higher values are confined to elevations below 2000 masl. Oxygen (δ(18)O) and hydrogen (δD) isotopic values range from -6.2 to +5.8‰ and -31.3 to +33.3‰, respectively, they are also highly variable in the rift floor where they attain maximum values. Fluoride base levels in the precursor vitreous volcanic rocks are higher (between 3750-6000 ppm) in minerals such as cordierite and muscovite while secondary minerals like illite and kaolinite have lower remnant fluoride (<1000 ppm). Thus, geochemical F(-) enrichment in regional groundwater is mainly due to a) rock alteration, i.e. through long residence times and natural discharge and/or enhanced leakages of deep seated geothermal water reservoirs, b) secondary concentration fortification of natural reservoirs through evaporation, through reduced recharge and/or enhanced abstraction and c) through additional enrichment of fluoride after volcanic emissions. The findings are useful to help improve water management in Naivasha as well as similar

  9. Groundwater fluoride enrichment in an active rift setting: Central Kenya Rift case study.

    PubMed

    Olaka, Lydia A; Wilke, Franziska D H; Olago, Daniel O; Odada, Eric O; Mulch, Andreas; Musolff, Andreas

    2016-03-01

    Groundwater is used extensively in the Central Kenya Rift for domestic and agricultural demands. In these active rift settings groundwater can exhibit high fluoride levels. In order to address water security and reduce human exposure to high fluoride in drinking water, knowledge of the source and geochemical processes of enrichment are required. A study was therefore carried out within the Naivasha catchment (Kenya) to understand the genesis, enrichment and seasonal variations of fluoride in the groundwater. Rocks, rain, surface and groundwater sources were sampled for hydrogeochemical and isotopic investigations, the data was statistically and geospatially analyzed. Water sources have variable fluoride concentrations between 0.02-75 mg/L. 73% exceed the health limit (1.5mg/L) in both dry and wet seasons. F(-) concentrations in rivers are lower (0.2-9.2mg/L) than groundwater (0.09 to 43.6 mg/L) while saline lake waters have the highest concentrations (0.27-75 mg/L). The higher values are confined to elevations below 2000 masl. Oxygen (δ(18)O) and hydrogen (δD) isotopic values range from -6.2 to +5.8‰ and -31.3 to +33.3‰, respectively, they are also highly variable in the rift floor where they attain maximum values. Fluoride base levels in the precursor vitreous volcanic rocks are higher (between 3750-6000 ppm) in minerals such as cordierite and muscovite while secondary minerals like illite and kaolinite have lower remnant fluoride (<1000 ppm). Thus, geochemical F(-) enrichment in regional groundwater is mainly due to a) rock alteration, i.e. through long residence times and natural discharge and/or enhanced leakages of deep seated geothermal water reservoirs, b) secondary concentration fortification of natural reservoirs through evaporation, through reduced recharge and/or enhanced abstraction and c) through additional enrichment of fluoride after volcanic emissions. The findings are useful to help improve water management in Naivasha as well as similar

  10. 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.

  11. Constraints on rift thermal processes from heat flow and uplift

    NASA Technical Reports Server (NTRS)

    Morgan, P.

    1983-01-01

    The implications of heat flow data available from five major Cenozoic continental rift systems for the processes of continental rifting are discussed, and simple thermal models of lithospheric thinning which predict uplift are used to further constrain the thermal processes in the lithosphere during rifting. Compilations of the heat flow data are summarized and the salient results of these compilations are briefly discussed. The uplift predictions of the slow and rapid thinning models, in which thinning is assumed to occur at a respectively slower and faster rate than heat can be conducted into the lithosphere, are presented. Comparison of uplift rates with model results indicates that the lithosphere is in a state between the two models. While uplift is predicted to continue after thinning has ceased due to thermal relaxation of the lithosphere, the rapid thinning model is always predicted to apply to surface heat flow, and an anomaly in this flow is not predicted to develop until after thinning has stopped.

  12. 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

  13. Accumulation of fossil fuels and metallic minerals in active and ancient rift lakes

    USGS Publications Warehouse

    Robbins, E.I.

    1983-01-01

    A study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of processes that form rift valleys with those that take place in and around rift lakes. The deposition of the precursors of petroleum, gas, oil shale, coal, phosphate, barite, Cu-Pb-Zn sulfides, and uranium begins with erosion of uplifted areas, and the consequent input of abundant nutrients and solute loads into swamps and tectonic lakes. Hot springs and volcanism add other nutrients and solutes. The resulting high biological productivity creates oxidized/reduced interfaces, and anoxic and H2S-rich bottom waters which preserves metal-bearing organic tissues and horizons. In the depositional phases, the fine-grained lake deposits are in contact with coarse-grained beach, delta, river, talus, and alluvial fan deposits. Earthquake-induced turbidites also are common coarse-grained deposits of rift lakes. Postdepositional processes in rifts include high heat flow and a resulting concentration of the organic and metallic components that were dispersed throughout the lakebeds. Postdepositional faulting brings organic- and metal-rich sourcebeds in contact with coarse-grained host and reservoir rocks. A suite of potentially economic deposits is therefore a characteristic of rift valleys. ?? 1983.

  14. Which mantle below the active rift segments in Afar?

    NASA Astrophysics Data System (ADS)

    Pik, Raphael; Stab, Martin; Ancellin, Marie-Anne; Sarah, Medynski; Cloquet, Christophe; Vye-Brown, Charlotte; Ayalew, Dereje; Chazot, Gilles; Bellahsen, Nicolas; Leroy, Sylvie

    2014-05-01

    The evolution of mantle sources beneath the Ethiopian volcanic province has long been discussed and debated with a long-lived controversy in identifying mantle reservoirs and locating them in the mantle. One interpretation of the isotopic composition of erupted lavas considers that the Afar mantle plume composition is best expressed by recent lavas from Afar and Gulf of Aden (e.g. Erta Ale, Manda Inakir and the 45°E torus anomaly on the Gulf of Aden) implying that all other volcanics (including other active segments and the initial flood basalt province) result from mixing of this plume component with additional lithospheric and asthenospheric components. A completely opposite view considers that the initial Oligocene continental flood basalts best represent the isotopic composition of the Afar mantle plume, which is subsequently mixed in various proportions with continental lithospheric mantle for generating some of the specific signature of Miocene and Quaternary volcanics. The precise and correct identification of mantle components involved in the generation of magmas is of particular importance because this is the only way to document the participation of mantle during extension and its potential role in break-up processes. In this contribution we provide new isotopic data for central Afar and we revisit the whole data set of the Ethiopian volcanic province in order to: (i) precisely identify the distinct mantle components implicated and (ii) discuss their location and evolution not only considering geochemical mixings, but also taking into account additional characteristics of erupted magmatic suites (volumes, location and relationships with amount of extension and segmentation). This new interpretation of geochemical data allows reconsidering the evolution of mantle in the course of rift evolution. In terms of mantle sources, two populations of active segments are frontally opposed in the volcanic province: those that share exactly the same composition with

  15. Role of mass wasting processes in the modification of oceanic rift valley morphology

    SciTech Connect

    Keith, D.J.; Fox, P.J.; Karson, J. A.

    1985-01-01

    During the last eight years field investigations using the high resolution capabilities of submersibles and deep-towed cameras have been conducted along the rift valley of the Mid-Cayman Rise, the western and eastern intersections of the Kane Transform Fault and the Mid-Atlantic Ridge and the eastern intersection of the Oceanographer Transform Fault. These 3 sites are representative of the range of tectonic environments which are characteristic of slowly accreting plate boundaries. Photographic and observational data collected from within these natural laboratories reveal important geomorphic information concerning the temporal and spatial evolution of volcanic constructional and fault-bounded terrain in response to mass wasting processes. The results of this investigation indicate that sedimentary processes significantly influence the development of oceanic lithosphere soon after its creation and continues to do so with increasing geologic age out to approximately 2 m.y. The data indicate that the rift valley floor distal from transform faults is dominated by a hummocky, volcanic morphology that is rapidly degraded by hyaloclastic mass wasting activity. With the evolution of the rift walls into the rift mountains, photographic data indicates that the processes associated with dislodgement and gravitational transport do not cease to operate but work much more infrequently relative to the tectonically active lower slopes.

  16. Investigating feedbacks between surface processes and tectonics in rift settings using numerical models

    NASA Astrophysics Data System (ADS)

    Beucher, Romain; Huismans, Ritske

    2015-04-01

    The Earth's landscape is a product of complex feedbacks between tectonics, driven by plate motion, and climate, which control the erosional processes. Interactions between erosion, tectonics and climate form, modify or destroy geomorphic features while the transfer of mass resulting from erosion and sedimentation affects isostasy and the mechanical behaviour of the lithosphere. Evolution of extensional basins and rifted continental margins creates significant topography, which in turn can change precipitation and erosion patterns. Transfer of sediments from the margins into the basin may change the stress pattern in the crust and thus affect the geometry of the rift. Modelling the complexity of such a system requires an integrated approach looking at interactions between tectonics and surface processes on a range of spatial and temporal scales. We use high-resolution numerical experiments coupling a 2D upper-mantle-scale thermo-mechanical model with a plan-form 2D surface processes model (SPM) to investigate the factors controlling the style of deformation. The experiments consist in simple extension models involving lithosphere with variable thickness (normal-like lithosphere to thick cratonic-like lithosphere) and explore the effects of rheological and compositional variability of the layer components of the crust and the lithosphere. We also explore different values of erosion efficiency together with different pattern of precipitation, including orographic effects. Tomography and geochemistry evidences suggest compositional stratification of the lithosphere. We also explore the effect of a depleted lower lithosphere (compositionally less dense than sublithospheric mantle) on the rift geometry and the effect of the isostatic responses in terms of uplift or subsidence on the surrounding topography. The models provide a basis to discuss the type of interactions between erosion and tectonics in rift settings. Preliminary results show that if erosion does play a

  17. Groundwater origin and flow dynamics in active rift systems - A multi-isotope approach in the Main Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Bretzler, Anja; Osenbrück, Karsten; Gloaguen, Richard; Ruprecht, Janina S.; Kebede, Seifu; Stadler, Susanne

    2011-05-01

    SummaryThis study aims to investigate groundwater recharge and flow patterns in tectonically active rift systems, exemplified by a case study in the Main Ethiopian Rift. The chosen approach includes the investigation of hydrochemical parameters and environmental isotopes ( 3H, δ 2H, δ 18O, δ 13C-DIC, 14C-DIC, 87Sr/ 86Sr). Apparent groundwater ages were determined by radiocarbon dating after correction of 14C-DIC using a modified δ 13C-mixing model and further validation using geochemical modelling with NETPATH. Hydrochemical and isotopic data indicate an evolutionary trend existing from the escarpments towards the Rift floor. Groundwater evolves from tritium-containing and hence recently recharged Ca-HCO 3-type water on the escarpments to tritium-free Na-HCO 3 groundwater dominating deep Rift floor aquifers. Correspondingly, rising pH and HCO3- values coupled with increasingly enriched δ 13C signatures point to hydrochemical evolution of DIC and beginning dilution of the carbon isotope signature by other carbon sources, related to a diffuse influx of mantle CO 2 into the groundwater system. Especially thermal groundwater sampled near the most recent fault zones in the Fantale/Beseka region displays clear influence of mantle CO 2 and increased water-rock interaction, indicated by a shift in δ 13C and 87Sr/ 86Sr signatures. The calculation of apparent groundwater ages revealed an age increase of deep groundwater from the escarpments to the Rift floor, complying with hydrochemical evolution. Within the Rift, samples show a relatively uniform distribution of apparent 14C ages of ˜1800 to ˜2800 years, with the expected down-gradient aging trend lacking, contradicting the predominant intra-rift groundwater flow described in existing transect-based models of groundwater flow. By combining hydrochemical and new isotopic data with knowledge of the structural geology of the Rift, we improve the existing groundwater flow model and propose a new conceptual model by

  18. 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.

  19. Factors controlling the final depth of active and failed continental rift zones

    NASA Astrophysics Data System (ADS)

    Thybo, H.; Elesin, Y.; Artemieva, I. M.

    2012-12-01

    Rift zones are elongated, narrow tectonic depressions in the Earth's surface which with time become filled with sediments and volcanics. Rifting processes may lead to break-up of continental plates to form new oceanic lithosphere. Subsidence of rift basins is caused by thinning of the crust and lithospheric mantle, together with thermal relaxation and isostatic compensation for the extra load of sediments. It is generally believed that the final depth of rift basins is primarily controlled by the amount of stretching. However, we show that the relative rheological strength of faults inside and outside rift zones exerts substantial control on the volume of the final rift basin (by more than a factor of 3) even for the same amount of extension (total or inside the rift zone). This surprising result is mainly caused by irreversible deepening of the rift graben during stretching due to lower crustal flow when the faults in the rift zone are weak, whereas the effect is negligible for strong faults. Relatively strong faults inside the rift zone lead to substantial stretching of adjacent crust, and we find that long term stretching outside the main rift zone may explain the formation of wide continental margins, which are now below sea level. We also demonstrate that fast syn-rift erosion/sedimentation rates can increase the final volume of rift basins by up to a factor of 1.7 for weak crustal faults, whereas this effect is insignificant for strong faults inside the rift zone. These findings have significant implications for estimation of stretching factors, tectonic forces, and geodynamic evolution of sedimentary basins around failed rift zones.

  20. Continental rifting - Progress and outlook

    NASA Technical Reports Server (NTRS)

    Baker, B. H.; Morgan, P.

    1981-01-01

    It is noted that in spite of the flood of new data on continental rifts in the last 15 years, there is little consensus about the basic mechanisms and causes of rifting. The remarkable similarities in rift cross sections (shown in a figure), are considered to suggest that the anomalous lithospheric structure of rifts is more dependent on lithosphere properties than the mode of rifting. It is thought that there is a spectrum of rifting processes for which two fundamental mechanisms can be postulated: an active mechanism, whereby thermal energy is transmitted into the lithosphere from the underlying asthenosphere, and a passive mechanism by which mechanical energy is transmitted laterally through the lithosphere as a consequence of plate interactions at a distance. In order to permit the concept of the two fundamentally different mechanisms to be tested, a tentative classification is proposed that divides rifts into two basic categories: active rifting and passive rifting. Here, the magnitude of active rifting will depend on the rate at which lithosphere moves over the thermal source, with rifts being restricted to stationary or slow-moving plates.

  1. ENAM: A community seismic experiment targeting rifting processes and post-rift evolution of the Mid Atlantic US margin

    NASA Astrophysics Data System (ADS)

    Van Avendonk, H. J.; Magnani, M. B.; Shillington, D. J.; Gaherty, J. B.; Hornbach, M. J.; Dugan, B.; Long, M. D.; Lizarralde, D.; Becel, A.; Benoit, M. H.; Harder, S. H.; Wagner, L. S.; Christeson, G. L.

    2014-12-01

    The continental margins of the eastern United States formed in the Early Jurassic after the breakup of supercontinent Pangea. The relationship between the timing of this rift episode and the occurrence of offshore magmatism, which is expressed in the East Coast Magnetic Anomaly, is still unknown. The possible influence of magmatism and existing lithospheric structure on the rifting processes along margin of the eastern U.S. was one of the motivations to conduct a large-scale community seismic experiment in the Eastern North America (ENAM) GeoPRISMS focus site. In addition, there is also a clear need for better high-resolution seismic data with shallow penetration on this margin to better understand the geological setting of submarine landslides. The ENAM community seismic experiment is a project in which a team of scientists will gather both active-source and earthquake seismic data in the vicinity of Cape Hatteras on a 500 km wide section of the margin offshore North Carolina and Virginia. The timing of data acquisition in 2014 and 2015 facilitates leveraging of other geophysical data acquisition programs such as Earthscope's Transportable Array and the USGS marine seismic investigation of the continental shelf. In April of 2014, 30 broadband ocean-bottom seismometers were deployed on the shelf, slope and abyssal plain of the study site. These instruments will record earthquakes for one year, which will help future seismic imaging of the deeper lithosphere beneath the margin. In September and October of 2014, regional marine seismic reflection and refraction data will be gathered with the seismic vessel R/V Marcus Langseth, and airgun shots will also be recorded on land to provide data coverage across the shoreline. Last, in the summer of 2015, a land explosion seismic refraction study will provide constraints on the crustal structure in the adjacent coastal plain of North Carolina and Virginia. All seismic data will be distributed to the community through IRIS

  2. Active volcanism on Venus in the Ganiki Chasma rift zone

    NASA Astrophysics Data System (ADS)

    Shalygin, E. V.; Markiewicz, W. J.; Basilevsky, A. T.; Titov, D. V.; Ignatiev, N. I.; Head, J. W.

    2015-06-01

    Venus is known to have been volcanically resurfaced in the last third of solar system history and to have undergone a significant decrease in volcanic activity a few hundred million years ago. However, fundamental questions remain: Is Venus still volcanically active today, and if so, where and in what geological and geodynamic environment? Here we show evidence from the Venus Express Venus Monitoring Camera for transient bright spots that are consistent with the extrusion of lava flows that locally cause significantly elevated surface temperatures. The very strong spatial correlation of the transient bright spots with the extremely young Ganiki Chasma, their similarity to locations of rift-associated volcanism on Earth, provide strong evidence for their volcanic origin and suggests that Venus is currently geodynamically active.

  3. Long-term coupling and feedbacks between surface processes and tectonics during rifting

    NASA Astrophysics Data System (ADS)

    Theunissen, Thomas; Huismans, Ritske S.

    2016-04-01

    Whereas significant efforts have been made to understand the relationship between mountain building and surfaces processes, limited research has been done on the relationship between surface processes and extensional tectonics. Here we present high-resolution 2-D coupled tectonic-surface processes modeling of extensional basin formation. The main aim is to find out how erosion and deposition affect the deformation in extensional systems. We test the combined effects of crustal rheology and varying surface process efficiency (erodibility, sea level) on structural style of rift and passive margin formation. The results show that both erosion of rift flank areas and basin deposition enhance localization of crustal deformation. Frictional-plastic extensional shear zones accumulate more deformation during a longer period of time, and loading of offshore basins can generate crustal ductile flow. In extreme cases sediment deposition delays lithospheric rupture. These mechanisms are enhanced when fluvial erosion, transport and deposition are efficient. We show that removal of mass from rift flanks and sedimentary loading in the basin area provide a first order feedback with tectonic deformation and control on rifted margin tectonic-morphology. However, surface processes do not change the first order mode of rifting, which is largely controlled by crustal rheology. Rift escarpment morphology is function of paleo-topography and sea level. Variation of strain localization in natural rift systems correlates with the observed behavior and suggests similar feedbacks as demonstrated by the forward numerical models.

  4. InSAR Observations of Magmatic Processes in the East African Rift

    NASA Astrophysics Data System (ADS)

    Biggs, Juliet; Anthony, Elizabeth; Ebinger, Cynthia; Amelung, Falk; Gourmelen, Noel

    2010-05-01

    The role of magma in accommodating extension and it's relationship to fault-based extension in continental rifting is poorly understood. Here we present observations of the temporal and spatial evolution of surface displacements resulting from magmatic processes in the East African Rift. A systematic InSAR surveys have detected geodetic activity at six of the volcanoes in the East African Rift. In Kenya, subsidence of 2-5 cm occurred at Suswa and Menengai during 1997-2000, ~9cm of uplift at Longonot in 2004-2006 and ~21 cm of uplift at Paka during 2006-2007. The deformation is episodic, and no deformation was observed at these volcanoes during other time-periods. The best-fitting source models for each episode is inflation or deflation of a horizontal lensoid at a depth of 2-5 km. The episodic nature of the activity, its lack of correlation with seasons, and the preferred source geometry are all consistent with activity in the volatile-rich cap to a crystal-rich magma chamber beneath each of the 4 volcanoes. A seismic swarm occurred in Northern Tanzania from July 14 to August 4 2007. Using InSAR images from Envisat (IS2 and IS6) and ALOS, we show that the seismic swarm was accompanied by 1) subsidence that can be attributed to ~40 cm of normal motion on a NE striking fault, 2) the intrusion of ~2.4 m wide dyke, 3) deflation of a point source magma chamber and 4) collapse of a shallow graben. The large number of available SAR images allows us to examine the sequence and time-dependent behaviour of these processes and relationship between diking and faulting.

  5. Rapid spatiotemporal variations in rift structure during development of the Corinth Rift, central Greece

    NASA Astrophysics Data System (ADS)

    Nixon, Casey W.; McNeill, Lisa C.; Bull, Jonathan M.; Bell, Rebecca E.; Gawthorpe, Robert L.; Henstock, Timothy J.; Christodoulou, Dimitris; Ford, Mary; Taylor, Brian; Sakellariou, Dimitris; Ferentinos, George; Papatheodorou, George; Leeder, Mike R.; Collier, Richard E. LI.; Goodliffe, Andrew M.; Sachpazi, Maria; Kranis, Haralambos

    2016-05-01

    The Corinth Rift, central Greece, enables analysis of early rift development as it is young (<5 Ma) and highly active and its full history is recorded at high resolution by sedimentary systems. A complete compilation of marine geophysical data, complemented by onshore data, is used to develop a high-resolution chronostratigraphy and detailed fault history for the offshore Corinth Rift, integrating interpretations and reconciling previous discrepancies. Rift migration and localization of deformation have been significant within the rift since inception. Over the last circa 2 Myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift, but this transition did not occur synchronously along strike. Isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620-340 ka, characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at ~30 m/kyr. Since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin, demonstrated by lateral growth of discrete depocenters at ~40 m/kyr. A single central depocenter formed by circa 130 ka, indicating full fault linkage. These results indicate that rift localization is progressive (not instantaneous) and can be synchronous once a rift border fault system is established. This study illustrates that development processes within young rifts occur at 100 kyr timescales, including rapid changes in rift symmetry and growth and linkage of major rift faults.

  6. Morpho-structural evolution of a volcanic island developed inside an active oceanic rift: S. Miguel Island (Terceira Rift, Azores)

    NASA Astrophysics Data System (ADS)

    Sibrant, A. L. R.; Hildenbrand, A.; Marques, F. O.; Weiss, B.; Boulesteix, T.; Hübscher, C.; Lüdmann, T.; Costa, A. C. G.; Catalão, J. C.

    2015-08-01

    The evolution of volcanic islands is generally marked by fast construction phases alternating with destruction by a variety of mass-wasting processes. More specifically, volcanic islands located in areas of intense regional deformation can be particularly prone to gravitational destabilisation. The island of S. Miguel (Azores) has developed during the last 1 Myr inside the active Terceira Rift, a major tectonic structure materializing the present boundary between the Eurasian and Nubian lithospheric plates. In this work, we depict the evolution of the island, based on high-resolution DEM data, stratigraphic and structural analyses, high-precision K-Ar dating on separated mineral phases, and offshore data (bathymetry and seismic profiles). The new results indicate that: (1) the oldest volcanic complex (Nordeste), composing the easternmost part of the island, was dominantly active between ca. 850 and 750 ka, and was subsequently affected by a major south-directed flank collapse. (2) Between at least 500 ka and 250 ka, the landslide depression was massively filled by a thick lava succession erupted from volcanic cones and domes distributed along the main E-W collapse scar. (3) Since 250 kyr, the western part of this succession (Furnas area) was affected by multiple vertical collapses; associated plinian eruptions produced large pyroclastic deposits, here dated at ca. 60 ka and less than 25 ka. (4) During the same period, the eastern part of the landslide scar was enlarged by retrogressive erosion, producing the large Povoação valley, which was gradually filled by sediments and young volcanic products. (5) The Fogo volcano, in the middle of S. Miguel, is here dated between ca. 270 and 17 ka, and was affected by, at least, one southwards flank collapse. (6) The Sete Cidades volcano, in the western end of the island, is here dated between ca. 91 and 13 ka, and experienced mutliple caldera collapses; a landslide to the North is also suspected from the presence of a

  7. 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, N.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.

  8. Feedback between magmatic, tectonic and glacial processes in the West Antarctic Rift System (Invited)

    NASA Astrophysics Data System (ADS)

    Rocchi, S.

    2010-12-01

    swarms related to the main NW-SE discontinuities. (3) From the late Miocene to Present, the mantle flow led to normal faulting of the collapsing rift shoulder, which favored the rise of magmas building up large volcanoes along N-S normal-transtensional faults. This evolution of the WARS tectonic-magmatic activity is coeval with the main episodes of ice sheet development in Antarctica. The two main episodes of climate worsening, at the Eocene-Oligocene and early-mid Miocene transitions, were alternated with advances and retreats of ice sheets, with significant volume and thickness changes, that could be invoked as effective in triggering magma generation. It is thus possible to envisage a feedback system involving the WARS Cenozoic geological processes. The strike-slip rift regime affects magma genesis as well as segmentation and uplift of the Transantarctic Mountains (geodynamic forcing on magmatism and tectonics). Surface uplift affects landscape and temperature (tectonic forcing on environment). Mountainous landscape triggers glaciation (geomorphological forcing on climate), and climate affects temperature, hence erosive potential of glaciers (climate forcing on environment). Finally, ice load-unload cycles could affect magma genesis (glacial forcing on magmatism).

  9. Uppermost mantle (Pn) velocity model for the Afar region, Ethiopia: an insight into rifting processes

    NASA Astrophysics Data System (ADS)

    Stork, A. L.; Stuart, G. W.; Henderson, C. M.; Keir, D.; Hammond, J. O. S.

    2013-04-01

    The Afar Depression, Ethiopia, offers unique opportunities to study the transition from continental rifting to oceanic spreading because the process is occurring onland. Using traveltime tomography and data from a temporary seismic deployment, we describe the first regional study of uppermost mantle P-wave velocities (VPn). We find two separate low VPn zones (as low as 7.2 km s-1) beneath regions of localized thinned crust in northern Afar, indicating the existence of high temperatures and, potentially, partial melt. The zones are beneath and off-axis from, contemporary crustal magma intrusions in active magmatic segments, the Dabbahu-Manda-Hararo and Erta'Ale segments. This suggests that these intrusions can be fed by off-axis delivery of melt in the uppermost mantle and that discrete areas of mantle upwelling and partial melting, thought to characterize segmentation of the uppermost mantle at seafloor spreading centres, are initiated during the final stages of break-up.

  10. Faulting processes during early-stage rifting: seismicity analysis of the 2009-2010 Northern Malawi earthquake sequence

    NASA Astrophysics Data System (ADS)

    Gaherty, J. B.; Shillington, D. J.; Shuler, A. E.; Kapanje, W.; Chindandali, P.; Nooner, S. L.; Ebinger, C. J.; Nyblade, A.; Kalindekafe, L.; Pritchard, M. E.; Scholz, C. A.

    2010-12-01

    One of the most poorly understood aspects of the rifting process is early-stage rift initiation. Core questions concern how deformation nucleates and evolves throughout the lithosphere, both along strike and through time, and how magma participates in extension. In December 2009, a rare sequence of earthquakes initiated within the nascent, southern portion of the East African Rift (EAR) system in northern Malawi’s Karonga district; eleven earthquakes with Mw > 4.5 occurred over a 12-day period. The largest events range from Mw 5.8-6.0, and appear to occur on one or more shallow normal faults located in the hanging wall well above the primary rift border fault. They thus differ from nearly all other moderate-size earthquakes within the Western Rift of the EAR, which appear to be on major border faults, and they imply a critical role for hanging-wall faults in accommodating early-stage rifting. Using teleseismic and regional recordings of the largest events, and recordings of aftershocks from a temporary (4-month) local network comprising six stations, we evaluate the nature of faulting during early-stage rifting. The zeroth-order (automated) detection and location of over ~1000 aftershocks recorded on our temporary network between Jan-May 2010 is consistent with faulting in the hanging wall, with the bulk of the seismicity suggesting a west-dipping normal fault shallower than about 10 km. Surface breaks in the Karonga region have vertical offsets up to ~0.65 m, and are continuous along strike for up to 17 km. However, there is a broad distribution of epicenters extending both north and south of Karonga, as well as events significantly deeper than 10 km. The spatial distribution of events is strongly suggestive of multiple faults interacting to produce the observed deformation. More speculatively, the presence of events at depths > 25 km, and the abundant distribution of events up into the Rungwe volcanic province in southern Tanzania hint at either a potential

  11. Geochemical and geochronological constraints on the origin and evolution of rocks in the active Woodlark Rift of Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Zirakparvar, Nasser Alexander

    Tectonically active regions provide important natural laboratories to glean information that is applicable to developing a better understanding of the geologic record. One such area of the World is Papua New Guinea, much of which is situated in an active and transient plate boundary zone. The focus of this PhD research is to develop a better understanding of rocks in the active Woodlark Rift, situated in Papua New Guinea's southernmost reaches. In this region, rifting and lithospheric rupture is occurring within a former subduction complex where there is a history of continental subduction and (U)HP metamorphism. The lithostratigraphic units exposed in the Woodlark Rift provide an opportunity to better understand the records of plate boundary processes at many scales from micron-sized domains within individual minerals to regional geological relationships. This thesis is composed of three chapters that are independent of one another but are all related to the overall goal of developing a better understanding of the record of plate boundary processes in the rocks currently exposed in the Woodlark Rift. The first chapter, published in its entirety in Earth and Planetary Science Letters (2011 v. 309, p. 56 - 66), is entitled 'Lu-Hf garnet geochronology applied to plate boundary zones: Insights from the (U)HP terrane exhumed within the Woodlark Rift'. This chapter focuses on the use of the Lu-Hf isotopic system to date garnets in the Woodlark Rift. Major findings of this study are that some of the rocks in the Woodlark Rift preserve a Lu-Hf garnet isotopic record of initial metamorphism and continental subduction occurring in the Late Mesozoic, whereas others only preserve a record of tectonic processes related to lithospheric rupture during the initiation of rifting in the Late Cenozoic. The second chapter is entitled 'Geochemical and geochronological constraints on the origin of rocks in the active Woodlark Rift of Papua New Guinea: Recognizing the dispersed

  12. Active fault systems of the Kivu rift and Virunga volcanic province, and implications for geohazards

    NASA Astrophysics Data System (ADS)

    Zal, H. J.; Ebinger, C. J.; Wood, D. J.; Scholz, C. A.; d'Oreye, N.; Carn, S. A.; Rutagarama, U.

    2013-12-01

    H Zal, C Ebinger, D. Wood, C. Scholz, N. d'Oreye, S. Carn, U. Rutagarama The weakly magmatic Western rift system, East Africa, is marked by fault-bounded basins filled by freshwater lakes that record tectonic and climatic signals. One of the smallest of the African Great Lakes, Lake Kivu, represents a unique geohazard owing to the warm, saline bottom waters that are saturated in methane, as well as two of the most active volcanoes in Africa that effectively dam the northern end of the lake. Yet, the dynamics of the basin system and the role of magmatism were only loosely constrained prior to new field and laboratory studies in Rwanda. In this work, we curated, merged, and analyzed historical and digital data sets, including spectral analyses of merged Shuttle Radar Topography Mission topography and high resolution CHIRP bathymetry calibrated by previously mapped fault locations along the margins and beneath the lake. We quantitatively compare these fault maps with the time-space distribution of earthquakes located using data from a temporary array along the northern sector of Lake Kivu, as well as space-based geodetic data. During 2012, seismicity rates were highest beneath Nyiragongo volcano, where a range of low frequency (1-3 s peak frequency) to tectonic earthquakes were located. Swarms of low-frequency earthquakes correspond to periods of elevated gas emissions, as detected by Ozone Monitoring Instrument (OMI). Earthquake swarms also occur beneath Karisimbi and Nyamuragira volcanoes. A migrating swarm of earthquakes in May 2012 suggests a sill intrusion at the DR Congo-Rwanda border. We delineate two fault sets: SW-NE, and sub-N-S. Excluding the volcano-tectonic earthquakes, most of the earthquakes are located along subsurface projections of steep border faults, and intrabasinal faults calibrated by seismic reflection data. Small magnitude earthquakes also occur beneath the uplifted rift flanks. Time-space variations in seismicity patterns provide a baseline

  13. Mantle compensation of active metamorphic core complexes at Woodlark rift in Papua New Guinea.

    PubMed

    Abers, Geoffrey A; Ferris, Aaron; Craig, Mitchell; Davies, Hugh; Lerner-Lam, Arthur L; Mutter, John C; Taylor, Brian

    2002-08-22

    In many highly extended rifts on the Earth, tectonic removal of the upper crust exhumes mid-crustal rocks, producing metamorphic core complexes. These structures allow the upper continental crust to accommodate tens of kilometres of extension, but it is not clear how the lower crust and underlying mantle respond. Also, despite removal of the upper crust, such core complexes remain both topographically high and in isostatic equilibrium. Because many core complexes in the western United States are underlain by a flat Moho discontinuity, it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating. These processes should decouple upper-crust extension from that in the mantle. In contrast, here we present seismic observations of metamorphic core complexes of the western Woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension. These core complexes are actively being exhumed at a rate of 5-10 km Myr(-1), and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density, as indicated by low seismic velocities. We conclude that, at least in this case, the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon, but must involve mantle extension.

  14. Mid-lithospheric Discontinuity Beneath the Malawi Rift, Deduced from Gravity Studies and its Relation to the Rifting Process.

    NASA Astrophysics Data System (ADS)

    Njinju, E. A.; Atekwana, E. A.; Mickus, K. L.; Abdelsalam, M. G.; Atekwana, E. A.; Laó-Dávila, D. A.

    2015-12-01

    The World Gravity Map satellite gravity data were used to investigate the lithospheric structure beneath the Cenozoic-age Malawi Rift which forms the southern extension of the Western Branch of the East African Rift System. An analysis of the data using two-dimensional (2D) power spectrum methods indicates the two distinctive discontinuities at depths of 31‒44 km and 64‒124 km as defined by the two steepest slopes of the power spectrum curves. The shallower discontinuity corresponds to the crust-mantle boundary (Moho) and compares well with Moho depth determined from passive seismic studies. To understand the source of the deeper discontinuity, we applied the 2D power spectrum analysis to other rift segments of the Western Branch as well as regions with stable continental lithospheres where the lithospheric structure is well constrained through passive seismic studies. We found that the deeper discontinuity corresponds to a mid-lithospheric discontinuity (MLD), which is known to exist globally at depths between 60‒150 km and as determined by passive seismic studies. Our results show that beneath the Malawi Rift, there is no pattern of N-S elongated crustal thinning following the surface expression of the Malawi Rift. With the exception of a north-central region of crustal thinning (< 35 km), most of the southern part of the rift is underlain by thick crust (~40‒44 km). Different from the Moho, the MLD is shallower beneath the axis of the Malawi Rift forming a N-S trending zone with depths of 64‒80 km, showing a broad and gentle topography. We interpret the MLD as representing a sharp density contrast resulting from metasomatized lithosphere due to lateral migration along mobile belts of hot mantle melt or fluids from a distant plume and not from an ascending asthenosphere. These fluids weaken the lithosphere enhancing rift nucleation. The availability of satellite gravity worldwide makes gravity a promising technique for determining the MLD globally.

  15. Active Tectonics In The Rukwa Rift (sw Tanzania): A Study of The Potential For Large Earthquakes In A Continental Rift.

    NASA Astrophysics Data System (ADS)

    Kervyn, F.

    The Rukwa rift is a deep sedimentary basin that is considered as a tectonic trans- fer zone between the Tanganyika and the Malawi troughs. The tectonic evolution of the depression is controlled by the reactivation of proterozoic structures and started with the deposition of the permo-triasic Karoo sediments. In the southeast, the rift is divided into two facing half graben separated by a Precambrian horst, whereas its northwestern part has a more symmetrical graben structure. Although most of the vertical displacement is accommodated by the Lupa eastern boundary fault, onshore shallow seismic profiles have confirmed the co-occurrence of intrabasin synthetic- and strike-slip faults within the sub surface sediments. Both normal and dextral strike-slip movement are indeed observed in the basin in response to the E-W to WNW-SSE ex- tension. The region has a moderate seismic activity and the earthquakes magnitude is generally below M 6.5. However, a M 7.4 earthquake occurred in the Rukwa region in 1910 but its exact location remains uncertain. The current research aimed at the identi- fication of active faults within the recent deposits of the basin by the combination in a GIS of radar interferometric data with topographical and geological maps, geophysical data, and field observations. Radar interferometry (InSAR) was found to be especially suitable for DEM computation in low relief areas where available topographic data are limited in accuracy. Numerous topographic lineaments were observed on InSAR DEM, and follow two main directions, both oblique to the main NW-SE trend of the rift. On the one hand, the GIS analysis confirms that the observed lineaments corre- spond to real natural alignment such like the drainage for example, and are therefore not related to atmospheric artefacts. On the other hand, the field observations revealed that in most cases, the topographic lineaments are very subtle and difficult to identify. However, direct correlations with tectonic

  16. High Fluoride and Geothermal Activities In Continental Rift Zones, Ethiopia

    NASA Astrophysics Data System (ADS)

    Weldesenbet, S. F.; Wohnlich, S.

    2012-12-01

    The Central Main Ethiopian Rift basin is a continental rift system characterized by volcano-tectonic depression endowed with huge geothermal resource and associated natural geochemical changes on groundwater quality. Chemical composition of groundwater in the study area showed a well defined trend along flow from the highland and escarpment to the rift floor aquifer. The low TDS (< 500mg/l) Ca-Mg-HCO3 dominated water at recharge area in the highlands and escarpments evolve progressively into Ca-Na-HCO3 and Na-Ca-HCO3 type waters along the rift ward groundwater flow paths. These waters finally appear as moderate TDS (mean 960mg/l) Na-HCO3 type and as high TDS (> 1000 mg/l) Na-HCO3-Cl type in volcano-lacustrine aquifers of the rift floor. High concentrations of fluoride (up to 97.2 mg/l) and arsenic (up to 98μg/l) are recognized feature of groundwaters which occur mostly in the vicinity of the geothermal fields and the rift lakes in the basin. Fluoride and arsenic content of dry volcaniclastic sediments close to these areas are in the range 666-2586mg/kg and 10-13mg/kg respectively. The relationship between fluoride and calcium concentrations in groundwaters showed negative correlation. Near-equilibrium state attained between the mineral fluorite (CaF2) and the majority of fluoride-rich (>30mg/l) thermal groundwater and shallow cold groundwater. This indicated that the equilibrium condition control the high concentration of fluoride in the groundwaters. Whereas undersaturation state of fluorite in some relatively low-fluoride (<30mg/l) thermal waters indicated a dilution by cold waters. Laboratory batch leaching experiments showed that fast dissolution of fluoride from the sediment samples suddenly leached into the interacting water at the first one hour and then remain stable throughout the experiment. The concentrations of leached fluoride from the hot spring deposits, the lacustrine sediments, and the pyroclastic rock are usually low (1% of the total or less than

  17. Volcanic evolution of an active magmatic rift segment on a 100 Kyr timescale: exposure dating of lavas from the Manda Hararo/Dabbahu segment of the Afar Rift

    NASA Astrophysics Data System (ADS)

    Medynski, S.; Williams, A.; Pik, R.; Burnard, P.; Vye, C.; France, L.; Ayalew, D.; Yirgu, G.

    2012-12-01

    the 2005 rifting episode. This second magmatic centre supplies magma to the remaining 2/3 of the segment, but scarcely impacts its Northern termination (where the Dabbahu activity predominates) - except during extraordinary events when dykes are long enough to reach those parts, as in 2005. The eruption ages of the different lava units correlates with their degrees of differentiation, allowing different magmatic cycles of about a few tens of years each to be distinguished. During the first recorded magmatic cycle (~70 ka to ~55 ka), Dabbahu is built of wide-spreading pāhoehoe flows around localised eruptive centres. The resulting topography of the volcanic edifice remains low, and is only slightly affected by rift-related fault activity, with the development of minor scarps. The second recorded magmatic cycle (~50 ka to ~20 ka) coincides with a strong development of Dabbahu topography - underlined by the change in lava morphology with well channelized 'a'ā flows since 50 ka. Tectonic activity also clearly increases over this period, with the initiation of the major fault scarps of the rift, which have been dated at around 35 ka. Our study underlines the role of the magma supply and availability beneath Dabbahu in the evolution both topographies of Dabbahu volcano and of the rift depression morphology.

  18. Magmatism in rifting and basin formation

    NASA Astrophysics Data System (ADS)

    Thybo, H.

    2008-12-01

    Whether heating and magmatism cause rifting or rifting processes cause magmatic activity is highly debated. The stretching factor in rift zones can be estimated as the relation between the initial and the final crustal thickness provided that the magmatic addition to the crust is insignificant. Recent research demonstrates substantial magmatic intrusion into the crust in the form of sill like structures in the lowest crust in the presently active Kenya and Baikal rift zones and the DonBas palaeo-rift zone in Ukraine. This result may be surprising as the Kenya Rift is associated with large amounts of volcanic products, whereas the Baikal Rift shows very little volcanism. Identification of large amounts of magmatic intrusion into the crust has strong implications for estimation of stretching factor, which in the case of Baikal Rift Zone is around 1.7 but direct estimation gives a value of 1.3-1.4 if the magmatic addition is not taken into account. This may indicate that much more stretching has taken place on rift systems than hitherto believed. Wide sedimentary basins may form around aborted rifts due to loading of the lithosphere by sedimentary and volcanic in-fill of the rift. This type of subsidence will create wide basins without faulting. The Norwegian- Danish basin in the North Sea area also has subsided gradually during the Triassic without faulting, but only few rift structures have been identified below the Triassic sequences. We have identified several mafic intrusions in the form of large batholiths, typically more than 100 km long, 20-40 km wide and 20 km thick. The associated heating would have lifted the surface by about 2 km, which may have been eroded before cooling. The subsequent contraction due to solidification and cooling would create subsidence in a geometry similar to basins that developed by loading. These new aspects of magmatism will be discussed with regard to rifting and basin formation.

  19. 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

  20. Thermal and compaction processes in a young rifted basin containing evaporites: Gulf of Lions, France

    SciTech Connect

    Burrus, J.; Audebert, F. )

    1990-09-01

    The Gulf of Lions (northwestern Mediterranean) is a young Oligocene-Miocene rifted basin characterized by the presence of a thick evaporitic sequence. Observed data (surface heat flow, well temperatures, geopressures below the evaporites, crustal thinning) are used to model (1) the basin-forming mechanisms, and (2) the intrasedimentary processes that influence the temperature history, and thus, the maturation of source rocks. A two-dimensional finite-difference basin model, Temispack, simulates the sedimentation, compaction, and overpressures related to the low permeability of the post-rift sequence, the heat transfer from the rifted lithosphere into the sediments, and the level or maturation by using a kinetic approach. The classic passive margin model, which relates the history of heat flow and the rate of subsidence to a uniform thinning factor, yields subsidence and heat flow predictions that agree with observations. The recently accelerated subsidence observed in the deep margin is probably an effect of recent regional compression rather than the consequence of alternative rifting mechanisms. The history of maturation of organic matter was influenced by the variation of the regional heat flow due to the rifting, the effect of sedimentation (or blanketing), and the variability of the lithologies and porosities. The undercompaction of sediments underlying the evaporites is accounted for by the model, which suggests that hydraulic fracturing is taking place at present in the overpressured sediments. Fluid flow associated with the compaction disequilibrium has negligible thermal effect. Undercompacted shales below the evaporites have lower thermal diffusivity and thus higher temperatures, which has a significant effect on maturation. 15 figs., 4 tabs.

  1. San Andres Rift, Nicaraguan Shelf: A 346-Km-Long, North-South Rift Zone Actively Extending the Interior of the "Stable" Caribbean Plate

    NASA Astrophysics Data System (ADS)

    Carvajal, L. C.; Mann, P.

    2015-12-01

    The San Andres rift (SAR) is an active, 015°-trending, bathymetric and structural rift basin that extends for 346 km across the Nicaraguan platform and varies in bathymetric width from 11-27 km and in water depth from 1,250 to 2,500 m. We used four 2D regional seismic lines tied to two offshore, industry wells located west of the SAR on the Nicaraguan platform to map normal faults, transfer faults, and possibly volcanic features with the rift. The Colombian islands of San Andres (26 km2) and Providencia (17 km2) are footwall uplifts along west-dipping, normal fault bounding the eastern margin of the rift. Mapping indicates the pre-rift section is Late Cretaceous to Oligocene in age and that the onset of rifting began in the early to middle Miocene as shown by wedging of the Miocene and younger sedimentary fill controlled by north-south-striking normal faults. Structural restorations at two locations across the rift shows that the basin opened mainly by dip-slip fault motions producing a total, east-west extension of 18 km in the north and 15 km in the south. Structural restoration shows the rift formed on a 37-km-wide, elongate basement high - possibly of late Cretaceous, volcanic origin and related to the Caribbean large igneous province. Previous workers have noted that the SAR is associated with province of Pliocene to Quaternary seamounts and volcanoes which range from non-alkaline to mildly alkaline, including volcanic rocks on Providencia described as andesites and rhyolites. The SAR forms one of the few recognizable belts of recorded seismicity within the Caribbean plate. The origin of the SAR is related to Miocene and younger left-lateral displacement along the Pedro Banks fault to the north and the southwestern Hess fault to the south. We propose that the amount of left-lateral displacement that created the rift is equivalent to the amount of extension that formed it: 18-20 km.

  2. Crustal structure during active rifting in the central Salton Trough, California, constrained by the Salton Seismic Imaging Project (SSIP)

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Seismic refraction and reflection travel times from the Salton Seismic Imaging Project (SSIP) were used to constrain crustal structure during active continental rifting in the central Salton Trough, California. SSIP, funded by NSF and USGS, acquired seismic data in and across the Salton Trough in 2011 to investigate rifting processes at the northern end of the Gulf of California extensional province and earthquake hazards at the southern end of the San Andreas Fault system. Seven lines of refraction and low-fold reflection data were acquired onshore, two lines and a grid of airgun and OBS data were acquired in the Salton Sea, and onshore-offshore data were recorded. North American lithosphere in the central Salton Trough appears to have been rifted apart and replaced by new crust added by magmatism from below and sedimentation from above. Ongoing active rifting of this new crust is manifested by shallow (<10km depth) seismicity in the oblique Brawley Seismic Zone (connecting the Imperial and San Andreas transform faults), the small Salton Buttes volcanoes, and very high heat flow that enables geothermal energy production. Analyses of the onshore-offshore seismic line that extends along the axis of the Salton Trough, parallel to the direction of plate motion, constrains rifted crustal structure. Crystalline basement (~5 km/s) generally occurs at ~4 km depth, but is at 2-3 km depth in a localized region beneath the Salton Buttes and Salton Sea geothermal field. This crystalline rock is interpreted to be late Pliocene to Quaternary Colorado River sediment that has been metamorphosed by high heat flow to a depth of at least 10km. The shallower basement under the volcanic and geothermal field is due to more intense metamorphism and hydrothermal alteration in this region of extreme heat flow. Faster velocity (6.2-6.4 km/s) observed at 10-13 km depth might be the remains of ruptured pre-existing crust or might be produced by deeper magmatism. Seismic travel times indicate

  3. Topography, river network and recent fault activity at the margins of the Central Main Ethiopian Rift (East Africa)

    NASA Astrophysics Data System (ADS)

    Molin, Paola; Corti, Giacomo; Sembroni, Andrea

    2016-04-01

    Along its length, the Main Ethiopian Rift (MER) in East Africa records a transition from early fault-dominated morphology in the South to axial magma assisted-rifting typical of continental break-up in the North. It is one of the few locations on Earth offering a complete picture of the evolution of continental rifting and thus provides a unique opportunity to directly analyze how the drainage network reorganize under extensional tectonic forcing. In this paper we present a new analysis of the river network and relative landforms - complemented with a summary of recent geological data - at both rift margins of the Central MER, a key sector of the rift capturing the phase of drainage reorganization between incipient and mature rifting. This analysis shows that hydrography is strongly influenced by recent tectonics. Rectangular drainage patterns, windgaps, and lacustrine/swampy areas formed by structural dams document that the rivers are in continuous competition with fault activity. The irregular longitudinal profiles (with knickpoints/knickzones in correspondence with faults) also suggest that rivers are in a transient state of disequilibrium related to recent tectonic activity at rift margins, in agreement with previous geological and seismological data. A more regional analysis extended to the adjoining Northern and Southern MER indicates that rifting evolves from initial stages characterized by margins poorly incised by rivers with gentle channel gradients (except in correspondence with faults), to mature phases in which rift margins are highly incised by a well organized fluvial network composed by concave and steep rivers. Our regional analysis also indicates a stronger and/or more recent tectonic activity at the rift margins proceeding to the south, in line with previous models of rift development.

  4. Topography, river network and recent fault activity at the margins of the Central Main Ethiopian Rift (East Africa)

    NASA Astrophysics Data System (ADS)

    Molin, Paola; Corti, Giacomo

    2015-11-01

    Along its length, the Main Ethiopian Rift (MER) in East Africa records a transition from early fault-dominated morphology in the South to axial magma assisted-rifting typical of continental break-up in the North. It is one of the few locations on Earth offering a complete picture of the evolution of continental rifting and thus provides a unique opportunity to directly analyze how the drainage network reorganizes under extensional tectonic forcing. In this paper we present a new analysis of the river network and relative landforms-complemented with a summary of recent geological data-at both rift margins of the Central MER, a key sector of the rift capturing the phase of drainage reorganization between incipient and mature rifting. This analysis shows that hydrography is strongly influenced by recent tectonics. Rectangular drainage patterns, windgaps, and lacustrine/swampy areas formed by structural dams document that the rivers are in continuous competition with fault activity. The irregular longitudinal profiles (with knickpoints/knickzones in correspondence with faults) also suggest that rivers are in a transient state of disequilibrium related to recent tectonic activity at rift margins, in agreement with previous geological and seismological data. A more regional analysis extended to the adjoining Northern and Southern MER indicates that rifting evolves from initial stages characterized by margins poorly incised by rivers with gentle channel gradients (except in correspondence with faults), to mature phases in which rift margins are highly incised by a well organized fluvial network composed by concave and steep rivers. Our regional analysis also indicates a stronger and/or more recent tectonic activity at the rift margins proceeding to the south, in line with previous models of rift development.

  5. Distribution of fault activity in the early stages of continental breakup: an analysis of faults and volcanic products of the Natron Basin, East African Rift, Tanzania

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    Recent magmatic-tectonic crises in Ethiopia (e.g. 2005 Dabbahu rifting episode, Afar) have informed our understanding of the spatial and temporal distribution of strain in magmatic rifts transitioning to sea-floor spreading. However, the evolving contributions of magmatic and tectonic processes during the initial stages of rifting, is a subject of ongoing debate. The <5 Ma northern Tanzania and southern Kenya sectors of the East Africa Rift provide ideal locations to address this problem. We present preliminary findings from an investigation of fault structures utilizing aerial photography and satellite imagery of the ~35 km wide Natron rift-basin in northern Tanzania. Broad-scale structural mapping will be supplemented by field observations and 40Ar-39Ar dating of lava flows cut by faults to address three major aspects of magma-assisted rifting: (1) the relative timing of activity between the border fault and smaller faults distributed across the width of the rift; (2) time-averaged slip rates along rift-zone faults; and (3) the spatial distribution of faults and volcanic products, and their relative contributions to strain accommodation. Preliminary field observations suggest that the ~500 m high border fault system along the western edge of the Natron basin is either inactive or has experienced a reduced slip rate and higher recurrence interval between surface-breaking events, as evidence by a lack of recent surface-rupture along the main fault escarpments. An exception is an isolated, ~2 km-long segment of the Natron border fault, which is located in close proximity (< 5km) to the active Oldoinyo Lengai volcano. Here, ~10 m of seemingly recent throw is observed in volcaniclastic deposits. The proximity of the fault segment to Oldoinyo Lengai volcano and the localized distribution of fault-slip are consistent with magma-assisted faulting. Faults observed within the Natron basin and on the flanks of Gelai volcano, located on the eastern side of the rift, have

  6. Crustal Structure Across the Okavango Rift Zone, Botswana: Initial Results From the PRIDE-SEISORZ Active-Source Seismic Profile

    NASA Astrophysics Data System (ADS)

    Canales, J. P.; Moffat, L.; Lizarralde, D.; Laletsang, K.; Harder, S. H.; Kaip, G.; Modisi, M.

    2015-12-01

    The PRIDE project aims to understand the processes of continental rift initiation and evolution by analyzing along-axis trends in the southern portion of the East Africa Rift System, from Botswana through Zambia and Malawi. The SEISORZ active-source seismic component of PRIDE focused on the Okavango Rift Zone (ORZ) in northwestern Botswana, with the main goal of imaging the crustal structure across the ORZ. This will allow us to estimate total crustal extension, determine the pattern and amount of thinning, assess the possible presence of melt within the rift zone, and assess the contrasts in crustal blocks across the rift, which closely follows the trend of a fold belt. In November 2014 we conducted a crustal-scale, 450-km-long seismic refraction/wide-angle reflection profile consisting of 19 sources (shots in 30-m-deep boreholes) spaced ~25 km apart from each other, and 900 receivers (IRIS/PASSCAL "Texan" dataloggers and 4.5Hz geophones) with ~500 m spacing. From NW to SE, the profile crosses several tectonic domains: the Congo craton, the Damara metamorphic belt and the Ghanzi-Chobe fold belt where the axis of the ORZ is located, and continues into the Kalahari craton. The record sections display clear crustal refraction (Pg) and wide-angle Moho reflection (PmP) phases for all 17 of the good-quality shots, and a mantle refraction arrival (Pn), with the Pg-PmP-Pn triplication appearing at 175 km offset. There are distinct changes in the traveltime and amplitude of these phases along the transect, and on either side of the axis, that seem to correlate with sharp transitions across tectonic terrains. Initial modeling suggests: (1) the presence of a sedimentary half-graben structure at the rift axis beneath the Okavango delta, bounded to the SE by the Kunyere-Thamalakane fault system; (2) faster crustal Vp in the domains to the NW of the ORZ; and (3) thicker crust (45-50 km) at both ends of the profile within the Congo and Kalahari craton domains than at the ORZ and

  7. Active low-angle (?) normal faulting along the North Lunggar rift, western Tibet

    NASA Astrophysics Data System (ADS)

    Logan, M. A.; Taylor, M. H.; Styron, R. H.; Gosse, J. C.; Ding, L.; Yang, G.

    2012-12-01

    Here we present surface exposure ages of faulted fluvial terraces using cosmogenic nuclides from the North Lunggar rift. The Lunggar rift is one of seven major north-striking rift basins accommodating east-west directed extension on the Tibetan Plateau. The Lunggar rift in west-central Tibet is divided into two distinct north and south segments based on fault geometry. The North Lunggar range is bounded on its east side by a <40 degree dipping, ~N-striking normal fault. This normal fault is considered inactive as the main detachment is unconformably overlain by unfaulted moraines and alluvial fans. Farther into the hanging wall basin, approximately 6 km eastward, several fault scarps parallel the Lunggar detachment. Locally, active faulting is distributed in the hanging wall with as many as seven normal fault scarps accommodating active east-west directed extension. Recent activity of these smaller faults is apparent from cross-cut fluvial terraces that have been uplifted by as much as 75 m. The geomorphology and fault geometry of the North Lunggar rift are consistent with high-angle normal faults that sole into a single master detachment fault at depth. A high-resolution digital elevation model constructed from real-time kinematic-GPS data has made details of the geomorphology clear and allowed for precise measurements of geomorphic offsets across the fault scarps. We estimate the surface abandonment ages using the depth profiling approach with cosmogenic nuclides. Three cosmogenic depth profiles are being analyzed in this study with each depth profile consisting of five samples at varying depths in order to account for inheritance. Site 1 is the southernmost and is on the highest uplifted fluvial terrace and is being prepared for 10Be analysis. Site 2 comprises two depth profiles on the highest and intermediate uplifted terraces, respectively. Samples at site 2 have low quartz yields and are being prepared for 36Cl analysis. Combining the fault offsets and

  8. Dynamic magmatic processes at a continental rift caldera, observed using satellite geodesy

    NASA Astrophysics Data System (ADS)

    Lloyd, Ryan; Biggs, Juliet; Birhanu, Yelebe; Wilks, Matt; Gottsmann, Jo; Kendall, Mike; Lewi, Elias

    2016-04-01

    Large silicic calderas are a key feature of developing continental rifts, such as the Main Ethiopian Rift (MER), and are often observed to be deforming. Corbetti is one such example of a Holocene caldera in the MER that is undergoing deformation. However, the cause of the unrest, and the relationship to rift processes such as magma storage, transport and extension remain poorly understood. To investigate, we use InSAR (ascending and descending Cosmo-SkyMed data) and continuous GPS to observe the temporal and spatial evolution of sustained uplift at the Corbetti Caldera. Within the caldera, which was thought to have formed ~200 ka, there is evidence for numerous periods of resurgent volcanism in the form of plinian eruptions as well as effusive obsidian flows. How the sources of these varying styles of volcanism are reconciled at depth and in time is currently poorly constrained. Previous research has shown that pre-rift structures have a significant influence on the strain field, and hence on the magmatic and hydrothermal processes which drive it. The Cosmo-SkyMed data used in this study was specifically chosen such that each ascending image has a corresponding descending image acquired as contemporaneously as possible. This is necessary, given the rate of uplift, so as to reduce the number of assumptions when constructing time-series from multiple look directions, and when incorporating GPS data. We decompose the ascending and descending line-of-site deformation signals into vertical and east-west components and use finite source modeling to constrain the depth and geometry of the source of deformation. These results are then compared to available seismic, dynamic microgravity and magnetotelluric data to better understand this system, and how it is related to the volcanic hazard and local geothermal resources.

  9. Tectono-sedimentary processes at hyper-extended rifted margins: the Alpine and Pyrenean analogues

    NASA Astrophysics Data System (ADS)

    Masini, E.; Manatschal, G.; Mohn, G.; Tugend, J.

    2012-04-01

    The discovery of hyper-extended crust at deep-water rifted margins challenged the way of interpreting rifting and continental breakup. Indeed, syn-rift sedimentary basins, also referred to as either "supra-detachment basins" or "hyper-extended sag basins" are observed over thin hyper-extended crust that tapers oceanwards and is laterally replaced by exhumed subcontinental mantle. Studies performed off- and on-shore have shown that hyper-extended domains are formed by a complex interplay of exhumation faults leading to characteristic basin architectures. Despite of numerous interpretations of seismic sections across passive margins, the stratigraphic architecture of hyper-extended rift domains and their tectono-sedimentary evolution remain little understood. In this presentation, we present field-data from two well-preserved fossil hyper-extended domains exposed in the Alps and Pyrenees analogues that show some insights into the tectono-sedimentary evolution of hyper-extended rifted margins. Despite these two examples show a different sedimentary evolution, they exemplify how detachment systems can control accommodation space and sedimentary architecture. In both cases, the syn-rift sedimentary record can be studied in 3D allowing to evolutionary steps of supra-detachment basins to be identified. The study of these two field examples enables to identify key architectural elements of supra-detachment basins. These are detachment breakaway block and extensional allochthons, both controlling the first-order architecture of the basins. The former is the consequence of two successive low-angle detachment faults separated by a delaminated upper crustal block, whereas the latter results from the delamination of hanging-wall blocks and its emplacement over exhumed footwall. Exhumation along active detachment systems implies a very efficient creation of new real estate seafloor and results in a very specific stratigraphic record that includes from base to top: 1) a syn

  10. 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.

  11. Geodynamic significance of the TRM segment in the East African Rift: active tectonics and paleostress in western Tanzania

    NASA Astrophysics Data System (ADS)

    Delvaux, D.; Kervyn, F.; Macheyeki, A. S.; Temu, E. B.

    2012-04-01

    The Tanganyika-Rukwa-Malawi (TRM) rift segment in western Tanzania is a key sector for understanding the opening dynamics of the East African rift system (EARS). In an oblique opening model, it is considered as a dextral transfer fault zone that accommodates the general opening of the EARS in a NW-SE direction. In an orthogonal opening model, it accommodates pure dip-slip normal faulting with extension orthogonal to the rift segments and a general E-W extension for the entire EARS. We investigated the active tectonic architecture and paleostress evolution of the Ufipa plateau and adjacent Rukwa basin and in order to define their geodynamic role in the development of the EARS and highlight their pre-rift brittle tectonic history. The active fault architecture, fault-kinematic analysis and paleostress reconstruction show that the recent to active fault systems that control the rift structure develop in a pure extensional setting with extension direction orthogonal to the trend of the TRM segment. Two pre-rift brittle events are evidenced. An older brittle thrusting is related to the interaction between the Bangweulu block and the Tanzanian craton during the late Pan-African (early Paleozoic). It was followed by a transpressional inversion during the early Mesozoic. This inversion stage caused dextral strike-slip faulting along the fault systems that now control the major rift structures. It has been erroneously interpreted as related to the late Cenozoic EARS which instead is characterized by pure normal faulting.

  12. The formation of active protoclusters in the Aquila rift: a millimeter continuum view

    NASA Astrophysics Data System (ADS)

    Maury, A. J.; André, P.; Men'shchikov, A.; Könyves, V.; Bontemps, S.

    2011-11-01

    While most stars are believed to form in stellar clusters, the formation and early evolution of young stellar clusters is still largely unknown. Improving our knowledge of the earliest phases of clustered star formation is crucial for understanding the origin of the stellar initial mass function and the efficiency of the star formation process, which both play a key role in the evolution of galaxies. Here, we present an analysis of the Aquila rift complex which addresses the questions of the star formation rate (SFR), star formation efficiency (SFE) and typical lifetime of the Class 0 protostellar phase in two nearby cluster-forming clumps: the Serpens South and W40 protoclusters. We carried out a 1.2 mm dust continuum mapping of the Aquila rift complex with the MAMBO bolometer array on the IRAM 30 m telescope. Using a multi-scale source extraction method, we perform a systematic source extraction in our millimeter continuum map. Based on complementary data from the Herschel Gould Belt survey and Spitzer maps, we characterize the spectral energy distributions (SEDs) of the 77 mm continuum sources detected with MAMBO and estimate their evolutionary stages. Taking advantage of the comprehensive dataset available for the Serpens South region, spanning wavelengths from 2 μm to 1.2 mm, we estimate the numbers of young stellar objects (YSOs) at different evolutionary stages and find a ratio of Class 0 to Class I protostars N(0)/N(I) = 0.19-0.27. This low ratio supports a scenario of relatively fast accretion at the beginning of the protostellar phase, and leads to a Class 0 lifetime of ~4-9 × 104 yr. We also show that both the Serpens South and W40 protoclusters are characterized by large fractions of protostars and high SFRs ~ 20-50M⊙ Myr-1 pc-2, in agreement with the idea that these two nearby clumps are active sites of clustered star formation currently undergoing bursts of star formation, and have the potential ability to form bound star clusters. While the

  13. The Cenozoic volcanism in the Kivu rift: Assessment of the tectonic setting, geochemistry, and geochronology of the volcanic activity in the South-Kivu and Virunga regions

    NASA Astrophysics Data System (ADS)

    Pouclet, A.; Bellon, H.; Bram, K.

    2016-09-01

    The Kivu rift is part of the western branch of the East African Rift system. From Lake Tanganyika to Lake Albert, the Kivu rift is set in a succession of Precambrian zones of weakness trending NW-SE, NNE-SSW and NE-SW. At the NW to NNE turn of the rift direction in the Lake Kivu area, the inherited faults are crosscut by newly born N-S fractures which developed during the late Cenozoic rifting and controlled the volcanic activity. From Lake Kivu to Lake Edward, the N-S faults show a right-lateral en echelon pattern. Development of tension gashes in the Virunga area indicates a clockwise rotation of the constraint linked to dextral oblique motion of crustal blocks. The extensional direction was W-E in the Mio-Pliocene and ENE-WSW in the Pleistocene to present time. The volcanic rocks are assigned to three groups: (1) tholeiites and sodic alkali basalts in the South-Kivu, (2) sodic basalts and nephelinites in the northern Lake Kivu and western Virunga, and (3) potassic basanites and potassic nephelinites in the Virunga area. South-Kivu magmas were generated by melting of spinel + garnet lherzolite from two sources: an enriched lithospheric source and a less enriched mixed lithospheric and asthenospheric source. The latter source was implied in the genesis of the tholeiitic lavas at the beginning of the South-Kivu tectono-volcanic activity, in relationships with asthenosphere upwelling. The ensuing outpouring of alkaline basaltic lavas from the lithospheric source attests for the abortion of the asthenospheric contribution and a change of the rifting process. The sodic nephelinites of the northern Lake Kivu originated from low partial melting of garnet peridotite of the sub-continental mantle due to pressure release during swell initiation. The Virunga potassic magmas resulted from the melting of garnet peridotite with an increasing degree of melting from nephelinite to basanite. They originated from a lithospheric source enriched in both K and Rb, suggesting the

  14. Volcanic activities in the Southern part of East African rift initiation: Melilitites and nephelinites from the Manyara Basin (North Tanzania rift axis)

    NASA Astrophysics Data System (ADS)

    Baudouin, Celine; Parat, Fleurice; Tiberi, Christel; Gautier, Stéphanie; Peyrat, Sophie

    2016-04-01

    The East African Rift exposes different stages of plate boundary extension, from the initiation of the rift (North (N) Tanzania) to oceanic accretion (Afar). The N Tanzania rift-axis (north-south (S) trend) is divided into 2 different volcanic and seismic activities: (1) the Natron basin (N) with shallow seismicity and intense volcanism and (2) the Manyara basin (S) with deep crustal earthquakes and sparse volcanism. The Natron basin is characterized by extinct volcanoes (2 Ma-0.75 Ma) and active volcano (Oldoinyo Lengai) and a link between seismicity and volcanism has been observed during the Oldoinyo Lengai crisis in 2007. In the S part of the N Tanzanian rift, volcanoes erupted in the Manyara basin between 0.4 and 0.9 Ma. In this study, we used geochemical signature of magmas and deep fluids that percolate into the lithosphere beneath Manyara basin, to define the compositions of magmas and fluids at depth beneath the S part of the N Tanzania rift, compare to the Natron basin and place constrain on the volcanic and seismic activities. The Manyara basin has distinct volcanic activities with mafic magmas as melilitites (Labait) and Mg-nephelinites (carbonatite, Kwaraha), and more differentiated magmas as Mg-poor nephelinites (Hanang). Melilitites and Mg-nephelinites are primary magmas with olivine, clinopyroxene (cpx), and phlogopite recording high-pressure crystallization environment, (melilitites >4 GPa and Mg-nephelinites>1 GPa) with high volatile contents (whole rock: 0.7-4.6 wt% CO2, 0.1-0.3 wt% F and 0.1 wt% Cl). FTIR analyses of olivine constrained the water content of Labait and Kwaraha magmas at 0.1 and 0.4 wt% H2O, respectively. Geochemical modelling suggests that mafic magmas result from a low degree of partial melting (1-2%) of a peridotitic source with garnet and phlogopite (high Tb/Yb (>0.6) and Rb/Sr (0.03-0.12) ratio). Mg-poor nephelinites from Hanang volcano crystallized cpx, Ti-garnet, and nepheline as phenocrysts. Magmas result from fractional

  15. Incipient continental rifting: Insights from the Okavango Rift Zone, northwestern Botswana

    NASA Astrophysics Data System (ADS)

    Kinabo, Baraka Damas

    In this dissertation aeromagnetic, gravity, and Shuttle Radar Topography Mission Digital Elevation Model (SRTM DEM) data from the Okavango Rift Zone in northwest Botswana are used to map the distribution of rift and basement structures. The distribution of these structures provide useful insights into the early stages of continental rifting. The objectives of this study are (1) assessing the role of pre-existing structures on rift basin development, (2) characterizing the geometry of the nascent rift basins, (3) documenting fault growth and propagation patterns, and (4) investigating the border fault development. Potential field data especially aeromagnetic data are used to map out structures in the sediment covered basement, whereas SRTM DEM data express the surface morphology of the structures. The azimuth of rift faults parallel the orientation of the fold axes and the prominent foliation directions of the basement rocks. This indicates that pre-existing structures in the basement influenced the development of the rift structures. NE dipping faults consistently exhibit greater displacements than SE dipping faults, suggesting a developing half-graben geometry. Individual faults grow by along axis linkage of small segments that develop from soft linkage (under lapping to overlapping segments) to hard linkage (hooking, fused segments). Major rifts faults are also linking through transfer zones by the process of "fault piracy" to establish an immature border fault system. The relationships between scam heights and vertical throws reveal that the young and active faults are located outside the rift while the faults with no recent activities are in the middle suggesting that the rift is also growing in width. This study demonstrates the utility of potential field data and SRTM DEM to provide a 3-D view of incipient continental rifting processes such as fault growth and propagation.

  16. Rheological variations across an active rift system -- results from lithosphere-scale 3D gravity and thermal models of the Kenya Rift

    NASA Astrophysics Data System (ADS)

    Meeßen, Christian; Sippel, Judith; Cacace, Mauro; Scheck-Wenderoth, Magdalena; Fishwick, Stewart; Heine, Christian; Strecker, Manfred R.

    2015-04-01

    Due to its tectono-volcanic activity and economic (geothermal and petroleum) potential, the eastern branch of the East African Rift System (EARS) is one of the best studied extensional systems worldwide and an important natural laboratory for the development of geodynamic concepts on rifting and nascent continental break-up. The Kenya Rift, an integral part of the eastern branch of the EARS, has formed in the area of weak Proterozoic crust of the Mozambique mobile belt adjacent to the rheologically stronger Archean Tanzania craton. To assess the variations in lithospheric strength between different tectonic domains and their influence on the tectonic evolution of the region, we developed a set of structural, density, thermal and rheological 3D models. For these models we integrated multi-disciplinary information, such as published geological field data, sediment thicknesses, well information, existing structural models, seismic refraction and reflection data, seismic tomography, gravity and heat-flow data. Our main approach focused on combined 3D isostatic and gravity modelling. The resulting lithosphere-scale 3D density model provides new insights into the depth distribution of the crust-mantle boundary and thickness variations of different crustal density domains. The latter further facilitate interpretations of variations of lithologies and related physical rock properties. By considering lithology-dependent heat production and thermal conductivity, we calculate the conductive thermal field across the region of the greater Kenya Rift. Finally, the assessed variations in lithology and temperature allow deriving differences in the integrated strength of the lithosphere across the different tectonic domains.

  17. Distribution of deformation on an active normal fault network, NW Corinth Rift

    NASA Astrophysics Data System (ADS)

    Ford, Mary; Meyer, Nicolas; Boiselet, Aurélien; Lambotte, Sophie; Scotti, Oona; Lyon-Caen, Hélène; Briole, Pierre; Caumon, Guillaume; Bernard, Pascal

    2013-04-01

    Over the last 20-25 years, geodetic measurements across the Gulf of Corinth have recorded high extension rates varying from 1.1 cm/a in the east to a maximum of 1.6 cm/a in the west. Geodetic studies also show that current deformation is confined between two relatively rigid blocks defined as Central Greece (to the north) and the Peloponnesus to the south. Active north dipping faults (<1 Ma) define the south coast of the subsiding Gulf, while high seismicity (major earthquakes and micro-seismicity) is concentrated at depth below and to the north of the westernmost Gulf. How is this intense deformation distributed in the upper crust? Our objectives here are (1) to propose two models for the distribution of deformation in the upper crust in the westernmost rift since 1 Ma, and (2) to place the tectonic behaviour of the western Gulf in the context of longer term rift evolution. Over 20 major active normal faults have been identified in the CRL area based specific characteristics (capable of generating earthquakes M> 5.5, active in the last 1 M yrs, slip rate >0.5 mm/a). Because of the uncertainty related to fault geometry at depth two models for 3D fault network geometry in the western rift down to 10 km were constructed using all available geophysical and geological data. The first model assumes planar fault geometries while the second uses listric geometries for major faults. A model for the distribution of geodetically-defined extension on faults is constructed along five NNE-SSW cross sections using a variety of data and timescales. We assume that the role of smaller faults in accommodating deformation is negligible so that extension is fully accommodated on the identified major faults. Uncertainties and implications are discussed. These models provide estimates of slip rate for each fault that can be used in seismic hazard models. A compilation of onshore and offshore data shows that the western Gulf is the youngest part of the Corinth rift having initiated

  18. 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

    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

  19. Detection of Rift Valley fever viral activity in Kenya by satellite remote sensing imagery

    NASA Technical Reports Server (NTRS)

    Linthicum, Kenneth J.; Bailey, Charles L.; Davies, F. Glyn; Tucker, Compton J.

    1987-01-01

    Data from the advanced very high resolution radiometer on board the National Oceanic and Atmospheric Administration's polar-orbiting meteorological satellites have been used to infer ecological parameters associated with Rift Valley fever (RVF) viral activity in Kenya. An indicator of potential viral activity was produced from satellite data for two different ecological regions in Kenya, where RVF is enzootic. The correlation between the satellite-derived green vegetation index and the ecological parameters associated with RVF virus suggested that satellite data may become a forecasting tool for RVF in Kenya and, perhaps, in other areas of sub-Saharan Africa.

  20. Coastal and submarine instabilities distribution in the tectonically active SW margin of the Corinth Rift (Psathopyrgos, Achaia, Greece)

    NASA Astrophysics Data System (ADS)

    Simou, Eirini; Papanikolaou, Dimitrios; Lykousis, Vasilios; Nomikou, Paraskevi; Vassilakis, Emmanuel

    2014-05-01

    The Corinth Rift, one of the most active rifts in the world as local extension trending NE-SW reaches the amount of 14±2 mm/yr, corresponds to one of the largest zones of seismically active normal faulting. The formation, growth and migration southwards of the prevailing fault systems, which evolve simultaneously with the intense morphogenetic processes, are overprinted in the age, facies and thickness of the Plio-Pleistocene sequences constructing the south margin of the western Gulf of Corinth. The dominant fault blocks, defined by east-west trending, north dipping normal faults, are accompanied by several morphological features and anomalies, noticed in both the terrestrial and the marine environment. Our main aim has been to examine how the tectonic evolution, in combination with the attendant fierce erosional and sedimentary processes, has affected the morphology through geodynamic processes expressed as failures in the wider coastal area. High resolution multibeam bathymetry in combination with the available land surface data have contributed to submarine and subaerial morphological mapping. These have been used as a basis for the detection of all those geomorphic features that indicate instabilities probably triggered, directly or indirectly, by the ongoing active tectonic deformation. The interpretation of the combined datasets shows that the southwestern margin of the Corinth Rift towards Psathopyrgos fault zone is characterized by intense coastal relief and a narrow, almost absent, continental shelf, which passes abruptly to steep submarine slopes. These steep slope values denote the effects of the most recent brittle deformation and are related to coastal and submarine instabilities and failures. High uplift rates and rapid sedimentation, indicative of the regional high-energy terrestrial and submarine environment, are subsequently balanced by the transportation of the seafloor currents, especially where slope gradients decrease, disintegrating the

  1. Rift Valley fever dynamics in Senegal: a project for pro-active adaptation and improvement of livestock raising management.

    PubMed

    Lafaye, Murielle; Sall, Baba; Ndiaye, Youssou; Vignolles, Cecile; Tourre, Yves M; Borchi, Franc Ois; Soubeyroux, Jean-Michel; Diallo, Mawlouth; Dia, Ibrahima; Ba, Yamar; Faye, Abdoulaye; Ba, Taibou; Ka, Alioune; Ndione, Jacques-André; Gauthier, Hélène; Lacaux, Jean-Pierre

    2013-11-01

    The multi-disciplinary French project "Adaptation à la Fiévre de la Vallée du Rift" (AdaptFVR) has concluded a 10-year constructive interaction between many scientists/partners involved with the Rift Valley fever (RVF) dynamics in Senegal. The three targeted objectives reached were (i) to produce--in near real-time--validated risk maps for parked livestock exposed to RVF mosquitoes/vectors bites; (ii) to assess the impacts on RVF vectors from climate variability at different time-scales including climate change; and (iii) to isolate processes improving local livestock management and animal health. Based on these results, concrete, pro-active adaptive actions were taken on site, which led to the establishment of a RVF early warning system (RVFews). Bulletins were released in a timely fashion during the project, tested and validated in close collaboration with the local populations, i.e. the primary users. Among the strategic, adaptive methods developed, conducted and evaluated in terms of cost/benefit analyses are the larvicide campaigns and the coupled bio-mathematical (hydrological and entomological) model technologies, which are being transferred to the staff of the "Centre de Suivi Ecologique" (CSE) in Dakar during 2013. Based on the results from the AdaptFVR project, other projects with similar conceptual and modelling approaches are currently being implemented, e.g. for urban and rural malaria and dengue in the French Antilles. PMID:24258902

  2. Rift Valley fever dynamics in Senegal: a project for pro-active adaptation and improvement of livestock raising management.

    PubMed

    Lafaye, Murielle; Sall, Baba; Ndiaye, Youssou; Vignolles, Cecile; Tourre, Yves M; Borchi, Franc Ois; Soubeyroux, Jean-Michel; Diallo, Mawlouth; Dia, Ibrahima; Ba, Yamar; Faye, Abdoulaye; Ba, Taibou; Ka, Alioune; Ndione, Jacques-André; Gauthier, Hélène; Lacaux, Jean-Pierre

    2013-11-01

    The multi-disciplinary French project "Adaptation à la Fiévre de la Vallée du Rift" (AdaptFVR) has concluded a 10-year constructive interaction between many scientists/partners involved with the Rift Valley fever (RVF) dynamics in Senegal. The three targeted objectives reached were (i) to produce--in near real-time--validated risk maps for parked livestock exposed to RVF mosquitoes/vectors bites; (ii) to assess the impacts on RVF vectors from climate variability at different time-scales including climate change; and (iii) to isolate processes improving local livestock management and animal health. Based on these results, concrete, pro-active adaptive actions were taken on site, which led to the establishment of a RVF early warning system (RVFews). Bulletins were released in a timely fashion during the project, tested and validated in close collaboration with the local populations, i.e. the primary users. Among the strategic, adaptive methods developed, conducted and evaluated in terms of cost/benefit analyses are the larvicide campaigns and the coupled bio-mathematical (hydrological and entomological) model technologies, which are being transferred to the staff of the "Centre de Suivi Ecologique" (CSE) in Dakar during 2013. Based on the results from the AdaptFVR project, other projects with similar conceptual and modelling approaches are currently being implemented, e.g. for urban and rural malaria and dengue in the French Antilles.

  3. Non-extensivity and complexity in the earthquake activity at the West Corinth rift (Greece)

    NASA Astrophysics Data System (ADS)

    Michas, Georgios; Vallianatos, Filippos; Sammonds, Peter

    2013-04-01

    Earthquakes exhibit complex phenomenology that is revealed from the fractal structure in space, time and magnitude. For that reason other tools rather than the simple Poissonian statistics seem more appropriate to describe the statistical properties of the phenomenon. Here we use Non-Extensive Statistical Physics [NESP] to investigate the inter-event time distribution of the earthquake activity at the west Corinth rift (central Greece). This area is one of the most seismotectonically active areas in Europe, with an important continental N-S extension and high seismicity rates. NESP concept refers to the non-additive Tsallis entropy Sq that includes Boltzmann-Gibbs entropy as a particular case. This concept has been successfully used for the analysis of a variety of complex dynamic systems including earthquakes, where fractality and long-range interactions are important. The analysis indicates that the cumulative inter-event time distribution can be successfully described with NESP, implying the complexity that characterizes the temporal occurrences of earthquakes. Further on, we use the Tsallis entropy (Sq) and the Fischer Information Measure (FIM) to investigate the complexity that characterizes the inter-event time distribution through different time windows along the evolution of the seismic activity at the West Corinth rift. The results of this analysis reveal a different level of organization and clusterization of the seismic activity in time. Acknowledgments. GM wish to acknowledge the partial support of the Greek State Scholarships Foundation (IKY).

  4. Style of rifting and the stages of Pangea breakup

    NASA Astrophysics Data System (ADS)

    Frizon de Lamotte, Dominique; Fourdan, Brendan; Leleu, Sophie; Leparmentier, François; Clarens, Philippe

    2015-05-01

    Pangea results from the progressive amalgamation of continental blocks achieved at 320 Ma. Assuming that the ancient concept of "active" versus "passive" rifting remains pertinent as end-members of more complex processes, we show that the progressive Pangea breakup occurred through a succession of rifting episodes characterized by different tectonic evolutions. A first episode of passive continental rifting during the Upper Carboniferous and Permian led to the formation of the Neo-Tethys Ocean. Then at the beginning of Triassic times, two short episodes of active rifting associated to the Siberian and Emeishan large igneous provinces (LIPs) failed. The true disintegration of Pangea resulted from (1) a Triassic passive rifting leading to the emplacement of the central Atlantic magmatic province (200 Ma) LIP and the subsequent opening of the central Atlantic Ocean during the lowermost Jurassic and from (2) a Lower Jurassic active rifting triggered by the Karoo-Ferrar LIP (183 Ma), which led to the opening of the West Indian Ocean. The same sequence of passive then active rifting is observed during the Lower Cretaceous with, in between, the Parana-Etendeka LIP at 135 Ma. We show that the relationships between the style of rifts and their breakdown or with the type of resulting margins (as magma poor or magma dominated) are not straightforward. Finally, we discuss the respective role of mantle global warming promoted by continental agglomeration and mantle plumes in the weakening of the continental lithosphere and their roles as rifting triggers.

  5. Tectonomorphic evolution of Marie Byrd Land - Implications for Cenozoic rifting activity and onset of West Antarctic glaciation

    NASA Astrophysics Data System (ADS)

    Spiegel, Cornelia; Lindow, Julia; Kamp, Peter J. J.; Meisel, Ove; Mukasa, Samuel; Lisker, Frank; Kuhn, Gerhard; Gohl, Karsten

    2016-10-01

    The West Antarctic Rift System is one of the largest continental rifts on Earth. Because it is obscured by the West Antarctic Ice Sheet, its evolution is still poorly understood. Here we present the first low-temperature thermochronology data from eastern Marie Byrd Land, an area that stretches ~ 1000 km along the rift system, in order to shed light on its development. Furthermore, we petrographically analysed glacially transported detritus deposited in the marine realm, offshore Marie Byrd Land, to augment the data available from the limited terrestrial exposures. Our data provide information about the subglacial geology, and the tectonic and morphologic history of the rift system. Dominant lithologies of coastal Marie Byrd Land are igneous rocks that intruded (presumably early Paleozoic) low-grade meta-sedimentary rocks. No evidence was found for un-metamorphosed sedimentary rocks exposed beneath the ice. According to the thermochronology data, rifting occurred in two episodes. The earlier occurred between ~ 100 and 60 Ma and led to widespread tectonic denudation and block faulting over large areas of Marie Byrd Land. The later episode started during the Early Oligocene and was confined to western Pine Island Bay area. This Oligocene tectonic activity may be linked kinematically to previously described rift structures reaching into Bellingshausen Sea and beneath Pine Island Glacier, all assumed to be of Cenozoic age. However, our data provide the first direct evidence for Cenozoic tectonic activity along the rift system outside the Ross Sea area. Furthermore, we tentatively suggest that uplift of the Marie Byrd Land dome only started at ~ 20 Ma; that is, nearly 10 Ma later than previously assumed. The Marie Byrd Land dome is the only extensive part of continental West Antarctica elevated above sea level. Since the formation of a continental ice sheet requires a significant area of emergent land, our data, although only based on few samples, imply that extensive

  6. A numerical modelling approach to investigate the surface processes response to normal fault growth in multi-rift settings

    NASA Astrophysics Data System (ADS)

    Pechlivanidou, Sofia; Cowie, Patience; Finch, Emma; Gawthorpe, Robert; Attal, Mikael

    2016-04-01

    This study uses a numerical modelling approach to explore structural controls on erosional/depositional systems within rifts that are characterized by complex multiphase extensional histories. Multiphase-rift related topography is generated by a 3D discrete element model (Finch et al., Basin Res., 2004) of normal fault growth and is used to drive the landscape evolution model CHILD (Tucker et al., Comput. Geosci., 2001). Fault populations develop spontaneously in the discrete element model and grow by both tip propagation and segment linkage. We conduct a series of experiments to simulate the evolution of the landscape (55x40 km) produced by two extensional phases that differ in the direction and in the amount of extension. In order to isolate the effects of fault propagation on the drainage network development, we conduct experiments where uplift/subsidence rates vary both in space and time as the fault array evolves and compare these results with experiments using a fixed fault array geometry with uplift rate/subsidence rates that vary only spatially. In many cases, areas of sediment deposition become uplifted and vise-versa due to complex elevation changes with respect to sea level as the fault array develops. These changes from subaerial (erosional) to submarine (depositional) processes have implications for sediment volumes and sediment caliber as well as for the sediment routing systems across the rift. We also explore the consequences of changing the angle between the two phases of extension on the depositional systems and we make a comparison with single-phase rift systems. Finally, we discuss the controls of different erodibilities on sediment supply and detachment-limited versus transport-limited end-member models for river erosion. Our results provide insights into the nature and distribution of sediment source areas and the sediment routing in rift systems where pre-existing rift topography and normal fault growth exert a fundamental control on

  7. 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

  8. Impact of gravity processes on the initial post-rift stages of construction and evolution of a continental margin: Insights from the eastern Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Baurion, Celine; Gorini, Christian; Leroy, Sylvie; Migeon, Sebastien; Lucazeau, Francis; Bache, Francois; Zaragosi, Sebastien; Smit, Jeroen; Al-Toubi, Khalfan; dos Reis, Antonio

    2013-04-01

    The study of the post-rift sediment architecture and continental slope morphology leads to a reconstruction of the initial stages of formation and evolution of gravity-driven processes on the northern margin of the eastern Gulf of Aden. The slope-related features and associated deposits in the deep basin along this young passive margin are investigated through the analysis of a set of seismic-reflection and multibeam bathymetry data. This study demonstrates how preconditioning and triggering factors (tectonics, climate and eustatic variations) can interact and control the margin morphology and post-rift sediment architecture in a source-to-sink perspective. The combined geomorphological and stratigraphic study of this margin allows us to identify three morphological domains inherited from the structural segmentation. The monsoon climate combined with a major eustatic lowstand is proposed as the most likely set of factors preconditioning slope destabilisation on the whole margin. These factors also enhance the effect of the late post-rift uplift of the eastern morphological domain of the studied margin. The formation and distribution of the slope-related features are thus mainly controlled by active faults on the continental slope and the potential effect of bottom currents at the base of the continental slope. The oversteepening of the continental slope in the eastern domain of the studied margin is probably the main triggering factor controlling the generation of failure processes and subsequent canyon formation by upslope erosion. The analysis of canyon location and morphology along the uplifted part of the continental slope reveals the long-term influence of secondary slope-related features, contour currents and turbidite flows on the development of canyons. As a consequence of the late post-rift uplift that only affected the eastern part of the studied margin, huge volumes of sediment were accumulated in mass-transport complexes at the foot of numerous slope

  9. The deep structure of the South Atlantic rifted margins and the implications of the magmatic processes for the break-up

    NASA Astrophysics Data System (ADS)

    Becker, Katharina; Dieter, Franke; Trumbull, Robert; Schnabel, Michael; Heyde, Ingo; Schreckenberger, Bernd; Koopmann, Hannes; Bauer, Klaus; Jokat, Wilfried; Krawczyk, Charlotte

    2014-05-01

    The high velocity lower crust HVLC (Vp > 7km/s) together with seaward dipping reflectors (SDRs) and continental flood basalts are specific characteristics of volcanic rifted margins. The nature and origin of the HVLC is still under discussion. Here we provide a comprehensive study of the deep crustal structure of the South Atlantic rifted margins in which we focus on variations in the distribution and size of HVLC bodies along and across the margins. Two new and five existing refraction lines complemented by gravity models cover the area between the Rio Grande Rise - Walvis Ridge to the Falkland Agulhas Fracture Zone. Three seismic lines on the South American margin outline the change from a non-magmatic margin (lacking seaward dipping reflectors) in the south to a well-developed volcanic rifted margin off Uruguay in the north. While the HVLC exhibit a consistent increase in the cross-sectional area along both margins from South to North, we observe a major asymmetry across the margins. The African margin has about two-three times thicker and four times more voluminous HVLC than the South American margin. Importantly, the erupted lavas in the Etendeka-Paraná Provinces show the opposite asymmetry. Also the spatial position of the HVLC with regard to the inner SDRs varies consistently along both margins. Close to the Falkland Agulhas Fracture zone a small body of HVLC is not accompanied by seaward dipping reflectors. In the central segment, HVLC is centered under the SDRs inner wedge but in the north, HVLC also extends further seawards. These observations question a simple extrusive/intrusive relationship between SDRs and HVLC, and they imply differences in the timing of the HVLC formation during the rifting and break-up process. We conclude that the HVLC is predominantly a magmatic feature related mantle melting during break-up. Melt generation models suggest that the greater thickness of HVLC on the African margin is due to active upwelling combined with elevated

  10. Morphotectonics of the Tunka rift and its bordering mountains in the Baikal rift system, Russia

    NASA Astrophysics Data System (ADS)

    Shchetnikov, Alexander

    2016-11-01

    The Tunka section of the Baikal rift system presents a uniform alternation of the following neostructural forms: tilted horsts and asymmetrical block uplifts on the northern flank; the central system of the rift valleys; and the arched uplift of the southern flank. This is a standard set of morphostructural elements for the Baikal rift system. The main morphological feature of the Tunka rift is the strong inclination of its floor, ranging from 900 m to 200 km in general elevation above Lake Baikal. Such traits of recent geodynamics as volcanism, thermal activity, and seismicity are also different from other parts of the rift zone. All of these features of the Tunka rift are related to the deep structure of the rift zone. The peculiarities of the neotectonic structure of the Tunka rift, which are clearly expressed morphologically as is typical of the Baikal rift system, as well as its unique features are in accordance with deep geodynamic processes of the region. On the other hand, the development of the rift basin structures of the southwestern area near Baikal is complicated by inversion deformations. Local uplifts followed by deformations of the basin sedimentary cover and inverted morphostructures expressed in relief are fixed against the background of the general subsidence of blocks of the pre-Cenozoic basement grabens. The Tunka rift has repeatedly experienced inversion deformations throughout its history. The last wave of such deformations involved the southwestern region near Baikal in the second half of the late Pleistocene. During the Quaternary, the positive component prevailed in the whole range of vertical movements of the inter-rift and interbasin blocks; since the late Neogene, these structures have experienced a slow but steady uplift, accompanied by their extension at the expense of the bordering basins. The remote influence of the India-Asia collision on the formation of the southwestern section of the Baikal rift system is very significant and

  11. Petrologic constraints on rift-zone processes - Results from episode 1 of the Puu Oo eruption of Kilauea volcano, Hawaii

    USGS Publications Warehouse

    Garcia, M.O.; Ho, R.A.; Rhodes, J.M.; Wolfe, E.W.

    1989-01-01

    The Puu Oo eruption in the middle of Kilauea volcano's east rift zone provides an excellent opportunity to utilize petrologic constraints to interpret rift-zone processes. Emplacement of a dike began 24 hours before the start of the eruption on 3 January 1983. Seismic and geodetic evidence indicates that the dike collided with a magma body in the rift zone. Most of the lava produced during the initial episode of the Puu Oo eruption is of hybrid composition, with petrographic and geochemical evidence of mixing magmas of highly evllved and more mafic compositions. Some olivine and plagioclase grains in the hybrid lavas show reverse zoning. Whole-rock compositional variations are linear even for normally compatible elements like Ni and Cr. Leastsquares mixing calculations yield good residuals for major and trace element analyses for magma mixing. Crystal fractionation calculations yield unsatisfactory residuals. The highly evolved magma is similar in composition to the lava from the 1977 eruption and, at one point, vents for these two eruptions are only 200 m apart. Possibly both the 1977 lava and the highly evolved component of the episode 1 Puu Oo lava were derived from a common body of rift-zone-stored magma. The more mafic mixing component may be represented by the most mafic lava from the January 1983 eruption; it shows no evidence of magma mixing. The dike that was intruded just prior to the start of the Puu Oo eruption may have acted as a hydraulic plunger causing mixing of the two rift-zone-stored magmas. ?? 1989 Springer-Verlag.

  12. Modelling Rift Valley fever (RVF) disease vector habitats using active and passive remote sensing systems

    NASA Technical Reports Server (NTRS)

    Ambrosia, Vincent G.; Linthicum, K. G.; Bailey, C. L.; Sebesta, P.

    1989-01-01

    The NASA Ames Ecosystem Science and Technology Branch and the U.S. Army Medical Research Institute of Infectious Diseases are conducting research to detect Rift Valley fever (RVF) vector habitats in eastern Africa using active and passive remote-sensing. The normalized difference vegetation index (NDVI) calculated from Landsat TM and SPOT data is used to characterize the vegetation common to the Aedes mosquito. Relationships have been found between the highest NDVI and the 'dambo' habitat areas near Riuru, Kenya on both wet and dry data. High NDVI values, when combined with the vegetation classifications, are clearly related to the areas of vector habitats. SAR data have been proposed for use during the rainy season when optical systems are of minimal use and the short frequency and duration of the optimum RVF mosquito habitat conditions necessitate rapid evaluation of the vegetation/moisture conditions; only then can disease potential be stemmed and eradication efforts initiated.

  13. Broad Spectrum Antiviral Activity of Favipiravir (T-705): Protection from Highly Lethal Inhalational Rift Valley Fever

    PubMed Central

    Caroline, Amy L.; Powell, Diana S.; Bethel, Laura M.; Oury, Tim D.; Reed, Douglas S.; Hartman, Amy L.

    2014-01-01

    Background Development of antiviral drugs that have broad-spectrum activity against a number of viral infections would be of significant benefit. Due to the evolution of resistance to currently licensed antiviral drugs, development of novel anti-influenza drugs is in progress, including Favipiravir (T-705), which is currently in human clinical trials. T-705 displays broad-spectrum in vitro activity against a number of viruses, including Rift Valley Fever virus (RVFV). RVF is an important neglected tropical disease that causes human, agricultural, and economic losses in endemic regions. RVF has the capacity to emerge in new locations and also presents a potential bioterrorism threat. In the current study, the in vivo efficacy of T-705 was evaluated in Wistar-Furth rats infected with the virulent ZH501 strain of RVFV by the aerosol route. Methodology/Principal Findings Wistar-Furth rats are highly susceptible to a rapidly lethal disease after parenteral or inhalational exposure to the pathogenic ZH501 strain of RVFV. In the current study, two experiments were performed: a dose-determination study and a delayed-treatment study. In both experiments, all untreated control rats succumbed to disease. Out of 72 total rats infected with RVFV and treated with T-705, only 6 succumbed to disease. The remaining 66 rats (92%) survived lethal infection with no significant weight loss or fever. The 6 treated rats that succumbed survived significantly longer before succumbing to encephalitic disease. Conclusions/Significance Currently, there are no licensed antiviral drugs for treating RVF. Here, T-705 showed remarkable efficacy in a highly lethal rat model of Rift Valley Fever, even when given up to 48 hours post-infection. This is the first study to show protection of rats infected with the pathogenic ZH501 strain of RVFV. Our data suggest that T-705 has potential to be a broad-spectrum antiviral drug. PMID:24722586

  14. New Geophysical Results About the Relationship Between the Reelfoot Rift and the Rifted Margin of Laurentia

    NASA Astrophysics Data System (ADS)

    Guo, L.; Keller, G. R.

    2010-12-01

    The Reelfoot rift beneath the northern Mississippi embayment is an intracratonic graben system, which formed Early Cambrian time as a result of continental breakup, and has been subsequently reactivated by compressional or tensional stresses related to plate tectonic interactions. It strikes northeastward into the continent, and is approximately perpendicular to the rifted margin of the Laurentia that is shaped by the southeast-striking Alabama-Oklahoma transform fault. The northern section of the rift near the town of New Madrid, Missouri, was the site of three great 1811-1812 earthquakes, and it remains the most seismically active area east of the Rocky Mountains. However, the southern end of the rift is obscure, and the relationship between the Reelfoot rift and the rifted margin of Laurentia remains disputed. We analyzed the gravity and magnetic database for the region using new data enhancement techniques to shed some light on this relationship. We analyzed a large area to assess the regional geological structure. Complete Bouguer gravity data and and total magnetic intensity (TMI) data were assembled and gridded on a regular grid with spacing of 2km, the TMI data were then reduced to the magnetic pole. Then the data were processed with standard techniques to attenuate the high-frequency noise, and we analyzed the regional and residual anomalies. Specially, we calculated the tilt-angle derivatives of the data. We then calculated the directional horizontal derivatives of the tilt-angle derivatives both along and perpendicular to the strike of the rift. The maps of these derivatives clearly delineate the boundaries of the edges of the Reelfoot rift, the leading edge of the Ouachita thrust belt and the margin of Laurentia. The results of the preliminary processing indicate that the southern end of the rift is near the leading edge of the Ouachita thrust belt, which produces a more curvilinear shape for the Laurentian margin than the very linear Alabama

  15. A methodology to track temporal dynamics and rainfall thresholds of landslide processes in the East African Rift

    NASA Astrophysics Data System (ADS)

    Monsieurs, Elise; Jacobs, Liesbet; Kervyn, François; Kirschbaum, Dalia; d'Oreye, Nicolas; Derauw, Dominique; Kervyn, Matthieu; Nobile, Adriano; Trefois, Philippe; Dewitte, Olivier

    2015-04-01

    thresholds that trigger and control the different types of landslide in this region of the East African Rift. A specific attention will be given to the landslide processes in relation to volcanic activity and earthquakes.

  16. The evolution of rifting process in the tectonic history of the Earth

    NASA Technical Reports Server (NTRS)

    Milanovsky, E. E.; Nikishin, A. M.

    1985-01-01

    The continental rifting is the response of the lithosphere to the oriented tension. The distribution of viscosity in the lithosphere plays an essential role during all stages of the rifting. The viscosity is a function of the temperature, the lithostatic pressure, the rock composition, the deformation rate and other factors. The temperature is the most important factor. The vertical section of continental lithosphere of the rift zone may be divided into the following layers: the upper crust, in which brittle deformation prevails; the medialcrust, in which the role of plastic deformation increases; the lower crust, in which plastic deformation prevails; and the uppermost plastic part of the mantle overlapping asthenosphere. The depth of the boundaries in the crust layers are mainly controlled by the temperature.

  17. Venus - Volcanism and rift formation in Beta Regio

    NASA Astrophysics Data System (ADS)

    Campbell, D. B.; Head, J. W.; Harmon, J. K.; Hine, A. A.

    1984-10-01

    A new high-resolution radar image of Beta Regio, a Venus highland area, confirms the presence of a major tectonic rift system and associated volcanic activity. The lack of identifiable impact craters, together with the apparent superposition of the Theia Mons volcanic structure on the rift system, suggest that at least some of the volcanic activity occurred in relatively recent geologic time. The presence of topographically similar highland areas elsewhere on Venus (Aphrodite Terra, Dali Chasma, and Diana Chasma) suggests that rifting and volcanism are significant processes on Venus.

  18. Late Quaternary Deformation along the North Wuitaishan Fault of the Shanxi Graben System: Active Intracontinental Rifting in North China

    NASA Astrophysics Data System (ADS)

    Corley, J.; Cochran, W. J.; Hinrichs, N.; Ding, R.; Zhang, S.; Gomez, F.

    2012-12-01

    The Shanxi rift system in north China is an intracontinental rift zone which has been active since the late Tertiary. and has produced many destructive earthquakes in recorded history. This area is of particular interest for earthquake research because of the high seismicity levels in an intraplate setting. The Shanxi rift system is composed of NNE-oriented en-echelon half-graben basins controlled by normal faults. This study focuses on the north Wutaishan fault, which bounds the Wutai Mountains and the Xingding basin, located in the northern part of the Shanxi rift system. Quaternary tectonism is investigated using remotely-sensed imagery for mapping of large tectonically-influenced landforms, field investigations for ground truth, and structural analyses. Initial neotectonic mapping utilized stereoscopic Corona satellite imagery to differentiate between fluvial and agricultural terraces; Cartosat-based DEMs were used to correct altitude measurements of terrace heights and to analyze streams and other landforms for morphometric analysis. Fluvial terraces are used to reconstruct paleo-stream profiles of the Yangyan River and nearby tributaries to determine mountain uplift rates inferred from fluvial incision, basin extension rates, and possible warping of the footwall basin block. Field work provided ground truth for fluvial terrace altitude, type of terrace, and thicknesses of alluvial and loess deposits. Another aspect of the study involves development of structural cross-section to relate fault slip to regional tectonic strain. Fault kinematic analysis of micro-fault features found in bedrock were used to assess the Quaternary stress field. Results of this study have implications in the understanding of earthquake recurrence intervals and basin evolution in the Shanxi rift system and more generally, can improve the understanding of spatial and temporal variations of seismic events in intraplate settings.

  19. Incipient Crustal Stretching across AN Active Collision Belt: the Case of the Siculo-Calabrian Rift Zone (central Mediterranean)

    NASA Astrophysics Data System (ADS)

    Catalano, S.; Tortorici, G.; Romagnoli, G.; Pavano, F.

    2012-12-01

    increase in the elevation of the marine terraces of the last 600 ka. In the two analysed crustal blocks, the crustal doming with the related basin collapses and the concurrent tectonic motion without any back-stop can be respectively related to the thermal anomalies and to the drag forces exerted by a hot Mantle flow, triggered by the opening of a vertical slab windows at the southern edge of the Calabrian arc. This process has favored the migration of hot Mantle material towards the sectors of larger rollback of the subduction hinge, producing the spectacular tectonic uplift and active magmatism of the region. The increase of the volume of the Mantle wedged in the subduction zone of the Calabrian arc has been also effective to accommodate the differential roll-back between the adjacent segments of Nubia Plate, thus replacing the motion along the pre-existing transform faults dissecting the orogenic belt. In this frame, the deformation of the Hyblean-Etnean and the Peloritani-Aeolian blocks heralded the development of the two Sicily branches of the SCRZ. The delayed crustal stretching responsible for the propagation of the rift zone across the orogenic belt can be framed in the dynamic of a still active convergent margin if interpreted as the result of the sliding of the crust along the Ionian flank of the Mantle flow.

  20. 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).

  1. Crustal rifting and magmatic underplating in the Izu-Ogasawara (Bonin) intra-oceanic arc detected by active source seismic studies

    NASA Astrophysics Data System (ADS)

    Takahashi, N.; Kodaira, S.; Yamashita, M.; Miura, S.; Sato, T.; No, T.; Tatsumi, Y.; Kaneda, Y.

    2009-12-01

    Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has carried out seismic experiments using a multichannel reflection system and ocean bottom seismographs (OBSs) in the Izu-Ogasawara (Bonin)-Mariana (IBM) arc region since 2002 to understand growth process of continental crust. The source was an airgun array with a total capacity of 12,000 cubic inches and the OBSs as the receiver were deployed with an interval of 5 km for all seismic refraction experiments. As the results, we obtained crustal structures across the whole IBM arc with an interval of 50 km and detected the structural characteristics showing the crustal growth process. The IBM arc is one of typical oceanic island arc, which crustal growth started from subduction of an oceanic crust beneath the other oceanic crust. The arc crust has developed through repeatedly magmatic accretion from subduction slab and backarc opening. The volcanism has activated in Eocene, Oligocene, Miocene and Quaternary (e.g., Taylor, 1992), however, these detailed locations of past volcanic arc has been remained as one of unknown issues. In addition, a role of crustal rifting for the crustal growth has also been still unknown issue yet. Our seismic structures show three rows of past volcanic arc crusts except current arc. A rear arc and a forearc side have one and two, respectively. The first one, which was already reported by Kodaira et al. (2008), distributes in northern side from 27 N of the rear arc region. The second one, which develops in the forearc region next to the recent volcanic front, distributes in whole of the Izu-Ogasawara arc having crustal variation along arc direction. Ones of them sometimes have thicker crust than that beneath current volcanic front and no clear topographic high. Last one in the forearc connects to the Ogasawara Ridge. However, thickest crust is not always located beneath these volcanic arcs. The initial rifting region like the northern end of the Mariana Trough and the Sumisu

  2. 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

  3. Rapid onset of narrowing and along-strike propagation of an intra-arc rift: The Taupo Rift, New Zealand

    NASA Astrophysics Data System (ADS)

    Villamor, P.; Berryman, K. R.; Ellis, S. M.; Schreurs, G.; Wallace, L. M.; Leonard, G.; Langridge, R. M.; Nairn, I. A.

    2012-12-01

    Intra-arc active continental rifting occurs within the Taupo Rift in the North Island, New Zealand. Based on geological and geophysical evidence, we show that the Taupo rift has narrowed via inward and eastward migration of faulting (asymmetric narrowing) and propagated southwards along its axis. This evolution has occurred at relatively high rates of ~25 km/Ma (narrowing), ~ 7 to 15 km/Ma (eastward migration), and ~ < 200 to 275 km/Ma (southward propagation; rates only for the last ~ 340 kyr). The initial onshore narrow rift width is likely to be an effect of a narrow propagating rift from offshore. While several process are likely to influence rapid evolution, we propose that the main control on further rapid narrowing appears to be the presence of large heterogeneities in the crust that enable concentration of deformation, such as large magma bodies of the volcanic arc of Hikurangi subduction margin. The presence of these magma bodies localises faulting. Once faulting is localised it propagates along strike from the heterogeneity into non volcanic segments of the rift, which causes generalised narrowing. Temporal and spatial correlation between voluminous volcanic eruptions and major active faulting migration supports this model. Eastward migration of faulting also follows the eastward migration of the volcanic arc and is likely related to slab rollback. Finally, we show that southward propagation of rifting is linked to southward migration of the Hikurangi plateau and occurs episodically aided by voluminous local volcanism. The detailed recent spatial and temporal evolution of continental rifting in the Taupo Rift reveals the early stages of continental break-up and demonstrates fast evolution of rifting when aided by large scale volcanic processes such as rhyolitic supereruptions.

  4. Discovery of sublacustrine hydrothermal activity and associated massive sulfides and hydrocarbons in the north Tanganyika trough, East African Rift

    SciTech Connect

    Tiercelin, J.J.; Mondeguer, A. ); Thouin, C. ); Kalala, T. )

    1989-11-01

    Massive sulfides and carbonate mineral deposits associated with sublacustrine thermal springs were recently discovered along the Zaire side of the north Tanganyika trough, western branch of the East African Rift. This hydrothermal activity, investigated by scuba diving at a maximum depth of 20 m, is located at the intersection of major north-south normal faults and northwest-southeast faults belonging to the Tanganyika-Rukwa-Malawi (TRM) strike-slip fault zone. The preliminary results presented here come from analyses of sulfide deposits, hydrothermal fluids, and associated hydrocarbons that result from geothermal activity in this part of the East African Rift filled by a thick pile of sediment, the north Tanganyika trough.

  5. Hawaii Rifts

    SciTech Connect

    Nicole Lautze

    2015-01-01

    Rifts mapped through reviewing the location of dikes and vents on the USGS 2007 Geologic Map of the State of Hawaii, as well as our assessment of topography, and, to a small extent, gravity data. Data is in shapefile format.

  6. Rift basins - Origin, history, and distribution

    NASA Technical Reports Server (NTRS)

    Burke, K. C.

    1985-01-01

    Rifts are elongate depressions overlying places where the lithosphere has ruptured in extension. Where filled with sediment they may contain exploitable quantities of oil and gas. Because rits form in a variety of tectonic settings, it is helpful to define the particular tectonic environment in which a specific rift or set of rifts has developed. A useful approach has been to relate that environment to the Wilson Cycle of the opening and the closing of oceans. This appreciation of tectonic setting can help in better understanding of the depositional, structural and thermal history of individual rift systems. The global distribution of rifts can also be related to tectonic environment. For example, rifts associated with continental rupture at a temporary still-stand of a continent over the mantle convective system (rifts like those active in East Africa today) can be distinguished from those associated with continental collision (rifts like the Cenozoic rifts of China).

  7. New insights into continental rifting from a damage rheology modeling

    NASA Astrophysics Data System (ADS)

    Lyakhovsky, Vladimir; Segev, Amit; Weinberger, Ram; Schattner, Uri

    2010-05-01

    Previous studies have discussed how tectonic processes could produce relative tension to initiate and propagate rift zones and estimated the magnitude of the rift-driving forces. Both analytic and semi-analytic models as well as numerical simulations assume that the tectonic force required to initiate rifting is available. However, Buck (2004, 2006) estimated the minimum tectonic force to allow passive rifting and concluded that the available forces are probably not large enough for rifting of thick and strong lithosphere in the absence of basaltic magmatism (the "Tectonic Force" Paradox). The integral of the yielding stress needed for rifting over the thickness of the normal or thicker continental lithosphere are well above the available tectonic forces and tectonic rifting cannot happen (Buck, 2006). This conclusion is based on the assumption that the tectonic stress has to overcome simultaneously the yielding stress over the whole lithosphere thickness and ignore gradual weakening of the brittle rocks under long-term loading. In this study we demonstrate that the rifting process under moderate tectonic stretching is feasible due to gradual weakening and "long-term memory" of the heavily fractured brittle rocks, which makes it significantly weaker than the surrounding intact rock. This process provides a possible solution for the tectonic force paradox. We address these questions utilizing 3-D lithosphere-scale numerical simulations of the plate motion and faulting process base on the damage mechanics. The 3-D modeled volume consists of three main lithospheric layers: an upper layer of weak sediments, middle layer of crystalline crust and lower layer of the lithosphere mantle. Results of the modeling demonstrate gradual formation of the rift zone in the continental lithosphere with the flat layered structure. Successive formation of the rift system and associated seismicity pattern strongly depend not only on the applied tectonic force, but also on the healing

  8. Evolution of Jurassic carbonate platforms in an active transtensional rift: High Atlas of Morocco

    SciTech Connect

    Crevello, P.D.; Warme, J.E.; Septfontaine, M.; Burke, R.B.

    1987-05-01

    Carbonate platforms developed during the Early and Middle Jurassic on the margins of two seaways that are now exposed in the Middle and High Atlas, Morocco. The seaways occupied transtensional rifts, which, by Early Jurassic, had formed the segmented terrain on which the carbonate platforms established. Two episodes of platform development occurred in the High Atlas, both recording the effects of tectonic, eustatic, and sedimentologic controls on platform trends, morphologies, and internal stratigraphy: Lias (Sinemurian to Domerian) and Lias-Dogger (Aalenian to Bajocian). Along the borders of Mesetas, platform margins became established over basin-bounding faults, whereas platforms within the seaway developed on pre-existing or active structural highs. Fault-scarp margins, olistolith and mega-breccia emplacement, and drowned or foundered margins record the tectonic contribution to shaping the margins. Platform interior stratigraphy contains third- and fourth-order depositional sequences that reflect multiple sea level fluctuations during a gradual sea level rise. Structural highs in the interior of the seaways, some remaining active during the Lias, were capped or fringed by platform carbonates or, in deeper water, were covered by sponge-algal buildups. Basin lows filled with turbidites and limestone-marl cycles. Carbonate alluvial fans and neptunian dikes are associated with syntectonic wrenching within the seaway. The Liassic platform was inundated by a Toarcian drowning event, with shale deposition occurring across the southern platform, within the basin, and, locally, condensed sequences forming on intra-basin highs. Lias-Dogger platforms ceased with the transition to continental sedimentation, marking the end of the high Atlas seaway.

  9. Fault Orientations at Obliquely Rifted Margins: Where? When? Why?

    NASA Astrophysics Data System (ADS)

    Brune, Sascha

    2015-04-01

    setup is very simple (horizontally layered, no inherited faults, constant extension velocity and direction), its evolution exhibits a variety of fault orientations that are solely caused by the three-dimensionality of oblique rift systems. Allowing new insights on fault patterns of the proximal and distal margins, the model shows that individual fault populations are activated in a characteristic multi-phase evolution driven by lateral density variations of the evolving rift system. Moreover, the model depicts strain partitioning between rift-parallel and rift-perpendicular far-field velocity components that are accommodated by strike-slip faults in the rift centre and normal faults at the rift sides, respectively. Oblique extensional systems worldwide differ in many aspects and clearly one suit of models cannot explain all rifted margin structures at the same time. However, the distinct pattern of fault populations discussed in this study and their sequence of activity compares very well to previous studies of the Gulf of Aden and holds implications for many other rifted margins worldwide. Note that in nature, the resulting stress and fault pattern will also be affected by inherited heterogeneities, surface processes, as well as melting and dyke dynamics.

  10. Geodynamic significance of the TRM segment in the East African Rift (W-Tanzania): Active tectonics and paleostress in the Ufipa plateau and Rukwa basin

    NASA Astrophysics Data System (ADS)

    Delvaux, D.; Kervyn, F.; Macheyeki, A. S.; Temu, E. B.

    2012-04-01

    The Tanganyika-Rukwa-Malawi (TRM) rift segment in western Tanzania is a key sector for understanding the opening dynamics of the East African rift system (EARS). In an oblique opening model, it is considered as a dextral transfer fault zone that accommodates the general opening of the EARS in an NW-SE direction. In an orthogonal opening model, it accommodates pure dip-slip normal faulting with extension orthogonal to the rift segments and a general E-W extension for the entire EARS. The central part of the TRM rift segment is well exposed in the Ufipa plateau and Rukwa basin, within the Paleoproterozoic Ubende belt. It is also one of the most seismically active regions of the EARS. We investigated the active tectonic architecture and paleostress evolution of the Ufipa plateau and adjacent Rukwa basin and in order to define their geodynamic role in the development of the EARS and highlight their pre-rift brittle tectonic history. The active fault architecture, fault-kinematic analysis and paleostress reconstruction show that the recent to active fault systems that control the rift structure develop in a pure extensional setting with extension direction orthogonal to the trend of the TRM segment. Two pre-rift brittle events are evidenced. An older brittle thrusting is related to the interaction between the Bangweulu block and the Tanzanian craton during the late Pan-African (early Paleozoic). It was followed by a transpressional inversion during the early Mesozoic. This inversion stage is the best expressed in the field and caused dextral strike-slip faulting along the fault systems that now control the major rift structures. It has been erroneously interpreted as related to the late Cenozoic EARS which instead is characterized by pure normal faulting (our third and last stress stage).

  11. A Geochemical Study of Magmatic Processes and Evolution along the Submarine Southwest Rift zone of Mauna Loa Volcano, Hawaii

    NASA Astrophysics Data System (ADS)

    Rhodes, J. M.; Garcia, M. O.; Weis, D.; Trusdell, F. A.; Vollinger, M. J.

    2003-12-01

    Mauna Loa's southwest rift zone (SWR) extends for 102 km from its summit caldera, at an elevation of 4,170 m above sea level, to submarine depths of over 4,500 m. About 65% of the rift zone is subaerial and 35% submarine. Recent sampling with the Jason II submersible of the `mile-high' (1800 m) Ka Lae submarine landslide scarp and the deepest section of the rift zone, in conjunction with previous submersible and dredge-haul collecting, provides petrological and geochemical understanding of rift zone processes, as well as a record of Mauna Loa's eruptive history extending back about 400 ka. The major and trace element trends of the submarine lavas are remarkably similar to those of historical and young prehistoric lavas (<31 ka) erupted along the subaerial SWR. We take this to imply that magma-forming processes have remained relatively constant over much of the volcano's recorded eruptive history. However, the distribution of samples along these trends has varied, and is correlated with elevation. There are very few picrites (>12% MgO) among the subaerial lavas, and compositions tend to cluster around 6.8-8.0% MgO. In contrast, picritic lavas are extremely abundant in the submarine samples, increasing in frequency with depth, especially below 1200 m. These observations support earlier interpretations that the submarine lavas are derived directly from deeper levels in the magma column, and that magmas from a shallow, steady-state, magma reservoir are of uncommon at these depths. Isotopic ratios of Pb and Sr in the submarine lavas, in conjunction with Nb/Y and Zr/Nb ratios, extend from values that are identical with subaerial historical Mauna Loa lavas to lavas with markedly lower 87Sr/86Sr and higher 206Pb/204Pb isotopic ratios. As yet, we see no correlation with depth or age, but the implications are that, in the past, the plume source of Mauna Loa magmas was more variable than in the last 31 ka, and contained a greater proportion of the Kea component. *Team members

  12. Active fault segments as potential earthquake sources: Inferences from integrated geophysical mapping of the Magadi fault system, southern Kenya Rift

    NASA Astrophysics Data System (ADS)

    Kuria, Z. N.; Woldai, T.; van der Meer, F. D.; Barongo, J. O.

    2010-06-01

    Southern Kenya Rift has been known as a region of high geodynamic activity expressed by recent volcanism, geothermal activity and high rate of seismicity. The active faults that host these activities have not been investigated to determine their subsurface geometry, faulting intensity and constituents (fluids, sediments) for proper characterization of tectonic rift extension. Two different models of extension direction (E-W to ESE-WNW and NW-SE) have been proposed. However, they were based on limited field data and lacked subsurface investigations. In this research, we delineated active fault zones from ASTER image draped on ASTER DEM, together with relocated earthquakes. Subsequently, we combined field geologic mapping, electrical resistivity, ground magnetic traverses and aeromagnetic data to investigate the subsurface character of the active faults. Our results from structural studies identified four fault sets of different age and deformational styles, namely: normal N-S; dextral NW-SE; strike slip ENE-WSW; and sinistral NE-SW. The previous studies did not recognize the existence of the sinistral oblique slip NE-SW trending faults which were created under an E-W extension to counterbalance the NW-SE faults. The E-W extension has also been confirmed from focal mechanism solutions of the swarm earthquakes, which are located where all the four fault sets intersect. Our findings therefore, bridge the existing gap in opinion on neo-tectonic extension of the rift suggested by the earlier authors. Our results from resistivity survey show that the southern faults are in filled with fluid (0.05 and 0.2 Ωm), whereas fault zones to the north contain high resistivity (55-75 Ωm) material. The ground magnetic survey results have revealed faulting activity within active fault zones that do not contain fluids. In addition, the 2D inversion of the four aero-magnetic profiles (209 km long) revealed: major vertical to sub vertical faults (dipping 75-85° east or west); an

  13. Ocean-continent-transition and oceanic ridge structural evolution (eastern Gulf of Aden): Implications for rift to seafloor spreading processes

    NASA Astrophysics Data System (ADS)

    D'Acremont, E.; Leroy, S. D.; Beslier, M.; Autin, J.; Watremez, L.; Maia, M. A.; Gente, P.

    2009-12-01

    The rifting between Arabia and Somalia, which started around 35 Ma ago, is followed by oceanic accretion from at least 17.6 Ma leading to the present Gulf of Aden. The transition between the thinned continental and the oceanic crusts is characterized, in space and time, by an ocean-continent transition (OCT). Here, we use bathymetry, gravity, seismic reflection and magnetism from the Encens-Sheba and Encens cruises in order to constrain the structure and segmentation of the conjugate OCT as well as the oceanic ridge between two main fracture zones (Alula-Fartak and Socotra-Hadbeen). The segmentation of the initial oceanic spreading centers seem directly related to the margin structure. Then, magmatic processes and kinematics change strongly influenced the evolution of the segmentation. The OCT and the oceanic domain can be divided into two distinct areas in the study area. The Eastern area is characterized by an extremely thin OCT and oceanic crusts (< 4km), a ~30 km wide and tectonized OCT with isolated continental blocks and short axial segments. In the western area, thicker OCT and oceanic crusts (>5km), a ~15 km wide OCT with a volcanic ridge, and a 6 km thick underplated mafic body in the northern margin suggest a high melt supply. The magmatic supply observed in the western domain is probably due to an off-axis thermal anomaly located below the southern flank of the Sheba ridge, at 75 km east of the major Alula-Fartak transform fault. This suggests that the OCT and the axial ridge morphology of this domain are perturbed by post-rift volcanism, which is due to a combination of the spreading rate, a thermal anomaly, and the cold edge effect of the Alula-Fartak transform fault. The presence of the inherited Mesozoic basins (Jezar-Qamar-Gardafui basin) located on this western domain can also explain, the difference in both the structure and the nature of the OCT between the two domains. The nature of the OCT could be either (or both) exhumed lower crust or

  14. The role of inherited crustal structures and magmatism in the development of rift segments: Insights from the Kivu basin, western branch of the East African Rift

    NASA Astrophysics Data System (ADS)

    Smets, Benoît; Delvaux, Damien; Ross, Kelly Ann; Poppe, Sam; Kervyn, Matthieu; d'Oreye, Nicolas; Kervyn, François

    2016-06-01

    The study of rift basin's morphology can provide good insights into geological features influencing the development of rift valleys and the distribution of volcanism. The Kivu rift segment represents the central section of the western branch of the East African Rift and displays morphological characteristics contrasting with other rift segments. Differences and contradictions between several structural maps of the Kivu rift make it difficult to interpret the local geodynamic setting. In the present work, we use topographic and bathymetric data to map active fault networks and study the geomorphology of the Kivu basin. This relief-based fault lineament mapping appears as a good complement for field mapping or mapping using seismic reflection profiles. Results suggest that rifting reactivated NE-SW oriented structures probably related to the Precambrian basement, creating transfer zones and influencing the location and distribution of volcanism. Both volcanic provinces, north and south of the Kivu basin, extend into Lake Kivu and are connected to each other with a series of eruptive vents along the western rift escarpment. The complex morphology of this rift basin, characterized by a double synthetic half-graben structure, might result from the combined action of normal faulting, magmatic underplating, volcanism and erosion processes.

  15. Magma storage depths beneath an active rift volcano in Afar (Dabbahu), constrained by melt inclusion analyses, seismicity and Interferometric Synthetic Aperture Radar (INSAR)

    NASA Astrophysics Data System (ADS)

    Field, L.; Blundy, J.; Wright, T. J.; Yirgu, G.; Afar Consortium

    2010-12-01

    Dabbahu volcano is located at the northern end of the active Manda Hararo rift segment in western Afar, Ethiopia. In 2005 a major rifting episode began in the segment, which has been modelled as basalt dyke injections (1). Seismic activity, inflation and deflation have been recorded at the volcano. The aim of this research is to provide an insight into the history and evolution of a silicic magmatic centre in the rift, and to contribute to the wider aims of the NERC Afar Consortium to track the creation, migration, evolution and emplacement of magma from the asthenosphere to the crust. The volatile contents of rare melt inclusions trapped within phenocrysts of alkali feldspar, clinopyroxene and olivine from Dabbahu have been studied using secondary ion mass spectrometry. The host lavas are mildly peralkaline obsidians, which, based on field evidence and preliminary results from 40Ar-39Ar dating, represent the youngest samples on the volcano (<4 ka). Whilst the obsidian and pumice groundmass glasses are largely degassed, the H2O contents of the analysed inclusions are up to 5.8 wt%. CO2 contents are generally low; <462 ppm in the alkali feldspar-hosted inclusions, but higher values (up to 1457 ppm) have been found in the clinopyroxene-hosted inclusions. The pressure (and depth) of pre-eruptive magma storage beneath Dabbahu has been constrained using H2O and CO2 data, which suggest shallow magma storage at depths of ~1 - 5 km below the surface. These depths are consistent with observations from recorded seismicity and InSAR at Dabbahu. Seismicity has been recorded from deformation caused by deflation of the magma chamber following the 2005 dyke emplacement event (Oct 2005 - Apr 2006)(2) and InSAR has monitored deflation and subsequent steady inflation after this event. We show that melt inclusions accurately record a stable, shallow magma chamber as corroborated by remote sensing and geophysical observations at Dabbahu volcano. 1 Ayele et al. 2009 ‘September 2005

  16. Factors controlling depth of continental rift zones

    NASA Astrophysics Data System (ADS)

    Elesin, Y.; Artemieva, I.; Thybo, H.

    2012-04-01

    Subsidence of continental rift basins is caused by thinning of the crust and lithospheric mantle together with isostatic compensation for the extra load of sediments and thermal relaxation. It is generally believed that the final depth of rift basins is primarily controlled by the amount of stretching and that other processes, such as rheology and sedimentation, only have secondary influence. However, we show that the relative rheological strength of faults inside and outside rift zones exerts substantial control on the volume of the final rift basin (by more than a factor of 3) even for the same amount of extension (total or inside the rift zone). This surprising result is mainly caused by irreversible deepening of the rift graben during stretching due to lower crustal flow when the faults in the rift zone are weak, whereas the effect is negligible for strong faults. Relatively strong faults inside the rift zone lead to substantial stretching of adjacent crust, and we find that long term stretching outside the main rift zone may explain the formation of wide continental margins, which are now below sea level. We also demonstrate that fast syn-rift erosion/sedimentation rates can increase the final volume of rift basins by up to a factor of 1.7 for weak crustal faults, whereas this effect is insignificant for strong faults inside the rift zone. These findings have significant implications for estimation of stretching factors, tectonic forces, and geodynamic evolution of sedimentary basins around failed rift zones.

  17. Recent rift-related volcanism in Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Ferguson, David J.; Barnie, Talfan D.; Pyle, David M.; Oppenheimer, Clive; Yirgu, Gezahegn; Lewi, Elias; Kidane, Tesfaye; Carn, Simon; Hamling, Ian

    2010-04-01

    Rift zones are the most common magmatic environment on Earth. However opportunities to observe active rifting are rare, and consequently the volcanological characteristics of rift systems are not well understood. An ongoing phase of magmatic rifting along a section of the Red Sea system in Afar, Ethiopia, presents an exceptional opportunity to constrain relationships between volcanism and crustal growth. Here, by integrating analyses of satellite images (i.e. MODIS, OMI, ASTER, and ALI) with field observations, we characterise two recent (August 2007 and June 2009) basaltic fissure eruptions in Afar and evaluate the role and significance of volcanism in the rifting process. Both events were brief (36-72 h) and erupted 4.4-18 × 10 6 m 3 of lava from a fissure system 4-6.5 km in length. Data from the spaceborne Ozone Monitoring Instrument (OMI) suggests total SO 2 emissions for each eruption of 26 ± 5 kt (2007) and 34 ± 7 kt (2009), consistent with complete degassing of the erupted magma volumes. Using geodetic models for the intrusive activity in Afar we estimate the partitioning of magma between intrusive and extrusive components, up to July 2009, to be ˜ 180:1. Comparing the first-order volcanic characteristics and the intrusive-extrusive volume balance for the Afar volcanism with data from the 1975-1984 Krafla rifting cycle (Iceland) suggests that the volcanic flux in Afar will rise significantly over the next few years as the stresses are increasingly relieved by dyking, and subsequent dykes are able to propagate more easily to the surface. As a consequence, basaltic fissure eruptions in this section of the Afar rift will become of increasing large magnitude as the rifting event matures over the next 5-10 yr. Using available models of magmatic rifting we forecast the likely size and location of future eruptions in Afar.

  18. Stratigraphic Modelling of Continental Rifting

    NASA Astrophysics Data System (ADS)

    Mondy, Luke; Duclaux, Guillaume; Salles, Tristan; Thomas, Charmaine; Rey, Patrice

    2013-04-01

    Interlinks between deformation and sedimentation have long been recognised as an important factor in the evolution of continental rifts and basins development. However, determining the relative impact of tectonic and climatic forcing on the dynamics of these systems remains a major challenge. This problem in part derives from a lack of modelling tools capable of simulated high detailed surface processes within a large scale (spatially and temporally) tectonic setting. To overcome this issue an innovative framework has been designed using two existing numerical forward modelling codes: Underworld, capable of simulating 3D self-consistent tectonic and thermal lithospheric processes, and Tellus, a forward stratigraphic and geomorphic modelling framework dedicated to simulating highly detailed surface dynamics. The coupling framework enables Tellus to use Underworld outputs as internal and boundary conditions, thereby simulating the stratigraphic and geomorphic evolution of a realistic, active tectonic setting. The resulting models can provide high-resolution data on the stratigraphic record, grain-size variations, sediment provenance, fluvial hydrometric, and landscape evolution. Here we illustrate a one-way coupling method between active tectonics and surface processes in an example of 3D oblique rifting. Our coupled model enables us to visualise the distribution of sediment sources and sinks, and their evolution through time. From this we can extract and analyse at each simulation timestep the stratigraphic record anywhere within the model domain. We find that even from a generic oblique rift model, complex fluvial-deltaic and basin filling dynamics emerge. By isolating the tectonic activity from landscape dynamics with this one-way coupling, we are able to investigate the influence of changes in climate or geomorphic parameters on the sedimentary and landscape record. These impacts can be quantified in part via model post-processing to derive both instantaneous and

  19. Edaphics, active tectonics and animal movements in the Kenyan Rift - implications for early human evolution and dispersal

    NASA Astrophysics Data System (ADS)

    Kübler, Simon; Owenga, Peter; Rucina, Stephen; King, Geoffrey C. P.

    2014-05-01

    The quality of soils (edaphics) and the associated vegetation strongly controls the health of grazing animals. Until now, this has hardly been appreciated by paleo-anthropologists who only take into account the availability of water and vegetation in landscape reconstruction attempts. A lack of understanding the importance of the edaphics of a region greatly limits interpretations of the relation between our ancestors and animals over the last few million years. If a region lacks vital trace elements then wild grazing and browsing animals will avoid it and go to considerable length and take major risks to seek out better pasture. As a consequence animals must move around the landscape at different times of the year. In complex landscapes, such as tectonically active rifts, hominins can use advanced group behaviour to gain strategic advantage for hunting. Our study in the southern Kenya rift in the Lake Magadi region shows that the edaphics and active rift structures play a key role in present day animal movements as well as the for the location of an early hominin site at Mt. Olorgesailie. We carried out field analysis based on studying the relationship between the geology and soil development as well as the tectonic geomorphology to identify 'good' and 'bad' regions both in terms of edaphics and accessibility for grazing animals. We further sampled different soils that developed on the volcanic bedrock and sediment sources of the region and interviewed the local Maasai shepherds to learn about present-day good and bad grazing sites. At the Olorgesailie site the rift valley floor is covered with flood trachytes; basalts only occur at Mt. Olorgesailie and farther east up the rift flank. The hominin site is located in lacustrine sediments at the southern edge of a playa that extends north and northwest of Mt. Olorgesailie. The lakebeds are now tilted and eroded by motion on two north-south striking faults. The lake was trapped by basalt flows from Mt. Olorgesailie

  20. Thermomechanical models of the Rio Grande rift

    SciTech Connect

    Bridwell, R.J.; Anderson, C.A.

    1980-01-01

    Fully two-dimensional, coupled thermochemical solutions of a continental rift and platform are used to model the crust and mantle structure of a hot, buoyant mantle diapir beneath the Rio Grande rift. The thermomechanical model includes both linear and nonlinear laws of the Weertman type relating shear stress and creep strain rate, viscosity which depends on temperature and pressure, and activation energy, temperature-dependent thermal conductivity, temperature-dependent coefficient of thermal expansion, the Boussinesq approximation for thermal bouyancy, material convection using a stress rate that is invariant to rigid rotations, an elastically deformable crust, and a free surface. The model determines the free surface velocities, solid state flow field in the mantle, and viscosity structure of lithosphere and asthenosphere. Regional topography and crustal heat flow are simulated. A suite of symmetric models, assumes continental geotherms on the right and the successively increasing rift geotherms on the left. These models predict an asthenospheric flow field which transfers cold material laterally toward the rift at > 300 km, hot, buoyant material approx. 200 km wide which ascends vertically at rates of 1 km/my between 175 to 325 km, and spreads laterally away from the rift at the base of the lithosphere. Crustal spreading rates are similar to uplift rates. The lithosphere acts as stiff, elastic cap, damping upward motion through decreased velocities of 1 km/10 my and spreading uplift laterally. A parameter study varying material coefficients for the Weertman flow law suggests asthenospheric viscosities of approx. 10/sup 22/ to 10/sup 23/ poise. Similar studies predict crustal viscosities of approx. 10/sup 25/ poise. The buoyant process of mantle flow narrows and concentrates heat transport beneath the rift, increases upward velocity, and broadly arches the lithosphere. 10 figures, 1 table.

  1. Capturing magma intrusion and faulting processes during continental rupture: seismicity of the Dabbahu (Afar) rift

    NASA Astrophysics Data System (ADS)

    Ebinger, C. J.; Keir, D.; Ayele, A.; Calais, E.; Wright, T. J.; Belachew, M.; Hammond, J. O. S.; Campbell, E.; Buck, W. R.

    2008-09-01

    Continental rupture models emphasize the role of faults in extensional strain accommodation; extension by dyke intrusion is commonly overlooked. A major rifting episode that began in 2005 September in the Afar depression of Ethiopia provides an opportunity to examine strain accommodation in a zone of incipient plate rupture. Earthquakes recorded on a temporary seismic array (2005 October to 2006 April), direct observation of fault patterns and geodetic data document ongoing strain and continued dyke intrusion along the ~60-km long Dabbahu rift segment defined in earlier remote sensing studies. Epicentral locations lie along a ~3 km wide, ~50 km long swath that curves into the SE flank of Dabbahu volcano; a second strand continues to the north toward Gab'ho volcano. Considering the ~8 m of opening in the September crisis, we interpret the depth distribution of microseismicity as the dyke intrusion zone; the dykes rise from ~10 km to the near-surface along the ~60-km long length of the tectono-magmatic segment. Focal mechanisms indicate slip along NNW-striking normal faults, perpendicular to the Arabia-Nubia plate opening vector. The seismicity, InSAR, continuous GPS and structural patterns all suggest that magma injection from lower or subcrustal magma reservoirs continued at least 3 months after the main episode. Persistent earthquake swarms at two sites on Dabbahu volcano coincide with areas of deformation identified in the InSAR data: (1) an elliptical, northwestward-dipping zone of seismicity and subsidence interpreted as a magma conduit, and (2) a more diffuse, 8-km radius zone of shallow seismicity (<2 km) above a shadow zone, interpreted as a magma chamber between 2.5 and 6 km subsurface. InSAR and continuous GPS data show uplift above a shallow source in zone (2) and uplift above the largely aseismic Gab'ho volcano. The patterns of seismicity provide a 3-D perspective of magma feeding systems maintaining the along-axis segmentation of this incipient seafloor

  2. Late Quaternary high resolution sequence stratigraphy of an active rift, the Sperchios Basin, Greece: An analogue for subtle stratigraphic plays

    SciTech Connect

    Eliet, P.P.; Gawthorpe, R.L.

    1996-12-31

    The Sperchios Basin is an active asymmetric graben, bounded to the south by a major border fault system with major fault segments typically 20-30 km long. The basin is dominated by a major axial fluvio-deltaic system which enters the partially enclosed Maliakos Gulf to the east. Lateral sourced depositional systems within the basin comprise hanging-wall and footwall-derived alluvial fans and a narrow coastal plain along the footwall scarp bordering the Maliakos Gulf. High resolution seismic data from the Maliakos Gulf reveals three late Quaternary progradational parasequences sourced from axial and lateral depositional systems, with a regional late-Pleistocene transgressive surface dated at circa. 10 ka BP within the Maliakos Gulf. Differential subsidence of the late Pleistocene transgressive surface indicates marked variation in subsidence from 2.4 m ka{sup -1} at fault segment centers to 0.8 m ka{sup -1} at segment boundaries. The geometry and internal variability of each parasequence is controlled by the interplay of the local accommodation development and fluctuations in sediment supply and climatic conditions. The Sperchios Rift provides a modem analogue for subtle stratigraphic plays within ancient extensional basins. The study of controls on sediment source and transport patterns within active rifts has refined our appreciation of the controls on potential reservoir distribution and geometries.

  3. Late Quaternary high resolution sequence stratigraphy of an active rift, the Sperchios Basin, Greece: An analogue for subtle stratigraphic plays

    SciTech Connect

    Eliet, P.P. ); Gawthorpe, R.L. )

    1996-01-01

    The Sperchios Basin is an active asymmetric graben, bounded to the south by a major border fault system with major fault segments typically 20-30 km long. The basin is dominated by a major axial fluvio-deltaic system which enters the partially enclosed Maliakos Gulf to the east. Lateral sourced depositional systems within the basin comprise hanging-wall and footwall-derived alluvial fans and a narrow coastal plain along the footwall scarp bordering the Maliakos Gulf. High resolution seismic data from the Maliakos Gulf reveals three late Quaternary progradational parasequences sourced from axial and lateral depositional systems, with a regional late-Pleistocene transgressive surface dated at circa. 10 ka BP within the Maliakos Gulf. Differential subsidence of the late Pleistocene transgressive surface indicates marked variation in subsidence from 2.4 m ka[sup -1] at fault segment centers to 0.8 m ka[sup -1] at segment boundaries. The geometry and internal variability of each parasequence is controlled by the interplay of the local accommodation development and fluctuations in sediment supply and climatic conditions. The Sperchios Rift provides a modem analogue for subtle stratigraphic plays within ancient extensional basins. The study of controls on sediment source and transport patterns within active rifts has refined our appreciation of the controls on potential reservoir distribution and geometries.

  4. Fracturing and earthquake activity within the Prestahnúkur fissure swarm in the Western Volcanic Rift Zone of Iceland

    NASA Astrophysics Data System (ADS)

    Hjartardóttir, Ásta Rut; Hjaltadóttir, Sigurlaug; Einarsson, Páll; Vogfjörd, Kristín.; Muñoz-Cobo Belart, Joaquín.

    2015-12-01

    The Prestahnúkur fissure swarm is located within the ultraslowly spreading Western Volcanic Zone in Iceland. The fissure swarm is characterized by normal faults, open fractures, and evidence of subglacial fissure eruptions (tindars). In this study, fractures and faults within the Prestahnúkur fissure swarm were mapped in detail from aerial photographs to determine the extent and activity of the fissure swarm. Earthquakes during the last ~23 years were relocated to map the subsurface fault planes that they delineate. The Prestahnúkur fissure swarm is 40-80 km long and up to ~20 km wide. Most of the areas of the fissure swarm have been glacially eroded, although a part of it is covered by postglacial lava flows. The fissure swarm includes numerous faults with tens of meters vertical offset within the older glacially eroded part, whereas open fractures are found within postglacial lava flows. Comparison of relocated earthquakes and surface fractures indicates that some of the surface fractures have been activated at depth during the last ~23 years, although no dike intrusions have been ongoing. The existence of tindars nevertheless indicates that dike intrusions and rifting events do occur within the Prestahnúkur fissure swarm. The low-fracture density within postglacial lava flows and low density of postglacial eruptive fissures indicate that rifting episodes occur less often than in the faster spreading Northern Volcanic Zone.

  5. Identification of a precambrian rift through Missouri by digital image processing of geophysical and geological data

    NASA Technical Reports Server (NTRS)

    Guinness, E. A.; Arvidson, R. E.; Strebeck, J. W.; Schulz, K. J.; Davies, G. F.; Leff, C. E.

    1982-01-01

    A newly discovered feature in the midcontinent - a gravity low that begins at a break in the midcontinent gravity high in SE Nebraska, extends across Missouri in a NW-SE direction, and intersects the Mississippi Valley graben to form the Pascola arch - is discussed. The anomaly varies from 120 to 160 km in width, extends approximately 700 km, and is best expressed in southern Missouri, where it has a Bouguer amplitude of about -34 mGal. It is noted that the magnitude of the anomaly cannot be explained on the basis of a thickened section of Paleozoic sedimentary rock. The gravity data and the sparse seismic refraction data for the region are found to be consistent with an increased crustal thickness beneath the gravity low. It is thought that the gravity anomaly is probably the present expression of a failed arm of a rifting event, perhaps one associated with the spreading that led to or preceded formation of the granite and rhyolite terrain of southern Missouri.

  6. Mosquitoes and the Environment in Nile Delta Villages with Previous Rift Valley Fever Activity.

    PubMed

    Zayed, Abdelbaset B; Britch, Seth C; Soliman, Mohamed I; Linthicum, Kenneth J

    2015-06-01

    Egypt is affected by serious human and animal mosquito-borne diseases such as Rift Valley fever (RVF). We investigated how potential RVF virus mosquito vector populations are affected by environmental conditions in the Nile Delta region of Egypt by collecting mosquitoes and environmental data from 3 key governorates before and after 2012 seasonal flooding. We found that environmental effects varied among species, life stages, pre- and postflood groupings, and geographic populations of the same species, and that mosquito community composition could change after flooding. Our study provides preliminary data for modeling mosquitoes and mosquito-borne diseases in the Nile Delta region.

  7. Mosquitoes and the Environment in Nile Delta Villages with Previous Rift Valley Fever Activity.

    PubMed

    Zayed, Abdelbaset B; Britch, Seth C; Soliman, Mohamed I; Linthicum, Kenneth J

    2015-06-01

    Egypt is affected by serious human and animal mosquito-borne diseases such as Rift Valley fever (RVF). We investigated how potential RVF virus mosquito vector populations are affected by environmental conditions in the Nile Delta region of Egypt by collecting mosquitoes and environmental data from 3 key governorates before and after 2012 seasonal flooding. We found that environmental effects varied among species, life stages, pre- and postflood groupings, and geographic populations of the same species, and that mosquito community composition could change after flooding. Our study provides preliminary data for modeling mosquitoes and mosquito-borne diseases in the Nile Delta region. PMID:26181689

  8. 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.

  9. Classification of the rift zones of venus: Rift valleys and graben belts

    NASA Astrophysics Data System (ADS)

    Guseva, E. N.

    2016-05-01

    The spatial distribution of rift zones of Venus, their topographic configuration, morphometric parameters, and the type of volcanism associating with rifts were analyzed. This allowed the main characteristic features of rifts to be revealed and two different types of rift-forming structures, serving for classification of rift zones as rift valleys and graben belts, to be isolated. These structural types (facies) of rift zones are differently expressed in the relief: rift valleys are individual deep (several kilometers) W-shaped canyons, while graben belts are clusters of multiple V-shaped and rather shallow (hundreds of meters) depressions. Graben belts are longer and wider, as compared to rift valleys. Rift valleys are spatially associated with dome-shaped volcanic rises and large volcanos (concentrated volcanic sources), while graben belts do not exhibit such associations. Volcanic activity in the graben belts are presented by spacious lava fields with no apparent sources of volcanism. Graben belts and rift valleys were formed during the Atlian Period of geologic history of Venus, and they characterized the tectonic style of the planet at the late stages of its geologic evolution. Formation of this or that structural facies of the rift zones of Venus were probably governed by the thickness of the lithosphere, its rheological properties, and the development degree of the mantle diapirs associating with rift zones.

  10. From rifting to active spreading in the Lau Basin - Havre Trough backarc system (SW Pacific): Locking/unlocking induced by seamount chain subduction

    NASA Astrophysics Data System (ADS)

    Ruellan, E.; Delteil, J.; Wright, I.; Matsumoto, T.

    2003-05-01

    Associated with Pacific-Australia plate convergence, the Lau Basin - Havre Trough is an active back-arc basin that has been opened since ˜5.5 Ma by rifting and southward propagating oceanic spreading. Current back-arc opening rates decrease from 159 mm yr-1 in the northern Lau Basin to 15 mm y-1 in the southern Havre Trough. Major tectonic changes occur at the transition between Havre Trough rifting and full oceanic spreading of the Eastern Lau Spreading Center (ELSC), where the oblique-to-trench, westward subducting Louisville Seamount Chain (LSC) sweeps southwards along the Tonga trench. New swath bathymetry, seismic reflection data, and limited rock sampling in this area constrain a tectonic and kinematic back-arc model that incorporates the effects of LSC subduction. The ELSC, which extends southward to 24°55'S, forms a deep rift valley propagating southward through older, rifted arc basement. Present-day seismicity and fresh and fractured pillow lavas at 23°42'S are consistent with rift valley neovolcanism. Conversely, the northern Havre Trough has low seismicity and rifted volcanic basement ridges trending 25-45° oblique to the basin axis consistent with low levels of extensional tectonism and volcanism. This latter structural fabric is interpreted as an early stage of rifting that is now "locked" due to compression on the arc exerted by LSC subduction, while in the Lau Basin such effects have passed as the LSC swept along the Tonga Trench. It is proposed that the Lau-Havre back-arc opening is controlled by tectonic constraints exerted at the limits of the system by the LSC subduction, which determines the southward migration of the Tonga Arc pole of rotation and associated Lau Basin opening. A discrete three-stage back-arc opening evolution is proposed, comprising: (1) an initial phase of back-arc rifting along the whole length of the plate boundary, beginning at ˜6-5 Ma; (2) a subsequent phase, mostly present in the southern part of the back-arc domain

  11. Cenozoic rift formation in the northern Caribbean

    NASA Technical Reports Server (NTRS)

    Mann, P.; Burke, K.

    1984-01-01

    Rifts form in many different tectonic environments where the lithosphere is put into extension. An outline is provided of the distribution, orientation, and relative ages of 16 Cenozoic rifts along the northern edge of the Caribbean plate and it is suggested that these structures formed successively by localized extension as the Caribbean plate moved eastward past a continental promontory of North America. Evidence leading to this conclusion includes (1) recognition that the rifts become progressively younger westward; (2) a two-phase subsidence history in a rift exposed by upthrusting in Jamaica; (3) the absence of rifts east of Jamaica; and (4) the observation that removal of 1400 km of strike-slip displacement on the Cayman Trough fault system places the Paleogene rifts of Jamaica in an active area of extension south of Yucatan where the rifts of Honduras and Guatemala are forming today.

  12. Rift processes and crustal structure of the Amundsen Sea Embayment, West Antarctica, from 3D potential field modelling

    NASA Astrophysics Data System (ADS)

    Kalberg, Thomas; Gohl, Karsten; Eagles, Graeme; Spiegel, Cornelia

    2015-12-01

    The Amundsen Sea Embayment of West Antarctica is of particular interest as it provides critical geological boundary conditions in better understanding the dynamic behavior of the West Antarctic Ice Sheet, which is undergoing rapid ice loss in the Amundsen Sea sector. One of the highly debated hypothesis is whether this region has been affected by the West Antarctic Rift System, which is one of the largest in the world and the dominating tectonic feature in West Antarctica. Previous geophysical studies suggested an eastward continuation of this rift system into the Amundsen Sea Embayment. This geophysical study of the Amundsen Sea Embayment presents a compilation of data collected during two RV Polarstern expeditions in the Amundsen Sea Embayment of West Antarctica in 2006 and 2010. Bathymetry and satellite-derived gravity data of the Amundsen Sea Embayment complete the dataset. Our 3-D gravity and magnetic models of the lithospheric architecture and development of this Pacific margin improve previous interpretations from 2-D models of the region. The crust-mantle boundary beneath the continental rise and shelf is between 14 and 29 km deep. The imaged basement structure can be related to rift basins within the Amundsen Sea Embayment, some of which can be interpreted as products of the Cretaceous rift and break-up phase and some as products of later propagation of the West Antarctic Rift System into the region. An estimate of the flexural rigidity of the lithosphere reveals a thin elastic thickness in the eastern embayment which increases towards the west. The results are comparable to estimates in other rift systems such as the Basin and Range province or the East African Rift. Based on these results, we infer an arm of the West Antarctic Rift System is superposed on a distributed Cretaceous rift province in the Amundsen Sea Embayment. Finally, the embayment was affected by magmatism from discrete sources along the Pacific margin of West Antarctica in the Cenozoic.

  13. Factors controlling depth of continental rifts

    NASA Astrophysics Data System (ADS)

    Elesin, Y.; Artemieva, I. M.; Thybo, H.

    2011-12-01

    Rifting is a fundamental plate tectonic process, which forms elongated, narrow tectonic depressions in the Earth's surface and, eventually, may break continental plates to form new oceanic lithosphere. Subsidence of rift basins is caused by thinning of the crust and lithospheric mantle together with isostatic compensation for the extra load of sediments and thermal relaxation. It is generally believed that the final depth of rift basins is primarily controlled by the amount of stretching and that other processes only have secondary influence. However, we show that the relative rheological strength of faults inside and outside rift zones exerts substantial control on the volume of the final rift basin (by more than a factor of 3) even for the same amount of extension (total or inside the rift zone). This surprising result is mainly caused by irreversible deepening of the rift graben during stretching due to lower crustal flow when the faults in the rift zone are weak, whereas the effect is negligible for strong faults. Relatively strong faults inside the rift zone lead to substantial stretching of adjacent crust, and we find that long term stretching outside the main rift zone may explain the formation of wide continental margins, which are now below sea level. We also demonstrate that fast syn-rift erosion/sedimentation rates can increase the final volume of rift basins by up to a factor of 1.7 for weak crustal faults, whereas this effect is insignificant for strong faults inside the rift zone. These findings have significant implications for estimation of stretching factors, tectonic forces, and geodynamic evolution of sedimentary basins around failed rift zones.

  14. Influence of offset weak zones on the development of rift basins: Activation and abandonment during continental extension and breakup

    NASA Astrophysics Data System (ADS)

    Chenin, Pauline; Beaumont, Christopher

    2013-04-01

    We use numerical modelling to investigate reactivation of inherited Offset Weak Zones (OWZ) in continental crust and Mantle Weak Zones (MWZ) to form offset rift basins during continental rifting and breakup. Offset rift basins are basins that are set off/offset from the main rift/locus of breakup. Weak zones embedded in a stiff layer are preferentially and rapidly reactivated, whereas the same zones are either ignored or slowly reactivated when embedded in pliable layers. Here stiff implies a nonlinear flow law with a high stress exponent (n > ˜ 10,000), a plastic material, and pliable means a low stress exponent (n ˜ 2-5) as in ductile, power-law creep of rocks. Whether offset rift basins form during rifting of a composite lithosphere (i.e., comprising stiff and pliable layers) depends on the competition between necking instabilities that develop at the weak zones in the stiff layers, and the coupling between the stiff and pliable layers. Stiff/cratonic lithosphere results in early localization of the deformation at the MWZ, rapid necking and breakup without developing offset rift basins. In contrast, warm pliable lithosphere develops significant offset basins and has protracted rifting because the MWZ is now embedded in a pliable layer. We also investigate the influence of OWZ dip, sedimentation, and the sensitivity of reactivation to the distance from OWZ to the MWZ, and to the size of the MWZ. A tectonic rifting styles diagram is used to show that the model results agree with natural examples.

  15. Tag team tectonics: mantle upwelling and lithospheric heterogeneity ally to rift continents (Invited)

    NASA Astrophysics Data System (ADS)

    Nelson, W. R.; Furman, T.

    2013-12-01

    The configuration of continents we know today is the result of several billion years of active Wilson Cycle tectonics. The rifting of continents and subsequent development of ocean basins is an integral part of long-term planetary-scale recycling processes. The products of this process can be seen globally, and the East African Rift System (EARS) provides a unique view of extensional processes that actively divide a continent. Taken together with the adjoining Red Sea and Gulf of Aden, the EARS has experienced over 40 Ma of volcanism and ~30 Ma of extension. While early (pre-rift) volcanism in the region is attributed to mantle plume activity, much of the subsequent volcanism occurs synchronously with continental rifting. Numerous studies indicate that extension and magmatism are correlated: extension leads to decompression melting while magmatism accommodates further extension (e.g. Stein et al., 1997; Buck 2004; Corti 2012). Evaluation of the entire EARS reveals significant geochemical patterns - both spatial and temporal - in the volcanic products. Compositional variations are tied directly to the melt source(s), which changes over time. These variations can be characterized broadly by region: the Ethiopian plateau and Turkana Depression, the Kenya Rift, and the Western Rift. In the Ethiopian plateau, early flood basalt volcanism is dominated by mantle plume contributions with variable input from lherzolitic mantle lithosphere. Subsequent alkaline shield volcanism flanking the juvenile Main Ethiopian Rift records the same plume component as well as contributions from a hydrous peridotitic lithosphere. The hydrous lithosphere does not contribute indefinitely. Instead, young (< 2 Ma) volcanism taps a combination of the mantle plume and anhydrous depleted lithospheric mantle. In contrast, volcanism in the Kenya Rift and the Western Rift are derived dominantly from metasomatized lithospheric mantle rather than mantle plume material. These rifts lie in the mobile

  16. Discovery of Nascent Vents and Recent Colonization Associated with(Re)activated Hydrothermal Vent Fields by the GALREX 2011 Expedition on the Galápagos Rift

    NASA Astrophysics Data System (ADS)

    Shank, T. M.; Holden, J. F.; Herrera, S.; Munro, C.; Muric, T.; Lin, J.; Stuart, L.

    2011-12-01

    GALREX 2011 was a NOAA OER telepresence cruise that explored the diverse habitats and geologic settings of the deep Galápagos region. The expedition made12 Little Hercules ROV dives in July 2011.Abundant corals and a strong depth zonation of species (including deepwater coral communities) were found near 500 m depth on Paramount Seamount, likely influenced by past low sea level states, wave-cut terrace processes, and the historical presence of shallow reef structures. At fresh lava flows with associated (flocculent) hydrothermal venting near 88° W, now known as Uka Pacha and Pegasus Vent Fields, rocks were coated with white microbial mat and lacked sessile fauna, with few mobile fauna (e.g., bythograeid crabs, alvinocarid shrimp, polynoid worms, zoarcid fish, and dirivultid copepods). This suggests a recent creation of hydrothermal habitats through volcanic eruptions and/or diking events, which may have taken place over a 15 km span separating the two vent fields. The Rosebud vent field at 86°W was not observed and may have been covered with lava since last visited in 2005. A hydrothermal vent field near 86°W was discovered that is one of the largest vent fields known on the Rift (120m by 40m). Low-temperature vent habitats were colonized by low numbers of tubeworms including Riftia, Oasisia, and a potential Tevnia species (the latter not previously observed on the Galapagos Rift). Patches of tubeworms were observed with individuals less than 2cm in length, and the relatively few large Riftia had tube lengths near 70cm long. Large numbers of small (< 3cm long) bathymodiolin mussels lined cracks and crevices throughout the active part of the field. Live clams, at least four species of gastropod limpets, three species of polynoid polychaetes, juvenile and adult alvinocarid shrimp, actinostolid anemones, and white microbial communities were observed on the underside and vertical surfaces of basalt rock surfaces. There were at least 13 species of vent-endemic fauna

  17. A record of long-time rift activity and earthquake-induced ground effects in Pleistocene deposits of southern Tenerife (Canary Islands, Spain)

    NASA Astrophysics Data System (ADS)

    Buchner, Elmar; Kröchert, Jörg

    2009-09-01

    Faults with a well-defined strike direction that precisely coincides with the southern rift fault system occur in the study area in southern Tenerife. This fault system was generated contemporaneously with a chain of cinder cones ~948 ka. Open fractures in ignimbrites (~668 ka) and fossil beach deposits (~42 ka) of the El Médano area suggest that the rift-associated fault system was seismically active in the aftermath of the initial volcanic activity (~948 ka) and is probably still active. A second fault system striking perpendicular to the rift-related faults probably originates from a Holocene paleoearthquake of moderate intensity. Earthquake-induced ground effects in fossil beach deposits within the study area are consistent with seismically induced ground effects of several recent and well-documented earthquakes, as well as gravitational sliding triggered by an intense earthquake in the Nicoya Peninsula of Costa Rica in 1990. Both, the rift-associated and the earthquake-induced fault system, initially produced open fractures that were occupied by plants and subsequently stabilized by cementation, forming conspicuous sediment structures in fossil beach deposits of the El Médano site in southern Tenerife.

  18. Development regimes of rifted basins and criteria of their petroleum potential

    SciTech Connect

    Larchenkov, E. )

    1994-07-01

    The majority of great sedimentary basins were formed primarily by stretching and usually initiated by rifting. The evolution and development of intercontinental and passive margin rifted basins are discussed. Each basin type described (in the paper) is associated with either single or branched rift zones. The basin types are (1) unburied rift, i.e., recent rifts, or ancient rifts with post-rift stage without significant subsidence such as East Africa rifts, and Reconcavo basin in Brazil; and (2) buried rift zones where the post-rift stage is characterized by active subsidence and sediment accumulation. The basins often form by repeated rifting, and a rift zone of a different age may be completely or partially superimposed or separated from other rifting events.

  19. 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].

  20. 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.

  1. Volcanic history of the Colorado River extensional corridor: Active or passive rifting

    SciTech Connect

    Howard, K.A. )

    1993-04-01

    Magmatism and extension began nearly simultaneously in the Colorado River extensional corridor (CREC) between 34 and 35[degree] N. Initial eruptions of basanite at 23--19.5 Ma were low-volume but spanned a region now twice as wide as the 100-km-wide corridor. Extensional tilting of this age was local. A large flux of calc-alkaline basalt, andesite, dacite, and rhyolite was erupted at 22--18.5 Ma. They accumulated to average thicknesses of [approximately]1 km in the early CREC basin, and were accompanied by extensional tilting. Dike swarms, necks, and plutons represent intrusive equivalents. Plutons concentrate in the central belt of metamorphic core complexes, the most highly extended areas. Massive eruption at 18.5 Ma of the rhyolitic Peach Springs Tuff marked an ensuing lowered rate of volcanic output, a change to bimodal volcanism, much tilting and extension, and deposition of thick (to [approximately]2 km) synextensional clastic sediments 18--14 Ms. By 14--12 Ma, extensional tilting had largely ceased, and eruptions were sparse and basaltic only, as they have been since. Basalt compositions reveal changing patterns of trace-element composition that bear on sources. The early basanites have OIB-like compositions on spidergram plots, suggesting origin from the asthenosphere as would be expected from initiation of rifting driven by hot mantle upwelling. Basalts 20--12 Ma show low concentrations of Nb and Ta as in subduction-related arc magmas. Post-extensional basalts erupted 15--10 Ma exhibit a transition back toward primitive compositions seen in Quaternary alkalic basalts.

  2. Witnessing the birth of a new ocean? The first 6 years of the Dabbahu rifting episode, and other activity in Afar

    NASA Astrophysics Data System (ADS)

    Wright, T.; Ayele, A.; Barnie, T.; Belachew, M.; Calais, E.; Field, L.; Hamling, I.; Hammond, J.; Keir, D.

    2012-04-01

    Intense earthquake activity and a small rhyolitic eruption in September 2005 heralded the onset of an unprecedented period of geological activity in the Afar Depression. The seismic activity accompanied dyke intrusion in the upper 10 km of crust along 60 km of the Dabbahu (northern Manda-Hararo) Magmatic Segment (DMS) of the Nubia-Arabia plate boundary, a nascent seafloor spreading centre. InSAR observations of the resulting deformation showed that the initial dyke was up to 8 m thick, with a total volume of 2-2.5 km3. Urgency funding from the UK Natural Environmental Research Council (NERC) and US National Science Foundation (NSF) enabled us to deploy a local array of seismometers in October 2005, continuous GPS instruments in January 2006, and to acquire a dense time series of satellite radar images. The medium-term viability of these instruments was secured with major follow-on funding from NSF and NERC; these projects supported the collection and analysis of additional unique data sets, including data from a broader array of seismic and GPS instruments, magneto-telluric transects of the rift, airborne LiDAR, petrological sampling and micro-gravity work. The combination of these data has allowed us to quantify the processes associated with crustal growth at divergent plate boundaries for the first time. Here, we present a broad overview of geological activity in the Afar depression in the hyperactive 21st century. Activity in the DMS began after September 2000, when Gabho volcano at the north of the segment began uplifting, as its magma chamber, ~3 km below the surface, was replenished. It is likely that the inflation at Gabho ultimately triggered the onset of the Dabbahu rifting episode. The rifting episode began with intense seismicity at the northern end of the DMS, before jumping to the Ado Ale Volcanic Complex at the segment centre. This initial dyking was fed from shallow (~3 km) chambers at Gabho and Dabbahu as well as a deeper (~10 km) source at Ado Ale

  3. Volcanic rocks and processes of the Mid-Atlantic Ridge rift valley near 36 ° 49′ N

    USGS Publications Warehouse

    Hekinian, R.; Moore, J.G.; Bryan, W.B.

    1976-01-01

    The above relations indicate that the diverse lava types were erupted from a shallow, zoned magma chamber from fissures distributed over the width of the inner rift valley and elongate parallel to it. Differentiation was accomplished by cooling and crystallization of plagioclase, olivine, and clinopyroxene toward the margins of the chamber. The centrally located hills were built by the piling up of frequent eruption of mainly primitive lavas which also are the youngest flows. In contrast smaller and less frequent eruptions of more differentiated lavas were exposed on both sides of the rift valley axis.

  4. Drilling investigations of crustal rifting processes in the Salton Trough, California. Revision 1

    SciTech Connect

    Kasameyer, P.W.; Younker, L.W.; Newmark, R.L.; Duba, A.G.

    1986-01-01

    The results of CSDP activities in the Salton Sea Geothermal Field (SSGF) are briefly described, concentrating on a shallow heat flow survey, but also discussing preliminary results from the Salton Sea Scientific Drilling Program (SSSDP). The hypothesis that localized thermal zones are the source of all the heat in the Salton Trough is examined. (ACR)

  5. Drilling investigations of crustal rifting processes in the Salton Trough, California

    SciTech Connect

    Kasameyer, P.W.; Younker, L.W.; Newmark, R.L.; Duba, A.G.

    1986-01-01

    The paper describes the results of CSDP activities in the Salton Sea Geothermal Field (SSGF), concentrating on a shallow heat-flow survey, but also considering preliminary results from the Salton Sea Scientific Drilling Program (SSSDP). Whether the heat input rate to localized systems is high enough to account for the overall thermal budget of the Salton Trough is examined. (ACR)

  6. Seismically Articulating Kilauea Volcano's Active Conduits, Rift Zones, and Faults through HVO's Second Fifty Years

    NASA Astrophysics Data System (ADS)

    Okubo, P.; Nakata, J.; Klein, F.; Koyanagi, R.; Thelen, W.

    2011-12-01

    While seismic monitoring of active Hawaiian volcanoes began 100 years ago, the build-up of the U. S. Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO) seismographic network to its current configuration began in 1955, when Jerry Eaton established remote stations that telemetered data via landline to recorders at HVO. With network expansion through the 1960's, earthquake location and cataloging capabilities have evolved to afford a computer processed seismic catalog now spanning fifty years. Location accuracy and catalog completeness to smaller magnitudes have increased. Research and insights developed using HVO's seismic record have exploited the ability to seismically monitor volcanic activity at depth, to identify active regions within the volcanoes on the basis of computed hypocentral locations, to infer regions of magma storage by recognizing different families of volcanic earthquakes, and to forecast volcanic activity in both short and longer term from seismicity patterns. HVO's seismicity catalog was central to calculations of probabilistic seismic hazards. The ability to develop and implement additional analytical and interpretive capabilities has kept pace with improvements in both field and laboratory hardware and software. While the basic capabilities continue as part of HVO's core monitoring, additional interpretive capabilities now include adding details of volcanic and earthquake source regions, and viewing seismic data in juxtaposition with other observatory data streams. As HVO looks to its next century of volcano studies, research and development continue to shape the future. Broadband seismic recording at HVO has enabled extensive study by Chouet, Dawson, and co-workers of the relationship of very-long-period seismic sources beneath Kilauea's summit caldera to magma supply and transport. Recent upgrades have improved the ability to use these data in seismic cataloging and research. Data processing upgrades have bolstered the ability to

  7. Abrupt plate accelerations shape rifted continental margins

    NASA Astrophysics Data System (ADS)

    Brune, Sascha; Williams, Simon E.; Butterworth, Nathaniel P.; Müller, R. Dietmar

    2016-08-01

    Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth’s major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength–velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.

  8. Abrupt plate accelerations shape rifted continental margins.

    PubMed

    Brune, Sascha; Williams, Simon E; Butterworth, Nathaniel P; Müller, R Dietmar

    2016-08-11

    Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth's major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength--velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.

  9. Abrupt plate accelerations shape rifted continental margins.

    PubMed

    Brune, Sascha; Williams, Simon E; Butterworth, Nathaniel P; Müller, R Dietmar

    2016-08-11

    Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth's major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength--velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time. PMID:27437571

  10. A regional inventory of the landslide processes and the elements at risk on the Rift flanks west of Lake Kivu (DRC)

    NASA Astrophysics Data System (ADS)

    Maki Mateso, Jean-Claude; Monsieurs, Elise; Jacobs, Liesbet; Bagalwa Mateso, Luc; Fiama Bondo, Silvanos; Delvaux, Damien; Albino, Fabien; Kervyn, François; Dewitte, Olivier

    2016-04-01

    The Rift flanks west of Lake Kivu (DRC) are one of the Congolese regions most affected by fatal landslides. However, information on the distribution of these processes and their impact on society is still lacking. Here we present a first regional landslide inventory and the associated elements at risk. The inventory was conducted in an area of 5,700 km² in three administrative territories between the cities of Bukavu and Goma. The region is one of the most densely populated area of DRC with a density of up to 200 persons/km². The approach for the inventory relies on visual analysis of Google Earth imagery and a 5 m resolution DEM that we produced from TanDEM-X interferometry. Field validation was performed in target places accounting for 5% of the study area. More than 2,000 landslides were mapped and distinction was made between deep and shallow, and slide and flow processes. Average landslide area is 6 ha (max. = 430 ha). Geomorphological analysis of landslide distribution shows topographic, lithologic, climatic and seismic controls. For 600 randomly-selected landslides, elements at risk (house, road, cultivated land, river) were inventoried in the areas affected and potentially affected by the instabilities; 10% of the landslides are inhabited and 25% do not present any risk. Numerous landslides have caused direct and indirect damage in recent years. In some places, the impact of mining activities on slope stability can be important. Google Earth was the only way to locate the recent shallow failures triggered by known extreme rainfall events. This inventory is a first step towards the understanding of the landslide processes in the region. Further studies are needed to complete and validate the information, to better infer about the triggers, and to compute susceptibility and risk maps.

  11. Dynamic processes in the lithosphere leading to extension, rifting and basin formation

    NASA Astrophysics Data System (ADS)

    Anderson, Thomas

    2014-05-01

    -complexes may form in regions of coupled lithosphere within domains of ductile crust, as well as along the Alpine-Himalayan suture where contemporaneous ductile shearing and detachment are known. During westward movement, mantle displaced at the front of western NA, may form a swell that progresses eastward causing cratonal uplift and exhumation of cover strata. Co-genetic intra-continental unconformities record the passage of the mantle swell. Where mantle flows toward the wake of the moving plates, fluid pincers may push Africa and Eurasia closer thereby inducing subduction and adding to constriction recorded by uplift and cooling. De-coupling generally takes place after ca. 10 Ma of cooling of the oceanic lithosphere followed by resumption of subduction. The diverse movements of the Pacific and NA plates suggest that the motions of oceanic and continental plates are independent and that movements take place in response to different forces. Oceanic plates that passively move in response to gravitational forces acting upon the coolest parts of the slabs differ from large continental plates such as NA and Australia that may move in response to the effects of convecting mantle acting upon their lithospheric keels. Hiatuses between extensional episodes record decoupling of oceanic lithosphere from the overlying plate, a process that takes at least 10 m.y.

  12. US-Africa collaborative research on incipient continental rift zones

    NASA Astrophysics Data System (ADS)

    Atekwana, E. A.

    2007-12-01

    Since 1999, we have been conducting research in Botswana in collaboration with colleagues at the University of Botswana (UB). Recently, we have expanded our research activities to include the University of Zambia (UNZA). The goal of the collaborative efforts center on investigating geologic processes operating during the initial stages of continental extension. During student training, US students partner with peers from UB and UNZA to conduct field-based research within a multi-disciplinary framework focused on investigating the interplay between neotectonics and surficial processes due to rifting. The student projects are designed to: 1) assess the role of pre-existing structures on rift basin development; 2) determine fault kinematics and direction of rift extension; 3) characterize the geometry of the basins; 4) assess current models for fault growth and propagation and linkage to form border faults; 5) investigate environmental change information preserved in rift basin sediments; 6) determine how magma below the rift basin affects surface water chemical properties; and 7) develop tectonic and geologic models for the evolution of rift basins during the incipient stages of continental extension. Our goal is to provide is to improve research and education in developing countries while providing talented and motivated US students with hands-on field research experience in near surface geophysical surveying, field geologic mapping, GPS mapping, and geochemical and hydrogeologic techniques necessary for addressing basic research questions in the geosciences, as well as resources exploration (e.g., hydrocarbon, water resources, mineral, geothermal, etc.). Our US students acquire an enriching cultural experience, make personal contacts, and build relationships that will form the core of future international research collaborations. At the same time, project activities introduce the African students to state-of the art geophysical equipment and research methodologies

  13. Magmatism on rift flanks: Insights from ambient noise phase velocity in Afar region

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Weemstra, Cornelis; Leroy, Sylvie; Boschi, Lapo; Keir, Derek; Ren, Yong; Molinari, Irene; Ahmed, Abdulhakim; Stuart, Graham W.; Rolandone, Frédérique; Khanbari, Khaled; Hammond, James O. S.; Kendall, J. M.; Doubre, Cécile; Ganad, Ismail Al; Goitom, Berhe; Ayele, Atalay

    2015-04-01

    During the breakup of continents in magmatic settings, the extension of the rift valley is commonly assumed to initially occur by border faulting and progressively migrate in space and time toward the spreading axis. Magmatic processes near the rift flanks are commonly ignored. We present phase velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only the upper crust beneath the axial volcanic systems but also both upper and lower crust beneath the rift flanks where ongoing volcanism and hydrothermal activity occur at the surface. Magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process and may persist through to early seafloor spreading.

  14. Process involved in the propagation of rifts across the Ronne Ice Shelf, near the region of Hemmen Ice Rise

    NASA Technical Reports Server (NTRS)

    Larour, E.; Rignot, E.; Aubry, D.

    2002-01-01

    We use radar interferometric images collected by ERS-1 and Radarsat-1 to observe the rupture tip of rifts along Hemmen Ice Rise on the Ronne Ice Shelf, Antarctica. Interferograms generated in 1992 and 1997 allow us to observe the deformation of ice accumulated over respectively 9 and24 days. The results are an important step towards developing a better model of ice-shelf calving mechanism.

  15. Evidence of Ancient Rifts Beneath Texas

    NASA Astrophysics Data System (ADS)

    Irie, K.; Velasco, A. A.

    2011-12-01

    Continental rifts are defined as geological features where Earth's lithosphere is pulled away by surface expansion of the Earth. Their physiographic features include linear rift valleys associated with active volcanism. Many rifts fail to split a continent and ancient rifts that failed to split can be found by using seismic waves to image these ancient structures. Using seismic data collected by EarthScope USArray stations in Texas, we calculate teleseismic receiver functions and utilized surface wave dispersion curves to simultaneously invert for the velocity structure beneath each seismic station. With the calculated receiver functions, we generate maps to show preliminary 3-D crust/upper mantle boundary structure, the velocity ratio of P and S waves, and the S-wave velocity structure. We expect to characterize the ancient rift zones that exist in Texas and compare these results with the Rio Grande Rift in New Mexico. The goal for this comparison is to determine whether Rio Grande rift is still active or doomed to be another failed rift.

  16. A quantitative geomorphological approach to constraining the volcanic and tectonic evolution of the active Dabbahu rift segment, Afar, Ethiopia.

    NASA Astrophysics Data System (ADS)

    Medynski, Sarah; Pik, Raphaël; Burnard, Peter; Vye-Brown, Charlotte; Blard, Pierre-Henri; France, Lydéric; Dumont, Stéphanie; Grandin, Raphaël; Schimmelpfennig, Irene; Benedetti, Lucilla; Ayalew, Dereje; Yirgu, Gezahegn

    2013-04-01

    In the Afar depression (Ethiopia), extension is organised along rift segments that morphologically resemble oceanic rifts. Segmentation results from interactions between dyke injection and volcanism, as observed during the well-documented 2005 rifting event on the Dabbahu rift segment. This tectono-volcanic crisis was observed in detail via remote sensing techniques, providing invaluable information on the present-day tectonic - magmatic interplay during a sequence of dyke intrusions. However, lack of data remains on timescales of 1 to 100 kyr, the period over which the main morphology of the rift is acquired. The Dabbahu rift segment represents an ideal natural laboratory to study the evolution of rift morphology as a response to volcanic and tectonic influences. We use cosmogenic nuclides (3He and 36Cl) to determine the ages of young (<100 kyr) lava flows and to date the initiation and movement of fault scarps, which cut the lavas. Where possible, we analysed vertical profiles along fault scarps, in an attempt to distinguish individual tectonic events that offset the scarp, estimate their amplitudes and date the recurrence intervals. These geochronological constraints, combined with major & trace element compositions, field mapping and digital mapping (Landsat, ASTER and SPOT imagery), provide valuable insights on the magmatic and tectonic history of the segment. The results show that over the last 100 ka, the northern part of the Dabbahu segment was supplied by at least two different magma reservoirs, which can be identified from their distinctive chemistries. The main reservoir is located beneath Dabbahu volcano at the northern tip of the rift segment, and has been supplied with magma for at least 72 ka. The second reservoir is located further south on the rift axis and corresponds to the current mid-segment magma chamber, which was responsible for the 2005 rifting episode. Two magmatic cycles linked to the Dabbahu magma chamber were recorded, lasting 20-30 kyr

  17. Recent and Hazardous Volcanic Activity Along the NW Rift Zone of Piton De La Fournaise Volcano, La Réunion Island

    NASA Astrophysics Data System (ADS)

    Walther, G.; Frese, I.; Di Muro, A.; Kueppers, U.; Michon, L.; Metrich, N.

    2014-12-01

    Shield volcanoes are a common feature of basaltic volcanism. Their volcanic activity is often confined to a summit crater area and rift systems, both characterized by constructive (scoria and cinder cones; lava flows) and destructive (pit craters; caldera collapse) phenomena. Piton de la Fournaise (PdF) shield volcano (La Réunion Island, Indian Ocean) is an ideal place to study these differences in eruptive behaviour. Besides the frequent eruptions in the central Enclos Fouqué caldera, hundreds of eruptive vents opened along three main rift zones cutting the edifice during the last 50 kyrs. Two short rift zones are characterized by weak seismicity and lateral magma transport at shallow depth (above sea level). Here we focus on the third and largest rift zone (15km wide, 20 km long), which extends in a north-westerly direction between PdF and nearby Piton des Neiges volcanic complex. It is typified by deep seismicity (up to 30 km), emitting mostly primitive magmas, testifying of high fluid pressures (up to 5 kbar) and large-volume eruptions. We present new field data (including stratigraphic logs, a geological map of the area, C-14 dating and geochemical analyses of the eruption products) on one of the youngest (~6kyrs) and largest lava field (Trous Blancs eruption). It extends for 24km from a height of 1800 m asl, passing Le Tampon and Saint Pierre cities, until reaching the coast. The source area of this huge lava flow has been identified in an alignment of four previously unidentified pit craters. The eruption initiated with intense fountaining activity, producing a m-thick bed of loose black scoria, which becomes densely welded in its upper part; followed by an alternation of volume rich lava effusions and strombolian activity, resulting in the emplacement of meter-thick, massive units of olivine-basalt alternating with coarse scoria beds in the proximal area. Activity ended with the emplacement of a dm-thick bed of glassy, dense scoria and a stratified lithic

  18. Surface analogue outcrops of deep fractured basement reservoirs in extensional geological settings. Examples within active rift system (Uganda) and proximal passive margin (Morocco).

    NASA Astrophysics Data System (ADS)

    Walter, Bastien; Géraud, Yves; Diraison, Marc

    2014-05-01

    structures). Two field cases, located in Morocco and Uganda, allow us to investigate basement complexes at different stages of an extension process and give us analog geological data of similar fractured basement reservoirs. Border faults and associated fracture networks of an active rifting system propagated in Proterozoic basement rocks are analyzed in the Albertine rift system in Uganda. Brittle structures developed along a proximal passive margin of the Atlantic domain are analyzed in Proterozoic basements rocks in Western Anti-Atlas in Morocco.

  19. Subaerial exposure and drowning processes in a carbonate platform during the Mesozoic Tethyan rifting: The case of the Jurassic succession of Western Sicily (central Mediterranean)

    NASA Astrophysics Data System (ADS)

    Sulli, A.; Interbartolo, F.

    2016-01-01

    The Liassic carbonate platform succession outcropping at Monte Maranfusa (central Western Sicily) consists of a shallowing-upward sequence of peritidal carbonates, with Jurassic to Paleogene pelagic limestone and siliciclastic Tertiary covers above. The cyclic sequences of subtidal wackestones/packstones, intertidal microcrystalline carbonates with bird's-eye pores, and supratidal bioclastic grainstones are interbedded with dark layers of the following composition: 1) dark-gray, compact, and well-cemented limestone with blackish clasts, interpreted as calcretes (a type of carbonate soil) and 2) reddish calcite laminae, deformed by elongated cavities, filled with vadose silt, interpreted as paleokarst. This succession is crossed by almost vertical faults, of the Late Liassic to Miocene, which often coincides with neptunian dykes, filled by several generations of Toarcian-Early Miocene pelagic sediments. Another system of dykes, known as neptunian sills, filled by injected Upper Lias-Dogger pelagic sediments, lies parallel to the stratification. The parallel dykes were caused by the flexure of the platform during the Jurassic and presumably by a planar slip in the carbonate rocks, whereas neptunian dykes are caused by faulting episodes. Here, we present evidence that the dark layers in the Liassic succession of Monte Maranfusa, previously described by many authors only as parallel dykes, can actually be interpreted as a) neptunian sills, b) pedogenic calcretes, and c) paleospeleothems. Therefore, we found evidence of exposure/flooding intervals in the evolution of the carbonate platform during the Liassic, linked to different pulses in both the subsidence/tectonic activity and the sea-level oscillations. At the top, Fe-Mn crusts (hardgrounds) seal the carbonate platform succession, which is in turn overlain by condensed pelagic deposits, confirming its drowning during rifting processes.

  20. Variation in styles of rifting in the Gulf of California.

    PubMed

    Lizarralde, Daniel; Axen, Gary J; Brown, Hillary E; Fletcher, John M; González-Fernández, Antonio; Harding, Alistair J; Holbrook, W Steven; Kent, Graham M; Paramo, Pedro; Sutherland, Fiona; Umhoefer, Paul J

    2007-07-26

    Constraints on the structure of rifted continental margins and the magmatism resulting from such rifting can help refine our understanding of the strength of the lithosphere, the state of the underlying mantle and the transition from rifting to seafloor spreading. An important structural classification of rifts is by width, with narrow rifts thought to form as necking instabilities (where extension rates outpace thermal diffusion) and wide rifts thought to require a mechanism to inhibit localization, such as lower-crustal flow in high heat-flow settings. Observations of the magmatism that results from rifting range from volcanic margins with two to three times the magmatism predicted from melting models to non-volcanic margins with almost no rift or post-rift magmatism. Such variations in magmatic activity are commonly attributed to variations in mantle temperature. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present crustal-scale images across three rift segments. Over short lateral distances, we observe large differences in rifting style and magmatism--from wide rifting with minor synchronous magmatism to narrow rifting in magmatically robust segments. But many of the factors believed to control structural evolution and magmatism during rifting (extension rate, mantle potential temperature and heat flow) tend to vary over larger length scales. We conclude instead that mantle depletion, rather than low mantle temperature, accounts for the observed wide, magma-poor margins, and that mantle fertility and possibly sedimentary insulation, rather than high mantle temperature, account for the observed robust rift and post-rift magmatism.

  1. Variation in styles of rifting in the Gulf of California.

    PubMed

    Lizarralde, Daniel; Axen, Gary J; Brown, Hillary E; Fletcher, John M; González-Fernández, Antonio; Harding, Alistair J; Holbrook, W Steven; Kent, Graham M; Paramo, Pedro; Sutherland, Fiona; Umhoefer, Paul J

    2007-07-26

    Constraints on the structure of rifted continental margins and the magmatism resulting from such rifting can help refine our understanding of the strength of the lithosphere, the state of the underlying mantle and the transition from rifting to seafloor spreading. An important structural classification of rifts is by width, with narrow rifts thought to form as necking instabilities (where extension rates outpace thermal diffusion) and wide rifts thought to require a mechanism to inhibit localization, such as lower-crustal flow in high heat-flow settings. Observations of the magmatism that results from rifting range from volcanic margins with two to three times the magmatism predicted from melting models to non-volcanic margins with almost no rift or post-rift magmatism. Such variations in magmatic activity are commonly attributed to variations in mantle temperature. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present crustal-scale images across three rift segments. Over short lateral distances, we observe large differences in rifting style and magmatism--from wide rifting with minor synchronous magmatism to narrow rifting in magmatically robust segments. But many of the factors believed to control structural evolution and magmatism during rifting (extension rate, mantle potential temperature and heat flow) tend to vary over larger length scales. We conclude instead that mantle depletion, rather than low mantle temperature, accounts for the observed wide, magma-poor margins, and that mantle fertility and possibly sedimentary insulation, rather than high mantle temperature, account for the observed robust rift and post-rift magmatism. PMID:17653189

  2. Assessment and recommendations for two sites with active and potential aquaculture production in Rift Valley and Coast Provinces, Kenya

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Kenya has a long history of local fish consumption. The population in the Lake Victoria area (Rift Valley Province) Northwest of Nairobi and coastal communities (Coast Province) have historically included fish in their diet. Migration from villages to urban areas and increasing commerce has created ...

  3. Prediction, Assessment of the Rift Valley Fever Activity in East and Southern Africa 2006 - 2008 and Possible Vector Control Strategies

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Historical episodic outbreaks of Rift Valley fever (RVF) since the early 1950s have been associated with cyclical patterns (El Niño and La Niña) of El Niño Southern Oscillation (ENSO) phenomenon which results in elevated and widespread rainfall over the RVF endemic areas of Africa. Using satellite ...

  4. Prediction, Assessment of the Rift Valley fever Activity in East and Southern Africa 2006 - 2008 and Possible Vector Control Strategies

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Historical outbreaks of Rift Valley fever (RVF) since the early 1950s have been associated with cyclical patterns of the El Nino/Southern Oscillation (ENSO) phenomenon which results in elevated and widespread rainfall over the RVF endemic areas of Africa. Using satellite measurements of global and ...

  5. Olivine-liquid relations of lava erupted by Kilauea volcano from 1994 to 1998: Implications for shallow magmatic processes associated with the ongoing east-rift-zone eruption

    USGS Publications Warehouse

    Thornber, C.R.

    2001-01-01

    From 1994 through 1998, the eruption of Ki??lauea, in Hawai'i, was dominated by steady-state effusion at Pu'u 'O??'??o that was briefly disrupted by an eruption 4 km uprift at Np??au Crater on January 30, 1997. In this paper, I describe the systematic relations of whole-rock, glass, olivine, and olivine-inclusion compositions of lava samples collected throughout this interval. This suite comprises vent samples and tube-contained flows collected at variable distances from the vent. The glass composition of tube lava varies systematically with distance and allows for the "vent-correction" of glass thermometry and olivine-liquid KD as a function of tube-transport distance. Combined olivine-liquid data for vent samples and "vent-corrected" lava-tube samples are used to document pre-eruptive magmatic conditions. KD values determined for matrix glasses and forsterite cores define three types of olivine phenocrysts: type A (in equilibrium with host glass), type B (Mg-rich relative to host glass) and type C (Mg-poor relative to host glass). All three types of olivine have a cognate association with melts that are present within the shallow magmatic plumbing system during this interval. During steady-state eruptive activity, the compositions of whole-rock, glass and most olivine phenocrysts (type A) all vary sympathetically over time and as influenced by changes of magmatic pressure within the summit-rift-zone plumbing system. Type-A olivine is interpreted as having grown during passage from the summit magmachamber along the east-rift-zone conduit. Type-B olivine (high Fo) is consistent with equilibrium crystallization from bulk-rock compositions and is likely to have grown within the summit magma-chamber. Lower-temperature, fractionated lava was erupted during non-steady state activity of the Na??pau Crater eruption. Type-A and type-B olivine-liquid relations indicate that this lava is a mixture of rift-stored and summit-derived magmas. Post-Na??pau lava (at Pu'u 'O?? 'o

  6. Magnetotelluric and geomagnetic deep-sounding studies in rifts and adjacent areas: constraints on physical processes in the crust and upper mantle

    SciTech Connect

    Hermance, J.F.

    1982-01-01

    Deep electrical studies are reviewed along with other supportive geophysical/geological investigations of five of the major rift zones of the world: the Baikal rift, and Rhine graben, the East African rift, Iceland and the Rio Grande rift. All of these regions exhibit anomalously low values of electrical resistivity, density and seismic velocity, either within the crust itselt or at high levels in the mantle. Deep electrical studies support a model where ascending masses of material from the mantle are intimately coupled to the fractionation of a basalt melt and its accumulation at higher levels within the earth. In Iceland, an interplate rift, the accumulation and chilling of melt at the base of the crust apparently leads to a significant component of crustal underplating whereas in intraplate rifts such as the Rio Grande rift, the emplacement of basaltic magma at high levels may lead to extensive remelting of the crust, triggering eruptive episodes of silicic magmas.

  7. 3D Dynamics of Oblique Rift Systems: Fault Evolution from Rift to Break-up

    NASA Astrophysics Data System (ADS)

    Brune, S.

    2014-12-01

    Rift evolution and passive margin formation has been thoroughly investigated using conceptual and numerical models in two dimensions. However, the 2D assumption that the extension direction is perpendicular to the rift trend is often invalid. In fact, the majority of rift systems that lead to continental break-up during the last 150 My involved moderate to high rift obliquity. Yet, the degree to which oblique lithospheric extension affects first-order rift and passive margin properties like surface stress pattern, fault azimuths, and basin geometry, is still not entirely clear. This contribution provides insight in crustal stress patterns and fault orientations by applying a 3D numerical rift model to oblique extensional settings. The presented forward experiments cover the whole spectrum of oblique extension (i.e. rift-orthogonal extension, low obliquity, high obliquity, strike-slip deformation) from initial deformation to breakup. They are conducted using an elasto-visco-plastic finite element model and involve crustal and mantle layers accounting for self-consistent necking of the lithosphere. Even though the model setup is very simple (horizontally layered, no inherited faults), its evolution exhibits a variety of fault orientations that are solely caused by the interaction of far-field stresses with rift-intrinsic buoyancy and strength. Depending on rift obliquity, these orientations involve rift-parallel, extension-orthogonal, and intermediate normal fault directions as well as strike-slip faults. Allowing new insights on fault patterns of the proximal and distal margins, the model shows that individual fault populations are activated in a characteristic multi-phase evolution driven by lateral density variations of the evolving rift system. Model results are in very good agreement with inferences from the well-studied Gulf of Aden and provide testable predictions for other rifts and passive margins worldwide.

  8. Active Extensional Faulting at the Southern Half-Graben Belt of the Tepic-Zacoalco Rift, Western Mexico

    NASA Astrophysics Data System (ADS)

    Rosas-Elguera, J.; Ferrari, L.; Delgado, M.; Uribe, A.; Valdivia, L.; Castillo, R.

    2003-12-01

    In the past decade much debate has centered upon the kinematics and the mechanism of continental deformation in western Mexico and the motion of the Jalisco block relative to North America. Two distinct models have been proposed. The first one suggest a NW-motion of the Jalisco block that would implies a right-lateral faulting along the Tepic-Zacoalco rift (TZR). More recently others authors have documented a N-NE extensional tectonics active since late Miocene and suggested that the continental boundaries of the Jalisco block, are older structures reactivated by plate boundary forces. Studies on the crustal seismicity and the kinematics of Quaternary faults provide another constraint on the direction of motion between the Jalisco block and North America. On November 4, 5, 6, and 7, 1995, one month after the October 09, 1995, Manzanillo earthquake (Mw = 8.0), a swarm of small events was felt in the Amatlan de Ca¤as half-graben and recorded by the regional seismic network of Comision Federal de Electricidad. The coda magnitude of the largest event was Mc = 2.5-3.6 and the events were located depth ranging from 6 to 10 km. This seismic activity provoked that people from Pie de la Cuesta and Yerbabuena villages were evacuated. After that a seismic station equipped with an analogic seismograph MEQ-800 at Pie de la Cuesta was installed for three months. During the same time, October, 1995, some houses distributed along a WNW trend in Ameca city underwent severe damages, they are. The digital elevations model of the Ameca city suggest that several structures tectonics are shorter than 2 km are present in the area. The present direction of motion of the Rivera plate relative to North America plate along Middle America Trench has been estimated between N19° E to N48° E (e.g. Bandy et al., 1996). During the October 09, 1995, subduction-related earthquake (Mw = 8.0) a GPS network recorded a SW motion of the Jalisco block which could be associated to an elastic deformation

  9. The role of Variscan to pre-Jurassic active extension in controlling the architecture of the rifted passive margin of Adria: the example of the Canavese Zone (Western Southern Alps, Italy)

    NASA Astrophysics Data System (ADS)

    Succo, Andrea; De Caroli, Sara; Centelli, Arianna; Barbero, Edoardo; Balestro, Gianni; Festa, Andrea

    2016-04-01

    The Canavese Zone, in the Italian Western Southern Alps, represents the remnant of the Jurassic syn-rift stretching, thinning and dismemberment of the distal passive margin of Adria during the opening of the Penninic Ocean (i.e., Northern Alpine Tethys). Our findings, based on detailed geological mapping, structural analysis and stratigraphic and petrographic observations, document however that the inferred hyper-extensional dismemberment of this distal part of the passive margin of Adria, up to seafloor spreading, was favored by the inherited Variscan geometry and crustal architecture of the rifted margin, and by the subsequent Alpine-related strike-slip deformation. The new field data document, in fact, that the limited vertical displacement of syn-extensional (syn-rift) Jurassic faults was ineffective in producing and justifying the crustal thinning observed in the Canavese Zone. The deformation and thinning of the continental basement of Adria are constrained to the late Variscan time by the unconformable overlying of Late Permian deposits. Late Cretaceous-Early Paleocene and Late Cenozoic strike-slip faulting (i.e., Alpine and Insubric tectonic stages) reactivated previously formed faults, leading to the formation of a complex tectonic jigsaw which only partially coincides with the direct product of the Jurassic syn-rift dismemberment of the distal part of the passive margin of Adria. Our new findings document that this dismemberment of the rifted continental margin of Adria did not simply result from the syn-rift Jurassic extension, but was strongly favored by the inheritance of older (Variscan and post-Variscan) tectonic stages, which controlled earlier lithospheric weakness. The formation of rifted continental margins by extension of continental lithosphere leading to seafloor spreading is a complex and still poorly understood component of the plate tectonic cycle. Geological mapping of rifted continental margins may thus provide significant information to

  10. The Importance of Magmatic Fluids in Continental Rifting in East Africa

    NASA Astrophysics Data System (ADS)

    Muirhead, J.; Kattenhorn, S. A.; Ebinger, C. J.; Lee, H.; Fischer, T. P.; Roecker, S. W.; Kianji, G.

    2015-12-01

    The breakup of strong continental lithosphere requires more than far-field tectonic forces. Growing evidence for early-stage cratonic rift zones points to the importance of heat, magma and volatile transfer in driving lithospheric strength reduction. The relative contributions of these processes are fundamental to our understanding of continental rifting. We present a synthesis of results from geological, geochemical and geophysical studies in one of the most seismically and volcanically active sectors of the East African Rift (Kenya-Tanzania border) to investigate the role of fluids during early-stage rifting (<10 Ma). Xenolith data indicate that rifting initiated in initially thick lithosphere. Diffuse soil CO2 flux maxima occur in the vicinity of faults, with carbon isotope values exhibiting a mantle-derived signature. These faults feed aligned sets of hydrothermal springs, which have N2-He-Ar relative abundances also indicating a mantle-derived source. Geochemical and surface faulting information are integrated with subsurface imaging and fault kinematic data derived from the 38-station CRAFTI broadband seismic array. Teleseismic and abundant local earthquakes enable assessment of the state-of-stress and b-values as a function of depth. High Vp/Vs ratios and tomographic imaging suggest the presence of fluids in the crust, with high pore fluid pressures driving failure at lower tectonic stress. Together, these cross-disciplinary data provide compelling evidence that early-stage rifting in East Africa is assisted by fluids exsolved from deep magma bodies, some of which are imaged in the lower crust. We assert that the flux of deep magmatic fluids during rift initiation plays a key role in weakening lithosphere and localizing strain. High surface gas fluxes, fault-fed hydrothermal springs and persistent seismicity highlight the East African Rift as the ideal natural laboratory for investigating fluid-driven faulting processes in extensional tectonic environments.

  11. Spatial and Temporal Evolution of Eruptive Activity in a Youthful Extensional Setting: the Case of the Nyamulagira Volcanic Field, Western Branch of the East African Rift

    NASA Astrophysics Data System (ADS)

    Smets, B.; Kervyn, M.; d'Oreye, N.; Kervyn, F.

    2014-12-01

    Nyamulagira is the westernmost volcano of the Virunga volcanic province, in the western branch of the East African Rift. This shield volcano is one of the most active African volcanoes with one eruption every 1-4 year(s). Nyamulagira's eruptions usually occur along the flanks of the main edifice and in the lava plain, producing pyroclastic cone(s) and 10-20 km-long lava flows. Between 1913 and 1938, the activity was however restricted to the summit caldera, where lava fountains progressively gave birth to a lava lake, which disappeared in 1938 during the partial collapse of the summit caldera and the onset of a 2.5 years-long flank eruption. The location of flank eruptions and the orientation of the eruptive fissures are strongly influenced by the edifice loading, and by the NNW-SSE fracture network that crosses the main edifice and link it to the neighboring Nyiragongo volcano. But rift fault can also influence fissure orientations and cone alignments, especially for distal events. The flank eruptions typically have similar characteristics, lasting few days to few weeks, with an average of 20-30 days. Less frequently, flank eruptions can be larger and more complex, lasting several months and/or emitting much larger volumes of lava. By combining historical and recent observations, we suggest that magma overpressure at shallow depth is the main cause of flank events. Major eruptions seem to be related to a deeper source able to trigger large magma injections through deep structures, such as rift faults. Since April 2012, the activity of Nyamulagira is restricted to the summit caldera, with continuous and intense gas emissions and, since mid-2014, by lava fountains. This change in eruptive behavior, if it persists, may leads to the emergence of a new lava lake and may significantly decreases the frequency of flank events

  12. The formation of the south-eastern part of the Dniepr Donets Basin: 2-D forward and reverse modelling taking into account post-rift redeposition of syn-rift salt

    NASA Astrophysics Data System (ADS)

    Stovba, S. M.; Maystrenko, Yu. P.; Stephenson, R. A.; Kusznir, N. J.

    2003-02-01

    Forward and reverse modelling of structure and stratigraphy has been used to investigate the syn-rift (Late Devonian) and early post-rift (Carboniferous) evolution of the south-eastern part of the Dniepr-Donets Basin (DDB). Modelling was carried out with and without taking into consideration the withdrawal and surface extrusion of Devonian salt during the formation of salt diapirs. The great thickness of Carboniferous deposits can be explained by the superimposed actions of three processes: post-rift thermal subsidence, withdrawal of Devonian salt from the mother layer during phases of salt diapir activity, and regional subsidence of the East European Platform. The effects of other tectonic and/or non-tectonic processes are not required. Forward syn-rift modelling using the flexural cantilever model of sedimentary basin formation predicts the total syn-rift extension across the southeastern DDB to be approximately 65 km with a maximum β stretching factor of 2.4. Shallowing of the Moho during the syn-rift phase is estimated to be 15 km. The present-day Moho, after thermal subsidence and basin fill, is predicted to be 4-6 km shallower than surrounding regions. In the axial zone of the south-eastern DDB the thickness of the Devonian syn-rift sequence may have reached 7.5 km by the end of the rift stage. This is 3-3.5 km more than at present. The thicknesses reduction is due to the outflow of Devonian salt during post-rift periods of halokinetic activity in the early Visean, the middle Serpukhovian, and in the Early Permian. The withdrawal of salt from the mother layer produced additional accommodation space and up to 1.5-1.7 km of the total eventual thickness of the Carboniferous sedimentary succession can be explained as a result of this.

  13. 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.

  14. The Porcupine Basin: from rifting to continental breakup

    NASA Astrophysics Data System (ADS)

    Reston, Timothy; Gaw, Viola; Klaeschen, Dirk; McDermott, Ken

    2015-04-01

    Southwest of Ireland, the Porcupine Basin is characterized by axial stretching factors that increase southward to values greater than six and typical of rifted margins. As such, the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup, and in particular the processes of mantle serpentinisation, and the onset of detachment faulting. We have processed through to prestack depth migration a series of E-W profiles crossing the basin at different axial stretching factors and linked by a N-S profile running close to the rift axis. Our results constrain the structure of the basin and have implications for the evolution of rifted margins. In the north at a latitude of 52.25N, no clear detachment is imaged, although faults do appear to cut down into the mantle, so that serpentinisation may have started. Further south (51.75N), a bright reflection (here named P) cuts down to the west from the base of the sedimentary section, is overlain by small fault blocks and appears to represent a detachment fault. P may in part follow the top of partially serpentinized mantle: this interpretation is consistent with gravity modelling, with numerical models of crustal embrittlement and mantle serpentinization during extension and with wide-angle data (see posters of Prada and of Watremez). Furthermore, P closely resembles the S reflection west of Iberia, where such serpentinites are well documented. P develops where the crust was thinned to less than 3 km during rifting, again similar to S. Although overall the basin remains symmetrical, the consistent westward structural dip of the detachment implies that, at high stretching factors, extension became asymmetric. Analysis of the depth sections suggests that the detachment may have been active as a rolling hinge rooting at low-angle beneath the Porcupine Bank, consistent with the presence of a footwall of serpentinites. This requires very weak

  15. Rift Valley Fever Virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever virus (RVFV) is a mosquito-transmitted virus or arbovirus that is endemic in sub-Saharan Africa. In the last decade, Rift Valley fever (RVF) outbreaks have resulted in loss of human and animal life, as well as had significant economic impact. The disease in livestock is primarily a...

  16. New perspectives on the evolution of narrow, modest extension continental rifts: Embryonic core complexes and localized, rapid Quaternary extension in the Rio Grande rift, central New Mexico

    NASA Astrophysics Data System (ADS)

    Ricketts, J.; Karlstrom, K. E.; Kelley, S.

    2013-12-01

    Updated models for continental rift zones need to address the role and development of low-angle normal fault networks, episodicity of extension, and interaction of 'active and passive' driving mechanisms. In the Rio Grande rift, USA, low-angle normal faults are found throughout the entire length of the rift, but make up a small percentage of the total fault population. The low-angle Jeter and Knife Edge faults, for example, crop out along the SW and NE margins of the Albuquerque basin, respectively. Apatite fission track (AFT) age-elevation data and apatite (U-Th)/He (AHe) ages from these rift flank uplifts record cooling between ~21 - 16 Ma in the NE rift flank and ~20 - 10 Ma in the SW, which coincides with times of rapid extension and voluminous syntectonic sedimentation. The timing of exhumation is also similar to rift flanks farther north in active margins based on AFT data alone. In addition, synthetic faults in the hanging wall of each low-angle fault become progressively steeper and younger basinward, and footwall blocks are the highest elevation along the rift flanks. These observations are consistent with a model where initially high-angle faults are shallowed in regions of maximum extension. As they rotate, new intrabasinal faults emerge which also can be rotated if extension continues. These relationships are similarly described in mature core complexes, and if these processes continued in the Rio Grande rift, it could eventually result in mid-crustal ductily deformed rocks in the footwall placed against surficial deposits in the hanging wall across faults that have been isostatically rotated to shallow dips. Although existing data are consistent with highest strain rates during a pulse of extension along the entire length of the rift 20-10 Ma., GPS-constrained measurements suggest that the rift is still actively-extending at 1.23-1.39 nstr/yr (Berglund et al., 2012). Additional evidence for Quaternary extension comes from travertine deposits that are

  17. Off-axis volcanism in the Gregory rift, east Africa: implications for models of continental rifting

    SciTech Connect

    Bosworth, W.

    1987-05-01

    The largest volcanic centers of the Gregory rift occur in two belts located 100 to 150 km east and west of the axis of the rift valley. These off-axis volcanic belts include the highest peaks on the continent of Africa and are interpreted to lie above the intersection of low-angle detachment systems with the base of a regionally thinned lithosphere. These detachment systems are manifested at the surface as a series of breakaway zones and regional bounding faults that produce subbasins with half-graben form. The asymmetry of subbasins alternates along the rift axis, indicating that the polarity of the underlying active detachment systems also reverses. The detachments are separated laterally by regional oblique-slip accommodation zones typified by wrench-style tectonism. Off-axis from the rift, the detachments are inferred to merge along strike as they cut to the base of the lithosphere. This results in irregular but persistent paired zones of volcanism and lithospheric thinning off-axis from the rift proper. The development of major volcanic cones such as Mount Kilimanjaro may be controlled by the interaction of leaky accommodation zones with the regions of structurally thinned lithosphere. The central Kenya hot spot has produced the anomalous quantities of volcanic material that fills the Gregory rift and probably enhances the off-axis volcanism but does not directly control its location. The model proposed here for tectonic controls of volcanism in the Gregory rift may be applicable to Phanerozoic continental rifts in general.

  18. Tectono-Sedimentary Analysis of Rift Basins: Insights from the Corinth Rift, Greece

    NASA Astrophysics Data System (ADS)

    Gawthorpe, Robert; Ford, Mary

    2015-04-01

    Existing models for the tectono-sedimentary evolution of rift basins are strongly linked the growth and linkage of normal fault segments and localization of fault activity. Early stages of faulting (rift initiation phase) are characterized by distributed, short, low displacement fault segments, subdued fault-related topography and small depocentres within which sedimentation keeps pace with subsidence. Following linkage and displacement localization (rift climax phase), deformation if focused onto major, crustal-scale fault zones with kilometre-scale displacement. These major faults generate pronounced tilted fault-block topography, with subsidence rates that outpace sedimentation causing a pronounced change to deep-water deposition. Such models have been successful in helping to understand the gross structural and sedimentary evolution of rift basins, but recent work has suggested that pre-existing structures, normal fault interaction with pre-rift salt and antecedent drainage systems significantly alter this initiation-to-climax perspective of rift basin development. The E-W-striking, Pliocene-Pleistocene Corinth rift, central Greece, is an excellent natural laboratory for studying the tectono-sedimentary evolution of rift basins due to its young age, excellent onshore exposure of syn-rift structure and stratigraphy and extensive offshore seismic data. The rift cuts across the NW-SE-striking Hellenide mountain belt and has migrated northward and westward during its evolution. The Hellenide mountain belt significantly influences topography and drainage in the west of the rift. High topography and large antecedent drainage systems, focused along palaeovalleys, provided high sediment flux to NE-flowing alluvial systems that overfilled early-rift depocentres. Further east, away from the main antecedent drainage networks, contemporaneous deposits comprise deep-lacustrine turbidite channel and lobe complexes and basinal marls. Thus the stratigraphic expression within

  19. The importance of rift history for volcanic margin formation.

    PubMed

    Armitage, John J; Collier, Jenny S; Minshull, Tim A

    2010-06-17

    Rifting and magmatism are fundamental geological processes that shape the surface of our planet. A relationship between the two is widely acknowledged but its precise nature has eluded geoscientists and remained controversial. Largely on the basis of detailed observations from the North Atlantic Ocean, mantle temperature was identified as the primary factor controlling magmatic production, with most authors seeking to explain observed variations in volcanic activity at rifted margins in terms of the mantle temperature at the time of break-up. However, as more detailed observations have been made at other rifted margins worldwide, the validity of this interpretation and the importance of other factors in controlling break-up style have been much debated. One such observation is from the northwest Indian Ocean, where, despite an unequivocal link between an onshore flood basalt province, continental break-up and a hot-spot track leading to an active ocean island volcano, the associated continental margins show little magmatism. Here we reconcile these observations by applying a numerical model that accounts explicitly for the effects of earlier episodes of extension. Our approach allows us to directly compare break-up magmatism generated at different locations and so isolate the key controlling factors. We show that the volume of rift-related magmatism generated, both in the northwest Indian Ocean and at the better-known North Atlantic margins, depends not only on the mantle temperature but, to a similar degree, on the rift history. The inherited extensional history can either suppress or enhance melt generation, which can explain previously enigmatic observations.

  20. The importance of rift history for volcanic margin formation (Invited)

    NASA Astrophysics Data System (ADS)

    Collier, J.; Armitage, J. J.; Minshull, T. A.

    2010-12-01

    Rifting and magmatism are fundamental geological processes that shape the surface of our planet. A relationship between the two is widely acknowledged but its precise nature has eluded geoscientists and remained controversial. Two decades ago, largely based on detailed observations from the North Atlantic, a paradigm was established that identified mantle temperature as the primary factor. This idea has dominated our understanding with most authors seeking to explain observed variations in volcanic activity at rifted margins in terms of the mantle temperature at the time of breakup. However, as more detailed observations have been made at other rifted margins world-wide, the validity of this paradigm and the importance of other factors in controlling breakup style have been much debated. One such example is from the Northwest Indian Ocean where, despite an unequivocal link between an onshore flood basalt province, breakup and a hotspot track leading to an active ocean island volcano, the associated continental margins show little magmatism. Here we reconcile these observations by applying a numerical model that explicitly accounts for the effects of earlier episodes of extension. Our approach allows us to compare directly breakup magmatism generated at different locations and so isolate the key controlling factors. We show that the volume of rift-related magmatism generated, both in the Northwest Indian Ocean and at the better known North Atlantic margins, depends not only on the mantle temperature but, to a similar degree, on the rift history. Our work shows that the inherited extensional history can either suppress or enhance melt generation. We believe this explains previously enigmatic observations and brings a critical new understanding to models of continental breakup. Model predictions showing the importance of timing on the volume magmatism produced during continental break-up. Curves show peak igneous crustal thickness for the arrival of a 200 °C, 50 km

  1. Evaluation of the ongoing rifting and subduction processes in the geochemistry of magmas from the western part of the Mexican Volcanic Belt

    NASA Astrophysics Data System (ADS)

    Verma, Surendra P.; Pandarinath, Kailasa; Rivera-Gómez, M. Abdelaly

    2016-03-01

    A compilation of new and published geochemical data for 1512 samples of volcanic rocks from the western part of the Mexican Volcanic Belt was first subdivided according to the age group (136 samples of Miocene and 1376 samples of Pliocene-Holocene). Rocks of the younger group were then subdivided as Rift (1014 samples from the triple-rift system) and No Rift (362 samples outside of the triple-rift system) or Near Trench (937 samples) and Far Trench (439 samples) magmas. These subdivisions were considered separately as basic, intermediate, and acid magmatic rocks. The application of the conventional and multidimensional techniques confirmed the great tectonic and geochemical complexity of this region. The presence of oceanic-type basalts suggested to result from a mantle plume was not confirmed from the tectonomagmatic multidimensional diagrams. The Miocene rocks, which are present at the surface far from the Middle-America Trench, showed a likely continental rift setting in most diagrams for basic rocks and a continental arc setting for intermediate rocks. These differences can be explained in terms of the petrogenetic origin of the magmas. Unlike the current thinking, the triple-rift system seems to have influenced the chemistry of Pliocene-Holocene basic rocks, which indicated a continental rift setting. The Pliocene-Holocene intermediate and acid rocks, however, did not show such an influence. The Pliocene-Holocene basic rocks indicated a continental rift setting, irrespective of the Near Trench and Far Trench subdivision because numerous Near Trench rocks also lie in the triple-rift and graben systems. However, the intermediate rocks having a crustal component in their genesis indicated a continental arc (Near Trench) or a transitional arc to within-plate setting (Far Trench). The acid rocks having a crustal component also suggested a continental arc (Near Trench) or a transitional setting (Far Trench). The application of the tectonomagmatic multidimensional

  2. How Mountains Become Rifts

    NASA Astrophysics Data System (ADS)

    Buiter, S. J.; Tetreault, J. L.

    2015-12-01

    Rifting often initiates on former continental collision zones. For example, the present-day passive margins of the Atlantic and Indian Oceans formed after continental break-up occurred on relatively young and very old sutures, such as Morocco-Nova Scotia and East Antarctica-Australia, respectively. Rifts may localize on former collision zones for several reasons: orogens are thermally weak because of the increase in heat producing elements in their thicker crustal root, the inherited thrust faults form large-scale heterogeneities, and in the case of young sutures, extensional collapse of the orogen may help initiate rifting. We highlight the impact of collision zone inheritance on continental extension and rifted margin architecture using numerical experiments. We first explicitly prescribe collisional structures in the initial setup, such as increased crustal thickness and inherited thrust faults. Varying the prescribed structures results in different rift to break-up durations and margin widths. Our second series of experiments creates a collision zone through subduction and closure of an ocean. We confirm that post-collisional collapse is not a sufficient trigger for continental rifting and that a change in regional plate motions is required. When extension occurs, the weak former subduction interface and the elevated temperatures in the crustal nappe stack work in tandem as the main deformation localizers for continental rifting. Our experiments show that different approaches of initiating a continental rift result in different dynamics of the crust and mantle, thereby impacting rift geometry, rift to break-up duration, and exhumation of subduction-related sediments and oceanic crust.

  3. Petrofabrics of olivine in a rift axis and rift shoulder and their implications for seismic anisotropy beneath the Rio Grande rift

    NASA Astrophysics Data System (ADS)

    Park, Munjae; Jung, Haemyeong; Kil, Youngwoo

    2015-04-01

    Mantle-derived xenoliths associated with continental rifting can provide important information about the mantle structure and the physicochemical properties of deformation processes in the upper mantle. Metasomatized spinel peridotites from Adam's Diggings (AD) at a rift shoulder and Elephant Butte (EB) at a rift axis in the Rio Grande rift (RGR) were investigated to understand the deformation processes and seismic anisotropy occurring in the upper mantle. As determined through analysis of the lattice preferred orientation (LPO) of olivine by using a scanning electron microscope equipped with electron backscatter diffraction (SEM/EBSD), AD peridotites exhibited C-type LPO of olivine indicating a dominant slip system of (100)[001] at the rift shoulder, whereas EB peridotites exhibited A-type LPO indicating a dominant slip system of (010)[100] at the rift axis. Both geochemical data and microstructural observations indicate that the localized mantle enrichment processes, including melts with hydrous fluids, controlled multiple mantle metasomatisms and deformation of rocks under wet conditions (with olivine C-type LPO) at the rift shoulder (AD), whereas mantle depletion by decompression partial melting caused deformation of rocks under dry conditions (with olivine A-type LPO) at the rift axis (EB). These observations provide evidence for localized hydration and physicochemical heterogeneity of the upper mantle in the Rio Grande rift (RGR) zone. Seismic anisotropy observed beneath this zone can be attributed to the transtensional rupture, such as inhomogeneous stretching, and the petrofabrics of olivine beneath the study area.

  4. Evidence for alkaline igneous activity and associated metasomatism in the Reelfoot rift, south-central Midcontinent, U. S. A

    SciTech Connect

    Goldhaber, M.B.; Diehl, S.F.; Sutley, S.J. ); Flohr, M.J.K. )

    1993-03-01

    Alkaline igneous magmatism is commonly associated with intracontinental rifts such as the Reelfoot rift (RR). Direct evidence for alkaline magmatism in the area of the RR occurs as lamprophyre and syenite encountered in deep wells. The authors' new studies of lamprophyres and sedimentary rocks from wells in the region provide additional examples of alkaline magmatism and emphasize the effects of related metasomatism. Sedimentary rocks in the Dow Chemical No. 1 Garrigan well, which is not known to contain lamprophyre dikes, probably also were metasomatically altered, as they contain authigenic fluorapatite, Ce-phosphates, and other REE-rich minerals. Enrichments of incompatible and large ion lithophile elements commonly associated with alkaline magmatism occur in the New Madrid test well, near the crest of the Pascola Arch. The carbonate-free fraction of Paleozoic rocks in this well is highly enriched in Nb (500 ppm), Ba (> 5,000 ppm), La (500 ppm), Th (1,000 ppm), and F (2,400 ppm). Abundant inclusion-rich potassium-feldspar cement in a nearby well may also be the result of alkaline metasomatism. Fluorite and elevated F concentrations are found in several wells in the RR, and contrast with stratigraphically correlative platform carbonates of the Ozark uplift, which lack F enrichment. Well and spring water samples above the RR are enriched in fluorine (as much as 5,000 ppb) compared to samples away from the rift which typically have concentrations two orders of magnitude smaller. The data and observations are consistent with relatively widespread alkaline metasomatism, which was associated with the intrusion of alkaline magmas in the RR.

  5. Reconstructing Drainage Evolution in Response to Tectonic Deformation Along an Active Rift Margin Using Cosmogenic Exposure Age Dating of Desert Pavements

    NASA Astrophysics Data System (ADS)

    Guralnik, B.; Matmon, A.; Avni, Y.; Zilberman, E.; Fink, D.

    2009-04-01

    Fragmentation and rearrangement of drainage basins, and stream reversal occur in response to tectonic forcing such as subsidence of continental rift valleys and uplift of rift shoulders. We present new cosmogenic data from the central Negev Desert, southern Israel, that sheds light on the relations between the tectonic history of the western margins of the southern Dead Sea Rift (DSR) and drainage basin evolution since the Pliocene. In the Pliocene a major north-oriented river system drained the Negev into the Dead Sea basin and collected tributaries that originated east of the DSR and flowed westward across the central Negev. Tectonic deformation along the western margin of the DSR that began in the Pliocene caused regional eastward tilting and reversal of these tributaries by the early Pleistocene. Zero regional gradients which prevailed during the reversal stage, were accompanied by the accumulation of red beds and lake deposits, currently found on progressively lower elevations towards the rift. The present elevations of these deposits record Quaternary subsidence. To constrain the breakdown history of the Pliocene drainage system and reconstruct Quaternary subsidence, we sampled mature desert pavement from 13 abandoned alluvial surfaces associated with the Plio-Pleistocene deposits. From each surface, hundreds of chert clasts were collected and amalgamated into a single sample. Seven samples were collected from the highest windgaps along major water divides, in which remnants of the early Pleistocene surface are preserved. Five of these samples yielded exposure ages that range between 1.9 Ma and 1.5 Ma. These ages bracket the collapse of the Pliocene drainage basin and suggest the eastward migration of this process. Six other samples which yielded ages that range between 1.3 Ma and 0.5 Ma were collected from alluvial terraces inset into the early Pleistocene surface. They indicate stages of incision of the present drainage system. Under conditions of long

  6. Extension velocity partitioning, rheological crust-mantle and intra-crustal decoupling and tectonically inherited structures: consequences for continental rifting dynamics.

    NASA Astrophysics Data System (ADS)

    Wang, Kun; Mezri, Leila; Burov, Evgueni; Le Pourhiet, Laetitia

    2015-04-01

    We implemented series of systematic thermo-mechanical numerical models testing the importance of the rheological structure and extension rate partitioning for continental rift evolution. It is generally assumed that styles of continental rifting are mainly conditioned by the initial integrated strength of the lithosphere. For example, strong plates are expected to undergo extension in narrow rifting mode, while weak lithospheres would stretch in wide rifting mode. However, we show that this classification is largely insufficient because the notion of the integrated strength ignores the internal rheological structure of the lithosphere that may include several zones of crust-mantle or upper-crust-intermediate (etc) crust decoupling. As well, orogenic crusts characterizing most common sites of continental extension may exhibit inverted lithological sequences, with stronger and denser formerly lower crustal units on top of weaker and lighter upper crustal units. This all may result in the appearance of sharp rheological strength gradients and presence of decoupling zones, which may lead to substantially different evolution of the rift system. Indeed, strong jump-like contrasts in the mechanical properties result in mechanical instabilities while mechanical decoupling between the competent layers results in overall drop of the flexural strength of the system and may also lead to important horizontal flow of the ductile material. In particular, the commonly inferred concept of level of necking (that assumes the existence of a stationary horizontal stretching level during rifting) looses its sense if necking occurs at several distinct levels. In this case, due to different mechanical strength of the rheological layers, several necking levels develop and switch from one depth to another resulting in step-like variations of rifting style and accelerations/decelerations of subsidence during the active phase of rifting. During the post-rifting phase, initially decoupled

  7. InSAR observations of post-rifting deformation around the Dabbahu rift segment, Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Hamling, Ian J.; Wright, Tim J.; Calais, Eric; Lewi, Elias; Fukahata, Yukitoshi

    2014-04-01

    Increased displacement rates have been observed following manylarge earthquakes and magmatic events. Although an order of magnitude smaller than the displacements associated with the main event, the post-seismic or post-rifting deformation may continue for years to decades after the initial earthquake or dyke intrusion. Due to the rare occurrence of subaerial rifting events, there are very few observations to constrain models of post-rifting deformation. In 2005 September, a 60-km-long dyke was intruded along the Dabbahu segment of the Nubia-Arabia Plate boundary (Afar, Ethiopia), marking the beginning of an ongoing rifting episode. Continued activity has been monitored using satellite radar interferometry and data from global positioning system instruments deployed around the rift in response to the initial intrusion. Using multiple satellite passes, we are able to separate the rift perpendicular and vertical displacement fields around the Dabbahu segment. Rift perpendicular and vertical rates of up to 180 and 240 mm yr-1, respectively. Here, we show that models of viscoelastic relaxation alone are insufficient to reproduce the observed deformation field and that a large portion of the observed signal is related to the movement of magma within the rift segment. Our models suggest upper mantle viscosities of 1018-19 Pa s overlain by an elastic crust of between 15 and 30 km. To fit the observations, inflation and deflation of magma chambers in the centre of the rift and to the south east of the rift axis is required at rates of ˜0.13 and -0.08 km3 yr-1.

  8. 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.

  9. Evolution of the Main Ethiopian Rift in the frame of Afar and Kenya rifts propagation

    NASA Astrophysics Data System (ADS)

    Bonini, Marco; Corti, Giacomo; Innocenti, Fabrizio; Manetti, Piero; Mazzarini, Francesco; Abebe, Tsegaye; Pecskay, Zoltan

    2005-02-01

    The Main Ethiopian Rift (MER) has a complex structural pattern composed of southern, central, and northern segments. Ages of onset of faulting and volcanism apparently indicate a heterogeneous time-space evolution of the segments, generally referred to as a northward progression of the rifting process. New structural, petrological, and geochronological data have been used to attempt reconciling the evolution of the distinct MER segments into a volcanotectonic scenario accounting for the propagation of the Afar and the Kenya Rifts. In this evolutionary model, extension affected the Southern MER in the early Miocene (20-21 Ma) due to the northward propagation of the Kenya Rift-related deformation. This event lasted until 11 Ma, then deformation decreased radically and was resumed in Quaternary times. In the late Miocene (11 Ma), deformation focused in the Northern MER forming a proto-rift that we consider as the southernmost propagation of Afar. No major extensional deformation affected the Central MER in this period, as testified by the emplacement at 12-8 Ma of extensive plateau basalts currently outcropping on both rift margins. Significant rift opening occurred in the Central MER during the Pliocene (˜5-3 Ma) with the eruption of voluminous ignimbritic covers (Nazret sequence) exposed both on the rift shoulders and on the rift floor. The apparent discrepancy between the heterogeneous propagation of the three MER segments could be reconciled by considering the opening of Central MER and the later reactivation of the Southern MER as due to a southward propagation of rifting triggered by counterclockwise rotation of the Somalian plate starting around 10 Ma.

  10. Crustal Rheology and Rifted Margin Architecture: Comparing Iberia-Newfoundland, Central South Atlantic, and South China Sea

    NASA Astrophysics Data System (ADS)

    Brune, Sascha

    2015-04-01

    migration of the rift centre, which generates sequential fault activity within the brittle crust. Rift migration results from two processes: (i) Strain hardening takes place in the rift centre due to cooling of upwelling mantle material. (ii) The formation of a low viscosity crustal pocket adjacent to the rift centre is caused by heat transfer from the mantle and viscous strain softening of the lower crust. These mechanisms generate a lateral strength contrast that promotes rift migration in a steady-state manner forming a wide sliver of hyper-extended crust on one margins side, while the conjugate margin becomes narrow. In contrast to these Atlantic examples where wide margins are formed diachronously, the South China Sea evolved in wide rift mode. Here, several hundred kilometres of highly attenuated continental crust are deformed simultaneously during ~40 My of extension. Numerical modelling suggests that the presence of weak, ductile crust enabled the formation of two wide and symmetric margins. Independent indicators for a weak crust come from super-deep basins on the northern margin. These basins appear to be created after the end of active extension and with a significant deficit in brittle faulting, which suggests that subsidence was controlled by sediment loading and accommodated by lower crustal flow, a style of basin formation that is only possible in the presence of low crustal viscosity.

  11. Patterns of late Cenozoic volcanic and tectonic activity in the West Antarctic rift system revealed by aeromagnetic surveys

    USGS Publications Warehouse

    Behrendt, John C.; Saltus, R.; Damaske, D.; McCafferty, A.; Finn, C.A.; Blankenship, D.; Bell, R.E.

    1996-01-01

    Aeromagnetic surveys, spaced ???5 km, over widely separated areas of the largely ice- and sea-covered West Antarctic rift system, reveal similar patterns of 100- to 1700-nT, shallow-source magnetic anomalies interpreted as evidence of extensive late Cenozoic volcanism. We use the aeromagnetic data to extend the volcanic rift interpretation over West Antarctica starting with anomalies over (1) exposures of highly magnetic, late Cenozoic volcanic rocks several kilometers thick in the McMurdo-Ross Island area and elsewhere; continuing through (2) volcanoes and subvolcanic intrusions directly beneath the Ross Sea continental shelf defined by marine magnetic and seismic reflection data and aeromagnetic data and (3) volcanic structures interpreted beneath the Ross Ice Shelf partly controlled by seismic reflection determinations of seafloor depth to (4) an area of similar magnetic pattern over the West Antarctic Ice Sheet (400 km from the nearest exposed volcanic rock), where interpretations of late Cenozoic volcanic rocks at the base of the ice are controlled in part by radar ice sounding. North trending magnetic rift fabric in the Ross Sea-Ross Ice Shelf and Corridor Aerogeophysics of the Southeast Ross Transect Zone (CASERTZ) areas, revealed by the aeromagnetic surveys, is probably a reactivation of older rift trends (late Mesozoic?) and is superimposed on still older crosscutting structural trends revealed by magnetic terrace maps calculated from horizontal gradient of pseudogravity. Longwavelength (???100-km wide) magnetic terraces from sources within the subvolcanic basement cross the detailed survey areas. One of these extends across the Ross Sea survey from the front of the Transantarctic Mountains with an east-southeast trend crossing the north trending rift fabric. The Ross Sea-Ross Ice Shelf survey area is characterized by highly magnetic northern and southern zones which are separated by magnetically defined faults from a more moderately magnetic central zone

  12. Volcanism at rifts

    SciTech Connect

    White, R.S.; McKenzie, D.P.

    1989-07-01

    The earth's outer shell rifts continuously, stretching and splitting both on the ocean's floor and on continents. Every 30 million years or so the rifting becomes cataclysmic, releasing continent-size floods of magma. This paper explains that the same mechanism is at work in both cases, the difference being in the slightly hotter temperature of the parent mantle for spectacular volcanic outbursts. Two kinds of evidence are described: quantitative descriptions of rock melting and a wide range of observations made on the rifted edges of continents and in the oceans that have opened between them.

  13. Mesozoic and early Tertiary rift tectonics in East Africa

    NASA Astrophysics Data System (ADS)

    Bosworth, William

    1992-08-01

    A complex history of crustal extension occurred in east and central Africa during the Mesozoic and early Tertiary. Beginning in the Late Jurassic, this resulted in a large system of rifts, the Central African rift system, that spanned from central Sudan to southern Kenya. Late Jurassic rifting is best documented in the White and Blue Nile rifts of the Sudan, and records east-west extension in half-graben that were connected by large-scale shear zones and pull-apart basins. Early Cretaceous rifting re-activated Jurassic basins and spread to the large South Sudan rifts and Anza rift in Kenya. By the Late Cretaceous, the extension direction shifted to the NE-SW, and the presently observed large-scale rift geometry was established. In the early Tertiary, some Mesozoic basins were again reactivated, while other regions underwent wrench faulting and basin inversion. The large number of basins preserved in the Central African rift system can be used to construct an evolutionary model of continental rift tectonics. Early phases of extension at low strains produced alternating half-graben/accommodation zone geometries similar to those observed in most young and active continental rifts. At higher strains, some border faults were abandoned so that through-going, simpler active fault systems could evolve. This is interpreted as representing a switch from complex, oppositely dipping detachment structures, with strike dimensions of 50-150 km, to regional detachment structures that continue for hundreds of kilometers parallel to the rift. This change in the type of detachment was accompanied by a shift in the position of the subsidence away from the breakaway to a position focused further within the regional upper plate. Non-rotational, high angle, normal faulting dominates in the development of these late basin geometries. Deciphering similar rift basin histories from passive continental margins may, in many cases, exceed the limits of available reflection seismic data. East

  14. Structure of continental rifts: Role of older features and magmatism

    SciTech Connect

    Keller, G.R.

    1996-12-31

    Recent geological and geophysical studies in several continental rifts have begun to shed light on the details of the processes which govern the structural evolution of these important exploration targets. In Kenya and Tanzania, the classic East African rift has been the object of several investigations which reveal that its location follows the boundary (suture ?) between the Tanzanian craton (Archean) and Mozambiquan belt (Proterozoic), The Baikal rift also follows a similar boundary, and the Mid-continent rift of North America appears to do the same. Rifts themselves often act as zones of weakness which are reactivated by younger tectonic regimes. The classic North American example of this effect is the Eocambrian Southern Oklahoma aulacogen which was deformed to create the Anadarko basin and Wichita uplift in the late Paleozoic. The Central basin platform has a similar history although the original rift formed at {approximately}1,100Ma. Integration of geophysical data with petrologic and geochemical data from several rift zones has also provided a new picture of the nature and extent of magmatic modification of the crust. An interesting contradiction is that Phanerozoic rifts, except the Afar region, show little evidence for major magmatic modification of the crust whereas, at least in North America, many Precambrian rifts are associated with very large mafic bodies in the crust. The Kenya rift displays evidence for modification of the lower crust in a two-phase magmatic history, but upper crustal magmatic features are limited to local intrusions associated with volcanoes. In this rift, complex basement structure plays a much more important role than previously realized, and the geophysical signatures of basement structure and magmatism are easy to confuse. If this is also the case in other rifts, additional rift basins remain to be discovered.

  15. Structure of continental rifts: Role of older features and magmatism

    SciTech Connect

    Keller, G.R. )

    1996-01-01

    Recent geological and geophysical studies in several continental rifts have begun to shed light on the details of the processes which govern the structural evolution of these important exploration targets. In Kenya and Tanzania, the classic East African rift has been the object of several investigations which reveal that its location follows the boundary (suture ) between the Tanzanian craton (Archean) and Mozambiquan belt (Proterozoic), The Baikal rift also follows a similar boundary, and the Mid-continent rift of North America appears to do the same. Rifts themselves often act as zones of weakness which are reactivated by younger tectonic regimes. The classic North American example of this effect is the Eocambrian Southern Oklahoma aulacogen which was deformed to create the Anadarko basin and Wichita uplift in the late Paleozoic. The Central basin platform has a similar history although the original rift formed at [approximately]1,100Ma. Integration of geophysical data with petrologic and geochemical data from several rift zones has also provided a new picture of the nature and extent of magmatic modification of the crust. An interesting contradiction is that Phanerozoic rifts, except the Afar region, show little evidence for major magmatic modification of the crust whereas, at least in North America, many Precambrian rifts are associated with very large mafic bodies in the crust. The Kenya rift displays evidence for modification of the lower crust in a two-phase magmatic history, but upper crustal magmatic features are limited to local intrusions associated with volcanoes. In this rift, complex basement structure plays a much more important role than previously realized, and the geophysical signatures of basement structure and magmatism are easy to confuse. If this is also the case in other rifts, additional rift basins remain to be discovered.

  16. From orogenic collapse to rifting ; structures of the South China Sea

    NASA Astrophysics Data System (ADS)

    Pubellier, M.; Chan, L. S.; Chamot Rooke, N.; Shen, W.; Ringenbach, J. C.

    2009-04-01

    The opening of the South China Sea has been a matter of debate for many years because of its internal structure, the differences between the conjugate margins and the variations of rifting and spreading directions. Although it is considered as being a back-arc basin, it is not sitting directly above a subduction zone, and the rifting process lasted for an unusually long duration. Among the specific characteristics is the early phase of rifting which took place early in place of the former Yanshanian andean-type mountain range. This stage is marked by narrow basins filled with deformed conglomerate, and initiated around 70My ago within a framework where the oblique subduction marked by igneous activity and ductile wrench faults, was replaced by orogenic collapse. The rifting stage is marked by Eocene syntectonic normal faults and occasional volcanics centres and has proceeded from NW-SE to NS extension. The NW stretching created at least two aborted basins which remained at rift stage. Extension was followed by spreading from 33 to ~20 Ma in the South China Sea. The ocean floor spreading also changed direction to NW-SE with a propagator inside the Sunda shelf from 20 to 17My ago. However the propagator opening implies that deformation is also taken by rifting around a southern wedge which in turn created strain inside the thinned crust. Another extension parallel to the margin is also observed althought the spreading was in process. The southward motion of the southern conjugate margin was later accommodated by its subduction beneath the NW Borneo wedge until completion of the Proto South China Sea subduction. Variations of rifting spreading through time and variations of structural styles are discussed in terms of boundary forces acting to the SE.

  17. Surface deformation in volcanic rift zones

    USGS Publications Warehouse

    Pollard, D.D.; Delaney, P.T.; Duffield, W.A.; Endo, E.T.; Okamura, A.T.

    1983-01-01

    The principal conduits for magma transport within rift zones of basaltic volcanoes are steeply dipping dikes, some of which feed fissure eruptions. Elastic displacements accompanying a single dike emplacement elevate the flanks of the rift relative to a central depression. Concomitant normal faulting may transform the depression into a graben thus accentuating the topographic features of the rift. If eruption occurs the characteristic ridge-trough-ridge displacement profile changes to a single ridge, centered at the fissure, and the erupted lava alters the local topography. A well-developed rift zone owes its structure and topography to the integrated effects of many magmatic rifting events. To investigate this process we compute the elastic displacements and stresses in a homogeneous, two-dimensional half-space driven by a pressurized crack that may breach the surface. A derivative graphical method permits one to estimate the three geometric parameters of the dike (height, inclination, and depth-to-center) and the mechanical parameter (driving pressure/rock stiffness) from a smoothly varying displacement profile. Direct comparison of measured and theoretical profiles may be used to estimate these parameters even if inelastic deformation, notably normal faulting, creates discontinuities in the profile. Geological structures (open cracks, normal faults, buckles, and thrust faults) form because of stresses induced by dike emplacement and fissure eruption. Theoretical stress states associated with dilation of a pressurized crack are used to interpret the distribution and orientation of these structures and their role in rift formation. ?? 1983.

  18. Rift Valley Fever (RVF)

    MedlinePlus

    ... Outbreak resources, VHF information for specific groups, virus ecology, references... RVF Distribution Map Rift Valley Fever Transmission ... Outbreaks Outbreak Summaries RVF Distribution Map Resources Virus Ecology File Formats Help: How do I view different ...

  19. Metamorphic Tectonites and Differential Exhumation Reveal 3D Nature of Extension and Lower Crustal Flow in the Active Woodlark Rift, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Little, T. A.; Baldwin, S. L.; Fitzgerald, P. G.; Monteleone, B. D.; Peters, K. J.

    2004-12-01

    . Shear fabrics in MCCs closest to the Woodlark spreading ridges, including Normanby and Misima Islands are parallel to the NNE direction of 0.5-3.6 Ma Solomon Sea-Australia spreading. Farther west, lineations in the lower plates of the D'Entrecasteaux MCCs locally deflect ˜40-50° clockwise from this direction. This obliquity is interpreted to reflect inhomogeneous lower crustal extension to the west of the Woodlark spreading ridges. A rift corridor extending ˜100 km to the north of Goodenough and Fergusson Islands is defined by active normal faulting and subsidence of the Trobriand margin. We infer that a previously subducted, locally eclogite-bearing, slab of thinned Australian lower crust to the north of the islands is being pulled out from beneath this zone as it is being sinistrally sheared along its eastern edge. Today the rift zone steps ˜70 km south towards the Papuan Peninsula to define a right-step, an asymmetry that is enhanced by seafloor spreading east of ˜151.4° E. Published focal mechanisms suggest that N-S sinistral shear along the northern Woodlark rift is continuing. In central Normanby Island, ˜2 Ma andesites may have erupted along a transverse fault bounding the deeply exhumed D'Entrecasteaux corridor. Its offshore extension trends N along a major embayment in the Woodlark Rise.

  20. Composition and spatial evolution of mantle and fluids released beneath the active Southeast Mariana Forearc Rift: do they have arc or backarc basin signatures?

    NASA Astrophysics Data System (ADS)

    Ribeiro, J. M.; Stern, R. J.; Kelley, K. A.; Ishizuka, O.; Anthony, E. Y.; Ren, M.; Manton, W. I.; Ohara, Y.; Reagan, M. K.; Bloomer, S. H.

    2010-12-01

    Fluids of progressively changing composition are released from the subducting slab. Whereas the composition and effects of deep fluids are understood from studying arcs and backarc basin (BAB) lavas, those released at shallower depths beneath forearcs are less well known. Forearc rifts give us a unique opportunity to study the composition of ultra-shallow subduction-related fluids. At the southern end of the Mariana arc, the S.E. Mariana Forearc Rift (SEMFR), was discovered by HMR-1 sonar swath mapping (Martinez et al. 2000, JGR), and investigated in July 2008 by the manned submersible Shinkai 6500. The rift extends from the trench to the BAB spreading axis, where a magma chamber was recently documented (Becker et al., 2010, G-cubed). SEMFR is opening due to continued widening of the Mariana Trough BAB. Two suites of tholeiitic pillow lavas were recovered from the N.E. flank of the rift (dive 1096; slab depth ~ 30 ± 5 km), indicating recent magmatic activity. Dive 1096 lavas consist of upper primitive basalts (Mg# ≥ 60) and lower fractionated, basaltic andesites (Mg# < 60), separated by a thin sediment layer. Geochemical and isotopic studies show that these lavas were produced by extensive hydrous melting (≥ 15%) of a common depleted MORB-like mantle (Nb/Yb ~ 1, ɛNd ~ 9.3), likely S. Mariana BAB mantle, that interacted with < 3% metasomatic fluids. Thermobarometry constraints (Lee et al., 2009, EPSL) suggest that the primary melts equilibrated with the mantle at ~ 28 km, just above the slab, with a mean temperature ~1230°C. The fluid was enriched in fluid-mobile elements (Rb, Ba, K, U, Sr, Pb, Cs), mobilized from the ultra-shallow slab at low temperature, as well as melt-mobile elements (e.g. Th, LREE), released deeper and hotter. These fluids contribute 100% Cs, 97% Rb, 99% Ba, 69% Th, 74% U, 80% K, 83% Pb, 71% Sr, 45% La, 33% Ce, 20% Nd and 11% Sm to the magma. SEMFR lavas acquired BAB-like deep subduction component as well as arc-like ultra

  1. Volcanic architecture of the Afar Rift

    NASA Astrophysics Data System (ADS)

    Vye, C.; Smith, K.; Bateson, L.; Jordan, C.

    2010-12-01

    A new approach for rapidly mapping large volcanic areas has enabled identification of the spatial relationship between lava flows at the scale of single eruptive units, and the temporal development of faults associated with continental rifting. This integrated geological mapping approach involving remote sensing and three-dimensional image analysis has been applied to the Afar Region of the African Rift. We analyse topography and surface rock chemistry based false colour Landsat, ASTER and Lidar imagery within an immersive three-dimensional visualisation suite using SocetSet and Geovisionary software. This remote data is ground-proofed by the targeted field studies. This method is proving to be particularly successful in producing a subdivision of basaltic lava flows based on surface features and morphology of flow lobes where chemostratigraphic applications fail to identify individual eruption units. The high-resolution record has facilitated investigations of the style and size of fissure eruptions, their source, the processes affecting synchronous basaltic and felsic volcanic activity, and the style and duration of basaltic lava flow emplacement. The success of this technique is particularly significant when working in areas which are difficult to access, and may be applied in the future within environmentally or logistically challenging regions.

  2. Escarpment evolution on high-elevation rifted margins: Insights derived from a surface processes model that combines diffusion, advection and reaction

    NASA Technical Reports Server (NTRS)

    Kooi, Henk; Beaumont, Christopher

    1994-01-01

    Experiments with a surface processes model of large-scale (1-1000 km) long-term (1-100 m.y.) erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent. The mdoel describes changes in topographic form as a result of sumultaneous short- and long-range mass transport representing hillslope (diffusive) processes and fluvial transport (advection), repsectively. Fluvial entrainment is modeled as a first-order kinetic reaction which reflects the erodibility of the substrate, and therefore the fluvial system is not necessarily carrying at capacity. One dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are (1) antecedent topography/drainage; (2) the timesale (or equivalently a length scale) in the fluvial entrainment reaction; (3) the flexural response of the lithosphere to denudation; and (4) the relative efficiencies of the short- and long-range transport processes. When rainfall and substrate lithology are uniform, a significant amount of discharge draining over the escarpment top causes it to degrade. Only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions. An additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment. This corresponds to a substrate that is hard to detach by flucial erosion, and therefore to fluvial erosion that is not transport limited. Coninuous backtilting of an escarpment due ot flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide. It is essntial if the escarpment gradient is to be preserved during retreat in a uniform lithology. Low flexural rigidieties propote steep and slowly retreating escarpments. For given rainfall and substrate conditions, the morphology of a retraeating model escarpment is

  3. Syn-rift unconformities punctuating the lower-middle Cambrian transition in the Atlas Rift, Morocco

    NASA Astrophysics Data System (ADS)

    Álvaro, J. Javier; Ezzouhairi, Hassan; Clausen, Sébastien; Ribeiro, M. Luisa; Solá, Rita

    2015-04-01

    The Cambrian Tamdroust and Bab n'Ali Volcanic Complexes represent two magmatic episodes developed in the latest Ediacaran-Cambrian Atlas Rift of Morocco. Their rifting pulses were accompanied by accumulation of volcanosedimentary edifices (dominated by effusive lava flows in the former and explosive acidic aprons in the latter) associated with active tilting and uplift. Sealing of their peneplaned horst-and-graben palaeotopographies led to the onset of distinct onlapping geometries and angular discordances capping eroded basements ranging from the Ediacaran Ouarzazate Supergroup to the Cambrian Asrir Formation. Previous interpretations of these discordances as pull-apart or compressive events are revised here and reinterpreted in an extensional (rifting) context associated with active volcanism. The record of erosive unconformities, stratigraphic gaps, condensed beds and onlapping patterns across the traditional "lower-middle Cambrian" (or Cambrian Series 2-3) transition of the Atlas Rift must be taken into consideration for global chronostratigraphic correlation based on their trilobite content.

  4. Prediction, Assessment of the Rift Valley Fever Activity in East and Southern Africa 2006–2008 and Possible Vector Control Strategies

    PubMed Central

    Anyamba, Assaf; Linthicum, Kenneth J.; Small, Jennifer; Britch, Seth C.; Pak, Edwin; de La Rocque, Stephane; Formenty, Pierre; Hightower, Allen W.; Breiman, Robert F.; Chretien, Jean-Paul; Tucker, Compton J.; Schnabel, David; Sang, Rosemary; Haagsma, Karl; Latham, Mark; Lewandowski, Henry B.; Magdi, Salih Osman; Mohamed, Mohamed Ally; Nguku, Patrick M.; Reynes, Jean-Marc; Swanepoel, Robert

    2010-01-01

    Historical outbreaks of Rift Valley fever (RVF) since the early 1950s have been associated with cyclical patterns of the El Niño/Southern Oscillation (ENSO) phenomenon, which results in elevated and widespread rainfall over the RVF endemic areas of Africa. Using satellite measurements of global and regional elevated sea surface temperatures, elevated rainfall, and satellite derived-normalized difference vegetation index data, we predicted with lead times of 2–4 months areas where outbreaks of RVF in humans and animals were expected and occurred in the Horn of Africa, Sudan, and Southern Africa at different time periods from September 2006 to March 2008. Predictions were confirmed by entomological field investigations of virus activity and by reported cases of RVF in human and livestock populations. This represents the first series of prospective predictions of RVF outbreaks and provides a baseline for improved early warning, control, response planning, and mitigation into the future. PMID:20682905

  5. Prediction, assessment of the Rift Valley fever activity in East and Southern Africa 2006-2008 and possible vector control strategies.

    PubMed

    Anyamba, Assaf; Linthicum, Kenneth J; Small, Jennifer; Britch, Seth C; Pak, Edwin; de La Rocque, Stephane; Formenty, Pierre; Hightower, Allen W; Breiman, Robert F; Chretien, Jean-Paul; Tucker, Compton J; Schnabel, David; Sang, Rosemary; Haagsma, Karl; Latham, Mark; Lewandowski, Henry B; Magdi, Salih Osman; Mohamed, Mohamed Ally; Nguku, Patrick M; Reynes, Jean-Marc; Swanepoel, Robert

    2010-08-01

    Historical outbreaks of Rift Valley fever (RVF) since the early 1950s have been associated with cyclical patterns of the El Niño/Southern Oscillation (ENSO) phenomenon, which results in elevated and widespread rainfall over the RVF endemic areas of Africa. Using satellite measurements of global and regional elevated sea surface temperatures, elevated rainfall, and satellite derived-normalized difference vegetation index data, we predicted with lead times of 2-4 months areas where outbreaks of RVF in humans and animals were expected and occurred in the Horn of Africa, Sudan, and Southern Africa at different time periods from September 2006 to March 2008. Predictions were confirmed by entomological field investigations of virus activity and by reported cases of RVF in human and livestock populations. This represents the first series of prospective predictions of RVF outbreaks and provides a baseline for improved early warning, control, response planning, and mitigation into the future. PMID:20682905

  6. The rift to break-up evolution of the Gulf of Aden: Insights from 3D numerical lithospheric-scale modelling

    NASA Astrophysics Data System (ADS)

    Brune, Sascha; Autin, Julia

    2013-11-01

    The Gulf of Aden provides an ideal setting to study oblique rifting since numerous structural data are available onshore and offshore. Recent surveys showed that the spatio-temporal evolution of the Gulf of Aden rift system is dominated by three fault orientations: displacement-orthogonal (WSW), rift-parallel (WNW) and an intermediate E-W trend. The oldest parts of the rift that are exposed onshore feature displacement-orthogonal and intermediate directions, whereas the subsequently active necking zone involves mainly rift-parallel faults. The final rift phase recorded at the distal margin is characterised by displacement-orthogonal and intermediate fault orientations. We investigate the evolution of the Gulf of Aden from rift initiation to break-up by means of 3D numerical experiments on lithospheric scale. We apply the finite element model SLIM3D which includes realistic, elasto-visco-plastic rheology and a free surface. Despite recent advances, 3D numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved. We address this issue by proposing a simple post-processing method that uses the surface stress-tensor to evaluate stress regime (extensional, strike-slip, compressional) and preferred fault azimuth. The described method is applicable to any geodynamic model and easy to introduce. Our model reproduces the observed fault pattern of the Gulf of Aden and illustrates how multiple fault directions arise from the interaction of local and far-field tectonic stresses in an evolving rift system. The numerical simulations robustly feature intermediate faults during the initial rift phase, followed by rift-parallel normal faulting at the rift flanks and strike-slip faults in the central part of the rift system. Upon break-up, displacement-orthogonal as well as intermediate faults occur. This study corroborates and extends findings from previous analogue experiments of oblique rifting on lithospheric scale and allows new

  7. Geothermal resources of rifts: A comparison of the rio grande rift and the salton trough

    NASA Astrophysics Data System (ADS)

    Swanberg, Chandler A.

    1983-05-01

    The Rio Grande Rift and the Salton Trough are the best developed rift systems in the United States and both share many features common to rifts in general, including geothermal resources. These two rifts have different tectonic and magmatic histories, however, and these differences are reflected in the nature of their geothermal resources. The Salton Trough is a well developed and successful rift. It is the landward extension of the Gulf of California spreading center, which has separated Baja, California, from the remainder of Mexico. Quaternary silicic magmatization has occurred and several of the geothermal resources are associated with recent rhyolitic intrusions. Such resources tend to be high temperature (> 200°C). Greenschist facies metamorphism has been observed in several of the geothermal wells. Localized upper crustal melting is a distinct possibility and there is increasing speculation that very high temperature (> 300°C) geothermal fluids may underlie a large portion of the central trough at depths in excess of 4 km. Low temperature geothermal resources associated with shallow hydrothermal convection are less common and tend to be located on the flanks of the trough or in the Coachella Valley to the north of the zone of active rifting. In contrast, the Rio Grande Rift is less well developed. Recent volcanism consists primarily of mantle-derived basalts, which have not had sufficient residence time within the crust to generate significant crustal melting. The geothermal resources within the Rio Grande Rift do not correlate well with these young basalts. Rather, the quantity of geothermal resources are low temperature (< 100°C) and result from forced hydrothermal convection which discharges at constrictions within or at the end of the major sedimentary basins. High temperature resources are less common and the only discovered example is the Valles Caldera of northern New Mexico ( T = 250-300°C). The deep interiors of the sedimentary basins of the Rio

  8. Structural and stratigraphic evolution of the Anza rift, Kenya

    NASA Astrophysics Data System (ADS)

    Bosworth, William; Morley, Chris K.

    1994-09-01

    The Anza rift is a large, multi-phase continental rift basin that links the Lamu embayment of southern Kenya with the South Sudan rifts. Extension and deposition of syn-rift sediments are known to have commenced by the Neocomian. Aptian-Albian strata have, thus far, not been encountered during limited drilling campaigns and, in at least one well, are replaced by a significant unconformity. Widespread rifting occurred during the Cenomanian to Maastrichtian, and continued into the Early Tertiary. Marine waters appear to have reached the central Anza rift in the Cenomanian, and a second marine incursion may have occurred during the Campanian. As no wells have yet reached basement in the basinal deeps, the possibility exists that the Anza rift may have initiated in the Late Jurassic, in conjunction with extension to the south in the Lamu embayment and to the north in the Blue Nile rift of Sudan. Structural and stratigraphic evolution in the Anza rift followed a pattern that has now been inferred in several rift settings. Early phases of extension were accommodated by moderately dipping faults that produced large stratal rotations. Sedimentary environments were dominantly fluvial, with associated small lakes and dune fields. Volcanic activity is documented for the early Neocomian, but its extent is unknown. This initial style of deformation and sedimentation may have continued through several of the earliest pulses of rifting. By the Late Cretaceous, a new system of steeply dipping faults was established, that produced a deep basin without significant rotation of strata in the north, and only minor rotation in the south. This basin geometry favored the establishment of large, deep lakes, which occasionally were connected to the sea. The older basins were partly cannibalized during the sedimentary in-filling of these successor basins. Early Senonian volcanism was encountered in one well, and reflection seismic evidence suggests that one or more thick, regionally

  9. The Corinth Rift Laboratory, Greece (CRL) : A Multidisciplinary Near Fault Observatory (NFO) on a Fast Rifting System

    NASA Astrophysics Data System (ADS)

    Bernard, P.; Lyon-Caen, H.; Deschamps, A.; Briole, P.; Lambotte, S.; Ford, M.; Scotti, O.; Beck, C.; Hubert-Ferrari, A.; Boiselet, A.; Godano, M.; Matrullo, E.; Meyer, N.; Albini, P.; Elias, P.; Nercessian, A.; Katsonopoulou, D.; Papadimitriou, P.; Voulgaris, N.; Kapetanidis, V.; Sokos, E.; Serpetsidaki, A.; el Arem, S.; Dublanchet, P.; Duverger, C.; Makropoulos, K.; Tselentis, A.

    2014-12-01

    The western rift of Corinth (Greece) is one of the most active tectonic structures of the euro-mediterranean area. Its NS opening rate is 1.5 cm/yr ( strain rate of 10-6/yr) results into a high microseismicity level and a few destructive, M>6 earthquakes per century, activating a system of mostly north dipping normal faults. Since 2001, monitoring arrays of the European Corinth Rift Laboratory (CRL, www.crlab.eu) allowed to better track the mechanical processes at work, with short period and broad band seismometers, cGPS, borehole strainmeters, EM stations, …). The recent (300 kyr) tectonic history has been revealed by onland (uplifted fan deltas and terraces) and offshore geological studies (mapping, shallow seismic, coring), showing a fast evolution of the normal fault system. The microseismicity, dominated by swarms lasting from days to months, mostly clusters in a layer 1 to 3 km thick, between 6 and 9 km in depth, dipping towards north, on which most faults are rooting. The diffusion of the microseismicity suggests its triggering by pore pressure transients, with no or barely detected strain. Despite a large proportion of multiplets, true repeaters seem seldom, suggesting a minor contribution of creep in their triggering, although transient or steady creep is clearly detected on the shallow part of some majors faults. The microseismic layer may thus be an immature, downward growing detachment, and the dominant rifting mechanism might be a mode I, anelastic strain beneath the rift axis , for which a mechanical model is under development. Paleoseismological (trenching, paleoshorelines, turbidites), archeological and historical studies completed the catalogues of instrumental seismicity, motivating attempts of time dependent hazard assessment. The Near Fault Observatory of CRL is thus a multidisciplinary research infrastructure aiming at a better understanding and modeling of multiscale, coupled seismic/aseismic processes on fault systems.

  10. Surface displacements on faults triggered by slow magma transfers between dyke injections in the 2005-2010 rifting episode at Dabbahu-Manda-Hararo rift (Afar, Ethiopia)

    NASA Astrophysics Data System (ADS)

    Dumont, S.; Socquet, A.; Grandin, R.; Doubre, C.; Klinger, Y.

    2016-01-01

    The rifting episode that occurred in Dabbahu-Manda-Hararo (Ethiopia) between 2005 and 2010 during which 14 dyke intrusions were emitted, was a unique opportunity to study interactions between tectonic deformation and magmatic processes. While magmatism has been shown to control primarily the spatial and temporal distribution of dyke intrusions during this accretion sequence, the role of faults in accommodating plate spreading in rift segments is poorly understood. During interdyking periods, transient ground deformation due to magma movement is generally observed. Investigating such a small-scale deformation and in particular the movement along faults during these periods will help understanding the factors that trigger fault movement in magmatic rifts. We analyse fault activity during three interdyking periods: 2006 December-June (d0-d1), 2007 January-July (d5-d6) and 2009 November-January (d10-d11). The time-space evolution of surface displacements along ˜700 faults is derived from pairs of ascending and descending SAR interferograms. Surface slip distributions are then compared with codyking ground deformation fields. The results show that faults are mainly activated above the areas affected by magma emplacement during interdyking periods. A detailed analysis of brittle deformation during the six months following the 2005 September intrusion shows asymmetric deformation on the rift shoulders, with significant opening on faults located to the west of the dyke. We explain this feature by the activation of westward dipping pre-existing faults, with block rotations in between. In addition, we observe that the strip encompassing the activated faults narrows by 30 per cent from co- to interdyking period. This suggests that magma keeps migrating to shallower depths after the dyke intrusion. During a rifting episode, activation of faults in a pre-existing fracture network therefore seems to be mainly controlled by deep magma processes.

  11. Continental rifting: a planetary perspective

    SciTech Connect

    Muehlberger, W.R.

    1985-01-01

    The only inner planet that has abundant evidence of regional extension, and the consequent generation of rifts in the earth. The absence of plate motion on the other inner planets limits their rifts to localized bulges or subsidence areas. The rifting of oceanic lithosphere is seldom preserved in the geological record. Thus, such rifting must be inferred via plate tectonic interpretation: if there is rifting, then there must be subduction whose results are commonly well preserved. Modern continental rifts are found in many tectonic settings: continental breakup, extension transverse to collisional stresses, or wide regions of nearly uniform extension. Recognition of these settings in older rocks becomes more difficult the farther back in geologic time you travel. Rift basin fillings typically show rapid lateral and vertical facies and thickness changes, bimodal volcanism, and distinctive rift-drift sequences. Proterozoic rifts and aulacogens are well-documented in North America; ex. Keweenawan, western margin of Labrador fold belt, Belt-Uinta and the Wopmay-Athapuscow regions. Documented Archean rifts are rare. In Quebec, the truncated margin of the Minto craton bounded on the south by a 2.8 Ga greenstone belt implies an earlier rift event. The oldest proposed rift dated at 3.0 Ga contains the Pongola Supergroup in southeastern Africa. The presence of Archean dikes demonstrates a rigid crust and andesites as old as 3.5 Ga imply plate tectonics and thus, at least, oceanic rifting.

  12. Lithological Influences on Occurrence of High-Fluoride Waters in The Central Kenya Rift

    NASA Astrophysics Data System (ADS)

    Olaka, L. A.; Musolff, A.; Mulch, A.; Olago, D.; Odada, E. O.

    2013-12-01

    Within the East African rift, groundwater recharge results from the complex interplay of geology, land cover, geomorphology, climate and on going volcano-tectonic processes across a broad range of spatial and temporal scales. The interrelationships between these factors create complex patterns of water availability, reliability and quality. The hydrochemical evolution of the waters is further complex due to the different climatic regimes and geothermal processes going on in this area. High fluoridic waters within the rift have been reported by few studies, while dental fluorosis is high among the inhabitants of the rift. The natural sources of fluoride in waters can be from weathering of fluorine bearing minerals in rocks, volcanic or fumarolic activities. Fluoride concentration in water depends on a number of factors including pH, temperature, time of water-rock formation contact and geochemical processes. Knowledge of the sources and dispersion of fluoride in both surface and groundwaters within the central Kenya rift and seasonal variations between wet and dry seasons is still poor. The Central Kenya rift is marked by active tectonics, volcanic activity and fumarolic activity, the rocks are majorly volcanics: rhyolites, tuffs, basalts, phonolites, ashes and agglomerates some are highly fractured. Major NW-SE faults bound the rift escarpment while the rift floor is marked by N-S striking faults We combine petrographic, hydrochemistry and structural information to determine the sources and enrichment pathways of high fluoridic waters within the Naivasha catchment. A total of 120 water samples for both the dry season (January-February2012) and after wet season (June-July 2013) from springs, rivers, lakes, hand dug wells, fumaroles and boreholes within the Naivasha catchment are collected and analysed for fluoride, physicochemical parameters and stable isotopes (δ2 H, δ18 O) in order to determine the origin and evolution of the waters. Additionally, 30 soil and

  13. The Campbell Plateau (New Zealand) and its rifted margin: A history of its break-up process

    NASA Astrophysics Data System (ADS)

    Gohl, K.; Davey, F.; Davy, B.; Barker, D.; Uenzelmann-Neben, G.

    2003-04-01

    The Campbell Plateau is a large submarine continental plateau lying to the southeast of New Zealand and separated from the landmass of New Zealand by a series of deep extensional sedimentary basins and morphological deeps. Prior to about 110 Ma, the region (with New Zealand) formed part of Gondwana, fitting adjacent to eastern Ross Sea and western Marie Byrd Land, and coincident with an active subducting continental margin. At about 110 Ma, subduction at the margin ceased and extension of the region commenced. This extension continued until about 82 Ma when active seafloor spreading commenced forming the Southern Ocean. The Campbell Plateau has probably remained essentially submerged (presently at depths of about 500 m to 1000 m) since that time as extension has continued and little sedimentation has occurred on the Plateau due to the remoteness of sediment supply and ocean currents. The southeast margin of the Plateau, the conjugate margin to the western Marie Byrd Land margin of West Antarctica, has little sediment cover. Although the timing of break-up is relatively well documented, the processes that led to break-up and the subsequent evolution of the continental fragment forming Campbell Plateau are not well understood. The reason for the very steep southeastern margin is not known. Was it fast break-up, oblique break-up or erosion? Is a submarine plateau passive margin different to continental passive margin or just a result of differences in sediment supply? On the basis of potential field data, the Plateau can be divided into a series of crustal segments with varying crustal parameters. The Stokes and Campbell magnetic anomaly systems indicate extensive magmatic crustal provinces. What was the role of magmatism during break-up and subsequently? To answer these questions on the evolution of submarine continental plateaus, a geophysical survey was conducted across the Campbell Plateau in January-February 2003. The survey carried out a seismic (OBS) transect

  14. Prebiotic activation processes.

    NASA Technical Reports Server (NTRS)

    Lohrmann, R.; Orgel, L. E.

    1973-01-01

    Questions regarding the combination of amino acids and ribonucleotides to polypeptides and polynucleotides are investigated. Each of the reactions considered occurs in the solid state in plausible prebiotic conditions. Together they provide the basis for a unified scheme of amino acid and nucleotide activation. Urea, imidazole and Mg(++) are essential catalytic components of the reaction mixtures. However, these compounds could probably be replaced by other organic molecules.

  15. Rift initiation with volatiles and magma

    NASA Astrophysics Data System (ADS)

    Ebinger, Cynthia; Muirhead, James; Roecker, Steve; Tiberi, Christel; Muzuka, Alfred; Ferdinand, Rrichard; Mulibo, Gabrile; Kianji, Gladys

    2015-04-01

    Rift initiation in cratonic lithosphere remains an outstanding problem in continental tectonics, but strain and magmatism patterns in youthful sectors of the East African rift provide new insights. Few teleseisms occur in the Eastern rift arm of the East African rift system, except the southernmost sector in northern Tanzania where extension occurs in Archaean lithosphere. The change in seismic energy release occurs over a narrow along-axis zone, and between sectors with and without volcanoes in the central rift valley. Are these differences in strain behavior indicative of along-strike variations in a) rheology; b) strain transfer from border faults to magma intrusion zones; c) dike vs fault slip; and/or d) shallow vs deep magma chambers? We present time-space relations of seismicity recorded on a 38-station array spanning the Kenya-Tanzania border, focal mechanisms for the largest events during those time periods, and compare these to longer-term strain patterns. Lower crustal seismicity occurs along the rift length, including sectors on and off craton, and those with and without central rift valley volcanoes, and we see no clear along-strike variation in seismogenic layer thickness. One explanation for widespread lower crustal seismicity is high gas pressures and volatile migration from active metasomatism of upper mantle and magma degassing, consistent with very high volatile 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. Seismicity patterns indicate strain (and fluid?) transfer from the Manyara border fault to Gelai shield volcano (faulting, diking) via Oldoinyo Lengai volcano. Our focal mechanisms and Global CMTs from an intense fault-dike episode (2007) show a local, temporally stable, rotation from ~E-W extension to NE-SE extension in this linkage zone, consistent with longer term patterns recorded in vent and eruptive

  16. 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.

  17. Submarine thermal springs on the Galapagos Rift

    USGS Publications Warehouse

    Corliss, J.B.; Dymond, J.; Gordon, L.I.; Edmond, J.M.; Von Herzen, R. P.; Ballard, Richard D.; Green, K.; Williams, D.; Bainbridge, A.; Crane, K.; Van Andel, T. H.

    1979-01-01

    The submarine hydrothermal activity on and near the Galápagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new oceanic lithosphere at the Galápagos Rift in the first million years may be vented from thermal springs, predominantly along the axial ridge within the rift valley. The vent areas are populated by animal communities. They appear to utilize chemosynthesis by sulfur-oxidizing bacteria to derive their entire energy supply from reactions between the seawater and the rocks at high temperatures, rather than photosynthesis

  18. Submarine thermal sprirngs on the galapagos rift.

    PubMed

    Corliss, J B; Dymond, J; Gordon, L I; Edmond, J M; von Herzen, R P; Ballard, R D; Green, K; Williams, D; Bainbridge, A; Crane, K; van Andel, T H

    1979-03-16

    The submarine hydrothermal activity on and near the Galápagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new oceanic lithosphere at the Galápagos Rift in the first million years may be vented from thermal springs, predominantly along the axial ridge within the rift valley. The vent areas are populated by animal communities. They appear to utilize chemosynthesis by sulfur-oxidizing bacteria to derive their entire energy supply from reactions between the seawater and the rocks at high temperatures, rather than photosynthesis. PMID:17776033

  19. Stratigraphy and rifting history of the Mesozoic-Cenozoic Anza rift, Kenya

    SciTech Connect

    Winn, R.D. Jr.; Steinmetz, J.C. ); Kerekgyarto, W.L. )

    1993-11-01

    Lithological and compositional relationships, thicknesses, and palynological data from drilling cuttings from five wells in the Anza rift, Kenya, indicate active rifting during the Late Cretaceous and Eocene-Oligocene. The earlier rifting possibly started in the Santonian-Coniacian, primarily occurred in the Campanian, and probably extended into the Maastrichtian. Anza rift sedimentation was in lacustrine, lacustrine-deltaic, fluvial, and flood-basin environments. Inferred synrift intervals in wells are shalier, thicker, more compositionally immature, and more poorly sorted than Lower Cretaceous ( )-lower Upper Cretaceous and upper Oligocene( )-Miocene interrift deposits. Synrift sandstone is mostly feldspathic or arkosic wacke. Sandstone deposited in the Anza basin during nonrift periods is mostly quartz arenite, and is coarser and has a high proportion of probable fluvial deposits relative to other facies. Volcanic debris is absent in sedimentary strata older than Pliocene-Holocene, although small Cretaceous intrusions are present in the basin. Cretaceous sandstone is cemented in places by laumontite, possibly recording Campanian extension. Early Cretaceous history of the Anza basin is poorly known because of the limited strata sampled; Jurassic units were not reached. Cretaceous rifting in the Anza basin was synchronous with rifting in Sudan and with the breakup and separation of South America and Africa; these events likely were related. Eocene-Oligocene extension in the Anza basin reflects different stresses. The transition from active rifting to passive subsidence in the Anza basin at the end of the Neogene, in turn, records a reconfigured response of east African plates to stresses and is correlated with formation of the East Africa rift.

  20. Volcanic field elongation, vent distribution and tectonic evolution of continental rift: The Main Ethiopian Rift example

    NASA Astrophysics Data System (ADS)

    Mazzarini, Francesco; Le Corvec, Nicolas; Isola, Ilaria; Favalli, Massimiliano

    2015-04-01

    Magmatism and faulting operate in continental rifts and interact at a variety of scales, however their relationship is complex. The African rift, being the best example for both active continental rifting and magmatism, provides the ideal location to study the interplay between the two mechanisms. The Main Ethiopian Rift (MER), which connects the Afar depression in the north with the Turkana depression and Kenya Rift to the south, consists of two distinct systems of normal faults and its floor is scattered with volcanic fields formed by tens to several hundreds monogenetic, generally basaltic, small volcanoes and composite volcanoes and small calderas. The distribution of vents defines the overall shape of the volcanic field. Previous work has shown that the distribution of volcanic vents and the shape of a field are linked to its tectonic environment and its magmatic system. In order to distinguish the impact of each mechanism, we analyzed four volcanic fields located at the boundary between the central and northern MER, three of them (Debre Zeyit, Wonji and Kone) grew in the rift valley and one (Akaki) on the western rift shoulder. The elongation and shape of the fields were analyzed based on their vent distribution using the Principal Component Analysis (PCA), the Vent-to-Vent Distance (VVD), and the two dimensional symmetric Gaussian kernel density estimate methods. We extracted from these methods several parameters characterizing the spatial distribution of points (e.g., eccentricity (e), eigenvector index (evi), angular dispersion (Da)). These parameters allow to define at least three types of shape for volcanic fields: strong elongate (line and ellipse), bimodal/medium elongate (ellipse) and dispersed (circle) shapes. Applied to the natural example, these methods well differentiate each volcanic field. For example, the elongation of the field increases from shoulder to rift axis inversely to the angular dispersion. In addition, the results show that none of

  1. 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

    Ruleman, Cal; Grauch, V. J.

    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.

  2. Present-day kinematics of the East African Rift

    NASA Astrophysics Data System (ADS)

    Saria, E.; Calais, E.; Stamps, D. S.; Delvaux, D.; Hartnady, C. J. H.

    2014-04-01

    The East African Rift (EAR) is a type locale for investigating the processes that drive continental rifting and breakup. The current kinematics of this ~5000 km long divergent plate boundary between the Nubia and Somalia plates is starting to be unraveled thanks to a recent augmentation of space geodetic data in Africa. Here we use a new data set combining episodic GPS measurements with continuous measurements on the Nubian, Somalian, and Antarctic plates, together with earthquake slip vector directions and geologic indicators along the Southwest Indian Ridge to update the present-day kinematics of the EAR. We use geological and seismological data to determine the main rift faults and solve for rigid block rotations while accounting for elastic strain accumulation on locked active faults. We find that the data are best fit with a model that includes three microplates embedded within the EAR, between Nubia and Somalia (Victoria, Rovuma, and Lwandle), consistent with previous findings but with slower extension rates. We find that earthquake slip vectors provide information that is consistent with the GPS velocities and helps to significantly reduce uncertainties of plate angular velocity estimates. We also find that 3.16 Myr MORVEL average spreading rates along the Southwest Indian Ridge are systematically faster than prediction from GPS data alone. This likely indicates that outward displacement along the SWIR is larger than the default value used in the MORVEL plate motion model.

  3. Sulfide and silicate melt inclusions in the D. João de Castro Volcanic Seamount, a hydrothermally active area on the Terceira Rift, Azores

    NASA Astrophysics Data System (ADS)

    Marques, A. F. A.; Scott, S. D.; Madureira, P.; Rosa, C. J. P.; Lourenço, N.; Conceição, P.; TerRiftic Team

    2012-04-01

    The Azores plateau is a bathymetric high located in the North Atlantic encompassing a triple junction where the American, Eurasia and African plates meet. The Terceira Rift (TR), within the plateau, defines the Eurasia/African plate boundary and corresponds to a 550 Km long, ESE trending line of volcanic islands and seamounts (e.g. D. João de Castro - DJC) alternating with deep basins (e.g. Hirondelle - HIR) [1, 2]. Fresh basalts from the TR, in particular the DJC and HIR areas, were sampled from 2007 to 2009 [EMEPC 2007/2008/2009 cruises]. The team is now studying their melt inclusions [MI] in order to understand processes of magma evolution and mixing as well as the behavior of trace metals and volatiles in the pre-erupted magmas. Petrographic observations indicate that in DJC and HIR, basalts are porphyritic, vesicular, with a microcrystalline groundmass composed mostly of plagioclase laths ± olivine ± clinopyroxene ± skeletal Fe-Ti oxides ± glass. Phenocrysts are subhedral to euhedral with corroded rims. Clinopyroxene (aluminian diopside) is the main phenocryst phase, followed by olivine (Fo83 - DJC; Fo80 - HIR) and minor plagioclase (often as microphenocrysts). Incompatible trace elements in groundmass, glass, and exposed MI in clinopyroxene depict enriched patterns above the OIB field. REE patterns are similar in the groundmass and glass from DJC and HIR. Exposed MI from HIR depicts less enriched REE patterns than the groundmass, whilst DJC MI show similar REE patterns to the groundmass. MI were found in most mineral phases studied. They are distributed randomly (azonal) and appear glassy, partially devitrified or completely opaque with one or more vapor bubbles. Glassy to devitrified MI may show interpenetrating arrays of mineral phases, skeletal Fe-Ti oxides, and included euhedral Cr-spinel. Sulfide globules are common in clinopyroxene-hosted MI and are dispersed within the groundmass. Globules are small, less than 10 μm, and contain distinct mineral

  4. The Role of Rheological Weakening in the Formation of Narrow Rifts on Venus

    NASA Astrophysics Data System (ADS)

    Martone, Alexis; Montesi, Laurent

    2015-11-01

    The rift zones on Venus are remarkably similar to those seen on Earth, despite Venus’ current lack of plate tectonics. The Devana Chasma rift on Beta Regio accommodates extension in a narrow zone and is associated with volcanism. As a result, it has often been compared to the East African Rift (Burov and Gerya, 2014; Foster and Nimmo, 1996). It has been suggested that plate boundaries develop on Earth because an interconnected network of localized shear zones (areas of concentrated weakening) can form through the lithosphere (Regenauer-Lieb and Yuen, 2001). If Venusian rifts, such as Devana Chasma, are similar to terrestrial plate boundaries, then it is possible that shear zones should form in those locations.Montesi (2013) showed that water-bearing minerals, such as micas, which are probably not present on Venus, largely dominate weakening in the Earth’s crust. On Venus, melts are likely to play the role of the weak phase that allows for localization, due to its low viscosity relative to host rocks. Weakening due to grain size reduction is also possible if a dislocation-accommodated grain boundary sliding mechanism is active on Venus (Montesi, 2013).Rift stability for Venus-like conditions has been analyzed using the model of Buck (1991). This model links the evolution of lithospheric strength with the style of rifting (wide, narrow, or metamorphic core complex). The crust and mantle are assumed to be dry diabase and dry olivine, respectively (diabase rheological parameters are from Mackwell et. al. (1998), olivine rheological parameters are from Hirth and Kohlstedt (2003)). The crustal thickness and surface heat flux are varied based on estimated values from the literature (Nimmo and McKenzie, 1998; Buck, 2002). Without the inclusion of a weakening mechanism the large majority of model runs predict wide rifts developing. Adding a simplistic exponential decay to the lithospheric yield strength allows for more narrow rift formation to occur. Including explicit

  5. Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

    NASA Astrophysics Data System (ADS)

    Rooney, Tyrone O.; Mohr, Paul; Dosso, Laure; Hall, Chris

    2013-02-01

    The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ˜31 Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31 Ma and 22 Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22 Ma to 12 Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10 Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin

  6. Permian and Triassic Meliata-related rift and drift processes in Eastern Alps: middle and lower crust and its potential correlation with sedimentary units

    NASA Astrophysics Data System (ADS)

    Neubauer, Franz

    2016-04-01

    The poorly studied middle and lower crust (MLC) of passive continental margins, a key plate tectonic element, is often preserved in thick-skinned tectonic wedges of mountain belts. We studied the Permian to early Norian Meliata Ocean-related rift-characteristics of MLC and detached upper crust (UC) of the Austroalpine nappes of Eastern Alps (and Western Carpathians) with the aim to assess rift models, composition and temporal and spatial distribution of magmatism. We also compare the development of UC sedimentary units with similar successions of the eastern Southalpine unit. There, rifting started already during Early Permian, and a major extensional event occurred during late Early Permian separating Lower and Upper Permian carbonate platforms. In Austroalpine units, rifting occurred later, and the response of LMC to rifting includes acidic and subordinate, mostly earlier mafic magmatism, high-temperature/low-pressure metamorphism (ca. 0.46 GPa, 540 °C), and pure and simple shear deformation in shallow parts of MLC. In UC, the poorly dated rift-onset unconformity formed in Early or Middle Permian and resulted in N-S to NE-SW striking halfgrabens filled with up to 1.5 km thick terrestrial clastics. First marine ingressions occurred during latest Permian, since Anisian carbonate deposition dominated (loss of the clastic hinterland). We recently detected a break-up angular unconformity in central Northern Calcareous Alps (NCA) on top of tilted Lower Anisian Gutenstein Limestone and wedge-shaped Middle Triassic carbonates covered by Norian Dachstein Reef Limestone indicating the break-off and spreading in the Meliata oceanic tract. In Permian evaporites, polyhalite veins and grains crystallized between 235 and 225 Ma and at ca. 210 Ma testifying intense fluid flow along normal faults similar as Anisian/Ladinian strata-bound iron and Pb-Zn-Ba-F mineralizations do. In the underlying basement, we detected similar Permian to Triassic ductile shear zones with Ar

  7. Rift Valley Fever Review

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a disease of animals and humans that occurs in Africa and the Arabian Peninsula. A Phlebovirus in the family Bunyaviridae causes the disease that is transmitted by mosquitoes. Epidemics occur during years of unusually heavy rainfall that assessment models are being develo...

  8. Thermo-rheological aspects of crustal evolution during continental breakup and melt intrusion: The Main Ethiopian Rift, East Africa

    NASA Astrophysics Data System (ADS)

    Lavecchia, Alessio; Beekman, Fred; Clark, Stuart R.; Cloetingh, Sierd A. P. L.

    2016-08-01

    The Cenozoic-Quaternary Main Ethiopian Rift (MER) is characterized by extended magmatic activity. Although magmatism has been recognized as a key element in the process of continental breakup, the interaction between melts and intruded lithosphere is still poorly understood. We have performed a 2D thermo-rheological modeling study of continental crust incorporating rheological variations due to melt intrusion-related thermal perturbation. The model is calibrated based on the characteristics of lithologies occurring in the MER and its extensional history, and includes the effect of metamorphism and anatexis on crustal strength and rheological features. During Miocene early rift phases strain in the MER was mainly accommodated through rift border faults, whereas Pliocene-to-recent extension history is characterized by magma assisted rifting with most strain accommodated across magmatic segments in the rift axis. Consequently, very little strain is distributed in the old Pan-African to Paleogene crust during Pliocene to Holocene times. The magmatic activity along the rift axis created ≈ 20 km thick magmatic segments, with growth rate estimated to range from ≈ 3.5 mm yr- 1 to ≈ 6 mm yr- 1. Our model suggests that the strain transfer from Miocene rift border faults to magmatic segments was favored by a moderate increase in crustal strength, due to prograde metamorphism subsequent to the melt-induced thermal perturbation. Under such conditions, crustal stretching may not constitute an effective extension mechanism, thus strain may be preferentially accommodated by melt injection along hot, partially molten magmatic segments. Anatexis has been detected in our simulations, with melt fractions sufficient to break-up the crust solid framework and migrate. This determines local variations of rheological behavior and may induce seismicity. However, resulting melt percentages are not sufficient to induce widespread, crust-derived volcanic activity. Subsequently, volcanism

  9. 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

  10. The rift to drift evolution of the South China Sea

    NASA Astrophysics Data System (ADS)

    Ranero, Cesar R.; Cameselle, Alejandra; Franke, Dieter; Barckhausen, Udo

    2016-04-01

    Re-processing with modern algorithms of multichannel seismic reflection records from the South China Sea provide novel images on the crustal structure of the continental margin and its boundary zone with the oceanic crust (COB). The selected re-processed seismic lines strike perpendicular to the margins' trend and cross the entire basin, providing complementary images of conjugated rift segments of the NW, SW, and E sub-basins. Re-processed sections image the post-rift and syn-rift sediment, and fault-bounded basement blocks, often also intra-crustal fault reflections that together provide detailed information of the tectonic structural style during rifting. Further, the largest imaging improvement has been obtained in the delineation of -very often- clear fairly continuous reflections from the crust-mantle boundary across the continental margin into the oceanic crust. The images show how crustal thickness and structure change in parallel to changes in the tectonic style of the deformation during the evolution of the rift. The interpreted COB occurs in regions where the tectonic style displays the most noticeable changes from segments where extension is dominated by normal faulting to segments where faulting is comparatively minor and the crust shows fairly gentle lateral thickness variations; these latter segments are interpreted as oceanic crust. The identification of the continental and oceanic tectonic domains permits to study the along-strike evolution in rifting processes and rift segmentation. Also, the comparison of the tectonic structure of the conjugated flanks of the continental rift across the ocean basins is used to understand the last stages of rifting and the relative importance of tectonic extension and magmatism in final break up and spreading initiation. Although there is ample evidence of important volcanism in the images, with some spectacular large conical volcanoes formed over continental crust and numerous sill-like reflections in the

  11. Chalcophile element (Ni, Cu, PGE, and Au) variations in the Tamarack magmatic sulfide deposit in the Midcontinent Rift System: implications for dynamic ore-forming processes

    NASA Astrophysics Data System (ADS)

    Taranovic, Valentina; Ripley, Edward M.; Li, Chusi; Rossell, Dean

    2016-03-01

    correlation between Pt and Pd, and between individual IPGE. At a given Pt or Pd content, however, the semi-massive sulfide ores have higher IPGE contents than the disseminated sulfide samples. Modeling results show that the variations in PGE tenors (metals in recalculated 100 % sulfide) in the Tamarack magmatic sulfide deposit are mainly controlled by variable R factors (magma/sulfide-liquid mass ratios) during sulfide-liquid segregation and subsequent monosulfide solid solution (MSS) fractionation during cooling. The initial contents of Ir, Pt, and Pd in the parental magma, estimated from the metal tenors of the disseminated sulfides, are 0.2, 2, and 1.8 ppb, respectively, which are ˜1/5 of the values for the PGE-undepleted primitive basalts of the Midcontinent Rift System. The variations of PGE tenors in the semi-massive and massive sulfide ores can be explained by MSS fractional crystallization from sulfide liquids. Extreme variations in the PGE contents of the massive sulfides may also in part reflect metal mobility during post-crystallization hydrothermal processes. The higher PGE tenors for the disseminated sulfides in the CGO dike relative to those in the FGO Intrusion are consistent with formation in a dynamic conduit where the early sulfide liquids left in the conduit by the FGO magma were subsequently upgraded by the subsequent surge of the CGO magma. The relatively low PGE tenors for the semi-massive and massive sulfides can be explained by lack of such an upgrading process for the sulfide due to their distal locations in a migrating conduit.

  12. Chalcophile element (Ni, Cu, PGE, and Au) variations in the Tamarack magmatic sulfide deposit in the Midcontinent Rift System: implications for dynamic ore-forming processes

    NASA Astrophysics Data System (ADS)

    Taranovic, Valentina; Ripley, Edward M.; Li, Chusi; Rossell, Dean

    2016-10-01

    correlation between Pt and Pd, and between individual IPGE. At a given Pt or Pd content, however, the semi-massive sulfide ores have higher IPGE contents than the disseminated sulfide samples. Modeling results show that the variations in PGE tenors (metals in recalculated 100 % sulfide) in the Tamarack magmatic sulfide deposit are mainly controlled by variable R factors (magma/sulfide-liquid mass ratios) during sulfide-liquid segregation and subsequent monosulfide solid solution (MSS) fractionation during cooling. The initial contents of Ir, Pt, and Pd in the parental magma, estimated from the metal tenors of the disseminated sulfides, are 0.2, 2, and 1.8 ppb, respectively, which are ˜1/5 of the values for the PGE-undepleted primitive basalts of the Midcontinent Rift System. The variations of PGE tenors in the semi-massive and massive sulfide ores can be explained by MSS fractional crystallization from sulfide liquids. Extreme variations in the PGE contents of the massive sulfides may also in part reflect metal mobility during post-crystallization hydrothermal processes. The higher PGE tenors for the disseminated sulfides in the CGO dike relative to those in the FGO Intrusion are consistent with formation in a dynamic conduit where the early sulfide liquids left in the conduit by the FGO magma were subsequently upgraded by the subsequent surge of the CGO magma. The relatively low PGE tenors for the semi-massive and massive sulfides can be explained by lack of such an upgrading process for the sulfide due to their distal locations in a migrating conduit.

  13. Evidence of Gondwana early rifting process recorded by Resende-Ilha Grande Dike Swarm, southern Rio de Janeiro, Brazil

    NASA Astrophysics Data System (ADS)

    Guedes, Eliane; Heilbron, Monica; de Morisson Valeriano, Claudio; de Almeida, Julio César Horta; Szatmari, Peter

    2016-04-01

    Continental flood basalts and dike swarm have been related to continental breakup process through geological time. The Resende - Ilha Grande Dike swarm (RIGDS) located in the southeast Brazil, is related the Gondwana breakup and composed of dikes/sills intruded in Precambrian gneiss. The dikes have three distinguish orientations: NNW more inland; NS-NNE in the central segment and NE orientation in the coast line, consistent with Precambrian structural lineaments. The swarm comprises high-TiO2 tholeiitic basalts divided into three suites based on REE and Sr and Nd isotope data. The Resende and Volta Redonda suites present higher initial 87Sr/86Sr ratios between 0.7077 and 0.7065, while Angra dos Reis suite presents values of 0.7066 to 0.7057. Geochemical and isotopic data support the sub-continental lithospheric mantle (SCLM) as the main source for the high-TiO2 basalts. The suites heterogeneities are explained by different compositions of SCLM in accreted Precambrian terranes and/or different degree of partial melting and fractional. 40Ar/39Ar data indicate age interval between ca. 156 to 144 Ma for the swarm, older than the average for Gondwana breakup (ca. 130-120 Ma). The age interval places the RIGDS between the Karoo magmatism (181-178 Ma) and the Paraná-Etendeka magmatism (133-134 Ma) and indicates that extensional process affected the supercontinent prior the break-up.

  14. Forensic investigation of rift-to-drift transitions and volcanic rifted margins birth

    NASA Astrophysics Data System (ADS)

    Meyer, R.; Hertogen, J.

    2008-12-01

    Volcanic rifted margins (VRM) reflect excess magmatism generated during the rift-to-drift transition of a continental rift system evolving into a Mid-Ocean Ridge (MOR). As a result many VRM (e.g. NAIP and CAMP) are recognized as Large Igneous Provinces (LIP). The prominent structural characteristics of VRM are Continental Flood Basalts, High-Velocity Lower Crustal bodies (HVLC) and Seaward Dipping Reflector Sequences (SDRS). However, the causes of these anomalously high eruption rates and magma volumes are presently poorly understood. Controversial issue opinions are based on two competing hypotheses: 1) Mantle plume related mechanisms where the excess magmatism results from elevated mantle temperatures; and 2) Rift induced small scale convection processes causing temperature anomalies and enhancing the mantle rock flux through the melt window. Largely because of difficulties to sample oceanic basement at VRM -due to thick sediment covers- the composition of rift-to-drift transition magmas is generally poorly constrained. We reviewed the geodynamic histories and magma compositions from well known VRM (e.g. NE Australia, E USA, Madagascar) and compared these data with own geochemical data from different NE Atlantic tectono-magmatic VRM zones. These comparisons point to a consistent, general VRM formation model. This model has to explain the primary observation, that geological long periods of extension have been reported -in all investigated VRM areas- prior to the breakup. Extensional far field stress looks to be the main geodynamic cause for continental breakup. Small scale convection during the late phase of a continental rift system is probably the key process generating excess magmatism in LIP related to rift-to-drift transitions.

  15. Depositional and tectonic framework of the rift basins of Lake Baikal from multichannel seismic data

    USGS Publications Warehouse

    Hutchinson, D.R.; Golmshtok, A.J.; Zonenshain, L.P.; Moore, T.C.; Scholz, C.A.; Klitgord, Kim D.

    1992-01-01

    Recent multichannel seismic reflection data from Lake Baikal, located in a large, active, continental rift in central Asia, image three major stratigraphic units totalling 3.5 to 7.5 km thick in four subbasins. A major change in rift deposition and faulting between the oldest and middle-rift units probably corresponds to the change from slow to fast rifting. A brief comparison of the basins of Lake Baikal with those of the East African rift system highlights differences in structural style that can be explained by differences in age and evolution of the surrounding basement rocks. -from Authors

  16. Mid-Continent rift system - a frontier hydrocarbon province

    SciTech Connect

    Lee, C.K.; Kerr, S.D. Jr.

    1983-08-01

    Geophysical evidence in the Mid-Continent has led to delineation of a rift system active during the Proterozoic Y Era. The Mid-Continent rift system can be traced by the Mid-Continent gravity high and corresponding aeromagnetic anomaly signature from the surface exposure of the Keweenawan Supergroup in the Lake Superior basin southwest in the subsurface through Wisconsin, Minnesota, Iowa, Nebraska, and Kansas. The aeromagnetic anomaly signature of the rift trend discloses where these sediments have been preserved. Thick accumulations of upper Proterozoic sediments are indicated by both upward continuation of the aeromagnetic profiles across the rift trend and gravity models which incorporate: 1) a deep mafic body to create the narrow gravity high, 2) anomalously thick crust to account for the more regional gravity low, and 3) sedimentary accumulations on the Precambrian surface to explain the small-scale notches which occur within the narrow gravity high. Reflection seismic data are virtually unknown in the rift area; however, data recently acquired by COCORP across the southern end of the feature in Kansas provide evidence of thick stratified sequences in the rift valley. Studies of the East African rift have revealed that the tropical rift valley is an exceptionally fertile environment for deposition and preservation of kerogenous material. The Sirte, Suez, Viking, Dnieper-Donetz, and Tsaidam basins are just a few of the rift basins currently classed as giant producers. The existence of a rift basin trend with thick accumulations of preserved sediments, demonstrably organic rich, introduces the northern Mid-Continent US as a new frontier for hydrocarbon exploration.

  17. Geodynamic setting and geochemical signatures of Cambrian?Ordovician rift-related igneous rocks (Ossa-Morena Zone, SW Iberia)

    NASA Astrophysics Data System (ADS)

    Sánchez-García, T.; Bellido, F.; Quesada, C.

    2003-04-01

    An important rifting event, accompanied by massive igneous activity, is recorded in the Ossa-Morena Zone of the SW Iberian Massif (European Variscan Orogen). It likely culminated in the formation of a new oceanic basin (Rheic ocean?), remnants of which appear presently accreted at the southern margin of the Ossa-Morena Zone. Rifting propagated diachronously across the zone from the Early Cambrian to the Late Ordovician, but by Early Ordovician time, the existence of a significant tract of new ocean is evidenced by a breakup unconformity. Although early stages of rifting were not accompanied by mantle-derived igneous activity, a pronounced increase of the geothermal gradient is indicated by partial melting of metasedimentary protoliths in the upper and middle crust, and by coeval core-complex formation. Geochemistry of the main volume of igneous rocks, emplaced some million years later during more mature stages of rifting, suggests an origin in a variably enriched asthenospheric source, similar to that of many OIB, from which subsequent petrogenetic processes produced a wide range of compositions, from basalt to rhyolite. A tectonic model involving collision with, and subsequent overriding of, a MOR is proposed to account for the overall evolution, a present-day analogue for which lies in the overriding of the East Pacific Rise by North America and the rifting of Baja California.

  18. 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

  19. 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.

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

    PubMed

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

    2012-09-18

    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.

  1. Olivine-rich submarine basalts from the southwest rift zone of Mauna Loa Volcano: Implications for magmatic processes and geochemical evolution

    NASA Astrophysics Data System (ADS)

    Garcia, Michael O.; Hulsebosch, Thomas P.; Rhodes, J. Michael

    The east Ka Lae landslide on the submarine south flank of Mauna Loa exposed a 1.3 km thick section into the interior of its southwest rift zone. We sampled this section in four dredge hauls and four submersible dives and made a multibeam survey of the rift zone. New magnetic data and our observations and bathymetric results indicate that the axis of the southwest rift is two to three kilometers west of the present topographic high. Our submersible observations of old beach deposits and the low sulfur content of pillow-rim glasses indicate that this portion of the southwest rift zone has subsided >400 m. Olivine-rich basalts are extremely abundant along the submarine portion of Mauna Loa's southwest rift zone but their abundance decreases significantly in the upper parts of the two sections examined. This change probably occurred, ˜60 ka when Mauna Loa's eruption rate slowed and was unable to keep up with its subsidence rate. The dense magmas for these olivine-rich basalts were probably intruded into the deeper portions of the rift zones and erupted from its distal regions during periods of high magma supply. The preferential eruption of olivine-rich lavas on the flanks of Mauna Loa and other Hawaiian volcanoes is a strong indication that a density filter operates within these volcanoes. These lavas contain abundant euhedral, undeforrned olivine with high forsterite contents (typically 90%). Some of these olivines grew in magmas with 17.5 wt% MgO at temperatures of 1415°C, indicating that Hawaiian tholeiitic magmas are some of the most mafic and hottest magmas erupted during the Cenozoic. All of the submarine lavas have major element contents typical of Mauna Loa, but unlike its subaerial lavas, some of the submarine lavas have trace element and isotope ratios that overlap with those of Kilauea lavas. Thus, the source for Mauna Loa contained a Kilauea-like component that has been consumed during the last hundred thousand years, but the melt extraction conditions

  2. Magmatism on rift flanks: insights from Ambient-Noise Phase-velocity in Afar region

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Weemstra, Cornelis; Leroy, Sylvie; Boschi, Lapo; Ren, Yong; Ahmed, Abdulhakim; Keir, Derek; Stuart, Graham W.; Rolandone, Frédérique; Khanbari, Khaled; Hammond, James O. S.; Kendall, J. Michael; Doubre, Cécile; Ganad, Ismail Al

    2015-04-01

    During the breakup of continent in the presence of magma, strain is commonly assumed to initially occur by border faulting, and progressively migrate in space and time towards axial magma intrusion. Magmatic processes near the rift flanks are commonly ignored. We present phase-velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only upper crust beneath the axial volcanic systems, but also both upper and lower crust beneath rift flanks where ongoing volcanism and hydrothermal activity occurs at the surface. The results show that magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process, and may persist through to early seafloor spreading. Since ongoing flank magmatism during breakup impacts the thermal evolution of the lithosphere, it has implications for the subsidence history of the rift.

  3. East African Rift Valley, Kenya

    NASA Technical Reports Server (NTRS)

    1990-01-01

    This rare, cloud free view of the East African Rift Valley, Kenya (1.5N, 35.5E) shows a clear view of the Turkwell River Valley, an offshoot of the African REift System. The East African Rift is part of a vast plate fracture which extends from southern Turkey, through the Red Sea, East Africa and into Mozambique. Dark green patches of forests are seen along the rift margin and tea plantations occupy the cooler higher ground.

  4. Numerical models of oblique rifting: Quantifying the effect of shear

    NASA Astrophysics Data System (ADS)

    Brune, S.; Popov, A. A.; Sobolev, S. V.

    2011-12-01

    In many cases the initial stage of continental break-up was and is associated with oblique extension. That includes several conjugated margins in the Atlantic and Indian Ocean, as well as many recent rift systems, like Gulf of California, Ethiopia Rift and Dead Sea fault. Using three-dimensional, thermo-mechanical simulations and an analytical mechanical model we study the influence of oblique extension on the tectonic forces that are required to induce rifting. We find that oblique extension significantly facilitates the rift process. This is due to the fact that pure strike-slip deformation requires roughly two times less force in order to reach the plastic yield limit than rift-perpendicular extension. Other weakening processes like strain or strain-rate softening and shear heating are more efficient in strike-slip faults but are less important than high obliquity. The model shows that in the case of two competing rifts, with one perpendicular and one oblique to the direction of extension but otherwise having identical properties, the oblique rift zone attracts more strain so that continental break-up occurs there.

  5. Controls on magmatic cycles and development of rift topography of the Manda Hararo segment (Afar, Ethiopia): Insights from cosmogenic 3He investigation of landscape evolution

    NASA Astrophysics Data System (ADS)

    Medynski, S.; Pik, R.; Burnard, P.; Williams, A.; Vye-Brown, C.; Ferguson, D.; Blard, P.-H.; France, L.; Yirgu, G.; Seid, J. I.; Ayalew, D.; Calvert, A.

    2013-04-01

    Crustal extension at mature continental rifts and oceanic ridges occurs by a combination of normal faulting and magma injection, which interact to create rift morphology. Quantifying the relative roles of faulting and melt intrusion in accommodating extension at magmatic rifts remains difficult and requires studies at sufficient spatial and temporal scales to resolve the interaction between these processes. In this study we provide new chronological constraints based on cosmogenic exposure dating for the ˜100 kyr topographic evolution of a young and active magmatic rift segment in Afar, Ethiopia. We combine structural investigations, field mapping, geochemical analysis and cosmogenic 3He exposure dating of lava surfaces in order to investigate the interplay between volcanic activity and fault growth in the northern part of the axial depression, where the rift segment intersects a large stratovolcano. Our results allow us to determine the roles of the various magma reservoirs feeding this rift system and their interactions during accretion over the past 100 kyr. New age data for key lava units allow several magmatic cycles to be distinguished. Each cycle lasts 20-40 ka resulting in periods of high and low magma supply rate. The variations in magma supply rate at the segment extremity strongly affect the development of the rift depression, with the availability of melt controlling the morphological impact of faulting. Melts from different magma reservoirs feeding the segment are chemically distinct and geochemical analysis of lavas from the rift floor allows their respective contributions to maintaining magmatic accretion to be estimated. We propose that melts from the magma reservoir at the northern end of the segment contribute around one-third of the length of this portion of the segment, whereas the mid-segment reservoir is responsible for the remaining two-thirds of the segment accretion.

  6. Structure and evolution of a magmatic rift segment close to continental breakup

    NASA Astrophysics Data System (ADS)

    Medynski, Sarah; Pik, Raphaël; Burnard, Peter; Stab, Martin; Vye-Brown, Charlotte; France, Lyderic

    2014-05-01

    The continental-oceanic transition (OCT) at volcanic margins is often associated with large volcanic provinces and with specific tecto/volcanic structures, notably seaward dipping reflectors (SDRs), of which the origin and processes of formation are still debated. The Afar province corresponds to the very last observable stages of continental rifting before breakup. Interestingly, in this area where the continental crust remains ~20 km thick, magmatism and tectonic activity are already clearly expressed in well-individualised rift segments, presenting the morphological characteristics of mature oceanic ridges. The Afar region therefore represents an ideal natural laboratory to study the processes responsible for maintaining magmatic accretion and their links with the distribution of deformation in a pre-breakup context. We have focussed on the Dabbahu segment that experienced a diking event in 2005 that has been studied in detail. The subsequent tectono-volcanic crisis that lasted 5 years, has provided invaluable information on the mechanisms controlling dyke intrusion, the main process responsible for extension at the scale of a single rifting event. Our investigations focussed on the 1 to 100 kyrs timescales - a key period over which the main morphology of a rift/ridge segment is acquired, but which currently lacks chronological constraints. In order to bridge this gap in temporal evolution, our multidisciplinary approach combines surface exposure dating (3He and 36Cl) of lavas and fault scarps with major & trace element compositions and field/remote sensing mapping of the Dabbahu rift. Our results show that accretion is maintained by individual magmatic chambers, distributed along the active rift segment. There, we have identified magmatic cycles of 30-40 kyrs duration during which the magmas progressively differentiate, until the magmatic centre is abandoned. These cycles exert a strong control on the building of topography: the tectonic activity is subdued

  7. 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

  8. 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

  9. 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.

  10. Structural evolution history of the Red Sea Rift

    NASA Astrophysics Data System (ADS)

    D'Almeida, G. A. F.

    2010-05-01

    The Red Sea Rift has been an object of comprehensive studies by several generations of geologists and geophysicists. Many publications and open-file reports provide insights into the geological history of this rift. Paleogene and Cretaceous rocks, which are considered to be prerift, are locally exposed at the margins of the Red Sea Rift. At the same time, some evidence indicates that at least some of these rocks are related to the early stage of the evolution of the Red Sea Rift. The available geological data suggest that the Red Sea region started its active evolution in the Cretaceous. As follows from lithostratigraphic data, the Cretaceous-Paleogene trough that predated the Oligocene-Quaternary rift covered this region completely or partially. The pre-Oligocene magmatism and geological evidence show that the Cretaceous-Paleogene trough was of the rift type. The Cretaceous-Eocene and Oligocene-Quaternary phases of rifting were separated by an epoch of uplifting and denudation documented by the erosion surface and unconformity.

  11. Evolution, distribution, and characteristics of rifting in southern Ethiopia

    NASA Astrophysics Data System (ADS)

    Philippon, Melody; Corti, Giacomo; Sani, Federico; Bonini, Marco; Balestrieri, Maria-Laura; Molin, Paola; Willingshofer, Ernst; Sokoutis, Dimitrios; Cloetingh, Sierd

    2014-04-01

    Southern Ethiopia is a key region to understand the evolution of the East African rift system, since it is the area of interaction between the main Ethiopian rift (MER) and the Kenyan rift. However, geological data constraining rift evolution in this remote area are still relatively sparse. In this study the timing, distribution, and style of rifting in southern Ethiopia are constrained by new structural, geochronological, and geomorphological data. The border faults in the area are roughly parallel to preexisting basement fabrics and are progressively more oblique with respect to the regional Nubia-Somalia motion proceeding southward. Kinematic indicators along these faults are mainly dip slip, pointing to a progressive rotation of the computed direction of extension toward the south. Radiocarbon data indicate post 30 ka faulting at both western and eastern margins of the MER with limited axial deformation. Similarly, geomorphological data suggest recent fault activity along the western margins of the basins composing the Gofa Province and in the Chew Bahir basin. This supports that interaction between the MER and the Kenyan rift in southern Ethiopia occurs in a 200 km wide zone of ongoing deformation. Fault-related exhumation at ~10-12 Ma in the Gofa Province, as constrained by new apatite fission track data, occurred later than the ~20 Ma basement exhumation of the Chew Bahir basin, thus pointing to a northward propagation of the Kenyan rift-related extension in the area.

  12. A model for the three-dimensional evolution of continental rift basins, north-east Africa

    NASA Astrophysics Data System (ADS)

    Bosworth, William

    1994-12-01

    Large areas of north-east Africa were dominated by regional extension in the Late Phanerozoic. Widespread rifting occurred in the Late Jurassic, with regional extension culminating in the Cretaceous and resulting in the greatest areal extent and degree of interconnection of the west, central and north African rift systems. Basin reactivation continued in the Paleocene and Eocene and new rifts probably formed in the Red Sea and western Kenya. In the Oligocene and Early Miocene, rifts in Kenya, Ethiopia and the Red Sea linked and expanded to form the new east African rift system. This complex history of rifting resulted in failed rift basins with low to high strain geometries, a range of associated volcanism and varying degrees of interaction with older structures. One system, the Red Sea rift, has partially attained active seafloor spreading. From a comparison of these basins, a general model of three-dimensional rift evolution is proposed. Asymmetrical crustal geometries dominated the early phases of these basins, accompanied by low angle normal faulting that has been observed at least locally in outcrop. As rifting progressed, the original fault and basin forms were modified to produce larger, more through-going structures. Some basins were abandoned, others experienced reversals in regional dip and, in general, extension and subsidence became focused along narrower zones near the rift axes. The final transition to oceanic spreading was accomplished in the Red Sea by a change to high angle, planar normal faulting and diffuse dike injection, followed by the organization of an axial magma chamber.

  13. Imaging the lithosphere of rifted passive margins using waveform tomography: North Atlantic, South Atlantic and beyond

    NASA Astrophysics Data System (ADS)

    Lebedev, Sergei; Schaeffer, Andrew; Celli, Nicolas Luca

    2016-04-01

    Lateral variations in seismic velocities in the upper mantle reflect variations in the temperature of the rocks at depth. Seismic tomography thus provides a proxy for lateral changes in the temperature and thickness of the lithosphere. It can map the deep boundaries between tectonic blocks with different properties and age of the lithosphere. Our 3D tomographic models of the upper mantle and the crust at the Atlantic and global scales are constrained by an unprecedentedly large global dataset of broadband waveform fits (over one million seismograms) and provide improved resolution of the lithosphere, compared to other available models. The most prominent high-velocity anomalies, seen down to 150-200 km depths, indicate the cold, thick, stable mantle lithosphere beneath Precambrian cratons, including those in North America, Greenland, northern and eastern Europe, Africa and South America. The dominant, large-scale, low-velocity feature is the global system of mid-ocean ridges, with broader low-velocity regions near hotspots, including Iceland. Currently active continental rifts show highly variable expression in the upper mantle, from pronounced low velocities to weak anomalies; this correlates with the amount of magmatism within the rift zone. Rifted passive margins have typically undergone cooling since the rifting and show more subtle variations in their seismic-velocity structure. Their thermal structure and evolution, however, are also shaped by 3D geodynamic processes since their formation, including cooling by the adjacent cratonic blocks inland and heating by warm oceanic asthenosphere.

  14. Serpentized mantle at rifted margins: The Goban Spur example

    NASA Astrophysics Data System (ADS)

    Bullock, A. D.; Minshull, T. A.

    2002-12-01

    The crustal structure of rifted continental margins can tell us about the processes that operated from continental extension to eventual break-up and sea floor spreading. Variations between margins may record different processes operating during extension or indicate changes in the external geological controls such as mantle plume influence. Extension between Europe and North America began in the mid Cretaceous, dated at the Goban Spur-Flemish Cap rift as late Hauterivian-early Barremian (126-128 Ma) from deep sea drilling (DSDP leg 80) results on the Goban Spur margin. Marine magnetic anomaly 34 can be identified clearly on both margins and indicates that sea floor spreading began no later than 83 Ma. Syn-rift volcanism is limited to a 20 km basaltic body, with considerable lateral extent, at the foot of the continental slope, emplaced at the end of continental rifting. \

  15. An elastic wedge model for the development of coeval normal and thrust faulting in the Mauna Loa-Kilauea rift system in Hawaii

    NASA Astrophysics Data System (ADS)

    Yin, An; Kelty, T. K.

    2000-11-01

    A long-standing enigma of the Mauna Loa-Kilauea rift system in Hawaii is the coeval development of normal and thrust faults that are vertically partitioned. To address this question, we developed a simple elastic wedge model that explores plausible boundary conditions in terms of tractions for generating such a fault pattern. Analytical solutions that best simulate the observed faulting style and geodetically determined strain at the surface require that (1) the pore fluid pressure ratio within the wedge (λ) and along the basal decollement (λb,) must be exceedingly high (i.e., λ = λb= 0.90-0.95) and (2) a tensile stress of the order of 10-30 MPa must have existed in the very top part of the rift zone at the back side of the wedge-shaped rift flank. The high pore fluid pressure within the rift flank may be induced by pumping of fluids during emplacement of magma, whereas the high pore fluid pressure along the basal decollement may be caused by compaction of water-saturated sediments between the volcanic pile above and the oceanic floor below. Although the predicted tensile stress in the rift zone could be related to the presence of a relatively steep topographic slope, our results show that this is not a prerequisite. Therefore we attribute occurrence of tensile stress to either upward bending of the Hawaiian volcanic pile due to emplacement of magma, or inflation of a shallow magma chamber several kilometers beneath the surface. In any case, the results of our model indicate that magma emplacement in the shallow part of the rift zone may be a passive process, while the deep rift zone experiences forceful emplacement (i.e., active rifting via magma push).

  16. Kīlauea's Upper East Rift Zone: A Rift Zone in Name Only

    NASA Astrophysics Data System (ADS)

    Swanson, D. A.; Fiske, R. S.

    2014-12-01

    Kīlauea's upper east rift zone (UERZ) extends ~3 km southeastward from the summit caldera to the Koáe fault system, where it starts to bend into the main part of the ENE-trending rift zone. The UERZ lacks a distinct positive gravity anomaly (though coverage is poor) and any evidence of deformation associated with magma intrusion. All ground ruptures—and the Puhimau thermal area—trend ENE, crossing the UERZ at a high angle. Lua Manu, Puhimau, and Kóokóolau craters are the only surface evidence of the UERZ. Yet the UERZ is seismically active, and all magma entering the rest of the rift zone must pass through it. Rather than a rift zone in the traditional sense, with abundant dikes and ground ruptures along its trend, the UERZ cuts across the ENE structural grain and serves only as a connector to the rest of the rift zone, not a locus of dike formation along its length. The UERZ probably developed as a consequence of gradual SSE migration of the active part of the main east rift zone at the trailing edge of the south flank. During migration, a connection to the summit reservoir complex must be maintained; otherwise, the middle and lower east rift zone would starve and magma from Kīlauea's summit reservoir complex would have to go elsewhere. Over time, the UERZ lengthened and rotated clockwise to maintain the connection. Near the caldera, the UERZ may be widening westward as the summit reservoir complex migrates southward from the center of the caldera to its present position. A layered stress regime results in the upper 2-3 km mimicking the pervasive ENE structural grain of most of Kīlauea, whereas the underlying magmatic part of the UERZ responds to stresses related to SE magma transport. Magma intruding upward from the connector forms a dike that follows the ENE structural grain, as during the 1974 eruption. The active east rift zone has been migrating since ~100 ka, estimated by applying a 700-y extension rate across the Koa'e fault system to the ~6.5 km

  17. Numerical modeling of continental rifting: Implications for the East African Rift system

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    The East African Rift system (EARS) provides a unique system with juxtaposition of two contrasting yet simultaneously formed rift branches, the eastern, magma-rich, and the western, magma-poor, on either side of the old thick Tanzanian craton embedded into younger lithosphere. Here we take advantage of the improvements in our understanding of deep structures, geological evolution and recent kinematics, together with new cutting edge numerical modeling techniques to design a three-dimensional ultra-high resolution viscous plastic thermo-mechanical numerical model that accounts for thermo-rheological structure of the lithosphere and hence captures the essential geophysical features of the central EARS. Based on our experiments, we show that in case of the mantle plume seeded slightly to the northeast of the craton center, the ascending plume material is deflected by the cratonic keel and preferentially channeled along the eastern side of the craton, leading to formation of a large rift zone characterized by important magmatic activity with substantial amounts of melts derived from mantle plume material. This model is in good agreement with the observations in the EARS, as it reproduces the magmatic eastern branch and at the same time, anticlockwise rotation of the craton. However, this experiment does not reproduce the observed strain localization along the western margin of the cratonic bloc. To explain the formation of contrasting magmatic and amagmatic rift branches initiating simultaneously on either side of a non-deforming block as observed in the central EARS, we experimentally explored several scenarios of which three can be retained as specifically pertaining to the EARS: (1) The most trivial first scenario assumes rheologically weak vertical interface simulating the suture zone observed in the geological structure along the western border of the craton; (2) The second scenario involves a second smaller plume initially shifted in SW direction; (3) Finally, a

  18. Configuration and Correlation of Fluvial Terrace Deposits In the Lower Rio Salado Valley: A Record of Magmatic Uplift and Active Normal Faulting in the Rio Grande Rift

    NASA Astrophysics Data System (ADS)

    Sion, B. D.; Axen, G. J.; Phillips, F. M.; Harrison, B.

    2015-12-01

    The Rio Salado is a western tributary of the Rio Grande whose valley is flanked by six major terrace levels. The Rio crosses several active rift-related normal faults and the active, mid-crustal Socorro Magma Body (SMB; a sill at 19 km depth that is actively doming the land surface), providing an unusual opportunity to explore the effects of deep magma emplacement and active faulting on the terraces. Rio Salado terraces were mapped using a high-resolution DEM and digital color stereophotographs and were projected onto a valley-parallel vertical plane to construct longitudinal profiles. Three new soil pits were described to aid terrace correlation. A net incision rate of 0.41 ± 0.06 m/ka was inferred from the correlation of a major fill-cut terrace to the 122 ± 18 ka Airport surface ~25 km south of the Rio Salado. This incision rate is >1.5 times more rapid than estimated rates nearby or in other parts of New Mexico, but yields age estimates for other terraces that are consistent with soil development. Terrace gradients in the Rio Salado have increased through time, indicating either stream response to Rio Grande incision or footwall tilting from the Quaternary Loma Blanca fault (LBF). Two terraces in the LBF hanging wall are back-tilted relative to their footwall counterparts, suggesting a listric geometry for the LBF. However, two others (Qtf and Qtc) are east-tilted relative to their footwall counterparts. Both Qtf and Qtc merge eastward with the next youngest terrace in the flight, and Qtc is arched, consistent with an earlier episode of surface uplift above the SMB. Future work will involve (a) additional terrace mapping over the SMB, (b) cosmogenic 36Cl depth profile dating of the Rio Salado terraces to determine incision rates, allow regional terrace correlations, and constrain fault-slip slip rates and the record of SMB-related surface uplift, and (c) numerical modeling of SMB inflation constrained by uplift signals.

  19. High content image-based screening of a protease inhibitor library reveals compounds broadly active against Rift Valley fever virus and other highly pathogenic RNA viruses.

    PubMed

    Mudhasani, Rajini; Kota, Krishna P; Retterer, Cary; Tran, Julie P; Whitehouse, Chris A; Bavari, Sina

    2014-08-01

    High content image-based screening was developed as an approach to test a protease inhibitor small molecule library for antiviral activity against Rift Valley fever virus (RVFV) and to determine their mechanism of action. RVFV is the causative agent of severe disease of humans and animals throughout Africa and the Arabian Peninsula. Of the 849 compounds screened, 34 compounds exhibited ≥ 50% inhibition against RVFV. All of the hit compounds could be classified into 4 distinct groups based on their unique chemical backbone. Some of the compounds also showed broad antiviral activity against several highly pathogenic RNA viruses including Ebola, Marburg, Venezuela equine encephalitis, and Lassa viruses. Four hit compounds (C795-0925, D011-2120, F694-1532 and G202-0362), which were most active against RVFV and showed broad-spectrum antiviral activity, were selected for further evaluation for their cytotoxicity, dose response profile, and mode of action using classical virological methods and high-content imaging analysis. Time-of-addition assays in RVFV infections suggested that D011-2120 and G202-0362 targeted virus egress, while C795-0925 and F694-1532 inhibited virus replication. We showed that D011-2120 exhibited its antiviral effects by blocking microtubule polymerization, thereby disrupting the Golgi complex and inhibiting viral trafficking to the plasma membrane during virus egress. While G202-0362 also affected virus egress, it appears to do so by a different mechanism, namely by blocking virus budding from the trans Golgi. F694-1532 inhibited viral replication, but also appeared to inhibit overall cellular gene expression. However, G202-0362 and C795-0925 did not alter any of the morphological features that we examined and thus may prove to be good candidates for antiviral drug development. Overall this work demonstrates that high-content image analysis can be used to screen chemical libraries for new antivirals and to determine their mechanism of action and

  20. TDRS satellite over African Rift Valley, Kenya, Africa

    NASA Technical Reports Server (NTRS)

    1983-01-01

    This post deploy view of a TDRS satellite shows a segment of the African Rift Valley near Lake Baringo, Kenya, Africa (3.0S, 36.0E). The African Rift Valley system is a geologic fault having its origins in southern Turkey, through the near east forming the bed of the Jordan River, Gulf of Aqaba, the Red Sea and down through east Africa. The line of lakes and valleys of east Africa are the result of the faulting activity.

  1. Hypogene and supergene alteration of the zeolite-bearing pyroclastic deposits at Tell Rimah, Jordan, and rift-related processes along the Dead-Sea-Transform Fault System during the Quaternary

    NASA Astrophysics Data System (ADS)

    Dill, H. G.; Techmer, A.; Botz, R.; Dohrmann, R.; Kaufhold, S.

    2012-09-01

    The boundary between the Arabian and African plates, is marked in the Middle East by one of the most prominent deep-seated lineamentary structures, called the Dead-Sea-Transform Fault System (DSTFS). Structural and mineralogical processes related to the DSTFS were correlated with equivalent processes leading to the alteration of pyroclastic deposits of alkali-olivine basaltic to nepheline basaltic composition which formed during a time span of less than 0.5 Ma. The large deposit of Tell Rimah, Jordan, is operated for the exploitation of zeolites, tuffs, and as pozzolana raw material. Four discrete stages of mineralizations have been distinguished from each other within these volcanic-hosted mineral deposits. (1) Hypogene syneruptive alteration of pyroclastic rocks produced siliceous gels ("allophane"), smectite, analcime, and phillipsite in vesicles when the groundwater level was low in the rift basin of the DSTFS. The lake-level lowstand caused the fluid system in the pyroclastic cone to become self-sufficient and it has been considered as a closed hydrothermal system. (2) Periods of tectonic and magmatic quiescence grinded the detrital sedimentation in the rift basin to a halt, while triggering a supergene alteration in the eruptive cones on the adjacent Arabian Plate. (3) Epigenetic alteration affected the pyroclastic rocks in the distal part of the DSTFS as a result of a rising water level. The water gradually filled the pore space of the permeable pyroclastic deposits almost to completeness and caused meniscus and blocky cements of calcite, phillipsite and chabazite to develop. In the rift basin, contemporaneously with the alteration of the pyroclastic rocks, freshwater limestones formed on calcareous bedrocks. Ba and Mn minerals in these freshwater limestones were supplied by subaquatic brines. Subsequently, a drastic lowering of the lake water level in the DSTFS converted the system of subaquatic freshwater limestones into subaerial tufa and travertine. As

  2. Active faulting in apparently stable peninsular India: Rift inversion and a Holocene-age great earthquake on the Tapti Fault

    NASA Astrophysics Data System (ADS)

    Copley, Alex; Mitra, Supriyo; Sloan, R. Alastair; Gaonkar, Sharad; Reynolds, Kirsty

    2014-08-01

    We present observations of active faulting within peninsular India, far from the surrounding plate boundaries. Offset alluvial fan surfaces indicate one or more magnitude 7.6-8.4 thrust-faulting earthquakes on the Tapti Fault (Maharashtra, western India) during the Holocene. The high ratio of fault displacement to length on the alluvial fan offsets implies high stress-drop faulting, as has been observed elsewhere in the peninsula. The along-strike extent of the fan offsets is similar to the thickness of the seismogenic layer, suggesting a roughly equidimensional fault rupture. The subsiding footwall of the fault is likely to have been responsible for altering the continental-scale drainage pattern in central India and creating the large west flowing catchment of the Tapti river. A preexisting sedimentary basin in the uplifting hanging wall implies that the Tapti Fault was active as a normal fault during the Mesozoic and has been reactivated as a thrust, highlighting the role of preexisting structures in determining the rheology and deformation of the lithosphere. The slip sense of faults and earthquakes in India suggests that deformation south of the Ganges foreland basin is driven by the compressive force transmitted between India and the Tibetan Plateau. The along-strike continuation of faulting to the east of the Holocene ruptures we have studied represents a significant seismic hazard in central India.

  3. Anomalous subsidence at South China Sea rifted margin: Sediments digging their own hole

    NASA Astrophysics Data System (ADS)

    Brune, Sascha; Clift, Peter; Quinteros, Javier

    2015-04-01

    Rifted continental margins subside as a consequence of combined crustal thinning and mantle lithosphere cooling. While standard models predict a slowing of subsidence after the end of rifting, the deep basins on the northern margin of the South China Sea, notably the Baiyun Sag exhibit subsidence that accelerated several million years after the end of active extension. Additionally, backstripping analysis at the South China margin has shown that the amount of subsidence is much greater than that predicted from the degree of brittle upper crustal extension seen in seismic profiles. Here we explain these observations by linking climate change onshore and deformation of the crystalline crust offshore: Early Miocene monsoon intensification increased erosion and thus the sediment flux to offshore basins after the cessation of active extension. When the sediment load encountered the weak crust of the South China Sea margin, it induced lower crustal flow away from the basin axis so that the deep Baiyun basin was formed nearly without brittle extension. We corroborate this concept using seismic observations and backstripping techniques, as well as thermo-mechanical forward modeling. The numerical forward model is a 2D version of the finite element code SLIM3D. The code includes nonlinear temperature- and stress-dependent elasto-visco-plastic rheology and is able to reproduces a wide range of rift-related deformation processes such as flexure, lower crustal flow, and faulting. We find that two factors allow to satisfy the observational constraints: (1) Post-rift increase of sediment load: The East Asian Summer Monsoon strengthened around the start of the Miocene (~23 Ma), several million years after continental rupture. Changes in the flora of continental China date from around this time and sedimentation rates across continental margins and deltas in South and Southeast Asia increased, as might be expected under the influence of heavier precipitation driving faster erosion

  4. Career management: an active process.

    PubMed

    Mackowiak, J; Eckel, F M

    1985-03-01

    The self-assessment, goal-setting, and career-planning techniques of career management are discussed, and the organization's role in career management is discussed. Career management is a planned process, initiated and carried out by an individual with the assistance of others. Because work and nonwork activities are so interrelated, career and life management planning can maximize a pharmacist's personal success. The career- and life-management process begins with the development of a personal definition of success. A self-assessment must be made of one's values, needs, interests, and activities. The next step of the process involves setting goals and establishing a plan or strategy to achieve them. Establishing a career path requires researching alternate career goals. Career competencies are identified that can increase an employee's chances of success. The employer shares the responsibility for career development through coaching, job structuring, and keeping the employee aware of constraints. Through the integration of the roles of the individual and the organization in the career-management process, employees can optimize their contribution to an organization. Pharmacists can successfully manage their careers by applying the techniques of self-assessment, goal setting, and career planning. PMID:3985018

  5. Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study

    NASA Astrophysics Data System (ADS)

    Beltrando, Marco; Manatschal, Gianreto; Mohn, Geoffroy; Dal Piaz, Giorgio Vittorio; Vitale Brovarone, Alberto; Masini, Emmanuel

    2014-04-01

    Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-extended crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a pre-orogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments from the hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction mélange dynamics or deposition of sedimentary mélanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the

  6. Alkalic (ocean-island basalt type) and calc-alkalic volcanism in the Mexican volcanic belt: A case for plume-related magmatism and propagating rifting at an active margin?

    NASA Astrophysics Data System (ADS)

    Márquez, Alvaro; Oyarzun, Roberto; Doblas, Miguel; Verma, Surendra P.

    1999-01-01

    The Mexican volcanic belt has been traditionally regarded as a classic case of subduction-related calc-alkalic volcanism. However, a series of geologic, geophysical, and petrological arguments makes this simple relationship doubtful. A seismic gap beneath the belt, a large-scale mantle anomaly, a graben triple-junction domain, and the presence of volumetrically important oceanic-island basalt (OIB) volcanism throughout the belt suggest a more complex tectonic scenario involving plume- and subduction-related processes. We here propose a model involving the development of a propagating rift opening from west to east in response to plume activity. The process started in Miocene time within the western sector of the belt (Guadalajara) and gave rise to a graben triple junction and OIB-type and calc-alkalic volcanism. Extension and volcanism proceeded to the east, giving rise to progressively younger ages for the initiation of OIB-type volcanism: (1) Miocene in the west (e.g., Guadalajara), (2) Pliocene in the central zone (e.g., Michoacán-Guanajuato), and (3) Quaternary farther east (e.g., Chichinautzin). Geochemical evidence suggests that part of the modern calc-alkalic volcanism (e.g., Chichinautzin) may be derived from magma mixing between the OIB mafic magmas and silicic, crust-derived magmas. However, we do not preclude some influence of the subducting slab in the generation of other (e.g., Jorullo) calc-alkalic volcanic rocks. Our model suggests a currently unrooted upper plume attached to the subcontinental lithosphere, which defines a hot zone beneath the Mexican volcanic belt.

  7. The geology and geophysics of the Oslo rift

    NASA Technical Reports Server (NTRS)

    Ruder, M. E.

    1981-01-01

    The regional geology and geophysical characteristics of the Oslo graben are reviewed. The graben is part of a Permian age failed continental rift. Alkali olivine, tholefitic, and monzonitic intrusives as well as basaltic lavas outline the extent of the graben. Geophysical evidence indicates that rifting activity covered a much greater area in Skagerrak Sea as well as the Paleozoic time, possibly including the northern Skagerrak Sea as well as the Oslo graben itself. Much of the surficial geologic characteristics in the southern part of the rift have since been eroded or covered by sedimentation. Geophysical data reveal a gravity maximum along the strike of the Oslo graben, local emplacements of magnetic material throughout the Skagerrak and the graben, and a slight mantle upward beneath the rift zone. Petrologic and geophysical maps which depict regional structure are included in the text. An extensive bibliography of pertinent literature published in English between 1960 and 1980 is also provided.

  8. Lithospheric thinning beneath rifted regions of Southern California.

    PubMed

    Lekic, Vedran; French, Scott W; Fischer, Karen M

    2011-11-11

    The stretching and break-up of tectonic plates by rifting control the evolution of continents and oceans, but the processes by which lithosphere deforms and accommodates strain during rifting remain enigmatic. Using scattering of teleseismic shear waves beneath rifted zones and adjacent areas in Southern California, we resolve the lithosphere-asthenosphere boundary and lithospheric thickness variations to directly constrain this deformation. Substantial and laterally abrupt lithospheric thinning beneath rifted regions suggests efficient strain localization. In the Salton Trough, either the mantle lithosphere has experienced more thinning than the crust, or large volumes of new lithosphere have been created. Lack of a systematic offset between surface and deep lithospheric deformation rules out simple shear along throughgoing unidirectional shallow-dipping shear zones, but is consistent with symmetric extension of the lithosphere.

  9. Lithospheric thinning beneath rifted regions of Southern California.

    PubMed

    Lekic, Vedran; French, Scott W; Fischer, Karen M

    2011-11-11

    The stretching and break-up of tectonic plates by rifting control the evolution of continents and oceans, but the processes by which lithosphere deforms and accommodates strain during rifting remain enigmatic. Using scattering of teleseismic shear waves beneath rifted zones and adjacent areas in Southern California, we resolve the lithosphere-asthenosphere boundary and lithospheric thickness variations to directly constrain this deformation. Substantial and laterally abrupt lithospheric thinning beneath rifted regions suggests efficient strain localization. In the Salton Trough, either the mantle lithosphere has experienced more thinning than the crust, or large volumes of new lithosphere have been created. Lack of a systematic offset between surface and deep lithospheric deformation rules out simple shear along throughgoing unidirectional shallow-dipping shear zones, but is consistent with symmetric extension of the lithosphere. PMID:21979933

  10. Development of the Barents Sea rift and its influence on sedimentation and hydrocarbon formation

    NASA Astrophysics Data System (ADS)

    Balanyuk, Inna; Dmitrievsky, Anatoly; Shapovalov, Sergey; Chaikina, Olga

    2010-05-01

    A special attention is given to the geodynamic active zone of the Barents Sea rift. Its development was accompanied by vigorous tectonic activity, propagation of deep faults, deep fractured zones that played an important role in fluid dynamic and thermobaric regime of the whole region. Geodynamic development of the Barents Sea rift not only played a substantial role in formation of as unique oil and gas fields as Shtokman, Prirazlomnoe and others, but created prerequisites for possible gas outbursts into near-surface sediments that could result, in some cases, in hydrocarbon formation. All the Barents Sea deposits are situated in the epicenter of the rift and, most important, over the zone of listric faults intersection, which set up a knot system over the mantle diapir. It is confirmed by prospecting seismology. Intrusion of hot mantle matter with further cooling down of abnormal lense might be a possible cause of appearance and evolution of ultradeep depressions. A high "seismic stratification" of the lower crust (nearly reaching the basement surface) at time scale about 8 sec. is typical for the deepest part of the depression. Supposing the "seismic stratified" lower crust correspond to "basalt" layer, this area is nearly upper crust ("granitic-gneiss") free. This fact confurmes conception on development of "granite free gaps" in the depression basement. Thick blocks of "seismically transparent" upper crust corresponding to the "granitic-gneiss" layer are marked out within Kolsk-Kanin monocline. An abrupt thickness decrease and appearance of "stratified" areas takes place at the southern edge of the depression. A filling of the over-rift sag with sediments, revival of the faults and their effect on the filtration processes and gas hydrates formation took place in the South Barents Sea depression. Repeating activation of the fault blocks in the basement, especially during late Jurassic - early Cretaceous period contributed to formation of the structures related

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

    USGS Publications Warehouse

    Agar, S.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 (rift with concomitant shifts in depocentres. Within the hanging wall of the new western border fault, distinct segments control the location of drainage paths and syn-rift deposits. Morphology, sediment thicknesses and fault scarp amplitude indicate that a segmented rift flank graben has propagated southwards along the rift flank and is still actively fragmenting. These surficial data are used to constrain a model for the time-dependent topographic variations during progressive subsidence along a rift flank, involving the transfer of footwall units to hanging-wall domains. Rapid changes in border fault footwall relief in this model are associated with change in the active border fault location with widespread mass-wasting. The model shows that time-dependent histories need to be integrated with flexural uplift models for active normal faults. The active, syn-rift depositional systems of the Ol'khon region provide a valuable analogue for the early evolution of continental margins and the structural controls on syn-rift hydrocarbon sources and reservoirs.

  12. Buried Mesozoic rift basins of Moroccan Atlantic continental margin

    SciTech Connect

    Mohamed, N.; Jabour, H.; El Mostaine, M.

    1995-08-01

    The Atlantic continental margin is the largest frontier area for oil and gas exploration in Morocco. Most of the activity has been concentrated where Upper Jurassic carbonate rocks have been the drilling objectives, with only one significant but non commercial oil discovery. Recent exploration activities have focused on early Mesozoic Rift basins buried beneath the post-rift sediments of the Middle Atlantic coastal plain. Many of these basins are of interest because they contain fine-grained lacustrine rocks that have sufficient organic richness to be classified as efficient oil prone source rock. Location of inferred rift basins beneath the Atlantic coastal plain were determined by analysis of drilled-hole data in combination with gravity anomaly and aeromagnetic maps. These rift basins are characterized by several half graben filled by synrift sediments of Triassic age probably deposited in lacustrine environment. Coeval rift basins are known to be present in the U.S. Atlantic continental margin. Basin modeling suggested that many of the less deeply bored rift basins beneath the coastal plain are still within the oil window and present the most attractive exploration targets in the area.

  13. Thermo-mechanical modeling of continental rift evolution over mantle upwelling in presence of far-field stresses

    NASA Astrophysics Data System (ADS)

    Koptev, Alexander; Burov, Evgueni; Calais, Eric; Leroy, Sylvie; Gerya, Taras

    2016-04-01

    We conducted fully-coupled high resolution rheologically consistent 3D thermo-mechanical numerical models to investigate the processes of mantle-lithosphere interaction (MLI) in presence of preexisting far-field tectonic stresses. MLI-induced topography exhibits strongly asymmetric small-scale 3D features, such as rifts, flexural flank uplifts and complex faults structures. This suggests a dominant role of continental rheological structure and intra-plate stresses in controlling continental rifting and break-up processes above mantle upwelling while reconciling the passive (far-field tectonic stresses) versus active (plume-activated) rift concepts as our experiments show both processes in action. We tested different experiments by varying two principal controlling parameters: 1) horizontal extension velocity and 2) Moho temperature used as simplified indicator of the thermal and rheological lithosphere layering. An increase in the applied extension expectedly gives less localized deformation at lithospheric scale: the growth of external velocity from 1.5 mm/years to 6 mm/years leads to enlargement of the rift zones from 75-175 km to 150-425 km width. On the contrary, increasing of the lithospheric geotherm has an opposite effect leading to narrowing of the rift zone: the change of the Moho isotherm from 600°C to 800°C causes diminution of the rift width from 175-425 km to 75-150 km. Some of these finding are contra-intuitive in terms of usual assumptions. The models refer to strongly non-linear impact of far-field extension rates on timing of break-up processes. Experiments with relatively fast far-field extension (6 mm/years) show intensive normal fault localization in crust and uppermost mantle above the plume head at 15-20 Myrs after the onset of the experiment. When plume head material reaches the bottom of the continental crust (at 25 Myrs), the latter is rapidly ruptured (<1 Myrs) and several steady oceanic floor spreading centers develop. Slower (3 mm

  14. Recent seismicity of the East African Rift system and its implications

    NASA Astrophysics Data System (ADS)

    Kebede, Fekadu; Kulhánek, Ota

    1991-09-01

    The seismicity of the East African Rift system and southern Red Sea is studied here. Location of earthquake epicenters in East Africa shows that there is a seismicity gap in space and time between the Main Ethiopian Rift system and the eastern rift. However, distribution of earthquake epicenters together with the energy mapping suggest a continuity of seismic activity or stress field from the Main Ethiopian Rift system to the western rift system via the southernmost rifts of Ethiopia. In general (except for some earthquakes which occurred at different complex tectonics regions) mechanisms of earthquakes studied here show dominantly normal faulting suggesting that the rift system is an extensional zone on the continent. The presence of greater focal depth earthquakes to the southern part of the rift system may indicate that softer materials at a shallower depth are present in Afar and neighboring regions than in the remaining part of the East African Rift system. This interpretation is supported by other geophysical studies (low electrical resistivity and gravity data) performed in Afar. It is also supported by low and high stress drops found for the northern part (Afar depression) and southern part of the East African Rift system, respectively.

  15. The Active Mai'iu Low Angle Normal Fault, Woodlark Rift: Spatial and Temporal Slip Distributions, and Rider Block Abandonment Chronology.

    NASA Astrophysics Data System (ADS)

    Webber, S. M.; Little, T.; Norton, K. P.; Mizera, M.; Oesterle, J.; Ellis, S. M.

    2015-12-01

    Low-angle normal faults (LANFs) have induced debate due to their apparent non-Andersonian behavior and lack of significant seismicity associated with slip. Dipping ~21°, the Mai'iu Fault, Woodlark Rift is an active, rapidly slipping LANF located at the transition between continental extension and seafloor spreading. Based on campaign GPS data [Wallace et al., 2014] the Mai'iu Fault is thought to slip at 7-9 mm/yr, accommodating a large fraction of total basinal extension, although it is uncertain whether slip is seismic or aseismic. Surface geomorphology indicates that the fault scarp is not significantly eroded despite high rainfall and ~3000 m relief. We have obtained 15 rock samples (~5 m spacing) from the lowermost Mai'iu Fault scarp in order to determine Holocene slip rate and style over the last ~10 kyr using cosmogenic 10Be in quartz. This slip direction-parallel profile in exposed bedrock is supported by a suite of soil samples for 10Be analysis, which extend our temporal coverage. We model exposure age data in terms of slip rate and style by identification of discontinuities within the profile. Of particular interest is whether slip is seismic or aseismic. In addition we analyze the structure of conglomeratic strata and abandoned, back-rotated rider blocks in the Mai'iu Fault hanging wall, which record Quaternary splay faulting and tilting in response to sustained LANF slip. 20 quartz pebble samples were obtained from hanging wall conglomerates for the purpose of calculating cosmogenic burial (26Al/10Be) ages. These constrain the chronology of Quaternary hanging wall deformation. High-angle (~50°) faulting competes with LANF slip at <2 km depths, with high-angle faults cutting the main LANF and exposing footwall metabasalt up to 2 km north of the Mai'iu Fault. Past splay faulting is recorded in the progressive back-tilting and folding of the Gwoira rider block in a ~2 km deep depression in the corrugated Mai'iu fault plane. Our results provide new

  16. Tectonics of the South Georgia Rift

    NASA Astrophysics Data System (ADS)

    Heffner, David M.

    transfer zones of the SGR. The results of these studies suggest that tectonic inheritance of the Alleghanian suture played an important role in the rifting of Pangea and that tectonic inheritance may be an important process for the formation of an ocean basin.

  17. Magmatic cycles pace tectonic and morphological expression of rifting (Afar depression, Ethiopia)

    NASA Astrophysics Data System (ADS)

    Medynski, Sarah; Pik, Raphael; Burnard, Peter; Blard, Pierre-Henri

    2016-04-01

    Dyking and faulting at mid-oceanic ridges are concentrated in narrow axial volcanic zones due to focussing of both melt distribution and tectonic strain along the plate boundary. Due to the predominantly submarine location of oceanic ridges, the interplay between these processes remain poorly constrained in time and space. In this study, we use the Dabbahu-Manda Hararo (DMH) magmatic rift segment (MRS) (Afar, Ethiopia) to answers the long debated chicken-egg question about magmatic and tectonic processes in extensive context: which on comes first, and how those two processes interplay to finally form oceanic ridges? The DMH MRS is an oceanic ridge analogue and here we present quantitative slip rates on major and minor normal fault scarps for the past 40 kyr in the vicinity of a recent (September 2005) dike intrusion. Our data show that the long-term-vertical slip rates of faults that ruptured in 2005 are too low to explain the present rift topography and that the 2005 strain distribution is not the main stress accommodating mechanism in the DMH segment. Instead, we show that the axial valley topography is created by enhanced slip rates which occur only when the amount of magma available in magma reservoirs is limited, thus preventing dykes from reaching the surface. Our results suggest that development of the axial valley topography is regulated by the magma reservoir lifetime and, thus, to the magmatic cycles of replenishment/differentiation (< 100 ky). This implies that in the DMH rift system (with a magma supply typical of an intermediate spreading centre), significant topography of the axial rift valley is transient, and is expressed only when magma available in the reservoirs decreases. The absence of tilting on the rift margins over the last 200 kyr also suggests that amagmatic accommodation of extension is not required over this time period. Extension instead is accommodated by dykes injected laterally from multiple ephemeral reservoirs located along the DMH

  18. Concentration of strain in a marginal rift zone of the Japan backarc during post-rift compression

    NASA Astrophysics Data System (ADS)

    Sato, H.; Ishiyama, T.; Kato, N.; Abe, S.; Shiraishi, K.; Inaba, M.; Kurashimo, E.; Iwasaki, T.; Van Horne, A.; No, T.; Sato, T.; Kodaira, S.; Matsubara, M.; Takeda, T.; Abe, S.; Kodaira, C.

    2015-12-01

    Late Cenozoic deformation zones in Japan may be divided into two types: (1) arc-arc collision zones like those of Izu and the Hokkaido axial zone, and (2) reactivated back-arc marginal rift (BMR) systems. A BMR develops during a secondary rifting event that follows the opening of a back-arc basin. It forms close to the volcanic front and distant from the spreading center of the basin. In Japan, a BMR system developed along the Sea of Japan coast following the opening of the Japan Sea. The BMR appears to be the weakest, most deformable part of the arc back-arc system. When active rifting in the marginal basins ended, thermal subsidence, and then mechanical subsidence related to the onset of a compressional stress 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, in thin-skin style deformation. Shortening reached a maximum in the BMR system compared to other parts of the back-arc, suggesting that it is the weakest part of the entire system. We examined the structure of the BMR system using active source seismic investigation and earthquake tomography. The velocity structure beneath the marginal rift basin shows higher P-wave velocity in the upper mantle/lower crust which suggests significant mafic intrusion and thinning of the upper continental crust. The syn-rift mafic intrusive forms a convex shape, and the boundary between the pre-rift crust and the mafic intrusive dips outward. In the post-rift compressional stress regime, the boundary of the mafic body reactivated as a reverse fault, forming a large-scale wedge thrust and causing further subsidence of the rift basin. The driver of the intense shortening event along the Sea of Japan coast in SW Japan was the arrival of a buoyant young (15 Ma) Shikoku basin at the Nankai Trough. Subduction stalled and the backarc was compressed. As the buoyant basin cooled, subduction resumed, and the rate of

  19. Seismotectonics of Reelfoot rift basement structures

    SciTech Connect

    Dart, R.L.; Swolfs, H.S. )

    1993-03-01

    Contour maps of the Precambrian basement surface show major northwest-trending structural features within the boundaries of the northeast-oriented Reelfoot rift. These northwest-trending features, southeast of New Madrid, Missouri, consist of a trough flanked on the northeast by a 2-km-high ridge. These features correlate with similar features on an updated depth-to-magnetic basement map. The boundary between the trough and the ridge slopes gently to the southwest. The upward projection of this boundary into the overlying Paleozoic strata may be expressed on a structure-contour map of the Cambrian rocks. The vertical relief of this boundary on the younger datum is inferred to be about 1 km. This Precambrian trough-ridge structure may correlate with a southwest dipping, west-northwest-striking normal fault inferred by Schwalb (1982) to offset rocks of the Cambrian-Ordovician Knox Megagroup that subcrop at the Paleozoic surface. Schwalb (1982) inferred 1.22 km of vertical relief on this fault near the bootheel of Missouri. The nature and significance of this tectonic-structural boundary is unclear, but at the top of the Precambrian basement rocks, it coincides with the southwestern terminus of the New Madrid seismic zone (NMSZ) near the end of the Blytheville arch in northeastern Arkansas. Since the mid-1970's, when instrumental recording began, some of the earthquakes in the NMSZ having the largest magnitudes occurred in this area. The authors working hypothesis is that this trough-ridge structural boundary may concentrate stress and/or may be a barrier that defines the southwestern limit of the seismically active axial fault zone in the rift. Future study will concentrate on improving the understanding of the influence of rift-bounding faults on the lateral extent of this structure, as well as constructing a tectonic stress model of seismically active rift faults and this trough-ridge structure.

  20. Deformation, annealing, reactive melt percolation, and seismic anisotropy in the lithospheric mantle beneath the southeastern Ethiopian rift: Constraints from mantle xenoliths from Mega

    NASA Astrophysics Data System (ADS)

    Tommasi, Andréa; Baptiste, Virginie; Vauchez, Alain; Holtzman, Benjamin

    2016-07-01

    We explore the relations between deformation, annealing, and melt percolation during rifting and the effect of these processes on seismic anisotropy by analyzing the microstructures and crystal preferred orientations (CPO) in a suite of mantle xenoliths from Mega, in the southern end of the Ethiopian rift. Previous geochemical studies on these xenoliths showed evidence for interactions with variable melt types and volumes during the rifting process. The peridotites have dominantly coarse-porphyroclastic microstructures, but coarse granular or partially recrystallized microstructures also occur. The olivine CPO, characterized by orthorhombic to fiber-[100] patterns and moderate intensities, the common occurrence of (100) tilt walls, and the predominance of <0vw> rotation axes accommodating low angle misorientations in olivine support deformation by dislocation creep with dominant activation of the [100](010) system. Annealing (static recrystallization) of variable intensity followed this deformation. Modal enrichment in pyroxenes in > 60% of the studied peridotites corroborates extensive, but spatially heterogeneous reactive melt percolation leading to refertilization of the lithospheric mantle beneath the southern Ethiopian rift. The common interstitial shapes of the pyroxenes and lack of correlation between the pyroxenes and the olivine CPOs in many samples suggest that part of the refertilization is post-kinematic. However, there is no simple relation between reactive melt percolation and annealing of the olivine deformation microstructure. Comparison with data from other xenolith localities points to changes in the metasomatic imprint in the lithospheric mantle along the East African rift system correlated with the evolution in the rift maturity. Seismic properties averaged over all samples show typical lithospheric mantle patterns with fast propagation of P- and polarization of the fast S-waves parallel to the lineation. The anisotropy is moderate (< 6% for P

  1. Gas Geochemistry of Volcanic and Geothermal Areas in the Kenya Rift: Implications for the Role of Fluids in Continental Rifting

    NASA Astrophysics Data System (ADS)

    Lee, H.; Fischer, T. P.; Ranka, L. S.; Onguso, B.; Kanda, I.; Opiyo-Akech, N.; Sharp, Z. D.; Hilton, D. R.; Kattenhorn, S. A.; Muirhead, J.

    2013-12-01

    The East African Rift (EAR) is an active continental rift and ideal to investigate the processes of rift initiation and the breaking apart of continental lithosphere. Mantle and crust-derived fluids may play a pivotal role in both magmatism and faulting in the EAR. For instance, large quantities of mantle-derived volatiles are emitted at Oldoinyo Lengai volcano [1, 2]. Throughout the EAR, CO2-dominated volatile fluxes are prevalent [3, 4] and often associated with faults (i.e. Rungwe area, Tanzania, [5, 6]). The purpose of this study is to examine the relationship between volcanism, faulting and the volatile compositions, focusing on the central and southern Kenyan and northern Tanzanian section of the EAR. We report our analysis results for samples obtained during a 2013 field season in Kenya. Gases were sampled at fumaroles and geothermal plants in caldera volcanoes (T=83.1-120.2°C) and springs (T=40-79.6°C and pH 8.5-10) located near volcanoes, intra-rift faults, and a transverse fault (the Kordjya fault, a key fluid source in the Magadi rift) by 4N-NaOH solution-filled and empty Giggenbach bottles. Headspace gases were analyzed by a Gas Chromatograph and a Quadrupole Mass Spectrometer at the University of New Mexico. Both N2/Ar and N2/He ratios of all gases (35.38-205.31 and 142.92-564,272, respectively) range between air saturated water (ASW, 40 and ≥150,000) and MORB (100-200 and 40-50). In addition, an N2-Ar-He ternary diagram supports that the gases are produced by two component (mantle and air) mixing. Gases in the empty bottles from volcanoes and springs have N2 (90.88-895.99 mmom/mol), CO2 (2.47-681.21 mmom/mol), CH4 (0-214.78 mmom/mol), O2 (4.47-131.12 mmom/mol), H2 (0-35.78 mmom/mol), Ar (0.15-10.65 mmom/mol), He (0-2.21 mmom/mol), and CO (0-0.08 mmom/mol). Although some of the samples show an atmospheric component, CO2 is a major component in most samples, indicating both volcanoes and springs are emitting CO2. Gases from volcanoes are enriched in

  2. Continental breakup and UHP rock exhumation in action: GPS results from the Woodlark Rift, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Wallace, Laura M.; Ellis, Susan; Little, Tim; Tregoning, Paul; Palmer, Neville; Rosa, Robert; Stanaway, Richard; Oa, John; Nidkombu, Edwin; Kwazi, John

    2014-11-01

    show results from a network of campaign Global Positioning System (GPS) sites in the Woodlark Rift, southeastern Papua New Guinea, in a transition from seafloor spreading to continental rifting. GPS velocities indicate anticlockwise rotation (at 2-2.7°/Myr, relative to Australia) of crustal blocks north of the rift, producing 10-15 mm/yr of extension in the continental rift, increasing to 20-40 mm/yr of seafloor spreading at the Woodlark Spreading Center. Extension in the continental rift is distributed among multiple structures. These data demonstrate that low-angle normal faults in the continents, such as the Mai'iu Fault, can slip at high rates nearing 10 mm/yr. Extensional deformation observed in the D'Entrecasteaux Islands, the site of the world's only actively exhuming Ultra-High Pressure (UHP) rock terrane, supports the idea that extensional processes play a critical role in UHP rock exhumation. GPS data do not require significant interseismic coupling on faults in the region, suggesting that much of the deformation may be aseismic. Westward transfer of deformation from the Woodlark Spreading Center to the main plate boundary fault in the continental rift (the Mai'iu fault) is accommodated by clockwise rotation of a tectonic block beneath Goodenough Bay, and by dextral strike slip on transfer faults within (and surrounding) Normanby Island. Contemporary extension rates in the Woodlark Spreading Center are 30-50% slower than those from seafloor spreading-derived magnetic anomalies. The 0.5 Ma to present seafloor spreading estimates for the Woodlark Basin may be overestimated, and a reevaluation of these data in the context of the GPS rates is warranted.

  3. Structure of the southern Rio Grande rift from gravity interpretation

    NASA Technical Reports Server (NTRS)

    Daggett, P. H.; Keller, G. R.; Wen, C.-L.; Morgan, P.

    1986-01-01

    Regional Bouguer gravity anomalies in southern New Mexico have been analyzed by two-dimensional wave number filtering and poly-nomial trend surface analysis of the observed gravity field. A prominent, regional oval-shaped positive gravity anomaly was found to be associated with the southern Rio Grande rift. Computer modeling of three regional gravity profiles suggests that this anomaly is due to crustal thinning beneath the southern Rio Grande rift. These models indicate a 25 to 26-km minimum crustal thickness within the rift and suggest that the rift is underlain by a broad zone of anomalously low-density upper mantle. The southern terminus of the anomalous zone is approximately 50 km southwest of El Paso, Texas. A thinning of the rifted crust of 2-3 km relative to the adjacent Basin and Range province indicates an extension of about 9 percent during the formation of the modern southern Rio Grande rift. This extension estimate is consistent with estimates from other data sources. The crustal thinning and anomalous mantle is thought to result from magmatic activity related to surface volcanism and high heat flow in this area.

  4. P Wave Velocity Structure Beneath the Baikal Rift Axis

    NASA Astrophysics Data System (ADS)

    Brazier, R. A.; Nyblade, A. A.; Boman, E. C.

    2001-12-01

    Over 100 p wave travel times from the 1500 km en echelon Baikal Rift system are used in this study.The events range 3 to 13 degrees from Talaya, Russia (TLY) along the axis of southwest northeast trending rift in East Siberia. A Herglotz Wiechert inversion of these events resolved a crust of 6.4 km/s and a gradient in the mantle starting at 35 km depth and 7.7 km/s down to 200 km depth and 8.2 km/s. This is compatible with Gao et al,1994 cross sectional structure which cuts the rift at about 400km from TLY. The Baikal Rift hosts the deepest lake and is the most seismically active rift in the world. It is one of the few continental rifts, it separates the Siberian craton and the Syan-Baikal mobile fold belt. Two events, the March 21 1999 magnitude 5.7 earthquake 638 km from TLY and the November 13th 1995 magnitude 5.9 earthquake 863 km from TLY were modeled for there PnL wave structure using the discrete wavenumber method and the Harvard CMT solutions with adjusted depths from p-pP times. The PnL signals match well. A genetic algorithm will used to perturb the velocity structure and compare to a selection of the events between 3 and 13 degrees many will require moment tensor solutions.

  5. Exploring the contrasts between fast and slow rifting

    NASA Astrophysics Data System (ADS)

    Morgan, Jason P.; de Monserrat, Albert; White, Lloyd; Hall, Robert

    2016-04-01

    Researchers are now finding that extension sometimes occurs at rates much faster than the mean rates observed in the development of passive margins. Examples of rapid and ultra-rapid extension are found in several locations in Eastern Indonesia. This includes in northern and central Sulawesi as well as in eastern- and westernmost New Guinea. The periods of extension are associated with sedimentary basin growth as well as phases of crustal melting and rapid uplift. This is recorded through seismic imagery of basins offshore Sulawesi and New Guinea as well as through new field studies of the onshore geology in these regions. A growing body of new geochronological and biostratigraphic data provide some control on the rates of processes, indicating that rates of extension are typically at least twice as fast and potentially an order of magnitude faster than the fastest rates applied for more commonly studied rift settings (e.g. Atlantic opening, East African Rift, Australia-Antarctica opening). Here we explore a suite of experiments more appropriate for rifting episodes in Eastern Indonesia, and compare the evolution of these 'fast' (20-100 mm/year full rate) rifting models to experiments with the same crustal geometries rifting at ~5-20 mm/year. In particular, we explore to what depths hot lower crust and mantle can be exhumed by fast rifting, and whether we can produce the p-T-t paths implied by recent onshore geological studies.

  6. 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

  7. Evolution of post-rift sediment transport on a young rifted margin : Insights from the eastern part of the Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Baurion, C.; Gorini, C.; Leroy, S.; Lucazeau, F.; Migeon, S.

    2012-04-01

    The formation of gravity-driven sedimentary systems on continental rifted margins results from the interaction between climate, ocean currents and tectonic activity. During the early stages of margin evolution, the tectonic processes are probably as important as climate for the sedimentary architecture. Therefore, the young margins (ca. 35 Ma) of the Gulf of Aden provide the opportunity to evaluate the respective roles of monsoon and tectonic uplift in the formation and evolution through the post-rift period of gravity-driven deposits (Mass Transport Complexes (MTCs) and deep-sea systems) on the continental slopes and in the oceanic basin respectively. Here we present a combined geomorphologic and stratigraphic study of the northern margin (Oman and Yemen) and the southern margin (Socotra island), in which we classified and interpreted the gravity-driven processes, their formation and their evolution during the post-rift period. The interpretation of seismic lines reveals the presence of bottom currents since the drift phase, suggesting that the Gulf of Aden was connected to the world oceans at that time. An abrupt depositional change affected the eastern basin of the Gulf of Aden around 10 Ma or thereafter (Chron 5), characterised by the first occurrence of deep sea fans and an increase in the number of MTCs. The first occurrence of MTCs may be explained by the combined 2nd-3rd order fall of the relative sea-level (Serravalian/Tortonian transition). This variation of relative sea level combined with a climatic switch (Asian monsoon onset around 15 Ma and its intensification around 7-8 Ma) control the sediment flux. The youngest unit of the post-rift supersequence is characterised by a second important MTC occurrence that is restricted to the eastern part of the deep basin. This is caused by a late uplift of the northern and southern margins witnessed onshore by the presence of young stepped marine terraces.

  8. Rift-related active fault-system and a direction of maximum horizontal stress in the Cairo-Suez district, northeastern Egypt: A new approach from EMR-Technique and Cerescope data

    NASA Astrophysics Data System (ADS)

    Hagag, Wael; Obermeyer, Hennes

    2016-09-01

    An active fault system has been detected along the Cairo-Suez district in northeastern Egypt, applying the EMR-Technique using Cerescope. The E-W (old Mediterranean) and NW-SE (Red Sea-Gulf of Suez) fault-trends are estimated to have ongoing activity. Horizontal EMR-measurements indicate a NW to NNW orientation as a maximum horizontal stress direction (σ1), whereas an E-W orientation to has a secondary tendency. A simplified stress map for the Cairo-Suez district is constructed from the horizontal stress data measured at about 20 locations within the district. The mapped stresses will contribute to the stress data of the Cairo-Suez region on the world stress map (WSM). The present study results indicate rejuvenation of the inherited Mesozoic E-W oriented and Oligocene-Miocene rift-related NW-SE oriented faults. The transfer of rift-related deformation from Red Sea-Gulf of Suez region, which is currently undergoing an extensional stress regime in NE to NNE direction, would explain a seismotectonic activity of the Cairo-Suez district. These results are consistent with a present day NNW oriented compressional stresses attributed to a convergence between the African and Eurasian plates.

  9. Distributed Nubia-Somalia relative motion and dike intrusion in the Main Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Bendick, R.; McClusky, S.; Bilham, R.; Asfaw, L.; Klemperer, S.

    2006-04-01

    The Main Ethiopian Rift (MER) in central Ethiopia extended in the rift-normal direction at a mean rate of 4.0 +/- 0.9 mm yr-1 (1σ) during the period 1992-2003, nearly a factor of two slower than the opening rate estimated from global plate motion inversions. Rift opening near a geodetic array during this period was accommodated by a single dike injection event in 1993, spatially coincident with active magmatic segments, probably triggered by observed seismicity. Following dike injection, the crust in the rift relaxed as a layered medium, with a ~15-km-thick elastic lid over a viscous half space of 1018 Pa s. Diking, rather than normal faulting on rift-bounding faults, appears to be the predominant mechanism of extension in the MER, explaining the very low regional rates of moment release. The length scale and temporal behaviour of surface displacements require viscoelastic rheology in the rift.

  10. On the differences in continental rifting at the Earth, Mars and Venus

    NASA Technical Reports Server (NTRS)

    Nikishin, A. M.; Milanovsky, E. E.

    1985-01-01

    During the process of continental rifting on Earth, the lower ductile crust stretches, forming a neck, while the upper brittle crust is broken in blocks by faults, and the blocks sink down the thinned lower crust; if the stretching continues, the neck may break and a newly originated oceanic crust is formed at this place. The rift system structure depends on the depth of the boundary surface between the brittle crust and the ductile crust, the litospheric thickness, the tension value, etc.. The rigid brittle rifting when narrow necks form in the lower crust is characteristic of the contemporary Earth; on Mars the brittle rifting with large subsidence was characteristic of the Tharsis upland formation epoch. The ductile rifting is typical of the Venus. The differences in rheologic features of the lithospheres of different planets causes the variation in types of rifting.

  11. 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

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

    PubMed

    Passarelli, L; Rivalta, E; Shuler, A

    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

  13. 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.

  14. How Complex is Orogeny? the Role of Rift Inheritance in the Evolution of the Western Alps

    NASA Astrophysics Data System (ADS)

    Beltrando, M.; Mohn, G.; Manatschal, G.

    2012-12-01

    Numerical and conceptual models of the evolution of convergent plate margins normally rely on paleogeographic reconstructions consisting exclusively of (1) oceanic lithosphere, made of mantle peridotites, mafic crust and post-rift sediments and (2) continental lithosphere, with subcontinental mantle and a 20-30 km thick crust, overlain by pre-, syn- and post- rift sediments. However, lithological associations characteristic of the high-pressure part of Alpine-type orogenic belts often fail to match either end members. As a result, 'anomalous' lithological associations, including Paleozoic continental basement directly in contact with Jurassic ophiolites and/or post-rift sediments, have generally been attributed to complex subduction/orogenic dynamics, responsible for chaotic mixing of continental and oceanic lithologies. In contrast to this commonly held view, recent studies have shown that a large part of the apparent complexity of the axial zone of the Western Alps is inherited from Jurassic rifting, since most of the subducted Tethyan lithosphere originally consisted of variably serpentinized subcontinental mantle locally overlain by slivers of continental basement. This conclusion is based on a number of observations, including (1) the consistency of the lithostratigraphic architecture over large areas, despite Alpine deformation, excluding chaotic mixing during subduction/exhumation, (2) widespread pre-metamorphic brittle deformation in continental basement and ultramafics, indicating that they were juxtaposed by fault activity prior to the deposition of post-rift sediments, (3) the presence of clasts of basement rocks in the neighboring sediments, indicating the original proximity of the different lithologies, (4) the common Alpine tectonometamorphic evolution of the different lithologies from the oldest preserved deformation/metamorphic stages. The basement-cover relationships documented in the Western Alps are typical of present day magma-poor rifted

  15. Length and Timescales of Rift Faulting and Magma Intrusion: The Afar Rifting Cycle from 2005 to Present

    NASA Astrophysics Data System (ADS)

    Ebinger, Cynthia; Ayele, Atalay; Keir, Derek; Rowland, Julie; Yirgu, Gezahegn; Wright, Tim; Belachew, Manahloh; Hamling, Ian

    2010-05-01

    Although fault and magmatic processes have achieved plate spreading at mid-ocean ridges throughout Earth's history, discrete rifting episodes have rarely been observed. This paper synthesizes ongoing seismic, structural, space-based geodetic, and petrologic studies from the subaerial Red Sea rift in Ethiopia where a major rifting episode commenced in September 2005. Our aims are to determine the length and timescales of magmatism and faulting, the partitioning of strain between faulting and magmatism, and their implications for the maintenance of along-axis segmentation. Most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the Dabbahu-Manda Hararo rift segment. Strain is accommodated primarily by axial dike intrusions fed from mid-segment magma chamber(s). These findings show that episodic (approximate century interval), rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation. The scale (˜65 km × 8 km) and intensity of crustal deformation (˜6 m), as well as the volume of intrusive and extrusive magmatism (>3 km3), provokes a re-evaluation of seismic and volcanic hazards in subaerial rift zones.

  16. The Midcontinent rift system in Kansas

    SciTech Connect

    Berendsen, P. . Kansas Geological Survey)

    1993-03-01

    A sequence of rift-related mafic volcanic rocks, volcanoclastic-, and clastic sedimentary rocks are recognized in cuttings and cores from about seventy wells in Kansas. The age (1,097.5 Ma) for gabbro in the Poersch [number sign]1 well in northern Kansas, as well as the general petrographic characteristics of the sedimentary rocks throughout the area favors a correlation with established Keweenawan stratigraphy in the Lake Superior region. Rift-related northeast-trending faults and older northwest-trending faults divide the area up into a number of orthogonal fault blocks or basins. Depending upon the tectonic history of the individual basin all or part of the Keweenawan section may be preserved. It is believed that large amounts of Keweenawan clastic sedimentary rock were eroded from the nemaha uplift east of the central graben of the rift and transported in an easterly direction. Prior to deposition of Paleozoic rocks the area was peneplaned. Correlation of various stratigraphic units over any distance is complicated by tectonic activity occurring at several times during the Precambrian and Paleozoic. Stratabound or stratiform deposits can occur both in the Precambrian as well as the overlying Paleozoic rocks. The possibility of massive sulfides to occur in the mafic intrusive rocks must not be excluded. In the core from the Poersch [number sign]1 well sulfides are recognized in gabbroic sills or dikes. Dark, fissile shale, similar to the Nonesuch Shale in the [number sign]1--4 Finn well averages 0.75% organic carbon. Thermal maturation within the rift probably ranges from within the oil window to over maturity.

  17. 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.

  18. Observational constraints and models for conjugate North Atlantic volcanic rifted margins formation

    NASA Astrophysics Data System (ADS)

    Huismans, R. S.; Faleide, J. I.; Planke, S.

    2009-12-01

    The amount of breakup-related magmatism in the northeast Atlantic cannot be explained by passive decompression melting of sub-lithospheric mantle with a normal potential temperature. Three competing end-member hypotheses are proposed for the formation of this excess magmatic activity: 1) excess magmatism results from elevated mantle potential temperatures associated with mantle plume processes, 2) rifting induced small scale convection at the base of the lithosphere enhances the flux of material through the melt window during rifting and mid-oceanic ridge spreading, and 3) mantle heterogeneities with lower melt temperatures of the melt source may produce larger than expected magmatic productivity. We review observations and present forward numerical models of passive margin formation including melt production. Key characteristics that require explanation include: 1) igneous crustal thickness versus mean igneous seismic velocity relationships for magmatic underplated bodies on the Norwegian-Greenland conjugate margins that indicate a cooling trend and seismic velocities that point to a moderate or non-existent mantle thermal anomaly during and after breakup; 2) rapid along margin variations in the observed igneous crustal thickness that suggest a local lithospheric control on melt productivity; 3) significant asymmetries in melt production existing between the Norwegian-Greenland conjugate margins; 4) post rift magmatic activity 30 my after breakup with igneous seismic velocity indicating normal mantle temperature; 5) anomalous low magmatic productivity in the oceanic Norway basin following moderate excess productivity that suggests anomalous low mantle temperatures following breakup, 6) anomalous mantle lithosphere thinning in the distal margin in the late syn-rift, 7) uplift and erosion of highly thinned crustal blocks in the distal margin. Forward numerical models of passive margin formation and melt productivity suggest the following key controls: 1) rifting

  19. Next-generation Geotectonic Data Analysis: Using pyGPlates to quantify Rift Obliquity during Supercontinent Dispersal

    NASA Astrophysics Data System (ADS)

    Butterworth, Nathaniel; Brune, Sascha; Williams, Simon; Müller, Dietmar

    2015-04-01

    Fragmentation of a supercontinent by rifting is an integral part of plate tectonics, yet the dynamics that govern the success or failure of individual rift systems are still unclear. Recently, analytical and thermo-mechanical modelling has suggested that obliquely activated rifts are mechanically favoured over orthogonal rift systems. Hence, where two rift zones compete, the more oblique rift proceeds to break-up while the less oblique one stalls and becomes an aulacogen. This implies that the orientation and shape of individual rift systems affects the relative motion of Earth's continents during supercontinent break-up. We test this hypothesis using the latest global plate tectonic reconstructions for the past 200 million years. The analysis is performed using pyGPlates, a recently developed Python library that allows script-based access to the plate reconstruction software GPlates. We quantify rift obliquity, extension velocity and their temporal evolution for all small-scale rift segments that constituted a major rift system during the last 200 million years. Boundaries between continental and oceanic crust (COBs) mark the end of rifting and the beginning of sea floor spreading, which is why we use a global set of updated COBs in order to pinpoint continental break-up and as a proxy for the local trend of former rift systems. Analysing the entire length of all rift systems during the last 200 My, we find a mean obliquity of ~40° (measured as the angle between extension direction and local rift trend normal), with a standard deviation of 25°. More than 75% of all rift segments exceeded an obliquity of 20° highlighting the fact that oblique rifting is the rule, not the exception. More specifically, East and West Gondwana split along the East African coast with a mean obliquity of 45°. While rifting of the central and southern South Atlantic segment involved a low obliquity of 10°, the Equatorial Atlantic opened under a high angle of 60°. The separation of

  20. Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted Rheic margin to active Pacific margin

    NASA Astrophysics Data System (ADS)

    Keppie, J. Duncan; Dostal, Jaroslav; Murphy, J. Brendan; Nance, R. Damian

    2008-12-01

    Paleozoic rocks in southern Mexico occur in two terranes, Oaxaquia (Oaxacan Complex) and Mixteca (Acatlán Complex) that appear to record: (1) Ordovician rifting on the southern margin of the Rheic Ocean, (2) passive drifting with Amazonia during the Silurian, (3) Devonian-Permian subduction beneath southern Mexico producing an arc complex that was partially removed by subduction erosion, subjected to HP metamorphism and Mississippian extrusion into the upper plate, followed by reestablishment of a Permian arc. In the Oaxaquia terrane, the 920-1300 Ma basement is unconformably overlain by a ˜ 200 m uppermost Cambrian-lowest Ordovician shelf sequence containing Gondwanan fauna (Tiñu Formation), unconformably overlain by 650 m of shallow marine-continental Carboniferous sedimentary rocks containing a Midcontinent (USA) fauna. In the Mixteca terrane, the low-grade Paleozoic sequence is composed of: (a) a ?Cambrian-Ordovician clastic sequence intruded by ca. 480-440 Ma bimodal, rift-related igneous rocks; and (b) a latest Devonian-Permian shallow marine sequence (> 906 m) consisting of metapsammites, metapelites and tholeiitic mafic volcanic rocks. High pressure (HP) metamorphic rocks in the Mixteca terrane consists of: (i) a Cambro-Ordovician rift-shelf intruded by bimodal rift-related intrusions that are similar to the low-grade rocks; (ii) periarc ultramafic rocks, and (iii) arc and MORB rocks. The Ordovician granitoids contain concordant inherited zircons that range in age from ca. 900 to 1300 Ma, indicating a source in the Oaxacan Complex. Concordant ages of detrital zircons in both the low- and high-grade Cambro-Ordovician metasedimentary rocks indicate a provenance in local Ordovician plutons and/or ca. 1 Ga Oaxacan basement, and distal northwestern Gondwana sources with a unique source in the 900-750 Ma Goiás magmatic arc within the Brasiliano orogen. These data combined with the rift-related nature of the Cambro-Ordovician rocks are most consistent with an

  1. 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

  2. Parallel Activation in Bilingual Phonological Processing

    ERIC Educational Resources Information Center

    Lee, Su-Yeon

    2011-01-01

    In bilingual language processing, the parallel activation hypothesis suggests that bilinguals activate their two languages simultaneously during language processing. Support for the parallel activation mainly comes from studies of lexical (word-form) processing, with relatively less attention to phonological (sound) processing. According to…

  3. Late Pleistocene-Holocene Faulting History Along the Northern El Carrizal Fault, Baja California Sur, Mexico: Earthquake Recurrence at a Persistently Active Rifted Margin

    NASA Astrophysics Data System (ADS)

    Maloney, S. J.; Umhoefer, P. J.; Arrowsmith, J. R.; Gutiérrez, G. M.; Santillanez, A. U.; Rittenour, T. R.

    2007-12-01

    , the sediments appear to be faulted by 3 to 4 earthquakes. The two most recent events may have experienced 1-2 m of offset each. Map relationships suggest that this faulted unit is correlative to a fluvial unit near Bonfil that yields an optical age of early Holocene. This would imply that the two most recent events at the quarry are Holocene in age. Based on preliminary OSL data and total measured offset at trench sites, the slip rates are estimated to be from 0.1 to 0.5 mm/yr. In summary, earthquake recurrence and slip rate may increase towards the north along the El Carrizal fault, consistent with footwall geologic mapping. If the El Carrizal fault abides by the rules of normal fault geometry and fault offset, it is likely that the fault is at a maximum total offset offshore of San Juan de la Costa, a prediction to be tested in the summer of 2008 by offshore imaging using Compressed High Intensity Radar Pulse (CHIRP). Improving the earthquake record along the El Carrizal fault will prove beneficial for understanding the seismic hazards to the city of La Paz. Furthermore, quantifying faulting rates throughout the southwestern margin of the Gulf of California will improve our understanding of the rift-to-drift process and how that process has operated along an oblique-divergent plate margin.

  4. 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).

  5. [Rift Valley fever].

    PubMed

    Pépin, M

    2011-06-01

    Rift Valley Fever (RVF) is a zoonotic arbovirosis. Among animals, it mainly affects ruminants, causing abortions in gravid females and mortality among young animals. In humans, RVF virus infection is usually asymptomatic or characterized by a moderate fever. However, in 1 to 3% of cases, more severe forms of the disease (hepatitis, encephalitis, retinitis, hemorrhagic fever) can lead to the death of infected individuals or to major sequels. The RVF virus (Bunyaviridae, genus Phlebovirus) was identified for the first time in the 1930s in Kenya. It then spread over almost all African countries, sometimes causing major epizootics/epidemics. In 2000, the virus was carried out of Africa, in the Middle East Arabian Peninsula. In 2007-2008, Eastern-African countries, including Madagascar, reported significant episodes of RVF virus, this was also the case for the Comoros archipelago and the French island of Mayotte. This ability to spread associated with many vectors, including in Europe, and high viral loads in infected animals led the health authorities worldwide to warn about the potential emergence of RVF virus in areas with a temperate climate. The awareness has increased in recent years with climate changes, which may possibly modify the vector distribution and competence, and prompted many RVF virus-free countries to better prepare for a potential implantation of RVF.

  6. The MOZART Project - MOZAmbique Rift Tomography

    NASA Astrophysics Data System (ADS)

    Fonseca, J. F.; Chamussa, J. R.; Domingues, A.; Helffrich, G. R.; Fishwick, S.; Ferreira, A. M.; Custodio, S.; Brisbourne, A. M.; Grobbelaar, M.

    2012-12-01

    Project MOZART (MOZAmbique Rift Tomography) is an ongoing joint effort of Portuguese, Mozambican and British research groups to investigate the geological structure and current tectonic activity of the southernmost tip of the East African Rift System (EARS) through the deployment of a network of 30 broad band seismic stations in Central and Southern Mozambique. In contrast with other stretches of the EARS to the North and with the Kapvaal craton to the West and South, the lithosphere of Mozambique was not previously studied with a dense seismographic deployment on account of past political instability, and many questions remain unanswered with respect to the location and characteristics of the EARS to the south of Tanzania. In recent years, space geodesy revealed the existence of three microplates in and off Mozambique - Victoria, Rovuma, Lwandle - whose borders provide a connection of the EARS to the South West Indian Ridge as required by plate tectonics. However, the picture is still coarse concerning the location of the rift structures. The 2006 M7 Machaze earthquake in Central Mozambique highlighted the current tectonic activity of the region and added a further clue to the location of the continental rift, prompting the MOZART deployment. Besides helping unravel the current tectonics, the project is expected to shed light on the poorly known Mesoproterozoic structure described by Arthur Holmes in 1951 as the Mozambique Belt, and on the mechanisms of transition from stable craton to rifted continental crust, through the development of a tomographic model for the lithosphere. The MOZART network is distributed South of the Zambezi river at average inter-station spaces of the order of 100 km and includes four stations across the border in South Africa. Data exchange was agreed with AfricaArray. The deployment proceeded in two phases in March 2011, and November and December 2011. Decommissioning is foreseen for August 2013. We report preliminary results for this

  7. Late Miocene termination of tectonic activity on the detachment in the Alaşehir Rift, Western Anatolia: Depositional records of the Göbekli Formation and high-angle cross-cutting faults

    NASA Astrophysics Data System (ADS)

    Sen, Fatih

    2016-04-01

    sandstones in place of siltstone-claystone intercalations indicates fluvial environment to be controlled flat plains during depositional time of the levels in Göbekli Formation. Depositional records of the Göbekli Formation indicate that its deposition was formed in dissimilar depositional environments with different tectonic activity. The evidence for the unconformity is hidden in the depositional records of the Göbekli formation. The lower levels of the Göbekli formation were formed as alluvial-fan deposits under influence of tectonic activity of the Karadut detachment fault and low-angle antithetic and synthetic normal faults. The middle and upper sections of the sequence were however formed as fluvial controlled flat plain deposits without any tectonic activity of the Karadut detachment fault during early Pliocene. The Miocene sediments were overall deposited under the influence of the Karadut detachment system; however the detachment ceased its activity during the deposition of the middle and upper levels of the Göbekli formation. This view is also supported by Plio-Quaternary sediments which were deposited under the influence of high angle faults, thereby, leading to the unconformity between the Miocene and Plio-Quaternary sediments and a two-stage rift system.

  8. The First Prediction of a Rift Valley Fever Outbreak

    NASA Technical Reports Server (NTRS)

    Anyamba, Assaf; Chretien, Jean-Paul; Small, Jennifer; Tucker, Compton J.; Formenty, Pierre; Richardson, Jason H.; Britch, Seth C.; Schnabel, David C.; Erickson, Ralph L.; Linthicum, Kenneth J.

    2009-01-01

    El Nino/Southern Oscillation (ENSO) related anomalies were analyzed using a combination of satellite measurements of elevated sea surface temperatures, and subsequent elevated rainfall and satellite derived normalized difference vegetation index data. A Rift Valley fever risk mapping model using these climate data predicted areas where outbreaks of Rift Valley fever in humans and animals were expected and occurred in the Horn of Africa from December 2006 to May 2007. The predictions were subsequently confirmed by entomological and epidemiological field investigations of virus activity in the areas identified as at risk. Accurate spatial and temporal predictions of disease activity, as it occurred first in southern Somalia and then through much of Kenya before affecting northern Tanzania, provided a 2 to 6 week period of warning for the Horn of Africa that facilitated disease outbreak response and mitigation activities. This is the first prospective prediction of a Rift Valley fever outbreak.

  9. Conjugate volcanic rifted margins, spreading and micro-continent: Lessons from the Norwegian-Greenland Sea

    NASA Astrophysics Data System (ADS)

    Gernigon, L.; Blischke, A.; Nasuti, A.; Sand, M.

    2014-12-01

    We have acquired and processed new aeromagnetic data that covers the entire Norway Basin oceanic spreading system located between the Møre volcanic rifted margin and its (intermediate) conjugate system, the Jan Mayen microcontinent (JMMC). The new compilation allows us to revisit its entire structure and spreading evolution from the Early Eocene breakup to the Late Oligocene abortion of the Aegir Ridge. We here discuss the dynamics of conjugate volcanic (rifted) margin formation and reconstruct the subsequent spreading evolution of the Norway Basin until its abortion. We have estimated a new set of Euler poles of rotation for the Norway Basin derived from more than 88,000 km of new magnetic profiles. The new compilation confirms that a fan-shaped spreading evolution of the Norway Basin was particularly active before the cessation of seafloor spreading and abortion of the Aegir Ridge. The Norway Basin shows a more complex system of micro-plates and asymmetric segments locally affected by episodic ridge jumps. The new observations have implications for the syn- and post-breakup stratigraphic and rifted-margin tectonic development of the JMMC and its conjugate margins. In particular, an important Mid-Eocene geodynamic event at around magnetic chron C21r is recognized in the Norway Basin. This event coincides with the onset of diking and rifting between the proto-JMMC and the East Greenland margin, leading to a second phase of breakup in the Norwegian-Greenland Sea ~26 My later in the Oligocene. Restored in its pre-breakup configuration, the new surveys also allow us to discuss further the tectonic and crustal evolution of the conjugate volcanic rifted margins and associated transform and oblique segments. The applicability of magma-poor concepts, off Norway, for example, remains questionable for us. The significant amount of breakup magmatism, the huge amount of pre-breakup sag sedimentation and the presence of thinned and preserved continental crust without the

  10. Mesozoic Rifting in the German North Sea

    NASA Astrophysics Data System (ADS)

    Lutz, R.; Jähne, F.; Arfai, J.

    2013-12-01

    The Central Graben is the southernmost expressions of the Mesozoic North Sea rift system that includes the Viking Graben, Moray Firth-Witch Ground grabens and the Horda-Egersund half graben. In the southern North Sea the Central Graben extends across the Dutch and the German exclusive economic zones. The structure of the Central Graben in German territorial waters was mapped in great detail in 2D and 3D seismic data and the stratigraphy has been constraint by borehole data. We provide a detailed review of the rifting activity in the German North Sea sector both in time and space and the link between rifting and salt movement. Major rifting activity started in the Central Graben during the Late Triassic and peaked during the Late Jurassic when extensive rift grabens formed, further influenced by halokinetic movements. First subsidence in the Central Graben area appears in the Early Triassic. This is documented by thickness variations in the sedimentary strata from the Triassic to the Jurassic. Remarkably thick sediments were deposited during the Late Triassic along the eastern border fault of the Central Graben and in the Late Jurassic sediments accumulated along graben-wide extensional faults and in rim-synclines of salt-structures. A basin inversion commenced in the Late Cretaceous resulting in an erosion of wide portions of Lower Cretaceous rocks or even complete removal in some parts. The area to the east of the Central Graben faced a completely different evolution. In this area major rifting activity initiated already in the Early to Middle Triassic. This is evident from huge packages of Middle Buntsandstein to Muschelkalk (Middle Triassic) sediments in the Horn Graben. Jurassic doming, forming the Mid-North Sea High, resulted in almost complete erosion of Lower and Middle Jurassic sediments in the central German North Sea. Sedimentation continued during the Early and Late Cretaceous. The Glückstadt Graben, which is a structure located farther east has a

  11. Extension across the Laptev Sea continental rifts constrained by gravity modeling

    NASA Astrophysics Data System (ADS)

    Mazur, S.; Campbell, S.; Green, C.; Bouatmani, R.

    2015-03-01

    The Laptev Shelf is the area where the Gakkel Ridge, an active oceanic spreading axis, approaches a continental edge, causing a specific structural style dominated by extensive rift structures. From the latest Cretaceous to the Pliocene, extension exerted on the Laptev Shelf created there several deep subsided rifts and high-standing basement blocks. To understand syn-rift basin geometries and sediment supply relationships across the Laptev Shelf, accurate extension estimates are essential. Therefore, we used 2-D gravity modeling and 3-D gravity inversion to constrain the amount of crustal stretching across the North America-Eurasia plate boundary in the Laptev Shelf. The latest Cretaceous-Cenozoic extension in that area is partitioned among two rift zones, the Laptev Rift System and the New Siberian Rift. These rifts were both overprinted on the Eurasian margin that had been stretched by 190-250 km before the Late Cretaceous. While the Laptev Rift System, connected to the Gakkel Ridge, reveals increasing extension toward the shelf edge (190-380 km), the New Siberian Rift is characterized by approximately uniform stretching along strike (110-125 km). The architecture of the Laptev Rift System shows that the finite extension of about 500 km is sufficient to entirely eliminate crystalline continental crust. In the most stretched rift segment, continental mantle is exhumed at the base of the Late Mesozoic basement. The example of the Laptev Rift System shows that extension driven by divergent plate movement is a sufficient cause to produce almost complete continental breakup without an increased heat input from the asthenospheric mantle.

  12. Crustal Structure at a Young Continental Rift: A Receiver Function Study from Lake Tanganyika

    NASA Astrophysics Data System (ADS)

    Hodgson, I. D. S.; Illsley-Kemp, F.; Gallacher, R. J.; Keir, D.; Ebinger, C. J.; Drooff, C.; Khalfan, M.

    2015-12-01

    Lake Tanganyika, in western Tanzania, spans a large section of the Western rift yet there are very few constraints on bulk crustal and upper mantle structure. The Western rift system has no surface expression of magmatism, which is in stark contrast to the Eastern branch. This observation is difficult to reconcile with the approximately coeval initiation of rifting of the two branches. The variation in the nature of rifting provides a perfect setting to test current hypotheses for the initiation of continental breakup and early-stage development of continental rifts. The deployment of a seismic network of 13 broadband instruments on the south eastern shore of Lake Tanganyika, for 16 months, between 2014 and 2015 provides a unique opportunity to investigate extensional processes in thick continental lithosphere. We present here results from a P to S receiver function study that provides information on bulk crustal Vp/Vs ratio along the rift; a property that is sensitive to the presence of magmatic intrusions in the lower crust. Additionally this method allows us to map variations in crustal thickness both parallel and perpendicular to the rift axis. These results thus provide unprecedented insight into the large-scale mechanics of early-stage continental rifting along the non-volcanic Western rift.

  13. Drilling to investigate processes in active tectonics and magmatism

    NASA Astrophysics Data System (ADS)

    Shervais, J.; Evans, J.; Toy, V.; Kirkpatrick, J.; Clarke, A.; Eichelberger, J.

    2014-12-01

    Coordinated drilling efforts are an important method to investigate active tectonics and magmatic processes related to faults and volcanoes. The US National Science Foundation (NSF) recently sponsored a series of workshops to define the nature of future continental drilling efforts. As part of this series, we convened a workshop to explore how continental scientific drilling can be used to better understand active tectonic and magmatic processes. The workshop, held in Park City, Utah, in May 2013, was attended by 41 investigators from seven countries. Participants were asked to define compelling scientific justifications for examining problems that can be addressed by coordinated programs of continental scientific drilling and related site investigations. They were also asked to evaluate a wide range of proposed drilling projects, based on white papers submitted prior to the workshop. Participants working on faults and fault zone processes highlighted two overarching topics with exciting potential for future scientific drilling research: (1) the seismic cycle and (2) the mechanics and architecture of fault zones. Recommended projects target fundamental mechanical processes and controls on faulting, and range from induced earthquakes and earthquake initiation to investigations of detachment fault mechanics and fluid flow in fault zones. Participants working on active volcanism identified five themes: the volcano eruption cycle; eruption sustainability, near-field stresses, and system recovery; eruption hazards; verification of geophysical models; and interactions with other Earth systems. Recommended projects address problems that are transferrable to other volcanic systems, such as improved methods for identifying eruption history and constraining the rheological structure of shallow caldera regions. Participants working on chemical geodynamics identified four major themes: large igneous provinces (LIPs), ocean islands, continental hotspot tracks and rifts, and

  14. Kilauea east rift zone magmatism: An episode 54 perspective

    USGS Publications Warehouse

    Thornber, C.R.; Heliker, C.; Sherrod, D.R.; Kauahikaua, J.P.; Miklius, Asta; Okubo, P.G.; Trusdell, F.A.; Budahn, J.R.; Ridley, W.I.; Meeker, G.P.

    2003-01-01

    On January 29 30, 1997, prolonged steady-state effusion of lava from Pu'u'O'o was briefly disrupted by shallow extension beneath Napau Crater, 1 4 km uprift of the active Kilauea vent. A 23-h-long eruption (episode 54) ensued from fissures that were overlapping or en echelon with eruptive fissures formed during episode 1 in 1983 and those of earlier rift zone eruptions in 1963 and 1968. Combined geophysical and petrologic data for the 1994 1999 eruptive interval, including episode 54, reveal a variety of shallow magmatic conditions that persist in association with prolonged rift zone eruption. Near-vent lava samples document a significant range in composition, temperature and crystallinity of pre-eruptive magma. As supported by phenocryst liquid relations and Kilauea mineral thermometers established herein, the rift zone extension that led to episode 54 resulted in mixture of near-cotectic magma with discrete magma bodies cooled to ???1100??C. Mixing models indicate that magmas isolated beneath Napau Crater since 1963 and 1968 constituted 32 65% of the hybrid mixtures erupted during episode 54. Geophysical measurements support passive displacement of open-system magma along the active east rift conduit into closed-system rift-reservoirs along a shallow zone of extension. Geophysical and petrologic data for early episode 55 document the gradual flushing of episode 54 related magma during magmatic recharge of the edifice.

  15. Rifting on Venus: Implications for lithospheric structure

    NASA Technical Reports Server (NTRS)

    Banerdt, W. B.; Golombek, M. P.

    1985-01-01

    Lithospheric strength envelopes on Venus are reviewed and their implications for large scale rifting are discussed. Their relationship to crustal thicnesses and thermal gradients are explored. Also considered are the implications of a theory for rift formation.

  16. A new model for the formation of linear rift zones on oceanic island volcanoes

    NASA Astrophysics Data System (ADS)

    Kluegel, A.; Walter, T. R.

    2003-04-01

    ) is oriented nearly perpendicular to the rifts of adjacent Kilauea rather than parallel to them. We conclude that the effect of spreading and buttressing on large volcanic edifices may diverge laterally, where rifting between two edifices is (i) perpendicular to their line of contact if edifice overlap is minor developed, but (ii) parallel to this line if one volcano is only an attached part of a larger edifice. Once formed, these rifts stabilize themselves by alternating constructive and destructive processes.

  17. Tectonostratigraphic development of the Interior Sudan rifts, Central Africa

    NASA Astrophysics Data System (ADS)

    McHargue, Tim R.; Heidrick, Tom L.; Livingston, Jack E.

    1992-10-01

    In the Muglad, Melut and Blue Nile rift basins of Interior Sudan three major episodes of rifting, concomitant subsidence and nonmarine/nonvolcanic sedimentation are recognized. These three rifting cycles, which spanned 140 to 95 Ma (Fl), 95 to 65 Ma (F2), and 65 to 30 Ma (F3), resulted in the accumulation of up to 5400, 4200 and 5400 m of sediments, respectively. In the Muglad Basin, the best known and largest of the Sudan rift basins, each rifting cycle consists of (1) a basal sandstone unit (at least near rift margins), that is followed by (2) an upward coarsening section of lacustrine shale grading through marginal lacustrine mudstone and sandstone into fluvial mudstone and sandstone, and (3) a capping blanket of fluvial and alluvial sandstone. The shale-dominated portions of these cycles were deposited in a closed-drainage basin during active faulting. The fluvial and alluvial blanket sands were deposited in an open-drainage basin during the thermal sag phase following each tectonic cycle. The Early Cretaceous F1 intracontinental rifts of Interior Sudan were linked to major rifts/spreading centres in the Proto-South Atlantic by the dextral WSW-trending Central African Shear Zone and to the Indian Ocean via the NW-trending Anza rift in Kenya. In the Muglad Basin, F1 deformation involved high strain rates, rapid syn-rift crustal stretching and subsidence, and the formation of deep, fault-bounded tensional and transtensional pull-apart basins. During the F2 and F3 deformations, the rates of subsidence and stretching were much lower and were focused within smaller geographic areas. Structural elements include asymmetric half-grabens and less common full-grabens with central highs. The three superimposed tectonic episodes resulted in the subsidence of NNW- to NW-trending rift sub-basins; this gave rise to a wide variety of normal fault geometries, displacements, and growth histories. Planar domino-style and listric normal F1 fault arrays are modeled. The rotated F1

  18. Magmatic cycles pace tectonic and morphological expression of rifting (Afar depression, Ethiopia)

    NASA Astrophysics Data System (ADS)

    Medynski, S.; Pik, R.; Burnard, P.; Dumont, S.; Grandin, R.; Williams, A.; Blard, P.-H.; Schimmelpfennig, I.; Vye-Brown, C.; France, L.; Ayalew, D.; Benedetti, L.; Yirgu, G.

    2016-07-01

    The existence of narrow axial volcanic zones of mid-oceanic ridges testifies of the underlying concentration of both melt distribution and tectonic strain. As a result of repeated diking and faulting, axial volcanic zones therefore represent a spectacular topographic expression of plate divergence. However, the submarine location of oceanic ridges makes it difficult to constrain the interplay between tectonic and magmatic processes in time and space. In this study, we use the Dabbahu-Manda Hararo (DMH) magmatic rift segment (Afar, Ethiopia) to provide quantitative constraints on the response of tectonic processes to variations in magma supply at divergent plate boundaries. The DMH magmatic rift segment is considered an analogue of an oceanic ridge, exhibiting a fault pattern, extension rate and topographic relief comparable to intermediate- to slow-spreading ridges. Here, we focus on the northern and central parts of DMH rift, where we present quantitative slip rates for the past 40 kyr for major and minor normal fault scarps in the vicinity of a recent (September 2005) dike intrusion. The data obtained show that the axial valley topography has been created by enhanced slip rates that occurred during periods of limited volcanism, suggestive of reduced magmatic activity, probably in association with changes in strain distribution in the crust. Our results indicate that the development of the axial valley topography has been regulated by the lifetimes of the magma reservoirs and their spatial distribution along the segment, and thus to the magmatic cycles of replenishment/differentiation (<100 kyr). Our findings are also consistent with magma-induced deformation in magma-rich rift segments. The record of two tectonic events of metric vertical amplitude on the fault that accommodated the most part of surface displacement during the 2005 dike intrusion suggests that the latter type of intrusion occurs roughly every 10 kyr in the northern part of the DMH segment.

  19. Characterising Antarctic and Southern Ocean Lithosphere with Magnetic and Gravity Imaging of East Antarctic Rift Systems

    NASA Astrophysics Data System (ADS)

    Vaughan, A. P.; Kusznir, N. J.; Ferraccioli, F.; Jordan, T. A.; Purucker, M. E.; Golynsky, A. V.; Rogozhina, I.

    2012-12-01

    Since the International Geophysical Year (1957), a view has prevailed that the lithospheric structure of East Antarctica is relatively homogeneous, forming a geological block of largely cratonic nature, consisting of a mosaic of Precambrian terranes, stable since the Pan-African orogeny ~500 million years ago. Recent recognition of a continental-scale rift system cutting the East Antarctic interior indicates that this is incorrect, and has crystallised an alternative view of much more recent geological activity with important implications for tectonic reconstructions and controls on ice sheet formation and stability. The newly defined East Antarctic Rift System appears to extend from at least the South Pole to the continental margin at the Lambert Rift, a distance of 2500 km. This is comparable in scale to the well-studied East African rift system. New analysis of RadarSat data pioneered by Golynsky & Golynsky indicates that further rift zones may extend the East Antarctic Rift System into widely distributed extension zones within the continent. We have carried out a pilot study, using a newly developed gravity inversion technique with existing public domain satellite data, which shows that East Antarctica consists of distinct crustal thickness provinces with anomalously thick areas separated by thin, possibly rifted crust and overall high average thickness. Understanding the nature of crustal thickness in East Antarctica is critical because: 1) Better understanding of crustal thickness in Antarctica, especially along the ocean-continent transition (OCT), will make it possible to improve the plate reconstruction fit between Antarctica, Australia and India in Gondwana and also refine constraints on how and when these continents separated; 2) crustal thickness provinces can be used to aid supercontinent reconstructions and provide new assessments of the influence of basement architecture and mechanical properties on rifting processes; 3) tracking rift zones through

  20. Syn-Rift Stratigraphic Architecture Reveals the Growth History of a Sub-basinal Fault Population in the Outer Moray Firth, North Sea

    NASA Astrophysics Data System (ADS)

    Kane, K.; Gupta, S.; Trudgill, B.; Johnson, H.

    2003-12-01

    Processes of normal fault propagation and linkage are recorded in the stratigraphic record by syn-rift sedimentary deposits that fill the generated accommodation volume. Using 3D seismic stratigraphic analysis, supported by well log and core interpretation, we investigate how the growth of an intrabasinal fault population led to the progressive development of an extensional sub-basin in the Moray Firth rift arm of the North Sea. The North Halibut Graben sub-basin has an E-W to WNW-ESE orientation and formed through the interaction of two main structural trends during late Jurassic rifting. E-W trending structural barriers bound the sub-basin to the north (Tartan and Petronella Ridges) and south (Halibut Horst Spur) whilst major NE-SW trending structures occur at the eastern margin. Spatial and temporal changes in syn-rift stratigraphic architecture reflect the history of faulting within the North Halibut Graben sub-basin. Fault parallel seismic profiles and intra-syn rift isochron maps demonstrate how faults initially developed as separate segments and subsequently linked to form longer strands through progressive growth and propagation. They also provide clear evidence that a major change in the structural framework occurred during rifting, supporting earlier studies advocating sequential rather than synchronous normal fault activity. The syn-rift sequence can be divided into at least two phases based on shifts in sedimentary packages and reorganistation of sequence thicknesses. Isochron maps illustrate that from late Oxfordian times (syn-rift phase I), early syn-rift sedimentation was controlled solely by NE-SW trending faults at the eastern margin of the basin. Strain was initially accommodated across several distributed, highly segmented faults but, with progressive linkage, stress became localised on one or two major through-going fault strands whilst shorter surrounding segments were switched off. From early-mid Volgian times we observe a progressive switch

  1. Beta Regio rift system on Venus: Geologic interpretation of Magellan images

    NASA Technical Reports Server (NTRS)

    Nikishin, A. M.; Bobina, N. N.; Borozdin, V. K.; Burba, G. A.

    1993-01-01

    Magellan SAR images and altimetric data were used to produce a new geologic map of the Northern part of Beta Regio within the frames of C1-30N279 mapsheet. It was part of our contributions into C1-formate geologic mapping efforts. The original map is at 1:8,000,000 scale. The rift structures are typical for Beta Regio on Venus. There are many large uplifted tessera areas on Beta upland. They occupy areas of higher topography. These tessera are partly burried by younger volcanic cover of plain material. These observations show that Beta upland was formed mainly due to lithospheric tectonical uplifting, and only partly was constructed by volcanic activity. A number of rift valleis traverse Beta upland and spread to the surrounding lowlands. The largest rift crosses Beta N to S. Typical width of rifts is 40 to 160 km. Rift valleis in this region are structurally represented by crustal grabens and half-grabens. There are symmetrical and asymmetrical rifts. A lot of them have shoulder uplifts with the relative high up to 0.5-1 km and width 40 to 60 km. Preliminary analysis of the largest rift valley structural cross-sections leads to the conclusion that it originated due to a 5-10 percent crustal extension. The prominent shield volcano - Theia Mons - is located at the center of Beta rift system. It could be considered as the surface manifestation of the upper mantle hot spot. Most of the rift belts are located radially to Theia Mons. The set of these data leads to conclusion that Beta rift system has an 'active-passive' origin. It was formed due to the regional tectonic lithospheric extension. Rifting was accelerated by the upper mantle hot spot located under the center of passive extension (under Beta Regio).

  2. Applications of surface geology models to subsurface interpretations in continental rifted basins

    SciTech Connect

    Charpentier, P.; Jarrige, J.; Richert, J. )

    1990-05-01

    Field geology surveys done from 1980 to 1987 along the Gulf of Suez, Red Sea, and in the East African rift led us to (1) compare the two in-extension domains in terms of geometry, mechanism and timing of deformation to point out the structural and sedimentary elements useful to interpret seismic and well data and (2) propose geodynamic models to perform a more efficient exploration in other rifted basins. Field observations show that the fault pattern is controlled by the inherited fabric of the basement, which is reactivated during the rifting process. This fabric defines the location of the oblique-to-the-rift transfer zones which cut the deformed area in losangic blocks limited by antithetic normal faults parallel to the rift. Transfer zones exhibit either strike-slip faults or local compressive structures, or horst-and-graben pattern. The interference between the normal faults and the transfer zones induces the typical zigzag pattern in which petroleum traps will have specific setting. These synsedimentary deformations have a direct influence on the reservoir facies changes. The mechanical evolution is characterized by multistage tectonic deformations in which the doming generally approved as a first rifting initiation is not necessary to explain the observed extension. Sinking of the central trough and uplift of the rift shoulders represents the last stage of the rifting process due only to thermal subsidence. This process is important in hydrocarbon generation and migration.

  3. Mode of rifting in magmatic-rich setting: Tectono-magmatic evolution of the Central Afar rift system

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    Observation of deep structures related to break-up processes at volcanic passive margins (VPM) is often a troublesome exercise: thick pre- to syn-breakup seaward-dipping reflectors (SDR) usually mask the continent-ocean boundary and hide the syn-rift tectonic structures that accommodate crustal stretching and thinning. Some of the current challenges are about clarifying 1) if tectonic stretching fits the observed thinning and 2) what is the effect of continuous magma supply and re-thickening of the crust during extension from a rheological point of view? The Afar region in Ethiopia is an ideal natural laboratory to address those questions, as it is a highly magmatic rift that is probably close enough to breakup to present some characteristics of VPM. Moreover, the structures related to rifting since Oligocene are out-cropping, onshore and well preserved. In this contribution, we present new structural field data and lavas (U-Th/He) datings along a cross-section from the Ethiopian Plateau, through the marginal graben down to the Manda-Hararo active rift axis. We mapped continent-ward normal fault array affecting highly tilted trapp series unconformably overlain by tilted Miocene (25-7 Ma) acid series. The main extensional and necking/thinning event took place during the end of this Miocene magmatic episode. It is itself overlain by flat lying Pliocene series, including the Stratoid. Balanced cross-sections of those areas allow us to constrain a surface stretching factor of about 2.1-2.9. Those findings have the following implications: - High beta factor constrained from field observations is at odd with thinning factor of ~1.3 predicted by seismic and gravimetric studies. We propose that the continental crust in Central Afar has been re-thickened by the emplacement of underplated magma and SDR. - The deformation in Central Afar appears to be largely distributed through space and time. It has been accommodated in a 200-300 km wide strip being a diffuse incipient

  4. Hydrocarbon potential of intracratonic rift basins

    SciTech Connect

    Baker, D.G.; Derksen, S.J.

    1984-09-01

    Significant world oil reserves have been added in recent years from rift system. Examples of petroliferous rift basins may be found on nearly every major continent. As our understanding of the mechanisms of sedimentation and structure in rift basins grows, more rift systems will be found. With a few notable exceptions, rifts that have been explored in the past are those that formed along continental margins. These contain marine sediments, and the conditions of source rock, sediment type, depositional environment, and structural style are well-known exploration concepts. Intracratonic rift systems containing continental sediments, and also because of the problems perceived to accompany continental sedimentation. A good modern analog is the East African rift system. Several companies have made significant oil discoveries in different components of the Central African rift system. Average daily production for 1982 from the basins associated with the Benue trough was 107.928 BOPD. In the Abu Gabra rift component, where Marathon is currently exploring, Chevron has drilled approximately 60 wells. Nineteen of these were discoveries and tested an average rate per well of 3,500 BOPD. The Abu Gabra rift may contain up to 10 billion bbl of oil. Research indicates that this type of rift system is present in other areas of the world. Ongoing worldwide exploration has shown that intracratonic rift basins have the potential to make a significant contribution to world oil reserves.

  5. Dike injection and magma mixing in Kenya rift volcanoes

    NASA Astrophysics Data System (ADS)

    Anthony, E. Y.; Espejel, V.; Biggs, J.

    2009-12-01

    A nexus of volcanoes in the rift graben at approximately the latitude of Nairobi consist of central vent trachyte, phonolite, and peralkaline rhyolite and cinder cone and fissure-fed flows of basalt to benmoreite. The volcanoes are referred to as the Central Kenya Peralkaline Province (CKPP, Macdonald and Scaillet, 2006, Lithos 91, 59-73) and formed by a combination of processes including fractional crystallization, magma mixing, and volatile transport (Ren et al., 2006, Lithos 91, 109-124; Macdonald et al., 2008, JPet 49, 1515-1547). This presentation focuses on magma mixing for trachytes and phonolites for Suswa rocks, which are the southernmost part of the CKPP. We also explore the contribution of magma process studies to the interpretation of recent geodetic data, which indicate inflation/deflation of up to 21 cm for Kenyan volcanoes from 1997 to present (Biggs et al., 2009, Geology, in press). Incontrovertible evidence for magma mixing is found in field evidence, where a basaltic trachyandesite ash horizon is found interbedded with syncaldera trachyte (Skilling, 1993, J. Geol. Society London 150, 885-896), hand-specimen and thin-section petrography, and disequilibrium mineral chemistry. Precaldera lavas contain a homogeneous group of anorthoclase crystals with An content 6% or less. Syncaldera samples contain this same group and two other populations: polysynthetic twinned labradorite and andesine and anorthoclase with An content of 17%. Textures for all three groups indicate disequilibrium. Postcaldera flows contain the high and low An anorthoclase populations but lack the polysynthetic twinned labradorite and andesine. These observations suggest a model of injection of mafic magmas via diking into shallow trachtytic magma systems. Recent geodetic studies of dike injection and subsequent seismic/volcanic activity in both Ethiopia and Lengai point to the ongoing importance of these processes to rift evolution in East Africa.

  6. 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

    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.

  7. The role of rifting in the evolution of the continental margin of Eastern Asia: Geophysical evidence

    NASA Astrophysics Data System (ADS)

    Rodnikov, A. G.; Rodnikova, R. D.; Zorina, Yu. G.

    1992-08-01

    The role of rift processes is analysed in the structural evolution of the continental margins of Eastern Asia including the Indo-China Peninsula and North China plain. Paleoreconstructions were made for the Indo-China Peninsula to characterize individual stages of rifting covering the Late Cretaceous-Eocene, Oligocene-Middle Miocene and Late Pliocene-Early Quaternary epochs. The rifting of continental margins occurred synchronously with spreading processes in marginal seas, whereas the formation of rift structures in the North China plain was concurrent with the formation of a deep-water basin of the Philippine Sea. The development of asthenospheric diapire led to crustal extension and was responsible for the formation of rift structures in marginal seas and continental margins.

  8. East Antarctic rifting triggers uplift of the Gamburtsev Mountains.

    PubMed

    Ferraccioli, Fausto; Finn, Carol A; Jordan, Tom A; Bell, Robin E; Anderson, Lester M; Damaske, Detlef

    2011-11-16

    The Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 1958. The preservation of Alpine topography in the Gamburtsevs may reflect extremely low long-term erosion rates beneath the ice sheet, but the mountains' origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long rift system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1 Gyr ago), was preserved as in some old orogens and was rejuvenated during much later Permian (roughly 250 Myr ago) and Cretaceous (roughly 100 Myr ago) rifting. Much like East Africa, the interior of East Antarctica is a mosaic of Precambrian provinces affected by rifting processes. Our models show that the combination of rift-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that rifting and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere.

  9. East Antarctic rifting triggers uplift of the Gamburtsev Mountains.

    PubMed

    Ferraccioli, Fausto; Finn, Carol A; Jordan, Tom A; Bell, Robin E; Anderson, Lester M; Damaske, Detlef

    2011-11-17

    The Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 1958. The preservation of Alpine topography in the Gamburtsevs may reflect extremely low long-term erosion rates beneath the ice sheet, but the mountains' origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long rift system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1 Gyr ago), was preserved as in some old orogens and was rejuvenated during much later Permian (roughly 250 Myr ago) and Cretaceous (roughly 100 Myr ago) rifting. Much like East Africa, the interior of East Antarctica is a mosaic of Precambrian provinces affected by rifting processes. Our models show that the combination of rift-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that rifting and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere. PMID:22094700

  10. Geochemical anomalies in the bottom waters of the Tadjoura rift zone, Gulf of Aden

    SciTech Connect

    Demina, L.L.; Tambiev, S.B.

    1987-04-01

    The study of geochemical fields and geochemical anomalies is a necessary part of exploration for ore deposits on the ocean floor. Geochemical processes related to hydrothermal activity occurring at the boundary between different media are of special interest. About 10 years ago, concentrations of suspended iron 20 times greater than those in the overlying waters were found in the bottom waters of the Mid-Atlantic Rift near 26/sup 0/N. A number of reports then appeared, showing that in other rift zones as well, the bottom/water interface is characterized by elevated concentrations of iron, manganese, zinc, copper, nickel, and other elements. Thus the present writers were persuaded that the minor elements in bottom waters can serve as indicators of hydrothermal flux to the ocean floor. In carrying out investigations of this kind, one must study the distribution of the metals not only in the bottom waters, but through the entire depth of the sea, so that anomalies can be detected and localized against the level of the background concentrations. In addition, to obtain information on the sources of the metals, one must determine not just the total contents, but also distinguish the metals in solution and in suspension. Results are discussed. The observations clarify the relations between dissolved and suspended forms of metals in weakly mineralized waters above oceanic rifts. 13 references, 5 figures, 5 tables.

  11. Neogene Development of the Terror Rift, western Ross Sea, Antarctica

    NASA Astrophysics Data System (ADS)

    Sauli, C.; Sorlien, C. C.; Busetti, M.; De Santis, L.; Wardell, N.; Henrys, S. A.; Geletti, R.; Wilson, T. J.; Luyendyk, B. P.

    2015-12-01

    Terror Rift is a >300 km-long, 50-70 km-wide, 14 km-deep sedimentary basin at the edge of the West Antarctic Rift System, adjacent to the Transantarctic Mountains. It is cut into the broader Victoria Land Basin (VLB). The VLB experienced 100 km of mid-Cenozoic extension associated with larger sea floor spreading farther north. The post-spreading (Neogene) development of Terror Rift is not well understood, in part because of past use of different stratigraphic age models. We use the new Rossmap seismic stratigraphy correlated to Cape Roberts and Andrill cores in the west and to DSDP cores in the distant East. This stratigraphy, and new fault interpretations, was developed using different resolutions of seismic reflection data included those available from the Seismic Data Library System. Depth conversion used a new 3D velocity model. A 29 Ma horizon is as deep as 8 km in the south, and a 19 Ma horizon is >5 km deep there and 4 km-deep 100 km farther north. There is a shallower northern part of Terror Rift misaligned with the southern basin across a 50 km right double bend. It is bounded by steep N-S faults down-dropping towards the basin axis. Between Cape Roberts and Ross Island, the Oligocene section is also progressively-tilted. This Oligocene section is not imaged within northern Terror Rift, but the simplest hypothesis is that some of the Terror Rift-bounding faults were active at least during Oligocene through Quaternary time. Many faults are normal separation, but some are locally vertical or even reverse-separation in the upper couple of km. However, much of the vertical relief of the strata is due to progressive tilting (horizontal axis rotation) and not by shallow faulting. Along the trend of the basin, the relief alternates between tilting and faulting, with a tilting margin facing a faulted margin across the Rift, forming asymmetric basins. Connecting faults across the basin form an accommodation zone similar to other oblique rifts. The Neogene basin is

  12. 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.

  13. Dyke intrusion dynamics during the ongoing rifting episode in Afar

    NASA Astrophysics Data System (ADS)

    Jacques, E.; Grandin, R.; Nercessian, A.; Ayele, A.; Keir, D.; Doubre, C.; Socquet, A.; Lemarchand, A.

    2010-12-01

    Dyke intrusion is one of the main processes of crustal accretion occurring along magma-assisted divergent plate boundaries, in particular along Mid-Ocean Ridges. Seismology, through the analysis of earthquake migration coeval to dyke intrusion, is one of the few tools, with geodesy, which allows studying the dynamics of this fundamental process. In Afar (Ethiopia), the major Dabbahu-Manda Hararo rifting episode started in September 2005 with the intrusion a 60 km-long, 5 m-wide mega-dyke. Between June 2006 and May 2010, 13 smaller dykes (~10 km long) were emplaced, giving an outstanding opportunity to perform such studies. A few weeks after the rifting episode commenced, a regional seismological network of broadband stations was installed in Afar (part of a multinational project including UK, US and Ethiopia). In November 2007, in collaboration with the Geophysical Observatory of Addis Ababa University, IPGP (Institut de Physique du Globe de Paris) installed a local telemetered seismological network including 5 short-period stations around the southern half of the Dabbahu-Manda Hararo rift. We focus here on the microseismicity related to five dyke intrusions, which occurred between June 2006 and February 2009. The dykes propagated away from the Walis magma reservoir (WMR, 12.3°N, 40.6°E) at velocities ranging from 0.5km/h to 2km/h. Earthquake migrations during dyke intrusions involved a narrow (less than 5 km wide), seismically-active strip around the dykes, and seem to show the following pattern: first, near the tip of the propagating dyke, probably in the damage zone, small to moderate earthquakes are detected. Subsequent, more energetic earthquakes may have been triggered only after sufficient dyke inflation had occurred. Earthquake sequences, which occurred north of WMR, migrated faster (~2km/h) than those that took place south of WMR (0.5-1km/h), and liberated a greater amount of seismic energy. This is in keeping with results of normal stress modeling

  14. Along-axis transition between narrow and wide rifts: Insights from 3D numerical experiments

    NASA Astrophysics Data System (ADS)

    Koptev, Alexander; Calais, Eric; Burov, Evgueni; Leroy, Sylvie; Gerya, Taras

    2016-04-01

    Based on performed high-resolution rheologically consistent three-dimensional thermo-mechanical numerical models, we show that there is a significant difference in the influence of the rheological profile on rifting style in the case of dominant active (plume-activated) rifting compared to dominant passive (far-field tectonic stresses) rifting. Narrow rifting, conventionally attributed to cold strong lithosphere in passive rifting mode, may develop in weak hot ultra-stretched lithosphere during active rifting, after plume impingement on a tectonically pre-stressed lithosphere. In that case, initially ultra-wide small-amplitude rift patterns focus, in a few Myr, in large-scale faults that form a narrow rift. Also, wide rifting may develop during ultra-slow spreading of strong lithosphere, and "switch" to the narrow rifting upon plume impingement. For further understanding the mechanisms behind the interactions between the mantle plume and far-field stresses in case of realistic horizontally heterogeneous lithosphere, we have tested our models on the case of the central East African Rift system (EARS). The EARS south of the Ethiopian Rift Valley bifurcates in two branches (eastern, magma-rich and western, magma-poor) surrounding the strong Tanzanian craton. Broad zones of low seismic velocity observed throughout the upper mantle beneath the central part of the EARS are consistent with the spreading of a deep mantle plume. The extensional features and topographic expression of the Eastern rift varies significantly north-southward: in northern Kenya the area of deformation is very wide (some 150-250 km in E-W direction), to the south the rift narrows to 60-70 km, yet further to the south this localized deformation widens again. Here we investigate this transition between localized and wide rifting using thermo-mechanical numerical modeling that couples, in a dynamic sense, the rise of the upper mantle material with the deformation of the African lithosphere below the

  15. Structural style of the Turkana Rift, Kenya

    SciTech Connect

    Dunkelman, T.J.; Karson, J.A.; Rosendahl, B.R.

    1988-03-01

    Multifold seismic reflection and geologic mapping in part of the eastern branch of the East African Rift system of northern Kenya reveal a major rift structure containing at least 3 km of Neogene sediment fill beneath Lake Turkana. This includes a series of half-graben basins, with centrally located quaternary volcanic centers, which are linked end-to-end by structural accommodation zones. Whereas the geometry of rifting is similar to that of the nonvolcanic western branch of the East African Rift system, the Turkana half-grabens are much smaller and may reflect extension of a thinner lithosphere or development of more closely spaced fracture patterns during rift evolution, or both.

  16. Geodetic measurements and numerical models of rifting in Northern Iceland for 1993-2008

    NASA Astrophysics Data System (ADS)

    Ali, S. T.; Feigl, K. L.; Carr, B. B.; Masterlark, T.; Sigmundsson, F.

    2014-03-01

    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 the NVZ using a 3-D numerical model in order to explain synthetic aperture radar data acquired by the ERS and Envisat satellite missions between 1993 and 2008. The deformation is non-linear in time over the observed interval. The observed deformation can be explained by a combination of three processes, including: (i) secular plate spreading between the North American and Eurasian plates at a rate of 18.2 mm yr-1, (ii) viscoelastic relaxation following the Krafla Fires rifting episode between 1975 and 1984 and (iii) inflation/deflation of shallow magma chambers beneath the Theistareykir and Krafla central volcanoes. We minimize the misfit between the observed and modelled values of the range change gradient, averaged over all samples, using a simulated annealing algorithm that uses a first-order Taylor series to approximate the fitting function. The calibration parameters include the locking depth of the plate boundary and the rheological properties of the lower crust and mantle. The 68-per cent confidence intervals for the parameters in the solution that best fits the data are: (i) a locking depth of 8.0 to 9.5 km, (ii) a viscosity of 19 to 49 EPa.s (1 EPa.s =1018 Pa.s) in the lower crust at depths between 8 and 24 km and (iii) a viscosity of 5 to 9 EPa.s in the upper mantle below 24 km.

  17. Intermittent upwelling of asthenosphere beneath the Gregory Rift, Kenya

    SciTech Connect

    Tatsumi, Yoshiyuki Kyoto Univ. ); Kimura, Nobukazu ); Itaya, Tetsumaru ); Koyaguchi, Takehiro ); Suwa, Kanenori )

    1991-06-01

    K-Ar dates and chemical compositions of basalts in the Gregory Rift, Kenya, demonstrate marked secular variation of lava chemistry. Two magmatic cycles characterized by incompatible element relative depletion are recognized; both occurring immediately after the peak of basaltic volcanism and coeval with both trachyte/phonolite volcanism and domal uplift of the region. These cycles may be attributed to increasing degree of partial melting of mantle source material in association with thinning of the lithosphere by thermal erosion through contact with hot upwelling asthenospheric mantle. Cyclic variation in asthenosphere upwelling may be considered an important controlling process in the evolution of the Gregory Rift.

  18. Phanerozoic Rifting Phases And Mineral Deposits

    NASA Astrophysics Data System (ADS)

    Hassaan, Mahmoud

    2016-04-01

    connected with NW,WNW and N-S faults genetically related to volcano-hydrothermal activity associated the Red Sea rifting. At Sherm EL-Sheikh hydrothermal manganese deposit occurs in Oligocene clastics within fault zone. Four iron-manganese-barite mineralization in Esh-Elmellaha plateau are controlled by faults trending NW,NE and nearly E-W intersecting Miocene carbonate rocks. Barite exists disseminated in the ores and as a vein in NW fault. In Shalatee - Halaib district 24 manganese deposits and barite veins with sulphide patches occur within Miocene carbonates distributed along two NW fault planes,trending 240°and 310° and occur in granite and basalt . Uranium -lead-zinc sulfide mineralization occur in Late Proterozoic granite, Late Cretaceous sandstones, and chiefly in Miocene clastic-carbonate-evaporate rocks. The occurrences of uranium- lead-zinc and iron-manganese-barite mineralization have the characteristic features of hypogene cavity filling and replacement deposits correlated with Miocene- Recent Aden volcanic rocks rifting. In western Saudi Arabia barite-lead-zinc mineralization occurs at Lat. 25° 45' and 25° 50'N hosted by Tertiary sediments in limestone nearby basaltic flows and NE-SW fault system. The mineralized hot brines in the Red Sea deeps considered by the author a part of this province. The author considers the constant rifting phases of Pangea and then progressive fragmentation of Western Gondwana during the Late Carboniferous-Lias, Late Jurassic-Early Aptian, Late Aptian - Albian and Late Eocene-Early Miocene and Oligocene-Miocene, responsible for formation of the mineral deposits constituting the M provinces. During these events, rifting, magmatism and hydrothermal activities took place in different peri-continental margins.

  19. Flexural analysis of uplifted rift flanks on Venus

    NASA Technical Reports Server (NTRS)

    Evans, Susan A.; Simons, Mark; Solomon, Sean C.

    1992-01-01

    Knowledge of the thermal structure of a planet is vital to a thorough understanding of its general scheme of tectonics. Since no direct measurements of heat flow or thermal gradient are available for Venus, most estimates have been derived from theoretical considerations or by analog with the Earth. The flexural response of the lithosphere to applied loads is sensitive to regional thermal structure. Under the assumption that the yield strength as a function of depth can be specified, the temperature gradient can be inferred from the effective elastic plate thickness. Previous estimates of the effective elastic plate thickness of Venus range from 11-18 km for the foredeep north of Uorsar Rupes to 30-60 km for the annular troughs around several coronae. Thermal gradients inferred for these regions are 14-23 K km(exp -1) and 4-9 K km(exp -1) respectively. In this study, we apply the same techniques to investigate the uplifted flanks of an extensional rift. Hypotheses for the origin of uplifted rift flanks on Earth include lateral transport of heat from the center of the rift, vertical transport of heat by small-scale convection, differential thinning of the lithosphere, dynamical uplift, and isostatic response to mechanical uploading of the lithosphere. The 1st hypothesis is considered the dominant contributor to terrestrial rift flanks lacking evidence for volcanic activity, particularly for rift structures that are no longer active. In this study, we model the uplifted flanks of a venusian rift as the flexural response to a vertical end load.

  20. Stress perturbation associated with the Amazonas and other ancient continental rifts

    USGS Publications Warehouse

    Zoback, M.L.; Richardson, R.M.

    1996-01-01

    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.

  1. 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.

  2. Melting during late-stage rifting in Afar is hot and deep.

    PubMed

    Ferguson, D J; Maclennan, J; Bastow, I D; Pyle, D M; Jones, S M; Keir, D; Blundy, J D; Plank, T; Yirgu, G

    2013-07-01

    Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup, modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning. Also disputed is the extent to which plate stretching or thinning is uniform or varies with depth with the entire continental lithospheric mantle potentially being removed before plate rupture. Here we show that the extensive magmatism during rifting along the southern Red Sea rift in Afar, a unique region of sub-aerial transition from continental to oceanic rifting, is driven by deep melting of hotter-than-normal asthenosphere. Petrogenetic modelling shows that melts are predominantly generated at depths greater than 80 kilometres, implying the existence of a thick upper thermo-mechanical boundary layer in a rift system approaching the point of plate rupture. Numerical modelling of rift development shows that when breakup occurs at the slow extension rates observed in Afar, the survival of a thick plate is an inevitable consequence of conductive cooling of the lithosphere, even when the underlying asthenosphere is hot. Sustained magmatic activity during rifting in Afar thus requires persistently high mantle temperatures, which would allow melting at high pressure beneath the thick plate. If extensive plate thinning does occur during breakup it must do so abruptly at a late stage, immediately before the formation of the new ocean basin.

  3. Geoscience Methods Lead to Paleo-anthropological Discoveries in Afar Rift, Ethiopia

    NASA Astrophysics Data System (ADS)

    WoldeGabriel, Giday; Renne, Paul R.; Hart, William K.; Ambrose, Stanley; Asfaw, Berhane; White, Tim D.

    2004-07-01

    With few exceptions, most of the hominid evolutionary record in Africa is closely associated with the East African Rift System. The exceptions are the South African and Chadian hominids collected from the southern and west-central parts of the continent, respectively. The Middle Awash region stands alone as the most prolific paleoanthropological area ever discovered (Figure 1). Its paleontological record has yielded over 13,000 vertebrate fossils, including several hominid taxa, ranging in age from 5.8 Ma to the present. The uniqueness of the Middle Awash hominid sites lies in their occurrence within long, > 6 Ma volcanic and sedimentary stratigraphic records. The Middle Awash region has yielded the longest hominid record yet available. The region is characterized by distinct geologic features related to a volcanic and tectonic transition zone between the continental Main Ethiopian and the proto-oceanic Afar Rifts. The rift floor is wider-200 km-than other parts of the East African Rift (Figure 1). Moreover, its Quaternary axial rift zone is wide and asymetrically located close to the western margin. The fossil assemblages and the lithostratigraphic records suggest that volcanic and tectonic activities within the broad rift floor and the adjacent rift margins were intense and episodic during the late Neogene rift evolution.

  4. 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

  5. Seismic Migration Imaging of the Lithosphere beneath the Afar Rift System, East Africa

    NASA Astrophysics Data System (ADS)

    Lee, T. T. Y.; Chen, C. W.; Rychert, C.; Harmon, N.

    2015-12-01

    The Afar Rift system in east Africa is an ideal natural laboratory for investigating the incipient continental rifting, an essential component of plate tectonics. The Afar Rift is situated at the triple junction of three rifts, namely the southern Red Sea Rift, Gulf of Aden Rift and Main Ethiopian Rift (MER). The ongoing continental rifting at Afar transitions to seafloor spreading toward the southern Red Sea. The tectonic evolution of Afar is thought to be influenced by a mantle plume, but how the plume affects and interacts with the Afar lithosphere remains elusive. In this study, we use array seismic data to produce high-resolution migration images of the Afar lithosphere from scattered teleseismic wavefields to shed light on the lithospheric structure and associated tectonic processes. Our preliminary results indicate the presence of lithospheric seismic discontinuities with depth variation across the Afar region. Beneath the MER axis, we detect a pronounced discontinuity at 55 km depth, characterized by downward fast-to-slow velocity contrast, which appears to abruptly deepen to 75 km depth to the northern flank of MER. This discontinuity may be interpreted as the lithosphere-asthenosphere boundary. Beneath the Ethiopian Plateau, on the other hand, a dipping structure with velocity increase is identified at 70-90 km depth. Further synthesis of observations from seismic tomography, receiver functions, and seismic anisotropy in the Afar region will offer better understanding of tectonic significance of the lithospheric discontinuities.

  6. Comparing Two Profiles: The Amazing Size of the Rift Valleys.

    ERIC Educational Resources Information Center

    Housepian, Jean

    1983-01-01

    Describes an activity for grade 7-9 students previously introduced to topographic maps and profile lines. Two profiles of equal scale are used to help students appreciate the tremendous size of mid-ocean rift valleys. Procedures and examples of completed profiles are provided. (JN)

  7. Lake-level rise in the late Pleistocene and active subaquatic volcanism since the Holocene in Lake Kivu, East African Rift

    NASA Astrophysics Data System (ADS)

    Ross, Kelly Ann; Smets, Benoît; De Batist, Marc; Hilbe, Michael; Schmid, Martin; Anselmetti, Flavio S.

    2014-09-01

    The history of Lake Kivu is strongly linked to the activity of the Virunga volcanoes. Subaerial and subaquatic volcanoes, in addition to lake-level changes, shape the subaquatic morphologic and structural features in Lake Kivu's Main Basin. Previous studies revealed that volcanic eruptions blocked the former outlet of the lake to the north in the late Pleistocene, leading to a substantial rise in the lake level and subsequently the present-day thermohaline stratification. Additional studies have speculated that volcanic and seismic activities threaten to trigger a catastrophic release of the large amount of gases dissolved in the lake. The current study presents a bathymetric mapping and seismic profiling survey that covers the volcanically active area of the Main Basin at a resolution that is unprecedented for Lake Kivu. New geomorphologic features identified on the lake floor can accurately describe related lake-floor processes for the first time. The late Pleistocene lowstand is observed at 425 m depth, and volcanic cones, tuff rings, and lava flows observed above this level indicate both subaerial and subaquatic volcanic activities during the Holocene. The geomorphologic analysis yields new implications on the geologic processes that have shaped Lake Kivu's basin, and the presence of young volcanic features can be linked to the possibility of a lake overturn.

  8. Focal mechanisms of earthquake multiplets in the western part of the Corinth Rift (Greece): influence of the velocity model and constraints on the geometry of the active faults

    NASA Astrophysics Data System (ADS)

    Godano, Maxime; Deschamps, Anne; Lambotte, Sophie; Lyon-Caen, Hélène; Bernard, Pascal; Pacchiani, Francesco

    2014-06-01

    The composite fault plane solutions for 24 large multiplets recorded in the western part of the Corinth Rift between 2000 and 2007 are computed by jointly inverting P polarities and Sv/P, Sh/P, Sv/Sh amplitude ratios of the direct waves. The fault plane solutions are determined using 1-D and 3-D velocity models. Solutions computed with the 3-D velocity model are preferred to the ones computed with the 1-D model because overall, 3-D solutions have a better score function. They correspond essentially to E-NE/W-SW and W-NW/E-SE striking normal faults, which is consistent with the N-S extensional/vertical shortening tectonic regime of the area. For 15 multiplets, one of the nodal planes is similar to the plane delineated by the earthquakes. It is then possible to determine which nodal plane is the fault plane. The analysis of the fault plane solutions highlights a clear decrease of their dip with depth and towards the north. Several multiplets with steeply dipping fault planes (50°-60°) located at depths of 7-8 km are clearly located at the base of onshore and offshore faults that crop out close to the south border of the Corinth Gulf, indicating that these faults are steep down to 7-8 km depth. To the north, multiplets underline a low angle north-dipping structure (20°-30°) on which steep north-dipping faults could take root.

  9. Petrogenesis of coeval sodic and potassic alkaline magmas at Spanish Peaks, Colorado: Magmatism related to the opening of the Rio Grande rift

    NASA Astrophysics Data System (ADS)

    Lord, A. Brooke Hamil; McGregor, Heath; Roden, Michael F.; Salters, Vincent J. M.; Sarafian, Adam; Leahy, Rory

    2016-07-01

    Approximately coeval, relatively primitive (∼5-10% MgO with exception of a trachyandesite) alkaline mafic dikes and sills at or near Spanish Peaks, CO are divided into relatively sodic and potassic varieties on the basis of K2O/Na2O. Many of these dikes are true lamprophyres. In spite of variable alkali element ratios, the alkaline rocks share a number of geochemical similarities: high LIL element contents, high Ba and similar Sr, Nd and Hf isotope ratios near that of Bulk Earth. One important difference is that the potassic rocks are characterized by lower Al2O3 contents, typically less than 12 wt.%, than the sodic dikes/sills which typically have more than 13 wt.% Al2O3, and this difference is independent of MgO content. We attribute the distinct Al2O3 contents to varying pressure during melting: a mica-bearing, Al-poor vein assemblage for the potassic magmas melted at higher pressure than an aluminous amphibole-bearing vein assemblage for the sodic magmas. Remarkable isotopic and trace element similarities with approximately contemporaneous, nearby Rio Grande rift-related basalts in the San Luis Valley, indicate that the magmatism at Spanish Peaks was rift-related, and that lithosphere sources were shared between some rift magmas and those at Spanish Peaks. High Zn/Fe ratios in the Spanish Peaks mafic rocks point to a clinopyroxene- and garnet-rich source such as lithosphere veined by pyroxenite or eclogite. Lithospheric melting was possibly triggered by foundering of cool, dense lithosphere beneath the Rio Grande rift during the initiation of rifting with the potassic parent magmas generated by higher pressure melting of the foundered lithosphere than the sodic parent magmas. This process, caused by gravitational instability of the lithosphere (Elkins-Tanton, 2007) may be common beneath active continental rifts.

  10. Massive and prolonged deep carbon emissions associated with continental rifting

    NASA Astrophysics Data System (ADS)

    Lee, Hyunwoo; Muirhead, James D.; Fischer, Tobias P.; Ebinger, Cynthia J.; Kattenhorn, Simon A.; Sharp, Zachary D.; Kianji, Gladys

    2016-02-01

    Carbon from Earth’s interior is thought to be released to the atmosphere mostly via degassing of CO2 from active volcanoes. CO2 can also escape along faults away from active volcanic centres, but such tectonic degassing is poorly constrained. Here we use measurements of diffuse soil CO2, combined with carbon isotopic analyses to quantify the flux of CO2 through fault systems away from active volcanoes in the East African Rift system. We find that about 4 Mt yr-1 of mantle-derived CO2 is released in the Magadi-Natron Basin, at the border between Kenya and Tanzania. Seismicity at depths of 15-30 km implies that extensional faults in this region may penetrate the lower crust. We therefore suggest that CO2 is transferred from upper-mantle or lower-crustal magma bodies along these deep faults. Extrapolation of our measurements to the entire Eastern rift of the rift system implies a CO2 flux on the order of tens of megatonnes per year, comparable to emissions from the entire mid-ocean ridge system of 53-97 Mt yr-1. We conclude that widespread continental rifting and super-continent breakup could produce massive, long-term CO2 emissions and contribute to prolonged greenhouse conditions like those of the Cretaceous.

  11. Gas-oil fluids in the formation of travertines in the Baikal rift zone

    NASA Astrophysics Data System (ADS)

    Tatarinov, A. A.; Yalovik, L. I.; Shumilova, T. G.; Kanakin, S. V.

    2016-07-01

    Active participation of gas-oil fluids in the processes of mineral formation and petrogenesis in travertines of the Arshan and Garga hot springs is substantiated. The parageneses of the products of pyrolytic decomposition and oxidation of the gas-oil components of hydrothermal fluids (amorphous bitumen, graphite-like CM, and graphite) with different genetic groups of minerals crystallized in a wide range of P-T conditions were established. Travertines of the Baikal rift zone were formed from multicomponent hydrous-gas-oil fluids by the following basic mechanisms of mineral formation: chemogenic, biogenic, cavitation, fluid pyrometamorphism, and pyrolysis.

  12. Discussion on final rifting evolution and breakup : insights from the Mid Norwegian - North East Greenland rifted system

    NASA Astrophysics Data System (ADS)

    Peron-Pinvidic, Gwenn; Terje Osmundsen, Per

    2016-04-01

    In terms of rifted margin studies, the characteristics of the distal and outer domains are among the today's most debated questions. The architecture and composition of deep margins are rarely well constrained and hence little understood. Except from in a handful number of cases (eg. Iberia-Newfoundland, Southern Australia, Red Sea), basement samples are not available to decipher between the various interpretations allowed by geophysical models. No consensus has been reached on the basement composition, tectonic structures, sedimentary geometries or magmatic content. The result is that non-unique end-member interpretations and models are still proposed in the literature. So, although these domains mark the connection between continents and oceans, and thus correspond to unique stages in the Earth's lithospheric life cycle, their spatial and temporal evolution are still unresolved. The Norwegian-Greenland Sea rift system represents an exceptional laboratory to work on questions related to rifting, rifted margin formation and sedimentary basin evolution. It has been extensively studied for decades by both the academic and the industry communities. The proven and expected oil and gas potentials led to the methodical acquisition of world-class geophysical datasets, which permit the detailed research and thorough testing of concepts at local and regional scales. This contribution is issued from a three years project funded by ExxonMobil aiming at better understanding the crustal-scale nature and evolution of the Norwegian-Greenland Sea. The idea was to take advantage of the data availability on this specific rift system to investigate further the full crustal conjugate scale history of rifting, confronting the various available datasets. In this contribution, we will review the possible structural and sedimentary geometries of the distal margin, and their connection to the oceanic domain. We will discuss the definition of 'breakup' and introduce a first order conceptual

  13. Initiation of the western branch of the East African Rift coeval with the eastern branch

    NASA Astrophysics Data System (ADS)

    Roberts, E. M.; Stevens, N. J.; O'Connor, P. M.; Dirks, P. H. G. M.; Gottfried, M. D.; Clyde, W. C.; Armstrong, R. A.; Kemp, A. I. S.; Hemming, S.

    2012-04-01

    The East African Rift System transects the anomalously high-elevation Ethiopian and East African plateaux that together form part of the 6,000-km-long African superswell structure. Rifting putatively developed as a result of mantle plume activity that initiated under eastern Africa. The mantle activity has caused topographic uplift that has been connected to African Cenozoic climate change and faunal evolution. The rift is traditionally interpreted to be composed of two distinct segments: an older, volcanically active eastern branch and a younger, less volcanic western branch. Here, we show that initiation of rifting in the western branch began more than 14 million years earlier than previously thought, contemporaneously with the eastern branch. We use a combination of detrital zircon geochronology, tephro- and magnetostratigraphy, along with analyses of past river flow recorded in sedimentary rocks from the Rukwa Rift Basin, Tanzania, to constrain the timing of rifting, magmatism and drainage development in this part of the western branch. We find that rift-related volcanism and lake development had begun by about 25million years ago. These events were preceded by pediment development and a fluvial drainage reversal that we suggest records the onset of topographic uplift caused by the African superswell. We conclude that uplift of eastern Africa was more widespread and synchronous than previously recognized.

  14. Simple shear detachment fault system and marginal grabens in the southernmost Red Sea rift

    NASA Astrophysics Data System (ADS)

    Tesfaye, Samson; Ghebreab, Woldai

    2013-11-01

    The NNW-SSE oriented Red Sea rift, which separates the African and Arabian plates, bifurcates southwards into two parallel branches, southeastern and southern, collectively referred to as the southernmost Red Sea rift. The southern branch forms the magmatically and seismo-tectonically active Afar rift, while the less active southeastern branch connects the Red Sea to the Gulf of Aden through the strait of Bab el Mandeb. The Afar rift is characterized by lateral heterogeneities in crustal thickness, and along-strike variation in extension. The Danakil horst, a counterclockwise rotating, narrow sliver of coherent continental relic, stands between the two rift branches. The western margin of the Afar rift is marked by a series of N-S aligned right-lateral-stepping and seismo-tectonically active marginal grabens. The tectonic configuration of the parallel rift branches, the alignment of the marginal grabens, and the Danakil horst are linked to the initial mode of stretching of the continental crust and its progressive deformation that led to the breakup of the once contiguous African-Arabian plates. We attribute the initial stretching of the continental crust to a simple shear ramp-flat detachment fault geometry where the marginal grabens mark the breakaway zone. The rift basins represent the ramps and the Danakil horst corresponds to the flat in the detachment fault system. As extension progressed, pure shear deformation dominated and overprinted the initial low-angle detachment fault system. Magmatic activity continues to play an integral part in extensional deformation in the southernmost Red Sea rift.

  15. The Supra-Detachment Sedimentary Evolution of hyper-extended Rifted Margins: the Example of the Alpine Tethys Fossil Analogue Exposed in the Alps (SE Switzerland)

    NASA Astrophysics Data System (ADS)

    Masini, Emmanuel; Mohn, Geoffroy; Manatschal, Gianreto; Ghienne, Jean-François; Lafont, François

    2010-05-01

    Rifting processes leading to the formation of deep-water hyper-extended, magma-starved rifted margins are poorly constrained due to the lack of data from present-day rifted margins. In many present-day examples the classical models remain unable to predict the observed tectonic, sedimentary and subsidence history. Therefore, finding good field analogues, which preserve primary relationships between structures and syn-tectonic sediments is a key to understand the timing and evolution of these margins. Our study focuses on the Jurassic Adriatic rifted margin, which is preserved in the Central Alps in SE Switzerland. The margin architecture can be restored and rift structures with their associated syn- to post- tectonic sedimentary sequences can be mapped through three main paleogeographic domains. A "proximal domain" with classical fault bounded basins, a "necking zone" dominated by thinning structures, and a "distal domain" with exhumed crustal rocks overlain by extensional allochthons that pass oceanwards into an exhumed subcontinental mantle domain. The low syn-rift sedimentation rate of the whole Adriatic margin gives an opportunity to study the exhumation history and its related tectono-sedimentary record. In our poster we constrain the tectono-sedimentary evolution of a supra-detachment rift basin, which is preserved between the necking zone and the exhumed subcontinental mantle in the so-called ocean continent transition. The syn-tectonic record can be divided into four main stratigraphic units on top of an evolving detachment system: (1) the Bardella Fm records the delamination of the hanging-wall blocks (pre-rift platform) on top of an active detachment system, leading to the formation of local debris-flows in fault bounded basins. (2) the Saluver A Fm records the first exhumation of the detachment and its footwall (the basement) which becomes an efficient new source feeding the debris-flows. (3) the Saluver B Fm consists of the onset of axial sand

  16. Geologic Mapping of the Juno Chasma Quadrangle, Venus: Establishing the Relation Between Rifting and Volcanism

    NASA Technical Reports Server (NTRS)

    Senske, D. A.

    2008-01-01

    To understand the spatial and temporal relations between tectonic and volcanic processes on Venus, the Juno Chasma region is mapped. Geologic units are used to establish regional stratigraphic relations and the timing between rifting and volcanism.

  17. How oblique extension and structural inheritance control rift segment linkage: Insights from 4D analogue models

    NASA Astrophysics Data System (ADS)

    Zwaan, Frank; Schreurs, Guido

    2016-04-01

    the extension direction. This occurs when rifts are laterally sufficiently far apart and local effects probably overrule the far-field stresses. Our CT- and PIV-analyses will reveal this surprising effect in more detail. The influence of rift-connecting seeds (model series 1) on rift interaction is limited. Only when they are oriented some 30° or more oblique to the extension direction, can they be activated. In most of these cases oblique-slip fault zones (transfer zones) form along the rift-connecting weak zone, linking the rift segments. Transfer zone structures depend on the angle between the seed orientation and extension direction: the higher the angle, the wider the fault zone. However, these observations are only valid under dextral oblique extension conditions; none of our rift-connecting weak zones (connecting right-stepping rift segments) are activated when sinistral oblique extension is applied. Still our models show how structural inheritance can control the orientation and structuration of transfer zones between rift segments that later on might evolve into oceanic transform faults. REFERENCE Zwaan, F., Schreurs, G., Naliboff, J., Buiter, S.J.H. (in revision) Insights into the effects of oblique extension on continental rift interaction from 3D analogue and numerical models.

  18. Buried troughs, grabens and rifts in Sudan

    NASA Astrophysics Data System (ADS)

    Salama, R. B.

    The deep lineaments and shear patterns of Sudan follow two main directions :NNW (Red Sea trend) and ENE (Gulf of Aden trend). Precambrian mobile belts trend NE and NW. Palaezoic(?) sediments occupy NE-SW aligned grabens. Mesozoic continental sediments with NW paleotrends were deposited in two major depressions also aligned NW. Cainozoic uplift and volcanism was associated with domal uplifts along NE and SE axes. Fracturing and faulting in NW and NE directions resulted in the formation of NW-SE fault bounded grabens ranging in depth from 1 to 11 km. Extending from the western boundaries of Sudan to the eastern borders with Ethiopia, the Sudanese Cainozoic rift systemforms the largest rift system in Africa and includes: (a) Bahr E1 Arab Rift, (b) White Nile Rift, (c) Blue Nile Rift, (d) River Atbara Rift and (e) Wadi El Kuu Rift. The grabens and trouhs within the rift system form the main groundwater basins of Sudan. The discovery of oil in three of these rifts will encourage the exploration for oil in the others and a search for similar structures in the northern areas of Sudan.

  19. Initiation and development of the Kivu rift segment in Central Africa by reactivating un-favorably oriented structural weaknesses

    NASA Astrophysics Data System (ADS)

    Delvaux, Damien; Smets, Benoît

    2015-04-01

    The Kivu rift region forms the central segment of the western branch of the East African rift system, between the northern termination of the Tanganyika rift and the southern extension of the Edward-George rift. Its structure and geological evolution has been revised in the light of a compilation of existing data on earthquake epicenters, focal depth, focal mechanisms, thermal springs and neotectonic faults. It has long been shown that the link between the Kivu rift basin and the Northern termination of the Tanganyika rift basin forms an accommodation zone in which the Rusizi tectonic depression occupies a central place (Ebinger, 1989). In addition, our compilation suggests that the NNE-trending Kivu rift basin and the N-S northern half of the Tanganyika rift basin initiated as separated, partly overlapping and differently oriented basins. The orientation and development of the Kivu rift basin was controlled by an inferred Mid-Proterozoic crustal shear zone and a Pan-African reverse fault front. It was not optimally oriented with the general (first-order) stress field characterized by roughly E-W extension. In a later stage, the more optimally N-S oriented North Tanganyika basin progressed towards the North and connected to Kivu rift in its middle in a region now occupied by the town of Bukavu. This accommodation zone is marked by Quaternary volcanism, warm thermal springs, frequent and relatively shallow seismicity. The southwestern part of the Kivu rift became progressively abandoned but it is still seismically active and hosts a number of warm thermal springs. This particular architecture influences the present-day stress field. This work is a contribution to the Belgian GeoRisCA project. Ebinger, C.J. 1989. Geometric and kinematic development of border faults and accommodation zones, Kivu-Rusizi Rift, Africa. Tectonics, 8, 117-133

  20. Extensional tectonics and collapse structures in the Suez Rift (Egypt)

    NASA Technical Reports Server (NTRS)

    Chenet, P. Y.; Colletta, B.; Desforges, G.; Ousset, E.; Zaghloul, E. A.

    1985-01-01

    The Suez Rift is a 300 km long and 50 to 80 km wide basin which cuts a granitic and metamorphic shield of Precambrian age, covered by sediments of Paleozoic to Paleogene age. The rift structure is dominated by tilted blocks bounded by NW-SE normal faults. The reconstruction of the paleostresses indicates a N 050 extension during the whole stage of rifting. Rifting began 24 My ago with dikes intrusions; main faulting and subsidence occurred during Early Miocene producing a 80 km wide basin (Clysmic Gulf). During Pliocene and Quaternary times, faulting is still active but subsidence is restricted to a narrower area (Present Gulf). On the Eastern margin of the gulf, two sets of fault trends are predominant: (1) N 140 to 150 E faults parallel to the gulf trend with pure dip-slip displacement; and (2) cross faults, oriented NOO to N 30 E that have a strike-slip component consistent with the N 050 E distensive stress regime. The mean dip cross fault is steeper (70 to 80 deg) than the dip of the faults parallel to the Gulf (30 to 70 deg). These two sets of fault define diamond shaped tilted block. The difference of mechanical behavior between the basement rocks and the overlying sedimentary cover caused structural disharmony and distinct fault geometries.

  1. Mapping of the major structures of the African rift system

    NASA Technical Reports Server (NTRS)

    Mohr, P. A. (Principal Investigator)

    1973-01-01

    The author has identified the following significant results. ERTS-1 imagery of the African rift system has already proved of great value in structural geological studies. One of the interesting megastructures expressed on the imagery occurs some 40 km east of the eastern margin of the main Ethiopian rift, in Arussi province, and extending between latitude 71/2 and 81/4 deg N. The Badda-Encuolo ridge proves to have been a line of major Tertiary volcanism and probably supplied the thick Trap Series flood basalt sequence exposed farther east in the canyons of the Webi Shebeli drainage system. The ridge itself was built up by the waning activity of the Sagatu line of volcanism. Serendipitious has been the discovery on Mt. Badda of several deeply glaciated valleys, many of which show clearly on the ERTS-1 imagery. It seems that Mt. Badda was one of the most important glacial centers in eastern Africa during the Pleistocene. Three major late-Tertiary trachytic centers lie between the Badda-Encuolo ridge and the rift valley. The relationships of these three volcanoes to each other and to the rift faulting is revealed for the first time by the ERTS-1 imagery, as is the form of the cladera of Baltata and the crater of Chilalo.

  2. Seismicity of the Earth 1900-2013 East African Rift

    USGS Publications Warehouse

    Hayes, Gavin P.; Jones, Eric S.; Stadler, Timothy J.; Barnhart, William D.; McNamara, Daniel E.; Benz, Harley M.; Furlong, Kevin P.; Villaseñor, Antonio; Hayes, Gavin P.; Jones, Eric S.; Stadler, Timothy J.; Barnhart, William D.; McNamara, Daniel E.; Benz, Harley M.; Furlong, Kevin P.; Villaseñor, Antonio

    2014-01-01

    Rifting in East Africa is not all coeval; volcanism and faulting have been an ongoing phenomenon on the continent since the Eocene (~45 Ma). The rifting began in northern East Africa, and led to the separation of the Nubia (Africa) and Arabia plates in the Red Sea and Gulf of Aden, and in the Lake Turkana area at the Kenya-Ethiopia border. A Paleogene mantle superplume beneath East Africa caused extension within the Nubia plate, as well as a first order topographic high known as the African superswell which now includes most of the eastern and southern sectors of the Nubia plate. Widespread volcanism erupted onto much of the rising plateau in Ethiopia during the Eocene-Oligocene (45–29 Ma), with chains of volcanoes forming along the rift separating Africa and Arabia. Since the initiation of rifting in northeastern Africa, the system has propagated over 3,000 km to the south and southwest, and it experiences seismicity as a direct result of the extension and active magmatism.

  3. Innovative tephra studies in the East African Rift System

    NASA Astrophysics Data System (ADS)

    WoldeGabriel, Giday; Hart, William K.; Heiken, Grant

    Geosciences investigations form the foundation for paleoanthropological research in the East African Rift System. However, innovative applications of tephra studies for constraining spatial and temporal relations of diverse geological processes, biostratigraphic records, and paleoenvironmental conditions within the East African Rift System were fueled by paleoanthropological investigations into the origin and evolution of hominids and material culture. Tephra is a collective, size-independent term used for any material ejected during an explosive volcanic eruption.The East African Rift System has become a magnet for paleoanthropological research ever since the discovery of the first hominids at Olduvai Gorge, in Tanzania, in the 1950s [Leakey et al., 1961]. Currently, numerous multidisciplinary scientific teams from academic institutions in the United States and Western Europe make annual pilgrimages for a couple of months to conduct paleoanthropological field research in the fossil-rich sedimentary deposits of the East African Rift System in Ethiopia, Kenya, and Tanzania. The field expedition consists of geological, paleontological, archaeological, and paleoenvironmental investigations.

  4. Helium isotopes in fluids of the Baykal rift zone

    SciTech Connect

    Polyak, B.G.; Prasolov, E.M.; Tolstikhin, I.N.; Kozlovtseva, S.V.; Kononov, V.I.; Khutorskoy, M.D. All-Union Petroleum Geologic Exploration Research Inst., Saint Petersburg Kola Research Center, Apatity )

    1993-02-01

    The isotopic composition of helium in subterranean fluids from 34 locations in southern Siberia and northern Mongolia has been measured. The ratio [sup 3]He/[sup 4]He ranges from radiogenic ([approximately]10[sup [minus]8]) to mantle ([approximately]10[sup [minus]5]) values. For the most part, the hydrocarbon fluids of the Irkutsk amphitheater contain only radiogenic helium; this distinguishes the amphitheater from ancient platforms. In contrast, fluids from the Baykal rift zone are distinguished by the presence of helium with a mantle component; this component reaches almost 100% in thermal fluids in the Tunka basin. The ratios CH[sub 4]/[sup 3]He and (CO[sub 2] + CH[sub 4])/[sup 3]He suggest that the methane and carbon dioxide in the rift zone fluids may be partially magmatogenic; in the Irkutsk amphitheater, on the other other hand, such gases originated as a result of purely crustal processes. The [sup 3]He/[sup 4]He ratio varies along the length of the Baikal rift zone, with decreasing values correlating with decreasing heat flow, increasing thickness of crust, and decreasing dimensions of rift basins. 39 refs., 4 figs., 3 tabs.

  5. Rifting to spreading in the southern Lau Basin: Variations within the transition zone

    NASA Astrophysics Data System (ADS)

    Watanabe, M.; Okino, K.; Kodera, T.

    2010-11-01

    The Lau Basin and Havre Trough are back-arc basins related to Pacific-Australian plate convergence. Seafloor spreading occurs in the Lau Basin whereas the Havre Trough is in a rifting stage. At present, the spreading propagator's tip lies at the southern end of the Valu Fa Ridge (VFR) at 22°40'S. Studying this propagation process provides an opportunity to characterize the evolution of rifting to the initiation of seafloor spreading which is fundamental to back-arc basin development. New geophysical data of the southern Lau Basin reveals that as spreading propagates south, it evolves in a discrete style south of 22°40'S. The propagation axis lies along the eastern margin of the basin, where the well defined, linear VFR loses its identifying morphology. Topography in this eastern zone is characterized by grabens separated by short narrow ridges. High backscatter intensity indicates tectonic and magmatic activity in this eastern area. Mantle Bouguer anomalies (MBA) increase southwards from the VFR to form an elevated MBA area extending west from the currently active area. This indicates eastward migration of active rifting, during which the arc crust was extremely thinned. High magnetization is observed in a left-stepping pattern south of the VFR. We interpret this pattern as discrete segments that characterize the initiation of the spreading stage. There is no evidence of a single, continuous spreading axis like that which characterizes the central and northern Lau Basin. The magnetization highs are discrete and are observed in areas where deformation and magmatism are focused. They are offset relative to the VFR, though they generally follow the same north-south trend as the VFR.

  6. Processed sweet corn has higher antioxidant activity.

    PubMed

    Dewanto, Veronica; Wu, Xianzhong; Liu, Rui Hai

    2002-08-14

    Processed fruits and vegetables have been long considered to have lower nutritional value than the fresh produce due to the loss of vitamin C during processing. Vitamin C in apples has been found to contribute <0.4% of total antioxidant activity, indicating most of the activity comes from the natural combination of phytochemicals. This suggests that processed fruits and vegetables may retain their antioxidant activity despite the loss of vitamin C. Here it is shown that thermal processing at 115 degrees C for 25 min significantly elevated the total antioxidant activity of sweet corn by 44% and increased phytochemical content such as ferulic acid by 550% and total phenolics by 54%, although 25% vitamin C loss was observed. Processed sweet corn has increased antioxidant activity equivalent to 210 mg of vitamin C/100 g of corn compared to the remaining 3.2 mg of vitamin C in the sample that contributed only 1.5% of its total antioxidant activity. These findings do not support the notion that processed fruits and vegetables have lower nutritional value than fresh produce. This information may have a significant impact on consumers' food selection by increasing their consumption of fruits and vegetables to reduce the risk of chronic diseases. PMID:12166989

  7. Active faulting and seismicity across the SW Gulf of California plate margin: anomalous rifting at slow geologic rates 2-3 m.y. after spreading initiated

    NASA Astrophysics Data System (ADS)

    Arrowsmith, R.; Busch, M. M.; Umhoefer, P. J.; Kent, G.; Driscoll, N.; Martínez-Gutiérrez, G.; Maloney, S. J.; Buchanan, B.

    2009-12-01

    Onshore and offshore paleoseismology provides new constraints on late Quaternary to Present deformation rates across the SW margin of the Gulf of California plate boundary at the latitude of La Paz, Baja California Sur, Mexico. Here we report the results of a 2008 CHIRP survey of offshore faults and what it tells us about the combined onshore - offshore faulting patterns. From west to east the Carrizal - Centenario, San Juan de los Planes (SJP) - La Gata to Espiritu Santo faults are being studied in detail, and to date the San Jose del Cabo (SJC) fault is being studied in reconnaissance. GPS results suggest rates of motion across the marginal fault array of ~1 mm/year while recent data from Alarcon rise suggests 46 mm/yr on the main plate boundary. Onshore studies suggest 0.1 - 0.3 mm/yr along the Carrizal and Los Planes faults in the late Quaternary from paleoseismic excavations, uplifted marine terraces, and geomorphic analysis. Geologic estimates and fault patterns with faults splaying and ending onshore to the south suggest that faulting rates may be higher to the north offshore. CHIRP and onshore data indicate three faults across La Paz Bay that form one system along the rift escarpment. Similar data suggest a second fault system including the onshore Los Planes and La Gata faults that form a complex step over to the NW to the Espiritu Santo fault, along which a M6.2 earthquake occurred in 1995. The third fault system in the transect across the plate margin are the Cerralvo trough faults along which the 1969 M5.6 earthquake probably occurred. It is not clear why the residual 1-2% of plate motion is being accomodated in the margin. The large topographic gradient across the plate boundary and dominantly down-to-Gulf normal faults suggest that potential gravitational energy may be a factor. Low sedimentation rates in the arid region are presumably even slower than faulting rates and result in relief being maintained and semi-starved basins.

  8. Landscape of a Glaciated Rift Flank: Structure of the Transantarctic Mountains From the Royal Society Range to the Churchill Mountains

    NASA Astrophysics Data System (ADS)

    Demyanick, E.; Wilson, T. J.

    2006-12-01

    Multiphase tectonic activity has shaped the regional landscape of the Transantarctic Mountains in Paleozoic, Jurassic, and Cenozoic times. The Antarctic ice sheets have covered the continent for the last 34 m.y. and coeval glacial activity and tectonism have affected mountain landscape evolution. Large-scale linear morphologic features within the mountains are controlled by bedrock structure. Satellite imagery and digital elevation models (DEMs) have been used in this study to map regional, linear morphologic trends along the mountain chain between the Royal Society Range and the Churchill Mountains. ASTER imagery was mosaicked and processed to enhance linear features and lineament analysis was employed to quantify major trends in the mountains. These trends were then compared to geologic maps and other structural and geomorphic data for the area. A low-resolution DEM derived from topographic map contours and new, higher- resolution DEMs created from ASTER imagery were used to create contour, shaded relief, slope steepness, and slope aspect maps. Geomorphology was assessed using these maps and related to mapped lineaments. Linear features that parallel known rift-related faults, or form typical rift-fault patterns, were used to develop a structural model for the architecture of this sector of the Transantarctic Mountains rift flank.

  9. Rift-to-collision transition recorded by tectonothermal evolution of the northern Pyrenees

    NASA Astrophysics Data System (ADS)

    Vacherat, Arnaud; Mouthereau, Frédéric; Pik, Raphaël.; Bellahsen, Nicolas; Gautheron, Cécile; Bernet, Matthias; Daudet, Maxime; Balansa, Jocelyn; Tibari, Bouchaib; Pinna Jamme, Rosella; Radal, Julien

    2016-04-01

    The impact of rift-related processes on tectonic and thermal evolution of collisional orogens is poorly documented. Here, we study the northern Pyrenees, a region that has preserved a geological record of the transition from rifting to collision. Using modeling of new low-temperature thermochronological data, including fission track and (U-Th)/He on apatite and zircon, we propose a temporal reconstruction of the inversion of the European rifted margin. Our data confirm that rifting and related cooling started in the Late Paleozoic-Triassic. Throughout the Jurassic and Early Cretaceous the European margin recorded slow heating during postrift subdsidence. Modeling of thermochronological data allows distinguishing subsidence and denudation controlled by south dipping normal faults in granitic massifs that reflect a second episode of crustal thinning at 130-110 Ma. Following onset of convergence at 83 Ma, shortening accumulated into the weak and hot Albian-Cenomanian rift basins floored by both hyperextended continental crust and exhumed subcontinental mantle. The lack of cooling during this initial stage of convergence is explained by the persistence of a high geothermal gradient. The onset of exhumation-related cooling is recognized in the whole Pyrenean region at 50-35 Ma. This timing reveals that the main phase of mountain building started when hyperextended rift basins closed and collision between proximal domains of the rifted margin occurred.

  10. Preserved record of initial rifting stages in the Betic collisional orogen (Spain). Palaeogeographic implications for the western Neo-Tethyan domain

    NASA Astrophysics Data System (ADS)

    Martin-Rojas, I.; Somma, R.; Delgado, F.; Estévez, A.; Iannace, A.; Perrone, V.; Zamparelli, V.

    2009-04-01

    The rifting process that lead to continental break-up of the Pangea continent has been well characterised in the Jurassic successions, as the main extension occurred during this time span. However, data from the initial Triassic rifting phases are scarce and based on indirect evidence. Here, we report on a wide range of features preserved in the Internal Zones of the Betic Cordillera collisional orogen (SE Spain): syn-sedimentary normal faults overstepped by younger strata, angular unconformities, slumps, intraformational breccias, pronounced thickness variations of lithological units, mafic igneous intrusions and Mississippi Valley-type ore deposits. Our biostratigraphical data (mainly based on foraminifers, algae and bivalves) allowed us to attribute most of the above-said features to the Ladinian-Carnian transition. We interpret these features as a direct record of the initial Triassic phase of continental rifting, preserved in the stratigraphic record because they were not reactivated during the Alpine orogeny. From a tectonic standpoint the features recorded in the study area appear to point to an active extension context for the Western Mediterranean sector during the Ladinian, as opposed to a sinistral transform proposed in some palaeotectonic models. This extensional area could connect the intracontinental rifts of eastern North America with the western Neo-Tethyan marine domain.

  11. Triassic rift-related sedimentary basins in northern Chile (24° 29°S)

    NASA Astrophysics Data System (ADS)

    Suarez, M.; Bell, C. M.

    1992-10-01

    Triassic rocks in northern Chile (latitude 24°-29°S) include marine and continental rift-related sedimentary deposits, associated with basaltic, andesitic, and silicic volcanic rocks. Five main successions include the deposits of two marine basins (Profeta and San Félix) separated by three continental basins (Cifuncho, La Ternera, and La Coipa). The marine strata include turbidites and debris flow deposits interpreted as coarse-grained fan-delta sediments. The continental sediments include lacustrine fan delta, open lake, braided river, alluvial fan, and sabkha deposits. The widespread fan-delta systems (both marine and lacustrine), together with abrupt lateral and vertical facies discontinuities and large-scale depositional cycles, are indicative of rift-controlled sedimentation. The associated magmatic activity indicates that this rifting was the product of subduction-related extension or strike-slip movement on the active plate margin. Triassic rifting was followed in Late Triassic to Early Jurassic times by widespread thermotectonic subsidence.

  12. CASERTZ aeromagnetic data reveal late Cenozoic flood basalts (?) in the West Antarctic rift system

    USGS Publications Warehouse

    Behrendt, John C.

    1994-01-01

    The late Cenozoic volcanic and tectonic activity of the enigmatic West Antarctic rift system, the least understood of the great active continental rifts, has been suggested to be plume driven. In 1991-1992, as part of the CASERTZ (Corridor Aerogeophysics of the Southeast Ross Transect Zone) program, an ~25 000 km aeromagnetic survey over the ice-covered Byrd subglacial basin shows magnetic "texture' critical to interpretations of the underlying extended volcanic terrane. The aeromagnetic data reveal numerous semicircular anomalies ~100-1100 nT in amplitude, interpreted as having volcanic sources at the base of the ice sheet; they are concentrated along north-trending magnetic lineations interpreted as rift fabric. The CASERTZ aeromagnetic results, combined with >100 000 km of widely spaced aeromagnetic profiles, indicate at least 106 km3 of probable late Cenozoic volcanic rock (flood basalt?) in the West Antarctic rift beneath the ice sheet and Ross Ice Shelf. -from Authors

  13. Cenozoic extension in the Kenya Rift from low-temperature thermochronology: Links to diachronous spatiotemporal evolution of rifting in East Africa

    NASA Astrophysics Data System (ADS)

    Torres Acosta, Verónica; Bande, Alejandro; Sobel, Edward R.; Parra, Mauricio; Schildgen, Taylor F.; Stuart, Finlay; Strecker, Manfred R.

    2015-12-01

    The cooling history of rift shoulders and the subsidence history of rift basins are cornerstones for reconstructing the morphotectonic evolution of extensional geodynamic provinces, assessing their role in paleoenvironmental changes and evaluating the resource potential of their basin fills. Our apatite fission track and zircon (U-Th)/He data from the Samburu Hills and the Elgeyo Escarpment in the northern and central sectors of the Kenya Rift indicate a broadly consistent thermal evolution of both regions. Results of thermal modeling support a three-phased thermal history since the early Paleocene. The first phase (~65-50 Ma) was characterized by rapid cooling of the rift shoulders and may be coeval with faulting and sedimentation in the Anza Rift basin, now located in the subsurface of the Turkana depression and areas to the east in northern Kenya. In the second phase, very slow cooling or slight reheating occurred between ~45 and 15 Ma as a result of either stable surface conditions, very slow exhumation, or subsidence. The third phase comprised renewed rapid cooling starting at ~15 Ma. This final cooling represents the most recent stage of rifting, which followed widespread flood-phonolite emplacement and has shaped the present-day landscape through rift shoulder uplift, faulting, basin filling, protracted volcanism, and erosion. When compared with thermochronologic and geologic data from other sectors of the East African Rift System, extension appears to be diachronous, spatially disparate, and partly overlapping, likely driven by interactions between mantle-driven processes and crustal heterogeneities, rather than the previously suggested north-south migrating influence of a mantle plume.

  14. 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.

  15. Magmatic Versus Amagmatic Rifting in the East African Rift System from Pn and Sn Tomography

    NASA Astrophysics Data System (ADS)

    O'Donnell, J. P.; Nyblade, A.

    2014-12-01

    Geodynamic models of rifting currently rely on the mechanism of hot mantle upwelling and decompressional melting to weaken lithospheric rock to the degree that rifting can initiate. However, many rift segments worldwide are apparently amagmatic. The East African Rift System is a prime example, with large sections of the system subaerially amagmatic. We seek to address the question of whether these apparently amagmatic rift segments merely lack a surficial expression of magmatism which exists at depth, or whether rifting is genuinely amagmatic. Based on regional earthquakes recorded by the Tanzania Broadband Seismic Experiment, the Kenya Broadband Seismic Experiment, the AfricaArray East African Seismic Experiment and several permanent GSN stations, we probe for uppermost mantle melt signatures along the East African Rift System using P- and S-wave speed ratios derived from Pn and Sn tomography. Pn- and Sn-velocity models, and their ratio which can be diagnostic of the presence of fluids, will be presented.

  16. Timing the Initiation of Extension Along the Shanxi Rift, China, With Implications for the Cenozoic Tectonics of Asia

    NASA Astrophysics Data System (ADS)

    Webb, A.; Yin, A.; Burnard, P.; Blythe, A.

    2002-12-01

    The Shanxi rift accommodates east-west extension along the eastern margin of the Ordos block in the North China craton. Previous workers have assumed a Paleogene initiation age for the southern part of the Shanxi rift, because Paleogene sediments occur in the graben basin. However, these sediments also occur along the rift shoulder in the footwalls of major rift-bounding faults, implying Paleogene sediments may be pre-rift. The central and northern parts of the Shanxi rift contain only Neogene sediments, suggesting a Neogene initiation age. Three hypotheses for the timing of rift initiation are proposed: (1) the Shanxi rift initiated in the Paleogene, (2) the rift initiated in the Neogene, and (3) the southern part of the Shanxi rift initiated first in the Paleogene, and subsequent rifting propagated to the north in the Neogene. These hypotheses are being tested by (i) (U-Th) He dating of apatite and (ii) fission track dating of apatite and zircon from east-west transects across graben-bounding normal faults along the Shanxi rift. East-west extension is active across east Asia, occurring in the Himalaya, Tibet, along the northwest and east margins of the Ordos Block, the Hangay extensional system, and in the Lake Baikal region. These systems may be induced by a common dynamic cause because they share the same extension direction. Genetic relations among these systems can be tested by determining (1) kinematic relationships and (2) timing of rifting. To the south the Shanxi rift terminates at the left-slip Qinling fault, which may connect with the left-slip Kunlun fault to the west via a large contractional step-over structure. Both faults are important structures accommodating the Indo-Asian collision in the past 10 m.y. The Kunlun fault marks the northern boundary of active north-trending rifts in Tibet. If the Kunlun fault does kinematically connect with the Qinling fault, these relationships suggest a dynamic link between active east-west extension in Tibet and

  17. Rifting and Post-Rift Reactivation of The Eastern Sardinian Margin (Western Tyrrhenian Back-Arc Basin) Evidenced by the Messinian Salinity Crisis Markers and Salt Tectonics

    NASA Astrophysics Data System (ADS)

    Gaullier, V.; Chanier, F.; Vendeville, B.; Lymer, G.; Lofi, J.; Sage, F.; Maillard, A.; Thinon, I.

    2014-12-01

    The Eastern Sardinian margin formed during the opening of the Tyrrhenian Sea, a back-arc basin created by continental rifting and oceanic spreading related to the eastward migrating Apennine subduction system from middle Miocene to Pliocene times. We carried out the "METYSS" project aiming at better understanding the Miocene-Pliocene relationships between crustal tectonics and salt tectonics in this key-area, where rifting is pro parte coeval with the Messinian Salinity Crisis (MSC, 5.96-5.33 Ma) and Messinian salt décollement creates thin-skinned tectonics. Thereby, 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 quantifying vertical and horizontal movements. Our mapping of the Messinian Erosion Surface and of Messinian Upper and Mobile Units shows that a rifted basin already existed by the Messinian times, revealing a major pre-MSC rifting episode across the entire domain. Because salt tectonics can create fan-shaped geometries in sediments, syn-rift deposits have to be carefully re-examined in order to decipher the effects of crustal tectonics (rifting) and salt tectonics. Our data surprisingly showed that there are no clues for Messinian syn-rift sediments along the East-Sardinia Basin and Cornaglia Terrace, hence no evidence for rifting after Late Tortonian times. Nevertheless, widespread deformation occurred during the Pliocene and is attributed to post-rift reactivation. 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

  18. Fault-controlled hydration of the upper mantle during continental rifting

    NASA Astrophysics Data System (ADS)

    Bayrakci, G.; Minshull, T. A.; Sawyer, D. S.; Reston, T. J.; Klaeschen, D.; Papenberg, C.; Ranero, C.; Bull, J. M.; Davy, R. G.; Shillington, D. J.; Perez-Gussinye, M.; Morgan, J. K.

    2016-05-01

    Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems. Serpentinized mantle rocks are found beneath the sea floor at slow- to ultraslow-spreading mid-ocean ridges and are thought to be present at about half the world’s rifted margins. Serpentinite is also inferred to exist in the downgoing plate at subduction zones, where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults. Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that sea water reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth.

  19. Tectonic controls on rift basin morphology: Evolution of the northern Malawi (Nyasa) rift

    NASA Technical Reports Server (NTRS)

    Ebinger, C. J.; Deino, A. L.; Tesha, A. L.; Becker, T.; Ring, U.

    1993-01-01

    Radiometric (K-Ar and Ar-40/Ar-39) age determinations of volcanic and volcaniclastic rocks, combined with structural, gravity, and seismic reflection data, are used to constrain the age of sedimentary strata contained within the seismically and volcanically active northern Malawi (Nyasa) rift and to characterize changes in basin and flank morphologies with time. Faulting and volcanism within the Tukuyu-Karonga basin began at approximately 8.6 Ma, when sediments were deposited in abroad, initially asymmetric lake basin bounded on its northeastern side by a border fault system with minor topographic relief. Extensions, primarily by a slip along the border fault, and subsequent regional isostatic compensation led to the development of a 5-km-deep basin bounded by broad uplifted flanks. Along the low-relief basin margin opposite border fault, younger stratigraphic sequences commonly onlap older wedge-shaped sequences, although their internal geometry is often progradational. Intrabasinal faulting, flankuplift, and basaltic and felsic volcanism from centers at the northern end of the basin became more important at about 2.5 Ma when cross-rift transfer faults developed to link the Tukuyu-Karonga basin to the Rukwa basin. Local uplift and volcanic construction at the northern end of the basin led to a southeastward shift in the basin's depocenter. Sequence boundaries are commonly erosional along this low-relief (hanging wall) margin and conformable in the deep lake basin. The geometry of stratigraphic sequences and the distribution of the erosion indicate that horizontal and vertical crustal movements both across and along the length of the rift basin led to changes in levels of the lake, irrespective of paleoclimatic fluctuations.

  20. Water management problems in the Ethiopian rift: Challenges for development

    NASA Astrophysics Data System (ADS)

    Ayenew, Tenalem

    2007-06-01

    The Ethiopian rift is characterized by many perennial rivers and lakes occupying volcano-tectonic depressions with highly variable hydrogeological setting. The rift lakes and rivers were the focal points for relatively large-scale water resources development. They are used for irrigation, soda abstraction, commercial fish farming, recreation and support a wide variety of endemic birds and wild animals. Ethiopia's major mechanized irrigation farms and commercial fishery are confined within the rift. A few of the lakes have shrunk as a result of excessive abstraction of water; others expanded due to increased surface runoff and groundwater flux from percolated over-irrigated fields and active tectonism. Excessive land degradation and deforestation have also played a role. Human factors, in combination with the natural conditions of climate and geology have influenced the water quality. The chemistry of some of the lakes has been changed dramatically. This paper tries to present the challenges of surface water resources development with particular reference to environmental problems caused in the last few decades. The methods employed include field hydrological mapping supported by aerial photograph and satellite imagery interpretations, hydrometeorological and hydrochemical data analysis and catchment hydrological modeling. A converging evidence approach was adapted to reconstruct the temporal and spatial variations of lake levels and the hydrochemistry. The result revealed that the major changes in the rift valley are related mainly to recent improper utilization of water and land resources in the rivers draining the rift floor and the lakes' catchment, and to direct lake water abstraction, aggravated intermittently by natural factors (climate and tectonism). These changes appear to have grave environmental consequences, which demand urgent integrated basin-wide water management practice.

  1. Ground deformation across the Corinth rift from 22 years of GPS observations

    NASA Astrophysics Data System (ADS)

    Briole, Pierre

    2013-04-01

    shows a co-seismic displacement in January 2010. The extension rate at all stations except Efpalio is steady over the ten years period. The velocities determined at approximately a hundred network points (1st order and 2nd order observed twice or more) show no temporal variation during the sampled period except the co-seismic of the large 1995 Aigion earthquake. The southern side of the rift behaves as a rigid body with less than 1mm/yr internal deformation except around the Psathopirgos fault. Most of the extension, more than 12 mm/yr at the longitude of Trizonia, occurs offshore in the centre of the rift. The northern side of the rift is less rigid, with 3 mm/yr accommodated between Trizonia and Lidoriki. The points located along the northern shore between Nafpaktos and Itea show a westward (or clockwise) component with respect to the overall velocity field. No significant deformation is observed in the area located between Nafpaktos and the eastern termination of the Trichonis lake and the block located between Etoliko, Thermo, Lidoriki and Nafpaktos has less than 1mm/yr internal deformation. At the western termination of the Psathopirgos fault both GPS and SAR interferometry show the existence of localized deformation in the first few kilometres inland that becomes progressively dominated by right lateral strike slip corresponding probably to the northern termination of the crustal discontinuity activated more to the southwest during the M=6.4 June 8, 2008 Andravida earthquake. No vertical motion is detected at campaign points except at the Drepano lighthouse northwest of the Psathopirgos fault. Further steps forward in the knowledge of the deformation of this exceptional area during the next few decades require among others the deployment of a few ten of permanent GPS stations across the main actives structures on both sides of the rift and at its western termination around Patras, a complete analysis of the available and future InSAR data and fusion with the GPS

  2. East Antarctic rifting triggers uplift of the Gamburtsev Mountains

    USGS Publications Warehouse

    Ferraccioli, F.; Finn, Carol A.; Jordan, Tom A.; Bell, Robin E.; Anderson, Lester M.; Damaske, Detlef

    2011-01-01

    The Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 1958. The preservation of Alpine topography in the Gamburtsevs may reflect extremely low long-term erosion rates beneath the ice sheet, but the mountains’ origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long rift system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1 Gyr ago), was preserved as in some old orogens and was rejuvenated during much later Permian (roughly 250 Myr ago) and Cretaceous (roughly 100 Myr ago) rifting. Much like East Africa, the interior of East Antarctica is a mosaic of Precambrian provinces affected by rifting processes. Our models show that the combination of rift-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that rifting and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere.

  3. Rio Grande rift: problems and perspectives

    SciTech Connect

    Baldridge, W.S.; Olsen, K.H.; Callender, J.F.

    1984-01-01

    Topics and ideas addressed include: (1) the regional extent of the Rio Grande rift; (2) the structure of the crust and upper mantle; (3) whether the evidence for an axile dike in the lower crust is compelling; (4) the nature of faulting and extension in the crust; and (5) the structural and magmatic development of the rift. 88 references, 5 figures.

  4. Detection and Response for Rift Valley fever

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever is a viral disease that impacts domestic livestock and humans in Africa and the Middle East, and poses a threat to military operations in these areas. We describe a Rift Valley fever Risk Monitoring website, and its ability to predict risk of disease temporally and spatially. We al...

  5. Distribution of contemporary crustal deformation and mechanisms for extension in the Woodlark Rift: insights from GPS

    NASA Astrophysics Data System (ADS)

    Wallace, L. M.; Ellis, S. M.; Tregoning, P.; Little, T. A.; Palmer, N.

    2012-12-01

    The Woodlark Rift, southeastern Papua New Guinea, is a classic example of a rift transitioning from continental rifting to seafloor spreading, and is also the site of exhumation of the world's youngest Ultra-High Pressure (UHP) terranes. Prior to now, very little GPS data existed to constrain the kinematics of contemporary rifting, and the relationship of modern-day rifting to exhumation of the young UHP terranes. We present results from GPS campaign measurements at ~45 sites throughout the southeastern Papua New Guinea region, from GPS campaigns conducted in 2009, 2010, and 2012. Our results suggest that most of the modern-day extensional deformation has shifted southward towards the north coast of the PNG mainland, away from the locus of UHP exhumation in the D'Entrecasteaux Islands, although a few mm/yr of active extension remains in the region of UHP rock exhumation. This is consistent with modelling studies that predict a shift in the locus of extension away from the locus of UHP exhumation during the final, waning stages of UHP exhumation. Rates of total extension in the Woodlark Rift increase from west to east from several mm/yr (in the far western Woodlark Rift) to >20 mm/yr further east, due to clockwise rotation of microplates in the region about nearby poles of rotation. We will discuss the implications that our kinematic modelling of the GPS data, earthquake slip vector data, and geological data have for the large-scale driving mechanisms behind rifting in southeast PNG. Our results favour a model where rapid microplate rotation (at 2-3 degrees/Myr) and rifting in the Woodlark Basin is a consequence of strong slab pull forces from extremely rapid subduction (6-13 cm/yr) at the New Britain and San Cristobal trenches further to the north.

  6. The tectonic evolution of the southeastern Terceira Rift/São Miguel region (Azores)

    NASA Astrophysics Data System (ADS)

    Weiß, B. J.; Hübscher, C.; Lüdmann, T.

    2015-07-01

    The eastern Azores Archipelago with São Miguel being the dominant subaerial structure is located at the intersection of an oceanic rift (Terceira Rift) with a major transform fault (Gloria Fault) representing the westernmost part of the Nubian-Eurasian plate boundary. The evolution of islands, bathymetric highs and basin margins involves strong volcanism, but the controlling geodynamic and tectonic processes are currently under debate. In order to study this evolution, multibeam bathymetry and marine seismic reflection data were collected to image faults and stratigraphy. The basins of the southeastern Terceira Rift are rift valleys whose southwestern and northeastern margins are defined by few major normal faults and several minor normal faults, respectively. Since São Miguel in between the rift valleys shows an unusual W-E orientation, it is supposed to be located on a leaky transform. South of the island and separated by a N120° trending graben system, the Monacco Bank represents a N160° oriented flat topped volcanic ridge dominated by tilted fault blocks. Up to six seismic units are interpreted for each basin. Although volcanic ridges hamper a direct linking of depositional strata between the rift and adjacent basins, the individual seismic stratigraphic units have distinct characteristics. Using these units to provide a consistent relative chrono-stratigraphic scheme for the entire study area, we suggest that the evolution of the southeastern Terceira Rift occurred in two stages. Considering age constrains from previous studies, we conclude that N140° structures developed orthogonal to the SW-NE direction of plate-tectonic extension before ~ 10 Ma. The N160° trending volcanic ridges and faults developed later as the plate tectonic spreading direction changed to WSW-ENE. Hence, the evolution of the southeastern Terceira Rift domain is predominantly controlled by plate kinematics and lithospheric stress forming a kind of a re-organized rift system.

  7. A High Resolution Geophysical Study of the Offshore Western Gulf of Corinth Rift

    NASA Astrophysics Data System (ADS)

    McNeill, L.; Cotterill, C.; Stefatos, A.; Henstock, T.; Bull, J.; Collier, R.; Papatheodorou, G.; Georgiopoulou, A.; Ferentinos, G.

    2003-12-01

    The western Gulf of Corinth has generated recent debate in terms of distribution of extensional strain, interactions between active faults and fault geometry. Onshore data suggest that faults do not accommodate extensional strain of the magnitude suggested by geodetic measurements. Recently acquired high resolution geophysical data in the western Gulf of Corinth, including Reson Seabat 8160 50 kHz multibeam bathymetry (with sidescan collected simultaneously) and sparker and boomer seismic profiles, will allow a detailed study of faulting relationships, fault propagation history and associated sedimentological processes. Multibeam data indicate the complex axial and tributary channel pattern of the gulf as well as revealing the surface expression of active faults on both margins and within the basin centre. Several fault tips are evident, including the Aigion fault which has been surveyed in great detail with boomer lines spaced between 25-100 m. The fault tip is complex with multiple synthetic and antithetic splays. The post-lowstand transgressive surface is clearly imaged and therefore fault growth rates can be established. Gas-related features are common, including pockmarks and mud volcanoes. To the east, displacement on the eastern tip of the Eliki fault decreases rapidly offshore and a splay of the Derveni fault is observed. A major S-dipping antithetic fault opposite the Eastern Eliki fault has clear bathymetric expression and is locally associated with a prominent basement ridge. This fault may make a significant contribution to extensional strain in this part of the rift. In the centre of the basin, sediments are deformed by multiple minor faults with seafloor displacement. Ultimately, high resolution offshore interpretations can be integrated with regional datasets and existing data (e.g., geomorphic, paleoseismological and sedimentological) onshore and used to better assess rift deformation models, rift evolution and local seismic hazards.

  8. Hydrothermal petroleum from lacustrine sedimentary organic matter in the East African Rift.

    PubMed

    Simoneit, B R; Aboul-Kassim, T A; Tiercelin, J J

    2000-03-01

    Cape Kalamba oil seeps occur at the south end of the Ubwari Peninsula, at the intersection of faults controlling the morphology of the northern basin of the Tanganyika Rift, East Africa. Oil samples collected at the surface of the lake 3-4 km offshore from Cape Kalamba have been studied. The aliphatic hydrocarbon and biomarker compositions, with the absence of the typical suite of polynuclear aromatic hydrocarbons, indicate an origin from hydrothermal alteration of immature microbial biomass in the sediments. These data show a similarity between a tar sample from the beach and the petroleum from the oil seeps, and confirm that the source of these oils is from organic matter consisting mainly of bacterial and degraded algal biomass, altered by hydrothermal activity. The compositions also demonstrate a < 200 degrees C temperature for formation/generation of this hydrothermal petroleum, similar to the fluid temperature identified for the Pemba hydrothermal site located 150 km north of Cape Kalamba. The 14C age of 25.6 ka B.P. obtained for the tar ball suggests that Pleistocene lake sediments could be the source rock. Hydrothermal generation may have occurred slightly before 25 ka B.P., during a dry climatic environment, when the lake level was lower than today. These results also suggest that the Cape Kalamba hydrothermal activity did not occur in connection with an increased flux of meteoric water, higher water tables and lake levels as demonstrated in the Kenya Rift and for the Pemba site. Hydrothermal petroleum formation is a facile process also in continental rift systems and should be considered in exploration for energy resources in such locales. PMID:17654787

  9. Hydrothermal petroleum from lacustrine sedimentary organic matter in the East African Rift.

    PubMed

    Simoneit, B R; Aboul-Kassim, T A; Tiercelin, J J

    2000-03-01

    Cape Kalamba oil seeps occur at the south end of the Ubwari Peninsula, at the intersection of faults controlling the morphology of the northern basin of the Tanganyika Rift, East Africa. Oil samples collected at the surface of the lake 3-4 km offshore from Cape Kalamba have been studied. The aliphatic hydrocarbon and biomarker compositions, with the absence of the typical suite of polynuclear aromatic hydrocarbons, indicate an origin from hydrothermal alteration of immature microbial biomass in the sediments. These data show a similarity between a tar sample from the beach and the petroleum from the oil seeps, and confirm that the source of these oils is from organic matter consisting mainly of bacterial and degraded algal biomass, altered by hydrothermal activity. The compositions also demonstrate a < 200 degrees C temperature for formation/generation of this hydrothermal petroleum, similar to the fluid temperature identified for the Pemba hydrothermal site located 150 km north of Cape Kalamba. The 14C age of 25.6 ka B.P. obtained for the tar ball suggests that Pleistocene lake sediments could be the source rock. Hydrothermal generation may have occurred slightly before 25 ka B.P., during a dry climatic environment, when the lake level was lower than today. These results also suggest that the Cape Kalamba hydrothermal activity did not occur in connection with an increased flux of meteoric water, higher water tables and lake levels as demonstrated in the Kenya Rift and for the Pemba site. Hydrothermal petroleum formation is a facile process also in continental rift systems and should be considered in exploration for energy resources in such locales.

  10. 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.

  11. Propagated rifting in the Southwest Sub-basin, South China Sea: Insights from analogue modelling

    NASA Astrophysics Data System (ADS)

    Ding, Weiwei; Li, Jiabiao

    2016-10-01

    How the South China Sea rifted has long been a puzzling question that is still debated, particularly with reference to the Southwest Sub-basin (SWSB). Analogue modelling remains one of the most useful tools for testing rift models and processes. Here, we present and discuss a series of analogue modelling experiments designed to investigate the rifting process of the SWSB. Convincing geophysical results were compiled to provide realistic constraints to test the experimental results and interpretations. A heterogeneous lithosphere model with a varied lithospheric structure showed tectono-morphological features similar to the natural case of the SWSB, indicating that the initial thermal condition and rheological stratification of the lithosphere should have a dominant effect on the rifting process of the SWSB. Rigid tectonic blocks existed in the continental margin, such as the Macclesfield Bank and the Reed Bank, and they played important roles in both the shaping of the continent-ocean boundary and the coupling between the crust and mantle. The initial thermal condition and rheological stratification of the lithosphere under the South China Sea controlled the propagated rifting process of the SWSB. Extension was centred on the deep troughs between the rigid blocks, and the break-up occurred in these areas between them. The westward rifting propagation is best explained with a heterogeneous lithosphere model characterized by varied lithospheric structure, and it was responsible for producing the V-shaped configuration of the SWSB.

  12. Middle Stone Age starch acquisition in the Niassa Rift, Mozambique

    NASA Astrophysics Data System (ADS)

    Mercader, Julio; Bennett, Tim; Raja, Mussa

    2008-09-01

    The quest for direct lines of evidence for Paleolithic plant consumption during the African Middle Stone Age has led scientists to study residues and use-wear on flaked stone tools. Past work has established lithic function through multiple lines of evidence and the spatial breakdown of use-wear and microscopic traces on tool surfaces. This paper focuses on the quantitative analysis of starch assemblages and the botanical identification of grains from flake and core tools to learn about human ecology of carbohydrate use around the Niassa woodlands, in the Mozambican Rift. The processing of starchy plant parts is deduced from the occurrence of starch assemblages that presumably got attached to stone tool surfaces by actions associated with extractive or culinary activities. Specifically, we investigate starch grains from stone tools recently excavated in northern Mozambique at the site of Mikuyu; which presumably spans the middle to late Pleistocene and represents similar sites found along the Malawi/Niassa corridor that links East, Southern, and Central Africa. Starch was extracted and processed with a diverse tool kit consisting of scrapers, cores, points, flakes, and other kinds of tools. The microbotanical data suggests consumption of seeds, legumes, caryopses, piths, underground storage organs, nuts, and mesocarps from more than a dozen families. Our data suggest a great antiquity for starch use in Africa as well as an expanded diet and intensification.

  13. Prolonged post-rift magmatism on highly extended crust of divergent continental margins (Baiyun Sag, South China Sea)

    NASA Astrophysics Data System (ADS)

    Zhao, Fang; Alves, Tiago M.; Wu, Shiguo; Li, Wei; Huuse, Mads; Mi, Lijun; Sun, Qiliang; Ma, Benjun

    2016-07-01

    Three-dimensional (3D) seismic, borehole and geochemical data reveal a prolonged phase of post-rift magmatism on highly extended crust of the Baiyun Sag, South China Sea. Two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the South China Sea. Biostratigraphic data from exploration wells BY7-1 and BY2, complemented by K-Ar datings from core samples, confirm that magmatic activity in the Baiyun Sag occurred in two main stages: (1) a first episode at the base of the Miocene (23.8 Ma); and (2) a second episode occurring at the end of the Early Miocene (17.6 Ma). The relative location of volcanic complexes in the Baiyun Sag, and their stratigraphic position, reveals prolonged magmatism inboard of the ocean-continent transition zone during continental breakup. We suggest that magmatism in the Baiyun Sag reflects progressive continental breakup in the South China Sea, with the last volcanic episode marking the end of a breakup sequence representing the early post-rift tectonic events associated with the continental breakup process. Seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup.

  14. Coulomb stress evolution in the Shanxi rift system, North China, since 1303 associated with coseismic, post-seismic and interseismic deformation

    NASA Astrophysics Data System (ADS)

    Li, Bin; Sørensen, Mathilde Bøttger; Atakan, Kuvvet

    2015-12-01

    The Shanxi rift system is one of the most active intraplate tectonic zones in the North China Block, resulting in devastating seismicity. Since 1303, the rift has experienced fifteen Ms ≥ 6.5 earthquakes. Aiming at a better understanding of Coulomb stress evolution and its relationship with the seismicity in the rift system, we investigated the Coulomb stress changes due to coseismic slip and post-seismic relaxation processes following strong earthquakes as well as the interseismic tectonic loading since the 1303 Hongdong Ms = 8.0 earthquake. Our calculation applies a specified regional medium model, takes the gravity effect into account and uses the fault geometry of the next event as the receiver fault in a given calculation. Our results show that nine out of 12 Ms ≥ 6.5 earthquakes since the 1303 Hongdong earthquake and more than 82 per cent of small-medium instrumental events after the 1989 Datong-Yanggao Ms = 6.1 earthquake fall into the total stress increased areas. Our results also reveal the different roles of the coseismic, post-seismic and interseismic Coulomb stress changes in the earthquake triggering process in the Shanxi rift system. In a short period after a strong event, the stress field changes are dominated by coseismic Coulomb stress due to sudden slip of the ruptured fault, while in the long term, the stress field is mainly dominated by the accumulation of interseismic tectonic loading. Post-seismic stress changes play an important role by further modifying the distribution of stress and therefore cannot be ignored. Based on the current stress status in the Shanxi rift system, the Linfen basin, southern and northern Taiyuan basin, Xinding basin and the north part of the rift system are identified as the most likely locations of large events in the future. The results of this study can provide important clues for the further understanding of seismic hazard in the Shanxi rift system and thus help guiding earthquake risk mitigation efforts in

  15. An integrated geophysical analysis of the upper crust of the southern Kenya rift

    NASA Astrophysics Data System (ADS)

    Simiyu, Silas M.; Keller, G. Randy

    2001-12-01

    Previous interpretations of seismic data collected by the Kenya Rift International Seismic Project (KRISP) experiments indicate the presence of crustal thickening within the rift valley area beneath the Kenya dome, an uplift centred on the southern part of the Kenya rift. North of the dome, these interpretations show thinning of the crust and an increase in crustal extension. To the south near the Kenya/Tanzania border, crustal thinning associated with the rift is modest. Our study was aimed at further investigating crustal structure from this dome southwards via a detailed analysis focused on upper crustal structure. We used results from surface geological mapping, drill hole data from water wells and geothermal exploration wells, KRISP 85 seismic data for a profile across the rift, KRISP 85 and 90 seismic data for a profile along the rift axis and KRISP 94 seismic data for a profile crossing southernmost Kenya to constrain gravity modelling and construction of integrated models of crustal structure. Our integrated analysis produced the following results concerning the structure and evolution of the southern Kenya rift: (1) the graben master faults are consistently located along the western margin of the rift valley, and there is no evidence for half-graben polarity reversals for a distance of about 300km (2) there is no axial (north-south) crustal symmetry with respect to the apex of the Kenya dome, and the crustal thickness may be as much related to pre-rift crustal type and thickness as it is to crustal thickening and modification by magmatic processes; (3) the pre-existing lithospheric contrast between the Archaean and Proterozoic basement terranes played a significant role in the location and structural geometry of the rift; (4) south of latitude 1°S, low velocities and densities observed under the western flank of the rift probably represent reworked Archaean Tanzanian craton; (5) magmatic modification of the upper crust is modest except near the major

  16. Matching conjugate volcanic rifted margins: 40Ar/ 39Ar chrono-stratigraphy of pre- and syn-rift bimodal flood volcanism in Ethiopia and Yemen

    NASA Astrophysics Data System (ADS)

    Ukstins, Ingrid A.; Renne, Paul R.; Wolfenden, Ellen; Baker, Joel; Ayalew, Dereje; Menzies, Martin

    2002-05-01

    40Ar/ 39Ar dating of mineral separates and whole-rock samples of rhyolitic ignimbrites and basaltic lavas from the pre- and syn-rift flood volcanic units of northern Ethiopia provides a temporal link between the Ethiopian and Yemen conjugate rifted volcanic margins. Sixteen new 40Ar/ 39Ar dates confirm that basaltic flood volcanism in Ethiopia was contemporaneous with flood volcanism on the conjugate margin in Yemen. The new data also establish that flood volcanism initiated prior to 30.9 Ma in Ethiopia and may predate initiation of similar magmatic activity in Yemen by ˜0.2-2.0 Myr. Rhyolitic volcanism in Ethiopia commenced at 30.2 Ma, contemporaneous with the first rhyolitic ignimbrite unit in Yemen at ˜30 Ma. Accurate and precise 40Ar/ 39Ar dates on initial rhyolitic ignimbrite eruptions suggest that silicic flood volcanism in Afro-Arabia post-dates the Oligocene Oi2 global cooling event, ruling out a causative link between these explosive silicic eruptions (with individual volumes ≥200 km 3) and climatic cooling which produced the first major expansion of the Antarctic ice sheets. Ethiopian volcanism shows a progressive and systematic younging from north to south along the escarpment and parallel to the rifted margin, from pre-rift flood volcanics in the north to syn-rift northern Main Ethiopian Rift volcanism in the south. A dramatic decrease in volcanic activity in Ethiopia between 25 and 20 Ma correlates with a prominent break-up unconformity in Yemen (26-19 Ma), both of which mark the transition from pre- to syn-rift volcanism (˜25-26 Ma) triggered by the separation of Africa and Arabia. The architecture of the Ethiopian margin is characterized by accumulation and preservation of syn-rift volcanism, while the Yemen margin was shaped by denudational unloading and magmatic starvation as the Arabian plate rifted away from the Afar plume. A second magmatic hiatus and angular unconformity in the northern Main Ethiopian Rift is evident at 10.6-3.2 Ma, and is

  17. Pre-existing discontinuities and oblique rifting in the Kenya Rift: Comparisons with analogue models.

    NASA Astrophysics Data System (ADS)

    Rolet, J.; Gloaguen, R.; Gloaguen, R.; Dooley, T.; McClay, K.

    2001-12-01

    Oblique rift structures such as the SSE-trending Aswa Transverse Zone in the Kenya rift are poorly understood and are rarely taken into account in geometric and kinematic models for the origin of this rift zone. However, remote sensing demonstrates that transverse structures are quite numerous and have a significant influence on the geometry and segmentation of the rift and the development of faults within or at the boundaries of the oblique zones. The importance of these transverse zones varies depending on their orientation and position with respect to the main rift. The origin of these oblique zones can be directly related to pre-existing fabrics and shear zones in the Precambrian basement and thus act as mechanically distinct structural domains during later extensional events. In order to assess the importance and role of these oblique structures we used optical (SPOT, LANDSAT) and microwave (RADARSAT, ERS) data combined with field observations and measurements. Collected structural data were then compared with scaled physical models of orthogonal and oblique rifting in order to refine the rift model. The data and comparison with physical models suggest that these transverse zones are best described as oblique rift zones where the rift border faults are parallel to the basement grain whereas intra-rift fault systems form orthogonal to the extension direction. This model also implies that the present day extension direction in Kenya is oriented E-W.

  18. Early growth of Kohala volcano and formation of long Hawaiian rift zones

    USGS Publications Warehouse

    Lipman, P.W.; Calvert, A.T.

    2011-01-01

    Transitional-composition pillow basalts from the toe of the Hilo Ridge, collected from outcrop by submersible, have yielded the oldest ages known from the Island of Hawaii: 1138 ?? 34 to 1159 ?? 33 ka. Hilo Ridge has long been interpreted as a submarine rift zone of Mauna Kea, but the new ages validate proposals that it is the distal east rift zone of Kohala, the oldest subaerial volcano on the island. These ages constrain the inception of tholeiitic volcanism at Kohala, provide the first measured duration of tholeiitic shield building (???870 k.y.) for any Hawaiian volcano, and show that this 125-km-long rift zone developed to near-total length during early growth of Kohala. Long eastern-trending rift zones of Hawaiian volcanoes may follow fractures in oceanic crust activated by arching of the Hawaiian Swell in front of the propagating hotspot. ?? 2011 Geological Society of America.

  19. 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

  20. Processing of Color Words Activates Color Representations

    ERIC Educational Resources Information Center

    Richter, Tobias; Zwaan, Rolf A.

    2009-01-01

    Two experiments were conducted to investigate whether color representations are routinely activated when color words are processed. Congruency effects of colors and color words were observed in both directions. Lexical decisions on color words were faster when preceding colors matched the color named by the word. Color-discrimination responses…

  1. Observed rift propagation in the Larsen C Ice Shelf from Sentinel 1-A radar data

    NASA Astrophysics Data System (ADS)

    Jansen, Daniela; Helm, Veit; Neckel, Niklas; Luckman, Adrian; Bevan, Suzanne

    2016-04-01

    The Larsen C Ice Shelf is the most northerly of the remaining major Antarctic Peninsula ice shelves and is vulnerable to changes in both to ocean and atmospheric forcing. It is the largest ice shelf in the region and its loss would lead to a significant drawdown of ice from the Antarctic Peninsula Ice Sheet. There have been observations of widespread thinning, melt ponding in the northern inlets, and, in the northern part, a speed-up in ice flow, all processes which have been linked to former ice shelf collapses. Previous studies have also highlighted the vulnerability of Larsen C Ice Shelf to specific potential changes in its geometry including a retreat from the Bawden and Gipps Ice Rise. In a change from the usual pattern, a northwards-propagating rift from Gipps Ice Rise has recently advanced towards the center of the ice shelf. It is now more than halfway towards calving a large section of the ice shelf and continues to widen. We followed the rift propagation on MODIS and Landsat imagery and, during the austral winter 2015, on Sentinel-1A radar data. Due to the very cloudy weather conditions during the austral Summer 2015 / 2016 the Sentinel data became an essential part of the monitoring. By calculating differential interferograms it was possible to clearly identify the active tip of the rift, which was not always obvious on the Landsat images. Further, surface velocities were derived from recent Sentinel-1A acquisitions by means of offset intensity tracking. In order to investigate a possible speed-up of the ice shelf we extended the study area to the north including Bawden ice rise.

  2. Gravity-driven structures and rift basin evolution: Rio Muni Basin, offshore equatorial West Africa

    SciTech Connect

    Turner, J.P.

    1995-08-01

    Offshore Equatorial Guinea, west Africa, gravity-driven nappes, more than 1 km thick and 15 km from head to toe, provide key evidence in reconstructing the late synrift: evolution of this part of the South Atlantic margin basin system. Furthermore, Aptian-Cenomanian carbonate and clastic rocks in the nappes` allochthonous hanging walls are attracting interest as a new exploration play in west Africa. The nappes exhibit a range of geometries that suggest they share many of the same deformation processes as thin-skin thrust and linked extensional fault systems. Not only are these structures significant in their own right, representing a rare example of gravity tectonics in the virtual absence of major halokinesis, but their presence may record an other-wise undetectable process active during the transition from a rift basin to a passive continental margin. A review of Equatorial Guinea in its pre-Atlantic configuration, alongside neighboring basins in Brazil (the Sergipe-Alagoas basin) and Gabon, suggests that gravity gliding was sustained by a relatively steep, westward paleoslope promoted by east-ward offset of the locus of thermal uplift from the rift basin (i.e., a simple shear model of basin formation). In contrast to gravity-driven structures in most postrift settings, the Equatorial Guinea nappes developed at the close of the Aptian-Albian synrift episode in response to a growing bathymetric deep caused by rapid subsidence outpacing restricted sedimentation.

  3. Cenozoic rift tectonics of the Japan Sea

    SciTech Connect

    Kimura, K.

    1988-08-01

    The Japan Sea is one of the back-arc basins in trench-arc systems bordering the western Pacific. Recent paleomagnetic works suggest the Japan Sea opened during early to middle Miocene. Radiometric and microfossil ages of the Cenozoic onland sequences in the Japanese Islands elucidate the rift tectonics of the Japan Sea. The rifting history is summarized as follows: nonmarine volcanic formations of prerift stage before 50 Ma, rift-onset unconformity at 40 Ma, nonmarine volcanic formations of synrift stage 20-33 Ma, breakup unconformity 19 Ma showing the opening of the Japan Sea, marine volcanic and sedimentary formations of synrift stage 14.5-18 Ma, beginning of regional subsidence 14.5 Ma corresponding to the end of the Japan Sea opening, marine sedimentary formations of postdrift stage after 14.5 Ma. Rifting is not limited to the synrift stage but is continued to the syndrift stage. Rifting led to a horst-and-graben structure. Thus, the Cenozoic onland sequences in the Japanese Islands are suited for a study of rift tectonics because the sequences were subaerially exposed by the late Miocene-Holocene island-arc tectonics. Rift tectonics cannot be studied as easily in most Atlantic-type passive margins.

  4. Seismological investigation of the Okavango Rift, Botswana

    NASA Astrophysics Data System (ADS)

    Yu, Youqiang

    The mechanisms of rifting have been intensively investigated using geological and geophysical techniques beneath mature rift zones. However, current understanding on the earliest stages of rifting is seriously limited. Here we employ recently archived data from 17 broadband seismic stations traversing northern Botswana to conduct the first shear wave splitting and mantle transition zone (MTZ) studies within the Okavango Rift Zone (ORZ). The ORZ is an incipient continental rift situated at the terminal of the southwestern branch of the East African Rift System. The resulting normal MTZ thickness and consistently rift-parallel fast polarizations imply an absence of significant thermal anomalies in the upper mantle, ruling out the role of mantle plumes in the initiation of the ORZ. The observed anisotropy beneath the ORZ and adjacent areas is mainly attributed to the relative movement between the lithosphere and asthenosphere with regional contributions from fabrics in the lithosphere and flow deflection by the bottom of the lithosphere. Our observations imply that the initiation and development of the ORZ can be initiated following a passive mode from the consequences of relative movements between the South African block and the rest of the African plate along a zone of lithospheric weakness between the Congo and Kalahari cratons. In addition, an approach was developed to effectively remove the near surface reverberations in the resulting receiver functions, decipher the P-to-S converted phases associated with the Moho discontinuity, and thus resolve sub-sediment crustal structure beneath stations sitting on a low-velocity sedimentary layer.

  5. U-Th disequilibria constraints on physical and chemical erosion processes and rates in soils from the Lake Natron-Lake Magadi (Gregory Rift Valley) drainage area vs hydrology/paleohydrology and bedrock lithology

    NASA Astrophysics Data System (ADS)

    Hillaire-Marcel, Claude

    2015-04-01

    This presentation is a tribute to my former PhD student, the late Dr. Christian Goetz (1960-1991), who carried out intensive field and laboratory investigations on actinides in soils and sediments from lakes Manyara and Natron (Tanzania), and Magadi (Kenya) during his doctoral studies. Soils developed on granites from the Precambrian plateau, west of Gregory Rift, as well as those developed on the trachitic floor of the Rift yield nearly similar patterns, with U-leached (and Th-enriched) upper horizons vs source rocks. They differ from each other by i) the much higher [U]/[Th] mass ratio of soil over the granitic (~ 0.3) vs the trachitic (~ 0.1) basements (both near secular equilibrium), and ii) the greater decay of 230Th-excesses (230Thxs) in top soils over trachites, pointing to much older soils and/or lower leaching rates at the Rift floor. This difference seems related to the more arid conditions prevailing in the deeper part of the Rift. In contrast, soils developed on the basaltic walls of the Rift, characterized by abundant spring water from the basalt aquifer and a dense vegetation, depict a three-stage U-Th isotope evolution, with bedrock at near secular radioactive equilibrium and a [U]/[Th] ratio of ~ 0.2. It is overlain by a U-depleted horizon (with up to 75% relative losses in U), then topped by a low Eh, organic-matter rich layer, with evidence for a secondary uptake of U. The high 230Thxs observed in the U-leached horizon point to relatively fast U-leaching rates and/or "young" soil pattern. The present-day clays deposited in Lake Natron point to a Precambrian granitic plateau origin (through major rivers flowing eastwards towards the lake), whereas clay supplies from the rift escarpment basalts are carried with hydrothermal seepages towards Lake Magadi. U-Th measurements in early diagenetic minerals (phosphates) and clays (smectites) deposited during late Pleistocene high-lake levels (≥ 12 ka BP) provide robust constraints on source-rocks and

  6. Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data

    SciTech Connect

    Trehu, A.; Shay, J. ); Morel-a-l'Huissier, P.; Milkereit, B. ); Meyer, R.; Jefferson, T.; Shih, X.R. ); Karl, J. ); Mereu, R.; Epili, D. ); Sexton, J.; Wendling, S. ); Hajnal, Z.; Chan, W.K. ); Hutchison, D. )

    1991-04-01

    The authors present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore-offshore large-aperture seismic and gravity data. High velocities obtained within a highly reflective half-graben that was imaged on coincident seismic reflection data demonstrate the dominantly magic composition of the graben fill and constrain its total thickness to be at least 30 km. Strong wide-angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55-60 km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high-velocity lower crust interpreted to result from syn-rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half-graben. A second region of thick lower crust is found approximately 100 km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre-rift Moho is probably within or above the high-velocity lower crust, whereas the post-rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.

  7. Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data

    USGS Publications Warehouse

    Trehu, Anne M.; Morel-a-l'Huissier, Patrick; Meyer, R.; Hajnal, Z.; Karl, J.; Mereu, R. F.; Sexton, J.; Shay, J.; Chan, W. K.; Epili, D.; Jefferson, T.; Shih, X. R.; Wendling, S.; Milkereit, B.; Green, A.; Hutchinson, Deborah R.

    1991-01-01

    We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore-offshore large-aperture seismic and gravity data. High velocities obtained within a highly reflective half-graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide-angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55–60km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high-velocity lower crust interpreted to result from syn-rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half-graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre-rift Moho is probably within or above the high-velocity lower crust, whereas the post-rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.

  8. Formation of hyperextended rifted margins: Insights from flexural isostatic structural-stratigraphic modeling and observations from present-day rifted margins

    NASA Astrophysics Data System (ADS)

    Mohn, Geoffroy; Kusznir, Nick; Manatschal, Gianreto

    2014-05-01

    The understanding of how continental lithosphere extends, thins and ruptures leading to the formation of a new divergent plate boundary represent a fundamental question in Earth Sciences. In particular the mechanisms controlling the extreme pre-breakup stretching and thinning of the continental crust and lithosphere, documented at many present-day rifted margins, are still poorly known. Many questions remain of the fundamental processes controlling the extensional deformation of the continental crust and lithosphere, including fault geometries and their evolution in space and time, the occurrence of decoupling horizons within the continental crust and the importance of depth-dependent lithosphere thinning processes. We investigate the control of these key factors on continental crust and lithosphere thinning processes by combining seismic reflection and drill-hole observations from present-day Iberia-Newfoundland rifted margins with flexural isostatic forward tectonic and stratigraphic modeling. These observations made at present-day rifted margins constrain the input parameters used in the flexural isostatic forward modeling. At the same time, the forward tectonic and stratigraphic modeling provides validation of the interpretation of the seismic reflection data. Through this modeling, we produce isostatically and thermally balanced sections reproducing the geometries observed along the Iberia-Newfoundland rifted margins. Our results suggest that crustal and lithospheric thinning results from the combination of both pure- and simple-shear deformation. The model predicts the critical role of intra-crustal decoupling horizons confirming the importance of depth-dependent thinning through polyphased rifting events.

  9. Calcium chloride brines: The vital component in the hydrothermal brine-hydrothermal ore deposit-evaporite-basinal brine cycle in continental rift basins

    SciTech Connect

    Hardie, L. . Dept. of Earth and Planetary Science)

    1992-01-01

    Nonmarine evaporites are forming today in chloride-rich saline lakes in a number of arid continental rift and strike-slip basins that are characterized by upwelling of subsurface CaCl[sub 2]-bearing brines driven by forced convection of cool basinal brines or by free convection of hydrothermal brines which reach the surface as brine springs. The compositions of these upwelling brines are distinctively different from that of seawater or typical continental waters due primarily to their high proportion of Ca and low proportion of SO[sub 4]. The most viable explanation for the CaCl[sub 2] composition of these upwelling brines is the interaction between hot convecting groundwaters and bedrock at or above zeolite facies temperatures, as for example occurs in the modern Salton Sea basin. Such upwelling CaCl[sub 2] brines in extensional fault basins can explain the puzzling chemical composition of MgSO[sub 4]-poor potash evaporites, the least understood of all ancient salt deposits. In this regard it is suggested that the following cyclic succession of processes occurs in active continental rift basins during a magmatically-driven thermal event: (1) hydrothermal convection of the ambient porewaters in the rift sediments, (2) dissolution of buried evaporites and hydrothermal metamorphism of the rift sediments, (3) hydrothermal ore deposition in fault-related fractures and within the rift sediments, (4) upwelling brine springs add CaCl[sub 2] and KCl components to the surface lake waters, which on evaporation produce MgSO[sub 4]-poor potash evaporites, (5) decay of the thermal event leads to cool down of the hot brines, which now migrate gravitationally to the deeper parts of the basin to become static Na-Ca-Cl basinal brines.

  10. The activated sludge process: Fundamentals of operation

    SciTech Connect

    Junkins, R.; Deeny, K.J.; Eckhoff, T.H.

    1983-01-01

    The procedures given here - based on extensive and intensive experience. Background information on process mechanics is followed by detailed consideration of control and troubleshooting practices. Contents: PREFACE AND INTRODUCTION; PROCESS MECHANICS; Basic Mechanism of Activated Sludge Systems; Formation of Activated Sludge; Growth of Microorganisms; Classifications of Microorganisms: Type, Environment, Age; Solids Separation and Return; FACTORS AFFECTING OPERATION; Raw Wastewater Strength; Dissolved Oxygen; pH; Temperature; Nutrients; Toxicity; Mixing; Detention Time; Hydraulics; PROCESS MODIFICATIONS; Conventional; Complete Mix; Contact-Stabilization; Extended Aeration; Others; PROCESS MONITORING; Visual; Analytical Indicators; OPERATIONAL CONTROL; Sludge Volume Index; Sludge Age; Mean Cell Residence Time; Food/Microorganism Ratio; Organic Loading Rate; Solids Loading Rate; Clarifier Overflow Rate; Weir Overflow Rate; Sludge Recycle Rate, Sludge Wastage Rate; Chemical Feed Rate; TROUBLESHOOTING; Low BOD Removal; Low D.O. in Aeration Baisn; Poor Settling; PLANT START-UP; Introduction; Pre Start-up Checkup; Wastewater Analysis; Seed Screening; Process Checklist; Mechanical Checklist; Familiarization and Training; Start-up; Seeding; Process Monitoring; Transition; Typical Start-up Problems; Foaming; Settling Problems; Low BOD Removal; INDEX.

  11. Flexural modeling of the midcontinent rift

    SciTech Connect

    Nyquist, J.E.; Wang, H.F.

    1988-08-10

    A basement profile obtained from seismic reflection data has been used to constrain a two-dimensional flexural model of basin formation for the Midcontinent Rift at a latitude of 45/sup 0/25'N. Model parameters included the thickness of the elastic plate, the basin width, and the maximum basin thickness. Modeling suggests that flexure produced a deep narrow basin along the rift axis and that the crust was thinned at the time of basin formation to an elastic thickness of 9.6 km for a plate ruptured by rifting, or 5.7 km for an unbroken plate, with corresponding maximum basin thicknesses of 14 km and 16 km respectively. The plate thickness depends most strongly on the basin width and is well constrained by the seismic data, although erosion may have narrowed the basin. The maximum basin thickness is poorly constrained because of the lack of seismic data for depths greater than about 10 km and because the strata at the center of the rift have been disturbed by a postrift compressional event which produced the St. Croix horst. Despite uncertainty about the basin thickness, the load required to flex the crust to produce the Midcontinent Rift basin is too large to be attributed to the weight of the central flood basalts unless the basin subsided into a fluid less dense than the solidified basalts. On the basis of seismic refraction data and by analogy with other rifts, we hypothesize that a magnetic ''rift pillow'' intruded in the lower crust. The basaltic pillow subsequently solidified to produce a large, high-velocity region in the lower crust, centered under the rift axis, as determined from deep seismic refraction. This crystallization and cooling may be responsible for the ''sag'' phase of rift evolution, as evidenced by laterally widespread occurrence of postvolcanic sediments. copyright American Geophysical Union 1988

  12. Speech perception as an active cognitive process

    PubMed Central

    Heald, Shannon L. M.; Nusbaum, Howard C.

    2014-01-01

    One view of speech perception is that acoustic signals are transformed into representations for pattern matching to determine linguistic structure. This process can be taken as a statistical pattern-matching problem, assuming realtively stable linguistic categories are characterized by neural representations related to auditory properties of speech that can be compared to speech input. This kind of pattern matching can be termed a passive process which implies rigidity of processing with few demands on cognitive processing. An alternative view is that speech recognition, even in early stages, is an active process in which speech analysis is attentionally guided. Note that this does not mean consciously guided but that information-contingent changes in early auditory encoding can occur as a function of context and experience. Active processing assumes that attention, plasticity, and listening goals are important in considering how listeners cope with adverse circumstances that impair hearing by masking noise in the environment or hearing loss. Although theories of speech perception have begun to incorporate some active processing, they seldom treat early speech encoding as plastic and attentionally guided. Recent research has suggested that speech perception is the product of both feedforward and feedback interactions between a number of brain regions that include descending projections perhaps as far downstream as the cochlea. It is important to understand how the ambiguity of the speech signal and constraints of context dynamically determine cognitive resources recruited during perception including focused attention, learning, and working memory. Theories of speech perception need to go beyond the current corticocentric approach in order to account for the intrinsic dynamics of the auditory encoding of speech. In doing so, this may provide new insights into ways in which hearing disorders and loss may be treated either through augementation or therapy. PMID

  13. A Numerical and Analogue Study of Dike Ascent in Asymmetric Continental Rift Zones

    NASA Astrophysics Data System (ADS)

    Schierjott, J.; Maccaferri, F.; Acocella, V.; Rivalta, E.

    2015-12-01

    In continental rift zones, tectonic extension generates deep topographic depressions, typically graben or half-graben structures, confined by large border faults. Volcanism may be distributed within, at the border and outside of the depressions, and the mechanisms controlling this distribution are debated. Recently, Maccaferri et al. (2014) proposed that the reorientation of the principal stresses linked to crustal thinning and overall crustal mass redistribution in rift zones modifies the expected trajectory of ascending magma pockets and plays a fundamental role in the distribution of volcanism at the surface. However, the model does not explain why volcanism is asymmetric in most continental rift zones. The goal of this study is to investigate the relation between the characteristic distribution of volcanism at the surface, the distribution and geometry of magma storage at depth, and the observed geometric asymmetry of the grabens at most rift zones. By using a boundary element model for dike propagation and analogue laboratory experiments we evaluate the ascent path of magmatic dikes in asymmetric continental rifts.We find that the position of the magma source along the cross section of the rift and its spatial extent and the asymmetry of the graben cross section are the most important factors controlling one-sided volcanic activity at surface. For dikes starting beneath the rift's center, the more asymmetric the rift structure the more likely is asymmetric volcanic activity. Dikes are deflected to the shallow rift side and no volcanism develops on the deep side or only focused in one spot. However, if the position of the magma ponding region is offset towards the deep side of the graben, the dikes tend to emerge on the rift shoulder adjacent to such deep side. To a minor extent, also the starting depth of the dikes, any topographic loading on the graben flanks due to flank uplift and the background tectonic stress impact the surface distribution of volcanism

  14. Rift to drift transition in Siberian Arctic and its impact on continental margin architecture

    NASA Astrophysics Data System (ADS)

    Drachev, S. S.

    2003-04-01

    The East Siberian Arctic Continental Margin (ESAM) represents a rare case of rifting to spreading transition. Present-day geodynamics of this plate tectonic interplay is characterized by a very slow plate divergence in the Laptev Sea as this regions is located just landward of the slowest spreading center worldwide (the Gakkel Ridge), close to the pole of North American/Eurasian plate rotation. However the existing geological and geophysical data, mainly seismic reflection and potential field data, allow conclusion that this situation has been far different in the past. Just after its formation at the end of Late Cretaceous through a series of plate convergence and folding episodes the crust of the ESAM has been strongly modified by an intense rifting. The earliest rift episode took place eastward of the present Laptev Sea, in the East Siberian Sea and probably Chukchi seas, where presently abandoned rifts are stretched landward along the principal weakened zones in the ESAM basement. This rifting might have been related to a spreading episode in the Amerasia Basin and perhaps was triggered by a mantle plume ca. 120 mln. yr. ago (De Long and Franz Joseph Land basalts). Outer parts of the ERAM might have also been rifted away to create marginal blocks, as the Arlis and Chukchi plateau. Second rift event was clearly related to the opening of the Eurasia Basin, preceding it and remaining active through the Cenozoic. The rift to drift transition has been taking place in a huge, “dry” and still active Laptev Rift System, which is a landward projection of the Gakkel Ridge spreading axis. This extension had a major effect on the western ERAM causing strong normal faulting and crustal thinning, up to 70% in some places. However, total crustal extension in the Laptev Rift System is considerably smaller than a value of total opening of the Eurasia Basin, so the spreading is not completely accommodated by the rifting. It may be speculated that a major portion of this

  15. Do melt-rich shear zones lubricate rift flanks? 3-D spatial gradients in anisotropy beneath the East African Rift in Ethiopia

    NASA Astrophysics Data System (ADS)

    Holtzman, B. K.; Gaherty, J. B.; Kendall, J.; Stuart, G.

    2006-12-01

    Melt-enhanced weakening of the mantle may be essential for a continent to break apart and rift. A primary means of understanding this aspect of the dynamics of rifting (and other geodynamic settings) is the interpretation of seismic anisotropy in terms of melt distribution and/or mantle flow direction. In recent rock- deformation experiments, the relationships between flow direction and seismic fast direction are complicated by the presence of water (e.g. Katayama et al., 2004) and segregated melt (Holtzman et al. 2003). In the latter, deviatoric stress drives melt to organize into networks of melt-rich shear zones. Scaling from laboratory to upper mantle conditions predicts characteristic network spacings of <1 km, less than wavelengths of measured seismic waves; thus they should cause significant seismic anisotropy. Measurable gradients in anisotropy may be diagnostic indicators of gradients in melt segregation. Because melt-rich shear zones will weaken the mantle on a large scale, the presence of such anisotropy gradients would map to gradients in viscosity. To map melt distribution beneath the rift and test for the presence or absence of this process, we combine measurements of seismic velocities beneath the East African Rift in Ethiopia with modeling of elastic properties of aligned, segregated melt and olivine fabric. Analysis of SKS phases has shown that fast directions parallel magmatic centers in the rift and splitting magnitudes are largest near the rift flanks, consistent with the hypothesis above (e.g. Kendall et al., 2006). Preliminary analysis of Love-Rayleigh differential times across the rift are consistent with a vertical fast axis, suggesting the presence of a vertically aligned, rift parallel melt phase down to the solidus (Pilidou et al., 2005). We expand on these results by applying a cross-correlation procedure to precisely estimate relative amplitude and phase of surface waves traversing the rift. Data are derived from the EAGLE and Ethiopia

  16. The magmatic budget of Atlantic type rifted margins: is it related to inheritance?

    NASA Astrophysics Data System (ADS)

    Manatschal, Gianreto; Tugend, Julia; Picazo, Suzanne; Müntener, Othmar

    2016-04-01

    In the past, Atlantic type rifted margins were either classified as volcanic or non-volcanic. An increasing number of high quality reflection and refraction seismic surveys and drill hole data show a divergent style of margin architecture and an evolution in which the quantity and distribution of syn-rift magmatism is variable, independently of the amount of extension. Overgeneralized classifications and models assuming simple relations between magmatic and extensional systems are thus inappropriate to describe the formation of rifted margins. More recent studies show that the magmatic evolution of rifted margins is complex and cannot be characterized based on the volume of observed magma alone. On the one hand, so-called "non-volcanic" margins are not necessarily amagmatic, as shown by the results of ODP drilling along the Iberia-Newfoundland rifted margins. On the other hand, magma-rich margins, such as the Norwegian, NW Australian or the Namibia rifted margins show evidence for hyper-extension prior to breakup. These observations suggest that the magmatic budget does not only depend on extension rates but also on the composition and temperature of the decompressing mantle. Moreover, the fact that the magmatic budget may change very abruptly along strike and across the margin is difficult to reconcile with the occurrence of plumes or other deep-seated large-scale mantle phenomena only. These overall observations result in questions on how magmatic and tectonic processes are interacting during rifting and lithospheric breakup and on how far inheritance may control the magmatic budget during rifting. In our presentation we will review results from the South and North Atlantic and the Alpine Tethys domain and will discuss the structural and magmatic evolution of so-called magma-rich and magma-poor rifted margins. In particular, we will try to define when, where and how much magma forms during rifting and lithospheric breakup. The key questions that we aim to address

  17. Erosion rates along fault scarps and rift-shoulder environments in central and northern Kenya: Insights from new 10Be-derived basin-wide erosion rates

    NASA Astrophysics Data System (ADS)

    Torres-Acosta, V.; Strecker, M. R.; Schildgen, T. F.; Wittmann, H.; Scherler, D.; Bookhagen, B.

    2011-12-01

    The Kenya Rift is typical example of an active continental rift zone and is a fundamental part of the East African Rift system. The rift valley plays a central role in archiving the relationships between sedimentation, erosion, and climate in the region. However, the links between surface processes (i.e., erosion, sedimentation) and tectonic setting are currently poorly understood. In this study we analyze to what degree tectono-geomorphic setting and/or climatic characteristics control erosion rates in the region. We extract morphometric characteristics of the rift flanks and the plateau surface from SRTM 90-m resolution digital elevation data. We rely on calibrated, satellite-derived Tropical Rainfall Measurement Mission (TRMM 2B31) rainfall to characterize the different climatic compartments throughout the study region. We calculate specific stream power amounts using integrated rainfall as discharge amounts. Next, we analyze the relation between cosmogenic radionuclide (CRN) basin-wide erosion rates and climatic and geomorphic parameters. We determined erosion rates from twenty-six river sand samples acquired from along the flanks of the Elgeyo Escarpment (northern section of western rift flank), the Nguruman Escarpment (southern section of western flank), the Tirr Tirr Plateau (north), the Kapute Plains, and the Suguta Valley. Catchment-wide erosion rates range from 0.001 to 0.1 mm/y across the different climatic compartments. Comparisons to catchment climate and topographic characteristics suggest that more than 60% of variation in erosion can be explain by specific stream power amounts using rainfall as discharge component. The catchment-averaged normalized channel steepness index, which doesn't take into account variations in precipitation, explains only 42% of the variation in erosion rates. These observations demonstrate that the strong spatial variations in erosion rates are largely controlled by both catchment morphology and climatic gradients. In

  18. 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

    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

  19. Process for preparing active oxide powders

    DOEpatents

    Berard, Michael F.; Hunter, Jr., Orville; Shiers, Loren E.; Dole, Stephen L.; Scheidecker, Ralph W.

    1979-02-20

    An improved process for preparing active oxide powders in which cation hydroxide gels, prepared in the conventional manner are chemically dried by alternately washing the gels with a liquid organic compound having polar characteristics and a liquid organic compound having nonpolar characteristics until the mechanical water is removed from the gel. The water-free cation hydroxide is then contacted with a final liquid organic wash to remove the previous organic wash and speed drying. The dried hydroxide treated in the conventional manner will form a highly sinterable active oxide powder.

  20. Enhanced multistatic active sonar signal processing.

    PubMed

    Zhao, Kexin; Liang, Junli; Karlsson, Johan; Li, Jian

    2013-07-01

    Multistatic active sonar systems involve the transmission and reception of multiple probing sequences and can achieve significantly enhanced performance of target detection and localization through exploiting spatial diversity. This paper mainly focuses on two signal processing aspects of such systems, namely, enhanced range-Doppler imaging and improved target parameter estimation. The main contributions of this paper are (1) a hybrid dense-sparse method is proposed to generate range-Doppler images with both low sidelobe levels and high accuracy; (2) a generalized K-Means clustering (GKC) method for target association is developed to associate the range measurements from different transmitter-receiver pairs, which is actually a range fitting procedure; (3) the extended invariance principle-based weighted least-squares method is developed for accurate target position and velocity estimation. The effectiveness of the proposed multistatic active sonar signal processing techniques is verified using numerical examples.

  1. Rifting to drifting transition of the Southwest Subbasin of the South China Sea

    NASA Astrophysics Data System (ADS)

    Song, Taoran; Li, Chun-Feng

    2015-09-01

    Continental rupturing process and related dynamics on the onset of seafloor spreading remain poorly understood in the opening of the South China Sea. To constrain the timing and cause of major tectonic events, we focus on the rifting-to-drifting transition of the Southwest Subbasin, which has very wide extended continental margins. By carefully interpreting rifting structures and carbonate platforms and reefs, we distinguished two major unconformities, i.e., the breakup unconformity (BRU) and the mid-Miocene unconformity, in the two conjugate margins of the Southwest Subbasin. The age of the BRU in our study area is near the Oligocene/Miocene boundary (~23 Ma). Pre-stack depth migration of a recently acquired multichannel reflection seismic profile reveals complex structures and strong lateral velocity variations associated with a 3.5 km thick syn-rifting sequence developed right at the continent-ocean boundary (COB) of the Southwest Subbasin. This syn-rifting sequence is bounded landwards by a large seaward dipping fault, and tapers out seawards. An erosional truncation, which represents the mid-Miocene unconformity landwards but the older breakup unconformity on the seaward side, occurred at the top of this sequence. The overall transitional deformation style from the rifting to drifting suggests a successive episode of rifting, faulting, compression, tilting, and erosion at the COB during the crustal thinning and mantle upwelling. Localized thick syn-rifting deposition and early deposition beneath the BRU in the oceanic domain exist only at the seaward concave part of the COB, indicating discrete rifting and seafloor spreading prior to the buildup of a unified spreading center for the entire subbasin.

  2. Deformation during the 1975-1984 Krafla rifting crisis, NE Iceland, measured from historical optical imagery

    NASA Astrophysics Data System (ADS)

    Hollingsworth, James; Leprince, SéBastien; Ayoub, FrançOis; Avouac, Jean-Philippe

    2012-11-01

    We measure the displacement field resulting from the 1975-1984 Krafla rifting crisis, NE Iceland, using optical image correlation. Images are processed using the COSI-Corr software package. Surface extension is accommodated on normal faults and fissures which bound the rift zone, in response to dike injection at depth. Correlation of declassified KH-9 spy and SPOT5 satellite images reveals extension between 1977-2002 (2.5 m average opening over 80 km), while correlation of aerial photos between 1957-1990 provide measurements of the total extension (average 4.3 m opening over 80 km). Our results show ˜8 m of opening immediately north of Krafla caldera, decreasing to 3-4 m at the northern end of the rift. Correlation of aerial photos from 1957-1976 reveal a bi-modal pattern of opening along the rift during the early crisis, which may indicate either two different magma sources located at either end of the rift zone (a similar pattern of opening was observed in the 2005 Afar rift crisis in East Africa), or variations in rock strength along the rift. Our results provide new information on how past dike injection events accommodate long-term plate spreading, as well as providing more details on the Krafla rift crisis. This study also highlights the potential of optical image correlation using inexpensive declassified spy satellite and aerial photos to measure deformation of the Earth's surface going back many decades, thus providing a new tool for measuring Earth surface dynamics, e.g. glaciers, landsliding, coastal erosion, volcano monitoring and earthquake studies, when InSAR and GPS data are not available.

  3. Active voltammetric microsensors with neural signal processing.

    SciTech Connect

    Vogt, M. C.

    1998-12-11

    Many industrial and environmental processes, including bioremediation, would benefit from the feedback and control information provided by a local multi-analyte chemical sensor. For most processes, such a sensor would need to be rugged enough to be placed in situ for long-term remote monitoring, and inexpensive enough to be fielded in useful numbers. The multi-analyte capability is difficult to obtain from common passive sensors, but can be provided by an active device that produces a spectrum-type response. Such new active gas microsensor technology has been developed at Argonne National Laboratory. The technology couples an electrocatalytic ceramic-metallic (cermet) microsensor with a voltammetric measurement technique and advanced neural signal processing. It has been demonstrated to be flexible, rugged, and very economical to produce and deploy. Both narrow interest detectors and wide spectrum instruments have been developed around this technology. Much of this technology's strength lies in the active measurement technique employed. The technique involves applying voltammetry to a miniature electrocatalytic cell to produce unique chemical ''signatures'' from the analytes. These signatures are processed with neural pattern recognition algorithms to identify and quantify the components in the analyte. The neural signal processing allows for innovative sampling and analysis strategies to be employed with the microsensor. In most situations, the whole response signature from the voltammogram can be used to identify, classify, and quantify an analyte, without dissecting it into component parts. This allows an instrument to be calibrated once for a specific gas or mixture of gases by simple exposure to a multi-component standard rather than by a series of individual gases. The sampled unknown analytes can vary in composition or in concentration, the calibration, sensing, and processing methods of these active voltammetric microsensors can detect, recognize, and

  4. Structural evolution of the southern transfer zone of the Gulf of Suez rift, Egypt

    NASA Astrophysics Data System (ADS)

    Abd-Allah, Ali M. A.; Abdel Aal, Mohamed H.; El-Said, Mohamed M.; Abd El-Naby, Ahmed

    2014-08-01

    We present a detailed study about the initiation and reactivations of Zeit-El Tor transfer zone, south Gulf of Suez rift, and its structural setting and tectonic evolution with respect to the Cretaceous-Cenozoic tectonic movements in North Egyptian margin. NE trending zone of opposed-dipping faults (22 km wide) has transferred the NE and SW rotations of the sub-basins in central and south Gulf of Suez rift, respectively. The evolution of this zone started by reactivation of the NE oriented late Neoproterozoic fractures that controlled the occurrence of Dokhan Volcanics in the rift shoulders. Later, the Syrian Arc contraction reactivated these fractures by a sinistral transpression during the Late Cretaceous-Eocene time. N64°E extension of the Oligo-Miocene rift reactivated the NE fractures by a sinistral transtension. During this rifting, the NE trending faults forming the transfer zone were more active than the rift-bounding faults; the Upper Cretaceous reverse faults in the blocks lying between these NE trending faults were rotated; and drape-related reverse faults and the positive flower structures were formed. Tectonic inversion from contraction to extension controlled the distribution and thickness of the Upper Cretaceous-Miocene rocks.

  5. Formation of the Shanxi Rift in North China: The control of preexisting lithospheric weakness

    NASA Astrophysics Data System (ADS)

    Lin, F.; Liu, M.; Ye, J.

    2012-12-01

    The Shanxi Rift is an active seismic zone in North China, developed mainly since Pliocene (~5 Ma). Its formation has been associated with the Indo-Asian collision; other hypothesized causes include a regional extensional stress field associated with subduction of the western pacific plate and mantle upwelling under the North China Plain. However, these mechanisms do not explain why the rift system did not form along the western boundary of the North China Plain, where lithospheric thickness changes sharply from more than 150 km under the Ordos block and the Taihangshan Mountains to the west, to less than 70 km under the North China Plain. We have used a viscoplastic finite element model to explore the conditions for localized rifting in North China. Our results show that, for all the hypothesized causes, the preferred site of rifting would be along the boundary zone of changing lithospheric thickness. The only way to initiate the Shanxi rift in its current location, which is between the Ordos block and the Taihangshan Mountains with thick lithosphere, is to have preexisting lithospheric weakening there. This lithospheric weakness was likely formed during the collision between the Easter North China block and the Western North China block during the Paleoproterozoic (~1.8 Ga). Hence the ancient tectonic event still controls the young continental rifting.

  6. Origin of three-armed rifts in volcanic islands: the case of El Hierro (Canary Islands)

    NASA Astrophysics Data System (ADS)

    Galindo Jiménez, Inés; Becerril Carretero, Laura; Martí Molist, Joan; Gudmundsson, Agust

    2015-04-01

    Rifts zones in volcanic oceanic islands are common structures that have been explained through several theories/models. However, despite all these models it is as yet unclear whether it is the intense intrusive activity or the sector collapses that actually control the structural evolution and geometry of oceanic-island rift zones. Here we provide a new hypothesis to explain the origin and characteristics of the feeding system of oceanic-island rift zones based on the analysis of more than 1700 surface, subsurface (water galleries), and submarine structural data from El Hierro (Canary Islands). El Hierro's geological structure is primarily controlled by a three-armed rift-zone, the arms striking NE, WSW and S. Between the rift axes there are three valleys formed during huge landslides: El Golfo, El Julan, and Las Playas. Our results show: (1) a predominant NE-SW strike of structural elements, which coincides with the main regional trend of the Canary Archipelago as a whole; (2) a clear radial strike distribution of structural elements for the whole volcanic edifice (including submarine flanks) with respect to the centre of the island; (3) that the rift zones are mainly subaerial structures and do not propagate through the submarine edifice; (4) that it is only in the NE rift that structures have a general strike similar to that of the rift as a whole, and; (5) that in the W and S rifts there is not clear main direction, showing the structural elements in the W rift a fan distribution coinciding with the general radial pattern in the island as a whole. Based on these data, we suggest that the radial-striking structures reflect comparatively uniform stress fields that operated during the constructive episodes, mainly conditioned by the combination of overburden pressure, gravitational spreading, and magma-induced stresses. By contrast, in the shallower parts of the edifice, that is, the NE-SW, N-S and WNW-ESE-striking structures, reflect local stress fields related

  7. The development of the Midcontinent Rift in the context of rapid paleogeographic change

    NASA Astrophysics Data System (ADS)

    Swanson-Hysell, N.; Vaughn, A. A.; Mustain, M. R.; Feinberg, J. M.

    2012-12-01

    Despite being active for >20 million years and resulting in the thinning of pre-rift crust by a factor of 3 or more, the 1.1 Ga Midcontinent Rift failed to dismember the Laurentian craton. This failure resulted in the preservation of a thick record of rift-related volcanic and sedimentary rocks that give geoscientists a powerful window into the development of this ancient rift. Most models for the development of the Midcontinent Rift attribute its origin to the upwelling and decompression melting of a mantle plume. On the basis of the great volume of generated magma and interpretation of geochemical data, it is argued that the early stage plateau flood basalts of the rift (~1110-1105 Ma) and the main stage volcanics that erupted into the central basin (~1100-1095 Ma) were both dominated by plume-sourced melts. However, this model needs to be reconciled with paleomagnetic data from rift volcanics that reveal a significant decrease in inclination between the early and main stage volcanics. New data we have developed from 90+ flows of the early stage Osler Volcanic Group bolster evidence from the succession at Mamainse Point that this change in inclination is the result of fast equatorward plate motion during the early stage and into the main stage of rift volcanism. Even with >20° of latitudinal motion from the time of initial volcanism to eruption of the thick main stage volcanics, magmatism was largely confined to the same geographic region in a relatively narrow central basin. If a long-lived plume was in a fixed position relative to Earth's spin axis, the large relative motion of Laurentia would make it unable to continue to be a source of melt to the rift. Two possible explanations to reconcile a plume-contribution in the main stage with this latitudinal change are: (1) That the active contribution from an underlying plume was limited to the early stage of volcanism, but substantial volume of material accreted to the lithosphere that was subsequently sampled

  8. Activation of consolidation processes of alumina ceramics

    NASA Astrophysics Data System (ADS)

    Matrenin, S. V.; Zenin, B. S.; Tayukin, R. V.

    2016-02-01

    The methods for activating sintering ceramics based on Al2O3 by mechanical activation in the planetary mill, by adding in the mixture of nanopowders (NP) Al, Al2O3, and submicron powder TiO2, and by applying the technology of spark plasma sintering (SPS) are developed. It has been shown that adding the nanopowder up to 20 wt. % Al2O3 in a coarse powder α-Al2O3 activates the sintering process resulting in increased density and hardness of the sintered alumina ceramics. Substantial effect of increasing density of alumina ceramics due to adding the submicron powder TiO2 in the compound of initial powder mixtures has been established.

  9. NW Africa post-rift tectonics: fieldwork constraints from an "unfitting" anticline in west Morocco

    NASA Astrophysics Data System (ADS)

    Fernández-Blanco, David; Gouiza, Mohamed

    2015-04-01

    The evolution of the Moroccan Atlantic rifted margin is marked by a period of abnormal and excessive early post-rift subsidence during the Late Jurassic-Early Cretaceous affecting the proximal coastal basins, the continental shelf and the distal deep basins, which acted coevally to km-scale uplift and erosion of large domains to the east. The tectonics of the uplift event are still unclear, as it took place 30 to 50 Myr after lithospheric breakup between Morocco and Nova Scotia and prior to the Atlas/Alpine contraction, which gave rise to the Atlas and the Rif mountain belts. The Essaouira-Haha basin, located on the coastal plain of the Atlantic rifted margin of Morocco, and bounded by two uplifted Paleozoic basement highs (i.e. the Massif Ancien of Marrakech, to the east, and the Jebilet, to the northeast), is an ideal location to investigate the tectonic processes that might have triggered these vertical movements. Although most of the deformation observed in the basin is classically attributed to Upper Cretaceous halokinesis and Neogene Atlas contraction, recent works have shown the existence of contractional structures. We carry out a structural analysis of the Jbel Amsittene Anticline, located in the middle of the Essaouira-Haha basin to investigate the tectonics of its formation and its relationship with the above-mentioned exhumation. We show structural field data along several cross-sections transecting the anticline, and characterize a salt-cored fault propagation fold verging north, with a Triassic salt acting as a detachment plane. Regional kinematic indicators and structures show overall NNW-SSE to NNE-SSW shortening and active tectonics during the postrift phase, as indicated by syn-tectonic wedges seen for the Late Jurassic to Early Cretaceous period. These facts discard the "salt-drives-tectonics" theory to let "tectonic-drives-salt" one to rise, and point to factors other than small-cell mantle convection acting during the evolution of the Moroccan

  10. Graben formation during the Bárðarbunga rifting event in central Iceland

    NASA Astrophysics Data System (ADS)

    Ruch, Joël; Jónsson, Sigurjón; Wang, Teng; Xu, Wenbin; Trippanera, Daniele

    2015-04-01

    On the 16th of August 2014, an intense seismic swarm was detected at the Bárðarbunga caldera (central Iceland), which migrated to the east and then to the northeast during the following days. The swarm, highlighting magma propagation pathway from the caldera, migrated laterally during the following two weeks over 40 km. By the end of August, a volcanic eruption had started along a north-south oriented fissure located ~45 km from the caldera. Here we focus on the near-field deformation related to the dike emplacement in the shallow crust, which generated in few days an 8 km long by 0.8 km wide graben (depression) structure. The new graben extends from the northern edge of the Vatnajökull glacier and to the north to the eruptive fissure. We analyze the temporal evolution of the graben by integrating structural mapping using multiple acquisitions of TerraSAR-X amplitude radar images, InSAR and ground-truth data with GPS and structural measurements. Pixel-offset tracking of radar amplitude images shows clearly the graben subsidence, directly above the intrusion pathway, of up to 6 meters in the satellite line-of-sight direction. We installed a GPS profile of 15 points across the graben in October 2014 and measured its depth up to 8 meters, relative to the flanks of the graben. Field structural observations show graben collapse structures that typically accompany dike intrusions, with two tilted blocks dipping toward the graben axis, bordered by two normal faults. Extensive fractures at the center of the graben and at the graben edges show a cumulative extension of ~8 meters. The formation of the graben was also accompanied by strong seismic activity locally, constraining the time frame period of the main graben formation subsidence. Our results show a rare case of a graben formation captured from space and from ground observations. Such structures are the dominant features along rift zones, however, their formation remain poorly understood. The results also provide

  11. Soils, slopes and source rocks: Application of a soil chemistry model to nutrient delivery to rift lakes

    NASA Astrophysics Data System (ADS)

    Harris, Nicholas B.; Tucker, Gregory E.

    2015-06-01

    The topographic evolution of rift basins may be critical to the deposition of lacustrine source rocks such as the organic-rich Lower Cretaceous shales of the South Atlantic margin. Soils have been proposed as a key link between topography and source rock deposition by providing nutrients for the algae growth in rift lakes. Decreasing topographic relief from active rift to late rift has several effects on soils: soils become thicker and finer, erosion of dead surface and soil organic matter decreases, and the fractionation of precipitation between runoff and infiltration may favor increased infiltration. This hypothesis is tested by application of CENTURY, a complex box model that simulates transfer of nutrients within soil pools. The model is first applied to a rainforest soil, with several parameters individually varied. Infiltration experiments show that the concentrations of C, N and P in groundwater decrease rapidly as infiltration decreases, whether due to increased slope or to decreased precipitation. Increased erosion of surface plant litter and topsoil results in substantially decreased nutrient concentrations in groundwater. Increased sand content in soil causes an increase in nutrient concentration. We integrate these variables in analyzing topographic swathes from the Rio Grande Rift, comparing the southern part of the rift, where topography is relatively old and reduced, to the northern rift. C and P concentrations in groundwater increase as slope gradient decreases, resulting in substantially larger C and P concentrations in groundwater in the southern rift than the northern rift. Nitrogen concentrations in groundwater depends on whether infiltration varies as a function of slope gradient; in experiments where the fraction of infiltrated precipitation decreased with increasing slope, N concentrations was also substantially higher in the southern rift; but in experiments where that fraction was held constant, N concentrations was lower in the southern

  12. Structure and stratigraphy of the Rukwa rift

    NASA Astrophysics Data System (ADS)

    Kilembe, Elias A.; Rosendahl, Bruce R.

    1992-08-01

    Combining recently acquired multifold seismic data with well and gravity information and field mapping, a comprehensive picture of the structure and stratigraphy of the Rukwa rift has emerged. The Rukwa rift lies between the Tanganyika and Nyasa (Malawi) rifts in the western branch of the East African rift system in southwest Tanzania. The Rukwa rift is a NW-trending half-graben basin that is 350 km long and 40 km wide. Unlike the neighboring Tanganyika and Nyasa rifts, there is no evidence of half-graben polarity reversals in the Rukwa rift. The NW-trending boundary fault system lies on the northeastern side of the basin and comprises a series of listric faults. Most internal faults also show listric forms and trend N-S, oblique to the boundary fault. The basal sedimentary section is the Permo-Triassic (Karroo) Sequence. This is overlain by the Red Bed Sandstone Sequence, in which both Mesozoic and Tertiary fauna have been found. The Cenozoic Lake Bed Sequence is the highest unit and covers nearly all of the present basin. Sediment thicknesses commonly reach 7 km and attain a maximum of 12 km at the southeastern end of the basin. The Lake Bed Sequence is the thickest unit in the main depocentre, but the Karroo Sequence is often the thickest unit on the shoaling side of the half-graben. The Rukwa rift is here interpreted to have evolved as a strike- to oblique-slip pull-apart basin, based on numerous indications of NW-trending strike-slip faulting.

  13. Formation of hyperextended rifted margins: Insights from flexural isostatic forward tectonic stratigraphic modeling and observations from fossil analogues

    NASA Astrophysics Data System (ADS)

    Mohn, G.; Kusznir, N. J.; Manatschal, G.

    2013-12-01

    The understanding of how continental lithosphere thins and ruptures leading to the formation of a new plate boundary represent a fundamental question in Earth Sciences. In particular the mechanisms controlling the extreme thinning of the continental crust, documented at many present-day rifted margins, are still poorly known. Many questions remain of the fundamental processes controlling the extensional deformation of the continental crust and lithosphere, including fault geometries and their evolution in space and time, the occurrence of decoupling horizons within the continental crust and the importance of depth dependent thinning processes. In this contribution, we aim to investigate the control of these key factors on continental crust and lithosphere thinning processes by coupling field observations from fossil analogues of rifted margins with flexural isostatic forward tectonic and stratigraphic modeling. Remnants of the Alpine Tethys rifted margins preserved in the Alpine orogeny in Western Europe represent an ideal natural laboratory providing direct access to rifted continental margin structures. This natural laboratory provides insights on fault geometry and kinematics, subsidence history and as well the nature of the basement rocks involved in rifting processes. These observations represent the input parameters to constrain the flexural isostatic forward modeling. Through this modeling, we produce isostatically balanced sections reproducing the geometries observed in the fossil analogues. Our results suggest that crust and lithosphere hyperextension results from the combination of both pure and simple shear deformation. The model predicts the critical role of intra-crustal decoupling horizons confirming the importance of depth-dependent thinning through polyphased rifting events. This ongoing work includes the analysis and modeling of present-day rifted margin examples such as the Iberia-Newfoundland rifted margins. These modeling results give critical

  14. Lower Crustal Seismicity, Volatiles, and Evolving Strain Fields During the Initial Stages of Cratonic Rifting

    NASA Astrophysics Data System (ADS)

    Lambert, C.; Muirhead, J.; Ebinger, C. J.; Tiberi, C.; Roecker, S. W.; Ferdinand-Wambura, R.; Kianji, G.; Mulibo, G. D.

    2014-12-01

    The volcanically active East African rift system in southern Kenya and northern Tanzania transects thick cratonic lithosphere, and comprises several basins characterized by deep crustal seismicity. The US-French-Tanzania-Kenya CRAFTI project aims to understand the role of magma and volatile movement during the initiation and evolution of rifting in cratonic lithosphere. Our 38-station broadband network spans all or parts of fault-bounded rift segments, enabling comparison of lithospheric structure, fault kinematics, and seismogenic layer thickness with age and proximity to the deeply rooted Archaen craton. Seismicity levels are high in all basins, but we find profound differences in seismogenic layer thickness along the length of the rift. Seismicity in the Manyara basin occurs almost exclusively within the lower crust, and in spatial clusters that have been active since 1990. In contrast, seismicity in the ~ 5 My older Magadi basin is localized in the upper crust, and the long border fault bounding the west side of the basin is seismically inactive. Between these two basins lies the Natron rift segment, which shows seismicity between ~ 20 and ~2 km depth, and high concentrations at Oldoinyo Lengai and Gelai volcanoes. Older volcanoes on the uplifted western flank (e.g., Ngorongoro) experience swarms of activity, suggesting that active magmatism and degassing are widespread. Focal mechanisms of the frequent earthquakes recorded across the array are spatially variable, and indicate a stress field strongly influenced by (1) Holocene volcanoes, (2) mechanical interactions between adjacent rift basins, and (3) a far-field ESE-WNW extensional stress regime. We explore the spatial correlation between zones of intense degassing along fault systems and seismicity, and examine the influence of high gas pressures on lower and upper crustal seismicity in this youthful cratonic rift zone.

  15. The Rift Valley Fever virus protein NSm and putative cellular protein interactions

    PubMed Central

    2012-01-01

    Rift Valley Fever is an infectious viral disease and an emerging problem in many countries of Africa and on the Arabian Peninsula. The causative virus is predominantly transmitted by mosquitoes and high mortality and abortion rates characterize outbreaks in animals while symptoms ranging from mild to life-threatening encephalitis and hemorrhagic fever are noticed among infected humans. For a better prevention and treatment of the infection, an increased knowledge of the infectious process of the virus is required. The focus of this work was to identify protein-protein interactions between the non-structural protein (NSm), encoded by the M-segment of the virus, and host cell proteins. This study was initiated by screening approximately 26 million cDNA clones of a mouse embryonic cDNA library for interactions with the NSm protein using a yeast two-hybrid system. We have identified nine murine proteins that interact with NSm protein of Rift Valley Fever virus, and the putative protein-protein interactions were confirmed by growth selection procedures and β-gal activity measurements. Our results suggest that the cleavage and polyadenylation specificity factor subunit 2 (Cpsf2), the peptidyl-prolyl cis-trans isomerase (cyclophilin)-like 2 protein (Ppil2), and the synaptosome-associated protein of 25 kDa (SNAP-25) are the most promising targets for the NSm protein of the virus during an infection. PMID:22838834

  16. No thermal anomalies in the mantle transition zone beneath an incipient continental rift: evidence from the first receiver function study across the Okavango Rift Zone, Botswana

    NASA Astrophysics Data System (ADS)

    Yu, Y.; Liu, K. H.; Moidaki, M.; Reed, C. A.; Gao, S. S.

    2015-08-01

    Mechanisms leading to the initiation and early-stage development of continental rifts remain enigmatic, in spite of numerous studies. Among the various rifting models, which were developed mostly based on studies of mature rifts, far-field stresses originating from plate interactions (passive rifting) and nearby active mantle upwelling (active rifting) are commonly used to explain rift dynamics. Situated atop of the hypothesized African Superplume, the incipient Okavango Rift Zone (ORZ) of northern Botswana is ideal to investigate the role of mantle plumes in rift initiation and development, as well as the interaction between the upper and lower mantle. The ORZ developed within the Neoproterozoic Damara belt between the Congo Craton to the northwest and the Kalahari Craton to the southeast. Mantle structure and thermal status beneath the ORZ are poorly known, mostly due to a complete paucity of broad-band seismic stations in the area. As a component of an interdisciplinary project funded by the United States National Science Foundation, a broad-band seismic array was deployed over a 2-yr period between mid-2012 and mid-2014 along a profile 756 km in length. Using P-to-S receiver functions (RFs) recorded by the stations, the 410 and 660 km discontinuities bordering the mantle transition zone (MTZ) are imaged for the first time. When a standard Earth model is used for the stacking of RFs, the apparent depths of both discontinuities beneath the Kalahari Craton are about 15 km shallower than those beneath the Congo Craton. Using teleseismic P- and S-wave traveltime residuals obtained by this study and lithospheric thickness estimated by previous studies, we conclude that the apparent shallowing is the result of a 100-150 km difference in the thickness of the lithosphere between the two cratons. Relative to the adjacent tectonically stable areas, no significant anomalies in the depth of the MTZ discontinuities or in teleseismic P- and S-wave traveltime residuals are

  17. Mechanical response of the south flank of kilauea volcano, hawaii, to intrusive events along the rift systems

    USGS Publications Warehouse

    Dvorak, J.J.; Okamura, A.T.; English, T.T.; Koyanagi, R.Y.; Nakata, J.S.; Sako, M.K.; Tanigawa, W.T.; Yamashita, K.M.

    1986-01-01

    Increased earthquake activity and compression of the south flank of Kilauea volcano, Hawaii, have been recognized by previous investigators to accompany rift intrusions. We further detail the temporal and spatial changes in earthquake rates and ground strain along the south flank induced by six major rift intrusions which occurred between December 1971 and January 1981. The seismic response of the south flank to individual rift intrusions is immediate; the increased rate of earthquake activity lasts from 1 to 4 weeks. Horizontal strain measurements indicate that compression of the south flank usually accompanies rift intrusions and eruptions. Emplacement of an intrusion at a depth greater than about 4 km, such as the June 1982 southwest rift intrusion, however, results in a slight extension of the subaerial portion of the south flank. Horizontal strain measurements along the south flank are used to locate the January 1983 east-rift intrusion, which resulted in eruptive activity. The intrusion is modeled as a vertical rectangular sheet with constant displacement perpendicular to the plane of the sheet. This model suggests that the intrusive body that compressed the south flank in January 1983 extended from the surface to about 2.4 km depth, and was aligned along a strike of N66??E. The intrusion is approximately 11 km in length, extended beyond the January 1983 eruptive fissures, which are 8 km in length and is contained within the 14-km-long region of shallow rift earthquakes. ?? 1986.

  18. Impact of rheological layering on rift asymmetry

    NASA Astrophysics Data System (ADS)

    Jaquet, Yoann; Schmalholz, Stefan M.; Duretz, Thibault

    2015-04-01

    Although numerous models of rift formation have been proposed, what triggers asymmetry of rifted margins remains unclear. Parametrized material softening is often employed to induce asymmetric fault patterns in numerical models. Here, we use thermo-mechanical finite element models that allow softening via thermal weakening. We investigate the importance of lithosphere rheology and mechanical layering on rift morphology. The numerical code is based on the MILAMIN solver and uses the Triangle mesh generator. Our model configuration consists of a visco-elasto-platic layered lithosphere comprising either (1) only one brittle-ductile transition (in the mantle) or (2) three brittle-ductile transitions (one in the upper crust, one in the lower crust and one in the mantle). We perform then two sets of simulations characterized by low and high extensional strain rates (5*10-15 s-1, 2*10-14 s-1). The results show that the extension of a lithosphere comprising only one brittle-ductile transition produces a symmetric 'neck' type rift. The upper and lower crusts are thinned until the lithospheric mantle is exhumed to the seafloor. A lithosphere containing three brittle-ductile transitions favors strain localization. Shear zones at different horizontal locations and generated in the brittle levels of the lithosphere get connected by the weak ductile layers. The results suggest that rheological layering of the lithosphere can be a reason for the generation of asymmetric rifting and subsequent rift morphology.

  19. 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.

  20. Evidences of a lithospheric fault zone in the Sicily Channel continental rift (southern Italy) from instrumental seismicity data

    NASA Astrophysics Data System (ADS)

    Calò, M.; Parisi, L.

    2014-10-01

    Sicily Channel is a portion of Mediterranean Sea, between Sicily (Southern Italy) and Tunisia, representing a part of the foreland Apennine-Maghrebian thrust belt. The seismicity of the region is commonly associated with the normal faulting related to the rifting process and volcanic activity of the region. However, certain seismic patterns suggest the existence of some mechanism coexisting with the rifting process. In this work, we present the results of a statistical analysis of the instrumental seismicity and a reliable relocalization of the events recorded in the last 30 yr in the Sicily Channel and western Sicily using the Double Difference method and 3-D Vp and Vs tomographic models. Our procedure allows us to discern the seismic regime of the Sicily sea from the Tyrrhenian one and to describe the main features of an active fault zone in the study area that could not be related to the rifting process. We report that most of the events are highly clustered in the region between 12.5°-13.5°E and 35.5°-37°N with hypocentral depth of 5-40 km, and reaching 70 km depth in the southernmost sector. The alignment of the seismic clusters, the distribution of volcanic and geothermal regions and the location of some large events occurred in the last century suggest the existence of a subvertical shear zone extending for least 250 km and oriented approximately NNE-SSW. The spatial distribution of the seismic moment suggests that this transfer fault zone is seismically discontinuous showing large seismic gaps in proximity of the Ferdinandea Island, and Graham and Nameless Bank.

  1. Benue trough and the mid-African rift system

    SciTech Connect

    Thomas, D.

    1996-01-29

    Large areas of the Anambra and Gongola basins have distinct petroleum exploration problems: a geologically persistent high geothermal gradient that promoted Cretaceous source rock maturation into the gas phase very early on; intrusive lead-zinc mineralization veins attributed to the Senonian igneous and folding event; and meteoric water-flushing along the periphery of the basins. From preliminary analysis, these basins have to be considered high risk for the discovery of commercial oil accumulations. On the other hand, the petroleum potential of the Bornu basins seems favorable. This Nigerian northernmost rift basin continues into the Kanem basin of western Chad, which has proven oil accumulations in Coniacian deltaic sands. Cretaceous paleofacies is considered to be relatively continuous throughout both basins. Paleo-geothermal history is also considered to be similar, although some igneous activity is recorded in the Bornu basin (Senonian?). There is a very real possibility of kerogen-rich non-marine basal Albo-Aptian basin fill lacustrine source rocks, as found in the Doba basin, could be present in the deepest sections of the Nigerian rift basins. Due to the depths involved, no well is expected to penetrate the incipient graben-fill stage sequences; however, possible oil migration from these tectono-stratigraphic units would certainly enhance the petroleum potential of cooler sections of the rift system. As opposed to interpreted thermogenic gas which seems to be prevalent in the Anambra basin.

  2. A continental rift model for the La Grande greenstone belt

    NASA Technical Reports Server (NTRS)

    Skulski, T.; Hynes, A.; Liu, M.; Francis, D.; Rivard, B.; Stamatelopoulou-Seymour, K.

    1986-01-01

    Stratigraphic relationships and the geochemistry of volcanic rocks contrain the nature and timing of the tectonic and magmatic processes in the pre-deformational history of the La Grande greenstone belt in the Superior Province of north-central Quebec. The lowermost supracrustals in this belt are obscured by syntectonic granitoid intrusives. The supracrustal succession in the western part of the belt consists of a lower sequence of immature clastic sediments and mafic volcanoclastics, overlain by pillowed and massive basalts. Further east, along tectonic strike, a lower sequence of mafic volcanoclastics and immature clastic sediments is overlain by a thick sequence of pillowed and massive basalts, and resedimented coarse clastic sediments and banded iron formation. These are overlain by assive basaltic andesites, andesites and intermediate volcanoclastics intercalated with immature clastic sediments. In contrast, in the eastern part of the belt lenses of felsic volcanics and volcanoclastics occur at the base of the succession and pillowed and massive basalts are overlain by komatiites at the top. The La Grande greenstone belt can be explained as the product of continental rifting. The restricted occurence of komatiites, and eastwardly directed paleocurrents in clastic sediments in the central part of the belt are consistent with rifting commencing in the east and propagating westward with time. The increase in depth of emplacement and deposition with time of the lower three units in the central part of the belt reflects deposition in a subsiding basin. These supracrustal rocks are believed to represent the initial rift succession.

  3. Erosion of Terrestrial Rift Flank Topography: A Quantitative Study

    NASA Technical Reports Server (NTRS)

    Weissel, Jeffrey K.

    1999-01-01

    Many rifted or passive continental margins feature a seaward-facing erosional escarpment which abruptly demarcates deeply weathered, low relief, interior uplands from a deeply incised, high relief coastal zone. It is generally accepted that these escarpments originate at the time of continental rifting and propagate inland through the elevated rift flank topography at rates on the order of 1 km/Myr over the course of a margin's history. Considering the length of passive margins worldwide and an average rift<