These are representative sample records from Science.gov related to your search topic.
For comprehensive and current results, perform a real-time search at Science.gov.
1

Continental rifting: a planetary perspective  

SciTech Connect

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

Muehlberger, W.R.

1985-01-01

2

Evolution of Oblique Rifting on the Main Ethiopian Rift  

NSDL National Science Digital Library

Movie showing the evolution of oblique rifting in analogue models (from Corti, 2008, Nature Geosc). Obliquity in this model is 30° (angle between the normal to the rift axis and the direction of extension). Note the two-phase rift evolution with a first phase of boundary fault activity and basin subsidence, followed by activation of en-echelon arranged internal faults obliquely cutting the rift floor.

Corti, Giacomo

3

Rift Valley Fever Review  

Technology Transfer Automated Retrieval System (TEKTRAN)

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

4

Volcanism at Rifts.  

ERIC Educational Resources Information Center

Investigates the nature of catastrophic volcanism and the rifting process. Describes two kinds of evidence: quantitative descriptions of rock melting and a wide range of observations. Discusses examples of continent growth in the North Atlantic, India and the Seychelles islands, and the South Atlantic. (YP)

White, Robert S.; McKenzie, Dan P.

1989-01-01

5

Continental rifts and mineral resources  

SciTech Connect

Continental rifts are widespread and range in age from the present to 3 b.y. Individual rifts may form parts of complex systems as in E. Africa and the Basin and Range. Rifts have originated in diverse environments such as arc-crests, sites of continental collision, collapsing mountain belts and on continents at rest over the mantle circulation pattern. Continental rift resources can be classified by depth of origin: For example, in the Great Dike, Norilsk and Mwadui magma from the mantle is the host. At shallower depths continental crust partly melted above mafic magma hosts ore (Climax, Henderson). Rift volcanics are linked to local hydrothermal systems and to extensive zeolite deposits (Basin and Range, East Africa). Copper (Zambia, Belt), zinc (Red Dog) and lead ores (Benue) are related to hydrothermal systems which involve hot rock and water flow through both pre-rift basement and sedimentary and volcanic rift fill. Economically significant sediments in rifts include coals (the Gondwana of Inida), marine evaporites (Lou Ann of the Gulf of Mexico) and non-marine evaporites (East Africa). Oil and gas in rifts relate to a variety of source, reservoir and trap relations (North Sea, Libya), but rift-lake sediment sources are important (Sung Liao, Bo Hai, Mina, Cabinda). Some ancient iron ores (Hammersley) may have formed in rift lakes but Algoman ores and greenstone belt mineral deposits in general are linked to oceanic and island arc environments. To the extent that continental environments are represented in such areas as the Archean of the Superior and Slave they are Andean Arc environments which today have locally rifted crests (Ecuador, N. Peru). The Pongola, on Kaapvaal craton may, on the other hand represent the world's oldest preserved, little deformed, continental rift.

Burke, K. (Univ. of Houston, TX (United States). Geosciences Dept.)

1992-01-01

6

Algunas aplicaciones de Cinematica Computacional a Navegacion  

E-print Network

Algunas aplicaciones de Cinem´atica Computacional a Navegaci´on Finat, J. Viloria, A., Gonzalo, M. Fuentes, L. Resumen La Cinem´atica Computacional es una extensi´on natural de la Geometr´ia Computacional´on computacional de estos objetivos requiere disponer de estructuras de datos que puedan adaptarse a m

Llanos, Diego R.

7

Granular mechanics and rifting  

NASA Astrophysics Data System (ADS)

Numerical models have proved useful in the interpretation of seismic-scale images of rifted margins. In an effort to both test and further illuminate predictions of numerical models, workers have made some strides using map-scale field relations, microstructures, and strain analyses. Yet, fundamental predictions of modeling and tectonic restorations are not able to capture critical observations. For example, many models and interpretations call on continuous faults with restorable kinematic histories. In contrast, s-reflectors and other interpreted shear fabrics in the middle crust tend to be discontinuous and non-planar across a margin. Additionally, most rift-evolution models and interpretations call on end-member ductile flow laws over a range of mechanical and thermal conditions. In contrast, field observations have found that a range of "brittle" fault rocks (e.g., cataclasites and breccias) form in the deeper crust. Similarly, upper crustal materials in deep basins and fault zones can deform through both distributed and localized deformation. Altogether, there appears to be reason to bring a new perspective to aspects of the structural evolution of rifted margins. A granular mechanics approach to crustal deformation studies has several important strengths. Granular materials efficiently localize shear and exhibit a range of stick-slip behaviors, including quasi-viscous rheological responses. These behaviors emerge in discrete element models, analog-materials experiments, and natural and engineered systems regardless of the specific micromechanical flow law. Yet, strictly speaking, granular deformation occurs via failure of frictional contacts between elastic grains. Here, we explore how to relate granular-mechanics models to mesoscale (outcrop) structural evolution, in turn providing insight into basin- and margin- scale evolution. At this stage we are focusing on analog-materials experiments and micro-to-mesoscale observations linking theoretical predictions to structural geological observations. With this combined approach we seek to establish characteristic length scales such as grain sizes and shear zone thicknesses, and time-scales such as stick-slip event dynamics. This would allow us to define a flow law at the mesoscale from comparing the experimental results and the field observations. This rheology could eventually be used to model the strain localization history of rifted margins

Reber, Jacqueline E.; Hayman, Nicholas W.; Lavier, Luc L.

2013-04-01

8

Rifts in Spreading Wax Layers  

E-print Network

We report experimental results on the rift formation between two freezing wax plates. The plates were pulled apart with constant velocity, while floating on the melt, in a way akin to the tectonic plates of the earth's crust. At slow spreading rates, a rift, initially perpendicular to the spreading direction, was found to be stable, while above a critical spreading rate a "spiky" rift with fracture zones almost parallel to the spreading direction developed. At yet higher spreading rates a second transition from the spiky rift to a zig-zag pattern occurred. In this regime the rift can be characterized by a single angle which was found to be dependent on the spreading rate. We show that the oblique spreading angles agree with a simple geometrical model. The coarsening of the zig-zag pattern over time and the three-dimensional structure of the solidified crust are also discussed.

Rolf Ragnarsson; J. Lewis Ford; Christian D. Santangelo; Eberhard Bodenschatz

1995-10-19

9

Kinematics of Rift-Parallel Deformation Along the Rukwa Rift, Western Branch, and Main Ethiopian Rift  

NASA Astrophysics Data System (ADS)

The East African Rift System spans N-S ~5000 km and currently experiences E-W extension. Previous kinematic studies of the EARS delineated 3 relatively rigid sub-plates (Victoria, Rovuma, and Lwandle) between the Nubian and Somalian plates. GPS observations of these block interiors confirm the rigid plate model, but we also detect a systematic along-rift deformation pattern at GPS stations located within rift zones bounding the western Victoria block and continuing north between the Nubian and Somalian plates. Here we present a kinematic model of present-day rift-parallel deformation along the Western branch, Rukwa Rift, and Main Ethiopian Rift constrained by a new GPS solution, earthquake slip vectors, and mapped active fault structures. We test the roles of block rotation, elastic deformation, and anelastic deformation by varying block geometry, fault slip distribution parameters, estimating permanent strain rate, and scoring each model with GPS observations. We also explore how the present-day deformation patterns relate to longer-term paleostress indicators. Observations of slickensides and offsets in seismic reflection profiles in the northern Western branch (Albertine rift) indicate a change from ~NNE trending normal faulting to include strike-slip motion within the past 7 My that may be related to previously studied stress changes in the Turkana rift. Preliminary results from the kinematic modeling demonstrate simple elastic strain accumulation on major border faults cannot explain an observed systematic northward component in GPS velocities relative to the Victoria block and the Nubian plate.

Stamps, D.; Koehn, D.; Burke, K. C.; d'Oreye, N.; Saria, E.; Xu, R.

2013-12-01

10

Cenozoic rifting in the West Antarctic Rift System  

NASA Astrophysics Data System (ADS)

The West Antarctic Rift System (WARS) experienced two episodes of Cenozoic rifting. Seafloor spreading at the Adare spreading axis, north of the Ross Sea, from Middle Eocene to Late Oligocene time (43 - 26 Ma), was directly linked with motions within the WARS. For this time interval, marine magnetic anomalies within the Adare Basin and structural features within the Ross Sea constrain the motion between East and West Antarctica. During this episode, widespread intrusive activity took place in the continental part of the rift. Subsequent Late Oligocene until present-day (26 - 0 Ma) extension was characterized by a transition to volcanic activity. Yet, the details of extension during this episode have been poorly resolved. We present preliminary results of new seismic reflection and seafloor mapping data acquired on geophysical cruise 07-01 aboard the R/VIB Nathaniel Palmer in the northern part of the rift. Our results suggest that the style of deformation changed from spreading-related faulting into diffuse normal faulting (tilted blocks) that trend NE-SW with little resultant E-W extension. Recent volcanism is distributed throughout but tends to align with the NE-SW trend, into a localized zone. Formation of the Terror Rift, Ross Sea, within the same time frame suggests that the pole of rotation has changed its position, reflecting a change in the relative magnitudes of tensile stresses along the rift. Moreover, this change was accompanied with a sharp decrease of extension rates.

Granot, R.; Cande, S. S.; Stock, J. M.; Clayton, R. W.; Davey, F. J.

2007-12-01

11

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

12

Midcontinent rift system: a frontier hydrocarbon province  

Microsoft Academic Search

The Mid-continent rift system can be traced by the Mid-continent geophysical anomaly (MGA) from the surface exposure of the Keweenawan Supergroup in the Lake Superior basin southwest in the subsurface through Wisconsin, Minnesota, Iowa, Nebraska, and Kansas. Outcrop and well penetrations of the late rift Keweenawan sedimentary rocks reveal sediments reflecting a characteristic early continental rift clastic sequence, including alluvial

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

1984-01-01

13

Current rifting episode in north Iceland  

Microsoft Academic Search

A major rifting episode is now occurring in north Iceland. This started on 20 December 1975, with a basaltic eruption, an exceptionally intense earthquake swarm and movement on an 80-km segment of the plate boundary. Inflation and deflation of the Krafla caldera indicate upwelling of magma and injection into the rift zone. Historical records show that similar episodic rifting occurs

Axel Bjrnsson; Kristjn Saemundsson; Pll Einarsson; Eysteinn Tryggvason; Karl Grnvold

1977-01-01

14

Structural style of the Turkana Rift, Kenya  

SciTech Connect

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

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

1988-03-01

15

Structural style of the Turkana Rift, Kenya  

NASA Astrophysics Data System (ADS)

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

Dunkelman, Thomas J.; Karson, Jeffrey A.; Rosendahl, Bruce R.

1988-03-01

16

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

17

Dynamic processes controlling evolution of rifted basins  

NASA Astrophysics Data System (ADS)

The extension of the lithosphere, controlling the development of rifted basins, is driven by a combination of plate-boundary forces, frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system. Although mantle plumes are not a primary driving force of rifting, they play an important secondary role by weakening the lithosphere and by controlling the level of rift-related volcanic activity. A distinction between "active" and "passive" rifting is only conditionally justified. The extension of the lithosphere, depending on its rate and magnitude, and the potential temperature of the asthenosphere, can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere. In rift systems, the level and timing of volcanic activity is highly variable. The lack of volcanic activity implies "passive" rifting. An initial "passive" rifting stage can be followed by a more "active" one during which magmatism plays an increasingly important role. Magmatic destabilization of the Moho may account for the frequently observed discrepancy between upper and lower crustal extension factors. Combined with evidence for thermal thinning of the mantle-lithosphere, this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models. The structural style of rifts is controlled by the rheological structure of the lithosphere, the availability of crustal discontinuities that can be tensionally reactivated, the mode (orthogonal or oblique) and amount of extension, and the lithological composition of pre- and syn-rift sediments. Simple-shear extension prevails in rifts that subparallel the structural grain of the basement. Pure-shear extension is typical for rifts cross-cutting the basement grain. Pre-existing crustal and mantle-lithospheric discontinuities contribute to the localization of rift systems. The duration of the rifting stage of extensional basins is highly variable. Stress field changes can cause abrupt termination of rifting. In major rift systems, progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts. Depending on constraints on lateral block movements, crustal separation can be achieved after as little as 9 My and as much as 280 My of rifting activity. Syn-rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere, its magmatic inflation and thermal attenuation of the mantle-lithosphere. Post-rift basin subsidence is governed by thermal reequilibration of the lithosphere-asthenosphere system. Deep-seated thermal anomalies related to syn-rift pull-up of the asthenosphere-lithosphere boundary have decayed after 60 My by about 65% and after 180 My by about 95%. The magnitude of post-rift subsidence is a function of the rift-induced thermal anomaly and crustal density changes, the potential temperature of the asthenosphere and initial water depths. Intraplate stresses can have an overprinting effect on post-rift subsidence. Stretching factors derived from post-rift subsidence analyses must be corrected for such effects.

Ziegler, Peter A.; Cloetingh, Sierd

2004-01-01

18

The diverging volcanic rift system  

NASA Astrophysics Data System (ADS)

Eruptions and volcano internal growth are mostly fed by dykes. The comprehension of the control factors on dyke paths is fundamental for the assessment of areas prone to vent formation and to the general understanding of how volcanoes work. We analyse an understudied magma path system; field data of nine volcanoes show they have a rectilinear rift zone in the central part passing into fan-arranged dykes at the two opposite volcano flanks. The geological, geomorphological and structural characteristics of these volcanoes and their substrate suggest that the formation of these "diverging rifts" is not specifically linked to substrate lithology and mechanical behaviour. The studied volcanoes have elongation < 0.88 and V > 10 km3 (mostly > 300 km3). Eight volcanoes have the central rift that is normal to the regional tectonic least principal stress (?3reg) and in one case it is sub-perpendicular. Field data have been combined with scaled analogue modelling, suggesting that if the ?3reg is oblique to the volcano elongation axis, dyke geometry in the edifice axial zone is controlled by elongation and thus by local gravity ?3, but dyke strike becomes perpendicular to ?3reg when dykes intrude the more external areas of the volcano. If a dyke is injected under the volcano flanks with slope inclination > 50, it attains a geometry parallel to the slope. At lower slope inclinations at the edifice terminations, magma paths diverge outwards and crosscut slopes at high angle. Our data are in agreement with the assumption that regional tectonic stresses can affect large volcanoes up to the summit area guiding the development of a rectilinear thoroughgoing rift, both in extensional and transtensional regimes. The diverging pattern takes place due to reorientation of the local stress field guided by topography only when dyke inception localizes laterally respect to the edifice axis.

Tibaldi, A.; Bonali, F. L.; Corazzato, C.

2014-01-01

19

Lithospheric structure of the Rio Grande rift.  

PubMed

A high-resolution, regional passive seismic experiment in the Rio Grande rift region of the southwestern United States has produced new images of upper-mantle velocity structure and crust-mantle topography. Synthesizing these results with geochemical and other geophysical evidence reveals highly symmetric lower-crustal and upper-mantle lithosphere extensional deformation, suggesting a pure-shear rifting mechanism for the Rio Grande rift. Extension in the lower crust is distributed over a region four times the width of the rift's surface expression. Here we propose that the laterally distributed, pure shear extension is a combined effect of low strain rate and a regionally elevated geotherm, possibly abetted by pre-existing lithospheric structures, at the time of rift initiation. Distributed extension in the lower crust and mantle has induced less concentrated vertical mantle upwelling and less vigorous small-scale convection than would have arisen from more localized deformation. This lack of highly focused mantle upwelling may explain a deficit of rift-related volcanics in the Rio Grande rift compared to other major rift systems such as the Kenya rift. PMID:15729338

Wilson, David; Aster, Richard; West, Michael; Ni, James; Grand, Steve; Gao, Wei; Baldridge, W Scott; Semken, Steve; Patel, Paresh

2005-02-24

20

Rift Valley Fever Outbreak, Southern Mauritania, 2012  

PubMed Central

After a period of heavy rainfall, an outbreak of Rift Valley fever occurred in southern Mauritania during SeptemberNovember 2012. A total of 41 human cases were confirmed, including 13 deaths, and 12 Rift Valley fever virus strains were isolated. Moudjeria and Temchecket Departments were the most affected areas. PMID:24447334

Sow, Abdourahmane; Faye, Ousmane; Ba, Yamar; Ba, Hampath; Diallo, Diawo; Faye, Oumar; Loucoubar, Cheikh; Boushab, Mohamed; Barry, Yahya; Diallo, Mawlouth

2014-01-01

21

Dynamics of crustal rifting in NE Iceland  

Microsoft Academic Search

Magnetotelluric measurements have revealed a crustal thickness of 8-10 km in the axial rift zone of NE Iceland and above the proposed hot spot in central east and north Iceland. The crust thickens with age and is 20-30 km thick in the older Tertiary areas to the east and west of the axial rift zone. It also thickens toward north

Axel Bjrnsson

1985-01-01

22

The Effect of Continental Rifting on Lithospheric Fabric: Evidence From the Mid-Continent Rift  

NASA Astrophysics Data System (ADS)

The Mid-Continent Rift (MCR) is a major feature of the North American continent: a 1.1 Ga rift that failed to develop into an ocean basin. Though the crustal expression of the rift is preserved, it is impossible to determine from crustal evidence the nature of the lithospheric contribution to the rifting process. The installation of teleseismic instrumentation through the Superior Province Rifting Earthscope Experiment (SPREE) is allowing investigation of the lithosphere beneath the MCR, which will help in addressing questions about the initiation, propagation, and failure of the rift structure. We focus on observing the strength and orientation of lithospheric fabric through measurements of the splitting of teleseismic SK(K)S waves at instruments in and near the rift axis, using the method of Silver and Chan (1991) to find the set of parameters that optimally restores linear particle motion. Our results show that the fast direction varies only subtly across the study area, with the exception of localized outliers. The fast direction is close to the direction of absolute plate motion, but shows greater scatter within the MCR itself. Split times show strong variations (from near-zero to 1.5 s), with lower values within the rift; the Nipigon Embayment stands out as a particularly low-anisotropy region. These measurements suggest that the rifting process thinned the lithosphere or reset its fabric, indicating significant lithospheric participation in the rifting process.

Ola, O. B.; Frederiksen, A. W.

2013-12-01

23

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

24

Ensayo sobre equilibrio general computable: Teora y aplicaciones  

Microsoft Academic Search

Este documento de trabajo presenta un resumen de la literatura conceptual y aplicada sobre modelos de equilibrio general computables (EGC). Se examinan las principales caractersticas de los EGC, las formas en que se construyen, los datos requeridos y mtodos de calibracin, as como sus ventajas y desventajas. Luego se discuten sus principales aplicaciones tanto en el mundo como en Chile.

Ral ORyan; Carlos J. de Miguel; Sebastin Miller

2000-01-01

25

Experto Universitario Java Enterprise Aplicaciones AJAX y REST  

E-print Network

Experto Universitario Java Enterprise Spring Sesión 4 Aplicaciones AJAX y REST #12;Experto Universitario Java Enterprise Spring © 2012-2013 Depto. Ciencia de la Computación e IA AJAX y REST con Spring Puntos a tratar · AJAX con Spring MVC · Enviar fragmentos de HTML/objetos al cliente · Recibir objetos

Escolano, Francisco

26

East African Rift Valley Links for Learning  

NSDL National Science Digital Library

This web page contains links to a collection of resources devoted to the East African Valley Rift, with emphasis on environmental issues and concerns. It lists web addresses by subtopics, in outline form, with a short description to assist the viewer in searching for information. Topics include The Rift Valley, Stromboli online, African volcanoes, the Alid page, paleontology, and many more. The creator of this collection has checked uRLs provided for content and guarantees them to be quality web pages.

Moyra/MysticPC

27

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

28

12.2.2. Institut d'Aplicacions Computacionals de Codi Comunitari de la UIB L'Institut d'Aplicacions Computacionals de Codi Comunitari (IAC3  

E-print Network

12.2.2. Institut d'Aplicacions Computacionals de Codi Comunitari de la UIB (IAC3 ) L'Institut d'Aplicacions Computacionals de Codi Comunitari (IAC3 ) sorgeix dins l'àmbit dels instituts propis de la UIB, el juliol de 2008

Oro, Daniel

29

Institut d'Aplicacions Computacionals de Codi Comunitari de la UIB (IAC3 L'Institut d'Aplicacions Computacionals de Codi Comunitari (IAC3  

E-print Network

Institut d'Aplicacions Computacionals de Codi Comunitari de la UIB (IAC3 ) L'Institut d'Aplicacions Computacionals de Codi Comunitari (IAC3 ) sorgeix dins l'àmbit dels instituts propis de la UIB, el juliol de 2008

Oro, Daniel

30

11.3.2. Institut d'Aplicacions Computacionals de Codi Comunitari (IAC3 L'Institut d'Aplicacions Computacionals de Codi Comunitari (IAC3  

E-print Network

11.3.2. Institut d'Aplicacions Computacionals de Codi Comunitari (IAC3 ) L'Institut d'Aplicacions Computacionals de Codi Comunitari (IAC3 ) es crea en l'àmbit de la Universitat de les Illes Balears (UIB) el

Oro, Daniel

31

Institut d'Aplicacions Computacionals de Codi Comunitari de la UIB (IAC3 L'Institut d'Aplicacions Computacionals de Codi Comunitari (IAC3  

E-print Network

Institut d'Aplicacions Computacionals de Codi Comunitari de la UIB (IAC3 ) L'Institut d'Aplicacions Computacionals de Codi Comunitari (IAC3 ) sorgeix en l'àmbit dels instituts propis de la UIB, el juliol de 2008

Oro, Daniel

32

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

SciTech Connect

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

Bosworth, W.

1987-05-01

33

Inuence of rift obliquity on fault-population systematics: results of experimental clay models  

E-print Network

. Rift obliquity is related to the acute angle, a, between the rift trend and the displacement direction develop in the center of the rift zone, one approximately rift-parallel and the other displacement in oblique continental rift basins. At the rift margins, tensional stresses are modulated and reoriented

Clifton, Amy Elizabeth

34

Anisotropy beneath a highly extended continental rift  

NASA Astrophysics Data System (ADS)

have employed shear wave splitting techniques to image anisotropy beneath the D'Entrecasteaux Islands, in southeastern Papua New Guinea. Our results provide a detailed picture of the extending continent that lies immediately ahead of a propagating mid-ocean ridge tip; we image the transition from continental to oceanic extension. A dense shear wave splitting data set from a 2010 to 2011 passive-source seismic deployment is analyzed using single and multichannel methods. Splitting delay times of 1-1.5 s are observed and fast axes of anisotropy trending N-S, parallel to rifting direction, predominate the results. This trend is linked to lattice-preferred orientation of olivine, primarily in the shallow convecting mantle, driven by up to 200 km of N-S continental extension ahead of the westward-propagating Woodlark Rift. This pattern differs from several other continental rifts that evince rift-strike-parallel fast axes and is evident despite the complex recent tectonic history. We contend that across most of this rift, the unusually high rate and magnitude of extension has been sufficient to produce a regime change to a mid-ocean-ridge-like mantle fabric. Stations in the south of our array show more complex splitting that might be related to melt or to complex inherited structure at the edge of the extended region.

Eilon, Zachary; Abers, Geoffrey A.; Jin, Ge; Gaherty, James B.

2014-03-01

35

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 chelon 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, Ren C.; Rolet, JoL.; Huchon, Philippe; Guillou, Herv

2011-02-01

36

Variation in styles of rifting in the Gulf of California.  

PubMed

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

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

2007-07-26

37

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

NASA Astrophysics Data System (ADS)

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

Guo, L.; Keller, G. R.

2010-12-01

38

MidContinent rift system: a frontier hydrocarbon province  

Microsoft Academic Search

The Mid-Continent rift system can be traced by the Mid-Continent geophysical anomaly (MGA) from the surface exposure of the Keweenawan Supergroup in the Lake Superior basin southwest in the subsurface through Wisconsin, Minnesota, Iowa, Nebraska, and Kansas. Outcrop and well penetrations of the late rift Keweenawan sedimentary rocks reveal sediments reflecting a characteristic early continental rift clastic sequence, including alluvial

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

1984-01-01

39

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 45 km maximum elevation marks one flank and extends from

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

1991-01-01

40

Kinematics of the South Atlantic rift  

NASA Astrophysics Data System (ADS)

The South Atlantic rift basin evolved as branch of a large Jurassic-Cretaceous intraplate rift zone between the African and South American plates during the final breakup of western Gondwana. While the relative motions between South America and Africa for post-breakup times are well resolved, many issues pertaining to the fit reconstruction and particular the relation between kinematics and lithosphere dynamics during pre-breakup remain unclear in currently published plate models. We have compiled and assimilated data from these intraplated rifts and constructed a revised plate kinematic model for the pre-breakup evolution of the South Atlantic. Based on structural restoration of the conjugate South Atlantic margins and intracontinental rift basins in Africa and South America, we achieve a tight fit reconstruction which eliminates the need for previously inferred large intracontinental shear zones, in particular in Patagonian South America. By quantitatively accounting for crustal deformation in the Central and West African rift zone, we have been able to indirectly construct the kinematic history of the pre-breakup evolution of the conjugate West African-Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic Pre-salt sag basin and the So Paulo High. We model an initial E-W directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities until Upper Hauterivian times (?126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial ?17 Myr-long stretching episode the Pre-salt basin width on the conjugate Brazilian and West African margins is generated. An intermediate stage between 126.57 Ma and Base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE-SW. From Base Aptian onwards diachronous lithospheric breakup occurred along the central South Atlantic rift, first in the Sergipe-Alagoas/Rio Muni margin segment in the northernmost South Atlantic. Final breakup between South America and Africa occurred in the conjugate Santos-Benguela margin segment at around 113 Ma and in the Equatorial Atlantic domain between the Ghanaian Ridge and the Piau-Cear margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of this conjugate passive margins systems and can explain the first order tectonic structures along the South Atlantic and possibly other passive margins.

Heine, C.; Zoethout, J.; Mller, R. D.

2013-01-01

41

Seismicity, deformation and seismic hazard in the western rift of Corinth: New insights from the Corinth Rift Laboratory (CRL)  

Microsoft Academic Search

This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10km, defining an active layer, gently dipping

P. Bernard; H. Lyon-Caen; P. Briole; A. Deschamps; F. Boudin; K. Makropoulos; P. Papadimitriou; F. Lemeille; G. Patau; H. Billiris; D. Paradissis; K. Papazissi; H. Castarde; O. Charade; A. Nercessian; A. Avallone; F. Pacchiani; J. Zahradnik; S. Sacks; A. Linde

2006-01-01

42

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

NASA Astrophysics Data System (ADS)

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

lvaro, J. Javier; Ezzouhairi, Hassan; Clausen, Sbastien; Ribeiro, M. Luisa; Sol, Rita

2014-11-01

43

Implications of new gravity data for Baikal Rift zone structure  

NASA Technical Reports Server (NTRS)

Newly available, 2D Bouguer gravity anomaly data from the Baikal Rift zone, Siberia, indicate that this discrete, intracontinental rift system is regionally compensated by an elastic plate about 50 km thick. However, spectral and spatial domain analyses and isostatic anomaly calculations show that simple elastic plate theory does not offer an adequate explanation for compensation in the rift zone, probably because of significant lateral variations in plate strength and the presence of subsurface loads. Our results and other geophysical observations support the interpretation that the Baikal Rift zone is colder than either the East African or Rio Grande rift.

Ruppel, C.; Kogan, M. G.; Mcnutt, M. K.

1993-01-01

44

Flexural rift flank uplift at the Rio Grande rift, New Mexico  

NASA Astrophysics Data System (ADS)

Like other Cenozoic continental rifts, the Rio Grande rift in Colorado and New Mexico exhibits prominent flanking uplifts. Several driving stresses and thermal-mechanical processes have been proposed to explain the origin of rift flank relief, which can be modeled to infer lithospheric structure. Although we have identified multiple uplift styles at the Rio Grande rift, only one rangethe Sacramento Mountainsis attributable to flexural upwarping of the lithosphere, the process most suitable for geodynamic modeling and interpretation. We demonstrate that two common assumptions in such modeling potentially introduce serious errors. First, presuming only one mechanism acts to uplift the flanks is inappropriate; various forces influence flank topography at different depths and wavelengths and no single one is dominant. Second, the end-member boundary conditions of complete mechanical continuity or discontinuity (broken plate) at the range-bounding normal fault are, in general, not applicable at rift flanks. We examine alternative analytic plate flexure solutions by comparing them to finite element models of footwall flexure at a normal fault in a two-dimensional elastic plate undergoing extension. These simulations indicate that broken plate fits to rift flanks underestimate the plate thickness unless the uplift is large (at least 1 km), which promotes decoupling between the hanging wall and footwall. If denudation dominates the flank unloading, as may commonly be the case, the best-fit broken plate thickness error can be even greater. Our flexural analysis of the Sacramento Mountains suggests that the Pecos River Valley originated as a flexural down warp adjacent to the rift flank. Sensitivity tests of least-squares fits to the Sacramento Mountains imply typical plate thickness errors of <20%, although in extreme cases the combined errors may be 50%. The average effective elastic lithosphere thickness is 23 km. We find that elastic-plastic models of rift flank flexure are unable to provide meaningful constraints on the thermal structure of continental lithosphere.

Brown, C. David; Phillips, Roger J.

1999-12-01

45

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

46

Structure and kinematics of the Taupo Rift, New Zealand  

NASA Astrophysics Data System (ADS)

structure and kinematics of the continental intra-arc Taupo Rift have been constrained by fault-trace mapping, a large catalogue of focal mechanisms (N = 202) and fault slip striations. The mean extension direction of ~137 is approximately orthogonal to the regional trend of the rift and arc front (? = 84 and 79, respectively) and to the strike of the underlying subducting Pacific Plate. Bending and rollback of the subduction hinge strongly influence the location, orientation, and extension direction of intra-arc rifting in the North Island. In detail, orthogonal rifting (? = 85-90) transitions northward to oblique rifting (? = 69-71) across a paleovertical-axis rotation boundary where rift faults, extension directions, and basement fabric rotate by ~20-25. Toward the south, extension is orthogonal to normal faults which are parallel to, and reactivate, steeply dipping basement fabric. Basement reactivation facilitates strain partitioning with a portion of margin-parallel motion in the overriding plate mainly accommodated east of the rift by strike-slip faults in the North Island Fault System (NIFS). Toward the north where the rift and NIFS intersect, ~4 mm/yr strike slip is transferred into the rift with net oblique extension accommodating a component of margin-parallel motion. The trend and kinematics of the Taupo Rift are comparable to late Miocene-Pliocene intra-arc rifting in the Taranaki Basin, indicating that the northeast strike of the subducting plate and the southeast extension direction have been uniform since at least 4 Ma.

Seebeck, Hannu; Nicol, Andrew; Villamor, Pilar; Ristau, John; Pettinga, Jarg

2014-06-01

47

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

USGS Publications Warehouse

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

Hall, Jerome; Wilson, Terry; Henrys, Stuart

2007-01-01

48

Contrasting conditions of rift and off-rift silicic magma origin on Iceland  

NASA Astrophysics Data System (ADS)

controlling the origin of silicic magmas on Iceland are poorly constrained. Here we present new data on H2O content, pressure, temperature, oxygen fugacity, and oxygen isotope composition of rhyolites from Askja, rfajkull, and Hekla volcanoes. All these parameters correlate with tectonic (rift and off-rift) setting of the volcanoes. Askja rift rhyolites originate through extensive assimilation of high-temperature hydrothermally altered crust (?18O < 2) at shallow depths (?1.8 km). These rhyolites are hot (935-1008C), relatively dry (H2O < 2.7 wt%), and oxidized (QFM = +1.4). Cooler (874-902C), wet (H2O = 4-6.3 wt%), and non-oxidized (~QFM to QFM-1) off-rift rhyolites (rfajkull, Hekla) originate through differentiation deeper in the crust (?4 km) with almost no or little assimilation of high-T, altered crust, as reflected by slightly lower to normal ?18O values (5.2-6). Although off-rift rhyolites predominate during the Holocene, older silicic rocks on Iceland primarily formed in a rift setting possibly analogous to the oldest continental crust on Earth.

Schattel, Nadine; Portnyagin, Maxim; Golowin, Roman; Hoernle, Kaj; Bindeman, Ilya

2014-08-01

49

Proterozoic Midcontinent Rift System, an overview  

Microsoft Academic Search

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

S. D. Kerr; S. M. Landon

1992-01-01

50

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

51

The 1973 Ethiopian-Rift geodimeter survey  

NASA Technical Reports Server (NTRS)

Remeasurement of the Adama, Lake Langana, and Arba Minch (Lake Margherita) geodimeter networks in 1973 has enabled Mohr's interpretation concerning possible surface ground deformation in the Ethiopian rift to be considerably developed. Extension appears to have occurred across the Mojjo-Adama horst at a rate of about 1 cm yr/1. The opposing rims of the Adama graben have not moved significantly relative to one another (between 1969 and 1973), but stations on the sliced graben floor show possible movement with a large rift-trend component. In the Wolenchiti quadrilateral, significant movement of station RABBIT is confirmed, but the radical change of vector (that of 1970-1971 to that of 1971-1973) casts doubt on a tectonic cause and seems to indicate that stations on steep hillslopes are liable to be unstable. South of the quadrilateral and east of the Adama graben, alternating rift-trend zones of extension and shortening appear to coexist. In the Lake Langana network, significant movements of the order of 0.5 cm yr/1 are directed perpendicular to the rift floor faulting.

Mohr, P. A.

1974-01-01

52

Reemergence of Rift Valley Fever, Mauritania, 2010  

PubMed Central

A Rift Valley fever (RVF) outbreak in humans and animals occurred in Mauritania in 2010. Thirty cases of RVF in humans and 3 deaths were identified. RVFV isolates were recovered from humans, camels, sheep, goats, and Culex antennatus mosquitoes. Phylogenetic analysis of isolates indicated a virus origin from western Africa. PMID:24447381

Faye, Ousmane; Ba, Hampath; Ba, Yamar; Freire, Caio C.M.; Faye, Oumar; Ndiaye, Oumar; Elgady, Isselmou O.; Zanotto, Paolo M.A.; Diallo, Mawlouth

2014-01-01

53

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

54

Rift Valley fever, Mayotte, 2007-2008.  

PubMed

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

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

2009-04-01

55

RIFT VALLEY FEVER POTENTIAL, ARABIAN PENINSULA  

Technology Transfer Automated Retrieval System (TEKTRAN)

Rift Valley fever (RVF) was first confirmed outside of Africa in September 2000. This outbreak, which occurred in southwestern coastal Saudi Arabia and neighboring coastal areas of Yemen, followed elevated rainfall levels in nearby highlands which flooded the coastal areas, providing ideal environm...

56

Diagnostic approaches for Rift Valley Fever  

Technology Transfer Automated Retrieval System (TEKTRAN)

Disease outbreaks caused by arthropod-borne animal viruses (arboviruses) resulting in significant livestock and economic losses world-wide appear to be increasing. Rift Valley fever (RVF) virus (RVFV) is an important arbovirus that causes lethal disease in cattle, camels, sheep and goats in Sub-Saha...

57

Rift Valley fever: A neglected zoonotic disease?  

Technology Transfer Automated Retrieval System (TEKTRAN)

Rift Valley fever (RVF) is a serious viral disease of animals and humans in Africa and the Middle East that is transmitted by mosquitoes. First isolated in Kenya during an outbreak in 1930, subsequent outbreaks have had a significant impact on animal and human health, as well as national economies. ...

58

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

59

Development regimes of rifted basins and criteria of their petroleum potential  

SciTech Connect

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

Larchenkov, E. (Odessa State Univ. (Ukraine))

1994-07-01

60

Controls on (anomalous) topography in rifted margin settings  

NASA Astrophysics Data System (ADS)

Contrasting end members of volcanic and non-volcanic passive margin formation show a large variability in basin shape and structure, subsidence history, and associated topographic evolution of the onshore rifted margins. The large range of structural style and associated topography of these systems imply a strong variability in the underlying thermo-mechanical conditions at the time of rifting. Rift - passive margin styles ranging from narrow to ultra wide are explained using forward numerical models with varying rheological structure, with strong crust lithosphere leading to narrow rift formation associated with highly elevated rift shoulders and conversely weak crust lithosphere resulting in highly stretched wide rifted conjugate margins and little flank morphology. In some cases rifted margins appear to indicate the formation of anomalous post rift topography. A number of mechanisms including small-scale convective removal of the lower lithosphere, lithosphere counter-flow, and dynamic topography, have been invoked to explain the anomalous topography. Forward numerical models are used to predict the magnitude and characteristic topography associated with each of these mechanisms and to evaluate their potential for explaining these apparent anomalous characteristics of rifts and rifted margins.

Huismans, Ritske S.

2014-05-01

61

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

62

Kinematics of the South Atlantic rift  

NASA Astrophysics Data System (ADS)

The South Atlantic rift basin evolved as a branch of a large Jurassic-Cretaceous intraplate rift zone between the African and South American plates during the final break-up of western Gondwana. While the relative motions between South America and Africa for post-break-up times are well resolved, many issues pertaining to the fit reconstruction and particularly the relation between kinematics and lithosphere dynamics during pre-break-up remain unclear in currently published plate models. We have compiled and assimilated data from these intraplated rifts and constructed a revised plate kinematic model for the pre-break-up evolution of the South Atlantic. Based on structural restoration of the conjugate South Atlantic margins and intracontinental rift basins in Africa and South America, we achieve a tight-fit reconstruction which eliminates the need for previously inferred large intracontinental shear zones, in particular in Patagonian South America. By quantitatively accounting for crustal deformation in the Central and West African Rift Zones, we have been able to indirectly construct the kinematic history of the pre-break-up evolution of the conjugate west African-Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic pre-salt sag basin and the So Paulo High. We model an initial E-W-directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities from 140 Ma until late Hauterivian times (?126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial ?14 Myr-long stretching episode the pre-salt basin width on the conjugate Brazilian and west African margins is generated. An intermediate stage between ?126 Ma and base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE-SW. From base Aptian onwards diachronous lithospheric break-up occurred along the central South Atlantic rift, first in the Sergipe-Alagoas/Rio Muni margin segment in the northernmost South Atlantic. Final break-up between South America and Africa occurred in the conjugate Santos-Benguela margin segment at around 113 Ma and in the equatorial Atlantic domain between the Ghanaian Ridge and the Piau-Cear margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of these conjugate passive-margin systems and can explain the first-order tectonic structures along the South Atlantic and possibly other passive margins.

Heine, C.; Zoethout, J.; Mller, R. D.

2013-08-01

63

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

64

The Midcontinent rift system in Kansas  

SciTech Connect

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

Berendsen, P. (Univ. of Kansas, Lawrence, KS (United States). Kansas Geological Survey)

1993-03-01

65

MidContinent rift: new frontier in an old area  

Microsoft Academic Search

The Mid-Continent rift (MCR) is a 2000-km-long intracontinental feature of middle Proterozoic age (1.1 Ga) that extends from Kansas northeastward through the Lake Superior basin and then southeastward through the lower peninsula of Michigan. The authors believe that rift-related marginal basins overlying axial basins and other structures associated with this feature may locally be prospective within four geographically identifiable rift

D. M. Jr. Davidson; M. G. Jr. Mudrey

1986-01-01

66

Seismicity of the Galapagos 95.5W propagating rift  

Microsoft Academic Search

An array of ocean bottom seismometers was deployed for 21 days about the 95.5W Galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation. Seismicity was fiarly constant (10-17 events per day) with no indication of swarm activity. Hypocenters were determined for 304 earthquakes, the largest of which

Patricia A. Cooper; Phillip D. Milholland; Frederick K. Duennebier

1987-01-01

67

Structural and stratigraphic evolution of the Anza rift, Kenya  

NASA Astrophysics Data System (ADS)

The Anza rift is a large, multi-phase continental rift basin that links the Lamu embayment of southern Kenya with the South Sudan rifts. Extension and deposition of syn-rift sediments are known to have commenced by the Neocomian. Aptian-Albian strata have, thus far, not been encountered during limited drilling campaigns and, in at least one well, are replaced by a significant unconformity. Widespread rifting occurred during the Cenomanian to Maastrichtian, and continued into the Early Tertiary. Marine waters appear to have reached the central Anza rift in the Cenomanian, and a second marine incursion may have occurred during the Campanian. As no wells have yet reached basement in the basinal deeps, the possibility exists that the Anza rift may have initiated in the Late Jurassic, in conjunction with extension to the south in the Lamu embayment and to the north in the Blue Nile rift of Sudan. Structural and stratigraphic evolution in the Anza rift followed a pattern that has now been inferred in several rift settings. Early phases of extension were accommodated by moderately dipping faults that produced large stratal rotations. Sedimentary environments were dominantly fluvial, with associated small lakes and dune fields. Volcanic activity is documented for the early Neocomian, but its extent is unknown. This initial style of deformation and sedimentation may have continued through several of the earliest pulses of rifting. By the Late Cretaceous, a new system of steeply dipping faults was established, that produced a deep basin without significant rotation of strata in the north, and only minor rotation in the south. This basin geometry favored the establishment of large, deep lakes, which occasionally were connected to the sea. The older basins were partly cannibalized during the sedimentary in-filling of these successor basins. Early Senonian volcanism was encountered in one well, and reflection seismic evidence suggests that one or more thick, regionally extensive igneous sills were intruded, probably during the Early Tertiary. The change in rift style from early, strongly rotational, shallow basins to late, non-rotational, deep basins has been observed in the southern Gulf of Suez/northern Red Sea, the Southwestern Turkana/northern Kenyan rift, and at Anza. It therefore takes place in rifts in variable tectonic settings, with a wide range of volcanic activity and, presumably, with different driving mechanisms. The shift in deposition in each case is away from early rift-bounding faults toward the half-graben flexural margins, further in-board to the upper structural plate. This suggests at least some component of regional simple shear in the deformation history of the rifted lithosphere, either via broad shear zones or at discrete detachment surfaces.

Bosworth, William; Morley, Chris K.

1994-09-01

68

The MOZART Project - MOZAmbique Rift Tomography  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

69

Mesozoic Rifting in the German North Sea  

NASA Astrophysics Data System (ADS)

The Central Graben is the southernmost expressions of the Mesozoic North Sea rift system that includes the Viking Graben, Moray Firth-Witch Ground grabens and the Horda-Egersund half graben. In the southern North Sea the Central Graben extends across the Dutch and the German exclusive economic zones. The structure of the Central Graben in German territorial waters was mapped in great detail in 2D and 3D seismic data and the stratigraphy has been constraint by borehole data. We provide a detailed review of the rifting activity in the German North Sea sector both in time and space and the link between rifting and salt movement. Major rifting activity started in the Central Graben during the Late Triassic and peaked during the Late Jurassic when extensive rift grabens formed, further influenced by halokinetic movements. First subsidence in the Central Graben area appears in the Early Triassic. This is documented by thickness variations in the sedimentary strata from the Triassic to the Jurassic. Remarkably thick sediments were deposited during the Late Triassic along the eastern border fault of the Central Graben and in the Late Jurassic sediments accumulated along graben-wide extensional faults and in rim-synclines of salt-structures. A basin inversion commenced in the Late Cretaceous resulting in an erosion of wide portions of Lower Cretaceous rocks or even complete removal in some parts. The area to the east of the Central Graben faced a completely different evolution. In this area major rifting activity initiated already in the Early to Middle Triassic. This is evident from huge packages of Middle Buntsandstein to Muschelkalk (Middle Triassic) sediments in the Horn Graben. Jurassic doming, forming the Mid-North Sea High, resulted in almost complete erosion of Lower and Middle Jurassic sediments in the central German North Sea. Sedimentation continued during the Early and Late Cretaceous. The Glckstadt Graben, which is a structure located farther east has a similar evolution as the Horn Graben with high sediment thicknesses deposited during the Triassic. This indicates that initiation of rifting started in the central German North Sea already during the Early Triassic and subsequently migrated to the west into the Central Graben. Before the Triassic thick layers of Permian rock salt (Upper Rotliegend and Zechstein) were deposited in the Central European Basin System, which spans from the UK across the Netherlands, southern Denmark, Germany, and into Poland. Salt movements resulted in the formation of salt diapirs, salt pillows, salt walls and intrusions into faults. Analyses of rim-synclines of salt diapirs reveals that most of the salt structures inside the German Central Graben had a main phase of growth during the Late Jurassic while the salt structures situated toward the southeast had their main phase of growth during the Triassic. The final products of the project are accessible at www.geopotenzial-nordsee.de/.

Lutz, R.; Jhne, F.; Arfai, J.

2013-12-01

70

InSAR observations of 2007 Tanzania rifting episode reveal mixed fault and dyke extension in an immature continental rift  

NASA Astrophysics Data System (ADS)

In the early stages of continental rifting, extension takes place by normal faulting, while in mature continental rifts dyke intrusion dominates. Little is known about the nature of the transition between fault-controlled and dyke-controlled extension or about the processes in an intermediate setting. Here, we present observations of the temporal and spatial evolution of surface displacements during the 2007 July 14-August 4 rifting episode in Northern Tanzania, an immature section of the East African Rift. The ground deformation initiated with subsidence that can be attributed to ~40 cm of normal motion on a NE striking fault. Following July 17, deformation was dominated by the intrusion of ~7-km-long dyke. Dyke opening increased gradually to a total of ~2.4 m. From July 21, the collapse of a shallow graben above the fault dominated the near-field displacements. Comparison to the 2007 Dabbahu dyke, Afar, which occurred in a more mature rift, shows an order-of-magnitude scale difference in dyke length. Using numerical models of dyke propagation, we attribute this to the size and depth of the magma chamber; in immature rifts the thick crust and slow spreading rate favour small, deep magma chambers, forming short, buried dykes, whereas in mature rifts the thinner crust and faster spreading rate favour large, shallow magma chambers and long, erupting dykes. Observing the pattern of active processes in the East African Rift is key to understanding the development of rift systems and passive margins elsewhere.

Biggs, Juliet; Amelung, Falk; Gourmelen, Noel; Dixon, Timothy H.; Kim, Sang-Wan

2009-10-01

71

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

72

Basement rift control on oil production in eastern Kansas  

Microsoft Academic Search

Improved understanding of the central North American rift system (CNARS) offers a new interpretation of the basement structure in certain parts of the Mid-Continent. In eastern Kansas, basement structure can be shown to control oil production from some producing fields. Structural control includes rotated blocks along faults created by horst and graben tectonics typically associated with rift zones. A distinctive

1983-01-01

73

New Vectors of Rift Valley Fever in West Africa  

Microsoft Academic Search

After an outbreak of Rift Valley fever in Southern Mauritania in 1987, entomologic studies were conducted in a bordering region in Sngal from 1991 to 1996 to identify the sylvatic vectors of Rift Valley fever virus. The virus was isolated from the floodwater mosquitoes Aedes vexans and Ae. ochraceus. In 1974 and 1983, the virus had been isolated from Ae.

D. Fontenille; M. Traore-Lamizana; M. Diallo; J. Thonnon; J. P. Digoutte; H. G. Zeller

1998-01-01

74

Assessing the extent of carbonate deposition in early rift settings  

E-print Network

in this review article are all from East Africa: Lakes Turkana, Bogoria, Natron and Magadi, Manyara on microbialites and tufa, to be expected in early rift settings. Early rift lake examples exam- ined, and Tanganyika. Other lake examples include four from the western United States (Great Salt Lake and high lake

Purkis, Sam

75

Mid-Continent rift system - a frontier hydrocarbon province  

SciTech Connect

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

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

1983-08-01

76

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

77

Crustal structure across the transition from rifting to spreading: the Woodlark rift system of Papua New Guinea  

Microsoft Academic Search

The Woodlark rift system is one of the few places where active ocean basin formation can be studied. Within this rift system, continental extension rates are some of the fastest on the planet, and extension progresses eastwards to full seafloor spreading. We use results from a recent passive seismic experiment to address the role of magmatism prior to the onset

Aaron Ferris; Geoffrey A. Abers; Barry Zelt; Brian Taylor; Steve Roecker

2006-01-01

78

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

79

The Cenozoic evolution of the Roer Valley Rift System integrated at a European scale  

Microsoft Academic Search

The Roer Valley Rift System (RVRS) is located between the West European rift and the North Sea rift system. During the Cenozoic, the RVRS was characterized by several periods of subsidence and inversion, which are linked to the evolution of the adjacent rift systems. Combination of subsidence analysis and results from the analysis of thickness distributions and fault systems allows

Laurent Michon; Ronald T. Van Balen; Olivier Merle; Henk Pagnier

2003-01-01

80

Papers presented to the Conference on the Processes of Planetary Rifting  

NASA Technical Reports Server (NTRS)

The basic problems of processes of planetary rifting are addressed from the following viewpoints: (1) speculation as to the origin and development of rifts; (2) rifts on other planets; (3) tectonics; (4) geology; (5) chemistry of the lithosphere; (6) physics of the lithosphere; and (7) resources associated with rifting. The state of ignorance on the subject and its remedy is debated.

1981-01-01

81

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

82

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

83

The eastern arm of the Midcontinent Rift: Progress and problems  

SciTech Connect

The extent and nature of the Midcontinent Rift System (MCR) was initially determined by potential-field mapping and extrapolation of geologic information from the Lake Superior region. Early interpretation suggested a rift origin which is well supported by deep crustal reflection seismic data and isotopic evidence from the related volcanic rocks that became available during the past decade. A rift origin of the eastern arm of the MCR was corroborated by sub-Phanerozoic drilling into the clastic sediment and volcanic rocks in the McClure-Sparks drill hole located on a massive anticlinal feature in the Precambrian rocks mapped by seismic reflection data. Subsequent seismic profiling further detailed the character of the rift. However, these studies also indicate that the eastern arm is unlike the western, e.g., adjacent clastic rock basins are absent, late-stage compressional features are present, but definite evidence for high-angle reverse faulting is missing, and volcanic basins are not continuous. The termination of this arm of the rift also remains problematic. There is no direct evidence of the rift SE of the McClure-Sparks hole in central Michigan. Geophysical anomalies and deep drilling in the Howell anticline region suggest that the 1,100 Ma old rift is covered by Grenville-age thrusts. If the rift extends farther to the SE, its nature must have been altered by the Grenville orogeny. The hypothesized extension across Ohio east of the Grenville Front is unsupported by seismic reflection profiling and anomaly modeling. Grabens identified at the basement surface in Ohio and to the south are of unknown age, but appear to be more clearly related to late-stage Grenville activity and/or continuation of Eocambrian rifts of the Mississippi Embayment.

Hinze, W.J. (Purdue Univ., West Lafayette, IN (United States). Dept. of Earth and Atmospheric Sciences)

1994-04-01

84

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 dcollement 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

85

Deep seismic sounding in the Turkana depression, northern Kenya Rift  

NASA Astrophysics Data System (ADS)

Existing models of the structure and evolution of the Kenya Rift such as pure shear lithospheric extension, extension by simple shear or rift development by diapiric upwelling of an asthenolith, were based upon surface geology and a few geophysical (mainly gravity and seismic) data sets. Since the knowledge of the lithospheric structure plays an important role to distinguish between these different models, the Kenya Rift International Seismic Project was conducted in 1990 in the area of the Kenya Rift. The project involved a detailed multi-fold refraction wide-angle reflection line in the northern Kenya Rift along the western shore of Lake Turkana which is the most prominent feature of the so-called Turkana depression. Under-water explosions were used as sources and a good signal-to-noise ratio was obtained allowing the identification of arrivals from the crust and upper mantle. The most important result of modelling the data is a crustal thickness of 21 km in the area of Lake Turkana, whereas in the area of the Kenya dome the crustal thickness is 34 km. This points to an extension not previously expected in the northern Kenya Rift. The Moho discontinuity appears as a transition zone in the northern part of the profile, whereas in the southern part it changes to a first-order discontinuity. The largest vertical and lateral heterogeneities are observed in the rift infill displaying basins of varying thickness. The upper and lower crust which are separated by the Conrad discontinuity (the velocity changes from 6.2 to 6.4 km/s here) show very little lateral heterogeneity. The P-wave velocity in the upper mantle was modelled to be 7.7 km/s and thus can be distinguished from that in the southern Kenya Rift, where a velocity of 7.5 km/s was observed. The crustal structure of the graben supports a model of a southerly propagation of the rift.

Gajewski, Dirk; Schulte, Andreas; Riaroh, Don; Thybo, Hans

1994-09-01

86

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

87

Two-stage Early Cretaceous rifting of the Otway Basin margin of southeastern Australia: Implications for rifting of the Australian southern margin  

NASA Astrophysics Data System (ADS)

The Otway Basin of southeastern Australia rifted from Antarctica in two distinct stages in the Early Cretaceous. The first stage began around 140 Ma and ended between 110 and 120 Ma and formed an onshore rift; the rift faults typically detached within basement and were oriented between east-west and east-northeast. On the Crayfish Platform, these first-stage rift structures are preserved. The second stage of rifting at about 95 Ma, just prior to sea-floor spreading, had fault trends between east-southeast and southeast. It was less extensive in area, and the zone of highest extension was localized near the junction with oceanic crust. Geohistory and tectonic analyses indicate that both stages of rifting affected the whole southern margin. In the west, however, the first rift began at about 160 Ma, and first-stage trends persisted during the second rift.

Williamson, P. E.; Swift, M. G.; O'Brien, G. W.; Falvey, D. A.

1990-01-01

88

Teleseismic body wave tomography within a highly extended continental rift: the Woodlark Rift, Papua New Guinea  

NASA Astrophysics Data System (ADS)

The Woodlark Rift, Papua New Guinea, has been a region of westward-propagating continental extension for 6-8 Ma, grading westward from seafloor spreading to newly thinned continent. The D'Entrecasteaux Islands (DIs) lie immediately to the west of the youngest spreading centres in continental crust that has undergone 140-190 km of extension. These islands are dominated by metamorphic core complexes (MCCs) containing 5-6 Ma ultra-high pressure (UHP) coesite-eclogite exhumed at ~20 mm/yr coeval with extension. An array of 31 PASSCAL broadband seismometers and 8 broadband OBSs was installed around the region from 2010-2011 to investigate the thinned continent close to the onset of seafloor spreading. We present results of a teleseismic P- and S- wave tomography study that images the mantle beneath the rapidly extending continent. Preliminary observations include strong azimuthal dependence of differential travel times, indicating significant lateral velocity variations and inferred thermal gradients. Using Ps receiver functions and SsPmP reflections, we estimate variations in Moho depth to correct for the crustal effect on travel times. We observe large (>1s) travel time delays beneath the DIs in both P and S arrivals, while stations on the Trobriand Islands and Papuan Peninsula exhibit travel time deficits of 1-2 s. This indicates that lithosphere is thinnest beneath the DIs, along the axis of the rift, in agreement with the location of Quaternary volcanism and consistent with results from surface waves [Ge et al., AGU2013 abstract] and a previous, lower-resolution tomographic study nearby. There is also evidence for moderately thinned lithosphere in the basin immediately south of the DIs. We have previously established strong, spreading-parallel anisotropy from SKS splitting caused by mantle olivine fabric beneath the DIs and the Trobriand Platform, inferred to represent asthenospheric flow in response to rifting. Detailed tomography will reveal how thinning of mantle lithosphere varies with depth and distance from the rift axis, and possibly other features related to UHP exhumation. We anticipate an improved understanding of the kinematics of lithospheric thinning, and the transition in deep structure between rifted continent and adjacent spreading centers. Differential teleseismic P-wave travel times from cross-correlation of 192 earthquakes between March 2010 and August 2011, with crustal correction applied assuming Vp_crust = 6.2 km/s. Delay times at each station are plotted by back azimuth.

Eilon, Z.; Abers, G. A.; Jin, G.; Kim, Y.; Gaherty, J. B.

2013-12-01

89

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

90

Low lower crustal velocity across Ethiopia: Is the Main Ethiopian Rift a narrow rift in a hot craton?  

USGS Publications Warehouse

[1] The Main Ethiopian Rift (MER) is a classic narrow rift that developed in hot, weak lithosphere, not in the initially cold, thick, and strong lithosphere that would be predicted by common models of rift mode formation. Our new 1-D seismic velocity profiles from Rayleigh wave/receiver function joint inversion across the MER and the Ethiopian Plateau indicate that hot lower crust and upper mantle are present throughout the broad region affected by Oligocene flood basalt volcanism, including both the present rift and the adjacent Ethiopian Plateau hundreds of kilometers from the rift valley. The region of hot lithosphere closely corresponds to the region of flood basalt volcanism, and we interpret that the volcanism and thermal perturbation were jointly caused by impingement of the Afar plume head. Across the affected region, Vs is 3.6-3.8 km/s in the lowermost crust and ???4.3 km/s in the uppermost mantle, both ??0.3 km/s lower than in the eastern and western branches of the East African Rift System to the south. We interpret the low Vs in the lower crust and upper mantle as indicative of hot lithosphere with partial melt. Our results lead to a hybrid rift mode, in which the brittle upper crust has developed as a narrow rift along the Neoproterozoic suture between East and West Gondwana, while at depth lithospheric deformation is distributed over the broad region (??400 km wide) thermally perturbed by the broad thermal upwelling associated with the Afar plume head. Development of both the East African Rift System to the south (in cold, strong lithosphere) and the MER to the north (in hot, weak lithosphere) as narrow rifts, despite their vastly different initial thermal states and depth-integrated lithospheric strength, indicates that common models of rift mode formation that focus only on temperature, thickness, and vertical strength profiles do not apply to these classic continental rifts. Instead, inherited structure and associated lithospheric weaknesses are the primary control on the mode of extension. ?? 2009 by the American Geophysical Union.

Keranen, K.M.; Klemperer, S.L.; Julia, J.; Lawrence, J. F.; Nyblade, A.A.

2009-01-01

91

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

92

Preferential rifting of continents - A source of displaced terranes  

NASA Technical Reports Server (NTRS)

Lithospheric rifting, while prevalent in the continents, rarely occurs in oceanic regions. To explain this preferential rifting of continents, the total strength of different lithospheres is compared 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 three. Also, a thickened crust can halve the total strength of normal continental lithosphere. Because the weakest area acts as a stress guide, any rifting close to an ocean-continent boundary would prefer a continental pathway. This results in the formation of small continental fragments or microplates that, once accreted back to a continent during subduction, are seen as displaced terranes. In addition, the large crustal thicknesses associated with suture zones would make such areas likely locations for future rifting episodes. This results in the tendency of new oceans to open along the suture where a former ocean had closed.

Vink, G. E.; Morgan, W. J.; Zhao, W.-L.

1984-01-01

93

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

TERC. Center for Earth and Space Science Education

2003-01-01

94

Serpentized mantle at rifted margins: The Goban Spur example  

NASA Astrophysics Data System (ADS)

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

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

2002-12-01

95

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

96

The geology and geophysics of the Oslo rift  

NASA Technical Reports Server (NTRS)

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

Ruder, M. E.

1981-01-01

97

Plate-Tectonic Setting of the Mid-Continent Rift  

NASA Astrophysics Data System (ADS)

The 1.1 Ga Mid-Continent Rift (MCR) in North America is often viewed as formed by isolated midplate volcanism and extension. 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. The West and Central African rifts (WCAR), a failed rift system, formed as part of the Early Cretaceous regional extension during the separation of Africa and South America. Once seafloor spreading was established along the entire margin, extension soon stopped within Africa, leaving a failed rift. Shortly afterwards, ~85-80 Ma, motion reversed on some of the normal faults of the WCAR, probably associated with compression due to the collision of Africa with Europe. While it was active, the MCR likely resembled the present East African rift system, which has been extending for about the last 30 my, with one end in the Gulf of Aden and Red Sea having largely evolved to seafloor spreading, while the other has diffuse continental extension. Thus rather than viewing the MCR as an isolated, failed, extensional zone within a stable continent, it may have been part of a larger rift system formed during a short extensional phase within the ~1.3-~0.98 Ga Grenville orogeny, perhaps extending eastward of the Grenville Front. This extensional phase would have ended on the MCR before the continental collisions that produced the Grenville front, the most inland Grenville deformation. Later reverse faulting on the MCR normal faults due to compression, perhaps from collisions around Rodina's margins, would not be unexpected because the MCR would be a relatively weak intraplate zone due to higher crustal temperatures and faults. This reactivation might be similar to the Paleozoic deformation in the central US due to the collisions at the margins of North America.

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

2012-12-01

98

Anatomy of the Midcontinent Rift beneath Lake Superior  

NASA Astrophysics Data System (ADS)

The structure and geometry of the 1.1-b.y.-old Midcontinent Rift system under Lake Superior is interpreted from 20 seismic reflection profiles recorded during the early and mid-1980s. The seismic data reveal that rift basins under Lake Superior are variable in depth and are partially filled with Keweenawan age sediments to depths of 7 km or more and volcanic flows to depths of 36 km. These rift basins form a continuous and sinuous feature that widens in the Allouez Basin and Marquette Basin in the western and central lake and narrows between White Ridge and the Porcupine Mountains. The rift basin bends southeast around the Keweenaw Peninsula, widens to about 100 km as it extends into the eastern half of Lake Superior, and exists the lake with its axis in the vicinity of Au Sable Point in Pictured Rocks National Lake Shore, about 50 km northeast of Munising, Michigan. The axis of the rift may exit the western end of the lake near Chequamegon Bay in Wisconsin. However, lack of data in that area limits interpretation at this time. Prior to late-stage reverse-faulting, a continuous basin of more uniform thickness was present beneath the lake. Crustal extension during rifting of approximately 50 km was followed by plate convergence and crustal shortening of approximately 30 km, with the major component of thrust from the southeast. Crustal shortening occurred after development of rift grabens and their filling with lava flows, but before deposition of the final sag basin sediments. Integration of information obtained from outcrops with data reported here indicates that the Lake Superior section of the rift is associated with as many as three major boundary faults.

Thompson, M. D.; McGinnis, L. D.; Ervin, C. P.; Mudrey, M. G.

99

Lithospheric modification during crustal extension in the Main Ethiopian Rift  

NASA Astrophysics Data System (ADS)

Quaternary lavas erupted in zones of tectonomagmatic extension within the Main Ethiopian Rift (MER) preserve details of lithospheric structure in the East African Rift System. Despite observed source heterogeneity, basalts, trachybasalts, and basaltic trachyandesites erupted in the Wonjii Fault Belt (WFB) and the Silti-Debre Zeyit Fault Zone (SDFZ) form coherent fractionation paths dominated by variable removal of observed phenocryst phases. Crustal assimilation is not widespread, though it is observed at the southern end of the WFB where both fault belts merge; farther north, assimilation of cumulate phases related to fractional crystallization of previous magmas is identified. Shallow fractionation conditions (1 kbar) within the WFB do not change from north to south. In contrast, lavas erupted within the contemporaneous SDFZ fractionate at various crustal depths. These results indicate a better developed magmatic system beneath the WFB where magmas rose quickly before undergoing more significant fractionation at near surface levels and a less developed system beneath the SDFZ. The distribution of magmatism and extant geophysical data indicate thinned crust and a single rift-centered zone of magmatic activity northeast of 830'N, consistent with a transitional lithosphere between continental and oceanic settings. Southwest of 830'N, thicker crust and rift-marginal axes of extension suggest lithosphere with continental affinities. The WFB is propagating southward in response to extension within the Red Sea Rift; the northward propagating SDFZ is related to rifting within the East African Rift System. This region records the unification of two rift systems, requiring care in interpreting the MER as simply transitional between continental and oceanic environments.

Rooney, Tyrone; Furman, Tanya; Bastow, Ian; Ayalew, Dereje; Yirgu, Gezahegn

2007-10-01

100

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

Microsoft Academic Search

The Terror Rift is a zone of post-middle Miocene faulting and volcanism along the western margin of the West Antarctic Rift System. A new seismic data set from NSF geophysical cruise NBP04-01, integrated with the previous dataset to provide higher spatial resolution, has been interpreted in this study in order to improve understanding of the architecture and history of the

Jerome Hall; Terry Wilson; Stuart Henrys

2007-01-01

101

Rifting and volcanism: Examples from volcanic rifted and magma-poor margins based on multichannel seismic data  

NASA Astrophysics Data System (ADS)

Great efforts in the research of passive rifted margins in the last decades highlighted also that lots of open questions remain. A considerable controversy exists about the role of the mantle during rifting and the subsequent formation of oceanic crust and about the interaction of mantle and surface processes, i.e. the precise nature of volcanism in the rifting process. There are two end-member extremes of passive rifted margins. Volcanic rifted margins evolve by a combination of extension, and extensive extrusive flood volcanism over short time periods during breakup, manifested in reflection seismic data as seaward dipping reflectors. These margins are commonly related to mantle plumes; however, in the past years this has been questioned. Magma-poor rifted margins in contrast show wide extensional features as rotated faults blocks and detachment surfaces near the base of the continental crust, but limited magmatism that in addition seems to be delayed to post-breakup. In this presentation examples from three locations that are less frequently cited in the discussion about (either magma-poor or volcanic) rifted margins will be shown: The Laptev Sea margin in the Arctic Ocean, where the active Arctic mid-oceanic ridge meets continental lithosphere at a high angle, the southernmost South Atlantic with well expressed conjugate volcanic rifted margins in a comparably "simple" configuration, potentially influenced by a mantle plume, the Tristan hot spot, and the South China Sea that may represent an intermediary form of continental extension between the end member extremes. The role of (hot-spot related) volcanism during break-up will be discussed for the three example margins that evolved in the Early Cretaceous, the Paleocene and the Oligocene, respectively.

Franke, D.

2012-04-01

102

Rifting, Volcanism, and the Geochemical Character of the Mantle Beneath the West Antarctic Rift System (Invited)  

NASA Astrophysics Data System (ADS)

The West Antarctic Rift System (WARS) is one of the largest extensional alkali volcanic provinces on Earth, but the mechanisms responsible for generating the massive amounts of its associated magmatism remain controversial. The failure of both passive and active decompression melting models to adequately explain the observed lava volumes has prompted debate about the relative roles of thermal plume-related melting and ancient subduction-related flux melting. 40Ar/39Ar dating and geochemical analyses of the lavas, as well as volatile and trace-element determinations of olivine-hosted melt inclusions shed light on the relationship between rifting and volcanism, and also improve our understanding of the geochemical character of the mantle beneath the WARS. Results show that the magmatism post-dates the main phase of extension along the Terror Rift within the WARS, which supports a decompression-melting model without the benefit of a significant thermal anomaly. However, the observed large magma volumes seem to require a volatile-fluxed mantle, a notion supported by a long history of subduction (>500 Myr) along the paleo-Pacific margin of Gondwana. In fact, the legacy of that subduction may manifest itself in the high H2O concentrations of olivine-hosted melt inclusions (up to 3 wt% in preliminary results from ion probe measurements). The major oxide compositions of lavas in the WARS are best matched to experimental melts of garnet pyroxenite and carbonated peridotite sources. The Pb and Nd isotopic systems are decoupled from each other, suggesting removal of fluid-mobile elements from the mantle source possibly during the long history of subduction along this Gondwana margin. Extremely unradiogenic 187Os/188Os ranging to as low as 0.1081 0.0001 hints at the involvement of lithospheric components in generation of magmas in the WARS.

Mukasa, S. B.; Aviado, K. B.; Rilling-Hall, S.; Bryce, J. G.; Cabato, J.

2013-12-01

103

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

SciTech Connect

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

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

1995-03-01

104

Mississippi embayment syncline: A reactivation of the Reelfoot rift zone  

SciTech Connect

Contour maps of the tops of the Paleozoic, Cretaceous, and the Eocene Porters Creek Clay sections were compiled using depth data obtained from oil, gas, and water wells which are located in six states: Tennessee, Arkansas, Mississippi, Missouri, Illinois and Indiana. All these strata are warped into the broad syncline of the Mississippi embayment. An analysis of the structural relations between the Mississippi embayment syncline and the underlying Reelfoot rift zone shows that these two structures are not coaxial; instead, their axes diverge by about 20[degree]. Late Cretaceous and early Tertiary depocenters within the embayment are not located along the rift zone. The known distribution of igneous intrusions within the embayment corresponds better to the embayment synclinal axis than to the rift axis. Therefore the authors infer that the Mississippi embayment may not have formed simply as a result of reactivation of the Reelfoot rift during the late Cretaceous and early Eocene, as was previously suggested. The formation of the Mississippi embayment syncline, its overall shape, and its relative position are probably the result of the interaction of at least two processes: (1) the cooling of Mesozoic magma intrusions, initiating subsidence; and (2) continuous loading due to sediment deposition. The distribution of modern strike-slip seismicity extends along the axis of the Reelfoot rift zone, indicating that the rift has been reactivated as a strike-slip fault system. The youngest strata that were warped into the Mississippi embayment syncline are late Eocene in age. Thus, the latest reactivation of the Reelfoot rift responsible for the present earthquakes must postdate the Late Eocene.

Li, Y. (Memphis State Univ., TN (United States). Center for Earthquake Research and Information); Dart, R.L. (Geological Survey, Denver, CO (United States))

1993-03-01

105

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

E-print Network

Lluvia, Caudal, Calidad dela isla. Red de monitoreo de Lluvia, Caudal, Calidad de Agua, acuíferos, etcAplicaciones de Arc GISAplicaciones de Arc GIS en Recursos de Aguaen Recursos de Agua Alejandra predominantemente eventos de alta intensidad de lluvia.intensidad de lluvia. En promedio un huracán durante los

Gilbes, Fernando

106

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 ~ N100E Nubia-Somalia motion.

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

2011-01-01

107

Refining the footwall cooling history of a rift flank uplift, Rio Grande rift, New Mexico  

NASA Astrophysics Data System (ADS)

Apatite fission track (AFT) and (U-Th)/He data from the Sandia Mountains and Hagan embayment provide new insights into the thermal and tectonic evolution of the eastern flank of the Rio Grande rift in northern New Mexico. AFT and (U-Th)/He data reveal rapid cooling in the Sandia Mountains between 22 and 17 Ma, followed by a decrease in cooling rate at 16 to 14 Ma that temporally corresponds to a hiatus in deposition in the Albuquerque basin. A second increase in cooling rate at approximately 14 Ma was followed by continued slow cooling until present. Cooling ages from Jurassic to Permian sandstones in the Hagan embayment northeast of the Sandia Mountains are used to constrain the thermal conditions in Oligocene time that are necessary to map cooling histories into exhumation histories, thereby providing a limit on the amount of section removed during rift flank development. Thermal modeling, geologic constraints, and low-temperature thermochronology are used to demonstrate that the heat flow in the Sandia Mountain region was at least 25 mW/m2 higher during Oligocene time compared to today. Furthermore, at least 3.1 km of material has been exhumed from the Sandia Mountains and 2.4 km of rock uplift occurred during flexural tilting of the block since middle Miocene time.

House, M. A.; Kelley, S. A.; Roy, M.

2003-10-01

108

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

109

East Antarctic rifting triggers uplift of the Gamburtsev Mountains  

USGS Publications Warehouse

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

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

2011-01-01

110

Quaternary volcanic centres of the Turkana Rift, Kenya  

NASA Astrophysics Data System (ADS)

Three small islands (North, Central and South Island) rise above the surface of Lake Turkana in northern Kenya. They represent the tops of Quaternary volcanic centres aligned along the axis of the Turkana Rift. They are links in the continuous chain of axial volcanic centres that run the length of the Eastern Branch of the East African Rift and represent the most recent additions to the architecture of the Rift. Mapping and petrographic studies reveal that the three island volcanic centres have many similarities in their general stratigraphy, structure, and geologic history; however, there are significant differences in relative lithologic abundances and compositions among them. The island volcanic centres are situated near the centres of each of a series of half-graben that are linked together along the length of the Turkana Rift. Estimates of the volumes of basaltic and intermediate volcanic components coupled with likely fractionation histories suggest that significant mafic-ultramafic intrusive complexes underlie the island volcanic centres. These are probably just the most recent additions to magmatically thickened, but mechanically thinned crust in this section of the East African Rift.

Karson, Jeffrey A.; Curtis, Patchin C.

1994-01-01

111

Remote sensing studies and morphotectonic investigations in an arid rift setting, Baja California, Mexico  

E-print Network

The Gulf of California and its surrounding land areas provide a classic example of recently rifted continental lithosphere. The recent tectonic history of eastern Baja California has been dominated by oblique rifting that began at ~12 Ma. Thus...

El-Sobky, Hesham Farouk

2009-05-15

112

Submarine thermal springs on the Gala??pagos Rift  

USGS Publications Warehouse

The submarine hydrothermal activity on and near the Gala??pagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new oceanic lithosphere at the Gala??pagos Rift in the first million years may be vented from thermal springs, predominantly along the axial ridge within the rift valley. The vent areas are populated by animal communities. They appear to utilize chemosynthesis by sulfur-oxidizing bacteria to derive their entire energy supply from reactions between the seawater and the rocks at high temperatures, rather than photosynthesis. Copyright ?? 1979 AAAS.

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

1979-01-01

113

A groundwater convection model for Rio Grande rift geothermal resources  

NASA Technical Reports Server (NTRS)

It has been proposed that forced convection, driven by normal groundwater flow through the interconnected basins of the Rio Grande rift is the primary source mechanism for the numerous geothermal anomalies along the rift. A test of this concept using an analytical model indicates that significant forced convection must occur in the basins even if permeabilities are as low as 50-200 millidarcies at a depth of 2 km. Where groundwater flow is constricted at the discharge areas of the basins forced convection can locally increase the gradient to a level where free convection also occurs, generating surface heat flow anomalies 5-15 times background. A compilation of groundwater data for the rift basins shows a strong correlation between constrictions in groundwater flow and hot springs and geothermal anomalies, giving strong circumstantial support to the convection model.

Morgan, P.; Harder, V.; Daggett, P. H.; Swanberg, C. A.

1981-01-01

114

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 (N105-110E) and the rift floor (N90-95E), 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

115

Relative Timing of CAMP, Rifting, Continental Breakup, and Basin Inversion: Tectonic Significance  

E-print Network

1 Relative Timing of CAMP, Rifting, Continental Breakup, and Basin Inversion: Tectonic Significance in eastern North America provides a temporal benchmark for assessing the relative timing of rifting, drift, and geochronological data favor a diachronous rift-drift transition (seafloor spreading began earlier in the south

116

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

117

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

118

Mid-Continent rift: new frontier in an old area  

SciTech Connect

The Mid-Continent rift (MCR) is a 2000-km-long intracontinental feature of middle Proterozoic age (1.1 Ga) that extends from Kansas northeastward through the Lake Superior basin and then southeastward through the lower peninsula of Michigan. The authors believe that rift-related marginal basins overlying axial basins and other structures associated with this feature may locally be prospective within four geographically identifiable rift segments: Kansas, Iowa, Minnesota, and Michigan. Source is the most problematic aspect of exploration potential along this structure. Calculations of source rock availability leads us to predict ultimate recoverable reserves of less than 5 billion bbl of oil or 15 tcf of gas alone the MCR. Source beds are probably restricted in areal extent, and they are old (Proterozoic-lower Paleozoic). Both organic and inorganic sources are possible. Adequate volumes of reservoir rock have been identified in all rift segments. The most promising hosts are rift-related clastics of Proterozoic age, or more extensive basal Paleozoic sands that locally occur in marginal basins along and in sheets adjacent to the MCR. Migration is assumed to have occurred under normal hydrodynamic conditions (load) with close spatial proximity between source and reservoir units. Permeability problems related to diagenesis are not anticipated. The authors postulate that both structural and stratigraphic traps probably occur. Fault-related structures vary in style within the various rift segments: rotated, half-graben in Kansas; overthrust and normal in Iowa; and high-angle reverse in Minnesota and Michigan. Unconformities involving Proterozoic and possibly Paleozoic units may occur in all segments. Seals along faulted structures are assumed to be gouge-related and their effectiveness will depend on the abundance and types of clay within individual faults.

Davidson, D.M. Jr.; Mudrey, M.G. Jr.

1986-05-01

119

Petroleum system of the Shelf Rift Basin, East China Sea  

SciTech Connect

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 rocks and reservoired in sandstone at the Pinghu field to the north of the study area provides an known petroleum system analogue. In the Shelf Rift Basin, synrift high-amplitude parallel reflections within the graben axes correlate with coaly siltstone strata and are interpreted as coastal plain and possibly lacustrine facies with source rock potential. Synrift clinoform seismic facies prograding from the northwest footwall correlate with non-marine to marginal marine conglomerate, sandstone and siltstone, and are interpreted as possible delta or fan-delta facies with reservoir potential although porosity and permeability is low within the Wenzhou 6-1-1 well. Post-rift thermal sag sequences are characterized by parallel and relatively continuous seismic reflections and locally developed clinoform packages. These facies correlate with porous and permeable marine sandstone and siltstone. Shales of potential sealing capacity occur within marine flooding intervals of both the synrift and post-rift sequences. Traps consist of differentially rotated synrift fill, and post-rift inversion anticlines. Major exploration risk factors include migration from the synrift coaly source rocks to the post-rift porous and permeable sandstones, and seismic imaging and drilling problems associated with extensive Tertiary igneous intrusions.

Cunningham, A.C.; Armentrout, J.M.; Prebish, M. [Mobil Oil Corp., Dallas, TX (United States)] [and others

1996-12-31

120

Continental rifting and the origin of Beta Regio, Venus  

NASA Technical Reports Server (NTRS)

Topographic maps based on Pioneer Venus altimetry suggest that Beta Regio, an elevated feature centered at 27 deg N, 282 deg E, is analogous to domes associated with continental rift systems on earth. This interpretation is consistent with the commonly quoted analogy between the East African rift system and the topography of the region from Beta Regio southward to Phoebe Regio. If Beta Regio is a dome, major structural uplift of the crust of Venus is implied, suggesting a more dynamic upper mantle than would be the case if Beta Regio were simply a large volcanic construct.

Mcgill, G. E.; Steenstrup, S. J.; Barton, C.; Ford, P. G.

1981-01-01

121

Distribution of Rift Valley fever among cattle in Zambia.  

PubMed

In the present study, 1,421 cattle in 32 herds within nine districts, which are important cattle-producing centers in the nine provinces of Zambia, were tested for Rift Valley fever by the indirect immunofluorescence assay. One hundred and forty-seven cattle (10.5%) in 28 herds (88.9%) in the nine districts tested were positive for Rift Valley fever implying a country-wide distribution. In districts associated with flood plains and/or "dambos" (low lying areas of perpetual flooding), high herd and individual positive rates (100% and > 10%, respectively) were found, suggesting a significance of these features in the distribution of the disease. PMID:9559442

Samui, K L; Inoue, S; Mweene, A S; Nambota, A M; Mlangwa, J E; Chilonda, P; Onuma, M; Morita, C

1997-04-01

122

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

123

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 N95E extension on the axial Wonji faults, a N105E to N110E 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

124

Geothermal measurements in the northern Red Sea: Implications for lithospheric thermal structure and mode of extension during continental rifting  

Microsoft Academic Search

The northern Red Sea is a continental rift in the process of transition from continental to oceanic rifting. We present 191 new heat flow measurements from the northern Red Sea forming three traverses across the water covered portion of the rift. The heat flow across the rift systematically increases from values of about 125 mW\\/m2 seaward of the coasts to

Fernando Martinez; James R. Cochran

1989-01-01

125

Characterization and quantification of the neotectonics of the Roer Valley rift system from high precision digital elevation model analysis  

Microsoft Academic Search

The Roer Valley Rift System (RVRS) is located in the northern prolongation of the Upper Rhine Graben. During the Cenozoic, the RVRS evolution was influenced by two different rift systems situated in the North and in the South (the North Sea rift system and the West European rift, respectively). During the last decades, the intense seismicity revealed the continuous activity

L. Michon; R. T. van Balen

2003-01-01

126

Characterization and quantification of active faulting in the Roer valley rift system based on high precision digital elevation models  

Microsoft Academic Search

The Roer valley rift system (RVRS) is located in the northern prolongation of the upper Rhine Graben. During the Cenozoic, the evolution of the RVRS was influenced by two different rift systems situated in the North and in the South (the North Sea rift system and the West European rift system, respectively). During the last decades, moderate seismicity revealed the

Laurent Michon; Ronald T. Van Balen

2005-01-01

127

Large-scale variation in lithospheric structure along and across the Kenya rift  

USGS Publications Warehouse

The Kenya rift is one of the classic examples of a continental rift zone: models for its evolution range from extension of the lithosphere by pure shear1, through extension by simple shear2, to diapiric upwelling of an asthenolith3. Following a pilot study in 19854, the present work involved the shooting of three seismic refraction and wide-angle reflection profiles along the axis, across the margins, and on the northeastern flank of the rift (Fig. 1). These lines were intended to reconcile the different crustal thickness estimates for the northern and southern parts of the rift4-6 and to reveal the structure across the rift, including that beneath the flanks. The data, presented here, reveal significant lateral variations in structure both along and across the rift. The crust thins along the rift axis from 35 km in the south to 20 km in the north; there are abrupt changes in Mono depth and uppermost-mantle seismic velocity across the rift margins, and crustal thickening across the boundary between the Archaean craton and PanAfrican orogenic belt immediately west of the rift. These results suggest that thickened crust may have controlled the rift's location, that there is a decrease in extension from north to south, and that the upper mantle immediately beneath the rift may contain reservoirs of magma generated at greater depth.

Prodehl, C.; Mechie, J.; Kaminski, W.; Fuchs, K.; Grosse, C.; Hoffmann, H.; Stangl, R.; Stellrecht, R.; Khan, M.A.; Maguire, P.K.H.; Kirk, W.; Keller, G.R.; Githui, A.; Baker, M.; Mooney, W.; Criley, E.; Luetgert, J.; Jacob, B.; Thybo, H.; Demartin, M.; Scarascia, S.; Hirn, A.; Bowman, J.R.; Nyambok, I.; Gaciri, S.; Patel, J.; Dindi, E.; Griffiths, D.H.; King, R.F.; Mussett, A.E.; Braile, L.W.; Thompson, G.; Olsen, K.; Harder, S.; Vees, R.; Gajewski, D.; Schulte, A.; Obel, J.; Mwango, F.; Mukinya, J.; Riaroh, D.

1991-01-01

128

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

PubMed

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 lithosphere during the initial stages of rifting. Here we show that most of the strain during the July-August 2007 seismic crisis in the weakly extended Natron rift, Tanzania, was released aseismically. Deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping. This event provides compelling evidence for strain accommodation by magma intrusion, in addition to slip along normal faults, during the initial stages of continental rifting and before significant crustal thinning. PMID:19079058

Calais, Eric; d'Oreye, Nicolas; Albaric, Julie; Deschamps, Anne; Delvaux, Damien; Dverchre, Jacques; Ebinger, Cynthia; Ferdinand, Richard W; Kervyn, Franois; Macheyeki, Athanas S; Oyen, Anneleen; Perrot, Julie; Saria, Elifuraha; Smets, Benot; Stamps, D Sarah; Wauthier, Christelle

2008-12-11

129

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

130

Seismicity of the northern part of the Kenya Rift Valley  

NASA Astrophysics Data System (ADS)

During the first eight months of 1981 earthquake data were recorded during a passive seismic experiment (KRISP 81) in northern Kenya. An eight station, small aperture, short period seismic array was located on the eastern margin of the Rift at 1.7N, 37.3E. Two single-point, three component stations were also located north and west of the array, forming a triangular network with approximately 150 km length sides. 2329 events were recorded during the 231 days of recording. A preliminary micro-earthquake seismicity map of the central and northern parts of the country has been produced, using a uniform half space velocity model derived from the analysis of apparent velocities, azimuths and P-S times of event arrivals at the small aperture array. Events located within the Rift show a marked north-south linearity extending from Lakes Bogoria and Baringo in the south, into the Sugata Valley to the north. Around the southern part of Lake Turkana the seismicity becomes more diffuse. However, there is little seismic activity associated with the broad zone of splay faulting that exists in northern Kenya. The seismicity observed along the axis of the Rift suggests a continuation to about 2.5N of the tectonic style observed over the apex of the Kenya dome. A relatively quiet zone separates the activity within the Rift from a second, diffuse, north-south zone of seismicity approximately 150 km further to the east.

Pointing, A. J.; Maguire, P. K. H.; Khan, M. A.; Francis, D. J.; Swain, C. J.; Shah, E. R.; Griffiths, D. H.

1985-07-01

131

Environmental assessment of the East African Rift Valley lakes  

Microsoft Academic Search

An assessment of the East African Rift Valley lakes was initiated by the United Nations Environment Programme (UNEP) with funding from Global Environment Facility as part of the Global International Waters Assessment (GIWA). The purpose of GIWA was to produce globally comparable assessments and examine stresses on international waters: marine, coastal and fresh; surface and groundwaters. The assessment of the

Eric O. Odada; Daniel O. Olago; Fred Bugenyi; Kassim Kulindwa; Jerome Karimumuryango; Kelly West; Micheni Ntiba; Shem Wandiga; Peninah Aloo-Obudho; Pius Achola

2003-01-01

132

Potential Effects of Rift Valley Fever in the United States  

Technology Transfer Automated Retrieval System (TEKTRAN)

Rift Valley fever virus (RVFV) has been the cause of disease outbreaks throughout Africa and the Arabian Peninsula, and the infection often results in heavy economic costs through loss of livestock. If RVFV, which is common to select agent lists of the US Department of Health and Human Services and ...

133

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

134

Monitoring Volcanic Gases on Kilauea's East Rift Zone  

USGS Multimedia Gallery

Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course. Instrumentation at this site measures ambient concentration of noxious sulfur...

135

Monitoring Volcanic Gases on Kilauea's East Rift Zone II  

USGS Multimedia Gallery

Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course. Instrumentation at this site measures ambient concentration of noxious su...

136

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 MERthe 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

137

Rift Valley Fever Overview and Recent Developments at USDA  

Technology Transfer Automated Retrieval System (TEKTRAN)

Rift Valley fever (RVF) is a mosquito-borne viral disease with significant health and economic impacts to domestic animals and humans in much of sub-Saharan Africa. Human infections are believed to occur mainly from mosquito bites and from infectious aerosols. The available strategies for protection...

138

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

139

Forecast and Outbreak of Rift valley fever in Sudan, 2007  

Technology Transfer Automated Retrieval System (TEKTRAN)

Background Rift Valley fever (RVF) outbreaks occur during heavy rainfall in various sub-Saharan countries including Kenya, Somalia, and Tanzania and more recently in Saudi Arabia and Yemen. Given the wide geographic and ecological range of RVF virus, it is necessary to monitor large areas for condit...

140

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 Boulgue; Pascal Gente; Jol Rolet; Christophe Coussement; Karl O. Stetter; Robert Huber; Sony Buku; Wafula Mifundu

1993-01-01

141

Seismicity of the Earth 1900-2013 East African Rift  

USGS Publications Warehouse

Rifting in East Africa is not all coeval; volcanism and faulting have been an ongoing phenomenon on the continent since the Eocene (~45 Ma). The rifting began in northern East Africa, and led to the separation of the Nubia (Africa) and Arabia plates in the Red Sea and Gulf of Aden, and in the Lake Turkana area at the Kenya-Ethiopia border. A Paleogene mantle superplume beneath East Africa caused extension within the Nubia plate, as well as a first order topographic high known as the African superswell which now includes most of the eastern and southern sectors of the Nubia plate. Widespread volcanism erupted onto much of the rising plateau in Ethiopia during the Eocene-Oligocene (4529 Ma), with chains of volcanoes forming along the rift separating Africa and Arabia. Since the initiation of rifting in northeastern Africa, the system has propagated over 3,000 km to the south and southwest, and it experiences seismicity as a direct result of the extension and active magmatism.

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

2014-01-01

142

Remote Sensing for Biodiversity Conservation of the Albertine Rift  

E-print Network

and processes is the leading cause of biodiversity loss and extinction at local, regional, and global scales183 10 Remote Sensing for Biodiversity Conservation of the Albertine Rift in Eastern Africa Samuel of biodiversity conservation is understanding how environmental factors influence species abundance

Wang, Y.Q. "Yeqiao"

143

Masirah Graben, Oman: A hidden Cretaceous rift basin  

Microsoft Academic Search

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

W. H. Beauchamp; A. C. Ries; M. P. Coward

1995-01-01

144

Sesin 3 -Sistemas Distribuidos y Aplicaciones Mdicas 89 Escuela de Computacin Universidad Central de Venezuela UCV  

E-print Network

Central de Venezuela UCV #12;II Simposio Científico y Tecnológico en Computación 201290 Escuela de Computación Universidad Central de Venezuela UCV #12;Sesión 3 - Sistemas Distribuidos y Aplicaciones Médicas 91 Escuela de Computación Universidad Central de Venezuela UCV #12;II Simposio Científico y

Coto, Ernesto

145

Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana  

NASA Astrophysics Data System (ADS)

We use aeromagnetic and Bouguer gravity data from the incipient Okavango Rift Zone (ORZ), the surrounding Archean cratons and Proterozoic mobile belts in northwestern Botswana to estimate the depth to the Curie isotherm and the crustal thickness beneath the rift. Imaging the depth to the base of magnetic sources is important in understanding and in constraining the thermal structure of the continental crust in zones of incipient continental rifting where no other data are available to image the crustal thermal structure. Our objective was to determine if there are thermal perturbations within the lithosphere under this young rift that might be suggestive of the presence of magmatic processes associated with rift initiation. The top and bottom of the magnetized crust were calculated using the two dimensional (2D) power-density spectrum analyses and three dimensional (3D) inversion of the total field magnetic data in overlapping windows of 1o x 1o. We found out that the Curie Point Depth (CPD) estimates vary between ~7 and ~23 km. The deepest CPD values (16 to 23 km) occur within the cratons which have lithospheric thickness greater than 200 km. Within the orogenic mobile belts, the CPD estimates range between 15 and 17 km. The shallowest CPD values of 7 - 14 km occur beneath the ORZ. In the northeastern part of ORZ with more developed rift structures, hot springs are known to occur. The shallowest CPD values, high heat flow and thin crust occur within a zone of ~60 km along the southeastern boundary of the ORZ and coincide with a strong NE-trending basement fabric, possibly acting as conduits for upward migration of hot fluids. These observations suggest the existence of a possible thermal anomaly within the lithosphere at shallow crustal depths beneath ORZ. We interpret the thermal anomaly as resulting from an infiltration front of melts/fluids due to ascending asthenosphere. Additional detailed tomographic imaging to be obtained as part of the Project for Rift Initiation, Development and Evolution experiments is needed to confirm this proposition. ..

Leseane, Khumo

146

Seismic structure of the uppermost mantle beneath the Kenya rift  

NASA Astrophysics Data System (ADS)

A major goal of the Kenya Rift International Seismic Project (KRISP) 1990 experiment was the determination of deep lithospheric structure. In the refraction/wide-angle reflection part of the KRISP effort, the experiment was designed to obtain arrivals to distances in excess of 400 km. Phases from interfaces within the mantle were recorded from many shotpoints, and by design, the best data were obtained along the axial profile. Reflected arrivals from two thin (< 10 km), high-velocity layers were observed along this profile and a refracted arrival was observed from the upper high-velocity layer. These mantle phases were observed on record sections from four axial profile shotpoints so overlapping and reversed coverage was obtained. Both high-velocity layers are deepest beneath Lake Turkana and become more shallow southward as the apex of the Kenya dome is approached. The first layer has a velocity of 8.05-8.15 km/s, is at a depth of about 45 km beneath Lake Turkana, and is observed at depths of about 40 km to the south before it disappears near the base of the crust. The deeper layer has velocities ranging from 7.7 to 7.8 km/s in the south to about 8.3 km/s in the north, has a similar dip as the upper one, and is found at depths of 60-65 km. Mantle arrivals outside the rift valley appear to correlate with this layer. The large amounts of extrusive volcanics associated with the rift suggest compositional anomalies as an explanation for the observed velocity structure. However, the effects of the large heat anomaly associated with the rift indicate that composition alone cannot explain the high-velocity layers observed. These layers require some anisotropy probably due to the preferred orientation of olivine crystals. The seismic model is consistent with hot mantle material rising beneath the Kenya dome in the southern Kenya rift and north-dipping shearing along the rift axis near the base of the lithosphere beneath the northern Kenya rift. This implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lithosphere due to stretching in the north.

Keller, G. R.; Mechie, James; Braile, L. W.; Mooney, W. D.; Prodehl, Claus

1994-09-01

147

Two-stage magmatism during the evolution of the transitional Ethiopian rift  

NASA Astrophysics Data System (ADS)

The Ethiopian rift marks the transition between continental rifting and incipient seafloor spreading. The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) included a 400 km-long cross-rift profile with 97 broadband passive seismometers with the aim to investigate the change from mechanical to magmatic extension by defining the lithospheric structure and extent of magmatism beneath the rift. Complimentary studies of P-wave receiver functions, shear-wave splitting and teleseismic earthquake arrival times show that the lithospheric structure is inherently different beneath the north-western rift flank, rift valley and south- eastern rift flank, with contrasting crustal thickness and composition, upper mantle velocity and lithospheric anisotropy. Two stages of magmatic addition are interpreted: 1) a 6--18 km-thick underplate lens at the base of the crust, which probably formed synchronous with an Oligocene flood basalt event (and therefore pre-dates the adjacent rifting by ~20 Myr); and 2) a 20--30 km-wide zone of intense dyking and partial melt, which most likely pervades the entire crust beneath the rift valley and marks the locus of current rift extension. Furthermore, Precambrian collision-related lithospheric fabric is proposed to be the main source of the strong anisotropy that is observed along the entire cross-rift profile, which may be augmented by magmatism beneath the rift. An active, followed by a passive magma-assisted rifting model that is controlled by a combination of far-field plate stresses, the pre-existing lithospheric framework and magmatism is invoked to explain the rift evolution.

Cornwell, D. G.; England, R. W.; Maguire, P. K.; Kendall, M.; Stuart, G. W.

2008-12-01

148

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

149

Crustal structure beneath the Kenya Rift from axial profile data  

NASA Astrophysics Data System (ADS)

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 P n 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 P n 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 P g 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.

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

1994-09-01

150

Rifting and volcanism: The formation of rift-related unconformities and the timing of volcanism in the Laptev Sea rift, the South China Sea, and the southern South Atlantic based on multichannel seismic data  

NASA Astrophysics Data System (ADS)

There are two end-member extremes of passive rifted margins. Volcanic rifted margins evolve by a combination of extension, and extensive extrusive flood volcanism over short time periods during breakup, manifested in reflection seismic data as seaward dipping reflectors. These margins are commonly related to mantle plumes; however, in the past years this has been questioned. Magma-poor rifted margins in contrast show wide extensional features as rotated faults blocks and detachment surfaces near the base of the continental crust, but limited magmatism that in addition seems to be delayed to post-breakup. In this presentation examples from three locations that are less frequently cited in the discussion about (either magma-poor or volcanic) rifted margins will be discussed: The Laptev Sea margin in the Arctic Ocean, where the active Arctic mid-oceanic ridge meets continental lithosphere at a high angle, the southernmost South Atlantic with well expressed conjugate volcanic rifted margins in a comparably "simple" configuration, potentially influenced by a mantle plume, the Tristan hot spot, and the South China Sea that may represent an intermediary form of continental extension between the end member extremes. The idea of a controlling role of (hot-spot related) magmatism during breakup on the rift evolution is questioned for the three margins that evolved in the Early Cretaceous, the Paleocene and the Oligocene, respectively. Instead much more top-down control on the rift evolution and rift-related magmatism than previously thought is suggested.

Franke, D.

2012-12-01

151

Mildly peraluminous high-silica granites in a continental rift: the Drammen and Finnemarka batholiths, Oslo Rift, Norway  

Microsoft Academic Search

The peraluminous Drammen batholith (650 km2) is the largest granite complex within the mainly alkaline province of the Permo-Carboniferous Oslo Rift, and peraluminous to metaluminous granites are also present in the southern part of the otherwise alkaline Finnemarka complex (125 km2). The emplacement of the Drammen granite, and probably most of the other biotite granite complexes, predate the alkaline syenites

Reidar G. Trnnes; Alan D. Brandon

1992-01-01

152

The origin of high bicarbonate and fluoride concentrations in waters of the Main Ethiopian Rift Valley, East African Rift system  

Microsoft Academic Search

Thermal waters in the Main Ethiopian Rift Valley are characterized by high Na, bicarbonate and fluoride concentrations, and near-neutral to alkaline pH. Sodium, bicarbonate and fluoride are positively correlated in the waters. The principal reason for the bicarbonate in the area is the high rate of carbon dioxide outgassing. This, combined with acid volcanics, geothermal heating, low Ca and low

Berhanu Gizaw

1996-01-01

153

Geochronological and geochemical assessment of Cenozoic volcanism from the Terror Rift region of the West Antarctic Rift System  

NASA Astrophysics Data System (ADS)

The work presented in this dissertation explains results from three different methods to determine the relation between tectonism and rift-related volcanism in the Terror Rift region of the West Antarctic Rift System (WARS). Alkaline lavas from seven submarine features, Beaufort Island and Franklin Islands, and several locations near Mt Melbourne were dated by 40Ar/39Ar geochronology and analyzed for elemental and isotopic chemical signatures. Each chapter addresses a different aspect of the hypothesis that the presence of volatiles, primarily H2O or CO2, in the magma source has led to anomalously high volumes of magmatism after rift-related decompressional melting rather than requiring an active mantle plume source. Chapter 2 provides the temporal framework, illustrating that the sampled features range in age from 6.7 Ma to 89 ka, post-dating the main Miocene age phase of Terror Rift extension. Chapter 3 illustrates the traditional enriched elemental and isotopic chemical signatures to support the overall homogeneity of these lavas and previously analyzed areas of the WARS. This chapter also provides a new model for the generation of the Pb isotopic signatures consistent with a history of metasomatism in the magma source. Chapter 4 provides an entirely new chemical dataset for the WARS. The first platinum group element (PGE) abundances and extremely unradiogenic Os isotopic signatures of Cenozoic lavas from Antarctica provide the strongest evidence of melting contributions from a lithospheric mantle source. The combined results from these three studies consistently support the original hypothesis of this dissertation. New evidence suggests that WARS related lavas are not related to a mantle plume(s) as previously proposed. Instead, they are generated by passive, decompressional melting of a source, likely a combination of the asthenospheric and lithospheric mantle, which has undergone previous melting events and metasomatism.

Rilling, Sarah E.

154

Colorado Basin Structure and Rifting, Argentine passive margin  

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

155

Preliminary analysis of SPREE Flexible Array data for lithospheric heterogeneity below the Midcontinent Rift  

NASA Astrophysics Data System (ADS)

The Midcontinent Rift (MCR) is a 1.1 Ga failed rift which now lies in the stable interior of North America. During about 20 Myr of extension and rifting, a volume of igneous rock was produced that is sufficient to form a Red Sea-sized ocean floor. Yet for reasons unknown, rifting ceased before full-fledged seafloor spreading could begin. The area has been stable for at least the past billion years and displays no present-day seismicity. Continental- and regional-scale tomographic models show no velocity anomaly that significantly correlates with the rift, but the used data lack resolution near the MCR. Little evidence seems to have remained of the thin lithosphere that must have allowed the asthenosphere to ascend and melt during rifting. Furthermore, it is unknown how rifting and post-rift stabilization have affected the lithosphere. The Superior Province Rifting Earthscope Experiment (SPREE) seismic array was deployed in April of 2011 to address the apparent absence of anomalies in the lithospheric and sub-lithospheric mantle. beneath the MCR. We present a preliminary analysis of continental S and surface wave train data recorded during the first six months of the SPREE deployment. We then make inferences about the structure and heterogeneity of the lithosphere and asthenosphere below the most-prominent, western arm of the Midcontinent Rift by comparing the observed waveform data to synthetic seismograms for a subset of possible lithosphere-asthenosphere boundary configurations.

Wolin, E.; Van der Lee, S.

2011-12-01

156

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

157

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

158

Contour mapping of relic structures in the Precambrian basement of the Reelfoot rift, North American midcontinent  

USGS Publications Warehouse

A new contour map of the basement of the Reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features. The major features are two asymmetric intrarift basins, bounded by three structural highs, and the rift margins. The basins are oriented normal to the northeast trend of the rift. Two of the highs appear to be ridges of undetermined width that extend across the rift. The third high is an isolated dome or platform located between the basins. The minor features are three linear structures of low relief oriented subparallel to the trend of the rift. Two of these, located within the rift basins, may divide the rift basins into paired subbasins. These mapped features may be the remnants of initial extensional rifting, half graben faulting, and basement subsidence. The rift basins are interpreted as having formed as opposing half graben, and the structural highs are interpreted as having formed as associated accommodation zones. Some of these features appear to be reactivated seismogenic structures within the modem midcontinent compressional stress regime. A detailed knowledge of the geometries of the Reelfoot rift's basement features, therefore, is essential when evaluating their seismic risk potential.

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

1998-01-01

159

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

160

Deformational models of rifting and folding on Venus  

NASA Astrophysics Data System (ADS)

Features of presumed tectonic origin on Venus are reviewed, and lithospheric strength envelopes are derived based on laboratory measurements of the deformational properties of crustal and subcrustal rocks, extrapolated to conditions appropriate to Venus. Models for rifting and folding are developed that use this lithospheric structure and take into account both brittle and ductile yielding as well as finite elastic strength. For both rifting and folding, structures with characteristic widths and spacings are predicted whose size depends on the thickness of the lithosphere, density contrast, and elastic properties of the layer. Finally, the model predictions are compared with the widths and spacings of observed tectonic features, and it is concluded that they are consistent with a relatively strong mantle layer separated from a thin brittle surface layer by a ductile lower crust. These results allow constraints to be placed on the crustal thickness and thermal gradient on Venus.

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

1988-05-01

161

Potential for Autoimmune Pathogenesis of Rift Valley Fever Virus Retinitis  

PubMed Central

Rift Valley Fever (RVF) is a significant threat to human health because it can progress to retinitis, encephalitis, and hemorrhagic fever. The timing of onset of Rift Valley Fever virus (RVFV) retinitis suggests an autoimmune origin. To determine whether RVFV retinitis is associated with increased levels of IgG against retinal tissue, we measured and compared levels of IgG against healthy human eye tissue by immunohistochemical analysis. We found that serum samples from RVFV-exposed Kenyans with retinitis (n = 8) were slightly more likely to have antibodies against retinal tissue than control populations, but the correlation was not statistically significant. Further investigation into the possible immune pathogenesis of RVFV retinitis could lead to improved therapies to prevent or treat this severe complication. PMID:23918215

Newman-Gerhardt, Shoshana; Muiruri, Samuel; Muchiri, Eric; Peters, Clarence J.; Morrill, John; Lucas, Alexander H.; King, Charles H.; Kazura, James; LaBeaud, Angelle Desiree

2013-01-01

162

Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes  

NASA Astrophysics Data System (ADS)

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.

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

2014-01-01

163

Longitudinal strike-slip faults in oceanic rifting: a mesostructural study from western to southeastern Iceland  

NASA Astrophysics Data System (ADS)

Mesostructural analysis carried out in several localities of western, southern-central and southeastern Iceland shows an unexpectedly frequent occurrence of strike-slip faults parallel to, or lying at relatively small angles with, the axis of rifting. Some faults can be interpreted in terms of shear parallel to the rifts, whereas others form conjugate systems referable to rift-parallel compression. Axial shear can be explained by the accommodation of differences in the directions of tectonic extension between different segments of the axial rift system and subordinately by lateral (rift-parallel) displacements between the major lithospheric plates. Axial compression may be attributable to rift-oblique transform shear or to mantle transport parallel to the spreading axis compensating local deficiencies in mantle upwelling. The data regard 1974 fault surfaces, mainly small-scale.

Passerini, P.; Marcucci, M.; Sguazzoni, G.; Pecchioni, E.

1997-01-01

164

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

NASA Astrophysics Data System (ADS)

The ~1.1 Ga Mid-Continent 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 geometry and timing of Precambrian rifting events including the MCR's extension to southwest Alabama along the East Continent Gravity High, southern Appalachian rocks having Amazonian affinities, and recent interpretation of large igneous provinces in Amazonia.

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

2014-05-01

165

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

166

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

167

Seismicity of oceanic and continental riftsa geodynamic approach  

Microsoft Academic Search

Two major kinds of divergent structuresoceanic and intracontinental riftswere compared in principal seismic and tectonic characteristics. First, the role of main components of the mid-oceanic ridges (MOR) was estimated for the whole Earth. We considered two levels of the MOR segmentation. The first-order structures are the segments of MOR between triple junctions and the second-order structures are a transform faults

P. O. Sobolev; D. V. Rundquist

1999-01-01

168

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

169

Crustal structure on the northeastern flank of the Kenya rift  

NASA Astrophysics Data System (ADS)

The KRISP flank line E converges with the Kenya rift at an angle of about 45 and is approximately parallel to the older Anza graben to the north. The depth to the basement is almost zero along the entire onshore part of the profile with higher velocities at the southeastern end indicative of extensive Precambrian gabbroic intrusions in the upper crust. The Moho shallows steadily from about 35 km at the southeastern end of the profile to about 24 km under Lake Turkana. Even though the Moho rises fairly steadily, there is significant heterogeneity in the crust above it. This shows that the extension is unevenly distributed between the upper and the lower crust. The Moho is laminated and variably reflective. Compared to the KRISP cross-line D further south, the crust is unexpectedly thin and shows extension increasing in a northerly direction. This extension is probably not associated with the Anza and Kenya rifting but with the profile's position on the slope of the Kenya dome. The indications are that there is a relatively abrupt change to a 20-km Moho depth near the Lake Turkana Central shotpoint. This change to a mid-rift crustal thickness occurs not at the postulated margin at the southeastern shore of Lake Turkana but at least 50 km further to the northwest. We suggest that the position of this margin may need to be redefined. The P n velocity is quite high at 8.1 km/s. This may indicate either a cold upper mantle or anisotropy. An upper-mantle reflector has been identified between 15 and 20 km below the Moho. It dips gently away from the rift.

Prodehl, C.; Jacob, A. W. B.; Thybo, H.; Dindi, E.; Stangl, R.

1994-09-01

170

CRUSTAL STRUCTURE ON THE NORTHEASTERN FLANK OF THE KENYA RIFT  

Microsoft Academic Search

The KRISP flank line E converges with the Kenya rift at an angle of\\u000a about 45 degrees and is approximately parallel to the older Anza graben\\u000a to the north. The depth to the basement is almost zero along the entire\\u000a onshore part of the profile with higher velocities at the southeastern\\u000a end indicative of extensive Precambrian gabbroic intrusions in the

C. Prodehl; AWB JACOB; H THYBO; E DINDI; R STANGL

1994-01-01

171

Petrological evidence regarding the evolution of the Kenya Rift Valley  

NASA Astrophysics Data System (ADS)

The compositions of mafic volcanic rocks of the Kenya Rift Valley indicate either generation, or final equilibration, of magmas within the lithosphere. The dominant lithologies in the mantle sources are spinel- and garnet-lherzolites and melt generation spanned the spinel-garnet transition. Modally metasomatised sources have been less important, at least in Quaternary times. The sources are depleted relative to the sources of oceanic island basalts and heterogeneous as regards minor and trace element distribution. Variable degrees of partial melting (always < 3%) and retention of minor phases diversify magma compositions. Primary magmas are rare in the rift zone. All major sequences show evidence of extensive polybaric fractionation within the upper mantle and lower crust. Inter-crustal fractionation has commonly been accompanied by assimilation and the development of silica-(over)saturated liquids. Fractionation at high crustal levels has generated a wide range of mugearitic, trachytic and phonolitic magmas. Crustal anatexis has locally resulted in the formation of peralkaline rhyolites. Generation of the enormous volumes of rift volcanics must also have generated huge volumes of mafic and ultramafic cumulates, especially at or near the Moho. It is possible that, in the most magmatically active areas, more than 20% of the thickness of the crust is due to magmatic underplating and crustal intrusion.

Macdonald, Ray

1994-09-01

172

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

173

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

174

Seismological constraints on lithospheric structure beneath rifted margins  

NASA Astrophysics Data System (ADS)

There is considerable variation in both topography and crustal architecture along passive margins worldwide. However, the variations in lithospheric mantle structure are less well studied. This is, perhaps, in part due to the technical challenge for offshore-onshore passive seismology and also the lower resolution obtained in most mantle studies, particularly when compared to detailed imaging of the crust available from reflection seismology. The available large scale observations of mantle structure (predominately from surface waves), and crustal structure (from receiver functions) for the continental region adjacent to the margins are reviewed. Results for Africa and Australia show clear correlations between the mantle structure and the present day topography of the margin, and this relationship is explored from a worldwide perspective. Seismic studies can also provide information on lithospheric thickness at the margin, which can be used as an additional constraint for the thermal modelling of basin structure. In this case the limitations include the depth resolution of the method, and the particular proxy used to extract a lithospheric thickness estimate from a seismic model. Perhaps most importantly, is to remember that these seismological observations tell us only the existing structure. The challenge remains how to decipher whether the present structures relate to inherited pre-rift architecture; to alteration of lithospheric mantle during rifting, or to much later post rift changes associated with separate tectonic events.

Fishwick, Stewart

2014-05-01

175

Rifting and early tectonic evolution of the equatorial Atlantic  

SciTech Connect

The Equatorial Atlantic forms a giant transform connecting the Central and the South Atlantic between northwest Africa and Brazil. Improved onshore mapping and offshore seismic and drilling by Petrobras permitted the differentiation of the following phases in the tectonic evolution of the Equatorial Atlantic: (1) Basaltic volcanism along the eastern and southern margins of the Guyanan Shield during Triassic-Jurassic time as North America separated from Gondwana. (2) During Early Cretaceous time, clockwise rotation of South America caused rifting east of Fortaleza and compression and arching of the basement further west. Subsequently, the rift propagated westward along the whole length of the Equatorial Atlantic. (3) From Albian time on, there has been right-lateral divergent movement between northwest Africa and South America. (4) This movement was interrupted by intense compression in the Late Cretaceous resulting from convergence between southwest Europe and Africa, causing shortening and southward thrusting of the rift sequence. This compression was alleviated by left-lateral movement along the intracontinental Sobral transform.

Szatmari, P.; Zanotto, O.; Francolin, J.; Wolff, S.

1985-01-01

176

Ouachita trough: Part of a Cambrian failed rift system  

NASA Astrophysics Data System (ADS)

Pre-flysch (Cambrian-Mississippian) strata of the Ouachita Mountains of Arkansas and Oklahoma include two main sandstone lithofacies: (1) a craton-derived lithofacies made up largely of mature medium- to coarse-grained quartzose and carbonate detritus and, in some units, sediment eroded from exposed basement rocks and (2) an orogen-derived facies made up mainly of fine-grained quartzose sedimentary and metasedimentary debris and possibly, in lower units, a volcaniclastic component. Paleocurrent and distribution patterns indicate that detritus of facies I in the Benton uplift was derived from north and detritus of facies II throughout the Ouachitas was derived from south and east of the depositional basin. Overall sedimentological results suggest that the Ouachita trough was a relatively narrow, two-sided basin throughout most and probably all of its existence and never formed the southern margin of the North American craton. Regional comparisons suggest that it was one of several basins, including the Southern Oklahoma aulacogen, Reelfoot Rift, Illinois Basin, and Rome trough, that formed as a Cambrian failed rift system 150 to 250 m.y. after initial rifting along the Appalachian margin of the North American craton.

Lowe, Donald R.

1985-11-01

177

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 plumetriple-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 plumetriple-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

178

Rift zone reorganization through flank instability in ocean island volcanoes: an example from Tenerife, Canary Islands  

Microsoft Academic Search

The relationship between rift zones and flank instability in ocean island volcanoes is often inferred but rarely documented. Our field data, aerial image analysis, and 40Ar\\/39Ar chronology from Anaga basaltic shield volcano on Tenerife, Canary Islands, support a rift zoneflank instability relationship. A single rift zone dominated the early stage of the Anaga edifice (~64.5Ma). Destabilization of the northern sector

T. R. Walter; V. R. Troll; B. Cailleau; A. Belousov; H.-U. Schmincke; F. Amelung; P. v. d. Bogaard

2005-01-01

179

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

180

Seismic structure of the Central US crust and shallow upper mantle: Uniqueness of the Reelfoot Rift  

NASA Astrophysics Data System (ADS)

Using seismic surface waves recorded with Earthscope's Transportable Array, we apply surface wave imaging to determine 3D seismic velocity in the crust and uppermost mantle. Our images span several Proterozoic and early Cambrian rift zones (Mid-Continent Rift, Rough Creek Graben-Rome trough, Birmingham trough, Southern Oklahoma Aulacogen, and Reelfoot Rift). While ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences, the Reelfoot Rift is unique in its association with low mantle seismic velocity. Its mantle low-velocity zone (LVZ) is exceptionally pronounced and extends down to at least 200 km depth. This LVZ is of variable width, being relatively narrow (?50 km wide) within the northern Reelfoot Rift, which hosts the New Madrid Seismic Zone (NMSZ). We hypothesize that this mantle volume is weaker than its surroundings and that the Reelfoot Rift consequently has relatively low elastic plate thickness, which would tend to concentrate tectonic stress within this zone. No other intraplate ancient rift zone is known to be associated with such a deep mantle low-velocity anomaly, which suggests that the NMSZ is more susceptible to external stress perturbations than other ancient rift zones.

Pollitz, Fred F.; Mooney, Walter D.

2014-09-01

181

Rift to post-rift evolution of a ``passive'' continental margin: the Ponta Grossa Arch, SE Brazil  

NASA Astrophysics Data System (ADS)

Low-temperature thermochronology was applied at the Brazilian passive continental margin in order to understand and reconstruct the post-rift evolution since the break-up of southwestern Gondwana. Thermochronological data obtained from apatite fission-track analysis of Neoproterozoic metamorphic and Paleozoic to Mesozoic siliciclastic rocks as well as Mesozoic dikes and alkaline intrusions from the Ponta Grossa Arch provided ages between 66.2 (1.3) and 5.9 (0.8) Ma. These data clearly indicate a post-rift reactivation during Late Cretaceous and Paleogene times. Integrating the results of older thermochronological studies, the reactivation of the southeastern Brazilian margin could be described in three main phases related to the rift to post-rift evolution of SE Brazil. Furthermore, the spatial distribution of age data indicates the presence of two age groups: a NE age-group (NE of Curitiba), with ages around 20 Ma and a SW age-group (Curitiba and NW) with ages of around 50 Ma. The change of ages follows the NW-SE trending So Jernimo-Curiva fault zone that can be traced offshore into the southern end of the Santos basin. Within the Santos basin, this lineament ends up to the salt occurrence in the south and seams to play a major role in the structural evolution of the Santos basin and the Rio Grande Rise. Sedimentological studies in the Santos basin evidenced that the transport direction changed in Miocene from WNW to WNW/NNW. During the Oligocene and earlier, the sediments were transported mainly from southeastwards to the direction of the Curitiba area into the Santos basin. Within the Miocene, an additional transport direction from an area north of Curitiba developed.

Franco-Magalhaes, A. O. B.; Hackspacher, P. C.; Glasmacher, U. A.; Saad, A. R.

2010-10-01

182

Mantle deformation during rifting: Constraints from quantitative microstructural analysis of olivine from the East African Rift (Marsabit, Kenya)  

NASA Astrophysics Data System (ADS)

Lherzolitic and harzburgitic peridotite xenoliths from the Marsabit volcanic field of the East-African Rift, Kenya, display a range of porphyroclastic to ultramylonitic textures that record intense deformation and strain localisation during decompression, cooling and exhumation of subcontinental mantle lithosphere during the early stages of continental rifting. Quantitative microstructural analysis by electron backscatter diffraction of both bulk fabrics and intragrain low-angle boundaries have been applied to these xenoliths to establish how deformation has evolved during this exhumation history. Bulk rock fabric analysis indicates the operation of (001)[100] (E-type) as the dominant slip system in the constituent olivines, with only one porphyroclastic xenolith recording the development of the classical (010)[100] (A-type) fabric. A weak E-type fabric is also present within the fine-grained matrix of the mylonitic and ultramylonitic xenoliths. Low-angle boundaries, preserved within individual olivine grains within the different xenoliths indicate the evolution of intra-grain slip systems from (010)[100] (A-slip) and (001)[100] (E-slip) in porphyroclastic peridotites, to dominant (001)[100] (E-slip) in the protomylonite and mylonite and dominant (100)[001] (C-slip) in the ultramylonite. Bulk fabric and intragrain low-angle boundary analyses are therefore concordant and indicate a systematic evolution of olivine slip systems from A-type to E-type to C-type during strain localisation associated with cooling during mantle exhumation. Both approaches confirm the dominant activity of the (001)[100] E-type slip system during rifting. However, the ultramylonites also provide the first evidence of [001] slip in such an environment. The activation of these two slip systems in olivine might be a useful indicator of an extensional rift margin setting in other peridotites.

Kaczmarek, Mary-Alix; Reddy, Steven M.

2013-11-01

183

Postspreading rifting in the Adare Basin, Antarctica: Regional tectonic consequences  

NASA Astrophysics Data System (ADS)

Extension during the middle Cenozoic (43-26 Ma) in the north end of the West Antarctic rift system (WARS) is well constrained by seafloor magnetic anomalies formed at the extinct Adare spreading axis. Kinematic solutions for this time interval suggest a southward decrease in relative motion between East and West Antarctica. Here we present multichannel seismic reflection and seafloor mapping data acquired within and near the Adare Basin on a recent geophysical cruise. We have traced the ANTOSTRAT seismic stratigraphic framework from the northwest Ross Sea into the Adare Basin, verified and tied to DSDP drill sites 273 and 274. Our results reveal three distinct periods of tectonic activity. An early localized deformational event took place close to the cessation of seafloor spreading in the Adare Basin (24 Ma). It reactivated a few normal faults and initiated the formation of the Adare Trough. A prominent pulse of rifting in the early Miocene (17 Ma) resulted in normal faulting that initiated tilted blocks. The overall trend of structures was NE-SW, linking the event with the activity outside the basin. It resulted in major uplift of the Adare Trough and marks the last extensional phase of the Adare Basin. Recent volcanic vents (Pliocene to present day) tend to align with the early Miocene structures and the on-land Hallett volcanic province. This latest phase of tectonic activity also involves near-vertical normal faulting (still active in places) with negligible horizontal consequences. The early Miocene extensional event found within the Adare Basin does not require a change in the relative motion between East and West Antarctica. However, the lack of subsequent rifting within the Adare Basin coupled with the formation of the Terror Rift and an on-land and subice extension within the WARS require a pronounced change in the kinematics of the rift. These observations indicate that extension increased southward, therefore suggesting that a major change in relative plate motion took place in the middle Miocene. The late Miocene pole of rotation might have been located north of the Adare Basin, with opposite opening sign compared to the Eocene-Oligocene pole.

Granot, R.; Cande, S. C.; Stock, J. M.; Davey, F. J.; Clayton, R. W.

2010-08-01

184

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

185

Late Paleoproterozoic rift-related magmatic rocks in the North China Craton: Geological records of rifting in the Columbia supercontinent  

NASA Astrophysics Data System (ADS)

Late Paleoproterozoic (1.84?1.62 Ga) magmatic rocks including dykes/sills/intrusions and volcanic rocks occur throughout the North China Craton (NCC), which is considered to be part of the Columbia supercontinent by ca. 1.9-1.85 Ga. On the basis of petrogeochemical data, these magmatic rocks can be classified into three major magma types: HN (Nb/La > 0.8, Ce/Nb = 1.7?3, (Th/Nb)N = 0.6?1.2), MN (Nb/La = 0.8-0.5, Ce/Nb = 3?5, (Th/Nb)N = 0.9?3.5) and LN (Nb/La < 0.5, Ce/Nb = 5?80, (Th/Nb)N = 1?60). The geochemical variation of the MN and LN rocks can be explained by lithospheric contamination of asthenosphere- (or plume-) derived magmas, whereas the parental magmas of the HN rocks did not undergo, during their ascent, pronounced lithospheric contamination. These magmatic rocks exhibit at least two characteristics: (1) most displaying a spectrum of compositions from mafic to silicic; (2) forming in an intracontinental rift setting. This Late Paleoproterozoic rift-related magmatism is the most distinguishing feature of the rifting of the Columbia supercontinent.

Xia, Linqi; Xia, Zuchun; Xu, Xueyi; Li, Xiangmin; Ma, Zhongping

2013-10-01

186

The timing of uplift, volcanism, and rifting peripheral to the Red Sea: a case for passive rifting?  

USGS Publications Warehouse

Prior to the formation of the Red Sea the northeastern Afro/Arabian continent had low relief and was largely below sea level from the Late Cretaceous to the early Oligocene. The events leading to the formation of the Red Sea followed the sequence (1) alkaline volcanism and rifting beginning about 30-32 Ma affecting a narrow linear zone in the continent, (2) rotational block faulting and detachment faulting, well underway by 25 Ma, (3) gabbro and diorite magmatism, andesite to rhyolite volcanism, and fine-grained nonmarine sedimentation in the rift between 20 and 25 Ma, (4) fine-grained marine sedimentation in the rift as the early shelves started to subside in the middle Miocene, and (5) uplift of the adjacent continents (about 3 km) and subsidence of the shelves (about 4 km) between 13.8 and 5 Ma. The youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the Proterozoic Arabian Shield that range in age from 13.8 to 568 Ma. The youngest of these ages, coupled with the present high relief along the Arabian escarpment and published heat flow measurements, indicate that 2.5-4 km uplift has occurred in the last 13.8 m.y. -from Authors

Bohannon, R.G.; Naeser, C.W.; Schmidt, D.L.; Zimmermann, R.A.

1989-01-01

187

The evolving contribution of border faults and intra-rift faults in early-stage East African rifts: insights from the Natron (Tanzania) and Magadi (Kenya) basins  

NASA Astrophysics Data System (ADS)

In the early stages of continental rifting, East African Rift (EAR) basins are conventionally depicted as asymmetric basins bounded on one side by a ~100 km-long border fault. As rifting progresses, strain concentrates into the rift center, producing intra-rift faults. The timing and nature of the transition from border fault to intra-rift-dominated strain accommodation is unclear. Our study focuses on this transitional phase of continental rifting by exploring the spatial and temporal evolution of faulting in the Natron (border fault initiation at ~3 Ma) and Magadi (~7 Ma) basins of northern Tanzania and southern Kenya, respectively. We compare the morphologies and activity histories of faults in each basin using field observations and remote sensing in order to address the relative contributions of border faults and intra-rift faults to crustal strain accommodation as rifting progresses. The ~500 m-high border fault along the western margin of the Natron basin is steep compared to many border faults in the eastern branch of the EAR, indicating limited scarp degradation by mass wasting. Locally, the escarpment shows open fissures and young scarps 10s of meters high and a few kilometers long, implying ongoing border fault activity in this young rift. However, intra-rift faults within ~1 Ma lavas are greatly eroded and fresh scarps are typically absent, implying long recurrence intervals between slip events. Rift-normal topographic profiles across the Natron basin show the lowest elevations in the lake-filled basin adjacent to the border fault, where a number of hydrothermal springs along the border fault system expel water into the lake. In contrast to Natron, a ~1600 m high, densely vegetated, border fault escarpment along the western edge of the Magadi basin is highly degraded; we were unable to identify evidence of recent rupturing. Rift-normal elevation profiles indicate the focus of strain has migrated away from the border fault into the rift center, where faults pervasively dissect 1.2-0.8 Ma trachyte lavas. Unlike Natron, intra-rift faults in the Magadi basin exhibit primarily steep, little-degraded fault scarps, implying greater activity than Natron intra-rift faults. Numerous fault-associated springs feed water into perennial Lake Magadi, which has no surface drainage input, yet survives despite a high evaporation rate that has created economically viable evaporite deposits. Calcite vein-filled joints are common along fault zones around Lake Magadi, as well as several cm veins around columnar joints that imply isotropic expansion of the fracture network under high pressures of CO2-rich fluids. Our work indicates that the locus of strain in this portion of the EAR transfers from the border fault to the center of the rift basin some time between 3 and 7 million years after rift initiation. This transition likely reflects the evolving respective roles of crustal flexure and magma budget in focusing strain, as well as the hydrothermal fluid budget along evolving fault zones.

Muirhead, J.; Kattenhorn, S. A.; Dindi, E.; Gama, R.

2013-12-01

188

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

USGS Publications Warehouse

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

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

1998-01-01

189

Petrological constraints on the crustal structure under rift zones  

NASA Astrophysics Data System (ADS)

When magmatism occurs in extensional settings, the transfer and release of heat by magma transport and storage may control the thermal structure of the crust and uppermost mantle underlying the rift zone. The composition and mineralogy of the material added to the solid crust during magmatism is also dictated by the conditions of crystallisation. The temperature, pressure and composition of this material controls the physical response of the crust on timescales relevant both for geophysical imaging of the deep structure of rift zones and for the dynamical development of the structure of zones of extension. Examination of the petrology of the products of rift zone magmatism can be used to provide constraints on the depth distribution of crystallisation and the composition of the solid material accreted to the crust and should be a key consideration in geophysical and dynamical investigations of rift zones. A number of complementary petrological techniques have been used to determine crystallisation depths under the rift zones of Iceland. These techniques include: 1) a parametrisation of clinopyroxene-liquid equilibrium; 2) a parametrisation of the composition of liquid in joint equilibrium with the phases olivine, plagioclase and clinopyroxene; 3) assessment of the order of appearance of phases on the liquidus; 4) the relationship between melt CO2 content and pressure. However, the application of each technique is dependent upon a number of assumptions about the achievement of equilibrium between phases and also the interpolation of parametrisations between experimental conditions. Most rift zone eruptions carry large crystals that have formed in the magma storage zone and these crystals are often referred to as phenocrysts, a term that implies equilibrium between the crystals and the melts that carry them to the surface for eruption. However, careful examination of the trace element composition of Icelandic clinopyroxene phenocrysts shows that they are rarely in equilibrium with their carrier liquids. In order to avoid significant systematic errors in estimates of crystallisation depths it is therefore necessary to use both the major and trace element composition of the clinopyroxenes to establish the range of potential equilibrium liquids at the depth of crystallisation and to identify if such liquids correspond to the composition of lavas from the same magmatic system. Existing parametrisations of experimental equilibrium between clinopyroxene and basaltic liquid are prone to large errors at crustal pressures. In order to address this problem, a series of experiments was conducted using a primitive Icelandic basalt composition as a starting material. These experiments indicate that the parametrisations can robustly be used to establish the importance of crystallisation close to the Moho under Iceland, at depths of over 20 km. It is likely that crystallisation also occurs in the middle and lower crust, generating cumulate gabbro material. However, systematic errors in available parameterisations of clinopyroxene-liquid equilibrium hinder barometry over this pressure range. In order to better establish crystallisation conditions for rift zone magmas, further experimental constraints for crustal pressures are required, along with development of more systematic studies of the volatile contents of melt inclusions.

Maclennan, J.; Gaetani, G. A.; Hartley, M. E.; Neave, D.; Winpenny, B.

2012-12-01

190

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 N080E 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

191

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

192

The Mesozoic rift basins of eastern North America: Potential reservoir or Explorationist's folly  

Microsoft Academic Search

Mesozoic rift basins are found on the East Coast of North America from Georgia to Nova Scotia. The basins formed as a result of extensional activity associated with the breakup of Pangaea. The internal geometry of the basins includes a depositional sequence ranging from coarse fanglomerates to fine-grained siltstones and argillites. Since these Mesozoic rift basins were first studied, they

1991-01-01

193

Grenville/MCR 5/30/14 Did the Grenville Orogeny make the Midcontinent Rift fail?  

E-print Network

Grenville/MCR 5/30/14 1 1 Did the Grenville Orogeny make the Midcontinent Rift The Midcontinent Rift (MCR) is traditionally considered to have formed by midplate extension and volcanism from ~1 and faulting in the Lake Superior region, where MCR rocks are exposed and structures have been seismically

Stein, Seth

194

Internal structure and deformation of an accommodation zone in the northern part of the Suez rift  

NASA Astrophysics Data System (ADS)

Detailed structural study of the eastern (onshore) part of the Gharandal accommodation zone that separates the northern (SW-dipping) and central (NE-dipping) half grabens of the Suez rift helpeddecipher the internal structure and deformation of accommodation zones of continental rifts. This 60 km-wide zone is affected by pure normal faulting. The NE-dipping faults of the northern half graben extend southward into the accommodation zone where they interfinger with SW-dipping faults extending from the central half graben. These two sets of rift-parallel faults form several horsts and grabens in the accommodation zone. Areas dipping parallel to the northern or southern half grabens form several intermixing dip domains in the accommodation zone. Smaller-scale accommodation of dip between these dip domains proceeds by the development of rift-parallel folds (twist zones). In contrast to the southern accommodation zone of the Suez rift, the internal structure of the Gharandal accommodation zone is believed to be representative of accommodation zones in regions (a) unaffected by prerift structures lying at high angles to the rift; and (b) experiencing relatively small extension. Accommodation zones in areas having pre-rift structures lying at high angle to the rift have relatively narrow width and are characterized by transverse, strike-slip faults. Strike-slip movement on these faults is related to the torsional strain resulting from the opposite tilt directions and transport of fault blocks of adjacent half grabens.

Moustafa, Adel R.

1996-01-01

195

Multiscale dynamics of a normal fault system: The western rift of Corinth  

E-print Network

Multiscale dynamics of a normal fault system: The western rift of Corinth Pascal Bernard, IPGP S, 2010 - en-echelon large normal fault system dipping north -younger, western rift : higher strain rate Normal faults rooting on a major detachment: - low angle dip of microseismicity - NS localized gradient

Cerveny, Vlastislav

196

Geometry and scaling relations of a population of very small rift-related normal faults  

E-print Network

Geometry and scaling relations of a population of very small rift-related normal faults Roy W normal faults within the Solite Quarry of the Dan River rift basin range in length from a few millimetres and scaling re- lations of a population of exceptionally well exposed, small (L 1.25 m) normal faults within

197

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

E-print Network

The Tethyan plume: geochemical diversity of Middle Permian basalts from the Oman rifted margin H are exposed in Oman within either the Hawasina nappes or the para-autochtonous Arabian platform exposed-related basalts; Middle Permian; Neo-Tethys; Arabian rifted margin; Hawasina Nappes; Oman; Isotopic chemistry 1

Demouchy, Sylvie

198

Lower crustal earthquakes near the Ethiopian rift induced by magmatic processes  

E-print Network

high for brittle failure to occur. Here we present accurately located earthquakes in central EthiopiaLower 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

Brest, Université de

199

Rifting of the plate boundary in North Iceland 1975-1978  

Microsoft Academic Search

A rifting episode started in 1975 on the accreting plate boundary in North Iceland after 100 years of quiescence. Horizontal extension of some 3 m has been observed in the Krafla caldera and the associated 80 km long fissure swarm. The rifting occurs periodically in short active pulses at a few months intervals. Between these active pulses, continuous inflation of

Axel Bjrnsson; Gunnar Johnsen; Sven Sigurdsson; Gunnar Thorbergsson; Eysteinn Tryggvason

1979-01-01

200

Inflation along Kilauea's Southwest Rift Zone in 2006 David Myer a,  

E-print Network

Inflation along Kilauea's Southwest Rift Zone in 2006 David Myer a, , David Sandwell a , Benjamin results showing the first crustal inflation along the southwest rift zone at Kilauea volcano in over 20, and after inflation shows that a swarm of shallow earthquakes (zb5 km) signaled the beginning of the uplift

Sandwell, David T.

201

The origin and geologic evolution of the East Continent Rift Basin  

SciTech Connect

The East Continent Rift Basin (ECRB) is a newly recognized, dominantly sedimentary-volcanic Proterozoic rift basin that apparently represents the southern extension of the Keweenawan Midcontinent Rift. The ECRB extends from central Michigan at least as far south as south-central Kentucky. The inferred age of the rift fill is approximately 1,000 Ma. Evidence supporting a rift origin for the ECRB includes: interbedding of continental flood basalts and felsic volcanics with siliciclastics; sedimentary fill consisting of distal, arid-climate alluvial fan sediments that lack metamorphic lithologies; close proximity and similar lithologic succession to the Keweenawan rift-fill rocks of the Michigan Basin; and inferred marginal block faulting of Granite-Rhyolite Province rocks near the western edge of the ECRB. ECRB evolution is interpreted as follows: (1) formation of Granite-Rhyolite Province rocks (1,500--1,340 Ma); (2) Keweenawan crustal extension and rifting with development of central mafic complexes, emplacement of volcanic rocks, and deposition of siliciclastic fill from eroded marginal Granite-Rhyolite Province tilted fault blocks (ca 1,000 Ma); (3) overthrusting of the Grenville allochthon and associated foreland thrusting and folding of the rift sequence rocks together with deposition of foreland basin sediments (975---890 Ma); (4) Late Proterozoic erosional removal of the foreland basin sediments and interpreted wrench faulting along the Grenville Front (post-975 to pre-570 Ma); and (5) tectonic inversion, with the ECRB area remaining relatively high during major cambrian subsidence in central Kentucky (590--510 Ma).

Drahovzal, J.A. (Univ. of Kentucky, Lexington, KY (United States). Kentucky Geological Survey)

1992-01-01

202

Rifting, recurrent landsliding and Miocene structural reorganization on NW-Tenerife (Canary Islands)  

Microsoft Academic Search

We studied mechanisms of structural destabilization of ocean island flanks by considering the linkage between volcano construction and volcano destruction, exemplified by the composite Teno shield volcano on Tenerife (Canary Islands). During growth, Tenerife episodically experienced giant landslides, genetically associated with rifting and preferentially located between two arms of a three-armed rift system. The deeply eroded late Miocene Teno massif

T. Walter; H.-U. Schmincke

2002-01-01

203

Modern and ancient mineralization in the Salton Trough Rift  

SciTech Connect

The Salton Trough of SW North America is an active continental rift, the landward extension of the divergent tectonics of the Gulf of California. Shallow magmatic heat sources, thick porous sediments, tectonic activity and saline lakes interact to yield a variety of Pleistocene to modern hydrothermal systems. The oldest mineralization, the fish Creek evaporite, is a CASO[sub 4] deposit formed by a pre-rift Tertiary marine incursion. 4--5 million years ago the prograding Colorado River delta bisected the Trough, influencing the character of Pliocene and younger hydrothermal activity. The northern part of the Trough became a closed basin filled intermittently by large freshwater lakes. Along the W margin of the rift lies the Modoc hot spring gold deposit. This deposit occurs at the intersection of a range-front growth fault with fossil lake levels, suggesting paleohydrologic control by ancient lakes. Active geothermal systems within the Trough include low-T systems such as Heber and East Mesa, localized along high-angle faults where shallow groundwaters are conductively heated above basement highs. These blind systems have no surface expression and only moderate geophysical anomalies. High-T (> 250 C) active systems occur in sediment filled pull-apart basins developed over spreading center fragments (e.g., Salton Sea, Brawley, Cerro Prieto). These systems exhibit high heat flow, strong gravity and magnetic anomalies, and often have surface manifestations such as Quaternary volcanoes and thermal features. Many contain hot metalliferous brines that have evolved in the saline lake environment of the northern Trough.

McKibben, M.A. (Univ. California, Riverside, CA (United States). Dept. Earth Sciences)

1992-01-01

204

Stratigraphy, structure, and extent of the East Continent Rift Basin  

SciTech Connect

The proven existence of pre-Mt. Simon sedimentary rocks named the Middle Run Formation in southwestern Ohio led to the establishment of the Cincinnati Arch Consortium, a joint industry-government partnership to investigate the areal extent, nature, and origin of this new unit. Utilizing available well, seismic, and potential-field data, the consortium has shown that the Middle Run was deposited in a Precambrian rift basin, named the East Continent Rift Basin (ECRB). These data indicate the ECRB assemblage consists of a large folded and faulted wedge of interlayered volcanic and sedimentary rocks, unconformably overlain by Cambrian strata. This wedge is estimated to be thickest (up to about 22,000 feet) on the western edge, where it is in fault contact with Grenville Province rocks. To the west, the ECRB may extend as far as central Illinois and postdates the Precambrian Granite-Rhyolite Province rocks. The contact between the ECRB and this older province appears to be in part an angular unconformity and in part block faulted. The northern limit of this basin was not encountered in the study area; this may indicate a connection with the Midcontinent Rift in Michigan. In central Kentucky, the boundary conditions are more complex. It appears that the ECRB is constricted between a large block of the Granite-Rhyolite Province to the west and the Grenville Front on the east. Large Cambrian extensional structures (Rough Creek Graben and Rome Trough) were overprinted on the Granite-Rhyolite and Grenville Provinces. The ECRB may have acted as a stable block between these Cambrian features. The relationships of the ECRB to overlying Paleozoic features may be profound. Indeed, the ECRB may prove to be the reason for the very existence of the Cincinnati and Kankakee Arches.

Wickstrom, L.H. (Cincinnati Arch Consortium and Ohio Geological Survey, Columbus, OH (United States))

1992-01-01

205

Early Continental Rifting of the South China Sea  

NASA Astrophysics Data System (ADS)

Combined two years (2007 and 2008) of OBS and MCS studies in the northern slope of the South China Sea, we suggest that the early rifting, probably during 60 - 30 mabp, is an asymmetrical Atlantic-type continental rifting. The crust thin out from 35-40 km of possible continental crust to about 10-15 km of typical oceanic crust. Along the continent-ocean boundary, we observe an intrusion of the high P-wave velocity (about 7.5-8.0 km/sec). This is possible of mantle exhumation as comparable to other Atlantic-type continental margins. The OBS result is revealed by the gravity data. Along the upper layers of the continental crust as well as the oceanic crust, the MCS and multi-beam bathymetry data show that they are covered by numerous submarine seamounts. This probably relate to a volcanic origin of the Cenozoic sea-floor spreading during 30-15 mabp as mapped by previous magnetic anomalies in this region. The sea-floor spreading spread apart in the central, NW and SW sub-basins with several different episodes. Lack of the deep crustal data in the southern slope of the South China Sea, particularly around the Sprately area, the interpretation is speculative. However, several very large-size atolls (150 - 200 km in diameter), such as the Chen-Ho, Shun-Zu, Chung-Yeh and Chiu-Cheng fringing reefs, are sub-parallel located along the south margins. We interpret that these are the upper portions of the continental rifting. Combined the two tectonic stories in the northern and southern slope of the South China Sea, we believe that it is in consistent with the complicate nature of the South China Sea crust.

Lee, C.; Chiu, M.; Chan, C.

2010-12-01

206

Geometria Diferencial i Aplicacions MAT-GDA/92/1 Lecture notes on KdV hierarchies  

E-print Network

Geometria Diferencial i Aplicacions MAT-GDA/92/1 Lecture notes on KdV hierarchies of Gelfand and Dickey and present the corresponding bihamiltonian structure. Keywords: KdV hierarchy 2 we present the hierarchy of KdV equations in terms of the PDO formalism and construct the infinite

Batlle, Carles

207

Uplift, rifting and related geomorphological evolution of the Ethiopian volcanic province: what do we really know ? (Invited)  

Microsoft Academic Search

The East African Rift System (EARS) is particularly famous because sediments trapped in some of the rift depressions have recorded a major piece of the late Cenozoic mammalian and hominin evolution, as well as associated proxies of climate and environmental changes. The present day landscape along the EARS is spectacular and characterized by axial rift valleys surrounded by variously extended

R. Pik; D. Ayalew; G. Yirgu

2010-01-01

208

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

209

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

210

An epidemic of Rift Valley fever in Egypt  

PubMed Central

During the epidemic of Rift Valley fever (RVF) that occurred in Egypt and other areas of North Africa in 1977, the virus was isolated from various species of domestic animal and rats (Rattus rattus frugivorus) as well as man. The highest number of RVF virus isolates were obtained from sheep; only one isolate was recovered from each of the other species tested, viz. cow, camel, goat, horse, and rat. RVF virus was reisolated from both camel and horse sera, apparently for the first time. PMID:314355

Imam, Imam Z. E.; Karamany, R. El; Darwish, Medhat A.

1979-01-01

211

Endemic transmission of Rift Valley Fever in Senegal.  

PubMed

Rift Valley fever (RVF) is an expanding zoonotic disease transmitted from ruminant to ruminant by Culicidae mosquitoes. In 2004, a longitudinal serological survey was performed on small ruminants in the Ferlo are (Senegal) to study RVF transmission and compared the results with those obtained from the same study in 2003. The results confirm that the disease is endemic and that the spatial transmission of RVF is highly heterogeneous. The virus could be maintained during dry season by transovarian transmission in Aedes vexans. Further studies are needed to improve the understanding of the epidemiological cycle of RVF in this region to implement adapted surveillance measures. PMID:19548898

Chevalier, V; Thiongane, Y; Lancelot, R

2009-12-01

212

Probing the age and temperature of rifting in Afar  

NASA Astrophysics Data System (ADS)

Rifting along the southern part of the Red Sea margin in NE Africa (leading to formation of Afar) has been closely associated with magmatic activity since the initiation of extension at around ~ 25Ma. Numerous active volcanoes are currently found along rift zones here and magma intrusion into the crust has potentially accommodated significant amounts of extension. This extensive present-day volcanism has been linked to elevated mantle temperature, perhaps due to a thermal plume, or as a consequence of passive flow in the mantle beneath the extending lithosphere. Geochemical evidence for basaltic lavas erupted in Afar have been used to suggest that mantle temperatures are in the range 1370 to 1490C, and that the region is currently experiencing late stage rifting. Analysis of changes in shear wave seismic velocities and relative travel time tomography suggests mantle temperatures are within a similar range, yet the region has greater similarities to a young spreading centre. The range in potential temperature estimates is however very large, with different implications for the volcanic history of the region and hence timing of break-up. Rather than focusing a single observable, we use a relatively straight forward model of extension and decompression melting to predict the seismic-velocity and attenuation structure of the asthenosphere and lithosphere, synthetic receiver functions as a result of this seismic structure, crustal thickness as a result of decompression and finally the melt composition. From this combined study we find that melt composition and seismic structure are dependent on both temperature and time. If mantle potential temperature is 1350C then both the seismic structure and melt composition can be matched if the duration of extension is more than 30 Myr. However this is longer than the estimated duration of extension from plate reconstructions, and given the low rate of extension in Afar, this cold model only generates up to 5 km of igneous crust. If mantle potential temperature is 1450C then both the seismic structure and melt composition can be matched after 22 Myr of extension. Furthermore, igneous crustal thickness is in excess of 10 km. Therefore, the most simple explanation is that Afar is a developing continental rift zone where melting is enhanced by a warm mantle.

Armitage, John; Goes, Saskia; Ferguson, David; Hammond, James; Calais, Eric

2014-05-01

213

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

214

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

NASA Astrophysics Data System (ADS)

The configuration of continents we know today is the result of several billion years of active Wilson Cycle tectonics. The rifting of continents and subsequent development of ocean basins is an integral part of long-term planetary-scale recycling processes. The products of this process can be seen globally, and the East African Rift System (EARS) provides a unique view of extensional processes that actively divide a continent. Taken together with the adjoining Red Sea and Gulf of Aden, the EARS has experienced over 40 Ma of volcanism and ~30 Ma of extension. While early (pre-rift) volcanism in the region is attributed to mantle plume activity, much of the subsequent volcanism occurs synchronously with continental rifting. Numerous studies indicate that extension and magmatism are correlated: extension leads to decompression melting while magmatism accommodates further extension (e.g. Stein et al., 1997; Buck 2004; Corti 2012). Evaluation of the entire EARS reveals significant geochemical patterns - both spatial and temporal - in the volcanic products. Compositional variations are tied directly to the melt source(s), which changes over time. These variations can be characterized broadly by region: the Ethiopian plateau and Turkana Depression, the Kenya Rift, and the Western Rift. In the Ethiopian plateau, early flood basalt volcanism is dominated by mantle plume contributions with variable input from lherzolitic mantle lithosphere. Subsequent alkaline shield volcanism flanking the juvenile Main Ethiopian Rift records the same plume component as well as contributions from a hydrous peridotitic lithosphere. The hydrous lithosphere does not contribute indefinitely. Instead, young (< 2 Ma) volcanism taps a combination of the mantle plume and anhydrous depleted lithospheric mantle. In contrast, volcanism in the Kenya Rift and the Western Rift are derived dominantly from metasomatized lithospheric mantle rather than mantle plume material. These rifts lie in the mobile belts flanking the Archean Tanzanian craton, suggesting the lithosphere in these regions has a complex geodynamic history that provides an avenue of weakness for rift development around the craton. In the Tanzanian portion of the Kenya Rift, highly sodic mafic and carbonatitic lavas dominate both the rift valleys and rift shoulders. Mantle xenoliths from this region show petrographic and isotopic evidence for ancient and recent metasomatic events. In the Western Rift, highly potassic mafic lavas dominate the volcanic landscape in the rift valley and rift margins. The lithospheric mantle underlying this region is highly metasomatized, as evidenced by the presence of clinopyroxenite, websterite and glimmerite xenoliths and the absence of peridotite. The highly fusible metasomatized mantle beneath both the Kenya and Western rifts enables lithospheric melting with comparatively minor P-T perturbation. Regardless of which process begot the other, lithospheric thinning promotes melting while volcanism promotes lithospheric weakness and enables rift propagation. When viewed holistically, both mantle plume activity and lithospheric heterogeneities have played a vital role in the initiation and subsequent evolution of the rifting throughout eastern Africa.

Nelson, W. R.; Furman, T.

2013-12-01

215

A morphotectonic study of an extensional fault zone in a magma-rich rift: the Baringo Trachyte Fault System, central Kenya Rift  

Microsoft Academic Search

The Baringo Trachyte Fault System is located within the central Kenya Rift and forms part of a NS-trending linked extensional fault network. This fault system bounds to the west the 8km deep Baringo Basin which itself lies within the axial valley of the central Kenya Rift. It mainly affects a middle Pleistocene trachytic dome (510ka), the so-called Baringo Trachyte (BT).

B Le Gall; J.-J Tiercelin; J.-P Richert; P Gente; N. C Sturchio; D Stead; C Le Turdu

2000-01-01

216

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

NASA Astrophysics Data System (ADS)

The East African Rift (EAR) is an active continental rift and ideal to investigate the processes of rift initiation and the breaking apart of continental lithosphere. Mantle and crust-derived fluids may play a pivotal role in both magmatism and faulting in the EAR. For instance, large quantities of mantle-derived volatiles are emitted at Oldoinyo Lengai volcano [1, 2]. Throughout the EAR, CO2-dominated volatile fluxes are prevalent [3, 4] and often associated with faults (i.e. Rungwe area, Tanzania, [5, 6]). The purpose of this study is to examine the relationship between volcanism, faulting and the volatile compositions, focusing on the central and southern Kenyan and northern Tanzanian section of the EAR. We report our analysis results for samples obtained during a 2013 field season in Kenya. Gases were sampled at fumaroles and geothermal plants in caldera volcanoes (T=83.1-120.2C) and springs (T=40-79.6C and pH 8.5-10) located near volcanoes, intra-rift faults, and a transverse fault (the Kordjya fault, a key fluid source in the Magadi rift) by 4N-NaOH solution-filled and empty Giggenbach bottles. Headspace gases were analyzed by a Gas Chromatograph and a Quadrupole Mass Spectrometer at the University of New Mexico. Both N2/Ar and N2/He ratios of all gases (35.38-205.31 and 142.92-564,272, respectively) range between air saturated water (ASW, 40 and ?150,000) and MORB (100-200 and 40-50). In addition, an N2-Ar-He ternary diagram supports that the gases are produced by two component (mantle and air) mixing. Gases in the empty bottles from volcanoes and springs have N2 (90.88-895.99 mmom/mol), CO2 (2.47-681.21 mmom/mol), CH4 (0-214.78 mmom/mol), O2 (4.47-131.12 mmom/mol), H2 (0-35.78 mmom/mol), Ar (0.15-10.65 mmom/mol), He (0-2.21 mmom/mol), and CO (0-0.08 mmom/mol). Although some of the samples show an atmospheric component, CO2 is a major component in most samples, indicating both volcanoes and springs are emitting CO2. Gases from volcanoes are enriched in CH4 and H2, denoting their sources are reduced (oxygen-poor) magma chambers or hydrothermal systems. 40Ar/36Ar ratios (average of all samples=299.15) are similar to our air standard value (299.654.05), however, some volcanoes (~308.75) and springs (~321.96) have slightly higher ratios. The springs with elevated 40Ar imply that both the intra-rift (Lake Bogoria) and transverse (Lake Magadi) faults are possibly pathways to carry volatiles from deep sources to the surface. In future work, we will carry out wet chemistry and ion chromatography analyses of the NaOH solutions, measure 3He/4He ratios and complete C, N, and S isotope analyses to further constrain fluid sources and migration processes. [1] Fischer et al., 2009, Nature 459. [2] de Moor et al., 2013, EPSL 361. [3] Sawyer et al., 2008, G-cubed 9. [4] Tassi et al., 2009, G-cubed 10. [5] Barry et al., 2013, Chem Geol 339. [6] de Moor et al., 2013, Chem Geol 339.

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

2013-12-01

217

ALVIN investigation of an active propagating rift system, Galapagos 95.5?? W  

USGS Publications Warehouse

ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5?? W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.-1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seafloor spreading center, an extensively faulted and fissured volcanic ridge, develops ~ 10 km (~ 200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed. ?? 1992 Kluwer Academic Publishers.

Hey, R.N.; Sinton, J.M.; Kleinrock, M.C.; Yonover, R.N.; MacDonald, K.C.; Miller, S.P.; Searle, R.C.; Christie, D.M.; Atwater, T.M.; Sleep, N.H.; Johnson, H. Paul; Neal, C.A.

1992-01-01

218

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 Baa 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 Baa do Filipe Complex was affected by a major flank collapse towards the SW, again removing most of the edifice. (4) The remnants of the Baa 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

219

Comparison of the Tanganyika, Malawi, Rukwa and Turkana Rift zones from analyses of seismic reflection data  

NASA Astrophysics Data System (ADS)

Northwest-southeast extension has opened the East African Rift System along two main branches, the Western and Eastern Branches. Rift zones along the Western Branch are marked by narrow lakes floored by thick piles of fluvial clastic and 'pelagic' sediment. Magmatism is restricted to a few small areas in the 'arches' between the lakes. In contrast, rift zones along the Eastern Branch are largely filled with volcanic and volcaniclastic materials and magmatism is generally perceived to be an integral part of the rifting process. In an attempt to sort out the significance and meaning of these and other differences, we have compared multifold seismic data from three Western Branch rift zones (Tanganyika, Rukwa and Malawi) and one Eastern Branch zone (Turkana). The Tanganyika and Malawi Rift Zones are composed of half-graben basins linked in complex ways by accommodation zones which generally trend oblique to the rift axes, and sometimes oblique to the extension direction. Half-grabens alternate basinal polarities where the rift crosses Proterozoic dislocation zones. Complex fault geometries are associated with some accommodation zones; elsewhere faults are almost exclusively planar. Sedimentary fill reaches at least 4-5 km and the section is mostly Cenozoic in age. Patches of Permo-Triassic sedimentary rocks are believed to occur within both rift zones. The Rukwa Rift is a pull-apart zone that connects the northern (Livingstone) basin of Lake Malawi to the Kalemie Basin in central Lake Tanganyika. The entire pull-apart system may be a series of down-to-the-east half-grabens. An accommodation zone develops along a short stretch of the Rukwa Rift, but no full polarity reversal occurs. The break-away faults of the Livingstone, Rukwa and Kalemie basins are essentially coincident with the Proterozoic Rukwa dislocation zone, which sub-parallels the inferred extension direction. Fault geometries in the Rukwa Rift are markedly listric, especially in the pre-Cenozoic section. Sedimentary fill ranges in age from pre-Karroo through Cenozoic and locally exceeds 10 km in thickness. The Turkana Rift is composed of short, linear, NNE-trending normal fault segments that are offset in a left-lateral sense by numerous, NW-SE trending transfer faults, linking facing border fault segments together. The overall trend of the rift zone is oblique to the opening direction, like the Tanganyika and Malawi cases, but the border fault segments are sub-perpendicular. Fault geometries are highly variable, but flower structures associated with transfer faults predominate. Igneous activity is ubiquitous and appears to be localized along the transfer faults. Basin fill reaches 4-5 km in thickness and is dominated by fluvial clastic, volcaniclastic and volcanic materials. The structural differences within the Tanganyika-Rukwa-Malawi system stem mainly from the modifying effects of pre-rift anistropies on strain expressions. Fundamentally, this system is a NW-SE trending series of single-polarity pull-apart basins. At the two ends of the pull-apart zone, the rift is deflected into more N-S trending basins which have a high tendency to alternate polarities along strike. This explanation does not account for the differences in fault forms between the Tanganyika-Malawi (planar) and Rukwa (listric) Rifts. For the time being, we presume these differences arise from systematic differences between Tanganyika-Malawi and Rukwa in the age ranges of the fill and/or the maximum depths of seismic imaging. Rifting in Turkana is profoundly different than in the Tanganyika-Rukwa-Malawi sub-branch and seems to involve a softer, more ductile, more organized style of extension which may be closer to the ideal case. In a thermal sense, rifting has progressed further in Turkana than along the Western Branch zones. This does not preclude original, fundamental difference in the thermal states of two branches.

Rosendahl, Bruce R.; Kilembe, Elias; Kaczmarick, Kurt

1992-10-01

220

Oil exploration in nonmarine rift basins of interior Sudan  

SciTech Connect

In early 1975 Chevron Overseas Petroleum Inc. commenced a major petroleum exploration effort in previously unexplored interior Sudan. With the complete cooperation of the Sudanese Government, Chevron has acquired a vast amount of geologic and geophysical data during the past 9 years. These data include extensive aeromagnetic and gravity surveys, 25,000 mi (40,200 km) of seismic data, and the results of 66 wells. This information has defined several large rift basins which are now recognized as a major part of the Central African rift system. The sedimentary basins of interior Sudan are characterized by thick Cretaceous and Tertiary nonmarine clastic sequences. Over 35,000 ft (10,600 m) of sediment have been deposited in the deepest trough, and extensive basinal areas are underlain by more than 20,000 ft (6100 m) of sediment. The depositional sequence includes thick lacustrine shales and claystones, flood plain claystones, and lacustrine, fluvial, and alluvial sandstones and conglomerates. Those lacustrine claystones which were deposited in an anoxic environment provide oil-prone source rocks. Reservoir sandstones have been found in a wide variety of nonmarine sandstone facies. The extensional tectonism which formed these basins began in the Early Cretaceous. Movement along major fault trends continued intermittently into the Miocene. This deformation resulted in a complex structural history which led to the formation of several deep fault-bounded troughs, major interbasin high trends, and complex basin flanks. This tectonism has created a wide variety of structures, many of which have become effective hydrocarbon traps.

Schull, T.J.

1984-04-01

221

Kinematic problem of oblique rift zones in Iceland  

SciTech Connect

Regional distribution of surficial isochrons on Iceland locally lacks the oceanic-ridge subparallel alignment that is characteristic of the adjoining North Atlantic sea floor. Strikes of Icelandic lava isochrons generally reflect dips imparted by continued lava accumulation, subsidence, and burial. The angle between trends of central volcano-related fissure swarms, dikes, and faults, and regionally-adjacent approximate isochrons, defines the Obliquity Angle (OA). The OA is measured from the isochron to the extensional structures, and may vary from 0/sup 0/ to 90/sup 0/; cw angles are (+), ccw (-). Three large-OA neovolcanic zones are present in Iceland: the WNW-trending zone is central Iceland; the ENE zone along Reykjanes peninsula; and, possibly, the WNW and largely submarine Tjornes Fracture Zone (TFZ). The TFZ shares characteristics of a highly-oblique rift zone and a transform-related shear zone. In north-central Iceland, OA's along 7-11 Ma isochrons vary continuously from +40/sup 0/ to 90/sup 0/ to -50/sup 0/, over 130 km from West to East. As subaerial isochrons on Iceland range up to 90/sup 0/ to spatially-associated extensional structures, isochron trends locally parallel presumed plate-flow lines (N8OW); offshore, isochrons are approximately normal to flow lines. However extension in central Iceland occurs within the long-lived, WNW-trending oblique rift zone, with little or no commensurate extension and volcanism parallel to structural strike to the South and North, constitutes a major kinematic problem.

Jancin, M.

1985-01-01

222

ALVIN-SeaBeam studies of the Sumisu Rift, Izu-Bonin arc  

NASA Astrophysics Data System (ADS)

Bimodal volcanism, normal faulting, rapid sedimentation, and hydrothermal circulation characterize the rifting of the Izu-Bonin arc at 31N. Analysis of the zigzag pattern, in plan view, of the normal faults that bound Sumisu Rift indicates that the extension direction (080 10) is orthogonal to the regional trend of the volcanic front. Normal faults divide the rift into an inner rift on the arc side, which is the locus for maximum subsidence and sedimentation, and an outer rift further west. Transfer zones that link opposing master faults and/or rift flank uplifts further subdivide the rift into three segments along strike. Volcanism is concentrated along the ENE-trending transfer zone which separates the northern and central rift segments. The differential motion across the zone is accommodated by interdigitating north-trending normal faults rather than by ENE-trending oblique-slip faults. Volcanism in the outer rift has built 50-700 m high edifices without summit craters whereas in the inner rift it has formed two multi-vent en echelon ridges (the largest is 600 m high and 16 km long). The volcanism is dominantly basaltic, with compositions reflecting mantle sources little influenced by arc components. An elongate rhyolite dome and low-temperature hydrothermal deposits occur at the en echelon step in the larger ridge, which is located at the intersection of the transfer zone with the inner rift. The chimneys, veins, and crusts are composed of silica, barite and iron oxide, and are of similar composition to the ferruginous chert that mantles the Kuroko deposits. A 1.2-km transect of seven ALVIN heat flow measurements at 3048.5'N showed that the inner-rift-bounding faults may serve as water recharge zones, but that they are not necessarily areas of focussed hydrothermal outflow, which instead occurs through the thick basin sediments. The rift basin and arc margin sediments are probably dominated by permeable rhyolitic pumice and ash erupted from submarine arc calderas such as Sumisu and South Sumisu volcanoes.

Taylor, B.; Brown, G.; Fryer, P.; Gill, J. B.; Hochstaedter, A. G.; Hotta, H.; Langmuir, C. H.; Leinen, M.; Nishimura, A.; Urabe, T.

1990-10-01

223

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

NASA Astrophysics Data System (ADS)

The Mre margin is the result of a long period of rifting events influenced by different tectonic processes. The last extension phase of the Mre 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 Mre 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 Mre volcanic rifted margin and adjacent Jan Mayen corridor. In the proximal domain of the Mre 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 Mre Basin. Our model suggests that the Mre 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

224

Extension on rifted continental margins: Observations vs. models.  

NASA Astrophysics Data System (ADS)

Mapping the signature of extensional deformation on rifted margins is often hampered by thick sedimentary or volcanic successions, or because salt tectonics makes sub-salt seismic imaging challenging. Over the past 20 years the literature is witnessing that lack of mapable faults have resulted in a variety of numerical models based on the assumption that the upper crust takes little or no extensional thinning, while the observed reduction of crustal thickness is taken up in the middle and lower crust, as well as in the mantle. In this presentation two case studies are used to highlight the difference that 3D seismic data may have on our understanding. The small patches of 3D resolution data allow us to get a glance of the 'real' signature of extensional faulting, which by analogy can be extrapolate from one margin segment to the next. In the South Atlantic salt tectonics represents a major problem for sub-salt imaging. The conjugate margins of Brazil and Angola are, however, characterized by pronounced crustal thinning as documented by crustal scale 2D reflection and refraction data. Off Angola the 3D 'reality' demonstrates that upper crustal extension by faulting is comparable to the full crustal, as well as lithospheric thinning as derived from refraction data and basin subsidence analysis. The mapped faults are listric low angle faults that seem to detach at mid crustal levels. 2D seismic has in the past been interpreted to indicate that almost no extensional faulting can be mapped towards the base of the so-called 'sag basin'. The whole concept of the 'sag basin', often ascribed to as crustal thinning without upper crustal deformation, is in fact related to this 'lack of observation', and furthermore, have caused the making of different types of dynamic models attempting to account for this. In the NE Atlantic significant Paleocene extensional faulting is locally seen adjacent to the 50 to more than 200 km wide volcanic cover on each side of the breakup axis. The associated amount of lateral motion on these, mainly listric, normal faults represents several tens of km. These observations contrast with the general lack of observed faults along volcanic margins due to the overall problem with sub-basalt imaging. A variety of models with respect to mode and duration of extension, including narrow and fast breakup, melt generation by small scale convection, and different modes of mantle flow have been suggested. The interesting aspect is that it is all based on features we can't see. Both study areas clearly points towards the importance of improved seismic imaging, a need for revised understanding of strain rates and strain partitioning during rift development, and the necessity of moving from 2D cross section modeling to more realistic 3D spatial distribution of rift elements and subsequent break-up processes. One important aspect is that both volcanic and non-volcanic margins are rifted margins formed by a protracted rift development.

Skogseid, Jakob

2014-05-01

225

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

226

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

227

Pliocene Cinder Cone Chains Adjacent to the Main Ethiopian Rift: Precursors to Rift-based Quaternary Zones of Focused Magmatic Intrusion?  

NASA Astrophysics Data System (ADS)

The Main Ethiopian rift (MER) is the premier example of rifting that is actively extending a continent. A key target in understanding the transition from continental rifting to oceanic spreading are the Quaternary linear magmatic-tectonic belts that extend discontinuously along the rift north of 6N (Wonji Fault Belt and Silti-Debre Zeyit Fault Zone). However, uncertainties remain as to how these zones of focused magmatic intrusion initiate. Possible precursors to the rift floor Quaternary linear magmatic-tectonic belts are preserved in the central MER. Here, Pliocene linear magmatic chains adjacent to the rift on eastern and western plateaus share many of the same morphological features as the later rift-floor magmatic-tectonic belts, and allow for a temporal probe of mantle sources and magmatic plumbing systems of the evolving rift. The Pliocene Akaki belt is located along the western Ethiopian plateau margin adjacent to the rift-floor Quaternary Silti-Debre Zeyit Fault Zone. Magmas within the Akaki belt extend to some of the most primitive recorded in the rift (up to ~14% MgO), and follow an augite-dominated fractionation path that is consistent with crystal fractionation at deep crustal levels. This interpretation is supported by geophysical evidence of melt present in the lower crust in this region. The magmatic plumbing system of the Akaki magmas is inferred to be similar to that of the adjacent Silti-Debre Zeyit Fault Zone. Both magmatic belts are dominated by fractionation at deep levels within the crust, and have abundant megacrysts of augite, feldspar and olivine - evidence of magma stalling. By comparison, lavas from the Wonji Fault Belt - the regionally dominant magmatic belt located adjacent to the eastern rift margin - have fractionated at much shallower levels and have few megacrysts. These data suggest a dominantly spatial control of the magma plumbing systems in the rift, with little temporal changes along the western rift margin during the Pliocene- Quaternary magmatic episodes. The trace element characteristics of more primitive (>7% MgO) Akaki lavas are comparable to other MER magmas, but show slightly elevated values of Ba/Rb and weakly negative K anomalies in primitive-mantle normalized plots. We interpret these characteristics as evidence for small quantities of amphibole in the mantle source of the Akaki lavas. By contrast, minor phlogopite is present in the source of adjacent Quaternary Silti-Debre Zeyit Fault Zone lavas, implying a temporal shift in mantle source lithologies of magmas along the western rift margin. The basis of this temporal shift and the origin of these contrasting source lithologies remains uncertain. While the geochemical signature of amphibole in the source of a magma indicates some contribution from the lithospheric mantle, the origin of this amphibole is equivocal and could be equally derived from modern or ancient metasomatic events. Easily fusible phases such as amphibole and phlogopite may be important in melt generation, and future isotopic characterization of the Akaki lavas will help constrain unresolved questions surrounding the origin of these phases and their role in magmatism in the central MER.

Rooney, T. O.; Yirgu, G.; Zimbelman, J. R.; Grosfils, E. B.; Ramsey, M.; Movsesian, E.

2012-12-01

228

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

229

Transient cracks and triple junctions induced by Cocos-Nazca propagating rift  

NASA Astrophysics Data System (ADS)

The Galapagos triple junction is a ridge-ridge-ridge triple junction where the Cocos, Nazca, and Pacific plates meet around the Galapagos microplate (GMP). On the Cocos plate, north of the large gore that marks the propagating Cocos-Nazca (C-N) Rift, a 250-km-long and 50-km-wide band of NW-SE-trending cracks crosscuts the N-S-trending abyssal hills of the East Pacific Rise (EPR). These appear as a succession of minor rifts, accommodating some NE-SW extension of EPR-generated seafloor. The rifts successively intersected the EPR in triple junctions at distances of 50-100 km north of the tip of the C-N Rift. We proposed a simple crack interaction model to explain the location of the transient rifts and their junction with the EPR. The model predicts that crack locations are controlled by the stress perturbation along the EPR, induced by the dominant C-N Rift, and scaled by the distance of its tip to the EPR (Schouten et al., 2008). The model also predicts that tensile stresses are symmetric about the C-N Rift and thus, similar cracks should have occurred south of the C-N Rift prior to formation of the GMP about 1 Ma. There were no data at the time to test this prediction. In early 2009 (AT 15-41), we mapped an area on the Nazca plate south of the C-N rift out to 4 Ma. The new bathymetric data confirm the existence of a distinctive pattern of cracks south of the southern C-N gore that mirrors the pattern on the Cocos plate until about 1 Ma, and lends support to the crack interaction model. The envelope of the symmetric cracking pattern indicates that the distance between the C-N Rift tip and the EPR varied between 40 and 65 km during this time (1-4 Ma). The breakdown of the symmetry at 1 Ma accurately dates the onset of a southern plate boundary of the GMP, now Dietz Deep Rift. At present, the southern rift boundary of the GMP joins the EPR with a steep-sided, 80 km long ridge. This ridge releases the stress perturbation otherwise induced along the EPR by elastic interaction with the C-N Rift and prevents the formation of minor rifts of the type in the North of the C-N Rift. However, the seafloor displays traces of rifts formed as the Dietz Deep Rift was approaching the EPR. In fact, the present day ridge appears to have originated as one of these minor rifts, probably stabilized by enhanced magma supply from a nearby volcano at the southwestern end of Dietz Deep.

Schouten, H.; Smith, D. K.; Zhu, W.; Montesi, L. G.; Mitchell, G. A.; Cann, J. R.

2009-12-01

230

Lithostratigraphy and source potential of Keweenawan rocks in Mid-Continent rift  

SciTech Connect

In the aftermath of Amoco's deep test in the Mid-Continent rift, exploration moves slowly forward in this potentially vast hydrocarbon province. Important questions are being asked about the source potential of the 1 billion year-old lacustrine shale known only from its presence in the Lake Superior district. Specifically, is there enough source rock distributed in the rift to generate significant reserves Did maturation occur so long ago that subsequent tectonism and erosion preclude its present existence Did thermal activity associated with rifting destroy the source potential in the rift If not, can the authors determine what portions of the rift are most prospective Although few data are available to answer such questions, petrographic studies coupled with stratigraphic and burial history modeling of the Keweenawan source from Michigan offer important information to the explorationists. Results indicate that significant amounts of source rock are likely available throughout the Mid-Continent rift. Additionally, the rift may be partitioned into oil-prone and gas-prone regions.

Yarus, J.M.; Leponard, J.E.; Merisch, M.D.

1989-03-01

231

Lattice-particle simulation of stress patterns in a Rwenzori-type rift transfer zone  

NASA Astrophysics Data System (ADS)

A new 3D spring lattice computer model has been developed and used to calculate the stress-field in the vicinity of a rift transfer zone. The numerical setup is based on the Rwenzori block, a transfer zone in the Western Branch of the East African Rift Valley. The study has two closely related, yet independent aims: primarily to gain insight into the pattern and the causes of the stress field in the Rwenzori area. The second aim is the evaluation of the model itself, based on a comparison of the model results with local geological structures. The simulations calculate the stress in the brittle part of the crust, at the topographic surface and at a depth of 10 km. The model does therefore not rely on the rate of the continental extension, but only on the amount. The generated stress maps display the maximum and minimum principal normal stress. The stress is calculated for a total extension of 7.5 km, a value based on common estimates for the local rift extension. The local stress field is created by the interplay of the plate driven far-field stress and the mechanics of the rift system. The propagation of the rift system leads thus to stress rotations and to characteristic stress patterns. An important mechanism is lithospheric bending caused by rift flank uplift, which reverses the far field stress in rift vicinity. The stress-field resulting from the simulations explains several previously unexplained structural features encountered in the Rwenzori block.

Sachau, Till; Koehn, Daniel; Passchier, Cees

2011-11-01

232

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

233

Curie Point Depth Estimates Beneath the Incipient Okavango Rift Zone, Northwest Botswana  

NASA Astrophysics Data System (ADS)

We investigated the regional thermal structure of the crust beneath the Okavango Rift Zone (ORZ), surrounding cratons and orogenic mobile belts using the Curie Point Depth (CPD) estimates. Estimating the depth to the base of magnetic sources is important in understanding and constraining the thermal structure of the crust in zones of incipient continental rifting where no other data are available to image the crustal thermal structure. Our objective was to determine if there are any thermal perturbations within the lithosphere during rift initiation. The top and bottom of the magnetized crust were calculated using the two dimensional (2D) power-density spectra analysis and three dimensional (3D) inversions of the total field magnetic data of Botswana in overlapping square windows of 1degree x 1 degree. The calculated CPD estimates varied between ~8 km and ~24 km. The deepest CPD values (16-24 km) occur under the surrounding cratons and orogenic mobile belts whereas the shallowest CPD values were found within the ORZ. CPD values of 8 to 10 km occur in the northeastern part of ORZ; a site of more developed rift structures and where hot springs are known to occur. CPD values of 12 to 16 km were obtained in the southwestern part of the ORZ where rift structures are progressively less developed and where the rift terminates. The results suggests possible thermal anomaly beneath the incipient ORZ. Further geophysical studies as part of the PRIDE (Project for Rift Initiation Development and Evolution) project are needed to confirm this proposition.

Leseane, K.; Atekwana, E. A.; Mickus, K. L.; Mohamed, A.; Atekwana, E. A.

2013-12-01

234

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

NASA Astrophysics Data System (ADS)

Observation of deep structures related to break-up processes at volcanic passive margins (VPM) is often a troublesome exercise: thick pre- to syn-breakup seaward-dipping reflectors (SDR) usually mask the continent-ocean boundary and hide the syn-rift tectonic structures that accommodate crustal stretching and thinning. Some of the current challenges are about clarifying 1) if tectonic stretching fits the observed thinning and 2) what is the effect of continuous magma supply and re-thickening of the crust during extension from a rheological point of view? The Afar region in Ethiopia is an ideal natural laboratory to address those questions, as it is a highly magmatic rift that is probably close enough to breakup to present some characteristics of VPM. Moreover, the structures related to rifting since Oligocene are out-cropping, onshore and well preserved. In this contribution, we present new structural field data and lavas (U-Th/He) datings along a cross-section from the Ethiopian Plateau, through the marginal graben down to the Manda-Hararo active rift axis. We mapped continent-ward normal fault array affecting highly tilted trapp series unconformably overlain by tilted Miocene (25-7 Ma) acid series. The main extensional and necking/thinning event took place during the end of this Miocene magmatic episode. It is itself overlain by flat lying Pliocene series, including the Stratoid. Balanced cross-sections of those areas allow us to constrain a surface stretching factor of about 2.1-2.9. Those findings have the following implications: - High beta factor constrained from field observations is at odd with thinning factor of ~1.3 predicted by seismic and gravimetric studies. We propose that the continental crust in Central Afar has been re-thickened by the emplacement of underplated magma and SDR. - The deformation in Central Afar appears to be largely distributed through space and time. It has been accommodated in a 200-300 km wide strip being a diffuse incipient plate boundary until the formation of present-day magmatic segments. - The difference in tectono-magmatic style between Central Afar (distributed extension and thick crust) and Northern Afar, i.e. Erta Ale segment (narrow graben, thin crust) may be explained by the difference of magma volume (extruded & underplated) brought to the crust during extension. Magma supply in Central Afar allows the crust to be stretched without subsequent thinning despite high degree of extension. - Presence or absence of thinned crust does not necessarily announce break-up. It may occur in both Central and Northern Afar, depending upon a sudden change in magmatic regime. The striking difference between the two tectono-magmatic styles of Central and Northern Afar are probably due to a combination of: 1) magma supply that affects both crustal thickness and rheology, 2) the amount of extension that may be higher in Central Afar, 3) the distance to the magmatic province, and 4) the presence of an early syn-rift transfer/transform between the two segments that might have controlled the distribution of magmatic activity.

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

2014-05-01

235

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

236

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

NASA Astrophysics Data System (ADS)

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

Tesfaye, Samson; Ghebreab, Woldai

2013-11-01

237

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

238

Distribution of contemporary crustal deformation and mechanisms for extension in the Woodlark Rift: insights from GPS  

NASA Astrophysics Data System (ADS)

The Woodlark Rift, southeastern Papua New Guinea, is a classic example of a rift transitioning from continental rifting to seafloor spreading, and is also the site of exhumation of the world's youngest Ultra-High Pressure (UHP) terranes. Prior to now, very little GPS data existed to constrain the kinematics of contemporary rifting, and the relationship of modern-day rifting to exhumation of the young UHP terranes. We present results from GPS campaign measurements at ~45 sites throughout the southeastern Papua New Guinea region, from GPS campaigns conducted in 2009, 2010, and 2012. Our results suggest that most of the modern-day extensional deformation has shifted southward towards the north coast of the PNG mainland, away from the locus of UHP exhumation in the D'Entrecasteaux Islands, although a few mm/yr of active extension remains in the region of UHP rock exhumation. This is consistent with modelling studies that predict a shift in the locus of extension away from the locus of UHP exhumation during the final, waning stages of UHP exhumation. Rates of total extension in the Woodlark Rift increase from west to east from several mm/yr (in the far western Woodlark Rift) to >20 mm/yr further east, due to clockwise rotation of microplates in the region about nearby poles of rotation. We will discuss the implications that our kinematic modelling of the GPS data, earthquake slip vector data, and geological data have for the large-scale driving mechanisms behind rifting in southeast PNG. Our results favour a model where rapid microplate rotation (at 2-3 degrees/Myr) and rifting in the Woodlark Basin is a consequence of strong slab pull forces from extremely rapid subduction (6-13 cm/yr) at the New Britain and San Cristobal trenches further to the north.

Wallace, L. M.; Ellis, S. M.; Tregoning, P.; Little, T. A.; Palmer, N.

2012-12-01

239

Distributed deformation ahead of the Cocos-Nazca Rift at the Galapagos triple junction  

NASA Astrophysics Data System (ADS)

The Galapagos triple junction is not a simple ridge-ridge-ridge (RRR) triple junction. The Cocos-Nazca Rift (C-N Rift) tip does not meet the East Pacific Rise (EPR). Instead, two secondary rifts form the link: Incipient Rift at 240?N and Dietz Deep volcanic ridge, the southern boundary of the Galapagos microplate (GMP), at 110?N. Recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the GMP (?1.5 Ma). South of C-N Rift a band of northeast-trending cracks cuts EPR-generated abyssal hills. It is a mirror image of a band of cracks previously identified north of C-N Rift on the same age crust. In both areas, the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the EPR spreading center and cut eastward into pre-existing topography. Each crack formed a short-lived triple junction until it was abandoned and a new crack and triple junction initiated nearby. Between 2.5 and 1.5 Ma, the pattern of cracking is remarkably symmetric about C-N Rift providing support for a crack interaction model in which crack initiation at the EPR axis is controlled by stresses associated with the tip of the westward-propagating C-N Rift. The model also shows that offsets of the EPR axis may explain times when cracking is not symmetric. South of C-N Rift, cracks are observed on seafloor as old as 10.5 Ma suggesting that this triple junction has not been a simple RRR triple junction during that time.

Smith, Deborah K.; Schouten, Hans; Zhu, Wen-lu; Montsi, Laurent G. J.; Cann, Johnson R.

2011-11-01

240

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

241

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 4.60.3 mm/yr. While, the Nubia and Arabia plates are moving with 330.15mm/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

242

The mode of rifting of the Tyrrhenian Sea  

NASA Astrophysics Data System (ADS)

The back-arc evolution of the Tyrrhenian Sea has mainly been attributed to the roll-back towards the south-east of the subducting Ionian plate that could have provided the bulk of the space required for this extension. The Tyrrhenian Sea is a triangular basin characterized by two large bathyal basins (Vavilov and Marsili basins) that are covered by some hundred meters of sediments, and a number of peri- Tyrrhenian basins filled by thousands of meters of clastic and/or volcaniclastic sediments. The stratigraphic record of these basins offers an opportunity to study the timing and kinematics of the basin-forming faults that are relevant for the creation of a model on the opening of the Tyrrhenian Sea. Basin analysis was performed using interpretation of seismic reflection profiles and well logs. The interpretation of these data was made using seismic and sequence stratigraphy and structural geology in a GIS-dedicated environment. The sequence stratigraphy interpretation of the deepest wells were performed using discontinuities and trends in wireline log pattern. Systems tracts and transgressive-regressive cycles were identified in well log succession and seismic profiles. The sequence stratigraphy approach allows the identification of 4th-order depositional sequences (100 ka). The geologic evolution, in terms of age of basin formation, style of deformation, timing of activity of the fault bounding basins, tectonic subsidence, post-rift infill and volcanic activity, was analyzed for several peri-Tyrrhenian basins. The study reconstructed the three-dimensional architecture of the peri-Tyrrhenian basins and illustrated the link between the bathyal basin and the Tyrrhenian margin. We document that during the evolution of the Tyrrhenian region several basins opened contemporaneously with different direction of extension and a progressive change in rifting direction occurred along the Campania Margin. The mode of rifting of the Tyrrhenian Sea was characterized by different styles of the extensional basins and detachment faulting. Taking into account the published geological data, we propose a kinematic evolution of the Tyrrhenian basin over the last 12 Ma.

Milia, Alfonsa; Torrente, Maurizio M.

2014-05-01

243

A Comparison of Embryonic and Mature Volcanic Passive Margins. Main Ethiopian Rift vs North Atlantic Margins.  

NASA Astrophysics Data System (ADS)

The North Atlantic margins and the Main Ethiopian Rift system are considered, on the basis of geochemical and geophysical data, to record rifting of continental lithosphere above a mantle plume. Such rift systems are frequently referred to as volcanic passive margins. Rifting in the NE Atlantic occurred c.55 Ma ago and in Ethiopia it began between 15 and 18 Ma ago and continues to the present day. Consequently, the Main Ethiopian rift and its extension into the Afar volcanic province is considered to represent an embryonic passive margin system in which magmatic processes associated with the initiation of ocean spreading are beginning to dominate over tectonic rifting. Present day magmatism is concentrating in elongate en-echlon zones that have developed at the centre of the tectonic rift system in the last 2 Ma both at the surface and in the upper and mid crust. In contrast the older margins of the North Atlantic have stabilised following post-rift subsidence. Their structure is dominated by extended crust intruded by large bodies of magmatic underplate and overlain by seaward dipping reflectors. Subsequent burial and the problems of sub-basalt imaging mean that detailed structure is obscured A comparison between the embryonic structure of the MER and the older/developed NE Atlantic margin yields a stark contrast in lithospheric structure, particularly in the volume of magmatic rock identified. This suggests that an acceleration in the rate of magma production must accompany the actual break-up in order for the Main Ethiopian rift to develop into a full volcanic margin. Currently, there is very little magmatism in the Main Ethiopian rift but it has been argued that this is sufficient for break-up to proceed. If this is the case it is likely that a margin more closely similar to non-volcanic margins would result. Alternatively, the large pulse of magmatism, such as is recorded in the North Atlantic is either the result of rifting above a mantle plume, meaning that the plume plays only a small role in the initial stages of rifting, or there was an acceleration of magmatic activity in the North Atlantic that coincided with the late stage of break- up.

England, R.; Cornwell, D.; Maguire, P.

2006-12-01

244

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

245

Rift Valley Fever during Rainy Seasons, Madagascar, 2008 and 2009  

PubMed Central

During 2 successive rainy seasons, January 2008 through May 2008 and November 2008 through March 2009, Rift Valley fever virus (RVFV) caused outbreaks in Madagascar. Human and animal infections were confirmed on the northern and southern coasts and in the central highlands. Analysis of partial sequences from RVFV strains showed that all were similar to the strains circulating in Kenya during 20062007. A national cross-sectional serologic survey among slaughterhouse workers at high risk showed that RVFV circulation during the 2008 outbreaks included all of the Malagasy regions and that the virus has circulated in at least 92 of Madagascars 111 districts. To better predict and respond to RVF outbreaks in Madagascar, further epidemiologic studies are needed, such as RVFV complete genome analysis, ruminant movement mapping, and surveillance implementation. PMID:20507747

Andriamandimby, Soa Fy; Randrianarivo-Solofoniaina, Armand Eugne; Jeanmaire, Elisabeth M.; Ravololomanana, Lisette; Razafimanantsoa, Lanto Tiana; Rakotojoelinandrasana, Tsanta; Razainirina, Josette; Hoffmann, Jonathan; Ravalohery, Jean-Pierre; Rafisandratantsoa, Jean-Thophile; Rollin, Pierre E.

2010-01-01

246

Modeling the Spatial Spread of Rift Valley Fever in Egypt  

PubMed Central

Rift Valley fever (RVF) is a severe viral zoonosis in Africa and the Middle East that harms both human health and livestock production. It is believed that RVF in Egypt has been repeatedly introduced by the importation of infected animals from Sudan. In this paper, we propose a three-patch model for the process by which animals enter Egypt from Sudan, are moved up the Nile, and then consumed at population centers. The basic reproduction number for each patch is introduced and then the threshold dynamics of the model are established. We simulate an interesting scenario showing a possible explanation of the observed phenomenon of the geographic spread of RVF in Egypt. PMID:23377629

Gao, Daozhou; Cosner, Chris; Cantrell, Robert Stephen; Beier, John C.; Ruan, Shigui

2013-01-01

247

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

248

A model for the structure, composition and evolution of the Kenya rift  

NASA Astrophysics Data System (ADS)

The seismic refraction-wide-angle reflection experiments carried out in 1985 and 1990 in the Kenya rift (KRISP '85 and KRISP '90) show major crustal thickness variations both along and across the rift. Along the rift axis crustal thickness varies from 35 km in the south beneath the Kenya dome to 20 km in the north beneath the Turkana region. Due to the distribution of crustal thickness beneath the rift flanks, it can be stated that the major amount of variation in crustal thickness along the rift axis is due to the Tertiary rifting episode. The northwards decrease in crustal thickness can be correlated with changes in surface topography (northwards decrease), rift width (northwards increase), surface estimates of extension (5-10 km in the south and 35-40 km in the north) and Bouguer gravity, the regional northwards increase of which can be explained entirely by the change in crustal thickness. Below the 750 km long axial rift profile, uppermost mantle P n velocities are low, being 7.5-7.7 km/s. However, under the northern part of the rift two layers with velocities of 8.1 km/s and 8.3 km/s are embedded in the low-velocity mantle material at 40-45 km and 60-65 km depth, respectively. In contrast, the wide-angle data show that beneath the Kenya dome, in the southern part of the rift, low mantle velocities occur down to at least 65 km depth. This mantle velocity structure is indicative of the depth to the onset of melting being at least 65 km beneath the northern part of the rift and thus not being shallower than the depth (45-50 km) to the onset of melting under the Kenya dome to the south. A profile across the rift north of the Kenya dome at the latitude of Lake Baringo shows that the low uppermost mantle P n velocity of 7.5-7.7 km/s and crustal thinning of 5-10 km is confined to below the surface expression of the rift. An abrupt change in Moho depths and P n velocities occurs as the rift boundaries are crossed. Beneath the rift flanks, normal P n velocities of 8.0-8.2 km/s occur. The presence of hot mantle material beneath the Kenya dome since the onset of volcanism here at 15-20 Ma is still compatible with the abrupt change in mantle P-wave velocities as the rift boundaries are crossed. Petrological interpretation of the seismic velocities indicates a few (up to 5) percent basaltic melt in the mantle below the rift except in the two layers with velocities greater than 8.0 km/s under the northern part of the rift where some crystal orientation (anisotropy) is necessary. Below about 45-50 km depth beneath the southern part of the rift the magma could exist as in situ partial melt. The above results, taken together with results from teleseismic studies, petrology and surface geology, indicate anomalously hot mantle material appearing below the present site of the Kenya rift at about 20-30 Ma. The active uprising of this anomalously hot mantle material since this time has given rise to widespread volcanism along the whole length of the rift and has modified the crust beneath the rift by mafic igneous underplating and intrusion, especially into the basal crustal layer. Accompanying the uprise of the anomalously hot mantle material minor crustal extension (5-10 km) has occurred beneath the Kenya dome in the southern part of the rift where crustal thickness is large (35 km). Under the Turkana region in the northern part of the rift, a greater amount of extension (35-40 km) has taken place and the crustal thickness is small (20 km), although the depth to the onset of melting under the northern part of the rift is, if anything, greater than under the southern part of the rift.

Mechie, J.; Keller, G. R.; Prodehl, C.; Khan, M. A.; Gaciri, S. J.

1997-09-01

249

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

250

Climatic influence on rift structure: Axial rift evolution in Guaymas Basin during the past 250,000 years  

NASA Astrophysics Data System (ADS)

Newly collected, high-resolution seismic profiles in the Southern Trough of Guaymas Basin display temporal changes in axial graben structure; ages of these structures correlate to glacial-interglacial cycles over the past quarter millennium. During glacial periods the southern spreading trough in Guaymas Basin is overwhelmed with turbidites, whereas during interglacials, extension out-paces sedimentation and the trough deepens. Glacial low-stand shedding--caused by lower sea level, shelf exposure, and enhanced precipitation--results in the erosion of shelf sediments and transportation of these to the deeper basins in the Gulf. Major offshore canyon systems directly southwest of the Southern Trough capture eroded sediments from the exposed continental shelf and funnel these sediments into the axial graben as turbidites. In contrast, the higher sea level and reduced precipitation characteristic of interglacials limits the introduction of terrestrial sediments into the axial trough; sedimentation during these periods predominately consists of redistributed pelagic sediments swept into the trough by tidal and bottom currents. These sediments, dominated by diatomaceous ooze, appear as acoustically transparent layers in seismic profiles, whereas turbidites composed of fluvial and reworked shelf material appear as high-amplitude reflector packages. Systematic changes between the two sediment types are apparent on seismic profiles within the Southern Trough of Guaymas Basin as well as Carmen Basin. In addition, the regional extent of the youngest acoustically transparent layer imaged with multichannel seismic profiling approximately matches modern distribution and deposition patterns of sediments dominated by diatomaceous ooze, further supporting our interpretation that the acoustically transparent layers represent sediment packages composed mainly of pelagic material. Plotting paleo-temperature and sea-level curves along the MCS profile crossing the Southern Trough shows a striking correlation between climatic cycles, sedimentation rate, acoustic character of sediment packages, and changes in rift structure. Off-axis intrusion of young sills masks past trough floor structures resulting in only the paleo-rift flanks being imaged on seismic profiles. Assuming a constant spreading rate of 45 km/My and the relationship between different sediment packages and changes in climate, sediments imaged on off-axis flanks in the Southern Trough of Guaymas Basin record the evolution of the axial graben and flanks during the past 250,000 years. The relationship between climatic change and rift structure observed in the Southern Trough, Guaymas Basin may provide insight into how sedimentation and axial structure of a young intercontinental ocean basin have been modulated by climate change.

Kluesner, J. W.; Lonsdale, P.; Kirtland, S. E.

2009-12-01

251

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.

252

Style of extensional tectonism during rifting, Red Sea and Gulf of Aden  

USGS Publications Warehouse

Geologic and geophysical studies from the Arabian continental margin in the southern Red Sea and LANDSAT analysis of the northern Somalia margin in the Gulf of Aden suggest that the early continental rifts were long narrow features that formed by extension on closely spaced normal faults above moderate- to shallow-dipping detachments with break-away zones defining one rift flank and root zones under the opposing rift flank. The rift flanks presently form the opposing continental margins across each ocean basin. The detachment on the Arabian margin dips gently to the west, with a breakaway zone now eroded above the deeply dissected terrain of the Arabian escarpment. A model is proposed in which upper crustal breakup occurs on large detachment faults that have a distinct polarity. -from Author

Bohannon, R.G.

1989-01-01

253

Heat flow in the Nipigon arm of the Keweenawan rift, northwestern Ontario, Canada  

E-print Network

by Keweenawan diabase sills. It has been interpreted as a failed arm of the ca. 1100 Ma Keweenawan rift. Six new diabase sills ca. 1109 Ma [Davis and Sutcliffe, 1985; Sutcliffe, 1991]. Prior to the Keweenawan event

Long, Bernard

254

Modeling suggests that oblique extension facilitates rifting and continental break-up  

E-print Network

simulta- neously in a rift zone, but propagates in a certain direction, as was the case for the opening by efficient heating, and by mechanical strength reduction [Buck, 2007]. The impingement of man- tle plumes

Kaus, Boris

255

Workshop on The Rio Grande Rift: Crustal Modeling and Applications of Remote Sensing  

NASA Technical Reports Server (NTRS)

The elements of a program that could address significant earth science problems by combining remote sensing and traditional geological, geophysical, and geochemical approaches were addressed. Specific areas and tasks related to the Rio Grande Rift are discussed.

Blanchard, D. P. (editor)

1980-01-01

256

Potential for North American Mosquitoes (Diptera: Culicidae) to Transmit Rift Valley Fever Virus  

Technology Transfer Automated Retrieval System (TEKTRAN)

To determine which biting insects should be targeted for control should Rift Valley fever virus (RVFV) be detected in North America, we evaluated Culex erraticus, Culex erythrothorax, Culex pipiens, Culex quinquefasciatus, Culex tarsalis, Aedes dorsalis, Aedes vexans, Anopheles quadrimaculatus, and ...

257

Three-Dimensional (3D) Structure of the Malawi Rift from Remote Sensing and Geophysics Data  

NASA Astrophysics Data System (ADS)

The Malawi rift is a Cenozoic aged rift representing the southernmost segment of the Western Branch of the East African Rift System (EARS). This rift extends over 900 km from the Rungwe volcanic province (Tanzania) in the north to the Urema graben (Mozambique) to the south, with an average width of 50km. It traverses a complex array of Proterozoic orogenic belts of different ages and Permo-Triassic (Karoo) and cretaceous graben systems. The rift's depth is between 3 to 5km partitioned between the topographic escarpment and the sediments fill. The basin's subsidence reflects accumulation of sediments and rift flank uplift. Regardless of its importance in understanding rift tectonics, especially in Africa, the three-dimensional (3D) geometry of the rift is not fully understood. This research presents results from detailed analysis of Digital Elevation Model (DEM) extracted from the Shuttle Radar Topography Mission (SRTM) data to map surface morphological expressions of the entire basin. These results are compared with available seismic data to provide along-strike and at depth variation of the geometry of the border fault systems, nature of rift segmentation and alternation of the polarity of half-grabens, and the partitioning of displacement between exposed and sub-surface border faults. Our results show the following: (1) Surface expression of border faults show that, unlike the typical half-graben en-echelon rift model, where half-graben segments with opposite polarity are linked together through accommodation zones indicative of soft linkage, the Malawi rift shows along-strike segmentation by changing geometry from half-graben to full graben geometry. A half-graben with specific polarity passes through a full-graben geometry before giving place to a half-graben with the opposite polarity. The length of half-gaben and graben segments becomes shorter as the rift progresses from north to south, and this is accompanied by a decrease in displacement within border faults. This geometry is indicative of the propagation of border faults through hard linkage. (2) The continuation of border faults at the subsurface show patterns consistent with those observed at the surface. At the sub-surface, the general trend of rift segmentation, formation of full grabens at the end of each segment, and the decreases in the length of the segments from north to south is consistent with observations at the surface. This suggests the homogeneity of strain accommodation throughout the depth of border faults. (3) Zones of segmentation of the Malawi rift coincide with regions where the pre-existing structures (both the Proterozoic basement and the Karoo grabens) are at high angle to the trend of the rift whereas well-developed border faults of the basin coincides with N-trending pre-existing structures sub-parallel to the rift.

Salmi, Haifa S. Al; Abdelsalam, Mohamed G.

2014-05-01

258

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

259

Inflation rates, rifts, and bands in a p?hoehoe sheet flow  

USGS Publications Warehouse

The margins of sheet flowsp?hoehoe lavas emplaced on surfaces sloping Inflation and rift-band formation is probably cyclic, because the pattern we observed suggests episodic or crude cyclic behavior. Furthermore, some inflation rifts contain numerous bands whose spacing and general appearances are remarkably similar. We propose a conceptual model wherein the inferred cyclicity is due to the competition between the fluid pressure in the flow's liquid core and the tensile strength of the viscoelastic layer where it is weakestin inflation rifts. The viscoelastic layer consists of lava that has cooled to temperatures between 800 and 1070 C. This layer is the key parameter in our model because, in its absence, rift banding and stepwise changes in the flow height would not occur.

Hoblitt, Richard P.; Orr, Tim R.; Heliker, Christina; Denlinger, Roger P.; Hon, Ken; Cervelli, Peter F.

2012-01-01

260

East Antarctic Rift Systems - key to understanding of Gondwana break-up  

NASA Astrophysics Data System (ADS)

The results of analysis of radio-echo sounding surveys, the RADARSAT satellite data, magnetic and gravity information give evidence that East Antarctica contains 13 riftogenic systems and/or large linear tectonic structures. Among known and suggested rifts of East Antarctica the Lambert rift has a pivotal position and it manifests oneself as symmetry axis. Six additional systems are revealed on both sides of it and any one of them possesses special features in geologic and geomorphologic aspects. In most cases they inherited the anisotropy of long-lived cratonic blocks. Riftogenic and/or large linear tectonic structures along the East Antarctica coastal regions are distributed with a steady regularity with average distance between them about 650 km. For six (7) structures from 13 (Lambert, Jutulstraumen-Penckskket, Vestfjella, Mellor-Slessor (Bailey), Wilkes Basin, Gaussberg (?) and Rennick) there is a distinct spatial coupling with trough complexes of the Beacon Supergroup and their subsequent reactivation in Late Jurassic - Permian time when the East Gondwana started break-up. Rift system of the Lambert-Amery Glaciers and Prydz Bay is related to Mesozoic extension events and it inherited structures of Paleozoic grabens. The total length of the rift system exceeds 4000 km of the same scale as largest the World rift belts. The length of the western branch of the Lambert rift that includes the Mellor rift and graben-like structures of the Bailey and Slessor glaciers exceeds 2300 km. Results of radio-echo sounding investigation of the subglacial Aurora Basin allow to suggest that this large basin of sub-meridian extension is underlain by an extensive (> 1000 km) riftogenic structure that is running towards the Transantarctic Mountains where it forms a triple junction with the eastern branch of the Lambert rift and structures of the Wilkes Basin. It is hereby proposed that Aurora-Scott rift is formed by complex system of sub-parallel depressions divided by fragmentary horsts. The spatial correlation of the Aurora-Scott rift system, Permian basins of the Western Australia margin and coal-bearing basins in Rajmahal Hills allows suggesting that this East Antarctic structure was also formed during Permian time and about the existence of triple junction rift systems (Aurora-Scott, Perth, Rajmahal) in the pre-breakup Gondwana.

Golynsky, D. A.; Golynsky, A. V.

2012-04-01

261

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

262

Rift to Post-rift evolution of a "passive" continental margin: The Ponta Grossa Arch, SE Brazil  

NASA Astrophysics Data System (ADS)

Low-temperature thermochronology was applied at the Brazilian passive continental margin in order to understand and reconstruct the post-rift evolution since the break-up of southwestern Gondwana. Thermochronological data obtained from apatite fission-track analysis of Neoproterozoic metamorphic and Paleozoic to Mesozoic siliciclastic rocks as well as Mesozoic dikes from the Ponta Grossa Arch provided ages between 66.2 (1.3) and 5.9 (0.8) Ma. These data clearly indicate a post-rift reactivation during the Late Cretaceous and Paleogene. Integrating the results of older thermochronological studies, the reactivation of the southeastern Brazilian margin could be described in three main phases. Furthermore, the spatial distribution of age data indicate a NE-age group (NE of Curitiba) of about 20 Ma and a SW-age group (Curitiba and NW) of about 50 Ma. The change of ages follows the NW-SE trending So Jernimo-Curiva fault zone that can be traced offshore into the southern end of the Santos basin. Within the Santos basin these lineament terminates the salt occurrence in the south. It seams to play a major role in the structural evolution of the Santos basin and the Rio Grande Rise. Sedimentological studies in the Santos basin evidenced that the transport direction changed in Miocene time. During the Oligocene and earlier the sediments were transported mainly from the direction of the "Curitiba area" into the Santos basin. Within the Miocene an additional transport direction from an area north of Curitiba developed.

Franco-Magalhaes, Ana. O. B.; Hackspacher, Peter C.; Glasmacher, Ulrich A.; Saad, A. R.

2010-05-01

263

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

264

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

265

Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data  

USGS Publications Warehouse

We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore-offshore large-aperture seismic and gravity data. High velocities obtained within a highly reflective half-graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide-angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (5560km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high-velocity lower crust interpreted to result from syn-rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half-graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre-rift Moho is probably within or above the high-velocity lower crust, whereas the post-rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.

Trehu, Anne M.; Morel-a-l'Huissier, Patrick; Meyer, R.; Hajnal, Z.; Karl, J.; Mereu, R. F.; Sexton, J.; Shay, J.; Chan, W. K.; Epili, D.; Jefferson, T.; Shih, X. R.; Wendling, S.; Milkereit, B.; Green, A.; Hutchinson, Deborah R.

1991-01-01

266

Rift-Drift, Seafloor Spreading, and Subduction Tectonics of Albanian Ophiolites  

Microsoft Academic Search

Albanian ophiolites in the Dinaride-Albanide-Hellenide mountain system in the Balkan Peninsula represent oceanic remnants of the Mesozoic Pindos-Mirdita basin and structurally overlie the peripheral tectonic units, composed mainly of volcanosedimentary rift assemblages and conjugate passive margin sequences. These tectonic units and the ophiolites collectively record the rift-drift, seafloor spreading, and subduction evolution of the Pindos-Mirdita basin, which developed as a

Yildirim Dilek; Minella Shallo; Harald Furnes

2005-01-01

267

Geology of the Malawi Rift: kinematic and tectonosedimentary background to the Chiwondo Beds, northern Malawi  

Microsoft Academic Search

Kinematic analysis of Neogene and Quaternary faults demonstrates that the direction of extension in the Malawi Rift rotated from east-northeast to southeast. Rift development commenced at about 86 Ma with normal faulting causing erosion and re-sedimentation of Mesozoic deposits and formation of half-grabens bounded by northwest-, north-, and northeast-striking normal faults. Continued normal faulting led to block faulting and basin

Uwe Ring; Christian Betzler

1995-01-01

268

Deformation during the 1975-1984 Krafla rifting crisis, NE Iceland, measured from historical optical imagery  

NASA Astrophysics Data System (ADS)

We measure the displacement field resulting from the 1975-1984 Krafla rifting crisis, NE Iceland, using optical image correlation. Images are processed using the COSI-Corr software package. Surface extension is accommodated on normal faults and fissures which bound the rift zone, in response to dike injection at depth. Correlation of declassified KH-9 spy and SPOT5 satellite images reveals extension between 1977-2002 (2.5 m average opening over 80 km), while correlation of aerial photos between 1957-1990 provide measurements of the total extension (average 4.3 m opening over 80 km). Our results show 8 m of opening immediately north of Krafla caldera, decreasing to 3-4 m at the northern end of the rift. Correlation of aerial photos from 1957-1976 reveal a bi-modal pattern of opening along the rift during the early crisis, which may indicate either two different magma sources located at either end of the rift zone (a similar pattern of opening was observed in the 2005 Afar rift crisis in East Africa), or variations in rock strength along the rift. Our results provide new information on how past dike injection events accommodate long-term plate spreading, as well as providing more details on the Krafla rift crisis. This study also highlights the potential of optical image correlation using inexpensive declassified spy satellite and aerial photos to measure deformation of the Earth's surface going back many decades, thus providing a new tool for measuring Earth surface dynamics, e.g. glaciers, landsliding, coastal erosion, volcano monitoring and earthquake studies, when InSAR and GPS data are not available.

Hollingsworth, James; Leprince, SBastien; Ayoub, FranOis; Avouac, Jean-Philippe

2012-11-01

269

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

Microsoft Academic Search

Mooney, W.D., Andrews, M.C., Ginzburg, A., Peters, D.A. and Hamilton, R.M., 1983, Crustal structure of the northern Mississippi Embayment and a comparison with other continental rift zones. In: P. Morgan and B.H. Baker (Editors), Processes of Continentat Rifting Tecronophysics, 94: 327-348. Previous geological and geophysical investigations have suggested that the Mississippi Embayment is the site of a Late Precambrian continental

W. D. MOONEY; M. C. ANDREWS; A. GINZBURG; D. A. PETERS; R. M. HAMILTON

1983-01-01

270

Further remarks on the interpretation of the KRISP 90 cross-rift line  

Microsoft Academic Search

During the Kenya Rift International Seismic Project (KRISP 90) a 450-km east-west seismic profile was shot across the rift in the vicinity of the Equator. Reflectivity modelling of some of the P- and S-phases combined with results derived from the ray-trace forward model enable more information to be extracted from the data set. The Pg phase at the western end

R. Masotti; P. K. H. Maguire; J. Mechie

1997-01-01

271

On rates and styles of late volcanism and rifting on Venus  

NASA Astrophysics Data System (ADS)

We investigated the possibility of significant variations in the rates and styles of volcanism and rifting during the time postdating the formation of regional plains on Venus. We analyzed the age relations of all known impact craters >=30 km in diameter (183 craters) with the neighboring geologic units. Of these we selected 164 craters which were superposed on regional plains and determined if post-regional-plains (PRP) volcanics and/or PRP rift structures were present in the crater vicinity and if these craters postdated the volcanics and rift structures or predated them. In 53 cases it was possible to determine these relations. On the basis of these relationships, it was found that the general rates of volcanism and rifting during PRP time were close to constant or at least had no drastic changes. This implies a significant change in the rates of volcanism and rifting in the vicinity of the boundary between PRP time and the preceding time, which is marked by the formation of the global wrinkle-ridge network. It was also found that (1) the role of rift-associated volcanism during PRP time was close to constant or slightly decreasing, (2) the role of corona-related volcanism was noticeably decreasing, and (3) the role of noncorona, hot spot volcanism was proportionally increasing. The latter changes may be due to thickening of the lithosphere during PRP time. The results imply also that at most if not all places where PRP rifting occurred, the rift-associated fracturing, when started, continued for a significant part of PRP time. Our conclusions are valid both (1) for the case of a globally synchronous transition from emplacement of regional plains to the PRP regime and (2) for the case of such a transition occurring at different times in different areas of Venus.

Basilevsky, Alexander T.; Head, James W.

2002-06-01

272

East African rift and northeast lineaments: continental spreadingtransform system?  

NASA Astrophysics Data System (ADS)

Seafloor spreading and transform faulting processes are also likely to be operative during continental rifting events. Continental lines of old weakness oriented at high angles to the direction of continental rifting may be reactivated by transform faulting. These older continental transform faults, which predate and accomodate the rifting, will continue to propagate as younger oceanic transform faults as the rift develops into seas and oceans. This model is applied to the East African Rift which is postulated to be a continental spreading rift that is accommodated by east-northeast continental transform lineaments that are reactivated older crustal defects of appropriate orientation. At least five continental transform lineaments can be tentatively identified by empirical best fits to oceanic transform directions of the South Atlantic Ocean and to various continental African northeast-trending structures: (1) Cape Town-Maputo (CT-LM); (2) Orange River-Beira (OR-B); (3) Luderitz-Lindi (L-Li); (4) Walvis Bay-Mombasa-Mogadishu (WB-M-Mo); and (5) Luanda-Afar (Lu-Af). As these postulated lineaments are perennial deep seated crustal defects they may also control the development of mineral deposits.

Katz, M. B.

273

Structural evolution of the southern transfer zone of the Gulf of Suez rift, Egypt  

NASA Astrophysics Data System (ADS)

We present a detailed study about the initiation and reactivations of Zeit-El Tor transfer zone, south Gulf of Suez rift, and its structural setting and tectonic evolution with respect to the Cretaceous-Cenozoic tectonic movements in North Egyptian margin. NE trending zone of opposed-dipping faults (22 km wide) has transferred the NE and SW rotations of the sub-basins in central and south Gulf of Suez rift, respectively. The evolution of this zone started by reactivation of the NE oriented late Neoproterozoic fractures that controlled the occurrence of Dokhan Volcanics in the rift shoulders. Later, the Syrian Arc contraction reactivated these fractures by a sinistral transpression during the Late Cretaceous-Eocene time. N64E extension of the Oligo-Miocene rift reactivated the NE fractures by a sinistral transtension. During this rifting, the NE trending faults forming the transfer zone were more active than the rift-bounding faults; the Upper Cretaceous reverse faults in the blocks lying between these NE trending faults were rotated; and drape-related reverse faults and the positive flower structures were formed. Tectonic inversion from contraction to extension controlled the distribution and thickness of the Upper Cretaceous-Miocene rocks.

Abd-Allah, Ali M. A.; Abdel Aal, Mohamed H.; El-Said, Mohamed M.; Abd El-Naby, Ahmed

2014-08-01

274

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

275

Microplate rotation in northeast Brazil during South Atlantic rifting: Analogies with the Sinai microplate  

NASA Astrophysics Data System (ADS)

The Early Cretaceous northeast Brazilian Sergipe microplate, formed at the northern end of the South Atlantic rift between South America and Africa, closely resembles the modern Sinai microplate at the northern end of the Red Sea in size, shape, and relative motion. Both formed where east-northeast trending transverse shear zones arrested northward rift propagation, causing the Tucano-Recncavo and Gulf of Suez rifts to fail and be replaced by northeast-trending leaky transforms (Sergipe-Alagoas and Dead Sea transforms) as the new paths of continental breakup. Bordered by the failed rift, the leaky transform, and the transverse shear zone, both microplates were rotated counterclockwise by drag along their eastern transform margins. Rotation thrust the edge of the Sergipe microplate over part of its northern border, creating the Arcoverde thrust wedge. The northwest-trending Vaza-Barris fault sheared the microplate, transferring the rift and evaporite sequence from the Sergipe-Alagoas to the Gabon continental margin. In Albian time, heating of the lithosphere in the Cabo igneous province near Recife permitted the South Atlantic rift to propagate across the Arcoverde thrust wedge, completing continental breakup.

Szatmari, Peter; Milani, Edison J.

1999-12-01

276

Variation in magma volume along the two arms of the Midcontinent Rift System  

NASA Astrophysics Data System (ADS)

The 2000km-long Midcontinent Rift System (MCRS) has two major arms meeting in the Lake Superior region. One extends southwestward at least as far as central Kansas, and the other extends southeastward through Michigan. Gravity and magnetic anomalies delimit the rift zone because the highly magnetic and dense mafic igneous rocks filling the central grabens of the rift system have strong susceptibility and density contrasts with adjacent rock formations. Because the rift lies hidden beneath gently dipping Phanerozoic sedimentary rocks except in the Lake Superior region, most models of rift structure have been extrapolated from the few areas that have seismic reflection data. A fundamental question is how the two arms differ. The west arm is more pronounced than the east arm in the gravity data. Whether this difference is due to the east arm being buried by the Michigan Basin or reflects magma volume has implications for the formation of the MCRS. Existing gravity models are not standardized across the rift and therefore cannot be used to directly compare the arms. We have thus conducted gravity modeling with a uniform approach and find that the west arm has significantly more magma and that the magma volume along the west arm increases toward Lake Superior. These results imply that there was more spreading along the western arm and that the spreading on this arm decreased southward, consistent with the arms being boundaries of a microplate rotating with respect to the Superior province with its rotation pole to the southwest.

Merino, M.; Keller, G. R.; Stein, S. A.

2012-12-01

277

Formation of the Shanxi Rift in North China: The control of preexisting lithospheric weakness  

NASA Astrophysics Data System (ADS)

The Shanxi Rift is an active seismic zone in North China, developed mainly since Pliocene (~5 Ma). Its formation has been associated with the Indo-Asian collision; other hypothesized causes include a regional extensional stress field associated with subduction of the western pacific plate and mantle upwelling under the North China Plain. However, these mechanisms do not explain why the rift system did not form along the western boundary of the North China Plain, where lithospheric thickness changes sharply from more than 150 km under the Ordos block and the Taihangshan Mountains to the west, to less than 70 km under the North China Plain. We have used a viscoplastic finite element model to explore the conditions for localized rifting in North China. Our results show that, for all the hypothesized causes, the preferred site of rifting would be along the boundary zone of changing lithospheric thickness. The only way to initiate the Shanxi rift in its current location, which is between the Ordos block and the Taihangshan Mountains with thick lithosphere, is to have preexisting lithospheric weakening there. This lithospheric weakness was likely formed during the collision between the Easter North China block and the Western North China block during the Paleoproterozoic (~1.8 Ga). Hence the ancient tectonic event still controls the young continental rifting.

Lin, F.; Liu, M.; Ye, J.

2012-12-01

278

Some remarks on the structure and geodynamics of the Kenya Rift  

NASA Astrophysics Data System (ADS)

Since 1985 the KRISP working group has recorded a number of seismic profiles and carried out array experiments in the Kenya Rift valley. Long range profiling has shown that across the Rift at latitude 0.5, the crust thins from 40 km beneath the western flank to 30 km beneath the graben proper, and thickens again to 35 km beneath the eastern flank. The crustal thickness beneath the graben decreases northwards from 35 km beneath the culmination of the Kenya dome to 20 km under Lake Turkana, suggesting a change to a highly extended terrain in the northern Kenya Rift. Furthermore, the anomalous low mantle velocities of 7.5-7.6 km/s suggest that the upper mantle immediately beneath the Rift may contain reservoirs of magmas, which were generated at greater depth. The anomalous mantle under the graben proper, as seen by the tomographic imaging, is characterized by a large, steep walled low velocity zone with a velocity decrease of at least 10% in the central part between 65 and 100 km depth which reaches down to more than 150 km. A comparison of the shape and size of the observed Bouguer anomaly with the LVZ suggests that the upper mantle beneath the Rift probably includes some 5% partial melt, concentrated in the areas with lowest velocities. These results provide new constraints for geodynamic models of the evolution of the Kenya Rift.

Achauer, U.; Maguire, P. K. H.; Mechie, J.; Green, W. V.; Krisp Working Group

1992-10-01

279

The Historic and Future Roles of Magmatism and Underplate Beneath the Northern Main Ethiopian Rift  

NASA Astrophysics Data System (ADS)

The magmatic Northern Main Ethiopian Rift (NMER) is believed to represent the transitional stage between continental and oceanic rifting, with recent studies from the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) project concluding that magmatic processes dominate the rifting. 133 broadband seismometers were deployed in three separate arrays across the NMER at a spacing of between 5 and 30 km and receiver functions have been analyzed using H-? stacking, forward modelling and inversion techniques. H-? stacking results reveal a consistent V_p/V_s ratio of 1.75--1.80 and Moho depth of 38--40 km along 125 km of the south-eastern rift flank. Beneath the rift valley, the V_p/V_s ratios rise to over 1.90 with the Moho depth thinning to 30--35 km. The average crustal V_p/V_s ratio varies between 1.80 and 2.00 to the north-west of the rift. A lower crustal or upper mantle layer is confirmed beneath the north-western rift flank from the observation of a double P- to S-wave conversion, as identified by seismic refraction and interpreted as underplate. This layer thins from ~15 km to less than 10 km from NW to SE into the rift and is likely to have a V_p/V_s ratio of between 1.75 and 1.95. Beneath this layer, evidence for an upper mantle discontinuity exists, that shallows into the rift. These results, together with further receiver function analyses, indicate that significant magma has been added to the base of the crust and throughout the crust on the north-western rift flank and throughout the crust beneath the rift valley. It is unlikely that magma has been added beneath the south-eastern rift flank. The maximum crustal thinning and surface expression of the present-day rifting occur at the location where the underplate layer is at its thinnest or where there is no evidence for its existence. This may indicate that the underplate emplacement phase occurred and strengthened the crust beneath the Ethiopian Plateau and rifting followed by magmatic processes exploited the relatively thin and weak crust (aided by a pre-existing lithospheric suture or weak zone). The receiver function results are combined with and compared to shear-wave splitting, gravity, seismic tomography and controlled-source seismic results to build a 3D model of the location, volume, nature and emplacement history of magmatic addition to the base of the crust beneath the Ethiopian Plateau. We examine how this underplate layer affects the current magmatically-driven rifting in the NMER in terms of crustal thinning, recent magmatic addition throughout the whole crust and upper mantle partial melting.

Cornwell, D. G.; England, R. W.; Maguire, P. K.; Stuart, G. W.

2006-12-01

280

Combined Seismic and Geodetic Observations of Dike Injection in the Dabbahu Rift Segment, Afar, Ethiopia  

NASA Astrophysics Data System (ADS)

As rifting proceeds to breakup, strain localizes to discrete, narrow rift segments by a combination of diking and faulting. However, the controls on the kinematics and dynamics of rifting episodes are poorly understood. We integrate new seismic and geodetic data from the Dabbahu rift segment, Afar, to constrain the location of magma sources, and the spatial/temporal variability of diking and faulting just prior to the onset of sea-floor spreading. Since 4Ma, strain in north Afar has localized to 60-km-long rift segments that are similar in size and morphology to slow-spreading mid-ocean ridge segments. In June2006 vigorous deformation occured near AdoAle volcano, in the center of the Dabbahu rift segment. InSAR shows 2.5m opening along a 10km-long zone, with no evidence of deflation of shallow chambers beneath central volcanoes. Deformation is consistent with injection of a 2m- wide dike at 1-10km depth. Seismic and GPS data constrain the spatial/temporal evolution of deformation. Starting midday 17th June, ML<3 earthquakes occur at the southern end of the deforming zone. After 1hr, earthquake merge into tremor, increase in magnitude, and migrate 10km NNW over 4hrs. Earthquakes are likely caused by inflation induced tension at the tip of a laterally propagating dike. Prior to June, broad deformation is observed beneath AdoAle, consistent with inflation of a source zone 10-15km deep beneath the central portion of the rift segment. The June06 dike is the first of 6 major intrusions in the Dabbahu rift since Sep05, a pattern similar to the 1975-84 rifting event in Krafla, Iceland. Our results show that during continental rupture strain is accommodated by repeated injection of dikes and induced faulting. Magma may be sourced directly to the center of the rift segment from reservoirs in the upper mantle and/or lower crust and delivered into the upper 10km by the lateral injection of discrete dikes.

Keir, D.; Ayele, A.; Baker, E.; Belachew, M.; Calais, E.; Ebinger, C.; Hamling, I.; Lewi, E.; Jacques, E.; Rowland, J.; Wright, T.

2007-12-01

281

Combined Seismic and Geodetic Observations of Dike Injection in the Dabbahu Rift Segment, Afar, Ethiopia  

NASA Astrophysics Data System (ADS)

As rifting proceeds to breakup, strain localizes to discrete, narrow rift segments by a combination of diking and faulting. However, the controls on the kinematics and dynamics of rifting episodes are poorly understood. We integrate new seismic and geodetic data from the Dabbahu rift segment, Afar, to constrain the location of magma sources, and the spatial/temporal variability of diking and faulting just prior to the onset of sea-floor spreading. Since 4Ma, strain in north Afar has localized to 60-km-long rift segments that are similar in size and morphology to slow-spreading mid-ocean ridge segments. In June2006 vigorous deformation occured near AdoAle volcano, in the center of the Dabbahu rift segment. InSAR shows 2.5m opening along a 10km-long zone, with no evidence of deflation of shallow chambers beneath central volcanoes. Deformation is consistent with injection of a 2m- wide dike at 1-10km depth. Seismic and GPS data constrain the spatial/temporal evolution of deformation. Starting midday 17th June, ML<3 earthquakes occur at the southern end of the deforming zone. After 1hr, earthquake merge into tremor, increase in magnitude, and migrate 10km NNW over 4hrs. Earthquakes are likely caused by inflation induced tension at the tip of a laterally propagating dike. Prior to June, broad deformation is observed beneath AdoAle, consistent with inflation of a source zone 10-15km deep beneath the central portion of the rift segment. The June06 dike is the first of 6 major intrusions in the Dabbahu rift since Sep05, a pattern similar to the 1975-84 rifting event in Krafla, Iceland. Our results show that during continental rupture strain is accommodated by repeated injection of dikes and induced faulting. Magma may be sourced directly to the center of the rift segment from reservoirs in the upper mantle and/or lower crust and delivered into the upper 10km by the lateral injection of discrete dikes.

Keir, D.; Ayele, A.; Baker, E.; Belachew, M.; Calais, E.; Ebinger, C.; Hamling, I.; Lewi, E.; Jacques, E.; Rowland, J.; Wright, T.

2004-12-01

282

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

283

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

NASA Astrophysics Data System (ADS)

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 and magnitude makeup. Suggestion of some deeper sourced gases having migrated through fractures in the rift zones is advanced. Also differences among the samples rift zones are presented.

Sidle, W. C.

1984-09-01

284

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

NASA Astrophysics Data System (ADS)

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

Chenin, Pauline; Beaumont, Christopher

2013-04-01

285

Traces of extension in continental lithosphere: Towards a seismic image of the Mid-Continent Rift  

NASA Astrophysics Data System (ADS)

The centre of the North American continent preserves tectonic events ranging from the Archean to the Proterozoic. One major late-stage event was the failed Mid-Continent Rift (MCR), a ca. 1.1 Ga episode of extension and magmatism along a ca. 2000 km path through the Great Lakes and environs. The rifting process never reached the point of developing an ocean basin, and so the MCR presumably preserves a snapshot of the rifting process at both crustal and lithospheric levels. The in-progress Superior Province Rifting Earthscope Experiment (SPREE) is a major multi-institution deployment of Earthscope Flexible Array instruments designed to augment the Transportable Array deployment as it crosses the MCR, both by extending coverage into Canada along the north shore of Lake Superior, and by concentrating a dense deployment of instruments along and across the terrestrial portion of the MCR in Minnesota and Wisconsin. Multiple seismic imaging techniques will be then used to obtain crustal and lithospheric images of the MCR, and so examine questions regarding the origin, propagation, and failure of the rift, and its relationship to the contemporaneous Grenville orogen. Though SPREE itself is in its early stages, some novel work has been done with pre-SPREE data. Notably, we present new tomographic and shear-wave splitting constraints on the region immediately west of the MCR. New results include delineation of the southern and western limits of the anomalous Western Superior lithosphere, which may have been particularly resistant to rifting; a novel low-velocity channel striking WNW-ESE beneath Minnesota and the Dakotas, which may represent an unrecognized failed branch of the MCR; and very weak lithospheric fabric beneath the unusual Minnesota River Valley terrane, which may reflect rifting-related disruption of previously-existing fabric, or a different tectonic origin to the remainder of the Superior. We will also present some preliminary travel-time and splitting results from early-stage SPREE data.

Frederiksen, A. W.; Bollmann, T. A.; Darbyshire, F. A.; Jurdy, D. M.; Revenaugh, J.; Stein, S. A.; van der Lee, S.; Wiens, D. A.; Wysession, M. E.

2012-12-01

286

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; Villagmez, Diego; Putlitz, Benita; Chiaradia, Massimo

2014-03-01

287

Plate kinematics of the Afro-Arabian Rift System with emphasis on the Afar Depression, Ethiopia  

NASA Astrophysics Data System (ADS)

This work utilizes the Four-Dimensional Plates (4DPlates) software, and Differential Interferometric Synthetic Aperture Radar (DInSAR) to examine plate-scale, regional-scale and local-scale kinematics of the Afro-Arabian Rift System with emphasis on the Afar Depression in Ethiopia. First, the 4DPlates is used to restore the Red Sea, the Gulf of Aden, the Afar Depression and the Main Ethiopian Rift to development of a new model that adopts two poles of rotation for Arabia. Second, the 4DPlates is used to model regional-scale and local-scale kinematics within the Afar Depression. Most plate reconstruction models of the Afro-Arabian Rift System relies on considering the Afar Depression as a typical rift-rift-rift triple junction where the Arabian, Somali and Nubian (African) plates are separating by the Red Sea, the Gulf of Aden and the Main Ethiopian Rift suggesting the presence of "sharp and rigid" plate boundaries. However, at the regional-scale the Afar kinematics are more complex due to stepping of the Red Sea propagator and the Gulf of Aden propagator onto Afar as well as the presence of the Danakil, Ali Sabieh and East Central Block "micro-plates". This study incorporates the motion of these micro-plates into the regional-scale model and defined the plate boundary between the Arabian and the African plates within Afar as likely a diffused zone of extensional strain within the East Central Block. Third, DInSAR technology is used to create ascending and descending differential interferograms from the Envisat Advanced Synthetic Aperture Radar (ASAR) C-Band data for the East Central Block to image active crustal deformation related to extensional tectonics and volcanism. Results of the DInSAR study indicate no strong strain localization but rather a diffused pattern of deformation across the entire East Central Block.

Bottenberg, Helen Carrie

288

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

289

An Assembly Model of Rift Valley Fever Virus  

PubMed Central

Rift Valley fever virus (RVFV) is a bunyavirus endemic to Africa and the Arabian Peninsula that infects humans and livestock. The virus encodes two glycoproteins, Gn and Gc, which represent the major structural antigens and are responsible for host cell receptor binding and fusion. Both glycoproteins are organized on the virus surface as cylindrical hollow spikes that cluster into distinct capsomers with the overall assembly exhibiting an icosahedral symmetry. Currently, no experimental three-dimensional structure for any entire bunyavirus glycoprotein is available. Using fold recognition, we generated molecular models for both RVFV glycoproteins and found significant structural matches between the RVFV Gn protein and the influenza virus hemagglutinin protein and a separate match between RVFV Gc protein and Sindbis virus envelope protein E1. Using these models, the potential interaction and arrangement of both glycoproteins in the RVFV particle was analyzed, by modeling their placement within the cryo-electron microscopy density map of RVFV. We identified four possible arrangements of the glycoproteins in the virion envelope. Each assembly model proposes that the ectodomain of Gn forms the majority of the protruding capsomer and that Gc is involved in formation of the capsomer base. Furthermore, Gc is suggested to facilitate intercapsomer connections. The proposed arrangement of the two glycoproteins on the RVFV surface is similar to that described for the alphavirus E1-E2 proteins. Our models will provide guidance to better understand the assembly process of phleboviruses and such structural studies can also contribute to the design of targeted antivirals. PMID:22837754

Rusu, Mirabela; Bonneau, Richard; Holbrook, Michael R.; Watowich, Stanley J.; Birmanns, Stefan; Wriggers, Willy; Freiberg, Alexander N.

2012-01-01

290

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

291

Middle Stone Age starch acquisition in the Niassa Rift, Mozambique  

NASA Astrophysics Data System (ADS)

The quest for direct lines of evidence for Paleolithic plant consumption during the African Middle Stone Age has led scientists to study residues and use-wear on flaked stone tools. Past work has established lithic function through multiple lines of evidence and the spatial breakdown of use-wear and microscopic traces on tool surfaces. This paper focuses on the quantitative analysis of starch assemblages and the botanical identification of grains from flake and core tools to learn about human ecology of carbohydrate use around the Niassa woodlands, in the Mozambican Rift. The processing of starchy plant parts is deduced from the occurrence of starch assemblages that presumably got attached to stone tool surfaces by actions associated with extractive or culinary activities. Specifically, we investigate starch grains from stone tools recently excavated in northern Mozambique at the site of Mikuyu; which presumably spans the middle to late Pleistocene and represents similar sites found along the Malawi/Niassa corridor that links East, Southern, and Central Africa. Starch was extracted and processed with a diverse tool kit consisting of scrapers, cores, points, flakes, and other kinds of tools. The microbotanical data suggests consumption of seeds, legumes, caryopses, piths, underground storage organs, nuts, and mesocarps from more than a dozen families. Our data suggest a great antiquity for starch use in Africa as well as an expanded diet and intensification.

Mercader, Julio; Bennett, Tim; Raja, Mussa

2008-09-01

292

Fluid flow in fault zones from an active rift  

NASA Astrophysics Data System (ADS)

The geometry and hydraulic properties of fault zones are investigated for Mesozoic greywacke basement and Miocene sandstone from 37 km of tunnels in the southern Taupo Rift, New Zealand. Localised groundwater inflows occur almost exclusively (?90%) within, and immediately adjacent to, fault zones. Fault zones in the contrasting lithologies comprise fault rock, small-scale faults, and fractures with thicknesses of 0.01-110 m approximating power law distributions and bulk permeabilities of 10-9-10-12 m2. Variability in fault zone structure results in a highly heterogeneous distribution of flow rates and locations. Within basement 80% of the flow rate occurs from fault zones ?10 m wide, with 30% of the total localised flow rate originating from a single fault zone (i.e. consistent with the golden fracture concept). No simple relationships are found between flow rates and either fault strike or hydraulic head, with ?50% of fault zones in any given orientation flowing. A general positive relationship does however exist between fault zone thickness and maximum flow rate. Higher flow rates from larger fault zones may arise because these structures have greater dimensions and are more likely (than smaller faults) to be connected to other faults in the system and the ground surface.

Seebeck, H.; Nicol, A.; Walsh, J. J.; Childs, C.; Beetham, R. D.; Pettinga, J.

2014-05-01

293

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

294

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

295

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

296

Rift Valley Fever Virus Infection in Golden Syrian Hamsters  

PubMed Central

Rift Valley fever virus (RVFV) is a formidable pathogen that causes severe disease and abortion in a variety of livestock species and a range of disease in humans that includes hemorrhagic fever, fulminant hepatitis, encephalitis and blindness. The natural transmission cycle involves mosquito vectors, but exposure can also occur through contact with infected fluids and tissues. The lack of approved antiviral therapies and vaccines for human use underlies the importance of small animal models for proof-of-concept efficacy studies. Several mouse and rat models of RVFV infection have been well characterized and provide useful systems for the study of certain aspects of pathogenesis, as well as antiviral drug and vaccine development. However, certain host-directed therapeutics may not act on mouse or rat pathways. Here, we describe the natural history of disease in golden Syrian hamsters challenged subcutaneously with the pathogenic ZH501 strain of RVFV. Peracute disease resulted in rapid lethality within 2 to 3 days of RVFV challenge. High titer viremia and substantial viral loads were observed in most tissues examined; however, histopathology and immunostaining for RVFV antigen were largely restricted to the liver. Acute hepatocellular necrosis associated with a strong presence of viral antigen in the hepatocytes indicates that fulminant hepatitis is the likely cause of mortality. Further studies to assess the susceptibility and disease progression following respiratory route exposure are warranted. The use of the hamsters to model RVFV infection is suitable for early stage antiviral drug and vaccine development studies. PMID:25607955

Scharton, Dionna; Van Wettere, Arnaud J.; Bailey, Kevin W.; Vest, Zachary; Westover, Jonna B.; Siddharthan, Venkatraman; Gowen, Brian B.

2015-01-01

297

The Paleogene pre-rift to syn-rift succession in the Dhofar margin (northeastern Gulf of Aden): Stratigraphy and depositional environments  

NASA Astrophysics Data System (ADS)

The Paleogene deposits on the northern passive margin of the Gulf of Aden record the transition from the pre-rift to the syn-rift stages of the southern Arabian plate margin. In southern Oman (Dhofar Region), the relative continuity of the sedimentary record offers the possibility to investigate the early deformation phases of the Aden rift system. A new detailed sedimentological and biostratigraphic analysis of the Cuisian to Rupelian deposits of the Dhofar region allows to define a second-order transgressive-regressive cycle, that can be further subdivided into four third-order sequences between the Late Cuisian and the Early Rupelian time. The sequence stratigraphy established in this study has major implications for the understanding of the time equivalent deposits described in the eastern Arabian plate and illustrates the polyphased history of the initiation of the Aden Gulf rift system. The first two depositional sequences are controlled by a phase of deformation that only affects the eastern Oman margin, in relation with the tectonic activity at the Arabian-Indian plate boundary, during the Late Cuisian-Middle Lutetian. The last two depositional sequences record a westward migration of the deformation within the eastern realm of the proto-Gulf of Aden from the Bartonian. Priabonian uplift resulted in the basinward shift of the depositional system followed by a phase of tectonic subsidence that is recorded by the aggradation of lacustrine deposits in localized fault bounded basins. A subsequent major regional relative sea level fall related to domal uplift is recorded by terrigenous deposits (lower part of the Ashawq Formation) prior to the main phase of syn-rift tectonic subsidence (upper part of the Ashawq and Mughsayl formations) in Rupelian-Chattian times.

Robinet, J.; Razin, P.; Serra-Kiel, J.; Gallardo-Garcia, A.; Leroy, S.; Roger, J.; Grelaud, C.

2013-11-01

298

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

299

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

300

Cenozoic thermal, mechanical and tectonic evolution of the Rio Grande rift  

NASA Technical Reports Server (NTRS)

Two areas of New Mexico which exhibit complex but similar Cenozoic histories of extensional tectonism are analyzed. The first study area is the Basin and Range province and southern Rio Gande rift in southern New Mexico; the second study area is the central Rio Grande rift in central and northern New Mexico, the southern San Luis basin, the Espanola basin, and the Albuquerque basin. Two phases of extension were identified: the first phase which began in mid-Oligocene was characterized by local high-strain extension events, low-angle faulting, and the development of broad, shallow basins, all indicating an approximately NE-SW + or - 25 deg extension direction; the later phase which occurred primarily in the late Miocene, was characterized by synchronous, high-angle faulting, resulting in large vertical strains which produced the modern Rio Grande rift morphology. Extension direction was approximately E-W. Geotherms were estimated and lithospheric strength curves were calculated for these two phases of extension. A high geotherm was deduced for the early phase resulting in a shallow crustal brittle transition, and insignificant mantle strength. The lithosphere subsequently cooled, resulting in a significant zone of mantle strength beneath the Moho. It is concluded that the interrelationship among regional and local prerifting, synrifting, and postrifting events in the Rio Grande rift attests to the fact that the rifting (in the region studied) should be considered in the context of other geologic events.

Morgan, P.; Seager, W. R.; Golombek, M. P.

1986-01-01

301

Crustal structure of the Southern Rio Grande rift determined from seismic refraction profiling  

NASA Technical Reports Server (NTRS)

As part of a major cooperative seismic experiment, a series of seismic refraction profiles have been recorded in south-central New Mexico with the goal of determining the crustal structure in the southern Rio Grande rift. The data gathered greatly expand the seismic data base in the area, and consist of three interlocking regional profiles: a reversed E-W line across the rift, an unreversed N-S axial line, and an unreversed SW-SE line. The reversed E-W line shows no significant dip along the Moho (32 km thick crust) and a 7.7 km/s Pn velocity. Results from the N-S axial line and the NW-SE line indicate an apparent Pn velocity of 7.95 km/s and significant dip along the Moho with crustal thinning toward the south and southeast. When interpreted together, these data indicate a crustal thinning in the southern rift of 4-6 km with respect to the northern rift and the adjacent Basin and Range province, and establish the regional Pn velocity to be approximately 7.7 km/s. These results suggest that the Rio Grande rift can be identified as a crustal feature separate and distinct from the Basin and Range province.

Sinno, Y. A.; Keller, G. R.; Harder, S. H.; Daggett, P. H.; Morgan, P.

1986-01-01

302

The Pantelleria graben (Sicily Channel, Central Mediterranean): An example of intraplate passive rift  

NASA Astrophysics Data System (ADS)

We present new high-resolution swath bathymetric data and multichannel seismic profiles acquired in the Pantelleria graben, one of the three main tectonic depressions forming the Sicily Channel Rift Zone. This region experienced a Late Miocene-Early Pliocene continental extension with the development of NW-trending, fault-bounded troughs, later accompanied by widespread volcanic manifestations. Data support the interpretation that the Pantelleria graben evolution was dominated by two tectonic phases: A lithospheric-scale continental rifting (Early Pliocene), in which the whole graben was formed, and a successive phase (Late Pliocene-Pleistocene) characterized by a magma-assisted extensional mechanism. Ascending magmas within the graben floor seem to migrate from the S-E sector of the depression toward the N-W sector, which is almost entirely floored by igneous material, and where the volcanic edifice of the Pantelleria Island is emerged. The volcanic activity is presently concentrated north of the Pantelleria Island. The tectonic evolution of the Pantelleria graben, characterized by the chronological sequence of events: rifting-doming-volcanism, can be referred to as a 'passive' rifting model. Crustal stretching, and subsequent fault development and rifting within the Pelagian block, may have been controlled by slab-pull forces of the northward-subducting African slab.

Civile, D.; Lodolo, E.; Accettella, D.; Geletti, R.; Ben-Avraham, Z.; Deponte, M.; Facchin, L.; Ramella, R.; Romeo, R.

2010-07-01

303

Rift border system: The interplay between tectonics and sedimentation in the Reconcavo basin, northeastern Brazil  

SciTech Connect

A geometric and depositional model is proposed to explain the tectonic and sedimentary evolution of the main border of the Reconcavo basin. The architecture of the rift margin is characterized by a rift border system constituted by (1) a master fault, (2) a step, and (3) a clastic wedge. This footwall-derived clastic wedge is interpreted as alluvial fans and fan deltas composed of conglomerates that interfinger with hanging-wall strata. The analysis of the vertical distribution of coarse-grained components of this wedge suggests that its composition is geographically controlled, and no regular inverted stratigraphy is commonly described for this type of succession. During an initial lacustrine phase, turbidites accumulated farther from and parallel to the rift margin. The mapping of marker beds that bound these lacustrine turbidite deposits may be used to infer major periods of clastic influx and, therefore, to correlate with periods of fault-related subsidence or climatic fluctuations in the depositional basin and erosion of the sediment source area. Periods of limited back-faulting and basin expansion toward the main border are distinguished through patterns of progradation and aggradation indicating progressive retreat of the rift border and younging; in the footwall direction. The overall evolution of the rift border seems to be related to extension, block rotation, hanging-wall subsidence, and footwall uplift associated with the initial master fault, with limited propagation of faults away from the basin into the footwall.

Magnavita, L.P.; Silva, T.F. da [Petrobras/E & P - BA, Bahia (Brazil)

1995-11-01

304

Volcanics and structure of the famous narrowgate rift: Evidence for cyclic evolution: AMAR 1  

SciTech Connect

A near-bottom photographic survey on the Mid-Atlantic Ridge from the FAMOUS region south to Transform B reveals structural and volcanologic evidence for an alternating widening and narrowing rift valley. Extension wedges appear to be propagating south from Transform A and north from Transform B. These merge at Mt. Mars, where the rift valley is narrowest and shallowest (Narrowgate), giving the valley a symmetrical hourglass shape. Three major volcanic en echelon lineaments trend N10 /sup 0/E, 10/sup 0/ oblique to the strike of the inner walls. Faulting and fissuring are antisymmetrical across the rift valley in contrast to the major extension. This contrasting antisymmetry reflects the surficial distribution of stress across crust of varying strengths, whereas the large-scale wedge phenomenon determines the periodicity of transition from a narrow valley to a wide valley. Using vector analysis, the extension wedges are propagating at about 9 cm/yr toward each other. At this rate a narrow rift valley will be fully transformed into wide rift in 0.64 m.y.

Crane, K.; Ballard, R.D.

1981-06-10

305

Synrift sedimentation in the Gulf of Suez rift controlled by eustatic sea level variations  

SciTech Connect

Laterally persistent stratigraphic variations in the southern Gulf of Suez rift indicate that eustatic variations in sea level predominate over local tectonic effects in controlling Neogene basin-fill sedimentation. Low sea level allows extensive erosion of tilt-block crests and rift shoulders, sending clastic aprons into the intervening subbasins and allowing evaporite deposition. High sea level minimizes clastic input, allowing marls and shales to build up off structure and reefs to form on and around tilt blocks. Thus variations in sediment character indicate relative sea level. Early rifting events in the upper Oligocene are marked by deposition of continental red beds. Overlying lowest Miocene clastics and evaporites are cut by a lower Burdigalian unconformity, indicating a minor transgression in the Aquitanian. Above a disconformity, laterally varying clastics and evaporites suggest regression followed by intermittent shallow-water conditions. A higher unconformity is overlain by thick cyclic evaporites representing periodic flooding and drying of the rift, a result of sea level remaining close to the height of the Suez sill to the north. A major Messinian unconformity cuts the section, indicating major regression, and is overlain by largely clastic sediments of both continental and marine affinities, showing rapid sea level fluctuations. Regional synrift sedimentation has been controlled more by eustatic sea level change, modified by the Suez sill to the north, than by tectonic movements within the rift.

Perry, S.K.; Schamel, S.

1985-01-01

306

Structure and stratigraphy of the Turkana rift from seismic reflection data  

NASA Astrophysics Data System (ADS)

A multichannel seismic survey was conducted on Lake Turkana, a rift lake occupying a portion of the Eastern Branch of the East African Rift System. This survey revealed the presence of a significant rift structure beneath the lake, the extent and structural details of which had been previously unknown. The lake is underlain by a series of half-graben basins which alternate in polarity along the axis of the lake, and are linked end to end by zones of structural high. These half-graben often have Quaternary volcanic centers located near their latitudinal midpoints. Half-graben basins beneath Turkana deepen progressively to the north, attaining thicknesses of up to 4 km. A deep seismic reflector has been identified, and is believed to correspond to the top of a thick series of Mio-Pliocene flood volcanics. Overlying reflectors are thought to correspond to fluvial, lacustrine and volcano-clastic sediments that are Plio-Pleistocene to Recent in age. The style of rifting beneath Lake Turkana is similar to that observed beneath Lake Tanganyika, which occupies a portion of the Western Branch of the East African Rift System. The most noticeable differences are the abundant volcanism and smaller half-graben size associated with Lake Turkana. These differences may be attributed to the warmer, thinner lithosphere underlying the Eastern Branch as compared to that underlying the Western Branch.

Dunkelman, Thomas J.; Rosendahl, Bruce R.; Karson, Jeffrey A.

307

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

308

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

309

Oligo-Miocene syn-rift and Miocene post-rift sedimentary records: the tectono-stratigraphic development of the northern proximal margin of the Gulf of Aden  

NASA Astrophysics Data System (ADS)

The northern margin of the Gulf of Aden results from the Oligo-Miocene rifting (34Ma) leading to continental break-up and the oceanic spreading since the Burdigalian (17.6 Ma). We, here, investigate in detail the tectono-stratigraphy development of the Ashawq Graben belonging to the proximal part of northern margin (southern Oman, Dhofar). This graben exhibits sedimentary records of syn-rift and post-rift sequences, so-called Dhofar (Rupelian-Burdigalian) and Fars (middle Miocene-Pliocene) Groups respectively. Analyzing the deposit conditions and sequences geometries provide fundamental inputs for the whole margin understanding. An accurate sedimentological and biostratigraphical analysis evidences two second-order sea level cycles corresponding to the syn-rift and the post-rift units separated by an erosive surface with paleo-karst cavities. The first stage of the rifting expresses as a regional uplift which led to set up of an early Oligocene mix platform system (Ashawq Fm., Shizar Mb.) overlying the proximal platform (Aydim Fm.) and continental (Zalumah Fm.) system deposit of the late Eocene to earliest Oligocene time. Then, the rift extension process during early Oligocene leads to verticals movements along normal faults and increase of the accommodation rate in the Ashawq graben. Such increase of accommodation is fully compensated by an important carbonate production leading to the aggradation of a thick reefal carbonate platform (Ashawq Fm., Nakhlit Mb.). An acceleration of the extension processes during late Oligocene time reaches an increase of the tectonic subsidence associated to the partial drowning and collapsing of the platform and to the set up of carbonate gravity-flow deposits in a deep basin (Mughsayl Fm.). In the most proximal realm, the sedimentation rate attempts to compensate the accommodation rate resulting in a differential aggradation of the reefal carbonate platform, sometimes in the form of patch reef. At the early Miocene time, the progradation of a conglomeratic fan delta system testifies the decrease of the accommodation rate and a strong basinward system shift controlled by a general uplift of the margin. This surrection phase leading to a subaerial exposure is interpreted as the consequence of the continental breakup at the Burdigalian time (17.6 Ma) and the set up of Ocean-Continent Transition (OCT). At the middle Miocene time, new subsidence phase is associated with a partial marine incursion and to the set up of proximal shallow marine carbonate deposits (Adawnib Fm.) and the lateral equivalent conglomeratic alluvial fan deposits (Nar Fm.). This post-rift unit records a progressive decrease of the tectonic activity, which may be related to the migration of the deformation towards the distal margin up to the oceanic spreading in the Gulf of Aden. Late deformation phases (erosive paleo-surface at the top the post-rift conglomerates, preservation of uplifted paleo-beach deposits) may imply a large-scale geodynamic processes.

Robinet, J.; Razin, P.; Serra Kiel, J.; Gallardo Garcia, A.; Grelaud, C.; Roger, J.; Leroy, S.; Malaval, M.

2012-04-01

310

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

311

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

312

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

313

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; Jrg Keller

2010-01-01

314

Tectonics of the Nereus Deep, Red Sea: A deep-tow investigation of a site of initial rifting  

Microsoft Academic Search

The Nereus Deep (23N) lies in the central portion of the Red Sea, in a region which marks a transition between the nearly continuous axial rift valley of the southern Red Sea and the northern Red Sea, where a well defined axial rift is absent. The deep-tow survey and associated heat flow measurements reported here show that the Nereus Deep

John D. Bicknell; Ken C. MacDonald; Stephen P. Miller; Peter F. Lonsdale; Keir Becker

1986-01-01

315

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

316

Twenty-five years of geodetic measurements along the Tadjoura-Asal rift system, Djibouti, East Africa  

E-print Network

years of geodetic measurements along the Tadjoura-Asal rift system, Djibouti, East Africa, J. GeophysTwenty-five years of geodetic measurements along the Tadjoura- Asal rift system, Djibouti, East Africa Christophe Vigny,1 Jean-Bernard de Chabalier,2 Jean-Claude Ruegg,2 Philippe Huchon,3 Kurt L. Feigl

Vigny, Christophe

317

ACCEPTED MANUSCRIPT Depositional model and stratigraphic architecture of rift climax Gilbert-type fan deltas (Gulf of  

E-print Network

ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT Depositional model and stratigraphic architecture of rift). Abstract Facies, depositional model and stratigraphic architecture of Pleistocene giant Gilbert-type fan to the Middle Group of the Corinth Rift infill and their stratigraphic development was strongly influenced

Paris-Sud XI, Université de

318

Facies relationships and sedimentation in large rift lakes and implications for hydrocarbon exploration: Examples from lakes Turkana and Tanganyika  

Microsoft Academic Search

Cohen, A. S., 1989. Facies relationships and sedimentation in large rift lakes and implications for hydrocarbon exploration: examples from lakes Turkana and Tanganyika. Palaeogeogr., Palaeoclimatol., Palaeoecol., 70: 65-80. Two African lakes, Turkana and Tanganyika, illustrate a spectrum of sediments and facies architectures which can occur in large rift lacustrine basins. They demonstrate contrasts between sediments deposited in a semi-arid, moderately

Andrew S. Cohen

1989-01-01

319

The Mesozoic rift basins of eastern North America: Potential reservoir or Explorationist's folly  

SciTech Connect

Mesozoic rift basins are found on the East Coast of North America from Georgia to Nova Scotia. The basins formed as a result of extensional activity associated with the breakup of Pangaea. The internal geometry of the basins includes a depositional sequence ranging from coarse fanglomerates to fine-grained siltstones and argillites. Since these Mesozoic rift basins were first studied, they have not been considered to be likely spots for hydrocarbon accumulations. Recently, geologists have reconsidered these Mesozoic basins and have developed a more synergistic approach that suggests that many of these rift basins might be suitable targets for exploration. By analogy, these Mesozoic basins are correlative to similar basins in northwestern Africa, where significant reserved of oil and natural gas have been developed. The similarity between the productive basins in northwestern Africa and the Mesozoic basins of North America and their proximity to major markets provides sufficient rationale to further investigate these basins.

Pyron, A.

1991-08-01

320

Shear source multiazimuth near- and far-offset vertical seismic profiles, Asal rift, Republic of Djibouti  

NASA Astrophysics Data System (ADS)

A three-component borehole seismic data set obtained in the Asal rift zone shows that azimuthal anisotropy may be present with the fast propagation direction aligned with the present-day rift axis. This is seen on the near- and far-offset vertical seismic profile (VSP) data from the two source points lying parallel to the rift axis. The third orthogonal source point gives propagation times characteristic of near-horizontal paths along the detected slow direction. The reflected VSP wave field contains both P and S energy. The main P and S reflections correlate well and correspond to thin clay and siltstone layers interspersed within the lava flow series. Some evidence exists for a charge with depth of the amount or direction of the anisotropy found in the uppermost 400 m.

Hirn, Alfred; Magnier, Sophie-Adelaide

1992-12-01

321

Episodic rifting of phanerozoic rocks in the victoria land basin, Western ross sea, antarctica.  

PubMed

Multichannel seismic-reflection data show that the Victoria Land-basin, unlike other sedimentary basins in the Ross Sea, includes a rift-depression 15 to 25 kilometers wide that parallels the Transantarctic Mountains and contains up to 12 kilometers of possible Paleozoic to Holocene age sedimentary rocks. An unconformity separates the previously identified Cenozoic sedimentary section from the underlying strata of possible Mesozoic and Paleozoic age. Late Cenozoic volcanic rocks intrude into the entire section along the eastern flank of the basin. The Victoria Land basin is probably part of a more extensive rift system that has been active episodically since Paleozoic time. Inferred rifting and basin subsidence during Mesozoic and Cenozoic time may be associated with regional crustal extension and uplift of the nearby Transantarctic Mountains. PMID:17753283

Cooper, A K; Davey, F J

1985-09-13

322

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

323

Episodic rifting of phanerozoic rocks in the Victoria Land basin, Western Ross Sea, Antarctica  

USGS Publications Warehouse

Multichannel seismic-reflection data show that the Victoria Land basin, unlike other sedimentary basins in the Ross Sea, includes a rift-depression 15 to 25 kilometers wide that parallels the Transantarctic Mountains and contains up to 12 kilometers of possible Paleozoic to Holocene age sedimentary rocks. An unconformity separates the previously identified Cenozoic sedimentary section from the underlying strata of possible Mesozoic and Paleozoic age. Late Cenozoic volcanic rocks intrude into the entire section along the eastern flank of the basin. The Victoria Land basin is probably part of a more extensive rift system that has been active episodically since Paleozoic time. Inferred rifting and basin subsidence during Mesozoic and Cenozoic time may be associated with regional crustal extension and uplift of the nearby Transantarctic Mountains.

Cooper, A. K.; Davey, F.J.

1985-01-01

324

Geophysical and morpho-tectonic study of the transition between seafloor spreading and continental rifting, western Woodlark Basin, Papua New Guinea  

Microsoft Academic Search

Two major morpho-tectonic domains, separated by a major transfer zone, are described at the transition between seafloor spreading and continental rifting in the western Woodlark Basin, off-shore eastern Papua New Guinea. The oceanic domain comprises new oceanic crust formed during the Bruhnes Epoch, older transitional crust and the rifted continental margins. Two rift branches are recognized within the continental domain.

Vladimir Benes; Natasha Bocharova; Eduard Popov; Steven D. Scott; Lev Zonenshain

1997-01-01

325

Receiver functions analysis in Northern Tanzania to understand the earliest stage of rifting  

NASA Astrophysics Data System (ADS)

The East African Rift (EAR) is the site of stretching and breakup of the lithosphere in response to a combination of regional pulling forces and mantle upwellings. Deformation results from complex interactions between magmatic intrusions, faulting, asthenospheric dynamism and far field stresses. It thus involves both deep processes and local inherited fabrics. In the frame of two international projects CRAFTI (NSF) and CoLiBrEA (ANR), we gather our skills to lead a multidisciplinary project in order to characterize the factors involved in continental rifting. We target the first 5 My of a magmatic rift initiating in thick (>150 km) continental lithosphere, where we can directly image and detect fault and magma interactions, the role of inherited and rheological heterogeneities of the lithosphere on rift localisation. We deployed 35 broadband seismic stations in Natron and Ngorongoro areas in January 2013 to characterize crustal and mantle structures of the rift. The stations were equipped by 3 component sensors and Reftek Recorders to continuously record teleseisms as well as local seismicity. We present here a receiver function analyse on the teleseismic events recorded during the first 6 months of the experiment. Both P- and S-waves receiver functions were proceeded to document the modification of the crust and the mantle due to plate stretching and magmatic processes. The Vp/Vs ratio informs on the state of the crust, which is affected by magmatic and fluids intrusions at different depths. The S-wave receiver function gives insight into the lithosphere state and the nature of the mantle beneath the rift (archean or plume affected).

Tiberi, C.; Albaric, J.; Deschamps, A.; Deverchere, J.; Ebinger, C. J.; Ferdinand, R. W.; Gautier, S.; Lambert, C.; Msabi, M.; Mtelela, K.; Muzuka, A.; Perrot, J.; Rasendra, N.; Roecker, S. W.; Rodzianko, A.; Witkin, E.

2013-12-01

326

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

327

The mechanism of post-rift fault activities in Baiyun sag, Pearl River Mouth basin  

NASA Astrophysics Data System (ADS)

Post-rift fault activities were often observed in deepwater basins, which have great contributions to oil and gas migration and accumulation. The main causes for post-rift fault activities include tectonic events, mud or salt diapirs, and gravitational collapse. In the South China Sea continental margin, post-rift fault activities are widely distributed, especially in Baiyun sag, one of the largest deepwater sag with its main body located beneath present continental slope. During the post-rift stage, large population of faults kept active for a long time from 32 Ma (T70) till 5.5 Ma (T10). Seismic interpretation, fault analysis and analogue modeling experiments indicate that the post-rift fault activities in Baiyun sag between 32 Ma (T70) and 13.8 Ma (T30) was mainly controlled by gravity pointing to the Main Baiyun sag, which caused the faults extensive on the side facing Main Baiyun sag and the back side compressive. Around 32 Ma (T70), the breakup of the continental margin and the spreading of the South China Sea shed a combined effect of weak compression toward Baiyun sag. The gravity during post-rift stage might be caused by discrepant subsidence and sedimentation between strongly thinned sag center and wing areas. This is supported by positive relationship between sedimentation rate and fault growth index. After 13.8 Ma (T30), fault activity shows negative relationship with sedimentation rate. Compressive uplift and erosion in seismic profiles as well as negative tectonic subsiding rates suggest that the fault activity from 13.8 Ma (T30) to 5.5 Ma (T10) might be controlled by the subductive compression from the Philippine plate in the east.

Sun, Zhen; Xu, Ziying; Sun, Longtao; Pang, Xiong; Yan, Chengzhi; Li, Yuanping; Zhao, Zhongxian; Wang, Zhangwen; Zhang, Cuimei

2014-08-01

328

The Midcontinent rift system and the Precambrian basement in southern Michigan  

SciTech Connect

The Precambrian basement within Michigan consists of at least three provinces, each characterized by distinctive potential field anomalies: (1) the Eastern Granite-Rhyolite Province (EGRP) in the south, (2) the Grenville Province in the southeast and (3) the Penokean Province to the north. Also located within the basement is the Mid-Michigan rift (MMR), which is the eastern arm of the Midcontinent rift system (MRS). Southwest and parallel to the MMR is a series of linear positive gravity anomalies which has been referred to as the Ft. Wayne rift (FWR) and the Southwest Michigan Anomaly (SWMA). The EGRP, which is characterized by undeformed and unmetamorphosed rhyolite to dacite and epizonal granites, was emplaced ca. 1510--1450 Ma. However, the EGRP may be comprised of several terranes of varying extent and origin based on analysis of potential field data and rock and mineral ages. The MMR and the FWR/SWMA are characterized by linear arrays of positive magnetic and gravity anomalies, which are probably due to thick accumulations of mafic igneous rocks within the rifts. The extent and trends of the FWR/SWMA have been largely inferred from geophysical data with a presumption of the age of about 1,100 Ma. The continuation of the MMR southward into Ohio and Kentucky as a sequence of gravity highs is questionable and needs further resolution. The FWR/SWMA may be part of the East Continent Rift Basin (ECRB). The ECRB, which is a large complex of related rift basins of Keweenawan age (1300 --1100 Ma), may be an extension of the MRS but it is not physically continuous with it. The ECRB lies to the west of the Grenville Front and extends at least from northwest Ohio to central Kentucky. Extensions of the ECRB north and south are speculative.

Smith, W.A. (Western Michigan Univ., Kalamazoo, MI (United States). Dept. of Geology)

1994-04-01

329

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

330

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-chelon 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

331

Recent seismic activity of the Kivu Province, Western Rift Valley of Africa  

NASA Astrophysics Data System (ADS)

The Kivu Province is located at the junction between the well-defined Ruzizi Valley to the south and the Lake Amin Trough to the north. In this zone, the Rift Valley is characterized by the highest uplift and by complex dislocations of the crust, accompanied by the most intensive volcanism of the East African Rift System. In this paper, we show the recent state of the seismic activity of this zone in connection with the seismic activity generated by the volcanoes Nyiragongo and Nyamuragira. The pattern of cumulative energy release by these volcanoes shows a steplike increase that is believed to be a precursor of volcanic eruptions.

Zana, N.; Kamba, M.; Katsongo, S.; Janssen, Th.

1989-11-01

332

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.

T. Bornhorst

333

Deformation during the 1975-84 Krafla rifting crisis, NE Iceland, measured by optical image correlation  

NASA Astrophysics Data System (ADS)

In this study we combine results from optical image correlation of SPOT, KH-9 spy satellite and aerial photos, EDM data and high resolution topographic data to better constrain the 3D deformation associated with the 1975-84 Krafla rifting crisis, NE Iceland. Inversion of the various geodetic datasets yields new volumes for the amount of material injected into the crust during this rifting crisis. Correlation of aerial photos from 1957 and 1990 for the middle section of the 2 km-wide Krafla fissure swarm, along with DEM differencing of their respective 1957 and 1990 DEM's (extracted using photogrammetric techniques), provides constraints on the full 3D displacement field spanning the entire rifting period. Elastic dislocation modeling of this displacement data is then used to determine the geometry of faulting and diking in the crust. In contrast to leveling data from the northern end of the fissure swarm (Rubin, et al., 1988), we find that dikes do not extend into the upper 1-2 km, where extension is accommodated primarily by faulting in the fissure swarm. Dislocation modeling of a 4 m-wide dike injected between 2 km and 6 km in the crust produces a maximum surface strain which reaches the elastic yield limit for rock (derived from laboratory experiments of deformed granite) at two points spanning a 2 km-wide zone above the dike, and which corresponds with the location of the major rift-bounding faults of the Krafla fissure swarm. If dikes extend nearer to the surface, the predicted fissure zone width would be correspondingly smaller (consistent with the southern-end of the fissure swarm), while deeper diking produces a wider fissure swarm (consistent with the northern-end of the fissure swarm). The apparent northward increase in depth of diking is consistent with the flexural effects of rift-margin topography (Behn, et al., 2006); increased flexure in the south, where the Krafla caldera is located, results in the promotion of shallow diking, where as subdued topography in the north promotes deeper diking. Correlation of aerial photos between 1957 and 1976 (during the early stages of the rifting crisis) indicate 2 m extension, which is localized on faults along the northern end of the fissure swarm. No fault slip occurs in the central section of the fissure swarm during the same period, suggesting extension in the north during the early stages of rifting may result from dike injections sourced from the north (possibly offshore), rather than the Krafla caldera to the south. A similar variation in magmatic source region was also observed during the 2005-2009 Afar rifting crisis in East Africa.

Hollingsworth, J.; Leprince, S.; Avouac, J.; Ayoub, F.

2011-12-01

334

Orogenic bending around a rigid Proterozoic magmatic rift beneath the Central Appalachian Mountains  

NASA Astrophysics Data System (ADS)

The transition zone between the modern northern and southern Appalachian Mountains is located in Pennsylvania, where the structural orientation of the fold-and-thrust belt changes from north-south to east-west, and the orogeny narrows significantly. Vintage studies of wide-angle reflection and temporary broadband seismic data suggest that the crust beneath the ?8 km of foreland basin sequences was thickened and heavily intruded around the margins of and beneath a failed Neoproterozoic rift. We use receiver function analysis of broadband seismic data recorded by additional and permanent stations, along with forward and inverse modeling of Bouguer gravity data to constrain the geometry and depth extent of mafic intrusion and underplating in the rift, as well as the role of this Proterozoic heterogeneity on the location and geometry of the curvature of the Appalachian orogen. The receiver function analyses suggest that the crust is ?47-49 km thick beneath the ancient rift, about 5-7 km thicker than the surrounding area. Inverse models of gravity data indicate that the ?300 km-long zone of thickened, high density crust is bounded on both the NW and SE sides by steep contacts; its shorter NE and SW margins are also steep contacts interpreted as crustal-scale faults. Forward modeling of the gravity data, constrained by the receiver function crustal thickness estimates, sparse seismic reflection data and Euler deconvolution solutions, implies that the Proterozoic rift has been heavily intruded as well as thickened by a 7-10 km mafic underplate. Its margins appear to have been sheared along NE-striking fault zones that parallel Appalachian thrust sheet transport directions. These combined results suggest that the mid- and lower-crust of the Proterozoic rift was enriched with pyroxene, which strengthened the crust locally and localized compressional strain along its margins during the North American-African collision. Compressional strain in the pre-Appalachian crystalline crust to the southwest and northeast of the Proterozoic rift may have been more distributed, leading to the formation of oblique-slip faults orthogonal to the axes of folds, and leading to the curvature in the Pennsylvania salient. Additionally, the thrust sheets are stacked most thickly in front of the rift, suggesting that the rift served as a backstop during collision.

Benoit, Margaret H.; Ebinger, Cynthia; Crampton, Melanie

2014-09-01

335

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

336

Depositional setting and hydrocarbon source potential of the Miocene Gulf of Suez syn-rift evaporites  

SciTech Connect

The Red Sea rift basin and its northern continuation, the Gulf of Suez, has experienced continuous deposition of marine evaporites throughout much of its development from the early Miocene to the Pliocene resulting in the accumulation of up to 5 km of evaporite strata in the rift. In this paper, the geologic history of these evaporites are discussed, along with their petroleum source rock potential. The authors hypothesize that rapid deposition of organic matter occurred during episodic storms and freshening events in which a less saline surface layer developed.

Richardson, M.; Arthur, M.A.; Quinn, J.S.; Whelan, J.K.; Katz, B.J. (Univ. of Rhode Island, Narragansett (USA))

1988-08-01

337

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 N10-20E, and are therefore oblique to the main direction of Mid-Miocene border faults (N35E). 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 N10-20E 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

338

The development of the East African Rift system in north-central Kenya  

NASA Astrophysics Data System (ADS)

Between 1980 and 1986 geological surveying to produce maps on a scale of 1:250,000 was completed over an area of over 100,000 km 2 in north-central Kenya, bounded by the Equator, the Ethiopian border and longitudes 36 and 38 E. The Gregory Rift, much of which has the structure of an asymmetric half-graben, is the most prominent component of the Cenozoic multiple rift system which extends up to 200 km to the east and for about 100 km to the west, forming the Kenya dome. On the eastern shoulder and fringes two en echelon arrays of late Tertiary to Quaternary multicentre shields can be recognized: to the south is the Aberdares-Mount Kenya-Nyambeni Range chain and, to the north the clusters of Mount Kulal, Asie, Huri Hills and Marsabit, with plateau lavas and fissure vents south of Marsabit in the Laisamis area. The Gregory Rift terminates at the southern end of Lake Turkana. Further north the rift system splays: the arcuate Kinu Sogo fault zone forms an offset link with the central Ethiopian Rift system. In the rifts of north-central Kenya volcanism, sedimentation and extensional tectonics commenced and have been continuous since the late Oligocene. Throughout this period the Elgeyo Fault acted as a major bounding fault. A comparative study of the northern and eastern fringes of the Kenya dome with the axial graben reinforces the impression of regional E-W asymmetry. Deviations from the essential N-trend of the Gregory Rift reflect structural weaknesses in the underlying Proterozoic basement, the Mozambique Orogenic Belt: thus south of Lake Baringo the swing to the southeast parallels the axes of the ca. 620 Ma phase folds. Secondary faults associated with this flexure have created a "shark tooth" array, an expression of en echelon offsets of the eastern margin of the Gregory Rift in a transtensional stress regime: hinge zones where major faults intersect on the eastern shoulder feature intense box faulting and ramp structures which have counterparts in the rift system in southern Ethiopia. The NE- and ENE-trending fissures of the eastern fringes of the Kenya dome, notably in the Meru-Nyambeni areaand in the Huri and Marsabit shields, parallel late orogenic structures dated at around 580-480 Ma. Alkaline trends characterize the petrochemistry of the Cenozoic volcanics: In the Gregory Rift, voluminous Miocene alkali basalts, associated with hawaiite/mugearite lavas, define a trend culminating in the Miocene flood phonolites of the eastern shoulderand in the trachyphonolites, trachytes and peralkaline rhyolites, with associated pyroclastics, in central volcanoes such as Korosi, Paka and Silali. Such trends may manifest in the products of a single volcanic centre, also regionally on a broadly cyclic basis. On the eastern flanks of the Kenya dome the flood phonolites are less evident, but the same alkaline trends dominate the lava sequences, supplemented by nephelinitic extrusives in parts of the Nyambeni Range and in the Laisamis area. Results from recent seismicity surveys in the Laisamis area indicate that crustal extension may be currently active on the eastern fringes of the Kenya dome, but manifest at greater depths than in the axial Gregory Rift-Lake Turkana zone: a correlation is suggested with the ultra-alkaline petrochemistry of some of the eastern multicentre shields.

Hackman, B. D.; Charsley, T. J.; Key, R. M.; Wilkinson, A. F.

1990-11-01

339

Middle to late cenozoic magmatism of the southeastern Colorado plateau and central Rio Grande rift (New Mexico and Arizona, U.S.A.) : a model for continental rifting  

USGS Publications Warehouse

The region of the present Rio Grande rift and southeastern Colorado Plateau underwent lithospheric extension during middle to late Cenozoic deformation affecting the entire southwestern U.S. Lithospheric mantle was disrupted, and in many regions displaced or replaced by asthenospheric mantle at depths from which basaltic magmas were derived and erupted to the surface. Study of the igneous rocks erupted or intruded during this deformation yields insights into processes of magmatism associated with extension of continental lithosphere. Magmatic rocks associated with an early (late Oligocene-early Miocene) ductile phase of extension are dominantly basaltic andesites and related, calc-alkaline intermediate to silicic derivative rocks. Mafic magmas were probably derived from isotopically "enriched" lithospheric mantle. Igneous rocks associated with a later (middle Miocene-Holocene), more brittle phase of extension include widespread basaltic rocks and localized central volcanoes of intermediate to silicic composition. Isotopic compositions of mafic rocks, which include both tholeiitic and alkalic basalts, correlate strongly with tectonic setting and lithospheric structure. Basalts erupted in areas of greatest crustal extension, such as the central and southern rift and Basin and Range province, were derived from isotopically "depleted" (correlated with "asthenospheric") mantle. Also, isotopic compositions of Pliocene to Holocene basalts are slightly more depleted than those of Miocene basalts, suggesting that subcrustal lithospheric mantle was thinned during late Miocene extension. Intermediate rocks of the central volcanoes formed by a complex combination of processes, probably dominated by fractional crystallization and by assimilation of upper and lower crust in isolated, small magma chambers. The petrologic, geochemical, and isotopic data are compatible with a model, derived first from geophysical data, whereby lithosphere is thinned beneath the central rift and southeastern Colorado Plateau, with greatest thinning centered beneath the axis of the rift. A lithospheric model involving uniform-sense simple shear does not appear compatible with the data as presently understood. ?? 1991.

Baldridge, W.S.; Perry, F.V.; Vaniman, D.T.; Nealey, L.D.; Leavy, B.D.; Laughlin, A.W.; Kyle, P.; Bartov, Y.; Steinitz, G.; Gladney, E.S.

1991-01-01

340

Increased mantle heat flow with on-going rifting of the West Antarctic rift system inferred from characterisation of plagioclase peridotite in the shallow Antarctic mantle  

NASA Astrophysics Data System (ADS)

The lithospheric, and shallow asthenospheric, mantle in Southern Victoria Land are known to record anomalously high heat flow but the cause remains imperfectly understood. To address this issue plagioclase peridotite xenoliths have been collected from Cenozoic alkalic igneous rocks at three localities along a 150 km transect across the western shoulder of the West Antarctic rift system in Southern Victoria Land, Antarctica. There is a geochemical, thermal and chronological progression across this section of the rift shoulder from relatively hot, young and thick lithosphere in the west to cooler, older and thinner lithosphere in the east. Overprinting this progression are relatively more recent mantle refertilising events. Melt depletion and refertilisation was relatively limited in the lithospheric mantle to the west but has been more extensive in the east. Thermometry obtained from orthopyroxene in these plagioclase peridotites indicates that those samples most recently affected by refertilising melts have attained the highest temperatures, above those predicted from idealised dynamic rift or Northern Victoria Land geotherms and higher than those prevailing in the equivalent East Antarctic mantle. Anomalously high heat flow can thus be attributed to entrapment of syn-rift melts in the lithosphere, probably since regional magmatism commenced at least 24 Myr ago. The chemistry and mineralogy of shallow plagioclase peridotite mantle can be explained by up to 8% melt extraction and a series of refertilisation events. These include: (a) up to 8% refertilisation by a N-MORB melt; (b) metasomatism involving up to 1% addition of a subduction-related component; and (c) addition of ~ 1.5% average calcio-carbonatite. A high MgO group of clinopyroxenes can be modelled by the addition of up to 1% alkalic melt. Melt extraction and refertilisation mainly occurred in the spinel stability field prior to decompression and uplift. In this region mantle plagioclase originates by a combination of subsolidus recrystallisation during decompression within the plagioclase stability field and refertilisation by basaltic melt.

Martin, A. P.; Cooper, A. F.; Price, R. C.

2014-03-01

341

Earthquake-induced barium anomalies in the Lisan Formation, Dead Sea Rift valley, Israel  

E-print Network

Earthquake-induced barium anomalies in the Lisan Formation, Dead Sea Rift valley, Israel Amitai Editor: L. Stixrude Keywords: barium Dead Sea seismite earthquake Lisan recurrence rate Prominent barium The present paper is a rst report of prominent barium concen- tration anomalies in seismically perturbed

Marco, Shmuel "Shmulik"

342

Formation and evolution of the Nova Scotian rifted margin: Evidence from deep seismic reflection data  

Microsoft Academic Search

A deep marine seismic reflection profile was obtained across the Mesozoic rifted continental margin off Nova Scotia, eastern Canada. This profile crosses the Scotian Basin, one of the deepest basins on the margin of eastern North America, and it complements other deep crustal seismic data on this margin. The seismic data have been interpreted in conjunction with gravity anomaly and

C. E. Keen; D. P. Potter

1995-01-01

343

Formation and evolution of the Nova Scotian rifted margin: Evidence from deep seismic reflection data  

Microsoft Academic Search

A deep marine seismic reflection profile was obtained across the Mesozoic rifted continental margin off Nova Scotia, eastern Canada. This profile crosses the Seotian Basin, one of the deepest basins on the margin of eastern North America, and it complements other deep crustal seismic data on this margin. The seismic data have been interpreted in conjunction with gravity anomaly and

C. E. Keen; D. P. Potter

1995-01-01

344

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

E-print Network

A Fusion-Inhibiting Peptide against Rift Valley Fever Virus Inhibits Multiple, Diverse Viruses of America Abstract For enveloped viruses, fusion of the viral envelope with a cellular membrane is critical for a productive infection to occur. This fusion process is mediated by at least three classes of fusion proteins

345

Potential for North American Mosquitoes to Transmit Rift Valley Fever Virus  

Technology Transfer Automated Retrieval System (TEKTRAN)

The recent outbreaks of disease caused by Rift Valley fever virus (RVFV) in Kenya, Mauritania, Yemen, Tanzania, Somalia, and Madagascar indicate the potential for RVFV to cause severe disease in both humans and domestic animals and its potential to be introduced into new areas, including North Ameri...

346

USDA, ARS, ABDRL Research on Countermeasures for Rift Valley Fever Virus  

Technology Transfer Automated Retrieval System (TEKTRAN)

The United State Department of Agriculture, Agriculture Research Service has recently established research program to address countermeasures for of Rift Valley fever (RVF) virus (RVFV). The recent outbreak in Kenya, Tanzania and Somalia demonstrates the impact this virus can have on human and live...

347

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

348

Present-day kinematics of Eastern African Rift tectonic plates using continuous and episodic GPS data  

Microsoft Academic Search

This paper focuses on the tectonic setup of the East African Rift, a complex system that extends from the Afar region where the triple junction between Somalia, Nubia and Arabia plates is located to the South West Indian Ridge where the Nubia and Antarctica meets with a third tectonic unit - possibly the so-called Lwandle block which is considered by

R. M. Fernandes; J. P. Ferreira; J. L. Quembo; L. Combrinck; C. M. Kamamia; A. Al-Aydrus; Y. Boodhoo

2009-01-01

349

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

350

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

Federal Register 2010, 2011, 2012, 2013, 2014

...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

351

Geometry and scaling relations of a population of very small rift-related normal faults  

Microsoft Academic Search

Exceptionally well exposed normal faults within the Solite Quarry of the Dan River rift basin range in length from a few millimetres to a few metres and are possibly the smallest visible faults studied to date. Displacement is greatest at or near the center of isolated faults and decreases toward the fault tips. Relay structures form between closely overlapping faults.

Roy W. Schlische; Scott S. Young; Rolf V. Ackermann; Anupma Gupta

1996-01-01

352

Blood Meal Analysis of Mosquitoes Involved in a Rift Valley fever Outbreak  

Technology Transfer Automated Retrieval System (TEKTRAN)

Background: Rift Valley fever (RVF) is a zoonosis of domestic ruminants in Africa. Bloodfed mosquitoes collected during the 2006-2007 RVF outbreak in Kenya were analyzed to determine the virus infection status and animal source of the bloodmeals. Bloodmeals from individual mosquito abdomens were sc...

353

Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode  

Microsoft Academic Search

Seafloor spreading centres show a regular along-axis segmentation thought to be produced by a segmented magma supply in the passively upwelling mantle. On the other hand, continental rifts are segmented by large offset normal faults, and many lack magmatism. It is unclear how, when and where the ubiquitous segmented melt zones are emplaced during the continental rupture process. Between 14

Tim J. Wright; Cindy Ebinger; Juliet Biggs; Atalay Ayele; Gezahegn Yirgu; Derek Keir; Anna Stork

2006-01-01

354

Factors Affecting the Ability of American Mosquitoes to Transmit Rift Valley Fever Virus  

Technology Transfer Automated Retrieval System (TEKTRAN)

The recent outbreaks of disease caused by Rift Valley fever virus (RVFV) in Kenya, Mauritania, Yemen, Tanzania, Somalia, and Madagascar indicate the potential for RVFV to cause severe disease in both humans and domestic animals and its potential to be introduced into new areas, including North Ameri...

355

The Great Rift Valleys of Pangea in Eastern North America Volume 1, Tectonics, Structure, and Volcanism  

E-print Network

Errata for The Great Rift Valleys of Pangea in Eastern North America Volume 1, Tectonics, Structure with the synthetic APW path for North America of Courtillot, Besse, and Theveniaut (1994). #12; minus precipitation (E-P) for the modern land plus ocean surface (Crowley and North 1991), which has

Olsen, Paul E.

356

CURRENT ISSUES AND CONCERNS REGARDING RIFT VALLEY FEVER, AN EMERGING VIRUS THREAT  

Technology Transfer Automated Retrieval System (TEKTRAN)

Rift Valley fever (RVF) virus is a mosquito-borne zoonotic hemorrhagic disease that causes 100% abortions in cattle, sheep, and goats and is often fatal to young animals. Though currently confined mainly to Africa this disease could be introduced into the U.S. and spread via mosquitoes at least as ...

357

Rift Valley fever virus structural proteins: expression, characterization and assembly of recombinant proteins  

Microsoft Academic Search

BACKGROUND: Studies on Rift Valley Fever Virus (RVFV) infection process and morphogenesis have been hampered due to the biosafety conditions required to handle this virus, making alternative systems such as recombinant virus-like particles, that may facilitate understanding of these processes are highly desirable. In this report we present the expression and characterization of RVFV structural proteins N, Gn and Gc

Li Liu; Polly Roy

2008-01-01

358

Seismicity In The Aigion Area As Seen By The Corinth Rift Seismological Network  

Microsoft Academic Search

Since April 2000, a network of 8 seismological stations, upgraded to 12 stations in June 2001, is recording continuously the microseismicity of the Aigion area, in the Corinth Rift. The level of activity has been high, the earthquakes occuring mostly in swarms. Several small seismic crisis, lasting from few days to few months have oc- cured within the network. The

H. Lyon-Caen; K. Makropoulos; P. Papadimitriou; A. Deschamps; P. Bernard; F. Pacchianni; S. Bourouis; H. Castarede; F. Cornet; G. Patau

2002-01-01

359

Vector Competence of Selected African Mosquito (Diptera: Culicidae) Species for Rift Valley Fever Virus  

Technology Transfer Automated Retrieval System (TEKTRAN)

Outbreaks of Rift Valley fever (RVF) in Egypt, Yemen, and Saudi Arabia have indicated the potential for this disease to spread from its enzootic areas in sub-Saharan Africa. Because little is known about the potential for most African mosquito species to transmit RVF virus (RVFV), we conducted stud...

360

Neoproterozoic strata of southeastern Idaho and Utah: record of Cryogenian rifting and glaciation  

E-print Network

Chapter 38 Neoproterozoic strata of southeastern Idaho and Utah: record of Cryogenian rifting and glaciation PAUL KARL LINK1* & NICHOLAS CHRISTIE-BLICK2 1 Department of Geosciences, Idaho State University@isu.edu) Abstract: Neoproterozoic strata in southeastern Idaho and Utah include the ,766 Ma Uinta Mountain Group

Christie-Blick, Nicholas

361

Assessment of U.S. Agriculture Sector and Human Vulnerability to a Rift Valley Fever Outbreak  

E-print Network

on the assessment of the U.S. agricultural sector and human vulnerability to a Rift Valley Fever (RVF) outbreak and the value of a select set of alternative disease control strategies. RVF is a vector-borne, zoonotic disease that affects both livestock and humans...

Hughes, Randi Catherine

2011-08-08

362

Remelting in caldera and rift environments and the genesis of hot, ''recycled'' rhyolites  

Microsoft Academic Search

Large and small volume rhyolites are generated in calderas and rift zones, inheriting older and isotopically diverse crystal populations from their volcanic predecessors. Low-d 18 O values in many of these rhyolites suggest that they were derived from the remelting of solid, hydrothermally altered by meteoric water protoliths that were once close to the surface, but become buried by caldera

A. G. Simakin; I. N. Bindeman

2012-01-01

363

Spreading process of the northern Mariana Trough: Rifting-spreading transition at 22N  

E-print Network

southward from incipient rifting to seafloor spreading within this region. This study aims to clarify of seafloor spreading after the transition. The new data set includes swath bathymetry with side- scan images is proved by seafloor-spreading fabric in the bathymetry, clear magnetic lineations, and the bull

Utrecht, Universiteit

364

Evolution of magma-poor continental margins from rifting to seafloor spreading.  

PubMed

The rifting of continents involves faulting (tectonism) and magmatism, which reflect the strain-rate and temperature dependent processes of solid-state deformation and decompression melting within the Earth. Most models of this rifting have treated tectonism and magmatism separately, and few numerical simulations have attempted to include continental break-up and melting, let alone describe how continental rifting evolves into seafloor spreading. Models of this evolution conventionally juxtapose continental and oceanic crust. Here we present observations that support the existence of a zone of exhumed continental mantle, several tens of kilometres wide, between oceanic and continental crust on continental margins where magma-poor rifting has taken place. We present geophysical and geological observations from the west Iberia margin, and geological mapping of margins of the former Tethys ocean now exposed in the Alps. We use these complementary findings to propose a conceptual model that focuses on the final stage of continental extension and break-up, and the creation of a zone of exhumed continental mantle that evolves oceanward into seafloor spreading. We conclude that the evolving stress and thermal fields are constrained by a rising and narrowing ridge of asthenospheric mantle, and that magmatism and rates of extension systematically increase oceanward. PMID:11557977

Whitmarsh, R B; Manatschal, G; Minshull, T A

2001-09-13

365

Complex Records of Environmental Character and Change in Neogene African Rift Basins  

Microsoft Academic Search

The Neogene rift basins of East Africa preserve complex environmental records encompassing the effects of tectonics and climate along with the influences of volcanism, landscape succession and biotic change. Detailed reconstruction of individual basin evolution, along with the integration of parallel basin histories can allow the isolation of controlling factors, particularly when they are represented by distinctive signatures. Major Neogene

C. S. Feibel

2004-01-01

366

SATELLITE-BASED RIFT VALLEY FEVER FORECASTS PREDICT A LARGE YELLOW FEVER EPIDEMIC IN SUDAN, 2005  

Technology Transfer Automated Retrieval System (TEKTRAN)

Sustained, heavy East African rainfall is associated with Aedes spp. breeding in Kenyan grasslands and Rift Valley Fever (RVF) epidemics. Validated RVF forecasts use satellite measurements of vegetation greenness (which increases after heavy rains) and other eco-climate indicators. These models may ...

367

Violence and Exodus in Kenya's Rift Valley, 2008: Predictable and Preventable?  

Microsoft Academic Search

This article offers a preliminary analysis of the outbreak of violence in the Rift Valley Province of Kenya, over January and February 2008, following the national elections of December 2007. Maps of the earliest phase of the violence are reproduced to illustrate the sequencing and location of conflict. The causes of the violence are explored through discussion of historical patterns

David Anderson; Emma Lochery

2008-01-01

368

ELSEVIER Tectonophysics 294 (1998) 2142 Seismic record of tectonic evolution and backarc rifting in the southern  

E-print Network

describe systematically a complete cross-section from the East China Sea continental shelf to the Ryukyu Pleistocene, pre-rift deposits of the Shimajiri Group accumulated over a wide region from the East China Sea Suyehiro a , Asahiko Taira a a Ocean Research Institute, University of Tokyo, Nakano-ku, Tokyo 164, Japan b

Lin, Andrew Tien-Shun

369

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

370

Petrology and geochemistry of the Tertiary Suez rift volcanism, Sinai, Egypt  

NASA Astrophysics Data System (ADS)

The Tertiary basaltic rocks of Southwestern Sinai, situated along the Wadi Nukhul-Wadi Matullah-Wadi El Tayiba join, were selected to study the Gulf of Suez rift related-lavas and their geochemical and petrological relation with the rifting process. Whole rock samples were studied petrographically and analysed for major and trace elements. The samples from dykes, sills and flows from multiple magmatic events display a large variety in texture and in modal mineral compositions. They range from olivine dolerites and olivine-bearing basalts to vitrophyric, texturally heterogeneous basalts and crystal lithic tuffs. The transitional tholeiitic basalts display low compatible element concentrations and an enrichment of the whole spectrum of the incompatible elements. Major, trace and Rare Earth Element data suggest that the melts formed by 5% melting of mantle peridotite at the spinel-garnet transition zone (80-90 km depth), in the presence of 2-4% residual garnet. During the melt ascent, the fractionating phases were olivine, clinopyroxene and, to a lesser extent, plagioclase. Thermobarometric calculations indicate the presence of two crystallization levels beneath the Gulf of Suez rift: a shallower stage at 15-20 km and a deeper stage at depths of 25-30 km. The mantle source consists of streaks and blobs of enriched mantle, preserved in the geochemical signatures of these rocks. The enriched mantle sources melted preferentially compared to the surrounding ambient mantle and thus led to a preferential enrichment of the sources of the Gulf of Suez rift.

Shallaly, N. A.; Beier, C.; Haase, K. M.; Hammed, M. S.

2013-11-01

371

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

372

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, mcclintock@berkeley.edu Received on July 15, 2009; accepted on December 14, 2009 Urban agriculture (UA relocalization, urban agriculture (UA) is sprouting up in the empty spaces of post-industrial landscapes

373

Dike-induced faulting in rift zones of Iceland and Afar  

Microsoft Academic Search

Geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the Earth's surface. Elastic modeling of the vertical ground-surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at

Allan M. Rubin; David D. Pollard

1988-01-01

374

Riparian Rifts in the Space-Time Continuum Noel P. Gurwick1  

E-print Network

1 2 3 4 5 6 7 8 9 Riparian Rifts in the Space-Time Continuum Noel P. Gurwick1 , Peter M. Groffman 15 Keywords: Riparian zone, denitrification, groundwater, aquifer, roots, subsurface, subsurface, and riparian zones sit atop the list of N removal hot spots in the landscape (Hill 1996). Looking across

Gurwick, Noel P.

375

Utility of Antibody Avidity for Rift Valley Fever Virus Vaccine Potency and Immunogenicity Studies  

Technology Transfer Automated Retrieval System (TEKTRAN)

Disease outbreaks caused by arthropod-borne animal viruses (arboviruses) resulting in significant livestock and economic losses world-wide appear to be increasing. Rift Valley fever (RVF) virus is an important arbovirus that causes lethal disease in cattle, camels, sheep and goats in sub-Saharan Afr...

376

Rift Valley fever risk map model and seroprevalence in selected wild ungulates and camels from Kenya  

Technology Transfer Automated Retrieval System (TEKTRAN)

Since the first isolation of Rift Valley fever virus (RVFV) in the 1930s, there have been multiple epizootics and epidemics in animals and humans in sub-Saharan Africa. Prospective climate-based models have recently been developed that flag areas at risk of RVFV transmission in endemic regions based...

377

GEOLOGY, GEOCHEMISTRY, AND STRATIGRAPHY OF THE LEMUDONG'O FORMATION, KENYA RIFT VALLEY  

Microsoft Academic Search

The Lemudong'o Formation is defined here as part of a late Miocene to Late Pleistocene sequence of stratified lavas, air-fall and waterlain tuffs, lacustrine, alluvial, and fluvial sediments, and paleosols, that crop out over an approximately 25 3 50 km area on the western margin of the southern Kenyan Rift Valley, approximately 100 km west of Nairobi. The study area

STANLEY H. AMBROSE; CHRISTOPHER M. NYAMAI; ELIUD M. MATHU

378

Pupils' Environmental Awareness and Knowledge: A Springboard for Action in Primary Schools in Kenya's Rift Valley  

ERIC Educational Resources Information Center

This study was carried out with 276 standard eight pupils in eleven primary schools in the rural town of Narok in Kenya's Rift Valley. It evaluated their awareness of key environmental issues in their local area and their knowledge about the causes, effects and solutions pertaining to these environmental issues. A descriptive research design was

Mutisya, Sammy M.; Barker, Miles

2011-01-01

379

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

380

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

381

Rifting process and thermal evolution of the continental margin of Eastern Canada determined from subsidence curves  

Microsoft Academic Search

The subsidence history of the Nova Scotia and Labrador shelves, determined from deep exploratory well data, indicates that these regions underwent extension during rifting in the Early Jurassic and Late Cretaceous, respectively, and have since subsided passively due to conductive cooling of the lithosphere. The timing of the extension process is consistent with the ages of oldest sea floor adjacent

L. Royden; C. E. Keen

1980-01-01

382

3D Graph Visualization with the Oculus Rift Virtual Graph Reality  

E-print Network

3D Graph Visualization with the Oculus Rift Virtual Graph Reality Farshad Barahimi, Stephen Wismath, possibly with a stereographic effect such as shutter glasses, or anaglyphically, · displayed in a virtual reality environment such as a CAVE, or · printed as a physical model with a 3D printer. Early studies

Wismath, Stephen

383

Experimental infection of the African buffalo with the virus of Rift Valley fever  

Microsoft Academic Search

Five African buffalo(Syncercus caffer) were inoculated with pantropic Rift Valley fever virus; 1 of 2 pregnant animals aborted. Four of the 5 developed a viraemia persisting for 48 h with an antibody response comparable to that found in cattle after RVF infection. No neutralising antibody was found in a series of some 114 feral buffalo sera from different parts of

F. G. Davies; L. Karstad

1981-01-01

384

Master1RservoirsGologiquesDynamiquedesBassins-MichelSranne Post-rift tectonics on passive margins  

E-print Network

stress => plate-tectonic-driven deformation 85Ma #12;20/11/ 47 Master1Réservoirs20/11/ 37 Master1RéservoirsGéologiquesDynamiquedesBassins-MichelSéranne Post-rift tectonics for gravitational tectonics load Tilting=slope Low friction decollement f(load, slope, friction) #12;20/11/ 39

Demouchy, Sylvie

385

New Diagnostic Tools for Detecting Rift Valley Fever (RVF) and other Arboviruses  

Technology Transfer Automated Retrieval System (TEKTRAN)

The outbreak of West Nile virus in the United Sates and the recent outbreak of Rift Valley fever (RVF) virus in East Africa have highlighted the need for validated early detection tools for arthropod-borne animal diseases. The Arthropod-Borne Animal Diseases Research Laboratory (ABADRL) has been inv...

386

Potential for North American mosquitoes to transmit Rift Valley fever virus  

Technology Transfer Automated Retrieval System (TEKTRAN)

The recent outbreaks of disease caused by Rift Valley fever virus (RVFV) in Kenya, Mauritania, Yemen, Tanzania, Somalia, and Madagascar indicate the potential for RVFV to cause severe disease in both humans and domestic animals and its potential to be introduced into new areas, possibly even North A...

387

Immunohistochemical Detection of Rift Valley Fever Virus with Non-Infectious, Recombinant Viral Protein Antibodies  

Technology Transfer Automated Retrieval System (TEKTRAN)

Rift Valley fever virus (RVFV) causes re-emerging disease outbreaks and abortion storms in mature cattle, sheep, and goats, and can cause 100% mortality in young animals. The spread of this exotic, insect transmitted virus is of particular concern because of its widely recognized potential for being...

388

Mercury in fish from three rift valley lakes (Turkana, Naivasha and Baringo), Kenya, East Africa  

Microsoft Academic Search

Total mercury (THg) concentrations were measured for various fish species from Lakes Turkana, Naivasha and Baringo in the rift valley of Kenya. The highest THg concentration (636 ng g?1 wet weight) was measured for a piscivorous tigerfish Hydrocynus forskahlii from Lake Turkana. THg concentrations for the Perciformes species, the Nile perch Lates niloticus from Lake Turkana and the largemouth bass

L. M. Campbell; O. Osano; R. E Hecky; D. G Dixon

2003-01-01

389

Pore water chemistry of an alkaline rift valley lake: Lake Turkana, Kenya  

Microsoft Academic Search

Lake Turkana is the largest closed basin lake in the African rift system. It has evolved through the past 5000 years to become a moderately alkaline lake. Previous mass balance argument suggest that sulfate is removed from the lake by sulfate reduction in the sediments, and that the lake is accumulating in chloride, sodium, and alkalinity. Studies of pore water

T. E. Cerling; T. C. Johnson; J. D. Halfman; G. Lister

1985-01-01

390

Significant crustal thinning beneath the Baikal rift zone: New constraints from receiver function analysis  

E-print Network

Significant crustal thinning beneath the Baikal rift zone: New constraints from receiver function 2004; accepted 6 October 2004; published 30 October 2004. [1] Thinning of the crust of more than 10 km in contradicting amount of thinning, ranging from almost none to more than 10 km. We measure crustal thickness

Gao, Stephen Shangxing

391

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

392

Geochemical studies of abyssal lavas recovered by DSRV Alvin from Eastern Galapagos Rift, Inca Transform, and Ecuador Rift: 2. Phase chemistry and crystallization history  

NASA Astrophysics Data System (ADS)

A diverse suite of lavas recovered by DSRV Alvin from the eastern Galapagos rift and Inca transform includes mid-ocean ridge tholeiitic basalts (MORB), iron- and titanium-enriched basalts (FeTi basalts), and abyssal andesites. Rock types transitional in character (ferrobasalts and basaltic andesites) were also recovered. The most mafic glassy basalts contain plagioclase, augite, and olivine as near-liquidus phases, whereas in more fractionated basalts, pigeonite replaces olivine and iron-titanium oxides crystallize. Plagioclase crystallizes after pyroxenes and iron-titanium oxides in andesites, possibly due to increased water contents or cooling rates. Apatite phenocrysts are present in some andesitic glasses. Ovoid sulfide globules are also common in many lavas. Compositional variations of phenocrysts in glassy lavas reflect changes in magma chemistry, temperature of crystallization, and cooling rate. The overall chemical variations parallel the chemical evolution of the lava suite and are similar to those in other fractionated tholeiitic complexes. Elemental partitioning between plagioclase-, pyroxene-, and olivine-glass pairs suggests that equilibration occurred at low pressure in a rather restricted temperature range. Various geothermometers indicate that the most primitive MORB began to crystallize between 1150 and 1200C with fo2 < 10-7 atm. Coexisting iron-titanium oxides in more evolved lavas yield temperatures 1025C to as low as 910C withfo2 from 10-8 to 10-12 atm. PH 2 o could have been as high as 1 kbar during andesite crystallization. Compositions of the lavas from the Galapagos rift follow the experimentally determined (1 atm-QFM) liquid line of descent. Least squares calculations for the major elements indicate that the entire suite of lavas can be produced by fractional crystallization of successive residual liquids from a MORB parent magma. FeTi basalts represent 30-65 cumulative weight percent crystallization of plagioclase, augite, and olivine. An additional 30-50% fractionation of pyroxenes, plagioclase, titanomagnetite, and possible apatite is required to generate andesite from FeTi basalt liquids. The presence of partially resorbed mafic xenocrysts in some andesites, FeTi basalt inclusions in these xenocrysts, high-silica glass inclusions in basaltic andesites, and the transitional chemistry of basaltic andesites are evidence that some magma mixing occurred during crystal fractionation. The diversity of lava types recovered at single dive sites suggests that low-pressure fractional crystallization is a very efficient process beneath the eastern Galapagos rift and that isolated magma bodies must be present at shallow levels beneath the accretionary locus. Voluminous FeTi basalts erupted at the rift-transform intersection are genetically related to the rift lavas, but their restricted chemistry reflects different thermal and tectonic controls on their petrogenesis.

Perfit, Michael R.; Fornari, Daniel J.

1983-12-01