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1

Rifting in the Proterozoic  

NASA Astrophysics Data System (ADS)

Proterozoic rifts at various stages of extension have been involved in different types of endogenous activity. As a result, the following evolutionary gradation of riftogenous structures is recorded: Aulacogens—continental paleorifts that are not affected by metamorphism and granite formation; intracontinental ensialic folded systems which have originated from continental rifts of higher permeability that passed through monocyclic intrageosyncline evolution including granite formation and metamorphism (Gudsonides, Kibarides, Timanides, etc.); intracontinental ensimatic folded systems which have originated from rifts where spreading has terminated (Red Sea type), and which passed through intracontinental monocyclic evolution (Hoggar-Atacorides, Sayano-Enisey system, etc.), and marginal folded belts which formed along ancient continental margins and which underwent polycyclic orthogeosyncline evolution of the accretionary type (Arabian-Nubian shield, etc.). Proterozoic riftogenous structures are closely related in their evolution with zones of tectonothermal reworking of ancient basement (Mozambique belt, Stanovoy belt, etc.) which are domains of dispersed rifting. During the first half of the Proterozoic, only intracontinental riftogenous structures existed. Real oceanic openings and plate tectonic events did not occur before the Middle Riphean. This reflects the evolution of rifting as well as tectonic processes as a whole. Proterozoic riftogenous folded structures and zones of basement reworking predetermined the location of rifts and oceans of Mesozoic and Cenozoic age, e.g., the Stanovoy area (Mesozoic rifts), the Baikalides (Baikal rift system), the Mozambique belt (East African rift system), the Arabian-Nubian area (Red Sea rift), and the Protoatlantic folded system within Gondwanaland (South Atlantic Ocean).

Bozhko, N. A.

1987-11-01

2

Volcanism at rifts  

SciTech Connect

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.

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

1989-07-01

3

Intracontinental Rifts As Glorious Failures  

NASA Astrophysics Data System (ADS)

Rifts: "Elongate depressions overlying places where the lithosphere has ruptured in extension" develop in many environments because rocks are weak in extension (Sengor 2nd edn. Springer Encycl. Solid Earth Geophys.). I focus on intra-continental rifts in which the Wilson Cycle failed to develop but in which that failure has led to glory because rocks and structures in those rifts throw exceptional light on how Earth's complex continental evolution can operate: The best studied record of human evolution is in the East African Rift; The Ventersdorp rifts (2.7 Ga) have yielded superb crustal-scale rift seismic reflection records; "Upside-down drainage" (Sleep 1997) has guided supra-plume-head partial melt into older continental rifts leading Deccan basalt of ~66Ma to erupt into a Late Paleozoic (~ 300Ma) rift and the CAMP basalts of ~201 Ma into Ladinian, ~230 Ma, rifts. Nepheline syenites and carbonatites, which are abundant in rifts that overlie sutures in the underlying mantle lithosphere, form by decompression melting of deformed nepheline syenites and carbonatites ornamenting those sutures (Burke et al.2003). Folding, faulting and igneous episodes involving decompression melting in old rifts can relate to collision at a remote plate margin (Guiraud and Bosworth 1997, Dewey and Burke 1974) or to passage of the rift over a plume generation zone (PGZ Burke et al.2008) on the Core Mantle Boundary (e.g.Lake Ellen MI kimberlites at ~206 Ma).

Burke, K.

2012-12-01

4

Lake Superior Rift basins  

NASA Astrophysics Data System (ADS)

Sedimentary basins of late Precambrian age have been identified beneath Lake Superior using seismic reflection profiles leased by Argonne National Laboratory, Argonne, Ill., from Grant Norpac, Inc. [McGinnis et al., 1989]. These data, along with 650 km of Great Lakes International Multidisciplinary Program for Crustal Evolution (GLIMPCE) profiles [Behrendt et al., 1988], are being used to develop an understanding of failed rift processes, from initial plate separation, through basin evolution, to final quiescence.

McGinnis, L. D.

5

East African Rift Valley, Kenya  

NASA Technical Reports Server (NTRS)

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.

1990-01-01

6

The midcontinent rift system  

E-print Network

FAULT EDGE OF ARCHEAt~ PAL~OZOIC MINN a2 o 200 W|S KAN MO oHIO Fiaure 3 Generalized geologic map showing major features of the prerift Precambrian basement and principal geologic units associated with the midcontinent rift system. Random "=" represent... of the crust is greatly reduced. The gravity and magnetic anomalies of the MGA are correlative in a general sense--that is, major maxima and minima are spatially coincident (Figures 1, 2); however, in detail the anomalies are commonly not correlated. Gravity...

Van Schmus, W. R.; Hinze, W. J.

1985-01-01

7

Frameworks de desarrollo de Aplicaciones -Spring  

E-print Network

. Acceso a datos apuntes traspas ejercicios plantillas 3. Spring MVC apuntes traspas ejercicios plantillas 4. Aplicaciones AJAX y REST con Spring MVC apuntes traspas ejercicios Frameworks de desarrollo de;5. Validaci�n e internacionalizaci�n en Spring MVC apuntes traspas ejercicios plantillas 6. Acceso remoto

Escolano, Francisco

8

Aplicaciones del láser de diodo en Odontología  

Microsoft Academic Search

Resumen: El láser de diodo tiene numerosas aplicaciones en la especialidad de Cirugía Bucal siendo utilizado preferentemente para realizar intervenciones quirúr- gicas sobre los tejidos blandos siempre que no impliquen un excesivo sangrado. En Endodoncia, Implantología Bucofacial y Periodoncia se emplea por su importante efecto bactericida. También se utiliza en procedimientos de blanqueamiento denta- rio. Es importante controlar adecuadamente el

Antonio Jesús

2005-01-01

9

Relationships between pre-rift structure and rift architecture in Lakes Tanganyika and Malawi, East Africa  

Microsoft Academic Search

Continental rift systems are rips in plates caused by focusing of extensional stresses along some zone. In the same way that tensile cracks in the side of a brick building generally follow the mortar between bricks, rifts initially follow the weakest pathways in the pre-rift materials. There has even been a suggestion that the occurrence of rifts is controlled by

J. Versfelt; B. R. Rosendahl

1989-01-01

10

Inversion tectonics during continental rifting: The Turkana Cenozoic rifted zone, northern Kenya  

E-print Network

Inversion tectonics during continental rifting: The Turkana Cenozoic rifted zone, northern Kenya B: The Turkana Cenozoic rifted zone, northern Kenya, Tectonics, 24, TC2002, doi:10.1029/2004TC001637. 1 tectonics occurs also quite widely in continental rifts, and areas of inversion in the reference Cenozoic

Brest, Université de

11

The East African rift system  

NASA Astrophysics Data System (ADS)

This overview paper considers the East African rift system (EARS) as an intra-continental ridge system, comprising an axial rift. It describes the structural organization in three branches, the overall morphology, lithospheric cross-sections, the morphotectonics, the main tectonic features—with emphasis on the tension fractures—and volcanism in its relationships with the tectonics. The most characteristic features in the EARS are narrow elongate zones of thinned continental lithosphere related to asthenospheric intrusions in the upper mantle. This hidden part of the rift structure is expressed on the surface by thermal uplift of the rift shoulders. The graben valleys and basins are organized over a major failure in the lithospheric mantle, and in the crust comprise a major border fault, linked in depth to a low angle detachment fault, inducing asymmetric roll-over pattern, eventually accompanied by smaller normal faulting and tilted blocks. Considering the kinematics, divergent movements caused the continent to split along lines of preexisting lithospheric weaknesses marked by ancient tectonic patterns that focus the extensional strain. The hypothesis favored here is SE-ward relative divergent drifting of a not yet well individualized Somalian plate, a model in agreement with the existence of NW-striking transform and transfer zones. The East African rift system comprises a unique succession of graben basins linked and segmented by intracontinental transform, transfer and accommodation zones. In an attempt to make a point on the rift system evolution through time and space, it is clear that the role of plume impacts is determinant. The main phenomenon is formation of domes related to plume effect, weakening the lithosphere and, long after, failure inducing focused upper mantle thinning, asthenospheric intrusion and related thermal uplift of shoulders. The plume that had formed first at around 30 Ma was not in the Afar but likely in Lake Tana region (Ethiopia), its almost 1000 km diameter panache weakening the lithosphere and preparing the later first rifting episode along a preexisting weak zone, a Pan-African suture zone bordering the future Afar region. From the Afar, the rift propagated afterward from north to south on the whole, with steps of local lithospheric failure nucleations along preexisting weak zones. These predisposed lines are mainly suture zones, in which partial activation of low angle detachment faults reworked former thrust faults verging in opposite directions, belonging to double verging ancient belts. This is responsible for eventual reversal in rift asymmetry from one basin to the next. Supposing the plume migrated southward, or other plumes emplaced, the rift could propagate following former weaknesses, even outside areas influenced by plumes. This view of rift formation reconciles the classical models: active plume effect triggered the first ruptures; passive propagations of failure along lithospheric scale weak zones were responsible for the onset of the main rift segments. Various other aspects are shortly considered, such as tectonics and sedimentation, and relationships of the 'cradle of Mankind' with human evolution. By its size, structure and occurrence of oceanic lithosphere in the Afar, the EARS can be taken as a model of the prelude of oceanic opening inside a continent.

Chorowicz, Jean

2005-10-01

12

Cenozoic rift formation in the northern Caribbean  

NASA Technical Reports Server (NTRS)

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.

Mann, P.; Burke, K.

1984-01-01

13

Parga Chasma: Coronae and Rifting on Venus  

NASA Technical Reports Server (NTRS)

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].

Smrekar, S. E.; Stofan, E. R.; Buck, W. R.; Martin, P.

2005-01-01

14

Heat flow in the Kenya rift zone  

NASA Astrophysics Data System (ADS)

An understanding of the processes of continental rifting is fundamental to understanding the evolution of the continents. Considerable evidence exists to suggest that continental rift zones are associated with high heat flow and elevated lithospheric geotherms, but direct heat-flow measurements from young rifts do not clearly define surface heat-flow anomalies associated with deep-seated thermal processes in these rifts. The first detailed compilation of heat-flow data from the Neogene Kenya rift is presented here. Heat-flow data are presented from traditional heat-flow determinations in water drill-holes, from bottom-hole-temperature measurements in oil wells, and from heat-flow estimates from groundwater silica data. These data define generally low heat flow on the flanks of the Kenya rift, with high, but variable heat flow on the rift floor. There is a spatial association among high heat-flow values, Quaternary volcanism and faulting, and hydrothermal manifestations on the rift floor. We interpret these results to suggest that any deep-seated thermal anomaly associated with the Kenya rift has not yet been conducted to the surface. The high heat-flow values are interpreted to result from heat advected into the axial rift zone with local redistribution of this heat by hydrothermal convection. Normal to moderately high heat flow was measured in eastern Kenya between the rift zone and the coast. The regional heat flow in eastern Kenya is interpreted to be normal, with local shallow modification by groundwater flow eastward from the Kenya dome. These interpretations support a model of relatively young evolution of the asthenospheric anomaly beneath the Kenya rift zone, with the age of heating of the mantle at the Mono no older than about 10 Ma.

Wheildon, J.; Morgan, Paul; Williamson, K. H.; Evans, T. R.; Swanberg, C. A.

1994-09-01

15

The geothermal fields of the Kenya rift  

NASA Astrophysics Data System (ADS)

From the standpoint of geothermal energy, Kenya's resources are due to the presence of the Kenya rift which is part of the East African rift system. Geological, geophysical and geothermal studies indicate that Neogene volcanic activity has led to the presence of near surface heat generating sources. Geothermal fields of the Kenya rift occur in two types of environments. The main geothermal fields are associated with Quaternary volcanoes. The second type is associated with fissures that are related to active fault zones. In either case, these fields are dissected by numerous rift faults that give rise to a number of geothermal springs and fumaroles.

Riaroh, Don; Okoth, William

1994-09-01

16

Rift Valley fever in Namibia, 2010.  

PubMed

During May-July 2010 in Namibia, outbreaks of Rift Valley fever were reported to the National Veterinary Service. Analysis of animal specimens confirmed virus circulation on 7 farms. Molecular characterization showed that all outbreaks were caused by a strain of Rift Valley fever virus closely related to virus strains responsible for outbreaks in South Africa during 2009-2010. PMID:24274469

Monaco, Federica; Pinoni, Chiara; Cosseddu, Gian Mario; Khaiseb, Siegfried; Calistri, Paolo; Molini, Umberto; Bishi, Alec; Conte, Annamaria; Scacchia, Massimo; Lelli, Rossella

2013-12-01

17

Cenozoic rift tectonics of the Japan Sea  

SciTech Connect

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.

Kimura, K.

1988-08-01

18

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

SciTech Connect

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.

Sanford, A.; Balch, R. [New Mexico Inst. of Mining and Technology, Socorro, NM (United States); House, L.; Hartse, H. [Los Alamos National Lab., NM (United States)

1995-12-01

19

Internet y sus aplicaciones al sector turstico E. Furio -M. Alonso Matilde Alonso Prez  

E-print Network

Internet y sus aplicaciones al sector turístico E. Furio -M. Alonso - 1 - Matilde Alonso Pérez-00119439,version1-24Dec2006 #12;Internet y sus aplicaciones al sector turístico E. Furio -M. Alonso - 2 - Resumen: El objetivo de este trabajo es analizar las aplicaciones que Internet tiene en el sector

Paris-Sud XI, Université de

20

The 1974 Ethiopian rift geodimeter survey  

NASA Technical Reports Server (NTRS)

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.

Mohr, P.

1977-01-01

21

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

NASA Astrophysics Data System (ADS)

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.

Kinabo, Baraka Damas

22

Drilling to Resolve the Evolution of the Corinth Rift  

NASA Astrophysics Data System (ADS)

The initiation and evolution of continental rifting, ultimately leading to rifted margin and ocean basin formation, are major unanswered questions in solid Earth-plate tectonics. Many previous insights have come from mature rifted margins where activity has ceased or from computer models. The Gulf of Corinth Rift in central Greece presents an ideal laboratory for the study of young, highly active rifting that complements other rift zones (e.g., the East African and Gulf of California rifts). Exposure and preservation of syn-rift stratigraphy, high rates of extension, and an existing network of offshore seismic data offer a unique opportunity to constrain the rift history and basin development at exceptionally high resolution in the Gulf of Corinth.

McNeill, Lisa; Sakellariou, Dimitris; Nixon, Casey

2014-05-01

23

Erosion and rift dynamics: new thermomechanical aspects of post-rift evolution of extensional basins  

NASA Astrophysics Data System (ADS)

In this study, we investigate thermo-mechanical consequences of erosion of rift shoulders. Conventional models imply post-rift cooling that results in subsidence and strengthening of the lithosphere. Existing models neglect geodynamic consequences of surface processes (erosion and sedimentation). According to sedimentologic and geomorphologic data, surface processes modify the topography and thickness of sedimentary infill at rates comparable with the rates of the tectonic uplift/subsidence (few 0.1 mm/y). Consequently, a coupling between the surface and tectonic processes can be expected. An increase of the sedimentary load leads to localised inelastic weakening of the lithosphere. At the same time, erosional unloading of rift shoulders leads to local strengthening and flexural rebound. Subsidence of the rift "neck" (strongest layer of the thinned lithosphere) and uplift of the rift shoulders create pressure gradients sufficient to drive ductile flow in the low-viscosity lower crust. This flow, directed outward from the centre of the basin might facilitate uplift of the rift shoulders. It may even drive some post-rift "extension". In the limiting case of slow erosion and sedimentation rates, gravitational stresses can reverse the flow, resulting in a retardation of basin subsidence rate, homogenisation of the crustal thickness, accelerated collapse of the shoulders and in some post-rift "compression". These effects significantly change predictions of basin evolution inferred from the conventional back-stripping models.

Burov, E.; Cloetingh, S.

1997-07-01

24

Three-dimensional laboratory modelling of rifting: application to the Baikal Rift, Russia  

E-print Network

-branch configuration of the Baikal rift system only by introducing a weak zone in the area of Lake Baikal. Such a zone to the tension direction. In nature and in the model, rifting starts in the central branch corresponding to Lake Baikal. The modelling also predicts the formation of a fourth oblique f NS-trending branch to the south

Déverchère, Jacques

25

Helium isotope ratios in Ethiopian Rift basalts  

NASA Astrophysics Data System (ADS)

Helium isotope ratios were measured in olivine and pyroxene phenocrysts from basalts of the Ethiopian Rift Valley and Afar Depression between 6° and 15°N and 37° and 43°E. 3He/ 4He ratios range from 6 to 17 times the atmospheric value (R A = 1.4 × 10 -6), that is, from ratios less than typical MORB (depleted mantle) helium (R/R A= 8 ± 1) to ratios similar to high- 3He hotspots and to the Yellowstone hotspot (R/R A= 16.5). The high 3He/ 4He ratios occur all along the Ethiopian Rift and well up into the Afar Depression, with a maximum value of 17.0 R A at 8°N in the Rift Axis and a high value of 14.2 R A in the central Tat'Ali sector of the Afar Depression. The ratios decrease to MORB-like values near the edge of the Red Sea, and to sub-MORB ratios (5-6 R A) at the northern end of the Rift (Zula Peninsula) and at the southern end, at lakes Abaya and Chamo. The Ethiopian Rift provides the only continental hotspot terrain in which helium isotope ratios can be compared in detail between volcanic lavas and associated geothermal and volcanic gases, a primary motivation for this work. Comparison with our previously measured ratios in fluids and gases (range 2-15 R A) shows excellent agreement in the areas sampled for both lavas and fluids, and indicates that high-temperature volcanic fluids can be used for establishing helium isotope signatures in such terrains. The high- 3He values in both fluids and basalts show that a Primitive Mantle (PM) component is required and that a Lower Mantle High- 3He plume is strongly involved as a driving force in the rifting process of the East African Rift System.

Scarsi, P.; Craig, H.

1996-11-01

26

Crustal tomographic imaging of a transitional continental rift: the Ethiopian rift  

NASA Astrophysics Data System (ADS)

In this study we image crustal structure beneath a magmatic continental rift to understand the interplay between crustal stretching and magmatism during the late stages of continental rifting: the Main Ethiopian Rift (MER). The northern sector of this region marks the transition from continental rifting in the East African Rift to incipient seafloor spreading in the southern Red Sea and western Gulf of Aden. Our local tomographic inversion exploits 172 broad-band instruments covering an area of 250 × 350 km of the rift and adjacent plateaux. The instruments recorded a total of 2139 local earthquakes over a 16-month period. Several synthetic tests show that resolution is good between 12 and 25 km depth (below sea level), but some horizontal velocity smearing is evident along the axis of the Main Ethiopian Rift below 16 km. We present a 3-D P-wave velocity model of the mid-crust and present the first 3-D Vp/Vs model of the region. Our models show high P-wave velocities (6.5 km s-1) beneath the axis of the rift at a depth of 12-25 km. The presence of high Vp/Vs ratios (1.81-1.84) at the same depth range suggest that they are cooled mafic intrusions. The high Vp/Vs values, along with other geophysical evidence, suggest that dyking is pervasive beneath the axis of the rift from the mid-crustal depths to the surface and that some portion of partial melt may exist at lower crustal depths. Although the crustal stretching factor across the Main Ethiopian Rift is ~1.7, our results indicate that magma intrusion in narrow zones accommodates a large proportion of extensional strain, with similarities to slow-spreading mid-ocean ridge processes.

Daly, E.; Keir, D.; Ebinger, C. J.; Stuart, G. W.; Bastow, I. D.; Ayele, A.

2008-03-01

27

Off rift and on rift volcanism along the southern most extremity of the Reykjanes Ridge.  

NASA Astrophysics Data System (ADS)

In August-September 2013 R/V Marcus G Langseth conducted a geophysical survey of the southern Reykjanes Ridge and flanks to the Bight transform fault including the first orthogonally spreading segment to the south. The objectives were to better understand how the Reykjanes Ridge replaced the earlier transform fault-dominated structure. The survey acquired full-coverage multibeam bathymetry of some 90,000 km2 and acoustic backscatter imagery and coincident gravity and magnetic profiles. The Rift axis of the RR is defined by a rift valley, striking 36° NE, and deepens from N to S towards the Bight transform fault. Volcanism along the rift axis is characterized by en-echelon volcanic ridges striking 14°NE and rising some 400-1000 m above the valley floor, single circular volcanic sea mounts 400-600 m high, lava flow sheets and craters. Fissures and faults are not very prominent with in the rift valley. However, at both sides bounding the rift valley, fissure, faults and uplifting of the crust is a dominant feature. Surprisingly numerous volcanic edifices are observed on the faulted crust drifting away from the plate boundary. Further these volcanic edifices do not all show any faulting and have cone shape forms, indicating more explosive activity than within the rift. The volcanic edifices range in size from 2-3 km at the base to some hundreds of meters. Backscatter analysis shows that in general the volcanic edifices have higher values than the surrounding basement. These vents are observed as far as 100 km from the rifting center. High backscatter along with little or no faulting indicates that these off rift volcanic vents are younger than the basement they are resting on, thus manifesting that volcanism is not solely confined to the active rift boundary in the area. The segment south of Bight transform fault is highly dotted by these off rift volcanic vents

Hoskuldsson, Armann; Martinez, Fernando; Hey, Richard

2014-05-01

28

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

NASA Astrophysics Data System (ADS)

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.

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

2014-04-01

29

Earthquake clusters in Corinth Rift  

NASA Astrophysics Data System (ADS)

Clusters commonly occur as main shock-aftershock (MS-AS) sequences but also as earthquake swarms, which are empirically defined as an increase in seismicity rate above the background rate without a clear triggering main shock earthquake. Earthquake swarms occur in a variety of different environments and might have a diversity of origins, characterized by a high b-value in their magnitude distribution. The Corinth Rift, which was selected as our target area, appears to be the most recent extensional structure, with a likely rate of fault slip of about 1cm/yr and opening of 7mm/yr. High seismic activity accommodates the active deformation with frequent strong (M?6.0) events and several seismic excitations without a main shock with clearly discriminative magnitude. Identification of earthquake clusters that occurred in this area in last years and investigation of their spatio-temporal distribution is attempted, with the application of known declustering algorithms, aiming to associate their occurrence with certain patterns in seismicity behavior. The earthquake catalog of the National Hellenic Seismological Network is used, and a certain number of clusters were extracted from the dataset, with the MS-AS sequences being distinguished from earthquake swarms. Spatio-temporal properties of each subset were analyzed in detail, after determining the respective completeness magnitude. This work was supported in part by the THALES Program of the Ministry of Education of Greece and the European Union in the framework of the project entitled "Integrated understanding of Seismicity, using innovative Methodologies of Fracture mechanics along with Earthquake and non-extensive statistical physics - Application to the geodynamic system of the Hellenic Arc, SEISMO FEAR HELLARC".

Mesimeri, Maria; Papadimitriou, Eleftheria; Karakostas, Vasilios; Tsaklidis, George

2013-04-01

30

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

NASA Astrophysics Data System (ADS)

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

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

2011-02-01

31

Rifting Attractor Structures in the Baikal Rift System: Location and Effects  

NASA Astrophysics Data System (ADS)

The current geodynamics and tectonophysics of the Baikal rift system (BRS) as recorded in lithospheric stress and strain are discussed in the context of self organization of nonlinear dissipative dynamic systems and nonlinear media. The regional strain field inferred from instrumental seismic moment and fault radius data for almost 70,000 MLH ? 2.0 events of 1968 through 1994 shows a complex pattern with zones of high strain anisotropy in the central part and both flanks of the rift system (the South Baikal, Hovsgöl, and Muya rift basins, respectively). The three zones of local strain anisotropy highs coincide with domains of predominantly vertical stress where earthquakes of different magnitudes are mostly of normal slip geometry. Pulse-like reversals of principal stresses in the high-strain domains appear to be nonlinear responses of the system to subcrustal processes. In this respect, the BRS lithosphere is interpreted in terms of the self organization theory as a geological dissipative system. Correspondingly, the domains of high strain anisotropy and stress change, called rifting attractor structures (RAS), are the driving forces of its evolution. The location and nonlinear dynamics of the rifting attractors have controlled lithospheric stress and strain of the rift system over the period of observations, and the same scenario may have been valid also in the Mesozoic-Cenozoic rifting history. The suggested model of a positive-feedback (fire-like) evolution of nonlinear dynamical systems with rifting attractors opens a new perspective on the current geodynamics and tectonophysics of the Baikal rift system.

Klyuchevskii, Anatoly V.

2014-07-01

32

Geophysical studies of the West Antarctic Rift System  

Microsoft Academic Search

The West Antarctic rift system extends over a 3000 × 750 km, largely ice covered area from the Ross Sea to the base of the Antarctic Peninsula, comparable in area to the Basin and Range and the East African rift system. A spectacular rift shoulder scarp along which peaks reach 4–5 km maximum elevation marks one flank and extends from

J. C. Behrendt; W. E. LeMasurier; A. K. Cooper; F. Tessensohn; A. Tréhu; D. Damaske

1991-01-01

33

A finite amplitude necking model of rifting in brittle lithosphere  

NASA Astrophysics Data System (ADS)

We formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic-plastic layer overlying a fluid substrate. A perfectly plastic rheology is a continuum description of faulting in rift zones. Two important aspects of rift evolution are illustrated by this model: the evolution of the rift width as extension proceeds and the finite strain that occurs. A region at yield initially develops with a width determined by the thickness of the brittle layer, and the internal deformation within this yield zone is proportional to the topographic slope. As extension proceeds, the surface within the rift subsides, and the width of the subsiding yield zone decreases. At any stage of rifting, material in regions just outside the yield zone is deformed but no longer deforming. The width of these deformed regions increases with increasing extension. Vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone, creating flanking uplifts. The external force required to maintain active rifting increases with the amount of lithospheric stretching, indicating that rifting is a quasi-static, stable process. Because the yield zone will revert to elastic behavior if the external force causing extension is removed, the model predicts that the rift depression and flanking uplifts will be preserved after extension stops. Our simple mechanical model demonstrates the inherent relationship among graben formation, lithospheric thinning, and rift shoulder uplift in rift zones.

Lin, Jian; Parmentier, E. M.

1990-04-01

34

A Multidisciplinary Investigation of Rio Grande Rift Deformation  

Microsoft Academic Search

The Rio Grande rift is the easternmost actively deforming province of the western margin of North America. Geologic observations suggest the character of rifting changes from north to south, with a narrow rift marked by linear topographic depressions in Colorado and northern New Mexico grading to a broad ``basin and range'' expression in south-central New Mexico. Reasons behind the variable

A. R. Lowry; A. F. Sheehan; M. Roy; E. Jones; S. Nerem

2005-01-01

35

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

SciTech Connect

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.

Winn, R.D. Jr.; Steinmetz, J.C. (Marathon Oil Co., Littleton, CO (United States)); Kerekgyarto, W.L. (Marathon Oil Co., Houston, TX (United States))

1993-11-01

36

Longitudinal evolution of Suez rift structure, Egypt  

SciTech Connect

A three-dimensional study of the structure of the Suez Rift has been carried out using field and subsurface data in an attempt to determine the role of transverse faults and the longitudinal evolution of the rift. As in most intracontinental rifts, the structure of the Gulf of Suez area is governed by normal faults and tilted blocks, whose crests constitute the main target of exploratory wells. The fault pattern consists of two major sets of trends: (1) longitudinal faults parallel with the rift axis and created in an extensional regime, trending east-northeast-west-southwest, and (2) transverse faults with north-south to north-northeast-south-southwest dominant trend. The transverse faults are inherited passive discontinuities, whereas most of the longitudinal faults were created during the Neogene in a purely extensional regime. Both sets were simultaneously active, producing a zigzag pattern and rhombic-shaped blocks. The transverse faults can show horizontal strike-slip components and act as relays between major normal faults.

Colletta, B.; Le Quellec, P.; Letouzey, J.; Moretti, I.

1988-01-01

37

Late Paleozoic Rifting in northern Pakistan  

NASA Astrophysics Data System (ADS)

Metasedimentary rocks exposed in the eastern Peshawar basin and the southern Swat region of northern Pakistan provide evidence for late Paleozoic continental rifting. The onset of extensional tectonics in the Early Carboniferous is indicated by north derived clasts in the Jafar Kandao Formation eroded from thermally induced uplifts of parts of the formerly passive margin of Gondwana. Rift highlands were eroded until they were inundated during the Middle Carboniferous. Renewed uplift accompanied the eruption of basaltic lava flows during the Early Permian. Uplift along south dipping, northeast striking normal faults during the Carboniferous was accompanied by alkaline magmatism represented by the Shewa-Shahbazgarhi and Warsak porphyries and Koga syenite. Geochemistry of basaltic flows (now amphibolites) and intrusions associated with Permian uplift is similar to the coeval Panjal volcanics of northwestern India and indicates rift zone magmatism. Postrifting thermal subsidence led to the deposition of Upper Triassic marine carbonate rocks which unconformably overlie the rift basalts. A similar tectonic history in central Afghanistan suggests continuity between the two regions prior to the opening of the Neo-Tethys.

Pogue, Kevin R.; Dipietro, Joseph A.; Khan, Said Rahim; Hughes, Scott S.; Dilles, John H.; Lawrence, Robert D.

1992-08-01

38

Rift Valley Fever, Mayotte, 2007-2008  

PubMed Central

After the 2006–2007 epidemic wave of Rift Valley fever (RVF) in East Africa and its circulation in the Comoros, laboratory case-finding of RVF was conducted in Mayotte from September 2007 through May 2008. Ten recent human RVF cases were detected, which confirms the indigenous transmission of RFV virus in Mayotte. PMID:19331733

Giry, Claude; Gabrie, Philippe; Tarantola, Arnaud; Pettinelli, Francois; Collet, Louis; D'Ortenzio, Eric; Renault, Philippe; Pierre, Vincent

2009-01-01

39

Submarine Thermal Springs on the Galapagos Rift  

Microsoft Academic Search

The submarine hydrothermal activity on and near the Galapagos 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

John B. Corliss; Jack Dymond; Louis I. Gordon; John M. Edmond; Richard P. von Herzen; Robert D. Ballard; Kenneth Green; David Williams; Arnold Bainbridge; Kathy Crane; Tjeerd H. van Andel

1979-01-01

40

Tertiary arc rifting in northern Luzon, Philippines  

Microsoft Academic Search

The North Luzon terrane (NLT), comprising the section of Luzon north of the Philippine Fault, is one of the largest arc terranes in the Philippine Archipelago. Numerous features suggest that the NLT is a late Oligocene to early Miocene analogue for the processes in the modern intra-arc rift zone at the northern end of the Mariana Trough. First, the NLT

Federico F. Florendo

1994-01-01

41

How do volcanic rift zones relate to flank instability? Evidence from collapsing rifts at Etna  

NASA Astrophysics Data System (ADS)

Volcanic rift zones, characterized by repeated dike emplacements, are expected to delimit the upper portion of unstable flanks at basaltic edifices. We use nearly two decades of InSAR observations excluding wintertime acquisitions, to analyze the relationships between rift zones, dike emplacement and flank instability at Etna. The results highlight a general eastward shift of the volcano summit, including the northeast and south rifts. This steady-state eastward movement (1-2 cm/yr) is interrupted or even reversed during transient dike injections. Detailed analysis of the northeast rift shows that only during phases of dike injection, as in 2002, does the rift transiently becomes the upper border of the unstable flank. The flank's steady-state eastward movement is inferred to result from the interplay between magmatic activity, asymmetric topographic unbuttressing, and east-dipping detachment geometry at its base. This study documents the first evidence of steady-state volcano rift instability interrupted by transient dike injection at basaltic edifices.

Ruch, Joel; Pepe, Susi; Casu, Francesco; Acocella, Valerio; Neri, Marco; Solaro, Giuseppe; Sansosti, Eugenio

2012-10-01

42

Structural inheritance, segmentation, and rift localization in the Gulf of Aden oblique rift  

NASA Astrophysics Data System (ADS)

The structural evolution of the Gulf of Aden passive margins was controlled by its oblique divergence kinematics, inherited structures, and the Afar hot spot. The rifting between Arabia and Somalia started at 35 Ma just before the hot spot paroxysm (at 30Ma) and lasted until 18Ma, when oceanic spreading started. Fieldwork suggests that rift parallel normal faults initiated in the (future) distal margins, after a first stage of distributed rifting, and witness the rift localization, as confirmed by 4-layer analogue models. These faults arise either from crust or lithosphere scale buoyancy forces that are strongly controlled by the mantle temperature under the influence of the Afar hot spot. This implies a transition from a distributed mode to a localized one, sharper, both in space and time, in the West (close to the hot spot) than in the East (far away from the hot spot). In this framework, first order transform F.Z. are here (re-) defined by the fact that they deform continental crust. In the Gulf of Aden, as well as in other continental margins, it appears that these F.Z. are often, if not always, located at continental transfer or "transform" fault zones. Our detailed field-study of an offshore transfer fault zone in the southeastern Gulf of Aden (Socotra Island) shows that these structures are long-lived since early rifting until post rift times. During the early rifting, they are inherited structures reactivated as oblique normal faults before accommodating strike-slip motion. During the Ocean-Continent Transition (OCT) formation ("post syn-rift" times), a significant uplift occurred in the transfer fault zone footwall as shown by stratigraphic and LT thermochronology data. Second order transform F.Z. are defined as deforming only the OCT, thus initiated at the moment of its formation. In the western Gulf of Aden, the hot spot provoked a rift localization strongly oblique to the divergence and, as a consequence, several second order transform F.Z. formed (as well as third order ones that initiated after the onset of oceanic spreading). In the East, the second and third order segmentation is less pronounced as both the OCT and ridge segments are sub-perpendicular to the divergence. During post-rift times, plate reorganization led to oceanic propagator development and second/third transform F.Z. migration along with deformation and vertical movements and normal displacement along the first order transform F.Z. Finally, during Quaternary times, the evolving boundary conditions of the Arabian plate probably also induced vertical movements along the margins.

Bellahsen, Nicolas; Leroy, Sylvie; Autin, Julia; d'Acremont, Elia; Razin, Philippe; Husson, Laurent; Pik, Raphael; Watremez, Louise; Baurion, Celine; Beslier, Marie-Odile; Khanbari, Khaled; Ahmed, Abdulhakim

2013-04-01

43

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

NASA Astrophysics Data System (ADS)

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.

Meyer, R.; Hertogen, J.

2008-12-01

44

Seismic anisotropy and mantle flow beneath the Baikal rift zone  

NASA Astrophysics Data System (ADS)

SEISMIC studies have shown that continental rifts such as Lake Baikal and the Great Rift Valley of East Africa are like mid-ocean rifts in that they lie above broad regions of asthenospheric upvvarp of much greater extent than the surface expression of rifting1-4. The direction of mantle flow in such regions can be investigated using the seismic anisotropy created by flow-induced orientation of mantle olivine crystals5-8. Seismic studies of the Mid-Atlantic Ridge have revealed upwelling mantle flow beneath the ridge and flow normal to the ridge axis on either side8-10. Here we present results from an array of seismic stations across the Baikal rift zone in southern Siberia. The splitting in arrival times of SKS seismic waves indicates that the upper mantle beneath the rift zone is anisotropic, with the fast direction (which reflects the direction of mantle flow) being horizontal and normal to the rift axis. This suggests that the broad upwarp associated with this continental rift is caused by similar mantle flow to that at mid- ocean rifts. This may help to elucidate the processes involved in continental rifting.

Gao, S.; Davis, P. M.; Liu, H.; Slack, P. D.; Zorin, Yu. A.; Mordvinova, V. V.; Kozhevnikov, V. M.; Meyer, R. P.

1994-09-01

45

Evidence for a Nascent Rift in South Sudan: Westward Extension of the East African Rift System?  

NASA Astrophysics Data System (ADS)

Joint inversion of seismic and gravity data of eastern Africa reveals a low seismic wave velocity arm stretching from the southern Main Ethiopian rift westward in an east-west direction that has not been noticed in earlier work. The zone of low velocities is located in the upper mantle and is not overlain by a known structural rift expression. We analyzed the local pattern of seismicity and the stresses in the African plate to interpret this low velocity arm. The zone of low velocities is located within the Central African Fold Belt, which dissects the northern and southern portions of the African continent. It is seismically active with small to intermediate sized earthquakes occurring in the crust. Seven earthquake solutions indicate (oblique) normal faulting and low-angle normal faulting with a NS to NNW-SSE opening direction, as well as strike-slip faulting. This pattern of deformation is typically associated with rifting. The present day stress field in northeastern Africa reveals a tensional state of stress at the location of the low velocity arm with an opening direction that corresponds to the earthquake data. We propose that the South Sudan low velocity zone and seismic center are part of an undeveloped, nascent rift arm. The arm stretches from the East African Rift system westward.

Maceira, M.; Van Wijk, J. W.; Coblentz, D. D.; Modrak, R. T.

2013-12-01

46

The Pathogenesis of Rift Valley Fever  

PubMed Central

Rift Valley fever (RVF) is an emerging zoonotic disease distributed in sub-Saharan African countries and the Arabian Peninsula. The disease is caused by the Rift Valley fever virus (RVFV) of the family Bunyaviridae and the genus Phlebovirus. The virus is transmitted by mosquitoes, and virus replication in domestic ruminant results in high rates of mortality and abortion. RVFV infection in humans usually causes a self-limiting, acute and febrile illness; however, a small number of cases progress to neurological disorders, partial or complete blindness, hemorrhagic fever, or thrombosis. This review describes the pathology of RVF in human patients and several animal models, and summarizes the role of viral virulence factors and host factors that affect RVFV pathogenesis. PMID:21666766

Ikegami, Tetsuro; Makino, Shinji

2011-01-01

47

The pathogenesis of Rift Valley fever.  

PubMed

Rift Valley fever (RVF) is an emerging zoonotic disease distributed in sub-Saharan African countries and the Arabian Peninsula. The disease is caused by the Rift Valley fever virus (RVFV) of the family Bunyaviridae and the genus Phlebovirus. The virus is transmitted by mosquitoes, and virus replication in domestic ruminant results in high rates of mortality and abortion. RVFV infection in humans usually causes a self-limiting, acute and febrile illness; however, a small number of cases progress to neurological disorders, partial or complete blindness, hemorrhagic fever, or thrombosis. This review describes the pathology of RVF in human patients and several animal models, and summarizes the role of viral virulence factors and host factors that affect RVFV pathogenesis. PMID:21666766

Ikegami, Tetsuro; Makino, Shinji

2011-05-01

48

Rifted Margins: Building Blocks of Later Collision  

Microsoft Academic Search

\\u000a Rifted margins develop by the splitting apart of continents and form the trailing edges of ocean basins, to be incorporated\\u000a into collision zones when those basins eventually close. Magma-poor margins are dominated by tectonic processes, including\\u000a crustal extension and thinning, mantle serpentinization and the unroofing of broad expanses of lithospheric mantle. Magma\\u000a rich margins may follow a similar history until

T. Reston; G. Manatschal

49

Proterozoic Midcontinent Rift System, an overview  

SciTech Connect

The Middle and Late Proterozoic Midcontinent Rift System (MRS) extends across the middle US, from Lake Superior through Wisconsin, Minnesota, Iowa and Nebraska into Kansas on the southwest limb and across upper and lower Michigan on the southeast limb. Exploration for oil and gas generated over 7,000 miles of seismic, a leasehold of near seven million acres, but only three test wells. The initial extension of the MRS was marked by filling with layered basalt. Thickness documented by GLIMPCE suggests crustal separation was nearly achieved. The thick dense basalts and thinned pre-rift crust provide high amplitude gravity anomalies which characterize the rift trend. Extension slowed and eventually ceased, creating a sag phase during which clastic sediments were deposited, including sapropelic shale and siltstone, fluvial sandstones and siltstones, and fluvial/alluvial conglomerates. Tectonic inversion to compressional and transpressional forces occurred late in rift history, possibly during part of the period of clastic fill. The MRS trend is highly segmented, with varied tectonic styles, suggesting complex stress systems in its development. The Nonesuch Formation is marine or lacustrine siltstone and shale containing sufficient organic matter to be an effective source rock for oil and gas. Similar facies have been identified along the extent of the western limb, in the subsurface in Minnesota, Iowa and Kansas. TOC values are as high as 3% and maturity ranges from peak oil to advanced. Surface seeps, fluid inclusions, mud log shows and modeling indicate the potential for multiple episodes of generation. Potential reservoir rocks have been identified and seals are present as lacustrine and fluvial shales and possible evaporites. The MRS remains a relatively unexplored frontier hydrocarbon province with giant field potential in the heart of North America.

Kerr, S.D.; Landon, S.M.

1992-01-01

50

The South China sea margins: Implications for rifting contrasts  

Microsoft Academic Search

Implications regarding spatially complex continental rifting, crustal extension, and the subsequent evolution to seafloor spreading are re-examined for the northern and southern-rifted margins of the South China Sea. Previous seismic studies have shown dramatic differences in the present-day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south China. Although the total crustal

Dennis E. Hayes; Susan S. Nissen

2005-01-01

51

Anomalous deep earthquakes beneath the East African Rift: evidence for rift induced delamination of the lithosphere?  

NASA Astrophysics Data System (ADS)

The over 5000 m high Rwenzori Mountains are situated within the western branch of the East African Rift System, at the border between Uganda and the Democratic Republic of Congo. They represent a basement block within the rift valley whose origin and relation to the evolution of the EARS are highly puzzling. During 2006/2007 a network of 27 seismological stations was operated in this area to investigate crustal and upper mantle structure in conjunction with local seismicity. The data analysis revealed unexpectedly high microseismic activity. On average more than 800 events per month could be located with magnitudes ranging from 0.5 to 5.1. Hypocentral depths go as deep as 30 km with a pronounced concentration of activity at a depth of about 15 km. This presentation focuses on a cluster of seven earthquakes that were located at anomalous depths between 53 and 60 km. According to our present knowledge these are the deepest events so far observed within the EARS and the African Plate. Their origin might be connected to magmatic intrusions. However, the existence of earthquakes at this depth is enigmatic, especially within a rifting regime were one expects hot and weak material close to the surface, which is not capable of seismogenic deformation. We think that these events are closely related to the evolution of the Rwenzoris. A recent hypothesis to explain the extreme uplift of the Rwenzori Mountains is rift induced delamination (RID) of mantle lithosphere that is captured between two approaching rift segments. By numerical modelling we show that the RID-process is also able to bring material that is cold and brittle enough to release seismic energy into greater depth. Therefore the RID-mechanism gives a consistent explanation for the detected deep events as well as for the uplift of a mountain block in a rift setting.

Lindenfeld, Michael; Rümpker, Georg; Schmeling, Harro; Wallner, Herbert

2010-05-01

52

Tertiary arc rifting in northern Luzon, Philippines  

NASA Astrophysics Data System (ADS)

The North Luzon terrane (NLT), comprising the section of Luzon north of the Philippine Fault, is one of the largest arc terranes in the Philippine Archipelago. Numerous features suggest that the NLT is a late Oligocene to early Miocene analogue for the processes in the modern intra-arc rift zone at the northern end of the Mariana Trough. First, the NLT has bifurcating magmatic arcs sharing similar magmatic histories. These include the Northern Sierra Madre (NSM) and Cordillera Central (CC) magmatic arcs, which are separated by the Cagayan basin but which are linked in the Caraballo Range to the south. The rock record indicates that the NSM, CC, and Caraballo Ranges were active arcs in late Eocene to late Oligocene time. Second, seismic reflection and well data indicate that the Cagayan basin formed by extensional faulting in late Oligocene to early Miocene time. Third, alkalic arc magmatism, recognized to be a precursor of intra-arc rifting in modern settings, occurred at the juncture of the NSM and CC arcs in late Oligocene to early Miocene time. Fourth, oceanic crust, represented by the Itogon ophiolite, formed at the southwestern end of the Cagayan basin in late Oligocene to early Miocene time. Major and trace element chemistry show that the Itogon sheeted dikes have tholeiitic arc and backarc basin basalt affinities. The rock record and geophysical offshore data suggest that the NLT was developing in an island arc system above the subducting West Philippine plate in late Eocene time. Rifting occurred in the island arc from late Oligocene to early Miocene time but did not mature into backarc spreading, most likely because of the collision of the Benham Rise, a basaltic rise in the West Philippine basin, with the NLT. The arc rifting in the NLT may be another manifestation of the extensional tectonism that affected most of Southeast Asia in late Oligocene to early Miocene time, during which the South China and Southeast Sulu basins formed. Subsequent to arc rifting, the history of the NLT has been linked to the subduction of the South China plate along the Manila Trench. The structural history of the Cagayan basin and magmatic history of the southern CC suggest that the subduction in the Manila Trench at the latitude of the NLT began about 15 Ma.

Florendo, Federico F.

1994-06-01

53

Lake Baikal - A Touchstone for Global Change and Rift Studies  

NSDL National Science Digital Library

This is a United States Geological Survey (USGS) fact sheet about the Lake Baikal rift system. This site provides a good general overview of this rift system, illustrating its importance to the overall study of plate tectonics. The Lake Baikal rift system is a modern analogue for formation of ancient Atlantic-type continental margins. It tells us the first chapter in the story of how continents separate and ultimately develop into ocean basins like the Atlantic Ocean. Continental rifting is an important component of plate tectonics theory.

54

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

NASA Astrophysics Data System (ADS)

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

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

2007-05-01

55

Controls on rifting in Africa and the regional tectonic model for the Nigeria and East Niger rift basins  

NASA Astrophysics Data System (ADS)

Since early Mesozoic times, three phases of rifting have occurred in Africa and are related to distinct phases of breakup of Gondwana. These contrasting rift episodes have provided an insight to the extent to which plate tectonic processes and more localised mechanical anisotropy processes within the African lithosphere have influenced rifting. Gravity modelling of the Nigeria and East Niger rift basins shows the extent and nature of the broad (regional) positive Bouguer anomaly associated with these rifts. The removal of this regional anomaly allows the delineation of the (residual) negative Bouguer anomaly which reflects the lateral extent and thickness of Mesozoic and Cenozoic sediments. This residual anomaly is interpreted using iterative three dimensional modelling, by incorporating a density-depth function derived from well logs. Results indicate that an extensive basin complex exists in eastern Niger with sedimentary sections greater than 7 km in depth and are in good agreement with the aeromagnetic data. A simple estimate of crustal extension across the East Niger rift basin indicates that up to 58 km of crustal stretching has occurred, placing an upper limit on the amount of sinistral strike-slip movement required within the Benue Trough to open the East Niger rift. A similar strike-slip and extension rift geometry is observed elsewhere in the West and Central African rift system which indicates that the Cretaceous period was an important time for strike-slip tectonics. Changes in the differential opening of the Central and South Atlantic Oceans during the Cretaceous can adequately explain the large strike-slip displacements and associated rift basins in West and North Central Africa and are considered here to more closely reflect the initial rift processes that shaped the continental margin of Africa than those associated with the modern East African rift system.

Fairhead, J. D.; Green, C. M.

56

Rifting and breakup in the South China Sea  

NASA Astrophysics Data System (ADS)

The magma-poor or intermediate magmatic South China Sea is a natural laboratory for studying rifting and breakup. The basin shows an irregular triangular shape with a SW pointing apex, which manifests a preceding propagating rift. The earliest phase of rifting started in the Early Paleocene when a Mesozoic convergent margin changed to extension. After about 30 Million years of rifting, breakup in the major eastern subbasin of the SCS occurred in the Early Oligocene but rifting continued and subsequent breakup of the southwest subbasin took place in the Late Oligocene. The wide Early Cenozoic South China Sea rift preserves the initial rift architecture at the distal margins. Seismic reflection data imaging conjugate crustal sections at the South China Sea margins result in a conceptual model for rift-evolution at conjugate magma-poor margins in time and space. Most distinct are regular undulations in the crust-mantle boundary. Individual rift basins are bounded to crustal blocks by listric normal faults on either side. Moho uplifts are distinct beneath major rift basins, while the Moho is downbended beneath crustal blocks, with a wavelength of undulations in the crust-mantle boundary that approximately equals the thickness of the continental crust. Most of the basin-bounding faults sole out within the middle crust. At the distal margins, detachment faults are located at a mid-crustal level where a weak zone decouples crust and mantle lithosphere during rifting. The lower crust in contrast is interpreted as being strong. Only in the region within about 50 km from the continent-ocean transition (COT) we suggest that normal faults reach the mantle, enabling potentially a coupling between the crust and the mantle. Here, at the proximal margins detachment fault dip either seaward or landward. This may indicate the presence of exhumed mantle bordering the continental margins. Post-rift shallow-water platform carbonates indicate a delay in subsidence during rifting in the South China Sea. We propose that this is an inherent process in highly-extended continental margins and a common origin may be the influx of warm asthenospheric material into initially cool sub-lithospheric mantle. On a crustal-scale largely symmetric process predominate in the initial rifting stage. At the future COT either of the rift basin bounding faults subsequently penetrates the entire crust, resulting in asymmetry at this location. However, asymmetric deformation which is controlled by large scale detachment faulting is confined to narrow areas and does not result in a margin-wide simple-shear model. Rather considerable along-margin variations are suggested resulting in alternating "upper and lower plate" margins.

Franke, Dieter; Savva, Dimitri; Pubellier, Manuel; Steuer, Stephan; Mouly, Benoit; Auxietre, Jean-Luc; Meresse, Florian; Chamot-Rooke, Nicolas

2014-05-01

57

Geophysical glimpses into the Ferrigno Rift at the northwestern tip of the West Antarctic Rift System  

NASA Astrophysics Data System (ADS)

The West Antarctic Rift System (WARS) forms one of the largest continental rift systems on Earth. The WARS is of major significance as it forms the lithospheric cradle for the marine-based and potentially unstable West Antarctic Ice Sheet (WAIS). Seismic refraction, reflection, aeromagnetic, gravity and drilling in the Ross Sea have revealed most of what we know about its structure, tectonic and magmatic patterns and sedimentary basins. Aerogeophysical research and passive seismic networks have considerably extended our knowledge of the WARS and its influence on the overlying WAIS in the Siple Coast and Amundsen Sea Embayment (ASE) regions. The Bellingshausen Sea Embayment region has however remained largely unexplored, and hence the possible extent of the WARS in this sector has remained poorly constrained. Here we use a combination of reconnaissance ground-based and airborne radar observations, airborne gravity, satellite gravity and aeromagnetic data to investigate the WARS in the Bellingshausen Sea Embayment, in the area of the Ferrigno Ice Stream (Bingham et al., 2012, Nature). This region is of high significance, as it one of the main sectors of the WAIS that is currently exhibiting rapid ice loss, thought to be driven primarily by oceanic warming. Assessing geological controls on subice topography and ice dynamics is therefore of prime importance in this part of the WAIS. Ground-based and airborne radar image a subglacial basin beneath the Ferrigno Ice Stream that is up to 1.5 kilometres deep and that connects the ice-sheet interior to the Bellingshausen Sea margin. We interpret this basin as a narrow, glacially overdeepened rift basin that formed at the northwestern tip of the WARS. Satellite gravity data cannot resolve such a narrow rift basin but indicate that the crust beneath the region is likely thinned, lending support to the hypothesis that this area is indeed part of the WARS. Widely-spaced aeromagnetic data image a linear low along the inferred Ferrigno rift, but provide no evidence for high-amplitude aeromagnetic anomalies, typically associated with Cenozoic magmatism within the WARS. However, the reconnaissance character of these data, do not enable us to rule out the presence of magmatism within this part of the rift and cannot disclose the potential greater variability in subglacial geology either. Bingham et al. (2012) proposed the glacially overdeepened Ferrigno rift basins provided major controls for a palaeo-ice stream on the adjacent continental shelf during glacial maxima. The palaeo-ice stream, in turn, eroded the 'Belgica' trough, which today routes warm open ocean water back to the ice front to potentially reinforce dynamic thinning. Dynamic thinning in the Bellingshausen Sea region appears to be steered back to the ice-sheet interior along the Ferrigno rift system. We conclude that detailed aerogeophysical studies of the inferred rift basins that cut across the WAIS margin in the Bellingshausen Sea sector are a high priority to: a) better comprehend the structure and the tectono-magmatic evolution of the WARS and; b) to test the hypothesis that these rifts play a key role in rapidly transmitting oceanic-driven change inland, potentially promoting accelerated ice-sheet instability.

Bingham, Robert; Ferraccioli, Fausto

2014-05-01

58

Exploring the Environment: Rift Valley Fever  

NSDL National Science Digital Library

Students will study the nature and transmission of the Rift Valley fever virus to determine ways to prevent an outbreak. This is one of several interdisciplinary problem-based learning modules for high school and middle school students addressing real-world problems in environmental earth science. Teacher pages include module notes, software information, and a teacher-to-teacher message board to share/borrow ideas for planning, facilitating, and assessing information. Situations and very clear instructions are given on how to implement problem-based learning in the classroom.

University, Classrooms O.; Program, Nasa L.; University, Wheeling J.

2005-01-01

59

Volcanic highlands in the South Atlantic rift zone  

NASA Astrophysics Data System (ADS)

The morphostructure of the segment between the Cardno and St. Helen transform fracture zones is studied in the rift zone of the South Atlantic slow-spreading mid-oceanic ridge (SAMOR). It was found that it is atypical of similar ridges because of the absence of an evolved rift valley. The rift zone in the transverse section is a cupola with flat slopes, whose surface is divided by volcanic massifs, plateau-like valleys, and unclear ridges and valleys. The entire morphostructure (a cupola-like regional pedestal and the listed relief forms of the second order) indicates its volcanic origin, and the rift zone in this segment is a volcanic high-land. This conclusion is supported by seismic and magnetic data. Because other (not all) SAMOR segments contain the rift valley, the results of this study indicate alternation of the tectonic and magmatic morphostructures along the entire rift zone and identification of its scales is the most important task of the morphostructural study of the SAMOR rift zone. Determination of geodynamic regimes on the basis of the results of morphostructural studies of the rift zone will arise from the solution of this task.

Il'in, A. V.

2014-04-01

60

Dynamics of rifting and modes of extension on icy satellites  

Microsoft Academic Search

A simple numerical model of extension in icy satellite shells is developed. Thinning of the ice weakens the shell, promoting further extension. If lateral flow in the lower part of the shell is unimportant, extension is opposed and wide rifts are generated; if lateral flow is rapid, localized extension is favored and narrow rifts are produced. Thick shells or high

F. Nimmo

2004-01-01

61

Tectonic maps of the Ethiopian rift system, and an apology  

Microsoft Academic Search

The Ethiopian rift system, inclusive of Afar, has become of very great interest to volcanologists and structural geologists, more particularly since the advent of plate tectonic theory. As is well-known, this theory predicts t hat the floor of Afar must be underlain by some form of neo-oceanic crust, where the Red Sea, Gulf of Aden and A frican rift systems

P. A. Mohr

1972-01-01

62

PROPOSAL TITLE Evolution of Rift and Rifted Margin Sedimentary Basins: Numerical Investigation of Tectonics, Sedimentation, and Salt-  

E-print Network

of Tectonics, Sedimentation, and Salt- Related Structures of the Atlantic Canada Margin and Elsewhere will focus on: crustal deformation during rifting and rift reactivation; salt tectonics induced by thin of salt and the development of finite amplitude salt structures. 3) The role of differential sedimentary

Beaumont, Christopher

63

Extension in the Rio Grande rift.  

USGS Publications Warehouse

A positive gravity anomaly along the axis of the Rio Grande rift reflects a volume of anomalous mass added at the base of the crust and intruded into the crust. Part of this volume can be associated with vertical uplift of the crust. The remainder of this anomalous volume, plus the volume of surficial graben fill, can be associated with horizontal crustal extension. The volume of crustal uplift in the Rio Grande rift is unknown, but this term can be eliminated by means of an independent equation provided by assumption of generalized isostatic equilibrium. The volume and mass equations combined provide a solution for extension of the crust in terms of the following parameters: total anomalous mass deficiency in the mantle lithosphere, total anomalous mass excess in the crust and its density contrast, total anomalous mass deficiency of surficial graben fill and its density contrast, and the volume of material eroded from the uplift. Using standard density estimates and masses determined by equivalent-source modeling of gravity profiles, I obtained 1-km extension at 37oN (Colorado-New Mexico border), 13- km extension at 35oN (Albuquerque, New Mexico), and 24-km extension at 33oN in S New Mexico.-Author

Cordell, L.

1982-01-01

64

Strain accommodation by slow slip and dyking in a youthful continental rift, East Africa  

Microsoft Academic Search

Continental rifts begin and develop through repeated episodes of faulting and magmatism, but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented. In highly evolved rifts, tensile stresses from far-field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions. These rifting crises are rarely observed in thick

Eric Calais; Nicolas D'Oreye; Julie Albaric; Anne Deschamps; Damien Delvaux; Jacques Déverchère; Cynthia Ebinger; Richard W. Ferdinand; François Kervyn; Athanas S. Macheyeki; Anneleen Oyen; Julie Perrot; Elifuraha Saria; Benoît Smets; D. Sarah Stamps; Christelle Wauthier

2008-01-01

65

Evolution, distribution, and characteristics of rifting in southern Ethiopia  

NASA Astrophysics Data System (ADS)

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.

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

2014-04-01

66

Magma-compensated crustal thinning in continental rift zones.  

PubMed

Continental rift zones are long, narrow tectonic depressions in the Earth's surface where the entire lithosphere has been modified in extension. Rifting can eventually lead to rupture of the continental lithosphere and creation of new oceanic lithosphere or, alternatively, lead to formation of wide sedimentary basins around failed rift zones. Conventional models of rift zones include three characteristic features: surface manifestation as an elongated topographic trough, Moho shallowing due to crustal thinning, and reduced seismic velocity in the uppermost mantle due to decompression melting or heating from the Earth's interior. Here we demonstrate that only the surface manifestation is observed at the Baikal rift zone, whereas the crustal and mantle characteristics can be ruled out by a new seismic profile across southern Lake Baikal in Siberia. Instead we observe a localized zone in the lower crust which has exceptionally high seismic velocity and is highly reflective. We suggest that the expected Moho uplift was compensated by magmatic intrusion into the lower crust, producing the observed high-velocity zone. This finding demonstrates a previously unknown role for magmatism in rifting processes with significant implications for estimation of stretching factors and modelling of sedimentary basins around failed rift structures. PMID:19212408

Thybo, H; Nielsen, C A

2009-02-12

67

Composition of the crust beneath the Kenya rift  

USGS Publications Warehouse

We infer the composition of the crust beneath and on the flanks of the Kenya rift based on a comparison of the KRISP-90 crustal velocity structure with laboratory measurements of compressional-wave velocities of rock samples from Kenya. The rock samples studied, which are representative of the major lithologies exposed in Kenya, include volcanic tuffs and flows (primarily basalts and phonolites), and felsic to intermediate composition gneisses. This comparison indicates that the upper crust (5-12 km depth) consists primarily of quartzo-feldspathic gneisses and schists similar to rocks exposed on the flanks of the rift, whereas the middle crust (12-22 km depth) consists of more mafic, hornblende-rich metamorphic rocks, probably intruded by mafic rocks beneath the rift axis. The lower crust on the flanks of the rift may consist of mafic granulite facies rocks. Along the rift axis, the lower crust varies in thickness from 9 km in the southern rift to only 2-3 km in the north, and has a seismic velocity substantially higher than the samples investigated in this study. The lower crust of the rift probably consists of a crust/mantle mix of high-grade metamorphic rocks, mafic intrusives, and an igneous mafic residuum accreted to the base of the crust during differentiation of a melt derived from the upper mantle. ?? 1994.

Mooney, W.D.; Christensen, N.I.

1994-01-01

68

Tectonic Framework of the Kachchh Rift Basin  

NASA Astrophysics Data System (ADS)

Evaluation of available geological data has allowed us to determine the tectonic framework of the Kachchh rift basin (KRB), the host to the 1819 Kachchh (MW 7.8), 1956 Anjar ( M 6.0) and the recent January 26, 2001 Bhachau (MW 7.6) earthquakes. The ~ 500 km x 200 km east-west trending KRB was formed during the Mesozoic following the break-up of Gondwanaland. It is bounded to the north and south by the Nagar Parkar and Kathiawar faults which separate it from the Precambrian granitic rocks of the Indian craton. The eastern border is the Radanpur-Barmer arch (defined by an elongate belt of gravity highs) which separates it from the early Cretaceous Cambay rift basin. KRB extends ~ 150 km offshore to its western boundary, the continental shelf. Following India's collision with Eurasia, starting ~ 50 MY ago, there was a stress reversal, from an extensional to the (currently N-S) compressional regime. Various geological observations attest to continuous tectonic activity within the KRB. Mesozoic sediments were uplifted and folded and then intruded by Deccan trap basalt flows in late Cretaceous. Other evidence of continuous tectonic activity include seismically induced soft sediment deformation features in the Upper Jurassic Katrol formation on the Kachchh Mainland and in the Holocene sequences in the Great Rann. Pleistocene faulting in the fluvial sequence along the Mahi River (in the bordering Cambay rift) and minor uplift during late Quaternary at Nal Sarovar, prehistoric and historic seismicity associated with surface deformation further attest to ongoing tectonic activity. KRB has responded to N-S compressional stress regime by the formation of east-west trending folds associated with Allah Bund, Kachchh Mainland, Banni, Vigodi, Katrol Hills and Wagad faults. The Allah Bund, Katrol Hill and Kachchh Mainland faults were associated with the 1819, 1956 and 2001 earthquakes. Northeast trending Median High, Bhuj fault and Rajkot-Lathi lineament cut across the east-west trending faults. This tectonic framework suggests that KRB provides a local weak zone in the otherwise stable continental Indian craton and the observed seismicity occurs in response to the N-S compression.

Talwani, P.; Gangopadhyay, A. K.

2001-05-01

69

Rifting Processes and Regional Sections across the South China Sea  

NASA Astrophysics Data System (ADS)

The South China Sea developed from magma-poor rifting to subsequent seafloor spreading in the Paleogene. With dimensions of more than 1000x1000 km attenuated continental crust it is close to the Basin and Range province in the United States. The area is particularly well suited for studying the transition from rifting to seafloor spreading because the marginal basin is relatively young and thus it likely preserves differences in subsidence and thermal history resulting from rifting. After only 10 to 15 Ma of seafloor spreading the margins are still close enough to each other to allow detailed conjugate studies from onshore to offshore. We address the widely discussed questions about the role and the evolution of detachment faults in combination with the nature and behavior of the lower crust. The key points which are debated are the fact that rifting continues after the formation of first oceanic crust, the role of volcanism in the rifting process and the fact that extreme crustal thinning occurred under shallow marine or even sub-aerial conditions. The classical representation of magma-poor margins showing pre-, syn- and postrift sediment architecture over uniformly stretched continental crust is modified to acknowledge the occurrence of high-angle listric faults related to fault-bounded rift basins and the presence of extremely thinned crust close to the transition to oceanic crust. A décollement zone is interpreted between the brittle upper crust and the ductile lower crust. The models need to take into account the long-lasting Paleogene rifting period during which continental or very shallow marine conditions persisted. In this presentation, conjugate pairs of rifted margins are compared regarding symmetries in architecture, synrift infill and vertical position during the rifting process.

Pubellier, M.; Franke, D.; Meresse, F.; Savva, D.; Steuer, S.; Auxietre, J.; Aurelio, M.; Chan, L. S.; Wong, P.

2012-12-01

70

Guidebook to Rio Grande rift in New Mexico  

USGS Publications Warehouse

Discusses the details of geologic features along the rift zone. Included are short papers on topics relative to the overall region. These papers and the road logs are of special interest to any one pursuing further study of the rift. This book is a comprehensive guide to the middle and late Cenozoic geology of the Rio Grande region of Colorado and New Mexico. Though initially used on field trips for the International Symposium on Tectonics and Magmatism of the Rio Grande rift, the guidebook will be useful to anyone interested in the Cenozoic history of the 600-mi-long area extending from central Colorado to El Paso, Texas.

Compiled by Hawley, J. W.

1978-01-01

71

Regional magnetic anomaly constraints on continental rifting  

NASA Technical Reports Server (NTRS)

Radially polarized MAGSAT anomalies of North and South America, Europe, Africa, India, Australia and Antarctica demonstrate remarkably detailed correlation of regional magnetic lithospheric sources across rifted margins when plotted on a reconstruction of Pangea. These major magnetic features apparently preserve their integrity until a superimposed metamorphoric event alters the magnitude and pattern of the anomalies. The longevity of continental scale magnetic anomalies contrasts markedly with that of regional gravity anomalies which tend to reflect predominantly isostatic adjustments associated with neo-tectonism. First observed as a result of NASA's magnetic satellite programs, these anomalies provide new and fundamental constraints on the geologic evolution and dynamics of the continents and oceans. Accordingly, satellite magnetic observations provide a further tool for investigating continental drift to compliment other lines of evidence in paleoclimatology, paleontology, paleomagnetism, and studies of the radiometric ages and geometric fit of the continents.

Vonfrese, R. R. B.; Hinze, W. J.; Olivier, R.; Bentley, C. R.

1985-01-01

72

Rift Valley Fever, Sudan, 2007 and 2010  

PubMed Central

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

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

2013-01-01

73

Middle Miocene to Pleistocene sedimentary record of rift evolution in the southern Albert Rift (Uganda)  

NASA Astrophysics Data System (ADS)

This study presents an almost complete Middle Miocene to Pleistocene sequence of synrift sediments in the western branch of the East African Rift. The studied succession is exposed in several patches on an eastward tilted block between the northern tip of the Rwenzori Block and the eastern shoulder of the Albert Rift. In this position, it reaches a maximum thickness of 600 m of which 350 m have been logged systematically by analysing lithofacies and sediment architecture. Stratigraphic subdivision of the succession relies on published biostratigraphic data of endemic mollusc associations and their correlation across East Africa. The synrift sediments encountered are siliciclastics ranging from clay to coarse gravel with gypsum and ferrugineous interlayers or impregnations. Lithofacies and architectural analysis indicate alluvial plain, delta plain, nearshore, delta front, or lacustrine depositional environments. Based on the vertical stacking pattern, prograding and retrograding trends of the depositional environments, and climatic indicators (e.g. conservation of feldspar, gypsum, and/or iron hydroxide precipitation), four evolutionary phases can be distinguished: (i) a first phase between ca. 14.5 and 10.0 Ma is characterised by bedload-dominated fluvial environment with massive sandy to gravelly bedforms, feldspar-rich sands, rare iron impregnations and relatively low accommodation space. This phase is interpreted as pre- and early synrift sedimentation under a semiarid climate. (ii) From ca. 10.0 to 4.5 Ma predominantly fine-grained siliciclastics were deposited in a distal fluvial plain to lacustrine setting characterised by limited accommodation space. Fluctuation of thin beds, dominance of clay and frequent iron impregnations point to a more humid climate with seasonality and weak tectonic activity. (iii) During the third phase between 4.5 and 2.0 Ma delta plain and nearshore deposits with frequent ferrugineous impregnations and rich mollusc associations occurred, indicating a humid period with lake-level highstands and accelerated subsidence. (iv) During the final sedimentary interval between 2.0 and 1.5 Ma gravel units reoccurred with less iron- but more carbonate and gypsum impregnations, and arkosic sandstones. This phase recorded a general aridisation trend most probably caused by the upcoming rain barrier of the Rwenzori Mountains together with accelerated rift-flank uplift and strong subsidence of the rift floor. The results of this study are of particular importance for delineating key controls on sedimentation in the Albert Rift.

Roller, S.; Hornung, J.; Hinderer, M.; Ssemmanda, I.

2010-10-01

74

Structural development of Sumisu Rift, Izu-Bonin Arc  

NASA Astrophysics Data System (ADS)

Geophysical swath mapping, multichannel seismic profiling, and ocean drilling data are used to document the structural evolution of Sumisu Rift and to analyze the pattern of strain resulting from extension of an intraoceanic island arc. The ˜120-km-long, 30-50-km-wide Sumisu Rift is bounded to the north and south by structural and volcanic highs west of the Sumisu and Torishima calderas and longitudinally by curvilinear border fault zones with both convex and concave dip slopes. The zig-zag pattern of normal faults (average strikes 337° and 355°) indicates extension oriented 076°±10°, orthogonal to the volcanic arc. Three oblique transfer zones divide the rift along strike into four segments with different fault trends and uplift/subsidence patterns. Differential strain across the transfer zones is accommodated by interdigitating, rift-parallel faults and sometimes by cross-rift volcanism, rather than by strike- or oblique-slip faults. From estimates of extension (2-5 km), the age of the rift (˜2 Ma), and the accelerating subsidence, we infer that Sumisu Rift is in the early synrift stage of back arc basin formation. Following an early sag phase, half graben formed with synthetically faulted, structural rollovers facing large-offset (2-2.5 km throw) border fault zones. In the three northern rift segments the largest faults are on the arc side and dip 60°-75°W, whereas in the southern segment they are on the west side and dip 25°-50°E. The present "full graben" stage is dominated by hanging wall antithetic faulting, basin widening by footwall collapse, and a concentration of subsidence in an inner rift. The hanging wall collapses, but not necessarily as a result of border fault propagation from adjacent rift segments. Whereas the border faults may penetrate the Theologically weak lithosphere (Te ? 3 km), many of the hanging wall and footwall collapse structures are detached only a few kilometers below the seafloor. Back arc volcanism, usually erupted along faults, occurs in the rift and along the protoremnant arc during both stages. Where drilled, the arc margin has been uplifted 1.1±0.5 km concurrently with ˜1.1 km of rift basin subsidence. Extremely high sedimentation rates, up to 6 m/kyr in the inner rift, have kept pace with synrift faulting, created a smooth basin floor, and resulted in sediment thicknesses that mimic the differential basin subsidence. A linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting. Rifts are better developed between the arc edifices; intrusions may be accommodating extensional strain adjacent to the arc volcanoes. No obvious correlations are observed between the rift structures and preexisting cross-arc trends.

Taylor, Brian; Klaus, Adam; Brown, Glenn R.; Moore, Gregory F.; Okamura, Yukinobu; Murakami, Fumitoshi

1991-09-01

75

Observe an animation of volcanism along a rift zone  

NSDL National Science Digital Library

Here is an animation for middle and high school students that depicts volcanism at a mid-ocean ridge. An introductory paragraph describes what occurs at a rift zone as two plates diverge and magma repeatedly moves up through the rift. The animation consists of two coordinated movie clips--one that shows a cross section of a rift zone and another that provides a view from above the rift. These clips show the volcanism and seafloor spreading that take place as two oceanic plates move apart. Students are supplied with control buttons to play, pause, and scroll through the animation. Labels indicate the key features in the animation. Copyright 2005 Eisenhower National Clearinghouse

Education, Terc. C.; Littell, Mcdougal

2003-01-01

76

Thermal and mechanical development of the East African Rift System  

E-print Network

The deep basins, uplifted flanks, and volcanoes of the Western and Kenya rift systems have developed along the western and eastern margins of the 1300 km-wide East African plateau. Structural patterns deduced from field, ...

Ebinger, Cynthia Joan

1988-01-01

77

From Lithospheric Thickening and Divergent Collapse to Active Continental Rifting  

E-print Network

From Lithospheric Thickening and Divergent Collapse to Active Continental Rifting Patrice F. Rey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.5 Lithospheric geometry through time . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5, the Alboran Sea, and the Basin and Range Province suggest that continental lithosphere following gravitational

Rey, Patrice F.

78

An updated global earthquake catalogue for stable continental regions: Reassessing the correlation with ancient rifts  

USGS Publications Warehouse

We present an updated global earthquake catalogue for stable continental regions (SCRs; i.e. intraplate earthquakes) that is available on the Internet. Our database contains information on location, magnitude, seismic moment and focal mechanisms for over 1300 M (moment magnitude) ??? 4.5 historic and instrumentally recorded crustal events. Using this updated earthquake database in combination with a recently published global catalogue of rifts, we assess the correlation of intraplate seismicity with ancient rifts on a global scale. Each tectonic event is put into one of five categories based on location: (i) interior rifts/taphrogens, (ii) rifted continental margins, (iii) non-rifted crust, (iv) possible interior rifts and (v) possible rifted margins. We find that approximately 27 per cent of all events are classified as interior rifts (i), 25 per cent are rifted continental margins (ii), 36 per cent are within non-rifted crust (iii) and 12 per cent (iv and v) remain uncertain. Thus, over half (52 per cent) of all events are associated with rifted crust, although within the continental interiors (i.e. away from continental margins), non-rifted crust has experienced more earthquakes than interior rifts. No major change in distribution is found if only large (M ??? 6.0) earthquakes are considered. The largest events (M ??? 7.0) however, have occurred predominantly within rifts (50 per cent) and continental margins (43 per cent). Intraplate seismicity is not distributed evenly. Instead several zones of concentrated seismicity seem to exist. This is especially true for interior rifts/taphrogens, where a total of only 12 regions are responsible for 74 per cent of all events and as much as 98 per cent of all seismic moment released in that category. Of the four rifts/taphrogens that have experienced the largest earthquakes, seismicity within the Kutch rift, India, and the East China rift system, may be controlled by diffuse plate boundary deformation more than by the presence of the ancient rifts themselves. The St. Lawrence depression, Canada, besides being an ancient rift, is also the site of a major collisional suture. Thus only at the Reelfoot rift (New Madrid seismic zone, NMSZ, USA), is the presence of features associated with rifting itself the sole candidate for causing seismicity. Our results suggest that on a global scale, the correlation of seismicity within SCRs and ancient rifts has been overestimated in the past. Because the majority of models used to explain intraplate seismicity have focused on seismicity within rifts, we conclude that a shift in attention more towards non-rifted as well as rifted crust is in order. ?? 2005 RAS.

Schulte, S.M.; Mooney, W.D.

2005-01-01

79

An integrated geophysical study of the northern Kenya rift  

NASA Astrophysics Data System (ADS)

The Kenyan part of the East African rift is among the most studied rift zones in the world. It is characterized by: (1) a classic rift valley, (2) sheer escarpments along the faulted borders of the rift valley, (3) voluminous volcanics that flowed from faults and fissures along the rift, and (4) axial and flank volcanoes where magma flow was most intense. In northern Kenya, the rift faults formed in an area where the lithosphere was weakened and stretched by Cretaceous-Paleogene extension, and in central and southern Kenya, it formed along old zones of weakness at the contact between the Archean Tanzania craton and the Proterozoic Mozambique orogenic belt. Recent geophysical investigations focused on the tectonic evolution of the East African rift and on exploration for geothermal energy in the southern portion of the Kenyan rift provide considerable information and insight on the structure and evolution of the lithosphere. In the north, a variety of other data exist. However, the lack of an integrated regional analysis of these data was the motivation for this study. Our study began with the collection and compilation of gravity data, and then we used the seismic refraction results from the Kenya Rift International Seismic Project (KRISP), published seismic reflection data, aeromagnetic data, and geologic and drilling data as constraints in the construction of integrated gravity models. These models and gravity anomaly maps provide insight on spatial variations in crustal thickness and upper mantle structure. In addition, they show the distribution of basins and help characterize the distribution of magmatism along the axis of the northern sector of the rift. Our main observations are the following: (1) the region of thinning and anomalous mantle widens northward in agreement with previous studies showing that the crust thins from about 35 km in the south to 20 km in the north; (2) as observed in the south, gravity highs observed along the axis are due to mafic bodies associated with the main volcanic centers, densification of the upper crust due to diking, and horst blocks where Precambrian basement has been brought to or near the surface; (3) the axis of the rift is marked by a series of high amplitude magnetic anomalies whose wavelengths are less than 2.5 km, the positive anomalies coinciding closely with known Quaternary volcanoes.

Mariita, Nicolas O.; Keller, G. Randy

2007-06-01

80

Crustal structure of central Lake Baikal: Insights into intracontinental rifting  

USGS Publications Warehouse

The Cenozoic rift system of Baikal, located in the interior of the largest continental mass on Earth, is thought to represent a potential analog of the early stage of breakup of supercontinents. We present a detailed P wave velocity structure of the crust and sediments beneath the Central Basin, the deepest basin in the Baikal rift system. The structure is characterized by a Moho depth of 39-42.5 km; an 8-km-thick, laterally continuous high-velocity (7.05-7.4 km/s) lower crust, normal upper mantle velocity (8 km/s), a sedimentary section reaching maximum depths of 9 km, and a gradual increase of sediment velocity with depth. We interpret the high-velocity lower crust to be part of the Siberian Platform that was not thinned or altered significantly during rifting. In comparison to published results from the Siberian Platform, Moho under the basin is elevated by <3 km. On the basis of these results we propose that the basin was formed by upper crustal extension, possibly reactivating structures in an ancient fold-and-thrust belt. The extent and location of upper mantle extension are not revealed by our data, and it may be offset from the rift. We believe that the Baikal rift structure is similar in many respects to the Mesozoic Atlantic rift system, the precursor to the formation of the North Atlantic Ocean. We also propose that the Central Baikal rift evolved by episodic fault propagation and basin enlargement, rather than by two-stage rift evolution as is commonly assumed.

ten Brink, U.S.; Taylor, M.H.

2002-01-01

81

Geodynamic Evolution of the Southern Flank of the Corinth Rift  

Microsoft Academic Search

The Gulf of Corinth is the most seismically active area of Europe extending in a N-S direction at a rate of 12mm\\/yr. Many studies have focused on currently active normal faults, which bound the southern flank of the Corinth rift. However, many prominent faults, that are considered no longer active, outcrop over a broader area along with their syn-rift sedimentary

D. Jousselin; S. Bourlange; M. Ford; C. Le Carlier; S. Rohais

2005-01-01

82

The sensitivity of East African rift lakes to climate fluctuations  

Microsoft Academic Search

Sequences of paleo-shorelines and the deposits of rift lakes are used to reconstruct past climate changes in East Africa.\\u000a These recorders of hydrological changes in the Rift Valley indicate extreme lake-level variations on the order of tens to\\u000a hundreds of meters during the last 20,000 years. Lake-balance and climate modeling results, on the other hand, suggest relatively\\u000a moderate changes in

Lydia A. OlakaEric; Eric O. Odada; Martin H. Trauth; Daniel O. Olago

2010-01-01

83

Regional Hydrothermal Cooling During the Initiation of Continental Rifting  

NASA Astrophysics Data System (ADS)

Thermochronologic data from the uplifted flanks of the Red Sea clearly indicate at least two distinct phases of regional cooling: the first at about 32 Ma and the second at about 21 Ma according to Omar and Steckler (1995). The latter phase correlates well with the rapid subsidence in the rift and uplift of the flanks. Thus this cooling data is generally interpreted to result from tectonic denudation. The earlier cooling is more problematic, since sediments of the same age are not clearly identified in rift basins. A different explanation for the early cooling episode is suggested by numerical models of lithospheric stretching. Those models show that rift initiation can result in a very broad region of lithospheric downbending. This bending produces small magnitude (several percent) surface extension of the sides of the rift. Eventually, these rift flanks are bent in the opposite sense and uplifted. The earlier phase of extensional strain may create a system of cracks and faults of sufficient permeability to allow significant water circulation. Hydrothermal flow could produce cooling of rocks as deep as 5-10 km. The cooling would be recorded in low-temperature thermochrologic systems such as apatite fission tracks. Model thermal histories for different rift and hydrothermal flow histories are compared to fission track data from the Red Sea region.

Buck, W. R.

2001-12-01

84

Seismic Observations From the Afar Rift Dynamics Project: Preliminary Results  

NASA Astrophysics Data System (ADS)

Following the 2005 Dabbahu rifting event in Afar, 9 broadband seismometers were installed around the active rift segment to study the microseismicity associated with this and subsequent dyking events. These recorded more than one year of continuous data. In March 2007, 41 stations were deployed throughout Afar and the adjacent rift flanks as part of a large multi-national, collaboration involving universities and organisations from the UK, US and Ethiopia. This abstract describes the crustal and upper mantle structure results of the first 19 months of data. Bulk crustal structure has been determined using the H-k stacking of receiver functions and thickness varies from ~45 km on the rift margins to ~16 km beneath the northeastern Afar stations. Estimates of Vp/Vs show normal continental crust values (1.7-1.8) on the rift margins, and very high values (2.0-2.2) in Afar. A study of seismic noise interferometry is in early stages, but inversions using 20 s Green's function estimates, with some control from regional surface waves, show evidence for thin crustal regions around the recently rifted Dabbahu segment. To improve our understanding of the physical and compositional properties of the crust and locate regions of high attenuation (an indicator of melt), we determine attenuation (Q) using t* values measured from spectra of P wave arrivals. We present whole path attenuation from source to receiver, which will provide a starting point for a future tomographic inversion. SKS-wave splitting results show sharp changes over small lateral distances (40° over <30 km), with fast directions overlying the Dabbahu segment aligning parallel with the recent diking. This supports ideas of melt dominated anisotropy beneath the Ethiopian rift. Seismic tomography inversions show that in the top 150 km low velocities mimic the trend of the seismicity in Afar. The low velocity anomalies extend from the main Ethiopian rift NE, towards Djibouti, and from Djibouti NW towards the Dabbahu segment. Outside of these linear regions the velocities are relatively fast. Below ~250 km the anomaly broadens to cover most of the Afar region with only the rift margins remaining fast. The seismic studies will be integrated with results from other areas of the consortium project (e.g., Magneto- tellurics, GPS, insar, gravity, petrology, geochemistry), enabling us to develop a greater understanding of rifting beneath an area of incipient oceanic spreading.

Hammond, J. O.; Guidarelli, M.; Belachew, M.; Keir, D.; Ayele, A.; Ebinger, C.; Stuart, G.; Kendall, J.

2008-12-01

85

Miocene rifting of Fuerteventura (Canary Islands)  

NASA Astrophysics Data System (ADS)

The older geological units of the volcanic island of Fuerteventura (Canary Islands), i.e., the so-called basal complex and the lower part of the subaerial volcanic rocks, show abundant structures indicative of a long-lived period of Miocene tectonic activity. These structures include faults, dike swarms, kilometer-scale folds, tilted sequences, and fissural and central volcanic edifices. A detailed structural study, based on geological mapping and the use of fault slip inversion techniques and statistical analysis of dike orientation, has allowed the identification of three separated Miocene deformation phases: M-D1, M-D2, and M-D3. The average extension directions determined for these phases are NW-SE, NNE-SSW, and ENE-WSW, respectively. A model of oceanic lithosphere rifting is proposed to account for this deformation history. A buoyant, anomalous sublithosphere mantle triggered the extension in the lithosphere beneath Fuerteventura, isolating it during the early and middle Miocene from the plate-scale collision regime predominant in the NW corner of the African plate.

FernáNdez, Carlos; Casillas, Ramón; GarcíA Navarro, Encarnación; GutiéRrez, Margarita; Camacho, Manuel A.; Ahijado, Agustina

2006-12-01

86

Fault architecture in the Main Ethiopian Rift and comparison with experimental models: Implications for rift evolution and Nubia-Somalia kinematics  

NASA Astrophysics Data System (ADS)

The Main Ethiopian Rift (MER) offers a complete record of the time-space evolution of a continental rift. We have characterized the brittle deformation in different rift sectors through the statistical analysis of a new database of faults obtained from the integration between satellite images and digital elevation models, and implemented with field controls. This analysis has been compared with the results of lithospheric-scale analogue models reproducing the kinematical conditions of orthogonal and oblique rifting. Integration of these approaches suggests substantial differences in fault architecture in the different rift sectors that in turn reflect an along-axis variation of the rift development and southward decrease in rift evolution. The northernmost MER sector is in a mature stage of incipient continental rupture, with deformation localised within the rift floor along discrete tectono-magmatic segments and almost inactive boundary faults. The central MER sector records a transitional stage in which migration of deformation from boundary faults to faults internal to the rift valley is in an incipient phase. The southernmost MER sector is instead in an early continental stage, with the largest part of deformation being accommodated by boundary faults and almost absent internal faults. The MER thus records along its axis the typical evolution of continental rifting, from fault-dominated rift morphology in the early stages of extension toward magma-dominated extension during break-up. The extrapolation of modelling results suggests that a variable rift obliquity contributes to the observed along-axis variations in rift architecture and evolutionary stage, being oblique rifting conditions controlling the MER evolution since its birth in the Late Miocene in relation to a constant post ca. 11 Ma ~ N100°E Nubia-Somalia motion.

Agostini, Andrea; Bonini, Marco; Corti, Giacomo; Sani, Federico; Mazzarini, Francesco

2011-01-01

87

Plume-rift interaction in the Deccan volcanic province  

NASA Astrophysics Data System (ADS)

It is widely accepted that the grand volcanic episode of the Deccan in India was a consequence of the passage of the northerly drifting Indian subcontinent over the Réunion starting plume in the Late Cretaceous. This plume also produced the three-rift Cambay triple junction, the three arms being the West Coast graben belt, the Narmada-Tapi rift zone and the Cambay rift. Deccan-related alkalic magmatism both preceded and followed the main tholeiitic phase of ˜ 65 Ma ago by about 3 MY. The uniformly tholeiitic thick basalt sequence of the Western Ghats was derived from direct melting of the plume head, but the tholeiitic-alkalic magmatism of the three rift belts was due to melting of the hydrous lithospheric mantle due to variable amounts of rifting. Any lithospheric loading mechanism for generating post-tholeiite alkalic lavas does not seem feasible for the Deccan; nor can it explain the pre-tholeiite alkaline magmatism in the Deccan. Similarly, conductive heating of the lithosphere by the plume cannot have generated the alkalic melts. Some amount of actual rifting did occur. Direct melt supply from the plume is not necessary for magmatism in the rifts, though it seems to have occurred in some cases. Our documentation of indigenous magmatism for the three rift belts calls into question any attempts at inferring the size of the plume head from synchroneity in geochronologic data alone, and we negate the possibility or necessity of large-distance transport of magmas from the plume through the lithosphere, to feed the rifts. Recent evidence indicates that alkalic magmatism north of the main Deccan outcrop preceded the main tholeiite phase by ˜ 3 MY. The Cambay graben was active 3.5 MY before the main tholeiite phase. As the alkalic complexes at the northern end of this graben are 68.5 Ma in age (so far the oldest well-proven Deccan-related rocks), they are the first expression of the Réunion plume, and the borehole picrites from the triple junction area, until now conventionally regarded as the oldest, can no longer be considered so. Accurate radiometric dating of these picrites is therefore much needed.

Sheth, H. C.; Chandrasekharam, D.

1997-02-01

88

Field Studies of Geothermal Reservoirs Rio Grande Rift, New Mexico  

SciTech Connect

The Rio Grande rift provides an excellent field laboratory to study the nature of geothermal systems in an extensional environment. Much of the geologic complexity that is found in the Basin and Range is absent because the rift is located on cratonic crust with a thin and well-characterized Phanerozoic stratigraphy and tectonic history. On the other hand, the Neogene thermo-tectonic history of the rift has many parallels with the Basin and Range to the west. The geology of the southern Rio Grande rift is among the best characterized of any rift system in the world. Also, most geologic maps for the region are rather unique in that detailed analyses of Quaternary stratigraphic and surficial unit are added in concert with the details of bedrock geology. Pleistocene to Holocene entrenchment of the Rio Grande and tributaries unroofs the alteration signatures and permeability attributes of paleo outflow plumes and upflow zones, associated with present-day, but hidden or ''blind,'' hydrothermal systems at Rincon and San Diego Mountain.

James C Witcher

2002-07-30

89

Hydrogeochemical and lake level changes in the Ethiopian Rift  

NASA Astrophysics Data System (ADS)

The Ethiopian Rift is characterized by a chain of lakes varying in size, hydrological and hydrogeological settings. The rift lakes and feeder rivers are used for irrigation, soda extraction, commercial fish farming and recreation, and support a wide variety of endemic birds and wild animals. The level of some lakes shows dramatic changes in the last few decades. Lakes Abiyata and Beseka, both heavily impacted by human activities, show contrasting lake level trends: the level of Abiyata has dropped by about 5 m over three decades while Beseka has expanded from an area of 2.5-40 km 2 over the same span of time. Changes in lake levels are accompanied by dilution in ionic concentration of lake Beseka and increase in salinity of lake Abiyata. Although the principal hydrogeochemical process in the rift lakes is controlled by the input and output conditions and carbonate precipitation, anthropogenic factors such as water diversion for irrigation and soda ash extraction played important role. The recent changes appear to have grave environmental consequences on the fragile rift ecosystem, which demands an integrated basin-wide water management practice. This paper demonstrates the drastic changes of lake levels and associated changes in lake chemistry of the two studied lakes. It also gives the regional hydrogeochemical picture of the other rift lakes that do not show significant response due to climate change and human impact.

Alemayehu, Tamiru; Ayenew, Tenalem; Kebede, Seifu

2006-01-01

90

Patterns of Rift Valley fever activity in Zambia.  

PubMed

An hypothesis that there was an annual emergence of Rift Valley fever virus in Zambia, during or after the seasonal rains, was examined with the aid of sentinel cattle. Serum samples taken during 1974 and 1978 showed evidence of epizootic Rift Valley fever in Zambia, with more than 80% positive. A sentinel herd exposed from 1982 to 1986 showed that some Rift Valley fever occurred each year. This was usually at a low level, with 3-8% of the susceptible cattle seroconverting. In 1985-6 more than 20% of the animals seroconverted, and this greater activity was associated with vegetational changes--which could be detected by remote-sensing satellite imagery--which have also been associated with greater virus activity in Kenya. PMID:1547835

Davies, F G; Kilelu, E; Linthicum, K J; Pegram, R G

1992-02-01

91

Flexural analysis of uplifted rift flanks on Venus  

NASA Technical Reports Server (NTRS)

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.

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

1992-01-01

92

Magnetotelluric pilot study in the Rio Grande Rift, southwest USA  

NASA Astrophysics Data System (ADS)

A magnetotelluric (MT) pilot study consisting of approximately 25 stations distributed in and around the Rio Grande Rift of the southwest United States was carried out in the summer of 2012. Both broadband (100 Hz to 1000 s) and long-period (up to 10 000 s) MT data were collected across two profiles that run perpendicular to the rift axis near Denver, Colorado and Taos, New Mexico, respectively. Time-domain EM data was also collected at each site to account for galvanic distortion in the near-surface. The tectonic forces and rheologic properties behind the initiation and propagation of the rift are poorly understood. Surface mapping of volcanism, normal faulting and sedimentary basins reveals a narrow band of crustal deformation confined to a region in close proximity to the rift axis while geophysical results suggest that deformation is distributed across a much broader and deeper region of the lithosphere. In particular, seismic tomography shows low seismic wave speeds into the lower crust and upper mantle. The magnetotelluric technique is a well-proven passive electromagnetic method that allows for the detection of apparent resistivity at a wide range of depth scales. Complimenting the seismic results with MT data will provide important new information on the geologic and geophysical properties that control the rifting process in this low-strain rate environment. Properties to which the MT method is particular sensitive include temperature, fluid content, and mineral alteration. Preliminary results from this most recent survey are encouraging, showing good data quality up to 10 000 s. In an important precursor to full 2D modeling, the magnetotelluric phase tensor has been used to assess the dimensionality of the electrical resistivity structure at depth. This pilot study provides proof of concept for a much larger magnetotelluric experiment planned to take place in the Rio Grande Rift in 2013.

Feucht, D. W.; Bedrosian, P. A.; Sheehan, A. F.

2012-12-01

93

US-Africa collaborative research on incipient continental rift zones  

NASA Astrophysics Data System (ADS)

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 that will result in capacity building in the African nations involved. In this presentation, we will provide our perspective on both the opportunities provided and challenges faced while conducting basic research in sub-Saharan Africa.

Atekwana, E. A.

2007-12-01

94

Distribution of Quaternary deformation in the central Main Ethiopian Rift, East Africa  

NASA Astrophysics Data System (ADS)

The Main Ethiopian Rift (MER) is a narrow continental rift characterized by an along-axis variation in rift evolution, with early stages in the south evolving to incipient breakup in the north. Although distribution and style of Quaternary volcanotectonic deformation is well known in the northern rift sector, knowledge of these characteristics is comparatively less constrained southward. In this paper we present the results of a field structural study carried out to better constrain the time-space distribution of faulting in the central sector of the MER (central MER). The new field structural data coupled with new 14C radiometric dating of faulted rocks suggest a localization of faulting at both rift margins of the central MER, where radiometric dating of faulted material has allowed establishing a Late Pleistocene-Holocene activity of border faults. Conversely, in-rift faulting (Wonji Fault Belt (WFB)) is subordinate highlighting a major difference with the northern sector of the MER where deformation is essentially accommodated in the axial zone. This is consistent with an along-axis variation in rift evolution, showing the central MER less evolved than the northern rift sector. Inversion of cumulative fault slip data reveals a variation in the extension direction between the rift margins (N105°-110°E) and the rift floor (N90°-95°E), which accords well with the current Nubia-Somalia plate kinematics. The variation in extension direction across the rift could manifest a slip partitioning between the boundary faults and in-rift WFB faults.

Agostini, Andrea; Bonini, Marco; Corti, Giacomo; Sani, Federico; Manetti, Piero

2011-08-01

95

Milankovich Cycles in Early Mesozoic Rift Basinsof Eastern North America Provide Physical Stratigraphy  

E-print Network

as the present Lake Tanganyika of East Africa or Baikalof Siberia, both of which are also rift valley lakesMilankovich Cycles in Early Mesozoic Rift Basinsof Eastern North America Provide Physical) whichfill thesebasinsisthe chronology of the rifting history of the Atlantic Ocean. Recent analysis by Paul

Olsen, Paul E.

96

Calving of large tabular icebergs from ice shelf rift systems Ian Joughin1,2  

E-print Network

Calving of large tabular icebergs from ice shelf rift systems Ian Joughin1,2 and Douglas R. Mac large icebergs to calve from the Ross Ice Shelf, Antarctica. Time series of rift geometries indicate geometry. Both the observations and model suggest that rift opening, and, thus, tabular-iceberg calving

Boyce, C. Kevin

97

Analogue models of oblique rifting in a cold1 lithosphere2  

E-print Network

1 Analogue models of oblique rifting in a cold1 lithosphere2 3 4 5 1,2,3 Julia Autin, 1,2 Nicolas New lithospheric analogue models of oblique rifting presented here capture the main11 characteristics lithospheric weakness (model B) and another with no weakness zone14 (model A). Both oblique rifts have

Boyer, Edmond

98

Intermittent upwelling of asthenosphere beneath the Gregory Rift, Kenya  

SciTech Connect

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.

Tatsumi, Yoshiyuki (Univ. of Tasmania (Australia) Kyoto Univ. (Japan)); Kimura, Nobukazu (Kyoto Univ. (Japan)); Itaya, Tetsumaru (Okayama Univ. of Science (Japan)); Koyaguchi, Takehiro (Kumamoto Univ. (Japan)); Suwa, Kanenori (Nagoya Univ. (Japan))

1991-06-01

99

Sensitivity of the East African rift lakes to climate variability  

NASA Astrophysics Data System (ADS)

Lakes in the East African Rift have provided excellent proxies to reconstruct past climate changes in the low latitudes. The lakes occupy volcano-tectonic depressions with highly variable climate and hydrological setting, that present a good opportunity to study the climatic and hydrogeological influences on the lake water budget. Previous studies have used lake floor sediments to establish the sensitivity of the East African rift lakes. This study focuses on geomorphology and climate to offer additional or alternative record of lake history that are key to quantifying sensitivity of these lakes as archives to external and internal climatic forcings. By using the published Holocene lake areas and levels, we analyze twelve lakes on the eastern arm of the East African rift; Ziway, Awassa, Turkana, Suguta, Baringo, Nakuru, Elmenteita, Naivasha, Natron, Manyara and compare with Lake Victoria, that occupies the plateau between the east and the western arms of the rift. Using the SRTM data, Hypsometric (area-altitude) analysis has been used to compare the lake basins between latitude 80 North and 30 South. The mean elevation for the lakes, is between 524 and 2262 meters above sea level, the lakes' hypsometric integrals (HI), a measure of landmass volume above the reference plane, vary from 0.31 to 0.76. The aridity index (Ai), defined as Precipitation/ Evapotranspiration, quantifies the water available to a lake, it encompasses land cover and climatic effects. It is lowest (arid) in the basin between the Ethiopian rift and the Kenyan rift and at the southern termination of the Kenyan Rift in the catchments of lake Turkana, Suguta, Baringo and Manyara with values of 0.55, 0.43, 0.43 and 0.5 respectively. And it is highest (wet) in the catchments of, Ziway, Awassa, Nakuru and Naivasha as 1.33,1.03 and 1.2 respectively, which occupy the highest points of the rift. Lake Victoria has an index of 1.42 the highest of these lakes and receives a high precipitation. We use a simple model written on a Matlab code to illustrate the lake volume and area response to climate of surficialy closed, graben shaped and panshaped lake basins. From preliminary results, lake basins that are sensitive to climate variability have a high HI and high aridity index, which will be presented in this conference

Olaka, L.; Trauth, M. H.

2009-04-01

100

Stress and slip partitioning during oblique rifting: comparison between data from the Main Ethiopian Rift and laboratory experiments  

NASA Astrophysics Data System (ADS)

Oblique rifting in the central and northern Main Ethiopian Rift (MER) has resulted in a complex structural pattern characterized by two differently oriented fault systems: a set of NE-SW-trending boundary faults and a system of roughly NNE-SSW-oriented fault swarms affecting the rift floor (Wonji faults). Boundary faults formed oblique to the regional extension vector, likely as a result of the oblique reactivation of a pre-existing deep-seated rheological anisotropy, whereas internal Wonji faults developed sub-orthogonal to the stretching direction. Previous works have successfully reconciled this rift architecture and fault distribution with the long-term plate kinematics; however, at a more local scale, fault-slip and earthquake data reveal significant variations in the orientation the minimum principal stress and related fault-slip direction across the rift valley. Whereas fault measurements indicate a roughly N95°E extension on the axial Wonji faults, a N105°E to N110°E directed minimum principal stress is observed along boundary faults. Both fault-slip data and analysis of seismicity indicate a roughly pure dip-slip motion on the boundary faults, despite their orientation (oblique to the regional extension vector) should result in an oblique displacement. To shed light on the process driving the variability of data derived from fault-slip (and seismicity) analysis we present crustal-scale analogue models of oblique rifting, deformed in a large-capacity centrifuge by using materials and boundary conditions described in several previous modeling works. As in these previous works, the experiments show the diachronous activation of two fault systems, boundary and internal, whose pattern strikingly resemble that observed in previous lithospheric-scale modeling, as well as that described in the MER. Internal faults arrange in two different, en-echelon segments connected by a transfer zone where strike-slip displacement dominates. Whereas internal faults develop roughly orthogonal to the extension direction, boundary faults form oblique to the imposed stretching vector: as a group, the faults follow the rift trend, controlled by a pre-existing weak anisotropy, but individually they form oblique to both the rift margin and the extension vector. Detailed analysis of fault displacements suggest that whereas the average displacement on single internal faults is consistent with the imposed direction of extension, slip on boundary faults does not parallel this direction; the average motion on these faults is orthogonal to the faults, resulting in a roughly pure dip-slip motion. This gives rise to a marked difference in fault-slip direction between internal faults (where slip orientation follow the regional extension) and boundary faults (where displacement is oblique to the "regional" extension). A similar scenario is observed for the reconstructed direction of the minimum principal stress that follows the regional stress field within the rift and is re-oriented at rift margins. Minor counterclockwise block rotations accommodate the different slip along the different fault systems. The model-to-nature striking is striking in terms of fault orientation, stress and slip orientation and its across-axis variations. The analogue models thus allows explaining the across-axis variability observed in natural fault-slip and earthquake data. Modeling results support that boundary faults form in response to a local stress re-orientation imposed by a deep seated anisotropy: their displacement trajectories deviate from those imposed by the regional extension, resulting in a pure dip-slip motion in an overall oblique rifting kinematics, as observed in other sectors of the East African Rift. Conversely, internal faults -which form later and affect a weaker, more uniform lithosphere- respond directly to the regional extension direction resulting in a fault slip sub-parallel to the Nubia-Somalia motion. Minor counterclockwise block rotations are required to accommodate the difference in slip along the different fault systems.

Corti, G.; Philippon, M.; Sani, F.; Keir, D.

2012-04-01

101

Trivialidad de nible de familias de aplicaciones de nibles en estructuras o-minimales.  

E-print Network

Trivialidad de nible de familias de aplicaciones de nibles en estructuras o-minimales. por Jesus estructuras o- minimales. Por otra parte, durante mis estancias en Rennes, he tenido la oportunidad de conocer. Si esta memoria tiene una estructura, digamos, \\legible", se debe fundamentalmente a su detallado e

Ruiz, Jesús M.

102

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

USGS Publications Warehouse

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

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

1992-01-01

103

Preferential Rifting of Continents: A Source of Displaced Terranes  

Microsoft Academic Search

Lithospheric rifting, while prevalent in the continents, rarely occurs in oceanic regions. To explain this preferential rifling of continents, we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth. Comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3. Also, a thickened crust can

Gregory E. Vink; W. Jason Morgan; Wu-Ling Zhao

1984-01-01

104

REVIEW Open Access Towards a better understanding of Rift Valley  

E-print Network

). Subsequent outbreaks with human cases have been reported in South Africa [6] and the Nile Valley from Sudan (2007) [14], Sudan (2007­2008) [16], Madagascar (2008­ 2009) [17], South Africa (2008, 2009, and 2010REVIEW Open Access Towards a better understanding of Rift Valley fever epidemiology in the south

Paris-Sud XI, Université de

105

Mapping of the major structures of the African rift system  

NASA Technical Reports Server (NTRS)

The author has identified the following significant results. The new fault map of the main Ethiopian rift, based on aerial photo compilations, generally agrees well with the maps produced from ERTS-1 imagery. Characteristically, the ERTS-1 imagery shows some of the major faults to be more extensive than realized from ground studies, though due to the angle of sun illumination some east-facing fault scarps are not easily discernible on the imagery. The Corbetti caldera, shows up surprisingly poor on the imagery, and is shown to be an adjunct to an older, larger caldera now occupied by Lakes Awassa and Shallo. The lithological boundaries mapped by De Paola in the rift are difficult to discern on the ERTS-1 imagery. On the Somalian plateau, east of the rift, a denuded caldera has been identified as the source of much of the lavas of the Batu Mountains. Further south, ERTS-1 imagery amplifies the structural and lithological mapping of the Precambrian rocks of the Shakisso-Arero area, and of the Kenya-Ethiopia border region. For the first time with some certainty, it is now possible to say that on the evidence of the ERTS-1 imagery, the Western Rift does not continue northeast beyond the Sudan-Uganda border, and is thus not to be sought in western Ethiopia.

Mohr, P. A. (principal investigator)

1973-01-01

106

Sensitivity of the East African rift lakes to climate variability  

Microsoft Academic Search

Lakes in the East African Rift have provided excellent proxies to reconstruct past climate changes in the low latitudes. The lakes occupy volcano-tectonic depressions with highly variable climate and hydrological setting, that present a good opportunity to study the climatic and hydrogeological influences on the lake water budget. Previous studies have used lake floor sediments to establish the sensitivity of

L. Olaka; M. H. Trauth

2009-01-01

107

Hydrothermal vents in Lake Tanganyika, East African, Rift system  

Microsoft Academic Search

Sublacustrine hydrothermal vents with associated massive sulfides were discovered during April 1987 at Pemba and Cape Banza on the Zaire side of the northern basin of Lake Tanganyika, East African Rift system. New investigations by a team of ten scuba divers during the multinational (France, Zaire, Germany, and Burundi) TANGANYDRO expedition (August-October 1991) found hydrothermal vents down to a depth

Jean-Jacques Tiercelin; Catherine Pflumio; Maryse Castrec; Jacques Boulégue; Pascal Gente; Joël Rolet; Christophe Coussement; Karl O. Stetter; Robert Huber; Sony Buku; Wafula Mifundu

1993-01-01

108

Volcanic rifting at Martian grabens Daniel Me`ge,1  

E-print Network

with terrestrial rifts. The DEMs were obtained from Mars Observer Laser Altimeter, Viking Orbiter stereo images. The ambient extensional stress field induced by regional body forces is combined at depth with decompression grabens that have been altered by further collapse modified grabens. Modified grabens include grabens

Mege, Daniel

109

Seismicity of the Baikal rift system from regional network observations  

NASA Astrophysics Data System (ADS)

In the paper we report the state-of-the-art of seismicity study in the Baikal rift system and the general results obtained. At present, the regional earthquake catalog for fifty years of the permanent instrumental observations consists of over 185,000 events. The spatial distribution of the epicenters, which either gather along well-delineated belts or in discrete swarms is considered in detail for different areas of the rift system. At the same time, the hypocenters are poorly constrained making it difficult to identify the fault geometry. Clustered events like aftershock sequences or earthquake swarms are typical patterns in the region; moreover, aftershocks of M ? 4.7 earthquakes make up a quarter of the whole catalog. The maximum magnitude of earthquakes recorded instrumentally is MLH7.6 for a strike-slip event in the NE part of the Baikal rift system and MLH6.8 for a normal fault earthquake in the central part of the rift system (Lake Baikal basin). Predominant movement type is normal faulting on NE striking faults with a left lateral strike-slip component on W-E planes. In conclusion, some shortcomings of the seismic network and data processing are pointed out.

Radziminovich, N. A.; Gileva, N. A.; Melnikova, V. I.; Ochkovskaya, M. G.

2013-01-01

110

The protracted development of focused magmatic intrusion during continental rifting  

NASA Astrophysics Data System (ADS)

transition from mechanical thinning toward focused magmatic intrusion during continental rifting is poorly constrained; the tectonically active Main Ethiopian Rift (MER) provides an ideal study locale to address this issue. The presence of linear magmatic-tectonic belts in the relatively immature central MER may indicate that the transition from mechanical to magmatic rifting is more spatially distributed and temporally protracted than has previously been assumed. Here we examine lava geochemistry and vent distribution of a Pliocene-Quaternary linear magmatic chain along the western margin of the central MER—the Akaki Magmatic Zone. Our results show limited variability in parental magma that evolve in a complex polybaric fractionation system that has not changed significantly over the past 3 Ma. Our results suggest the following: (1) channeling of plume material and the localization of shear- or topography-induced porosity modulates melt intrusion into the continental lithosphere. (2) Pre-existing lithospheric structures may act as catalysts for intrusion of magmas into the lithospheric mantle. (3) The midcrustal to upper crustal strain regime dictates the surface orientation of volcanic vents. Therefore, although linear magmatic belts like those in the central MER may young progressively toward the rift axis and superficially resemble oceanic style magmatism, they actually represent prebreakup magmatism on continental crust. The oldest linear magmatic belts observed seismically and magnetically at the edge of the ocean basins thus may not, as is often assumed, actually mark the onset of seafloor spreading.

Rooney, Tyrone O.; Bastow, Ian D.; Keir, Derek; Mazzarini, Francesco; Movsesian, Emily; Grosfils, Eric B.; Zimbelman, James R.; Ramsey, Michael S.; Ayalew, Dereje; Yirgu, Gezahegn

2014-06-01

111

Aedes Mosquito Saliva Modulates Rift Valley Fever Virus Pathogenicity  

E-print Network

Aedes Mosquito Saliva Modulates Rift Valley Fever Virus Pathogenicity Alain Le Coupanec1 , Divya) is a severe mosquito-borne disease affecting humans and domestic ruminants. Mosquito saliva contains compounds responses may facilitate virus infection. Indeed, Aedes mosquito saliva played a crucial role in the vector

Boyer, Edmond

112

Centrifuge models simulating magma emplacement during oblique rifting  

Microsoft Academic Search

A series of centrifuge analogue experiments have been performed to model the mechanics of continental oblique extension (in the range of 0° to 60°) in the presence of underplated magma at the base of the continental crust. The experiments reproduced the main characteristics of oblique rifting, such as (1) en-echelon arrangement of structures, (2) mean fault trends oblique to the

Giacomo Corti; Marco Bonini; Fabrizio Innocenti; Piero Manetti; Genene Mulugeta

2001-01-01

113

Heat transport by groundwater flow during the Baikal rift evolution  

Microsoft Academic Search

A two-dimensional modelling study of sedimentation, fluid flow, and heat flow in the Baikal rift basin undergoing flank uplift and basin subsidence has been performed in order to understand the impact of these processes on the surface heat flow signal. Heat flow anomalies of different scales and magnitudes have been observed at the sediment surface of the lake Baikal basin,

Jeffrey Poort; Oleg Polyansky

2002-01-01

114

Masirah Graben, Oman: A hidden Cretaceous rift basin  

SciTech Connect

Reflection seismic data, well data, geochemical data, and surface geology suggest that a Cretaceous rift basin exists beneath the thrusted allochthonous sedimentary sequence of the Masirah graben, Oman. The Masirah graben is located east of the Huqf uplift, parallel to the southern coast of Oman. The eastern side of the northeast-trending Huqf anticlinorium is bounded by an extensional fault system that is downthrown to the southeast, forming the western edge of the Masirah graben. This graben is limited to the east by a large wedge of sea floor sediments and oceanic crust, that is stacked as imbricate thrusts. These sediments/ophiolites were obducted onto the southern margin of the Arabian plate during the collision of the Indian/Afghan plates at the end of the Cretaceous. Most of the Masirah graben is covered by an allochthonous sedimentary sequence, which is complexly folded and deformed above a detachment. This complexly deformed sequence contrasts sharply with what is believed to be a rift sequence below the ophiolites. The sedimentary sequence in the Masirah graben was stable until further rifting of the Arabian Sea/Gulf of Aden in the late Tertiary, resulting in reactivation of earlier rift-associated faults. Wells drilled in the Masirah graben in the south penetrated reservoir quality rocks in the Lower Cretaceous Natih and Shuaiba carbonates. Analyses of oil extracted from Infracambrian sedimentary rocks penetrated by these wells suggest an origin from a Mesozoic source rock.

Beauchamp, W.H. [Cornell Univ., Ithaca, NY (United States); Ries, A.C. [Ries-Coward Associates Ltd., Caversham (United Kingdom); Coward, M.P. [Imperial College, London (United Kingdom)] [and others

1995-06-01

115

The Pongola structure of southeastern Africa - The world's oldest preserved rift?  

NASA Technical Reports Server (NTRS)

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.

Burke, K.; Kidd, W. S. F.; Kusky, T. M.

1985-01-01

116

Water management problems in the Ethiopian rift: Challenges for development  

NASA Astrophysics Data System (ADS)

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.

Ayenew, Tenalem

2007-06-01

117

Crustal structure beneath the Kenya Rift from axial profile data  

USGS Publications Warehouse

Modelling of the KRISP 90 axial line data shows that major crustal thinning occurs along the axis of the Kenya Rift from Moho depths of 35 km in the south beneath the Kenya Dome in the vicinity of Lake Naivasha to 20 km in the north beneath Lake Turkana. Low Pn velocities of 7.5-7.7 km/s are found beneath the whole of the axial line. The results indicate that crustal extension increases to the north and that the low Pn velocities are probably caused by magma (partial melt) rising from below and being trapped in the uppermost kilometres of the mantle. Along the axial line, the rift infill consisting of volcanics and a minor amount of sediments varies in thickness from zero where Precambrian crystalline basement highs occur to 5-6 km beneath the lakes Turkana and Naivasha. Analysis of the Pg phase shows that the upper crystalline crust has velocities of 6.1-6.3 km/s. Bearing in mind the Cainozoic volcanism associated with the rift, these velocities most probably represent Precambrian basement intruded by small amounts of igneous material. The boundary between the upper and lower crusts occurs at about 10 km depth beneath the northern part of the rift and 15 km depth beneath the southern part of the rift. The upper part of the lower crust has velocities of 6.4-6.5 km/s. The basal crustal layer which varies in thickness from a maximum of 2 km in the north to around 9 km in the south has a velocity of about 6.8 km/s. ?? 1994.

Mechie, J.; Keller, G.R.; Prodehl, C.; Gaciri, S.; Braile, L.W.; Mooney, W.D.; Gajewski, D.; Sandmeier, K.-J.

1994-01-01

118

Structural and environmental controls on Antarctic ice shelf rift propagation inferred from satellite monitoring  

NASA Astrophysics Data System (ADS)

Iceberg calving from ice shelves accounts for nearly half of the mass loss from the Antarctic Ice Sheet, yet our understanding of this process is limited. The precursor to iceberg calving is large through-cutting fractures, called "rifts," that can propagate for decades after they have initiated until they become iceberg detachment boundaries. To improve our knowledge of rift propagation, we monitored the lengths of 78 rifts in 13 Antarctic ice shelves using satellite imagery from the Moderate Resolution Imaging Spectroradiometer and Multiangle Imaging Spectroradiometer between 2002 and 2012. This data set allowed us to monitor trends in rift propagation over the past decade and test if variation in trends is controlled by variable environmental forcings. We found that 43 of the 78 rifts were dormant, i.e., propagated less than 500 m over the observational interval. We found only seven rifts propagated continuously throughout the decade. An additional eight rifts propagated for at least 2 years prior to arresting and remaining dormant for the rest of the decade, and 13 rifts exhibited isolated sudden bursts of propagation after 2 or more years of dormancy. Twelve of the fifteen active rifts were initiated at the ice shelf fronts, suggesting that front-initiated rifts are more active than across-flow rifts. Although we did not find a link between the observed variability in rift propagation rate and changes in atmospheric temperature or sea ice concentration correlated with, we did find a statistically significant correlation between the arrival of tsunamis and propagation of front-initiated rifts in eight ice shelves. This suggests a connection between ice shelf rift propagation and mechanical ocean interaction that needs to be better understood.

Walker, C. C.; Bassis, J. N.; Fricker, H. A.; Czerwinski, R. J.

2013-12-01

119

The effect of thermal weakening and buoyancy forces on rift localization: Field evidences from the Gulf of Aden oblique rifting  

E-print Network

The effect of thermal weakening and buoyancy forces on rift localization: Field evidences from Buoyancy forces Analog models On the basis of field and geophysical data, analog and numerical models, we here discuss the role of buoyancy forces arising from thickness variations in the lithosphere during

Husson, Laurent

120

Aerosol and Cloud Microphysical Characteristics of Rifts and Gradients in Maritime Stratocumulus Clouds  

NASA Technical Reports Server (NTRS)

A cloud rift is characterized as a large-scale, persistent area of broken, low reflectivity stratocumulus clouds usually surrounded by a solid deck of stratocumulus. A rift observed off the coast of Monterey Bay, California on 16 July 1999 was studied to compare the aerosol and cloud microphysical properties in the rift with those of the surrounding solid stratus deck. Variables measured from an instrumented aircraft included temperature, water vapor, and cloud liquid water. These measurements characterized the thermodynamic properties of the solid deck and rift areas. Microphysical measurements made included aerosol, cloud drop and drizzle drop concentrations and cloud condensation nuclei (CCN) concentrations. The microphysical characteristics in a solid stratus deck differ substantially from those of a broken, cellular rift where cloud droplet concentrations are a factor of 2 lower than those in the solid cloud. Further, CCN concentrations were found to be about 3 times greater in the solid cloud area compared with those in the rift and aerosol concentrations showed a similar difference as well. Although drizzle was observed near cloud top in parts of the solid stratus cloud, the largest drizzle rates were associated with the broken clouds within the rift area. In addition to marked differences in particle concentrations, evidence of a mesoscale circulation near the solid cloud rift boundary is presented. This mesoscale circulation provides a mechanism for maintaining a rift, but further study is required to understand the initiation of a rift and the conditions that may cause it to fill.

Sharon, Tarah M.; Albrecht, Bruce A.; Jonsson, Haflidi H.; Minnis, Patrick; Khaiyer, Mandana M.; Van Reken, Timothy; Seinfeld, John; Flagan, Rick

2008-01-01

121

Fault evolution in the Potiguar rift termination, Equatorial margin of Brazil  

NASA Astrophysics Data System (ADS)

The transform shearing between South American and African plates in the Cretaceous generated a series of sedimentary basins on both plate margins. In this study, we use gravity, aeromagnetic, and resistivity surveys to identify fault architecture and to analyse the evolution of the eastern Equatorial margin of Brazil. Our study area is the southern onshore termination of the Potiguar rift, which is an aborted NE-trending rift arm developed during the breakup of Pangea. The Potiguar rift is a Neocomian structure located in the intersection of the Equatorial and western South Atlantic and is composed of a series of NE-trending horsts and grabens. This study reveals new grabens in the Potiguar rift and indicates that stretching in the southern rift termination created a WNW-trending, 10 km wide and ~40 km long right-lateral strike-slip fault zone. This zone encompasses at least eight depocenters, which are bounded by a left-stepping, en-echelon system of NW- to EW-striking normal faults. These depocenters form grabens up to 1200 m deep with a rhomb-shaped geometry, which are filled with rift sedimentary units and capped by post-rift sedimentary sequences. The evolution of the rift termination is consistent with the right-lateral shearing of the Equatorial margin in the Cretaceous and occurs not only at the rift termination, but also as isolated structures away from the main rift.

de Castro, D. L.; Bezerra, F. H. R.

2014-10-01

122

Sediment geochemistry and tectonic setting: Application of discrimination diagrams to early stages of intracontinental rift evolution, with examples from the Okavango and Southern Tanganyika rift basins  

Microsoft Academic Search

In this article, we have applied discriminant diagrams and bivariate plots for tectonic setting to Quaternary sediments from the East African Rift System (EARS). Sediment samples used in this study represent two different phases in early stage intracontinental rift evolution: the alluvial fan of the nascent Okavango system and a lacustrine basin within the relatively more mature Tanganyika system. The

P. Huntsman-Mapila; J.-J. Tiercelin; M. Benoit; S. Ringrose; S. Diskin; J. Cotten; C. Hémond

2009-01-01

123

Interepidemic Rift Valley Fever Virus Seropositivity, Northeastern Kenya  

PubMed Central

Most outbreaks of Rift Valley fever (RVF) occur in remote locations after floods. To determine environmental risk factors and long-term sequelae of human RVF, we examined rates of previous Rift Valley fever virus (RVFV) exposure by age and location during an interepidemic period in 2006. In a randomized household cluster survey in 2 areas of Ijara District, Kenya, we examined 248 residents of 2 sublocations, Gumarey (village) and Sogan-Godud (town). Overall, the RVFV seropositivity rate was 13% according to immunoglobulin G ELISA; evidence of interepidemic RVFV transmission was detected. Increased seropositivity was found among older persons, those who were male, those who lived in the rural village (Gumarey), and those who had disposed of animal abortus. Rural Gumarey reported more mosquito and animal exposure than Sogan-Godud. Seropositive persons were more likely to have visual impairment and retinal lesions; other physical findings did not differ. PMID:18680647

Muchiri, Eric M.; Ndzovu, Malik; Mwanje, Mariam T.; Muiruri, Samuel; Peters, Clarence J.; King, Charles H.

2008-01-01

124

Aplicaciones de Arc GISAplicaciones de Arc GIS en Recursos de Aguaen Recursos de Agua  

E-print Network

Aplicaciones de Arc GISAplicaciones de Arc GIS en Recursos de Aguaen Recursos de Agua Alejandra 14: 440 personas/km2 vrs India y Jap�n 338vrs India y Jap�n 338 perper/km2./km2. El uso del agua se ha incrementado un 1.6% de 1970 aEl uso del agua se ha incrementado un 1.6% de 1970 a 1995 con 2

Gilbes, Fernando

125

Rifted continental margins: The case for depth-dependent extension  

NASA Astrophysics Data System (ADS)

Even though many basic properties of non-volcanic rifted margins are predicted by uniform extension of the lithosphere, uniform extension fails to explain other important characteristics. Particularly significant discrepancies are observed at: 1) the Iberia-Newfoundland conjugate margins (Type I), where large tracts of continental mantle lithosphere are exposed at the seafloor, and at 2) ultra-wide central South Atlantic margins (Type II) where continental crust spans wide regions below which it appears that lower crust and mantle lithosphere were removed. Neither corresponds to uniform extension in which crust and mantle thin by the same factor. Instead, either the crust or mantle lithosphere has been preferentially removed during extension. We show that the Type I and II styles are respectively reproduced by dynamical numerical lithospheric stretching models (Models I-A/C and II-A/C) that undergo depth-dependent extension. In this notation A and C imply underplating of the rift zone during rifting by asthenosphere and lower cratonic lithosphere, respectively. We also present results for models with a weak upper crust and strong lower crust, Models III-A/C, to show that lower crust can also be removed from beneath the rift zone by horizontal advection with the mantle lithosphere. From the model results we infer that these Type I, II, and III margin styles are controlled by the strength of the mid/lower crust, which determines the amount of decoupling between upper and lower lithosphere during extension and the excision of crust or mantle. We also predict the styles of sedimentary basins that form on these margins as a test of the concepts presented.

Huismans, Ritske S.; Beaumont, Christopher

2014-12-01

126

Undiscovered petroleum of the Brazilian Interior Rift Basins  

SciTech Connect

Brazil is estimated to contain 52% of the undiscovered oil and gas resources of South America, outside of Venezuela and Colombia. The Reconcavo, Tucano-Jatoba, and Tacutu interior rift basins of Brazil are investigated in this paper, the first in a series dealing with undiscovered petroleum of South America (exclusive of Venezuela and Colombia). Preliminary estimates of the undiscovered petroleum resources are 0.273 billion barrels of oil (BBO) and 1.234 trillion cubic feet of gas (TCFG) for the four basins. A review of the 1983-1993 history of petroleum reserves in South America indicated that an initial consensus estimate of 0.27 BBO and 4.15 TCFG (mean values) by the World Energy Program Group at the U.S. Geological Survey is too low. An adjusted range of probabilities is recalculated from original identified reserves, resulting in revised means values of 0.29 BBO and 4.15 TCGF for the four interior rift basins. Of these four basins, the Reconcavo is in a mature stage of exploration with 80 fields; the other rift basins, as well as most of the other basins in brazil, have not been as extensively investigated. In the Reconcavo basin, the principal plays are found in the pre-rift fault blocks, and in lower Cretaceous turbidites and sand lenses. Three gas fields are known in the southern Tucano subbasin. The three subbasins (southern, central, and northern) and the adjacent Jatoba basin become progressively less prospective northward because of lower levels of source-rock-maturation. The plays in these basins are similiar to those of the Reconcavo. The main play in the small Tacutu basin of northwestern Brazil is in deltaic sandstones; fractured basement rocks and volcanics constitute a minor play. The best reservoirs may be expected on the margins of the basin, although the reservoir seals are absent or poorly developed. 12 refs., 11 figs., 5 tabs.

Kingston, J. [Geological Survey. Palo Alto, CA (United States); Matzko, J.R. [Geological Survey, Reston, VA (United States)

1995-05-01

127

Erosion of Terrestrial Rift Flank Topography: A Quantitative Study  

NASA Technical Reports Server (NTRS)

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 flank plateau height of several hundred meters, it is clear that sediment eroded from passive margins is an important component of the mass flux from continents to oceans through geologic time. The overall goal of the research reported here is to develop a quantitative understanding of the kinematics of escarpment propagation across passive margins and the underlying geological processes responsible for this behavior. Plateau-bounding escarpments in general exhibit two basic forms depending on the direction of surface water drainage on the plateau interior relative to the escarpment. Where surface water flows away from the escarpment, the escarpment takes the form of subdued embayments and promontories, such that its overall trend remains fairly straight as it evolves with time. Where upland streams flow across the escarpment, it takes the form of dramatic, narrow gorges whose heads appear to propagate up the plateau drainage systems as large-scale knickpoints. From work on the Colorado Plateau, Schmidt (1987) noted that the Colorado River is located much closer to the Grand Canyon's south rim, a drainage divide escarpment, than to the north rim, which is a gorge-like escarpment. The main implication is that the gorge-like form might be associated with higher long-term average erosion rates compared to the drainage divide escarpment type.

Weissel, Jeffrey K.

1999-01-01

128

Petroleum system of the Shelf Rift Basin, East China Sea  

Microsoft Academic Search

The Tertiary section of the Oujioang and Quiontang Depressions of the East China Sea Basin consists of at least eight rift-related depositional sequences identified seismically by regionally significant onlap and truncation surfaces. These sequences are calibrated by several wells including the Wenzhou 6-1-1 permitting extrapolation of petroleum system elements using seismic facies analysis. Gas and condensate correlated to non-marine source

A. C. Cunningham; J. M. Armentrout; M. Prebish

1996-01-01

129

Eocene to Miocene geometry of the West Antarctic Rift System  

Microsoft Academic Search

Tectonic models for the Late Cretaceous\\/Tertiary evolution of the West Antarctic Rift System range from hundreds of kilometres of extension to negligible strike-slip displacement and are based on a variety of observations, as well as kinematic and geodynamic models. Most data constraining these models originate from the Ross Sea\\/Adare Trough area and the Transantarctic Mountains. We use a new Antarctic

R. D. Müller; K. Gohl; S. C. Cande; A. Goncharov; A. V. Golynsky

2007-01-01

130

Deformation Distribution and Type in the Main Ethiopian Rift  

NASA Astrophysics Data System (ADS)

The Main Ethiopian Rift central graben is magmatically segmented and seismically active. These magmatic segments are about 50 km long and up to 20 km wide, oriented about N10-20, and therefore oblique to the main direction of Mid-Miocene border faults (N35E). We use the association of the high spectral resolution of Landsat data and the very high spatial resolution of digitized aerial photographs to characterize and quantify the deformation along the rift. Several Digital Elevation Models of metric resolution have been produced at different locations along the segments in order to quantify and describe the fault deformation. We have analysed the Landsat data (texture analysis, PCA, classifications) in order to refine the existing geological maps and to propose a relative chronology of the deformation. The deformation within the segments varies from their centres, usually highly faulted and mainly rhyolitic and trachytic calderas (Gedemsa, Bosetti, Fantale, Dofan ), to their tips where the deformation is brittle at the surface and associated to recent basaltic flows and aligned cones. There we propose that the atypical (by their morphologies and Displacement/Length ratios) faults are dyke induced (see Gloaguen et al., same session). The overlap of the segments, the cross-cuttings of faults on the rift margins and the associated seismic activity are in agreement with the longitudinal growth of the magmatic segments via along-axis magma injection.

Kurz, T.; Gloaguen, R.; Ebinger, C.; Casey, M.; Abebe, B.

2003-12-01

131

The Sagatu Ridge dike swarm, Ethiopian rift margin. [tectonic evolution  

NASA Technical Reports Server (NTRS)

A swarm of dikes forms the core of the Sagatu Ridge, a 70-km-long topographic feature elevated to more than 4000 m above sea level and 1500 m above the level of the Eastern (Somalian) plateau. The ridge trends NNE and lies about 50 km east of the northeasterly trending rift-valley margin. Intrusion of the dikes and buildup of the flood-lava pile, largely hawaiitic but with trachyte preponderant in the final stages, occurred during the late Pliocene-early Pleistocene and may have been contemporaneous with downwarping of the protorift trough to the west. The ensuing faulting that formed the present rift margin, however, bypassed the ridge. The peculiar situation and orientation of the Sagatu Ridge, and its temporary existence as a line of crustal extension and voluminous magmatism, are considered related to a powerful structural control by a major line of Precambrian crustal weakness, well exposed further south. Transverse rift structures of unknown type appear to have limited the development of the ridge to the north and south.

Mohr, P. A.; Potter, E. C.

1976-01-01

132

The rifting to spreading transition in the Red Sea  

NASA Astrophysics Data System (ADS)

The transition from continental rifting to seafloor spreading is presently occurring at only a few places on Earth, such as the Red Sea or the Woodlark Basin. Competing theories for how spreading begins (either by quasi-instantaneous formation of a whole spreading segment or by initiation of spreading at multiple discrete “nodes” separated by thinned continental lithosphere) have been put forward. The major evidence for the nodes theory comes from the Red Sea and geophysical surveys carried out there in the “multi-deeps region” during the 1970's and 1980's. We present new high-resolution multibeam bathymetric information over the same region, which, when combined with acoustic backscatter data, seafloor sampling and magmatic geochemical information appears to provide no support for the nodes model. We show that, although the discrete deeps undoubtedly exist, they are not separated from one another by tectonic boundaries but rather represent “windows” onto a continuous spreading axis which is locally inundated and masked by massive slumping of sediments and evaporites from the rift flanks. The geophysical data that was previously used to support the presence of continental crust between the “nodes” can be equally well explained by processes related to the sedimentary blanketing and sub-sedimentary hydrothermal alteration. A single, “quasi-instantaneous segment formation” model would appear to be all that is required to explain observations from present-day rifting/spreading transitions globally.

Augustin, Nico; Devey, Colin W.; van der Zwan, Froukje M.; Feldens, Peter; Tominaga, Masako; Bantan, Rashad A.; Kwasnitschka, Tom

2014-06-01

133

Exploration in Mid-Continent geophysical anomaly: rift or riffraff  

SciTech Connect

Geologic and geophysical exploration for oil and gas in the Mid-Continent geophysical anomaly (MGA) resulted in acquiring millions of acres and thousands of miles of seismic data in areas previously considered nonprospective for economic hydrocarbon production. Interpreted to be a rift, this 700-mi feature extends from northern Kansas through Lake Superior into Michigan. Factors influencing major company interest in the feature include: oil occurring in Keweenawan sediments from Michigan; gravity and magnetic data suggesting that clastic-filled basins are present; and extensive folding and faulting of Precambrian rock units seen on seismic sections. Little attention has been given to other important factors, including lithostratigraphy, thermal and burial history of reservoir and source rocks, and distribution of observed gravity and magnetic anomalies throughout the rift. Although such data are sparse or highly susceptible to interpretation, any ensuing model should attempt to explain the observations. The observed data constrain exploration modeling within certain boundaries. These constraints are: (1) proximity of the reservoir rock to the potential source rock; (2) different thermal histories for the flanks of the rift versus the center; and (3) high versus low densities and magnetic susceptibilities in the central part versus the flanking positions of the MGA south of Lake Superior.

Yarus, J.M.; Hinks, A.

1986-05-01

134

Intraplate stresses, nonthermal subsidence, and fluid regimes in rifted basins  

SciTech Connect

Short-term fluctuations of the level of intraplate stresses modulate the shape of rifted basins and therefore affect the stratigraphic record. We investigated the effect of such stress variations on fluid flow and sedimentation patterns in sedimentary basins using a dynamic numerical model which combines the stretching mechanism for rifted-basin formation with the lithospheric necking concept. The results show that increases in the level of compressive intraplate stresses strongly influence the hydrodynamic regime during the postrift phase of sedimentary basins by causing an increase of meteoric water influx and compactional driven flow. These short-term perturbations in fluid-flow patterns occur simultaneously with subsidence rates far in excess of those predicted by thermal models. The results of the modeling enable us to discriminate the effect of tectonically induced distortions of the basin shape (and associated differential vertical motions in rifted basins) on the fluid-flow regime from perturbations induced by changes in eustatic sea level. We discuss implications of the modeling predictions for maturation and migration of hydrocarbons and diagenesis. Episodic diagenetic and expulsion events can be explained by short-term changes in the magnitude of nonthermal subsidence.

Van Balen, R.T.; Cloetingh, A.P.L. (Vrije Universiteit, Amsterdam (Netherlands))

1993-09-01

135

Indirect detection of subsurface outflow from a rift valley lake  

NASA Astrophysics Data System (ADS)

Naivasha, highest of the Kenya (Gregory) Rift Valley lakes, has no surface outlet. However, unlike other Rift lakes it has not become saline despite high potential evaporation rates, which indicates that there must be some subsurface drainage. The fate of this outflow has been the subject of speculation for many years, especially during the general decline in lake water level during the 1980's. Particularly to the south of the lake, there are few opportunities to obtain information from direct groundwater sampling. However, the stable isotopic composition of fumarole steam from late Quaternary volcanic centres in the area has been used to infer groundwater composition. Using a simple mixing model between Rift-flank groundwater and highly-evaporated lakewater, this has enabled subsurface water flow to be contoured by its lakewater content. By this method, outflow can still be detected some 30 km to the south of the lake. Stable isotope data also confirm that much of the steam used by the local Olkaria geothermal power station is derived from lakewater, though simple balance considerations show that steam use cannot alone be responsible for the fall in lake level observed during the 1980's.

Darling, W. G.; Allen, D. J.; Armannsson, H.

1990-02-01

136

Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes  

PubMed Central

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

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

2014-01-01

137

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

PubMed

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

Passarelli, L; Rivalta, E; Shuler, A

2014-01-01

138

A model for Iapetan rifting of Laurentia based on Neoproterozoic dikes and related rocks  

USGS Publications Warehouse

Geologic evidence of the Neoproterozoic rifting of Laurentia during breakup of Rodinia is recorded in basement massifs of the cratonic margin by dike swarms, volcanic and plutonic rocks, and rift-related clastic sedimentary sequences. The spatial and temporal distribution of these geologic features varies both within and between the massifs but preserves evidence concerning the timing and nature of rifting. The most salient features include: (1) a rift-related magmatic event recorded in the French Broad massif and the southern and central Shenandoah massif that is distinctly older than that recorded in the northern Shenandoah massif and northward; (2) felsic volcanic centers at the north ends of both French Broad and Shenandoah massifs accompanied by dike swarms; (3) differences in volume between massifs of cover-sequence volcanic rocks and rift-related clastic rocks; and (4) WNW orientation of the Grenville dike swarm in contrast to the predominately NE orientation of other Neoproterozoic dikes. Previously proposed rifting mechanisms to explain these features include rift-transform and plume–triple-junction systems. The rift-transform system best explains features 1, 2, and 3, listed here, and we propose that it represents the dominant rifting mechanism for most of the Laurentian margin. To explain feature 4, as well as magmatic ages and geochemical trends in the Northern Appalachians, we propose that a plume–triple-junction system evolved into the rift-transform system. A ca. 600 Ma mantle plume centered east of the Sutton Mountains generated the radial dike swarm of the Adirondack massif and the Grenville dike swarm, and a collocated triple junction generated the northern part of the rift-transform system. An eastern branch of this system produced the Long Range dike swarm in Newfoundland, and a subsequent western branch produced the ca. 554 Ma Tibbit Hill volcanics and the ca. 550 Ma rift-related magmatism of Newfoundland.

Burton, William C.; Southworth, Scott

2010-01-01

139

Basin evolution, organization of faulting and the distribution of displacement within the Gulf of Corinth rift  

NASA Astrophysics Data System (ADS)

The Gulf of Corinth is a rare example of continental rifting in its initial stages of development, with an extremely dense network of marine geophysical data collected over the past two decades, making it an ideal case study for investigating early rift evolution. Through the integration of numerous seismic reflection surveys, totalling ~3930 km of seismic profiles and covering a range of frequencies (both high resolution seismic and multi-channel seismic, analogue and digital), we present: 1. a refined chronostratigraphic model for the syn-rift sediments that have been deposited in the developing offshore Corinth basin over the past ~1-2 Ma and 2. a detailed rift fault network with confirmed locations, lengths, fault interactions and development, and details of recent displacement. Our results show that chronostratigraphic models from the West Eratini basin are coherent with models from the central part of the rift. We divide the rift stratigraphy into two sequences: a late rift sequence comprising recent interbedded marine-lacustrine sediments deposited over the last ~600 kyr, and a thick early rift sequence with deposits up to ~1-2 Ma of contrasting seismic and sedimentological character. The late rift sequence is divided into six packages and can be correlated with 100 kyr glacio-eustatic cycles. We identify multiple unconformities, including a basin wide unconformity that separates the early and late rift sequences. The unconformities are attributed to differences in fault development and basin subsidence pattern along the rift. Combining the refined chronostratigraphic model with the detailed fault network allows us to: a) determine relative timings of fault activity and basin development; b) estimate absolute fault displacements both spatially and temporally at high resolution (e.g. for each interpreted 100 kyr package); c) calculate sediment flux into the basin during each stratigraphic time interval and spatial distribution of syn-rift sediment through time; and d) compare recent fault displacement and activity with seismicity to assess seismogenic hazard and fault interaction.

Nixon, C. W.; McNeill, L. C.; Henstock, T.; Bull, J. M.; Bell, R. E.; Christodoulou, D.; Papatheodorou, G.; Taylor, B.; Ferentinos, G.; Sakellariou, D.; Lykousis, V.; Sachpazi, M.; Ford, M.; Goodliffe, A. M.; Leeder, M.; Gawthorpe, R. L.; Collier, R. E.; Clements, B.

2013-12-01

140

Seismic Investigations of an Accommodation zone in the Northern Rio Grande Rift, New Mexico, USA  

NASA Astrophysics Data System (ADS)

Seismic reflection and refraction data acquired in the Rio Grande rift near Santa Fe, New Mexico, in 2009 and 2010 by the SAGE (Summer of Applied Geophysical Experience) program imaged the La Bajada fault (LBF) and strata offset across the associated, perpendicular Budagher fault (BF). The LBF is a major basin-bounding normal fault, offset down to the west; the smaller BF is an extensional fault that breaks the hanging wall ramp of the LBF. We chose this area because it is in a structurally complex region of the rift, comprising a small sub-basin and plunging relay ramps, where north-trending, en echelon basin-bounding faults (including the LBF) transfer crustal extension laterally between the larger Española (to north) and Albuquerque rift basins. Our data help determine the precise location and geometry of the poorly exposed LBF, which, near the survey location, offsets the rift margin vertically about 3,000 m. When integrated with industry reflection data and other SAGE seismic, gravity, and magnetotelluric surveys, we are able to map differences in offset and extension laterally (especially southward) along the fault. We interpret only about 200 m of normal offset across the BF. Our continuing work helps define multiple structural elements, partly buried by syn-rift basin-filling sedimentary rocks, of a complex intra-rift accommodation zone. We are also able to discriminate pre-Eocene (Laramide) from post-Miocene (rift) structures. Our data help determine the amount of vertical offset of pre-rift strata across structural elements of the accommodation zone, and depth and geometry of basin fill. A goal is to infer the kinematic development of this margin of the rift, linkages among faults, growth history, and possible pre-rift structural controls. This information will be potentially useful for evaluation of resources, including oil and/or gas in pre-rift strata and ground water in Late Miocene to Holocene rift-filling units.

Baldridge, W. S.; Valdes, J.; Nedorub, O.; Phrampus, B.; Braile, L. W.; Ferguson, J. F.; Benage, M. C.; Litherland, M.

2010-12-01

141

Half graben filling processes in the early phase of continental rifting: The Miocene Namurungule Formation of the Kenya Rift  

NASA Astrophysics Data System (ADS)

The Miocene Namurungule Formation crops out on the eastern flank of the Kenya Rift, representing basin fill that developed in association with Miocene rifting. The formation is characterized by large volumes of volcaniclastics supplied passively by pyroclastic fall. Facies analysis reveals that this formation consists mostly of lacustrine delta deposits with minor alluvial fan deposits at its base. The conspicuous occurrence of a flood plain facies with infrequent channel fill deposits in the lower part of this formation suggest that the drainage area was limited in the early stage of deposition. Pyroclastic fall would therefore have been an important source of sediment during the early stages of rift development. The delta deposits are divided into two distinct successions based on lithological characteristics, separated by a thick pyroclastic layer in the middle part of the formation. The stacking pattern of the lower succession is retrogradational, whereas the upper succession is characterized by a pile of prograding bodies. Both delta successions are interpreted to have accumulated in an underfilled basin. The change in depositional mode from the lower to upper is considered to be due to a change in the balance between the sedimentation rate and the rate of lake-level rise. Assuming constant sediment supply, the apparent difference in flooding scale between the lower and upper successions is attributed to the topographical widening of the basin flat. Similar successions also occur in another basin in the East African Rift (Ngorora Formation, central Kenya), in which pyroclastic sediments are dominant and upward decrease in sedimentation rate is recognized. The differing in the stacking patterns between the Namurungule Formation and the Ngorora formation is probably induced by basin width. The narrow Namurungule basin seems to have been sensitive to increased sediment supply owing to the expansion of the catchment area.

Saneyoshi, Mototaka; Nakayama, Katsuhiro; Sakai, Tetsuya; Sawada, Yoshihiro; Ishida, Hidemi

2006-04-01

142

Hydrothermal vents is Lake Tanganyika, East African Rift system  

SciTech Connect

Sublacustrine hydrothermal vents with associated massive sulfides were discovered during April 1987 at Pemba and Cape Banza on the Zaire side of the northern basin of Lake Tanganyika, East African Rift system. New investigations by a team of ten scuba divers during the multinational (France, Zaire, Germany, and Burundi) TANGANYDRO expedition (August-October 1991) found hydrothermal vents down to a depth of 46 m along north-trending active faults bounding the Tanganyika rift on the western side. Temperatures from 53 to 103 {degrees}C were measured in hydrothermal fluids and sediments. Veins of massive sulfides 1-10 cm thick (pyrite and marcasite banding) were found associated with vents at the Pemba site. At Cape Banza, active vents are characterized by 1-70-cm-high aragonite chimneys, and there are microcrystalline pyrite coatings on the walls of hydrothermal pipes. Hydrothermal fluid end members show distinctive compositions at the two sites. The Pemba end member is a NaHCO{sub 3}-enriched fluid similar to the NaHCO{sub 3} thermal fluids form lakes Magadi and Bogoria in the eastern branch of the rift. The Cape Banza end member is a solution enriched in NaCl. Such brines may have a deep-seated basement origin, as do the Uvinza NaCl brines on the eastern flank of the Tanganyika basin. Geothermometric calculations have yielded temperatures of fluid-rock interaction of 219 and 179 {degrees}C in the Pemba and Cape Banza systems, respectively. Abundant white or reddish-brown microbial colonies resembling Beggiatoa mats were found surrounding the active vents. Thermal fluid circulation is permitted by opening of cracks related to 130{degrees}N normal-dextral faults that intersect the north-south major rift trend. The sources of heat for such hydrothermal systems may relate to the existence of magmatic bodies under the rift, which is suggested by the isotopic composition of carbon dioxide released at Pemba and Cape Banza. 21 refs., 2 figs.

Tiercelin, J.J. [Universite de Bretagne Occidentale, Brest (France)] [Universite de Bretagne Occidentale, Brest (France); Pflumio, C.; Castrec, M. [Universite Paris VI, Paris (France)] [and others] [Universite Paris VI, Paris (France); and others

1993-06-01

143

The Chukchi Borderland: a Sediment-starved Rifted Continental Margin  

NASA Astrophysics Data System (ADS)

The origin and geologic structure of the Chukchi Borderland region, approximately 650 by 400 km in size, has been the subject of speculation since the earliest ice island research groups discovered its existence more than 60 years ago. Multichannel seismic reflection and refraction data acquired between 2007 and 2011, together with legacy seismic data show fragments of high-standing basement (continental) horsts. The structure is draped with less than a kilometer of sediment. Between the high-standing blocks are deep grabens with locally tilted but mostly flat-lying deposits generally only 1-2 km thick. Northwind Escarpment, along the eastern boundary of the Borderland, is a 600-km-long fault adjacent to the deeply subsided and hyper-extended crust of the Canada Basin to the east. The long, linear, sub-parallel orientation of the major structures (including Northwind Escarpment) is consistent with transtensional deformation of the Borderland. The general paucity of thick sediments indicates a sediment-starved environment. Both the North Chukchi Basin on the west and an unnamed deeply buried valley east on the Beaufort margin provide sediment-routing conduits through which sediment by-passed the Borderland throughout much of the Cretaceous history of the growing Brooks Range to the south. Canada Basin deposits also show strata thicken towards the southwest, suggesting sediment influx via the deeply buried valley on the Beaufort margin. On the northeastern side of the Canada Basin, the region is underlain by horst and graben structures with orientations similar to the Chukchi Borderland, but the intervening valleys are filled with as much as two km of sediment and the entire feature is buried beneath another 2 km of post-rift sediment. The similarity of structural styles on both sides of the Canada Basin suggests that this style of transtensional rifting could have been widespread during the early extension of this part of the Arctic and perhaps the Chukchi Borderland and parts of the Canadian Arctic Archipelago margin were conjugates prior to rifting. Seismic data also show that volcanism associated with the High-Arctic Large Igneous Province to the north has intruded or flowed over the northern parts of the Borderland. The Chukchi Borderland, because of its lack of sedimentary cover, offers a unique window into the early rifting history of the Canada Basin and the transition from rifted to hyper-extended continental crust.

Hutchinson, D. R.; Houseknecht, D.; Mosher, D. C.; Hart, P. E.; Jackson, H. R.; Lebedeva-Ivanova, N. N.; Shimeld, J.; Chian, D.

2013-12-01

144

Lithospheric structural controls on magma composition: the Kenya Rift  

NASA Astrophysics Data System (ADS)

Lithospheric structure, as delineated by geophysics, plays a fundamental role in both felsic and mafic magmatic compositions in the Kenya Rift. With respect to the mafic rocks, there are, first, silica-undersaturated basanites of the Chyulu Hills. This location is off-axis to the rift, where the lithosphere is thick. The lavas have been modeled as high-pressure, small degree partial melts. This origin contrasts to that for the silica-saturated transitional basalts, basaltic trachy-andesites, and andesites in the axis of the rift. These magmas were generated by higher degrees of partial melt and are also much more evolved, with Mg numbers approximately 40 to 50. The lavas have seen substantial crystal fractionation prior to eruption. An important component of lithospheric structure within the rift axis is the Kenya Dome: it is an area of thick crust and high elevation and heat flow. The crust is made thicker by a 6.8 km/sec lower crustal layer. Immediately below this crust is a very slow upper mantle. Velocities become more lithospheric to the south of the Kenya Dome in the vicinity of Suswa. This lithosphere then thickens southward into Tanzania. The felsic central volcanoes of the rift, which are significant geothermal targets, reflect these lithospheric variations. Eburru and Olkaria are both centered on the Kenya Dome. Eburru is pantellerite and can be modeled as resulting from crystallization of silica-saturated basalt. Olkaria is comendite and resulted from fusion of lower crustal syenite. That we find such distinct petrogenesis for two closely spaced volcanoes indicates that this area of very warm mantle has the temperatures necessary to generate high degree partial melt magmas, which evolve into pantellerites, and also fuse the lower crust. Suswa, which is the southernmost volcano and in the area where lithosphere thickens, is composed on phonolites, which can be modeled as resulting from crystallization of silica-undersaturated mafic parents. Presumably, the thicker lithosphere lends itself to generation of high-pressure, low degree partial melts similar to those found in the off-axis Chyulu Hills.

Omenda, P. A.; Simiyu, S.; Anthony, E. Y.; Keller, G. R.; Dean, R. L.

2001-12-01

145

Motion in the north Iceland volcanic rift zone accommodated by bookshelf faulting  

NASA Astrophysics Data System (ADS)

Along mid-ocean ridges the extending crust is segmented on length scales of 10-1,000km. Where rift segments are offset from one another, motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis. Where segments overlap, non-transform offsets with a variety of geometries accommodate shear motions. Here we use micro-seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north Iceland. Between the rift segments, we identify a series of faults that are aligned sub-parallel to the orientation of the main rift. These faults slip through left-lateral strike-slip motion. Yet, movement between the overlapping rift segments is through right-lateral motion. Together, these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf-faulting mechanism, named after its resemblance to a tilting row of books on a shelf. The faults probably reactivated existing crustal weaknesses, such as dyke intrusions, that were originally oriented parallel to the main rift and have since rotated about 15° clockwise. Reactivation of pre-existing, rift-parallel weaknesses contrasts with typical mid-ocean ridge transform faults and is an important illustration of a non-transform offset accommodating shear motion between overlapping rift segments.

Green, Robert G.; White, Robert S.; Greenfield, Tim

2014-01-01

146

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

USGS Publications Warehouse

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.

Agar, S.M.; Klitgord, Kim D.

1995-01-01

147

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)

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

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

2008-07-01

148

Seismicity within a propagating ice shelf rift: the relationship between icequake locations and ice shelf structure  

USGS Publications Warehouse

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.

Heeszel, David S.; Fricker, Helen A.; Bassis, Jeremy N.; O'Neel, Shad; Walter, Fabian

2014-01-01

149

Dynamics of rifting and modes of extension on icy satellites Department of Earth Sciences, University College London, London, UK  

E-print Network

Dynamics of rifting and modes of extension on icy satellites F. Nimmo1 Department of Earth Sciences: Europa, Ganymede, extension, faulting, ice Citation: Nimmo, F. (2004), Dynamics of rifting and modes

Nimmo, Francis

150

Asthenospheric flow and origin of volcanism in the Baikal Rift area Sergei Lebedev a,, Thomas Meier b  

E-print Network

: continental rift; mantle flow; anisotropy; hotspot; plume; plum-pudding 1. Introduction Rifting within mantle models; we propose that sub-horizontal asthenospheric flow from beneath cratons is a common cause

Utrecht, Universiteit

151

Rifted continental margins: geometric control on crustal architecture and melting  

NASA Astrophysics Data System (ADS)

A new model is provided for the distribution of magma-poor and magma-rich rifted margins. The South Atlantic, Central Atlantic, North Atlantic - Arctic (Eurasia Basin), and Red Sea all are magma-rich at their distal ends and magma-poor at their proximal ends (with respect to their poles of rotation). The well-known architectural zonation across fully developed magma-poor margins (limited crustal stretching, hyperextension, exhumed mantle, oceanic crust) is also observed along the lengths of many margins at the super-regional scale. Zones of exhumed mantle, marking magma-poor margin, can be mapped for thousands of kilometers. Likewise can zones of seaward dipping reflectors (SDR) marking magma-rich margins. At this scale, the age of the oceanic crust becomes younger in the direction of the rotation pole, implying that the continents ruptured by rift tip propagation (and rotation pole propagation). Propagation is also manifested by the age of pre-break-up magmatism, break-up unconformity, and margin uplift. Hence, the classic cross-sectional depiction of margin evolution has a third dimension. The degree of melting follows the same pattern. At the distal end of e.g. the South Atlantic, SDR zones are wide and gradually thin toward the rotation pole. Eventually exhumed mantle takes over, marking the transition to the magma-poor margins, which remain to the proximal end of rifting. SDR zones also thin laterally from ca 10-15 km thickness at the continent-ocean boundary (COB) to ca 7 km thick oceanic crust beyond the SDRs. Outcrop data demonstrate that also exhumed mantle contains up to ca 12% melt, infiltrated in the peridotites. Thus, melting is largest at the distal ends near the COB, and decreases both laterally toward the evolving ocean and along strike toward the rift tip. Accepting that continents are rigid to a first order, the linear rate of extension at any given location along an evolving rift and ocean, is governed by the angular rate of opening, the distance to the rotation pole, and the rate of propagation of the pole. For a fixed angular rate, the linear extension rate increases away from the pole. Numerical models reveal that both mantle temperature and rate of extension can govern the degree of melting. However, the above empirical observations suggest that to a first order the rifted margin architecture, including the degree of melting, is governed by the linear rate of extension, which is a direct outcome of geometric rules of plate tectonics. Rapid pole propagation, or a pole jump, will induce a rapid increase in the linear rate. Magma-rich margins seem to form when continents break at a high extension/strain rate caused by rapid propagation; this occurs at the distal end of a rupturing plate. Our testable model questions the common ad hoc introduction of mantle plumes to explain "excess" melting along magma-rich margins. This does not rule out that mantle heterogeneities may exist, but such heterogeneities appear second order when it comes to generating magma-rich margins.

Lundin, Erik; Redfield, Tim; Peron-Pinvidic, Gwenn

2014-05-01

152

Stress perturbation associated with the Amazonas and other ancient continental rifts  

USGS Publications Warehouse

The state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts (specifically a dense rift pillow in the lower crust) may modify substantially the regional stress field. Both shallow (2.0-2.6 km depth) breakout data and deep (20-45 km depth) crustal earthquake focal mechanisms indicate a N to NNE maximum horizontal compression in the vicinity of the Paleozoic Amazonas rift in central Brazil. This compressive stress direction is nearly perpendicular to the rift structure and represents a ???75?? rotation relative to a regional E-W compressive stress direction in the South American plate. Elastic two-dimensional finite element models of the density structure associated with the Amazonas rift (as inferred from independent gravity modeling) indicate that elastic support of this dense feature would generate horizontal rift-normal compressional stresses between 60 and 120 MPa, with values of 80-100 MPa probably most representative of the overall structure. The observed ???75?? stress rotation constrains the ratio of the regional horizontal stress difference to the rift-normal compressive stress to be between 0.25 and 1.0, suggesting that this rift-normal stress may be from 1 to 4 times larger than the regional horizontal stress difference. A general expression for the modification of the normalized local horizontal shear stress (relative to the regional horizontal shear stress) shows that the same ratio of the rift-normal compression relative to the regional horizontal stress difference, which controls the amount of stress rotation, also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress. The potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike-slip stress regimes. In the Amazonas rift case, because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift-normal compression to be between 0.25 and 1.0, our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced (for ratios >0.5) or enhanced (for ratios <0.5). Additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust-faulting stress regime in the vicinity of the Amazonas rift. A rift-normal stress associated with the seismically active New Madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike-slip faults parallel to the axis of the rift, although this results in a 20-40% reduction in the local horizontal shear stress within the seismic zone. Sparse stress data in the vicinity of the seismically quiescent Midcontinent rift of the central United States suggest a stress state similar to that of New Madrid, with the local horizontal shear stress potentially reduced by as much as 60%. Thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows. The modeling and analysis here demonstrate that rift-normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension.

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

1996-01-01

153

Evidence for cenozoic rifting in Thailand from gravity modeling  

E-print Network

EVIDENCE FOR CENOZOIC' RIFTING IN THAILAND FROihl C, 'RAVITY MODEI, ING A Thesis by TIFFANY A. 0 HNS TAD Submitted to the Office of Clradnate Studies of Texas ASM Hniversity in partial fulfillment of the requirements for the degree of MASTER... OF SC'IENC'E MAY 1990 ihfajor Subject: Clpopllyslcs FVIDENCE FOR CENOZOIC RIFTIiUG IN THAILAND FROI'I GRAVITY MODELING A Thesis I'IFFANY A. OHNSTAD Approved as to style and content by: Robert J. ltlcCabe (Co-Chair of Committee) Steven Harder...

Ohnstad, Tiffany A.

2012-06-07

154

Structural aspects of Mid-Continent rift system in Kansas  

SciTech Connect

The Humboldt fault zone and the faulted Abilene anticline are among many prominent north-northeast to south-southwest-trending structures in northeastern Kansas that occur in Paleozoic rocks as a result of renewed movement of faults associated with the Mid-Continent rift system (MRS). The Humboldt fault zone consists of a number of anastomosing fault segments with high-angle, normal or reverse displacements of up to 600 m. Most of this movement occurred during the Late Mississippian to Early Pennsylvanian. Domal culminations, some of which bear oil, and rhomboid-shaped grabens, are recognized.

Berendsen, P.; Newell, K.D.; Blair, K.P. (Univ. of Kansas, Lawrence (USA))

1989-08-01

155

Evidence of Active Rifts in The Southwest United States Using Geophysical Inversion of Seismic Data  

NASA Astrophysics Data System (ADS)

A continental rift is a linear zone where the lithosphere has become thinner due to extensional forces associated with plate tectonic. Many of these rifts are still active, such as the East Africa rift, while others appear to be failed rifts. We investigate the southern section of the Rio Grande Rift using seismic data collected by USArray stations, and extend the analysis into Texas to investigate the differences between active and failed rifts. We collect multiple geophysical data sets, including receiver functions, surface waves, and teleseismic P- and S- wave travel times, and perform a joint inversion to determine 1-D S-wave velocity structure. This joint inversion is based on constrained optimization that introduces a structural constraint over the inversion model. From the 1-D models, we interpolate layers of the S-wave velocity to create a 3-D velocity model. These results will allow us to analyze and locate any possible active or failed rifts and make a correlation with data from the Rio Grande rift.

Celis, S. H.; Velasco, A. A.; Gonzalez-Huizar, H.; Sosa, A.

2013-05-01

156

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

E-print Network

junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal-oceanic axes of extension. Ã? 2012 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Early quantitative may also localize strain and magmatism during rifting (e.g., Ebinger and Sleep, 1998; van Wijk et al

Paris-Sud XI, Université de

157

The Rhine graben rift system-plate tectonics and transform faulting  

Microsoft Academic Search

The Rhinegraben is the central segment of a rift system which traverses Western Europe from the North Sea to the Mediterranean. Rifting, which had started in the Middle Eocene, was preceded by early-Teriary nephelinitic eruptions. The center of maximum subsidence shifted along the graben axis from south to north during geologic times. The maximum thickness of sedimentary fill is observed

J. H. Illies

1972-01-01

158

A kinematic model for the East African Rift D. Sarah Stamps,1  

E-print Network

that includes three subplates (Victoria, Rovuma, and Lwandle) between Nubia and Somalia. Continental rifting boundary between Nubia and Somalia, is often cited as a modern archetype for rifting and continental., 2002; Prawirodirdjo and Bock, 2004; Fernandes et al., 2004] have obtained significantly different Nubia-Somalia

Nocquet, Jean-Mathieu

159

Lower crustal earthquakes near the Ethiopian rift induced by magmatic processes  

E-print Network

Lower crustal earthquakes near the Ethiopian rift induced by magmatic processes Derek Keir School crustal earthquakes are commonly observed in continental rifts at depths where temperatures should be too high for brittle failure to occur. Here we present accurately located earthquakes in central Ethiopia

Brest, Université de

160

The 12 September 1999 Upper East Rift Zone dike intrusion at Kilauea Volcano, Hawaii  

E-print Network

The 12 September 1999 Upper East Rift Zone dike intrusion at Kilauea Volcano, Hawaii P. Cervelli,1 seaward sliding of Kilauea's south flank. INDEX TERMS: 7280 Seismology: Volcano seismology (8419); 8035 and associated volcanic tremor began at Kilauea Volcano. The swarm was concentrated along the east rift zone near

Segall, Paul

161

The lithostratigraphy of the Karoo supergroup in the Kilombero Rift Valley, Tanzania  

NASA Astrophysics Data System (ADS)

The Kilombero Rift Valley is located in central eastern Tanzania. The 20-40 km wide valley is elongated in a north-south direction. In the study area, it is filled with Karoo (Permian) and Quaternary siliciclastic sediments. The Karoo succession of the Kilombero Rift Valley, with a thickness of more than 6000 m, has been studied in great detail. Based on new structural, sedimentological, palæontological and petrological information, a revision of the stratigraphical subdivision is pertinent. The former main original stratigraphical outline is still valid, but the different units' subdivision and thickness distributions have been highly adjusted. The succession can be divided into four groups (Ndeke, Msolwa, Ruhembe, Mikumi), which in detail have been further subdivided into eight formations. The different formations show compositions that can be related to the different stages of tectonic and sedimentological developments in both syn- and post-rifting settings, each with a characteristic sequential configuration. The Ndeke Group reflects the alluvial fan deposits of a syn-rifting phase. The Msolwa Group reflects post-rifting conditions in the succeeding tectonic/depositional cycle 3. The Ruhembe Group represents syn- and post-rifting stages related to Late Permian to Early Triassic rejuvenation and the possible succeeding marine transgression of the rift. The shift in lithological composition, sedimentary facies and directions of transportation between the Msolwa and Ruhembe Groups, may be related to tectonic/depositional phase 4 of East Africa. This late post-rifting phase was capped by the fluvial deposits of the Mikumi Group.

Dypvik, H.; Hankel, O.; Nilsen, O.; Kaaya, C.; Kilembe, E.

2001-04-01

162

Failed Rifts: They May Have Failed but They are Still interesting  

NASA Astrophysics Data System (ADS)

The term failed rift has been used in the literature since the early days of plate tectonics. Successful rifts form oceans and the resulting passive margins often bury much of the information that could be used to deduct their evolution. There are many examples of failed rifts (aulacogens) on the continents that are well studied partly because they subside and basins develop that contain natural resources. It is not clear why some rifts fail. For example, the Mid-Continent rift in central North America was extremely active magmatically and very little of the original crust remains in the Lake Superior region. On the other hand, the West and Central African Rift System extends entirely across Africa from the Atlantic to the Indian Ocean. It is associated with only modest magmatism but is associated with a large number of basins. In many cases, a successful rift is preceded by a failed one inboard of it. Finally, many failed rifts have been reactivated one or more times in an intraplate setting and some pose seismic hazards today.

Keller, G. R.

2012-12-01

163

Modelling of rift propagation on Ronne Ice Shelf, Antarctica, and sensitivity to climate change  

Microsoft Academic Search

The calving of icebergs from large Antarctic ice shelves is controlled mainly by the formation and propagation of rifts originating from the side margins of the ice shelf and local areas of grounding. Using InSAR, we observe the evolution of rifts along Hemmen Ice Rise, on Ronne Ice Shelf, Antarctica prior to the large calving event of October 1998. We

E. Larour; E. Rignot; D. Aubry

2004-01-01

164

A new vertebrate fauna from the Cretaceous Red Sandstone Group, Rukwa Rift Basin, Southwestern Tanzania  

Microsoft Academic Search

The Rukwa Rift Basin Project was initiated to conduct exploratory field paleontology in poorly sampled terrestrial strata in southern and western Tanzania. Here we report the discovery of a series of new fossiliferous localities from Red Sandstone Group deposits in the Rukwa Rift Basin. These localities contain a diverse Cretaceous terrestrial\\/freshwater vertebrate fauna that consists of members of several major

Patrick M. O’Connor; Michael D. Gottfried; Nancy J. Stevens; Eric M. Roberts; Sifa Ngasala; Saidi Kapilima; Remigius Chami

2006-01-01

165

Gravity analysis of Parga and Hecate chasmata: Implications for rift and corona formation  

Microsoft Academic Search

The two largest rift systems on Venus, Parga, and Hecate chasmata contain one third of all coronae. We map variations in elastic thickness and apparent depth of compensation (ADC) in these two regions using the admittance function for gravity and topography. We examine the relationship between rifting and coronae by comparing lithospheric structure with corona characteristics including volcanism, topographic shape,

Suzanne E. Smrekar; Trudi Hoogenboom; Ellen R. Stofan; Paula Martin

2010-01-01

166

The distribution and implications of heat flow from the gregory rift in kenya  

NASA Astrophysics Data System (ADS)

Data from geothermal mapping along the Kenyan Gregory Rift zone using ground and aerial infrared techniques indicate that an average of 11-30 MW/km are advectively emitted along the rift. The bulk of the advected heat is lost through the central part of the Kenyan dome. Two other secondary geothermal sites are at the northern and southern limits of the rift adjacent to rift bifuration zones. These three geothermal sites are separated by ? 300 km. Active thermal fields occur only {1}/{7} as frequently as rift volcanoes. Geothermal spacing could be controlled by the length of the heat source at the lithosphere-upper mantle boundary, the thickness of the crust and the length of the rift segment between major structural breaks. At faster spreading centers the heat source is probably more continuous and following Elder's models should produce upwelling domes much more frequently than along the East African Rift. Advective cooling on one section of the East Pacific Rise at 21°N indicates that domes should appear every 40-80 km. If this is the case then approximately four times more heat is given off along a kilometer of East Pacific Rise crest as it is emitted along the Gregory Rift.

Crane, Kathleen; O'Connell, Suzanne

1983-05-01

167

A Contribution to the Further Investigation of McKenzie's Rifting Model  

Microsoft Academic Search

We have continued the investigation of generalized geodynamic McKenzie's model of rifting which was not developed exhaustively. Filling this want, we have examined the rift basin and heat flow evolution under different regimes of spreading. Within the mathematical simulation, three model versions of finite extension of the lithosphere were studied and compared: instantaneous, accelerated and with a constant rate. In

V. S. Sheplev; V. V. Reverdatto

168

RIFT-BASIN STRUCTURE AND ITS INFLUENCE ON SEDIMENTARY SYSTEMS MARTHA OLIVER WITHJACK AND ROY W. SCHLISCHE  

E-print Network

A-1 RIFT-BASIN STRUCTURE AND ITS INFLUENCE ON SEDIMENTARY SYSTEMS MARTHA OLIVER WITHJACK AND ROY W: Rift basins are complex features defined by several large-scale structural components including faulted-scale structures also develop within rift basins. These include basement-involved and detached normal faults

169

ICESat's new perspective on ice shelf rifts: The vertical dimension H. A. Fricker, J. N. Bassis, and B. Minster  

E-print Network

detail over features such as crevasses, through-cutting rifts, ice shelf margins, ice fronts, icebergs and grounding zones. In this paper we focus on rifts; another paper in this Special Section consid- ers icebergs for the calving of large tabular icebergs. Rift propa- gation rates on the Amery Ice Shelf have been shown

Boyce, C. Kevin

170

Was the Midcontinent Rift part of a successful seafloor-spreading episode?  

NASA Astrophysics Data System (ADS)

~1.1 Ga Midcontinent Rift (MCR), the 3000 km long largely buried feature causing the largest gravity and magnetic anomaly within the North American craton, is traditionally considered a failed rift formed by isolated midplate volcanism and extension. We propose instead that the MCR formed as part of the rifting of Amazonia (Precambrian northeast South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established. A cusp in Laurentia's apparent polar wander path near the onset of MCR volcanism, recorded by the MCR's volcanic rocks, likely reflects the rifting. This scenario is suggested by analogy with younger rifts elsewhere and consistent with the MCR's extension to northwest Alabama along the East Continent Gravity High, southern Appalachian rocks having Amazonian affinities, and recent identification of contemporaneous large igneous provinces in Amazonia.

Stein, Carol A.; Stein, Seth; Merino, Miguel; Randy Keller, G.; Flesch, Lucy M.; Jurdy, Donna M.

2014-03-01

171

Transtensional rifting in the late Proto-Gulf of California near Bahia Kino, Sonora, Mexico  

NASA Astrophysics Data System (ADS)

The Gulf of California provides an active rift example to test the role of rift obliquity in continental rupture. Continental rifts require focused strain to rupture and form an ocean basin. Strike-slip faults are ubiquitous in oblique rifts and focused transtensional strain adjacent to these faults may be a catalyst for rupture. To test this hypothesis, I completed structural mapping, fault-kinematic analysis, geochronology, basin analysis, and paleomagnetism of pre- and syn-rift rocks exposed in coastal Sonora, Mexico. Sedimentary basins record ˜16 km of west-northwest-directed transtension across the Kino-Chueca Shear Zone. Onset of transtension in the study area is estimated to be ca. 7 Ma and lasted for approximately 1 million years. This represents a significant portion (˜28%) of plate boundary deformation over this time interval. Dextral shear was progressively localized within this zone of extensional deformation, and together shear and extension acted to focus lithospheric-scale strain into a narrower zone.

Bennett, Scott Edmund Kelsey

172

Spatial variation of primordial 3-He in crustal fluids along the East-African Rift system (the Ethiopian and the Kenya Rift section)  

NASA Technical Reports Server (NTRS)

(3)He/(4)He compositions are presented for groundwater samples from the Ethiopian segment of the East-Afrikan Rift and from its northern extension, the adjacent Afar region (Djibuti). Helium isotope data are compared to those obtained previously from the Gregory Rift, south of Ethiopia. The distribution pattern of mantle-derived volatiles along the entire East-African-Rift (-from south Kenya to Djibuti-) is discussed and their sources are identified. Helium isotope ratios (R) for samples from the Ethiopian part of the Rift range from 6.3 to 16.0 times the atmospheric ratio (Ra=1.4 x 10(exp -6) and thus show together with a MOR component a considerable hotspot helium component. These mantle helium concentrations are comparable to those observed in groundwaters and volcanic rocks from the Afar plume region in Djibuti. Here R/Ra values range from 9 to 13 times the atmospheric composition, with mantle-derived helium concentrations being higher than at spreading ocean ridges. R/Ra values from Ethiopia and Djibuti are entirely different from those observed in groundwaters at the southerly extending Gregory Rift in Kenya, where R/Ra values scatter between 0.5 and 6. At the northernmost part of the Gregory Rift, close to Ethiopia mantle helium contents are slightly higher, with R/Ra-values varying between 6.5 and 8.0.

Griesshaber, E.; Weise, S.; Darling, G.

1994-01-01

173

Rifting, heat flux, and water availability beneath the catchment of Pine Island Glacier  

NASA Astrophysics Data System (ADS)

The West Antarctic Rift System (WARS) is a major rift system that developed in the Cretaceous and Cenozoic. It forms the lithsopheric cradle for the marine-based, and potentially unstable West Antarctic Ice Sheet (WAIS). Determining the geological boundary conditions beneath the WAIS and in particular geothermal heat flux may help model its response to external climatic forcing. However, in the Amundsen Sea Embayment sector of WAIS, where major glaciers such as Pine Island and Thwaites are rapidly changing today, fundamental properties such as geothermal heat flux to the base of the ice sheet have remained poorly constrained due to sparse geophysical data coverage and the lack of drilling sites. New crustal thickness estimates derived from airborne gravity data (Jordan et al., 2010, GSA Bul.), are interpreted to show a continuation of the WARS beneath Pine Island Glacier, and suggest two phases of continental rifting affected this region. Here we explore the impact of continental rifting on geothermal heat flux variations and basal water availability beneath Pine Island Glacier. Using 1D thermal models of rift evolution, we assess geothermal heat flux configurations resulting from either single or two-phase rifting and explore the dependency on the age of rifting and pre-rift setting. Additionally, 1D glaciological models were implemented to predict the changes in subglacial water distribution created by different rifting models. Our modelling reveals that geothermal heat-flux beneath the WAIS is critically sensitive to rift age and evolution and has the potential to significantly alter basal conditions if it continued to be active in the Neogene as some recent geological interpretations suggest.

Jordan, T. A.; Ferraccioli, F.; Hindmarsh, R. C.

2012-04-01

174

Construction and destruction of a volcanic island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores  

NASA Astrophysics Data System (ADS)

There is a great lack of knowledge regarding the evolution of islands inside active oceanic rifts, in particular the meaning of the different evolutionary steps. Therefore, we conducted an investigation in Graciosa Island, which lies at the northwestern end of the Terceira Rift in the Azores Triple Junction, with the objective of constraining the evolution of the island in terms of volcanic growth and mass wasting, in particular the meaning and age of the destruction events. From digital elevation model (DEM) analysis, stratigraphic and tectonic observations, K/Ar dating on key samples, and available bathymetry and gravity data, we propose that Graciosa comprises five main volcanic complexes separated by major unconformities related to large scale mass wasting: (1) The older volcanic edifice (Serra das Fontes Complex) grew until ca. 700 ka, and was affected by a major flank collapse towards the southwest, which removed the whole SW flank, the summit and a part of the NE flank. (2) The Baía do Filipe Complex developed between at least 472 ka and 433 ka in two different ways: in the SW (presently offshore) as a main volcano, and in the NE unconformably over the sub-aerial remnants of the Serra das Fontes Complex, as secondary volcanic edifices. (3) The Baía do Filipe Complex was affected by a major flank collapse towards the SW, again removing most of the edifice. (4) The remnants of the Baía do Filipe Complex were covered in unconformity by the Serra Dormida Complex between ca. 330 and 300 ka, which in turn was unconformably covered by the younger Basaltic Cover Complex between ca. 300 ka and 214 ka. These two units were affected by a third major sector collapse that removed the whole western flank, the summit and part of the eastern flank of the Serra Dormida and Basaltic Cover complexes. (5) Despite the relatively young age of Graciosa, the collapse scars are not well preserved, and not active anymore. (6) A central-type volcano has been growing since at least 60 ka at the southeastern end of the island, inside the scar left by a fourth sector collapse towards the SE, which affected most previous complexes. Contemporaneously, parasitic strombolian cones formed all over the island. Despite the location of Graciosa inside the active Terceira Rift, the new data indicate that the evolution of the island has been driven by a competition between volcano growth and repeated destruction by catastrophic sector collapses, rather than by slow incremental faulting associated with the tectonics of the rift.

Sibrant, A. L. R.; Marques, F. O.; Hildenbrand, A.

2014-09-01

175

Young Stellar Object Candidates in the Aquila Rift Region  

NASA Astrophysics Data System (ADS)

Using the 2m telescope of the Turingia State Observatory at Tauten-berg (TLS), imaging observations in 3 wavebands (H ?, R and I) are performed in the 16 fields in the Aquila Rift region. The observed fields cover about 7 square degrees. Excluding the 3 fields with unqualified data, the photometrical analysis is made for the remaining 13 fields, from which point sources are identified, and finally 7 H ? emission-line star candidates are identified by color-color diagrams. The 7 candidates are located in five fields. Three of them are located near the Galactic plane, while the galactic latitudes of the rest are greater than 4°. The 2 M ASS counterparts of the point sources are identified, and the properties of the 7 H ? emission-line star candidates are further analyzed by using the two-color diagrams. It is found that the near-infrared radiation from these H ? emission-line star candidates has no obvious infrared excess, one of them even falls on the main-sequence branch. This indicates that the H ?-emissive young stellar objects (YSOs) are not always accompanied with the infrared excess, and that the results of the H ? emission line observation and the infrared excess observation are mutually supplemented. If the 7 H ? emission-line star candidates are regarded as YSO candidates, then the number of YSOs in the Aquila Rift region is quite small. The further confirmation of these candidates needs subsequent spectral observations.

Zhang, Miao-miao; Wang, Hong-chi; Stecklum, B.

2010-10-01

176

Generation of Continental Rifts, Basins and Swells by Lithosphere Instabilities  

NASA Astrophysics Data System (ADS)

Domal uplifts, volcanism, basin formation and rifting have often struck the same continent in different areas at the same time. Their characteristics and orientations are difficult to reconcile with mantle convection or tectonic forces and suggest a driving mechanism that is intrinsic to the continent. The rifts seem to develop preferentially at high angles to the edge of the continent whereas swells and basins seem confined to the interior. Another intriguing geometrical feature is that the rifts often branch out in complicated patterns at their landward end. In Western Africa, for example, magmatic activity currently occurs in a number of uplifted areas including the peculiar Cameroon Volcanic Line that stretches away from the continental margin over about 1000 km. Magmatic and volcanic activity has been sustained along this line for 70 My with no age progression. The mantle upwelling that feeds the volcanoes is not affected by absolute plate motions and hence is attached to the continent. The Cameroon Volcanic Line extends to the Biu swell to the North and the Jos plateau to the West defining a striking Y-shaped pattern. This structure segues into several volcanic domes including the Air, the Hoggar, the Darfur, the Tibesti and the Haruj domes towards the Mediterranean coast. Another example is provided by North America, where the late Proterozoic-early Ordovician saw the formation of four major basins, the Michigan, Illinois, Williston and Hudson Bay, as well as of major rifts in southern Oklahoma and the Mississipi Valley within a short time interval. At the same time, a series of uplifts developed, such as the Ozark and Nashville domes. Motivated by these observations, we have sought an explanation in the continental lithosphere itself. We describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent. We carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above and describe the peculiar horizontal planform that developed. Dynamical behaviour depends on three dimensionless numbers, a Rayleigh number for the unstable block, a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block. Within the block, instability develops in two different ways in an outer annulus and in an inner region. In the outer annulus, upwellings and downwellings take the form of radial rolls spaced regularly. In the interior region, the planform adopts the more familiar form of polygonal cells. Translated to geological conditions, such instabilities should manifest themselves as linear rifts striking at a right angle to the continent-ocean boundary and an array of domal uplifts, volcanic swells and basins in the continental interior. The laboratory data lead to simple scaling laws for the dimensions and spacings of the convective structures. For the sub-continental lithospheric mantle, these dimensions and distances take values in the 500-1000 km range, close to geological examples. The large intrinsic buoyancy of Archean lithospheric roots prevents this type of instability, which explains why the widespread volcanic activity that currently affects Western Africa is confined to post-Archean domains.

Milelli, L.; Fourel, L.; Jaupart, C. P.

2012-12-01

177

Geology and Petrology of the Southeast Mariana Forearc Rift  

NASA Astrophysics Data System (ADS)

The southernmost Mariana convergent margin is tectonically and magmatically very active, with submarine arc volcanoes that are sub-parallel to the Malaguana-Gadao Ridge backarc spreading center at ~110km from the trench axis. This activity reflects widening of the S. Mariana Trough. Stretching formed 3 southeast-facing, broad rifts extending from the trench to an extinct arc volcano chain (~80km from the trench axis) that is mostly composed of outcrops and fragments of pillow lavas partially covered by sediments. The 3 rifts comprise the S.E. Mariana Forearc Rift (SEMFR) and are 50-56km long and 3600 to 8200m deep, with axial valleys that narrow near the extinct arc. We studied the SEMFR using one Shinkai 6500 dive in 2008 and two Shinkai 6500 dives and 7 deep-tows in 2010. Near the trench, the SEMFR flanks are very steep and dominated by talus slopes of lava, fine-grained gabbro, diabase and peridotite, sometimes covered by thin volcaniclastic sediments. Few outcrops of pillow lavas, lava flows and volcaniclastics are observed, strongly suggesting that SEMFR morphology is dominated by faulting and landsliding. Lava outcrops are smoother and better preserved towards the extinct arc, suggesting that magmatic activity dominates that part of the rift. 40Ar-39Ar ages of 3 SEMFR lavas are 3.0-3.7Ma, so post-magmatic rifting is younger than ~3Ma. SEMFR pillow lavas are vesicular and microporphyritic with crystallite-rich glassy rinds, indicating they erupted underwater at near-liquidus conditions. In contrast, the lava flows are more crystallized and less vesicular. SEMFR lavas exhibit similar ranges in mineral composition with 2 kinds of plagioclase (An>80% and An<80%), clinopyroxene (Mg#?80% and Mg#<80%), olivine (Fo>90 and Fo<90), suggesting magma mixing. Gabbroic rocks are slightly altered and have olivine and clinopyroxene compositions similar to those of the lavas, but contain less anorthitic plagioclase with a wider range in composition (An20-70) than the lavas. One sample of the extinct arc lava is vesicular and aphyric, consistent with near-liquidus eruption conditions. SEMFR lavas are low-K calk-alkaline basalts to andesites, whereas lava from the extinct arc is tholeiitic basaltic andesite similar to those erupted from the Malaguana-Gadao Ridge. These lavas are enriched in LILE and depleted in HFSE, consistent with subduction zone settings. Their similar Yb-normalized trace element patterns (Pearce et al., 2005), with different enrichment in LILE, suggest that SEMFR lavas were produced from a relatively homogenous mantle source that interacted with different proportions of subduction-derived components. Only SEMFR lavas sampled near the extinct arc exhibit different petrographic and geochemical features (higher abundances in olivine and clinopyroxene, lower HREE patterns, lower Nb/Yb and higher Ba/Th), indicating that a more depleted mantle source interacted with a greater subduction input.

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

2011-12-01

178

Tectono-sedimentary development of the (Permian) Karoo sediments in the Kilombero Rift Valley, Tanzania  

NASA Astrophysics Data System (ADS)

The Kilombero Rift Valley in Tanzania belongs to the East African Rift System and contains a sedimentary sequence of Karoo age that is more than 6000 m thick. The present Kilombero Rift basin is most likely of Cenozoic origin. Nevertheless, within the present rift, the Karoo sequences were formed in an earlier rift basin characterised by westerly-dipping boundary faults, i.e. with a different polarity than the present one. The Karoo sequences of Permian age comprise three formations in the Kilombero Rift (Ndeke Beds, Calcareous Beds and Ruhembe Beds). These units are made up of terrigenous, mainly siliciclastic sediments. The succession is well correlated with other East African Karoo units, and displays a rift-controlled sedimentation. The Ndeke Beds represent debris flow, braided stream and floodplain deposits within an alluvial fan system, building out from well developed, high relief border faults, situated to the east in a westward-facing half-graben. The present results point towards a half-graben of about 80 km length and at least 40 km width. There seems to have been only one major early rifting phase, which triggered the deposition of the Ndeke Beds. Most of the Ndeke Beds and the succeeding Calcareous Beds represent a post-rift filling phase. In this development, the alluvial fans formed the marginal parts of a basin, which, through a succeeding topographic levelling and the development towards a low relief landscape, were dominated by lacustrine sedimentation. The overlying, mainly fluviodeltaic Ruhembe Beds reflect a late Karoo tectonic activity. This event is displayed in well-developed debris flow zones near the top of the Karoo succession in the Kilombero Rift basin. Our stratigraphical investigation has not discovered any sedimentary units between the Karoo and Quaternary. However, Mesozoic Beds have been described from the western rifts and the coastal areas of Tanzania, and may also be present in the central areas of the Kilombero Rift Basin. The Cenozoic rift phase is clearly reflected in the present topographic relief, where two synthetic, eastwards facing half-grabens are separated by a well developed, high relief accommodation zone (the Gombati Transfer Zone). The accommodation zone displays a complex tectonic signature due to block faulting and rotation of the strata, between prominent dextral transfer faults. This pattern is in great contrast to the minor tectonic disturbances found in the sediments, both to the north and south of the transfer zone. The switch in half-graben polarity and tectonic setting from the Karoo to the Cenozoic situation, clearly demonstrate that even if the tectonic lineaments tend to follow the same zones of weakness, their polarity and internal configuration is far from straightforward.

Nilsen, O.; Dypvik, H.; Kaaya, C.; Kilembe, E.

1999-08-01

179

Determination and implication of Lower Crustal Bodies in the Møre rifted margin (offshore Norway)  

NASA Astrophysics Data System (ADS)

The Møre margin is the result of a long period of rifting events influenced by different tectonic processes. The last extension phase of the Møre margin was accompanied by the onset of massive magmatic activity leading to the emplacement of seaward dipping reflector sequences (SDRS). However the previous events were probably characterized by a smaller amount of magmatism, like for a classic magma poor rifted margin. Several refraction studies across the Møre rifted margin have also revealed the occurrence of high velocity lower crustal bodies (LCB) under the distal SDRS wedge but also in more proximal positions of the margin (e.g. Kvarven et al., 2012). The nature, age and location of these LCBs are still questionable and represent key and primary parameters to understand the tectonic and crustal evolution of the volcanic rifted margin. Nature, age and geometry of these LCBs have major implications on the rheological and thermal evolution of the rift/margin system. In light of 2D potential field modelling combined with reflection and refraction seismic data, we reinvestigated the crustal nature of the Møre volcanic rifted margin and adjacent Jan Mayen corridor. In the proximal domain of the Møre volcanic rifted margin, our study shows that the LCBs most likely represent inherited crustal bodies and are not necessarily made of rift-related serpentinised mantle as previously proposed. In the distal margin, our preferred interpretation suggests that the outer LCBs are still made of relics of pre-rift lower continental crustal rocks, more or less intruded and/or underplated by Tertiary magmatic rocks. The seismic, magnetic and gravity data do not easily support large scale exhumation of serpentinised mantle in the inner and outer parts of the Møre Basin. Our model suggests that the Møre rift system evolved through a significant Late Jurassic-Cretaceous thinning phase. This rift episode led to a thin continental crust with pre rift lower crustal rocks but without perennial domains of exhumed and serpentinised mantle. This significant extensional event preceded a Late Cretaceous-Early Tertiary rift episode which is characterised by different and independent magmatic and lithospheric processes leading to breakup (e.g. diking and lithospheric plumbing).

Nirrengarten, Michael; Gernigon, Laurent; Manatschal, Gianreto

2014-05-01

180

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)

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 cut by multiple tensile vein sets along the western margin of the Albuquerque basin in the Lucero uplift. At this location, U-series ages on travertine deposits are used to calculate strain rates at this location. These strain rates (15-105 nstr/yr) are higher than both the modern strain rates as well as the average long-term strain rates (3-14 nstr/yr) obtained from restored cross-sections across different basins in the rift. To explain these observations, we propose a model involving high fluid pressures, which promote the formation of tensile veins that are oriented with respect to the modern day stress field in the rift. These regions of anomalously-high strain need not be widespread, and are only active on timescales of the hydraulic system, but they are nevertheless an underappreciated mechanism of progressive extension in the rift. Berglund, H.T., Sheehan, A.F., Murray, M.H., Roy, M., Lowry, A.R., Nerem, R.S., and Blume, F., 2012, Distributed deformation across the Rio Grande Rift, Great Plains, and Colorado Plateau: Geology, v. 40, p. 23-26.

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

2013-12-01

181

Elastic Lithosphere Thickness and Heat Flux Estimates from Rift Valley Topography: Coracis Fossae, Mars  

NASA Astrophysics Data System (ADS)

The Coracis Fossae in the Thaumasia region on Mars are two several hundred kilometer long and ˜50 km wide extensional structures. Their complex morphology, fractured graben floors and segmented border faults, which are arranged in en echelon pattern, suggest that they are Martian analogues to terrestrial rift systems. At Coracis Fossae's NE segment rift flank uplift is most pronounced, the rift shoulders having heights of more than 1000 m with respect to the surrounding planes. We model the uplift by fitting a flexed broken plate to the topography data obtained by the Mars Orbiter Laser Altimeter. Thus, the elastic thickness at the time of rifting is constrained to 10.3 - 12.5 km. Assuming a diabase composition of the crust, this corresponds to a thermal gradient of 27 - 33 K km-1. Investigating the key surface units associated with the rifting, the time of rift formation is determined by measuring their crater size-frequency distribution and comparing the results to an impact cratering chronology model. The time of rifting is thus constrained to 3.5 - 3.9 Gyr b.p. Given the fault block topography and elastic thickness, the stresses acting on the bounding faults which support the topography may be calculated. We estimate that at the Coracis Fossae the faults need not withstand stresses in excess of 5 MPa, a value comparable to terrestrial faults. We take this weakness as an indication that the faults are or have been in contact with liquid water below the surface.

Grott, M.; Hauber, E.; Werner, S. C.; Kronberg, P.; Neukum, G.

2005-08-01

182

Structural geology of the African rift system: Summary of new data from ERTS-1 imagery. [Precambrian influence  

NASA Technical Reports Server (NTRS)

ERTS imagery reveals for the first time the structural pattern of the African rift system as a whole. The strong influence of Precambrian structures on this pattern is clearly evident, especially along zones of cataclastic deformation, but the rift pattern is seen to be ultimately independent in origin and nature from Precambrian tectonism. Continuity of rift structures from one swell to another is noted. The widening of the Gregory rift as its northern end reflects an underlying Precambrian structural divergence, and is not a consequence of reaching the swell margin. Although the Western Rift is now proven to terminate at the Aswa Mylonite Zone, in southern Sudan, lineaments extend northeastwards from Lake Albert to the Eastern Rift at Lake Stefanie. The importance of en-echelon structures in the African rifts is seen to have been exaggerated.

Mohr, P. A.

1974-01-01

183

Form and growth of an embryonic continental rift: InSAR observations and modelling of the 2009 western Arabia rifting episode  

NASA Astrophysics Data System (ADS)

A magma-driven rifting episode occurred at the Harrat Lunayyir (Harrat Al-shaqa) volcanic field, western Arabia, between 2009 April and July. It was accompanied by a swarm of more than 4000 M > 2 earthquakes, the largest ever documented in that region, with a peak Mw 5.7 shock on May 19. We combine Interferometric Synthetic Aperture Radar (InSAR) measurements and elastic modelling with seismic moment calculations to resolve the evolution of surface deformation associated with this event. Phase discontinuities and low-coherence lineaments are used to infer the location of the main active structures during the various deformation stages and descending-track interferograms that span the entire period are inverted to resolve the slip and opening distributions along two graben-bounding normal faults and a dyke, respectively. Assuming negligible rift-parallel displacements, we combine ascending- and descending-track interferograms to derive the vertical and rift-perpendicular deformation, which add up to a maximum surface extension of 1.5 m across the rift and subsidence of 0.8 m. The far-field deformation is dominated by the dyke opening, whereas the near-field displacements are mostly associated with movements along the faults. The cumulative seismic moment released during the entire swarm period accounts for about 14 per cent of the total geodetic moment, compared to about 55 per cent at the 2007 Gelai (Tanzania) and about 8 per cent at the 2005 Manda Hararo-Dabbahu (Afar) events. We propose that the differences in moment partitioning ratios are due to the different crustal and seismogenic layer thicknesses in the three regions and represent different stages in the evolution of a volcanic rift. The Gelai event represents the most juvenile stage of rifting, the Dabbahu event represents the most evolved and the Harrat Lunayyir event represents a rift that is intermediate between the two in its degree of maturity.

Baer, Gidon; Hamiel, Yariv

2010-07-01

184

Geodetic determination of plate velocity vector in the Ethiopia Rift  

NASA Astrophysics Data System (ADS)

The Red sea, Gulf of Aden and the Main Ethiopian rift form a triple junction in the Afar Region. Although the East African Rift, the divergent plate boundary between Nubian and Somalia, is often cited as a modern archetype for rifting and continental breakup, its current kinematics is the least known of all major plate boundaries. Moreover, geodetic datum in such tectonically active area is subject to distortion that increases with time. Therefore, a close study of the positions and velocities of reference stations in such tectonic active areas is necessary, if one wants to have high precise geodetic measurement for any developmental activity. In this study phase and pseudo-range GPS measurements were processed to derive the daily solutions of positions in reference to the ITRF05. This solution from 8 continuous stations in Ethiopia, with a length of 0.75 to 2.67 years, is then combined into a cumulative solution with position and velocity estimates. Here a method that combines GPS observation data from 2007 to 2009 to estimate time-dependent motion of stations in a region of active deformation is implemented. First, observations were analysed separately to produce loosely constrained estimates of station positions and coordinate system parameters which are then combined with appropriate constraints to estimate velocities and co-seismic displacements. The result archived gives a good insight about the velocity at which the three major plates, namely the Nubian, Arabian and Somalia plates are moving with respect to each other. The study shows the relative velocity between Nubia and Somalia plates with 4.6±0.3 mm/yr. While, the Nubia and Arabia plates are moving with 33±0.15mm/yr.Moreover; positions of stations are computed with high precession for any future reference purpose. Due to short duration of measurements at some stations further observation are recommended to compute positions and velocity fields after all stations have data at least for two years time. Key words: GPS, Space geodesy, ITRF05, Deformation

Boku, E.; Teklemariam, E.; Rivalta, E.

2011-12-01

185

The development of extension and magmatism in the Red Sea rift of Afar  

NASA Astrophysics Data System (ADS)

Despite the importance of continental breakup in plate tectonics, precisely how extensional processes such as brittle faulting, ductile plate stretching, and magma intrusion evolve in space and time during the development of new ocean basins remains poorly understood. The rifting of Arabia from Africa in the Afar depression is an ideal natural laboratory to address this problem since the region exposes subaerially the tectonically active transition from continental rifting to incipient seafloor spreading. We review recent constraints on along-axis variations in rift morphology, crustal and mantle structure, the distribution and style of ongoing faulting, subsurface magmatism and surface volcanism in the Red Sea rift of Afar to understand processes ultimately responsible for the formation of magmatic rifted continental margins. Our synthesis shows that there is a fundamental change in rift morphology from central Afar northward into the Danakil depression, spatially coincident with marked thinning of the crust, an increase in the volume of young basalt flows, and subsidence of the land towards and below sea-level. The variations can be attributed to a northward increase in proportion of extension by ductile plate stretching at the expense of magma intrusion. This is likely in response to a longer history of localised heating and weakening in a narrower rift. Thus, although magma intrusion accommodates strain for a protracted period during rift development, the final stages of breakup are dominated by a phase of plate stretching with a shift from intrusive to extrusive magmatism. This late-stage pulse of decompression melting due to plate thinning may be responsible for the formation of seaward dipping reflector sequences of basalts and sediments, which are ubiquitous at magmatic rifted margins worldwide.

Keir, Derek; Bastow, Ian D.; Pagli, Carolina; Chambers, Emma L.

2013-11-01

186

Kinematic modelling of large-scale structural asymmetry across the Dead Sea Rift  

NASA Astrophysics Data System (ADS)

The Dead Sea Rift (DSR) is characterized by large-scale topographic and structural asymmetries: the rift's eastern side is flexed upward toward the axis and its overall shape resembles an uplifted shoulder; the western side of the rift is flexed down toward the axis and its overall shape resembles a wide arch. We use a kinematic model of the lithosphere to explain the cumulative deformation of the pre-rift topography in response to two tectonic processes: normal faulting due to lithospheric extension and isostatic uplift. The model considers the sum relief of three surfaces: relief that existed prior to the formation of the rift (initial topography), relief created by slip along a curved normal boundary fault (kinematic topography), and relief created by isostatic response of the lithosphere to this faulting and to an additional unmodelled load (isostatic topography). The model predicts the observed structure across the rift only when we considered a significant additional load, comparable in magnitude to the load induced by the kinematic topography. The additional load reflects the negative mass anomaly of the 8-10-km-deep Dead Sea Basin, which is filled with unconsolidated sediments. By constraining the model with the structural observations, we determined that the extension perpendicular to the rift axis lies in the range 1-4 km in the northern half of the rift and 2-8 km in the southern half. The model also explains other observations across the DSR, such as the configuration of the three present-day regional drainage systems and the observed low-magnitude upward deflection of the Moho beneath the rift.

Wdowinski, S.; Zilberman, Ezra

1996-12-01

187

Crustal structure of the northern mississippi embayment and a comparison with other continental rift zones  

USGS Publications Warehouse

Previous geological and geophysical investigations have suggested that the Mississippi Embayment is the site of a Late Precambrian continental rift that was reactivated in the Mesozoic. New information on the deep structure of the northern Mississippi Embayment, gained through an extensive seismic refraction survey, supports a rifting hypothesis. The data indicate that the crust of the Mississippi Embayment may be characterized by six primary layers that correspond geologically to unconsolidated Mesozoic and Tertiary sediments (1.8 km/s), Paleozoic carbonate and clastic sedimentary rocks (5.9 km/s), a low-velocity layer of Early Paleozoic sediments (4.9 km/s), crystalline upper crust (6.2 km/s), lower crust (6.6 km/s), modified lower crust (7.3 km/s), and mantle. Average crustal thickness is approximately 41 km. The presence and configuration of the low-velocity layer provide new evidence for rifting in the Mississippi Embayment. The layer lies within the northeast-trending upper-crustal graben reported by Kane et al. (1981), and probably represents marine shales deposited in the graben after rifting. The confirmation and delineation of a 7.3 km/s layer, identified in previous studies, implies that the lower crust has been altered by injection of mantle material. Our results indicate that this layer reaches a maximum thickness in the north-central Embayment and thins gradually to the southeast and northwest, and more rapidly to the southwest along the axis of the graben. The apparent doming of the 7.3 km/s layer in the north-central Embayment suggests that rifting may be the result of a triple junction located in the Reelfoot Basin area. The crustal structure of the Mississippi Embayment is compared to other continental rifts: the Rhinegraben, Limagnegraben, Rio Grande Rift, Gregory Rift, and the Salton Trough. This comparison suggests that alteration of the lower crust is a ubiquitous feature of continental rifts. ?? 1983.

Mooney, W.D.; Andrews, M.C.; Ginzburg, A.; Peters, D.A.; Hamilton, R.M.

1983-01-01

188

An integrated geophysical analysis of the upper crust of the southern Kenya rift  

NASA Astrophysics Data System (ADS)

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 Quaternary volcanic centres that produce a series of isolated axial gravity highs; (6) the other element of the axial gravity high is an intrarift horst block that extends along the axis of most of the rift valley.

Simiyu, Silas M.; Keller, G. Randy

2001-12-01

189

Arc rifting of the Carolina terrane in northwestern South Carolina  

SciTech Connect

Recent mapping and whole-rock geochemistry studies demonstrate that mafic metavolcanic rocks found along the boundary between the exotic Carolina terrane and the Inner Piedmont formed in a subduction-related volcanic arc and do not represent the Iapetan suture. Mafic metavolcanic rocks are spatially and genetically related to zoned mafic-ultramafic intrusive complexes. These rocks are similar to those found in other ancient and modern volcanic island arcs where ankaramites and picrites are well known, and they are locally associated with zoned complexes, e.g., Sierra Foothills-Klamath Mountains of the western U.S. Cordillera. The authors propose that prior to accretion to Laurentia in the early to middle Paleozoic, the Carolina arc terrane underwent an episode of intra-arc rifting which allowed primitive arc magmas to ascend and erupt without significant crystal fractionation or lithospheric assimilation. This interpretation may help resolve some stratigraphic problems in the eastern part of the Carolina terrane (Caroline slate belt).

Dennis, A.J. (Univ. of South Carolina, Aiken (USA)); Shervais, J.W. (Univ. of South Carolina, Columbia (USA))

1991-03-01

190

Localized crustal deformation in the Godavari failed rift, India  

NASA Astrophysics Data System (ADS)

Six years of GPS measurements of crustal deformation in the Godavari failed rift (GFR) of stable India plate suggest very localized deformation. Elsewhere, all along the GFR the deformation is very low (<1.5 mm/yr). Localized deformation (up to 3.3±0.5 mm/yr) at least at two sites, implying compression on steep faults located on the southern margin of the GFR, is coincident with the region characterized by high level low-magnitude seismicity of past six years and implies strain accumulation for future moderate to strong magnitude earthquake in the region. The localized deformation is consistent with the view about deformation in such regions where seismicity migrates and deformation rate changes with time.

Mahesh, P.; Gahalaut, V. K.; Catherine, J. K.; Ambikapathy, A.; Kundu, Bhaskar; Bansal, Amit; Chadha, R. K.; Narsaiah, M.

2012-06-01

191

Rift Valley Fever Outbreaks in Mauritania and Related Environmental Conditions  

PubMed Central

Four large outbreaks of Rift Valley Fever (RVF) occurred in Mauritania in 1998, 2003, 2010 and 2012 which caused lots of animal and several human deaths. We investigated rainfall and vegetation conditions that might have impacted on RVF transmission over the affected regions. Our results corroborate that RVF transmission generally occurs during the months of September and October in Mauritania, similarly to Senegal. The four outbreaks were preceded by a rainless period lasting at least a week followed by heavy precipitation that took place during the second half of the rainy season. First human infections were generally reported three to five weeks later. By bridging the gap between meteorological forecasting centers and veterinary services, an early warning system might be developed in Senegal and Mauritania to warn decision makers and health services about the upcoming RVF risk. PMID:24413703

Caminade, Cyril; Ndione, Jacques A.; Diallo, Mawlouth; MacLeod, Dave A.; Faye, Ousmane; Ba, Yamar; Dia, Ibrahima; Morse, Andrew P.

2014-01-01

192

Basin evolution and the distribution of strain within the Gulf of Corinth rift  

NASA Astrophysics Data System (ADS)

The Gulf of Corinth is a classic young active continental rift initiating <5 Ma and with current extension rates up to 15 mm/yr. The modern rift (ca. 1-2 Myr old) has been studied extensively both onshore and offshore. In this paper we bring together the results of study of the offshore rift with existing onshore data to generate a model for how the modern rift has tectonically evolved, how strain is distributed across and along the rift, how slip on individual major faults controlling rift basin subsidence has changed over relatively short timescales (e.g. <0.5 Myr) and how extension in the upper crust through fault displacement compares with whole crustal extension over the history of the rift. The results indicate that the rift stratigraphy is divided into two units (pre- and post- ca. 0.5Ma). The two units indicate markedly different rift basin geometry during these two time periods. Two separated depocentres 20-50 km long were created controlled by N- and S-dipping faults before ca. 0.5 Ma, while since ca. 0.5 Ma a single depocentre (80 km long) has been controlled by several connected N-dipping faults, with maximum subsidence focused between the two older depocentres. Thus isolated but nearby faults can persist for timescales ca. 1 Ma and form major basins before becoming linked. There is a general evolution towards a dominance of N-dipping faults; however, in the western Gulf strain is distributed across several active N- and S-dipping faults throughout rift history, producing a more complex basin geometry. Examination of extension at a larger spatial and temporal scale suggests that uniform pure shear extension without the need for a significant N-S dipping detachment fault is a viable extension mechanism for at least the western rift where constraints are greater. These results also indicate that the present day strain distribution indicated by GPS data cannot have persisted over the lifetime of the modern rift. We are now building on these studies by performing a major data integration exercise of all available seismic reflection (both conventional and high resolution, digital and analogue) and bathymetric data within the Gulf. This integrated dataset will allow us to refine and improve the syn-rift chronostratigraphic model (based on sequence stratigraphic interpretation) for the Gulf, to correlate this stratigraphic interpretation throughout the Gulf, and to interpret the rift fault network at a much great level of detail than before. We will use these results to refine locations of proposed ocean drilling boreholes which may ultimately provide critical absolute chronological and environmental information. In addition the results will allow us to develop models of rift fault activity, growth and interaction, a detailed history of rift evolution in time and space, and identify the roles of tectonics and climate on sediment flux into the rift and landscape development.

Bell, Rebecca; McNeill, Lisa; Nixon, Casey; Henstock, Timothy; Bull, Jonathan; Christodoulou, Dimitris; Papatheodorou, George; Taylor, Brian; Ferentinos, George; Sakellariou, Dimitris; Lykousis, Vasilis; Sachpazi, Maria; Ford, Mary; Goodliffe, Andrew; Leeder, Mike; Gawthorpe, Robert; Collier, Richard; Clements, Benjamin

2013-04-01

193

Modeling fault kinematics, segment interaction and transfer zone geometry as a function of pre-existing fabrics: the Albertine rift, East African Rift System.  

NASA Astrophysics Data System (ADS)

This study focuses on the development of the Rwenzori Mountains, an uplift horst block within the northern-most segment of the western branch of the East African Rift System (EARS). Attention is drawn to the role of pre-existing crustal weaknesses left behind by Proterozoic mobile belts that pass around cratonic Archean shields namely the Tanzanian Craton to the southeast and the Congo craton to the northwest. We study how the southward propagating sub-segment of the rift that contains Lake Albert to the north interacts with the northward propagating sub-segment that contains the lakes Edward and George and how this interaction produces the structural geometries observed within and around the Rwenzori horst block. Analogue experiments are used to simulate behavior of the upper crust with pre-cut rubber strips of varying overstep/overlap, placed oblique and/or orthogonal to the extension vector. The points of connection to the basal sheet present velocity discontinuities to localize deformation below the sand. Surface geometry of the developing rifts and section cuts are used to study the kinematics that result from the given boundary conditions. In general we try to model two parallel rifts that propagate towards each other and interact. Results show that greater overstep of rifts produces an oblique shear-dominated transfer zone with deep grabens (max.7.0km) in the adjoining segments. Smaller overlap ends in extension-dominated transfer, offset rift segments without oblique transfer faults to join two adjacent rift arms and produces moderately deep grabens (max.4.6km). When overlap doubles the overstep (SbR5), rifts propagate sub-orthogonal to the extension direction in a rotation-dominated transfer and form shallow valleys (max.2.9km). Whether a block like the Rwenzori Mountains is captured and rotates, depends on the overlap/overstep ratio where the rotation direction of a captured block is determined by the sense of overlap (right- or left-lateral). Fault orientation, fault kinematics and block rotation (once in play) re-enforce each other, and depending on the local kinematics different parts of a captured block can be rotated by different amounts but in the same general direction. The results are compared with the natural scenario. Keywords: Albertine rift; Analogue; Extension; Kinematics; Transfer zone

Aanyu, Kevin; Koehn, Daniel

2010-05-01

194

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

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

195

Epidemiological study of Rift Valley fever virus in Kigoma, Tanzania.  

PubMed

Rift Valley fever virus (RVFV) is an acute, zoonotic viral disease caused by a Phlebovirus, which belongs to the Bunyaviridae family. Among livestock, outbreaks of the disease are economically devastating. They are often characterised by large, sweeping abortion storms and have significant mortality in adult livestock. The aim of the current study was to investigate RVFV infection in the Kigoma region, which is nestled under the hills of the western arm of the Great Rift Valley on the edge of Lake Tanganyika, Tanzania. A region-wide serosurvey was conducted on non-vaccinated small ruminants (sheep and goats, n = 411). Sera samples were tested for the presence of anti-RVFV antibodies and viral antigen, using commercial enzyme-linked immunosorbent assay and reverse transcriptase polymerase chain reaction, respectively. The overall past infections were detected in 22 of the 411 animals, 5.4% (Confidence Interval (CI) 95% = 3.5% - 8.1%). The Kigoma rural area recorded the higher seroprevalence of 12.0% (CI 95% = 7.3% - 18.3%; p < 0.0001), followed by Kibondo at 2.3% (CI 95% = 0.5% - 6.5%; p > 0.05) and the Kasulu district at 0.8% (CI 95% = 0.0% - 4.2%; p > 0.05). The prevalence was 12.5% and 4.7% for sheep and goats, respectively. Reverse transcriptase polymerase chain reaction results indicated that only eight samples were found to be positive (n = 63). This study has confirmed, for the first time, the presence of the RVFV in the Kigoma region four years after the 2007 epizootic in Tanzania. The study further suggests that the virus activity exists during the inter-epizootic period, even in regions with no history of RVFV. PMID:25005550

Kifaro, Emmanuel G; Nkangaga, Japhet; Joshua, Gradson; Sallu, Raphael; Yongolo, Mmeta; Dautu, George; Kasanga, Christopher J

2014-01-01

196

Metavolcanics trace the development of ocean rifting, SW Svalbard  

NASA Astrophysics Data System (ADS)

This study investigates late Neoproterozoic metavolcanics from southwestern Spitsbergen to determine the evolution of rifting and the relationship with continental breakup. A number of exposures of metavolcanics are connected with the regional Torellian unconformity, which is associated with Late Neoproterozoic orogenesis - post 640 Ma. This unconformity divides the older metasedimentary sequence of the Deilegga Group from the younger Sofiebogen metasedimentary Group. Samples have been collected from three areas from south to the north: Wedel Jarlsberg Land including the Werenskiöldbreen area (Jens Erikfjellet Formation) and the Chamberlindalen area as well as pillow lavas from Nordenskiöld Land, all belonging to the Sofiebogen Group. The rocks from the Werenskiöldbreen area are classified as subalkaline basalt to basaltic andesite, while metavolcanics from Chamberlindalen are alkaline basalts, whereas rocks from Nordenskiöld Land are tholeiitic basalts. All of them are affected by metamorphism under a minimum of greenschist facies conditions. The immobile REE are used throughout, because they are least affected by metamorphism. The REE profiles show flat patterns for samples from Nordenskiöld Land, whereas the rest of the samples are enriched in LREE. The trace element geochemistry indicates LREE-enrichment for samples from the Werenskiöldbreen area (Lan/Smn 2.1 - 5.7) and Chamberlindalen (Lan/Smn = 1.8 - 3.2), but LREE-depletion for Nordeskiöld metabasalts (Lan/Smn = 1.0 - 1.5). The Smn/Ybn ratio is low for metavolcanics from the Werenskiöldbreen area and from Nordenskiöld Land, while rocks of Chamberlindalen have elevated Smn/Ybn. The metavolcanics from SW Svalbard indicate an increasingly depleted character from south to north. We believe this reflects increasing degrees of melting as rifting develops, signaling continental break-up. This research is supported by NCN No. 2012/05/N/ST10/03594

Goluchowska, Karolina; Barker, Abigail; Manecki, Maciej; Majka, Jaros?aw; Czerny, Jerzy

2013-04-01

197

Molecular biology and genetic diversity of Rift Valley fever virus  

PubMed Central

Rift Valley fever virus (RVFV), a member of the family Bunyaviridae, genus Phlebovirus, is the causative agent of Rift Valley fever (RVF), a mosquito-borne disease of ruminant animals and humans. The generation of a large sequence database has facilitated studies of the evolution and spread of the virus. Bayesian analyses indicate that currently circulating strains of RVFV are descended from an ancestral species that emerged from a natural reservoir in Africa when large-scale cattle and sheep farming were introduced during the 19th century. Viruses descended from multiple lineages persist in that region, through infection of reservoir animals and vertical transmission in mosquitoes, emerging in years of heavy rainfall to cause epizootics and epidemics. On a number of occasions, viruses from these lineages have been transported outside the enzootic region through the movement of infected animals or mosquitoes, triggering outbreaks in countries such as Egypt, Saudi Arabia, Mauritania and Madagascar, where RVF had not previously been seen. Such viruses could potentially become established in their new environments through infection of wild and domestic ruminants and other animals and vertical transmission in local mosquito species. Despite their extensive geographic dispersion, all strains of RVFV remain closely related at the nucleotide and amino acid level. The high degree of conservation of genes encoding the virion surface glycoproteins suggests that a single vaccine should protect against all currently circulating RVFV strains. Similarly, preservation of the sequence of the RNA-dependent RNA polymerase across viral lineages implies that antiviral drugs targeting the enzyme should be effective against all strains. Researchers should be encouraged to collect additional RVFV isolates and perform whole-genome sequencing and phylogenetic analysis, so as to enhance our understanding of the continuing evolution of this important virus. This review forms part of a series of invited papers in Antiviral Research on the genetic diversity of emerging viruses. PMID:22710362

Ikegami, Tetsuro

2013-01-01

198

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)

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 structures (faults, fissures, ...) were recog- nized in some places. From the satellite imagery, the northwestern part of the basin is marked by the succession of paleo-shorelines evidencing the lake level fluctuations. The highest level was dated to 10.000 yrs BP, when the Rukwa was supposed to flow into lake Tanganyika. The corresponding shoreline is cut by a fault, ~ 40 km long, striking NNW-SSE, evidenced on the InSAR DEM and confirmed by the LANDSAT and ASTER optical imagery. Although a possible strike-slip component, the 5 to 10 1 meters of vertical offset from one to the other side of the fault would yield to a slip rate of 0.5 U 1mm/yr. 2

Kervyn, F.

199

From surface observations to in depth structures and plate kinematics in oblique rifts. Insights from the Main Ethiopian Rift.  

NASA Astrophysics Data System (ADS)

The Main Ethiopian Rift (MER) is the oblique NE-SW trending plate boundary between Nubia and Somalia that developed above an inherited lithospheric-scale weak zone, the Mozambique Ocean Suture Zone (MOSZ). In this area, plate's kinematics has been estimated from various sources including: GPS and seismic data, spreading rate estimates, magnetic anomaly & paleostress reconstruction from field and seismic data. These various data sets give a range for the orientation of the stretching direction between N105° and 115°. However, in the MER, it is shown that strain is strongly partitioned between boundary and internal faults. Far field stress is re-oriented along the boundary faults that are activated in pure extension. This observation questions the use of paleostress reconstructions based on fault slip data and focal mechanisms to estimate the direction of plate motion. Detailed analysis of fault orientation and fault kinematics in analogue model illustrates that strain partitioning is triggered by the geometry of the deep-seated weak zone and that fault orientations give a better insight on the direction of stretching than paleostress tensors. Moreover, our model allows the recovering of the far field stress direction and the orientation of the weak zone in depth from surface observation of fault trend data. Applying this model to surface data of the MER give a different stretching direction for the formation of the boundary and the internal fault suggesting a clockwise rotation of Somalia. Also the model gives major constraints on the direction of the deep-seated weak zone. Its orientation is evolving from N62° in the Northern MER, to N18° in the Wide Rifted Zone (passing from N35° and N25° for the Central MER and the Southern MER).

Philippon, Melody; Sokoutis, Dimitrios; Willingshofer, Ernst; Corti, Giacomo; Bonini, Marco; Sani, Federico; Cloetingh, Sierd

2013-04-01

200

Geometry and growth of an inner rift fault pattern: the Kino Sogo Fault Belt, Turkana Rift (North Kenya)  

NASA Astrophysics Data System (ADS)

A quantitative analysis is presented of the scaling properties of faults within the exceptionally well-exposed Kino Sogo Fault Belt (KSFB) from the eastern part of the 200-km-wide Turkana rift, Northern Kenya. The KSFB comprises a series of horsts and grabens within an arcuate 40-km-wide zone that dissects Miocene-Pliocene lavas overlying an earlier asymmetric fault block. The fault belt is ˜150 km long and is bounded to the north and south by transverse (N50°E and N140°E) fault zones. An unusual feature of the fault system is that it accommodates very low strains (<1%) and since it is no older than 3 Ma, it could be characterised by extension rates and strain rates that are as low as ˜0.1 mm/yr and 10 -16 s -1, respectively. Despite its immaturity, the fault system comprises segmented fault arrays with lengths of up to 40 km, with individual fault segments ranging up to ˜9 km in length. Fault length distributions subscribe to a negative exponential scaling law, as opposed to the power law scaling typical of other fault systems. The relatively long faults and segments are, however, characterised by maximum throws of no more than 100 m, providing displacement/length ratios that are significantly below those of other fault systems. The under-displaced nature of the fault system is attributed to early stage rapid fault propagation possibly arising from reactivation of earlier underlying basement fabrics/faults or magmatic-related fractures. Combined with the structural control exercised by pre-existing transverse structures, the KSFB demonstrates the strong influence of older structures on rift fault system growth and the relatively rapid development of under-displaced fault geometries at low strains.

Vétel, William; Le Gall, Bernard; Walsh, John J.

201

Bookshelf faulting and horizontal block rotations between overlapping rifts in southern Afar  

SciTech Connect

Lateral slip on initially rift-parallel normal faults may be a particularly efficient mechanism to accommodate strain between overlapping oceanic rifts. It occurs in southern Afar, where clockwise block rotations result from distributed dextral shear between the overlapping Ghoubbet Asal-Manda Inakir and Manda Hararo-Abhe Bad rifts. Faulting observed during the 1969, Serdo earthquakes and on SPOT images is consistent with the shear being taken up by left-lateral slip on steep NW-SE striking faults, which formed as normal faults before extensional strain became localized in the two rifts. This bookshelf faulting accounts quantitatively for the 14.5{degree} {plus minus}7.5{degree} rotation documented by paleomagnetism in the 1.8 {plus minus}0.4 Ma old Afar stratoid basalts, given the 17.5 {plus minus}5 mm/yr rate of separation between Arabia and Somalia.

Tapponnier, P.; Armijo, R.; Manighetti, I.; Courtillot, V. (Institut de Physique du Globe de Paris (France))

1990-01-01

202

Studies on Rift Valley fever in some African murids (Rodentia: Muridae).  

PubMed

Brains, spleens and livers of 2214 murids, 27 shrews and 7 dormice, trapped at 7 sites in Rhodesia, were tested in 277 pools for the presence of Rift Valley Fever virus. There were no isolations of Rift Valley Fever, but 69 isolations of an unidentified virus were obtained. Sixteen out of 867 sera had low-titre haemagglutination-inhibition activity against Rift Valley Fever antigen, but only one out of 1260 sera had neutralizing antibody. The evidence suggests that murids fail to encounter infection in nature and are unlikely to play a role in circulation and dissemination of Rift Valley Fever virus. Four out of seven widely distributed species of muried, Rhabdomys pumilio, Saccostomys campestris, Aethomys chrysophilus and Lemniscomys griselda, were shown to be capable of circulating amounts of virus likely to be infective for mosquitoes. PMID:632561

Swanepoel, R; Blackburn, N K; Efstratiou, S; Condy, J B

1978-04-01

203

CASERTZ aeromagnetic data reveal late Cenozoic flood basalts (?) in the West Antarctic rift system  

USGS Publications Warehouse

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

Behrendt, J.C.

1994-01-01

204

Deep structure and origin of the Baikal rift zone Dapeng Zhao a,, Jianshe Lei a  

E-print Network

-8577, Japan b Department of Geology, Kansas State University, Manhattan, KS 66506, USA Received 28 May 2005 earthquakes with magnitudes larger than 6.5 have occurred within this area [8]. The Baikal rift is more than

Gao, Stephen Shangxing

205

Continuity of Mid-Oceanic Ridge and Rift Valley in the Southwestern Indian Ocean Confirmed  

Microsoft Academic Search

The existence of a continuous, rifted, mid-oceanic ridge in the southwestern Indian Ocean, previously predicted by us, has been confirmed by soundings taken by the research vessel Vema during the expedition now in progress.

Maurice Ewing; Bruce C. Heezen

1960-01-01

206

Shear wave velocity and radial anisotropy along the Rio Grande rift  

NASA Astrophysics Data System (ADS)

We have determined shear wave velocity and radial anisotropy beneath the Rio Grande rift by analyzing ambient seismic noise recorded at the USArray Transportable Array in New Mexico. The results reveal a variable degree of lithosphere extension along the rift. Magma chambers are imaged as significant low velocity anomalies under the Albuquerque volcano in the shallow crust and beneath Socorro in the mid-crust. The central and southern rift is characterized by high velocity anomaly in the lower crust and uppermost mantle, reflecting residual materials after the extraction of melt, and by strong radial anisotropy with VSH > VSV in the mid to lower crust, implying horizontal alignment of crustal minerals due to the vigorous extensional deformation. However, low velocity anomaly and small radial anisotropy are observed in the rift in northern New Mexico, suggesting the presence of partial melt in the lower crust and uppermost mantle and a weak lithosphere extension.

Li, A.; Fu, Y. V.

2011-12-01

207

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

208

Failure was not an option- the Mid-Continent Rift system succeeded  

NASA Astrophysics Data System (ADS)

The 1.1 Ga Mid-Continent Rift (MCR) in North America is often viewed as a failed rift formed by isolated midplate volcanism and extension within the ~1.3-~0.98 Ga Grenville orogeny. An alternative view is suggested by analogy with younger and morphologically similar rift systems, whose plate tectonic settings are more easily understood because their surroundings - including seafloor with magnetic anomalies - have not been deformed or destroyed by subsequent collisions and rifting events. In this view, the MCR was part of a larger plate boundary rifting event that resulted in a successful episode of seafloor spreading. This view is motivated by various pieces of evidence. The MCR rifting looks much like rigid plate block motion, such as associated with the West Central African Rift systems formed during the Mesozoic breakup of Africa and South America and the ongoing rifting in the East African Rift region with seafloor spreading in the Gulf of Aden and the Red Sea. This view explains the affinities of the Grenville-age rocks in the central and southern Appalachians to Amazonia rather than Canadian Grenville-age Appalachian rocks. The MCR extends farther to the south than traditionally assumed along the East Continental Gravity High (a buried feature from Ohio to Alabama). This failed portion of the rift system connected to the rift successfully separating Laurentia and Amazonia. The seafloor spreading separating Amazonia from Laurentia may explain the former's relative motion toward Greenland and Baltica. This model is consistent with some of the ~1.1 Ga geological events in Amazonia. A change in the apparent polar wander path for Laurentia during the period of volcanism of the MCR could be attributed to this plate reconfiguration. The extensional phase on the MCR may have ended because motion was taken up by seafloor spreading between Laurentia and Amazonia rather ending due to another continental collision. Later reverse faulting on the MCR normal faults due to compression, perhaps from collisions around Rodinia's margins, would not be unexpected because the MCR would be a relatively weak intraplate zone due to higher crustal temperatures and faults.

Merino, M.; Stein, C. A.; Stein, S. A.; Keller, G. R.; Flesch, L. M.; Jurdy, D. M.

2013-12-01

209

Impact of lithospheric heterogeneities on continental rifting evolution: Constraints from analogue modelling on South Atlantic margins  

NASA Astrophysics Data System (ADS)

Lithospheric-scale experiments integrated with restored crustal transects are used to study the evolution of the Central Segment (confined between the Rio Grande Fracture Zone to the south and the Chain Fracture Zone to the north) of the South Atlantic margin. The presence of crustal inhomogeneities, located within the Brazilian Santos and Campos basins, have been analysed and modelled in order to better understand their effects on the rift evolution and resulting structural architecture of the conjugate rifted margins. The results show that heterogeneities located within the lower crust can have a remarkable impact on the along-margin segmentation promoting articulated basins with horsts and grabens in response to a relative “strong” rheology, and focused and deeper basins related to a relatively “weak” rheology on the equivalent parts of the conjugate pairs. In particular, at the early-stage rift evolution the deformation is concentrated at the inner margin where, in the presence of a weak lower crust rheology, a main deep listric half-graben fault and associated thick and wedge-shaped syn-rift basin sequences are developed. A strong lower crust rheology, instead, gives rise to more planar, rotated, domino-type faulted basins with thinner sequences directly controlled by the individual fault-blocks. At the late-stage rift evolution, once the effects of the initial crustal rheology inhomogeneities are reduced due to the lithosperic thinning process, the outer margin records a late syn-rift sequence which shows comparable thicknesses for both cases of lower crust rheologies. This tectono-stratigraphic evolution of the rifting process gives rise to along-margin alterations in symmetry versus asymmetry of the width and structural architecture. The performed analogue modelling experiments also indicate that during the rifting evolution pieces of brittle mantle are preserved and could be elevated beneath the developed upper crustal structures, giving rise to complicated predictions for the along-margin heat-flow.

Cappelletti, A.; Tsikalas, F.; Nestola, Y.; Cavozzi, C.; Argnani, A.; Meda, M.; Salvi, F.

2013-11-01

210

Rates of volcanic activity along the southwest rift zone of Mauna Loa volcano, Hawaii  

Microsoft Academic Search

Flow by flow mapping of the 65-km-long anbaerial part of the southwest rift zone and adjacent flanks of Mauna Loa Volcano, Hawaii, and about 50 new14C dates on charcoal from beneath these flows permit estimates of rates of lava accumulation and volcanic growth over the past 10,000 years. The sequence of historic eraptions along the southwest rift zone, beginning in

P. W. Lipman

1980-01-01

211

Rates of volcanic activity along the southwest rift zone of Mauna Loa volcano, Hawaii  

Microsoft Academic Search

Flow by flow mapping of the 65-km-long anbaerial part of the southwest rift zone and adjacent flanks of Mauna Loa Volcano,\\u000a Hawaii, and about 50 new14C dates on charcoal from beneath these flows permit estimates of rates of lava accumulation and volcanic growth over the past\\u000a 10,000 years.\\u000a \\u000a The sequence of historic eraptions along the southwest rift zone, beginning in

P. W. Lipman

1980-01-01

212

The late Cenozoic Viviparidae (Mollusca, Gastropoda) of the Albertine Rift Valley (Uganda—Congo)  

Microsoft Academic Search

During late Cenozoic pre-rift times the viviparid genus Bellamya was probably confined to eastern Africa while in the Congo\\u000a Basin occurred the genera Neothauma and Kaya (a new genus described herein). During the Pliocene, Kaya became extinct and\\u000a Neothauma became a relict in Lake Tanganyika. All African rift lakes formed during or after the Pliocene were colonised by\\u000a populations of

D. Van Damme; M. Pickford

1998-01-01

213

Shortening deformation of the back-arc rift basin in the central northern Honshu, Japan  

Microsoft Academic Search

The Pacific plate is being subducted beneath northern Honshu, Japan, forms a classical example of trench-arc-back arc system. The compressional stress, perpendicular to the northern Honshu arc, has produced the shortening deformation in the Miocene back arc rift basins since the Pliocene. Two narrow up-rift zones run parallel to the arc: Dewa hills on the west and Ou Backbone range

N. Kato; H. Sato

2003-01-01

214

Petroleum geology of Cretaceous-Tertiary rift basins in Niger, Chad, and Central African Republic  

SciTech Connect

This overview of the petroleum geology of rift basins in Niger, Chad, and Central African Republic (CAR) is based on exploration work by Exxon and partners in the years 1969-1991. The work included 50,000 km of modern reflection seismic, 53 exploration wells, 1,000,000 km[sup 2] of aeromagnetic coverage, and about 10,500 km of gravity profiles. The results outline ten Cretaceous and Tertiary rift basins, which constitute a major part of the West and Central African rift system (WCARS). The rift basins derive from a multiphased geologic history dating from the Pan-African (approximately 750-550 Ma) to the Holocene. WCARS in the study area is divided into the West African rift subsystem (WAS) and the Central African rift subsystem (WAS) and the Central African rift subsystem (CAS). WAS basins in Niger and Chad are chiefly extensional, and are filled by up to 13,000 m of Lower Cretaceous to Holocene continental and marine clastics. The basins contain five oil (19-43[degrees]API) and two oil and gas accumulations in Upper Cretaceous and Eocene sandstone reservoirs. The hydrocarbons are sourced and sealed by Upper Cretaceous and Eocene marine and lacustrine shales. The most common structural styles and hydrocarbon traps usually are associated with normal fault blocks. CAS rift basins in Chad and CAR are extensional and transtensional, and are filled by up to 7500 m of chiefly Lower Cretaceous continental clastics. The basins contain eight oil (15-39[degrees]API) and one oil and gas discovery in Lower and Upper Cretaceous sandstone reservoirs. The hydrocarbons are sourced by Lower Cretaceous shales and sealed by interbedded lacustrine and flood-plain shales. Structural styles range from simple fault blocks through complex flower structures. The main hydrocarbon traps are in contractional anticlines. Geological conditions favor the discovery of potentially commercial volumes of oil in WCARS basins, of Niger, Chad and CAR. 108 refs., 24 figs., 4 tabs.

Genik, G.J. (Exxon Exploration Co., Houston, TX (United States))

1993-08-01

215

Modeling the 2-D seismic velocity structure across the Kenya rift  

NASA Astrophysics Data System (ADS)

A 460-km-long seismic refraction/wide-angle reflection profile across the East African rift in Kenya has been interpreted using a travel-time inversion method to calculate a two-dimensional crustal and uppermost mantle seismic velocity model. The derived model is consistent with the crustal structure determined by independent interpretation of axial (along the rift) and flank (near the eastern end of the cross profile) data sets. The velocity model indicates that the Kenya rift at this location (near the Equator) is a relatively narrow (about 100 km wide) feature from surface expression (fault-bounded basins) to upper-mantle depths. A 5-km-deep, sediment- and volcanic-filled basin is present beneath the rift valley. Seismic velocities in the underlying crust are slightly higher directly beneath the rift valley than in the adjacent terranes. Additionally, the crust thins by about 8 km (to a thickness of about 30 km) in a 100-km-wide zone beneath the rift valley and anomalously low upper-mantle seismic velocity (Pn ? 7.6 km/s) is present only beneath the thinned crust and extends to depths of greater than 120 km.

Braile, L. W.; Wang, B.; Daudt, C. R.; Keller, G. R.; Patel, J. P.

1994-09-01

216

Quantification and restoration of extensional deformation along the Western Iberia and Newfoundland rifted margins  

NASA Astrophysics Data System (ADS)

Many recent papers describe the structure of the Iberia and Newfoundland rifted margins; however, none of them propose kinematic restorations of the complete rift system to quantify the amount of extension necessary to exhume mantle and initiate seafloor spreading. In our study, we use two pairs of cross sections considered as conjugate lines: one across the Galicia Bank-Flemish Cap and the other across the Southern Iberia Abyssal Plain-Flemish Pass. Both transects have been imaged by reflection- and refraction-seismic methods and have been drilled during Ocean Drilling Program Legs 103, 149, 173, and 210. Drilling penetrated parts of the rift stratigraphy and the underlying basement. The cross sections can therefore be considered as the best-documented conjugate transects across present-day hyperextended, magma-poor rifted margins. The aim of this paper is threefold: (1) provide a detailed description of the crustal architecture of the two conjugate sections, (2) define the extensional structures and their ages, and (3) quantify the amount of strain and strain rate accommodated along these lines. This paper proposes a quantitative description of extension along the Iberia-Newfoundland rift system and discusses the limitations and problems in quantifying extensional deformation along hyperextended rifted margins.

Sutra, Emilie; Manatschal, Gianreto; Mohn, Geoffroy; Unternehr, Patrick

2013-08-01

217

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

USGS Publications Warehouse

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.

Robbins, E. I.

1983-01-01

218

Morphostructural evidence for Recent/active extension in Central Tanzania beyond the southern termination of the Kenya Rift.  

NASA Astrophysics Data System (ADS)

The southern tip zone of the Kenya Rift on the eastern branch of the East African System is usually thought to occur in the so-called North Tanzanian Divergence. In this region, the narrow (50 km-wide) axial graben of southern Kenya splays southwards, via a major EW-trending volcanic lineament, into a 200 km-wide broad rifted zone with three separate arms of normal faulting and tilted fault blocks (Eyasi, Manyara and Pangani arms from W to E). Remote sensing analysis from Central Tanzania demonstrates that rift morphology exists over an area lying 400 km beyond the southern termination of the Kenya Rift. The most prominent rift structures are observed in the Kilombero region and consist of a 100 km-wide range of uplifted basement blocks fringed to the west by an E-facing half-graben inferred to reach depths of 6-8 km from aeromagnetic dataset. Physiographic features (fault scarps), and river drainage anomalies suggest that the present-day rift pattern in the Kilombero extensional province principally results from Recent/Neogene deformation. That assumption is also supported by the seismogenic character of a number of faults. The Kilombero half-graben is superimposed upon an earlier rift system, Karoo in age, which is totally overprinted and is only evidenced from its sedimentary infill. On the other hand, the nature and thickness of the inferred Neogene synrift section is still unknown. The Kilombero rifted zone is assumed to connect northwards into the central rift arm (Manyara) of the South Kenya Rift via a seismically active transverse fault zone that follows ductile fabrics within the Mozambican crystalline basement. The proposed rift model implies that incipient rifting propagates hroughout the cold and strong crust/lithosphere of Central Tanzania along Proterozoic (N140=B0E) basement weakness zones and earlier Karoo (NS)rift structures. A second belt of Recent-active linked fault/basins also extends further East from the Pangani rift arm to the offshore Zanzibar-Kerimbas graben system. The structural connection of the Kilombero rifted zone with the Lake Malawi rift further south is also envisaged and should imply the link of the eastern and western branchs of the East African Rift System south of the Tanzanian craton.

Le Gall, B.; Rolet, J.; Gernigon, L.; Ebinger, C.; Gloaguen, R.

2003-04-01

219

The Corinth Rift Laboratory (CRL) strainmeters: calibration and data analysis  

NASA Astrophysics Data System (ADS)

The Gulf of Corinth (Greece) is one of the most seismic regions in Europe, producing some strong earthquakes in the decades, 1 to 1.5 cm/yr of north-south extension, and frequent seismic swarms. This structure is a 110 km long, N110E oriented graben bounded by systems of very recent normal faults. The Corinth Rift Laboratory (CRL) project is concentrated in the western part of the rift, around the city of Aigion, where instrumental seismicity and strain rate is highest. The CRL Network is made up about fifteen seismic stations as well as tiltmeters, strainmeters or GPS in order to study the local seismicity, and to observe and model the short and long term mechanics of the normal fault system. The instrumental seismicity in the Aigion zone clearly shows a strong concentration of small earthquakes between 5 and 10 km. In order to study slow transient deformation,two borehole strainmeters have been installed in the area (Trizonia, Monasteraki). We focus here on the one installed in the Trizonia island, which is continuously recording the horizontal strain at 150 m depth with a short term resolution better than 10-9. The dominant signal is the earth and sea tidal effects (few 10-7 strain), this one is modulated by the mechanical effects of the free oscillations of the Gulf with periods between 8 and 40 min. The barometric pressure fluctuations acts in combination with the mean sea level variation at longer periods and both effects are not independant. The comparison between the strain data and the two forcing signals exhibits a non zero phase delay of the sea-level which is increasing with period. We estimate a transfer function after few correlation iterations for each forcing signal but the physical interpretation of the sea-level function is still unclear. As the strainmeter is at 150 m depth, below the shoreline, a sea water percolation on land would increase the effect of sea level fluctuation, and be more efficient at longer periods. The dilatometer response to crustal sources, made possible recently through the record of the coseismic strain steps of the 2010, M=5.3 local events, shows a fast relaxation (hours). This suggests poor coupling to the rock, due to cementation problems. The sensor would then act dominantly as a pore pressure sensor. Taking into account the related transfer function, together with the one of the external signals, allow us to study accurately the residual signal in order to find slow transient signatures, especially during the reported seismic swarms. We also present here the first analysis of the Monasteraki strainmeter records after correction of the fast long term drift (10-7/day).

Canitano, A.; Bernard, P.; Linde, A. T.; Sacks, S. I.; Boudin, F.

2010-12-01

220

The Role of Rift Obliquity in Formation of the Gulf of California  

NASA Astrophysics Data System (ADS)

The Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to large-offset normal faults in adjacent pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic basins. The Gulf of California rift has accommodated oblique divergence of the Pacific and North America tectonic plates in northwestern Mexico since Miocene time. Due to its infancy, the rifted margins of the Gulf of California preserve a rare onshore record of early continental break-up processes from which to investigate the role of rift obliquity in strain localization. Using new high-precision paleomagnetic vectors from tectonically stable sites in north-central Baja California, I compile a paleomagnetic transect of Miocene ignimbrites across northern Baja California and Sonora that reveals the timing and distribution of dextral shear associated with inception of this oblique rift. I integrate detailed geologic mapping, basin analysis, and geochronology of pre-rift and syn-rift volcanic units to determine the timing of fault activity on Isla Tiburon, a proximal onshore exposure of the rifted North America margin, adjacent to the axis of the Gulf of California. The onset of strike-slip faulting on Isla Tiburon, ca. 8 - 7 Ma, was synchronous with the onset of transform faulting along a significant length of the nascent plate boundary within the rift. This tectonic transition coincides with a clockwise azimuthal shift in Pacific-North America relative motion that increased rift obliquity. I constrain the earliest marine conditions on southwest Isla Tiburon to ca. 6.4 - 6.0 Ma, coincident with a regional latest Miocene marine incursion in the northern proto-Gulf of California. This event likely flooded a narrow, incipient topographic depression along a ˜650 km-long portion of the latest Miocene plate boundary and corresponds in time and space with formation of a newly-constrained ˜50-100 kilometer-wide transtensional belt of focused strike-slip faulting, basin formation, and rotating crustal blocks. This proto-Gulf of California shear zone, embedded within the wider Mexican Basin and Range extensional province and connected to the San Andreas fault in southern California, hosted subsequent localization of the plate boundary and rupture of the continental lithosphere.

Bennett, Scott Edmund Kelsey

221

The development of the Midcontinent Rift in the context of rapid paleogeographic change  

NASA Astrophysics Data System (ADS)

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 during the main stage. While such a scenario could explain the geochemical signature, it removes an active plume as a heat source contributing to voluminous melt generation. (2) That the North American plate and a deep-seated mantle plume traveled in unison to lower latitudes as a result of large-scale true polar wander. An interpretation of an active contribution from a plume throughout rift development implicitly favors this true polar wander hypothesis.

Swanson-Hysell, N.; Vaughn, A. A.; Mustain, M. R.; Feinberg, J. M.

2012-12-01

222

Extreme uplift of the Rwenzori Mountains in the East African Rift, Uganda: Structural framework and possible role of glaciations  

Microsoft Academic Search

The >5-km-high Rwenzori Mountains in the East African Rift form a promontory on the rift shoulder and are an extreme expression of rift-mountain uplift. This study reports the pattern and the kinematics of major faults in the Rwenzori region. The fault pattern is characterized by a first set of N\\/NNE-striking normal faults that are offset by mainly NE- to E-striking

Uwe Ring

2008-01-01

223

Toarcian–Kimmeridgian depositional cycles of the south-western Morondava Basin along the rifted continental margin of Madagascar  

Microsoft Academic Search

After rifting and final breakup of Gondwana along the former East-African-Antarctic Orogen during the Toarcian–Aalenian, passive\\u000a margins formed around the Proto-Indian Ocean. Sedimentological and stratigraphic studies in the southern Morondava Basin contribute\\u000a to an improved reconstruction of palaeoenvironmental changes during the syn-rift and post-rift margin formation. Depositional\\u000a models based on outcrop and literature data in combination with subsurface data sets

Markus Geiger; Günter Schweigert

2006-01-01

224

Advances in Rift Valley Fever Research: Insights for Disease Prevention  

PubMed Central

Purpose of review The purpose of the study was to review recent research on Rift Valley fever virus (RVFV) infection, encompassing four main areas: epidemiology and outbreak prediction, viral pathogenesis, human diagnostics and therapeutics, and vaccine and therapeutic candidates. Recent findings RVFV continues to extend its range in Africa and the Middle East. Better definition of RVFV-related clinical syndromes and human risk factors for severe disease, combined with early-warning systems based on remote-sensing, simplified rapid diagnostics, and tele-epidemiology, hold promise for earlier deployment of effective outbreak control measures. Advances in understanding of viral replication pathways and host cell-related pathogenesis suggest means for antiviral therapeutics and for more effective vaccination strategies based on genetically engineered virus strains or subunit vaccines. Summary RVFV is a significant health and economic burden in many areas of Africa, and remains a serious threat to other parts of the world. Development of more effective methods for RVFV outbreak prevention and control remains a global health priority. PMID:20613512

LaBeaud, A. Desiree; Kazura, James W.; King, Charles H.

2011-01-01

225

Security Conference in Europe Exposes Rifts between US, Allies  

NSDL National Science Digital Library

A high-level security conference in Munich this weekend exposed rifts between the US and its European allies on the issues of a US missile defense system and the deployment of a "rapid reaction" European Union force. The US missile defense system took center stage over the weekend as Secretary of Defense Donald Rumsfeld announced the Bush administration's plan to go ahead with development and deployment, unilaterally if necessary. Deliberately dropping "national" or "US" from his references to the system, Rumsfeld offered to develop it with European allies and extend its protection to their countries as well. Nonetheless, the European response was guarded and cautiously negative. German Chancellor Gerhard Schroeder warned the US against "overly hasty and early determinations" about deploying missile defenses. The reaction of Russia was more pointed, referring to the possible development of a new arms race that could extend even into outer space (the Bush administration is considering a laser defense system) and the system's violation of the 1972 Anti-Ballistic Missile Treaty. Rumsfeld dismissed European fears as unfounded and suggested that the relationship with Russia needed to be renegotiated on the ground of current threats, rather than outdated cold war assumptions. In the meantime, talks at the conference also revealed some US concern over current European Union plans to develop a "rapid reaction" force that could operate independently of NATO. Turkey's opposition to such a force, combined with the US's concerns, may help to ultimately kill the project.

Charbonneau, David D.

2001-01-01

226

Probable rift origin of the Canada basin, Arctic Ocean  

USGS Publications Warehouse

Formation of the Canada basin by post-Triassic rifting seems the most workable and logical hypothesis with information available. Speculated counterclockwise rotation of the Alaska-Chukchi continental edge best rationalizes the complex geology of northern Alaska, whereas a single continental block before the Jurassic makes the best palinspastic fit for Arctic America. The Arctic Ocean is the focus of present-day spreading and probably was the focus of earlier stages of spreading in which spread of the Canada basin would be an initial stage. If the Atlantic formed by seafloor spreading, spread of the Canada basin is probable because analogies between the Arctic and Atlantic edges indicate a common origin for the ocean basins. Late Cretaceous and younger deflections of the Cordillera in the Arctic and diabasic emplacements in the northern Arctic Islands may reflect later stages of spreading. Pre-Mesozoic plate tectonism may be represented by the widespread Proterozoic diabasic emplacements in the Canadian Arctic and by the Franklinian-Innuitian tract where the volcanogenic rocks and deformation resulted not from a classical eugeosyncline-miogeosyncline couple but from the junction of a mid-Paleozoic continental edge and another plate on closure of a pre-Arctic Ocean.

Tailleur, Irvin L.

1973-01-01

227

Probable rift origin of Canada Basin, Arctic Ocean  

USGS Publications Warehouse

Formation of the Canada basin by post-Triassic rifting seems the most workable and logical hypothesis on the basis of available information. Speculated counterclockwise rotation of the Alaska-Chukchi continental edge best rationalizes the complex geology of northern Alaska, whereas the assumption that a single continental block was present before the Jurassic makes the best palinspastic fit for Arctic America. The Arctic Ocean is the focus of present-day spreading and probably was the focus of earlier stages of spreading in which spread of the Canada basin would have been an initial stage. Spread of the Canada basin is probable if the Atlantic formed by sea-floor spreading, because analogies between the Arctic and Atlantic edges indicate a common origin for the ocean basins. Late Cretaceous and younger deflections of the Cordillera in the Arctic and diabasic emplacements in the northern Arctic Islands may reflect later stages of spreading. Pre-Mesozoic plate tectonism may be represented by the widespread Proterozoic diabasic emplacements in the Canadian Arctic and by the Franklinian-lnnuitian tract, where the volcanogenic rocks and deformation resulted not from a classical eugeosyncline-miogeosyncline couple, but from the junction of a mid-Paleozoic continental edge and another plate on closure of a pre-Arctic Ocean.

Tailleur, Irvin L.

1973-01-01

228

Climate Change Affects the East African Rift Valley Lakes  

NASA Astrophysics Data System (ADS)

Over the last 100 years, air temperatures in eastern African have been warming consistent with the global average temperature increase. This has led to warmer water temperatures in the East African Rift Valley lakes, increasing the stability of the water column. Subsequently, there has been a reduction in the upwelling of deep nutrient-rich waters that are the primary source of nutrients for most of these lakes. There were decreases in surface water N and P and increases in the Si:P ratio over the past 70 years for Lakes Malawi, Tanganyika, Edward, and Albert. The lower nutrient concentrations in the surface waters were associated with reduced algal biomass and increased water clarity. The consistent, regional-scale changes among these lakes provide strong evidence that climate warming may be having a large negative affect on these unique tropical lakes. A decrease in primary productivity of 20% has been indicated for Lake Tanganyika, which would be associated with a 30% decrease in fisheries yields. The human implications of such subtle, but progressive, environmental changes are potentially dire in this densely populated region of the world, where these large lakes are an important nutritional and economic resource.

O'Reilly, C. M.; Plisnier, P.; Cohen, A. S.

2004-12-01

229

Rift Valley fever outbreak--Kenya, November 2006-January 2007.  

PubMed

In mid-December 2006, several unexplained fatalities associated with fever and generalized bleeding were reported to the Kenya Ministry of Health (KMOH) from Garissa District in North Eastern Province (NEP). By December 20, a total of 11 deaths had been reported. Of serum samples collected from the first 19 patients, Rift Valley fever (RVF) virus RNA or immunoglobulin M (IgM) antibodies against RVF virus were found in samples from 10 patients; all serum specimens were negative for yellow fever, Ebola, Crimean-Congo hemorrhagic fever, and dengue viruses. The outbreak was confirmed by isolation of RVF virus from six of the specimens. Humans can be infected with RVF virus from bites of mosquitoes or other arthropod vectors that have fed on animals infected with RVF virus, or through contact with viremic animals, particularly livestock. Reports of livestock deaths and unexplained animal abortions in NEP provided further evidence of an RVF outbreak. On December 20, an investigation was launched by KMOH, the Kenya Field Epidemiology and Laboratory Training Program (FELTP), the Kenya Medical Research Institute (KEMRI), the Walter Reed Project of the U.S. Army Medical Research Unit, CDC-Kenya's Global Disease Detection Center, and other partners, including the World Health Organization (WHO) and Médecins Sans Frontières (MSF). This report describes the findings from that initial investigation and the control measures taken in response to the RVF outbreak, which spread to multiple additional provinces and districts, resulting in 404 cases with 118 deaths as of January 25, 2007. PMID:17268404

2007-02-01

230

Observations on rift valley fever virus and vaccines in Egypt  

PubMed Central

Rift Valley Fever virus (RVFV, genus: Phlebovirus, family: Bunyaviridae), is an arbovirus which causes significant morbidity and mortality in animals and humans. RVFV was introduced for the first time in Egypt in 1977. In endemic areas, the insect vector control and vaccination is considering appropriate measures if applied properly and the used vaccine is completely safe and the vaccination programs cover all the susceptible animals. Egypt is importing livestock and camels from the African Horn & the Sudan for human consumption. The imported livestock and camels were usually not vaccinated against RVFV. But in rare occasions, the imported livestock were vaccinated but with unknown date of vaccination and the unvaccinated control contacts were unavailable for laboratory investigations. Also, large number of the imported livestock and camels are often escaped slaughtering for breeding which led to the spread of new strains of FMD and the introduction of RVFV from the enzootic African countries. This article provide general picture about the present situation of RVFV in Egypt to help in controlling this important disease. PMID:22152149

2011-01-01

231

Persistence of Rift Valley fever virus in East Africa  

NASA Astrophysics Data System (ADS)

Rift Valley fever virus (RVFv) is a mosquito-borne pathogen of livestock, wildlife and humans that causes severe outbreaks in intervals of several years. One of the open questions is how the virus persists between outbreaks. We developed a spatially-explicit, individual-based simulation model of the RVFv transmission dynamics to investigate this question. The model, is based on livestock and mosquito population dynamics. Spatial aspects are explicitly represented by a set of grid cells that represent mosquito breeding sites. A grid cell measures 500 by 500m and the model considers a grid of 100 by 100 grid cells; the model thus operates on the regional scale of 2500km2. Livestock herds move between grid cells, and provide connectivity between the cells. The model is used to explore the spatio-temporal dynamics of RVFv persistence in absence of a wildlife reservoir in an east African semi-arid context. Specifically, the model assesses the importance of local virus persistence in mosquito breeding sites relative to global virus persistence mitigated by movement of hosts. Local persistence is determined by the length of time the virus remains in a mosquito breeding site once introduced. In the model, this is a function of the number of mosquitoes that emerge infected and their lifespan. Global persistence is determined by the level of connectivity between isolated grid cells. Our work gives insights into the ecological and epidemiological conditions under which RVFv persists. The implication for disease surveillance and management are discussed.

Gachohi, J.; Hansen, F.; Bett, B.; Kitala, P.

2012-04-01

232

A statistical model of Rift Valley fever activity in Egypt  

PubMed Central

Rift Valley fever (RVF) is a viral disease of animals and humans and a global public health concern due to its ecological plasticity, adaptivity, and potential for spread to countries with a temperate climate. In many places, outbreaks are episodic and linked to climatic, hydrologic, and socioeconomic factors. Although outbreaks of RVF have occurred in Egypt since 1977, attempts to identify risk factors have been limited. Using a statistical learning approach (lasso-regularized generalized linear model), we tested the hypotheses that outbreaks in Egypt are linked to (1) River Nile conditions that create a mosquito vector habitat, (2) entomologic conditions favorable to transmission, (3) socio-economic factors (Islamic festival of Greater Bairam), and (4) recent history of transmission activity. Evidence was found for effects of rainfall and river discharge and recent history of transmission activity. There was no evidence for an effect of Greater Bairam. The model predicted RVF activity correctly in 351 of 358 months (98.0%). This is the first study to statistically identify risk factors for RVF outbreaks in a region of unstable transmission. PMID:24581353

Hassan, Ali N.; Beier, John C.

2014-01-01

233

Lacustrine deposits in rifted deep basins of Yellow Sea  

SciTech Connect

The central Yellow Sea is a typical intracratonic rifted basin that consists of 4 major depressions bounded by aligned listric faults along horst blocks of uplifted basement (Kunsan, West Kunsan, Yellow Sea sub-basins, and Central Trough). The depressions are half grabens caused by pull-apart extensional stresses. Core analysis and micropaleotologic study indicate that more than 5 km of lacustrine sediments were accumulated in the central part of the West Kunsan basin. Two distinctive sedimentary successions are recognized in the core descriptions: alternation of reddish-brown siltstones and sandstones containing evaporites and marlstones, and an overlying progradational sequence including minor limestone beds in the lower part of the sequence. The progradational sequence is interpreted as lacustrine deltaic deposits. Abundant palynofloral occurrence of freshwater green algae, Pediastrum, and absence of marine fauna such as dinoflagellates are also supporting evidence for a lacustrine environment. The lithofacies and tectonic framework of the Yellow Sea are very similar to those of Cretaceous lacustrine sediments of the Korea Peninsula onshore and Pohai coastal basin in China.

Han, J.H.

1985-02-01

234

Efficacy of three candidate Rift Valley fever vaccines in sheep.  

PubMed

Rift Valley fever virus (RVFV) is a mosquito-transmitted Bunyavirus that causes high morbidity and mortality among ruminants and humans. The virus is endemic to the African continent and the Arabian Peninsula and continues to spread into new areas. The explosive nature of RVF outbreaks requires that vaccines provide swift protection after a single vaccination. We recently developed several candidate vaccines and here report their efficacy in lambs within three weeks after a single vaccination. The first vaccine comprises the purified ectodomain of the Gn structural glycoprotein formulated in a water-in-oil adjuvant. The second vaccine is based on a Newcastle disease virus-based vector that produces both RVFV structural glycoproteins Gn and Gc. The third vaccine comprises a recently developed nonspreading RVFV. The latter two vaccines were administered without adjuvant. The inactivated whole virus-based vaccine produced by Onderstepoort Biological Products was used as a positive control. Five out of six mock-vaccinated lambs developed high viremia and fever and one lamb succumbed to the challenge infection. A single vaccination with each vaccine resulted in a neutralizing antibody response within three weeks after vaccination and protected lambs from viremia, pyrexia and mortality. PMID:22449427

Kortekaas, J; Antonis, A F G; Kant, J; Vloet, R P M; Vogel, A; Oreshkova, N; de Boer, S M; Bosch, B J; Moormann, R J M

2012-05-14

235

Reservoir, seal, and source rock distribution in Essaouira Rift Basin  

SciTech Connect

The Essaouira onshore basin is an important hydrocarbon generating basin, which is situated in western Morocco. There are seven oil and gas-with-condensate fields; six are from Jurassic reservoirs and one from a Triassic reservoir. As a segment of the Atlantic passive continental margin, the Essaouira basin was subjected to several post-Hercynian basin deformation phases, which resulted in distribution, in space and time, of reservoir, seal, and source rock. These basin deformations are synsedimentary infilling of major half grabens with continental red buds and evaporite associated with the rifting phase, emplacement of a thick postrifting Jurassic and Cretaceous sedimentary wedge during thermal subsidence, salt movements, and structural deformations in relation to the Atlas mergence. The widely extending lower Oxfordian shales are the only Jurassic shale beds penetrated and recognized as potential and mature source rocks. However, facies analysis and mapping suggested the presence of untested source rocks in Dogger marine shales and Triassic to Liassic lacustrine shales. Rocks with adequate reservoir characteristics were encountered in Triassic/Liassic fluvial sands, upper Liassic dolomites, and upper Oxfordian sandy dolomites. The seals are provided by Liassic salt for the lower reservoirs and Middle to Upper Jurassic anhydrite for the upper reservoirs. Recent exploration studies demonstrate that many prospective structure reserves remain untested.

Ait Salem, A. (ONAREP, Rabat (Morocco))

1994-07-01

236

Tectonic and magmatic patterns in the Jutulstraumen rift (?) region, East Antarctica, as imaged by high-resolution aeromagnetic data  

NASA Astrophysics Data System (ADS)

The Jutulstraumen ice stream in western Dronning Maud Land may conceal a Jurassic continental rift. Delineating the geometry and the magmatic patterns of this inferred glaciated rift in East Antarctica is important to improve our understanding of the regional tectonic and magmatic processes associated with Gondwana break-up. A high-resolution aeromagnetic survey provides new insights over the largely buried tectonic and magmatic patterns of the Jutulstraumen area. Prominent NE-SW oriented aeromagnetic trends are detected over the Jutulstraumen. These trends delineate major inherited structural boundaries, active in Grenvillian (about 1.1 Ga) and Pan-African times (about 500 Ma), which appear to strongly control the location of the later Jurassic rift. The postulated eastern flank of the rift is marked by a broad positive anomaly over H. U. Sverdrupfjella. Buried Grenvillian age rocks may be the source of the long-wavelength anomaly. However, the higher frequency components correlate with granitoids of late Pan-African age. The inferred western flank of the rift features short-wavelength anomalies over the Borgmassivet and Ahlmannryggen areas, indicating a considerably greater extent of mid-Proterozoic tholeiitic sills than apparent in outcrop. In contrast, aeromagnetic signatures suggest that alkaline plutons, which relate to Jurassic rifting, are restricted to outcrop areas along the eastern rift flank. The prominent magnetic low over the Jutulstraumen indicates either a largely amagmatic rift, or perhaps subglacial sediments within the rift basin.

Ferraccioli, F.; Jones, P. C.; Curtis, M. L.; Leat, P. T.; Riley, T. R.

2005-08-01

237

Spatial Variation of Primordial 3-He in Crustal Fluids Along the East-African Rift System (Ethiopian and the Kenya Rift Section).  

National Technical Information Service (NTIS)

(3)He/(4)He compositions are presented for groundwater samples from the Ethiopian segment of the East-African Rift and from its northern extension, the adjacent Afar region (Djibuti). Helium isotope data are compared to those obtained previously from the ...

E. Griesshaber, S. Weise, G. Darling

1994-01-01

238

563Studinger et al.--Airborne geophysical mapping of the West Antarctic Rift System Mesozoic and Cenozoic extensional tectonics of the West Antarctic Rift  

E-print Network

and Cenozoic extensional tectonics of the West Antarctic Rift System from high-resolution airborne geophysical shoulder close to the Whitmore Mountains block might have been reactivated during regional Cenozoic right Byrd Land (Fig. 1). In contrast, models of the Mesozoic and Cenozoic evolution of the region between

Studinger, Michael

239

Temporal compositional variation of early syn-rift rhyolites along the western margin of the southern Red Sea and northern Main Ethiopian rift  

Microsoft Academic Search

New structural and geochronological analyses indicate that the felsic volcanism at the top of the Oligocene flood basalt sequences along the western margin of the Afar Depression marks the onset of rifting in the southern Red Sea. Geochronological studies show a pattern of riftward migration of the locus of magmatism and faulting from 28 Ma to present-day. Preliminary geochemical analyses

D. Ayalew; E. Bourdon; G. Yirgu; C. Ebinger; E. Wolfenden

2003-01-01

240

Permo-Triassic anatexis, continental rifting and the disassembly of western Pangaea  

NASA Astrophysics Data System (ADS)

Crustal anatectites are frequently observed along ocean-continent active margins, although their origins are disputed with interpretations varying between rift-related and collisional. We report geochemical, isotopic and geochronological data that define an ~ 1500 km long belt of S-type meta-granites along the Andes of Colombia and Ecuador, which formed during 275-223 Ma. These are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon U-Pb dates ranging between 240 and 223 Ma. A model is presented which places these rocks within a compressive Permian arc setting that existed during the amalgamation of westernmost Pangaea. Anatexis and mafic intrusion during 240-223 Ma are interpreted to have occurred during continental rifting, which culminated in the formation of oceanic crust and initiated the break-up of western Pangaea. Compression during 275-240 Ma generated small volumes of crustal melting. Rifting during 240-225 Ma was characterized by basaltic underplating, the intrusion of tholeiitic basalts and a peak in crustal melting. Tholeiitic intrusions during 225-216 Ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust, and we assign this period to the onset of continental drift. Dissected ophiolitic sequences in northern Colombia yield zircon U-Pb dates of 216 Ma. The Permo-Triassic margin of Ecuador and Colombia exhibits close temporal, faunal and geochemical similarities with various crustal blocks that form the basement to parts of Mexico, and thus these may represent the relict conjugate margin to NW Gondwana. The magmatic record of the early disassembly of Pangaea spans ~ 20 Ma (240-216 Ma), and the duration of rifting and rift-drift transition is similar to that documented in Cretaceous-Tertiary rift settings such as the West Iberia-Newfoundland conjugate margins, and the Taupo-Lau-Havre System, where rifting and continental disassembly also occurred over periods lasting ~ 20 Ma.

Cochrane, Ryan; Spikings, Richard; Gerdes, Axel; Ulianov, Alexey; Mora, Andres; Villagómez, Diego; Putlitz, Benita; Chiaradia, Massimo

2014-03-01

241

Ordovician earliest Silurian rift tholeiites in the Acatlán Complex, southern Mexico: Evidence of rifting on the southern margin of the Rheic Ocean  

NASA Astrophysics Data System (ADS)

The Acatlán Complex of southern Mexico is a vestige of a Paleozoic Ocean inferred to be either the Cambro-Ordovician Iapetus and/or the Ordovician-Carboniferous Rheic oceans. Ordovician granitoids in the complex have been interpreted as either the products of dehydration melting, arc or rift magmatism, however, the geochemistry of felsic rocks is inconclusive. The geochemistry of a recently discovered, major, Ordovician-earliest Silurian mafic igneous suite associated with these granitoids is critical to defining the tectonic setting of the igneous event, and to paleogeographic reconstructions. Such data from three areas in the Acatlán Complex (Xayacatlán, Patlanoaya, and Cuaulote) document tholeiitic, within-plate characteristics with a source in either primitive mantle or mantle previously modified by subduction-related magmatism possibly combined with crustal contamination. This, combined with their occurrence as a dike swarm intruding rift-passive margin clastic sedimentary rocks, indicates rifting of a continental margin. Mafic dikes at Xayacatlán yielded a concordant U-Pb TIMS zircon age of 442 ± 1 Ma and a 40Ar/ 39Ar hornblende plateau age of 434 ± 3 Ma. The age of mafic magmatism at other localities is defined by the ages of associated granitoids intruded at ca. 461 Ma and by the age of the youngest detrital zircon in the host rocks: 496 ± 25 Ma at Patlanoaya. Previously published age data suggest that this igneous event may have started earlier at 478 ± 5 Ma (Early Ordovician). Although the life spans of the Iapetus and Rheic oceans overlap during the Ordovician, subduction and collision characterize the former, whereas the latter was in a rift-drift mode. Thus, this Ordovician-earliest Silurian magmatism is more consistent with rifting along the southern margin of the Rheic Ocean. Initiation of rifting at ca. 480 Ma is contemporaneous with separation of peri-Gondwanan terranes, such as Avalonia and Carolinia, from Amazonia-Oaxaquia. Subsequent rifting may be analogous to Mesozoic-Cenozoic western North America where transtensional deformation is related to dispersal of terranes (e.g. Baja California and Cretaceous Baja British Columbia) leading to a gradual collapse of the inboard continental margin.

Keppie, J. Duncan; Dostal, Jaroslav; Miller, Brent V.; Ramos-Arias, M. A.; Morales-Gámez, Miguel; Nance, R. Damian; Murphy, J. Brendan; Ortega-Rivera, Amabel; Lee, J. W. K.; Housh, T.; Cooper, P.

2008-12-01

242

Modeling the tectonic development of the Tucano and Sergipe-Alagoas rift basins, Brazil  

NASA Astrophysics Data System (ADS)

The Tucano and Sergipe-Alagoas basins of northeast Brazil form part of a regional extensional basin system that was operative during the Mesozoic breakup of South America and Africa with both basins developing on Precambrian crust. The synchronous development of the rift basins suggest that they are genetically linked in space and time. Whereas the offshore Sergipe-Alagoas basin is characterized by a substantial thickness of post-rift sediment, the Tucano basin either failed to develop or at least preserve a significant thickness of post-rift sediment. Observed primary porosities within both pre- and syn-rift Tucano basin sediment imply that significant post-rift sedimentation never occurred. Failure to develop significant post-rift subsidence has important thermal and mechanical implications for the reaction of the lithosphère to rifting and can be explained in terms of: (1) depth-dependent lithospheric extension in which intracrustal detachments allow the extension of the crust to be decoupled from the thinning of the lithospheric mantle, (2) small rates of extension that allow the lithosphere to cool during rifting, and/or (3) lithospheric rifting during which the flexural strength of the lithosphere remains high. With respect to points (2) and (3), forward modeling demonstrates that finite rifting rates over a 20-25 m.y. period are insufficient to cool the lithosphere to the point where post-rift subsidence fails to develop. An interesting complication arises when the flexural strength of the lithosphere remains large during rifting: it tends to suppress the vertical motions of the lithosphere, such as those engendered by the cooling of the lithosphere following rifting, thereby reducing significantly the amplitude of the post-rift subsidence. Thus, the lack of post-rift sedimentation within a basin does not necessarily imply that extension has been limited to the crust. From our kinematic modeling of the Tucano basin, the observed negative free-air and Bouguer gravity anomalies (- 120 mGals) suggest that the flexural strength of the lithosphere has remained sufficiently large to maintain the load of the sediment. We can model successfully the observed Tucano basin architecture and gravity anomaly irrespective of whether we assume depth-dependent or depth-independent lithospheric extension primarily because the flexural strength of the lithosphere tends to "buffer" or suppress the amplitude of the post-rift subsidence. In contrast, the adjacent Sergipe-Alagoas basin is associated with low-amplitude gravity anomalies which may reflect a resetting of flexural strength during rifting. Prior to rifting, however, there was no appreciable elevation difference between the Tucano and Sergipe-Alagoas regions (as might be induced by a hot-spot for example) implying that the flexural strength of the lithosphere was similar. The total thickness of the syn- and post-rift sediments within the Sergipe-Alagoas basin is about the same as the thickness of rift-phase sediments in the Tucano basin. Thus, the amount of crustal extension responsible for each basin was similar. However, since the Sergipe-Alagoas basin contains 4-5 km of post-rift sediments, lithospheric mantle thinning in this region must have been significantly greater than the crustal extension to overcome the effects of "flexural bufferring". The need for lithospheric mantle thinning to be greater that crustal extension in the Sergipe-Alagoas region was also a result obtained by modeling the development of the ocean/continent boundary between Brazil and Africa. From our coupled kinematic and rheological model of lithospheric extension, we predict that the ocean/continent boundary should form preferentially at the location of greatest crustal and lithospheric mantle thinning, that is, within the region of maximum depth-independent lithospheric extension. The ocean/continent boundary formed east of the Sergipe-Alagoas basin despite the fact that the Tucano basin represents the region of greatest crustal thinning. If extension had been uniform with dep

Karner, Garry D.; Egan, Stuart S.; Weissel, Jeffrey K.

1992-12-01

243

Sediment infill within rift basins: Facies distribution and effects of deformation: Examples from the Kenya and Tanganyika Rifts, East Africa  

SciTech Connect

Oil is known from lacustrine basins of the east African rift. The geology of such basins is complex and different depending on location in the eastern and western branches. The western branch has little volcanism, leading to long-lived basins, such as Lake Tanganyika, whereas a large quantity of volcanics results in the eastern branch characterized by ephemeral basins, as the Baringo-Bogoria basin in Kenya. The Baringo-Bogoria basin is a north-south half graben formed in the middle Pleistocene and presently occupied by the hypersaline Lake Bogoria and the freshwater Lake Baringo. Lake Bogoria is fed by hot springs and ephemeral streams controlled by grid faults bounding the basin to the west. The sedimentary fill is formed by cycles of organic oozes having a good petroleum potential and evaporites. On the other hand, and as a consequence of the grid faults, Lake Baringo is fed by permanent streams bringing into the basin large quantities of terrigenous sediments. Lake Tanganyika is a meromictic lake 1470 m deep and 700 km long, of middle Miocene age. It is subdivided into seven asymmetric half grabens separated by transverse ridges. The sedimentary fill is thick and formed by organic oozes having a very good petroleum potential. In contrast to Bogoria, the lateral distribution of organic matter is characterized by considerable heterogeneity due to the existence of structural blocks or to redepositional processes.

Tiercelin, J.J.; Lezzar, K.E. (Universite de Bretagne Occidentale, Brest (France)); Richert, J.P. (Elf Aquitaine, Pau (France))

1994-07-01

244

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

NASA Astrophysics Data System (ADS)

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

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

2011-06-01

245

Crustal Deformation Field Around Rift Zone In Southeastern Afar Derived From Jers-1/in-sar  

NASA Astrophysics Data System (ADS)

Afar is one of the major active rift zones recognized on the ground and located around the triple junction of Arabia, Somalia and Nubian plates. Afar is one of the major rift zones recognized on the ground. The crustal deformation of Afar has been deduced from paleomagnetism, geology and seismology by many scientists. The current crustal deformation must be detected by geodetic measurements. Ruegg et al. (J. Geophys. Res., 1984) showed the crustal deformation across the Asal-Ghoubbet rift with rate of about 60 mm/yr extension derived from triangulation and trilateration. Walpersdorf et al. (J. Geodyn., 1999) show the opening between South Djibouti and Yemen with rate of 16 mm/yr by GPS surveys. Denser observations are required for detailed crustal deformation, however it is difficult to construct such observation network because of harsh environment. The geodetic application of remote sensing is useful in this region, and we apply JERS-1 SAR interferometry in southeastern Afar, which is one of the most active deformation area. In this study, we use six SAR scenes observed from 1996/5/20 to 1997/5/7, and generate five interferograms; these repeat cycles are 88 (2 pairs), 176, 264, 352 days. First, we generate the digital elevation model (DEM) from two 88 repeat cycle pairs applying the multiple pass SAR interferometry method by Kwok and Fahnestock (IEEE Trans. Geosci. Remote Sensing, 1996). Next, the topographic fringes of all pairs are removed using the DEM. The crustal deformation derived from SAR interferometry increases with expanding repeat cycle. Finally, the velocity field is estimated by fitting to linear trend for each pixel. The spreading rate of Asal-Ghoubbet rift derived from SAR interferometry is good agreement with that by Ruegg et al. (J. Geophys. Res., 1984). We can see the crustal deformation with the subsidence sense in the west of Asal-Ghoubbet rift. This suggests that the extension is distinguished in this area. The subsidence sense deformation is also seen along the Manda-Inakir rift, and we can see the V shape of subsidence area in the east end of the Manda-Inakir rift, which is transition zone from the Manda-Inakir rift to the Mak'Arrassou strike slip zone. It may be caused that the stress along the tectonic line from the Manda-Inakir rift to the Mak'Arrasou strike slip zone is gradually change from the extension to the strike slip.

Ozawa, T.; Nogi, Y.; Shibuya, K.

246

Modelling of sea floor spreading initiation and rifted continental margin formation  

NASA Astrophysics Data System (ADS)

Recent observations of depth dependent (heterogeneous) stretching where upper crustal extension is much less than that of the lower crust and lithospheric mantle at both non-volcanic and volcanic margins plus the discovery of broad domains of exhumed continental mantle at non-volcanic rifted margins are not predicted by existing quantitative models of rifted margin formation which are usually based on intra-continental rift models subjected to very large stretching factors. New conceptual and quantitative models of rifted margin formation are required. Observations and continuum mechanics suggest that the dominant process responsible for rifted continental margin formation is sea-floor spreading of the young ocean ridge, rather than pre-breakup intra-continental rifting. Simple fluid flow models of ocean ridge processes using analytical iso-viscous corner-flow demonstrate that the divergent motion of the upwelling mantle beneath the ocean ridge, when viewed in the reference frame of the young continental margin, shows oceanward flow of the lower continental crust and lithospheric mantle of the young rifted margin giving rise to depth dependent stretching as observed. Single-phase fluid-models have been developed to model the initiation of sea-floor spreading and the thermal, stretching and thinning evolution of the young rifted continental margin. Finite element fluid-flow modelling incorporating the evolving temperature dependent viscosity field on the fluid flow also show depth dependent stretching of the young continental margin. Two-phase flow models of ocean ridges incorporating the transport of both solid matrix and melt fluid (Spiegelman &Reynolds 1999) predict the divergent motion of the asthenosphere and lithosphere matrix, and the focusing of basaltic melt into the narrow axial zone spreading centre at ocean ridges. We are adapting two-phase flow models for application to the initiation of sea-floor spreading and rifted continental margin formation. iSIMM investigators are V Tymms, NJ Kusznir, RS White, AM Roberts, PAF Christie, N Hurst, Z Lunnon, CJ Parkin, AW Roberts, LK Smith, R Spitzer, A. Davies and A. Surendra, with funding from NERC, DTI, Agip UK, BP, Amerada Hess Ltd., Anadarko, Conoco, Phillips, Shell, Statoil, and WesternGeco.

Tymms, V. J.; Isimm Team

2003-04-01

247

Strain Distribution across the Terceira (Azores) Rift from 13 years of GPS data  

NASA Astrophysics Data System (ADS)

The Terceira Rift is the westernmost segment of the Nubia-Eurasia plate boundary. Despite the intense investigation carried out along the past decades based on geological, geophysical, and geodetic data, the interpretation of the behaviour of this plate boundary is still controversial in what concerns the location of the triple junction and the strain distribution across the Azores plateau, and in particular, the amount of extension taking place outside the Terceira Rift. This is particular difficult to study because geodetic displacements can only be measured on the islands and most of the processes occur offshore. In this work, we use GPS observations acquired between 1997 and 2010 to investigate the strain distribution along the Terceira Rift, The number and data-span of the existing permanent stations are still limited and so we combine their velocity field with the solutions derived from episodic data, which averages 7-8 years with 3-4 reoccupations. This allows the definition of mean velocities along a SW-NE transect, crossing Pico-Faial, S. Jorge and Terceira Islands, close to the direction of Eurasia Nubia relative motion, as described by regional geodetic models. Finally, the geodetic displacement field is compared with half space elastic modelling, to assess the probable location of rifting processes and the amount of extension taking place outside the Terceira Rift. This is a contribution from MAREKH (PTDC/MAR/108142/2008) FCT project.

Fernandes, R. M.; Miranda, J. M.; Luis, J. M.

2011-12-01

248

Rift migration explains continental margin asymmetry and crustal hyper-extension  

PubMed Central

When continents break apart, continental crust and lithosphere are thinned until break-up is achieved and an oceanic basin is formed. The most remarkable and least understood structures associated with this process are up to 200?km wide areas of hyper-extended continental crust, which are partitioned between conjugate margins with pronounced asymmetry. Here we show, using high-resolution thermo-mechanical modelling, that hyper-extended crust and margin asymmetry are produced by steady state rift migration. We demonstrate that rift migration is accomplished by sequential, oceanward-younging, upper crustal faults, and is balanced through lower crustal flow. Constraining our model with a new South Atlantic plate reconstruction, we demonstrate that larger extension velocities may account for southward increasing width and asymmetry of these conjugate magma-poor margins. Our model challenges conventional ideas of rifted margin evolution, as it implies that during rift migration large amounts of material are transferred from one side of the rift zone to the other. PMID:24905463

Brune, Sascha; Heine, Christian; Perez-Gussinye, Marta; Sobolev, Stephan V.

2014-01-01

249

Geodynamic modelling of non-volcanic rifted margins: sedimentation process and effects  

NASA Astrophysics Data System (ADS)

Non-volcanic rifted margins (NVRMs) are characterized by an anomalous lack of magmatism and a wide continental-ocean transition which has been interpreted as an expanse of exhumed and serpentinized mantle. NVRMs are represented all over the world as in Labrador Sea, Southeast Australia, Newfoundland, and West Iberian Margin (WIM), where a serpentinized peridotite ridge is found. Erosion and sedimentation are surficial processes that redistribute material along the margins, changing the forces along the margin and affecting its thermal structure. In order to study coupled mantle dynamics and sedimentation processes during rifting, we used a modified version of the dynamic 2D code MILAMIN (Dabrowski et al. 2008). Our modified version includes a free surface together with a free-surface stabilization algorithm to generate stable topographies, strain softening to simulate faulting, serpentinization, magmatism, erosion and sedimentation. Erosion and sedimentation algorithm is based on diffusion and transport equations in 1D. The code allow us to investigate how sedimentation process conditions the architecture of the margins during rifting and in which magnitude the width of the margins and the height of the rift shoulders are affected by surficial processes. Furthermore, we can study how sediments influence the temperature distribution and evolution and, ultimately, the rheology of the crust during rifting. We are also working on modelling sedimentation with high resolution meshes to try to reproduce break-up uncomformities and to study the thermal evolution of the sediments.

Andres-Martinez, Miguel; Perez-Gussinye, Marta; Phipps Morgan, Jason; Armitage, John; Monteiro da Silva, Rafael

2014-05-01

250

Seismic stratigraphy and subsidence history of the United Arab Emirates (UAE) rifted margin and overlying foreland basins  

NASA Astrophysics Data System (ADS)

Regional seismic reflection profiles and deep exploratory well data have been used to determine the structure and evolution of the United Arab Emirates (UAE) rifted margin and overlying foreland basins. We recognized three major tectonostratigraphic sequences in the seismic profiles. A lower rifted margin sequence which is characterised by an early syn-rift sequence of mainly Late Permian age that is overlain by a post-rift sequence of Triassic-Jurassic to Late Cretaceous age, and an upper foreland basin sequences of Aruma (Late Cretaceous) and Pabdeh (Palaeocene - Oligocene) Groups. Backstripping of the well data provides new constraints on the age of rifting, the amount of crustal and mantle extension, and the development of the UAE rifted margin and foreland basins. The tectonic subsidence and uplift history at the wells can be explained by a uniform extension model with at least two episodes of rifting punctuated by periods of relative tectonic quiescence and thermal subsidence. An initial age of rifting of 260 Ma and a stretching factor, ?, of 1.2 was followed by second period of stretching with an initial age of rifting of 180 Ma, and ?, of 1.1 (Fig. 1). This model accounts for the general exponential decrease observed in the backstripped tectonic subsidence. The model, fails, however, to completely explain the slow subsidence and uplift history of the margin during Late Triassic to Early Jurassic. We attribute this slow subsidence to combine effect of a sea-level fall and regional uplift which caused major unconformity particularly offshore western Abu Dhabi. The backstrip curves suggest that the transition from an extensional rifted margin to a compressional foreland basin occurred at ~90 Ma, which is within the range for the emplacement of the Semail Ophiolite (95-68 Ma). The history during this time is characterised by uplift followed by rapid subsidence. We attribute these differential vertical movements to orogenic loading and flexure of the Arabian rifted margin by the Semail Ophiolite and development of Aruma foreland basin

Ali, M. Y.; Watts, A. B.; Searle, M. P.

2011-12-01

251

Tectono-stratigraphic signature of multiphased rifting on divergent margins (deep-offshore southwest Iberia, North Atlantic)  

NASA Astrophysics Data System (ADS)

Regional 2D multichannel seismic, borehole, dredge and outcrop data, together with burial models for strata in southwest Iberia, are used to investigate the tectono-stratigraphic signature of multiphased rifting on divergent margins. Our burial model reveals that Mesozoic extension occurred during three main phases, each comprising distinct subsidence pulses separated by short-lived periods of crustal uplift. The importance of the three phases varies across discrete sectors of the margin, each one revealing similar depositional architectures and associated tectonic systems tracts: 1) the Rift Initiation phase, characterized by incipient subsidence and overall aggradation/progradation over a basal unconformity, 2) the Rift Climax phase, which marks maxima of tectonic subsidence and is characterized by retrogradation-progradation, and 3) the Late Rift phase, recording the progradational infill of the basin and the effects of eustasy. The Rift Initiation systems tracts comprise Sinemurian and late Callovian-early Oxfordian strata. Marine units in the Pliensbachian and Late Oxfordian-Kimmeridgian represent the Rift Climax phase, a period marked by the development of Maximum Flooding Surfaces. Late Rift deposits were identified in the Rhaetian-Hettangian, Toarcian-Bathonian and Kimmeridgian-Berriasian. The results of this work are important to the economic exploration of deep-offshore rift basins, as they reveal that sequence stratigraphy can be used to predict sedimentary facies distribution in more distal segments of such basins. Significantly, this work recognizes that multiple tectonic-stratigraphic (rift) cycles can occur on deep-offshore rift basins, from the onset of rift-related extension until continental break-up, a character that contrast to what is known from deep-sea drilling data from the distal margin of Northwest Iberia.

Pereira, Ricardo; Alves, Tiago M.

2012-08-01

252

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

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

253

Curcumin Inhibits Rift Valley Fever Virus Replication in Human Cells*  

PubMed Central

Rift Valley fever virus (RVFV) is an arbovirus that is classified as a select agent, an emerging infectious virus, and an agricultural pathogen. Understanding RVFV-host interactions is imperative to the design of novel therapeutics. Here, we report that an infection by the MP-12 strain of RVFV induces phosphorylation of the p65 component of the NF?B cascade. We demonstrate that phosphorylation of p65 (serine 536) involves phosphorylation of I?B? and occurs through the classical NF?B cascade. A unique, low molecular weight complex of the IKK-? subunit can be observed in MP-12-infected cells, which we have labeled IKK-?2. The IKK-?2 complex retains kinase activity and phosphorylates an I?B? substrate. Inhibition of the IKK complex using inhibitors impairs viral replication, thus alluding to the requirement of an active IKK complex to the viral life cycle. Curcumin strongly down-regulates levels of extracellular infectious virus. Our data demonstrated that curcumin binds to and inhibits kinase activity of the IKK-?2 complex in infected cells. Curcumin partially exerts its inhibitory influence on RVFV replication by interfering with IKK-?2-mediated phosphorylation of the viral protein NSs and by altering the cell cycle of treated cells. Curcumin also demonstrated efficacy against ZH501, the fully virulent version of RVFV. Curcumin treatment down-regulated viral replication in the liver of infected animals. Our data point to the possibility that RVFV infection may result in the generation of novel versions of host components (such as IKK-?2) that, by virtue of altered protein interaction and function, qualify as unique therapeutic targets. PMID:22847000

Narayanan, Aarthi; Kehn-Hall, Kylene; Senina, Svetlana; Lundberg, Lindsay; Van Duyne, Rachel; Guendel, Irene; Das, Ravi; Baer, Alan; Bethel, Laura; Turell, Michael; Hartman, Amy Lynn; Das, Bhaskar; Bailey, Charles; Kashanchi, Fatah

2012-01-01

254

A cluster of outflows in the Vulpecula Rift  

NASA Astrophysics Data System (ADS)

We present 12CO, 13CO and C18O (J=3-2) observations of a new cluster of outflows in the Vulpecula Rift with HARP on the James Clerk Maxwell Telescope (JCMT). The mass associated with the outflows, measured using the 12CO HARP observations and assuming a distance to the region of 2.3 kpc, is 129 M?, while the mass associated with the dense gas from C18O observations is 458 M? and the associated sub-millimeter (sub-mm) core has a mass of 327 ± 112 M? independently determined from Bolocam 1.1-mm data. The outflow-to-core mass ratio is therefore ˜0.4, making this region one of the most efficient observed thus far with more than an order of magnitude more mass in the outflow than would be expected based on previous results. The kinetic energy associated with the flows, 94 × 1045 erg, is enough to drive the turbulence in the local clump, and potentially unbind the local region altogether. The detection of SiO (J=8-7) emission towards the outflows indicates that the flow is still active, and not simply a fossil flow. We also model the spectral energy distributions (SEDs) of the four young stellar objects (YSOs) associated with the molecular material, finding them all to be of mid to early B spectral type. The energetic nature of the outflows and significant reservoir of cold dust detected in the sub-mm suggest that these intermediate mass YSOs will continue to accrete and become massive, rather than reach the main sequence at their current mass.

Mottram, J. C.; Brunt, C. M.

2012-02-01

255

Flexural uplift of rift flanks in central Greece  

NASA Astrophysics Data System (ADS)

Uplifts, with elevations of up to 2000 m and short wavelengths (30-35 km), flank three major grabens in central Greece: the Rio, Sparta, and Atalanti grabens. They are bordered on their landward sides by narrow basins oriented parallel to the graben axes: the Manesi and Trichonis basins at the Rio graben and Copais and Istiea basins at the Atalanti graben. The flexural origin of these uplift profiles is investigated here by using thin, broken plate models. It is demonstrated that the observed topography of the graben flanks is consistent with the upward deflection of elastic and viscoelastic plates in response to upward directed forces applied at the graben flanks. In order to evaluate to applicability of each model, their predictions are checked against the observations. The elastic predictive modeling fits well with the observed flexural wavelengths and the flanking seismicity. However, it fails to predict the graben widths and the inferred elastic layer thickness. In contrast, the viscoelastic model successfully explains the graben widths and the "back" basins as flexural hinterland basins and matches the seismological data and the time constraints of rifting. It is therefore suggested that flexural uplift with viscoelastic relaxation accounts for the building of the graben flanks. The invoked viscoelastic models constrain the effective elastic thickness of the plates at 10 km for the Rio graben and 15 km for the Sparta and Atalanti grabens, suggesting a low-viscosity lower crust. Furthermore, they predict low rates of tectonic uplift of the order of 0.1 mm a-1 for the Atalanti graben, intermediate rates of 0.24-0.37 mm a-1 for the Rio graben, and high rates of 0.7-0.9 mm a-1 for the Sparta graben. The latter are quite possibly overestimated since napping events, capable of producing high local relief, are traced normal to the modeled profiles.

Poulimenos, George; Doutsos, Theodor

1997-12-01

256

Evaporation estimation of rift valley lakes: comparison of models.  

PubMed

Evapotranspiration (ET) accounts for a substantial amount of the water flux in the arid and semi-arid regions of the World. Accurate estimation of ET has been a challenge for hydrologists, mainly because of the spatiotemporal variability of the environmental and physical parameters governing the latent heat flux. In addition, most available ET models depend on intensive meteorological information for ET estimation. Such data are not available at the desired spatial and temporal scales in less developed and remote parts of the world. This limitation has necessitated the development of simple models that are less data intensive and provide ET estimates with acceptable level of accuracy. Remote sensing approach can also be applied to large areas where meteorological data are not available and field scale data collection is costly, time consuming and difficult. In areas like the Rift Valley regions of Ethiopia, the applicability of the Simple Method (Abtew Method) of lake evaporation estimation and surface energy balance approach using remote sensing was studied. The Simple Method and a remote sensing-based lake evaporation estimates were compared to the Penman, Energy balance, Pan, Radiation and Complementary Relationship Lake Evaporation (CRLE) methods applied in the region. Results indicate a good correspondence of the models outputs to that of the above methods. Comparison of the 1986 and 2000 monthly lake ET from the Landsat images to the Simple and Penman Methods show that the remote sensing and surface energy balance approach is promising for large scale applications to understand the spatial variation of the latent heat flux. PMID:22303142

Melesse, Assefa M; Abtew, Wossenu; Dessalegne, Tibebe

2009-01-01

257

Evaporation Estimation of Rift Valley Lakes: Comparison of Models  

PubMed Central

Evapotranspiration (ET) accounts for a substantial amount of the water flux in the arid and semi-arid regions of the World. Accurate estimation of ET has been a challenge for hydrologists, mainly because of the spatiotemporal variability of the environmental and physical parameters governing the latent heat flux. In addition, most available ET models depend on intensive meteorological information for ET estimation. Such data are not available at the desired spatial and temporal scales in less developed and remote parts of the world. This limitation has necessitated the development of simple models that are less data intensive and provide ET estimates with acceptable level of accuracy. Remote sensing approach can also be applied to large areas where meteorological data are not available and field scale data collection is costly, time consuming and difficult. In areas like the Rift Valley regions of Ethiopia, the applicability of the Simple Method (Abtew Method) of lake evaporation estimation and surface energy balance approach using remote sensing was studied. The Simple Method and a remote sensing-based lake evaporation estimates were compared to the Penman, Energy balance, Pan, Radiation and Complementary Relationship Lake Evaporation (CRLE) methods applied in the region. Results indicate a good correspondence of the models outputs to that of the above methods. Comparison of the 1986 and 2000 monthly lake ET from the Landsat images to the Simple and Penman Methods show that the remote sensing and surface energy balance approach is promising for large scale applications to understand the spatial variation of the latent heat flux. PMID:22303142

Melesse, Assefa M.; Abtew, Wossenu; Dessalegne, Tibebe

2009-01-01

258

Seismic monitoring of the Olkaria Geothermal area, Kenya Rift valley  

NASA Astrophysics Data System (ADS)

Seismic monitoring of the Olkaria Geothermal area in the southern Rift Valley region of Kenya has been carried out since 1985. The initial purpose of this effort was to determine the background level of seismicity before full exploitation of the geothermal resource was started. This monitoring began with one seismic station. However, since May 1996, a seismic network comprising six stations was operated and focused mainly on the East Production Field. During the 5 months of network recording up to mid-September 1996, more than 460 local events originating within the Olkaria Geothermal area ( Ts- Tp<5 s) were recorded, out of which 123 were well-located. Also, 62 events were recorded at regional distances (5 s< Ts- Tp<40 s), and 44 events at teleseismic distance ( Ts- Tp>40 s). During this period, the local microseismicity was found to be continuous with swarms occurring every 4-5 days. Duration magnitudes based on the coda length did not exceed 3.0. Preliminary spectral analysis shows three kinds of seismic signals, with only the first type displaying well-defined P- and S-phases. The seismicity is mainly concentrated in the central area of the recording network, and the linear alignments in the epicenters are striking. A prominent alignment occurs along the Ololbutot fault zone extending from the northern end of the greater Olkaria volcanic complex to the south near the southern terminus of Hell's gorge. Two other prominent alignments occur along NW-SE trends that coincide with fault zones which have been detected by geological and gravity studies. Consequently, they are interpreted to be associated with fluid movement in the geothermal field. These preliminary results suggest that seismic monitoring will be useful to both monitor the field during production and to help site additional wells.

Simiyu, Silas M.; Keller, G. Randy

2000-01-01

259

Influence of pre-existing fabrics on fault kinematics and rift geometry of interacting segments: Analogue models based on the Albertine Rift (Uganda), Western Branch-East African Rift System  

NASA Astrophysics Data System (ADS)

This study aims at showing how far pre-existing crustal weaknesses left behind by Proterozoic mobile belts, that pass around cratonic Archean shields (Tanzania Craton to the southeast and Congo Craton to the northwest), control the geometry of the Albertine Rift. Focus is laid on the development of the Lake Albert and Lake Edward/George sub-segments and between them the greatly uplifted Rwenzori Mountains, a horst block located within the rift and whose highest peak rises to >5000 m above mean sea level. In particular we study how the southward propagating Lake Albert sub-segment to the north interacts with the northward propagating Lake Edward/George sub-segment south of it, and how this interaction produces the structures and geometry observed in this section of the western branch of the East African Rift, especially within and around the Rwenzori horst. We simulate behaviour of the upper crust by conducting sandbox analogue experiments in which pre-cut rubber strips of varying overstep/overlap connected to a basal sheet and oriented oblique and/or orthogonal to the extension vector, are placed below the sand-pack. The points of connection present velocity discontinuities to localise deformation, while the rubber strips represent ductile domain affected by older mobile belts. From fault geometry of developing rift segments in plan view and section cuts, we study kinematics resulting from a given set of boundary conditions, and results are compared with the natural scenario. Three different basal model-configurations are used to simulate two parallel rifts that propagate towards each other and interact. Wider overstep (model SbR3) produces an oblique transfer zone with deep grabens (max. 7.0 km) in the adjoining segments. Smaller overlap (model SbR4) ends in offset rift segments without oblique transfer faults to join the two, and produces moderately deep grabens (max. 4.6 km). When overlap doubles the overstep (model SbR5), rifts propagate sub-orthogonal to the extension direction and form shallow valleys (max. 2.9 km). Relative ratios of overlap/overstep between rift segments dictate the kind of transition zone that develops and whether or not a block (like the Rwenzoris) is captured and rotates; hence determining the end-member geometry. Rotation direction is controlled by pre-existing fabrics. Fault orientation, fault kinematics, and block rotation (once in play) reinforce each other; and depending on the local kinematics, different parts of a captured block may rotate with variable velocities but in the same general direction. Mechanical strength anisotropy of pre-structured crust only initially centres fault nucleation and propagation parallel to the grain of weakness of the basement, but at later stages of a protracted period of crustal extension, such boundaries are locally defied.

Aanyu, K.; Koehn, D.

2011-02-01

260

Exhumation, rift-flank uplift, and the thermal evolution of the Rwenzori Mountains determined by combined (U-Th)/He and U-Pb thermochronometry  

E-print Network

Rising over 5 km along the border of Uganda and the Democratic Republic of the Congo, the Rwenzori Mountains represent an extreme example of basement rift-flank uplift in the western branch of the East African Rift, a ...

MacPhee, Daniel

2006-01-01

261

Deep magma body beneath the summit and rift zones of Kilauea Volcano, Hawaii  

USGS Publications Warehouse

A magnitude 7.2 earthquake in 1975 caused the south flank of Kilauea Volcano, Hawaii, to move seaward in response to slippage along a deep fault. Since then, a large part of the volcano's edifice has been adjusting to this perturbation. The summit of Kilauea extended at a rate of 0.26 meter per year until 1983, the south flank uplifted more than 0.5 meter, and the axes of both the volcano's rift zones extended and subsided; the summit continues to subside. These ground-surface motions have been remarkably steady and much more widespread than those caused by either recurrent inflation and deflation of the summit magma chamber or the episodic propagation of dikes into the rift zones. Kilauea's magmatic system is, therefore, probably deeper and more extensive than previously thought; the summit and both rift zones may be underlain by a thick, near vertical dike-like magma system at a depth of 3 to 9 kilometers.

Delaney, P.T.; Fiske, R.S.; Miklius, A.; Okamura, A.T.; Sako, M.K.

1990-01-01

262

Evidence for a seismogenic upper mantle and lower crust in the Baikal rift  

SciTech Connect

The high level seismicity of the Baikal rift zone and its spatial distribution in dense swarms and belts provide an opportunity to study the seismogenic behavior of a continental lithosphere submitted to extension in an early stage. Using data from a regional seismological network, the authors analyze a significantly large set of events from an earthquake swarm located east of the nearly aseismic northern Baikal lake. They find that at least 10% of the well-constrained events are located in the lower crust or the uppermost mantle. The fault plane solutions of earthquakes within the crust define a NW-SE extensional stress regime perpendicular to the rift axis. Results confirm the idea that zones of continental extension may exhibit significant rigidity. The authors propose to infer a migration of deformation from the northern Baikal lake to an initially stronger part of the lithosphere, i.e. the Barguzin rift and its extension to the east.

Deverchere, J.; Houdry, F. (Univ. Paris 6 (France)); Diament, M. (Inst. de Physique du Globe, Paris (France)); Solonenko, N.V.; Solonenko, A.V. (Inst. of the Earth's Crust, Irkutsk (USSR))

1991-06-01

263

Siting of Large Volcanic Centers at Releasing Fault Stepovers, Walker Lane Rift  

NASA Astrophysics Data System (ADS)

The transtensional eastern boundary of the Sierran microplate (Walker Lane rift) represents the northernmost extension of the Gulf of California rift, and it forms an onland analog in several ways. It formed at the same time (about 12 Ma), by a similar mechanism: transtension within the thermally- and structurally-weakened axis of a subduction-related arc. The two segments show similar structural trends: NE oblique slip normal faults (Walker Lane) or seafloor spreading centers (Gulf of California), connected by long NNW strike slip faults. However, the process of continental rupture has not yet been completed in the Walker Lane, so the structural controls on transtensional rift volcanism can be directly observed on land. The Walker Lane segment also differs from the Gulf of California segment by showing a northward time-transgressive transition from arc rift magmatism to continental rift magmatism, following the northward migration of the Mendocino triple junction (MTJ). The effect of MTJ migration has been previously recognized in arc to rift geochemical transitions, but not in the timing of development of large arc volcanic fields. For the past ~11-12 Ma, the biggest arc rift and continental rift volcanic centers or fields have been sited on major releasing fault stepovers on the trailing edge of the Sierran microplate. Additionally, major transtensional arc rift centers or fields appear to have progressively migrated northward with time, in advance of the TMJ, although gaps exist in detailed map and age data. These large transtensional arc volcanic fields/centers are, from south to north (oldest to youngest): (1) A ~11 - 9 Ma arc volcanic field that lies along the Sierran crest and rangefront in the Sonora Pass - Bridgeport area of the central Sierra Nevada. Its transtensional structural setting and its size (~ 50 X 50 km) had not been appreciated prior to my field efforts with students, although a modest-sized caldera in this volcanic field had long been recognized ("Little Walker caldera" of Priest, 1979). At this center, "flood andesites" were erupted from 6-8 km long fault-controlled fissures and ponded in grabens, to thicknesses of 400 m, with single flows up to 25 km3 in volume. Total volume is difficult to estimate due to Pleistocene glacial erosion, but it is >200 km3. (2) The Ebbetts Pass center, which formed at ~5-4 Ma (dating in progress with Paul Renne, BGC). This large center had not been recognized prior to our mapping; it appears to be a complex central volcano with a large footprint (>16 km diameter, glacially eroded). Its original volume may be better estimated after its collapse deposits are mapped and dated, because it appears to have repeatedly collapsed into range-front half grabens. (3) The active Lassen arc volcanic center, which formed at <3.5 Ma in a transtensional environment "favorable to the development of major volcanic centers" (Muffler et al., 2008, EOS 8-53). The active Long Valley rift volcanic field south of the MTJ also formed in a releasing bend in the Walker Lane transtensional rift (since ~4.5 Ma); the structure of this field (Jayko and Bursik, in press) is remarkably similar to that of the ~11-9 Ma arc rift volcanic field at Sonora Pass (Busby, in press; both in Tectonics of Sedimentary Basins, Wiley Blackwell, 2012).

Busby, C.

2011-12-01

264

Geodetic measurement of horizontal deformation across the Rio Grande rift near Socorro, New Mexico.  

USGS Publications Warehouse

Trilateration surveys of a geodetic network across the Rio Grande rift near Socorro, New Mexico, in 1972, 73, 76 and 79 have failed to detect any significant strain accumulation. The surveys place an upper bound (95% confidence limit) of 1 mm/a (a = years) on EW spreading across the rift in 1972-79. There is marginal evidence from triangulation for an episode of EW spreading across the rift within the interval 1954-72. The trilateration network lies on the S flank of an uplift caused by magma intrusion into a midcrustal sill during this century according to Reilinger and Oliver. The horizontal deformation induced by sill inflation is sufficiently small that continued uplift during 1972-79 cannot be excluded by the observed absence of significant horizontal deformation.-Authors

Savage, J. C.; Lisowski, M.; Prescott, W. H.; Sanford, A. R.

1980-01-01

265

Introduction in New perspectives on Rio Grande rift basins: from tectonics to groundwater  

USGS Publications Warehouse

Basins of the Rio Grande rift have long been studied both for their record of rift development and for their potential as host of natural resources. Early workers described the basin geomorphology and the character of infilling sediments (e.g. Siebenthal, 1910; Bryan, 1938; Speigel and Baldwin, 1963), and subsequent research compilations provided general stratigraphic and tectonic overviews of rift basins and described their geophysical characteristics within the crust (Hawley, 1978; Riecker, 1979; Baldridge et al., 1984; Keller, 1986). Subsurface knowledge gained from hydrocarbon exploration activities coupled with detailed surface studies of basins and their flanking uplifts were presented in Geological Society of America (GSA) Special Paper 291, edited by Keller and Cather (1994a).

Hudson, Mark R.; Grauch, V. J. S

2013-01-01

266

Focal mechanisms, stress field and crustal rheology in the North Tanzanian Divergence (East African Rift) inferred from local seismicity analysis  

Microsoft Academic Search

We deployed a temporary local seismic network in the North Tanzanian Divergence (NTD) for 6 months in 2007 (35 stations, SEISMOTANZ'07 experiment). The region is characterized by major changes in the magmatic\\/tectonic nature of the rift, at the place where the eastern branch of the East African Rift enters the Tanzanian craton. More than 200 earthquakes were accurately located south

Julie Albaric; Maxime Godano; Jacques Déverchère; Julie Perrot; Anne Deschamps; Bernard Le Gall; Richard W. Ferdinand; Christian Sue; Carole Petit; Christel Tiberi

2010-01-01

267

The Lava sequence of the East African Rift escarpment in the Oldoinyo Lengai – Lake Natron sector, Tanzania  

Microsoft Academic Search

A 500m sequence of horizontal lava flows forms the Gregory rift escarpment of the western rift shoulder between Lake Natron and Oldoinyo Lengai. A detailed volcanic stratigraphy of this >1.2Ma evolution of the EAR in Northern Tanzania is presented. The sequence is formed by several distinct rock suites, with increasing alkalinity from base to top. Alkali olivine basalts of the

Florian Neukirchen; Thomas Finkenbein; Jörg Keller

2010-01-01

268

Cenozoic rifting and volcanism in eastern China: a mantle dynamic link to the IndoAsian collision?  

E-print Network

Cenozoic rifting and volcanism in eastern China: a mantle dynamic link to the Indo­Asian collision upwelling, rifting, and widespread Cenozoic volcanism in eastern China. D 2004 Elsevier B.V. All rights has been characterized by crustal shortening and mountain building throughout the Cenozoic (Allegre et

Liu, Mian

269

Genetic Analysis of Viruses Associated with Emergence of Rift Valley Fever in Saudi Arabia and Yemen, 2000-01  

Microsoft Academic Search

The first confirmed Rift Valley fever outbreak outside Africa was reported in September 2000, in the Ara- bian Peninsula. As of February 2001, a total of 884 hospitalized patients were identified in Saudi Arabia, with 124 deaths. In Yemen, 1,087 cases were estimated to have occurred, with 121 deaths. Laboratory diagnosis of Rift Valley fever virus (RVFV) infections included virus

Trevor Shoemaker; Carla Boulianne; Martin J. Vincent; Linda Pezzanite; Mohammed M. Al-Qahtani; Yagub Al-Mazrou; Ali S. Khan; Pierre E. Rollin; Robert Swanepoel; Thomas G. Ksiazek; Stuart T. Nichol

270

Evolution, pattern, and partitioning of deformation during oblique continental rifting: Inferences from lithospheric-scale centrifuge models  

Microsoft Academic Search

Oblique rifting is investigated through centrifuge experiments that reproduce extension of a continental lithosphere containing a preexisting weakness zone. During extension, this weakness localizes deformation, and different rift obliquity is obtained by varying its trend with respect to the stretching direction. Model results show that deformation is mostly controlled by the obliquity angle alpha (defined as the angle between the

Andrea Agostini; Giacomo Corti; Antonio Zeoli; Genene Mulugeta

2009-01-01

271

Control of the rheological structure of the lithosphere on the inward migration of tectonic activity during continental rifting  

Microsoft Academic Search

Analogue models are used to analyse the parameters controlling the evolution of extensional deformation in continental rifts. Models are deformed in a centrifuge and simulate the continental lithosphere floating and extending above a low-viscosity asthenosphere. Model results reproduce the typical evolution of deformation during continental narrow rifting, with early activation of large boundary faults and basin subsidence, followed by their

Giacomo Corti; Giorgio Ranalli; Genene Mulugeta; Andrea Agostini; Federico Sani; Alessandro Zugu

2010-01-01

272

Soil gases associated with rift zones in the Eastern Snake River Plain, Idaho, U.S.A  

Microsoft Academic Search

Soil gases have been measured, utilizing petroleum nearsurface exploration techniques, in the volcanic province of the Eastern Snake River Plain, In Idaho, U.S.A. The analyses of the soil atmosphere included light hydrocarbon gases, helium, hydrogen, and carbon dioxide. Samples were collected in and near recent basaltic rift zones. Characterization of rift zone soil gases has indicated variability of their compositional

W. C. Sidle

1984-01-01

273

Impact of lake-level changes on the formation of thermogene travertine in continental rifts: Evidence from Lake Bogoria,  

E-print Network

to drier conditions. Keywords Geothermal, hot spring, Kenya Rift, saline lake, travertine. Sedimentology: Evidence from Lake Bogoria, Kenya Rift Valley ROBIN W. RENAUT*, R. BERNHART OWEN , BRIAN JONESà, JEAN Europe´en de la Mer, Place Nicolas Copernic, 29280 Plouzane´, France **National Oil Corporation of Kenya

Konhauser, Kurt

274

Strength of the lithosphere and strain localisation in the Baikal rift Carole Petit , Evguene Burov, Christel Tiberi  

E-print Network

Strength of the lithosphere and strain localisation in the Baikal rift Carole Petit , Evguene Burov.P. Jaupart Keywords: effective elastic thickness Baikal rift strain localisation lithosphere We use previously published estimates of Moho and lithosphere­asthenosphere boundary (LAB) depths combined with a 3D

Déverchère, Jacques

275

A geophysical view of the Southeastern Brazilian margin at Santos Basin: Insights into rifting evolution  

NASA Astrophysics Data System (ADS)

This study investigates the rifting structures of Santos Basin at the Southeastern Brazilian margin, based on an integrated geophysical approach. Our aim is to constrain the crustal basement topography of central and northern Santos basin, the presence of magmatism and the role of inherited structures in space and time through the rifting processes. We present a new high resolution aeromagnetic dataset, which in correlation with gravity anomalies enables us to interpret the tectonic trends and crustal basement structures. We calculated the magnetic basement depth for the central and northern Santos Basin using power spectrum analysis. The obtained depths range between 2 and 9 kms, and are comparable with results from previous works. From our integrated study, three margin domains could be identified, which display distinct rifting structures and are characterized by important lateral variation along the margin. The proximal domain displays trends and magnetic basement blocks NE-SW oriented, i.e., parallel to inherited onshore crustal basement with an inflexion to E-W oriented trends; the necking domain is characterized by oblique magnetic basement highs and lows (E-W and NW-SE) and a structural trend change. The trends and magnetic basement highs are bounded by NW-SE negative anomalies, interpreted as transfer zones. Oceanwards at the distal domain, the lineaments and transfer zones show a progressive structural inflexion to ENE and E-W, sub-parallel to adjacent South Atlantic Fracture Zones. The observed crustal basement architecture and segmentation suggest the reactivation of pre-rift structures at the proximal margin and the obliquity of rifting relative to them. From the proximal domain oceanwards the structural pattern may reflect the passage from a “continental type” domain, where lithospheric inheritance controls the deformation, to a distal margin where this influence diminishes and “new” structural trends are formed. We propose that northern Santos Basin show evidences of an intensely deformed zone, where rift evolved under oblique extension, similar to that observed at transform margin segments.

Stanton, N.; Ponte-Neto, C.; Bijani, R.; Masini, E.; Fontes, S.; Flexor, J.-M.

2014-11-01

276

Seismicity at the Rwenzori Mountains, East African Rift: earthquake distribution, magnitudes and source mechanisms  

NASA Astrophysics Data System (ADS)

We have analysed the microseismic activity within the Rwenzori Mountains area in the western branch of the East African Rift. Seismogram recordings from a temporary array of up to 27 stations reveal approximately 800 events per month with local magnitudes ranging from -0.5 to 5.1. 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, where the mountains are in contact with the rift shoulders. The hypocentral depth distribution exhibits a pronounced peak of seismic energy release at 15 km depth. The maximum extent of seismicity ranges from 20 to 32 km and correlates well with Moho depths that were derived from teleseismic receiver functions. We observe two general features: (i) beneath the rift shoulders seismicity extends from the surface down to ca. 30 km depth; (ii) beneath the rift valley seismicity is confined to depths greater than 10 km. From the observations there is no indication for a crustal root beneath the Rwenzori Mountains. The magnitude frequency distribution reveals a b-value of 1.1, which is consistent with the hypothesis that part of the seismicity is caused by magmatic processes within the crust. Fault plane solutions of 304 events were derived from P-polarities and SV/P amplitude ratios. More than 70 % of the source mechanisms exhibit pure or predominantly 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. At the northernmost part of the region we observe a rotation of the T-axis trends to NEN-SWS, which may be indicative of a local perturbation of the regional stress field.

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

2012-05-01

277

Rift Basin Architecture & Evolution http://www.ldeo.columbia.edu/~polsen/nbcp/breakup... 1 of 13 7/19/06 3:05 PM  

E-print Network

Rift Basin Architecture & Evolution http://www.ldeo.columbia.edu/~polsen/nbcp/breakup... 1 of 13 7: Pangea Breakup 3.3.1. Rift Basin Architecture and Evolution Roy W. Schlische & Martha Oliver Withjack Department of Geological Sciences, Rutgers University, Piscataway, NJ 08854-8066 U.S.A. Rift basins have been

278

Tectonic development of the SW Arabian Plate margin within the central Arabian flank of the Red Sea rift system  

NASA Astrophysics Data System (ADS)

The Red Sea rift system is a prime example of continental rifting and has contributed significantly to our understanding of the geologic processes that manage the rupture of continental lithosphere. Using a combined geo- and thermochronometric approach, we explore the modes and mechanisms of rift margin development by studying Red Sea rift-related geologic products along the central Saudi Arabian flank of the rift system, north of Jeddah. We use apatite and zircon (U-Th)/He thermochronometry and whole-rock 40Ar/39Ar dating of basalt to define the spatiotemporal relationship between rift flank extensional structures and rift-related harrat volcanism. This technical approach permits the reconstruction of the tectonic margin from early rift architecture, to strain distribution during progressive rifting, and through subsequent whole-scale modifications of the rift flank due to thermal and isostatic factors. Constraints on the dynamics of rift flank deformation are achieved through the collection of geologic samples along long-baseline thermochronometric transects that traverse the entire Arabian shield from the coastal escarpment to the inland Paleozoic sedimentary cover sequences. Long-baseline transects resolve the timing of rift flank uplift and reveal the pattern of lithospheric modification during the rupturing of continental lithosphere. Locally, short-baseline elevation transects map the footwall exhumation of major normal faults that delineate both the modern rift margin and inland extensional basins such as the NW-trending Hamd-Jizil basin, a prominent syn-extensional basin comprised of two distinct half-grabens (Jizil and Hamd) located NW of Medina. Diffuse lithospheric extension during the Oligo-Miocene affected a widespread area well inboard from the modern rift margin; samples from footwall blocks that bound the inland Jizil and Hamd half-grabens yield apatite (U-Th)/He cooling ages of 14.7 ± 0.9 Ma and 24.5 ± 1.5 Ma, respectively. The mid-Miocene age is doubly-significant since it reveals not only a Red Sea rift signal 140 km inboard from the modern border fault complex but also underscores the importance of fault reactivation during progressive rifting since the Jizil half-graben is structurally delineated by faults formed within the Neoproterozoic Najd shear zone fabric. Oligo-Miocene apatite (U-Th)/He ages from Jabal Radwa, a pluton exhumed within the border fault complex, show that rift flank exhumation and extensional faulting occurred within the central portion of the Arabian rift flank penecontemporaneous to faulting in the Hamd-Jizil half-graben system. Within the rift flank fault systems, an absence of footwall exhumation ages younger than ~13 Ma confirms that major extensional faulting migrated basin-ward in the middle Miocene during a time of rift reorganization and the establishment of the Dead Sea-Gulf of Aqaba transform. However, minor deepening of the inland Hamd-Jizil basin continued through the late Miocene as relatively young, basin-internal faults cut 17 Ma - 10 Ma basalt flows from harrat Khaybar that had blanketed the region.

Szymanski, E.; Stockli, D. F.; Johnson, P. R.; Kattan, F. H.; Cosca, M. A.

2009-12-01

279

Anomalous Subsidence at the Ocean Continent Transition of the Gulf of Aden Rifted Continental Margin  

NASA Astrophysics Data System (ADS)

It has been proposed that some rifted continental margins have anomalous subsidence and that at break-up they were elevated at shallower bathymetries than the isostatic response predicted by classical rift models (McKenzie, 1978). The existence of anomalous syn- or early-post break-up subsidence of this form would have important implications for our understanding of the geodynamics of continental break-up and sea-floor spreading initiation. We have investigated subsidence of the young rifted continental margin of the eastern Gulf of Aden, focussing on the western Oman margin (break-up age 17.6 Ma). Lucazeau et al. (2008) have found that the observed bathymetry here is approximately 1 km shallower than the predicted bathymetry. In order to examine the proposition of an anomalous early post break-up subsidence history of the Omani Gulf of Aden rifted continental margin, we have determined the subsidence of the oldest oceanic crust adjacent to the continent-ocean boundary (COB) using residual depth anomaly (RDA) analysis corrected for sediment loading and oceanic crustal thickness variation. RDAs corrected for sediment loading using flexural backstripping and decompaction have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous subsidence of the Gulf of Aden rifted continental margin. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions of Crosby and McKenzie (2009). Non-zero RDAs at the Omani Gulf of Aden rifted continental margin can be the result of non standard oceanic crustal thickness or the effect of mantle dynamic topography or a non-classical rift and break-up model. Oceanic crustal basement thicknesses from gravity inversion together with Airy isostasy have been used to predict a "synthetic" gravity RDA, in order to determine the RDA contribution from non-standard oceanic crustal thickness. Gravity inversion, used to determine crustal basement thickness, incorporates a lithosphere thermal gravity anomaly correction and uses sediment thicknesses from 2D seismic data. Reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. The difference between the sediment corrected RDA and the "synthetic" gravity derived RDA gives the component of the RDA which is not due to variations in oceanic crustal thickness. This RDA corrected for sediment loading and crustal thickness variation has a magnitude between +600m and +1000m (corresponding to anomalous uplift) and is comparable to that reported (+1km) by Lucazeau et al. (2008). We are unable to distinguish whether this anomalous uplift is due to mantle dynamic topography or anomalous subsidence with respect to classical rift model predictions.

Cowie, Leanne; Kusznir, Nick; Leroy, Sylvie

2013-04-01

280

Fluids circulation during the Miocene rifting of the Penedès half-graben, NE Iberian Peninsula  

NASA Astrophysics Data System (ADS)

The Penedès half-graben, located in the north-western part of the Mediterranean, is a NE-SW oriented basin generated during the Miocene rifting. This graben is bounded to the northwest by the SE-dipping Vallès-Penedès fault, which places the Mesozoic rocks in contact with the Miocene basin-fill. The basin is filled with an up to 4 km thick succession of sediments divided into three lithostratigraphic units. From base to top: (1) a lower continental complex, (2) a continental to marine complex and (3) an upper continental complex. These units are covered by Pliocene deposits which onlap a Messinian regional erosive surface. The structural features within the Penedès half-graben allow defining three deformational phases during the Miocene rifting. The first, during the syn-rift, two successive stages of NE-SW normal faults were formed. The second, during the early post-rift, one stage of NE-SW normal faults and one minor compression phase with a dextral directional developed. The third, during the late post-rift, two successive stages of N-S trending extensional fractures (faults and joints) and one minor compression with a sinistral component developed. The fractures related to the syn-rift stage acted as conduits for meteoric fluids both, in the phreatic and in the vadose zone. During the early post-rift, Fe2+- rich fluids precipitated oxides along the NE-SW fault planes. The dextral directional faults served as conduits for meteoric fluids which reequilibrated totally the marine Miocene host rocks under the phreatic environment. The late post-rift stage was characterized by marine fluids upflowing through the N-S fractures, probably derived from the Miocene marine interval, which mixed with meteoric fluids producing dolomitization. The second set of N-S fractures served as conduits for meteoric fluids characterised by ?13C-depleted soil-derived CO2 attributed to precipitation in the vadose zone. The change from phreatic to vadose meteoric environment and the prevailing of these vadose conditions is interpreted as the falling-down of the meteoric water table related to a generalized sea level fall in the Mediterranean area during the Messinian and/or with the uplift of the basin.

Baqués, Vinyet; Travé, Anna; Cantarero, Irene

2013-04-01

281

Stratigraphy, Structure, and Ore Deposits of the Southern Limb of the Midcontinent Rift System  

NSDL National Science Digital Library

This site features an overview of the Midcontinent Rift system of North America, an area that extends for more than 2000 km northeasterly from Kansas, through the Lake Superior region, and then southeasterly through lower Michigan. This summary of the stratigraphy, structure, and mineralization of rift rocks provides an overview of the geologic history in northern Wisconsin and upper Michigan. Separate sections describe the tectonic history and structural features of the area, the stratigraphy of volcanic and sedimentary deposits, and the mineralization that produced rich copper and silver deposits. Information is supported by numerous citations while maps and diagrams help illustrate the concepts.

Bornhorst, T.; Woodruff, L.; Nicholson, S.; University, Michigan T.

282

Gulf of California Rift Structure and Magmatism: Guaymas Basin and Comparison With Southern Basins  

NASA Astrophysics Data System (ADS)

We present a detailed seismic velocity model of the crust from margin to margin across the Guaymas Basin and compare this model with basins in the southern Gulf of California. These results are based on data from a 2002 crustal-scale, active-source seismic experiment, where multi-channel and wide-angle seismic data were acquired along three flow-line transects across Guaymas Basin, Alarcon Basin, and between Puerto Vallarta and Cabo San Lucas. Each of these transects was instrumented with ocean-bottom seismometers spaced 10-15 km apart and similarly spaced seismometers on land recording the offshore shots. The seafloor morphology of the Gulf has suggested to many that the mode of extension transitions from localized extension followed by seafloor spreading (a narrow rift) in the southern Gulf to a wide-rift mode of distributed extension in the north. This is not the case, however. The thickly sedimented Guaymas basin is a narrow rift. Lithospheric necking occurred after ~150 km of extension, and a subsequent ~300 km of extension has been accommodated by the formation of new lithosphere. The spreading center has been robustly magmatic, forming new igneous crust 7 to 10 km thick. To the south, the Alarcon basin is a wider rift, experiencing ~290 km of extension, including rift relocations, prior to the onset of seafloor spreading (~160 km of extension), which produces crust ~6 km thick. Thus, total extension for these two basins is similar (~450 km), but the mode of extension and manifestation of magmatism are very different. Differences in sedimentation may provide an explanation for the variation in magmatism between these basins, with the thicker sediments in the north providing a thermal blanket that enables more complete melt extraction from the mantle. Magmatism does not provide a sufficient explanation for the difference in rift width, however, since rifting between Cabo San Lucas and Puerto Vallarta, just one segment south of Alarcon, was particularly narrow and amagmatic. These observations indicate that a continuous, along-axis transition in rift mode does not to exist in the Gulf of California. Rather, the mode of extension seems best correlated with geologic provinces on the Baja peninsula.

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

2005-12-01

283

Kinematics of the Ethiopian Rift and Absolute motion of Africa and Somalia Plates  

NASA Astrophysics Data System (ADS)

The Ethiopian Rift (ER), in the northern part of East African Rift System (EARS), forms a boundary zone accommodating differential motion between Africa and Somalia Plates. Its orientation was influenced by the inherited Pan-African collisional system and related lithospheric fabric. We present the kinematics of ER derived from compilation of geodetic velocities, focal mechanism inversions, structural data analysis, and construction of geological profiles. GPS velocity field shows a systematic eastward magnitude increase in NE direction in the central ER. In the same region, incremental extensional strain axes recorded by earthquake focal mechanism and fault slip inversion show ?N1000E orientation. This deviation between GPS velocity trajectories and orientation of incremental extensional strain is developed due to left lateral transtensional deformation. This interpretation is consistent with the en-échelon pattern of tensional and transtensional faults, the distribution of the volcanic centers, and the asymmetry of the rift itself. Small amount of vertical axis blocks rotation, sinistral strike slip faults and dyke intrusions in the rift accommodate the transtensional deformation. We analyzed the kinematics of ER relative to Deep and Shallow Hot Spot Reference Frames (HSRF). Comparison between the two reference frames shows different kinematics in ER and also Africa and Somalia plate motion both in magnitude and direction. Plate spreading direction in shallow HSRF (i.e. the source of the plumes locates in the asthenosphere) and the trend of ER deviate by about 27°. Shearing and extension across the plate boundary zone contribute both to the style of deformation and overall kinematics in the rift. We conclude that the observed long wavelength kinematics and tectonics are consequences of faster SW ward motion of Africa than Somalia in the shallow HSRF. This reference frame seems more consistent with the geophysical and geological constraints in the Rift. The faster SW motion of Africa with respect to Somalia plate is due to a possibly lower viscosity in the top asthenosphere (Low-Velocity Zone) beneath Africa. These findings have significant implications for the evolution of continental rifting in transtensional settings and provide evidence for the kinematics and tectonics of the Ethiopian rift in the context of the Africa-Somalia plate interaction in the mantle reference frame.

Muluneh, A. A.; Cuffaro, M.; Doglioni, C.

2013-12-01

284

Volcano spacings and lithospheric attenuation in the Eastern Rift of Africa  

NASA Technical Reports Server (NTRS)

The Eastern Rift of Africa runs the gamut of crustal and lithospheric attenuation from undeformed shield through attenuated rift margin to active neo-oceanic spreading zones. It is therefore peculiarly well suited to an examination of relationships between volcano spacings and crust/lithosphere thickness. Although lithospheric thickness is not well known in Eastern Africa, it appears to have direct expression in the surface spacing of volcanoes for any given tectonic regime. This applies whether the volcanoes are essentially basaltic, silicic, or alkaline-carbonatitic. No evidence is found for control of volcano sites by a pre-existing fracture grid in the crust.

Mohr, P. A.; Wood, C. A.

1976-01-01

285

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

NASA Astrophysics Data System (ADS)

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 (?2 H, ?18 O) in order to determine the origin and evolution of the waters. Additionally, 30 soil and rock samples were also collected and analysed for fluoride, and rock samples were subjected to petrographic investigations and X-ray diffraction. The fluoride levels in surface and groundwater for the dry season range from 0.019 - 50.14 mg/L, on average above the WHO permissible limit. The high fluoride occurs both in the lake and groundwater. Preliminary petrographic studies show considerable fluoride in micas. The study is on-going and plans to present the relative abundances of fluoride in the lithology as the sources and the fluoride enrichment pathways of the groundwater within the Central Kenya rift.

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

2013-12-01

286

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

NASA Astrophysics Data System (ADS)

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

Bois, C.

1993-11-01

287

Electromagnetic Exploration of the Exmouth and Voring Rifted Margins  

NASA Astrophysics Data System (ADS)

In this work, we present improvements to and the use of two marine electromagnetic exploration methods at the Exmouth and Voring Plateaus. First we discuss the marine controlled-source electromagnetic method (CSEM), specifically the formation and characteristics of custom binary and ternary waveforms as well as improvements to time-series processing which allow derivation of noise estimates. We then apply these improvements to the study of the Scarborough gas reservoir beneath the Exmouth Plateau off the northwest shelf of Australia, over which we collected the world's largest academic CSEM dataset. Using these data we (1) discuss methods for determining uncertainties in CSEM, (2) investigate the resolution of CSEM in an environment where multiple resistive geologic layers lie in close repose, (3) develop the prejudiced minimum norm as a regularization method in 1D CSEM inversion, discussing its strengths and limitations, and (4) develop the 1D/2D and 1D/3D inversion methods in which the forward model is 1D but the regularization is 2D or 3D. All of the new inversion methods derive more realistic results than 1D smooth inversion alone and may form a good starting point for higher dimensional inversion. Finally we target the deep structure of the Exmouth Plateau with marine magnetotellurics and compare the results to the Voring Plateau off the northwest shelf of Norway. Magnetotelluric data reveal that both plateaus host a body of unexpectedly low resistivity (? 0.1 Ohm m) at ˜10 km depth. We show that the depth of each body is consistent with known sills and theorize that the low resistivity is due to magnetite cumulates precipitated as layers in mafic-ultramafic sills. Both plateaus exhibit the features of volcanic passive margins in which the final continental rifting event was relatively quick and accompanied by voluminous volcanism. Our data are consistent with this explanation and suggest that a large volume of mafic material was injected into the crust forming a relatively interconnected network of conductive minerals.

Myer, David Gerard

288

Geochemical Studies of Abyssal Lavas Recovered by DSRV Alvin From Eastern Galapagos Rift, Inca Transform, and Ecuador Rift 3. Trace Element Abundances and Petrogenesis  

Microsoft Academic Search

Glassy to sparsely phyric submarine lavas were recovered from nine Alvin dive sites located along the eastern Galapagos rift and at the intersection with the Inca transform. Samples include quartz-normative tholeiitic basalts (MORB), numerous Fe- and Ti-enriched basalts (FeTi basalts) and a smaller number of oceanic andesires (55.9-64.3 wt % SiO2). MORB have light rare earth dement (REE) and large

Michael R. Perfit; Daniel J. Fornari; Alexander Malahoff; Robert W. Embley

1983-01-01

289

Galapagos rift at 86 /sup 0/W 5. Variations in volcanism, structure, and hydrothermal activity along a 30-kilometer segment of the rift valley  

SciTech Connect

A 30-km segment of the Galapagos Rift near 86 /sup 0/W has been mapped in detail using the Angus towed camera system, the submersible Alvin, and multi-narrowbeam sonar data. Recent volcanic activity and active hydrothermal circulation are evident along the entire length of the segment mapped. There are, however, clear along-strike variations in these processes which render previous two-dimensional models obsolete. Although alternate explanations are possible, eruptive sequences appear to begin with the outpouring of surface-fed sheet flows and end with more channelized pillow flows. In the western portion of the rift studied, sheet flows dominate with the entire valley floor covered by recent flows associated with a broad shield volcano. The eastern portion, on the other hand, is narrower; consisting primarily of less voluminous pillow flows of apparently the same youthful age. Three possible models for the volcanic evolution of this rift segment are presented. According to the first model, the extrusive portion of the crust is formed by a distinct volcanic episode, followed by a long period of volcanic quiescence. The volcanic phase begins with voluminous sheet flows emerging from numerous eruptive fissures, which in time evolve into a narrow pillow ridge. Farther along-strike, where the flows are smaller and the extrusive zone narrow, the marginal portions undergo continued fissuring and subsequent uplift to form marginal highs and lows. This deformational activity also affects the extrusive zone once volcanic activity ends, converting the distinctly lobate topography of the active period into highly lineated fault-controlled terrain. According to the second model, extension and volcanism can be viewed as a continuous process without major periods of volcanic quiescence. The initial lava flows of a new eruptive sequence fill low areas, frequently spilling over local sills and flooding much of the rift valley.

Ballard, R.D.; van Andel, T.H.; Holcomb, R.T.

1982-02-10

290

Deformation distribution and type in the Main Ethiopian Rift (MER): A remote sensing study  

NASA Astrophysics Data System (ADS)

The Main Ethiopian Rift (MER) is magmatically segmented and seismically active. The magmatic segments are arranged en echelon (e.g. Ebinger, C.J., Casey, M., 2001. Continental breakup in magmatic provinces: An Ethiopian example. Geology Society of America Bulletin, vol. 12, pp. 527-530). Furthermore they are about 50 km long and up to 20 km wide, orientated about N10°-20°E, and are therefore oblique to the main direction of Mid-Miocene border faults (N35°E). In this paper we propose that these segments not only represent magmatic but also tectonic segmentation. We use the association of the high spectral resolution of Landsat data (principal component analysis and classifications) and the very high spatial resolution of digitized aerial photographs to characterize and quantify the deformation along the rift. Additionally, several digital elevation models of metric resolution have been produced at different locations along the segments in order to quantify and describe the faulting. We define 4 tecto-magmatic segments (Gedemsa, Bosetti, Kone and Fantale), 40-70 km, 10-15 wide, arranged en echelon and N10°-20°E orientated. Each tecto-magmatic segment (TMS) is separated by 2-18 km wide domains exhibiting minor deformation. Furthermore, the TMS overlap in a N-S direction and show similar symmetrical deformation patterns. Within an individual segment deformation varies from its centre, where it is essentially magmatic, to its tips, where brittle deformation is predominant. The largest volumes of recent basalt are erupted in the centres and nearby. In contrast, recent magma injections are absent in the tip domains. It seems probable that dikes connect laterally to, and are fed by, the magmatic centres. Dikes are sub-parallel to the segment axis and therefore orientated normal to the recent extension direction. Within the TMS, aligned cones, atypical fault geometries, en echelon fault segments, and atypical displacement-length ratios all suggest that faulting is dike-induced. Along axis propagation of the TMS generate interactions between younger (Quaternary) intra-rift faults and older (Mid-Miocene) border faults. Thus, at the deformed tips, cross-cutting and long curved faults are produced. The fault interactions in the MER confirm that the border faults have been inactive during the recent stage of rifting. We discuss the implications of oblique rifting with moderate obliquity ( ? = 27°, angle between the extension direction and the trend perpendicular to the rift) at slow extension rates on active rifting processes associated with intermediate continental crust. The tecto-magmatic segmentation mirrors crustal segmentation beneath the rift axis, as indicated from 3-D-tomography (Daly, E., Keir, D., Ebinger, C., Stuart, G., Ayele, A., Waltham, D., 2007. Crustal structure of the northern Main Ethiopian Rift from a tomographic inversion of local earthquakes. Geophysical Journal International, in press; Keranen, K., Klemperer, S.L., Gloaguen, R., EAGLE Working Group, 2004. Three-dimensional seismic imaging of a protoridge axis in the Main Ethiopian rift. Geology 32 (11), 949-952). These long and narrow zones (40-70 km long and 10-15 km wide), where deformation is generated principally by diking and is associated with magmatic centres, have similarities with along-axis segmentation at mid ocean ridges. Consequently, deformation distribution and type in the MER can be considered the outcome of a transition between continental and active oceanic rifting.

Kurz, T.; Gloaguen, R.; Ebinger, C.; Casey, M.; Abebe, B.

2007-06-01

291

Why farm the city? Theorizing urban agriculture through a lens of metabolic rift  

E-print Network

Why farm the city? Theorizing urban agriculture through a lens of metabolic rift Nathan Mc, urban farming JEL Classifications: Q, R Introduction Part of the momentum surrounding food system, mcclintock@berkeley.edu Received on July 15, 2009; accepted on December 14, 2009 Urban agriculture (UA

292

First Evidence of Epithermal Gold Occurrences in the SE Afar Rift, Republic of Djibouti  

Microsoft Academic Search

The Republic of Djibouti, located at the SE part of the Afar volcanic Triangle, is characterized by intense tectonic and bimodal volcanic activity, and is emplaced over an earlier magmatic rift system, as old as 25-30 Ma. Each magmatic event is accompanied by hydrothermal activity. Few works have been so far published on hydrothermal mineralization in the Afar area. Mineralization

Nima Moussa; Yves Fouquet; Antoine Marie Caminiti; Bernard Le Gall; Joel Rolet; Marcel Bohn; Joel Etoubleau; Christophe Delacourt; Mohamed Jalludin

2010-01-01

293

Strain distribution across magmatic margins during the breakup stage: Seismicity patterns in the Afar rift zone  

NASA Astrophysics Data System (ADS)

Fault patterns record the strain history along passive continental margins, but geochronological constraints are, in general, too sparse to evaluate these patterns in 3D. The Afar depression in Ethiopia provides a unique setting to evaluate the time and space relations between faulting and magmatism across an incipient passive margin that formed above a mantle plume. The margin comprises a high elevation flood basalt province with thick, underplated continental crust, a narrow fault-line escarpment underlain by stretched and intruded crust, and a broad zone of highly intruded, mafic crust lying near sealevel. We analyze fault and seismicity patterns across and along the length of the Afar rift zone to determine the spatial distribution of strain during the final stages of continental breakup, and its relation to active magmatism and dike intrusions. Seismicity data include historic data and 2005-2007 data from the collaborative US-UK-Ethiopia Afar Geodynamics Project that includes the 2005-present Dabbahu rift episode. Earthquake epicenters cluster within discrete, 50 km-long magmatic segments that lack any fault linkage. Swarms also cluster along the fault-line scarp between the unstretched and highly stretched Afar rift zone; these earthquakes may signal release of stresses generated by large lateral density contrasts. We compare Coulomb static stress models with focal mechanisms and fault kinematics to discriminate between segmented magma intrusion and crank- arm models for the central Afar rift zone.

Brown, C.; Ebinger, C. J.; Belachew, M.; Gregg, T.; Keir, D.; Ayele, A.; Aronovitz, A.; Campbell, E.

2008-12-01

294

Rift Valley fever in Kenya: the presence of antibody to the virus in camels (Camelus dromedarius).  

PubMed Central

Five hundred and seventy-one camel sera collected after an epizootic of Rift Valley Fever were examined for antibody to the virus. Clinical disease had not been observed in cattle and sheep in the ecosystems shared with the camels. Positive sera with high titres of serum neutralizing antibody were found in 22% of camels at one of the seven sampling sites. PMID:3989285

Davies, F. G.; Koros, J.; Mbugua, H.

1985-01-01

295

A Fusion-Inhibiting Peptide against Rift Valley Fever Virus Inhibits Multiple, Diverse Viruses  

E-print Network

virus), Class II (Andes virus), or Class III (vesicular stomatitis virus) fusion proteins using, we show that infectivity can be inhibited for diverse, unrelated RNA viruses that have Class I (EbolaA Fusion-Inhibiting Peptide against Rift Valley Fever Virus Inhibits Multiple, Diverse Viruses

296

Earthquake depth distribution in the Baikal rift system and its rheological meaning  

Microsoft Academic Search

Accurate determination of the earthquake source depth within the crust is possible provided that dense seismic network is operating and reliable velocity models are available. These conditions are not implemented in the Baikal rift system that leads to the ambiguity of the results and confusion with their interpretations. Meanwhile the earthquake depth is of special importance for the study of

Natalia Radziminovich

2010-01-01

297

Variations in Mid-Continent Rift magma volumes consistent with microplate evolution  

E-print Network

; accepted 23 February 2013; published 28 April 2013. [1] Modeling of gravity data along the ~1.1 Ga failed arms. The volume of magma increases towards the Lake Superior region, consistent with magma flowing gravity map of the central United States (Figure 1). The rift formed at ~1.1 Ga, recorded by two pulses

Stein, Seth

298

77 FR 68783 - Prospective Grant of Exclusive License: Veterinary Vaccines for Rift Valley Fever Virus  

Federal Register 2010, 2011, 2012, 2013

...Health and Human Services (HHS), is thinking of giving a co-exclusive license in...constructs that contain complete deletions of critical virulence factors of Rift Valley Fever...vaccinated animals--a feature that is critical in agricultural settings. This...

2012-11-16

299

Age of Fault Movements in Tanzanian Sector of East African Rift System  

Microsoft Academic Search

THE Neogene volcanic province of northern Tanzania is a southerly extension of the more extensive volcanic areas of Ethiopia and Kenya. It stands astride the Eastern Rift Valley and represents a complex interplay of volcanic activity and widespread Earth movements with associated faulting. A general picture is that an older series of basaltic-trachytic shield volcanoes, together with smaller nephelinitic centres,

R. M. MacIntyre

1974-01-01

300

Propagation of rifting along the Arabia-Somalia plate boundary The Gulfs of Aden and Tadjoura  

Microsoft Academic Search

The localization and propagation of rifting between Arabia and Somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern Gulf of Aden and the Gulf of Tadjoura, using bathymetric, magnetic, seismological, and structural evidence. Large-scale, southwestward propagation of the Aden ridge, markedly oblique to the Arabia-Somalia relative motion vector, began about 30 Myr ago

Isabelle Manighetti; Paul Tapponnier; Vincent Courtillot; Sylvie Gruszow; Pierre-Yves Gillot

1997-01-01

301

The influence of large-scale mantle flow on the dynamics of the East African Rift  

NASA Astrophysics Data System (ADS)

Because the Nubia/Somalia plate system is almost entirely surrounded by spreading ridges in the Atlantic and Indian Oceans, Earth's mantle buoyancies are thought to play a key role in the dynamics of East Africa rifting. While this notion is supported by images of large-scale mantle structures beneath Africa from seismic tomography, the details of the force balance driving continental rifting are still debated. In particular, the contribution of gravitational potential energy (GPE) associated with continental topography in driving ~E-W extension across the East African Rift remains elusive. The Nubia/Somalia divergent boundary therefore presents a unique opportunity to quantify in detail plate-driving forces associated with 1) mantle buoyancies acting at the base of the African lithosphere and 2) lateral GPE gradients within the lithosphere. In this study we employ numerical models of the mantle/lithosphere system to test explicitly the contribution of mantle convection to the dynamics of rifting in East Africa. Preliminary results suggest that such contribution may be smaller than previously thought, and that forces arising from GPE gradients may control surface deformation in East Africa.

Stamps, D. S.; Iaffaldano, G.; Calais, E.

2011-12-01

302

Rifted continental margins are formed by progressive extension of the lithosphere.  

E-print Network

,a major seismic experi- ment was conducted in February­March 2005 in the eastern Black Sea Basin (Figure 1 cannot be well constrained.However,the Black Sea differs from these sediment-starved rifted margins in that the surrounding continent has provided an ongoing sediment supply. The Eastern Black Sea Basin The Black Sea

Shillington, Donna J.

303

Paleostress reconstructions and geodynamics of the Baikal region, Central Asia, Part 2. Cenozoic rifting  

Microsoft Academic Search

Investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses. It appears that changes of stress field with time are an important factor

Damien Delvaux; Rikkert Moeys; Gerco Stapel; Carole Petit; Kirill Levi; Andrei Miroshnichenko; Valery Ruzhich; Volodia San'kov

1997-01-01

304

The hydrothermal mounds of the Galapagos Rift: Observations with DSRV Alvin and detailed heat flow studies  

Microsoft Academic Search

Raging in height from gentle hills of less than a meter to steep-sided giants of more than 20 m, the mounds of the Galapagos Rift are spectacular hydrothermal features. Their internal temperatures have been measured at up to 13°C above the bottom water temperature, and total heat flow (conducted plus convected) can be several hundred to several thousand times the

David L. Williams; Kenneth Green; Tjeerd H. van Andel; Richard P. Von Herzen; Jack R. Dymond; Kathleen Crane

1979-01-01

305

Volcanic geology and eruption frequency, lower east rift zone of Kilauea volcano, Hawaii  

Microsoft Academic Search

Detailed geologic mapping and radiocarbon dating of tholeiitic basalts covering about 275 km2 on the lower east rift zone (LERZ) and adjoining flanks of Kilauea volcano, Hawaii, show that at least 112 separate eruptions have occurred during the past 2360 years. Eruptive products include spatter ramparts and cones, a shield, two extensive lithic-rich tuff deposits, aa and pahoehoe flows, and

Richard B Moore

1992-01-01

306

A geomorphological reconnaissance of the submarine part of the East Rift Zone of Kilauea Volcano, Hawaii  

Microsoft Academic Search

More than half of the intensely active East Rift Zone of Kilauea Volcano crops out underwater along the crest of the submarine Puna Ridge. I present multibeam bathmetry of the entire ridge, near-bottom photographic and sonar observations of the plunging crest of its deeper distal half, and seismic profiles across the ridge tip and the adjacent structural moat. Analysis of

Peter Lonsdale

1989-01-01

307

Pigment Analysis of Short Cores from the Central Ethiopian Rift Valley Lakes  

Microsoft Academic Search

Pigment analysis (Chlorophyll Derivatives — CD, and Total Carotenoids — TC) from surface and core sediments of three lakes: Langano, Abijata and Shalla in the Central Ethiopian Rift Valley are presented. The results show that pigment concentration is very low in modern sediments with CD generally higher than TC. This is in accordance with the present low productivity of the

M. U. Mohammed; R. Bonnefille; Kebede Seifu

308

On rates and styles of late volcanism and rifting on Venus Alexander T. Basilevsky  

E-print Network

On rates and styles of late volcanism and rifting on Venus Alexander T. Basilevsky Vernadsky the formation of regional plains on Venus. We analyzed the age relations of all known impact craters !30 km of such a transition occurring at different times in different areas of Venus. INDEX TERMS: 6295 Planetology: Solar

Head III, James William

309

A Cenozoic diffuse alkaline magmatic province (DAMP) in the southwest Pacific without rift or plume origin  

E-print Network

A Cenozoic diffuse alkaline magmatic province (DAMP) in the southwest Pacific without rift or plume of the Pacific Plate. A key to generating the Cenozoic magmatism is the combination of metasomatized lithosphere. The model may also provide a mechanism for warming south Pacific mantle and resulting Cenozoic alkaline

Müller, Dietmar

310

Wilson cycles, tectonic inheritance, and rifting of the North American Gulf of Mexico continental margin  

E-print Network

accommodated within a diffuse region adjacent to the oro- gen. This variation in location of rifting-Caledonian oro- gen (Ziegler, 1989). The association between the positions of continental breakup and older very little extensional deformation. Thus, the two margins differ in that the Ouachita oro- gen appears

Huerta, Audrey D.

311

Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia  

USGS Publications Warehouse

The extensive harrat lava province of Arabia formed during the past 30 million years in response to Red Sea rifting and mantle upwelling. The area was regarded as seismically quiet, but between April and June 2009 a swarm of more than 30,000 earthquakes struck one of the lava fields in the province, Harrat Lunayyir, northwest Saudi Arabia. Concerned that larger damaging earthquakes might occur, the Saudi Arabian government evacuated 40,000 people from the region. Here we use geologic, geodetic and seismic data to show that the earthquake swarm resulted from magmatic dyke intrusion. We document a surface fault rupture that is 8 km long with 91 cm of offset. Surface deformation is best modelled by the shallow intrusion of a north-west trending dyke that is about 10 km long. Seismic waves generated during the earthquakes exhibit overlapping very low- and high-frequency components. We interpret the low frequencies to represent intrusion of magma and the high frequencies to represent fracturing of the crystalline basement rocks. Rather than extension being accommodated entirely by the central Red Sea rift axis, we suggest that the broad deformation observed in Harrat Lunayyir indicates that rift margins can remain as active sites of extension throughout rifting. Our analyses allowed us to forecast the likelihood of a future eruption or large earthquake in the region and informed the decisions made by the Saudi Arabian government to return the evacuees. ?? 2010 Macmillan Publishers Limited. All rights reserved.

Pallister, J.S.; McCausland, W.A.; Jonsson, S.; Lu, Z.; Zahran, H.M.; El, Hadidy S.; Aburukbah, A.; Stewart, I.C.F.; Lundgren, P.R.; White, R.A.; Moufti, M.R.H.

2010-01-01

312

An unusual mantle source region for nephelinites from the Oslo Rift, Norway  

Microsoft Academic Search

We have analyzed the trace-element and isotopic composition of a series of lavas and their constituent pyroxenes and calcite from near the town of Skien in the Permo-Carboniferous Oslo Rift. The series decreases in alkalinity upsection: the lowermost lavas are nephelinites, followed by basanites and finally alkali basalts. This sequence is accompanied by decreasing light rare earth element (LREE), Th,

E. Y. Anthony; T. V. Segalstad; E.-R. Neumann

1989-01-01

313

Effect of strain localization on sub-rift upper mantle rheology and anisotropy  

Microsoft Academic Search

In order to explain a possible weak rheology of the uppermost mantle suggested by work on the focal mechanism distribution in the continental lithosphere, we have investigated microstructures, LPO and water contents using FTIR of peridotites from present-day (Baja California) and ancient (North Pyrenean) rift zones. The aim of this study is link microstructural analysis with the rheology and the

L. N. Palasse; M. Drury; R. Vissers

2007-01-01

314

Permeability, underpressures, and convection in the oceanic crust near the Costa Rica Rift, eastern equatorial pacific  

Microsoft Academic Search

In situ permeability and pore pressures were measured 200 m deep beneath the top of the oceanic crust at DSDP site 504B. These measurements have relevance for the transition from convective to conductive heat flow on the south flank of the Costa Rica Rift. Conventional 'slug' and constant rate injection tests were made below a hydraulic packer set at various

Roger N. Anderson; Mark D. Zoback

1982-01-01

315

Continental rift architecture and patterns of magma migration: A dynamic analysis based on centrifuge models  

Microsoft Academic Search

Small-scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement. Experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at Moho depths. Results suggest that surface deformation, which reflects the weakness zone geometry, exerts a major control on patterns of magma migration.

Giacomo Corti; Marco Bonini; Dimitrios Sokoutis; Fabrizio Innocenti; Piero Manetti; Sierd Cloetingh; Genene Mulugeta

2004-01-01

316

Modeling heterogeneous stretching during episodic or steady rifting of the continental lithosphere  

Microsoft Academic Search

The geometry of brittle-ductile extension and modes of normal faulting during steady or episodic rifting of the continental lithosphere are investigated by analogue modeling using the centrifuge technique. We use a frictional material to simulate the brittle behavior and various nonlinear bouncing putties to represent the ductile rheology of the stratified lithosphere. Steady stretching of the continental lithosphere supports rapid

Genene Mulugeta; Woldai Ghebreab

2001-01-01

317

Modelling the lithospheric rheology control on Cretaceous rifting in West Antarctica  

Microsoft Academic Search

Small-scale analogue models were used to investigate the process of Cretaceous orthogonal extension in the West Antarctic Rift System. The models considered the transition from the East Antarctic Craton to a weaker lithosphere, and the results support previous hypotheses about the strong control exerted by lateral variations in lithospheric structures on the process of extension. Strain was mostly accommodated at

Marco Bonini; Giacomo Corti; C. DelVentisette; Piero Manetti; Genene Mulugeta; Dimitrios Sokoutis

2007-01-01

318

Morphology of Europan bands at high resolution: A mid-ocean ridge-type rift mechanism  

E-print Network

Morphology of Europan bands at high resolution: A mid-ocean ridge-type rift mechanism Louise M on terrestrial mid-ocean ridges reveal several similarities, including axial troughs, subcircular hummocks, normal faults, and indications of symmetrical spreading. We conclude that terrestrial mid-ocean ridge

Head III, James William

319

Choice Issue Opens Rift in Missouri: Tax-Credit Scholarships Divide Black Caucus  

ERIC Educational Resources Information Center

The debate over a school choice bill in the Missouri legislature has opened a bitter rift among some of the state's top Black elected officials and reflects a larger divide among African-Americans over school choice nationwide. The bill, which is similar to programs that are growing in popularity in other states, would provide tax credits for…

Viadero, Debra

2006-01-01

320

Tectonic controls on the hydrogeology of the Rio Grande Rift, New Mexico  

Microsoft Academic Search

Mathematical modeling is used in this study to assess how tectonic movement of fault blocks and fault permeability influence the present-day and paleohydrogeology of the Rio Grande Rift near Socorro, New Mexico. Our analysis focuses on active and ancient groundwater flow patterns and hot spring development within the southern La Jencia and Socorro subbasins. The best agreement between model results

Brian J. Mailloux; Mark Person; Shari Kelley; Nelia Dunbar; Steve Cather; Luther Strayer; Peter Hudleston

1999-01-01

321

Hydrogeological aspects of fault zones on various scales in the Roer Valley Rift System  

E-print Network

Abstract Hydrogeological aspects of fault zones on various scales in the Roer Valley Rift System in the damage zone that has formed around the main fault plane shows in detail the impact of tectonic deformation on the hydrogeologic structure of the fault zone. Mechanisms like clay smearing and partic- ulate

Bense, Victor

322

A new class of ``pseudofaults'' and their bearing on plate tectonics: A propagating rift model  

Microsoft Academic Search

The pattern of magnetic anomaly offsets striking obliquely to the Blanco fracture zone near the Juan de Fuca spreading center appears to be incompatible with the rigid-plate hypothesis. Previous workers have thus called upon complex, or anomalous, mechanisms to explain the tectonic evolution of this area. According to the ``propagating rift'' model developed here, the basic observations that previous hypotheses

Richard Hey

1977-01-01

323

Mesozoic rift basins in western desert of Egypt, their southern extension and impact on future exploration  

SciTech Connect

Rift basins are a primary target of exploration in east, central, and west Africa. These intracratonic rift basins range in age from the Triassic to the Neogene and are filled with lagoonal-lacustrine sand-shale sequences. Several rift basins may be present in the Western Desert of Egypt. In the northeastern African platform, the Mesozoic Tethyan strand lines were previously interpreted to have limited southern extension onto the continent. This concept, based upon a relatively limited amount of subsurface data, has directed and focused the exploration for oil and gas to the northernmost 120 km of the Western Desert of Egypt. Recent well and geophysical data indicate a southerly extension of mesozoic rift basins several hundred kilometers inland from the Mediterranean Sea. Shushan/Faghur and Abu Gharadig/Bahrein basins may represent subparallel Mesozoic basins, trending northeast-southwest. Marine Oxfordian-Kimmeridgian sediments were recently reported from wells drilled approximately 500 km south of the present-day Mediterranean shoreline. The link of these basins with the Sirte basin to the southwest in Libya is not well understood. Exploration is needed to evaluate the hydrocarbon potential of such basins.

Taha, M.A. (Conoco, Cairo (Egypt))

1988-08-01

324

Modes of continental rifting as a function of ductile strain localization in the lithospheric mantle  

E-print Network

1 Modes of continental rifting as a function of ductile strain localization in the lithospheric Orléans cedex 2. Abstract Analogue and numerical models have shown that the strength of the lithospheric mantle controls the mode of lithosphere deformation. In extension, the presence or absence of a high

Paris-Sud XI, Université de

325

Magmatism at rift zones - The generation of volcanic continental margins and flood basalts  

Microsoft Academic Search

A simple model is developed which explains the occurrence of volcanic continental margins and flood basalts as a consequence of their association with nearby plumes that were active at the time of rifting. In the model, asthenosphere temperatures are increased by 100-150 C over large regions of the earth by heat advected upward in mantle plumes. The amount of partial

Robert White; Dan McKenzie

1989-01-01

326

The Tethyan plume: geochemical diversity of Middle Permian basalts from the Oman rifted margin  

E-print Network

are exposed in Oman within either the Hawasina nappes or the para-autochtonous Arabian platform exposed the Hawasina Nappes. This group differs from Groups 1 and 2 by its low to negative eNdi ( + 1.6 to Ã? 2). Group-related basalts; Middle Permian; Neo-Tethys; Arabian rifted margin; Hawasina Nappes; Oman; Isotopic chemistry 1

Demouchy, Sylvie

327

Finite Element Models for the deformation of the Askja volcanic complex and rift segment, Iceland  

NASA Astrophysics Data System (ADS)

The Askja volcanic complex and rift segment of Iceland's Northern Volcanic Zone has been continuously subsiding at an usually high rate for more than two decades. InSAR data compiled over the last decade reveal two distinct patterns of deformation for Askja: (1) a radially symmetric pattern of subsidence local to Askja's caldera that suggests decompression within a shallow magma reservoir and (2) an elongated pattern of subsidence that tracks the rift segment. Microgravity data suggest a mass loss from the region of the shallow reservoir and seismicity data reveal a relatively shallow brittle-ductile transition beneath the Askja rift segment. A simple model combining two vertically-aligned and contracting Mogi sources, one shallow (~3 km) and one deep (~20 km), in an elastic half space generally predicts the observed InSAR deformation. Subsidence along the Askja fissure swarm has also been attributed to effects of plate spreading across rheologically weak fissure swarms. The shallow contracting Mogi source and microgravity data are consistent with magma migration out of the shallow reservoir. Interpretations of the deep (~ 20 km) contracting source beneath the brittle ductile transition are more uncertain. We present an alternative model that combines magma extraction from a shallow, fluid-filled cavity (to simulate the shallow reservoir) with a plate spreading model having rheologic partitioning expected for the rift segment. This 3D finite element model (FEM) simulates an elastic upper crust and viscoelastic lower crust. Inspired by a recent model configuration presented by Pedersen et al. [EPSL, 2009], the simulated brittle-ductile transition shallows significantly beneath the rift segment, in accord with seismicity data. The FEM is driven by plate spreading at a constant rate, imposed as velocity boundary conditions, and specified mass flux from the shallow fluid-filled cavity. The magnitude of flux is a calibration parameter that is estimated from the InSAR data via inverse methods. Preliminary results suggest that this alternative model generally predicts the deformation fields observed for both the caldera (local) and rift segment (regional). However, the simulated shallow brittle ductile transition, combined with kinematic loading of plate spreading, accounts for much of the regional deformation originally attributed to magma migration out of a deep reservoir. This suggests that the estimated characteristics of magma extraction from the deep reservoir should be re-examined. The calibrated FEM accounts for multiple types of observations (both the local and regional subsidence patterns, microgravity data, seismicity data, and plate spreading) associated with active deformation of the Askja volcano complex and rift segment.

Dickinson, H.; Masterlark, T.; Feigl, K.; Pedersen, R.; Sigmundsson, F.

2009-12-01

328

Seismicity Patterns and Magmatic Processes in the Rwenzori Region, East-African Rift  

NASA Astrophysics Data System (ADS)

The 5000m high Rwenzori Mountains are situated within the western branch of the East African Rift System (EARS), at the border between Uganda and the Democratic Republic of Congo. They represent a basement block located within the rift valley whose origin and relation to the evolution of the EARS are highly puzzling. During a recent seismological campaign we located more than 800 earthquakes per month with magnitudes ranging from 0.5 to 5.1. Vertical sections across the northern parts of the Rwenzoris show, that west of the mountains (towards the rift valley) the focal depths range from 10 to 20 km, whereas the hypocentres go as deep as 30 km on the eastern side. This is in good agreement with Moho-depths derived from receiver functions and implies that all of these events are located within the crust. However, about 30 km east of the northern mountain ridge we located a cluster of 7 events that exhibit an anomalous depth of about 60 km. We can confidently locate these earthquakes within the mantle lithosphere beneath the rift. The existence of earthquakes at this depth is enigmatic, especially within a rifting regime were one expects hot and weak material relatively close to the surface. We think that these events are possibly related to the evolution of the Rwenzori Mountains. A recent hypothesis to explain the extreme uplift of the Rwenzori Mountains is rift induced delamination (RID) of mantle lithosphere. Here we show that the RID-process is indeed capable of explaining the seismic energy release in the mantle. However, in view of the specific hypocentral location of the event cluster, magmatic impregnation processes associated with dyke propagation into the mantle lithosphere may be a more realistic cause for seismic radiation at the observed depth. Crustal earthquakes northeast of the Rwenzori area are relocated with a double-difference algorithm to improve the spatial resolution of seismicity pattern. Several event clusters in the vicinity of the Fort Portal volcanic field form pipe-like structures with vertical extensions of 3 to 6 km and diameters of 1 to 2 km. In this region the rifting process is probably still in an early stage. The structures possibly indicate magmatic feeding channels through the crust that originate from the heated and impregnated lithospheric mantle.

Lindenfeld, M.; Rumpker, G.; Schmeling, H.; Wallner, H.

2010-12-01

329

Extension style in the Orphan Basin during the Mesozoic North Atlantic rifting  

NASA Astrophysics Data System (ADS)

The Orphan Basin, lying along the Newfoundland passive continental margin, has formed in Mesozoic time during the opening of the North Atlantic Ocean and the breakup of Iberia/Eurasia from North America. Regional deep seismic reflection profiles across the basin indicate that the Neoproterozoic basement has been affected by repeated extensional episodes between the Late Triassic/Jurassic and the Early Cretaceous. Deformation initiated in the eastern part of the Orphan basin in the Jurassic and migrated toward the west in the Early Cretaceous, resulting in numerous rift structures filled with Jurassic-Lower Cretaceous syn-rift successions and sealed by thick Upper Cretaceous-Cenozoic post-rift sediments. The seismic data show an extremely attenuated crust underneath the eastern and western part of the deep basin, forming two sub-basins associated with the development of rifting. The two sub-basins are separated by a wide structural high with a relatively thick crust and are bounded to the west by the continental shelf domain. Restoration of the Orphan Basin along a 2D crustal section (520 km long), yields a total amount of stretching of about 144 km, while the total crustal thinning indicates an extension of around 250 km, assuming mass conservation along the section and an initial crustal thickness of 28 km. Brittle deformation accommodated by normal faults is documented in the seismic profiles and affected essentially the present-day upper portion of the crust, and represents only 60% of the total extension which thinned the Orphan crust. The remaining crustal thinning must involve other deformation processes which are not (easily) recognizable in the seismic data. We propose two models that could explain discrepancies between brittle deformation and total crustal thinning during lithospheric extension. The first model assumes the reactivation of pre-rift inherited structures, which act as crustal-scale detachments during the early stages of rifting. The second model uses depth-dependent extension of a 20 km thick crust characterized by a strong upper crust and a weak lower crust. Both models raise secondary issues that are discussed around the order of rifting events and the original crustal thickness.

Gouiza, Mohamed; Hall, Jeremy

2013-04-01

330

Boundary separating the seismically active reelfoot rift from the sparsely seismic Rough Creek graben, Kentucky and Illinois  

USGS Publications Warehouse

The Reelfoot rift is the most active of six Iapetan rifts and grabens in central and eastern North America. In contrast, the Rough Creek graben is one of the least active, being seismically indistinguishable from the central craton of North America. Yet the rift and graben adjoin. Hazard assessment in the rift and graben would be aided by identification of a boundary between them. Changes in the strikes of single large faults, the location of a Cambrian transfer zone, and the geographic extent of alkaline igneous rocks provide three independent estimates of the location of a structural boundary between the rift and the graben. The boundary trends north-northwest through the northeastern part of the Fluorspar Area Fault Complex of Kentucky and Illinois, and has no obvious surface expression. The boundary involves the largest faults, which are the most likely to penetrate to hypocentral depths, and the boundary coincides with the geographic change from abundant seismicity in the rift to sparse seismicity in the graben. Because the structural boundary was defined by geologic variables that are expected to be causally associated with seismicity, it may continue to bound the Reelfoot rift seismicity in the future.

Wheeler, R. L.

1997-01-01

331

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

NASA Astrophysics Data System (ADS)

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.

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

2012-04-01

332

Impact of geodynamic development of the Barents Sea deep rift on evolving petroleum systems  

NASA Astrophysics Data System (ADS)

All the Barents Sea deposits are situated in the epicenter on active geodynamic development of the Barents Sea rift and, most important, over the zone of listric faults intersection, which consist a knot system over the mantle diapir. This 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 inner, 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 to the greatest deposits of the South Barents Sea depression. An extended field acoustic data collected in the Barents Sea led to understanding of general fundamental problems for all Arctic Seas and, first of all, the problem of Quaternary glaciations. An analysis of Eurasian-Arctic continental margin shows correspondence between the rift systems of the shelf with those of the ocean. This relation can be observed by an example of the central Arctic region. All the rift systems underlying the sediment basin are expressed in the sea bed relief as spacious and extensive graben valleys burnished by lobes. The rift structures and the sediment cover are connected by the systems of transversal (or oblique) faults. A study of the regional geologic structure of basins affords believing in the very complicated inner riftogenic structure of the base and the lower part of the sediment cover, which is represented by a combination of grabens and horsts bound by the same tectonic border in the form of high amplitude fault zones, and have a very high density of fault-fractured tectonics. In the same time, a branching of the rift structures related to rounding of large solid blocks of the crust, represented by Archaean or Baikal base prominences, takes place. Their frequent structural connection with the continental paleo-rift structures is ascertained, but the shelf rift system are not their straight prolongation and separated by base thresholds concealed under the sediment cover or by bulkheads expressed even in the modern relief. Filling of over rift flexure by sediments, revival of faults and their impact on flow processes and gas hydrate formation in under sea bed sediments have occurred in the South Barents depression. Geodynamic development of the Barents sea rift not only played a substantial role in formation of as unique fields as Shtokman, Prirazlomnoe and others, but created prerequisites for possible gas outbursts into near-surface sediments of the Barents Sea that could result, in some cases, in gas hydrate formation. Periodic activation of basic fault blocks, especially in late Jurassic - early Cretaceous time significantly contributed to formation of the structures related to the greatest fields of South Barents depression. The information on geodynamic development of the region, thermal convection and modern sedimentation enable assessment of the real potentiality for underwater works in the region, potential gas hydrates resources, and will help to assign a strategy of prospecting work, to range the known

Balanyuk, I.; Dmitrievsky, A.

2009-04-01

333

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

NASA Astrophysics Data System (ADS)

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 to the greatest deposits of this depression. An extended field acoustic data collected in the Barents Sea led to understanding of general fundamental problems for all Arctic Seas and, first of all, the problem of Quaternary glaciations. An analysis of Eurasian-Arctic continental margin shows correspondence between the rift systems of the shelf with those of the ocean. This relation can be observed by an example of the central Arctic region. All the rift systems underlying the sediment basin are expressed in the sea bed relief as spacious and extensive graben valleys burnished by lobes. The rift structures and the sediment cover are connected by the systems of transversal (or oblique) faults. A study of the regional geologic structure of basins affords believing in the very complicated inner riftogenic structure of the base and the lower part of the sediment cover, which is represented by a combination of grabens and horsts bound by the same tectonic border in the form of high amplitude fault zones, and have a very high density of fault-fractured tectonics. In the same time, a branching of the rift structures related to rounding of large solid blocks of the crust, represented by Archaean or Baikal base prominences, takes place. Their frequent structural connection with the continental paleo-rift structures is ascertained, but the shelf rift system are not their straight prolongation and separated by base thresholds concealed under the sediment cover or by bulkheads expressed even in the modern relief. The information on geodynamic development of the region, thermal convection and modern sedimentation enable assessment of the real potentiality for underwater works in the region, potential gas hydrates resources, and will help to assign a strategy of prospecting work, to range the known fields, to carry out a regional survey for engineering work.

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

2010-05-01

334

Paleomagnetic evidence of oblique rift localization in the Gulf of California  

NASA Astrophysics Data System (ADS)

Paleomagnetic analysis of extensive Miocene ignimbrites in northwest Mexico reveals the timing and distribution of dextral shear associated with the inception of the Gulf of California rift. We drilled new high-precision paleomagnetic reference sites for the regionally extensive 12.5 Ma Tuff of San Felipe (SF) and the 6.4 Ma Tuffs of Mesa Cuadrada (MC) in undeformed, mesa-top exposures in north-central Baja California, west of the rift-bounding San Pedro Martír fault system. The paleomagnetic remanence directions determined at these tectonically stable sites in central Baja California (DSF=212.4°, ISF=-3.0°, nSF=48; DMC=15.6°, IMC=56.2°, nMC=92) are consistent over tens of kilometers. These directions are counter-clockwise from directions measured at most previously published paleomagnetic sites within the rift, an indication that fault blocks within the rift have experienced variable clockwise vertical-axis rotation. Additionally, ?95 confidence cones at these new sites (SF=1.3°, MC=1.0°) are smaller than at the previously defined reference site at Mesa Cuadrada (SF=4.1°, MC=8.9°), where few cores were drilled (nSF=6, nMC=3). Comparisons of new paleomagnetic remanence directions from central Baja California with directions from previous intra-rift studies indicate clockwise vertical-axis rotations for SF and MC up to 76 ± 11° and 40 ± 3°, respectively. An important exception is the easternmost SF drill site in central Sonora, where no clockwise rotation has occurred since 12.5 Ma. Although SF is tilted gently to the west at this site, it appears to lie beyond the limit of clockwise vertical-axis rotation related to oblique rifting. The mean magnitudes of the rotation errors (bar?RSF= 4.3°, bar?RMC= 6.3°) using these new paleomagnetic sites are lower than using the previous reference sites at Mesa Cuadrada (bar?RSF=5.6°, bar?RMC=13.2°), owing mostly to the larger number of cores collected at the new reference sites. At 10 of 11 paired sites within the rift where both SF and MC are present in the same fault block, SF is rotated clockwise by a greater amount than MC. 7 of these 10 sites are precise enough to statistically isolate differential rotation. At paired sites, a weighted mean of 16° of clockwise rotation (maximum of 48°) occurred prior to 6.4 Ma. By weighting all paired site results by the differential rotation error, we estimate that 48% (locally 0% to 75%) of the net rotation occurred during the proto-Gulf time period (12.5 - 6 Ma) and prior to rift localization ca. 6 Ma. This early rotation occurred in a ~50-100 kilometer-wide belt of strike-slip faulting embedded within the wider Mexican Basin and Range extensional province, and connected to the San Andreas fault in southern California. Where extension is hosted within a strike-slip-dominated setting (i.e. oblique rift), the overall higher strain rates along shear zones and across the intervening extensional pull-apart basins may localize crustal thinning within an otherwise wide-rift setting, facilitating crustal rupture. By increasing strain rates and connecting areas of focused crustal thinning, strike-slip faulting may have catalyzed the subsequent formation of the Gulf of California

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

2012-12-01

335

Models of segmentation along the Terceira Rift: implications for the MAR evolution to the south of Azores  

NASA Astrophysics Data System (ADS)

Within the MAREKH project we investigate the Mid-Atlantic Ridge Evolution in the last 20 Ma between Kurchatov and Hayes. Our aim is to extend to the south previous kinematic studies to understand the most important kinematic changes and their impact on the segmentation pattern of the MAR and the formation of the Terceira Rift. The Terceira rift (TR) is frequently pointed out as unique ultraslow oblique spreading system differing from other oceanic rifts in that it cuts across relatively old and thick lithosphere. Its pattern of segmentation and the evidence for highly focused magmatism makes it comparable with the continental East African Rift. The analogy with the Main Ethiopian Rift (MER) is particularly remarkable since both the TR and the MER tectonic settings are characterized by two distinct sets of faults trending 20-30° apart. Findings from analogue modelling by Corti (2008) have shown that rift evolution and segmentation in the MER is controlled by rift obliquity independently of magmatic processes or changes in plate kinematics. In this work we explore the feasibility of a similar model in the Azores using finite element numerical modeling. The results improve our understanding on the processes of formation and activation of the observed pattern of faulting and rift segmentation in the Azores plateau, exploring in particular if the two sets of tectonic trends formed diachronically with activity switching from one to another or if they formed simultaneously like a mixed extensional-shear fracture network, establish the connection between the step-like pattern of the Terceira Rift and the segmentation of the MAR to the south of the triple junction and eventually provide constraints on the thermal structure and material properties by determining what combination of model set-ups match the observed patterns. This work was funded by FCT, PTDC/MAR/108142/2008 MAREKH research project. Pest-OE/CTE/LA0019/2011 - IDL

Neves, M. L.; Miranda, J. M.; Luis, J. M.

2011-12-01

336

Formation of hyperextended rifted margins: Insights from flexural isostatic forward tectonic stratigraphic modeling and observations from fossil analogues  

NASA Astrophysics Data System (ADS)

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 insights on processes controlling the formation of rifted margins.

Mohn, G.; Kusznir, N. J.; Manatschal, G.

2013-12-01

337

Facies distributions within contrasting structural components of a rift lake: Lake Tanganyika, Africa  

SciTech Connect

Lake Tanganyika is the most widely cited modern analog for interpreting ancient rift lakes; thus, understanding controls on its facies distribution is critical for refining stratigraphic models for rifts. Four recurrent margin types occur along the alternating half-graben structure of the lake: rift axes, platforms, escarpments, and accommodation zones. Data from study sites in the northern part of the lake suggest that predictable facies differences exist between these structural margin types. The rift axis site comprises a low-gradient, clastic (wave/current)-dominated deltaic system, with strong facies asymmetry and minor carbonate accumulations on raised benches. The platform margin site comprises a series of structurally controlled benches over which long, continuous facies tracts occur. Carbonate sands, muds, and shell gravel dominate; clastics are limited to moderate-sized silty deltas and long, narrow shoreface sands. The escarpment margin site is a steep-gradient system along which small ({lt}1 km{sup 2}) fan deltas alternate with cemented talus. The accommodation zone margin sites are also dominated by rugged structural relief, generally small fan deltas, and semicontinuous shoreface sand belts ({gt}5 km) onshore and poorly sorted silts offshore. TOC from fine-grained samples reflects the contrast in margin types. TOC values for the platform and rift axis range from 0.4 - 2.1 wt. % (avg. 1.3%), whereas accommodation zone and escarpment margin values range from 0.5-5.5% (avg. 3.0%). Acid insoluble sulfur shows a similar trend. Although all data are significantly correlated with depth, the relative area of the lake margin above and below the oxicline is directly controlled by the structural style of the lake margin.

Soreghan, M.J.; Cohen, A.S. (Univ. of Arizona, Tucson (United States))

1991-03-01

338

Magnetotelluric apparent conductivity and seismic p-wave tomography comparison, Rio Grande Rift, Southwestern USA  

NASA Astrophysics Data System (ADS)

During the summers of 2012 and 2013, magnetotelluric (MT) data was collected in three transects across the Rio Grande Rift in Colorado and New Mexico. Previous seismological studies, including both regional deployments and EarthScope USArray Transportable Array (TA), have shown that a wide area of upper mantle below the Rio Grande Rift is seismically slow, but the cause of this is unclear. MT has the potential to help reduce the ambiguity in determining the cause of the seismically slow anomaly. Electrical conductivity determined from MT and elastic parameters determined from seismic experiments have different sensitivities to temperature, partial melt, hydration and composition; together these methods can be used to constrain the physical, chemical, and thermal state of the lithosphere, and to investigate changes in it throughout the rift. These constraints can help distinguish between different rift opening models such as the rift unzipping south to north or being rotated open. We have used a cross correlation algorithm to pick teleseismic P-wave arrival times from twenty-two events recorded by TA stations in Colorado and New Mexico from 2008 - 2010. Our teleseismic picks were used to create maps and profiles of travel time residuals that can be compared with existing tomographic images and with our new MT data. We have forward modeled select MT data to produce 1-D models of conductivity versus depth, which can then be compared to the average travel time residuals at TA stations in proximity to the MT profile. The travel time residuals across the three MT lines are also qualitatively compared to cross-sections of the Schmandt & Humphreys (2010) P-wave tomography model.

Harding, J.; Sheehan, A. F.; Feucht, D. W.; Bedrosian, P.; O'Rourke, C. T.

2013-12-01

339

Water resource management and biodiversity conservation in the Eastern Rift Valley Lakes, Northern Tanzania  

NASA Astrophysics Data System (ADS)

The Eastern Rift Valley Lakes of East Africa and their watersheds have gone through significant anthropogenic changes over years. Several land use pressures and overexploitations of natural resources have eroded the biological and physical systems that support those resources. The principal objective of this study was to undertake a comprehensive water resource management problem analysis in the Eastern Rift Valley Lakes so as to highlight the current state of knowledge on key environmental and biodiversity problems, institutional capacities and needs to conserve biodiversity and water resources in the respective lakes. Two stages were be involved in data collection. The first stage involved literature search in libraries and documentation centres held in various institutions. Second stage involved the main fieldwork, which aimed at collecting secondary information from regional and districts offices situated within the basins in question. Findings from this study show that trends in the growth of human population, expansion of cropland and increase in livestock population in the Eastern Rift Valley Lakes zone indicate rapid increase over the next few decades. The pressure of this rapidly increasing population on the available resources will be too great to sustain desirable livelihood in the area. Even at the current rate of population increase, water resource utilisation in and around most Rift Valley Lakes is not sustainable. The intensification of agriculture through the application of fertilisers and pesticides will lead to the soil and water pollution, as is already happening in Mang’ola and Mto wa Mbu where irrigated farming is practised. Although a number of studies have been conducted in the Eastern Rift Valley Lakes and Wetlands in the Northern Tanzania, there are still a lot of issues which have not studied adequately.

Yanda, P. Z.; Madulu, N. F.

340

Numerical modeling of seismicity and geodynamics of the Kachchh rift zone, Gujarat, India  

NASA Astrophysics Data System (ADS)

The numerical block-and-fault model of lithosphere dynamics and seismicity (BAFD) is used to understand crustal motion and features of the observed seismicity in the Kachchh rift zone, Gujarat, Western India. The block-model allows simulating seismicity and geodynamics simultaneously unlike other modeling approaches for studying seismicity or geodynamics. The model structure of Kachchh rift zone is composed of seven major crustal blocks separated by fault planes. Based on the orientation of boundary crustal block movements, we develop a set of numerical experiments to analyze the spatial distribution of earthquakes, frequency-to-magnitude relationships, earthquake focal mechanisms, velocity field, and fault slip rates in the model. The main results of our modeling suggest that an NNW-SSE trending compression is a principal driving force in the Kachchh rift zone that explains basic features of the regional seismicity, direction of block motions, and the presence of an extensional stress regime associated with the Cambay rift zone. Large synthetic events occur on the fault segments associated with the Allah-Bund fault, Katrol hill fault and north Wagad fault which have been causative faults for the 1819 Mw7.7 Allah-Bund, 1956 Mw6.0 Anjar and 2001 Mw7.7 Bhuj earthquakes. The frequency-magnitude distribution for both synthetic seismicity and observed seismicity shows a similar slope. The focal mechanisms of the synthetic events are found to be consistent with those of earthquakes in the region. A special attention has been paid to study long-term and post-seismic deformations. Our results are in a qualitative agreement with the GPS post-seismic observations in the Kachchh rift zone. We infer that the observed seismicity and crustal block motions are a consequence of the dynamics of the entire regional fault and block system rather than that of a single causative fault only.

Vorobieva, Inessa; Mandal, Prantik; Gorshkov, Alexander

2014-11-01

341

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

SciTech Connect

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.

Burrus, J.; Audebert, F. (Institut Francais du Petrole, Rueil-Malmaison (France))

1990-09-01

342

Melt-induced seismic anisotropy and magma assisted rifting in Ethiopia: Evidence from surface waves  

NASA Astrophysics Data System (ADS)

The East African rift in Ethiopia is unique worldwide because it captures the final stages of transition from continental rifting to seafloor spreading. A recent study there has shown that magma intrusion plays an important role during the final stages of continental breakup, but the mechanism by which it is incorporated into the extending plate remains ambiguous: wide-angle seismic data and complementary geophysical tools such as gravity analysis are not strongly sensitive to the geometry of subsurface melt intrusions. Studies of shear wave splitting in near-vertical SKS phases beneath the transitional Main Ethiopian Rift (MER) provide strong and consistent evidence for a rift-parallel fast anisotropic direction. However, it is difficult to discriminate between oriented melt pocket (OMP) and lattice preferred orientation (LPO) causes of anisotropy based on SKS study alone. The speeds of horizontally propagating Love (SH) and Rayleigh (SV) waves vary in similar fashions with azimuth for LPO- and OMP-induced anisotropy, but their relative change is distinctive for each mechanism. This diagnostic is exploited by studying the propagation of surface waves from a suite of azimuths across the MER. Anisotropy is roughly perpendicular to the absolute plate motion direction, thus ruling out anisotropy due to the slowly moving African Plate. Instead, three mechanisms for anisotropy act beneath the MER: periodic thin layering of seismically fast and slow material in the uppermost ˜10 km, OMP between ˜20-75 km depth, and olivine LPO in the upper mantle beneath. The results are explained best by a model in which low aspect ratio melt inclusions (dykes and veins) are being intruded into an extending plate during late stage breakup. The observations from Ethiopia join a growing body of evidence from rifts and passive margins worldwide that shows magma intrusion plays an important role in accommodating extension without marked crustal thinning.

Bastow, I. D.; Pilidou, S.; Kendall, J.-M.; Stuart, G. W.

2010-06-01

343

September 2005 Manda Hararo-Dabbahu rifting event, Afar (Ethiopia): Constraints provided by geodetic data  

NASA Astrophysics Data System (ADS)

We provide a new set of complementary geodetic data for the 2005 rifting event of Afar (Ethiopia). Interferometric synthetic aperture radar and subpixel correlations of synthetic aperture radar and SPOT images allow us to deduce 3-D surface displacement unambiguously. We determine the geometry of the dike and neighboring magma chambers and invert for the distribution of opening of the dike, as well as slip on rift border faults. The volume of the 2005 dike (1.5-2.0 km3) is not balanced by sufficient volume loss at Dabbahu and Gabho volcanoes (0.42 and 0.12 km3, respectively). Taking into account the deflation of a suspected deep midsegment magma chamber simultaneously to dike intrusion produces a smoother opening distribution along the southern segment. Above the dike, faults slipped by an average 3 m, yielding an estimated geodetic moment of 3.5 × 1019 Nm, one order of magnitude larger than the cumulative seismic moment released during the earthquake swarm. Between Dabbahu and Ado'Ale volcanic complexes, significant opening occurred on the western side of the dike. The anomalous location of the dike at this latitude, offset to the east of the axial depression, may explain this phenomenon. A two-stage intrusion scenario is proposed, whereby rifting in the northern Manda Hararo Rift was triggered by magma upwelling in the Dabbahu area, at the northern extremity of the magmatic segment. Although vigorous dike injection occurred during the September 2005 event, the tectonic stress deficit since the previous rifting episode was not fully released, leading to further intrusions in 2006-2009.

Grandin, R.; Socquet, A.; Binet, R.; Klinger, Y.; Jacques, E.; de Chabalier, J.-B.; King, G. C. P.; Lasserre, C.; Tait, S.; Tapponnier, P.; Delorme, A.; Pinzuti, P.

2009-08-01

344

New Low-Temperature Thermochronology Reveals Contrasting Modes of Continental Extension Across the Sonoran Rifted Margin  

NASA Astrophysics Data System (ADS)

The Sonoran rifted margin extends 250 km from the western flanks of the Sierra Madre Occidental to the Gulf of California and contains a classic Basin and Range morphology that indicates "broad-rift" mode of continental extension. However, new low-temperature thermochronology reveals that the Sonoran rifted margin is also internally composed of at least two temporally and spatially distinct belts that display other distinct styles of extension. Mountain ranges that lie within a narrow belt (20 km wide) along the coast of the Gulf of California between Puerto Libertad and Bahia Kino yield highly discordant apatite fission track (AFT) ages that range from 5 to 54 Ma and likely reflect the strong tilting of these tectonic blocks. The widespread occurrence of AFT ages between 5 and 7 Ma, which are typically found in the deepest crustal levels of the tilt blocks, and the presence of Quaternary scarps indicate that extension in the coastal region largely occurred from late Miocene to recent times. We infer that this belt is dominated by a "narrow-rift" mode of extension where deformation has been focused to produce the Gulf depression. Well inland from the coast (175 km east) is a belt of metamorphic core complexes that extends more than 200 km from Magdalena to Mazatan and typically yields older and more concordant AFT ages from 14 to 23 Ma. However, the presence of ages as young as 8 to 11 Ma indicate that the "metamorphic-core-complex" mode of extension in this belt likely overlapped in time with the "narrow-rift" mode of extension in the Gulf of California. We conclude that the juxtaposition of major deformation belts each with different modes of continental extension reflects the diverse processes that have affected the Sonoran margin through time.

Kohn, B. P.; Fletcher, J. M.; Gleadow, A. J.; Calmus, T.; Nourse, J. A.

2003-12-01

345

Isotopic evidence for interaction between Öræfajökull mantle and the Eastern Rift Zone, Iceland  

NASA Astrophysics Data System (ADS)

The off-rift central volcano of Öræfajökull has very distinctive EM1-like isotopic compositions compared with other Icelandic lavas. New Pb-Nd-Sr isotopic data from Öræfajökull show strong correlations interpreted as a result of mixing. End-members are a depleted mantle source incorporating 0.5 % subduction-processed sediment and a mantle source with an isotopic signature similar to lavas of the Reykjanes Peninsula. Sr-Nd-Pb isotopic correlations of Icelandic Eastern Rift Zone (ERZ) lavas are almost completely distinct from those of the Reykjanes Peninsula and the Western Rift Zone (WRZ) and require a high-207Pb/204Pb, low-143Nd/144Nd end-member that resembles Öræfajökull compositions, which is very distinct from the enriched end-members suggested for the Reykjanes Peninsula and the WRZ. Given the similar depth and degree of melting at rift zones, variation in the observed enriched end-members between rift zones must indicate spatial variations in enriched mantle sources within the shallow mantle under Iceland rather than purely mixing of melts from a bi-lithological mantle. This is consistent with observations that the ERZ lavas erupted closest to Öræfajökull exhibit the most Öræfajökull-like isotopic compositions, implying that a homogenised Öræfajökull source with positive ?207Pb is focused under the Öræfajökull centre and its associated flank zone. This then mixes laterally with the dominant negative-?207Pb ERZ mantle source. Like Reykjanes Peninsula and WRZ lavas, the ERZ mantle source has strongly negative ?207Pb and low K/Nb (<170), and these provide evidence for a recycled oceanic crust contribution. The range in 206Pb/204Pb in mantle sources with negative ?207Pb was probably generated by heterogeneity in 206Pb/204Pb and ? in the recycled oceanic crust, which is the dominant source of incompatible elements in Icelandic lavas.

Manning, Christina J.; Thirlwall, Matthew F.

2014-01-01

346

Western closure of the Corinth Rift: Stratigraphy and structure of the Lakka fault block  

NASA Astrophysics Data System (ADS)

In the Corinth Gulf, seismicity is highest in the west, where the active Psathopyrgos-Neos Erineos-Aegion fault zone (PNEAFZ;30 km long, N dip) defines the south coast. To the south and SE the inactive early rift records N and NW migration of deformation since the Pliocene. When was the PNEAFZ initiated? How did it grow? What is the relevance of this fault zone within the full rift history? This paper presents new data for the onshore westernmost rift, indicating that it had a distinct early rifting history (Early to Middle Pleistocene) before being overprinted around 400 ka by the NW migrating Corinth rift. Two syn rift stratigraphic groups are recognised in the uplifted Lakka fault block in the footwall of the PNEAFZ. The youngest Galada group, comprises marine deposits and terraces that mainly document footwall uplift since initiation of the PNEAFZ at around 400-350 ka (Palyvos et al. 2010). The oldest sediments derived from the footwall of the Lakka fault are the 400-350 ka old Aravonitsa Gilbert delta (Palyvos et al. 2010), suggesting this fault is not significantly older than the PNEAFZ. The Galada group records a gradual eastward block tilting due to differential footwall uplift as the PNEAFZ propagated east. The underlying Profitis Ilias group, (pre 400 ka, < 600 m) is characterised by south and southeastward fining continental facies from coarse alluvial conglomerates in the immediate footwall of the Psathopyrgos fault (Rodini formation) passing east and south to fluvial sandstones and conglomerates (Salmoniko formation), to deltaic and shallow water sandstones interfingering with lacustrine marls, silts and fine sandstones with rare conglomerates and lignites (Synania formation). Faunal assemblages in the Synania formation indicate freshwater to brackish conditions with occasional marine levels and support an Early to Middle Pleistocene age. To the ESE, the Synania formation passes laterally and up into a 200 m succession of fine sandstones with rare conglomerates, lignites and mudstones (Koumaris formation). In the area of the Meganitis and Selinous rivers, the Profitis Ilias group underlies and may be partly laterally equivalent to the Early to Middle Pleistocene Middle group comprising Gilbert deltas sourced from the south. New data indicate that the Profitis Ilias group was a fluvio-lacustrine system sourced from the north. Accommodation was probably created by an, as yet, unidentified south dipping fault on the north side of or below the present Gulf. The same fluvio-lacustrine system may be traced into the Patras area, further west. The Profitis Ilias group depocentre may have been part of the NE limit of the Patras rift. The westernmost Corinth rift was therefore dominated by continental facies until late Middle Pleistocene. The Rion Straits did not open west to the Mediterranean until well after 400 Ka. Any marine influence in the earlier rift came from the east. Palyvos N., Mancini M., Sorel D., Lemeille F., Pantosti D., Julia R., Triantaphyllou M., De Martini P.M. (2010) - Geomorphological, stratigraphic and geochronologic evidence of fast Pleistocene coastal uplift in the westernmost part of the Corinth Gulf (Greece). Geological Journal, 45, 78-104.

Palyvos, Nikos; Ford, Mary; Mancini, Marco; Esu, Daniela; Girotti, Odoardo; Urban, Brigitte

2013-04-01

347

Sequence of rifting in Afar, Manda-Hararo rift, Ethiopia, 2005–2009: Time-space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling  

Microsoft Academic Search

Thirteen dike intrusions in the Manda Hararo rift, Afar (Ethiopia), from September 2005 to June 2009, studied using an extensive interferometric synthetic aperture radar (InSAR) data set, provide insight into the mechanics of a major active rift. Kinematic inversions of InSAR data reveal that dikes opened by 0.8–3.5 m at an average 5 km depth, with volumes of 0.04–0.2 km3

R. Grandin; A. Socquet; E. Jacques; N. Mazzoni; J.-B. de Chabalier; G. C. P. King

2010-01-01

348

Fault growth and propagation during incipient continental rifting: Insights from a combined aeromagnetic and Shuttle Radar Topography Mission digital elevation model investigation of the Okavango Rift Zone, northwest Botswana  

NASA Astrophysics Data System (ADS)

Digital Elevation Models (DEM) extracted from the Shuttle Radar Topography Mission (SRTM) data and high-resolution aeromagnetic data are used to characterize the growth and propagation of faults associated with the early stages of continental extension in the Okavango Rift Zone (ORZ), northwest Botswana. Significant differences in the height of fault scarps and the throws across the faults in the basement indicate extended fault histories accompanied by sediment accumulation within the rift graben. Faults in the center of the rift either lack topographic expressions or are interpreted to have become inactive, or have large throws and small scarp heights indicating waning activity. Faults on the outer margins of the rift exhibit either (1) large throws or significant scarp heights and are considered older and active or (2) throws and scarp heights that are in closer agreement and are considered young and active. Fault linkages between major fault systems through a process of "fault piracy" have combined to establish an immature border fault for the ORZ. Thus, in addition to growing in length (by along-axis linkage of segments), the rift is also growing in width (by transferring motion to younger faults along the outer margins while abandoning older faults in the middle). Finally, utilization of preexisting zones of weakness allowed the development of very long faults (>100 km) at a very early stage of continental rifting, explaining the apparent paradox between the fault length versus throw for this young rift. This study clearly demonstrates that the integration of the SRTM DEM and aeromagnetic data provides a 3-D view of the faults and fault systems, providing new insight into fault growth and propagation during the nascent stages of continental rifting.

Kinabo, B. D.; Hogan, J. P.; Atekwana, E. A.; Abdelsalam, M. G.; Modisi, M. P.

2008-06-01

349

Ages of pre-rift basement and synrift rocks along the conjugate rift and transform margins of the Argintine Precordillera and Laurentia  

USGS Publications Warehouse

New geochronologic data from basement rocks support the interpretation that the Argentine Precordillera (Cuyania) terrane was rifted from the Ouachita embayment of the Iapetan margin of Laurentia. New data from the Ozark dome show a range of ages in two groups at 1466 ± 3 to 1462 ± 1 Ma and 1323 ± 2 to 1317 ± 2 Ma, consistent with existing data for the Eastern Granite-Rhyolite province and Southern Granite-Rhyolite province, respectively. Similarly, a newly determined age of 1364 ± 2 Ma for the Tishomingo Granite in the Arbuckle Mountains confirms previously published analyses for this part of the Southern Granite-Rhyolite province. Along with previously reported ages from basement olistoliths in Ordovician slope deposits in the Ouachita embayment, the data for basement ages support the interpretation that rocks of the Southern Granite-Rhyolite province form the margin of Laurentian crust around the corner of the Ouachita embayment, which is bounded by the Ouachita rift and Alabama-Oklahoma transform fault. In contrast, both west and east of the corner of the Ouachita embayment, Grenville-Llano basement (approximately 1325–1000 Ma) forms the rifted margin of Laurentia. New U/Pb zircon data from basement rocks in the southern part of the Argentine Precordillera indicate crystallization ages of 1205 ± 1 Ma and 1204 ± 2 Ma, consistent with previously reported ages (approximately 1250–1000 Ma) of basement rocks from other parts of the Precordillera. These data document multiple events within the same time span as multiple events in the Grenville orogeny in eastern Laurentia, and are consistent with Grenville-age rocks along the conjugate margins of the Precordillera and Laurentia. Ages from one newly analyzed collection, however, are older than those from other basement rocks in the Precordillera. These ages, from granodioritic-granitic basement clasts in a conglomerate olistolith in Ordovician slope deposits, are 1370 ± 2 Ma and 1367 ± 5 Ma. These older ages from the Precordillera are consistent with indications that the Iapetan margin in the Ouachita embayment of Laurentia truncated the Grenville front and left older rocks of the Southern Granite-Rhyolite province (1390–1320 Ma) at the rifted margin. Chronostratigraphic correlations of synrift and post-rift sedimentary deposits on the Precordillera and on the Texas promontory of Laurentia document initial rifting in the Early Cambrian. Previously published data from synrift plutonic and volcanic rocks in the Wichita and Arbuckle Mountains along the transform-parallel intracratonic Southern Oklahoma fault system inboard from the Ouachita embayment document crystallization ages of 539–530 Ma. New data from synrift volcanic rocks in the Arbuckle Mountains in the eastern part of the Southern Oklahoma fault system yield ages of 539 ± 5 Ma and 536 ± 5 Ma, confirming the age of synrift volcanism.

Thomas, William A.; Tucker, Robert D.; Astini, Ricardo A.; Denison, Rodger E.

2012-01-01

350

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

NASA Astrophysics Data System (ADS)

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

Szatmari, P.

2012-04-01

351

Subsidence history, crustal structure and evolution of the Nogal Rift, Northern Somalia  

NASA Astrophysics Data System (ADS)

Seismic reflection profile, gravity anomaly, and biostratigraphic data from deep exploration wells have been used to determine the tectonic subsidence, structure and evolution of the Nogal basin, Northern Somalia, one of a number of ENE-WSW trending early Mesozoic rifts that formed prior to opening of the Gulf of Aden. Backstripping of biostratigraphic data at the Nogal-1 and Kali-1 wells provides new constraints on the age of rifting, and the amount of crustal and mantle extension. The tectonic subsidence and uplift history at the wells can be generally explained as a consequence of two, possibly three, major rifting events. The first event initiated in the Late Jurassic (~156 Ma) and lasted for ~10 Myr. We interpret the rift as a late stage event associated with the break-up of Gondwana and the separation of Africa and Madagascar. The second event initiated in the Late Cretaceous (~80 Ma) and lasted for ~20 Myr. This event probably correlates with a rapid increase in spreading rate on the ridges separating the African and Indian and African and Antarctica plates and a contemporaneous slowing down of Africa's plate motion. The backstripped tectonic subsidence data can be explained by a multi-rift extensional model with a stretching factor, ?, in the range 1.17-1.38. The third and most recent event occurred in the Oligocene (~32 Ma) and lasted for ~10 Myr. This rift only developed at the centre of the basin close to Nogal-1 well, and is related to the opening of the Gulf of Aden. The amount of crustal thinning inferred at the Kali-1 well is consistent with the results of Process-Oriented Gravity and Flexure (POGM) modelling, assuming an elastic thickness of ~30 km. The thinning at the Nogal-1 well, however, is greater by ~ 7 km than predicted suggesting that the basin may be locally underplated by magmatic material. Irrespective, POGM suggests the transition between thick crust beneath Northern Somalia to thin crust beneath the Indian Ocean forms a ~500 km wide zone which begins beneath the Kali-1 and Nogal-1 wells and ends beneath the upper continental rise.

Ali, M. Y.; Watts, A. B.

2013-12-01

352

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

NASA Astrophysics Data System (ADS)

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 during the active phases of the magma chamber, and are enhanced when the magmatic activity decreases. Our measurements show that the fault scarps have a continuous slip rate along the neo-volcanic zone, between 1-2 mm/yr. We also identify several individual palaeo-tectonic events of metric amplitude (very similar to the 2005 crisis) which suggest that the main process responsible for the building of the axial depression is indeed dike intrusions. This implies that in pre-breakup stages accretion is already organised and maintained by processes similar to those occurring in mature oceanic ridges. The question arises to know if these mechanisms, observed here on the scale of a few tens of kyrs, could be responsible for the creation of SDRs, implying these mechanisms operate on timescales of a few million years.

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

2014-05-01

353

Using Transportable Array and CREST data to define seismicity in the Rio Grande Rift  

NASA Astrophysics Data System (ADS)

Active low strain-rate deformation across the Rio Grande Rift within the Western U.S. presents the possibility that strain is released through widespread small magnitude earthquakes. In order to define seismicity across the rift and understand its tectonic significance, we construct a catalog of small magnitude earthquakes from 2007-2010. Colorado has never had a statewide seismic network, thus previous catalogs are limited to that from the USGS and from short-term temporary seismometer deployments. New Mexico seismicity is monitored with regional networks around Socorro and Carlsbad, though not statewide. This study is the first opportunity to locate events in a spatially and temporally comprehensive network from the USArray Transportable Array (TA) experiment supplemented by the Colorado Rockies Experiment and Seismic Transects (CREST) seismic experiment (Karlstrom et al., 2012). Our earthquake catalog will supplement the USArray Array Network Facility (ANF) catalog with smaller magnitude events and refined regional velocity models. We use earthquake detection and location programs to create a raw catalog of associated events and initial locations from 254 seismic stations surrounding the Rio Grande Rift. Detection processing and event association parameters have been chosen specifically to extend the magnitude threshold relative to the ANF catalog (from 3.0 to approximately 1.5) . Mine blasts will be removed by manual waveform identification from the final catalog. The catalog is compared with catalogs of existing small seismic networks near and in the rift, for example, the New Mexico Tech Seismic Network. Questions of interest include: 1) Can earthquakes be identified that are rift-related? 2) What do the patterns of seismicity tell us about the tectonics of the Rio Grande Rift? Preliminary results show event associations to ML 0.0, although magnitude completeness is higher, and several hundred events are identified each month in 2008. Tectonic events will be compared to possible induced seismic events in the Raton basin along the Colorado - New Mexico border and southeastern New Mexico. Magnitude-frequency relations and catalog completeness will be analyzed to further characterize seismic events.

Nakai, J.; Sheehan, A. F.; Bilek, S. L.

2013-12-01

354

Seismic Characterization of the June 17, 2007 East Rift Intrusion at Kilauea Volcano  

NASA Astrophysics Data System (ADS)

An early morning earthquake swarm on Sunday, June 17, 2007 signaled the beginning of a sequence of seismic and deformational activity consistent with an intrusion of magma in the upper East Rift of Kilauea volcano, Hawaii. This culminated in an outbreak of lava from a discontinuous 160 meter long fissure, approximately 6 km west of Pu`u `O`o and 13 km southeast of Kilauea's summit. Here we detail the seismic characteristics and observed deformation accompanying this magmatic intrusion and eruption. Seismic activity began at 0216 Hawaiian Standard Time (HST - UTC minus 10 hours) with 38 events greater than magnitude 2 and over 80 located earthquakes in the first two hours. These earthquakes were centered 1.5-2 km southwest of Mauna Ulu (9 km southeast of Kilauea summit) between 1.5 and 3 km deep. At the same time, tiltmeters at Kilauea's summit began indicating rapid deflation of the summit area. Starting at 0730 HST there was a pronounced eastward shift (by about 4 km) of the center of seismicity to a location just west of Makaopuhi crater. At this time GPS stations began to show extension across the rift in that area. Over the next 7-8 hours, the locus of seismicity continued to migrate eastward with over 70 events located at Makaopuhi crater. Over the next day more than 40 earthquakes occurred with locations ranging from the uppermost east rift to Makaopuhi crater. The next afternoon (June 18) there was another pronounced eastward shift in seismicity with 23 earthquakes, occurring between 1459-1600 HST, centered between Makaopuhi crater and Napau crater. Kilauea summit area tremor levels rose throughout the day on June 17, reaching a peak of 24 times background levels mid-day on the June 18, before beginning a slow decline. Tremor levels along the East Rift showed brief periods of strong activity (4-6 times background levels), and lava was discovered at the surface shortly after one such tremor episode at 0625 HST on June 19. Shortly before noon on the same day, Kilauea tiltmeters indicated an end to summit deflation, and GPS stations indicated a slowing of rift extension which had reached nearly 1 meter of across rift extension. This series of punctuated eastward migrations of seismicity (along with the observed deformational signals) provide a detailed record of the space-time evolution of the June 17 magmatic intrusion and eruption.

Wilson, D. C.; Uribe, J.; Kamibayashi, S.; Nakata, J.; Okubo, P.

2007-12-01

355

Migrating shoshonitic magmatism tracks Izu-Bonin-Mariana intra-oceanic arc rift propagation  

NASA Astrophysics Data System (ADS)

The southernmost Izu-Bonin arc and northernmost Mariana arc are characterized by K-rich and shoshonitic lavas, referred to as the alkalic volcano province (AVP). These compositions are unusual for intra-oceanic arcs and the interpretation of the AVP is controversial. Rifting to form the Mariana Trough back-arc basin occurs just south of the AVP although back-arc seafloor spreading has not begun. Here we report the results of dredge sampling of the West Mariana Ridge (WMR) in the region of rift propagation; this recovered exclusively medium K to shoshonitic basalts that show clear arc-like geochemical signatures. Ar-Ar ages of WMR shoshonitics systematically young northward. Age of c. 6 Ma was obtained at 21.5°N, c. 3 Ma at 23-23.5°N, and zero-age shoshonites occur on Io-to Island (formerly Iwo Jima) at 24.8°N. Shoshonitic magmatism migrated northward at 4.3 cm/year, in advance of northward-propagating Mariana Trough rifting. This implies that AVP shoshonitic magmatism manifests processes and sources that are uniquely associated with earliest back-arc basin rifting. High-precision Pb isotopic analyses reveal that WMR lavas form a single trend between 2 components, one with lower 206Pb/ 204Pb and high ?7/4 (arc-like), and another with high 206Pb/ 204Pb as well as low ? 7/4 and 8/4 (HIMU-like). These components could correspond respectively to subducted pelagic sediment and subducted seamounts and volcaniclastics with HIMU isotopic signature. These slab-derived components alone, however, cannot fully explain chemical characteristics of WMR shoshonitic lavas. These lavas require a component with high ?7/4 and high Ce/Pb, which is not likely to be either pelagic sediment or seamount volcanics. This component is only expressed when rifting begins, suggesting that it resides in enriched lithosphere or uppermost asthenosphere, which is easily melted due to decompression caused by rifting, when the lithosphere is first ruptured. This component might be linked to slow Vs anomalies in the mantle wedge beneath the AVP.

Ishizuka, O.; Yuasa, M.; Tamura, Y.; Shukuno, H.; Stern, R. J.; Naka, J.; Joshima, M.; Taylor, R. N.

2010-05-01

356

Rift Geometry and Evolution Associated with the Break-up of Pangea  

NASA Astrophysics Data System (ADS)

Diabase dikes related to the rifting of Pangea have been used to elucidate mechanisms by which the super-continent broke-up since the early 1970s. Subsequently, these dikes were related to each other and the massive sills and lava flows that make up the Central Atlantic Magmatic Province (CAMP). Our research focuses on the dikes and mechanisms of continental break-up in the southeastern United States. While, past studies indicate that the majority of Mesozoic diabase dikes within the Carolinas dominantly trend to the northwest, recent work in west-central South Carolina and the Piedmont of North Carolina has revealed numerous previously unmapped dikes with N and NE trends. These dikes have been related to the N and NE trending dikes located in the Northeastern United States via geochemical analyses. Cross-cutting relationships between dikes of different orientation has revealed an apparent pattern whereby the dikes were emplaced in the following order NW, N, and then NE-trending. Recent Ar39/Ar 40 dates suggest that these dikes were all intruded within a 2 million year window, indicating that the least compressive stress field (which would be perpendicular to dike orientation) rotated from NE-SW through E-W to NW-SE within this time period. Based on these observations, which are contrary to previous studies that attribute the northwest trending dikes in the Carolinas to a deep mantle plume, we constructed several finite element models to determine the source of the stress field change. These models were constructed to test the influence of geometry, rifting sequences, and location of force application on the orientation and evolution of stress fields in the Carolinas. Model results suggest that the direction from which rifting progressed has a strong effect on the stress field within the Carolinas and that it is unlikely that rifting began at the Blake Plateau as proposed by the plume hypothesis. Further, it is apparent that the geometry of the rifts themselves has a strong effect on the stress field within the continent, which suggests the need for further mapping of the final rifting events along the southeastern margin of the United States.

Debnam, C.; Beutel, E. K.

2004-12-01

357

Characterization and Mapping of the Gene Conferring Resistance to Rift Valley Fever Virus Hepatic Disease in WF.LEW Rats  

E-print Network

Rift Valley Fever Virus is a plebovirus that causes epidemics and epizootics in sub-Saharan African countries but has expanded to Egypt and the Arabian Peninsula. The laboratory rat (Rattus norvegicus) is susceptible to RVFV and has been shown...

Callicott, Ralph J.

2010-01-14

358

Discover the beauty of Taiwan...... 2-day Trip to National Scenic Area of Huadong Eastern Coast & Rift Valley  

E-print Network

& Rift Valley Ocean & mountain view attracts your eyes... Blue sky and white horses of sea, greeny forest the invasion is the exploding world of television. Their culture and tradition is disappearing. Well, there

Wong, Ngai-Ching

359

Evolution of oceanic margins : rifting in the Gulf of California and sediment diapirism and mantle hydration during subduction  

E-print Network

This thesis investigates three processes that control the evolution of oceanic margins. Chapter 2 presents seismic images of a ~2-km-thick evaporite body in Guaymas Basin, central Gulf of California. In rifts, evaporites ...

Miller, Nathaniel Clark

2013-01-01

360

Seismic anisotropy of the lithosphere/asthenosphere system beneath the Rwenzori region of the East-African Rift  

NASA Astrophysics Data System (ADS)

We present results from a temporary seismic network of 32 broad-band stations located around the Rwenzori region of the Albertine rift at the border between Uganda and DR Congo. The study aims to constrain seismic anisotropy and mantle deformation processes in relation to the formation of the rift zone. Shear-wave splitting measurements from local and teleseismic earthquakes are used to investigate the seismic anisotropy in the crust and upper mantle beneath the Rwenzori region. At most stations, shear-wave splitting parameters obtained from individual earthquakes exhibit only minor variations with backazimuth. We therefore employ a joint inversion of SKS waveforms to derive hypothetical one-layer parameters. The corresponding fast polarizations are generally rift-parallel and the average delay time is about 1 s. On the other hand, shear phases from local events within the crust are characterized by a bimodal pattern of fast polarizations and an average delay time of 0.04 s. This observation suggests that the dominant source region for seismic anisotropy beneath the rift is located within the mantle. We use finite-frequency waveform modeling to test different models of anisotropy within the lithosphere/asthenosphere system of the rift. The results show that the rift-parallel fast polarizations are consistent with HTI anisotropy caused by rift-parallel magmatic intrusions or lenses located within the lithospheric mantle - as it would be expected during the early stages of continental rifting. Furthermore, the short-scale spatial variations in the fast polarizations observed in the southern part of the study area can be explained by effects due to sedimentary basins of low isotropic velocity in combination with a shift in the orientation of anisotropic fabrics in the upper mantle. A uniform anisotropic layer in relation to large-scale asthenospheric mantle flow is less consistent with the observed splitting parameters.

Homuth, Benjamin; Löbl, Ulrike; Batte, Arthur; Link, Klemens; Kasereka, Celestine; Rümpker, Georg

2014-05-01

361

Evolution post-rift de la marge du Golfe du Lion en Languedoc : Une marge pas si passive que a !  

E-print Network

Evolution post-rift de la marge du Golfe du Lion en Languedoc : Une marge pas si passive que ça'objectifdel'excursionenLanguedocestd'observerlesstructuresgéologiques du rifting OligoMiocène du Golfe du Lion et d'analyser les évidences tectoniques, sédimentologiques-2Sep2013 #12; IntroductionsurleriftingoligomiocènedelamargeduGolfeduLion. Fig.I1:Evolution

Paris-Sud XI, Université de

362

The Late Paleozoic Southern Margin of the Siberian paleocontinent: transformation from an active continental margin to intracontinental rifting  

NASA Astrophysics Data System (ADS)

The large volcanoplutonic belt was formed on the southern margin of Siberian paleocontinent in the Early Carboniferous-Early Permian. Now it's stretched through whole Mongolia and the adjacent region of China. In the belt structure there are defined the successive rock complexes: the older one represented by differentiated basalt-andesite-rhyodacite series and younger bimodal complex of basalt-comendite-trachyrhyolite composition. The granodiorite-plagiogranite and diorite-monzonite-granodiorite plutonic massifs are associated with the former, while peralkaline granite massifs are characteristic of the latter. Geochronological results and geological relations between rocks of the bimodal and differentiated complexes showed first that rocks of the differentiated complex originated 350 to 330 Ma ago at the initial stage of forming of the marginal continental belt, linked with development active continental margin. This is evident from geochronological dates obtained for the Adzh-Bogd and Edrengiyn-Nuruu massifs and for volcanic associations of the complex. The dates are consistent with paleontological data. The bimodal association was formed later, 320 to 290 Ma ago. The time span separating formation of two igneous complexes ranges from several to 20-30 m.y. in different areas of the marginal belt. The bimodal magmatism was interrelated with rifting responsible for development of the Gobi-Tien Shan rift zone in the belt axial part and the Main Mongolian lineament along the belt northern boundary. Loci of bimodal rift magmatism likely migrated with time: the respective magmatic activity first initiated on the west of the rift system and then advanced gradually eastward with development of rift structures. Normal granitoids untypical but occurring nevertheless among the products of rift magmatism in addition to peralkaline massifs are assumed to have been formed, when the basic magmatism associated with rifting stimulated crustal anatexis and generation of crustal granitoid magmas under specific conditions of rifting within the active continental margin.

Kozlovsky, A. M.; Yarmolyuk, V. V.; Sal'Nikova, E. B.

2009-04-01

363

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)

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.

Atekwana, E. A.

2010-12-01

364

Wetlands as a Record of Climate Change and Hydrological Response in Arid Rift Settings  

NASA Astrophysics Data System (ADS)

Of all the terrestrial depositional settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. Lake deposits were the only rift component studied for records of long-term climatic change and for testing hypotheses of orbital forcing. Recently, the continuing quest for the paleontological and cultural records of human origins entombed in the sedimentary rocks of the East African Rift System raised questions concerning hydrologic and biologic response to climatic change. Additional issues are the impact of climate on paleolandscapes and the environmental stresses that might have affected human evolution. Other important indicators of rift hydrology, such as springs and wetlands are now emerging as viable records of climate change. Rift valley basins are shallow, hydrologically closed systems that are responsive to shifts in climate, and specifically sensitive to changes in the hydrologic budget (P-ET). Long term wet-dry cycles in the low latitudes are thought to be astronomically controlled, i.e. Milankovitch precession cycles (19-23 ka). In the tropics, precipitation (P) varies with changes in solar insolation which fluctuates <8-10 % over a cycle. Stronger insolation drives stronger summer monsoon maxima increasing P. Mean annual temperatures are high, but evapo-transpiration, ET (~ 2500 mm/yr) varies little. Consequently, during wetter periods regional groundwater reservoirs enlarge, the water table rises and springs and wetlands increase in number and in size compared to drier periods. Lake levels are known to fluctuate in response to change in hydrologic budget and wetlands appear to respond similarly. Springs and groundwater-fed wetlands are common, however the sources and sustainability of water or what geologic factors lead to the formation and longevity of wetlands is not well established. It appears that rainfall is trapped on topographic highs (rift fault blocks and volcanoes). This meteoric water infiltrates quickly through porous volcanic rocks and is stored in aquifers and released slowly. As a component of the rift hydrologic system, wetlands appear to be reliable indicators of rainfall fluctuations on both Milankovitch and sub-Milankovitch time scales. Wetland sediments are commoner in the geologic record during times of higher rainfall and are less common during drier periods. Modern arid rift wetland records are peats and organic-rich clay deposits that contain eolian-transported mineral matter, plant remains (e.g. roots, stems) pollen, phytoliths, diatoms, root casts, charcoal, carbonate and manganese-rich nodules, as well as copious evidence of bioturbation (plants and invertebrates to large vertebrate trampling). Older (Pleistocene) deposits that retain little original organic matter and plant remains are generally silicified, but otherwise the record is similar to modern wetlands. Records from Olduvai Gorge (1.85-1.75 Ma) contain springs and wetlands associated with stone tools. On shorter time-frames, a drought occurred during the Medieval Warm Period (MWP) and higher rainfall in East Africa during the Little Ice Age (700 BP) led to higher lake levels of Lake Naivasha and Lake Turkana (Mohamed et al. 1995; Verschuren et al. 2000) and the simultaneous initiation and expansion of Loboi Swamp in Baringo-Bogoria basin (Ashley et al. 2004). Therefore, a wetland as an indicator of climate change even on millennial-scale cycles is a viable option to lakes as paleoclimatic indicators in arid, low-latitude continental settings. Groundwater reservoirs provide a perennial water source for plants and animals (including hominins) in what might otherwise be a parched environment.

Ashley, G. M.

2004-12-01

365

Defmod - Parallel multiphysics finite element code for modeling crustal deformation during the earthquake/rifting cycle  

E-print Network

In this article, we present Defmod, a fully unstructured, two or three dimensional, parallel finite element code for modeling crustal deformation over time scales ranging from milliseconds to thousands of years. Defmod can simulate deformation due to all major processes that make up the earthquake/rifting cycle, in non-homogeneous media. Specifically, it can be used to model deformation due to dynamic and quasistatic processes such as co-seismic slip or dike intrusion(s), poroelastic rebound due to fluid flow and post-seismic or post-rifting viscoelastic relaxation. It can also be used to model deformation due to processes such as post-glacial rebound, hydrological (un)loading, injection and/or withdrawal of compressible or incompressible fluids from subsurface reservoirs etc. Defmod is written in Fortran 95 and uses PETSc's parallel sparse data structures and implicit solvers. Problems can be solved using (stabilized) linear triangular, quadrilateral, tetrahedral or hexahedral elements on shared or distribut...

Ali, S Tabrez

2014-01-01

366

Magmatic constraints on rift development and the uplift of the Rwenzori Mountains in western Uganda  

NASA Astrophysics Data System (ADS)

The northern end of the western branch of the East African rift valley in western Uganda is located at the junction between a Proterozoic belt to the south and continuous Archaean crust of the Congo-Tanzania craton to the north (1). The difficulty of rift propagation through the thick Archaean lithosphere may explain the lack of a connection with the eastern rift branch around Lake Turkana. The >5,000m-high Rwenzori Mountains form a block lodged between two rift arms at this junction (2). The location and composition of Pleistocene to Recent volcanic rocks can be used to constrain models for the uplift of the Rwenzori Mountains that explain uplift by delamination of a lithosphere block beneath the rift (3). Six small volcanic fields are located in the Toro Ankole area east and southeast of Rwenzori; they contain carbonatite either as lava flows or as components in lapilli tuffs, as well as unusual K2O-rich, SiO2-poor melt compositions akin to leucitites and potassic melilitites that increase in importance towards the south. Their major and trace element and Sr-, Nd-, Hf-, and Os-isotope compositions are consistent with a source composed not just of peridotite, but also containing mica- and clinopyroxene-rich ultramafic assemblages (4). The requirement for assemblages of this type is greater than for potassic melilitites and nephelinites further south in the rift, and this agrees with their greater abundance as xenoliths in the lavas of the Toro Ankole area. The xenoliths contain no evidence for metasomatic replacement of pre-existing peridotite minerals, but rather appear to be formed by crystallization as liquidus minerals on the walls of magma channelways within the upper mantle. Some are zoned and contain vein margins, indicating more than one episode of melt infiltration. Re-melting of such assemblages partially meets the geochemical requirements of the volcanics (4) and has been shown to produce melts with similar major element compositions in experiments (5). The xenolith-richest occurrence is in the Katwe-Kikorongo field, furthest west within the rift at the location purported to be the site of lithospheric delamination (3). This may indicate that mica pyroxenite dyke zones formed by alkaline melts rising along the western edge of the Tanzania craton root provide zones of weakness for later delamination events. The elevated CO2-contents of the western rift volcanics can be explained by low-degree partial melting of carbonate-bearing peridotite at depths of 120-150km (5), thus constraining the source of the volcanic melts in this area to be deeper than those of the Virunga field to the south, in keeping with the postulation of a cratonic lithosphere root just east of the Rwenzori Mountains. (1) Link et al (2010) Internat. J. Earth Sci. submitted (2) Koehn et al (2010) Internat. J. Earth Sci. in press (3) Wallner and Schmeling (2010) Internat. J. Earth Sci. in press (4) Rosenthal et al (2009) Earth Planet. Sci. Lett. 284, 236-248 (5) Lloyd, Edgar and Arima (1985) Contrib. Mineral. Petrol. 91, 321-329 (6) Foley et al (2009) Lithos 112S, 274-283

Foley, Stephen; Link, Klemens; Tiberindwa, John; Barifaijo, Erasmus

2010-05-01

367

The transition from rifting to spreading in the Red Sea: No sign of discrete spreading nodes?  

NASA Astrophysics Data System (ADS)

The rifting of a continent and its eventual splitting by the formation of an ocean basin is one of the decisive processes in plate tectonics - it is responsible for the formation of continental margins (home to most of the world's submarine hydrocarbon reserves) and has repeatedly divided and re-distributed the continental plates. The Red Sea is one of the few places on Earth where this process is presently occurring, with continental rifting occurring in the northern Red Sea (north of 23°N) and ocean floor creation in the south (south of 19.5°N). In the central region it has been proposed that the extension is accommodated in a series of discrete seafloor spreading cells in the so called "transition-" or "multi-deeps region". This suggestion is mainly based on interpretations of bathymetric, magnetic, gravimetric and seismic observations made in the late 1970's and mid 1980's at resolutions far below those possible today. During two major expeditions with the German and Dutch research vessels Poseidon (cruise #408) and Pelagia (cruise #350/51) in 2011 and 2012 we collected a continuous multibeam bathymetric dataset over and between the Red Sea deeps from the Thetis Deep at 23°N to the Red Sea Rift at 16.5°N, with a spatial acoustic resolution of 15-30 m over a total N-S distance of about 700 km. This data enables us to view in detail the bathymetric structures formed during extension and to make a detailed interpretation of the structural, tectonic, magmatic and sedimentary evolution of the Red Sea rift. Based on an analysis of the bathymetric datasets, combined with acoustic backscatter, shallow seismics, ground truthing sampling, and magmatic geochemical information, we arrive at a much simplier and less exotic model of the transition from rifting to spreading than previously proposed. Geomorphological features strongly indicate a continuous oceanic rift valley at least from Nereus Deep at 23°N southwards to the Danakil triple junction at 17°N. We can demonstrate that, although the previously described discrete deeps undoubtedly exist, they are not separated from one another by tectonic boundaries but rather represent a continuous deep spreading axis which is inundated and masked locally by massive sediment slumping or (in the case of the Red Sea evaporites) flow. This can also explain the relative paucity of large strike-slip earthquakes in the "transition region" - such earthquakes would be expected if the deeps were, as previously proposed, separated by transform- or "accommodation"-zones but are not required on a continuous spreading axis. The axis appears to become totally inundated with sediment and evaporites north of 23°N (Nereus Deep) - at what latitude seafloor spreading actually ceases is at present unknown. Geochemical indices of degree of melting vary in a systematic but not linear way along axis, with higher degrees of melting indicated for the axis around 18°N, falling off to the North and South of this. That the degree of melting falls off northwards (the direction in which rifting rate also decreases) is perhaps unsurprising, the decrease to the south seems to be related to the existence of a triple junction at 17°N and the partitioning of spreading between the Red Sea and the Danakil Rift.

Augustin, N.; Devey, C. W.; Feldens, P.; van der Zwan, F. M.; Bantan, R.; Kwasnitschka, T.

2012-12-01

368

Numerical Models of Salt Tectonics and Associated Thermal Evolution of Rifted Continental Margins  

NASA Astrophysics Data System (ADS)

Salt tectonics at rifted continental margins reflects the interplay between the geometry of the initial evaporite basin and subsequent mobilization of