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1

Continental Rifts  

NASA Astrophysics Data System (ADS)

Continental Rifts, edited by A. M. Quennell, is a new member of the Benchmark Papers in Geology Series, edited in toto by R. W. Fairbridge. In this series the individual volume editors peruse the literature on a given topic, select a few dozen papers of ostensibly benchmark quality, and then reorder them in some sensible fashion. Some of the original papers are republished intact, but many are chopped into “McNuggets™” of information. Depending upon the volume editor, the chopping process can range from a butchering job to careful and prudent pruning. The collecting, sifting, and reorganizing tasks are, of course, equally editor-sensitive. The end product of this series is something akin to a set of Reader's Digest of Geology.

Rosendahl, B. R.

2

Volcanism at rifts  

SciTech Connect

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

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

1989-07-01

3

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

4

The Northeast Brazilian Rift System  

Microsoft Academic Search

The northeast Brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the South Atlantic. These basins represent the locus of intersection of the Southern and Equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases. Similar data are not found

Renato M. Darros de Matos

1992-01-01

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

Rift Valley fever  

Microsoft Academic Search

Summary Rift Valley fever (RVF) is an arthropod-borne viral disease of ruminants, camels and humans. It is also a significant zoonosis which may be encountered as an uncomplicated influenza-like illness, but may also present as a haemorrhagic disease with liver involvement; there may also be ocular or neurological lesions. In animals, RVF may be inapparent in non-pregnant adults, but outbreaks

G. H. Gerdes

2004-01-01

7

Baikal Rift Zone: Intra-cratonic rifting without Moho uplift  

NASA Astrophysics Data System (ADS)

The Baikal Rift Zone is located in Siberia at the centre of the world's largest continental area. It provies a unique opportunity for studying the processes of ongoing continental rifting in an area with thick cratonic crust. The BEST project (Baikal Explosion Seismic Transects) aims at providing seismic velocity models of the crust and uppermost mantle across and along the strike of the Baikal Rift Zone. The project comprises two deep seismic profiles at the southern end of Lake Baikal: (1) a 360 km long, NS-trending profile across the rift zone from the Mongolian border to Cheremhovo, and (2) a 360 km long EW-trending profile along-strike of the rift zone at the northern shore of the lake into the Tunka depression to the Mongolian border. The seismic sources were 13 explosions in boreholes, airgun shots in the lake, and the supervibrator located at Babushkin near the cross point between the two profiles at the shore of the lake. The velocity models show a gently deepening Moho from the Siberian Craton into the Palaeozoic fold belt to the SE of Lake Baikal without any sign of Moho uplift around the more than 10 km thick sedimentary graben structure. Strong seismic reflectivity slightly offset to the NE from the rift zone indicates the presence of pronounced magmatic intrusions in the lower crust, despite of the non-volcanic appearance of much of the rift zone. These intrusions may have compensated rifting associated lower crustal thinning. Further there is no sign of any reduction of the seismic Pn wave velocity around the rift zone. These features indicates that the existing models of continental rifting may not be adequate for describing the underlying processes in thick, cratonic lithosphere.

Thybo, H.; Nielsen, C.; Jensen, M.; Suvorov, V. D.; Perchuc, E.

2006-12-01

8

Evolution of Rifting in Africa  

Microsoft Academic Search

THE recent report by Girdler et al.1 on the rift system of East Africa is of great interest because it presents a crustal model, based on new gravity observations, which suggests an intrusion of low velocity mantle material rising to the base of the sialic crust which underlies the eastern and western rift valleys of East Africa between 3° N

R. B. McConnell

1970-01-01

9

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

10

The Thaumasia "Rift", Mars -- Is It a Rift?  

NASA Astrophysics Data System (ADS)

We describe the morphology of a large and complex graben structure in the western Thaumasia region (the Thaumasia graben or "rift"). We consider fault geometries, determine extension, and discuss shortly possible models for its origin.

Hauber, E.; Kronberg, P.

2003-03-01

11

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.

2004-12-01

12

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

13

[Rift Valley fever].  

PubMed

Rift Valley Fever (RVF) is a zoonotic arbovirosis. Among animals, it mainly affects ruminants, causing abortions in gravid females and mortality among young animals. In humans, RVF virus infection is usually asymptomatic or characterized by a moderate fever. However, in 1 to 3% of cases, more severe forms of the disease (hepatitis, encephalitis, retinitis, hemorrhagic fever) can lead to the death of infected individuals or to major sequels. The RVF virus (Bunyaviridae, genus Phlebovirus) was identified for the first time in the 1930s in Kenya. It then spread over almost all African countries, sometimes causing major epizootics/epidemics. In 2000, the virus was carried out of Africa, in the Middle East Arabian Peninsula. In 2007-2008, Eastern-African countries, including Madagascar, reported significant episodes of RVF virus, this was also the case for the Comoros archipelago and the French island of Mayotte. This ability to spread associated with many vectors, including in Europe, and high viral loads in infected animals led the health authorities worldwide to warn about the potential emergence of RVF virus in areas with a temperate climate. The awareness has increased in recent years with climate changes, which may possibly modify the vector distribution and competence, and prompted many RVF virus-free countries to better prepare for a potential implantation of RVF. PMID:21295425

Pépin, M

2011-02-03

14

Thermochronological investigation of the timing of rifting and rift segmentation in the Gulf of Suez, Egypt  

Microsoft Academic Search

The Tertiary Gulf of Suez rift system is one of the best-studied continental rift systems and has inspired many fundamental geodynamic models for continental rifting. However, our limited knowledge of how extensional strain is spatially and temporally distributed has made it difficult to adequately evaluate models for the dynamic evolution of this rift. A critical aspect of constraining the evolution

W. Bosworth; D. F. Stockli

2006-01-01

15

The geometry of propagating rifts  

NASA Astrophysics Data System (ADS)

The kinematics of two different processes are investigated, both of which have been described as rift propagation. Courtillot uses this term to describe the change from distributed to localised extension which occurs during the early development of an ocean basin. The term localisation is instead used here to describe this process, to distinguish it from Hey's type of propagation. Localisation generally leads to rotation of the direction of magnetisation. To Hey propagation means the extension of a rift into the undeformed plate beyond a transform fault. Detail surveys of the Galapagos rift have shown that the propagating and failing rifts are not connected by a single transform fault, but by a zone which is undergoing shear. The principal deformation is simple shear, and the kinematics of this deformation are investigated in some detail. The strike of most of the lineations observed in the area can be produced by such deformation. The mode of extension on the propagating rift appears to be localised for some periods but to be distributed for others. Neither simple kinematic arguments nor stretching of the lithosphere with conservation of crust can account for the observed variations in water depth.

McKenzie, Dan

1986-03-01

16

Geophysical diagnosis of MidContinent rift  

Microsoft Academic Search

Of all recognized rift systems, the Mid-Continent rift may be the most geophysically analyzed of its genre. The data base associated with this 1900-km (1180-mi) long plate tectonic feature is evenly distributed between gravity, magnetics, and seismology. Since 1983, the western arm of the rift, traced from northeastern Kansas into the Lake Superior basin, has become an oil and gas

Dickas

1986-01-01

17

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

NASA Astrophysics Data System (ADS)

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

Corti, Giacomo

2009-09-01

18

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

19

Evolution of Rifting in Africa  

Microsoft Academic Search

Studies of seismicity, age of faulting, age of volcanoes and short and long wavelength gravity anomalies all indicate that the evolution of the East African rift system is related to attenuation and early stages of break-up of the African plate.

R. W. Girdler; J. D. Fairhead; R. C. Searle; W. T. C. Sowerbutts

1969-01-01

20

Cenozoic rift formation in the northern Caribbean  

NASA Astrophysics Data System (ADS)

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

Mann, P.; Burke, K.

1984-12-01

21

Factors controlling depth of continental rifts  

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

22

The Thaumasia "rift", Mars - is it a rift?  

NASA Astrophysics Data System (ADS)

We describe the morphology of a large and complex graben structure in western Thaumasia which was often ascribed to rifting by previous authors (the Thaumasia "rift" or TR). We consider possible fault geometries, determine extension, and discuss shortly possible models for its origin. The TR is characterized by a strong (half)graben asymmetry. The master fault system changes from the western border in the northern part to the eastern border in the southern part (at ˜21^oS). Several profiles across the TR display features that might indicate a listric master fault, including an overall halfgraben geometry, tilted blocks, and a curvature of the hanging wall which is characteristic of a rollover. For a listric fault, the depth D to a detachment can be determined from the surficial fault dip (?), the tilt of the graben floor (?), and the vertical offset (d). We measure a scarp height d of ˜2000 m and floor tilts ? between 0.9^o and 2.7^o. For ? = 60^o, we obtain values of D between ˜33 km and ˜67 km (? = 2.0^o and 1.0^o). Interestingly, these values correspond very well with recent estimations of the thickness of the elastic lithosphere T_e in S-Tharsis, as given by Zuber et al. (2000): Valles Marineris ˜60 km, Solis Planum ˜35 km. A listric W-dipping master fault in the middle and southern part of the TR might indicate gravitational gliding of an unstable part of the outward verging fold-and-thrust plateau margin towards W, i.e., toward the foreland of Thaumasia. However, slip along planar faults can also produce tilted graben floors and hanging wall flexure, so the observed morphology does not allow any firm statement about the fault geometry. Extension (assuming planar fault planes) was determined using the vertical displacement at faults. In the N, most of the extension occurred along a few major faults. In the S, it has been distributed among many smaller faults. Extension is 0.5 to 4.5 km (strain 1 to 3%). This is much less than 10 km, as previously calculated by other authors from scarp widths and shadows. While the structural geometry of the TR is more similar to classical rifts than that of Valles Marineris, there are better Martian analogues to terrestrial continental rifts, e.g., Tempe Fossae. Essential characteristics of continental rifts are: Regional domal uplift, crustal break-up, formation of through-going rift valleys, and rift-related volcanism. The structure and morpho-tectonics of the TR and the lack of extension-volcanism do not meet these criteria of terrestrial continental rifts. So far, the geodynamic processes that led to the formation of the TR are unclear (crustal break-down due to Thaumasia uplift? magma deficit near Syria Planum? a long-lived and late center of magmatectonic activity?).

Hauber, E.; Kronberg, P.

2003-04-01

23

Continental rift zones without Moho uplift  

NASA Astrophysics Data System (ADS)

Rifting is a fundamental plate tectonic process that creates elongated depressions in the Earth's surface, which become filled with sedimentary and volcanic material, as it is presently observed at the Baikal, East African, Rhine Graben and Rio Grande Rift Zones. All rifting models predict Moho uplift due to crustal thinning, and reduced seismic velocity in the uppermost mantle due to decompression or heating from the Earth's interior. However, recently acquired data from the presently active Baikal Rift zone in Siberia and the failed Dniepr-Donets rift zone in Ukraine are examples where there is no Moho topography that can related to the rifting process. Further, data from the Kenya Rift Zone shows sign of less Moho uplift than expected from the actual extension. At all these rift zones, we observe a localized zone in the lower crust which has exceptionally high seismic velocity and is highly reflective. We suggest that rift related crustal thinning took place, but the expected Moho up-warp was compensated by magmatic intrusion in the lower crust at the high-velocity zone. This finding has significant implications for modelling of the evolution of sedimentary basins around rift structures.

Thybo, H.; Lyngsie, S.; Nielsen, C.

2008-05-01

24

Rift to Post-rift evolution of a  

Microsoft Academic Search

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.

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

2010-01-01

25

Discontinuous and diachronous evolution of the Main Ethiopian Rift: Implications for development of continental rifts  

Microsoft Academic Search

The Main Ethiopian Rift (MER) is commonly considered the archetypal magma-assisted rift. Tomographic images of upper-mantle upwellings beneath the rift, aligned anisotropy beneath magmatic segments, and pervasive magmatic modification of the crust all indicate the importance of magmatic processes in present-day rift evolution. It has been suggested that this magmatic development is responsible for the straight and continuous path the

K. Keranen; S. L. Klemperer

2008-01-01

26

Discontinuous and diachronous evolution of the Main Ethiopian Rift: Implications for development of continental rifts  

NASA Astrophysics Data System (ADS)

The Main Ethiopian Rift (MER) is commonly considered the archetypal magma-assisted rift. Tomographic images of upper-mantle upwellings beneath the rift, aligned anisotropy beneath magmatic segments, and pervasive magmatic modification of the crust all indicate the importance of magmatic processes in present-day rift evolution. It has been suggested that this magmatic development is responsible for the straight and continuous path the rift cuts across the Ethiopian Plateau. We compile new evidence indicating that the MER is not as continuous and its development not as simple as previously believed. Significant lithospheric heterogeneities are evident in our compilation of recently acquired seismic, gravity, and geologic data. Numerical models of rift propagation in such heterogeneous lithosphere show that rift propagation may stall at rheological boundaries. We propose that the heterogeneities in the MER caused irregular rift propagation, resulting in a distinct discontinuity visible within the rift lithosphere. This discontinuity in structure spatially correlates to an apparent discontinuity in the age of extension between the northern MER and the central MER, lending support to our hypothesis. Our interpretation leads to a two-phase model of rift propagation in the MER, with initial rift development primarily controlled by lithospheric structure and a later phase during which magmatic processes are dominant. During the initial phase, rift propagation was irregular and at times stalled or was diverted away from the modern rift trend along pre-existing structures. Our model, while acknowledging the importance of magmatic processes in volcanic extensional regions, shows that even in this classic example of magma-assisted rifting, inherited lithospheric structure localized initial extension and controlled rift propagation. This early phase formed the template for future rift development and continental break-up.

Keranen, K.; Klemperer, S. L.

2008-01-01

27

Lithospheric structure of the Rio Grande rift  

Microsoft Academic Search

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

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

2005-01-01

28

Hydrothermal bitumen generated from sedimentary organic matter of rift lakes – Lake Chapala, Citala Rift, western Mexico  

Microsoft Academic Search

Lake Chapala is in the Citala Rift of western Mexico, which in association with the Tepic-Zacoalco and Colima Rifts, form the well-known neotectonic Jalisco continental triple junction. The rifts are characterized by evidence for both paleo- and active hydrothermal activity. At the south shore of the lake, near the Los Gorgos sublacustrine hydrothermal field, there are two tar emanations that

Pedro F. Zárate-del Valle; Bernd R. T. Simoneit

2005-01-01

29

Igneous Intrusion Control on Stress in Continental Rift Zones and the Impact on Rift Evolution  

Microsoft Academic Search

Igneous intrusions, both contemporaneous with and pre-dating continental rifting, strongly affect stress fields within and adjacent to the rift and may control major rift processes including future volcanism, segmentation, propagation, and overall evolution. The effect of intrusive bodies on stress in an extensional regime depends on the strength of the emplaced material versus the country rock, the greater the strength

E. K. Beutel

2009-01-01

30

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

31

Rio Grande rift: problems and perspectives  

SciTech Connect

Topics and ideas addressed include: (1) the regional extent of the Rio Grande rift; (2) the structure of the crust and upper mantle; (3) whether the evidence for an axile dike in the lower crust is compelling; (4) the nature of faulting and extension in the crust; and (5) the structural and magmatic development of the rift. 88 references, 5 figures.

Baldridge, W.S.; Olsen, K.H.; Callender, J.F.

1984-01-01

32

Geophysical diagnosis of Mid-Continent rift  

SciTech Connect

Of all recognized rift systems, the Mid-Continent rift may be the most geophysically analyzed of its genre. The data base associated with this 1900-km (1180-mi) long plate tectonic feature is evenly distributed between gravity, magnetics, and seismology. Since 1983, the western arm of the rift, traced from northeastern Kansas into the Lake Superior basin, has become an oil and gas exploration frontier. The target source and reservoir rocks are of Proterozoic Y (middle Keweenawan) age (1047 +/- 35 Ma minimum). Associated sedimentary and igneous rocks crop out only in northwestern Wisconsin and the adjacent upper peninsula of Michigan. Workers have developed various geophysical models for the Mid-Continent rift, and with the advent of economic interest, the classical compressive, central horst model has been questioned. Geophysical diagnosis of the Mid-Continent rift has gone through three stages. In stage I (late 1930s to 1978), the rift was discovered and identified, and its basic outline, geographic extent, and preliminary structural model were developed. In stage II (1978-1981), the Consortium for Continental Reflection Profiling conducted surveys in Kansas and Michigan, which suggested the rift was an extensional, subsiding trough. Now, in stage III (1984 to present), reflection seismology studies along the entire western area of the rift permit a new review. Examples of industrially acquired lines will be presented.

Dickas, A.B.

1986-05-01

33

Cenozoic rift formation in the northern Caribbean  

Microsoft Academic Search

Rifts form in many different tectonic environments where the lithosphere is put into extension. An outline is provided of the distribution, orientation, and relative ages of 16 Cenozoic rifts along the northern edge of the Caribbean plate and it is suggested that these structures formed successively by localized extension as the Caribbean plate moved eastward past a continental promontory of

Paul Mann; Kevin Burke

1984-01-01

34

Stratigraphic Modelling of Continental Rifting  

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

35

Two-stage rifting in the Kenya rift: implications for half-graben models  

NASA Astrophysics Data System (ADS)

The Kerio sub-basin in the northern Kenya rift is a transitional area between the southern Kenya rift, where crustal thickness is 30 km, and the northern Kenya rift, where crustal thickness is 20 km. The lack of data on the shallow crustal structure, geometry of rift-bounding faults, and rift evolution makes it difficult to determine if the crustal thickness variations are due to pre-rift structure, or along-axis variations in crustal stretching. We reprocessed reflection seismic data acquired for the National Oil Corporation of Kenya, and integrated results with field and gravity observations to (1) delineate the sub-surface geometry of the Kerio sub-basin, (2) correlate seismic stratigraphic sequences with dated strata exposed along the basin margins, and (3) use new and existing results to propose a two-stage rifting model for the central Kenya rift. Although a classic half-graben form previously had been inferred from the attitude of uppermost strata, new seismic data show a more complex form in the deeper basin: a narrow full-graben bounded by steep faults. We suggest that the complex basin form and the northwards increase in crustal thinning are caused by the superposition of two or more rifting events. The first rifting stage may have occurred during Palaeogene time contemporaneous with sedimentation and rifting in northwestern Kenya and southern Sudan. The distribution of seismic sequences suggests that a phase of regional thermal subsidence occurred prior to renewed faulting and subsidence at about 12 Ma after the eruption of flood phonolites throughout the central Kenya rift. A new border fault developed during the second rifting stage, effectively widening the basin. Gravity and seismic data indicate sedimentary and volcanic strata filling the basin are 6 km thick, with up to 4 km deposited during the first rifting stage.

Mugisha, F.; Ebinger, C. J.; Strecker, M.; Pope, D.

1997-09-01

36

Transient rifting north of the Galápagos Triple Junction  

NASA Astrophysics Data System (ADS)

Seafloor bathymetry north of the Galápagos microplate in the Eastern Pacific Ocean contains evidence for a sequence of short-lived rifts cross-cutting abyssal hills adjacent to the East Pacific Rise. These secondary rifts are sub-parallel to the Incipient Rift that marks the current triple junction at 2°40'N and are nearly perpendicular to the direction of opening between the Cocos and Nazca plates. Secondary rifts likely initiated from the EPR by lithospheric cracking. Eventually, their activity stopped and they were carried away from the triple junction as part of the Cocos plate. We developed and analyzed numerical models of rift interaction to understand the evolution of rift configuration in this area. By varying the geometry and locations of rifts, we constrain the factors that have led to the location and orientation of secondary rifts at the northern Galápagos Triple Junction. Interaction between secondary rifts and the Cocos-Nazca Rift results in ~ 10° clockwise rotation of the secondary rift propagation direction, as observed. Furthermore, if a rift has become detached from the East Pacific Rise, a zone of reduced tension is present ahead of the rift tip, prohibiting its connection to the East Pacific Rise. Two zones of tensile stress enhancement develop along the East Pacific Rise next to the detached rift and control where new cracks are likely to form. Although the magnitude of tensile stress enhancement is controlled by the gap between the detached rift and the East Pacific Rise, whether the new rift forms north or south of the detached rift is controlled by position of the Cocos-Nazca Rift. Therefore, the sequence of ancient rifts found northeast of the current triple junction at 2°40'N represents the natural consequence of rift disconnection events and their position records the kinematic history of the gap between the East Pacific Rise and the Cocos-Nazca Rift.

Mitchell, Garrett A.; Montési, Laurent G. J.; Zhu, Wenlu; Smith, Deborah K.; Schouten, Hans

2011-07-01

37

How many rifts are there in West Africa?  

Microsoft Academic Search

The West African Rift System has, for the last ten years, been thought to consist of five interconnected rifts extending from the Gulf of Guinea deep into the heart of Africa. Careful re-examination of the geophysical evidence makes it quite clear that there are only three interconnected rifts in West Africa; the Lower Benue Rift which extends to the northeast

S. J. Freeth

1984-01-01

38

New Insights into the Transition From Magmatic to Tectonic Rifting  

Microsoft Academic Search

Magma plays a major role in the development of many rifts and continental margins. This is particularly clear for some of the more recent continental rifts including the Afro-Arabian Rift System and the breakup of South America from Africa. We are interested in how magma, injected as dikes, may lead to weakening of the lithosphere so that rifting can proceed

R. W. Bialas; W. R. Buck; R. Qin

2008-01-01

39

Flexural modeling of the midcontinent rift  

SciTech Connect

A basement profile obtained from seismic reflection data has been used to constrain a two-dimensional flexural model of basin formation for the Midcontinent Rift at a latitude of 45/sup 0/25'N. Model parameters included the thickness of the elastic plate, the basin width, and the maximum basin thickness. Modeling suggests that flexure produced a deep narrow basin along the rift axis and that the crust was thinned at the time of basin formation to an elastic thickness of 9.6 km for a plate ruptured by rifting, or 5.7 km for an unbroken plate, with corresponding maximum basin thicknesses of 14 km and 16 km respectively. The plate thickness depends most strongly on the basin width and is well constrained by the seismic data, although erosion may have narrowed the basin. The maximum basin thickness is poorly constrained because of the lack of seismic data for depths greater than about 10 km and because the strata at the center of the rift have been disturbed by a postrift compressional event which produced the St. Croix horst. Despite uncertainty about the basin thickness, the load required to flex the crust to produce the Midcontinent Rift basin is too large to be attributed to the weight of the central flood basalts unless the basin subsided into a fluid less dense than the solidified basalts. On the basis of seismic refraction data and by analogy with other rifts, we hypothesize that a magnetic ''rift pillow'' intruded in the lower crust. The basaltic pillow subsequently solidified to produce a large, high-velocity region in the lower crust, centered under the rift axis, as determined from deep seismic refraction. This crystallization and cooling may be responsible for the ''sag'' phase of rift evolution, as evidenced by laterally widespread occurrence of postvolcanic sediments. copyright American Geophysical Union 1988

Nyquist, J.E.; Wang, H.F.

1988-08-10

40

The influence of preexisting structure on the evolution of the Cenozoic Malawi rift (East African rift system)  

Microsoft Academic Search

This paper analyzes the importance of preexisting structure for the evolution of the Cenozoic Malawi rift, which constitutes the southernmost part of the western branch of the East African rift system. Kinematic analyses demonstrate that the regional extension direction rotated clockwise from ENE to SE during rifting. Cenozoic rift faults (of dip-, oblique-, and strike-slip character) rejuvenated crustal structures whenever

Uwe Ring

1994-01-01

41

How Is Lower Crust Modified As A Neo-Rift Becomes A Paleo-Rift and Part Of The Craton?  

Microsoft Academic Search

The Southern Oklahoma Aulacogen (SOA), at the southern end of Laurentia (present coordinates), if behaving as neo-rifts, such as the Rio Grande Rift, presumably possessed a rift structure in the Cambrian with a continental thickness of about 28km. Seismic data, though sparse, suggest a present thickness of the SOA is about 45km, indistinguishable from adjacent rifted Proterozoic crust. By what

M. C. Gilbert

2004-01-01

42

Magma genesis by rifting of oceanic lithosphere above anomalous mantle: Terceira Rift, Azores  

Microsoft Academic Search

The Terceira Rift formed relatively recently (~1 Ma ago) by rifting of the old oceanic lithosphere of the Azores Plateau and is currently spreading at a rate of 2-4mm\\/a. Together with the Mid-Atlantic Ridge, the Terceira Rift forms a triple junction that separates the Eurasian, African, and American Plates. Four volcanic systems (São Miguel, João de Castro, Terceira, Graciosa), three

Christoph Beier; Karsten M. Haase; Wafa Abouchami; Marc-S. Krienitz; Folkmar Hauff

2008-01-01

43

Magma genesis by rifting of oceanic lithosphere above anomalous mantle: Terceira Rift, Azores  

Microsoft Academic Search

The Terceira Rift formed relatively recently (?1 Ma ago) by rifting of the old oceanic lithosphere of the Azores Plateau and is currently spreading at a rate of 2–4mm\\/a. Together with the Mid-Atlantic Ridge, the Terceira Rift forms a triple junction that separates the Eurasian, African, and American Plates. Four volcanic systems (São Miguel, João de Castro, Terceira, Graciosa), three

Christoph Beier; Karsten M. Haase; Wafa Abouchami; Marc-S. Krienitz; Folkmar Hauff

2008-01-01

44

Kinematics of the South Atlantic rift  

NASA Astrophysics Data System (ADS)

Extensional deformation along the South Atlantic, Central African and West Africa rift systems heralded in the breakup of the western half of the Gondwana supercontinent. The interplay between a complex series of intraplate rifts profoundly affected the spatio-temporal localisation of extensional deformation and the resulting geometries of the conjugate South Atlantic margins. We present a new plate kinematic model which quantitatively integrates lithosphere deformation and observations from both passive margins and intracontinental rifts in Africa and South America. Our model eliminates the need for previously inferred large offset continental shear zones in southern South America such as the Gastre Fault system while satisfying most observations of the onset of subsidence, deformation and rifting along the conjugate rifted margins and related marginal sedimentary basins. The model indicates that extension became focussed in the South Atlantic rift basin by the Earliest Cretaceous, continuing at low extensional velocities, predominantly controlled by African intraplate deformation while there was no deformation along the future Equatorial Atlantic. The Patagonian part of South America acted as an independent plate between Antarctica, Africa, and South America during the Latest Jurassic to Aptian times, hereby opening rift basins striking at high angles to the present-day South American margin. By about 125 Ma (Barremian/Aptian) deformation and subsequent weakening along the Equatorial Atlantic rift branch yielded a large increase in extensional velocities and a change in extension direction between South America and Africa, while lithospheric extension had progressed far enough to create the enigmatic sub-salt basins in the central South Atlantic. The change in spreading direction largely eliminated intraplate deformation in Patagonia and subsequently resulted in the onset of post-rift thermal subsidence in the southern South American basins. Rupture of extended continental lithosphere in the central South Atlantic followed a few million years after this major change in kinematics and is likely responsible for the observed first-order present day margin geometry. We compare our model-derived kinematics against predictions from forward numerical models of lithosphere extension along the conjugate West-African/Brazil and the Equatorial Atlantic margins. We conclude that the multi-velocity, multi-directional rift history can consistently explain the first order margin geometry observed in the South Atlantic. We further couple our plate kinematic model with predictive forward mantle flow models that integrate lithospheric deformation to test the influence of mantle-driven topography on the evolving rift.

Heine, C.; Brune, S.; Young, A.; Zoethout, J.; Flament, N.; Müller, R. D.

2012-04-01

45

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; Geolor.com

46

The Shear along the Dead Sea Rift  

Microsoft Academic Search

Recent surface and subsurface geological investigations in Israel and Jordan provide new data for the re-examination of Dubertret's (1932) hypothesis of the left-hand shear along the Dead Sea rift. It is found that while none of the pre-Tertiary sedimentary or igneous rock units extend right across the rift, all of them resume a reasonable palaeographical configuration once the east side

R. Freund; Z. Garfunkel; I. Zak; M. Goldberg; T. Weissbrod; B. Derin; F. Bender; F. E. Wellings; R. W. Girdler

1970-01-01

47

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

48

Tomographic Imaging of the Northern Ethiopian Rift - a Transition From Continental to Oceanic Rifting  

NASA Astrophysics Data System (ADS)

The EAGLE broadband array consisted of 30 sensors distributed over an 250 km x 250 km area of the northern Ethiopian Rift in order to study the crust and upper mantle structure of a region of transition from continental to oceanic rifting. Analyses of teleseismic relative residual travel times indicate a variation of the order of 1 sec; slower arrivals to the NW of the Rift on the uplifted Ethiopian Plateau and faster arrivals to the SE towards the Somali plate. Tomographic inversion of 3614 P-wave and 800 S-wave relative arrival times using the method of VanDecar images a narrow ˜70 km wide low velocity upwelling (1.5% P-wave velocity anomaly), well resolved to 300 km beneath the Rift in the southern part of the array. This anomaly broadens with depth northwards into Afar as asthenospheric (oceanic) rifting processes begin to take over from the lithospheric (continental) ones prevalent in the south. An off-rift axis E-W low velocity anomaly just south of Addis Ababa corresponds to the surface manifestation of a line of Quarternary volcanoes. Interestingly the maximum upper mantle low velocity anomaly is offset by about 25 km to the NW from the location of rift magmatic segments. S-wave anomalies show a similar rift structure to the P-wave results but are of twice the magnitude.

Bastow, I.; Stuart, G.; Kendall, J.; Ebinger, C.; Ayele, A.

2003-12-01

49

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

Microsoft Academic Search

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

L. Guo; G. R. Keller

2010-01-01

50

Continental rifting and upper mantle strength  

NASA Astrophysics Data System (ADS)

The uppermost mantle traditionally has been considered to be relatively strong compared to the crust. However, this view has recently been contested based on improved assessments of earthquake focal depths and considerations regarding effective elastic thickness, and caused much subsequent debate. Here we employ a high resolution, numerical model of continental rifting calibrated with topographic, gravimetric and seismological observations in order to constrain the strength of the uppermost mantle. Our model shows that a rift developing in the presence of a relatively strong uppermost mantle leads to lithospheric necking well beneath the Moho. This is consistent with previous models of lithospheric extension, but here we identify two important inconsistencies with observations from both recent and ancient continental rifts: 1): Necking beneath the Moho leads to downwards dislocation of the Moho beneath the rift. However, seismological studies always indicate that the Moho tends to shallow beneath rifts. 2): Deep necking additionally causes the development of regionally supported and unrealistically high flank uplifts together with associated extreme gravity anomalies. By modelling the thermo mechanical development of continental rift flanks at a range of lithosphere thicknesses and mantle strengths and comparing with topographic profiles from a global set of recent rift systems, we show that the strength of the uppermost mantle should be no more than 200 MPa. Our numerical model employs a self consistent representation of the lithosphere by including equilibrium small-scale convection which contributes with a relatively constant sub-crustal heat flux. Since the lithospheric mantle is colder and thus denser than the asthenosphere below, continental rifting requires a certain level of strength of the mantle in order to avoid unstable lithospheric delamination. We find this lower strength limit to be less than 20 MPa. Uppermost mantle strength in the range of 20-200 MPa is typically less than that of the crust and of previous estimates of upper mantle strength. Previous numerical studies require a strong uppermost mantle in order to obtain localized deformation as evident from narrow continental rifts. However, our model shows that rifts developing in weak mantle lithosphere, relative to the crust, indeed tend to become narrow.

Petersen, K. D.; Nielsen, S. B.; Stephenson, R.; Gerya, T.

2012-04-01

51

Evidence of Melt-Induced Seismic Anisotropy and Magma Assisted Rifting in the North Ethiopian Rift  

NASA Astrophysics Data System (ADS)

The complex process of the transition from continental to oceanic rifting remains poorly understood. The Northern Ethiopian Rift, being free from interference from other tectonic processes, is an ideal place to study the continental breakup process. With this in mind, the recent EAGLE experiment saw the deployment of 79 broadband seismometers over an area 250-350km centred on the Northern Ethiopian Rift. We investigate the signature of crustal and upper-mantle anisotropy in these data as it provides insights into rifting processes. Recent studies employing shear-wave splitting techniques provide strong and consistent evidence for a rift-parallel (NNE-SSW) fast anisotropic direction beneath the rift. The detailed characteristics of these observations imply a single anisotropic layer confined to the upper 100km. Surface-wave tomography shows that the fast rift-parallel directions persist to a depth of 400km beneath a broader area surrounding the rift. These observations eliminate a number of plausible causes of anisotropy including plate motion drag, radial mantle-flow in the Afar plume head, mantle flow perpendicular to the rift induced by the rifting process, or pre-existing frozen-in crystallographic fabric. The observed anisotropy is more likely to be caused by either channeled horizontal mantle-flow along the rift axis, which would cause the lattice preferred orientation (LPO) of olivine with the fast ? -axes paralleling the rift, or the presence of rift-aligned melt-filled pockets (MFP) in the mantle. % SKS-splitting results show that the distribution of the fast anisotropic orientation mimics closely the distribution of strain and magmatism in the rift, implying MFP-induced anisotropy. % However, the techniques employed by the studies to date do not provide a means of conclusively separating the two candidate causes of anisotropy. The speeds of horizontally propagating S v and S h waves vary in similar fashions with azimuth for LPO- and MFP-induced anisotropy ( S v ? cos({? }), S h ? cos({2? }), where ? is the azimuth measured from the rift axis). However, the the relative change in the two shear-wave velocities is distinctive for LPO- and MFP-induced anisotropy. This provides a powerful tool for distinguishing between the two candidate causes of anisotropy. % We present strong evidence for MFP-induced anisotropy beneath the rift in the depth range of 20--60~km, by showing that the azimuthal variation of the speeds of S v and S h waves propagating horizontally through the rift area is in good agreement with predicted anisotropy models for vertical rift-parallel melt-filled dykes.We obtain shear-velocity models by inverting the group and phase velocity dispersion experienced by a number of local and teleseismic Rayleigh and Love waves with total or inter-station propagation paths crossing the rift area with a variety of azimuths. By only using highly coherent waves for the phase-velocity measurements and by applying a phase-matching filtering technique in the group velocity extraction procedure we ensure that our measurements are free from bias introduced by scattering and noise and that our results are reliable and robust.

Pilidou, S.; Kendall, J.; Stuart, G.; Bastow, I.

2004-12-01

52

Post Rift Thermal Evolution of Extended Lithosphere  

NASA Astrophysics Data System (ADS)

An improved thermal model of the lithosphere extension is proposed and its influence in the petroleum system in sedimentary basins examined. The new model assumes existence of time dependent variation in the lithosphere thickness during the post-rift period, which was not take account in the formulation of the Mackenzie model (1978). In the present work we assume that the asymptotic growth of the lithosphere thickness, during the post-rift period, may be represented by an asymptotic relation of the type: L(t)=(L/?)+(L-L/?)erf(?*t) where L(t) is lithospheric thickness at post-rift time t, ? the stretching factor as defined in the McKenzie model, erf the error function and ? a suitable scaling constant. According to the above equation the syn-rift value of L is (L/?). For large times the thickness of the lithosphere approaches asymptotically the pre-rift value of L. The value of ? can be determined by calculating the time necessary for the stretched lithosphere (L/?) to return to original thickness. The results of numerical simulations indicate that the heat flux derived from the new model is substantially lower than the values predicted by the McKenzie model (see Figure 1). The new model has been calibrated using available information on evolution of thermal maturity indices for the Santos Basin, situated in the offshore area of southeast Brazil. Heat flow variations during the post stretch period, for the case in which the stretching factor is 2.5.

Cardoso, R. R.; Hamza, V. M.

2009-12-01

53

Genome Analysis of Rift Valley Fever Virus, Mayotte  

PubMed Central

As further confirmation of a first human case of Rift Valley fever in 2007 in Comoros, we isolated Rift Valley fever virus in suspected human cases. These viruses are genetically closely linked to the 2006–2007 isolates from Kenya.

Zeller, Herve; Grandadam, Marc; Caro, Valerie; Pettinelli, Francois; Bouloy, Michele; Cardinale, Eric; Albina, Emmanuel

2012-01-01

54

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

NASA Astrophysics Data System (ADS)

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

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

2011-02-01

55

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; González-Fernández, Antonio; Harding, Alistair J; Holbrook, W Steven; Kent, Graham M; Paramo, Pedro; Sutherland, Fiona; Umhoefer, Paul J

2007-07-26

56

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

57

Strain distribution in the East African Rift from GPS measurements  

Microsoft Academic Search

Rifting of continental lithosphere is a fundamental process that controls the growth and evolution of continents and the birth of ocean basins. Most rifting models assume that stretching results from far-field lithospheric stresses from plate motions, but there is evidence that asthenospheric processes play an active role in rifting, possibly through viscous coupling and\\/or the added buoyancy and thermal weakening

S. D. Stamps; E. Saria; E. Calais; D. Delvaux; C. Ebinger; L. Combrinck

2008-01-01

58

Structural evolution history of the Red Sea Rift  

Microsoft Academic Search

The Red Sea Rift has been an object of comprehensive studies by several generations of geologists and geophysicists. Many publications and open-file reports provide insights into the geological history of this rift. Paleogene and Cretaceous rocks, which are considered to be prerift, are locally exposed at the margins of the Red Sea Rift. At the same time, some evidence indicates

G. A. F. D'Almeida

2010-01-01

59

Middle Proterozoic to Cambrian Rifting, Central North America  

Microsoft Academic Search

The 1100 Ma Midcontinent Rift System in central North America, with its exposed Keweenawan volcanic and plutonic rocks in the Lake Superior region, is one of the major rift-related magmatic provinces in the world. Recent geophysical, petrologic, and geologic studies of the rift and related rocks have made the system one of the best studied structures of this type.Middle Proterozoic

W. Randall Van Schmus

1998-01-01

60

Thermomechanical models of the Rio Grande rift  

SciTech Connect

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

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

1980-01-01

61

Rifting and lower crustal reflectivity: A case study of the intracratonic Dniepr-Donets rift zone, Ukraine  

Microsoft Academic Search

Intracratonic rifting, caused by late Devonian extensional stresses in the East European Craton, created the largest rift zone in Europe, the Pripyat-Dniepr-Donets rift (southeast Ukraine). The rift basin is approximately 2000 km long, up to 170 km wide, and 22 km deep. Wide-angle refraction and reflection seismic data from the Donbas Basin deep seismic Refraction and Reflection Experiments (DOBRE'99) project

Stig B. Lyngsie; Hans Thybo; Rasmus Lang

2007-01-01

62

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 São 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 Piauí-Ceará 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.; Müller, R. D.

2013-01-01

63

Molecular Epidemiology of Rift Valley Fever Virus  

PubMed Central

Phylogenetic relationships were examined for 198 Rift Valley fever virus isolates and 5 derived strains obtained from various sources in Saudi Arabia and 16 countries in Africa during a 67-year period (1944–2010). A maximum-likelihood tree prepared with sequence data for a 490-nt section of the Gn glycoprotein gene showed that 95 unique sequences sorted into 15 lineages. A 2010 isolate from a patient in South Africa potentially exposed to co-infection with live animal vaccine and wild virus was a reassortant. The potential influence of large-scale use of live animal vaccine on evolution of Rift Valley fever virus is discussed.

Grobbelaar, Antoinette A.; Weyer, Jacqueline; Leman, Patricia A.; Kemp, Alan; Paweska, Janusz T.

2011-01-01

64

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 10 km, 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. Castarède; O. Charade; A. Nercessian; A. Avallone; F. Pacchiani; J. Zahradnik; S. Sacks; A. Linde

2006-01-01

65

Strain accommodation by magmatism and faulting as rifting proceeds to breakup: Seismicity of the northern Ethiopian rift  

Microsoft Academic Search

The volcanically active Main Ethiopian rift (MER) marks the transition from continental rifting in the East African rift to incipient seafloor spreading in Afar. We use new seismicity data to investigate the distribution of strain and its relationship with magmatism immediately prior to continental breakup. From October 2001 to January 2003, seismicity was recorded by up to 179 broadband instruments

Derek Keir; C. J. Ebinger; G. W. Stuart; E. Daly; A. Ayele

2006-01-01

66

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

Microsoft Academic Search

This study presents an almost complete Middle Miocene to Pleistocene sequence of synrift sediments in the western branch of\\u000a the East African Rift. The studied succession is exposed in several patches on an eastward tilted block between the northern\\u000a tip of the Rwenzori Block and the eastern shoulder of the Albert Rift. In this position, it reaches a maximum thickness

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

2010-01-01

67

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

68

The onshore northeast Brazilian rift basins: An early Neocomian aborted rift system  

Microsoft Academic Search

Early Cretaceous rift basins of northeastern Brazil illustrate key three-dimensional geometries of intracontinental rift systems, controlled mainly by the basement structures. These basins were formed and then abandoned during the early extension associated with the north-south-propagating separation of South America and Africa. During the early Neocomian, extensional deformation jumped from the easternmost basins (group 1: Sergipe Alagoas and Gabon basins;

Matos

1990-01-01

69

Tertiary arc rifting in northern Luzon, Philippines  

Microsoft Academic Search

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

Federico F. Florendo

1994-01-01

70

Unexpected Rift Valley Fever Outbreak, Northern Mauritania  

PubMed Central

During September–October 2010, an unprecedented outbreak of Rift Valley fever was reported in the northern Sahelian region of Mauritania after exceptionally heavy rainfall. Camels probably played a central role in the local amplification of the virus. We describe the main clinical signs (hemorrhagic fever, icterus, and nervous symptoms) observed during the outbreak.

El Mamy, Ahmed B. Ould; Baba, Mohamed Ould; Barry, Yahya; Isselmou, Katia; Dia, Mamadou L.; Hampate, Ba; Diallo, Mamadou Y.; El Kory, Mohamed Ould Brahim; Diop, Mariam; Lo, Modou Moustapha; Thiongane, Yaya; Bengoumi, Mohammed; Puech, Lilian; Plee, Ludovic; Claes, Filip; Doumbia, Baba

2011-01-01

71

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. ...

72

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...

73

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

74

Magma-assisted rifting in Ethiopia.  

PubMed

The rifting of continents and evolution of ocean basins is a fundamental component of plate tectonics, yet the process of continental break-up remains controversial. Plate driving forces have been estimated to be as much as an order of magnitude smaller than those required to rupture thick continental lithosphere. However, Buck has proposed that lithospheric heating by mantle upwelling and related magma production could promote lithospheric rupture at much lower stresses. Such models of mechanical versus magma-assisted extension can be tested, because they predict different temporal and spatial patterns of crustal and upper-mantle structure. Changes in plate deformation produce strain-enhanced crystal alignment and increased melt production within the upper mantle, both of which can cause seismic anisotropy. The Northern Ethiopian Rift is an ideal place to test break-up models because it formed in cratonic lithosphere with minor far-field plate stresses. Here we present evidence of seismic anisotropy in the upper mantle of this rift zone using observations of shear-wave splitting. Our observations, together with recent geological data, indicate a strong component of melt-induced anisotropy with only minor crustal stretching, supporting the magma-assisted rifting model in this area of initially cold, thick continental lithosphere. PMID:15650736

Kendall, J-M; Stuart, G W; Ebinger, C J; Bastow, I D; Keir, D

2005-01-13

75

Post Rift Thermal Evolution of Extended Lithosphere  

Microsoft Academic Search

An improved thermal model of the lithosphere extension is proposed and its influence in the petroleum system in sedimentary basins examined. The new model assumes existence of time dependent variation in the lithosphere thickness during the post-rift period, which was not take account in the formulation of the Mackenzie model (1978). In the present work we assume that the asymptotic

R. R. Cardoso; V. M. Hamza

2009-01-01

76

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

77

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, François; Collet, Louis; D'Ortenzio, Eric; Renault, Philippe; Pierre, Vincent

2009-04-01

78

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...

79

The Tectonics of the Albertlne Rift  

Microsoft Academic Search

IT has been represented to me that misconception with regard to this matter, so far as my own views are concerned, may arise in consequence of my passing, without comment, some remarks made by Prof. J. W. Gregory in his able review of Prof. Bailey Willis's book, ``Living Africa''.1 Gregory quotes my early description of the local rift valleys correctly

E. J. Wayland

1932-01-01

80

The rifted margin of Saudi Arabia  

Microsoft Academic Search

The structure of rifted continental margins has always been of great scientific interest, and now, with dwindling economic oil deposits, these complex geological features assume practical importance as well. The ocean-continent transition is, by definition, laterally heterogeneous and likely to be extremely complicated. The southernmost shotpoints (4, 5, and 6) in the U.S. Geological Survey seismic refraction profile in the

J. S. McClain; J. A. Orcutt

1984-01-01

81

Along-rift variations in style of deformation at the Red Sea rift in Afar  

NASA Astrophysics Data System (ADS)

Continental breakup and the transition to seafloor spreading is characterized by extensional faulting, thinning of the lithosphere and, at magmatic margins, voluminous intrusive and extrusive magmatism. It is difficult to discriminate between different mechanisms and timing of extension and magmatism at ancient continental margins because the continent-ocean transition is buried beneath thick layers of volcanic and sedimentary rocks and the tectonic activity that characterized breakup has ceased. Instead, the timing of these mechanisms is inferred from theoretical models or from the geological record preserved at the fully developed, ancient rifted margins. Ongoing rifting in the Red Sea rift in Afar offers a unique opportunity to address these problems because it exposes subaerially the transition between continental rifting and seafloor spreading. Here we present evidence from seismicity, InSAR, controlled source seismology, and volcanology for along-rift variations in style of deformation in Afar. We show that although intrusion of magma maintains crustal thickness during the early stages of the continent-ocean transition, subsidence of the rift below sea level, and eruption of voluminous basalt flows, is initiated by late-stage and rapid mechanical deformation (faulting in upper crust and stretching in the lower crust) of the heavily intruded, weakened plate just before the onset of seafloor spreading. We thus conclude that faulting, stretching and magma intrusion are each important, but at different times during breakup.

Keir, D.; Bastow, I.; Pagli, C.

2012-04-01

82

Introduction to Special Section on the Rio Grande Rift  

NASA Astrophysics Data System (ADS)

With the aid of a Penrose Conference in 1974 and an international rift conference held in 1978 in Santa Fe, New Mexico, the Rio Grande rift has become widely recognized as a major Cenozoic continental rift zone. As a result of the 1978 Santa Fe meeting, the American Geophysical Union published a special volume of papers concerned with the Rio Grande rift [Riecker, 1979], and the New Mexico Geological Society recently published another volume focused on this rift [Baldridge et al., 1984]. These volumes are a manifestation of the research activity which lead to the formation of the Rio Grande rift consortium whose purpose is to foster rift-related research and communication. This organization has sponsored several special sessions at geological and geophysical meetings and has generally increased the awareness of this important feature.

Keller, G. R.

1986-05-01

83

Continental rifting on Earth and Mars - A comparison  

NASA Astrophysics Data System (ADS)

The formation of continental rift systems on Earth is connected to prerift uplift generated by upwelling mantle plumes and extensional stresses which originate from remote plate boundary forces. Continental rifting and continental breakup on Earth are therefore intimately connected to Earth's plate tectonic environment. Recently, Martian candidate analogues to terrestrial continental rifts have been investigated in detail and it has been shown that the Tempe Fossae, Acheron Fossae and Thaumasia Highland Rifts bear many structural similarities to continental rifts on Earth. However, the question of the rift formation process has so far not been addressed and an active mechanism involving mantle plumes and local doming has usually been assumed. Rifts are also sometimes thought to be at least indirect evidence for plate tectonics, although the connection of Martian rifts to plate tectonic forces has so far not been discussed. We have investigated whether forces connected to plate movement are necessary to initiate rifting and show that lithosphere scale faulting at the Thaumasia Highland Rift is feasible even in the absence of mantle plumes or tensional plate-boundary forces. Rather, stresses originating from horizontal differences of the gravitational potential energy will be shown to be almost sufficient to induce rifting, supporting the hypothesis of a passive rifting mechanism in the Thaumasia Highlands. The emplacement of magma bodies in the upper crust could then sufficiently weaken the lithosphere to initiate lithosphere scale faulting and thus induce rifting. This hypothesis is in good agreement with the observation of rift-related volcanism as well as the fact that faults seem to initiate at volcanoes and propagate away from them before interconnecting. We conclude that rifting on Mars is feasible even if key factors connected to continental rifting on Earth, i.e. plate boundary forces and convection induced drag on the lower lithosphere, are absent. The absence of forces connected to plate tectonics is also consistent with the observed moderate extension of only a few kilometers. These values are typical for young terrestrial rifts (e.g., the Kenya-rift) and failed arms and suggest that large scale plate movement and subduction did not play a role in Martian rifting.

Grott, M.; Hauber, E.; Kronberg, P.

2007-08-01

84

Time and length scales of continental rifting: New perspectives for studies of rifting and rupture processes (Invited)  

NASA Astrophysics Data System (ADS)

Studies of active and ancient continental rift systems worldwide show consistent temporal and spatial patterns that are independent of geodynamic setting, as well as pre-existing crustal and lithospheric heterogeneities. These patterns indicate that the thermal and mechanical properties of the lithosphere are fundamentally important during rift initiation. The linkage of propagating, growing fault systems with maximum length proportional to plate strength leads to a regular basin segmentation pattern that is largely independent of pre-rift basement fabric. The mechanical stretching occurs with or without magma intrusion, which also introduces its own scaling and temporal factors. As stretching leads to more thinning, advective and conductive heat transfer from magmatic intrusions locally reduces the plate strength, and magma intrudes to shallower levels. Stresses will also localize along these hot, weak intrusion zones, maintaining the spatial location of magmatism during progressive rifting episodes, and potentially determining the location of plate rupture. Combined satellite geodesy and seismic studies provide new perspectives on the time scales of rifting processes, and motivate reconsideration of time-averaged strain patterns. Observations at continental and oceanic rifts show that tensile stresses from far-field plate motions accumulate over decades before being released during relatively short-lived rifting events. Strain release during magmatic rifting events can be largely aseismic. As Earthscope moves across the Mid-Continent rift to the rifted East Coast passive margin, the breadth and depth of new data sets provides many opportunities to address fundamental questions of continental rifting and rupture: What are the mechanisms and feedbacks controlling along-axis segmentation from rift initiation to breakup, and what are the timescales and rates of processes? How do these differ in magma-rich and magma-poor rift sectors? These questions can be addressed through comparisons between active and ancient rifts, and between magma-rich and magma-poor sectors.

Ebinger, C. J.

2009-12-01

85

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

86

Active Normal Fault Behaviour and Continental Rift Geometry in the Corinth Rift, Greece  

NASA Astrophysics Data System (ADS)

The Gulf of Corinth continental rift, central Greece extends at up to 15 mm/yr with regular M6+ earthquakes. However, rapid geodetic extension rates in the western rift cannot be accounted for by displacement on onshore faults alone, where slip rates determined from uplifted terraces and paleoseismological trenching are significantly lower. High resolution seismic reflection and multibeam bathymetric data were collected to survey offshore faults contributing to extension and quantify their displacement. In the western rift, a basement horst on the northern margin is uplifted by the N and S Eratini faults and the axial channel is fault-controlled. Subsided lowstand shorelines in the hangingwall of the N Eratini and the well-studied Aigion fault suggest that both faults have similar displacements. Summed extension from the four major faults across this part of the rift (Eliki, Sub-channel, S Eratini, N Eratini) is of the order of 8-12 mm/yr, thereby reconciling geologic and geodetic datasets. Geomorphology indicates that the rift geometry changes along axis, with a model of distributed deformation across multiple faults proposed for the western rift. The high resolution seismic data linked to sea level history within the gulf (isolated during lowstands) potentially allow changes in slip rate to be determined on a 10000 year timescale. These results compliment the often shorter (100's-10000's years) timescales of onshore fault trenching and uplifted terrace sequences in terms of temporal fault behaviour. Ultimately, seismic hazard can be refined based on fluctuations in past fault behaviour within the rift.

McNeill, L.; Cotterill, C.; Henstock, T.; Bull, J.; Stefatos, A.; Hicks, S.; Collier, R.; Papatheoderou, G.; Ferentinos, G.

2004-12-01

87

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

NASA Astrophysics Data System (ADS)

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

Meyer, R.; Hertogen, J.

2008-12-01

88

Structure of continental rifts: Role of older features and magmatism  

SciTech Connect

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

Keller, G.R. (Univ. of Texas, El Paso, TX (United States))

1996-01-01

89

The Pathogenesis of Rift Valley Fever  

PubMed Central

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

Ikegami, Tetsuro; Makino, Shinji

2011-01-01

90

Experimental Rift Valley fever in rhesus macaques  

Microsoft Academic Search

Summary Rift Valley fever (RVF) is a major cause of human morbidity and mortality in endemic areas of sub-Saharan Africa and has the potential to cause epidemic disease in receptive areas world-wide. In this study, a RVF viral isolate from the 1977 Egyptian epidemic (ZH-501) inoculated intravenously into rhesus macaques caused a benign viremic infection in most, but resulted in

C. J. Peters; D. Jones; R. Trotter; J. Donaldson; J. White; E. Stephen; T. W. Slone

1988-01-01

91

Oblique Rifting on Reykjanes Peninsula, SW Iceland  

Microsoft Academic Search

Reykjanes Peninsula offers the only on-shore outcrop of an currently active, oblique spreading zone of the Mid-Atlantic Ridge and thus, it is an excellent natural laboratory to investigate the structural geometries and the evolution of oblique rift zones. The Reykjanes Peninsula is characterised by dinstinct regions of (1) seismic activity along the current spreading axis and (2) discrete en-échelon fracture

Maria Helbig

92

Proterozoic Midcontinent Rift System, an overview  

SciTech Connect

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

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

1992-01-01

93

Early Paleozoic sedimentation in Reelfoot rift  

SciTech Connect

Analysis of subsurface data from deep tests drilled in the northern Mississippi embayment and southern Mid-Continent suggests that earliest Paleozoic sedimentation was dominated by the tectonic evolution of the Reelfoot rift. Throughout most of the Mid-Continent, the Upper Cambrian Lamotte (Mt. Simon) sandstone rests nonconformably on Precambrian basement and is overlain by the Bonneterre (Eau Claire) Formation. However, in the area of the Reelfoot rift, both the Lamotte and Bonneterre grade into thick, basinal shales that locally display evidence of episodic deposition of coarse clastics, perhaps on submarine fans. Moreover, two major sedimentary units are present beneath the Lamotte-Bonneterre basinal facies within the Reelfoot rift. Immediately underlying the Lamotte-Bonneterre shale is a carbonate stratum (probably dolomite) that thickens to more than 1,000 ft (300 m) along the axis of the basin in eastern Arkansas. Underlying this carbonate is a detrital unit that grades from arkosic sandstone near the northern terminus of the basin to a basinal shale southward. This basinal shale is at least several hundred feet thick near the axis of the basin. These two strata occupy the stratigraphic position of the Conasauga (Middle Cambrian) and Rome (Lower Cambrian) Formations of the southern Appalachians. The axial and transverse distribution of these strata suggests that the Reelfoot evolved as paired grabens or half grabens during the Early and Middle Cambrian. Subsequently, the Reelfoot remained the axis for more widespread subsidence and sedimentation throughout much of the Paleozoic.

Houseknecht, D.W.; Weaverling, P.H.

1983-09-01

94

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

95

Seismicity of oceanic and continental rifts—a geodynamic approach  

NASA Astrophysics Data System (ADS)

Two major kinds of divergent structures—oceanic and intracontinental rifts—were 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 and rift parts of MOR. The seismic catalogues NEIC and CMT were used to assess the seismic moment release. The seismic moment release was calculated another way using the global plate tectonic model NUVEL-1 and Brune's formulae. Comparison of these two values shows that the seismic coupling coefficient, ?, varies from 1 to 10% for most of MOR and is always higher for transform faults. Most of the deformation, therefore, is aseismic slip. Most seismicity of MOR is confined to transform faults. The energy contribution of transform faults is one to two orders magnitude higher than that of the rift, and increases with the spreading rate. There is a strong correlation between the seismic moment release of strike-slip faults and their total lengths. The correlation shows that the seismic moment release depends on the total transform area and confirms the simple thermal model of transform seismicity that was given by Burr and Solomon. The seismic moment release and the spreading rate have opposite patterns. For the rifts, there is an inverse correlation between the seismic moment of normal faults and spreading velocity, while it seems for transforms that these parameters are independent. Finally, these results show that the seismicity of transforms and rifts depends first of all on the thermal structure of oceanic lithosphere. In the case of continental rifts, one can distinguish in the degree of seismic activity depending on the stage of rifting. Hence, analysis of the continental seismicity requires the consideration of factors of a geological evolution that play practically no role in the case of oceanic lithosphere. The comparison of geological and seismic data for the East African region has allowed us to outline the regular changes of the seismic regime during development of the rift zone from the stage of incipient rift to mature oceanic rift. In the evolutionary series [intracontinental incipient rift]-[intracontinental mature rift]-[intercontinental rift]-[oceanic slow-spreading rift]-[oceanic fast-spreading rift], there is a gradual decrease of the role of rifts (sensu stricto, as tension structures) and increase of the role of strike-slip faults. Epicenters concentrate along major faults as well. The level of seismic energy becomes lower, although the rate of deformation increases.

Sobolev, P. O.; Rundquist, D. V.

1999-03-01

96

Physical characteristics and evolutionary trends of continental rifts  

SciTech Connect

Rifts may be defined as zones beneath which the entire lithosphere has ruptured in extension. They are widespread and occur in a variety of tectonic settings, and range up to 2,600 m.y. in age. The object of this review is to highlight characteristic features of modern and ancient rifts, to emphasize differences and similarities in order to help characterize evolutionary trends, to identify physical conditions favorable for initiation as well as termination of rifting, and to provide constraints for future modeling studies of rifting. Rifts are characterized on the basis of their structural, geomorphic, magmatic and geophysical features and the diverse character of these features and their evolutionary trends through time are discussed. Mechanisms of rifting are critically examined in terms of the physical characteristics and evolutionary trends of rifts, and it is concluded that while simple models can give valuable insight into specific processes of rifting, individual rifts can rarely, if ever, be characterized by well defined trends predicted by these models. More data are required to clearly define evolutionary trends, and the models require development to incorporate the effects of lithospheric heterogeneities and complex geologic histories.

Ramberg, I.B.; Morgan, P.

1984-01-01

97

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

Microsoft Academic Search

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

Sarah E. Rilling

2009-01-01

98

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

NASA Astrophysics Data System (ADS)

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

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

2007-05-01

99

Shear wave anisotropy of the Godavari rift in the south Indian shield: Rift signature or APM related strain?  

NASA Astrophysics Data System (ADS)

Shear wave splitting analysis using SKS, SKKS and S waveforms recorded at eight broadband seismic stations within and in the vicinity of the Proterozoic Godavari rift system in the south Indian shield yielded 104 new measurements of azimuthal anisotropy. Our results reveal that the delay times lie between 0.70 and 1.20 s, similar to the previous measurements from the Indian shield. The orientations of fast polarization directions at most of the stations within and beyond the rift are generally aligned orthogonal to the rift axis, consistent with asthenospheric flow in the extension direction. Absence of clear evidences for a preferentially thinner lithosphere beneath the Godavari rift or anomalously high heat flow values coupled with the fact that the NE oriented fast polarization directions are also close to the APM direction of the Indian plate, result in an ambiguity in interpreting the results as being due to strain related to plate motion or signatures of rifting. A thin lithosphere beneath the rift suggests that the source of anisotropy is likely to be in the asthenosphere. In such a scenario, either the mechanism of (1) active rifting where the asthenospheric flow drives the rifting or (2) passive rifting where mantle flow responds to lithospheric thinning caused by far field stresses or (3) present day APM related strain appear to be the plausible mechanisms for forging the observed anisotropy.

Kumar, Narendra; Kumar, M. Ravi; Singh, Arun; Solomon Raju, P.; Purnachandra Rao, N.

2010-08-01

100

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

101

Rifting and lower crustal reflectivity: A case study of the intracratonic Dniepr-Donets rift zone, Ukraine  

Microsoft Academic Search

Intracratonic rifting, caused by late Devonian extensional stresses in\\u000a the East European Craton, created the largest rift zone in Europe, the\\u000a Pripyat-Dniepr-Donets rift (southeast Ukraine). The rift basin is\\u000a approximately 2000 km long, up to 170 km wide, and 22 km deep.\\u000a Wide-angle refraction and reflection seismic data from the Donbas Basin\\u000a deep seismic Refraction and Reflection Experiments (DOBRE' 99)

Stig B. Lyngsie; Hans Thybo; Rasmus Lang

2007-01-01

102

Tectonic inheritance and continental rift architecture: Numerical and analogue models of the East African Rift system  

Microsoft Academic Search

The western branch of the East African Rift is composed of an arcuate succession of elongate asymmetric basins, which differ in terms of interaction geometry, fault architecture and kinematics, and patterns of uplift\\/subsidence and erosion\\/sedimentation. The basins are located within Proterozoic mobile belts at the edge of the strong Tanzanian craton; surface geology suggests that the geometry of these weak

Giacomo Corti; Jolante van Wijk; Sierd Cloetingh; Chris K. Morley

2007-01-01

103

Magma genesis by rifting of oceanic lithosphere above anomalous mantle: Terceira Rift, Azores  

NASA Astrophysics Data System (ADS)

The Terceira Rift formed relatively recently (˜1 Ma ago) by rifting of the old oceanic lithosphere of the Azores Plateau and is currently spreading at a rate of 2-4mm/a. Together with the Mid-Atlantic Ridge, the Terceira Rift forms a triple junction that separates the Eurasian, African, and American Plates. Four volcanic systems (São Miguel, João de Castro, Terceira, Graciosa), three of which are islands, are distinguished along the axis and are separated by deep avolcanic basins similar to other ultraslow spreading centers. The major element, trace element and Sr-Nd-Pb isotope geochemistry of submarine and subaerial lavas display large along-axis variations. Major and trace element modeling suggests melting in the garnet stability field at smaller degrees of partial melting at the easternmost volcanic system (São Miguel) compared to the central and western volcanoes, which appear to be characterized by slightly higher melting degrees in the spinel/garnet transition zone. The degrees of partial melting at the Terceira Rift are slightly lower than at other ultraslow mid-ocean ridge spreading axes (Southwest Indian Ridge, Gakkel Ridge) and occur at greater depths as a result of the melting anomaly beneath the Azores. The combined interaction of a high obliquity, very slow spreading rates, and a thick preexisting lithosphere along the axis probably prevents the formation and eruption of larger amounts of melt along the Terceira Rift. However, the presence of ocean islands requires a relatively stable melting anomaly over relatively long periods of time. The trace element and Sr-Nd-Pb isotopes display individual binary mixing arrays for each volcanic system and thus provide additional evidence for focused magmatism with no (or very limited) melt or source interaction between the volcanic systems. The westernmost mantle sources beneath Graciosa and the most radiogenic lavas from the neighboring Mid-Atlantic Ridge suggest a mantle flow from Graciosa toward the Mid-Atlantic Ridge and hence a flux of mantle material from one spreading axis into the other. The Terceira Rift represents a unique oceanic rift system situated within the thickened, relatively old oceanic lithosphere and thus exhibits both oceanic and continental features.

Beier, Christoph; Haase, Karsten M.; Abouchami, Wafa; Krienitz, Marc-S.; Hauff, Folkmar

2008-12-01

104

Understanding the Transition From Continental to Oceanic Rifting in the Northern Ethiopian Rift - the EAGLE Project  

NASA Astrophysics Data System (ADS)

A consortium of UK (Leeds, Leicester, Royal Holloway, Edinburgh,), US (Stanford, UTEP, Penn State,) and Ethiopian (Addis Ababa) universities are exploring the kinematics and dynamics of continental breakup through the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE), which aims to probe the crust and upper mantle structure between the Main Ethiopian (continental) and Afar (ocean spreading) rifts. EAGLE is a multi-disciplinary study centered around a set of passive and controlled-source seismic experiments, and incorporates additional magnetotelluric, gravity, GPS and petrological studies. The initial Phase I seismic experiment consisted of a deployment of 30 broadband seismometers for a period of 16 months (Oct. 2001 to Jan. 2003) over a 250 km x 250 km area of the rift valley and its uplifted flanks. P- and S-wave tompography from teleseismic traveltime residuals, SKS splitting analyses and receiver functions provide images of crust and deep earth structure. The Phase II seismic experiment consisted of a further 50 broadband instruments for a period of 4 months over a 200 km x 100 km area encompassing 4 magmatic segments in the Main Ethiopian Rift. These recordings have furthered our understanding of the location of active seismicity, fault plane mechanisms and segmentation of rift crustal structure. Phase III consisted of the deployment of a further 1100 seismic instruments during a controlled source seismic project involving 20 shots being fired into one 450 km cross-rift profile (Profile 1), one 450 km axial profile (Profile 2), and a dense 2D array of instruments in a 150 km diameter circle around the profiles1 intersection (Profile 3), all centered on the magmatically active Nazret region. The crust and upper mantle velocity models derived provide estimates of total crustal thinning across the rift, assess the role of basement in the location of major faults and magmatic segments, and determine whether significant underplating takes place. An 18 sounding magnetotelluric profile along the cross rift profile I provides additional constraints on the nature and distribution of crustal heterogeneity, and images melt accumulation zones. New gravity and geodetic information have also been acquired.

Stuart, G.

2003-12-01

105

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 Sénégal 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

106

Basin neotectonics of Lakes Edward and George, East African Rift  

Microsoft Academic Search

The Edward and George lake basins are located in the western branch of the East African Rift System (EARS). Both basins occupy half grabens with the main boundary fault located in the west and are separated by a high relief accommodation zone. This structural high formed where the NNE–SSW trend of the Miocene rifting and NW–SE oriented basement lineaments interfere.

Tine Lærdal; Michael R Talbot

2002-01-01

107

Strain distribution in the East African Rift from GPS measurements  

NASA Astrophysics Data System (ADS)

Rifting of continental lithosphere is a fundamental process that controls the growth and evolution of continents and the birth of ocean basins. Most rifting models assume that stretching results from far-field lithospheric stresses from plate motions, but there is evidence that asthenospheric processes play an active role in rifting, possibly through viscous coupling and/or the added buoyancy and thermal weakening from melt intrusions. The distribution of strain during rifting is a key observable to constrain such models but is however poorly known. The East African Rift (EAR) offers a unique opportunity to quantify strain distribution along and across an active continental rift and to compare a volcanic (Eastern branch) and a non-volcanic (Western branch) segment. In 2006, we established and first surveyed a network of 35 points across Tanzania and installed one continuous station in Dar Es Salaam (TANZ), followed in 2008 by a second occupation campaign. We present a preliminary velocity field for the central part of the EAR, spanning both the Western and Eastern rift branches. We compare our results with a recent kinematic model of the EAR (Stamps et al., GRL, 2008) and discuss its significance for understanding rifting processes.

Stamps, S. D.; Saria, E.; Calais, E.; Delvaux, D.; Ebinger, C.; Combrinck, L.

2008-12-01

108

Rio Grande Rift GPS Measurements 2006-2009  

Microsoft Academic Search

We use three years of measurements from 25 continuous GPS stations across the Rio Grande Rift in New Mexico and Colorado to estimate surface velocities, time series, baselines, and strain rates. The stations are part of the EarthScope Rio Grande Rift experiment, a collaboration between researchers at the University of Colorado at Boulder, the University of New Mexico, and Utah

H. Berglund; A. F. Sheehan; R. Nerem; J. Choe; A. R. Lowry; M. Roy; F. Blume; M. Murray

2009-01-01

109

Prevalence of Rift Valley Fever among Ruminants, Mayotte  

PubMed Central

Rift Valley fever threatens human and animal health. After a human case was confirmed in Comoros in 2007, 4 serosurveys among ruminants in Mayotte suggested that Rift Valley fever virus had been circulating at low levels since 2004, although no clinical cases occurred in animals. Entomologic and ecologic studies will help determine outbreak potential.

Pedarrieu, Aurelie; Guis, Helene; Defernez, Cedric; Bouloy, Michele; Favre, Jacques; Girard, Sebastien; Cardinale, Eric; Albina, Emmanuel

2012-01-01

110

The Mesoproterozoic Midcontinent Rift System, Lake Superior Region, USA  

NASA Astrophysics Data System (ADS)

Exposures in the Lake Superior region, and associated geophysical evidence, show that a 2000 km-long rift system developed within the North American craton ?1109-1087 Ma, the age span of most of the volcanic rocks. This system is characterized by immense volumes of mafic igneous rocks, mostly subaerial plateau basalts, generated in two major pulses largely by a hot mantle plume. A new ocean basin was nearly formed before rifting ceased, perhaps due to the remote effect of the Grenville continental collision to the east. Broad sagging/subsidence, combined with a system of axial half-grabens separated along the length of the rift by accommodation zones, provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post-rift clastic sediments, both along the rift axis and in basins flanking a central, post-volcanic horst. Pre-rift mature, quartzose sandstones imply little or no uplift prior to the onset of rift volcanism. Early post-rift red-bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings; the exception is one gray to black carbon-bearing lacustrine(?) unit. This early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red-bed, fluvial sediments with an extra-basinal provenance.

Ojakangas, R. W.; Morey, G. B.; Green, J. C.

2001-06-01

111

Dynamics of rifting and modes of extension on icy satellites  

Microsoft Academic Search

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

F. Nimmo

2004-01-01

112

Martian Canyons and African Rifts: Structural Comparisons and Implications.  

National Technical Information Service (NTIS)

The resistant parts of the canyon walls of the Martian rift complex Valled Marineris were used to infer an earlier, less eroded reconstruction of the major roughs. The individual canyons were then compared with individual rifts of East Africa. When measur...

H. Frey

1978-01-01

113

Present-day Kinematics of the East African Rift  

Microsoft Academic Search

The East African Rift (EAR), a ~5000 km-long series of seismically active structures that mark the divergent boundary between the Somalia and Nubia plates, is often cited as a modern archetype for rifting and continental breakup. Paradoxically, its current kinematics is the least well-known of all major plate boundaries, owing to its tremendous extent, difficult access, and lack of geodetic

D. S. Stamps; E. Calais; E. Saria; E. Mbede; C. Ebinger; D. Delvaux; F. Kervyn; L. Combrinck; C. Hartnady; J. Nocquet; R. Fernandes

2007-01-01

114

Prevalence of Rift Valley Fever among ruminants, Mayotte.  

PubMed

Rift Valley fever threatens human and animal health. After a human case was confirmed in Comoros in 2007, 4 serosurveys among ruminants in Mayotte suggested that Rift Valley fever virus had been circulating at low levels since 2004, although no clinical cases occurred in animals. Entomologic and ecologic studies will help determine outbreak potential. PMID:22607651

Cêtre-Sossah, Catherine; Pédarrieu, Aurélie; Guis, Hélène; Defernez, Cédric; Bouloy, Michèle; Favre, Jacques; Girard, Sébastien; Cardinale, Eric; Albina, Emmanuel

2012-06-01

115

Genome analysis of Rift Valley fever virus, Mayotte.  

PubMed

As further confirmation of a first human case of Rift Valley fever in 2007 in Comoros, we isolated Rift Valley fever virus in suspected human cases. These viruses are genetically closely linked to the 2006-2007 isolates from Kenya. PMID:22608405

Cêtre-Sossah, Catherine; Zeller, Hervé; Grandadam, Marc; Caro, Valérie; Pettinelli, François; Bouloy, Michèle; Cardinale, Eric; Albina, Emmanuel

2012-06-01

116

Experiments on rift zone evolution in unstable volcanic edifices  

Microsoft Academic Search

Large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses. Flank deformation is often caused by slip along a décollement within or underneath the volcanic edifice. We studied how such a stressed volcanic flank may bias the rift zone development. The influence of basal lubrication and lateral flank creep

Thomas R. Walter; Valentin R. Troll

2003-01-01

117

A plume head melting under a rifting margin  

Microsoft Academic Search

A large igneous province (LIP), in the form of a long narrow band of thickened oceanic crust, runs along the Atlantic margin of North America abutting the rifted continental shelf. We propose that this, like many other LIPs, has a mantle plume origin. There is evidence that when the central Atlantic Ocean opened the rift was underlain by the flattened

A. M. Leitch; G. F. Davies; M. Wells

1998-01-01

118

The Dead Sea Rift: lateral displacement and downfaulting phases  

Microsoft Academic Search

A sinistral strike slip of 105 km along the Dead Sea Rift is confirmed by the continuity in the restored trend of Eocene sedimentary belts. Yet, the similar characteristics on either side of the Rift between the Upper Miocene conglomerates, mudstones and lithographic limestones (particularly the Kefar Gil'adi Fm.) and the Pliocene marls, oolitic limestones and basalts (e.g., Bira Fm.,

A. Sneh

1996-01-01

119

Extension Across Valles Marineris and the Thaumasia "Rift", Mars  

NASA Astrophysics Data System (ADS)

We calculate extension from topographic profiles. We find the extension across the "Thaumasia Rift" (0.5-4.5 km), a large and complex graben sytem in the Claritas Fossae region, comparable to that across the Tempe Fossae Rift (2.5-3.1 km), but much lower than across Valles Marineris (9-20 km).

Zuschneid, W.; Hauber, E.; Kronberg, P.; Jaumann, R.

2003-03-01

120

Ethiopian Rift and Plateaus: Some Volcanic Petrochemical Differences  

Microsoft Academic Search

Volcanism on the Arabo-Ethiopian swell has accompanied the development of the three traversing spreading zones conjoining at Afar: the Red Sea, Gulf of Aden, and African rift systems. The Red Sea and Gulf of Aden floors are formed by oceanic tholerites, but Afar and the main Ethiopian rift show a wider range of more alkaline volcanics, related to slower crustal

P. A. Mohr

1971-01-01

121

High-Technology Investigations Of Rift Propogation And Plate Tectonics  

Microsoft Academic Search

Plate tectonics as originally formulated was so successful in revolutionizing geology that only very few modifications have been necessary. One of these modifications is the suggestion that spreading centers sometimes increase their length by propagation, progressively rifting apart preexisting lithospheric plates and reorganizing the Earth's plate boundary geometry. This small change to plate tectonic theory, known as the propagating rift

R. Hey

1983-01-01

122

Continental Rifting and the Implications For Plate Tectonic Reconstructions  

Microsoft Academic Search

Previous plate tectonic reconstructions have tried to recreate the pre-rifiing (Pangea) configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries. Such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting, that the continental boundaries are isochrons, and that the continents rift without distortion. This paper proposes a

Gregory E. Vink

1982-01-01

123

Exploring the Environment: Rift Valley Fever  

NSDL National Science Digital Library

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

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

2005-01-01

124

A fully dynamic model of continental rifting applied to the syn-rift evolution of sedimentary basins  

Microsoft Academic Search

A numerical technique has been developed to model dynamically lithosphere deformation and faulting. The method is based on the theory of generalised functions and has been used to investigate rifting processes in a two-layered lithosphere with up to four faults. Among other things, the method allows: (1) incorporation of the influence of syn-rift erosion and sedimentary loading on fault growth;

V. I. Starostenko; V. A. Danilenko; D. B. Vengrovitch; K. N. Poplavskii

1996-01-01

125

Magmatic lithospheric heating and weakening during continental rifting: A simple scaling law, a 2-D thermomechanical rifting model and the East African Rift System  

NASA Astrophysics Data System (ADS)

Continental rifting is accompanied by lithospheric thinning and decompressional melting. After extraction, melt is intruded at shallower depth thereby heating and weakening the lithosphere. In a feedback mechanism this weakening may assist rifting and melt production. A one-dimensional kinematic lithospheric thinning model is developed including decompressional melting and intrusional magma deposition. The intrusional heating effect is determined as a function of thinning rate and amount, melting parameters, potential temperature, and the depth range of emplacement. The temperature increases approximately proportionally to the square root of the thinning rate and to the square of the supersolidus potential temperature. Simple scaling laws are derived allowing predicting these effects and the surface heat flux for arbitrary scenarios. Two-dimensional thermomechanical extension models are carried out for a multicomponent (crust-mantle) two-phase (melt-matrix) system with a rheology based on laboratory data including magmatic weakening. In good agreement with the 1-D kinematic models it is found that the lithosphere may heat up by several 100 K. This heating enhances viscous weakening by one order of magnitude or more. In a feedback mechanism rifting is dynamically enforced, leading to a significant increase of rift induced melt generation. Including the effect of lateral focusing of magma toward the rift axis the laws are applied to different segments of the East African Rift System. The amount of intrusional heating increases with maturity of the rift from O(10 K) to up to 200 K or 400 K at the Afar Rift depending on the depth range of the magmatic emplacement.

Schmeling, Harro; Wallner, Herbert

2012-08-01

126

Endemic fluorosis in the Ethiopian Rift Valley.  

PubMed

Between 1977 and 1985, the fluoride content of drinking water and the incidence of endemic fluorosis were assessed and correlated in 16 large farms, villages and towns in the Ethiopian Rift Valley. The fluoride level of drinking-water collected from wells there ranged from 1.2 mg/litre to 36.0 mg/l (mean 10.0 mg/l). Dental fluorosis was observed in more than 80% of sampled children resident in the Rift Valley since birth, with maximum prevalence in the age-group 10-14 years; 32% of the children showed severe dental mottling. Males were affected more than females. Three areas, Wonji-Shoa, Alemtena and Samiberta, were identified as having cases of skeletal fluorosis. The highest incidence was at Wonji-Shoa sugar estates, where a linear relationship was observed between the development of crippling fluorosis, fluoride concentration of drinking-water, and period of exposure to it. The first cases of skeletal fluorosis there appeared among workers (98% males) who had been consuming water with fluoride content of more than 8ppm for over 10 years. Among 30 workers with crippling skeletal fluorosis, cervical radiculo-myelopathy was found to be the commonest incapacitating neurologic complication. As a preventive measure, low-fluoride surface water should be supplied for drinking wherever feasible; if this is not possible, the development of partial defluoridation should be considered. PMID:3433336

Haimanot, R T; Fekadu, A; Bushra, B

1987-07-01

127

Structural Evolution of the Incipient Okavango Rift Zone, NW Botswana  

NASA Astrophysics Data System (ADS)

Studies of the East African Rift System (EARS) and other continental rifts have significantly improved our understanding of rifting processes; however, we particularly lack studies of the embryonic stages of rift creation. The Okavango Rift Zone (ORZ), NW Botswana is one of few places worldwide where one can study the early stages of continental extension prior to the accumulation of significant amounts of sediments, volcanism, and multiphase deformation that obscure the investigation of these early time processes in more evolved continental rift zones. In this study, gravity and aeromagnetic data have been used to examine the initiation and development of the nascent ORZ. The Okavango basin in NW Botswana is located at the southern tip of the southwestern branch of the EARS. The rift is hosted within the Proterozoic fold and thrust belt of the Ghanzi-Chobe formation. Our objectives include (1) assessing the role of pre-existing structures on the development of rift faults and basin architecture, (2) Examining fault linkage patterns and boarder fault development, and (3) determining the shallow subsurface basin geometry. Aeromagnetic data from the ORZ suggest two main structural trends: 1) northeast-southwest (030- 070o) and 2) northwest - southeast (290 - 320o). The 030- 070o structures occur within the rift zone and throughout the surrounding basement. They form the main bounding fault system of this incipient rift. The NE - SW orientations of rift faults mirror the fold axes and foliation of the basement rocks, suggesting that the basement fabric played an important role in localizing the development of faults within the stress regime present during the initiation of this rift. Additionally, the greatest throw (~400- ~700 m) occurs along the Kunyere (NW dipping) and Tsau faults (SE dipping), defining a full graben as observed on gravity models. This differs from the half-graben model typical of most continental rift zones. Thus, it appears the basin geometry was strongly influenced by the position of these pre-existing faults. Evidence of fault linkage is seen along some of the faults. Linked segments of faults are well defined and some are > 200 km long. We suggest from this result that fault linkage and propagation occurred very early and prior to significant basin development. We conclude that basement fabric seems to be a controlling factor at least in the early stages of basin architecture and structural evolution of ORZ.

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

2005-05-01

128

Lithospheric thickness beneath the southern Kenya Rift: implications from basalt geochemistry  

Microsoft Academic Search

Geochemical data are reported for samples from the flanks and floor of the southern Kenya Rift Valley in the Lake Magadi area, and from two central volcanoes located within the rift valley. Rift lavas include samples of Singaraini and Ol Tepesi basalts on the eastern flank, Kirikiti basalts from the western flank, and plateau trachytes from the rift valley floor.

Anton P. Roex; Andreas Späth; Robert E. Zartman

2001-01-01

129

Kinematics of the Asal Rift (Djibouti) Determined from the Deformation of Fieale Volcano  

Microsoft Academic Search

Because of its subaerial exposure the Asal rift segment provides an exceptional opportunity to quantify the deformation field of an active rift and assess the contribution of tectonics and volcanism to rifting processes. The present topography of the Asal rift results from the tectonic dismemberment during the last 100,000 years of a large central volcanic edifice that formed astride the

Jean-Bernard de Chabalier; Jean-Philippe Avouac

1994-01-01

130

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

Microsoft Academic Search

We use clay models to simulate how fault population systematics vary as a function of rift obliquity. Rift obliquity is related to the acute angle, ?, between the rift trend and the displacement direction, so that the value of ? is inverse to the degree of obliquity. The range of azimuths in a fault population increases as rift obliquity increases

Amy E. Clifton; Roy W. Schlische; Martha O. Withjack; Rolf V. Ackermann

2000-01-01

131

What controls relay ramps and transfer faults within rift zones? Insights from analogue models  

Microsoft Academic Search

Structures within rift zones exhibit two main types of interaction relevant at the rift scale: relay ramps and transfer faults at high angle to the rift. Analogue experiments have been performed to investigate whether these types of interaction may be affected by differential extension along the rift. In these models, sand (brittle crust analogue) overlies two adjacent silicone (ductile crust

V. Acocella; P. Morvillo; R. Funiciello

2005-01-01

132

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

133

Seismic and geodetic constraints on magma emplacement in the Main Ethiopian rift and Red Sea rift in Afar  

NASA Astrophysics Data System (ADS)

The breakup of continents above anomalously warm mantle results in production of large volumes of melt that intrudes the lithosphere, and in so doing accommodates extensional strain. Despite the importance of magmatism during continental breakup, we have few constraints on how intrusion varies in both space and time, and how strain is partitioned between magma intrusion and faulting. We integrate seismic and geodetic data recorded during dike emplacement in the Red Sea rift and Main Ethiopian rift in the Afar depression. During June and July 2006, seismic and geodetic equipment captured rapid, lateral intrusion of two ~10-km-long, ~1-2-m-wide, rift axial dikes sourced from the center of the ~60-km-long Dabbahu segment in the Red Sea rift. Models of ground deformation determined using InSAR suggest shallow dike intrusion in the upper ~10 km of the Earth. Dike induced seismicity is characterised by ~4-5-hour-long, sub-horizontal migration of volcano-tectonic earthquakes along the dike length, but this rift parallel normal faulting accommodates less than 5% of the total strain. The June and July 2006 intrusions are part of the 2005-ongoing Dabbahu rifting episode, during which 13 discrete dikes, 2 accompanied by basaltic fissural eruptions, have accommodated nearly 10 m of rift opening along the rift segment. We also present data and models of deformation related to intrusion of magma beneath the northernmost Main Ethiopian rift, near Ayelu-Amoissa volcanoes, during May 2000. InSAR data and seismicity defines a ~10-km-long zone of deformation striking perpendicular to the rift, but spatially coincident with a ~40-km-right-stepping lateral-offset of the rift axis. Preliminary models of InSAR data show deformation consistent with ~5-10-km-deep dike intrusion, and models of seismic waveforms suggest the induced seismicity is dike parallel, rift perpendicular, normal faulting at ~7 km depth. The majority of the seismic moment release occurs during ~5 days of laterally migrating earthquakes and suggests the rate of magma emplacement is relatively slow. The anomalous location and orientation of deformation is consistent with accommodation of Arabia-Somalia plate separation along a paleo (~5-10-My-old) plate boundary, or strain during linkage of lateral stepping, en-echelon continental rift segments by magma intrusion. These dike intrusions suggest that strain accommodated by magma emplacement is a pervasive process leading to continental breakup above a mantle hotspot.

Keir, D.; Ayele, A.; Pagli, C.; Wright, T. J.; Hamling, I. J.; Guidarelli, M.; Ebinger, C. J.; Belachew, M.; Calais, E.

2009-12-01

134

Structural and Magmatic Evolution of Atlantic Type Rifted Margins  

NASA Astrophysics Data System (ADS)

The increasing number of high quality reflection and refraction seismic surveys and drill hole data in Atlantic type rifted margins show a divergent style of margin architecture and evolution in which quantity and distribution of syn-rift magmatism and fault structures are the most variable features. These observations led to an oversimplified classification of rifted margins as either volcanic or non-volcanic. Although this simple concept may lead to the idea that margins evolve either under the presence or absence of magma, the available data show that rifted margins are more complex and cannot be characterized based on the volume of observed magma alone. Indeed so called "non-volcanic" margins are not necessarily amagmatic, as shown by the results of ODP drilling along the Iberia-Newfoundland rifted margins. On the other hand, magma-rich margins, such as the Norwegian rifted margins, show evidence for hyper-extended crust, suggesting that there is more deformation before magma emplacement as previously proposed. This leads to the question about how magmatic and tectonic processes are interacting before, during and after continental breakup. In my presentation I will review results from the South and North Atlantic and will discuss the structural and magmatic evolution of so called magma-rich and magma-poor rifted margins. I will show that the commonly proposed end-member type margins, i.e. "non-magmatic" and "non-tectonic" margins do not exist and that rifted margins show a more complex poly-phase tectonic and magmatic evolution as previously suggested. However, it remains unclear whether decompression melting is the driving force, or rather the consequence of extension. In my presentation I will discuss to what extent variable amounts of magma may control the evolution and timing of continental breakup and control the architecture of Atlantic type rifted margins.

Manatschal, G.

2011-12-01

135

Tectonic Framework of the Kachchh Rift Basin  

NASA Astrophysics Data System (ADS)

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

Talwani, P.; Gangopadhyay, A. K.

2001-05-01

136

Biogeochemistry of Kenyan Rift Valley Lake Sediments  

NASA Astrophysics Data System (ADS)

The numerous lakes in the Kenyan Rift Valley show strong hydrochemical differences due to their varying geologic settings. There are freshwater lakes with a low alkalinity like Lake Naivasha on the one hand and very salt-rich lakes with high pH values like Lake Logipi on the other. It is known that the underlying lake sediments are influenced by the lake chemistry and by the microorganisms in the sediment. The aim of this work is to provide a biogeochemical characterization of the lake sediments and to use these data to identify the mechanisms that control lake chemistry and to reconstruct the biogeochemical evolution of each lake. The examined rift lakes were Lakes Logipi and Eight in the Suguta Valley, Lakes Baringo and Bogoria south of the valley, as well as Lakes Naivasha, Oloiden, and Sonachi on the Kenyan Dome. The porewater was analysed for different ions and hydrogen sulphide. Additionally, alkalinity and salinity of the lake water were determined as well as the cell numbers in the sediment, using fluorescent microscopy. The results of the porewater analysis show that the overall chemistry differs considerably between the lakes. In some lakes, concentrations of fluoride, chloride, sulphate, and/or hydrogen sulphide show strong concentration gradients with depth, whereas in other lakes the concentrations show only minor variations. Fluoride is present in all lakes; the lowest concentration is found in Lake Oloiden (60 - 90 mg/l), the highest one in Lake Bogoria (1,025 - 1,930 mg/l). The lakes show also large differences in sulphate concentrations. The values vary between 2 mg/l in Lake Baringo and 15,250 mg/l in Lake Eight. In all cores, sulphate concentration does not change significantly with depth; however, there is a distinct peak in each core, raising the question of synchronicity. As expected, chloride concentrations correlate with total salinity. There is no hydrogen sulphide present in the porewater of Lakes Naivasha, Baringo, and Oloiden, whereas in the other lakes, the hydrogen sulphide concentrations are decreasing with depth. Although in close proximity to each other, the Kenyan Rift Valley lakes offer the unique opportunity to study a wide range of geochemical environments and the associated biogeochemical processes.

Grewe, Sina; Kallmeyer, Jens

2013-04-01

137

Interaction Between Mantle Plume Processes and Surface Topography in Incipient Rift Systems, EARS  

Microsoft Academic Search

In an asymetric extensional setting such as the East African Rift System (EARS), the pronounced regional variations in topography are generally explained by two different models: a) a post-rift uplift resulting from mechanical relaxation that evokes an uplift of the rift shoulders, and b) a syn-rift uplift effected by mantle plume pushing during extension. However, an important pre-rift topographic feature

H. Wichura; R. Bousquet; R. Oberhänsli

2009-01-01

138

Rift Valley Fever, Sudan, 2007 and 2010  

PubMed Central

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

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

2013-01-01

139

Diagnostic approaches for rift valley Fever.  

PubMed

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 Africa. There is concern that this virus could spread because of global warming, increased animal trade or through bioterrorism. This paper discusses the current and developing approaches to diagnosis of RVF. Diagnostic assays are available for RVF, but availability can be limited and there is a need for global harmonization. Continued improvement of standard serological and viral genome amplification approaches, including new embedded/syndromic testing, biosensor, emerging virus detection and characterization technologies is needed. Copyright © 2013 by the International Alliance for Biological Standardization (IABS), Carouge-Geneva (Switzerland). PMID:23689885

Wilson, W C; Weingartl, H M; Drolet, B S; Davé, K; Harpster, M H; Johnson, P A; Faburay, B; Ruder, M G; Richt, J A; McVey, D S

2013-05-14

140

Molecular Biology of Rift Valley Fever Virus  

PubMed Central

Rift Valley fever virus (RVFV) causes large outbreaks of acute febrile and often fatal illness among humans and domesticated animals in sub-saharan Africa and the Arabian peninsula. RVFV is a member of the family Bunyaviridae, genus Phlebovirus. Like all members of this large virus family, it contains a three-segmented genome of negative/ambisense strand RNA, packaged into viral nucleocapsid protein, and enveloped by a lipid bilayer containing two viral glycoproteins. During the past years, there was an increased interest in RVFV epidemiology, molecular biology, and virulence mechanisms. Here, we will try to provide an overview over the basic features of this significant pathogen, and review the latest developments in this highly active research field.

Bouloy, Michele; Weber, Friedemann

2010-01-01

141

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

142

Continental rifting parallel to ancient collisional belts: an effect of the mechanical anisotropy of the lithospheric mantle  

Microsoft Academic Search

Analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process. Rift propagation is not random, but tends to follow the trend of the orogenic fabric of the plates, systematically reactivating ancient lithospheric structures. Continental rifts often display a clear component of strike–slip deformation, in particular in the early rifting

Andréa Tommasi; Alain Vauchez

2001-01-01

143

Lithospheric thinning beneath rifted regions of Southern California.  

PubMed

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

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

2011-10-06

144

Structure and Evolution of the Kenya Rift Valley  

Microsoft Academic Search

Geological evidence makes possible a reconstruction of events since Miocene times, with about 10 km of crustal separation and the pressure beneath the central rift of an upward intrusion from the mantle.

B. H. Baker; J. Wohlenberg

1971-01-01

145

Amagmatic Lithospheric Rifting as Expressed in the Red Sea (Invited)  

NASA Astrophysics Data System (ADS)

The Red Sea is an active analog for the rifting that produced most of the non-magmatic passive continental margins of the Atlantic and Indian Oceans. Specifically, it cuts through old (Pre-Cambrian) lithosphere well away from other plate boundaries and rifting is within 30° of orthogonal to the trend of the rift. Observations in the Red Sea therefore provide insights that can be applied to older, relict non-magmatic margins Although rifting in the Red Sea has been basically amagmatic, magmatism at two crucial times was important in defining the location of the rift and of initiating active extension and rifting. The onset of rifting was preceded by massive volcanism in Ethiopia and southern Yemen. The major volume of lava was erupted over a short period from 31-29.5 Ma. This event appears to have defined the location of the triple junction between the Red Sea, Gulf of Aden and Ethiopian rifts. Although rifting along the Gulf of Aden may have initiated at the time of the Afar volcanism, this magmatic episode was not accompanied by significant extension in the region that was to become the Red Sea and there is no sign of continuing extension in the Red Sea beginning at that time. The onset of rifting and extension in the Red Sea immediately follows a massive episode of dike intrusion that occurred over a short period of time at 24-21 Ma. Diking extended for 1700 km along the entire length of the Red Sea with no discernable temporal pattern. The diking event coincides with deposition of the first clearly syn-rift sediments at locations from the Gulf of Suez to Eritrea as well as the beginning of uplift and denudation. The diking event may have enabled rifting and extension in the Red Sea. There is no evidence of additional volcanism within the Red Sea rift from 21 Ma to the initiation of seafloor spreading. Seismic reflection and potential field data from the northern Red Sea show that large, rotated fault blocks of continental crust underlie the basin. Rifting is symmetric with the fault blocks stepping down to an axial depression. These blocks of continental crust extend continuously from the margins to the axial depression where seafloor spreading cells are becoming established. Exhumed mantle has not been recognized anywhere in the Red Sea. Mantle rocks exposed on Zabragad Island result from local compression rather than regional extension and unroofing. The structure and geologic development of the Red Sea is thus very different from that observed at the well-studied Iberia and Newfoundland conjugate margins, which are often cited as the archetype of non-magmatic rifting. Observations in the Red Sea thus raise the question of whether Iberia/Newfoundland should be considered as the type example, or whether it may actually be an extreme end member in a wide continuum of possible rifting histories.

Cochran, J. R.

2009-12-01

146

Martian Rifting in the Absence of Plate Tectonics?  

NASA Astrophysics Data System (ADS)

We investigate the stresses necessary to induce rifting in the Thaumasia Highlands. We show that forces connected to plate tectonics need not be invoked to explain the observed lithosphere-scale faulting and present an alternative scenario.

Grott, M.; Hauber, E.; Kronberg, P.

2007-07-01

147

The Large Thaumasia Graben, Mars — Is It a Rift?  

NASA Astrophysics Data System (ADS)

We describe the morphology of a large and complex extensional structure in the western Thaumasia region (the "Thaumasia graben"). We consider possible fault geometries, determine extension, and discuss the case for or against rifting.

Hauber, E.; Kronberg, P.

2003-07-01

148

Concentration of Rift Valley Fever and Chikungunya Viruses by Precipitation.  

National Technical Information Service (NTIS)

Simple and efficient methods for concentrating Rift Valley fever (RVF) virus and chikungunya (CHIK) virus are described. Ammonium sulfate, potassium sulfate, or alcohol was used as a precipitating agent and the precipitate was resuspended to volumes suita...

B. G. Manlandt F. Klein R. E. Lincoln R. R. Cockey

1970-01-01

149

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

150

Geodynamic Evolution of the Southern Flank of the Corinth Rift  

Microsoft Academic Search

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

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

2005-01-01

151

Cretaceous rift related magmatism in central-western South America  

Microsoft Academic Search

The Cretaceous–Paleocene Andean basin system of central-western South America, comprises northwestern Argentina and southwestern Bolivia. It is situated between 62°–68°W and 18°–27°S, but extends westward to northern Chile and northward to Bolivia and Peru. These basins have been interpreted as an aborted foreland rift. In a general sense, it may be possible to relate this rift to the opening of

J. G Viramonte; S. M Kay; R Becchio; M Escayola; I Novitski

1999-01-01

152

Structure of the southern Rio Grande rift from gravity interpretation  

NASA Astrophysics Data System (ADS)

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

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

1986-05-01

153

Topographic controls on dike injection in volcanic rift zones  

NASA Astrophysics Data System (ADS)

Dike emplacement in volcanic rift zones is often associated with the injection of "blade-like" dikes, which propagate long distances parallel to the rift, but frequently remain trapped at depth and erupt only near the tip of the dike. Over geologic time, this style of dike injection implies that a greater percentage of extension is accommodated by magma accretion at depth than near the surface. In this study, we investigate the evolution of faulting, topography and stress state in volcanic rift zones using a kinematic model for dike injection in an extending 2-D elastic-viscoplastic layer. We show that the intrusion of blade-like dikes focuses deformation at the rift axis, leading to the formation of an axial rift valley. However, flexure associated with the development of the rift topography generates compression at the base of the plate. If the magnitude of these deviatoric compressive stresses exceeds the deviatoric tensile stress associated with far-field extension, further dike injection will be inhibited. In general, this transition from tensile to compressive deviatoric stresses occurs when the rate of accretion in the lower crust is greater than 50-60% of the far-field extension rate. These results indicate that, over geologic timescales, the injection of blade-like dikes is a self-limiting process in which dike-generated faulting and topography result in an efficient feedback mechanism that controls the time-averaged distribution of magma accretion within the crust.

Behn, Mark D.; Buck, W. Roger; Sacks, I. Selwyn

2006-06-01

154

Buried Mesozoic rift basins of Moroccan Atlantic continental margin  

SciTech Connect

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

Mohamed, N.; Jabour, H.; El Mostaine, M. [ONREP, Rabat (Morocco)

1995-08-01

155

Internal structure and evolution of a volcanic rift system in the eastern North Atlantic: the Desertas rift zone, Madeira archipelago  

Microsoft Academic Search

The three elongated Desertas Islands form the top of a 60-km-long NW–SE-striking submarine ridge southeast of Madeira (NE Atlantic). The alignment of eruptive centres and parallel dyke swarms indicates that the islands represent a deeply eroded volcanic rift zone. Detailed field studies combined with 40Ar\\/39Ar age determinations and geochemical analyses reveal the internal structure and evolution of this rift, which

Stefanie Schwarz; Andreas Klügel; Paul van den Bogaard; Jörg Geldmacher

2005-01-01

156

The onshore northeast Brazilian rift basins: An early Neocomian aborted rift system  

SciTech Connect

Early Cretaceous rift basins of northeastern Brazil illustrate key three-dimensional geometries of intracontinental rift systems, controlled mainly by the basement structures. These basins were formed and then abandoned during the early extension associated with the north-south-propagating separation of South America and Africa. During the early Neocomian, extensional deformation jumped from the easternmost basins (group 1: Sergipe Alagoas and Gabon basins; group 2: Reconcavo, Tucano, and Jatoba basins) to the west, forming a series of northeast-trending intracratonic basins (group 3: Araripe, Rio do Peixe, Iguatu, Malhada Vermelha, Lima Campos, and Potiguar basins). The intracratonic basins of groups 2 and 3 consist of asymmetric half-grabens separated by basement highs, transfer faults, and/or accommodation zones. These basins are typically a few tens of kilometers wide and trend northeast-southwest, roughly perpendicular to the main extension direction during the early Neocomian. Preexisting upper crustal weakness zones, like the dominantly northeast-southwest-trending shear zones of the Brazilian orogeny, controlled the development of intracrustal listric normal faults. Internal transverse structures such as transfer faults (Reconcavo basin and onshore Potiguar basin) and accommodation zones (onshore Potiguar basin and Araripe basin) were also controlled by the local basement structural framework. Transverse megafaults and lithostructural associations controlled the three main rift trends. The megashear zones of Pernanbuco (Brazil)-Ngaundere (Africa) apparently behaved like a huge accommodation zone, balancing extensional deformation along the Reconcavo-Jatoba/Sergipe Alagoas-Gabon trends with simultaneous extension along the Araripe-Potiguar trend. The Sergipe Alagoas-Gabon trend and the Potiguar basin represent the site of continued evolution into a marginal open basin following early Neocomian deformation.

Matos, R. (Cornell Univ., Ithaca, NY (USA))

1990-05-01

157

Qp beneath the Rio Grande and East African Rift Zones  

NASA Astrophysics Data System (ADS)

In 1983 teleseismic short-period P waves were recorded in the southwest United States along a linear array which stretched 1000 km across the Rio Grande rift. In 1985 a similar experiment was conducted across the East African rift, in which stations were installed along a 600 km array which ran east-west across Kenya. We have developed a spectral comparison technique to calculate the attenuation at each station. For a given event we simultaneously solve for the effective source spectrum, which includes mantle path effects as well as instrument response, an amplitude factor, and an attenuation operator t* at each station. We use downward projection of P wave travel time residuals to define the lithosphere-asthenosphere boundary, which we infer to upwarp beneath both rifts. The velocity structure so determined is then used to determine the Q of the asthenosphere relative to that of the lower lithosphere. When we assume a lower lithosphere Q of 100, we obtain asthenospheric Q values of 57 beneath the Rio Grande rift and 26 beneath the East African rift, with corresponding velocity contrasts of -8% and -12%, respectively. The low Q values as well as low velocity and density contrasts support the interpretation that the upwarped asthenosphere contains partial melt and that beneath the East African rift the concentration is higher than beneath the Rio Grande. Comparison of the results with experiments on peridotite (Sato et al., 1989a, b) suggests that the asthenosphere is at homologous temperatures of 1.01 and 1.03 with corresponding partial melt fractions of 1% and 3% beneath the Rio Grande and East African rifts, respectively.

Halderman, Thomas P.; Davis, Paul M.

1991-06-01

158

Numerical Models of Multi-Velocity Rift Evolution  

NASA Astrophysics Data System (ADS)

Numerical models are a useful tool to combine present-day knowledge on plate kinematics and lithosphere rheology in order to reveal the complex relations between deformation modes, margin asymmetry and crustal hyperextension. We investigate how long rift duration and distinct successive extensional velocities influence lithospheric localization and final margin geometry. Our scenarios are computed using a thermo-mechanical, finite element model that includes elasto-visco-plastic rheology and a free surface. The model allows for high resolution of 1 km in a 2d setup with 500 km width and 200 km depth range. We highlight the importance of strain hardening for slow rifting (~ 5 mm/yr full extension velocity) where cooling of uplifted mantle material promotes continuous lateral migration of the rift center. Despite initially symmetric extension, strain hardening can lead both to significant late rift asymmetry and to crustal hyperextension. We use our model to understand the South Atlantic conjugate margin geometries. We thereby build upon a new plate kinematic model for the South Atlantic rift which integrates time-dependent information on crustal deformation within a global self-consistent plate rotation framework. Here, the intial continental separation between South America and Africa starts in the Early Cretaceous at low velocities, controlled by African intracontinental rifting. After 20-25 Ma of rifting, loss of lithospheric strength in the Equatorial Atlantic domain results in a significant increase in extensional velocities and a change in extensional direction from 120 Ma onwards. We investigate the impact of this multi-velocity extension history on the spatio-temporal margin evolution and compare our results with conjugate margin cross-sections at representative locations in the South Atlantic. We couple observations on continental extension from global scale plate tectonic models with high resolution, thermo-mechanical models of lithosphere deformation. This approach offers a powerful way of converging to robust regional tectonic models and link plate-scale kinematics to lithospheric deformation modeling and smaller scale tectonics analysis.

Brune, S.; Heine, C.

2012-04-01

159

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

NASA Astrophysics Data System (ADS)

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

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

2011-01-01

160

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

161

Stress re-orientation along zones of weak fabrics in rifts: An explanation for pure extension in ‘oblique’ rift segments?  

NASA Astrophysics Data System (ADS)

The Cenozoic East African Rift System (EAR) displays strong interaction between rift faults and zones of weakness in Precambrian basement and Late Carboniferous-Permian Karroo rifts. Assuming E-W regional extension, NW-SE and NE-SW trending rift segments following older zones of weakness are expected to show oblique extensional or transtensional displacement. Yet, the earthquake focal mechanism-derived modern maximum horizontal stress (Shmax) pattern in the Western Branch of the EAR displays similar orientations to dominant foliation direction in Proterozoic orogenic belts. The oblique segments are undergoing almost pure extension. Localized stress deflections near zones of strength anisotropy are commonly described in studies of geomechanics, in-situ stresses and fracture patterns. The correspondence between foliation and Shmax direction in the Western Branch suggests re-orientation of stress at a larger scale than has previously been recognized. The stress guide effect appears best developed in the Ubendian Belt-Rukwa Rift where the Shmax direction from the regional N-S direction to a NW-SE trend and NW-SE trending foliations are consistently steeply dipping. However, this effect is considered only one of a number of possible causes of stress deflection in the East African Rift. Precambrian basement foliation is not oriented consistently enough to re-orient stress on a large-scale everywhere (particularly the Kenya Rift). The effects of stress rotation along oblique fabrics are: 1) faults normally predicted to exhibit oblique slip are actually near pure dip-slip, 2) faults along oblique trends can be optimally oriented with respect to the (local) stress field, and develop in a zone of reduced cohesive shear strength (with respect to isotropic rocks), 3) where foliations are inclined from vertical the maximum principal stress will be similarly inclined leading to initiation of extensional faults at angles considerably lower than 60°, and 4) faults will strike parallel to the foliation but cross-cut foliations in cross-section.

Morley, C. K.

2010-09-01

162

Fault reactivation and rift localization: Northeastern Gulf of Aden margin  

NASA Astrophysics Data System (ADS)

The Gulf of Aden and the Sheba spreading ridge (Gulf of Aden) forms the southern boundary of the Arabian Plate. Its orientation (075°E) and its kinematics (about 030°E divergence) are interpreted as the result of an oblique rifting. In this contribution, a field study in the northeastern Gulf of Aden allows us to confirm the Oligo-Miocene synrift directions of extension and to precise the normal fault network geometry. The synrift extensions are 020°E and 160°E (possibly in this chronological order); the normal faults strike 070°E, 090°E, and 110°E. The results show that some characteristics are consistent with oblique rifting analogue models, while some others are not. Especially, fault reactivation of Mesozoic structures is shown to have occurred significantly at the beginning and during rifting. These data are therefore compared to analogue models of oblique reactivation, and this comparison demonstrates that fault reactivation played a key role during the early stage of the Gulf of Aden rifting. Finally, scenarios of the lithospheric evolution during the eastern Gulf of Aden opening (preexisting weaknesses in the lithosphere or not) are discussed to better constrain the deformation history of the northern margin. Especially, we show that rift localization processes may imply stress rotations through time.

Bellahsen, N.; Fournier, M.; D'Acremont, E.; Leroy, S.; Daniel, J. M.

2006-02-01

163

Field Studies of Geothermal Reservoirs Rio Grande Rift, New Mexico  

SciTech Connect

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

James C Witcher

2002-07-30

164

Seismic-refraction studies of the Afro-Arabian rift system — a brief review  

Microsoft Academic Search

The crustal and uppermost-mantle structure of major units of the Afro-Arabian rift system has been consecutively investigated by seismic-refraction surveys in the Jordan-Dead Sea rift, the Red Sea, the Afar depression and the East African rift of Kenya. With the exception of the Jordan-Dead Sea transform, the entire Afro-Arabian rift system is underlain by anomalous mantle with Pn-velocities less than

C. Prodehl; K. Fuchs; J. Mechie

1997-01-01

165

Volcanism, tectonism, sedimentation, and the paleoanthropological record in the Ethiopian Rift System  

Microsoft Academic Search

The Ethiopian Rift System consists of basins that are in different stages of evolu- tion. Some of the rift-related basins in southwestern Ethiopia are half-grabens that have not evolved to symmetrical rifts since the initiation of rifting here in the middle Miocene. These basins contain fossiliferous Pliocene-Pleistocene volcaniclastic sedi- ments and volcanic rocks and have been occupied by early hominid

Giday WoldeGabriel; Grant Heiken; Tim D. White; Berhane Asfaw; William K. Hart; Paul R. Renne

2000-01-01

166

Formation of the Rio Grande Rift and Upper Mantle Seismic Wave Velocity Anomaly  

Microsoft Academic Search

The Rio Grande rift extends from central Colorado through New Mexico where it merges with the southern Basin and Range province, to Chihuahua in Mexico. Rifting started in the Oligocene-early Miocene, and strain rates were probably very modest since the Late Miocene. The rift is currently active (strain rates ~1-2 mm\\/yr) but rifting is not accompanied by seismic activity. The

J. van Wijk; J. van Hunen; S. Goes

2007-01-01

167

Formation of the Rio Grande Rift and Upper Mantle Seismic Wave Velocity Anomaly  

Microsoft Academic Search

The Rio Grande rift extends from central Colorado through New Mexico where it merges with the southern Basin and Range province, to Chihuahua in Mexico. Rifting started in the Oligocene-early Miocene, and strain rates were probably very modest since the Late Miocene. The rift is currently active (strain rates ~1-2 mm\\/yr) but rifting is not accompanied by seismic activity. The

J. van Wijk; J. van Hunen; S. Goes

2004-01-01

168

Seismicity of the Woodlark-D'Entrecasteaux Rift System in Eastern Papua New Guinea  

Microsoft Academic Search

The Woodlark-D'Entrecasteaux rift system may be the fastest opening continental rift on the planet, accommodating 25-40 mm\\/yr extension in continental crust. Along strike, extension rates increase and rifting has progressed to full sea floor spreading. We report on the results from the first local earthquake survey in the region, across the transition from oceanic rifting to distributed continental extension. From

A. Ferris; B. C. Zelt; G. A. Abers; J. S. Floyd; B. Taylor; J. C. Mutter; A. Lerner-Lam

2002-01-01

169

Geology and palaeontology of the Late Miocene Middle Awash valley, Afar rift, Ethiopia  

Microsoft Academic Search

The Middle Awash study area of Ethiopia's Afar rift has yielded abundant vertebrate fossils (~10,000), including several hominid taxa. The study area contains a long sedimentary record spanning Late Miocene (5.3-11.2Myr ago) to Holocene times. Exposed in a unique tectonic and volcanic transition zone between the main Ethiopian rift (MER) and the Afar rift, sediments along the western Afar rift

Giday WoldeGabriel; Yohannes Haile-Selassie; Paul R. Renne; William K. Hart; Stanley H. Ambrose; Berhane Asfaw; Grant Heiken; Tim White

2001-01-01

170

From hyper-extended rifts to orogens: the example of the Mauléon rift basin in the Western Pyrenees (SW France)  

NASA Astrophysics Data System (ADS)

An integral part of plate tectonic theory is that the fate of rifted margins is to be accreted into mountain belts. Thus, rift-related inheritance is an essential parameter controlling the evolution and architecture of collisional orogens. Although this link is well accepted, rift inheritance is often ignored. The Pyrenees, located along the Iberian and European plate boundary, can be considered as one of the best places to study the reactivation of former rift structures. In this orogen the Late Cretaceous and Tertiary convergence overprints a Late Jurassic to Lower Cretaceous complex intracontinental rift system related to the opening of the North Atlantic. During the rifting, several strongly subsiding basins developed in the axis of the Pyrenees showing evidence of extreme crustal extension and even locale mantle exhumation to the seafloor. Although the exact age and kinematics of rifting is still debated, these structures have an important impact in the subsequent orogenic overprint. In our presentation we discuss the example of the Mauléon basin, which escaped from the most pervasive deformations because of its specific location at the interface between the western termination of the chain and the Bay of Biscay oceanic realm. Detailed mapping combined with seismic reflection, gravity data and industry wells enabled to determine the 3D rift architecture of the Mauléon basin. Two major diachronous detachment systems can be mapped and followed through space. The Southern Mauléon Detachment (SMD) develops first, starts to thin the crust and floors the Southern Mauléon sub-Basin (SMB). The second, the Northern Mauléon Detachment (SMD) is younger and controls the final crustal thinning and mantle exhumation to the north. Both constitute the whole Mauléon basin. Like at the scale of the overall Pyrenees, the reactivation of the Mauléon Basin increases progressively from west to east, which enables to document the progressive reactivation of an aborted hyper-extended rift system. In our presentation, we discuss the compressional reactivation of the rift structures by the study of dip sections across the basin, from weakly reactivated sections in the west to strongly reactivated sections in the east. Comparing the sections, it results that the compression reactivated the rift structures (mainly the detachment faults) and that this reactivation occurred in 2 steps. It corresponds to the reactivation through time of the NMB before the SMB. This evolution is in line with an early proto-subduction of the hyper-extended domain beneath the European plate whereas the NMB sediments are wedged, folded and thrust onto the Iberia and Europe margins ("thin-skin" tectonics). The second step occurs when the deformation started to migrate southward resulting in the formation of the axial Pyrenees nappe stack (thick-skin tectonics). These results suggest that the inherited rift structures strongly controlled the initial convergence. Future work will revisit the more reactivated Albian basins throughout the chain to investigate how far the results from western Pyrenees can be used to understand the Central and Eastern Pyrenees. Moreover, this field-oriented study can serve as an example of how rift structures may control style and timing of orogenic processes.

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

2011-12-01

171

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, R. D.; Green, K.; Williams, D.; Bainbridge, A.; Crane, K.; Van Andel, T. H.

1979-01-01

172

Patterns of Rift Valley fever activity in Zambia.  

PubMed Central

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

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

1992-01-01

173

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

NASA Astrophysics Data System (ADS)

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

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

2011-08-01

174

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

Microsoft Academic Search

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 California was investigated using an instrumented aircraft to compare the aerosol, cloud microphysical, and thermodynamic properties in the rift with those of the surrounding solid stratocumulus deck. The microphysical characteristics

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

2006-01-01

175

Structural evolution of the Abiquiu embayment, Rio Grande Rift: Implications for the development of transfer zones  

Microsoft Academic Search

The Abiquiu embayment is located along the boundary between the Colorado Plateau and the Rio Grande rift in north central New Mexico. It is an early rift basin bordered by the Canones fault system on its west side that is oblique to the regional trend of the Rio Grande rift and lies within a region where the polarity of the

R. T. Hicks; M. A. Murphy

2006-01-01

176

Effects of Mechanical Layering on Dike Emplacement, Faulting, and Surface Deformation in Rift Zones  

Microsoft Academic Search

All rift zones in volcanic islands contain normal faults and dikes as their main structural elements. During rifting episodes there is normally fault slip, graben development and dike emplacement. Field and geodetic studies, however, show that most dikes emplaced in rift zones do not reach the surface but rather become arrested at certain crustal depths. Geodetic measurements of the surface

A. Gudmundsson; I. Fjeldskaar

2003-01-01

177

Neotectonic Stress Analysis Of The Red Sea Rift By Finite Element Modeling  

Microsoft Academic Search

The Red Sea is a tectonic rift that was formed in the late Oligocene-early Miocene when the originally connected African and Arabian land masses broke apart. At first it was a continental rift, then, as Arabia drifted away, developed into an intercontinental system that today separates the independent Arabian plate from the African plate. The Red Sea rift is part

S. K. Dwivedi; D. Hayashi

2006-01-01

178

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

Microsoft Academic Search

The evolution of non-volcanic rifted margins is key to understanding continental breakup and the early evolution of some of the world's most productive hydrocarbon basins. However, the early stages of such rifting are constrained by limited observations on ancient heavily sedimented margins such as Newfoundland and Iberia. Ultraslow spreading ridges, however, provide a modern analogue for early continental rifting. Ultraslow

H. J. Dick; J. E. Snow

2009-01-01

179

Volcanic and Structural History of the NE Rift Zone of Tenerife, Canary Islands, Spain  

Microsoft Academic Search

The NE Rift of Tenerife is an excellent example of a persistent, recurrent rift, providing important evidence on the origin and dynamics of these major volcanic features. The rift developed in three successive, intense and relatively short eruptive stages (a few hundred ka), separated by longer periods of quiescence or reduced activity: A Miocene stage (7203+\\/-155ka), apparently extending the central

J. Carracedo; H. Guillou; E. R. Badiola; F. P. Torrado; V. Troll; A. Delcamp; A. R. Gonzalez

2008-01-01

180

US-Africa collaborative research on incipient continental rift zones  

NASA Astrophysics Data System (ADS)

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

Atekwana, E. A.

2007-12-01

181

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)

1996-01-01

182

Drilling the Newfoundland Margin in the Newfoundland-Iberia Rift  

NASA Astrophysics Data System (ADS)

Extension in the Newfoundland-Iberia rift began in the Late Triassic and culminated in seafloor spreading by the mid-Cretaceous. Magnetic anomalies M0-M3 are consistently identified and indicate that seafloor spreading commenced no later than Barremian-Aptian time. On both margins, however, crustal 'transition' zones up to about 150 km wide separate the known ocean crust from clearly continental crust, and the origin of this lithosphere is disputed. Deep-sea drilling (ODP legs 149 and 173) and geophysical work has documented that about half this width on the Iberia margin may be extended continental crust, depending on location along the margin, while the seaward part of the transition zone is unroofed mantle emplaced with apparently little associated magmatism. The Newfoundland side of the rift is undrilled but has significantly different deep structure and basement character across the transition zone. Newfoundland basement averages more than a kilometer shallower (corrected for sediment loading) than the Iberia conjugate, exhibits much less roughness, and is capped by a level and highly reflective sequence (below the 'U' reflection) that is missing off Iberia. These features suggest asymmetry in rift development, with the Newfoundland margin forming an upper plate (thinned continental crust) and Iberia comprising a lower plate (exhumed mantle). Leg 210 ODP drilling to about 2200 meters depth in the transition zone of the Newfoundland Basin is scheduled for 2003. It is intended to sample and log basement, the basement-U interval (pre- to synrift?), and the entire post-rift sequence. Primary objectives are to document the nature of basement, the level of magmatism on the Newfoundland margin, the origin and significance of the sub-U reflection sequence, the post-rift subsidence and sedimentation history, and the history of strain partitioning in the rift.

Tucholke, B. E.; Driscoll, N. W.; Holbrook, W. S.; Hopper, J. R.; Larsen, H. C.; Louden, K. E.; Minshull, T. A.; Sawyer, D. S.; Sibuet, J. C.; Srivastava, S. P.; Whitmarsh, R. B.

2001-12-01

183

Intermittent upwelling of asthenosphere beneath the Gregory Rift, Kenya  

SciTech Connect

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

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

1991-06-01

184

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

185

Recent geodynamics and evolution of the Moma rift, Northeast Asia.  

NASA Astrophysics Data System (ADS)

The Cenozoic Moma rift system is a major tectonic feature in northeast Russia. It is composed of a series of basins (Selennyakh, Kyrin,Lower Moma,Upper Moma,etc.) filled with up to one km thick and bounded by the Chersky Range (up to 3100 m high) on the southwest and the Moma Range (up to 2400 m high) on the northeast. Northeast of the Moma Range is the Indigirka-Zyryanka foreland basin, composed of thick, up to 2.5 km, Eocene, Oligocene, and Miocene coal-bearing sequences, while on the southwestern side of the Chersky Range there are a number of piedmont basins (Tuostakh, Upper Adycha, Derbeke, etc.) containing up to several hundred meters of Miocene and Oligocene coal-bearing deposits. Despite considerable study over the past half-century, there is considerable debate over the origin, present-day tectonics, and evolution of the Moma rift system. The Cenozoic deposits of the basins generally become younger from northwest to southeast with the exception of the Seimchan-Buyunda basin. In the northeast, fan-shaped coal-bearing basins (e.g., Nenneli, Olzhoi, Selennyakh, Uyandina, Tommot, and others) are filled with Miocene to Pliocene deposits, while basins in the southeast (e.g., Taskan) are filled with Neogene sediments. The Seimchan-Buyunda basin, however, has sediments of Oligocene age. The Moma rift system is reflected a major step in the gravity field, presumably separating denser rocks of the Kolyma-Omolon superterrain from somewhat less dense rocks of the Verkhoyansk fold belt (margin of the North Asian Craton). Analysis of travel-times of Pn and Pg waves from local earthquakes indicates an area of thinned crust (30-35 km) southwest of the Moma rift system, extending as a "tongue" from the Lena River delta and the Laptev Sea to the upper part of the Kolyma River, as compared to 40-45 km in the surrounding areas. This region of thinned crust also coincides with a region of high heat flow values measured in boreholes of the Chersky Range (up to 88 mW/m2). Hot springs with temperatures up to +20°C are found within the Moma and Selnnyakh basins proper.The crustal inhomogeneity is also reflected in the upper mantle as indicated by a 40° rotation of the Rayleigh wave polarization angle from teleseisms recorded at Tiksi that cross the Moma rift system as opposed to those that do not. Cenozoic volcanism, chemically similar to basalts and rhyolites from rift zones elsewhere is found in the Moma rift proper. Balagan-Tas is a basaltic cinder cone which has been dated at 286,000 years based on Ar-Ar dating, while Uraga-Khaya is an undated, presumed Quaternary, rhyolitic dome. All these factors indicate that the Moma rift system originated as a continental rift, probably as an extension of the Arctic (Gakkel) Mid-Ocean Ridge. At the present, however, compressional conditions prevail within the Moma rift zone. Seismicity is generally absent from the rift basins proper or their margins; most seismicity is concentrated to the southwest of the Moma rift basins along major strike-slip fault systems. Focal mechanisms of the largest earthquakes in the Chersky Range also all show transpression. Field mapping indicates that the majority of the faults mapped in the field are strike-slip, thrust and reverse faults (86%) with only a small number of normal faults (14%) and that the Cenozoic deposits within the Moma rift are intensely folded. Re-leveling surveys conducted along the Indigirka River, which cuts across the Moma rift system, reveal a moderate rate of presnt-day vertical uplift (up to +4 mm/yr). Thus, the Moma rift system is no longer acting as a rift, but is undergoing transpression. This conclusion is also supported by recent plate motion calculations based on GPS and VLBI data, as well as slip-vectors of earthquakes, which indicates that the Euler pole between North America and Eurasia is located around 68-70°N, near the coast of the Laptev Sea. This places the Moma rift system in a zone of convergence between North America and Eurasia; this geometry also supports the extrusion of the Okhotsk Sea plate. Poles of ro

Imaev, V. S.; Imaeva, L. P.; Kozmin, B. M.; Fujita, K. S.; Mackey, K. G.

2009-04-01

186

Continental rifting parallel to ancient collisional belts: an e¡ect of the mechanical anisotropy of the lithospheric mantle  

Microsoft Academic Search

Analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process. Rift propagation is not random, but tends to follow the trend of the orogenic fabric of the plates, systematically reactivating ancient lithospheric structures. Continental rifts often display a clear component of strike^ slip deformation, in particular in the early

Andrea Tommasi; Alain Vauchez

2001-01-01

187

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

188

Rift Valley fever: an uninvited zoonosis in the Arabian peninsula  

Microsoft Academic Search

Rift Valley fever (RVF) is an acute viral disease, affecting mainly livestock but also humans. The virus is transmitted to humans through mosquito bites or by exposure to blood and bodily fluids. Drinking raw, unpasteurized milk from infected animals can also transmit RVF. Routine vaccination of livestock in Africa has been prohibitively expensive, leading to endemicity of RVF in most

Hanan H Balkhy; Ziad A Memish

2003-01-01

189

Geochemical Overview of the East African Rift System  

Microsoft Academic Search

Mafic volcanics of the East African Rift System (EARS) record a protracted history of continental extension that is linked to mantle plume activity. The modern EARS traverses two post-Miocene topographic domes separated by a region of polyphase extension in northern Kenya and southern Ethiopia. Basaltic magmatism commenced ˜45 Ma in this highly extended region, while the onset of plume-related activity

T. Furman

2003-01-01

190

Innovative tephra studies in the East African Rift System  

NASA Astrophysics Data System (ADS)

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

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

191

Undiscovered Petroleum of the Brazilian Interior Rift Basins  

Microsoft Academic Search

Brazil is estimated to contain 52% of the undiscovered oil and gas resources of South America, outside of Venezuela and Colombia. The Reconcavo, Tucano-Jatoba, and Tacutu interior rift basins of Brazil are investigated in this paper, the first in a series dealing with undiscovered petroleum of South America (exclusive of Venezuela and Colombia). Preliminary estimates of the undiscovered petroleum resources

John Kingston; John R. Matzko

1995-01-01

192

Geodetic constraints on rifting processes in East Africa (Invited)  

NASA Astrophysics Data System (ADS)

Many features in rift zones and passive margins have been successfully explained by stretching models where lithospheric thinning and crustal extension are driven by far-field tectonic stresses. However, recent observations in East Africa show that magma intrusions can accommodate large amounts of strain during the initial stages of continental rifting and prior to significant crustal thinning. We will present a large-scale kinematic model for the East African Rift derived from space geodetic observations, earthquake slip vectors, transform azimuths, and spreading rates that includes three subplates (Victoria, Rovuma, and Lwandle) between Nubia and Somalia, with total opening increasing from ~1 mm/yr in southern Mozambique to 7 mm/yr in northern Ethiopia. We will use regional-scale geodetic results to show that far-field plate divergence in East Africa can involve a significant amount of strain accommodation by magma intrusions. We will argue that contributions other than tectonic forces are required to overcome the strength of the continental lithosphere and to initiate and sustain rifting in East Africa.

Calais, E.; Stamps, D. S.; Bennati, L.; Saria, E.; Flesch, L. M.; Freed, A. M.

2009-12-01

193

Rift Valley fever Entomology, Ecology, and Outbreak Risk Factors  

Technology Transfer Automated Retrieval System (TEKTRAN)

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease of domestic ruminants and humans in Africa. The disease is most severe in cattle, sheep, and goats, and it causes high mortality in young animals and abortion in adults. Exotic aanimal breeds from areas where RVF is not endemic tend to be ...

194

Predicting the Next Outbreak of Rift Valley Fever (RVF)  

Technology Transfer Automated Retrieval System (TEKTRAN)

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease of domestic ruminants in Africa. The disease is most severe in cattle, sheep, and goats, and it causes high mortality in young animals and abortion in adults. Exotic aanimal breeds from areas where RVF is not endemic tend to be more suscep...

195

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

Microsoft Academic Search

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

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

2010-01-01

196

Fault reactivation and rift localization: Northeastern Gulf of Aden margin  

Microsoft Academic Search

The Gulf of Aden and the Sheba spreading ridge (Gulf of Aden) forms the southern boundary of the Arabian Plate. Its orientation (075°E) and its kinematics (about 030°E divergence) are interpreted as the result of an oblique rifting. In this contribution, a field study in the northeastern Gulf of Aden allows us to confirm the Oligo-Miocene synrift directions of extension

N. Bellahsen; M. Fournier; E. d'Acremont; S. Leroy; J. M. Daniel

2006-01-01

197

Potential for Rift Valley to be Introduced into North America  

Technology Transfer Automated Retrieval System (TEKTRAN)

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease of domestic ruminants in Africa. The disease is most severe in cattle, sheep, and goats, causing mortality in young animals and abortion in adults. Human infection causes significant morbidity and mortality. RVF occurs in sub-Saharan Afri...

198

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...

199

Drilling the Aegion Fault of Corinth Rift Laboratory  

Microsoft Academic Search

The objective of the Corinth Rift Laboratory is to investigate in situ fault mechanics, its relationship with fluid flow and the related hazards. Continuous monitoring of strain, seismicity, fluid pressure and geochemistry is carried out at ground surface, sea bottom, as well as in boreholes intersecting active faults. This laboratory covers an area 30-km by 30-km and extends across the

F. H. Cornet

2003-01-01

200

Geodetic constraints on continental rifting along the Red Sea  

Microsoft Academic Search

We are using the Global Positioning System (GPS) to monitor and quantify patterns and rates of tectonic and magmatic deformation associated with active rifting of the continental lithosphere and the transition to sea floor spreading in the Red Sea. Broad-scale motions of the Nubian and Arabian plates indicate coherent plate motion with internal deformation below the current resolution of our

R. Reilinger; S. McClusky; A. Arrajehi; S. Mahmoud; A. Rayan; W. Ghebreab; G. Ogubazghi; A. Al-Aydrus

2006-01-01

201

Three-Dimensional Modelling of Rift Basin Evolution  

Microsoft Academic Search

Numerical simulation of faulting is gaining interest, typically utilising the Discrete Element Modelling (DEM) technique. As computational power increases, so does the ability to run larger and more complex experiments. This research aims to explore rift basin evolution, with emphasis placed on the interaction of multiple faults as they evolve. Importance is placed on the technical aspects of implementing a

S. M. Longshaw; E. Finch; M. Turner; R. L. Gawthorpe

2009-01-01

202

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

NASA Astrophysics Data System (ADS)

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

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

203

Helium isotopes in fluids of the Baykal rift zone  

SciTech Connect

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

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

1993-02-01

204

Mantle Dynamics Beneath the Rio Grande Rift and Colorado Plateau  

Microsoft Academic Search

We have inverted P and S wave teleseismic delay times recorded by the LA RISTRA (Colorado PLateau, Rio Grande Rift, Great Plains Seismic TRansect) array. The RISTRA array consisted of 57 broad band seismic stations most of which were deployed in a line extending 950 km from Lake Powell, Utah to Pecos, Texas. P, PKP, S, and SKS travel time

W. Gao; S. Grand; R. Gok; J. Ni; D. Wilson; R. Aster; J. Schlue; S. Baldridge; S. Semken

2002-01-01

205

The effect of volcanic constructs on rift fault patterns  

Microsoft Academic Search

Volcanoes in rift situations often show distinctive ``hour-glass'' fault patterns, with increased fault throw as the volcano is approached. We present analogue models that show that this is caused by an interaction of the regional stress field with that set up by the volcano mass. For faults to be reorientated there must be a ductile layer below the volcano. This

Benjamin van Wyk de Vries; Olivier Merle

1996-01-01

206

Serpentized mantle at rifted margins: The Goban Spur example  

Microsoft Academic Search

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

A. D. Bullock; T. A. Minshull

2002-01-01

207

High mantle temperatures following rifting caused by continental insulation  

NASA Astrophysics Data System (ADS)

The distribution of continents is thought to influence the temperature of the underlying mantle. Over geological timescales, insulation effects generate a build-up of heat that may cause increased magmatism, such as flood basalt volcanism, and ultimately rift the continents, causing them to break apart and new ocean basins to form. Here we use analyses of the major element geochemistry of lava samples collected from oceanic crust in the Atlantic and Pacific oceans to quantify the effect of continental insulation. The lavas formed at mid-ocean ridges following continental rifting and break up, and preserve a record of upper mantle temperatures over the past 170Myr. We find that samples from the Pacific Ocean--formed more than 2,000km from the nearest continental margin--do not record raised mantle temperatures. In contrast, samples from the Atlantic Ocean that formed close to the margin of the rifted continent reveal an upper mantle temperature immediately after continental rifting that was up to 150°C higher than the present-day average; mantle temperatures remained high for 60-70Myr. We conclude that the Atlantic thermal anomaly was created by continental insulation, and persisted in the mantle beneath the Atlantic Ocean long after the continental fragments had dispersed.

Brandl, Philipp A.; Regelous, Marcel; Beier, Christoph; Haase, Karsten M.

2013-05-01

208

Rifting, landsliding and magmatic variability in the Canary Islands  

Microsoft Academic Search

Rifts, probably the most influential structures in the geology of the Canary Islands, may also be responsible for the development of central felsic volcanoes, which are consistently nested in the collapse basins of the massive lateral collapses found in the Canaries. Three main types of post-collapse volcanism have been observed, particularly in the western Canaries: 1. Collapses followed by relatively

J. C. Carracedo; V. R. Troll; H. Guillou; E. R. Badiola; F. J. Pérez-Torrado; S. Wiesmaier; A. Delcamp; A. R. Gonzalez

2009-01-01

209

Seismic structure of the uppermost mantle beneath the Kenya rift  

USGS Publications Warehouse

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. ?? 1994.

Keller, G. R.; Mechie, J.; Braile, L. W.; Mooney, W. D.; Prodehl, C.

1994-01-01

210

Rio Grande Rift GPS Measurements 2006-2009  

NASA Astrophysics Data System (ADS)

We use three years of measurements from 25 continuous GPS stations across the Rio Grande Rift in New Mexico and Colorado to estimate surface velocities, time series, baselines, and strain rates. The stations are part of the EarthScope Rio Grande Rift experiment, a collaboration between researchers at the University of Colorado at Boulder, the University of New Mexico, and Utah State University. The network includes 5 east-west station profiles transecting the rift, with the southernmost line in southern New Mexico and the northernmost line in northern Colorado. Most of the stations have shallow-drilled braced monuments installed in 2006-2007 and will remain occupied until 2010-2011 or longer. We also estimate station coordinates and velocities from the 2001 and 2008 High Accuracy Reference Network (HARN) campaigns conducted in Colorado. Initial 72-hour observations were made in the summer of 2001 and were repeated in the summer of 2008. Data from regional Plate Boundary Observatory (PBO) GPS stations are included in the processing to increase station density and extend profiles further to the east and west of the Rio Grande Rift. We use GAMIT/GLOBK to process regional sub-networks that share several common sites well determined in the Stable North America Reference Frame (SNARF). These common sites are used as a tie between the sub-networks and SNARF. Our time series from the first three years of the experiment show excellent monument stability. We have solved for baseline distance as a function of time across each of these lines. Despite what might be expected for a rigid Colorado Plateau moving away from rigid North America about a pole near Colorado, we find no evidence of an increase in Rio Grande Rift opening to the south. Our results suggest that steady-state extension across the rift from northern Colorado to southern New Mexico has an upper bound less than ~1 mm/yr with strain rates less than ~20 nanostrain/yr, although these results are still preliminary. Station velocities will be further examined to quantify the rotation of the Colorado Plateau relative to the High Plains and investigate whether a propagating crack tip model is appropriate for the northern extent of the rift in Colorado.

Berglund, H.; Sheehan, A. F.; Nerem, R.; Choe, J.; Lowry, A. R.; Roy, M.; Blume, F.; Murray, M.

2009-12-01

211

Processing and interpretation of the gravity field of the East African Rift: implication for crustal extension  

NASA Astrophysics Data System (ADS)

Compilation of new and existing gravity data were undertaken to assess the nature of the crust beneath the East African Rift System. Using 3D gravity modeling code crustal model of gravity profiles across two sectors of the rift were computed. The results are discussed in light of the structure of the rift system. The results of the 3D modeling of gravity profiles across the two rift zones revealed northward thinning of the crust. The maximum crustal attenuation occurs beneath the Afar depression, indicating the Afar rift undergoes an intense fragmentation of the crust resulting from faulting and magmatic activity. However, our computed crustal thickness below the Afar depression falls within an upper bound compared to elsewhere below tectonically active rift zones. This can be explained in terms of crustal accretion resulting from an impact of the Afar mantle plume since ˜30 Ma ago. The residual gravity obtained using high-cut filtering techniques reveals significant density contrast between the northern and southern sectors of the rift. The northern part of the rift is characterized by regular patterns of positive gravity anomalies, which can be interpreted in terms of a zone of crustal thinning through which relatively dense materials have intruded the overlying crust. In contrast, south of the Main Ethiopian Rift, the anomalies are characterized by random patterns and low amplitudes. The along-rift-axis variation in gravity anomalies implies that the style of crustal deformation changed progressively, beginning with regionally distributed crustal deformation, such as the one we observe within the more juvenile and wider southern segment of the rift, to localized deformation within the active and narrow rift zones of the northern sector of the Ethiopian Rift. We suggest that the key parameters controlling along-rift-axis variation in gravity anomalies are the rate of crustal extension, faulting and magmatic activities.

Tessema, A.; Antoine, L. A. G.

2004-12-01

212

Comparing extension on multiple time and depth scales in the Corinth Rift, Central Greece  

NASA Astrophysics Data System (ADS)

The young (< 5 Ma) Corinth Rift is an ideal natural laboratory to investigate rift deformation mechanisms by comparing extension rates determined by various methods spanning different depth and time ranges. Corinth Rift geodetic extension rates averaged over 5-100 yr have been interpreted to increase from ˜5 mm yr-1 or less in the east to >10-15 mm yr-1 in the west. We quantify total upper-crust and whole-crust extension on three profiles across the Corinth Rift. Whole-crust extension is greater across the central rift (˜11-21 km) than across the western part of the rift (˜5-13 km). This correlates with the overall rift morphology, which shows maximum basement subsidence, sediment accumulation, rift width and greatest summed Late Quaternary fault displacements in the central basin, but contrasts with the pattern of geodetic extension rates which are greater to the west of the central basin. The E-W increase in strain rates interpreted from geodetic data cannot have persisted over rift history to produce the observed rift morphology. We suggest the discrepancy between short-term and long-term extension patterns is related to shifts in the loci of maximum extension due to fault growth and linkage during Corinth Rift history, and is likely a characteristic of rift development in general. Total upper-crust and whole-crust extension estimates in the western rift, where extension estimates are best constrained, are within error. We propose that uniform pure-shear extension is a viable extension mechanism in the western rift and crustal extension estimates do not require the existence of a major active N-S dipping detachment fault.

Bell, Rebecca E.; McNeill, Lisa C.; Henstock, Timothy J.; Bull, Jonathan M.

2011-08-01

213

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

214

The West Antarctic Rift System - some outstanding issues  

NASA Astrophysics Data System (ADS)

The West Antarctic Rift System (WARS) is the product of several phases of deformation since the Jurassic with an early history related to that of New Zealand and Tasmania. Several outstanding issues exist in reconstructing the evolution of the West Antarctic Rift System. An early (mid-Cretaceous) intracontinental extensional episode led to the broad crustal thinning of WARS. Recent models propose a 50 km thick crust prior to the onset of extension, but the broad evidence for this is equivocal, as it must have also affected Tasmania and southern New Zealand as well as Antarctica prior to break-up of this part of Gondwana. Several more recent extensional episodes, largely W-E extension, have been proposed, in the early, mid and late Cenozoic, that give rise to a series of N-S trending extensional sedimentary basins. How well are these episodes defined, spatially distributed and how do they relate to a further hypothesis that NW-SE dextral strike slip faulting has had a dominant influence on this extension and basin formation? WARS sedimentary basins are moderately well defined under the Ross Sea, although their age of origin is often not well constrained, but their extension under the Ross Ice Shelf is also controversial. Gravity data have been used to infer their existence but the limited seismic data available do not confirm these models. A direct relationship between the Transantarctic Mountains (TAM), a major mountain range forming the major rift margin of the WARS, and the extensional structures is also equivocal as a major sedimentary basin in the adjacent rift floor only occurs along part of the rift margin.

Davey, F. J.

2010-12-01

215

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

216

Water management problems in the Ethiopian rift: Challenges for development  

NASA Astrophysics Data System (ADS)

The Ethiopian rift is characterized by many perennial rivers and lakes occupying volcano-tectonic depressions with highly variable hydrogeological setting. The rift lakes and rivers were the focal points for relatively large-scale water resources development. They are used for irrigation, soda abstraction, commercial fish farming, recreation and support a wide variety of endemic birds and wild animals. Ethiopia’s major mechanized irrigation farms and commercial fishery are confined within the rift. A few of the lakes have shrunk as a result of excessive abstraction of water; others expanded due to increased surface runoff and groundwater flux from percolated over-irrigated fields and active tectonism. Excessive land degradation and deforestation have also played a role. Human factors, in combination with the natural conditions of climate and geology have influenced the water quality. The chemistry of some of the lakes has been changed dramatically. This paper tries to present the challenges of surface water resources development with particular reference to environmental problems caused in the last few decades. The methods employed include field hydrological mapping supported by aerial photograph and satellite imagery interpretations, hydrometeorological and hydrochemical data analysis and catchment hydrological modeling. A converging evidence approach was adapted to reconstruct the temporal and spatial variations of lake levels and the hydrochemistry. The result revealed that the major changes in the rift valley are related mainly to recent improper utilization of water and land resources in the rivers draining the rift floor and the lakes’ catchment, and to direct lake water abstraction, aggravated intermittently by natural factors (climate and tectonism). These changes appear to have grave environmental consequences, which demand urgent integrated basin-wide water management practice.

Ayenew, Tenalem

2007-06-01

217

The importance of rift history for volcanic margin formation.  

PubMed

Rifting and magmatism are fundamental geological processes that shape the surface of our planet. A relationship between the two is widely acknowledged but its precise nature has eluded geoscientists and remained controversial. Largely on the basis of detailed observations from the North Atlantic Ocean, mantle temperature was identified as the primary factor controlling magmatic production, with most authors seeking to explain observed variations in volcanic activity at rifted margins in terms of the mantle temperature at the time of break-up. However, as more detailed observations have been made at other rifted margins worldwide, the validity of this interpretation and the importance of other factors in controlling break-up style have been much debated. One such observation is from the northwest Indian Ocean, where, despite an unequivocal link between an onshore flood basalt province, continental break-up and a hot-spot track leading to an active ocean island volcano, the associated continental margins show little magmatism. Here we reconcile these observations by applying a numerical model that accounts explicitly for the effects of earlier episodes of extension. Our approach allows us to directly compare break-up magmatism generated at different locations and so isolate the key controlling factors. We show that the volume of rift-related magmatism generated, both in the northwest Indian Ocean and at the better-known North Atlantic margins, depends not only on the mantle temperature but, to a similar degree, on the rift history. The inherited extensional history can either suppress or enhance melt generation, which can explain previously enigmatic observations. PMID:20559385

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

2010-06-17

218

Galapagos Rift at 86°W: 3. Sheet flows, collapse pits, and lava lakes of the rift valley  

Microsoft Academic Search

It has long been known that pillow basalts are a dominant feature of oceanic volcanism on midocean ridges. Recent studies in the Cayman Trough, East Pacific Rise, and Galapagos Rift valley have shown that sheet flows are also an important component, especially on ridges with intermediate and fast opening rates. This paper describes the surface forms of such sheet flows

Robert D. Ballard; Robin T. Holcomb; Tjeerd H. van Andel

1979-01-01

219

Fluid history in hyper-extended rifted margins: Examples from the fossil Alpine and western Pyrenean rift systems and the present-day Iberia rifted continental margin.  

NASA Astrophysics Data System (ADS)

The evolution of deep-water, magma-poor rifted margins is intimately linked with complex and polyphase fault structures. These structures, known as detachment faults, are responsible for extreme crustal thinning and mantle exhumation. During the evolution of detachment faults fluid-rock interaction plays an important role, changing the chemical and physical properties of rocks. These processes likely have major implications for the strain localization and structural evolution of the margin. The change in rock chemistry and rheology is best indicated by the breakdown of feldspars and olivine into clay and serpentine minerals and the pervasive cementation and precipitation of quartz along the fault zones. Although the chemical and mineral reactions are well known, it is still unclear to what extent these reactions lead to changes in the overall rheology of the extending lithosphere and how they can affect the thermal evolution of the hyper-extended rifted margins. In order to answer to these questions it is important to understand the origin, timing, pathways and composition of the fluids generated during rifting. Are fluids solely of marine origin or do they have a metamorphic- or mantle-derived component? Can we determine the range of temperature and consequently at what depth these fluids are formed? And can we constrain the age of their migration? These questions can be addressed in the well-known hyper-extended rift systems such as the Alpine Tethys margins exposed in the Alps, the Mauléon basin in the Western Pyrenees and the Deep Iberia margin drilled and seismically imaged offshore Portugal. All of these rift settings show evidence for detachment systems associated with hyper-extension and mantle exhumation. The aim of this ongoing study is to characterize the fluid signature in hyper-extended domain in magma-poor rifted margins. Including different sites with different degrees of compressional and metamorphic overprint enables us to compare results and to define the general importance of fluid systems in the development of hyper-extended rifts systems. The first results show that in all three geological settings fluid percolation can be recognized in fault rocks linked to the detachment systems. Evidence for the presence of fluids comes from the analyses of hydration reactions in fault zones. In the Alps the major and trace elements show a gain in elements typical from mantle rocks (Mg, Ni, Cu, Co, V). In the Pyrenees, microstructural studies show that detachment faulting crossed a range of crustal depths providing constraints on the depths of fluid migration. Future analyses will focus on additional major and trace elements and isotopic ratios (Sr and B) of hydrated rocks recovered from these hyper-extended domains, which will be linked with the temporal and spatial evolution of the major detachment structures.

Pinto, Victor Hugo; Manatschal, Gianreto; Karpoff, Anne Marie; Masini, Emmanuel; Lemarchand, Damien; Hayman, Nicholas; Trow, Rudolph; Viana, Adriano

2013-04-01

220

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.

221

Magmatism During Rifting Controls the Polarity of Tilted Blocks  

NASA Astrophysics Data System (ADS)

Magma-poor rifts, such as non-volcanic passive continental margins (e.g. Galicia) and slow-spreading oceanic ridges (e.g. Mid-Atlantic Ridge), are composed of faulted crustal blocks that dip generally away from the rift axis. By contrast, magma-rich rifts, such as volcanic passive margins (e.g. Norway, Namibia and the obducted paleo-volcanic margin of Oman) and hotspot-influenced slow-spreading oceanic ridges (e.g. Iceland), are composed of faulted crustal blocks that dip generally towards the rift axis. At volcanic passive margins, these tilted blocks are overlain by syn-tectonic volcano-sedimentary sequences that appear on seismic profiles as packages of seaward-dipping reflectors (SDRs). They are associated with swarms of magmatic dikes and sills. On the basis of a detailed structural study of Iceland (Bourgeois et al. 2005, Geodinamica Acta 18:59-80), we demonstrate that, in magma-rich rifts, lithospheric stretching is accomodated in a long-term deformation strip, n x 100 km wide, by the development of successive roll-over structures controlled by growth-faults and underlain by shallow magma chambers. As a given roll-over structure progressively develops and tilts in response to lithospheric stretching, it is continuously covered by lavas erupted from the associated magma chamber and reaching the surface through dike swarms dominantly located along the growth fault. After a lifetime of a few My, this roll-over structure dies at the expense of the activation of a new, laterally offset, one. Correspondingly, such roll-over structures form successively at different places within a diffuse plate boundary n x 100 km wide. After several roll-over structures have developed and died, the overall structure of the long-term deformation strip is composed of faulted crustal blocks that generally dip towards the rift axis and that are covered by volcano-sedimentary sequences. Physical laboratory experiments conducted with analogue materials demonstrate that this peculiar mode of rifting and the polarity of tilted blocks are controlled by the ratio between the rate of lithospheric stretching and the rate of magma supply. When the rate of crustal thickening by magmatism is smaller than (or equal to) the rate of crustal thinning by lithospheric stretching (as in non-volcanic passive margins and in slow-spreading oceanic ridges) the resulting rift zone is composed of outward-tilted blocks. By contrast, when the rate of crustal thickening by magmatism is bigger than the rate of crustal thinning by lithospheric stretching (as in volcanic passive margins and in hotspot-influenced slow-spreading oceanic ridges), the resulting rift zone is composed of inward-tilted blocks.

Chauvet, F.; Bourgeois, O.; Dauteuil, O.

2009-12-01

222

Geochemical Overview of the East African Rift System  

NASA Astrophysics Data System (ADS)

Mafic volcanics of the East African Rift System (EARS) record a protracted history of continental extension that is linked to mantle plume activity. The modern EARS traverses two post-Miocene topographic domes separated by a region of polyphase extension in northern Kenya and southern Ethiopia. Basaltic magmatism commenced ˜45 Ma in this highly extended region, while the onset of plume-related activity took place ˜30 Ma with eruption of flood basalts in central Ethiopia. A spatial and temporal synthesis of EARS volcanic geochemistry shows progressive lithospheric removal (by erosion and melting) as the degree of rifting increases, with basalts in the most highly extended areas recording melting of depleted asthenosphere. Plume contributions are indicated locally in the northern half of the EARS, but are absent from the southern half. The geochemical signatures are compatible with a physical model in which the entire EARS is fed by a discontinuous plume emanating from the core-mantle boundary as the South African Superswell. Quaternary basaltic lavas erupted in the Afar triangle, Red Sea and Gulf of Aden define the geochemical signature attributed to the Afar plume (87Sr/86Sr 0.7034-0.7037, 143Nd/144Nd 0.5129-0.5130; La/Nb 0.6-0.9; Nb/U 40-50). These suites commonly record mixing with ambient upper mantle having less radiogenic isotopes but generally overlapping incompatible trace element abundances. Within the Ethiopian dome both lithospheric and sub-lithoshperic contributions can be documented clearly; lithospheric contributions are manifest in more radiogenic isotope values (87Sr/86Sr up to 0.7050) and distinctive trace element abundances (e.g., La/Nb <2.0, Nb/U > 10). The degree of lithospheric contribution is lowest within the active Main Ethiopian Rift and increases towards the southern margin of the dome. The estimated depth of melting (65-75 km) is consistent with geophysical observations of lithospheric thickness. In regions of prolonged volcanism the lithospheric contributions and estimated melting depths decrease through time, corresponding to a higher degree of rifting. In the Kenyan dome, including the western rift, the degree of extension is low and lithospheric melting is the dominant source for basaltic magmatism. Mafic lavas from these regions have generally lower MgO but higher contents of alkalis, P2O5 and many incompatible trace elements than are observed in the Ethiopian Rift. High values of 87Sr/86Sr, 207Pb/204Pb and Zr/Hf relative to other parts of the EARS indicate melting of metasomatized lithosphere. Melting in this area occurs at depths up to 100+ km, consistent with the thick crustal section observed seismically. Between the topographic domes, basalts from the Turkana region record melting at shallow levels ( ˜35 km) consistent with seismic evidence for nearly complete rifting of the crustal section. The geochemistry of these lavas is dominated by asthenospheric source materials, with only minor lithospheric involvement. Temporal evolution of EARS geochemistry reflects progressive rifting of the thick craton. This change is manifest within lavas that are interpreted as plume-derived, as Tb/Yb values decrease from 30 Ma through the present. The modern thermal anomaly associated with Afar volcanism does not appear to extend below the shallow mantle, but may reflect a large blob of deep mantle material that became stuck to Africa 30 Ma and has contributed to regional volcanism ever since. Relative contributions from this deep mantle source, shallow asthenosphere and lithosphere are controlled by the extent of rifting and cannot be predicted solely on the basis of surface topography.

Furman, T.

2003-12-01

223

Rifting to spreading in the Gulf of Aden  

NASA Astrophysics Data System (ADS)

The Gulf of Aden margins systems are volcanic to the West, where they are influenced by the Afar hotspot, and non-volcanic East of longitude 46°E. The combined use of magnetics, gravity, seismic reflection, field observations (tectonic and sedimentological) allowed us to obtain better constraints on the timing of continental rifting and seafloor spreading. From the Permo-Triassic to the Oligocene, the Arabian-African plate was subject to distributed extension, probably due, at least from the Cretaceous, to tensile stresses related to the subduction of the Tethysian slab in the north. In Late Eocene, 35 Ma ago, rifting started to localize along the future area of continental breakup. Initially guided by the inherited basins, continental rifting then occurred synchronously over the entire gulf before becoming localized on the northern and southern borders of the inherited grabens, in the direction of the Afar hot-spot. In the areas with non-volcanic margins (in the East), the faults marking the end of rifting trend parallel to the inherited grabens. Only the transfer faults cross-cut the inherited grabens, and some of these faults later developed into transform faults. The most important of these transform faults follow a Precambrian trend. Volcanic margins were formed in the West of the Gulf, up to the Guban graben in the south-east and as far as the southern boundary of the Bahlaf graben in the North-East. Seaward dipping reflectors (SDRs) can be observed on many oil-industry seismic profiles. The influence of the hotspot during rifting was concentrated on the western part of the gulf. Therefore, it seems that the western domain was uplifted and eroded at the onset of rifting, while the eastern domain was characterized by more continuous sedimentation. The phase of distributed deformation was followed by a phase of strain localization during the final rifting stage, just before formation of the Ocean-Continent Transition (OCT), in the most distal graben (DIM graben). About 20 Ma ago, the emplacement of the OCT started in the east with exhumation of the subcontinental mantle. Farther west, the system was heated up by the strong influence of the Afar hot-spot, which led to breakup with much less extension. In the Gulf of Aden (s.str), up to the Shukra El Sheik fracture zone, oceanic spreading started 17.6 Ma ago. West of this fracture zone, oceanic accretion started 10 Ma ago, and 2 Ma ago in the Gulf of Tadjoura. Post-rift deformation of the eastern margins of the Gulf of Aden can be seen in the distal and proximal domains. Indeed, the substantial post-rift uplift of these margins could be associated with either the continental break-up, or activity of the Afar hotspot and related volcanic/magmatic activity. The Afar plume is therefore important for several reasons. It allows the localization of deformation along the Red Sea/Aden system and the rapid opening of the Gulf after the continental break-up.

Leroy, S.; Razin, P.; Lucazeau, F.; D'Acremont, E.; Autin, J.; Watremez, L.; Robinet, J.; Baurion, C.

2011-12-01

224

Neoproterozoic rifting in the southern Georgina Basin, central Australia: Implications for reconstructing Australia in Rodinia  

NASA Astrophysics Data System (ADS)

A system of northwest striking Neoproterozoic rift basins underlies Paleozoic strata in the southern Georgina Basin of central Australia. Normal faults bounding these rift basins were selectively reactivated during the mid-Paleozoic Alice Springs Orogeny and are now expressed as high-angle reverse faults that invert the preexisting rift basins. Exhumed and eroded rift basin remnants are present in the hanging wall of the Oomoolmilla, Lucy Creek, Tarlton, and Toomba reverse faults, and rift basins may be preserved in the subsurface beneath the Toko Syncline and Burke River Structural Belt. Rift basin fill indicates two periods of extension: a major rift-forming episode between approximately 700 and 650 Ma (coeval with Sturtian glacial deposits) and a second episode of extension at approximately 600 Ma (coeval with Marinoan glacial deposits). This northwest striking rift system in central Australia supports results from other regions, indicating that the Neoproterozoic continental margin of Australia consisted of northwest striking rift segments offset by northeast striking transform faults. Such a configuration is geometrically incompatible with a Laurentian continental margin consisting of northeast striking rift segments and conflicts with reconstructions such as SWEAT and AUSWUS that match Australia with western Laurentia in the Rodinia supercontinent.

Greene, David C.

2010-10-01

225

How sediment promotes narrow rifting: Application to the Gulf of California  

NASA Astrophysics Data System (ADS)

If the gravitational stress changes related to crustal thinning are large enough to affect the mode of extension, then sedimentation should also affect that mode. We propose that the weight of sediments reduces the difference in crustal buoyancy forces caused by local crustal thinning, allowing a rift to extend more easily in a narrow rift mode. We examine the effect of deposition of large amounts of nonlocally derived sediment on extensional style using two-dimensional, regional-scale numerical experiments of extending thick continental crust with varied initial thermal conditions, geometries, rheological parameters, and sedimentation properties. Depending on initial model conditions, the onset of rifting in thick continental crust occurs in the narrow rifting, wide rifting, or core complex mode. With continued extension, all cases eventually transition to a narrow rifting mode. For a system in wide rift mode, moderate to fast sedimentation shortens the time needed to transition to a narrow rift. In the Gulf of California, changes in extensional style correlate with sediment thickness, with an earlier transition to narrow rifting in the north versus the south. We compare our model results to the extensional history of the Gulf of California, where sediments may have caused the northern and north central domain of the gulf (Delfin-Tiburon and Guaymas segments) to transition to narrow rifting before the south central domains (Alarcon segment).

Bialas, Robert W.; Buck, W. Roger

2009-08-01

226

Potential for autoimmune pathogenesis of rift valley Fever virus retinitis.  

PubMed

Abstract. 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

2013-08-05

227

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.

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

2008-01-01

228

Rift Valley Fever in Small Ruminants, Senegal, 2003  

PubMed Central

During the 2003 rainy season, the clinical and serologic incidence of Rift Valley fever was assessed in small ruminant herds living around temporary ponds located in the semi-arid region of the Ferlo, Senegal. No outbreak was detected by the surveillance system. Serologic incidence was estimated at 2.9% (95% confidence interval 1.0–8.7) and occurred in 5 of 7 ponds with large variations in the observed incidence rate (0%–20.3%). The location of ponds in the Ferlo Valley and small ponds were correlated with higher serologic incidence (p = 0.0005 and p = 0.005, respectively). Rift Valley fever surveillance should be improved to allow early detection of virus activity. Ruminant vaccination programs should be prepared to confront the foreseeable higher risks for future epidemics of this disease.

Lancelot, Renaud; Thiongane, Yaya; Sall, Baba; Diaite, Amadou; Mondet, Bernard

2005-01-01

229

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

230

Undiscovered petroleum of the Brazilian Interior Rift Basins  

SciTech Connect

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

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

1995-05-01

231

Age of volcanism and rifting in southwestern Ethiopia  

Microsoft Academic Search

It has been suggested that volcanism in the Ethiopian region of the Afro-Arabian Rift System has migrated with time, both laterally towards the present axial zone1-3 and longitudinally southwards from the Red Sea4,5. Field data and K-Ar isotopic ages from southwestern Ethiopia, summarised below, indicate that volcanism in this area began earlier than previously suspected, and that Quaternary volcanism was

A. Davidson; D. C. Rex

1980-01-01

232

Bridging the Rift -- Scientific Cooperation between Israel and Jordan  

NASA Astrophysics Data System (ADS)

Bridging the Rift is a project designed to develop scientific collaborations between scholars from Jordan and Israel. Over the past three years scholars from five Jordanian universities and all the Israeli universities have participated in research together. The fields of emphasis are Microbiology and Ecology. Joint field trips, research planning meetings, and laboratory studies have already been carried out. A physical institution spanning the Israeli-Jordanian border is planned as the home of this long-term collaboration.

Feldman, Marcus

2007-03-01

233

The large Thaumasia graben on Mars: Is it a rift?  

NASA Astrophysics Data System (ADS)

We investigate the morphology and topography of one of the largest fault-bounded tectonic structures on Mars, a complex, approximately N-S trending system of troughs and scarps at the western border of the Thaumasia plateau in the Claritas region (hereinafter referred to as ``Thaumasia graben,'' or TG). It is located between 15°S and 38°S latitude and at ~255°E longitude. No detailed investigation of its morphotectonic setting has been performed yet. The region is a complexly fractured area with a number of different fault sets, including simple and complex graben. The TG extends over more than 1000 km along its trend, averaging 100 km in width and 1.6 km in depth. Crustal extension is accommodated by the formation of a system of asymmetric graben, or halfgraben. On the basis of fault orientation and trough depth, the TG can be subdivided in a north-south direction into three segments. Except for the northernmost segment, the predominant master fault system is located along the eastern flank of the TG, highlighting the overall asymmetric architecture. Fault length segments vary from 50 to 90 km with observable displacements of 1.3-2.2 km. Crustal extension, inferred from gridded MOLA topography across scarp offsets, varies along trend between 0.5 km and ~4 km, assuming a fault dip of 60°. This is relatively moderate extension if compared to terrestrial continental rifts, but consistent with extension measured across the Tempe Rift on Mars. We find that the Thaumasia graben displays some characteristics which are common to terrestrial continental rifts, whereas other properties are distinctively not rift-like.

Hauber, E.; Kronberg, P.

2005-07-01

234

Benue trough and the mid-African rift system  

SciTech Connect

Large areas of the Anambra and Gongola basins have distinct petroleum exploration problems: a geologically persistent high geothermal gradient that promoted Cretaceous source rock maturation into the gas phase very early on; intrusive lead-zinc mineralization veins attributed to the Senonian igneous and folding event; and meteoric water-flushing along the periphery of the basins. From preliminary analysis, these basins have to be considered high risk for the discovery of commercial oil accumulations. On the other hand, the petroleum potential of the Bornu basins seems favorable. This Nigerian northernmost rift basin continues into the Kanem basin of western Chad, which has proven oil accumulations in Coniacian deltaic sands. Cretaceous paleofacies is considered to be relatively continuous throughout both basins. Paleo-geothermal history is also considered to be similar, although some igneous activity is recorded in the Bornu basin (Senonian?). There is a very real possibility of kerogen-rich non-marine basal Albo-Aptian basin fill lacustrine source rocks, as found in the Doba basin, could be present in the deepest sections of the Nigerian rift basins. Due to the depths involved, no well is expected to penetrate the incipient graben-fill stage sequences; however, possible oil migration from these tectono-stratigraphic units would certainly enhance the petroleum potential of cooler sections of the rift system. As opposed to interpreted thermogenic gas which seems to be prevalent in the Anambra basin.

Thomas, D. [Thomas and Associates, Hastings (United Kingdom)

1996-01-29

235

Patterns of relative positioning of oceanic rifts and transform faults  

NASA Astrophysics Data System (ADS)

The general patterns of relative arrangement of rift zones and transform faults in the ocean were studied. The geological-geophysical materials for the crests of mid-oceanic ridges fall into three categories: (1) data on structure of oceanic rift zones and transform faults; (2) bathymetric and geophysical maps for regions explored in detail; (3) bathymetric maps at 1:2,000,000 and 1:5,000,000. These materials were used in determining the parameters of 360 active transform faults. The quantitative characteristics of each of these faults were catalogued. Distance between transform faults was determined, lengths of active parts of these faults were ascertained, the angular relationships between rift zones and transform faults were calculated. It is concluded that the distribution of the quantitative characteristics of the geometry of the axis of spreading of plates in the ocean is clearly dependent on the spreading rate. The thermal regime of the lithosphere exerts a decisive influence on the geometry of the axis of spreading of plates.

Bocharova, N. Y.; Mirlin, Y. G.; Popov, K. V.

1986-01-01

236

Origins and implications of zigzag rift patterns on lava lakes  

NASA Astrophysics Data System (ADS)

The distinctive rift patterns observed on newly formed lava lakes are very likely a product of interaction between heat transfer (cooling of lava) and deformation of the solid crust in response to applied stresses. One common pattern consists of symmetric “zigzag” rifts separating spreading plates. Zigzags can be characterized by two measurable parameters: an amplitude A, and an angle ? between segments that make up the zigzags. Similar patterns are observed in analog wax experiments in which molten wax acts as cooling and solidifying lava. We perform a series of these wax experiments to find the relationship between ?, A, and the cooling rate. We develop a model to explain the observed relationships: ? is determined by a balance of spreading and solidification speeds; the amplitude A is limited by the thickness of the solid wax crust. Theoretical predictions agree well with experimental data; this enables us to scale the model to basaltic lava lakes. If zigzag rifts are observed on the surface of lava lakes, and if physical properties of the lava crust can be measured or inferred by other means, measurements of ? and A make it possible to calculate crust-spreading velocity and crust thickness.

Karlstrom, Leif; Manga, Michael

2006-06-01

237

Structural and Stratigraphic Evolution of the Rio Grande Rift, Northern New Mexico and Southern Colorado  

Microsoft Academic Search

The well-known Pliocene to Quaternary Rio Grande rift of northern New Mexico and southern Colorado is distinctly different from the Miocene rift, especially in structural style. Prior to approximately 21 Ma, there was little extension or rift-basin development. Uppermost Oligocene and Lower Miocene strata were deposited as broad volcaniclastic aprons, with no significant evidence of syn-depositional faulting, in contrast to

Raymond V. Ingersoll

2001-01-01

238

Interaction Between Mantle Plume Processes and Surface Topography in Incipient Rift Systems, EARS  

NASA Astrophysics Data System (ADS)

In an asymetric extensional setting such as the East African Rift System (EARS), the pronounced regional variations in topography are generally explained by two different models: a) a post-rift uplift resulting from mechanical relaxation that evokes an uplift of the rift shoulders, and b) a syn-rift uplift effected by mantle plume pushing during extension. However, an important pre-rift topographic feature can be documented in the Kenya rift. The pre-rift Yatta Plateau (13.5 Ma) is a phonolitic lava flow with a length of appr. 290 km that flowed eastward, away from the present-day eastern rift shoulder of the Kenya rift. Due to the combination of flow within a pre-existent river valley and later erosional processes the flow is now characterized by relief inversion. At present this river runs exactly parallel to its paleo-valley and erodes its western flank. We reconstruct the paleo-topography by estimating the pre-rift slope and assuming an active lava flow. Viscosities, derived from phonolitic and basaltic bulk rock compositions cooling down from their eruption temperature, permits the calculation of the lava flow velocity. The appliance of this method to unravel the history of this volcanic feature allows us to draw conclusions on the geometry of the central EARS and proves the existence of high topography and relief contrasts along its recent rift axis since appr. 14 Ma, prior to the extension. We propose that the topography is due to a pre-rift uplift caused by thermal expansion of the lithospheric rocks and effected by a heating mantle plume.

Wichura, H.; Bousquet, R.; Oberhänsli, R.

2009-04-01

239

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

240

Faulting Mode Characterization using fault attributes : Example of a nascent oceanic rift the Manda-Hararo rift in Afar (Ethiopia)  

NASA Astrophysics Data System (ADS)

The Manda-Hararo rift segment, located in the Afar depression, underwent a major dyke injection of 65 km long in September 2005, that initiated a rifting episode. From June 2006 to May 2010, 13 other successive dykes were intruded and monitored using InSAR and seismic surveys. Aside from its recent activity, the Manda-Hararo rift architecture shows some particularities which distinguish the segment North of the central magma chamber from the rest of the rift. This Northern segment shows a change of strike of the rift axis and of the overlying faults, as well as a marked asymmetry featured by high no-conjugated west-dipping scarps. These observations led to wonder how the Northern part of this rift has been integrated into the long-term evolution of the whole rift, and whether its deformation mode and fault growth processes might be influenced by the Dabbahu volcano. To address such questions, we focus our analysis on the scaling laws applied to the fault attributes such as fault length, fault scarps or spacing between adjacent faults. This study is based on a fault mapping which was done using optical images (SPOT and, QUICKBIRD images) together with SAR interferograms and coherence images. This map is divided into three regions to isolate the different sources of deformation : the Northern segment close to the Dabbahu volcano, the central one where the main magma reservoir is located and dyke intrusions occurred, and finally the southernmost one coinciding with the segment end. A first stage in determining the scaling law, and consequently the growth mode, consists in characterizing the displacement (Dmax) versus length (L) relationship. With our whole dataset and the different groups of segments defined previously, we observe a scattering suggesting no clear evidence for a linear trend associated with self-similar processes. A possible explanation for such observation in addition to the sampling issue would be a distributed mode of deformation (Soliva et al. 2008). Next, for each of these three regions, we determine the distribution law and discuss them in terms of fault growth processes and the possible role of the fragile thickness as a limiting factor. The center and southern regions tend to a gamma law (Davy 1993), unlike the Northern part where an exponential law seems to be more appropriate. Such observation in the Northern part of the rift mean that faulting would be distributed and scale dependent, when the central and southern parts would be characterized by a faulting mode closer from the transition localized-distributed. The analysis of scaling laws applied to fault attributes is also discussed in terms of inward-outward dipping faults and compared to the oceanic ridge models (Carbotte et al. 1990). These observations suggest a similar and more advanced stage of evolution for the Central and Southern part of the segment, unlike the Northern segment, which shows a less localized deformation. Indeed, the preferential zone of intrusion of the Northern segment seems to be able to laterally jump over time, as illustrated by the unexpected path taken by the September 2005 intrusion.

Dumont, Stéphanie; Socquet, Anne; Doubre, Cécile; Grandin, Raphaël; Klinger, Yann; Medynski, Sarah; Jacques, Eric

2013-04-01

241

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

NASA Astrophysics Data System (ADS)

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

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

2005-08-01

242

Two mantle plumes beneath the East African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts  

Microsoft Academic Search

Major and trace element and radiogenic isotope ratios (Sr, Nd and Pb) are presented for a suite of Neogene to Recent basalts (MgO>4 wt%) from the axial regions of the Kenya Rift. Samples have compositions ranging from hypersthene-normative basalt through alkali basalt to basanite and are a subset of a larger database in which compositions extend to nephelinite. A broadly

Nick Rogers; Ray Macdonald; J. Godfrey Fitton; Rhiannon George; Martin Smith; Barbara Barreiro

2000-01-01

243

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

Microsoft Academic Search

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

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

2010-01-01

244

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 São Jerônimo-Curiúva 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

245

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

Microsoft Academic Search

Low-temperature thermochronology was applied at the Brazilian passive continental margin in order to understand and reconstruct\\u000a the post-rift evolution since the break-up of southwestern Gondwana. Thermochronological data obtained from apatite fission-track\\u000a analysis of Neoproterozoic metamorphic and Paleozoic to Mesozoic siliciclastic rocks as well as Mesozoic dikes and alkaline\\u000a intrusions from the Ponta Grossa Arch provided ages between 66.2 (1.3) and

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

2010-01-01

246

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

247

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

248

Uplift and subsidence of the Suez rift: Constraints from fission-track analysis and sediment backstripping  

SciTech Connect

The Gulf of Suez is a Neogene rift that has evolved as one arm of the Sinai triple junction. The basement uplifts flanking the rift are larger than can be explained by uniform lithospheric extension. The timing of the regional heating required by the uplift has important implications for hydrocarbon maturation for the Gulf of Suez and rifts in general. The local geology indicates that the uplift did not predate rifting. Therefore, a regional subsidence and two-dimensional backstripping of the rift sediments were undertaken in conjunction with fission track analyses of the basement uplift. The initial rift deposits (Nukhul Formation) indicate slow extension during the earliest Miocene. The extension rate increased at the beginning of the deposition of the Rudeis Formation at approximately 19 Ma as the Gulf of Suez entered its main phase of rifting. By the end of the deposition of the Kareem Formation (approximately 14-15 Ma), most of the Africa-Arabia separation had transferred to the Gulf of Aqaba-Dead Sea transform. In order to determine the onset of the rift flank uplift relative to the rift history, 55 apatite fission track analyses were performed on samples from the basement of the eastern desert, on the western side of the rift. Apatite fission tracks record the thermal history of the samples for temperatures up to 125{degree}c. Apparent ages range from 382 to 11 Ma representing samples that have undergone various degrees of track annealing. Track length distributions clearly show the fading of tracks acquired prior to uplift in more deeply buried samples and the accumulation of long unannealed tracks subsequent to unroofing. The pattern of the track length vs. age distribution indicates that major uplift began simultaneously with the main phase of rifting at 19-20 Ma.

Steckler, M.S.; Omar, G.I.; Buck, W.R. (Lamont-Doherty Geological Observatory, Palisades, NY (USA))

1988-08-01

249

Stress perturbation associated with the Amazonas and other ancient continental rifts  

USGS Publications Warehouse

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

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

1996-01-01

250

Timing of East African Rift development in southern Ethiopia: Implication for mantle plume activity and evolution of topography  

Microsoft Academic Search

Accurate determination of rifting chronology and associateduplift is crucial to understanding the evolution of the EastAfrican Rift System (EARS) and for identifying the significanceof mantle plumes during continental breakup. This investigationof rift-related cooling along a major fault scarp in southernEthiopia, using (U-Th)\\/He thermochronometry, shows that riftingstarted not before 20 Ma. Therefore, there is an absence ofsignificant rift activity synchronous with

Raphaël Pik; Bernard Marty; Jean Carignan; Gezahegn Yirgu; Teklewold Ayalew

2008-01-01

251

Volcanic Rift Zone and Associated Cinder Cone Field in Utopia Planitia, Mars  

NASA Astrophysics Data System (ADS)

We have analyzed a small area in SW Utopia Planitia that shows striking similarities to rift zone volcanoes on Earth. The area exhibits eruptive fissures, cinder cones, and dyke swarms indicating rifting and magmatic activity prior to the deposition of VBF material.

Lanz, J. K.; Wagner, R.; Wolf, U.; Neukum, G.; Kröchert, J.

2010-03-01

252

Northern Red Sea: Nucleation of an oceanic spreading center within a continental rift  

Microsoft Academic Search

The northern Red Sea is an amagmatic continental rift in which an oceanic spreading center is beginning to develop. A new compilation of marine geophysical data permits delineation of the structure of the northern Red Sea and of the manner in which the transition from continental to oceanic extension is occurring in this rift. The margins of the northern Red

James R. Cochran

2005-01-01

253

Kinematic and thermal evolution of the Moroccan rifted continental margin: Doukkala-High Atlas transect  

NASA Astrophysics Data System (ADS)

The Atlantic passive margin of Morocco developed during Mesozoic times in association with the opening of the Central Atlantic and the Alpine Tethys. Extensional basins formed along the future continental margin and in the Atlas rift system. In Alpine times, this system was inverted to form the High and Middle Atlas fold-and-thrust belts. To provide a quantitative kinematic analysis of the evolution of the rifted margin, we present a crustal section crossing the Atlantic margin in the region of the Doukkala Basin, the Meseta and the Atlas system. We construct a post-rift upper crustal section compensating for Tertiary to present vertical movements and horizontal deformations, and we conduct numerical modeling to test quantitative relations between amounts and distribution of thinning and related vertical movements. Rifting along the transect began in the Late Triassic and ended with the appearance of oceanic crust at 175 Ma. Subsidence, possibly related to crustal thinning, continued in the Atlas rift in the Middle Jurassic. The numerical models confirm that the margin experienced a polyphase rifting history. The lithosphere along the transect preserved some strength throughout rifting with the Effective Elastic Thickness corresponding to an isotherm of 450°C. A mid-crustal level of necking of 15 km characterized the pre-rift lithosphere.

Gouiza, M.; Bertotti, G.; Hafid, M.; Cloetingh, S.

2010-10-01

254

Episodic spreading and rift propagation: New paleomagnetic and geochronologic data from the afar Nascent passive margin  

Microsoft Academic Search

A recent model proposes that afar is one of the few areas in the world where the formation of a passive margin by rift propagation can be observed directly. Forty sites spread over approximately 10,000 km2 in the Republic of Djibouti, between the Asal and Abhe rifts, have been sampled for paleomagnetic and geochronologic study. Most basalt samples have a

V. Courtillot; F. Landre; J. Achache; N. Bonhommet; R. Montigny; G. Féraud

1984-01-01

255

Rifting to Spreading Process along the Northern Continental Margin of the South China Sea  

Microsoft Academic Search

Understanding the development from syn-rift to spreading in the South China Sea (SCS) is important in elucidating the western Pacific's tectonic evolution because the SCS is a major tectonic constituent of the many marginal seas in the region. This paper describes research examining the transition from rifting to spreading along the northern margin of the SCS, made possible by the

Yukari Kido; Kiyoshi Suyehiro; Hajimu Kinoshita

2001-01-01

256

Buttressing and fractional spreading of Tenerife, an experimental approach on the formation of rift zones  

Microsoft Academic Search

The island of Tenerife is composed of three Miocene shields, which are centered by the Cañadas volcano since the Pliocene. Tenerife sits on more than 2 km thick oceanic sediments. Quaternary volcanism of the Cañadas series and giant landslides were principally confined to triaxial rift zones. The mechanism of triaxial rifting, however, has remained unclear. Physical analog models show that

Thomas R. Walter

2003-01-01

257

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

PubMed

Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup, modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning. Also disputed is the extent to which plate stretching or thinning is uniform or varies with depth with the entire continental lithospheric mantle potentially being removed before plate rupture. Here we show that the extensive magmatism during rifting along the southern Red Sea rift in Afar, a unique region of sub-aerial transition from continental to oceanic rifting, is driven by deep melting of hotter-than-normal asthenosphere. Petrogenetic modelling shows that melts are predominantly generated at depths greater than 80?kilometres, implying the existence of a thick upper thermo-mechanical boundary layer in a rift system approaching the point of plate rupture. Numerical modelling of rift development shows that when breakup occurs at the slow extension rates observed in Afar, the survival of a thick plate is an inevitable consequence of conductive cooling of the lithosphere, even when the underlying asthenosphere is hot. Sustained magmatic activity during rifting in Afar thus requires persistently high mantle temperatures, which would allow melting at high pressure beneath the thick plate. If extensive plate thinning does occur during breakup it must do so abruptly at a late stage, immediately before the formation of the new ocean basin. PMID:23823795

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

2013-07-01

258

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

Microsoft Academic Search

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, extensional tectonics, bedrock lithology, long-term climatic changes, and evolving surface processes have controlled the tectono-geomorphological evolution of the eastern part of the

Hesham Farouk El-Sobky

2007-01-01

259

Tectonics of the Jemez lineament in the Jemez Mountains and Rio Grande rift  

Microsoft Academic Search

The Jemez lineament is a NE trending crystal flaw that controlled volcanism and tectonism in the Jemez Mountains and the Rio Grande rift zone. The fault system associated with the lineament in the rift zone includes, from west to east, the Jemez fault zone southwest of Valles-Toledo caldera complex, a series of NE trending faults on the resurgent dome in

M. J. Aldrich

1986-01-01

260

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

Microsoft Academic Search

The opening of the South China Sea has been a matter of debate for many years because of its internal structure, the differences between the conjugate margins and the variations of rifting and spreading directions. Although it is considered as being a back-arc basin, it is not sitting directly above a subduction zone, and the rifting process lasted for an

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

2009-01-01

261

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

Microsoft Academic Search

Of all the terrestrial depositional settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. Lake deposits were the only rift component studied for records of long-term climatic change and for testing hypotheses of orbital forcing. Recently, the continuing quest for the paleontological and cultural records of human origins entombed

G. M. Ashley

2004-01-01

262

Relation of summit deformation to east rift zone eruptions on Kilauea Volcano, Hawaii  

SciTech Connect

An inverse relationship exists between the summit deflation of Kilauea, as recorded by summit tilt, and the elevation of associated eruptive vents on the East Rift Zone. This relationship implies that East Rift eruptions drain the summit magma reservior to pressure levels that are dependent on the elevation of the eruptive vents.

Epp, D.; Decker, R.W.; Okamura, A.T.

1983-07-01

263

East African Rift System (EARS) Plume Structure: Insights from Quaternary Mafic Lavas of Turkana, Kenya  

Microsoft Academic Search

Quaternary mafic lavas from Lake Turkana (northern Kenya) provide information on processes operating beneath the East African Rift in an area of anomalous lithospheric and crustal thinning. Inferred depths of melting beneath Turkana (15---20km) are shal- lower than those recorded elsewhere along the rift, consistent with the anomalously thin crustal section. The mafic lavas have elevated incompatible trace element contents

TANYA FURMAN; JULIA G. BRYCE; JEFFREY KARSON; ANNAMARIA IOTTI

2004-01-01

264

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

SciTech Connect

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

Charpentier, P.; Jarrige, J.; Richert, J. (Elf Aquitaine, Pau (France))

1990-05-01

265

Mantle transition zone discontinuities beneath the Baikal rift and adjacent areas  

Microsoft Academic Search

Like most other major continental rifts, the Baikal rift zone (BRZ) in Siberia is presumably underlain by a hot and partially molten mantle, which has a reduced seismic velocity relative to surrounding areas. Recent seismic tomography studies, however, gave conflicting results about the depth extent and even the existence of the low-velocity anomaly beneath the BRZ, suggesting that additional constraints

Kelly H. Liu; Stephen S. Gao

2006-01-01

266

Precambrian alluvial fan and braidplain sedimentation patterns: Example from the Mesoproterozoic Rjukan Rift Basin, southern Norway  

Microsoft Academic Search

An integrated approach of facies analysis, geochemistry and paleohydrology provides new insight into the sedimentology and paleogeography of alluvial sediments in the Precambrian. Here, alluvial fan and braidplain sedimentation patterns are documented in a Mesoproterozoic rift basin, called (known as) the Rjukan Rift Basin in southern Norway. The studied formation (Heddersvatnet Formation) consists of volcanoclastic breccias and conglomerates, cross-bedded sandstones

Juha Köykkä

2011-01-01

267

Shallow marine syn-rift sedimentation: Middle Jurassic Pelion Formation, Jameson Land, East Greenland  

Microsoft Academic Search

The Middle Jurassic Pelion Formation - Fossilbjerget Formation couplet of Jameson Land, East Greenland, is a well-exposed example of the Middle Jurassic inshore-offshore successions char- acteristic of the rifted seaways in the Northwest European - North Atlantic region. Early Jurassic deposition took place under relatively quiet tectonic conditions following Late Permian - earli- est Triassic and Early Triassic rift phases

Michael Engkilde; Finn Surlyk

268

The life cycle of continental rifting as a focus for U.S.-African scientific collaboration  

NASA Astrophysics Data System (ADS)

The East African Rift System (EARS) provides the unique opportunity found nowhere else on Earth, to investigate extensional processes from incipient rifting in the Okavango Delta, Botswana, to continental breakup and creation of proto-oceanic basins 3000 km to the north in the Afar Depression in Ethiopia, Eritrea, and Djibouti.The study of continental rifts is of great interest because they represent the initial stages of continental breakup and passive margin development, they are sites for large-scale sediment accumulation, and their geomorphology may have controlled human evolution in the past and localizes geologic hazards in the present. But there is little research that provides insights into the linkage between broad geodynamic processes and the life cycle of continental rifts: We do not know why some rifts evolve into mid-ocean ridges whereas others abort their evolution to become aulacogens. Numerous studies of the EARS and other continental rifts have significantly increased our understanding of rifting processes, but we particularly lack studies of the embryonic stages of rift creation and the last stages of extension when continental breakup occurs.

Abdelsalam, Mohamed G.; Atekwana, Estella A.; Keller, G. Randy; Klemperer, Simon L.

2004-11-01

269

Petrogenesis of silicic peralkaline rocks in the Ethiopian rift: Geochemical evidence and volcanological implications  

Microsoft Academic Search

Major, trace element and isotopic data for mafic to peralkaline silicic volcanic rocks from the northern sector of the main Ethiopian rift are discussed with the aim of placing constraints on processes of magma genesis and evolution and to present models for magma plumbing systems of rift volcanoes. Basalts straddle the subalkaline–alkaline boundary and exhibit important variations of incompatible element

A. Peccerillo; C. Donati; A. P. Santo; A. Orlando; G. Yirgu; D. Ayalew

2007-01-01

270

The western margin of the Rio Grande rift in northern New Mexico: An aborted boundary?  

Microsoft Academic Search

The northwestern margin of the Espanola basin, part of the Rio Grande rift in northern New Mexico, is characterized by a zone >17 km wide of oblique-slip faults that off-set upper Paleozoic and Mesozoic strata of the eastern Colorado Plateau from Eocene and younger sedimentary rocks of the rift. Along this margin, a reasonably complete section of pre- and synrift

W. S. Baldridge; L. W. Barile; Bin Wang

1994-01-01

271

Upper crustal structure in the vicinity of Lake Magadi in the Kenya Rift Valley region  

Microsoft Academic Search

In 1994, the Kenya Rift International Seismic Project (KRISP) conducted a large-scale seismic experiment which extended across southern Kenya from Lake Victoria to the Indian Ocean. One major goal of the KRISP 94 effort was to determine the upper crustal structure of the southern Kenya Rift Valley near Lake Magadi. Thus in this area, the seismic refraction experiment was designed

Silas M. Simiyu; G. Randy Keller

1998-01-01

272

Interactions of wind-transported snow with a rift in the Ross Ice Shelf, Antarctica  

Microsoft Academic Search

Rifts in ice shelves accumulate a mélange of snow and firn from above and marine ice from below, material that has been postulated to negatively influence iceberg calving. From measurements and modeling we show that a 100 m wide rift near the front of the Ross Ice Shelf captures all wind-transported snow traveling in saltation and a substantial fraction of

Katherine C. Leonard; L.-Bruno Tremblay; Douglas R. MacAyeal; Stanley S. Jacobs

2008-01-01

273

Volcano spacings and lithospheric attenuation in the Eastern Rift of Africa  

Microsoft Academic Search

The Eastern Rift of Africa runs the gamut of crustal and lithospheric attenuation from undeformed shield through attenuated rift margin to active neo-oceanic spreading zones. It is therefore peculiarly well suited to an examination of relationships between volcano spacings and crust\\/lithosphere thickness. Although lithospheric thickness is not well known in Eastern Africa, it appears to have direct expression in the

P. A. Mohr; C. A. Wood

1976-01-01

274

Formation of the double rift system in the Thaumasia Highlands, Mars  

NASA Astrophysics Data System (ADS)

The Thaumasia Highland Rifts are two complex graben structures in the Thaumasia Highland belt, located in the southern Thaumasia region, Mars. The rifts are arranged 90-130 km apart in a nearly subparallel setting and represent a very unusual style of crustal deformation. We have investigated the mechanism responsible for this peculiar style of rifting and found strain localization by volcanic weak zones to be a major factor for fault-pattern formation, indicating widespread prerift and synrift volcanic activity in the region. This result is consistent with the high heat flow during rift emplacement as estimated from rift flank uplift. Our results indicate that the onset of volcanism predates the rift formation process and that magmatism and the associated lithospheric weak zones control the rift locations. The simulations performed here are in good agreement with the observations for extension in the NW-SE to NNW-SSE direction, and we favor a formation scenario involving passive rifting in a magmatectonically active environment.

Grott, M.; Kronberg, P.; Hauber, E.; Cailleau, B.

2007-06-01

275

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

276

Chemical and isotope compositions of nitric thermal water of Baikal rift zone  

Microsoft Academic Search

Three types of hydrotherms (nitric, carbonaceous and methane) are distinguished within the Baikal Rift Zone. The unloading sites of nitric therms are mostly located in the central and north-eastern parts of the Rift. Several chemical types are found among nitric therms (Pinneker, Pisarsky, Lomonosov, 1968; Lomonosov, 1974, etc.). The formation of terms being various in chemical compositions is associated with

A. M. Plyusnin; M. K. Chernyavsky; E. G. Peryazeva

2010-01-01

277

North Sinai-Levant rift-transform continental margin  

SciTech Connect

The passive continental margin of northern Egypt and the Levant coast formed during the Early mesozoic as the relatively small Anatolia plate broke away from northern Africa. The oceanic basin of the eastern Mediterranean and the unusual right-angle bend in the North Sinai-Levant shelf margin are both products of plate separation along a rift-transform fracture system, the south arm of Tethys. The north-south trending Levant transform margin is considerably narrower than the east-west trending rift margin of northern Egypt. Both exhibit similar facies and depositional histories through the mid-Tertiary. Analysis of subsurface data and published reports of the regional stratigraphy point to a three-stage tectonic evolution of this passive margin. The Triassic through mid-Cretaceous was marked by crustal breakup followed by rapid rotational subsidence of the shelf margins about hinge lines located just south and east of the present shorelines. Reef carbonates localized on the shelf edge separated a deep marine basin to the north from a deltaic-shallow marine platform to the south and east. In the Late Cretaceous-Early Tertiary, inversion of earlier formed half-grabens produced broad anticlinal upwarps of the Syrian Arc on the shelf margin that locally influenced facies patterns. The episode of inversion corresponds with the onset of northward subduction of the Africa plate beneath southern Asia. Beginning in the Oligocene and continuing to the present, there has been renewed subsidence of the North Sinai shelf margin beneath thick, outward building clastic wedges. The source of this large volume of sediment is the updomed and erosionally stripped margins of the Suez-Red Sea Rift and the redirected Nile River.

Ressetar, R.; Schamel, S.; Travis, C.J.

1985-01-01

278

Origin of the Rheic Ocean: Rifting along a Neoproterozoic suture?  

NASA Astrophysics Data System (ADS)

The Rheic Ocean is widely believed to have formed in the Late Cambrian Early Ordovician as a result of the drift of peri-Gondwanan terranes, such as Avalonia and Carolina, from the northern margin of Gondwana, and to have been consumed in the Devonian Carboniferous by continent-continent collision during the formation of Pangea. Other peri-Gondwanan terranes (e.g., Armorica, Ossa-Morena, northwest Iberia, Saxo-Thuringia, Moldanubia) remained along the Gondwanan margin at the time of Rheic Ocean formation. Differences in the Neoproterozoic histories of these peri-Gondwanan terranes suggest the location of the Rheic Ocean rift may have been inherited from Neoproterozoic lithospheric structures formed by the accretion and dispersal of peri-Gondwanan terranes along the northern Gondwanan margin prior to Rheic Ocean opening. Avalonia and Carolina have Sm-Nd isotopic characteristics indicative of recycling of a juvenile ca. 1 Ga source, and they were accreted to the northern Gondwanan margin prior to voluminous late Neoproterozoic arc magmatism. In contrast, Sm-Nd isotopic characteristics of most other peri-Gondwanan terranes closely match those of Eburnian basement, suggesting they reflect recycling of ancient (2 Ga) West African crust. The basements of terranes initially rifted from Gondwana to form the Rheic Ocean were those that had previously accreted during Neoproterozoic orogenesis, suggesting the rift was located near the suture between the accreted terranes and cratonic northern Gondwana. Opening of the Rheic Ocean coincided with the onset of subduction beneath the Laurentian margin in its predecessor, the Iapetus Ocean, suggesting geodynamic linkages between the destruction of the Iapetus Ocean and the creation of the Rheic Ocean.

Brendan Murphy, J.; Gutierrez-Alonso, Gabriel; Damian Nance, R.; Fernandez-Suarez, Javier; Keppie, J. Duncan; Quesada, Cecilio; Strachan, Rob A.; Dostal, Jarda

2006-05-01

279

Evolution of the Newfoundland-Iberia conjugate rifted margins  

NASA Astrophysics Data System (ADS)

It is accepted that mildly extended sedimentary basins form by largely uniform thinning of continental lithosphere. No such consensus exists for the formation of highly extended conjugate rifted continental margins. Instead, a wide range of models which invoke differing degrees of depth-dependent thinning have been proposed. Much of this debate has focussed on the well-studied Newfoundland-Iberia conjugate margins. We have tackled the problem of depth dependency at this pair of margins in three steps. First, we have reconstructed water-loaded subsidence histories by making simple assumptions about changes in water depth through time. Secondly, we have used these reconstructed subsidence histories to determine the spatial and temporal variation of lithospheric strain rate. An inversion algorithm minimizes the misfit between observed and predicted subsidence histories and crustal thicknesses by varying strain rate as a smooth function of distance across the margin, depth through the lithosphere, and geologic time. Depth-dependent thinning is permitted but, crucially, our algorithm does not prescribe its existence or form. Given the absence of significant volumes of syn-rift magmatism, we have also applied a minimal melting constraint. Inverse modeling has yielded excellent fits to both reconstructed subsidence and crustal observations, which suggest that rifting occurred from ˜ 150-135 Ma and at rates of up to 0.3 Ma- 1. Strain rate distributions are depth-dependent, suggesting that lithospheric mantle thins over a wider region than the crust. Beneath highly extended parts of the margin, crustal strain rates greatly exceed lithospheric mantle strain rates. Thirdly, we have tested our strain rate histories by comparing the total horizontal extension with the amount of extension inferred from normal faulting patterns. Both values agree within error. We freely acknowledge that there are important uncertainties in reconstructing the subsidence histories of deep-water margins. Nevertheless, stratigraphic records remain the only, albeit imperfect means of determining how crust and lithospheric mantle thin through time and space.

Crosby, Alistair; White, Nicky; Edwards, Glyn; Shillington, Donna J.

2008-08-01

280

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.

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

2013-01-01

281

Structure of the basins of the White Sea rift systems  

NASA Astrophysics Data System (ADS)

For the first time, the structure of the sedimentary basins of the Late Proterozoic rift system in the White Sea is characterized based on a set of new marine geological geophysical data such as the results of the common depth point seismic method, gravity and magnetic data, and seismoacoustics. The main tectonic structures in the topography of the heterogeneous basement within the basin of the White Sea are distinguished and described. A structural tectonic scheme of the basement surface is presented. The thicknesses of the sediments are estimated and the stratigraphic confinement of the seismic units recognized is done.

Zhuravlev, V. A.; Shipilov, E. V.

2008-02-01

282

Gravitative Spreading Causes en-echelon Diking Along a Volcanic Rift Zone: Observations From Madeira and an Experimental Approach  

Microsoft Academic Search

Volcanic rift zones are typically defined by parallel-striking dike complexes and eruptive fissures resulting in morphological ridges. Here, we describe the unusual occurrence of a pronounced rift arm where dike orientation is clearly oblique to the morphological rift axis: Desertas Islands (Madeira Archipelago), surface expression of a 60 km long submarine ridge. The Desertas ridge intersects the easternmost long axis

A. Klügel; T. R. Walter; S. Schwarz

2002-01-01

283

Geometry of Sierrita fault and its bearing on tectonic development of the Rio Grande rift, New Mexico  

Microsoft Academic Search

The Sierrita fault forms part of the western boundary of the Rio Grande rift in north-central New Mexico. This fault is crucial in determining the kinematics and mechanics of rifting because it has deeper and better exposures than are seen elsewhere along the margins of the rift. It is a normal fault with a curved surface that is concave eastward.

Lee A. Woodward; Harvey R. Duchene

1975-01-01

284

Tectonic Inheritance, Continental Rifts and the Transition to Segmented Seafloor Spreading: Results From Analogue and Numerical Models  

Microsoft Academic Search

We use analogue and numerical models to investigate the role of an inherited structural grain on continental rifting and the implications for oceanic ridge segmentation after continental breakup. Our models are based on geological observations from the western branch of the East African Rift System. The basins of this rift system are located within Proterozoic mobile belts at the edge

G. Corti; J. W. van Wijk

2006-01-01

285

Shear-Wave Splitting due to Rifting and Precambrian Accretion of Ethiopia  

NASA Astrophysics Data System (ADS)

We have utilized a dataset from the broadband seismic experiment of Nyblade and Langston (EOS v.83 p. 405, 2002) for a shear-wave splitting analysis in Ethiopia. A total of twenty-five broadband seismic stations, widely distributed in various physiographic regions in Ethiopia, were used. Six stations were installed on the southeastern plateau, twelve stations on the western plateau, and seven stations inside the Rift Valley, which runs northeast-southwest and separates the western and eastern highlands. The distribution of the broadband stations in the present study spans a broad region and allows us to compare the results of shear-wave splitting analysis inside the rift and on the rift-bounding plateaus. Previous shear-wave splitting results in Kenya, located south of Ethiopia, show fast polarization azimuths sub-parallel to the strike of the rift which are interpreted to be the result of vertical magma-filled cracks in the lithosphere opening in the direction perpendicular to the extension direction (Gao et al., 1997; Barruol and Ismail, 2001). However, this orientation is also perpendicular to the collision direction for the Mozambique belt and thus consistent with the fast azimuth being the result of fossilized anisotropy in the Precambrian lithosphere. Results from an SKS splitting analysis in Ethiopia (Maguire et al., EOS 2003 in press; Kendall et al., this session) show the orientation of the fast polarization azimuth within the Main Ethiopian Rift to be approximately NNE-SSW. Similar to the results from Kenya, this is parallel to the volcanic centers in the rift and perpendicular to the geodetically determined opening direction. Splitting directions on the rift shoulders are closer to NE, parallel to both the rift-bounding faults (perpendicular to the inferred average Neogene opening direction) and to the Precambrian accretionary structures. Our preliminary result from Addis Ababa suggests a fast azimuth of N23E, sub-parallel to the rift orientation in agreement with previous results by other workers. Further south near Arbaminch, results suggest an azimuth of N6E possibly due to the shift of rift orientation from NE-SW to nearly N-S in this region. Stations 250 km NW of the rift axis (Debre Markos) and 250 km SE of the rift axis (Goba) both show more north-easterly trends (N37E and N21E, respectively). Because of the distance of these stations from the rift, we suggest that this NE fast splitting direction is more likely related to fossilized anisotropy in the Precambrian lithosphere than to Neogene rifting.

Gashawbeza, E.; Keranen, K.; Nyblade, A.; Klemperer, S. L.; Walker, K.

2003-12-01

286

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

NASA Astrophysics Data System (ADS)

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

Bennett, Scott Edmund Kelsey

287

Matching conjugate volcanic rifted margins: 40Ar\\/ 39Ar chrono-stratigraphy of pre- and syn-rift bimodal flood volcanism in Ethiopia and Yemen  

Microsoft Academic Search

40Ar\\/39Ar dating of mineral separates and whole-rock samples of rhyolitic ignimbrites and basaltic lavas from the pre- and syn-rift flood volcanic units of northern Ethiopia provides a temporal link between the Ethiopian and Yemen conjugate rifted volcanic margins. Sixteen new 40Ar\\/39Ar dates confirm that basaltic flood volcanism in Ethiopia was contemporaneous with flood volcanism on the conjugate margin in Yemen.

Ingrid A. Ukstins; Paul R. Renne; Ellen Wolfenden; Joel Baker; Dereje Ayalew; Martin Menzies

2002-01-01

288

Petrological constraints on melt generation beneath the Asal Rift (Djibouti) using quaternary basalts  

NASA Astrophysics Data System (ADS)

The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 56 new lava flows sampled along 10 km of the rift axis and 9 km off-axis (i.e., erupted within the last 620 kyr). Petrological and primary geochemical results show that most of the samples of the inner floor of the Asal Rift are affected by plagioclase accumulation. Trace element ratios and major element compositions corrected for mineral accumulation and crystallization show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. While FeO, Fe8.0, Zr/Y, and (Dy/Yb)N decrease from the rift shoulders to the rift axis, SiO2, Na/Ti, Lu/Hf increase and Na2O and Na8.0 are constant across the rift. These variations are qualitatively consistent with shallow melting beneath the rift axis and deeper melting for off-axis lava flows. Na8.0 and Fe8.0 contents show that beneath the rift axis, melting paths are shallow, from 81 ± 4 to 43 ± 5 km. These melting paths are consistent with adiabatic melting in normal-temperature fertile asthenosphere, beneath an extensively thinned mantle lithosphere. On the contrary, melting on the rift shoulders (from 107 ± 7 to 67 ± 8 km) occurred beneath thicker lithosphere, requiring a mantle solidus temperature 100 ± 40°C hotter. In this geodynamic environment, the calculated rate of lithospheric thinning appears to be 4.0 ± 2.0 cm yr-1, a value close to the mean spreading rate (2.9 ± 0.2 cm yr-1) over the last 620 kyr.

Pinzuti, Paul; Humler, Eric; Manighetti, Isabelle; Gaudemer, Yves

2013-08-01

289

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

290

Thermal and exhumation history of the central Rwenzori Mountains, Western Rift of the East African Rift System, Uganda  

Microsoft Academic Search

The Rwenzori Mountains (Mtns) in west Uganda are the highest rift mountains on Earth and rise to more than 5,000 m. We apply\\u000a low-temperature thermochronology (apatite fission-track (AFT) and apatite (U–Th–Sm)\\/He (AHe) analysis) for tracking the cooling\\u000a history of the Rwenzori Mtns. Samples from the central and northern Rwenzoris reveal AFT ages between 195.0 (±8.4) Ma and\\u000a 85.3 (±5.3) Ma, and

F. U. Bauer; U. A. Glasmacher; U. Ring; A. Schumann; B. Nagudi

2010-01-01

291

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

292

Hydrogeologic evolution and hydrocarbon potential, southern Rio Grande rift, Texas  

SciTech Connect

Geotectonic events chronicled in the regional rock record around Terlingua include the late Paleozoic Ouachita orogeny, crustal down-warping in the Cretaceous, Laramide uplift and deformation, mid-Tertiary igneous intrusions, and consequent rifting. During the Ouachita orogeny, immense volumes of fluid and gas were driven from impinged sediment masses as continental plates collided and formed an overthrust belt, leading to reservoir mixing, water-washing of oils, fault lubrication, and hydrocarbon devolatilization in places. Later, during more quiescent periods, mixed fluids and gases differentiated as new hydrocarbons were generated in overthrust regions where overburden pressures and temperatures were increased by structural overriding and deeper sediment burial. In the Cretaceous Period, additional petroleum source beds were deposited around Terlingua as a seaway covered the entire Mid-Continent region of North America. However, Laramide uplift and compression subsequently redistributed subsurface fluids and gases in the region, while also exposing much of the Cretaceous section to meteoric recharge and flushing. Mid-Tertiary intrusions followed, generating dry gas in contact with Paleozoic sequences around Terlingua, and distilling some petroleum from source beds in the Cretaceous section. Today, ongoing tectonism and periodic rifting along the Rio Grande continue to influence subsurface fluid and gas distribution in the Terlingua district.

Fallin, J.A. (Texas Water Development Board, Austin (USA))

1989-09-01

293

Towards a safe, effective vaccine for Rift Valley fever virus  

PubMed Central

Rift Valley fever virus (RVFV) is an important animal and human threat and leads to longstanding morbidity and mortality in susceptible hosts. Since no therapies currently exist to treat Rift Valley fever, it remains a public and animal health priority to develop safe, effective RVFV vaccines (whether for animals, humans, or both) that provide long-term protective immunity. In the evaluated article, Bhardwaj and colleagues describe the creation and testing of two successful vaccine strategies against RVFV, a DNA plasmid vaccine expressing Gn coupled to C3d, and an alpha-virus replicon vaccine expressing Gn protein. Both vaccines elicited strong neutralizing antibody responses, prevented morbidity and mortality in RVFV-challenged mice, and enabled protection of naive mice via passive antibody transfer from vaccinated mice. Both DNA and replicon RVFV vaccines have previously been shown to protect against RVFV challenge, but these results allow for direct comparison of the two methods and evaluation of a combined prime–boost method. The results also highlight the specific humoral and cell-mediated immune responses to vaccination.

LaBeaud, Desiree

2011-01-01

294

An epidemiological model of Rift Valley fever with spatial dynamics.  

PubMed

As a category A agent in the Center for Disease Control bioterrorism list, Rift Valley fever (RVF) is considered a major threat to the United States (USA). Should the pathogen be intentionally or unintentionally introduced to the continental USA, there is tremendous potential for economic damages due to loss of livestock, trade restrictions, and subsequent food supply chain disruptions. We have incorporated the effects of space into a mathematical model of RVF in order to study the dynamics of the pathogen spread as affected by the movement of humans, livestock, and mosquitoes. The model accounts for the horizontal transmission of Rift Valley fever virus (RVFV) between two mosquito and one livestock species, and mother-to-offspring transmission of virus in one of the mosquito species. Space effects are introduced by dividing geographic space into smaller patches and considering the patch-to-patch movement of species. For each patch, a system of ordinary differential equations models fractions of populations susceptible to, incubating, infectious with, or immune to RVFV. The main contribution of this work is a methodology for analyzing the likelihood of pathogen establishment should an introduction occur into an area devoid of RVF. Examples are provided for general and specific cases to illustrate the methodology. PMID:22924058

Niu, Tianchan; Gaff, Holly D; Papelis, Yiannis E; Hartley, David M

2012-08-13

295

Upwarp of anomalous asthenosphere beneath the Rio Grande rift  

USGS Publications Warehouse

Continental rifts are possible analogues of mid-ocean ridges, although major plate tectonic features are less clearly observed1. Current thermal models of mid-ocean ridges2-4 consist of solid lithospheric plates overlying the hotter, less viscous asthenosphere, with plate thickness increasing away from the ridge axis. The lithospheric lower boundary lies at or near the melting point isotherm, so that at greater depths higher temperatures account for lower viscosity, lower seismic velocities and possibly partial melting. Upwarp of this boundary at the ridge axis concentrates heat there, thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium. At slow spreading ridges, a major central graben forms owing to the mechanics of magma injection into the crust5. Topography, heat flow, gravity and seismic studies support these models. On the continents, a low-velocity channel has been observed, although it is poorly developed beneath ancient cratons6-9. Plate tectonic models have been applied to continental basins and margins10-12, but further similarities to the oceanic models remain elusive. Topographic uplift is often ascribed to Airy type isostatic compensation caused by crustal thickening, rather than thermal compensation in the asthenosphere. Here we discuss the Rio Grande rift, in southwestern United States. Teleseismic P-wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening. ?? 1984 Nature Publishing Group.

Parker, E. C.; Davis, P. M.; Evans, J. R.; Iyer, H. M.; Olsen, K. H.

1984-01-01

296

Continental Rifting Across the Alarcon Basin, Gulf of California  

NASA Astrophysics Data System (ADS)

In Fall 2002 seismic refraction data and multi-channel seismic reflection data were collected in the Gulf of California as part of the Margins Rupturing Continental Lithosphere (RCL) initiative. Across the Alarcon Basin 56 Ocean-Bottom Seismometers and 11 onshore Ref-Teks collected wide-angle refraction data over an 880 km transect, coincident with 660 km of reflection data. The refraction data has been used to construct an initial velocity model across the entire rifted margin. Our results show ~150 km of new oceanic crust of normal thickness (6.5- 7 km) and lower crustal velocity (~6.7 km/s). The continent-ocean transition seems relatively abrupt occurring over less than 50 km on both conjugate margins. The rifted margin across the Alarcon Basin seems symmetric: There is roughly 300 km of extended continental crust on both margins- characterized by normal faulting at the surface and crust thinned to an average of 15 km with an average velocity of 6.2 km/s. At the furthest extents of our model, underneath the Baja peninsula crust seems to be about 28km thick, and under mainland Mexico about 26 km. Assuming an initially uniform crust of 30 km thickness, slightly greater than the observed maximum thicknesses, and 2-D extension there has been 100% extension across both margins. This predicts a total tectonic separation of around 470 km, which agrees with previous estimates for the Gulf of California of 450-500 km.

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

2004-12-01

297

Rift Valley Fever: Recent Insights into Pathogenesis and Prevention?  

PubMed Central

Rift Valley fever virus (RVFV) is a zoonotic pathogen that primarily affects ruminants but can also be lethal in humans. A negative-stranded RNA virus of the family Bunyaviridae, this pathogen is transmitted mainly via mosquito vectors. RVFV has shown the ability to inflict significant damage to livestock and is also a threat to public health. While outbreaks have traditionally occurred in sub-Saharan Africa, recent outbreaks in the Middle East have raised awareness of the potential of this virus to spread to Europe, Asia, and the Americas. Although the virus was initially characterized almost 80 years ago, the only vaccine approved for widespread veterinary use is an attenuated strain that has been associated with significant pathogenic side effects. However, increased understanding of the molecular biology of the virus over the last few years has led to recent advances in vaccine design and has enabled the development of more-potent prophylactic measures to combat infection. In this review, we discuss several aspects of RVFV, with particular emphasis on the molecular components of the virus and their respective roles in pathogenesis and an overview of current vaccine candidates. Progress in understanding the epidemiology of Rift Valley fever has also enabled prediction of potential outbreaks well in advance, thus providing another tool to combat the physical and economic impact of this disease.

Boshra, Hani; Lorenzo, Gema; Busquets, Nuria; Brun, Alejandro

2011-01-01

298

Geology and Petrology of the Southeast Mariana Forearc Rift  

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

299

From Rifting to Ultra-High-Pressure Metamorphism  

NASA Astrophysics Data System (ADS)

In Alpine-type orogens Ultra-High-Pressure (UHP) rocks are often found associated with serpentinites. This juxtaposition is generally interpreted to originate in subduction channels, where the hydration of the mantle wedge overlying a subduction zone, leading to the buoyant rise of newly formed serpentinites, has the potential to drive the early phases of exhumation. A study of the lithostratigraphy of the Zermatt-Saas Zone, in the Lago di Cignana area (Western Alps) contradicts this hypothesis, suggesting that the UHP rocks were already juxtaposed with serpentinites prior to orogenesis and that pre-orogenic rift-related lithostratigraphic associations may be preserved during burial to UHP conditions and subsequent exhumation. The Zermatt-Saas Zone, in the Lago di Cignana area, consists of abundant serpentinized mantle, intruded by Jurassic gabbros and locally overlain by syn- and post- rift sediments. The Lago di Cignana syn- and post-rift metasediments crop out along the upper part of the Zermatt-Saas Zone, where slivers of continental crust and pre-rift-sediments are also found. The association of continental basement rocks, pre-rift sediments and ophiolites has generally been ascribed to the formation of a tectonic melange during the Alpine orogeny. Here, we report a new study on zircons from Permian plutonic rocks of the Etirol-Levaz continental basement slice, which crops out in Valtournenche, only a few hundred meters from Lago di Cignana, at the same structural level. Our study shows that a distinctive phase of zircon growth occurred at ca. 170-160 Ma. High U/Th ratios and zoning patterns suggest that zircons grew as a result of melt infiltration related to the intrusion of mafic magmas, also dated at ca. 170-160 Ma, in the underlying serpentinites. Therefore, the continental basement slices and the oceanic basement rocks were already juxtaposed in the Jurassic and they were probably part of an Ocean-Continent Transition Zone (OCTZ), where the Lago di Cignana sediments were deposited. Omphacite-bearing zircon rims from the Etirol-Levaz slice crystallized at 47.3±1.3 Ma, showing that the continental basement underwent the same Alpine history as the underlying serpentinites and gabbros and as the Lago di Cignana metasediments. Alpine tectonics resulted only in minor reworking of the Jurassic contacts, generally preserving the original geometry. The partial preservation on a regional scale of the original relationships between rock units that underwent subduction to UHP conditions indicates that (1) the association of serpentinites and UHP rock units does not necessarily derive from chaotic counter-flow in a subduction channel, but may also be an inherited feature from the OCTZ and (2) the process of tectonic burial and exhumation is not necessarily chaotic. Indeed, OCTZ's are placed in a favorable position to reach UHP conditions, following negatively buoyant oceanic lithosphere into subduction, and then be accreted to the orogen, in response to the arrival of more buoyant continental lithosphere, resisting subduction.

Beltrando, Marco; Rubatto, Daniela; Manatschal, Gianreto

2010-05-01

300

Upper Mantle Seismic Structure of the Northern Ethiopian Rift - a Region of Incipient Continental Breakup  

NASA Astrophysics Data System (ADS)

The northern Ethiopian rift forms the third arm of the Red Sea, Gulf of Aden triple junction and captures the transition from continental rifting in East Africa to oceanic sea-floor spreading in Afar. As rifting proceeds to sea-floor spreading, it is expected that fault controlled (mechanical) segmentation associated with the continental rift setting will be superceded by asthenospheric and magmatic processes, which are more prevalent in oceanic settings. The EAGLE passive networks consisted of 79 broadband seismometers over an area 250x250km centred on the Boset magmatic segment 70km SE of Addis Ababa, in the centre of the rift. We image upper-mantle structure between depths of 40-300km depth using teleseismic tomographic inversion of P and S wave travel-time data. Crustal structure, unresolvable in our inversions, is accounted for using an a priori crustal model which we derive from receiver function analyses of Moho P-to-S converted phases and from the results of recent EAGLE refraction experiments in Ethiopia. Analysis of relative arrival-time residuals shows that the rift flanks are asymmetric with arrivals associated with the south-eastern Somalian Plate faster ( ˜0.65s for the P waves; ˜2s for the S waves) than the north-western Nubian Plate. Our tomographic inversions image a 75km-wide tabular low velocity zone (Vp=-1.5%, Vs=-4%) between 40-300km depth, interpreted as upwelling in the upper mantle beneath the less evolved southern part of the rift. At depths of ˜100km, north of 9 degrees N this anomaly broadens towards the oceanic spreading centre of Afar where it appears to be connected to deeper low-velocity structure. Along axis, the low velocity upwelling beneath the rift is segmented, with low velocity material in the uppermost 100 km always offset to whichever side of the rift has the highest rift flank topography. Lateral velocity contrasts and comparison of P and S travel-times suggests high temperatures and partial melt in the upper-mantle beneath the rift. We propose, that magma emplacement has occured within the lithosphere, thereby allowing rifting in an area where large scale tectonic forces may be too small for amagmatic rifting to occur.

Bastow, I. D.; Stuart, G. W.; Kendall, M. J.; Ebinger, C. J.; Ayele, A.

2004-12-01

301

Influence of heterogeneities within the lithosphere on the deformation pattern of continental rift systems.  

NASA Astrophysics Data System (ADS)

Understanding how heterogeneities within the lithosphere influence the deformation pattern in continental rifts still remains a challenge and is of real importance to constrain continental break-up. We have selected the Main Ethiopian Rift in East Africa and the Rio Grande Rift in the south-western U.S. These two rifts are perfect natural laboratories to investigate the effect of inherited as they share similar structural characteristics but develop above different kinds of lithosphere-scale heterogeneities. From a structural point of view both rifts show similar length (1000km), width (50 to 70 km) and asymmetry. The Main Ethiopian rift is the NE-SW trending plate boundary between the Nubian and Somalian plates that has been developing for the past 11 Ma above a palaeo-Proterozoic lithospheric-scale weak zone re-heated by the Afar hotspot, whereas the Rio Grande Rift is the eastern "boundary" of the Basin & Range system which has been developing for the past 30 Ma in the frame of a westward-retreating Farallon subduction zone. However, the Rio Grande Rift shows evidence of low angle normal faulting whereas the Main Ethiopian Rift shows steeply dipping (with a mean close to 70°) normal faults. The Main Ethiopian Rift shows larger volume of erupted lavas than the Rio Grande Rift. Combined with a structural analyses of both rifts, we present here a series of 2D cross sections numerical models that allow better understanding of the influence of initial heterogeneities such as 1) the rheological state of the crust; 2) the presence of a crustal-scale to lithospheric-scale discrete weak or strong zone, 3) the effects of the presence of magma. We illustrate that rheological boundaries are not reactivated if the rheological contrast it too high, which is the case of the Rio Grande Rift that developed to the east of the North American Craton within thinned lithosphere. We also illustrate that the width of the weak zone do no have any influence on the exhumation of the asthenospheric mantle. The temperature at the base of the lithosphere is the parameter controlling the asthenosphere rising.

Philippon, Melody; Thieulot, Cedric; van Wijk, Jolante; Sokoutis, Dimitrios; Willingshofer, Ernst; Cloetingh, Sierd

2013-04-01

302

Do Processes of Rhyolite Genesis Change as Icelandic Rifts Drift off of the Plume?  

NASA Astrophysics Data System (ADS)

The abandoned Snaefellsnes rift zone in western Iceland was the on-land manifestation of the Mid-Atlantic Ridge between 15 and 7 Ma. The rift zone was abandoned at 7 Ma, after it had drifted westward off of the Iceland hotspot, generally interpreted as a mantle plume. The position of the abandoned rift was initially recognized as the axis of a regional syncline analogous to the syncline developed in response to active rifting. Previous paleomagnetic and geochronologic studies have confirmed the position of the abandoned rift axis. Recent seismic tomography shows that the abandoned rift is also characterized by relatively thin crust (<20 km, versus up to 46 km above the plume). In the context of supervising Keck Geology Consortium undergraduate research projects in northwestern Iceland in 2003 and 2004, I have studied several silicic centers erupted at different times along the northern Snaefellsnes rift. A compilation of preliminary geochemical data from the Skagi area near the rift reveals several interesting trends that bear on the origin of silicic magmas as activity in the rift was waning. The compositional spectrum of silicic rocks in this area is from dacite (67 wt.% SiO2) to rhyolite (75 wt.% SiO2). Positive correlation between Na2O and SiO2 is consistent with either fractionation or decreasing degrees of crustal melting to get from dacite to rhyolite. However, Zr correlates negatively with SiO2, consistent with zircon fractionation, but inconsistent with variation in the degree of melting unless zircon is present in the source, unlikely for the meta-basaltic crust of Iceland. Therefore, I suggest these rocks reflect extreme (>90%) fractionation of a basaltic parent. A similar argument was advanced by Furman et al. (1992, J. Pet., 1405-1445) for rhyolites at Austerhorn in eastern Iceland. Compelling arguments have been previously advanced that most rhyolites erupted in the modern rifts of Iceland are the products of crustal melting. The same has been argued for some older centers erupted from the Snaefellsnes rift (Kroksfjordur, 9-10 Ma?). I propose the hypothesis that as a rift drifts off of the plume, and becomes more like a normal mid-ocean ridge (thinner crust), rhyolite genesis by crustal melting becomes uncommon, and that what rhyolites are generated are the result of extreme fractionation of a basaltic parent. Ongoing studies will test this hypothesis by more detailed trace element and O-isotope studies and the systematic study of a series of silicic centers erupted at the northern Snaefellsnes rift over its history of activity.

Jordan, B. T.

2004-12-01

303

Matching conjugate volcanic rifted margins: 40Ar/ 39Ar chrono-stratigraphy of pre- and syn-rift bimodal flood volcanism in Ethiopia and Yemen  

NASA Astrophysics Data System (ADS)

40Ar/ 39Ar dating of mineral separates and whole-rock samples of rhyolitic ignimbrites and basaltic lavas from the pre- and syn-rift flood volcanic units of northern Ethiopia provides a temporal link between the Ethiopian and Yemen conjugate rifted volcanic margins. Sixteen new 40Ar/ 39Ar dates confirm that basaltic flood volcanism in Ethiopia was contemporaneous with flood volcanism on the conjugate margin in Yemen. The new data also establish that flood volcanism initiated prior to 30.9 Ma in Ethiopia and may predate initiation of similar magmatic activity in Yemen by ˜0.2-2.0 Myr. Rhyolitic volcanism in Ethiopia commenced at 30.2 Ma, contemporaneous with the first rhyolitic ignimbrite unit in Yemen at ˜30 Ma. Accurate and precise 40Ar/ 39Ar dates on initial rhyolitic ignimbrite eruptions suggest that silicic flood volcanism in Afro-Arabia post-dates the Oligocene Oi2 global cooling event, ruling out a causative link between these explosive silicic eruptions (with individual volumes ?200 km 3) and climatic cooling which produced the first major expansion of the Antarctic ice sheets. Ethiopian volcanism shows a progressive and systematic younging from north to south along the escarpment and parallel to the rifted margin, from pre-rift flood volcanics in the north to syn-rift northern Main Ethiopian Rift volcanism in the south. A dramatic decrease in volcanic activity in Ethiopia between 25 and 20 Ma correlates with a prominent break-up unconformity in Yemen (26-19 Ma), both of which mark the transition from pre- to syn-rift volcanism (˜25-26 Ma) triggered by the separation of Africa and Arabia. The architecture of the Ethiopian margin is characterized by accumulation and preservation of syn-rift volcanism, while the Yemen margin was shaped by denudational unloading and magmatic starvation as the Arabian plate rifted away from the Afar plume. A second magmatic hiatus and angular unconformity in the northern Main Ethiopian Rift is evident at 10.6-3.2 Ma, and is also observed throughout the Arabian plate in Jordanian, Saudi Arabian and Yemeni intraplate volcanic fields and is possibly linked to tectonic re-organization and initiation of sea floor spreading in the Gulf of Aden and the Red Sea at 10 and 5 Ma, respectively.

Ukstins, Ingrid A.; Renne, Paul R.; Wolfenden, Ellen; Baker, Joel; Ayalew, Dereje; Menzies, Martin

2002-05-01

304

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

305

The Rift Valley complex as a barrier to gene flow for Anopheles gambiae in Kenya.  

PubMed

Recent studies of Anopheles gambiae, the principal mosquito vector of malaria in Africa, suggested that the eastern Rift Valley and its surrounding areas act as a barrier to gene flow. To quantify the unique effect of these areas on gene flow, we measured genetic variation within and between populations from each side of the Rift. Low differentiation was measured between populations on each side of the Rift (mean FST < 0.008, mean RST < 0.002). However, high differentiation was measured across the Rift (mean FST = 0.104; mean RST = 0.032). Genetic diversity within populations was lower in eastern populations, suggesting that the effective population sizes (Ne) of these populations were lower than those of western populations. We partitioned the overall differentiation across the Rift into three factors: variation in Ne between populations contributed 7-20%; distance contributed 10-30%, and the remainder, corresponding to the unique effect of the Rift was 50-80%. The Rift's effect was highly significant based on FST. The greater sensitivity of FST in measuring differentiation indicated that drift and not mutation generated the differences between populations. Restricted gene exchange across several hundred kilometers on the face of intense human transportation implies that active mosquito dispersal is the major form of migration, and that migration is a multistep process, where step length is relatively short. PMID:10589511

Lehmann, T; Hawley, W A; Grebert, H; Danga, M; Atieli, F; Collins, F H

306

Upper mantle seismic anisotropy in the intra-continental Kachchh rift zone, Gujarat, India  

NASA Astrophysics Data System (ADS)

Shear wave splitting study of 411 SKS/SKKS phases covering backazimuth range of 13° to 305° recorded by 12 broadband stations in the Kachchh rift has led to estimates of fast axis orientations and splitting times for 118 good measurements. The average vector mean of fast axis orientation (86 ± 14°) corresponds to the E-W axis of the Kachchh rift and the delay time (~ 1.6 s) is attributed to the ~ 184 km-thick upper mantle layer with 4% anisotropy. The anisotropic character observed for the Kachchh rift (KR) is comparable to other continental rifts and these are related to the high-temperature, lattice-preferred orientation fabric of olivine, inherited from the mantle flows. The source of the rift-axis parallel anisotropy is traced to the rift-parallel flows within the 76 ± 6 km-thick lithosphere. Additionally, the rift-parallel pockets of partial melts also induce anisotropy within the asthenosphere. Both these are inherited from the plume-lithosphere interaction during the Deccan/Reunion plume episode (~ 65 Ma).

Mandal, Prantik

2011-08-01

307

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

308

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

NASA Astrophysics Data System (ADS)

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

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

2005-08-01

309

Geodynamic Evolution of the Southern Flank of the Corinth Rift  

NASA Astrophysics Data System (ADS)

The Gulf of Corinth is the most seismically active area of Europe extending in a N-S direction at a rate of 12mm/yr. Many studies have focused on currently active normal faults, which bound the southern flank of the Corinth rift. However, many prominent faults, that are considered no longer active, outcrop over a broader area along with their syn-rift sedimentary sequence. The normal fault network and related synrift succession have been mapped and studied at a scale of 1:25,000. The geometry of the fault network in 3D has been reconstructed in the gOcad 3D geomodelling package. Based on these detailed field data we propose a geodynamic model for the evolution of the region in three steps over the last two million years. From 2 to 1 Ma, extension affected a broad area at least twenty kilometers wide. Alluvial conglomerates and sandstones, deposited in grabens and half grabens, were derived from erosion of the pre-existing (Hellenic) high relief to the south in the Peloponnese massif. Around 1Ma, a regional uplift led to the migration of the depocenter to the north. New faults are activated closer to the coast, in places generating forced folds that tilted synrift fluvial formations northward over a 2-3 kilometers wide area. Considerable accommodation space was generated by these faults and, with an increased sediment supply from the south, a series of giant marine Gilbert deltas developed in their hangingwalls. Finally, from 0.5 Ma to present, the activation of a series of major new faults further to the north (near the current coastline) coincided with regional uplift of the south coastal belt, and initiated the formation of a second generation of Gilbert delta, currently building out into the gulf. The faults can be divided into three families. The main one (74 data) is oriented N110, dipping 45 to 60° to the north. It includes 5 major faults spaced 3 to 5 km from south to north, with a throw larger than 1000m. A secondary set (41 data) strikes N60, dipping to the NW, it includes only one fault with a throw larger than 500m; the other secondary set (43 data) is antithetic to the two first ones as it strikes N60 to N110, dipping to the south, the largest fault it includes has a throw of about 400m. Most faults are planar and the importance of listric faults seems minor in the rift history.

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

2005-12-01

310

Geodetic determination of plate velocity vector in the Ethiopia Rift  

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

311

Tectonic evolution at rift zones: Geodynamics and Numerical Modeling  

NASA Astrophysics Data System (ADS)

Tectonic evolution at rift zones is commonly considered symmetric along mid-ocean ridges, when modeling with relative plate motions and steady-state processes. However, the bathymetry of rift zones is generally asymmetric, being the eastern flank in average slightly shallower (100-300 m) than the western one. Also, based on surface wave tomographic models, shear wave velocities in the upper mantle indicate a difference between the western and eastern flanks of an oceanic basin. A better way to understand dynamics of the lithosphere at rift zones, and lithosphere/mantle interactions corresponds to absolute plate kinematic analyses, i.e., with respect to the mantle, modeling time-dependent tectonic processes. We performed numerical simulations of plate-driven mantle flow beneath mid-ocean ridges and we considered a time-dipendent flow induced by the relative motion of overlying rigid plates in an incompressible viscous mantle. In mantle reference frames, a net "westward" rotation of the lithosphere relative to the mantle can be observed, and we used velocities obtained in the hotspot reference frame, as boundary conditions. This implies that plates along a ridge, and the ridge itself, move toward the west but with different velocities, relative to the mantle, and the separation between plates triggers mantle upwelling. Numerical solutions for viscosity flow beneath plates that thicken with increasing age are presented. The mantle can be modeled as a viscous fluid, and its dynamics can be described using the Stokes equations. At a first approximation the fluid is considered Newtonian. A further step in the description of the phenomena would require the inclusion of thermal effects: in this case the fluid viscosity and density have to be considered as a function of the temperature. For solving both the Stokes equations and the thermal effects, a finite element approach has been adopted. Results show an asymmetric thickening of plates along the ridge, as suggested by geological a geophysical observations, and provide useful relationships between mantle temperature and thickness of the oceanic lithosphere.

Cuffaro, M.; Miglio, E.; Doglioni, C.

2010-12-01

312

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

313

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

314

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

NASA Astrophysics Data System (ADS)

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

Aanyu, Kevin; Koehn, Daniel

2010-05-01

315

Modeling the spatial spread of Rift Valley fever in Egypt.  

PubMed

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

2013-02-02

316

Spatial risk assessment of Rift Valley fever in Senegal.  

PubMed

Rift Valley fever (RVF) is broadening its geographic range and is increasingly becoming a disease of global importance with potentially severe consequences for human and animal health. We conducted a spatial risk assessment of RVF in Senegal using serologic data from 16,738 animals in 211 locations. Bayesian spatial regression models were developed with interpolated seasonal rainfall, land surface temperature, distance to perennial water bodies, and time of year entered as fixed-effect variables. Average total monthly rainfall during December-February was the most important spatial predictor of risk of positive RVF serologic status. Maps derived from the models highlighted the lower Senegal River basin and the southern border regions of Senegal as high-risk areas. These risk maps are suitable for use in planning improved sentinel surveillance systems in Senegal, although further data collection is required in large areas of Senegal to better define the spatial distribution of RVF. PMID:17627440

Clements, Archie C A; Pfeiffer, Dirk U; Martin, Vincent; Pittliglio, Claudia; Best, Nicky; Thiongane, Yaya

2007-01-01

317

Rare Wisconsin wildcat to target midcontinent rift Nonesuch shale  

SciTech Connect

This paper reports on a Michigan operator which is to start drilling a remote Midcontinent rift wildcat in nonproducing Wisconsin early in 1992. Terra Energy Ltd., Traverse City, Mich., is attempting to obtain a state permit to drill a rank 6,000 ft wildcat to Middle Proterozoic Copper Harbor conglomerate. The wildcat will evaluate oil possibilities in Middle Proterozoic Nonesuch shale, just above Copper Harbor, in Bayfield County, Wis., about 10 miles west of Ashland. The company hopes to spud by yearend 1991 using a rig and services from Michigan, says Steve Savoie, Terra geologist. The proposed location is about 280 miles northwest of the nearest production, in the northwestern Michigan basin. The proposed location is about 70 miles west of the White Pine mine in northwestern Michigan, where copper has been mined and noncommercial volumes of oil have leaked from Lower Nonesuch.

Petzet, G.A.

1991-10-21

318

Arc rifting of the Carolina terrane in northwestern South Carolina  

SciTech Connect

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

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

1991-03-01

319

Molecular biology and genetic diversity of Rift Valley fever virus  

PubMed Central

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

Ikegami, Tetsuro

2013-01-01

320

Dike injection and magma mixing in Kenya rift volcanoes  

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

321

Rift basin evolution and the growth of normal faults  

SciTech Connect

The three-dimensional displacement geometry associated with micro-normal faults in sedimentary strata closely resembles the pattern of subsidence and uplift following earthquakes on parts of normal fault systems. Both are similar to the large-scale geometry of half-graben sedimentary basins and their uplifted footwall blocks. These observations suggest that (1) half-graben evolve as a consequence of repeated slip events on basin-bounding normal faults and (2) normal fault systems grow in length through time as displacement accumulates. Furthermore, details of the evolution of basins and their border fault systems (BFS) should be recorded in the basin fill. Indeed, Triassic synrift strata progressively onlap basement along the margins of many eastern North American rift basins, implying that these basins grew in length and width. A progressive decrease in accumulation rates through the Triassic section is also consistent with basin growth. Forward modeling involving basin growth, however, indicates that both features need not always occur. On the largest scale, a typical Mesozoic rift basin approximates a large syncline plunging toward its associated BFS. This geometry suggests that cumulative fault displacement and associated basin subsidence were highest near the center of the map trace of the BFS. On a smaller scale, transverse growth folds in the hanging wall of the BFS are associated with a segmentation of the BFS: synclines formed where fault segment displacement was highest; anticlines formed where displacement was lower, generally at the boundaries of fault segments. Thus, along-strike variations in fault displacement occur on a variety of scales. In addition, the map trace of the basement-sediment contact is apparently unaffected by the transverse folds and the segmentation of the BFS, implying some form of regional linkage among the segments of the BFs and perhaps other faults within the basin.

Schlische, R.W. (Rutgers, The State Univ. of New Jersey, Piscataway, NJ (United States). Dept. of Geological Sciences)

1992-01-01

322

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

SciTech Connect

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

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

1990-01-01

323

Buttressing and fractional spreading of Tenerife, an experimental approach on the formation of rift zones  

NASA Astrophysics Data System (ADS)

The island of Tenerife is composed of three Miocene shields, which are centered by the Cañadas volcano since the Pliocene. Tenerife sits on more than 2 km thick oceanic sediments. Quaternary volcanism of the Cañadas series and giant landslides were principally confined to triaxial rift zones. The mechanism of triaxial rifting, however, has remained unclear. Physical analog models show that these rift zones may have formed by gravity-driven lateral escape of island segments, induced by loading of the deformable substratum. For experiments scaled to Tenerife, three adjacent sand cones were mounted onto viscous PDMS substratum. Gravitational spreading caused circumferential expansion of each cone, until a large edifice (Cañadas) was constructed in their center. The older cones now acted each as a buttress; radial fractures were overprinted by fractional spreading of the Cañadas edifice. This resulted in formation of three main extensional zones, resembling the triaxial rifting configuration of Tenerife.

Walter, Thomas R.

2003-03-01

324

Rifts of Deeply Eroded Hawaiian Basaltic Shields: A Structural Analog for Large Martian Volcanoes.  

National Technical Information Service (NTIS)

Recently derived morphologic evidence suggests that intrusive events have not only influenced the growth of young shield volcanoes on Mars but also the distribution of volatiles surrounding these volcanoes: in addition to rift zones and flank eruptions on...

M. D. Knight G. P. L. Walker P. J. Mouginis-mark S. K. Rowland

1988-01-01

325

Synthetic Vaccines: Test of Concepts and Application to Rift Valley Fever Virus. Phase 1.  

National Technical Information Service (NTIS)

Use of synthetic peptides, comprising both B and T cell determinants, is being considered a viable approach to vaccine development. We explored several aspects this strategy employing Rift Valley fever virus (RVFV) as the study model. The immunogenicity o...

M. S. Collett

1990-01-01

326

Biological and Antigenic Relationship between Rift Valley Fever Virus Strains Isolated in Egypt and Madagascar.  

National Technical Information Service (NTIS)

Rift Valley fever (RVF) is an arthropod-borne viral disease primarily affecting domestic ungulates but with occasional extension into human populations. Before 1977, the known distribution of the disease was limited to sub-saharan Africa, causing sporadic...

J. F. Saluzzo G. W. Anderson J. F. Smith D. Fontenille P. Coulanges

1989-01-01

327

Analysis of a Thaumasia Planum rift through automatic mapping and strain characterization of normal faults  

NASA Astrophysics Data System (ADS)

A new semi-automatic technique is presented to map and characterize tectonic features on Mars. Automatic strain estimation associated with normal faults is achieved for synthetic and real fault scarps on Mars. The application of this new technique to a small rift located in Thaumasia Planum allowed the segmentation of the rift. The defined segmentation corresponds to changes in the strikes of faults that delimitate rift areas with different architecture. The rift is formed by several pull-apart basins developed due to the reactivation of previously formed tectonic structures. The strain spatial distribution and the overall geometry are consistent with a roughly East-West left-lateral shear transfer zone between two different lithospheric blocks.

Alegre Vaz, David

2011-09-01

328

Radiating rifts and dyke swarms of the middle Paleozoic Yakutsk plume of eastern Siberian craton  

NASA Astrophysics Data System (ADS)

The eastern part of the Siberian craton is locus to a mid-Paleozoic large igneous province (LIP) distributed over an area of >0.7 × 106 km. This Yakutsk LIP originated from a mantle plume that caused early domal uplift, formation of 'triple junction' rifts (with uplift of their shoulders), associated mafic magmatism including volcanics, sills and a giant radiating dyke swarm, and associated diamondiferous kimberlites (Mirnyi, Nakyn and Dladyn-Alakit fields).The mafic magmas are dominantly tholeiites with patterns that are overall similar to ocean island basalts (OIB), and isotope geochemistry suggests a mixture of enriched and depleted sources. Geochemistry is consistent with plume-lithospheric interactions.The main axis of failed rifting along the E-W trending Vilyui rift represents reactivation of the Akitkan belt along which at 1.9 Ga was the proposed locus of collision between northern (Anabar) and southern (Aldan) superterranes, and subsequent aulacogen rifting in the Mesoproterozoic.

Kiselev, A. I.; Ernst, R. E.; Yarmolyuk, V. V.; Egorov, K. N.

2012-02-01

329

Tectonic heritage and intra-crustal decoupling: consequences for post-orogenic rift basin dynamics  

NASA Astrophysics Data System (ADS)

It is generally accepted that styles of continental rifting are conditioned by the integrated strength of the lithosphere. For example, strong plates would extend in narrow rifting mode, while weak lithospheres would undergo stretching in wide rifting mode. In case of post-orogenic extensional basins this interpretation might be insufficient because it is based on the assumption of "standard" rheological structure of the continental crust, in which creep activation temperature increases with depth, the yield strength profile is characterized by smooth vertical variations , and more basic, denser layers underlay less basic (quartz-rich) low density layers. Yet, the orogenic crusts are largely formed by nappe stacking where strong dense, former lower crustal units slide over less dense weaker former uppers crustal units, resulting in inversed density-rheological sequences. Over-thickened continental crusts are also prone to metamorphic phase changes leading to additional variations in density and rheological properties. As result, unusual, "inversed" rheological structures may form characterized by sharp mechanical contrasts between the lithological layers and by presence of stronger heavy units on top of weaker low density units. To investigate the consequences of such structural heritage and of the associated structural weakening, we implement thermo-mechanical viscous-elastic-plastic numerical models of post-orogenic rifting. The experiments show that the inverted post-orogenic structures favor rapid development of mechanical and gravitational instabilities leading to variety of basin morphologies. In particular, wide rifts can form in quite strong lithospheres and then "switch" to narrow rifting mode. In other cases several rift basins develop quasi simultaneously, or low-angle faults and crustal detachments form without necessity of specific rheological mechanisms. The presence of rheological decoupling zones between different crustal and mantle layers is also of primary importance because it results in overall drop of the flexural strength of the system and leads to important horizontal flow of the ductile material. In this case crustal necking occurs at different levels so that necking levels switch from one depth to another resulting in step-like variations of rifting style and in acceleration/deceleration of subsidence during the active phase of rifting. During the post-rifting phase, the initially decoupled rheological layers may "stick" together resulting in step-like strengthening of the lithosphere and deceleration of subsidence. Hence, the entire rift system may exhibit polyphase subsidence history, yet conditioned by the evolution of the internal structure and not by variation of external factors such as far-field forces or mantle dynamics.

Burov, E. B.; Huet, B.; Le Pourhiet, L.

2011-12-01

330

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 São Jerônimo-Curiúva 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

331

Oxygen isotope evidence for subduction and rift-related mantle metasomatism beneath the Colorado Plateau-Rio Grande rift transition  

NASA Astrophysics Data System (ADS)

Spinel lherzolite and pyroxenite xenoliths from the Rio Puerco Volcanic Field, New Mexico, were analyzed for oxygen isotope ratios by laser fluorination. In lherzolites, olivine ?18O values are high (+5.5‰), whereas ?18O values for pyroxenes are low (cpx=+5.1‰; opx=+5.4‰) compared to average mantle values. Pyroxenite ?18O values (cpx=+5.0‰; opx=+5.3‰) are similar to those of the lherzolites and are also lower than typical mantle oxygen isotope compositions. Texturally and chemically primary calcite in pyroxenite xenoliths is far from isotopic equilibrium with other phases, with ?18O values of +21‰. The isotopic characteristics of the pyroxenite xenoliths are consistent with a petrogenetic origin from mixing of lherzolitic mantle with slab-derived silicate and carbonatite melts. The anomalously low ?18O in the pyroxenes reflects metasomatism by a silicate melt from subducted altered oceanic crust, and high ?18O calcite is interpreted to have crystallized from a high ?18O carbonatitic melt derived from subducted ophicarbonate. Similar isotopic signatures of metasomatism are seen throughout the Rio Puerco xenolith suite and at Kilbourne Hole in the southern Rio Grande rift. The discrete metasomatic components likely originated from the subducted Farallon slab but were not mobilized until heating associated with Rio Grande rifting occurred. Oxygen diffusion modeling requires that metasomatism leading to the isotopic disequilibrium between calcite and pyroxene in the pyroxenites occurred immediately prior to entrainment. Melt infiltration into spinel-facies mantle (xenoliths) prior to eruption was thus likely connected to garnet-facies melting that resulted in eruption of the host alkali basalt.

Perkins, George B.; Sharp, Zachary D.; Selverstone, Jane

2006-06-01

332

Postepidemic Analysis of Rift Valley Fever Virus Transmission in Northeastern Kenya: A Village Cohort Study  

Microsoft Academic Search

BackgroundIn endemic areas, Rift Valley fever virus (RVFV) is a significant threat to both human and animal health. Goals of this study were to measure human anti-RVFV seroprevalence in a high-risk area following the 2006–2007 Kenyan Rift Valley Fever (RVF) epidemic, to identify risk factors for interval seroconversion, and to monitor individuals previously exposed to RVFV in order to document

A. Desirée LaBeaud; Samuel Muiruri; Laura J. Sutherland; Saidi Dahir; Ginny Gildengorin; John Morrill; Eric M. Muchiri; Clarence J. Peters; Charles H. King

2011-01-01

333

Quaternary oblique extensional tectonics in the Ethiopian Rift (Horn of Africa)  

Microsoft Academic Search

The Ethiopian Rift extends in a northeasterly direction, from Southern Ethiopia to the Afar region. It shows a complex fault pattern, characterised by the interplay of a N30°E—N40°E-trending border fault system with the Quaternary Wonji Fault Belt, which is constituted by right-stepping en-echelon N?S to N20°E trending faults. The Wonji Fault Belt affects mainly the rift floor, but it also

Mario Boccaletti; Marco Bonini; Roberto Mazzuoli; Bekele Abebe; Luigi Piccardi; Luigi Tortorici

1998-01-01

334

Evidence for small-scale mantle convection in the upper mantle beneath the Baikal rift zone  

Microsoft Academic Search

Inversion of teleseismic P wave travel time residuals collected along a 1280-km-long profile traversing the Baikal rift zone (BRZ) reveals the existence of an upwarped lithosphere\\/asthenosphere interface, which causes a travel time delay of about 1 s at the rift axis (``central high''). An area with early arrivals relative to the stable Siberian platform of up to 0.5 s is

Stephen S. Gao; Kelly H. Liu; Paul M. Davis; Phillip D. Slack; Yuliy A. Zorin; Valentina V. Mordvinova; Vladimir M. Kozhevnikov

2003-01-01

335

Evidence for small-scale mantle convection in the upper mantle beneath the Baikal rift zone  

Microsoft Academic Search

Inversion of teleseismic P wave travel time residuals collected along a 1280-km-long profile traversing the Baikal rift zone (BRZ) reveals the existence of an upwarped lithosphere\\/asthenosphere interface, which causes a travel time delay of about 1 s at the rift axis (“central high”). An area with early arrivals relative to the stable Siberian platform of up to 0.5 s is

Stephen S. Gao; Kelly H. Liu; Paul M. Davis; Phillip D. Slack; Yuliy A. Zorin; Valentina V. Mordvinova; Vladimir M. Kozhevnikov

2003-01-01

336

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

Microsoft Academic Search

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

N. Kato; H. Sato

2003-01-01

337

Petrology and Evolution of a Monoclinally-folded Paleo-rift Lava Sequence, Vatnsdalsfjall, Northern Iceland  

Microsoft Academic Search

The Hvammur-Breiden area of Vatnsdalsfjall in northern Iceland lies 15 km east of the axis of the abandoned Snaefellsnes rift zone that was an active spreading center from 15-7 Ma. It contains an unusual and exceptionally well exposed monoclinal flexure that deforms a thick volcanic sequence produced during the waning stages of rift-related volcanic activity. A sequence of thin basalt

K. C. Ackerly; R. A. Wobus

2004-01-01

338

Asthenospheric flow and origin of volcanism in the Baikal Rift area  

Microsoft Academic Search

The origin of low-volume, hotspot-like volcanism often observed in continental rift areas is debated, as is the nature of the flow in the mantle beneath. In this paper we assemble seismic constraints on the mantle flow below the Baikal Rift Zone. We combine new evidence from upper-mantle tomography and from a radially anisotropic shear-velocity profile measured beneath and SE of

Sergei Lebedev; Thomas Meier; Rob D. van der Hilst

2006-01-01

339

Lithospheric and Upper Mantle Structure of the Rio Grande Rift: Implications for Pure Shear Extension  

Microsoft Academic Search

Results from the Colorado Plateau-Rio Grande Rift-Great Plains seismic transect (LA RISTRA) experiment are consistent with a pure shear extension mechanism for the Rio Grande rift (RGR). LA RISTRA was a 950 km-long PASSCAL broadband seismic line with approximately 18 km station spacing deployed during 1999-2001 along a great circle from Lake Powell, UT to Pecos, TX, crossing the RGR

D. C. Wilson; R. Aster; J. Ni; W. Gao; S. Grand; W. S. Baldridge; S. Semken

2003-01-01

340

Normal faulting from simple shear rifting in South Tibet, using evidence from passive seismic profiling across the Yadong-Gulu Rift  

NASA Astrophysics Data System (ADS)

The Tibetan Plateau is undergoing north–south shortening accompanied by west–east extension, as evidenced by the widespread development of north–south trending normal faults, grabens and rifts. While the mode of the north–south shortening has been the main focus of most international studies, knowledge of the deep structure beneath South Tibet is required for understanding the mechanism of the west–east extension. The onset of the north–south trending normal faulting is commonly taken as an indicator that the Tibetan Plateau was uplifted to a near-maximum elevation before entering a collapsing stage. Here we report on the receiver functions of a seismological experiment across the northern segment of the Yadong-Gulu Rift (YGR), one of the youngest rifts in South Tibet. The migrated receiver function images reveal that the YGR is a high-angle normal fault characterized by a 5-km Moho rise from its western to eastern flank, together with distinct differences in the crustal structure and intracrustal seismic conversion patterns between the two flanks. This highly asymmetric lithospheric structure suggests whole-crustal extension controlled by a simple/general shear rifting mechanism. This simple/general shear rifting in the YGR is attributed to an eastward (horizontal) shear at the base of the upper crust, as evidenced by the observed Tibetan GPS velocity field and our observation of shear wave splitting discrepancy among the upper crust, lower crust and lithospheric mantle. We propose that in the YGR, simple shear rifting accommodates the northward injection of the Indian lithosphere, which may suggest that the onset of the north–south normal faulting does not indicate gravitational collapse of the Tibetan lithosphere.

Zhang, Zhongjie; Chen, Yun; Yuan, Xiaohui; Tian, Xiaobo; Klemperer, Simon L.; Xu, Tao; Bai, Zhiming; Zhang, Hongshuang; Wu, Jing; Teng, Jiwen

2013-10-01

341

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

NASA Astrophysics Data System (ADS)

Thinning of the crust of more than 10 km is a major feature of typical continental rifts such as the East African (EAR) and Rio Grande (RGR) rifts. However, numerous previous studies across the Baikal rift zone (BRZ), which has similar surface expressions and tectonic history, and more active seismicity relative to EAR and RGR, have resulted in contradicting amount of thinning, ranging from almost none to more than 10 km. We measure crustal thickness by stacking teleseismic receiver functions beneath 51 sites on the southern and central parts of the BRZ and adjacent Siberian Platform and Sayan-Baikal-Mongolian Foldbelt. Our measurements reveal that beneath the southern part of the Platform, the average crustal thickness is about 38 km, which is about 7 km thinner than that beneath the Foldbelt and the un-rifted part of the BRZ. The thinnest crust, 35 km, is found beneath the central part of the rift, and represents a significant thinning of about 10 km relative to the un-rifted parts of the BRZ.

Gao, Stephen S.; Liu, Kelly H.; Chen, Chizheng

2004-10-01

342

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

343

Basaltic volcanism in Ethiopia: Constraints on continental rifting and mantle interactions  

SciTech Connect

Middle to late Cenozoic mafic lavas from the Ethiopian volcanic province exhibit considerable chemical and isotopic diversity that is linked to eruption age and eruption location. These variations provide a geochemical framework in which continental rifting can be examined. Trace element and Sr, Nd, and Pb isotopic data are interpreted to indicate involvement of up to two depleted and two enriched mantle reservoirs throughout Cenozoic rift development in Ethiopia. Superimposed on the characteristics imparted by varying degrees of melting of these distinct reservoirs are the effects of crystal fractionation and, in some instances, crustal contamnation. Initial stages of Oligocene rifting and volcanism, as manifested by the rift-bounding plateau flood basalts, are attributed to asthenospheric upwelling and melting of a heterogeneous, enriched subcontinental lithospheric mantle. Mildly alkaline lavas were produced from an enriched source with characteristics similar to those of the inferred source of other mantle/minus/derived lavas and xenoliths from east Africa (LoNd array, EMI to HIMU). Contemporaneous tholeiitic lavas were derived from a source similar to that producing oceanic basalts from Samoa and the Society Islands (EMII). As lithospheric thinning and rifting continued into the Miocene, upwelling depleted asthenosphere (depleted OIB reservoir, PREMA) interacted with the lithospheric sources producing lavas with hybrid elemental and isotopic characteristics (11-6 Ma plateau and rift margin basalts).

Hart, W. K.; WoldeGabriel, G.; Walter, R. C.; Mertzman, S. A.

1989-06-10

344

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

NASA Astrophysics Data System (ADS)

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

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

2013-08-01

345

Three-Dimensional Modelling of Rift Basin Evolution  

NASA Astrophysics Data System (ADS)

Numerical simulation of faulting is gaining interest, typically utilising the Discrete Element Modelling (DEM) technique. As computational power increases, so does the ability to run larger and more complex experiments. This research aims to explore rift basin evolution, with emphasis placed on the interaction of multiple faults as they evolve. Importance is placed on the technical aspects of implementing a complex and large DEM, specifically parallelism and whether currently popular software libraries known as physics engines can be a useful tool in their development. The technique employed is a variation of Mora and Place’s Lattice-Solid Model (1994). We use a 3D block of spheres, initially packed into a defined volume, interconnected using spring-damper structures. Elemental position is controlled throughout the simulation using Newtonian physics, while underlying plate movement is introduced by applying a specific force to each element. The spring-damper components provide a compressive force in the system, restricting each element to its neighbours, until the point the spring-damper has reached a pre-defined maximum extension, at which point it breaks resulting in a fault. Physical calculations are performed using using a publically available engine. The technical implementation of a physics engine is markedly more advanced than that found in many DEM codes used to date, specifically that of the collision detection system, which is implemented as a broad and then narrow phase process, which is solved as a Linear Complementarity Problem. In order to make use of large, distributed computing resources, a method by which to split the DEM into smaller sub-DEMs and then calculate each on an individual CPU has been defined. Typically results are accurate to within four decimal places compared to idealised values; this is comparable to other integration techniques such as Verlet or RK4. Through splitting the model and using multiple instances of a physics engine, it has been possible to limit the scale of simulation to available computing hardware, rather than software libraries. Extension of the crust results in an initial phase of a large number of small faults over which extensional strain is accommodated. As extension progresses, these faults are noted to either link to become larger structures or become inactive and passively rotated in the hanging-wall and footwalls of neighbouring structures. This does not mean, however, that they cannot be reactivated later during rift development. As deformation within the system reaches the rift climax phase, extension is focused on a small number of large faults within the basin. The timing, location, linkage and strain accommodated by these structures is determined during extension and examined as it progresses in three-dimensions. It can also be concluded that using a physics engine in the design of a geological DEM can be beneficial for development time, speed of execution and ensuring physical correctness. Model splitting provides a neat and scalable method by which to distribute computation of a large DEM over multiple CPUs and memory resources.

Longshaw, S. M.; Finch, E.; Turner, M.; Gawthorpe, R. L.

2009-12-01

346

Rifting processes in the centre of Siberia revealed by BEST (Baikal Explosion Seismic Transects)  

NASA Astrophysics Data System (ADS)

The Baikal Rift Zone is located in Siberia at the centre of the world's largest continental area. It offers a unique opportunity for studying the processes of intra-cratonic rifting. The BEST project (Baikal Explosion Seismic Transects) comprises two deep seismic profiles at the southern end of Lake Baikal. The field project was carried out in October 2002 after a pilot project in September 2001. The aim of the project is to determine the crustal and upper mantle seismic velocity structure. The two profiles are: (1) a 360 km long, NS-trending profile across the rift zone from the Mongolian border to Cheremhovo, and (2) a 360 km long EW-trending profile along-strike of the rift zone at the northern shore of the lake into the Tunka depression to the Mongolian border. 180 Texan one-component seismographs were deployed along each profile. The primary seismic sources were 13 explosions in boreholes, each with a 500 to 3000 kg charge. Supplementary sources were airgun shots in the lake (one 100 l airgun on profile 1, one 30 l airgun on profile 2) and the supervibrator located at Babushkin near the cross point between the two profiles at the shore of the lake. We present the preliminary modelling results of the BEST data from tomographic inversion of first arrival times and 2D ray tracing modelling of the seismic velocity structure of the crust and uppermost mantle. The models show: (1) A pronounced sedimentary graben structure to depths of at least 10 km; (2) No significant crustal thinning below the rift zone; (3) Strong seismic reflectivity underneath the rift zone, indicative of pronounced magmatic intrusion into the lower crust, despite the non-volcanic appearance of much of the rift zone; (4) No sign of significant reduction of the seismic Pn wave velocity; and (5) No clear indication of continuation of the cratonic crust underneath the rift zone in disagreement with a recently published model of the rifting process. Instead the model is in agreement with a model in which the rift zone developed in the suture between the Siberian Craton and the Palaeozoic fold belt to the SE of Lake Baikal.

Nielsen, C. A.; Thybo, H.; Jensen, M. M.; Ross, A.; Suvorov, V. D.; Emanov, A.; Seleznev, V.; Tatkov, G.; Perchuc, E.; Gazcynski, E.

2003-12-01

347

Insights into rifting from SKS splitting and receiver functions: examples from Ethiopia.  

NASA Astrophysics Data System (ADS)

Rifting in northern Ethiopia represents a transition from continental- to incipient oceanic- rifting. In an effort to obtain a better understanding of the role of asthenospheric dynamics in this transition, we re-analyse and enhance SKS splitting at stations distributed along the E.African Rift. New observations of shear-wave splitting and receiver functions are investigated at two Ethiopian stations near Addis Ababa, one broad-band (FURI) and the other short-period (AAE) close to the rift. Stations on or near the E. African Rift show the polarisation of the fast shear-wave (? ) aligned parallel to the rift axis. However, the magnitude of the splitting increases northward and may be explained by the higher melt production observed in the Ethiopian part of the rift. Stations in Kenya show smallest splitting values ( ~ 1s.), whilst the Djibouti station, ATD, shows the largest splitting (1.6s.). The Ethiopian results show a constancy in ? t and ? with respect to backazimuth, thus suggesting a single anisotropic layer beneath the stations. There is no observed correlation in ? with APM direction. Shear-wave splitting in local events recorded at FURI also reveal anisotropy in the crust with a similar orientation to the SKS results. Receiver function analysis at FURI and AAE supports evidence from refraction experiments of thick crust ( ~38km) in the region of continental rifting. There is also evidence for a deeper interface, ~60km below Moho, in the broadband FURI data, but not in the short-period AAE data, thus suggesting this may not be a sharp interface. This interface may mark the base of the lithosphere in this region. Azimuthal asymmetry in the receiver functions suggests the layer is dipping away from the rift and anisotropy may be affecting the results. One interpretation is that the anisotropy is due to aligned melt in upper c.90km of lithosphere. A <1% volume fraction of melt aligned in vertical ellipsoidal pockets generates sufficient splitting to explain the data. The deeper interface seen in the receiver functions could mark the onset of melting so might not be a sharp seismic boundary. Alternatively, the anisotropy may be due to the alignment of olivine parallel to the ridge axis as material flows laterally to fill the gap caused by lithospheric extension. Assuming a uniform magnitude of anisotropy this would suggest a northward thickening anisotropic layer beneath the rift. Current broadband seismic experiments (1999-2003) in Ethiopia will allow us to test these ideas.

Ayele, A.; Kendall, M.; Stuart, G.

2001-12-01

348

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

349

Genetic features of petroleum systems in rift basins of eastern China  

USGS Publications Warehouse

Most oil-bearing basins in eastern China are Mesozoic-Cenozoic continental rifts which have played a habitat for oil and gas in China. Investigation of the petroleum systems may give a better understanding of the oil and gas habitats in these basins. Of the essential elements of the petroleum system, the source rock is the most important in rift basins. However, rift tectonic evolution controls all the essential elements and processes nevessary for a petroleum system. A four stage evolution model is suggested for the controls in the rift basin. A rift basin may consist of sub-basins, depressions, sub-depressions, and major, moderate, and minor uplifts. A depression or sub-depression has its own depocentre (mainly occupied by source rock) and all kinds of lacustrine sediments, and thus has all the essential elements of a petroleum system. However, only those depressions or sub-depressions which are rich in organic matter and deeply buried to generate oil and gas form petroleum systems. Immature oil, another characteristic, complicates the petroleum system in the rift basins. Three types of oil and gas habitats are described as a result of this analysis of the petroleum systems of the 26 largest oil and gas fields discovered in eastern China rift basins: uplifts between oil source centres are the most prospective areas for oil and gas accumulations, slopes connecting oil source centres and uplifts are the second, and the third type is subtle traps in the soil source centre.Most oil-bearing basins in eastern China are Mesozoic-Cenozoic continental rifts which have played a habitat for oil and gas in China. Investigation of the petroleum systems may give a better understanding of the oil and gas habitats in these basins. Of the essential elements of the petroleum system, the source rock is the most important in rift basins. However, rift tectonic evolution controls all the essential elements and processes necessary for a petroleum system. A four stage evolution model is suggested for the controls in the rift basin. A rift basin may consist of sub-basins, depressions, sub-depressions, and major, moderate, and minor uplifts. A depression or sub-depression has its own depocentre (mainly occupied by source rock) and all kinds of lacustrine sediments, and thus has all the essential elements of a petroleum system. However, only those depressions or sub-depressions which are rich in organic matter and deeply buried to generate oil and gas form petroleum systems. Immature oil, another characteristic, complicates the petroleum system in the rift basins. Three types of oil and gas habitats are described as a result of this analysis of the petroleum systems of the 26 largest oil and gas fields discovered in eastern China rift basins: uplifts between oil source centres are the most prospective areas for oil and gas accumulations, slopes connecting oil source centres and uplifts are the second, and the third type is subtle traps in the oil source centre.

Qiang, J.; McCabe, P. J.

1998-01-01

350

Earthquakes along the East African Rift System: A multi-scale, continent-wide perspective  

NASA Astrophysics Data System (ADS)

Based on a comprehensive dataset of precisely determined depths of 121 large to moderate-sized earthquakes along and near the entire East African Rift system (EARS), there are three distinct patterns in focal depths which seem to reflect progressive stages in the development of the largest active rift in the world. First, away from both ends of the western, younger branch of the EARS, very large (MW ?7) earthquakes occurred in the top 15 km of the crust where surficial expressions of rifting are yet to appear. Curiously there are unusually deep aftershocks reaching down to 33±4 km. Second, under well-developed but amagmatic rift segments, focal depths show a bimodal distribution, with peaks centered near depths of about 15±5 km and 35±5 km. This pattern is present both under the main axis of the EARS where rift basins have lengths approaching 1,000 km, and beneath isolated rift segments ten times shorter in length. Underside reflections off the Moho indicate that at least part of the second peak in seismicity is due to mantle earthquakes down to 44±4 km, attesting to high differential stress in the mantle lithosphere which is capable of accumulating seismogenic elastic strains (the “jelly sandwich” rheology). Third, beneath magmatic segments of well-developed rifts, seismicity is largely confined to the upper 15 km of the crust, akin to the pattern along mid-ocean ridges where plastic flow due to high temperature inhibits accumulation of shear stress deep in the lithosphere.

Yang, Z.; Chen, W.

2009-12-01

351

Earthquakes along the East African Rift System: A multiscale, system-wide perspective  

NASA Astrophysics Data System (ADS)

On the basis of a comprehensive data set of precisely determined depths of 121 large to moderate-sized earthquakes along and near the entire East African Rift System (EARS), there are three distinct patterns in focal depths which seem to correlate with progressive stages in the development of the largest active rift in the world. First, away from both ends of the western, younger branch of the EARS, very large (Mw ? 7) earthquakes occurred in the top 15 km of the crust where surficial expressions of rifting are yet to appear. Curiously, there are unusually deep aftershocks reaching down to 35 ± 3 km. Second, under well-developed but amagmatic rift segments, focal depths show a bimodal distribution, with peaks centered near depths of about 15 ± 5 km and 35 ± 5 km. This pattern is present both under the main axis of the EARS, where rift zone have lengths approaching 1000 km, and beneath rift units 10 times shorter in length. Underside reflections off the Moho indicate that at least part of the second peak in seismicity is due to mantle earthquakes down to 44 ± 4 km, attesting to high differential stress in the mantle lithosphere which is capable of accumulating seismogenic, elastic strain (the "jelly sandwich" rheology). Third, beneath magmatic segments of well-developed rifts, seismicity is largely confined to the upper 15 km of the crust as observed previously, akin to the pattern along mid-ocean ridges where plastic flow due to high temperature inhibits accumulation of shear stress deep in the lithosphere.

Yang, Zhaohui; Chen, Wang-Ping

2010-12-01

352

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

NASA Astrophysics Data System (ADS)

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

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

2009-04-01

353

Stress Rotation in the Kachchh Rift Zone, Gujarat, India  

NASA Astrophysics Data System (ADS)

Double difference relocations of the 1402 Kachchh events (2001 2006) clearly delineate two fault zones viz. south-dipping North Wagad fault (NWF) and almost vertical Gedi fault (GF). The relocated focal depths delineate a marked variation of 4 and 7 km in the brittle-ductile transition depths beneath GF and NWF, respectively. The focal mechanism solutions of 464 aftershocks (using 8 12 first motions) show that the focal mechanisms ranged between pure reverse and pure strike-slip except for a few pure dip-slip solutions. The stress inversions performed for five rectangular zones across the Kachchh rift reveal both clockwise and anticlockwise rotation (7 32°) in the ?1 orientation within the rupture zone, favoring a heterogeneous stress regime with an average N-S fault normal compression. This rotation may be attributed to the presence of crustal mafic intrusives (5 35 km depth) in the rupture zone of the 2001 Bhuj main shock. Results suggest a relatively homogeneous stress regime in the GF zone favoring strike-slip motion, with a fault normal N-S compression.

Mandal, Prantik

2008-07-01

354

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

355

[Rift Valley Fever: veterinary aspects and impact for human health].  

PubMed

Rift Valley fever (RVF) is an arboviral zoonosis affecting a wide range of animal species as well as humans. Clinical incidence in domestic ruminants is high with infection causing abortions in pregnant animals and high mortality rates in newborns. In humans, clinical disease appears in about 50% of infected individuals. Human illness is characterized by dengue-like symptoms with severe complications including encephalitis, retinitis, hemorrhagic fever and death occurring in 1 to 3% of cases. During epidemic outbreaks, transmission between animals or from animals to humans is mainly by direct contact with infected biological material. Under these conditions, mosquito transmission probably plays a greater role in maintaining the enzootic cycle and initiating epizootic and epidemic outbreaks during the periods of heavy rainfall. The last epidemic outbreak of RVF in Kenya, Somalia, Tanzania and Sudan in 2006-2007 killed more than 4,000 ruminants and 600 humans. After confirmed diagnosis of one human case in 2007 in Comoros, an epidemiological survey was carried out in ruminant livestock in Mayotte. Results indicated that the RVF virus has been circulating on the island since 2005. In addition, serum samples collected from patients presenting dengue-like symptoms confirmed approximately 10 cases of human infection in 2007-2008. These results suggest low-level circulation of the RVF virus in Mayotte with weak impact on human and animal health. An assessment of future risk for the island is presented. PMID:19725386

Cêtre-Sossah, C; Albina, E

2009-08-01

356

Sequence stratigraphy in Bohai rift basin lacustrine environments  

SciTech Connect

Sequence stratigraphic studies of the Bohai rift basin have established seven (7) unconformity-bounded seismic sequences in Paleogene lacustrine sediments. Studies of microfossils, sedimentary facies, and shoreline onlaps show these sequences can be further divided into twelve-eighteen transgression/regression events. Two of these events, marked by high contents of salinity and rises of lake level, can be correlated to the global eustatic rises at 48-44 and 35-30 MY. Others seem to be of mainly tectonic and paleoclimatic controlled and are time-diachronous with the global sea level variation. Distinct shelf breaks have not been observed on seismic, and the basin margins appear to have evolved from steep to gentle ramp geometry. Rapid lateral facies changes have been attributed to multi-delta inputs around basin. Systems tracts identified on seismic include fan deltas, deep lacustrine fans, and incised valleys in lowstands and regressive deltas in highstands. Contrary to open marine environments, the highstand systems tracts are represented by thick mudstone sections as a result of constantly ample supply of sediments. Drilling results have demonstrated that reservoir rock facies are mainly in highstand and highstand systems tracts, and source rock facies are largely in transgressive systems tracts.

Mi Lijun; Cao Wenxian (China Offshore Oil Bohai Corp., Tanggu, Tianjin (China)); Chen, P. (Amoco Production Co., Houston, TX (United States))

1996-01-01

357

Block rotations in the Rio Grande Rift, New Mexico  

SciTech Connect

Paleomagnetic data from 13 sites of intrusive and extrusive rocks associated with the Oligocene Espinaso Formation around the Ortiz mountains, New Mexico, indicate mean magnetic directions of I = 42.1 deg and D = 337.8 deg, with a corresponding pole position of 67.9 deg N latitude and 142.3 deg E longitude. From comparison with the expected Oligocene direction, 17.8 + or - 11.3 deg of counterclockwise rotation is found, and the similar rotations for Oligocene and Pliocene age rocks suggests that motion has occurred in the past 5 m.y. Data support the previously suggested diamond-shaped crustal block extending from the San Luis basin southward to the Albuquerque basin bounded by well-defined recently active fault zones. The size of the block, the counterclockwise nature of rotation and the timing of motion are supported by geologic and structural data. The counterclockwise rotation of the block has resulted in uplift at the acute ends of the block and subsidence at the obtuse ends, and the driving mechanism may be left slip along the rift. 47 references.

Brown, L.L.; Golombek, M.P.

1986-06-01

358

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

359

Observations on rift valley fever virus and vaccines in Egypt  

PubMed Central

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

2011-01-01

360

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.

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

2012-01-01

361

Climate-disease connections: Rift Valley Fever in Kenya.  

PubMed

All known Rift Valley fever(RVF) outbreaks in Kenya from 1950 to 1998 followed periods of abnormally high rainfall. On an interannual scale, periods of above normal rainfall in East Africa are associated with the warm phase of the El Niño/Southern Oscillation (ENSO) phenomenon. Anomalous rainfall floods mosquito-breeding habitats called dambos, which contain transovarially infected mosquito eggs. The eggs hatch Aedes mosquitoes that transmit the RVF virus preferentially to livestock and to humans as well. Analysis of historical data on RVF outbreaks and indicators of ENSO (including Pacific and Indian Ocean sea surface temperatures and the Southern Oscillation Index) indicates that more than three quarters of the RVF outbreaks have occurred during warm ENSO event periods. Mapping of ecological conditions using satellite normalized difference vegetation index (NDVI) data show that areas where outbreaks have occurred during the satellite recording period (1981-1998) show anomalous positive departures in vegetation greenness, an indicator of above-normal precipitation. This is particularly observed in arid areas of East Africa, which are predominantly impacted by this disease. These results indicate a close association between interannual climate variability and RVF outbreaks in Kenya. PMID:11426274

Anyamba, A; Linthicum, K J; Tucker, C J

2001-01-01

362

The 1789 Rifting Event in the Hengill Volcanic System, SW-Iceland  

NASA Astrophysics Data System (ADS)

A volcano-tectonic episode in South Iceland in the 18th century was initiated by rifting and eruptions along the 30 km long Laki fissure within the Eastern Volcanic Zone (EVZ) and an eruption at the northern Reykjanes Reykjanes Ridge in 1783, forming the island of Nýey. Severe earthquakes within the South Iceland Seismic Zone (SISZ) followed in 1784. The sequence ended in 1789, with a rifting event in the Hengill volcanic system, located at the junction of the Western Volcanic Zone (WVZ) with the SISZ. The Hengill system consists of a central volcano, Mt. Hengill, which is transected by a 60 km long SW-NE striking rift zone. The rift north of Mt. Hengill forms a 6 km wide graben, partly filled with the lake Thingvallavatn. The central volcano is marked by high volcanic production, occurrences of acid rocks and a high temperature geothermal field. Rifting events within the Hengill system are considered to be mainly associated with crustal dike propagation as dikes have only breached the surface in an eruption four times during the Holocene. Extension rate is signified by tension gashes of about 70 m aggregate width across the rift zone in a 10,000 year old lava and by a maximum throw of about 40 m at Thingvellir near its western margin. In 1789, settlements only existed at the distal ends of the Hengill rift zone, at Selvogur and Thingvellir. A fairly detailed contemporary description of the rifting event exists, written by the vicar of Thingvellir. He states that the rifting 1789 began in early June and lasted 10 days with considerable earthquake activity. The central part of the Thingvellir graben subsided and the lake transgressed beyond its former shore while the margins of the graben were elevated so that water wells ran dry. Trails across the main boundary faults of Almannagjá and Hrafnagjá became impassable for horses. Old surface fissures widened and new formed. Fault movement was also observed southwest of lake Thingvallavatn, where a fault subsidence was reported being equal to a person in height, obviously considered rather spectacular for its size. There is mentioning of new hot springs having formed south of Mt. Hengill and a collapse of houses at the southwest end of the rift zone in Selvogur. A peat bog at the northeastern shore of the lake overlain by lake mud at 1.4-1.5 m depth indicates the amount of subsidence during the rifting event. A man-made rockwall submerged near the north shore of lake Thingvallavatn lies at a depth of 2.6 m, of which about 0,2 m are probably due to latent creep spanning the last 2 centuries. As there is no mentioning of subsequent events within the Hengill system, the 1789 rifting is considered a singular event consisting of dike propagation from the Hengill central volcano into both segments of the rift zone.

Saemundsson, K.

2006-12-01

363

Receiver function study of the rifted margin of the Gulf Coast Plain: A Pilot Project  

NASA Astrophysics Data System (ADS)

The Gulf Coast Plain (GCP) is one of the deepest sedimentary basins in the world; as a result little is known about the basement structure beneath this region. The few refraction seismic investigations of the GCP found sediment depths of 15 to 20 km, underlain by severely attenuated continental or transitional crust. A 2-D refraction investigation along the Texas-Louisiana Border, found the crystalline continental crust to be as little as 10 km thick, underlain by an anomalous lower crust/upper mantle body interpreted to be depleted mantle; a “rift pillow” developed during the Late Paleozoic through Early Mesozoic rifting. This rift pillow coincides with the magnetic high that parallels the coast line along the entire Texas GCP. This poster will present new receiver functions (RF) results using data from stations of the EarthScope Backbone array throughout the GCP and a temporary pilot deployment of five broadband instruments between Junction, TX and San Antonio, TX that took place in 2008. The stations at Kingsville, TX (KVTX) and Hockley, TX (HKT) are close to the coast. Receiver functions for KVTX have a very strong P20s phase that is likely from the sediment-basement contact and a Moho Ps phase at about 45 km deep. This depth to the Moho is greater than might be expected for a coastal setting. The 45 km depth is, however, similar to that found for the base of the “rift pillow” observed in the Northern GCP. KVTX is located in the magnetic high anomaly that was associated with the rift pillow in the Northern GCP. The station at Junction (JCT) appears to have about a 42 km depth to the Moho which is typical of a station from the stable continental (cratonic) setting. The station at Nacogdoches, TX (NATX) is situated in older Gulf Coast sediments in a similar position relative to GCP rifting as would be expected a few km outboard of San Antonio. The 36 km depth to the Moho estimated for KVTX appears shallower than those Moho depths estimated for JCT and KVTX which could be the result of attenuated, rifted continental crust that is inboard of the anomalous “rift pillow”. Receiver functions (RFs) computed for the temporary deployment between JCT and San Antonio appear to have a uniform depth to Moho of about 45 km. These stations probably did not reach the point at which rifting thinned the crust. Refraction results from a line shot by Cram (1962) found about a 34 km depth to the Moho at a point about half way between San Antonio and the coastline. So the severe crustal attenuation appears to be outboard of San Antonio.

Brundrett, M.; Gurrola, H.; Pulliam, J.; Stern, R. J.; Anthony, E. Y.; Keller, G. R.; Gao, S. S.; Mickus, K. L.

2009-12-01

364

Normal Fault System of the Western Corinth Rift (Greece): 3D Geometry, Kinematics and Evolution  

NASA Astrophysics Data System (ADS)

We use an integrated approach from field analyses (stratigraphy, structural geology, seismicity) to 3D geomodelling of the Corinth rift to characterize the initiation and early stages of continental rifting. The Gulf of Corinth is a rapidly opening rift superimposed on the external Hellenides orogenic belt and lying above an active subduction zone. The early Corinth rift is today uplifted and spectacularly exposed along the southern margin of the Gulf. The Plio-Pleistocene syn-rift stratigraphy and normal fault system have been mapped across an area of 600 km2. Major normal faults are planar and have a low level of connectivity. Rare listric intra-basinal faults are observed. Fault strikes vary from N070° to N120° with an average strike of N110°. Fault dips, predominantly toward the north, vary from 45° to 65°. Extension directions vary from N355° to N020°. Total extension across the whole rift (offshore and onshore) is estimated to be 10km (b=1.2). Fault activity and depocentres migrated north with time as the rift narrowed. Syn-rift stratigraphy records three phases of rifting. Extension accelerated significantly at around 1.4 Ma and around 800-600 ka. Uplift of the south flank started sometime 800-600 ka. Field data alone are not sufficient to constrain a valid 3D model of geometry and displacement distribution on the normal fault network, in particular because of their irregular and often sparse distribution. 3D model construction therefore requires the integration of theoretical assumptions, in particular, (1) a coherent distribution of displacement across the fault network, (2) an acceptable ratio of displacement to fault length, (3) a consistent level of connectivity. In the initial onland model, faults surfaces are extended to a depth of 2 km. In contrast, the surface trace of active fault surfaces can be linked with earthquake nodal planes located at depths of 5-8 km depth. These models suggest that the north dipping zone of micro-seismicity in the western Gulf may not represent a extensional decollement zone.

Ford, M.; Le Carlier de Veslud, C.; Lyon-Caen, H.; Rohais, S.; Moullard, M.; Williams, E. A.

2008-12-01

365

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

366

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

367

Oligo-Miocene rift of Sardinia and the early history of the Western Mediterranean Basin  

NASA Astrophysics Data System (ADS)

The geodynamic evolution of the Western Mediterranean Basin, in spite of many studies, is still uncertain. There is some consensus for interpreting this basin as a kind of small oceanic marginal basin. Its opening has generally been related to a subduction process which was active during the Oligocene-Miocene somewhere east of Sardinia-Corsica1-7. As the margins of the basin are deeply buried below Miocene-to-present sediments, direct lithological and stratigraphical data which could explain the events responsible for its formation are rare8-10 or missing altogether. To obtain such data, detailed field studies have been undertaken in Sardinia (Fig. 1), and the first results are presented here. This approach is justified by the fact that in that island, Oligocene and Miocene sediments were deposited in a rift (fossa tettonica sarda of Verdabasso11), which is the easternmost arm of the complex rift system that affected the European plate during Oligocene and Miocene times. One of these arms evolved towards a small oceanic basin-the Western Mediterranean or Algero-Provençal Basin-while others such as the Gulf of Valencia and the Sardinia rift aborted and remained at the rift stage. Its exceptional exposures make it possible to examine the Sardinia rift to clarify the sequence of events which created it, and to establish a sedimentological model which we believe is directly applicable to the Western Mediterranean Basin.

Cherchi, A.

1982-08-01

368

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

369

Inland extent of the Weddell Sea Rift imaged by new aerogeophysical data  

NASA Astrophysics Data System (ADS)

The Weddell Sea Rift was a major focus for Jurassic extension and magmatism during the early stages of Gondwana break-up and underlies the Weddell Sea Embayment, which separates East Antarctica from a collage of crustal blocks in West Antarctica. Newly-collected aerogeophysical data over the catchments of Institute and Möller ice streams reveal the inland extent of the Weddell Sea Rift against the Ellsworth-Whitmore block and a hitherto unknown major left-lateral strike slip boundary between East and West Antarctica. Aeromagnetic and gravity anomalies define the regional subglacial extent of Proterozoic basement, Middle Cambrian rift-related volcanic rocks, Jurassic intrusions and sedimentary rocks of inferred post-Jurassic age. 2D and 3D magnetic depth-to-source estimates were used to help constrain joint magnetic and gravity models for the region. The models reveal that Proterozoic crust similar to that exposed at Haag Nunataks, extends southeast of the Ellsworth Mountains to the margin of the Coastal Basins. Thick granitic Jurassic intrusions are modelled at the transition between the Ellsworth-Whitmore block and the thinner crust of the Weddell Sea Rift and within the Pagano Shear Zone. The crust beneath the inland extension of the Weddell Sea Rift is modelled as being either ~ 4 km thinner compared to the adjacent Ellsworth-Whitmore block or as underlain by an up to 8 km thick mafic underplate.

Jordan, Tom A.; Ferraccioli, Fausto; Ross, Neil; Corr, Hugh F. J.; Leat, Philip T.; Bingham, Rob G.; Rippin, David M.; le Brocq, Anne; Siegert, Martin J.

2013-02-01

370

The western margin of the Rio Grande rift in northern New Mexico: An aborted boundary?  

SciTech Connect

The northwestern margin of the Espanola basin, part of the Rio Grande rift in northern New Mexico, is characterized by a zone >17 km wide of oblique-slip faults that off-set upper Paleozoic and Mesozoic strata of the eastern Colorado Plateau from Eocene and younger sedimentary rocks of the rift. Along this margin, a reasonably complete section of pre- and synrift Tertiary sediments is exposed. Combined interpretations of seismic reflection, seismic refraction, gravity, and geologic data acquired along a profile perpendicular to this boundary define the geometry of faulting, possible rotation of sedimentary units, and stratigraphy of rift fill. Vertical separation on the westernmost major fault, assumed to be the bounding fault between the rift and the Colorado Plateau, is <500 m; separation on other faults in the zone is <200 m. Thus the northwestern part of the Espanola basin ({open_quotes}Abiquiu embayment{close_quotes}) is a shallow platform rather than a deep rift basin. The embayment is separated from the main Espanola basin by the east-northeast-striking Embudo transfer fault, which appears to act as the northern bounding fault of the main basin. 42 refs., 10 figs., 3 tabs.

Baldridge, W.S. [Los Alamos National Lab., NM (United States)] Ferguson, J.F. [Univ. of Texas, Dallas, TX (United States); Barile, L.W.; Wang, Bin [Purdue Univ., West Lafayette, IN (United States)] [and others

1994-12-01

371

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

2009-12-01

372

Curcumin Inhibits Rift Valley Fever Virus Replication in Human Cells*  

PubMed Central

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

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

2012-01-01

373

Rift Valley fever: Real or perceived threat for Zambia?  

PubMed

Rift Valley fever (RVF) in Zambia was first reported in 1974 during an epizootic of cattle and sheep that occurred in parts of Central, Southern and Copperbelt Provinces. In 1990, the disease was documented in nine districts of the provinces of Zambia. In the last two decades, there have been no reports of RVF. This long period without reported clinical disease raises questions as to whether RVF is a current or just a perceived threat. To address this question, World Organisation for Animal Health (OIE) disease occurrence data on RVF for the period 2005-2010 in the Southern Africa Development Community (SADC) was analysed. From the analysis, it was evident that most countries that share a common border with Zambia had reported at least one occurrence of the disease during the period under review. Due to the absence of natural physical barriers between Zambia and most of her neighbours, informal livestock trade and movements is a ubiquitous reality. Analysis of the rainfall patterns also showed that Zambia received rains sufficient to support a mosquito population large enough for high risk of RVF transmission. The evidence of disease occurrence in nearby countries coupled with animal movement, and environmental risk suggests that RVF is a serious threat to Zambia. In conclusion, the current occurrence of RVF in Zambia is unclear, but there are sufficient indications that the magnitude of the circulating infection is such that capacity building in disease surveillance and courses on recognition of the disease for field staff is recommended. Given the zoonotic potential of RVF, these measures are also a prerequisite for accurate assessment of the disease burden in humans. PMID:23327389

Dautu, George; Sindato, Calvin; Mweene, Aaron S; Samui, Kenny L; Roy, Polly; Noad, Robert; Paweska, Janusz; Majiwa, Phelix A O; Musoke, Antony

2012-06-20

374

Innate Immune Response to Rift Valley Fever Virus in Goats  

PubMed Central

Rift Valley fever (RVF), a re-emerging mosquito-borne disease of ruminants and man, was endemic in Africa but spread to Saudi Arabia and Yemen, meaning it could spread even further. Little is known about innate and cell-mediated immunity to RVF virus (RVFV) in ruminants, which is knowledge required for adequate vaccine trials. We therefore studied these aspects in experimentally infected goats. We also compared RVFV grown in an insect cell-line and that grown in a mammalian cell-line for differences in the course of infection. Goats developed viremia one day post infection (DPI), which lasted three to four days and some goats had transient fever coinciding with peak viremia. Up to 4% of peripheral blood mononuclear cells (PBMCs) were positive for RVFV. Monocytes and dendritic cells in PBMCs declined possibly from being directly infected with virus as suggested by in vitro exposure. Infected goats produced serum IFN-?, IL-12 and other proinflammatory cytokines but not IFN-?. Despite the lack of IFN-?, innate immunity via the IL-12 to IFN-? circuit possibly contributed to early protection against RVFV since neutralising antibodies were detected after viremia had cleared. The course of infection with insect cell-derived RVFV (IN-RVFV) appeared to be different from mammalian cell-derived RVFV (MAM-RVFV), with the former attaining peak viremia faster, inducing fever and profoundly affecting specific immune cell subpopulations. This indicated possible differences in infections of ruminants acquired from mosquito bites relative to those due to contact with infectious material from other animals. These differences need to be considered when testing RVF vaccines in laboratory settings.

Nfon, Charles K.; Marszal, Peter; Zhang, Shunzhen; Weingartl, Hana M.

2012-01-01

375

MONITORING OF EARTHQUAKES ACTIVITIES ALONG THE SYRIAN RIFT SYSTEM ( LEFT-LATERAL) BY USING REMOTE SENSING AND GIS DATABASE  

Microsoft Academic Search

Northwest of Syria is part of one of the very active deformation belt on the Earth today. This area and the western part of Syria are located along the great rift system (Left-Lateral or African- Syrian Rift System). Those areas are tectonically active and caused a lot of seismically events. The AL-Ghab Graben complex is situated within this wide area

MOUTAZ DALATI

376

Oblique-slip deformation in extensional terrains: A case study of the lakes Tanganyika and Malawi Rift Zones  

Microsoft Academic Search

The East African Rift system (EAR) is the archetypal continental rift and a widely proposed analogue for the early stages of evolution of passive continental margins. The three-dimensional structure of parts of the EAR has been recently elucidated by a multifold seismic (MFS) survey of Lakes Tanganyika and Malawi (Project PROBE). Analysis of fault geometries displayed on the PROBE MFS

Deborah L. Scott; Michael A. Etheridge; Bruce R. Rosendahl

1992-01-01

377

Transition from passive to active rifting: Relative importance of asthenospheric doming and passive extension of the lithosphere  

Microsoft Academic Search

We present quantitative modeling results of the dynamic interplay of passive extension and active convective thinning of the mantle lithosphere beneath intracontinental rift zones investigating the relative importance of thermal buoyancy forces associated with asthenospheric doming and far-field intraplate stresses on the style of rifting. To this aim we employ a twodimensional numerical code based on a finite element method

Ritske S. Huismans; Yuri Y. Podladchikov; Sierd Cloetingh

2001-01-01

378

Lake-groundwater relationships and fluid-rock interaction in the East African Rift Valley: isotopic evidence  

Microsoft Academic Search

The assessment of water resources in the Rift Valley environment is important for population, agriculture and energy-related issues and depends on a good understanding of the relationship between freshwater lakes and regional groundwater. This can be hampered by the amount of fluid-rock interaction which occurs throughout the rift, obscuring original hydrochemical signatures. However, O and H stable isotope ratios can

W. George Darling; Berhanu Gizaw; Musa K. Arusei

1996-01-01

379

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

Microsoft Academic Search

The combined effects of stretching and magmatism permanently modify crustal structure in continental rifts and volcanic passive margins. The Red Sea-Gulf of Aden-Ethiopian rift triple junction zone provides a unique opportunity to examine incipient volcanic margin formation above or near an asthenospheric upwelling. We use gravity inversions and forward modelling to examine lateral variations in crust and upper mantle structure

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

2005-01-01

380

Upper mantle seismic structure beneath the Ethiopian hot spot: Rifting at the edge of the African low-velocity anomaly  

Microsoft Academic Search

The Miocene-Recent East African Rift in Ethiopia subaerially exposes the transitional stage of rifting within a young continental flood basalt province. As such, it is an ideal study locale for continental breakup processes and hot spot tectonism. We combine teleseismic traveltime data from 108 seismic stations deployed during two spatially and temporally overlapping broadband networks to present detailed tomographic images

I. D. Bastow; A. A. Nyblade; G. W. Stuart; T. O. Rooney; M. H. Benoit

2008-01-01

381

The crust and upper mantle beneath the Rio Grande Rift and surrounding regions inferred from ambient noise and earthquake tomography  

Microsoft Academic Search

The Rio Grande Rift (RGR), located between the Colorado Plateau and the Great Plains, is a major continental rift that has slowly opened over the past 30 My. The mechanism of extension and its relations to the structure of the crust and upper mantle remain incompletely understood. Previous seismic studies based on data from the LA RISTRA experiment imaged structures

W. Shen; Y. Yang; M. H. Ritzwoller; J. Pulliam; S. P. Grand

2009-01-01

382

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

383

The distribution of basaltic volcanism on Tenerife, Canary Islands: Implications on the origin and dynamics of the rift systems  

Microsoft Academic Search

One of the most characteristic features of volcanic islands is the existence of rift zones defined commonly as orientated eruptive fissures or parallel rows of elongate cinder cones and dyke swarms. Occasionally, these rifts can appear at the birth of the volcanic island and persist until the last episodes of its constructions, controlling the form and structure of the island

A. Geyer; J. Martí

2010-01-01

384

Crust and Upper Mantle Structure of the Colorado Plateau - Rio Grande Rift - Great Plains Transect Using Teleseismic Receiver Functions  

Microsoft Academic Search

We investigate the crust and upper mantle structure of the Colorado Plateau - Rio Grande Rift - Great Plains transect using receiver functions calculated from teleseismic arrivals recorded by project LA RISTRA (Colorado PLAteau, Rio Grande RIft, Great Plains Seismic TRAnsect). The Ristra array was a 57 station PASSCAL broadband seismic array extending 950 km from Lake Powell, UT to

D. C. Wilson; R. Aster; J. Ni; J. Schlue; S. Grand; S. Baldridge; S. Semken; W. Gao

2001-01-01

385

Evidence for a Major Late Precambrian Tectonic Event (RIFTING?) in the Eastern Midcontinent Region, United States  

NASA Astrophysics Data System (ADS)

Recently acquired gravity and aeromagnetic data delineate a large linear gravity anomaly which extends through eastern Kentucky and Tennessee and coincides with a zone of complex, high-amplitude magnetic anomalies. Basement lithologies in the area can be interpreted as a bimodal volcanic suite which is locally peralkaline in nature. These volcanics appear to be metamorphosed where they lie east of the Grenville front, suggesting they predate the Grenville metamorphic event. The available gravity, aeromagnetic, seismic refraction, and petrologic data, along with regional correlations, suggest that the best tectonic interpretation of these data is that a Keweenawan rift zone extended through the area. This rift can be roughly outlined by the gravity high, which is locally offset, suggesting the presence of transform faults. The boundaries of this rift have been locally reactivated and, in fact, a recent earthquake was located along its western boundary in northern Kentucky.

Keller, G. R.; Bland, A. E.; Greenberg, J. K.

1982-04-01

386

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

387

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

388

Oceanic rift propagation - a cause of crustal underplating and seamount volcanism  

SciTech Connect

We present the first seismic reflection data across a pseudofault zone, the trace of a propagating rift away from a spreading axis. Strong reflections from the crust-mantle transition are discontinuous across the pseudofault. Deeper reflections, which originate near the base of the crust formed at the propagator tip, dip beneath the older oceanic crust and become subhorizontal. They are interpreted to represent the lower limit of an underplated subcrustal plutonic complex. An anomalously smooth basement surface indicative of massive lava flows and a 1200-m-high seamount are above the underplated zone. The sill complex extends tens of kilometres ahead of, and off-axis from, the former location of the propagating rift, indicating that the magmatic supply to the propagator tip was unusually vigorous. Other seamounts in the northeast Pacific lie close to pseudofaults and may have formed as a result of rift propagation.

Calvert, A.J.; Hasselgren, E.A.; Clowes, R.M. (Univ. of British Columbia, Vancouver (Canada))

1990-09-01

389

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

SciTech Connect

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 change with depth of the amount or direction of the anisotropy found in the uppermost 400 m.

Hirn, A.; Magnier, S.A. (Institut de Physique du Globe, Paris (France))

1992-12-10

390

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

USGS Publications Warehouse

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

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

1990-01-01

391

Contrasted styles of rifting in the eastern Gulf of Aden: A combined wide-angle, multichannel seismic, and heat flow survey  

Microsoft Academic Search

Continental rifts and passive continental margins show fundamental along-axis segmentation patterns that have been attributed to one or a number of different processes: extensional fault geometry, variable stretching along strike, preexisting lithospheric compositional and structural heterogeneities, oblique rifting, and the presence or absence of eruptive volcanic centers. The length and width scales of the rift stage fault-bounded basin systems change

Sylvie Leroy; Francis Lucazeau; Elia d'Acremont; Louise Watremez; Julia Autin; Stéphane Rouzo; Nicolas Bellahsen; Christel Tiberi; Cynthia Ebinger; Marie-Odile Beslier; Julie Perrot; Philippe Razin; Frédérique Rolandone; Heather Sloan; Graham Stuart; Ali Al Lazki; Khalfan Al-Toubi; François Bache; Alain Bonneville; Bruno Goutorbe; Philippe Huchon; Patrick Unternehr; Khaled Khanbari

2010-01-01

392

Insights into initial stages of rifting from seismotectonics and SKS splitting in the North Tanzanian Divergence  

NASA Astrophysics Data System (ADS)

Magmatism and faulting are preponderant processes involved in continental rifting. Their interaction, relative importance, and dependence to the rheological properties of the lithosphere and to the timing of rifting, remain poorly known. To address this question, we have used the results from a seismological experiment, called SEISMO-TANZ (35 stations, broadband and enlarged-band), launched in the North Tanzanian Divergence (NTD) for 6 months in 2007. The region encompasses one of the youngest parts of the East African rift (EAR) and is characterized by the development of the rift into the Tanzanian craton. The NTD is often considered as non-volcanic compared to other places in EAR and the lithosphere is highly resistant. More than 2000 local earthquakes were recorded, highlighting active faults and one magmatic intrusion. Inherited structures play a key role as guides for dykes and slips. 26 Focal mechanisms (double-couple hypothesis) were obtained from P-wave polarities and indicate a transtensive deformation in the southern part of the region (Manyara rift). The stress inversion performed indicates a stable, well-determined ?3 axis striking ESE-WNW. From 25 teleseismic events recorded during the experiment, we have measured seismic anisotropy (SKS splitting) and present here our last results. Fast polarization directions are quite homogeneously NE-SW and delays times increase from the craton (W) to the Mozambique belt (E). Fossilized anisotropy and dykes or melt-filled lenses alignments would both explain the majority of these observations. We finally compare these results with other seismic anisotropy measurements made in EAR and with geodetic and seismotectonic analyses in order to better assess the origin of the strain pattern in this part of the rift, and to discuss the respective role of magmatism, faulting and fabrics in the extending lithosphere.

Albaric, J.; Barruol, G.; Deverchère, J.; Deschamps, A.; Perrot, J.; Tiberi, C.; Ferdinand, R. W.; Sue, C.; Le Gall, B.; Petit, C.

2010-12-01

393

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

394

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

NSDL National Science Digital Library

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

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

395

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

NASA Astrophysics Data System (ADS)

It has been proposed that some rifted continental margins have anomalous subsidence and that at break-up they were elevated at shallower bathymetries than the isostatic response predicted by classical rift models (McKenzie, 1978). The existence of anomalous syn- or early-pos