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1

Rift propagation  

NASA Technical Reports Server (NTRS)

A model for rift propagation which treats the rift as a crack in an elastic plate which is filled from beneath by upwelling viscous asthenosphere as it lengthens and opens. Growth of the crack is driven by either remotely applied forces or the pressure of buoyant asthenosphere in the crack and is resisted by viscous stresses associated with filling the crack. The model predicts a time for a rift to form which depends primarily on the driving stress and asthenosphere viscosity. For a driving stress on the order of 10 MPa, as expected from the topography of rifted swells, the development of rifts over times of a few Myr requires an asthenosphere viscosity of 10 to the 16th Pa s (10 to the 17th poise). This viscosity, which is several orders of magnitude less than values determined by postglacial rebound and at least one order of magnitude less than that inferred for spreading center propagation, may reflect a high temperature or large amount of partial melting in the mantle beneath a rifted swell.

Parmentier, E. M.; Schubert, G.

1989-01-01

2

Volcanism at rifts  

SciTech Connect

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

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

1989-07-01

3

Aplicaciones del láser de diodo en Odontología  

Microsoft Academic Search

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

Antonio Jesús

2005-01-01

4

Algunas aplicaciones de Cinematica Computacional a Navegacion  

E-print Network

Algunas aplicaciones de Cinem´atica Computacional a Navegaci´on Finat, J. Viloria, A., Gonzalo, M a un espacio en el que se consideran objetos m´oviles cuyas trayectorias se desean estimar. Este el seguimiento (segmentaci´on m´ovil), desarrollo de sistemas de control predictivos basados en la

Llanos, Diego R.

5

Continental rifting: a planetary perspective  

SciTech Connect

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

Muehlberger, W.R.

1985-01-01

6

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.

Giacomo Corti

7

Intracontinental Rifts As Glorious Failures  

NASA Astrophysics Data System (ADS)

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

Burke, K.

2012-12-01

8

Volcanism at Rifts.  

ERIC Educational Resources Information Center

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

White, Robert S.; McKenzie, Dan P.

1989-01-01

9

Rift Valley Fever  

NSDL National Science Digital Library

In this problem-based learning module, students focus on issues related to understanding the nature and transmission of the Rift Valley fever virus, a virus that causes the death of large numbers of livestock in Kenya and in much of sub-Saharan Africa, and can also infect humans. This module is part of Exploring the Environment.

2012-08-03

10

Modelado Conceptual de aplicaciones adaptivas y proactivas en OOH  

Microsoft Academic Search

La personalizacion de entornos ha suscitado un gran interes desde el punto de vista de los metodos de modelado conceptual de aplicaciones web. Ello es debido al efecto que tienen las pol´iticas de personalizacion sobre todo el ciclo de desarrollo de la aplicacion, desde la recogida de requisitos a la propia arquitectura de ejecucion. La inclusion de estrategias de personalizacion

Irene Garrigos; Cristina Cachero; Jaime Gomez

11

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

12

Rift basins - Origin, history, and distribution  

NASA Technical Reports Server (NTRS)

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

Burke, K. C.

1985-01-01

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

2011-06-01

14

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

Microsoft Academic Search

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

J. Versfelt; B. R. Rosendahl

1989-01-01

15

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

E-print Network

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

Paris-Sud XI, Université de

16

Arquitectura de Aplicaciones Multimedia Adaptables en Entornos Inalámbricos  

Microsoft Academic Search

Resumen— Las aplicaciones adaptables son un concepto clave a tener en cuenta para la transmisión de contenidos multimedia de tiempo real en entornos móviles e inalámbricos en los cuales las condiciones específicas que presenta la red son muy cambiantes y los problemas que aparecen no son únicamente debidos a la congestión -como ocurre en las redes fijas- sino debido también

Pedro M. Ruiz; Antonio F. Gómez-Skarmeta; Emilio J. García; Gregorio Martínez; Rafael Marín

17

Calving of large tabular icebergs from ice shelf rift systems  

Microsoft Academic Search

We used Interferometric Synthetic Aperture Radar to study the detachment process that allowed two large icebergs to calve from the Ross Ice Shelf, Antarctica. Time series of rift geometries indicate that rift widths increased steadily, whereas rift lengths increased episodically through several discrete rift-tip propagation events. We also conducted modeling experiments constrained by the observed rift geometry. Both the observations

Ian Joughin; Douglas R. MacAyeal

2005-01-01

18

Parga Chasma: Coronae and Rifting on Venus  

NASA Technical Reports Server (NTRS)

The majority of coronae (quasicircular volcano-tectonic features) are found along rifts or fracture belts, and the majority of rifts have coronae [e.g. 1,2]. However, the relationship between coronae and rifts remains unclear [3-6]. There is evidence that coronae can form before, after, or synchronously with rifts [3,4]. The extensional fractures in the rift zones have been proposed to be a result of broad scale upwelling and traction on the lower lithosphere [7]. However, not all rift systems have a significant positive geoid anomaly, as would be expected for an upwelling site [8]. This could be explained if the rifts lacking anomalies are no longer active. Coronae are generally accepted to be sites of local upwelling [e.g. 1], but the observed rifting is frequently not radial to the coronae and extends well beyond the coronae into the surrounding plains. Thus the question remains as to whether the rifts represent regional extension, perhaps driven by mantle tractions, or if the coronae themselves create local thinning and extension of the lithosphere. In the first case, a regional extension model should be consistent with the observed characteristics of the rifts. In the latter case, a model of lithospheric loading and fracturing would be more appropriate. A good analogy may be the propagation of oceanic intraplate volcanoes [9].

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

2005-01-01

19

Magmatism in a Cambrian Laurentian Plate Rift  

NASA Astrophysics Data System (ADS)

Evidences of the Cambrian Southern Oklahoma Aulacogen extend over 1000km from about Dallas out to the Uncompahgre Plateau in SW Colorado. The signature of this originally extensional feature can be traced geophysically, and in some places at the present surface, petrologically and temporally, by the presence of mafic rock. It appears to have been the intracontinental third arm of a plume-generated? triple junction which helped to dismember the southern part of Laurentia on the final break-up of a Neoproterozoic supercontinent. Other parts of Laurentia rifted away and are now found in the Precordillera of Argentina. Rift magmatism appears to have been concentrated nearer the plate edge during the breakup. Perhaps as much as 40,000 km3 of mostly subaerial silicic volcanics and shallow-seated granites overlay and filled the top of the rift in the area of SW Oklahoma. The rift fill below the silicic rocks is large, layered mafic complexes and smaller, layered, hydrous gabbros, the whole set appearing as a shallow AMCG complex. Unusually, direct rift sediments are not obvious. Furthermore, silicic and mafic rocks have identical Nd signatures. Finally, about 20 Ma after rifting ceased and later into the Paleozoic during sea incursion, overlying sediments are thickened 4X compared to equivalent units 100's of kms to the rift sides. This rift appears distinct from most modern rifts. Conclusions are 1) This was a hot, narrow rift; 2) Basaltic magmatism , not sedimentation, filled the rift; 3) Magmatic intensity varied along the rift strike; 4) Silicic rocks were generated mostly directly from new mantle-derived basalt liquids through fractionation, not melting of older crustal rocks; 5) Laurentian lithosphere was weak allowing centering of the Early/Middle Paleozoic large "Oklahoma" basin (pre-Anadarko) over the rift.

Gilbert, M. C.

2008-12-01

20

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

E-print Network

formed on continental crust, prior or after oceanic spreading [Ziegler, 1988]. Well developed examplesInversion tectonics during continental rifting: The Turkana Cenozoic rifted zone, northern Kenya B Gall, B., W. Ve´tel, and C. K. Morley (2005), Inversion tectonics during continental rifting

Brest, Université de

21

Un API para el desarrollo de aplicaciones paralelas 100% Java  

E-print Network

Un API para el desarrollo de aplicaciones paralelas 100% Java Antonio Talón Toledo 5º curso de://www.cs.rit.edu/~ark/pj.shtml) · PJ es software libre que se distribuye bajo los términos de la GNU General Public License (http sobre un cluster #12;· Web del autor: http://www.cs.rit.edu/~ark/pj.shtml · Descargas: binario y fuentes

Giménez, Domingo

22

Rift Valley fever outbreak, southern Mauritania, 2012.  

PubMed

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

Sow, Abdourahmane; Faye, Ousmane; Ba, Yamar; Ba, Hampathé; Diallo, Diawo; Faye, Oumar; Loucoubar, Cheikh; Boushab, Mohamed; Barry, Yahya; Diallo, Mawlouth; Sall, Amadou Alpha

2014-02-01

23

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

24

Sedimentology and significance of an early syn-rift paleovalley, Wadi Tayiba, Suez Rift, Egypt  

NASA Astrophysics Data System (ADS)

Wadi Tayiba is located along the western margin of the Hammam Faraun fault block, western Sinai, Egypt and is generally thought to contain exposures of the 'type-section' for late pre-rift to early syn-rift stratigraphy associated with the Oligo-Miocene Suez Rift. Previous studies have suggested a complex vertical succession of sedimentary facies characterise the late pre-rift to early syn-rift and imply major and abrupt variations in relative sea-level during this time. Detailed sedimentological and stratigraphic analysis of the Wadi Tayiba type-section presented in this study identifies not only a far simpler vertical facies succession than previously suggested but also the development of a major paleovalley system at the base of the early syn-rift succession. It is suggested that this subtle but significant feature is the cause of the complex vertical facies succession previously interpreted. It is concluded that continuous marine sedimentation and only moderate amplitude variations in relative sea-level occurred during the Eocene to Early Oligocene within at least this part of the Suez Rift. A major relative sea-level fall occurred during the middle Oligocene and a regionally developed erosional surface associated with this event marks the contact between late pre-rift and early syn-rift strata. The results of this study have major implications for sub-regional correlations of late pre-rift to early syn-rift stratigraphic units and resultant palaeogeographic reconstructions of the late pre-rift to early syn-rift period.

Jackson, Christopher Aiden-Lee

2008-09-01

25

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

26

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

SciTech Connect

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

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

1995-12-01

27

The synrift subsidence deficit at rifted margins  

NASA Astrophysics Data System (ADS)

Across rifted margins, the prerift continental crust thins from ~ 30 km, reaching zero at the continent-ocean transition (COT) beyond which either oceanic crust or unroofed mantle forms top basement. As a result of the crustal thinning, considerable subsidence is both expected and observed. However at several margins, subsidence appears to have occurred largely after rather than during rifting. Examples of such behaviour described in the literature include the West Iberia margin, the salt basins of the South Atlantic, and the Exmouth Plateau margin. This synrift subsidence deficit can be explained by crustal depth-dependent stretching, in which much of the crust is withdrawn after the end of rifting, but considerable problems arise with this model. They can however also be explained at magma-rich margins by thermal uplift during rifting, the addition of igneous intrusions to the lithosphere during rifting, and the partial depletion of the mantle. At magma-poor margins, mantle serpentinization has a similar effect, although as serpentinization can only occur once the entire curst has become brittle, this is likely to be important only at high degrees of stretching. An alternative explanation may be the influx of asthenosphere warmer than the relatively cool sublithospheric mantle observed beneath several continents and which is one explanation for the lack of melt at many rifted margins. These different models would thus imply some modification to the McKenzie model for lithospheric stretching, arising because of the geodynamic processes accompanying continental breakup. But it is also possible that synrift subsidence has been systematically underestimated if local water level was substantially below global sealevel. The presence of thick evaporites at many rifted margins indicates that this was true at the end of rifting. As rifting leading to continental breakup by definition occurs within a continent, it may be expected that the rift initially develops isolated from the global ocean, with consequently unusual salinities and water levels very different from global sealevel. In summary, there are thus several explanations for the synrift subsidence discrepancy which are to be expected at rifted margins formed during continental breakup.

Reston, T.

2009-04-01

28

Discussion of Continental Rifts and Their Structure  

NASA Astrophysics Data System (ADS)

When continental crust rifts, two chief modifications of that crust occur: 1)stretching of older, existing crust; 2)addition of new rift mass--sediments and mantle mafic units. However, paleorifts, such as the Cambrian Southern Oklahoma Aulacogen differ from neorifts, such as the East African. Much of this difference may be reflected in the nature of the lower rift crust. Stretching of the upper crust is accomplished primarily through faulting while the lower crust flows. Concurrently addition of sediments occurs in downdropped faulted blocks in the upper crust, and of mafic magmas risen and emplaced as intrusive layered complexes through the rift and as extrusive flows. All this happens in a regime of higher temperatures and higher heat flow. Consequences of this can include either melting of the stretched existing crust, or direct fractionation of rising mafic magma or melting of already crystallized mafic complexes, forming new silicic magmas. Geochemistry of these different magmatic bodies elucidates which of these possible processes seems dominant. Most geophysical studies of rifts have two results: 1)higher gravity anomalies indicating addition of new mafic masses, usually interpreted to be concentrated in the upper rift crust; and 2)seismic characteristics indicating crustal mottling and layering of the upper rift crust. What is not clearly indicated is nature of the lower crust, and of the mantle-crust contact (M discontinuity). Comparison of paleorifts and neorifts, and later geological history of paleorifts, suggests interesting interpretations of lower rift crust,especially in paleorifts, and some of the difficulties in sorting out answers.

Gilbert, M. C.

2011-12-01

29

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

NASA Astrophysics Data System (ADS)

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 of rifting and rift segmentation in the Gulf of Suez involves acquiring reliable geochronological constraints on extensional faulting. This study has commenced a systematic investigation of the timing and spatial distribution of rifting, lateral rift segmentation, and rift localization within the Gulf of Suez, Egypt, employing apatite and zircon (U-Th)/He thermochronometry. (U-Th)/He thermochronometric analysis of sample transects from exhumed fault blocks within the rift integrated with structural data will allow us to directly determine the timing, distribution, and magnitude of extension. The onset of major rifting (~24-19 Ma) in the Gulf of Suez was marked by the development of crustal domino-style tilt blocks and syn-rift deposition of the late Oligocene non-marine Abu Zenima Fm and non-marine to restricted marine Nukhul Fm. Development of the Gulf of Aqaba-Dead Sea transform cut off the rift from the Red Sea rift at an early extensional stage. Apatite (AHe) and zircon (ZHe) (U- Th)/He data were collected from basement and pre-rift sedimentary sample transects from the central and southern Sinai Peninsula portion and the Gebel El Zeit area in the southern Gulf of Suez as well as from basement samples from selected drill cores off Gebel El Zeit. Preliminary data exhibit partially reset ages trending as old as ~70 Ma (AHe) and ~450 Ma (ZHe) from shallower structural levels (Proterozoic basement and Phanerozoic cover sequence). Structurally deeper samples yield abundant AHe ages of ~22-24 Ma, indicative of rapid cooling and exhumation during the early Miocene. More basin-ward AHe samples are as young as ~17 Ma, recording continued early to middle Miocene extension in the Gulf of Suez. Detailed thermochronometric dating in progress should yield a more complete picture of the temporal and spatial distribution of extensional faulting in the Gulf of Suez.

Bosworth, W.; Stockli, D. F.

2006-12-01

30

Drilling to Resolve the Evolution of the Corinth Rift  

NASA Astrophysics Data System (ADS)

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

McNeill, Lisa; Sakellariou, Dimitris; Nixon, Casey

2014-05-01

31

The 1974 Ethiopian rift geodimeter survey  

NASA Technical Reports Server (NTRS)

The field techniques and methods of data reduction for five successive geodimeter surveys in the Ethiopian rift valley are enlarged upon, with the considered conclusion that there is progressive accumulation of upper crustal strain, consonant with on-going rift extension. The extension is restricted to the Quaternary volcanotectonic axis of the rift, namely the Wonji fault belt, and is occurring at rates of 3 to 6 mm/yr in the northern sector of the rift valley. Although this concurs with the predictions of platetectonic analysis of the Afar triple junction, it is considered premature to endorse such a concurrence on the basis of only 5 years of observations. This is underlined by the detection of local tectonic contractions and expansions associated with geothermal and gravity anomalies in the central sector of the rift valley. There is a hint of a component of dextral slip along some of the rift-floor fault zones, both from geological evidence and from the strain patterns detected in the present geodetic surveys.

Mohr, P.

1977-01-01

32

Deepening, and repairing, the metabolic rift.  

PubMed

This paper critically assesses the metabolic rift as a social, ecological, and historical concept describing the disruption of natural cycles and processes and ruptures in material human-nature relations under capitalism. As a social concept, the metabolic rift presumes that metabolism is understood in relation to the labour process. This conception, however, privileges the organisation of labour to the exclusion of the practice of labour, which we argue challenges its utility for analysing contemporary socio-environmental crises. As an ecological concept, the metabolic rift is based on outmoded understandings of (agro) ecosystems and inadequately describes relations and interactions between labour and ecological processes. Historically, the metabolic rift is integral to debates about the definitions and relations of capitalism, industrialism, and modernity as historical concepts. At the same time, it gives rise to an epistemic rift, insofar as the separation of the natural and social worlds comes to be expressed in social thought and critical theory, which have one-sidedly focused on the social. We argue that a reunification of the social and the ecological, in historical practice and in historical thought, is the key to repairing the metabolic rift, both conceptually and practically. The food sovereignty movement in this respect is exemplary. PMID:20645448

Schneider, Mindi; McMichael, Philip

2010-01-01

33

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

E-print Network

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

Déverchère, Jacques

34

The provenance and evolution of the Rjukan Rift Basin, Telemark, south Norway: The shift from a rift basin to an epicontinental sea along a Mesoproterozoic supercontinent  

Microsoft Academic Search

The Rjukan Rift Basin formed in the Mesoproterozoic (ca. 1.5–1.4Ga) within the margin of a supercontinent that was formed by the Laurentian and Fennoscandian shields. The basin evolved from a continental syn-rift stage to a marine post-rift stage. Great volumes of bimodal volcanic rocks characterize the syn-rift stage. During the syn-rift and post-rift stages, the Rjukan Rift Basin was filled

J. Lamminen; J. Köykkä

2010-01-01

35

Review article Rift Valley fever virus (Bunyaviridae: Phlebovirus)  

E-print Network

Review article Rift Valley fever virus (Bunyaviridae: Phlebovirus): an update on pathogenesis Africa (Received 5 February 2010; accepted 21 May 2010) Abstract ­ Rift Valley fever (RVF) virus fever / molecular epidemiology / vector / pathogenesis / diagnostic Table of contents 1. Introduction

Paris-Sud XI, Université de

36

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

37

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

38

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

NASA Astrophysics Data System (ADS)

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

Klyuchevskii, Anatoly V.

2014-07-01

39

Earthquake clusters in Corinth Rift  

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

40

Numerical modelling of quaternary deformation and post-rifting displacement in the AsalGhoubbet rift (Djibouti, Africa)  

E-print Network

­Ghoubbet rift (Djibouti, Africa) Rodolphe Cattin a,*, Ce´cile Doubre b,c , Jean-Bernard de Chabalier d. Introduction Located at the western tip of the Aden ridge, the Asal­Ghoubbet rift in Republic of Djibouti

Vigny, Christophe

41

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

42

Longitudinal evolution of the Suez rift structure (Egypt)  

Microsoft Academic Search

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

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

1988-01-01

43

Aedes Mosquito Saliva Modulates Rift Valley Fever Virus Pathogenicity  

E-print Network

Aedes Mosquito Saliva Modulates Rift Valley Fever Virus Pathogenicity Alain Le Coupanec1 , Divya contro^le, Centre IRD de Montpellier, Montpellier, France Abstract Background: Rift Valley fever (RVF of mosquito saliva in the transmission of Rift Valley fever virus (RVFV) has not been investigated. Objective

Boyer, Edmond

44

Mineralization related to rift systems: examples from the Gulf of Suez and the Dead Sea Rift  

NASA Astrophysics Data System (ADS)

The enriched source rocks for the mineralizations in the Timna Valley and the Um Bogma areas are located in specific stratigraphic units in the Cambrian and Carboniferous sedimentary basins, respectively. These basins were faulted later, during Late Cenozoic, by rift systems which locally followed ancient tectonic features. Epigenetic, hydrothermal activity that occurred along faults and lineaments, caused the final enrichment. Some of the mineralization elements, such as uranium in the Timna Valley, were leached during this epigenetic stage. It is suggested that the hydrothermal systems propagated from present rift margins to rift centers. The primary source rocks were most probably the anorogenic A-type granites of Late Precambrian age.

Beyth, M.

1987-09-01

45

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

46

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

47

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

Microsoft Academic Search

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

Katie M. Keranen; Simon L. Klemperer; Jordi Julia; Jesse F. Lawrence; Andy A. Nyblade

2009-01-01

48

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

49

Deformation signals in the currently-rifting Afar (Ethiopia) Rift measured with InSAR  

Microsoft Academic Search

A major rifting episode is currently occurring in the Dabbahu magmatic segment in the Afar region. The rifting episode began in September-October 2005, when continuous seismicity, a volcanic eruption and extensive diking occurred along the 60-km-long magmatic segment. Since then, nine additional dike intrusions occurred in the area and have been detected by radar interferometry (InSAR), with the most recent

C. Pagli; T. J. Wright; H. Wang; I. J. Hamling; D. Kier; M. Belachew; C. Ebinger

2008-01-01

50

Structure and kinematics of the Taupo Rift, New Zealand  

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

51

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

52

The biodiversity of the Albertine Rift  

Microsoft Academic Search

The Albertine Rift is one of the most important regions for conservation in Africa. It contains more vertebrate species than any other region on the continent and contains more endemic species of vertebrate than any other region on mainland Africa. This paper compiles all currently known species distribution information for plants, endemic butterfly species and four vertebrate taxa from the

Andrew J. Plumptre; Tim R. B. Davenport; Mathias Behangana; Robert Kityo; Gerald Eilu; Paul Ssegawa; Corneille Ewango; Danny Meirte; Charles Kahindo; Marc Herremans; Julian Kerbis Peterhans; John D. Pilgrim; Malcolm Wilson; Marc Languy; David Moyer

2007-01-01

53

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...

54

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

55

The 1973 Ethiopian-Rift geodimeter survey  

NASA Technical Reports Server (NTRS)

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

Mohr, P. A.

1974-01-01

56

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

57

Prediction of a Rift Valley fever Outbreak  

Technology Transfer Automated Retrieval System (TEKTRAN)

Using satellite measurements to detect elevated sea surface temperatures (SSTs) and subsequent elevated normalized difference vegetation index (NDVI) data in Africa, we predicted an outbreak of Rift Valley fever (RVF) in humans and animals in the Horn of Africa during September 2006-May 2007. We det...

58

Unexpected Rift Valley fever outbreak, northern Mauritania.  

PubMed

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. PMID:22000364

El Mamy, Ahmed B O; Baba, Mohamed Ould; Barry, Yahya; Isselmou, Katia; Dia, Mamadou L; El Kory, Mohamed O B; Diop, Mariam; Lo, Modou Moustapha; Thiongane, Yaya; Bengoumi, Mohammed; Puech, Lilian; Plee, Ludovic; Claes, Filip; de La Rocque, Stephane; Doumbia, Baba

2011-10-01

59

Reemergence of Rift Valley fever, Mauritania, 2010.  

PubMed

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

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

2014-02-01

60

Submarine Thermal Springs on the Galapagos Rift  

Microsoft Academic Search

The submarine hydrothermal activity on and near the Galapagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new

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

1979-01-01

61

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...

62

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

63

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)

1996-12-31

64

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

65

Rifting and shallow-dipping detachments, clues from the Corinth Rift and the Aegean  

NASA Astrophysics Data System (ADS)

The Corinth Rift is superimposed on the Hellenic nappe stack that formed at the expense of the Apulian continental crust above the subducting African slab. Extension started in the Pliocene and the major steep normal faults that control the geometry of the present-day rift were born very recently, some 600 kyr ago only. They root into a shallow-dipping zone of microseismicity recorded near the base of the upper crust. The significance of this seismogenic zone is debated. Considering the northward dip of the zone of microseismicity, the depth of microearthquakes and their focal mechanisms, we observe a strong similarity with the northern Cycladic detachments in terms of expected pressure, temperature conditions and kinematics. We herein show (1) that the formation of the Corinth Rift can be considered a part of a continuum of extension that started some 30-35 Ma in the Aegean and that was recently localised in a more restricted area, (2) that the present-day structure and kinematics of the Corinth Rift can be explained with a series of decollements relayed by steeper ramps that altogether formed a mechanically weak, crustal-scale detachment, and (3) that the deformation, fluid behaviour and metamorphic features seen in the northern Cycladic metamorphic core complexes can be good analogues of the processes at work below the Corinth Rift.

Jolivet, Laurent; Labrousse, Loïc; Agard, Philippe; Lacombe, Olivier; Bailly, Vivien; Lecomte, Emmanuel; Mouthereau, Frédéric; Mehl, Caroline

2010-03-01

66

The Pathogenesis of Rift Valley Fever  

PubMed Central

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

Ikegami, Tetsuro; Makino, Shinji

2011-01-01

67

The North Polar Spur and Aquila Rift  

NASA Astrophysics Data System (ADS)

Soft X-ray intensity at 0.89 keV along the North Polar Spur (NPS) is shown to follow the extinction law due to the interstellar gas in the Aquila Rift by analysing the ROSAT archival data, which proves that the NPS is located behind the rift. The Aquila-Serpens molecular clouds, where the X-ray optical depth exceeds unity, are shown to have a mean LSR velocity of v = 7.33 ± 1.94 km s-1, corresponding to a kinematic distance of r = 0.642 ± 0.174 kpc. Assuming a shell structure, a lower limit of the distance to NPS is derived to be 1.01 ± 0.25 kpc, with the shell centre being located farther than 1.1 kpc. Based on the distance estimation, we argue that the NPS is a Galactic halo object.

Sofue, Yoshiaki

2015-03-01

68

The Midcontinent rift system in Kansas  

SciTech Connect

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

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

1993-03-01

69

Crustal structure of central Lake Baikal: Insights into intracontinental rifting  

E-print Network

Crustal structure of central Lake Baikal: Insights into intracontinental rifting Uri S. ten Brink and Michael H. Taylor1 U.S. Geological Survey, Woods Hole, Massachusetts, USA Received 26 January 2001; revised 15 October 2001; accepted 25 October...; KEYWORDS: Lake Baikal, continental rifts, Newark basin, crustal extension, plate driving forces, seismic velocity structure 1. Introduction [2] Most aspects of the deformation of intracontinental rifts can be explained by a combination of several physical...

ten Brink, Uri S.; Taylor, Michael Halford

2002-07-16

70

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\\u000a publications and open-file reports provide insights into the geological history of this rift. Paleogene and Cretaceous rocks,\\u000a which are considered to be prerift, are locally exposed at the margins of the Red Sea Rift. At the same time, some evidence\\u000a indicates

G. A. F. d’Almeida

2010-01-01

71

The South China sea margins: Implications for rifting contrasts  

Microsoft Academic Search

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

Dennis E. Hayes; Susan S. Nissen

2005-01-01

72

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

Microsoft Academic Search

The Eastern Branch of the East African Rift System is well known in Ethiopia (Main Ethiopian Rift) and Kenya (Kenya or Gregory Rift) and is usually considered to fade away southwards in the North Tanzanian Divergence, where it splits into the Eyasi, Manyara and Pangani segments. Further towards the south, rift structures are more weakly expressed and this area has

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

2008-01-01

73

Lake Baikal - A Touchstone for Global Change and Rift Studies  

NSDL National Science Digital Library

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

74

Physical characteristics and evolutionary trends of continental rifts  

NASA Technical Reports Server (NTRS)

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

75

Evolution of the northern Main Ethiopian rift: birth of a triple junction  

Microsoft Academic Search

Models for the formation of the archetypal rift–rift–rift triple junction in the Afar depression have assumed the synchronous development of the Red Sea–Aden–East African rift systems soon after flood basaltic magmatism at 31 Ma, but the timing of intial rifting in the northern sector of the East African rift system had been poorly constrained. The aims of our field, geochronology,

Ellen Wolfenden; Cynthia Ebinger; Gezahegn Yirgu; Alan Deino; Dereje Ayalew

2004-01-01

76

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

77

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

NASA Astrophysics Data System (ADS)

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 process do we add 15km to the original SOA crust: underplating, eclogite-gabbro transformation, or deformation? This question has bearing on how we understand and interpret all paleo-rifts now a part of continental cores. Geology of the southern Midcontinent of North America does not show evidence of significant thermal events in the Phanerozoic. This effectively rules out underplating and phase transformation as a cause of change in M-discontinuity depth. Present SOA outcrops are in the Wichita Mountains of southwestern Oklahoma, part of the easternmost Ancestral Rockies. These outcrops are in the Wichita-Amarillo crustal block uplifted about 7km in the Pennsylvanian. The Anadarko Basin to the north went down about 7km. Large Pennsylvanian thrust faults in the upper brittle crust are documented. Thus it appears that compressive deformation may be able to account for the change in crustal thickness from neo-rift type to paleo-rift and craton type. However, the accommodation made in the lower crust may be more dramatic than deformation in the upper crust because shortening, and thickening of the order of 2X, is probably required. Comparisons with other paleo-rifts in North America, such as the Middle Proterozoic Midcontinent Rift and the NeoProterozoic Reelfoot Rift, show that their crustal thicknesses now also match their previously rifted margins. Can the same sequence, as seems to be the case with the SOA, apply to other paleo-rifts?

Gilbert, M. C.

2004-12-01

78

Numerical modelling of quaternary deformation and post-rifting displacement in the Asal–Ghoubbet rift (Djibouti, Africa)  

Microsoft Academic Search

Over the last three decades a host of information on rifting process relating to the geological and thermal structure, long-time scale deformation (Quaternary and Holocene) and rifting cycle displacement across the Asal–Ghoubbet rift has been made available. These data are interpreted with a two-dimensional thermo-mechanical model that incorporates rheological layering of the lithosphere, dyke inflation and faulting. Active fault locations

Rodolphe Cattin; Cécile Doubre; Jean-Bernard de Chabalier; Geoffrey King; Christophe Vigny; Jean-Philippe Avouac; Jean-Claude Ruegg

2005-01-01

79

Mesozoic Rifting in the German North Sea  

NASA Astrophysics Data System (ADS)

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

Lutz, R.; Jähne, F.; Arfai, J.

2013-12-01

80

Rifting and breakup in the South China Sea  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

81

Una aplicacion de conjuntos rugosos difusos en seleccion de caracteristicas para la mejora de metodos de  

E-print Network

empleado. De esta manera, es posible mejorar los resultados de muchas aplicaciones de miner´ia de datos´on de Caracter´isticas, Clasificador del Vecino M´as Cercano 1. Introducci´on La reducci´on de datos es un proceso que puede aplicarse en situaciones en las que haya que analizar una gran cantidad de datos

Granada, Universidad de

82

Uso del conocimiento de la arquitectura Fermi para mejorar el rendimiento en aplicaciones  

E-print Network

Uso del conocimiento de la arquitectura Fermi para mejorar el rendimiento en aplicaciones CUDA Yuri arquitectura Fermi de NVIDIA introduce nuevos criterios a la hora de selec- cionar los tama~nos y la geometr arquitectura para aplicar adecua- damente t´ecnicas de optimizaci´on de c´odigo. Fermi [2] [3] [4] es la

Llanos, Diego R.

83

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

84

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

85

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

NASA Astrophysics Data System (ADS)

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

Bingham, Robert; Ferraccioli, Fausto

2014-05-01

86

Volcanic highlands in the South Atlantic rift zone  

NASA Astrophysics Data System (ADS)

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

Il'in, A. V.

2014-04-01

87

Basement rift control on oil production in eastern Kansas  

Microsoft Academic Search

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

1983-01-01

88

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

89

Seismic images of magmatic rifting beneath the western branch of the East African rift  

NASA Astrophysics Data System (ADS)

We have performed a tomographic study using a joint data set that includes local and teleseismic events, recorded by a temporary network in the western branch of the East African rift system. From the travel time residuals, we derive a three-dimensional model of seismic P-wave velocity anomalies for the crust and upper mantle down to a depth of 80 km. Particular attention is paid to the verification of the inversion results by various resolution tests. The results show that the eastern rift shoulder is characterized by relatively high seismic velocities. Lower velocities are obtained beneath the entire length of the rift valley and the Rwenzori Mountains. A prominent feature is observed north-east of the mountain range: here we detected a vertically oriented, cylindrical low-velocity anomaly with maximum amplitudes in the middle crust and the upper mantle lithosphere. We suggest that this anomaly indicates reservoirs of molten material related to the ongoing rifting process within this segment of the rift. Just above this anomaly, at depths between 5 and 16 km, earthquake swarms exist. The observed reduction in P-wave velocity is used to provide constraints on the possible melt content and temperature anomaly in the uppermost mantle. The observed 3-5% P-velocity decrease can be explained by melt fraction up to 2%-3.3% or alternatively by a temperature increase of at least 248 to 376 K and even higher-temperature anomalies are possible if lower ambient temperatures in the reference mantle are assumed. Probably, the two effects act in combination.

Jakovlev, Andrey; Rümpker, Georg; Schmeling, Harro; Koulakov, Ivan; Lindenfeld, Michael; Wallner, Herbert

2013-11-01

90

Rifting consequences of three plate separation  

NASA Astrophysics Data System (ADS)

Submarine sedimentary basins parallel to the trends of passive continental margins are well explained by tensional processes between two lithospheric plates. This leaves open the enigma of many basins which strike across margins. We use examples from the North Atlantic to show that such basins may be formed during the initial rifting stage due to enhanced lithospheric thinning caused by the separation of three plates. We suggest that volcanics can be emplaced in these basins and, in the extreme case, where mantle temperature and consequently the degree of partial melting are particularly high, that basaltic flows can be thick enough to fill and hide the basins.

Sibuet, Jean-Claude; Srivastava, Shiri

1994-04-01

91

Mid-Continent rift system - a frontier hydrocarbon province  

SciTech Connect

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

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

1983-08-01

92

Petrological Constraints on Melt Generation Beneath the Asal Rift (Djibouti)  

NASA Astrophysics Data System (ADS)

The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 95 lava flows sampled along 10 km of the rift axis and 8 km off-axis (that is for the last 650 ky). The major element composition and the trace element ratios of aphyric basalts across the Asal Rift show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. FeO, Fe8.0, Sm/YbN and Zr/Y increase, whereas SiO2 and Lu/HfN decrease from the rift axis to the rift shoulders. These variations are qualitatively consistent with a shallower melting beneath the rift axis than off-axis and the data show that the melting regime is inconsistent with a passive upwelling model. In order to quantify the depth range and extent of melting, we invert Na8.0 and Fe8.0 contents of basalts based on a pure active upwelling model. Beneath the rift axis, melting paths are shallow, from 60 to 30 km. These melting paths are consistent with adiabatic melting in normal-temperature asthenosphere, beneath an extensively thinned mantle lithosphere. In contrast, melting on the rift shoulders occurred beneath a thick mantle lithosphere and required mantle solidus temperature 180°C hotter than normal (melting paths from 110 to 75 km). The calculated rate of lithospheric thinning is high (6.0 cm yr-1) and could explain the survival of a metastable garnet within the mantle at depth shallower than 90 km beneath the modern Asal Rift.

Pinzuti, P.; Humler, E.; Manighetti, I.; Gaudemer, Y.; Bézos, A.

2010-12-01

93

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

E-print Network

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

Beaumont, Christopher

94

Continental rift structures and dynamics with reference to teleseismic studies of the Rio Grande and East African rifts  

NASA Astrophysics Data System (ADS)

Teleseismic travel time residuals measured on 600-1000 km seismic arrays across the Rio Grande and East African rifts provide evidence for upper mantle low velocity zones beneath each rift. We propose that these zones arise from asthenosphere replacing the base of the lithosphere . Refraction studies show thinning of the crust below the rift axes. Regional uplift, which extends for hundreds of kilometers on either side of the axes, is isostatically compensated by negative density contrast at depth. Since the refraction results argue against compensation by crustal thickening, compensation within the upper mantle is suggested. Temperature effects alone do not explain the magnitudes of the reduced velocities. Instead, generation of a few percent of partial melt is the most probable source of the reduction. A passive extensional mechanism is investigated for the Rio Grande rift in which hot asthenosphere intrudes into the lithosphere , raising its temperature and generating moderate melting. It requires less than 1% extension to explain the melting; in contrast, a prodigious 26% would be required to raise the temperature from a pre-rifting continental geotherm to the solidus. Geological estimates based on palinspastic reconstruction find that extension is less than 10% over the uplift zone. A passive mechanism is possible if the pre-rifting lower lithosphere geotherm lies closer to the solidus than the geotherm of a stable continent. This hypothesis is supported by the observed high heat flow in the Rio Grande rift uplift region which, given the long time constant for thermal diffusion, reflects elevated temperatures at depth well before the onset of rifting. The elevated asthenosphere boundary, found teleseismically, lies directly beneath both rifts favoring a "pure shear" over the "simple shear" model of rifting. We propose a model for continental rifting in which the asthenosphere intrudes into dike-like structures in the lithosphere beneath the rift zone. This explains the observed upwarp of the Moho beneath the rift as asthenosphere intrudes the upper mantle and base of the crust. The forces associated with wedging apart of the upper mantle give rise to tensional and compressional stresses in the crust which form the graben and uplifted shoulders, respectively. Mantle xenoliths indicate brittle diking occurs in the upper mantle to depths of 70 km. Furthermore, evidence of asthenospheric intrusion from Neodymium ratios supports the role of asthenospheric advection in the rifting process.

Davis, Paul M.

1991-10-01

95

Papers presented to the Conference on the Processes of Planetary Rifting  

NASA Technical Reports Server (NTRS)

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

1981-01-01

96

Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology  

Microsoft Academic Search

The question of plate boundary forces and deep versus shallow asthenospheric uplift has long been debated in intracontinental rift areas, particularly in the Baikal rift zone, Asia, which is colder than other continental rifts. As previous gravity and teleseismic studies support the dominance of opposing mechanisms in the Baikal rift, we reconsidered both data sets and jointly inverted them. This

C. Tiberi; M. Diament; J. Déverchère; C. Petit-Mariani; V. Mikhailov; S. Tikhotsky; U. Achauer

2003-01-01

97

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

98

Geochemical signals of progressive continental rupture in the Main Ethiopian Rift  

Microsoft Academic Search

Mafic volcanics of the Main Ethiopian Rift record the development of magmatic rift segments during continental extension. The Ethiopian Rift is one arm of a triple junction that formed above a Paleogene mantle plume, concurrent with eruption of flood basalts ca. 30 Ma across northern Ethiopian and Yemen. The geochemistry of Ethiopian Rift lavas thus provides insight into processes associated

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

2003-01-01

99

Magma-compensated crustal thinning in continental rift zones.  

PubMed

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

Thybo, H; Nielsen, C A

2009-02-12

100

Evolution, distribution, and characteristics of rifting in southern Ethiopia  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

101

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

102

Dynamics of Rifting in two Active Rift Segments in Afar - Geodetic and Structural Studies - DoRA Project  

NASA Astrophysics Data System (ADS)

The DoRA project aims to conduct complementary studies in two volcano-tectonic rifts in the Afar Depression. In Northern Afar, the Wal’is Dabbahu Rift (WD, Ethiopia) is currently undergoing a major rifting episode. This event started in September 2005 with a significant seismic activity. InSAR data revealed the injection of a 65 km-long mega-dyke that opened by up to 8 m, the slip of numerous normal faults and opening of fissures, and a rhyolitic eruption. Similarly, the Asal-Ghoubbet Rift (AG, Djibouti) was affected in 1978 by a smaller episode of rifting associated with the intrusion of a 2 m wide dyke into the crust. Since then, a large catalog of geodetic data that includes recent InSAR time series reveals the importance of non-steady deformation controlling the rift dynamics. Our goal is to gain an understanding of such volcano-tectonic segments on several time scales, including the dyking period itself and the post-event period. The study of the behavior of the AG Rift during its whole post-rifting period offers an image at t+30 years of the WD segment, while keeping in mind important structural and scale differences. First, we propose to build a complete and accurate set of geodetic data (InSAR, cGPS, GPS), covering the period under study. With a narrow temporal sample window, we will precisely describe the aseismic slip affecting the normal faults of these rifts, the periods of sudden slip and/or slip acceleration but also measure the deformation associated with probable future dyke intrusion. Second, we aim to constrain the origin of these displacements and their relation with mass transfers within the crust. Series of gravity measurements will be pursue or initiated in both rifts. Third, the recording of seismic activity is essential to constrain the relative importance of seismic and aseismic deformation. This will also help to evaluate the thickness of the seismogenic layer. Together with structural data collected during a seismic survey in the AG Rift, these results will offer crucial constraints on modeling the rifting dynamics in order to test the relative influences of the rheology, the fault/dyke geometry and fluids on the rupture mechanics, the viscous relaxation, dyke intrusion/inflation and aseismic slip and their interactions. Our multidisciplinary approach should provide important new constraints on the dynamics of rifting along divergent plate boundaries, and ultimately, in other geodynamical contexts affected by aseismic fault slip transients.

Doubre, C.; Socquet, A.; Masson, F.; Jacques, E.; Grandin, R.; Nercessian, A.; Kassim, M.; Vergne, J.; Diament, M.; Hinderer, J.; Ayele, A.; Lewi, E.; Calais, E.; Peltzer, G.; Toussaint, R.; de Chaballier, J.; Ballu, V. S.; Luck, B.; King, G. C.; Vigny, C.; Cattin, R.; Tiberi, C.; Kidane, T.; Jalludin, M.; Maggi, A.; Dorbath, C.; Manatschal, G.; Schmittbuhl, J.; Le Moigne, N.; Deroussi, S.

2009-12-01

103

Rifting of Continental Interiors: Some New Geophysical Data and Interpretations  

NASA Astrophysics Data System (ADS)

Rifting is one of the major processes that affect the evolution of the continents. This process sometimes leads to continental breakup and the formation of new oceans, but more often does not. This is presumably due to extension not progressing sufficiently to form a new plate margin resulting in a structure, which remains isolated in an intra-plate environment. The Southern Oklahoma aulacogen is such a feature, and the continental portion of the East African rift system may be a modern example. As more detailed geophysical and geological studies of rifts have become available in recent years, a complex picture of rift structure and evolution has emerged. Global patterns that reveal the connections between lithospheric structure (deep and shallow), magmatism (amount and style), amount of extension, uplift, and older structures remain elusive. However, our geophysical studies of modern and paleo rifts in North America, East Africa, and Europe makes it possible to make some general observations: 1). Magmatism in rifts is modest without the presence of a (pre-existing?) thermal anomaly in the mantle. 2). Magmatic modification of the crust takes many forms which probably depend on the nature of older structures present and the state of the lithosphere when rifting is initiated (i.e. cold vs. hot; fertility), 3) There is no clear relation between amount of extension and the amount of magmatic modification of the crust. 4) Brittle deformation in the upper crustal is complex, often asymmetrical and older features often play important roles in focusing deformation. However on a lithospheric scale, rift structure is usually symmetrical. 5) A better understanding of rift processes is emerging as we achieve higher levels of integration of a wide variety of geoscience data.

Keller, G. R.

2005-12-01

104

Rifting Processes and Regional Sections across the South China Sea  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

105

The formation of the northwestern Dniepr-Donets Basin: 2-D forward and reverse syn-rift and post-rift modelling  

NASA Astrophysics Data System (ADS)

Forward and reverse modelling of structure and stratigraphy have been used to investigate the syn-rift (Devonian) and early post-rift (Carboniferous) evolution of the northwestern Dniepr-Donets Basin. Modelling shows that basin formation is consistent with Frasnian-Famennian rifting followed by post-rift subsidence starting in the early Tournaisian. Forward syn-rift modelling, using the flexural cantilever model of rift basin formation, satisfactorily models the observed syn-rift stratigraphic thicknesses and structure within the basin using total syn-rift extension of approximately 15 km in the region studied with a maximum ? stretching factor of 1.3. Forward structural and stratigraphic modelling suggests that the Devonian rifting, which formed the northwestern Dniepr-Donets Basin, was accompanied by regional uplift of the order of 300 m. Both forward and reverse (flexural backstripping) modelling of post-rift stratigraphy through the Carboniferous suggest that post-rift thermal subsidence, which commenced in the Tournaisian following Devonian rifting, was augmented by additional regional subsidence of the order of 300 m in middle Carboniferous times. It is suggested that this transient regional uplift event, which accompanied rifting and decayed in middle Carboniferous times, was generated by a mantle plume.

Kusznir, N. J.; Stovba, S. M.; Stephenson, R. A.; Poplavskii, K. N.

1996-12-01

106

Reassessment of the rifting process in the Western Corinth Rift from relocated seismicity  

NASA Astrophysics Data System (ADS)

The seismic activity in the western part of the Corinth Rift (Greece) over the period 2000-2007, monitored by a dense network of three-component stations, is analysed in terms of multiplets and high precision relocation using double difference techniques. This detailed analysis provides new insights into the geometry of faults at depth, the nature and the structure of the active zone at 6-8 km depth previously interpreted as a possible detachment, and more generally into the rifting process. The seismicity exhibits a complex structure, strongly varying along the rift axis. The detailed picture of the seismic zone below the rift indicates that its shallower part (at depths of 6-8 km) is 1-1.5 km thick with a complex microstructure, and that its deeper part (at depths of 9-12 km) gently dipping to the north (10-20°) is 0.1-0.3 km thick with a microstructure consistent with the general slope of the structure. Although the nature of this seismic zone remains an open question, the presence of seismicity beneath the main active area, the strong variability of the structure along the rift over short distances and the complex microstructure of the shallower part revealed by the multiplet analysis are arguments against the hypothesis of a mature detachment under the rift: this active zone more likely represents a layer of diffuse deformation. The geometry of the mapped active faults is not well defined at depth, as no seismicity is observed between 0 and 4 km, except for the Aigion Fault rooting in the seismic layer at 6 km depth with a dip of 60°. A distinct cloud of seismicity may be associated with the antithetic Kalithea Fault, on which the 1909 Fokis earthquake (Ms = 6.3) may have occurred. The link between the 1995 rupture (Ms = 6.2) and the faults known at the surface has been better constrained, as the relocated seismicity favours a rupture on an offshore, blind fault dipping at 30°, rather than on the deeper part of the East Helike Fault. Consequently, the 1995 event is expected to have decreased the Coulomb stress on the East Helike Fault. To explain these seismic observations along with the geodetic observations, a new mechanical model for the rifting process in this region is proposed, involving non-elastic, mostly aseismic uniform NS opening below the rift axis, coupled with the downward and northward growth of a yet immature detachment: the reported GPS rates would mainly result from this deep, silent source, and the seismicity would reveal the detachment position, not yet connected to the ductile lower crust. In such a model, the strong fluctuations of microseismicity would result from small strain instabilities, undetected by continuous GPS and possibly related to pore pressure transients.

Lambotte, S.; Lyon-Caen, H.; Bernard, P.; Deschamps, A.; Patau, G.; Nercessian, A.; Pacchiani, F.; Bourouis, S.; Drilleau, M.; Adamova, P.

2014-06-01

107

Regional magnetic anomaly constraints on continental rifting  

NASA Technical Reports Server (NTRS)

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

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

1985-01-01

108

Rift Valley Fever, Sudan, 2007 and 2010  

PubMed Central

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

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

2013-01-01

109

Quaternary volcano-tectonic activity of the western rift margin in the Soddo region, southern Main Ethiopian Rift  

NASA Astrophysics Data System (ADS)

The Main Ethiopian Rift (MER) in East Africa is considered the ideal place to analyze the process of continental rifting since along its length it records all the different stages of rifting. Indeed, whereas the southern parts of the rift are believed to record the early stages of extension, with deformation localized at large boundary faults, the northern rift sectors are in incipient break-up stage, with focused tectono-magmatic activity at the rift axis. However, although the distribution and style of Quaternary volcano-tectonic deformation have been described in detail in both the northern and central rift sectors, knowledge of these characteristics is comparatively less constrained southwards. Analysis of fault distribution and comparison with analogue models, together with the characteristics of the seismic activity, suggest localized deformation at the rift margins during the Quaternary, with almost absent axial tectono-magmatic activity. However, geological data are currently still scattered in the area. In this contribution we present new geological and geochemical data from the western rift margin in the Soddo region, north of Lake Abaya, between latitudes 7°10' and 6°30'. Despite the lack of major fault escarpments, the area is characterized by numerous normal faults, which are typically sigmoidal in shape and en-echelon arranged; associated to these faults is a widespread volcanic activity, with recent volcanic centers (domes, scoria cones, fissures, etc.) aligned along and strongly interacting with normal faults. The volcanic activity is distinctly bimodal, with intermediate terms completely lacking. Erupted products are represented by alkali basalt, trachybasalts and rare tholeiitic basalts in the 45% to 49% SiO2 variation interval and alkali rhyolites with subordinate trachytes in 65%-74% range. The new field data coupled with new radiometric dating of faulted rocks suggest a Late Quaternary-Holocene age of the volcano-tectonic activity at this rift margin. This, together with a subordinate axial faulting, supports previous models that predict an along-axis, north to south decrease in rift maturity in the MER. Consistent with previous analogue models, the general fault architecture is coherent with the regional plate kinematics that gives rise to conditions of low-obliquity rifting. However, inversion of fault-slip data reveals local variations in the paleo-stress field that could result from local stress reorientations and/or influence of volcanic activity.

Corti, G.; Sani, F.; Agostini, S.; Philippon, M.; Sokoutis, D.; Willingshofer, E.; Abebe, T.

2012-04-01

110

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

111

POST-RIFT UPLIFT OF THE RIFTED MARGIN OF THE GULF OF ADEN  

NASA Astrophysics Data System (ADS)

The Gulf of Aden is a young and narrow oceanic basin formed in Oligo-Miocene time between the rifted margins of the Arabian and Somalian plates. The distal margin and particularly the Ocean-Continent Transition (OCT) domain were previously studied considering a large set of data (Leroy et al., 2004; d'Acremont et al., 2005; d'Acremont et al., 2006; Autin, et al accepted). This study focus on the sedimentary cover identified on seismic reflection profiles collected during Encens-Sheba (2000) and Encens (2006) cruises. Sedimentary stratal pattern and seismic facies succession permit us to highlight a late tectonic event affecting the Dhofar margin. The understanding of facies and depositional sequences is a major challenge for the knowledge of the post-rift tectono-sedimentological evolution of the Gulf of Aden during the spreading. This study let us to distinguish three domains, which match to the structural segmentation inherited from the rifting episode of this margin. The sedimentary record is strongly controlled by a recent (quaternary to now) tectonic phase. Vertical movements lead to the formation of numerous instabilities on the continental slope and Mass-transport deposits (MTDs) on the lower slope and deep basin. The quaternary uplift rate increases eastward, toward the Socotra Hadbeen transform fault zone. The recurrence of the gravitational events shows that the margin history can be divided into active and passive periods since the beginning of the post-rifting evolution of North Aden (17-6 Ma). There is a main sedimentological switch in the studied zone around 7- 10 Ma. This major changes of sedimentation rate and facies types (slope-wash detritus, Mass-transport deposits MTDs, first occurrence of deep sea fans) is probably due to the uplift of the margin and climatic change (first occurrence of the Monsoon in this region). All the incision/erosion stages of continental slope (from slope instabilities set up to the formation of mature canyon) observed on the eastern part of the margin lead to an incision shaping model on a steep continental slope. Elevated topography observed along many passive margins suggests permanent uplift of rift flank. This configuration should be due to flexural isostatic rebound in response to mechanical unloading of the lithosphere during extension.

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

2009-12-01

112

Aplicacion de PCA y tecnicas bayesianas a la clasificacion de pixeles basada en color  

E-print Network

patrones de las clases de un clasificador bayesiano. El m´etodo se ilustra a trav´es de varios experimentos´agenes y tiene m´ultiples aplicaciones en campos como la detecci´on de defectos de fabricaci´on en procesos la secci´on V se comentar´an varios experimentos realizados y finalmente en la secci´on VI se ver

Granada, Universidad de

113

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

E-print Network

Computación Universidad Central de Venezuela UCV #12;Sesión 3 - Sistemas Distribuidos y Aplicaciones Médicas 91 Escuela de Computación Universidad Central de Venezuela UCV #12;II Simposio Científico y Tecnológico en Computación 201292 Escuela de Computación Universidad Central de Venezuela UCV #12;Sesión 3

Coto, Ernesto

114

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

USGS Publications Warehouse

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

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

2005-01-01

115

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

116

Observe an animation of volcanism along a rift zone  

NSDL National Science Digital Library

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

TERC. Center for Earth and Space Science Education

2003-01-01

117

Upper Mantle Pollution during Afar Plume^Continental Rift Interaction  

E-print Network

Upper Mantle Pollution during Afar Plume^Continental Rift Interaction TYRONE O. ROONEY1 , BARRY B UNIVERSITY, UNIVERSITY PARK, PA 16802, USA 5 LABORATOIRE DE GE¤ OLOGIE DE LYON, ECOLE NORMALE SUPE¤ RIEURE DE

Paris-Sud XI, Université de

118

Preferential rifting of continents - A source of displaced terranes  

NASA Technical Reports Server (NTRS)

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

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

1984-01-01

119

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

2011-11-11

120

Thermal and mechanical development of the East African Rift System  

E-print Network

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

Ebinger, Cynthia Joan

1988-01-01

121

Sociocultural and Economic Dimensions of Rift Valley Fever  

PubMed Central

Health researchers have advocated for a cross-disciplinary approach to the study and prevention of infectious zoonotic diseases, such as Rift Valley Fever. It is believed that this approach can help bring out the social determinants and effects of the zoonotic diseases for the design of appropriate interventions and public health policy. A comprehensive literature review using a systematic search strategy was undertaken to explore the sociocultural and economic factors that influence the transmission and spread of Rift Valley Fever. Although the findings reveal a paucity of social research on Rift Valley Fever, they suggest that livestock sacrificial rituals, food preparation and consumption practices, gender roles, and inadequate resource base for public institutions are the key factors that influence the transmission. It is concluded that there is need for cross-disciplinary studies to increase the understanding of Rift Valley Fever and facilitate appropriate and timely response and mitigation measures. PMID:25688166

Muga, Geoffrey Otieno; Onyango-Ouma, Washington; Sang, Rosemary; Affognon, Hippolyte

2015-01-01

122

The geology and geophysics of the Oslo rift  

NASA Technical Reports Server (NTRS)

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

Ruder, M. E.

1981-01-01

123

Geochemical evidence of mantle reservoir evolution during progressive rifting  

NASA Astrophysics Data System (ADS)

The Afar region in East Africa, which represents the triple junction of three well-exposed Cenozoic rift systems, is a pivotal domain in the study of rift evolution. The western margin of Afar, defined by a wide transitional region from plateau to rift floor, developed in response to the rifting of the Red Sea commencing shortly after the eruption of the ~31-29 Ma Ethiopian-Yemen flood basalts. The Oligocene lava sequence which covers this rift margin was fed from intensive diking. The dikes and the block-faulting and monoclinal warping that followed provide an opportunity to probe the geochemical reservoirs preserved in the magmatic record and the development of the rifting processes. Argon geochronology reveals that dikes along the western Afar margin span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major and trace element, and isotopic results (Sr-Nd-Pb-Hf) from these dikes demonstrate temporal geochemical heterogeneity defined by variable contributions from the Afar plume, depleted mantle and African lithosphere, consistent with studies of Quaternary basalts from the Ethiopian Rift. On a broader scale our results show that as the western Afar margin matures, the initially significant contribution from the Afar plume wanes in favor of shallow asthenospheric and lithospheric reservoirs. The early dikes, which are coincident with the initial weakening of the lithosphere in a magma-assisted rifting model, geochemically resemble the widespread plume-derived flood basalts and shields that constitute the Ethiopian Plateau. Subsequent diking is characterized by a lesser role for the Afar plume and greater contributions from the African lithosphere and depleted mantle. During the terminal stage of dike emplacement, where focused magmatic intrusion accommodated extension, a more significant fraction is derived from the depleted mantle and less of a lithospheric signature is evident. For the terminal stages of the evolution of the western Afar rift margin, our observations confirm a model where magma-generation processes become dominated by shallow decompression melting of the ambient asthenosphere during the continent-to-ocean transition.

Rooney, T. O.; Mohr, P.; Dosso, L.; Hall, C. M.

2010-12-01

124

Crustal structure of central Lake Baikal: Insights into intracontinental rifting  

USGS Publications Warehouse

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

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

2002-01-01

125

Wrinkle ridges, rifts and the tectonic history of Venus  

NASA Astrophysics Data System (ADS)

New digital mappings of the wrinkle ridges of the plains of Venus show large-scale wrinkle-ridge (WR) and rift distribution properties consistent with the swell-push model (SPM) of lithospheric stress. Comparisons of mapped tectonic features and the SPM suggest that the geometries of most rift zones and several large-scale WR systems are consistent with the present-day stress field.

Bilotti, F.; Suppe, J.

1997-03-01

126

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

127

The Meliata and Piemont-Ligurian rifted margins: stratigraphic record and tectonic evolution of polyphase rift systems  

NASA Astrophysics Data System (ADS)

The Late Permian to Late Jurassic paleogeographic evolution of the Alpine domain was strongly controlled by the formation of polyphase rift systems. If these rift systems are the result of a single, long lasting rifting event or if they are generated by two distinct rift pulses, is still a matter of debate. Recent studies seem to agree on the second hypothesis, supporting two distinct rift events: one Early-Middle Triassic (Meliata s.l.) and one Early to Middle Jurassic (Piemont-Liguria s.l.). Nevertheless major incertitudes arise on the interpretations of the evolution of the former rifting, which lead to multiple or single, continuous oceanic branches. This uncertainity is mainly due to the successive orogenic overprint related to the formation of the Alpine belt and of the Western Mediterranean domain. The aim of this work is to explore how rifting events are recorded by the stratigraphic and structural evolution using both the vast existing literature and own observations. Selected areas belonging to different paleogeographic domains in the Alpine realm (Southalpine, Brianconnais s.l. and Austroalpine) will be studied in order to define relevant time-marker levels to map and correlate the temporal and spatial evolution of rift events. With this "basinal" approach we point to major tectonic events, filtering smaller-scale tectonics and minor environmental controlling factors on sedimentation. Our final goal is to identify "fingerprints" for major rifting events that may reveal the location and timing of hyper-extended domains. The evaporitic successions, the development of thick carbonate platforms, their demise or drowning, the iron-manganese hardgrounds sedimentation that could represent a response of hydrothermal circulation associated with hyper-extension, may correspond to correlable and mappable residues of large-scale, hyper-extended rift events. This data, together with subsidence analysis, basement and volcanics data provide a major, well constrained framework that can be used to compare the evolution of these two fossil rift systems with that of present-day potential analogues.

Decarlis, Alessandro; Manatschal, Gianreto; Masini, Emmanuel

2013-04-01

128

Magmatic expression of lithospheric thinning across continental rifts  

NASA Astrophysics Data System (ADS)

Studies of magmatism associated with continental rifting have traditionally focused only on volcanism within the downfaulted axial zone and along its immediate flanks. Teleseismic travel-time delay studies during the last decade have confirmed the results of earlier gravity surveys of rifted areas, showing that thinning at the base of the continental lithosphere occurs throughout a zone up to about 10 times wider than the physiographic expression of the rift. It is, therefore, logical to consider rifting-related magmatism on the same scale. Potential sources of mafic magmas in rift zones are the thinned subcontinental lithospheric mantle (SCLM), the convecting mantle beneath the continental plate and mixtures of the two. Detailed elemental and radiogenic isotope geochemical studies show that, during the initial extension of continental rifts, the associated mafic magmatism tends to be: (1) relatively sodic and from predominantly convecting mantle sources at the rift axis; (2) relatively potassic and from predominantly lithospheric mantle sources at the margins of the thinned-plate zone. This underlying geochemical pattern is obscured in many instances by such processes as crustal contamination and magma mixing within open-system reservoirs. The mafic ultrapotassic component that provides a distinctive input to SCLM-source magmas appears to be largely fusible at temperatures well below the dry solidus of SCLM; so that, in some cases, prolonged magmatism at a site causes removal of most or all of the potassic lithosphere-source melt (as mafic ultrapotassic magmas or as a contribution to mixed-source melts) without destruction of that lithosphere segment as a geophysically defined unit. Such a zone of refractory lithosphere permits subsequent, recognisable, convecting mantle source melts to penetrate it and reach the surface. These principles are illustrated by discussion of the Neogene-Quaternary magmatism of the Rio Grande, East African, Rhine and Baikal rifts, in the context of the most recent published models of their geophysical structures to depths > 200 km. Teleseismic and gravity studies identify lithospheric thinning beneath the Rio Grande, East African and Baikal rifts across zones 700-800 km wide. The failure of the southern Rhine graben to show a similar deep seismic structure may be a result of efficient buoyant migration of low-viscosity mafic alkalic melt out of the underlying mantle during the 7 Ma period since magmatism ceased, causing seismically defined asthenosphere to revert to lithosphere. A 700 km geochemical traverse across the Rio Grande rift at ~ 37°N, focusing on Oligocene-Miocene magmatism minimally affected by post-genesis processes, shows a clear symmetrical pattern of relatively sodic volcanics at the rift axis and mafic, ultrapotassic magmatism on its outer flanks. The geochemistry of these contrasting magma types is consistent with the view that they originated predominantly within the convecting mantle and SCLM, respectively. The same geochemical pattern is detectable in the volcanism within the equatorial segment of the East African rift system but it is complicated in two zones: east of the Gregory rift and southwest of Lake Kivu, by the effects of previous Cretaceous-Palaeogene magmatism. Limited, published, appropriate, geochemical data show that regional compositional variation in the volcanics associated with the Baikal rift appear to fit the pattern proposed here as a general model for rifting-related magmatism.

Thompson, R. N.; Gibson, S. A.

1994-05-01

129

Internal structure and evolution of a North-Atlantic rift system: the Desertas rift zone, Madeira Archipelago  

NASA Astrophysics Data System (ADS)

The three Desertas Islands represent the top of a 60 km long submarine ridge southeast of Madeira. They dominantly consist of lava flows, cinder cones stacked one on another and steeply dipping, partly sheeted dike swarms and thus represent the deeply eroded interior of a well-developed volcanic rift zone. In order to reconstruct the evolution of the Desertas rift and to understand its volcanotectonic origin, we have undertaken detailed field studies of volcanic structures. It was found that the abundance of scoria cones, dikes and sills indicating proximity to eruptive centers increase along the rift towards south: The northern end is composed mainly of lava flows up to several meter thickness, whereas the central part shows an alternation of proximal pyroclastics and series of subhorizontal lava flows. The southern part is dominated by scoria and cinder cones and minor thin lava flows. Dikes, isolated or as swarms, were observed all along the islands' coasts. The most distinctive and most dense dike swarm, however, occurs in the southernmost region, and sills up to 30 m thickness are also more common in this area. We did not find any volcanological or petrological indication of a central caldera underlain by a shallow magma reservoir feeding the rift zone. Because of the distribution of volcanics and intrusive rocks, we propose that a major volcanic center was located at the southern end of the Desertas ridge, and that the rift zone successively developed towards Madeira. This model implies that the Desertas ridge represents a discrete volcanic system that became interconnected with Madeira by growth to the north-northeast. The evolution of the Desertas rift hence differs from that of caldera-centered, two-armed rift systems typical for Hawaiian volcanoes such as Kilauea. A conceivable explanation for the formation of the Desertas rift zone is a local gravitative stress field causing preferred extension in-between the two overlapping Madeira and Desertas edifices.

Schwarz, S.; Klügel, A.

2003-12-01

130

Numerical modelling of quaternary deformation and post-rifting displacement in the Asal-Ghoubbet rift (Djibouti, Africa) [rapid communication  

NASA Astrophysics Data System (ADS)

Over the last three decades a host of information on rifting process relating to the geological and thermal structure, long-time scale deformation (Quaternary and Holocene) and rifting cycle displacement across the Asal-Ghoubbet rift has been made available. These data are interpreted with a two-dimensional thermo-mechanical model that incorporates rheological layering of the lithosphere, dyke inflation and faulting. Active fault locations and geometry are mainly controlled by both thermal structure and magma intrusion into the crust. The distributed slip throughout the inner rift is related to the closeness of magma chamber, leading to additional stress into the upper thinned crust. Assuming a constant Arabia-Somalia motion of 11 mm/year, the variation of subsidence rate between the last 100 and 9 ka is associated with a decrease of the average injection rate from 10 to 5 mm/year. These values, about equal to the regional opening rate, suggest that both volcanism and tectonic play an equivalent role in the rifting process. Our modelled sequence of events gives one possible explanation for both vertical and horizontal displacements observed since the 1978 seismovolcanic crisis. Although part of the post-rifting deformation could be due to viscous relaxation, the high opening rate in the first years after the event and the abrupt velocity change in 1984-1986 argue for a large dyke inflation of 12 cm/year ending in 1985. The asymmetric and constant pattern of the GPS velocity since 1991 suggests that present post-rifting deformation is mainly controlled by fault creep and regional stretching. This study demonstrates the internal consistency of the data set, highlights the role of magmatism in the mechanics of crustal stretching and reveals a complex post-rifting process including magma injection, fault creep and regional stretching.

Cattin, Rodolphe; Doubre, Cécile; de Chabalier, Jean-Bernard; King, Geoffrey; Vigny, Christophe; Avouac, Jean-Philippe; Ruegg, Jean-Claude

2005-11-01

131

Seismic Observations From the Afar Rift Dynamics Project: Preliminary Results  

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

132

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

SciTech Connect

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

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

1995-03-01

133

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

134

Rift-drift evolution of the outer Norwegian margin  

NASA Astrophysics Data System (ADS)

Most of the tectonic and dynamic concepts on the evolution of rifted margins have been developed from either intra-continental rift basins or proximal margin usually characterised by small amounts of crustal thinning. Some of these continental margins also display a high level of volcanic activity along the continent-ocean transition (COT). In such a context, the tectonic evolution of the proto-breakup rift system of the outer Norwegian margin is still problematic, due to sub-basalt imaging and a poor knowledge of the mechanisms involved before, during and slightly after the onset of breakup. Regional analysis and interpretation of multichannel seismic data, potential field data, integrated with refined plate reconstruction and finite-element modelling have provided the opportunity to propose an updated tectonic model for the evolution and segmentation of the Norwegian margin and the early Norwegian-Greenland Sea oceanic domain. Timing of deformation and structural styles observed along the conjugates reflect lateral variations of the rifted system which is influenced by complex inherited features, late magma-tectonic processes and local plate instabilities. We show that the deep structures associated with the volcanic rifted margin are still controversial and not necessarily so magmatic. We have also attempted to investigate the role of localised magmatic intrusion in rift and breakup dynamics and compared the results with our geophysical data, offshore Norway. The thickness, composition and temperature of the underplated and/or intruded bodies seem to be important factors that control lithospheric stretching, basin temperature, rift structure, margin asymmetry and COT formation. We also document the early spreading history of the mid-Norwegian by means of two news recent aeromagnetic surveys which highlight a complex spreading evolution correlated with the onset of microcontinent formation (Jan Mayen microcontinent) and an atypical (mid-Eocene?) magmatic event documented to the west of the Vøring Marginal High.

Gernigon, Laurent; Carmen, Gaina; Tadashi, Yamasaki; Gwenn, Péron-Pinvidic; Odleiv, Olesen

2010-05-01

135

Evolution of Rift Fault Populations in 2- and 3-Dimentions  

NASA Astrophysics Data System (ADS)

Numerous faults are created during rifting and the collective properties and spatio-temporal evolution of those faults may give insight into the underlying rifting process. We adopt a numerical approach to address topics related to a fault populations in rifting environments. First, sets of normal faults are generated by side- and bottom-driven extension applied to a weak rift structure, similar to the set-up for many sand and clay analogue models. Mohr-Coulomb plasticity with strain weakening is assumed and we explore constraints on model parameters that make numerically generated faults compatible with analogue counterparts in terms of cross-sectional geometry. Cohesion reduction is known to be sufficient for strain localization but, interestingly, a reduction in friction angle appears to be needed to reproduce upward-concave faults seen in analogue models. Next, the emergence of ‘domains’ of faults with similar dips is studied with a fully 3D numerical model. Faults are seen to grow and interact with each other and we attempt to test ideas about what controls the origin of such dip domains. In particular, we test the stress reduction zone model, which holds that fault initiation is inhibited in the stress shadow of large faults. We also consider the effect of rift thermal structure on the pattern of faulting seen for both oblique and orthogonal rifting. Preliminary results suggest that a line of thin lithosphere, such as might result from repeated along-axis dike intrusions, can greatly alter the spacing and length of faults formed by rifting. An example fault population represented by localized plastic strain and the corresponding surface topography. Mohr-Coulomb plasticity with strain weakening is assumed. The domain has the dimension of 40×30×4 km and a resolution of 500 m. The longest sides are pulled at a full spreading rate of 2 cm/yr. The results are shown after about 38 kyr of extension.

Choi, E.; Buck, W. R.

2010-12-01

136

Crust and Mantle Structure of a Closed Rift System from the Superior Province Rifting Earthscope Experiment (SPREE) (Invited)  

NASA Astrophysics Data System (ADS)

The existence of the 1.1 Ga Mid-continent Rift System (MRS) in the Great Lakes region of North America is well known on account of its prominent gravity and magnetic anomalies. These elongated anomalies are associated with dense igneous rocks, which surface in sparse outcrops and are imaged in a handful of active source profiles. Part of the MRS cuts across the Archean Superior Craton while other parts cut through at least three different Proterozoic terranes, though there are indications that offsets between rift segments, such as the Belle Plaine Fault, may follow pre-existing terrane boundaries. The total volume of igneous rock imaged in active source data is consistently estimated as at least one million km3, which is enough for a sea floor of the size of the current Gulf of California, or five times the size of Lake Baikal. However, cessation of rifting and closure of the rift uplifted the igneous rocks along the axes, causing lateral gravity gradients of 150 mgal over 50 km between the gravity high above the uplifted igneous rift axis and the low above the sediment deposits in the original rift flanks. Our seismic experiment (SPREE) covers an area around a one thousand km long segment of the MRS. A long, interrupted line of stations follows the rift axis, another line cuts across this high gravity gradient, yet another line cuts across the Belle Plaine rift axis offset, and a TA-like station group north of Lake Superior complements surrounding Transportable Array coverage. The Superior Province Rifting Earthscope Flexible Array (FA) Experiment (SPREE) has been running for two years with a data return of over 96%. Preliminary SPREE and other analyses show puzzling low velocities along the rift axis and complex Moho structure beneath thickened crust. Other crustal features include a large diversity of sediments, from soggy Quaternary mud through meta-sedimentary Proterozoic rocks. At the time of writing we are quantifying the effects of this complex geological history on our seismic data and attempting to analyze and interpret the residual data in the context of this complex geologic and rich MRS studies history. We will present and discuss constraints from SPREE data on lithospheric structure beneath the MRS from receiver functions, noise analysis, surface waves, and teleseismic travel times.

van der Lee, S.; Wolin, E.; Bollmann, T. A.; Tekverk, K.

2013-12-01

137

Evolution and characteristics of continental rifting: Analog modeling-inspired view and comparison with examples from the East African Rift System  

NASA Astrophysics Data System (ADS)

The evolution and characteristics of narrow continental rifting are illustrated in this paper through a review of recent lithospheric-scale analog models of continental extension compared with selected examples from the East African Rift System. Rift location is controlled by reactivation of lithospheric-scale pre-existing weaknesses; in these areas, the initial phases of rifting correspond to the activation of few, large-offset boundary faults that accommodate basin subsidence, which can be at places strongly asymmetric. The plan-view geometry of rift faults is primarily related to the relative orientation of the lithospheric weakness with respect to the extension direction: orthogonal rifting gives rise to long, extension-orthogonal boundary faults with associated pronounced subsidence, whereas oblique rifting results in a general en-echelon arrangement of faults and basins with less subsidence. Inherited fabrics having variable orientation with respect to the rift trend may control rift architecture at both regional and local scales. In these initial phases, widespread magmatism may encompass the rift, with volcanic activity localized along major boundary faults, transfer zones and limited portions of the rift shoulders (off-axis volcanism). Progressive extension leads to a change in deformation style from the few, large-offset boundary faults at the rift margins to dense fault swarms - with limited vertical motions - affecting the rift floor where the magmatic activity is concentrated. In these areas of focused tectono-magmatic activity (the so-called magmatic segments) the thinned lithosphere is strongly modified and weakened by the extensive magma intrusion, and extension is facilitated and accommodated by a combination of magmatic intrusion, dyking and faulting. The feedback between strain localization, magma injection and lithospheric weakening is self-reinforcing, facilitating the rupture of the continental lithosphere. At this stage, magmatic segments (as for instance in the Northern Main Ethiopian Rift) act as incipient slow-spreading mid-ocean ridges, developing within a lithosphere that is transitional between continental and oceanic.

Corti, Giacomo

2012-02-01

138

Hydrogeochemical and lake level changes in the Ethiopian Rift  

NASA Astrophysics Data System (ADS)

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

Alemayehu, Tamiru; Ayenew, Tenalem; Kebede, Seifu

2006-01-01

139

East Antarctic rifting triggers uplift of the Gamburtsev Mountains  

USGS Publications Warehouse

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

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

2011-01-01

140

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

141

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

142

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

143

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

144

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

E-print Network

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

Olsen, Paul E.

145

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

Microsoft Academic Search

Oceanic island volcanoes commonly contain rift zones along which eruptive centers and parallel dike complexes are concentrated. Formation and orientation of rifts often remain enigmatic, however. Tectonic lineaments and regional zones of weakness facilitating magma ascent may be one reason of rift evolution, e.g. Sao Jorge (Azores) or Iceland. Alternatively, gravity tectonics of a volcano may cause formation of dike

A. Kluegel; T. R. Walter

2003-01-01

146

Basin-scale migration of continental-rift brines: Paleohydrologic modeling of the Dead Sea basin  

Microsoft Academic Search

It was suggested that brine of the Dead Sea rift has originated from a residual product of intensively evaporated seawater that invaded the rift, precipitated halite, and later interacted through dolomitization with the host rock during subsurface migration. Detection of this brine in many deep wells located at distances as far as 100 km away from the rift was attributed

Eyal Stanislavsky; Haim Gvirtzman

1999-01-01

147

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

148

Host Alternation Is Necessary to Maintain the Genome Stability of Rift Valley Fever Virus  

E-print Network

Host Alternation Is Necessary to Maintain the Genome Stability of Rift Valley Fever Virus Sara evolution. To test this hypothesis, we used Rift Valley fever virus (RVFV), an arbovirus that can Is Necessary to Maintain the Genome Stability of Rift Valley Fever Virus. PLoS Negl Trop Dis 5(5): e1156. doi

Paris-Sud XI, Université de

149

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

E-print Network

A Fusion-Inhibiting Peptide against Rift Valley Fever Virus Inhibits Multiple, Diverse Viruses (Class I, II, and III) based on the protein sequence and structure. For Rift Valley fever virus (RVFV KW, Taylor SL, et al. (2013) A Fusion-Inhibiting Peptide against Rift Valley Fever Virus Inhibits

150

An Unexpected Recurrent Transmission of Rift Valley Fever Virus in Cattle in a Temperate and Mountainous  

E-print Network

An Unexpected Recurrent Transmission of Rift Valley Fever Virus in Cattle in a Temperate States of America Abstract Rift Valley fever is an acute, zoonotic viral disease of domestic ruminants­2009. The goal of the present study was to evaluate the point prevalence of antibodies against Rift Valley Fever

Paris-Sud XI, Université de

151

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

E-print Network

superposed on regional plains and determined if post-regional-plains (PRP) volcanics and/or PRP rift of these relationships, it was found that the general rates of volcanism and rifting during PRP time were close and rifting in the vicinity of the boundary between PRP time and the preceding time, which is marked

Head III, James William

152

Submarine thermal springs on the Gala??pagos Rift  

USGS Publications Warehouse

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

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

1979-01-01

153

Extension and rifting: the Zeit region, Gulf of Suez  

NASA Astrophysics Data System (ADS)

A field analysis of faults and fractures in the Ras Gharib-Ras Gemsa region of the Gulf of Suez shows that the main Late Cenozoic extension occurred perpendicular to the rift axis. Three main types of dip-slip normal faults successively developed as the tilt of blocks bounded by antithetic normal faults increased. Determinations of the amount of extension from structural data are compatible with estimates made using subsidence data through a simplified model of lithospheric stretching. The uplift of rift shoulders is related in chronology and volume to the subsidence of the rift. The geometry of fault patterns and directions of extension suggests that the Late Cenozoic total movement corresponds to a counterclockwise rotation of 4-5° of Sinai relative to Africa, with a pole close to Cairo.

Angelier, J.

154

A groundwater convection model for Rio Grande rift geothermal resources  

NASA Technical Reports Server (NTRS)

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

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

1981-01-01

155

The First Prediction of a Rift Valley Fever Outbreak  

NASA Technical Reports Server (NTRS)

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

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

2009-01-01

156

Flexural analysis of uplifted rift flanks on Venus  

NASA Technical Reports Server (NTRS)

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

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

1992-01-01

157

Petroleum system of the Shelf Rift Basin, East China Sea  

SciTech Connect

The Tertiary section of the Oujioang and Quiontang Depressions of the East China Sea Basin consists of at least eight rift-related depositional sequences identified seismically by regionally significant onlap and truncation surfaces. These sequences are calibrated by several wells including the Wenzhou 6-1-1 permitting extrapolation of petroleum system elements using seismic facies analysis. Gas and condensate correlated to non-marine source rocks and reservoired in sandstone at the Pinghu field to the north of the study area provides an known petroleum system analogue. In the Shelf Rift Basin, synrift high-amplitude parallel reflections within the graben axes correlate with coaly siltstone strata and are interpreted as coastal plain and possibly lacustrine facies with source rock potential. Synrift clinoform seismic facies prograding from the northwest footwall correlate with non-marine to marginal marine conglomerate, sandstone and siltstone, and are interpreted as possible delta or fan-delta facies with reservoir potential although porosity and permeability is low within the Wenzhou 6-1-1 well. Post-rift thermal sag sequences are characterized by parallel and relatively continuous seismic reflections and locally developed clinoform packages. These facies correlate with porous and permeable marine sandstone and siltstone. Shales of potential sealing capacity occur within marine flooding intervals of both the synrift and post-rift sequences. Traps consist of differentially rotated synrift fill, and post-rift inversion anticlines. Major exploration risk factors include migration from the synrift coaly source rocks to the post-rift porous and permeable sandstones, and seismic imaging and drilling problems associated with extensive Tertiary igneous intrusions.

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

1996-12-31

158

Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana  

NASA Astrophysics Data System (ADS)

We used aeromagnetic and gravity data to investigate the thermal structure beneath the incipient Okavango Rift Zone (ORZ) in northwestern Botswana in order to understand its role in strain localization during rift initiation. We used three-dimensional (3-D) inversion of aeromagnetic data to estimate the Curie Point Depth (CPD) and heat flow under the rift and surrounding basement. We also used two-dimensional (2-D) power-density spectrum analysis of gravity data to estimate the Moho depth. Our results reveal shallow CPD values (8-15 km) and high heat flow (60-90 mW m-2) beneath a ~60 km wide NE-trending zone coincident with major rift-related border faults and the boundary between Proterozoic orogenic belts. This is accompanied by thin crust (<30 km) in the northeastern and southwestern parts of the ORZ. Within the Precambrian basement areas, the CPD values are deeper (16-30 km) and the heat flow estimates are lower (30-50 mW m-2), corresponding to thicker crust (~40-50 km). We interpret the thermal structure under the ORZ as due to upward migration of hot mantle fluids through the lithospheric column that utilized the presence of Precambrian lithospheric shear zones as conduits. These fluids weaken the crust, enhancing rift nucleation. Our interpretation is supported by 2-D forward modeling of gravity data suggesting the presence of a wedge of altered lithospheric mantle centered beneath the ORZ. If our interpretation is correct, it may result in a potential paradigm shift in which strain localization at continental rift initiation could be achieved through fluid-assisted lithospheric weakening without asthenospheric involvement.

Leseane, Khumo; Atekwana, Estella A.; Mickus, Kevin L.; Abdelsalam, Mohamed G.; Shemang, Elisha M.; Atekwana, Eliot A.

2015-02-01

159

Continental rifting and the origin of Beta Regio, Venus  

NASA Technical Reports Server (NTRS)

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

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

1981-01-01

160

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

161

Lithosphere Response to Intracratonic Rifting: Examples from Europe and Siberia  

NASA Astrophysics Data System (ADS)

Several cratons have experienced a significant modification of their crustal and mantle lithosphere structure during Phanerozoic large-scale lithosphere-mantle interactions. In Eurasia, the most prominent examples include the Dniepre-Donets rift in the East European craton, the Oslo graben in the Baltic shield, the Viluy rift and the Baikal rift in Siberia. Despite some similarities, mostly in the crustal structure, there are also significant differences in the lithospheric structure of these rifts. Besides, a large lithosphere-scale Riphean suture/rift runs across the East European craton. While this suture can be recognized in the crustal structure, it is not clearly seen in the structure of the lithospheric mantle. In contrast, Phanerozoic processes associated with emplacement of large magma volumes had a strong effect on modification of the lithosphere structure, primarily by infiltration of basaltic magmas and consequently in a change in mantle density and seismic velocities. Although kimberlite magmatism is commonly not considered as a rifting events, its deep causes may be similar to the mantle-driven rifting and, as a consequence, modification of mantle density and velocity structure may also be expected. We present a new model for the structure of the crust in an area that encompasses the East European craton, the West Siberian basin, and the Siberian cratons. The region includes a nearly continuous age record for lithosphere evolution over ca. 3.6-3.8 billion years. The crustal model is based on critically assessed results from various seismic studies, including reflection and refraction profiles and receiver function studies. We also use global shear-wave tomography models, gravity constraints based on GOCE data, and thermal models for the lithosphere to speculate on thermo-chemical heterogeneity of the mantle. An analysis of the lithosphere structural heterogeneity is presented in relation to geotectonic setting and mantle geodynamics based on interpretations of geophysical models. The results indicate that there is no simple rule for lithosphere modification by intracontinental rifting. The pattern of is controlled by the pre-existing tectonic setting and the intensity of lithosphere-mantle interaction. The results are summarized in a series of maps of lateral variations in lithosphere structure, including the depth to the LAB and compositional heterogeneity of the lithosphere as reflected in it seismic wave velocity and density structure.

Artemieva, I. M.; Thybo, H.; Herceg, M.; Cherepanova, Y. V.; Chemia, Z.; Cammarano, F.

2012-12-01

162

Deformation signals in the currently-rifting Afar (Ethiopia) Rift measured with InSAR  

NASA Astrophysics Data System (ADS)

A major rifting episode is currently occurring in the Dabbahu magmatic segment in the Afar region. The rifting episode began in September-October 2005, when continuous seismicity, a volcanic eruption and extensive diking occurred along the 60-km-long magmatic segment. Since then, nine additional dike intrusions occurred in the area and have been detected by radar interferometry (InSAR), with the most recent dike intrusion occurring between July 4-21, 2008 at the southern edge of the segment. We used radar images acquired by the European satellite, Envisat, in both descending and ascending orbits to form interferograms, spanning the time period from October 2005 to the present. The interferograms cover the main magmatic segments in the Afar region including Dabbahu, Hararo, Alayta and Erta Ale. The recorded interferograms were inverted using a least-squares method to obtain average deformation maps and a time series of incremental deformations. Preliminary results indicate that rapid deformation is currently confined to the Dabbahu region with no large deformation signals observed in the nearby magmatic segments. High rates of deformation are observed in the Dabbahu segment associated with shallow magma movements and possible visco-elastic relaxation. Outside the Dabbahu segment, the largest signal is a broad area subsiding at a steady rate of 10 cm/yr, located south east of the Dabbahu segment, and east of the Hararo segment. Our preliminary interpretation is that this represents deep flow of magma away from the Hararo segment and towards Dabbahu. The observed deformation patterns will also be compared to seismicity observed over the same time periods.

Pagli, C.; Wright, T. J.; Wang, H.; Hamling, I. J.; Kier, D.; Belachew, M.; Ebinger, C.

2008-12-01

163

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

USGS Publications Warehouse

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

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

1992-01-01

164

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

165

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

NASA Technical Reports Server (NTRS)

Rocks of the Pongola Supergroup form an elongate belt in the Archean Kaapvaal Craton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, including rapid lateral variations in thickness and character of sediments, volcanic rocks that are bimodal in silica content, coarse, basement derived conglomerates and thick sequences of shallow water sedimentary facies associations, it is suggested that the Pongola Supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in earth history reveals striking similarities, suggesting that the processes that formed this rift were not significantly different from those that form continental rifts today.

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

1985-01-01

166

Assessing the extent of carbonate deposition in early rift settings  

E-print Network

. His work concentrates on using technology to resolve seabed architecture of modern and ancient the Red Sea shows well advanced rifting with marine incursion and reef­skeletal sand development. AUTHORS Paul M. (Mitch) Harris Chevron Energy Technology Company, San Ramon, California; mitchharris

Purkis, Sam

167

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...

168

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 ...

169

Seismicity of the Baikal rift system from regional network observations  

NASA Astrophysics Data System (ADS)

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

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

2013-01-01

170

Hydrothermal vents in Lake Tanganyika, East African, Rift system  

Microsoft Academic Search

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

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

1993-01-01

171

Volcaniclastic alluvial fan sedimentation, northern Rio Grande rift  

Microsoft Academic Search

The Pliocene Puye Formation is a well-exposed, volcanogenic, alluvial fan sequence 150+ m thick, representing a range of volcaniclastic deposits (proximal, medial, and distal) that may be generated in response to long-lived, multicompositional (basaltic to rhyolitic) volcanism in a rift setting. The deposits are a composite of eruptives (effusives and pyroclastics) and epiclastics (reworked primary volcanics). An almost complete record

J. G. McPherson; D. B. Waresback; S. Self

1986-01-01

172

Spatial and temporal evolution of the Suez rift subsidence  

NASA Astrophysics Data System (ADS)

In an attempt to characterize the longitudinal evolution of the Suez rift, subsidence curves have been computed for 75 wells and 10 cross-sections distributed all along the rift. Tectonic and overall subsidence have been distinguished by means of the backstripping method showing the role of sedimentary loading effects. Regional maps and profiles of the tectonic subsidence indicate that, because of block tilting, transversal discrepancies are much more important than longitudinal ones. Along the axial zone no p3opagating phenomenon could be demonstrated, and the whole rift length is attained during the first 2 or 3 My as evidenced by the ubiquitous deposition of the basal Nukhul Fm of Aquitanian to Lower Burdigalian age. The tectonic subsidence history appears identical all along the axial trough during the first stages of rifting. It is characterized by two major events: (1) a rapid and strong subsidence between 20 and 15 My (Rudeis Fm), and (2) a tectonic quiescence between 15 and 5 My. Main longitudinal variations occur after 5 My, and during this period 3 zones may be distinguished: (1) a northern area (Darag Basin) where uplift is dominant, (2) a central part (most of the Gulf) with very low tectonic subsidence or quiescence and (3) a southern part (close to the Red Sea) where tectonic subsidence has restarted.

Moretti, I.; Colletta, B.

1987-02-01

173

Mapping of the major structures of the African rift system  

NASA Technical Reports Server (NTRS)

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

Mohr, P. A. (principal investigator)

1973-01-01

174

The importance of rift history for volcanic margin formation  

Microsoft Academic Search

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

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

2010-01-01

175

The protracted development of focused magmatic intrusion during continental rifting  

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

176

Masirah Graben, Oman: A hidden Cretaceous rift basin  

SciTech Connect

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

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

1995-06-01

177

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...

178

Seismicity of the Earth 1900-2013 East African Rift  

USGS Publications Warehouse

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

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

2014-01-01

179

Monitoring Volcanic Gases on Kilauea's East Rift Zone  

USGS Multimedia Gallery

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

180

Monitoring Volcanic Gases on Kilauea's East Rift Zone II  

USGS Multimedia Gallery

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

181

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...

182

Mapping of the major structures of the African rift system  

NASA Technical Reports Server (NTRS)

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

Mohr, P. A. (principal investigator)

1973-01-01

183

Masirah Graben, Oman: A hidden Cretaceous rift basin  

Microsoft Academic Search

Reflection seismic data, well data, geochemical data, and surface geology suggest that a Cretaceous rift basin exists beneath the thrusted allochthonous sedimentary sequence of the Masirah graben, Oman. The Masirah graben is located east of the Huqf uplift, parallel to the southern coast of Oman. The eastern side of the northeast-trending Huqf anticlinorium is bounded by an extensional fault system

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

1995-01-01

184

Observations on the epidemiology of Rift Valley fever in Kenya.  

PubMed Central

The epizootic range of Rift Valley fever in Kenya is defined from the results of virus isolations during epizootics, and form an extensive serological survey of cattle which were exposed during an epizootic. A study of the sera from a wide range of wild bovidae sampled immediately after the epizootic, showed that they did not act as reservoir or amplifying hosts for RVF. Virus isolation attempts from a variety of rodents proved negative. Rift Valley fever did not persist between epizootics by producing symptomless abortions in cattle in areas within its epizootic range. A sentinel herd sampled annually after an epizootic in 1968 revealed not one single seroconversion from 1969 to 1974. Certain forest and forest edge situations were postulated as enzootic for Rift Valley fever, and a small percentage of seroconversions were detected in cattle in these areas, born four years after the last epizootic. This has been the only evidence for the persistence of the virus in Kenya since 1968, and may be a part of the interepizootic maintenance cycle for Rift Valley fever in Kenya, which otherwise remains unknown. PMID:1058243

Davies, F. G.

1975-01-01

185

Rift Valley Fever Overview and Recent Developments at USDA  

Technology Transfer Automated Retrieval System (TEKTRAN)

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

186

Rifting between India and Madagascar - mechanism and isostasy  

NASA Astrophysics Data System (ADS)

The nature of isostasy along the Western Continental Margin of India (WCMI) and the Eastern Continental Margin of Madagascar (ECMM) and their conjugate nature are examined. The first part of the study deals with the estimation of effective elastic thickness ( Te) of the lithosphere for the two margins through cross-spectral analysis of gravity and bathymetry data. The results bring out comparable Te values of 8-15 km for the WCMI and 10-13 km for the ECMM, despite the WCMI having been traversed along strike by a hotspot trace. We have also compared topographic profiles across the two margins extending inland, to examine the rift-flank uplift experienced by the two landmasses abutting the margins, which bear a striking similarity. These results establish the symmetrical rifting mechanism resulting in the formation of the two margins, and indicate that despite the presence of a hotspot trace along one margin (i.e. the WCMI), the two margins have comparable isostatic compensation mechanisms with low Te values. In the second part of the study we have examined the possible mechanism for the rift-flank uplifts using a process-oriented approach of backstripping the sediments along a traverse across the WCMI and reconstructing the original rift flank topography by flexurally backstacking the amount of eroded material on the present-day topography of the Western Ghats. The backstack load configuration is taken from the amount of erosion that took place over the Western Ghats, obtained from AFTA (Apatite Fission Track Analysis) studies. The reconstructed rift time topography and bathymetry are used to get the Moho configuration, which was used to calculate the gravity anomaly, and is compared with the observed anomaly to derive a possible evolution model for the region. We have tried to examine the observed gravity anomaly through two different processes. In the first process, a Te=15 km for backstripping and backstacking, and a compensated underplating for the topography give a best fit model to the observed gravity. In the second process, a necking model with a deep level of necking (20 km) with Te=15 km and without any underplating explains the topography as well as the observed gravity. The nature and amount of the Western Ghat topography and cratonic age (Archaen) of the pre-rifted crust of western India and eastern Madagascar are more in favor of the rifting mechanism (necking model) rather than due to the passage of the Reunion hotspot (underplating model). This is evidenced by the observed increase in topography away from the influence of the Reunion hotspot trace as well as the presence of a similar topography in Madagascar.

Chand, Shyam; Subrahmanyam, C.

2003-05-01

187

Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana  

NASA Astrophysics Data System (ADS)

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

Leseane, Khumo

188

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

189

Sedimentology of rift climax deep water systems; Lower Rudeis Formation, Hammam Faraun Fault Block, Suez Rift, Egypt  

NASA Astrophysics Data System (ADS)

In most marine rift basins, subsidence outpaces sedimentation during rift climax times. Typically this results in sediment-starved hangingwall depocentres dominated by deep-marine mudstones, with subordinate local development of coarser clastics in the immediate hangingwall derived from restricted catchments on the immediate footwall scarp. To highlight the spatial variability of rift climax facies and the controls upon them, we have investigated the detailed three-dimensional geometry and facies relationships of the extremely well exposed Miocene, rift climax Lower Rudeis Formation in the immediate hangingwall to the Thal Fault Zone, Suez Rift, Egypt. Detailed sedimentological analyses allows the Lower Rudeis Formation to be divided into two contemporaneous depositional systems, (1) a laterally continuous slope system comprising, hangingwall restricted (< 250 m wide) slope apron, slope slumps, fault scarp degradation complex and laterally extensive lower slope-to-basinal siltstones, and (2) a localized submarine fan complex up to 1 km wide and extending at least 2 km basinward of the fault zone. Interpretation of individual facies, facies relationships and their spatial variability indicate that deposition in the immediate hangingwall to the Thal Fault occurred via a range of submarine concentrated density flows, surge-like turbidity flows, mass wasting and hemipelagic processes. Major controls on the spatial variability and stratigraphic architecture of the depositional systems identified reflect the influence of the steep footwall physiography, accommodation and drainage evolution associated with the growth of the Thal Fault. The under-filled nature of the hangingwall depocentre combined with the steep footwall gradient result in a steep fault-controlled basin margin characterised by either slope bypass or erosion, with limited coastal plain or shelf area. Sediment supply to the slope apron deposits is controlled in part by the evolution and size of small footwall drainage catchments. In contrast, the localized submarine fan is interpreted to have been fed by a larger, antecedent drainage network. The structural style of the immediate footwall is also believed to exert a control on facies development and stratigraphic evolution. In particular, fault scarp degradation is enhanced by fault propagation folding which creates basinward-dipping bedding planes in the pre-rift footwall strata that large pre-rift blocks slide on.

Leppard, Christopher W.; Gawthorpe, Rob L.

2006-09-01

190

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

191

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

USGS Publications Warehouse

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

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

1998-01-01

192

Possible axial extension in the Gulf of Suez rift (work hypothesis)  

NASA Astrophysics Data System (ADS)

The cross structure (SW-NE) of the Gulf of Suez is well documented through several studies on the rifting in the area, whereas less emphasis has been placed on the structural interpretation along the axial direction of the rift itself. The aeromagnetic data show the presence of two main trends: "clysmic" (N320°) and "cross" trend (N50°). The reflection seismic profiles covering the Gulf of Suez offshore area indicate an alternance of ranges and asymmetrical basins in the axial direction as well as across the rift. Well data demonstrate the presence of structural dips either in NW or SE directions both in the pre-rifting and syn-rifting sedimentary sequences deposited in these asymmetrical basins. In an overall rifting tectonic framework, to explain the structural configuration resulting from the above data along the axial direction would imply a possible extension parallel to the axis of the rifting itself. This extension component occurs along the pre-existing cross trend oblique to the rifting, while the Aqaba trend does not seem to play any role. The axial extension has been active since the time of early rifting and it appears to have been reactivated in more recent times. In a plate tectonic framework, the axial extension of the Gulf of Suez area implies a movement of the Sinai microplate oblique to the axis of the rifting.

Argenton, A.; Maccagni, A.

1988-10-01

193

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

NASA Technical Reports Server (NTRS)

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

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

2008-01-01

194

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

195

Albertine Rift, Uganda: Deformation-Sedimentation-Erosion relationships  

NASA Astrophysics Data System (ADS)

The Albertine Rift is the northern part of the western branch of the East African Rift that runs over a distance of around 2000 km from Lake Albert in the north to Lake Malawi in the south. Lake Albert Basin is assumed to be a classical half-graben initiated around 12 Ma and oriented NNW-SSW, with a major northwesterly bounding fault - the Bunia fault - located along the western Congolese shoreline (Ebinger, 1989; Pickford & al., 1993). The aim of this study is to understand the relationships between deformation, erosion, and sedimentation of the rift through time by restoring (1) the timing and amplitude of vertical movements (subsidence, uplift), (2) the geometry and paleo-environmental evolution (including climate) of the sedimentary infilling and (3) the geomorphological evolution of the surrounding area and associated erosion budget. Seismic data and outcrops studies suggest a much more complex history than previously described. (1) The age model, mainly based on mammal fossils (Pickford et al., 1993; Van Damme and Pickford, 2003), is debated, but the early stage of the rift is probably Middle Miocene. (2) No half-graben geometry has been characterized: the infilling consists of juxtaposed tabular compartments with sharp thicknesses variations along bounding faults, in response of either low rate extensional or combined strike-slip/extensional movements. The following onshore-offshore evolution is proposed: - Middle Miocene (~ 13 Ma) to Late Miocene (?): rifting 1 - differential subsidence along N60° faults - major deepening from fluvio-deltaic to deep lacustrine environments (maximum flooding at 8 Ma) - uplift, erosion and reworking of weathered profiles - first generation of pediments. - Late Miocene (?) to Late Pliocene (~ 3 Ma): quiescence phase - homogenous subsidence - lacustrine clays interbedded with sandy flood-lobes - uplift, erosion and reworking of ferruginous laterite (iron duricrusts) - second generation of pediments. - Late Pliocene (~ 3Ma) to Early Pleistocene (~ 2 Ma): rifting 2 - major uplift and growth of the Ruwenzori Mountains (5000 m of elevation) - differential subsidence - deltaic to wave-dominated coast (shoreface) sandy deposits - pediments degradation by fluvial erosion. - Middle-Late Pleistocene: late regional uplift and tilting - drainage inversion and present-day scarp formation.

Simon, Brendan; Guillocheau, François; Robin, Cécile; Dauteuil, Olivier; Nalpas, Thierry; Bourges, Philippe; Bez, Martine; Lays, Philippe

2014-05-01

196

Seismological Constraints on the Magmato-tectonic Behavior of the Asal-Ghoubbet Rift (Afar Depression, Republic of Djibouti) Since the Last 1978-Rifting Episode  

NASA Astrophysics Data System (ADS)

The Asal-Ghoubbet rift was the locus of a seismic and volcanic crisis in 1978 followed by 8 years of rapid opening (60 mm/yr) before returning to its long-term opening rate of 16 mm/yr. We analyze the space-time evolution of the seismicity that occurred in the rift between 1979 and 2001. The data recorded by the Djibouti Observatory provide only hypocentral locations before 1995 and P and S-wave arrival times since 1996. Additional data acquired during a five months experiment in 2000-2001 allowed us to determine a 3D-velocity model of the rift, used to precisely relocate post 1996 events. The 2545 small-magnitude earthquakes (Md ? 3.2) recorded in the rift since the 1978 crisis provide a negligible contribution to the total extension across the rift, which occurs essentially aseismically. The temporal evolution of the seismicity reveals two distinct phases consistent with those observed in the geodetic data. The post-crisis period (1979-1986) is characterized by large-magnitude earthquakes exclusively located below the northern rift shoulder. These events are associated with the contraction of the side of the rift resulting from the fast opening of the central dyke system. The subsequent period (1987-2001) corresponding to normal opening rate across the rift is characterized by a micro-seismicity essentially located below the major rift caldera (Fieale). Most recorded events during this period concentrate within the rift inner floor at the top of an aseismic, low velocity zone located below the Fiale caldera, which we interpret as hot material above the magma chamber. Outside from post-crisis periods, the seismicity tends to cluster in time in response to stress changes in the brittle layer induced by episodic magmatic movements.

Doubre, C.; Manighetti, I.; Bertil, D.; Dorbath, C.; Dorbath, L.; Jacques, E.

2004-12-01

197

Rifting and Subsidence in the Gulf of Mexico: Implications for Syn-rift, Sag, and Salt Sections, and Subsequent Paleogeography  

NASA Astrophysics Data System (ADS)

Thick (up to 5 km), rapid (<3 Ma), salt deposition is problematic for basin modelling because such accommodation cannot be thermal, yet GoM salt deposits (Late Callovian-Early Oxfordian) appear to be post-rift (most salt overlies planar sub-salt unconformities on syn-rift section). One possible solution is that the pre-drift GoM was a deep (~2 km) air-filled rift depression where basement had already subsided tectonically, and thus could receive up to 5 km of salt, roughly the isostatic maximum on exhumed mantle, hyper-thinned continent, or new ocean crust. ION-GXT and other seismic data along W Florida and NW Yucatán show that (1) mother salt was only 1 km thick in these areas, (2) that these areas were depositionally connected to areas of thicker deposition, and (3) the top of all salt was at global sea level, and hence the sub-salt unconformity along Florida and Yucatán was only 1 km deep by end of salt deposition. These observations fit the air-filled chasm hypothesis; however, two further observations make that mechanism highly improbable: (1) basinward limits of sub-salt unconformities along Florida/Yucatán are deeper than top of adjacent ocean crust emplaced at ~2.7 km subsea (shown by backstripping), and (2) deepest abyssal sediments over ocean crust onlap the top of distal salt, demonstrating that the salt itself was rapidly drowned after deposition. Study of global ION datasets demonstrates the process of "rapid outer marginal collapse" at most margins, which we believe is achieved by low-angle detachment on deep, landward-dipping, Moho-equivalent surfaces such that outer rifted margins are hanging walls of crustal scale half-grabens over mantle. The tectonic accommodation space produced (up to 3 km, < 3 Ma) can be filled by ~5 km of sag/salt sequences with little apparent hanging wall rifting. When salt (or other) deposition lags behind, or ends during, outer marginal collapse, deep-water settings result. We suggest that this newly identified, "outer marginal detachment phase", normally separates the traditional "rift" from "drift" stages during continental margin creation. Importantly, this 2-3 km of subsidence presently is neither treated as tectonic nor as thermal in traditional subsidence analysis; thus, Beta estimates may be excessive at many outer margins. Outer marginal collapse was probably eastwardly diachronous with initiation of spreading in the GoM. Additionally, recent paleo-climate studies suggest humid Early/Middle Jurassic conditions in equatorial GoM, hindering air-filled chasm development, but North America's northward flight into middle latitudes initiated Callovian aridity.

Pindell, J. L.; Graham, R.; Horn, B.

2013-05-01

198

Rifting of a multi-layered continental crust: dynamical and rheological controls on syn-rift subsidence rates  

NASA Astrophysics Data System (ADS)

We further develop the thermo-mechanical numerical model of continental rifting based on multi-layered brittle-elasto-ductile rheology. The continental lithosphere presents a system of alternating competent and weak layers with temperature, stress and strain-rate dependent properties. Contrary to common ideas, our model suggests that in such a system there is no permanent level of necking, but several necking planes, which migrate from one competent layer to another during syn-rift stage. At the first order, the subsidence and uplift history of the basin depend on the mode of rifting. The overlying lithosphere is extended either in passive way under the action of far-field extension force either in an active way in the presence of a mantelic plume. In the first case, an instability develop lately at the base of the lithosphere. Its ascent is accommodated by an already weaked lithosphere and thus its positive buoyancy is compensated at small scale producing accentuated uplift of rift shoulders. In the second case, the mantle lithosphere is still strong when the hot material arrives to its bottom. The positive buoyancy due to the hot plume is thus compensated in surface as a large flexural swell untill the ascending hot material weakens, extends and erodes the bottom of the competent lithospheric mantle. Yet, the final rift geometry also largely depends on the crustal structure. The presence of weak intermediate crustal layers changes the lithospheric response to under-pluming and far-field forces. Intra-crustal layers favour mechanical decoupling and lateral ductile flow between the competent layers which subside in the space created by extension. When resistance of one of the competent layers vanishes due to extension, the integrated strength of the lithosphere drops in a step-wise fashion resulting in accelerated subsidence phases. If the extended layer is initially found below the initial level of necking, this level switches to another discrete depth level. Dependent on the number of competent rheological layers, multiple subsidence/stagnation events may occur during syn-rift evolution. These complex behaviours are strongly controlled by the initial geotherm, rheological properties, extension rate, surface processes, and total amount of extension.

Le Pourhiet, L.; Burov, B.

2003-04-01

199

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. PMID:16318720

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

2005-01-01

200

Rift Valley fever virus: an unrecognized emerging threat?  

PubMed

Rift Valley fever virus (RVFV) is an arthropod-borne pathogen that often results in severe morbidity and mortality in both humans and livestock. As its geographic range continues to spread, it presents a real threat to naïve populations around the world by accidental introduction (e.g., the result of increased world travel) or a bioterror event. The lack of prophylactic and therapeutic measures, the potential for human-to-human transmission, and the significant threat to livestock associated with RVFV make infection with these pathogens a serious public health concern. Rift Valley fever epizootics and epidemics might rapidly overwhelm the capacities of the public health and veterinary medical communities to provide rapid diagnostic testing, distribution of countermeasures and adequate medical care. PMID:20421731

Mandell, Robert B; Flick, Ramon

2010-07-01

201

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

PubMed

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

Passarelli, L; Rivalta, E; Shuler, A

2014-01-01

202

Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes  

PubMed Central

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

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

2014-01-01

203

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

204

Seismicity of oceanic and continental rifts—a geodynamic approach  

Microsoft Academic Search

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

P. O. Sobolev; D. V. Rundquist

1999-01-01

205

Rifted continental margins: The case for depth-dependent extension  

NASA Astrophysics Data System (ADS)

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

Huismans, Ritske S.; Beaumont, Christopher

2014-12-01

206

Seismic Evidence for an Active Southern Rio Grande Rift  

NASA Astrophysics Data System (ADS)

Competing models exist to explain what caused the Earth’s crust to spread apart 29 million years ago to create a region known today as the Rio Grande Rift (RGR). The RGR extends from central Colorado through New Mexico to northern Mexico, near El Paso. A growing body of evidence shows that geologic activity still occurs in the RGR, with a continuation of faulting, seismicity and a small widening rate. We map of the seismic velocity structure and crustal thickness using data from the Rio Grande Rift Seismic TRAnsect (RISTRA) experiment and the EarthScope Transportable Array (USArray) dataset. In addition to the data we collected from the RISTRA experiment and USArray dataset, we also acquired receiver functions from the EarthScope Automatic Receiver Survey (EARS) website (http://www.earthscope.org/data) and waveform data from the Incorporated Research Institutes for Seismology (IRIS) Data Management Center (DMC). In particular, we requested seismograms from the IRIS DMC database where we acquired teleseismic events from Jan 2000 to Dec 2009. This includes 7,259 seismic events with a minimum magnitude of 5.5 and 106,389 continuous waveforms. This data was preprocessed (merged, rotated) using a program called Standing Order of Data (SOD). We computed receiver functions and receiver function stacks for all data in the Southern Rio Grande Rift (SRGR). We map the crustal thickness, seismic velocity, and mantle structure to better determine the nature of tectonic activity that is presently taking place and further investigate the regional extension of the Southern Rio Grande Rift (SRGR). Here we present results of the crustal and velocity structure using the kriging interpolation scheme and interpret our results in relation to southern RGR deformation and extension.

Thompson, L. E.; Velasco, A. A.

2010-12-01

207

Erosion of Terrestrial Rift Flank Topography: A Quantitative Study  

NASA Technical Reports Server (NTRS)

Many rifted or passive continental margins feature a seaward-facing erosional escarpment which abruptly demarcates deeply weathered, low relief, interior uplands from a deeply incised, high relief coastal zone. It is generally accepted that these escarpments originate at the time of continental rifting and propagate inland through the elevated rift flank topography at rates on the order of 1 km/Myr over the course of a margin's history. Considering the length of passive margins worldwide and an average rift flank plateau height of several hundred meters, it is clear that sediment eroded from passive margins is an important component of the mass flux from continents to oceans through geologic time. The overall goal of the research reported here is to develop a quantitative understanding of the kinematics of escarpment propagation across passive margins and the underlying geological processes responsible for this behavior. Plateau-bounding escarpments in general exhibit two basic forms depending on the direction of surface water drainage on the plateau interior relative to the escarpment. Where surface water flows away from the escarpment, the escarpment takes the form of subdued embayments and promontories, such that its overall trend remains fairly straight as it evolves with time. Where upland streams flow across the escarpment, it takes the form of dramatic, narrow gorges whose heads appear to propagate up the plateau drainage systems as large-scale knickpoints. From work on the Colorado Plateau, Schmidt (1987) noted that the Colorado River is located much closer to the Grand Canyon's south rim, a drainage divide escarpment, than to the north rim, which is a gorge-like escarpment. The main implication is that the gorge-like form might be associated with higher long-term average erosion rates compared to the drainage divide escarpment type.

Weissel, Jeffrey K.

1999-01-01

208

Dead Sea Rift Transect An interdisciplinary research project to study  

E-print Network

Scientific Technical Report STR09/08 The DESERT Group www.gfz-potsdam.deISSN 1610-0956 STR09/08TheDESERTGroup,DESERTDeadSeaRiftTransect Scientific Technical Report STR 09/08 DOI: 10.2312/GFZ.b103-09084 Deutsches GeoForschungsZentrum GFZ #12 Scientific Technical Report STR09/08 The DESERT Group Scientific Technical Report STR 09/08 DOI: 10.2312/GFZ

Bauer, Klaus

209

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

Microsoft Academic Search

Seismic reflection profiles from the Lake Baikal Rift reveal extensive details about the sediment thickness, structural geometry\\u000a and history of extensional deformation and syn-rift sedimentation in this classic continental rift. The Selenga River is the\\u000a largest single source of terrigenous input into Lake Baikal, and its large delta sits astride the major accommodation zone\\u000a between the Central and South basins

C. A. Scholz; D. R. Hutchinson

2000-01-01

210

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

NASA Astrophysics Data System (ADS)

Iceberg calving is a dominant mass loss mechanism for Antarctic ice shelves, second only to basal melting. An important process involved in calving is the initiation and propagation of through-penetrating fractures called rifts; however, the mechanisms controlling rift propagation remain poorly understood. To investigate the mechanics of ice shelf rifting, we analyzed seismicity associated with a propagating rift tip on the Amery Ice Shelf, using data collected during the austral summers of 2004-2007. We apply a suite of passive seismological techniques including icequake locations, back projection, and moment tensor inversion. We confirm previous results that show ice shelf rifting is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of 1 to 3 h. Even during periods of quiescence, we find significant deformation around the rift tip. Moment tensors, calculated for a subset of the largest icequakes (Mw > -2.0) located near the rift tip, show steeply dipping fault planes, horizontal or shallowly plunging stress orientations, and often have a significant volumetric component. They also reveal that much of the observed seismicity is limited to the upper 50 m of the ice shelf. This suggests a complex system of deformation that involves the propagating rift, the region behind the rift tip, and a system of rift-transverse crevasses. Small-scale variations in the mechanical structure of the ice shelf, especially rift-transverse crevasses and accreted marine ice, play an important role in modulating the rate and location of seismicity associated with the propagating ice shelf rifts.

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

2014-04-01

211

New map reveals origin and geology of North American mid-continent rift  

Microsoft Academic Search

New aeromagnetic data from the north central United States are helping geophysicists and geologists better understand the 1.1-billion-year-old mid-continent rift, one of the fundamental components of the Precambrian basement of North America.A detailed geologic map of part of the rift is being made and a myriad of new details concerning the history of rift subsidence, volcanism, sedimentation, and inversion are

W. F. Cannon; D. L. Daniels; S. W. Nicholson; J. Phillips; L. G. Woodruff; Val W. Chandler; G. B. Morey; T. Boerboom; K. R. Wirth; M. G. Mudrey Jr.

2001-01-01

212

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

USGS Publications Warehouse

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

Burton, William C.; Southworth, Scott

2010-01-01

213

Crustal shear wave velocity and radial anisotropy beneath the Rio Grande rift from ambient noise tomography  

NASA Astrophysics Data System (ADS)

Shear wave velocity and radial anisotropy beneath New Mexico are obtained from ambient seismic noise tomography using data from the Transportable Array. Besides the distinct seismic structure imaged across the Rio Grande rift from the Colorado Plateau to the Great Plains, both velocity and anisotropy models also reveal significant variations along the rift. The rift at Albuquerque is characterized by remarkably low velocity in the shallow crust, high velocity and strong positive anisotropy in the middle and lower crust, and low velocity in the upper mantle. These observations can be interpreted as magma accumulation in the shallow crust and significant mafic underplating in the lower crust with abundant melt supply from the hot mantle. We propose that the Albuquerque region has recently been experiencing the most vigorous extensional deformation in the rift. Positive anisotropy with Vsh > Vsv appears in the central and southern rifts with a stronger anisotropy beneath younger volcanoes, reflecting layering of magma intrusion due to past and recent rifting activities. The low velocities in the uppermost mantle are observed under high-elevation places, the Jemez Lineament, northern rift, and east rift boundary, implying that the buoyancy of hot mantle largely compensates the local high topography. Low mantle velocities appear at the boundary of the southern rift, corresponding to the large lithosphere thickness change, instead of the rift center, consistent with the prediction from the small-scale, edge-driven mantle convection model. We conclude that the edge-driven upper mantle convection is probably the dominant mechanism for the recent and current rifting and uplift in the Rio Grande rift.

Fu, Yuanyuan V.; Li, Aibing

2015-02-01

214

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

Microsoft Academic Search

The relationship between rift zones and flank instability in ocean island volcanoes is often inferred but rarely documented. Our field data, aerial image analysis, and 40Ar\\/39Ar chronology from Anaga basaltic shield volcano on Tenerife, Canary Islands, support a rift zone—flank instability relationship. A single rift zone dominated the early stage of the Anaga edifice (~6–4.5 Ma). Destabilization of the northern sector

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

2005-01-01

215

Structure and evolution of the volcanic rift zone at Ponta de São Lourenço, eastern Madeira  

NASA Astrophysics Data System (ADS)

Ponta de São Lourenço is the deeply eroded eastern end of Madeira’s east-west trending rift zone, located near the geometric intersection of the Madeira rift axis with that of the Desertas Islands to the southeast. It dominantly consists of basaltic pyroclastic deposits from Strombolian and phreatomagmatic eruptions, lava flows, and a dike swarm. Main differences compared to highly productive rift zones such as in Hawai’i are a lower dike intensity (50-60 dikes/km) and the lack of a shallow magma reservoir or summit caldera. 40Ar/39Ar age determinations show that volcanic activity at Ponta de São Lourenço lasted from >5.2 to 4 Ma (early Madeira rift phase) and from 2.4 to 0.9 Ma (late Madeira rift phase), with a hiatus dividing the stratigraphy into lower and upper units. Toward the east, the distribution of eruptive centers becomes diffuse, and the rift axis bends to parallel the Desertas ridge. The bending may have resulted from mutual gravitational influence of the Madeira and Desertas volcanic edifices. We propose that Ponta de São Lourenço represents a type example for the interior of a fading rift arm on oceanic volcanoes, with modern analogues being the terminations of the rift zones at La Palma and El Hierro (Canary Islands). There is no evidence for Ponta de São Lourenço representing a former central volcano that interconnected and fed the Madeira and Desertas rifts. Our results suggest a subdivision of volcanic rift zones into (1) a highly productive endmember characterized by a central volcano with a shallow magma chamber feeding one or more rift arms, and (2) a less productive endmember characterized by rifts fed from deep-seated magma reservoirs rather than from a central volcano, as is the case for Ponta de São Lourenço.

Klügel, Andreas; Schwarz, Stefanie; van den Bogaard, Paul; Hoernle, Kaj A.; Wohlgemuth-Ueberwasser, Cora C.; Köster, Jana J.

2009-08-01

216

Istopically Defined Source Reservoirs of Primitive Magmas in the East African Rift  

Microsoft Academic Search

Extension within the East African Rift is a function of the interaction between plume-driven uplift and far-field stresses associated with plate tectonic processes. Geochemical and isotopic investigation of primitive basalts from the Main Ethiopian Rift (MER) reveals systematic spatial variations in the contributions from distinct and identifiable source reservoirs that, in turn help identify the mechanisms by which along-axis rifting

T. O. Rooney; T. Furman; B. Hanan

2005-01-01

217

[Rift Valley Fever, a Veterinary and Medical Problem (author's transl)].  

PubMed

Rift Valley fever probably occurred in East and South Africa for many years, where it occasionally caused sever epizootics among sheep and to a less extent in cattle and goats. Particularly newborn animals fall victims to the disease, whereas the majority of pregnant ewes and cows abort. In recent years, the disease spread to the Sudan and reached the Nile delta in 1977, where it not only caused an epizootic but also an epidemic among the population, resulting in approximately 18,000 cases and nearly 600 deaths. During the summer of 1978, the disease was observed again so that the virus must have overwintered in the newly infested area. The disease in animals and subsequently in man, displaying four different clinical forms, is discussed. The limited knowledge of the pathogenesis and epidemiology is stressed particular attention being paid to the Egyptian epizootic and epidemic. Because of inadequate knowledge of the clinical features, pathogenesis and other data, the disease among cattle was not detected until from five to six months after it had broken out. The spread of Rift Valley fever to the north-eastern part of Africa forms a potential threat of dissemination of the virus to neighbouring countries, situated on the two adjacent continents of Asia and Europe, and it is a real one. International travel and movement of goods and animals as well as the political situation in that area involves the danger of the Rift Valley fever virus also being transferred to new areas and becoming established there. PMID:494210

van Tongeren, H A

1979-09-01

218

Early Cretaceous rifts of Western and Central Africa: an overview  

NASA Astrophysics Data System (ADS)

The structure and evolution of Early Cretaceous rift basins in Western and Central Africa are described. Two stages of rift development and fracturing have been identified: (1) from Neocomian to Early Aptian roughly E-W and NW trending troughs (Upper Benue, N Cameroon, S Chad, Sudan etc.) opened in response to a submeridian extensional regime in Central Africa while in Western Africa the N-S trending transsaharian fault zone acted as a sinistral wrench; (2) from Middle Aptian to Late Albian large northwest trending troughs (E Niger, Sudan, Sirte, etc.) opened in response to a northeast extensional regime while the Central African fault zone (from Benue to Sudan) exhibited strike-slip movements, generating pull-apart basins. These rift and fracture systems delimit three large blocks within the African plate: a Western block, an Arabian-Nubian block and an Austral block. The Arabian-Nubian block tends to separate from the two other blocks, migrating towards the north during the first stage of basin development and then towards the NE during the second stage. The opening of the Atlantic Ocean was the dominant driving force for the Western and Austral blocks while the Arabian-Nubian block probably moved in response to the opening of the Indian Ocean and to the evolution of the Tethyan margin.

Guiraud, René; Maurin, Jean-Christophe

1992-10-01

219

Mapping of the major structures of the African rift system  

NASA Technical Reports Server (NTRS)

The author has identified the following significant results. ERTS-1 imagery of the African rift system resolves the major Cenozoic faults, zones of warping, and the associated volcanism. It also clearly depicts the crustal grain of the Precambrian rocks where these are exposed. New structural features, or new properties of known features such as greater extent, continuity, linearity, etc., are revealed by the ERTS-1 imagery. This applies to the NE-SW fracture zones in Yemen, the Aswa mylonite zone at the northern end of the Western Rift, the Nandi fault of western Kenya, the arcuate faults of the Elgeyo escarpment in the Gregory rift, and hemi-basins of warped Tertiary lavas on the Red Sea margin of Yemen, matching those of the Ethiopian plateau-Afar margin. A tentative scheme is proposed, relating the effect on the pattern of Cenozoic faulting of the degree of obliquity to Precambrian structural trend. Some ground-mapped lithological boundaries are obscure on ERTS-1 imagery. The present approaches to mapping of Precambrian terrain in Africa may require radical revision with the input of satellite imagery.

Mohr, P. A. (principal investigator)

1972-01-01

220

Ouachita trough: Part of a Cambrian failed rift system  

NASA Astrophysics Data System (ADS)

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

Lowe, Donald R.

1985-11-01

221

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

SciTech Connect

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

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

1993-09-01

222

The rifting to spreading transition in the Red Sea  

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

223

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

NASA Technical Reports Server (NTRS)

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

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

1976-01-01

224

Present-day kinematics of the East African Rift  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

225

Structural evolution of the Rio Grande rift: Synchronous exhumation of rift flanks from 20-10 Ma, embryonic core complexes, and fluid-enhanced Quaternary extension  

NASA Astrophysics Data System (ADS)

The Rio Grande rift in Colorado and New Mexico is one of the well-exposed and well-studied continental rifts in the world. Interest in the rift is driven not only by pure scientific intrigue, but also by a desire and a necessity to quantify earthquake hazards in New Mexico as well as to assess various water related issues throughout the state. These motivating topics have thus far led to the publication of two Geological Society of America Special Publication volumes in 1994 and 2013. This dissertation aims at building on the wealth of previous knowledge about the rift, and is composed of three separate chapters that focus on the structural evolution of the Rio Grande rift at several different time and spatial scales. At the largest scale, apatite (U-Th)/He thermochronologic data suggest synchronous extension along the entire length of the Rio Grande rift in Colorado and New Mexico from 20-10 Ma, which is important for understanding and evaluating possible driving mechanisms which are responsible for the rift. Previous tectonic and magmatic events in western North America were highly influential in the formation of the Rio Grande rift, and the new thermochronologic data suggest that its formation may have been closely linked to foundering and removal of the underlying Farallon Plate. A fundamental result of rift development at these scales is a concentration of strain is some regions of the rift. In these regions of maximum extension, fault networks display a geometry involving both high- and low-angle fault networks. These geometries are similar to the early stages in the development of metamorphic core complexes, and thus these regions in the rift link incipient extensional environments to highly extended terranes. At shorter time scales, heterogeneous strain accumulation may be governed in part by fluids in fault zones. As an example, along the western edge of the Albuquerque basin, travertine deposits are cut by extensional veins that record anomalously high strain rates during the Quaternary at this location. The fluids that precipitated the travertine and calcite in veins also contain a small component of deeply-derived fluids such that surface extension in this part of the rift is coupled with processes at deeper levels. Together, these studies suggest that removal of the Farallon slab beneath Colorado and New Mexico may have been a primary mechanism establishing extension in the Rio Grande rift, while continued extension is heterogeneous in time and space and provides an important link between surface processes and processes that operate at mid-crustal levels.

Ricketts, Jason William

226

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

USGS Publications Warehouse

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

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

1995-01-01

227

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

NASA Astrophysics Data System (ADS)

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

Pollitz, Fred F.; Mooney, Walter D.

2014-09-01

228

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

229

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

NASA Astrophysics Data System (ADS)

Oceanic island volcanoes commonly contain rift zones along which eruptive centers and parallel dike complexes are concentrated. Formation and orientation of rifts often remain enigmatic, however. Tectonic lineaments and regional zones of weakness facilitating magma ascent may be one reason of rift evolution, e.g. Sao Jorge (Azores) or Iceland. Alternatively, gravity tectonics of a volcano may cause formation of dike swarms oriented parallel to the line of contact between overlapping volcanic edifices (e.g. Kilauea / Mauna Loa, Hawaiian Islands). We have evidence that spreading of overlapping edifices can produce two types and orientations of dike complexes. A direction perpendicular to classic "Kilauea type" rifts is typified in the pronounced rift zones of La Palma (Canary Islands) and Madeira/Desertas islands. We suggest that these rift systems formed by edifice coalescence with a main spreading zone perpendicular to the initial line of contact between two volcanoes. Intrusions and eruptions focused along the resulting rift connecting the once separated volcanic cones, which successively grew together. Based on experimental studies we show that this mechanism works if the edifices overlap at lower (submarine) slopes and are situated both on weak substratum. By mounting analogue sand piles onto a viscous PDMS substratum, the setups represented the presumed pre-rift situations at La Palma and Madeira with small initial cones adjacent to the larger shields. Gravitative spreading of these cones produced fractures that mimic the orientation of both islands' present rift zones. The results are in agreement with the observation of an apparently old submarine cone at the southern end of the La Palma rift zone. Likewise, on Madeira, the terminal parts of the Desertas rift arm and of a recently discovered submarine rift zone off the island are both marked by a concentration of eruptive centers. Our results may also provide a clue why the rift zone of Loihi seamount (Hawaii) is oriented nearly perpendicular to the rifts of adjacent Kilauea rather than parallel to them. We conclude that the effect of spreading and buttressing on large volcanic edifices may diverge laterally, where rifting between two edifices is (i) perpendicular to their line of contact if edifice overlap is minor developed, but (ii) parallel to this line if one volcano is only an attached part of a larger edifice. Once formed, these rifts stabilize themselves by alternating constructive and destructive processes.

Kluegel, A.; Walter, T. R.

2003-04-01

230

Tectonics and stratigraphy of the East Brazil Rift system: an overview  

NASA Astrophysics Data System (ADS)

The East Brazilian Rift system (Ebris) constitutes the northern segment of the South Atlantic rift system which developed during the Mesozoic breakup of South America and Africa. Following crustal separation in the Late Aptian, it evolved into a passive continental margin. Along the continental margin six basins are recognized, while three onshore basins form part of an aborted rift. Three continental syn-rift stratigraphic sequences are recognized, spanning Jurassic to Barremian times. The Jurassic (Syn-rift I) and Neocomian (Syn-rift II) phases were most active in the interior rift basins. During the Barremian (Syn-rift III), rift subsidence rates were twice as large as during the Neocomian (Syn-rift II), both in the interior rift and in the marginal rift segments, indicating that rift axis did not migrate from the interior to the marginal setting. Rift magmatism was centered on the southern EBRIS and peaked between 130 and 120 Ma during syn-rift phase II. Rift phase III was followed by a transitional marine, evaporitic megasequence of Aptian age, which directly overlies the rift unconformity and a marine drift megasequence which spans Albian to Recent times. During the Late Cretaceous, sedimentation rates responded to first-order eustatic sea-level fluctuations. Tertiary accelerated sedimentation rates can be related to local clastic supply which filled in spaces inherited from previous starved conditions. Between 60 and 40 Ma, post-rift magmatism, centered on the Abrolhos and Royal Charlotte banks, is probably related to development of a hot spot associated with the Vitória-Trindade Seamount Chain. Although crossing three distinct Precambrian tectono-thermal provinces, ranging from Archean through Late Proterozoic, rift structures follow a general NE trend, subparallel to the principal basement fabric. A NW-SE oriented stress field appears to be compatible with both Neocomian and Barremian phases of crustal extension. Profiles transverse to the rift axis indicate crustal stretching factors ranging between ? = 2.16 and 2.88. In the shallow portions of the rift, surface extension and crustal thinning seem to be compatible; however, in the deep portions of the basins, this relationship could not be tested. Reinterpretation of refraction profiles, north and south of the Walvis-São Paulo Ridge transform, indicates that seafloor spreading, from M3 anomaly to Aptian off Pelotas Basin, was taken up by crustal extension in the São Paulo Plateau. Differences in stretching rates may have been accommodated by extension across the Ponta Grossa Arch. The Early Aptian syn-rift/post-rift transition in the EBRIS marginal basins does not coincide with the onset of the drift phase during the Early Albian. This apparent discrepancy may be explained by a change from distributed margin-wide extension to a focused mode of extension near the future continent/ocean boundary.

Hung Kiang Chang; Kowsmann, Renato Oscar; Figueiredo, Antonio Manuel Ferreira; Bender, AndréAdriano

1992-10-01

231

The Kaena Ridge Submarine Rift Zone off Oahu, Hawaii  

NASA Astrophysics Data System (ADS)

Deep-water multibeam data was collected in the Kauai Channel between the islands of Kauai and Oahu during the years 2000 to 2002 using the R/V Roger Revelle in support of the NSF funded Hawaii Ocean Mixing Experiment (HOME) with M. Gregg and A. Chave as principal investigators. The bathymetric surveys covered the Kaena Ridge, possibly a large submarine rift zone extension of the Waianae Volcano that makes up the western half of the island of Oahu. The Kaena ridge is bordered by the Kauai Channel and debris avalanche deposit material to the west, the Waianae slump to the south, and the Kaena slump to the north. The 35-55 km wide crest of the Kaena ridge extends 75-100 km northwest from Kaena Point, the westernmost tip of Oahu. The length and morphology resemble that of the Hana Ridge, the submarine extension of the Haleakala east rift zone. The broad central portion of Kaena ridge is comprised of two apparent lava shields 15 km and 10 km in diameter, both rising 200 m above the ridge (approximately 2500 m above the basal seafloor) to within 880 m and 660 m of sea level, respectively. The large size and distant location of the westernmost cone in the middle of the Kauai Channel suggests that it might predate Waianae Volcano and represent a precursor to the island of Oahu in the Hawaiian Chain, aborted before it ever reached sea level. Several 1500 m high steep concave landslide scars facing the giant Waianae slump mark the southern side of the ridge. To the north, a series of four 15 to 30 km wide benches step down to the basal seafloor, forming the Kaena slump, which may represent a less developed version of the Waianae slump. A 42 km long and 22 km wide (at its center) narrow rift zone ridge, in the classic Hawaiian submarine rift zone style (e.g., Puna ridge), bifurcates from the main broad ridge near the distal end. The smaller ridge trends north-northwest and is covered with approximately a dozen volcanic cones along its axis. Its rough morphology is characteristic of pillow and lobate lava flows effusing from a central dike injected rift zone.

Smith, J. R.

2002-12-01

232

The Chukchi Borderland: a Sediment-starved Rifted Continental Margin  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

233

Hydrothermal vents is Lake Tanganyika, East African Rift system  

SciTech Connect

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

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

1993-06-01

234

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

235

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

USGS Publications Warehouse

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

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

1998-01-01

236

Fluoride and Geothermal Activities In Continental Rift Zones, Ethiopia  

NASA Astrophysics Data System (ADS)

The Central Main Ethiopian Rift basin is a continental rift system characterized by volcano-tectonic depression endowed with huge geothermal resource and associated natural geochemical changes on groundwater quality. Chemical composition of groundwater in the study area showed a well defined trend along flow from the highland and escarpment to the rift floor aquifer. The low TDS (< 500mg/l) Ca-Mg-HCO3 dominated water at recharge area in the highlands and escarpments evolve progressively into Ca-Na-HCO3 and Na-Ca-HCO3 type waters along the rift ward groundwater flow paths. These waters finally appear as moderate TDS (mean 960mg/l) Na-HCO3 type and as high TDS (> 1000 mg/l) Na-HCO3-Cl type in volcano-lacustrine aquifers of the rift floor. High concentrations of fluoride (up to 97.2 mg/l) and arsenic (up to 98?g/l) are recognized feature of groundwaters which occur mostly in the vicinity of the geothermal fields and the rift lakes in the basin. Fluoride and arsenic content of dry volcaniclastic sediments close to these areas are in the range 666-2586mg/kg and 10-13mg/kg respectively. The relationship between fluoride and calcium concentrations in groundwaters showed negative correlation. Near-equilibrium state attained between the mineral fluorite (CaF2) and the majority of fluoride-rich (>30mg/l) thermal groundwater and shallow cold groundwater. This indicated that the equilibrium condition control the high concentration of fluoride in the groundwaters. Whereas undersaturation state of fluorite in some relatively low-fluoride (<30mg/l) thermal waters indicated a dilution by cold waters. Laboratory batch leaching experiments showed that fast dissolution of fluoride from the sediment samples suddenly leached into the interacting water at the first one hour and then remain stable throughout the experiment. The concentrations of leached fluoride from the hot spring deposits, the lacustrine sediments, and the pyroclastic rock are usually low (1% of the total or less than the content in the sediment or rock) but strongly correlated with the concentrations in groundwaters in the local vicinity. The readily leachable hot spring deposits and local lacustrine sediments, which were leached easily as high as three fold of other sediments leachability, are considered as the reservoir for the potential fluoride contamination of the rift groundwater. Leaching of fluoride in the sub-surface system is simulated with sediment-packed column leached by flowing water and applying temporary interruption of flow during the experiment. The result indicated that a sharp increase of fluoride concentration (up to 58mg/kg) observed in leachates before one pore-volume of water eluted from the column. The concentration of leached fluoride consequently declined with the increased flowing pore-volume of water and finally the lowest concentrations of leached fluoride occurred in the end of the experiment. Flow interruption during column leaching experiment causes a noticeable fluoride concentration perturbation due to the heterogeneity of the sediment.

Weldesenbet, S. F.

2012-12-01

237

High Fluoride and Geothermal Activities In Continental Rift Zones, Ethiopia  

NASA Astrophysics Data System (ADS)

The Central Main Ethiopian Rift basin is a continental rift system characterized by volcano-tectonic depression endowed with huge geothermal resource and associated natural geochemical changes on groundwater quality. Chemical composition of groundwater in the study area showed a well defined trend along flow from the highland and escarpment to the rift floor aquifer. The low TDS (< 500mg/l) Ca-Mg-HCO3 dominated water at recharge area in the highlands and escarpments evolve progressively into Ca-Na-HCO3 and Na-Ca-HCO3 type waters along the rift ward groundwater flow paths. These waters finally appear as moderate TDS (mean 960mg/l) Na-HCO3 type and as high TDS (> 1000 mg/l) Na-HCO3-Cl type in volcano-lacustrine aquifers of the rift floor. High concentrations of fluoride (up to 97.2 mg/l) and arsenic (up to 98?g/l) are recognized feature of groundwaters which occur mostly in the vicinity of the geothermal fields and the rift lakes in the basin. Fluoride and arsenic content of dry volcaniclastic sediments close to these areas are in the range 666-2586mg/kg and 10-13mg/kg respectively. The relationship between fluoride and calcium concentrations in groundwaters showed negative correlation. Near-equilibrium state attained between the mineral fluorite (CaF2) and the majority of fluoride-rich (>30mg/l) thermal groundwater and shallow cold groundwater. This indicated that the equilibrium condition control the high concentration of fluoride in the groundwaters. Whereas undersaturation state of fluorite in some relatively low-fluoride (<30mg/l) thermal waters indicated a dilution by cold waters. Laboratory batch leaching experiments showed that fast dissolution of fluoride from the sediment samples suddenly leached into the interacting water at the first one hour and then remain stable throughout the experiment. The concentrations of leached fluoride from the hot spring deposits, the lacustrine sediments, and the pyroclastic rock are usually low (1% of the total or less than the content in the sediment or rock) but strongly correlated with the concentrations in groundwaters in the local vicinity. The readily leachable hot spring deposits and local lacustrine sediments, which were leached easily as high as three fold of other sediments leachability, are considered as the reservoir for the potential fluoride contamination of the rift groundwater. Leaching of fluoride in the sub-surface system is simulated with sediment-packed column leached by flowing water and applying temporary interruption of flow during the experiment. The result indicated that a sharp increase of fluoride concentration (up to 58mg/kg) observed in leachates before one pore-volume of water eluted from the column. The concentration of leached fluoride consequently declined with the increased flowing pore-volume of water and finally the lowest concentrations of leached fluoride occurred in the end of the experiment. Flow interruption during column leaching experiment causes a noticeable fluoride concentration perturbation due to the heterogeneity of the sediment.

Weldesenbet, S. F.; Wohnlich, S.

2012-12-01

238

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

239

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

USGS Publications Warehouse

Iceberg calving is a dominant mass loss mechanism for Antarctic ice shelves, second only to basal melting. An important known process involved in calving is the initiation and propagation of through-penetrating fractures called rifts; however, the mechanisms controlling rift propagation remain poorly understood. To investigate the mechanics of ice-shelf rifting, we analyzed seismicity associated with a propagating rift tip on the Amery Ice Shelf, using data collected during the Austral summers of 2004-2007. We investigated seismicity associated with fracture propagation using a suite of passive seismological techniques including icequake locations, back projection, and moment tensor inversion. We confirm previous results that show that seismicity is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of one to three hours. However, even during periods of quiescence, we find significant seismic deformation around the rift tip. Moment tensors, calculated for a subset of the largest icequakes (MW?>?-2.0) located near the rift tip, show steeply dipping fault planes, horizontal or shallowly plunging stress orientations, and often have a significant volumetric component. They also reveal that much of the observed seismicity is limited to the upper 50?m of the ice shelf. This suggests a complex system of deformation that involves the propagating rift, the region behind the rift tip, and a system of rift-transverse crevasses. Small-scale variations in the mechanical structure of the ice shelf, especially rift-transverse crevasses and accreted marine ice, play an important role in modulating the rate and location of seismicity associated with propagating ice shelf rifts.

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

2014-01-01

240

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

241

Rifted continental margins: geometric control on crustal architecture and melting  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

242

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

243

Compositional Similarities between Hot Mineral Springs in the Jordan and Suez Rift Valleys  

Microsoft Academic Search

THE chemical composition of the Hammam Farun hot spring (72° C) in the Suez Rift Valley has been found to be almost identical to that of the Tiberias Hot Springs (60° C) in the Jordan Rift Valley (Figs. 1 and 2 and Table 1). This finding is of vital importance in the evaluation and sorting out of various hypotheses that

Emanuel Mazor

1968-01-01

244

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

E-print Network

Modeling suggests that oblique extension facilitates rifting and continental break-up Sascha Brune; accepted 5 June 2012; published 2 August 2012. [1] In many cases the initial stage of continental break-up was and is associated with oblique rifting. That includes break-up in the Southern and Equatorial Atlantic, separation

Kaus, Boris

245

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

246

Large-scale flow of geofluids at the Dead Sea Rift H. Gvirtzmana,*, E. Stanislavskyb  

E-print Network

Large-scale flow of geofluids at the Dead Sea Rift H. Gvirtzmana,*, E. Stanislavskyb a Institute that has caused large-scale migration of brine and hydrocarbons at the Dead Sea Rift. Numerical simulations indicate the coexistence of two basin-scale groundwater flow systems, one atop the other, with opposite

Gvirtzman, Haim

247

The transition from diffuse to focused extension: Modeled evolution of the West Antarctic Rift system  

E-print Network

The transition from diffuse to focused extension: Modeled evolution of the West Antarctic Rift of extension are recognized in the West Antarctic Rift system (WARS). During the first stage, beginning with the East Antarctic craton. The transition to focused extension was roughly coeval with volcanic activity

Huerta, Audrey D.

248

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

249

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

E-print Network

and invert for the distribution of opening of the dike, as well as slip on rift border faults. The volume segment. Above the dike, faults slipped by an average 3 m, yielding an estimated geodetic moment of 3.5 Ã? stress deficit since the previous rifting episode was not fully released, leading to further intrusions

Nicolas, Chamot-Rooke

250

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

E-print Network

fissuring, diking and voluminous eruption of the Ethiopian- Yemen flood basalt pile. The dikes which fed the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood

Paris-Sud XI, Université de

251

The origin and geologic evolution of the East Continent Rift Basin  

SciTech Connect

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

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

1992-01-01

252

NATURAL DISASTERS AND HAZARDS IN THE LAKE KIVU BASIN, WESTERN RIFT VALLEY OF AFRICA  

Microsoft Academic Search

The lake Kivu with altitude (1462m) and maximum depth (485m) is the most highest in the Western Rift. The lake Kivu basin is among the most active region in this Rift, dominated by the mountains. The earthquakes are making a lot of damages in villages and cities in the lake Kivu basin. The earthquakes are also generating landslides on slopes

D. M. Wafula; M. Yalire; M. Kasereka; M. Ciraba; M. Kwetuenda

253

Identification of a magma chamber in the Ghoubbet-Asal rift (Djibouti) from a magnetotelluric experiment  

Microsoft Academic Search

A very broad band (10-3 to 104 Hz) magnetotelluric investigation of the axial zone of the Ghoubbet-Asal rift (Djibouti) has revealed a shallow (2-4 km) magma chamber which can be mapped in some detail. The suggested roof of the chamber is shallowest very close to the Ardoukoba volcano which was built during the November 1978 rifting episode.

Ngoc Pham van; Daniele Boyer; Jean-Louis Le Mouël; Vincent Courtillot

1981-01-01

254

Rift Valley Fever in Humans and Animals in Mayotte, an Endemic Situation?  

E-print Network

Rift Valley Fever in Humans and Animals in Mayotte, an Endemic Situation? Tinne Lernout1 *. , Eric on Arboviruses and Viral Haemorrhagic fevers, Institut Pasteur, Paris, France, 5 Laboratory of the Hospital revealed that Rift Valley Fever virus (RVFV) has been circulating on Mayotte for at least several years

Paris-Sud XI, Université de

255

A SAP30 Complex Inhibits IFN-b Expression in Rift Valley Fever Virus Infected Cells  

E-print Network

A SAP30 Complex Inhibits IFN-b Expression in Rift Valley Fever Virus Infected Cells Nicolas Le May1, Institut Pasteur, Paris, France Rift Valley fever virus (RVFV) nonstructural protein NSs acts as the major Valley Fever Virus infected cells. PLoS Pathog 4(1): e13. doi:10.1371/journal.ppat.0040013 Introduction

Paris-Sud XI, Université de

256

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

257

Origin of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin  

E-print Network

Origin of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin A. T and well data suggest that the West Taiwan basin developed by orogenic loading and flexure of a rift-type continental margin. The most likely source of the loading is Taiwan, where oblique convergence between

Watts, A. B. "Tony"

258

Rise, transport, and storage of magma during continental rupture: Constraints from the Afar rifting episode (Invited)  

Microsoft Academic Search

The production, storage, and eruption of magma within continental and oceanic rift zones shapes the structure and morphology of the plate boundary, and the buoyancy force of the magma adds to the tectonic forces driving plate divergence. Dikes transport magma from crustal and sub-crustal magma reservoirs, accommodate extensional strain, and impart a length scale to rift deformation. From seismic and

C. J. Ebinger; M. Belachew; D. M. Cote; D. Keir; J. V. Rowland; J. Hammond

2009-01-01

259

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

E-print Network

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

260

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

261

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

E-print Network

Twenty-five years of geodetic measurements along the Tadjoura- Asal rift system, Djibouti, East GPS sites covers the Republic of Djibouti. Additional points were also measured in Yemen and Ethiopia Djibouti, the Asal-Ghoubbet rift system accommodates 16 ± 1 mm/yr of opening perpendicular to the rift axis

Vigny, Christophe

262

Contemporary, holocene, and quaternary deformation of the Asal rift, Djibouti: implications for the mechanics of slow spreading ridges  

Microsoft Academic Search

Because the frequency and character of rifting events along mid-ocean ridges are largely unknown, how the repetition of such events gives rise to rift structures is unexplored. The Asal rift in the Afar depression of Djibouti, Africa, provides the world's best subaerial analogue for young slow spreading mid-ocean ridges. Seismic, geodetic, and field observations of a seismovolcanic event in 1978

Ross S. Stein; Pierre Briole; Jean-Claude Ruegg; Paul Tapponnier; Françoise Gasse

1991-01-01

263

Culex pipiens, an Experimental Efficient Vector of West Nile and Rift Valley Fever Viruses in the Maghreb Region  

E-print Network

Culex pipiens, an Experimental Efficient Vector of West Nile and Rift Valley Fever Viruses fever (WNF) and Rift Valley fever (RVF) are emerging diseases causing epidemics outside their natural birds as amplifying hosts, and horses and humans as accidental dead-end hosts. Rift Valley fever virus

Paris-Sud XI, Université de

264

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

SciTech Connect

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

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

1992-01-01

265

Rifting process of the Xihu Depression, East China Sea Basin  

NASA Astrophysics Data System (ADS)

Having a maximum depositional depth up to 15 km, the NE-trending Xihu Depression is evolved from a deep Paleogene continental margin rifting along the present-day East China Sea continental shelf. In this paper we use deep seismic reflection, well logging, and gravity data to synthesize early rifting process of the depression. To study the regional tectonic subsidence history we developed a computer program that performs backstripping and simultaneous inversion for stretching factor, initial lithospheric thickness and incipient age of thermal subsidence. Tectonic subsidence analyses of 40 industrial wells reveal nonuniform stretching — preferential crustal extension along the depocenter but preferential mantle extension at the western flank. Uniform extension may have occurred only somewhere between the central inversion zone and the western margin. In the early rifting stage, subdued initial subsidence or initial uplift is observed on several wells near the western margin, due possibly to flexural isostatic balance. The initial mechanical lithospheric thicknesses from subsidence inversion seem to vary from about 55 km to almost 130 km, suggesting significant longitudinal segmentation as well as transverse variation in the prerifting lithosphere. It is likely that the southern part of the Xihu Depression was developed initially on a region of relatively thin lithosphere, a statement conformable to geological evidences. NW-SE trending deep seismic profiling clearly reveals the deep basement structure of the depression, as well as the synrift and postrift sedimentary packages separated by a breakup unconformity. Crustal layer thicknesses are independently estimated from gravity modeling/inversion, tectonic subsidence analyses, and deep seismic profiling, respectively. These methods all arrive at very small present-day thicknesses of the crustal layer beneath the depocenter of the Xihu Depression.

Li, Chun-Feng; Zhou, Zuyi; Ge, Heping; Mao, Yunxin

2009-07-01

266

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

267

Evidence for cenozoic rifting in Thailand from gravity modeling  

E-print Network

; Thanasuthipitak, 1978: Barr and MacDouald. 198!) anil NN%V-SSE trencls. Suensilpong et al (1978) suggest the intelrnontane basins in. western Thailand also fornled in Late C'retaceous anil Earlv Tertiary times Ifnox anil IVakefleld (1983). Tapponnier et al (lc... be nsecl to provide insight into the overall problem of the timing anil origin of the basins The investigation will show thai the basins were formed in Early Tertiary tune, no later than Oligocene time, by rifting. Extension was initiated...

Ohnstad, Tiffany A.

1990-01-01

268

Structural aspects of Mid-Continent rift system in Kansas  

SciTech Connect

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

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

1989-08-01

269

Seismic investigation of the southern Rio Grande Rift  

NASA Astrophysics Data System (ADS)

Competing models exist to explain what caused the Earth's crust to spread apart 29 million years ago to create a region known today as the Rio Grande Rift (RGR). The RGR extends from central Colorado through New Mexico to northern Mexico, near El Paso. The RGR has different geologic features that distinguish it from most other valleys (e.g., the RGR was not cut by a river nor does a river branch upstream). A growing body of evidence shows that geologic activity still occurs in the RGR, with a continuation of faulting, seismicity and widening at a small rate of about 0.3 mm/yr (Woodward , 1977). We map of the seismic velocity structure and crustal thickness using data from the Rio Grande Rift Seismic TRAnsect (RISTRA) experiment and the EarthScope Transportable Array (USArray) dataset. In addition to the data we collected from the RISTRA experiment and USArray dataset, we also acquired receiver functions from the EarthScope Automatic Receiver Survey (EARS) website (http://www.earthscope.org/data) and waveform data from the Incorporated Research Institutes for Seismology (IRIS) Data Management Center (DMC). We requested seismograms from the IRIS DMC database where we acquired teleseismic events from Jan 2000 to Dec 2009. This includes 7,259 seismic events with a minimum magnitude of 5.5 and 106,389 continuous waveforms. This data was preprocessed (merged, rotated) using a program called Standing Order of Data (SOD). The RISTRA experiment and the USArray were designed to image crust and mantle structures by computing receiver functions for all data in the Southern Rio Grande Rift (SRGR). We map the crustal thickness, seismic velocity, and mantle structure for the sole purpose to better determine the nature of tectonic activity that is presently taking place and further investigate the regional extension of the Southern Rio Grande Rift (SRGR). Here we present preliminary results of the crustal and velocity structure using the kriging interpolation scheme seem stable and we are now able to clearly observe certain patterns we can use to interpret the southern RGR deformation and extension.

Thompson, Lennox E.

270

Probing the age and temperature of rifting in Afar  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

271

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

NASA Astrophysics Data System (ADS)

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

Nelson, W. R.; Furman, T.

2013-12-01

272

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

273

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

USGS Publications Warehouse

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

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

1992-01-01

274

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

NASA Astrophysics Data System (ADS)

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

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

2015-02-01

275

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

276

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

277

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

NASA Astrophysics Data System (ADS)

The East African Rift (EAR) is an active continental rift and ideal to investigate the processes of rift initiation and the breaking apart of continental lithosphere. Mantle and crust-derived fluids may play a pivotal role in both magmatism and faulting in the EAR. For instance, large quantities of mantle-derived volatiles are emitted at Oldoinyo Lengai volcano [1, 2]. Throughout the EAR, CO2-dominated volatile fluxes are prevalent [3, 4] and often associated with faults (i.e. Rungwe area, Tanzania, [5, 6]). The purpose of this study is to examine the relationship between volcanism, faulting and the volatile compositions, focusing on the central and southern Kenyan and northern Tanzanian section of the EAR. We report our analysis results for samples obtained during a 2013 field season in Kenya. Gases were sampled at fumaroles and geothermal plants in caldera volcanoes (T=83.1-120.2°C) and springs (T=40-79.6°C and pH 8.5-10) located near volcanoes, intra-rift faults, and a transverse fault (the Kordjya fault, a key fluid source in the Magadi rift) by 4N-NaOH solution-filled and empty Giggenbach bottles. Headspace gases were analyzed by a Gas Chromatograph and a Quadrupole Mass Spectrometer at the University of New Mexico. Both N2/Ar and N2/He ratios of all gases (35.38-205.31 and 142.92-564,272, respectively) range between air saturated water (ASW, 40 and ?150,000) and MORB (100-200 and 40-50). In addition, an N2-Ar-He ternary diagram supports that the gases are produced by two component (mantle and air) mixing. Gases in the empty bottles from volcanoes and springs have N2 (90.88-895.99 mmom/mol), CO2 (2.47-681.21 mmom/mol), CH4 (0-214.78 mmom/mol), O2 (4.47-131.12 mmom/mol), H2 (0-35.78 mmom/mol), Ar (0.15-10.65 mmom/mol), He (0-2.21 mmom/mol), and CO (0-0.08 mmom/mol). Although some of the samples show an atmospheric component, CO2 is a major component in most samples, indicating both volcanoes and springs are emitting CO2. Gases from volcanoes are enriched in CH4 and H2, denoting their sources are reduced (oxygen-poor) magma chambers or hydrothermal systems. 40Ar/36Ar ratios (average of all samples=299.15) are similar to our air standard value (299.65×4.05), however, some volcanoes (~308.75) and springs (~321.96) have slightly higher ratios. The springs with elevated 40Ar imply that both the intra-rift (Lake Bogoria) and transverse (Lake Magadi) faults are possibly pathways to carry volatiles from deep sources to the surface. In future work, we will carry out wet chemistry and ion chromatography analyses of the NaOH solutions, measure 3He/4He ratios and complete C, N, and S isotope analyses to further constrain fluid sources and migration processes. [1] Fischer et al., 2009, Nature 459. [2] de Moor et al., 2013, EPSL 361. [3] Sawyer et al., 2008, G-cubed 9. [4] Tassi et al., 2009, G-cubed 10. [5] Barry et al., 2013, Chem Geol 339. [6] de Moor et al., 2013, Chem Geol 339.

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

2013-12-01

278

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

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

279

Young Stellar Object Candidates in the Aquila Rift Region  

NASA Astrophysics Data System (ADS)

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

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

2010-10-01

280

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

NASA Astrophysics Data System (ADS)

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

Nirrengarten, Michael; Gernigon, Laurent; Manatschal, Gianreto

2014-05-01

281

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

282

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

283

Extension on rifted continental margins: Observations vs. models.  

NASA Astrophysics Data System (ADS)

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

Skogseid, Jakob

2014-05-01

284

Generation of Continental Rifts, Basins and Swells by Lithosphere Instabilities  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

285

Stable isotope variation in tooth enamel from Neogene hippopotamids: monitor of meso and global climate and rift dynamics on the Albertine Rift, Uganda  

NASA Astrophysics Data System (ADS)

The Neogene was a period of long-term global cooling and increasing climatic variability. Variations in African-Asian monsoon intensity over the last 7 Ma have been deduced from patterns of eolian dust export into the Indian Ocean and Mediterranean Sea as well as from lake level records in the East African Rift System (EARS). However, lake systems not only depend on rainfall patterns, but also on the size and physiography of river catchment areas. This study is based on stable isotope proxy data (18O/16O, 13C/12C) from tooth enamel of hippopotamids (Mammalia) and aims in unravelling long-term climate and watershed dynamics that control the evolution of palaeolake systems in the western branch of the EARS (Lake Albert, Uganda) during the Late Neogene (7.5 Ma to recent). Having no dietary preferences with respect to wooded (C3) versus grassland (C4) vegetation, these territorial, water-dependant mammals are particularly useful for palaeoclimate analyses. As inhabitants of lakes and rivers, hippopotamid tooth enamel isotope data document mesoclimates of topographic depressions, such as the rift valleys and, therefore, changes in relative valley depth instead of exclusively global climate changes. Consequently, we ascribe a synchronous maximum in 18O/16O and 13C/12C composition of hippopotamid enamel centred around 1.5-2.5 Ma to maximum aridity and/or maximum hydrological isolation of the rift floor from rift-external river catchment areas in response to the combined effects of rift shoulder uplift and subsidence of the rift valley floor. Structural rearrangements by ~2.5 Ma within the northern segment of the Albertine Rift are well constrained by reversals in river flow, cannibalisation of catchments, biogeographic turnover and uplift of the Rwenzori horst. However, a growing rain shadow is not obvious in 18O/16O signatures of the hippopotamid teeth of the Albertine Rift. According to our interpretation, this is the result of the overriding effect of evaporation on 18O/16O responding to aridification of the basin floor by a valley air circulation system through relative deepening of the valley. On the other hand, a synchronous arid pulse is not so clearly recorded in palaeosol data and mammalian fauna of the eastern branch of the EARS. This discrepancy indicates that rift mesoclimates may represent an underestimated aspect in previous palaeoclimate reconstructions from rift valley data and represent a clear limitation to attempts at global climate reconstructions. The results of this study also suggest that using 18O/16O data as a proxy to rain shadow evolution must take into account relative basin subsidence to properly document mountain range uplift.

Brachert, Thomas Christian; Brügmann, Gerhard B.; Mertz, Dieter F.; Kullmer, Ottmar; Schrenk, Friedemann; Jacob, Dorrit E.; Ssemmanda, Immaculate; Taubald, Heinrich

2010-10-01

286

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

287

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

NASA Technical Reports Server (NTRS)

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

Mohr, P. A.

1974-01-01

288

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

NASA Astrophysics Data System (ADS)

In the Afar depression (Ethiopia), extension is already organised along rift segments which morphologically resemble oceanic rifts. Segmentation here results from interactions between dyke injection and volcanism, as observed during the well documented 2005 event on the Dabbahu rift segment. During this tectono-volcanic crisis, a megadyke was injected, followed by 12 subsequent dike intrusions, sometimes associated with fissure flow eruptions. Despite the accurate surveying of the magmatic and tectonic interplay during this event via remote sensing techniques, there is a lack of data on timescales of 1 to 100 kyr, the period over which the main morphology of a rift is acquired. The Dabbahu rift segment represents an ideal natural laboratory to study the evolution of rift morphology as a response to volcanic and tectonic influences. It is possible to constrain the timing of fault growth relative to the infilling of the rift axial depression by lava flows, and to assess the influence of the different magma bodies involved in lava production along the rift-segment. We use cosmogenic nuclides (3He) to determine the ages of young (<100 kyr) lava flows and to date the initiation and movement of fault scarps which cut the lavas. Combined with major & trace element compositions, field mapping and digital cartography (Landsat, ASTER and SPOT imagery), the rift geomorphology can be linked to the magmatic and tectonic history defined by surface exposure dating. The results show that over the last 100 ka the Northern part of the Dabbahu segment was supplied by two different magma reservoirs which can be identified based on their distinctive chemistries. The main reservoir is located beneath Dabbahu volcano, and has been supplied with magma for at least 72 ka. This magmatic centre supplies magma to most of the northern third of the rift segment. The second reservoir is located further south, on the axis, close to the current mid-segment magma chamber, which was responsible for the 2005 rifting episode. This second magmatic centre supplies magma to the remaining 2/3 of the segment, but scarcely impacts its Northern termination (where the Dabbahu activity predominates) - except during extraordinary events when dykes are long enough to reach those parts, as in 2005. The eruption ages of the different lava units correlates with their degrees of differentiation, allowing different magmatic cycles of about a few tens of years each to be distinguished. During the first recorded magmatic cycle (~70 ka to ~55 ka), Dabbahu is built of wide-spreading p?hoehoe flows around localised eruptive centres. The resulting topography of the volcanic edifice remains low, and is only slightly affected by rift-related fault activity, with the development of minor scarps. The second recorded magmatic cycle (~50 ka to ~20 ka) coincides with a strong development of Dabbahu topography - underlined by the change in lava morphology with well channelized 'a'? flows since 50 ka. Tectonic activity also clearly increases over this period, with the initiation of the major fault scarps of the rift, which have been dated at around 35 ka. Our study underlines the role of the magma supply and availability beneath Dabbahu in the evolution both topographies of Dabbahu volcano and of the rift depression morphology.

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

2012-12-01

289

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

NASA Astrophysics Data System (ADS)

Updated models for continental rift zones need to address the role and development of low-angle normal fault networks, episodicity of extension, and interaction of 'active and passive' driving mechanisms. In the Rio Grande rift, USA, low-angle normal faults are found throughout the entire length of the rift, but make up a small percentage of the total fault population. The low-angle Jeter and Knife Edge faults, for example, crop out along the SW and NE margins of the Albuquerque basin, respectively. Apatite fission track (AFT) age-elevation data and apatite (U-Th)/He (AHe) ages from these rift flank uplifts record cooling between ~21 - 16 Ma in the NE rift flank and ~20 - 10 Ma in the SW, which coincides with times of rapid extension and voluminous syntectonic sedimentation. The timing of exhumation is also similar to rift flanks farther north in active margins based on AFT data alone. In addition, synthetic faults in the hanging wall of each low-angle fault become progressively steeper and younger basinward, and footwall blocks are the highest elevation along the rift flanks. These observations are consistent with a model where initially high-angle faults are shallowed in regions of maximum extension. As they rotate, new intrabasinal faults emerge which also can be rotated if extension continues. These relationships are similarly described in mature core complexes, and if these processes continued in the Rio Grande rift, it could eventually result in mid-crustal ductily deformed rocks in the footwall placed against surficial deposits in the hanging wall across faults that have been isostatically rotated to shallow dips. Although existing data are consistent with highest strain rates during a pulse of extension along the entire length of the rift 20-10 Ma., GPS-constrained measurements suggest that the rift is still actively-extending at 1.23-1.39 nstr/yr (Berglund et al., 2012). Additional evidence for Quaternary extension comes from travertine deposits that are cut by multiple tensile vein sets along the western margin of the Albuquerque basin in the Lucero uplift. At this location, U-series ages on travertine deposits are used to calculate strain rates at this location. These strain rates (15-105 nstr/yr) are higher than both the modern strain rates as well as the average long-term strain rates (3-14 nstr/yr) obtained from restored cross-sections across different basins in the rift. To explain these observations, we propose a model involving high fluid pressures, which promote the formation of tensile veins that are oriented with respect to the modern day stress field in the rift. These regions of anomalously-high strain need not be widespread, and are only active on timescales of the hydraulic system, but they are nevertheless an underappreciated mechanism of progressive extension in the rift. Berglund, H.T., Sheehan, A.F., Murray, M.H., Roy, M., Lowry, A.R., Nerem, R.S., and Blume, F., 2012, Distributed deformation across the Rio Grande Rift, Great Plains, and Colorado Plateau: Geology, v. 40, p. 23-26.

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

2013-12-01

290

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

291

Long-term cooling history of the Albertine Rift: new evidence from the western rift shoulder, D.R. Congo  

NASA Astrophysics Data System (ADS)

To determine the long-term landscape evolution of the Albertine Rift in East Africa, low-temperature thermochronology was applied and the cooling history constrained using thermal history modelling. Acquired results reveal (1) "old" cooling ages, with predominantly Devonian to Carboniferous apatite fission-track ages, Ordovician to Silurian zircon (U-Th)/He ages and Jurassic to Cretaceous apatite (U-Th-Sm)/He ages; (2) protracted cooling histories of the western rift shoulder with major phases of exhumation in mid-Palaeozoic and Palaeogene to Neogene times; (3) low Palaeozoic and Neogene erosion rates. This indicates a long residence time of the analysed samples in the uppermost crust, with the current landscape surface at a near-surface position for hundreds of million years. Apatite He cooling ages and thermal history models indicate moderate reheating in Jurassic to Cretaceous times. Together with the cooling age distribution, a possible Albertine high with a distinct relief can be inferred that might have been a source area for the Congo Basin.

Bauer, F. U.; Glasmacher, U. A.; Ring, U.; Grobe, R. W.; Mambo, V. S.; Starz, M.

2015-02-01

292

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

293

Brittle reactivation of ductile shear zones in NW Namibia in relation to South Atlantic rifting  

NASA Astrophysics Data System (ADS)

Rifting has occurred worldwide along preexisting mobile belts, which are therefore thought to control rift orientation on a large scale. On a smaller scale, shear zones within mobile belts are reactivated as rift faults. In NW Namibia, shear zones of the Neoproterozoic Kaoko Belt run subparallel to the present-day continental passive margin and are inferred to have been reactivated during the opening of the South Atlantic Ocean. However, the extent of this reactivation and the influence of the reactivated shear zones on South Atlantic rifting are largely unknown. A combined remote sensing and field study was conducted to quantify offsets that are a direct function of shear zone reactivation. The shear zones of the Kaoko Belt are partly overlain by the Paraná-Etendeka volcanic rocks, which were emplaced shortly before or simultaneously to the Atlantic rifting. Faulting within these volcanic rocks can be linked to synrift or postrift movements. Along the shear zones, downfaulting of the basalts is widespread along listric faults where half-graben form in the hanging wall. At three sites we could determine vertical offsets of ~1180 m, ~470 m, and ~70 m. Although many shear zones were reactivated as faults, these are isolated, and offsets are small, suggesting that reactivation occurred only as a side effect of the rifting and that the Kaoko Belt shear zones have not exerted a significant influence on the rift orientation.

Salomon, Eric; Koehn, Daniel; Passchier, Cees

2015-01-01

294

Basin-scale migration of continental-rift brines: Paleohydrologic modeling of the Dead Sea basin  

NASA Astrophysics Data System (ADS)

It was suggested that brine of the Dead Sea rift has originated from a residual product of intensively evaporated seawater that invaded the rift, precipitated halite, and later interacted through dolomitization with the host rock during subsurface migration. Detection of this brine in many deep wells located at distances as far as 100 km away from the rift was attributed to long-distance migration of the brine. The physical feasibility of such migration, which probably spanned the past 3 6 m.y., is quantitatively tested and verified in this study by using paleohydrologic modeling. The structural formation of the rift is described by a chronological sequence of geologic cross sections serving as the basis for hydrodynamic calculations, which assess the effects of the structure on fluid migration, salinity redistribution, and heat transport across the sedimentary basin. Results indicate that two basin-scale ground-water systems, one atop the other but with opposite flow directions, coexisted in the Dead Sea rift valley. The first is a topography-driven flow of meteoric water from the surrounding highlands toward the rift through relatively shallow aquifers (? 1 km). The second is a density-driven migration of the Dead Sea brine through deep aquifers (4 5 km) in the opposite direction. The configuration of these flow systems has changed during the structural evolution of the Dead Sea rift, illustrating the interrelationships among basin formation, paleohydrology, and paleogeochemistry.

Stanislavsky, Eyal; Gvirtzman, Haim

1999-09-01

295

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

NASA Astrophysics Data System (ADS)

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 negative correlation between Zr/Nb (<2-7) and Ce/Y (1-8) indicates derivation from a garnet-bearing mantle source region as a result of <3% melting. Isotope ratios in basalts from the axial regions of the Kenya Rift have 143Nd/ 144Nd=0.51300 to 0.51255, 87Sr/ 86Sr=0.7030 to 0.7055 and 206Pb/ 204Pb<18 to >20, broadly similar to values from OIB. The Kenya Rift cuts through basement of different ages and aspects of the composition of mafic magmas reflect the anisotropy of the underlying lithosphere. Specifically, those basalts from that part of the rift underlain by the Tanzanian craton (TC) have higher Ce/Y and lower Zr/Nb ratios than those erupted through the Panafrican Mozambique belt (MB) implying an origin either at greater depth or from a more trace element-enriched source region. Samples erupted through the zone of reactivated craton margin (RCM) share the characteristics of mafic lavas from both the craton and the mobile belt. MB samples have 143Nd/ 144Nd=0.5130-0.5127, 87Sr/ 86Sr=0.7030-0.7035 and 206Pb/ 204Pb=18.3-19.8, defining a steep negative trend on the Nd-Sr diagram and plotting close to the NHRL on conventional Pb isotope diagrams. By contrast TC and RCM samples have 143Nd/ 144Nd=0.5124-0.51275, 87Sr/ 86Sr=0.7035-0.7056 and 206Pb/ 204Pb=17.6-21.2, defining flat-lying arrays on Nd-Sr plots and a much greater scatter and spread on Pb isotope diagrams, with many analyses plotting above the NHRL. Both groups of analyses trend towards a common end member on a plot of 143Nd/ 144Nd against 87Sr/ 86Sr, at 143Nd/ 144Nd˜0.51275 and 87Sr/ 86Sr˜0.7035. These values are suggested to reflect the isotopic characteristics of the sub-lithospheric Kenyan mantle, inferred to be the Kenya mantle plume. Comparison with data from Afar suggest that the Kenya plume is distinct from the Afar plume, implying that the east African Rift is underlain by at least two distinct mantle plumes. Eocene and Oligocene basalts from southern Ethiopia bear a closer resemblance to the Kenyan basalts than to those from Afar and the Ethiopian plateau, suggesting that the Kenya plume has been active for at least 45 Ma. Migration of magmatism from southern Ethiopia southwards is consistent with the northeastward migration of the African plate over the Kenya plume over the past 50 My.

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

2000-03-01

296

Transition From Rift to Drift at Obliquely Divergent Continental Rifts: the Paired Rio Muni (W Africa) and NE Brazilian Margins  

NASA Astrophysics Data System (ADS)

We develop a synoptic model for the breakup and drift of a major sheared continental margin system: the Rio Muni basin and its NE Brazilian counterpart. It relates the evolution of crustal structure determined from seismic interpretation (including gravity-modelling of the deep-imaging PROBE dataset) to the cooling history of this margin system yielded from thermal history data (mainly apatite fission track analysis - AFTAr - and vitrinite reflectance data). Shear margins initiate as a leaky transform fracture system accommodating the oblique (i.e. non-orthogonal) divergence of opposing rifted continental margins. As such, the transition from continental breakup (i.e. rupture) to continental drift (i.e. ocean opening) at shear margins exhibits significant differences from that of the much better understood normally divergent rifts, where the spreading vector is normal to the strike of the opposing rift margins. For example, unlike at normally divergent rifts, continental breakup and drift are separate episodes in the early evolution of shear margins. In Rio Muni-NE Brazil, they are recorded by separate breakup and drift unconformities spanning a 15-20Ma. interval, the time taken for the ocean ridge to traverse the length of the margin before a continuous arm of oceanic crust separated Rio Muni from its Brazilian counterpart. In the Rio Muni basin, the c.70km-wide Ascension Fracture Zone (AFZ) exhibits oblique-slip faulting and synrift half-graben formation that accommodated oblique extension during the period leading up to and immediately following whole-lithosphere failure and continental breakup 117Ma. Gravity-modelling of PROBE seismic profiles reveals a land-locked precursor oceanic basin that preceded full ocean opening and which subsequently was stranded on the African margin. Its existence supports the idea of multiple rift suture lines accommodating episodic breakup and it emphasizes the significance of separate breakup and drift episodes at shear margins. Oblique extension is recorded also by strike- and oblique-slip fault geometry within the AFZ and buckling of Aptian synrift rocks in response to block rotation and local transpression. Principal cooling episodes affecting both margins identified from the AFTAr occurred in the Mid-Cretaceous 110-90Ma., following a period of high heatflow during which the geothermal gradient was more than twice that of the present day 25°C/km gradient, and 45-35Ma. coinciding with a major change in the pole of rotation at Chron 34 some 84Ma. Additional cooling episodes between 85-75Ma. and 10-2Ma. are restricted to the African side. Given that this margin system underwent breakup some 117Ma., it appears that thermal equilibrium was maintained throughout the initial breakup meaning that no significant uplift and erosion is recorded by the thermal history data during this period. Instead, Mid-Cretaceous cooling is interpreted as a response to the influence of i) the c.105Ma. St Helena Plume and ii) transpressional shortening and inversion that accompanied the breakup to drift period.

Turner, J. P.; Green, P. F.; Wilson, P. G.; Westbrook, G. K.; Lawrence, S.

2005-12-01

297

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

NASA Astrophysics Data System (ADS)

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

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

2013-11-01

298

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

USGS Publications Warehouse

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

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

1983-01-01

299

A NEW (or old) RIFT IN EASTERN CALIFORNIA  

NASA Astrophysics Data System (ADS)

The Eastern California Shear Zone (ECSZ) includes a broad network of right-lateral faults in the Mojave Desert and southern Basin-Ranges of California. Seven large earthquakes, including the 1992 M7.4 Landers and 1999 M7.1 Hector Mine earthquakes, have occurred within this zone in the last 60 years. This severe seismic activity is complicated by two puzzling facts: 1) the hypocenter of the Landers and Hector Mine earthquakes occur at shallow crustal depths (10±4 km); and 2) the aftershock patterns associated with these earthquakes cross cut the structural grain of the ECSZ at an acute angle. GPS data indicate that crustal rocks within the Mojave Desert segment of the ECSZ are moving as much as 14 mm/year to the northwest relative to Station FUNE in Death Valley; crustal rocks within the Basin-Ranges segment are moving 12.5 mm/year northwestward. Dokka and Travis (1990) concluded that the ECZS accommodates 20-25 percent of relative plate motion along the North American-Pacific plate margin inboard of the San Andreas Fault Zone. Nur and others (1993) reported that the large earthquakes may occur along a new fault system that cuts across the older faults; they named this young fault the Landers-Mojave earthquake line. Neither model, however, considers Quaternary volcanic centers in eastern CA and NV just east of and locally within the ECSZ. Rocks within these volcanic centers, including (from north to south) Lathrop Wells, Cima, Pisgah, Amboy, and Obsidian Butte, are younger than but chemically and isotopically similar to Neogene volcanic rocks that bound the rift province of the northern Gulf of California. The seismic and GPS data, combined with petrologic data from nearby volcanic centers, suggests that the ECZS is the active margin of an incipient transtensional rift as the Gulf of California propagates northward. Given the volume of volcanic rocks and prehistoric seismic history in California, it is not certain if this rift is growing or dying; only time will tell.

Calzia, J.

2009-12-01

300

Dyke intrusion dynamics during the ongoing rifting episode in Afar  

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

301

Which mantle below the active rift segments in Afar?  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

302

Formation of hyperextended rifted margins: Insights from flexural isostatic structural-stratigraphic modeling and observations from present-day rifted margins  

NASA Astrophysics Data System (ADS)

The understanding of how continental lithosphere extends, thins and ruptures leading to the formation of a new divergent plate boundary represent a fundamental question in Earth Sciences. In particular the mechanisms controlling the extreme pre-breakup stretching and thinning of the continental crust and lithosphere, documented at many present-day rifted margins, are still poorly known. Many questions remain of the fundamental processes controlling the extensional deformation of the continental crust and lithosphere, including fault geometries and their evolution in space and time, the occurrence of decoupling horizons within the continental crust and the importance of depth-dependent lithosphere thinning processes. We investigate the control of these key factors on continental crust and lithosphere thinning processes by combining seismic reflection and drill-hole observations from present-day Iberia-Newfoundland rifted margins with flexural isostatic forward tectonic and stratigraphic modeling. These observations made at present-day rifted margins constrain the input parameters used in the flexural isostatic forward modeling. At the same time, the forward tectonic and stratigraphic modeling provides validation of the interpretation of the seismic reflection data. Through this modeling, we produce isostatically and thermally balanced sections reproducing the geometries observed along the Iberia-Newfoundland rifted margins. Our results suggest that crustal and lithospheric thinning results from the combination of both pure- and simple-shear deformation. The model predicts the critical role of intra-crustal decoupling horizons confirming the importance of depth-dependent thinning through polyphased rifting events.

Mohn, Geoffroy; Kusznir, Nick; Manatschal, Gianreto

2014-05-01

303

Rift Valley Fever Outbreaks in Mauritania and Related Environmental Conditions  

PubMed Central

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

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

2014-01-01

304

Rupture Zones of Strong Earthquakes In The Corinth Rift  

NASA Astrophysics Data System (ADS)

Ruptures zones of the strong (M 8805; 6) earthquakes that occurred in the Corinth rift in the last three hundred years have been determined on the basis of aftershock epi- central distributions , intensity distributions and observations regarding seismogenic ground failures and tsunamis. The space U time distribution of the rupture zones indi- cates that (1) for time intervals of about 50yrs the rupture zones do not overlap; over- alpping appear, however, in longer time intervals , (2) there is a trend of the seismic activity to decrease westwards , and (3) particular regions constitute potential seis- mic gaps , like the Kiato UXylocastro region in the south coast of the Corinth Gulf, where the large 1402 earthquake occurred, and the Livadia U Desfina region where the A.D.361 and 551 large earthquakes possibly took place.

Papadopoulos, G. A.; Kouskouna, V.; Plessa, A.

305

Modeling the Spatial Spread of Rift Valley Fever in Egypt  

PubMed Central

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

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

2013-01-01

306

Neotectonics of the Roer Valley Rift System, the Netherlands  

NASA Astrophysics Data System (ADS)

The Roer Valley Rift System (RVRS) is located in the southern part of the Netherlands and adjacent parts of Gemany and Belgium. The last rifting episode of the RVRS started in the Late Oligocene and is still ongoing. The present-day seismic activity in the rift system is part of that last rifting episode. In this paper, the Quaternary tectonics of the RVRS are studied using the detailed stratigraphic record. Subsidence analyses show that three periods of subsidence can be discriminated during the Quaternary. A phase of rapid subsidence took place from the beginning of the Quaternary to the Upper Tiglian (˜1800 ka). This was followed by a phase of slow subsidence lasting until the Late Quaternary (˜500 ka). An acceleration in subsidence at the end of the Quaternary occurred in the central and northern parts of the RVRS (i.e. the Roer Valley Graben and the Peel Horst) during the last 500 ka. During the Quaternary, the most active fault zones in the RVRS are the Peel Boundary Fault zone and the Feldbiss Fault zone. Average displacements along these fault zones vary between 5 and 80 mm/ka. Periods of high and low displacement rates along faults can be discriminated. The magnitude of the subsidence rate in the central part of the RVRS, which in theory is caused by a combination of processes like faulting, cooling of the lithosphere and isostasy, is within the range of the rate of displacement along the major fault zones of the RVRS, which implies that the subsidence of the RVRS is to a large extent controlled by faulting. Along the wide and staggered Feldbiss Fault zone, the location of the largest displacement rate shifts during the Quaternary, whereas the Peel Boundary Fault zone, which is narrow and has a straight structure, is more stable in this respect. The present-day fault displacement rates inferred by geodetic measurements are two orders of magnitude larger than the rates inferred from the geological record. Such a large difference can be explained by a high variability of fault movements on a short time-scale due to fault-stress interactions. The stratigraphic record has preserved average displacement rates. Flexural analyses shows that the pattern of geodetically determined displacements is in accordance with the fault spacing in the fault zone. The NW-SE directed fault system active during the Quaternary and the Tertiary is inherited from the late stage of the Variscan orogeny. This fault system was also dominantly active during the Mesozoic and Early Cenozoic evolution of the RVRS. Lineament analysis of the topography indicates that apart from the dominant NW-SE-oriented faults, N-S and NE-SW directed faults are also prominent. These faults originate from the Caledonian tectonic phases. They have, however, no large displacements during the Mesozoic and Cenozoic. The fact that Paleozoic fault systems are reactivated during Quaternary and Tertiary indicates that these faults are fundamental weakness zones.

Houtgast, R. F.; van Balen, R. T.

2000-12-01

307

Rift Valley Fever outbreaks in Mauritania and related environmental conditions.  

PubMed

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

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

2014-01-01

308

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

309

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

PubMed

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

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

2014-01-01

310

Aedes Mosquito Saliva Modulates Rift Valley Fever Virus Pathogenicity  

PubMed Central

Background Rift Valley fever (RVF) is a severe mosquito-borne disease affecting humans and domestic ruminants. Mosquito saliva contains compounds that counteract the hemostatic, inflammatory, and immune responses of the host. Modulation of these defensive responses may facilitate virus infection. Indeed, Aedes mosquito saliva played a crucial role in the vector's capacity to effectively transfer arboviruses such as the Cache Valley and West Nile viruses. The role of mosquito saliva in the transmission of Rift Valley fever virus (RVFV) has not been investigated. Objective Using a murine model, we explored the potential for mosquitoes to impact the course of RVF disease by determining whether differences in pathogenesis occurred in the presence or absence of mosquito saliva and salivary gland extract. Methods C57BL/6NRJ male mice were infected with the ZH548 strain of RVFV via intraperitoneal or intradermal route, or via bites from RVFV-exposed mosquitoes. The virus titers in mosquitoes and mouse organs were determined by plaque assays. Findings After intraperitoneal injection, RVFV infection primarily resulted in liver damage. In contrast, RVFV infection via intradermal injection caused both liver and neurological symptoms and this route best mimicked the natural infection by mosquitoes. Co-injections of RVFV with salivary gland extract or saliva via intradermal route increased the mortality rates of mice, as well as the virus titers measured in several organs and in the blood. Furthermore, the blood cell counts of infected mice were altered compared to those of uninfected mice. Interpretation Different routes of infection determine the pattern in which the virus spreads and the organs it targets. Aedes saliva significantly increases the pathogenicity of RVFV. PMID:23785528

Le Coupanec, Alain; Babin, Divya; Fiette, Laurence; Jouvion, Grégory; Ave, Patrick; Misse, Dorothee; Bouloy, Michèle; Choumet, Valerie

2013-01-01

311

Molecular biology and genetic diversity of Rift Valley fever virus  

PubMed Central

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

Ikegami, Tetsuro

2013-01-01

312

Molecular biology and genetic diversity of Rift Valley fever virus.  

PubMed

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

Ikegami, Tetsuro

2012-09-01

313

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

NASA Astrophysics Data System (ADS)

The Rukwa rift is a deep sedimentary basin that is considered as a tectonic trans- fer zone between the Tanganyika and the Malawi troughs. The tectonic evolution of the depression is controlled by the reactivation of proterozoic structures and started with the deposition of the permo-triasic Karoo sediments. In the southeast, the rift is divided into two facing half graben separated by a Precambrian horst, whereas its northwestern part has a more symmetrical graben structure. Although most of the vertical displacement is accommodated by the Lupa eastern boundary fault, onshore shallow seismic profiles have confirmed the co-occurrence of intrabasin synthetic- and strike-slip faults within the sub surface sediments. Both normal and dextral strike-slip movement are indeed observed in the basin in response to the E-W to WNW-SSE ex- tension. The region has a moderate seismic activity and the earthquakes magnitude is generally below M 6.5. However, a M 7.4 earthquake occurred in the Rukwa region in 1910 but its exact location remains uncertain. The current research aimed at the identi- fication of active faults within the recent deposits of the basin by the combination in a GIS of radar interferometric data with topographical and geological maps, geophysical data, and field observations. Radar interferometry (InSAR) was found to be especially suitable for DEM computation in low relief areas where available topographic data are limited in accuracy. Numerous topographic lineaments were observed on InSAR DEM, and follow two main directions, both oblique to the main NW-SE trend of the rift. On the one hand, the GIS analysis confirms that the observed lineaments corre- spond to real natural alignment such like the drainage for example, and are therefore not related to atmospheric artefacts. On the other hand, the field observations revealed that in most cases, the topographic lineaments are very subtle and difficult to identify. However, direct correlations with tectonic structures (faults, fissures, ...) were recog- nized in some places. From the satellite imagery, the northwestern part of the basin is marked by the succession of paleo-shorelines evidencing the lake level fluctuations. The highest level was dated to 10.000 yrs BP, when the Rukwa was supposed to flow into lake Tanganyika. The corresponding shoreline is cut by a fault, ~ 40 km long, striking NNW-SSE, evidenced on the InSAR DEM and confirmed by the LANDSAT and ASTER optical imagery. Although a possible strike-slip component, the 5 to 10 1 meters of vertical offset from one to the other side of the fault would yield to a slip rate of 0.5 U 1mm/yr. 2

Kervyn, F.

314

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

Technology Transfer Automated Retrieval System (TEKTRAN)

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

315

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

316

2006 Nature Publishing Group Magma-maintained rift segmentation at continental  

E-print Network

of the Nubia­Arabia plate boundary. Rifting of Africa and Arabia during the past ,30 Myr produced the ,300-km similarities to crust beneath Iceland6,8 . Global plate reconstructions based on geological and geodetic data

Biggs, Juliet

317

Andean deformation and rift inversion, eastern edge of Cordillera Oriental (GuatequeMedina area), Colombia  

E-print Network

and the rift geometry. We have mapped a regional transect and restored a cross section. We have also reconciled evaporitic layer, which is locally emerald bearing, has acted as a regional detachment. The underlying

Paris-Sud XI, Université de

318

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

NASA Technical Reports Server (NTRS)

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

Blanchard, D. P. (editor)

1980-01-01

319

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

USGS Publications Warehouse

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

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

2012-01-01

320

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

E-print Network

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

Long, Bernard

321

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

USGS Publications Warehouse

The late Cenozoic volcanic and tectonic activity of the enigmatic West Antarctic rift system, the least understood of the great active continental rifts, has been suggested to be plume driven. In 1991-1992, as part of the CASERTZ (Corridor Aerogeophysics of the Southeast Ross Transect Zone) program, an ~25 000 km aeromagnetic survey over the ice-covered Byrd subglacial basin shows magnetic "texture' critical to interpretations of the underlying extended volcanic terrane. The aeromagnetic data reveal numerous semicircular anomalies ~100-1100 nT in amplitude, interpreted as having volcanic sources at the base of the ice sheet; they are concentrated along north-trending magnetic lineations interpreted as rift fabric. The CASERTZ aeromagnetic results, combined with >100 000 km of widely spaced aeromagnetic profiles, indicate at least 106 km3 of probable late Cenozoic volcanic rock (flood basalt?) in the West Antarctic rift beneath the ice sheet and Ross Ice Shelf. -from Authors

Behrendt, John C.

1994-01-01

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

Banderas Rift Zone: A plausible NW limit of the Jalisco Block  

NASA Astrophysics Data System (ADS)

Echo soundings recently made in Bahía de Banderas show that this region is a graben with steeply dipping walls and several basins; it is the offshore continuation of the Valle de Banderas graben, and of a branching rift (Río Ameca rift) originating in the Tepic-Zacoalco rift zone. The general trend of the three structures is ENE with some NE trending offsets, and they have a total length of 150 km; this Banderas Rift Zone is proposed as the NW limit of the Jalisco block. The existence of this limit suggests that there is another platelet, or block, between the Jalisco block and a portion of the Rivera plate, probably bounded by the Tres Marías escarpment, the Jalisco block and the North America plate.

Alvarez, Román

2002-10-01

324

Los biopolímeros como materiales para el desarrollo de productos en aplicaciones farmacéuticas y de uso biomédico Biopolymers as materials for developing products in pharmaceutical applications and biomedical uses  

Microsoft Academic Search

RESUMEN Los biopolímeros han sido ampliamente estudiados en aplicaciones farmacéuticas para modificar la liberación de principios activos, localización de los fármacos en su diana terapéutica, sobrepaso de barreras fisiológicas (tisulares y celulares) y la protección de agentes terapéuticos inestables a las condiciones fisiológicas presentes en las vías de administración menos invasivas. Así mismo, es notable la importancia en el uso

Manuel Guillermo; Rojas Cortés; Bibiana Margarita; Vallejo Díaz; Jairo Ernesto Perilla

2008-01-01

325

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

E-print Network

Motion in the north Iceland volcanic rift zone accommo- dated by bookshelf faulting Robert. G. GreenH, Robert. S. White & Tim Greenfield Published article: Green, R. G., White, R. S., Greenfield, T. Motion in the north Iceland volcanic rift zone... region and the RRISP experiment. Earthquake picks on the easternmost strike-slip fault are from Martens & White[28]. Double difference relocations were then computed using HypoDD[29]. The layered velocity model required by the program was generated...

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

2013-12-01

326

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

USGS Publications Warehouse

We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore-offshore large-aperture seismic and gravity data. High velocities obtained within a highly reflective half-graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide-angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55–60km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high-velocity lower crust interpreted to result from syn-rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half-graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre-rift Moho is probably within or above the high-velocity lower crust, whereas the post-rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.

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

1991-01-01

327

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

SciTech Connect

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

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

1993-08-01

328

Permo–Triassic intraplate magmatism and rifting in Eurasia: implications for mantle plumes and mantle dynamics  

Microsoft Academic Search

At the transition from the Permian to the Triassic, Eurasia was the site of voluminous flood-basalt extrusion and rifting. Major flood-basalt provinces occur in the Tunguska, Taymyr, Kuznetsk, Verkhoyansk–Vilyuy and Pechora areas, as well as in the South Chinese Emeishen area. Contemporaneous rift systems developed in the West Siberian, South Kara Sea and Pyasina–Khatanga areas, on the Scythian platform and

A. M Nikishin; P. A Ziegler; D Abbott; M.-F Brunet; S Cloetingh

2002-01-01

329

Deformation across the Asal-Ghoubbet rift, Djibouti, uplift and crustal extension 1979-1986  

Microsoft Academic Search

Geodetic networks of different sizes have been established since 1973 in the Asal-Ghoubbet rift in south-western Afar in order to study crustal movements across an extensive plate boundary. Partial or entire remeasurements of these networks during the 1979-1986 interval provide detailed data about strain accumulation and vertical movements after a seismo-volcanic rifting event that occurred in November 1978 along this

J. C. Ruegg; M. Kasser

1987-01-01

330

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

331

Propagation of an active rift in the Ross Ice Shelf, Antarctica  

NASA Astrophysics Data System (ADS)

Understanding propagation behavior of large rifts in ice shelves is important for understanding shelf adjustments to change and for parameterizing calving in models. We use satellite images and a comparison of two epochs of the MOA (MODIS Mosaic of Antarctica) to study the propagation of an active rift within a rift system near the western front of the Ross Ice Shelf. Between 1992 and 2012, the most upstream rift within the rift system propagated over 90 km. We observe large jumps in propagation, with average propagation rates exceeding 30-40 m/day over a period of measurement compared to periods of slower growth at 1-10 m/day. Two types of episodic propagation were observed, one driven by the orientation of the fracture plane relative to the stress field and the other driven by increasing fracture length. We use a numerical model to simulate recent propagation behavior of test fractures within a stress field. We investigate temporal connections between dynamic adjustments along flow of fractures and shelf-front geometry following the calving of B15 in early 2000. Our observations indicate longitudinal displacement of fractures following impact events at the shelf front. These events may have displaced the eastern tip of the active rift into a more favorable orientation relative to the stress field. Our observations and model simulations support findings on the Ross and other large ice shelves regarding the importance of lateral propagation and the roles of transverse compressive stress, fracture length, and material inhomogeneity in controlling propagation behavior.

LeDoux, C. M.; Hulbe, C. L.

2012-12-01

332

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

333

The North American Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling  

USGS Publications Warehouse

The Midcontinent rift system is a 1.1-b.y.-old structure extending from Kansas, through the Lake Superior region, and into southern Michigan. The rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments. For most of its extent it is buried beneath Paleozoic rocks but can be traced by its strong gravity and magnetic anomalies. Seismic reflection surveys by the Great Lakes International Multidisciplinary Program on Crustal Evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail. The reflection profiles across the rift reveal a deep, asymmetrical central graben whose existence and magnitude was not previously documented. They show that, in addition to crustal sagging documented by previous investigations, normal faulting played a major role in subsidence of the axial region of the rift. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. -from Authors

Cannon, W.F.

1989-01-01

334

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

335

Evolution of rifted continental margins: the case of the Gulf of Lions (Western Mediterranean Basin)  

NASA Astrophysics Data System (ADS)

The formation of rifted continental margins has long been explained by numerous physical models. However, field observations are still lacking to validate or constrain these models. This study presents major new observations on the broad continental margin of the Gulf of Lions, based on a large amount of varied data. Two contrasting regions characterize the thinned continental crust of this margin. One of these regions corresponds to a narrow rift zone (40-50 km wide) that was highly thinned and stretched during rifting. In contrast with this domain, a large part of the margin subsided slowly during rifting and then rapidly after rifting. The thinning of this domain cannot be explained by stretching of the upper crust. We can thus recognize a zonation of the stretching in both time and space. In addition, the Provencal Basin is characterized by a segmentation of the order of 100-150 km. These observations have important consequences on the formation and evolution of the Gulf of Lions margin. Independently of the geodynamic context, we can propose some general features that characterize the formation of rifted continental margins.

Bache, François; Olivet, Jean Louis; Gorini, Christian; Aslanian, Daniel; Labails, Cinthia; Rabineau, Marina

2010-05-01

336

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

NASA Astrophysics Data System (ADS)

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 H 2S-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.

Robbins, Eleanora Iberall

1983-05-01

337

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

338

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

339

Single grain U\\/Pb geochronology of detrital zircons from Midcontinent rift arkoses, NE Kansas: Implications for depositional history  

Microsoft Academic Search

The Midcontinent rift system in the subsurface south of the Lake Superior region has been well imaged by magnetic, gravity and seismic surveys, however only a few wells have penetrated and recovered core from rift-basin fill in this region. Texaco's exploratory Noel Poersch well [number sign]1 in northeastern Kansas, penetrated [approximately] 2,600 m of rift-related volcanic, igneous, and arkosic sedimentary

M. W. Martin; W. R. Van Schmus; P. Berendsen

1993-01-01

340

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

NASA Astrophysics Data System (ADS)

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

Bennett, Scott Edmund Kelsey

341

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

342

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

NASA Astrophysics Data System (ADS)

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 to obtain better resolution of shallow structures by employing a shorter seismic station spacing (1.5 km). In this study, these seismic data were used simultaneously with surface geology, drill hole data and gravity measurements to construct a relatively detailed model of the upper crust in the Lake Magadi region. This integrated analysis provided several significant results. At Lake Magadi, the rift floor is covered by low density-low velocity (2400 kg m -3 and 4.2 km s -1) sediments and volcanics that form a complex, asymmetric graben whose deepest portion (about 3.5 km) is found adjacent to the Nguruman Escarpment master fault to the west. This basin is about 2.0 km deep along its eastern margin and is divided into two sub-basins by a horst that runs along its axis, creating a localised gravity high. This simple structure was sufficient to produce the observed geophysical anomalies, so no pervasive magmatic modification of the upper crust was required. Beneath the rift basin, a 4 km thick layer, with relatively low velocities and densities, was found to extend from the eastern rift valley margin across the western rift flank to the vicinity of the Oloololo Scarp. This basement layer, with an average velocity of 5.6 km s -1 and a density of 2600 kg m -3, is intetpreted to represent a combination of quartzites and rocks fractured by the Neoproterozoic (Pan-African) collision of the Mozambique Belt and Tanzania Craton. Although Proterozoic rocks of the Mozambique Belt outcrop on both sides of the rift valley, the velocities, densities and thicknesses of the layers constituting the crystalline upper crust on the western rift flank are less than those on the eastern flank and similar to values found within the Tanzania Craton elsewhere. This suggests that the two rift flanks are located in different structural regimes (craton and mobile belt) with rifting having been localised along the boundary between them. From a deep crustal perspective, this boundary appears to be the western margin of the Mozambique Belt terrane, although rocks of this belt are found at the surface further west.

Simiyu, Silas M.; Randy Keller, G.

1998-11-01

343

Insights into rifting from shear wave splitting and receiver functions: an example from Ethiopia  

NASA Astrophysics Data System (ADS)

Seismic anisotropy beneath broad-band stations in the vicinity of the East African rift are compared with those on stable cratonic parts of Africa and Arabia. Such measurements offer potential constraints on rift processes, absolute plate motions (APM) and tectonic structure. New SKS shear wave splitting parameters are analysed beneath the broad-band stations of FURI and AAE (Ethiopia), BGCA (Central African Republic) and RAYN (Saudi Arabia). The number of events considered at the four stations varies from 13 to 32 and provides good azimuthal coverage. Stations on or near the rift show the polarization of the fast shear wave (?) aligned parallel to the rift axis. The magnitude of the splitting delay (?t) increases northward along the East African rift. Previously published measurements in Kenya show the smallest splitting value (1.0 s), whilst the Djibouti station, ATD, shows the largest splitting (1.6 s). The Ethiopian results (?t= 1.38 + 0.03 s, ?= 36°+ 1) show constancy in ?t and ? with respect to backazimuth, thus, suggesting a single anisotropic layer beneath the stations. There is no observed correlation of ? with APM direction. Less splitting (?t) is observed beneath cratonic parts of Africa. BGCA in central Africa shows splitting parallel to the inferred direction of transpression, not the APM direction. Receiver-function analysis at FURI and AAE supports evidence from refraction experiments of thick crust (ca 40 km) in the region of continental rifting, however, the analysis shows a deeper interface at a depth of 90 km, also. This interface may mark the base of the lithosphere in this region. One interpretation of the splitting results is that the anisotropy at the Ethiopian stations is the result of aligned melt in this upper 90 km of lithosphere. A < 1 per cent volume fraction of melt aligned in thin (aspect-ratio <0.03) vertical ellipsoidal pockets generates sufficient splitting to explain the data. Higher splitting magnitudes in the north correlates with higher melt production observed in the Ethiopian part of the rift. Alternatively, anisotropy may be the result of the alignment of olivine in the asthenosphere parallel to the ridge axis, as material flows laterally to fill the gap caused by lithospheric extension. This would either suggest a northward-thickening anisotropic layer beneath the rift or enhanced olivine alignment as a result of changes in strain. Whatever the mechanism, it appears that the anisotropy is sensitive to, and provides insight into, the transition from continental to oceanic rifting in the northern Ethiopian rift.

Ayele, Atalay; Stuart, Graham; Kendall, J.-Michael

2004-04-01

344

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

345

Modelizacion, control e implementacion de un procesador energetico paralelo para aplicacion en sistemas multisalida  

NASA Astrophysics Data System (ADS)

Cualquier sistema electronico que incluya un procesado o tratamiento de la senal, y ademas, algun tipo de actuador mecanico generalmente necesita, como minimo, dos tensiones diferentes de alimentacion. Excluyendo los sistemas de alimentacion distribuida, la solucion tecnica mas utilizada para proporcionar dos o mas tensiones consiste en las fuentes de alimentacion multisalida. En una fuente de alimentacion multisalida los diferentes circuitos que conforman cada salida comparten un mismo transformador de potencia optimizando coste, masa, y volumen. Las ventajas obtenidas con este procedimiento tienen en su contra el efecto que sobre cada salida individual provocan las demas en su conjunto debido, principalmente, a los efectos de los elementos parasitos de los componentes. Un cambio de carga en una de las salidas produce un transitorio que es visto por todas las demas como un efecto de impedancia cruzada, y al final del transitorio, la tension de cada salida es diferente respecto a la que tenian antes del transitorio. Este ultimo resultado se conoce como regulacion cruzada. La disminucion de los efectos de la regulacion cruzada ha sido objeto de estudio durante los ultimos anos. El objetivo ha sido el desarrollo de distintas estrategias que permiten, desde disminuir los efectos de la regulacion cruzada hasta los niveles deseables, a eliminarla completamente. El resultado final suele suponer una penalizacion sobre el diseno del sistema directamente proporcional al grado de regulacion a conseguir en las distintas salidas. Entre las soluciones propuestas para eliminar la regulacion cruzada las tecnicas de post-regulacion se han consolidado como la opcion mas aceptada ya que, pueden aplicarse a cualquier convertidor y no suponen ninguna complejidad adicional a la hora de plantear el diseno. En esta Tesis Doctoral se abordara el estudio de la tecnica conocida como postregulacion mediante transformador controlado, que si bien se ha empleado en convertidores resonantes, su modelizacion, y aplicacion en convertidores PWM, esta aun por estudiar y valorar. El primer Capitulo consiste en una breve introduccion al problema de la regulacion cruzada y la impedancia cruzada para posteriormente describir las tecnicas de post-regulacion actualmente mas empleadas, con especial atencion al post-regulador con transformador controlado. El Capitulo segundo trata del estudio de las caracteristicas estaticas del postregulador con transformador controlado. Partiendo de los estudios disponibles sobre el postregulador se plantean mejoras en su modo de actuacion y se discuten tres alternativas diferentes para controlar el transformador. Las dos primeras consisten en emplear un convertidor auxiliar Boost en sus dos modos de funcionamiento, continuo y discontinuo. La tercera consiste en controlar el transformador con una tension PWM directamente, sin filtrado. Finalmente se comprueba experimentalmente, para el estado estacionario, el funcionamiento del post-regulador para cada uno de los tres metodos de control. El Capitulo tercero trata de la dinamica de la salida controlada con el post-regulador cuando este emplea un convertidor auxiliar tipo Boost. Mediante la tecnica de promediado de variables de estado se propone el modelo de pequena senal, tanto para el modo continuo como para el modo discontinuo de funcionamiento del convertidor auxiliar. Los resultados mas significativos de esta seccion son las expresiones analiticas de las impedancias cruzadas y de la impedancia de la salida post-regulada. Como complemento al modelo de pequena senal se plantea un modelo de gran senal implementado sobre el simulador Pspice. Con este nuevo modelo se reproducen los resultados obtenidos con el modelo de pequena senal y ademas es posible simular los transitorios en las tensiones de salida ante cambios de carga. La modelizacion del convertidor cuando el transformador se controla con una tension PWM sin filtrar es el objetivo del Capitulo 4. En las secciones siguientes del Capitulo se plantea el correspondiente modelo de gran senal aplicado a un nuevo prototipo exper

Ferreres Sabater, Agustin

346

Magma paths at Piton de la Fournaise volcano: a synthesis of Hawaiian and Etnean rift zones  

NASA Astrophysics Data System (ADS)

On ocean basaltic volcanoes, magma transfer to the surface occurs along sub-vertical ascent from the mantle lithosphere through the oceanic crust and the volcanic edifice, eventually followed by lateral propagation along rift zones. We use a 17-years-long database of volcano-tectonic seismic events and a detailed mapping of the pyroclastic cones to determine the geometry and the dynamics of the magma paths intersecting the edifice of Piton de la Fournaise volcano. We show that the overall plumbing system, from about 30 km depth to the surface, is composed of two structural levels that feed distinct types of rift zones. The lower plumbing system has a southeastward (N120) orientation and permits magma transfer from the lithospheric mantle to the base of the La Réunion edifice (5 km bsl). The related rift zone is wide, linear, spotted by small to large pyroclastic cones and related lava flows and involving magma resulting from high-pressure fractionation of ol ± cpx and presents an eruption periodicity of around 200 years over the last 30 kyrs. Seismic data suggest that the long-lasting activity of this rift zone result from a regional NNE-SSW extension reactivating inherited lithospheric faults by the effect of underplating and/or thermal erosion of the mantle lithosphere. The upper plumbing system originates at the base of the edifice in the vertical continuity of the lower plumbing system. It feeds frequent (1 eruption every 9 months on average), short-lived summit and distal (flank) eruptions along summit and outer rift zones, respectively. Summit rift zones are short and present an orthogonal pattern restricted to the central active cone of Piton de la Fournaise whereas outer rift zones extend from inside the Enclos Fouqué caldera to the NE and SE volcano flanks. We show that the outer rift zones are genetically linked to the east flank seaward displacements, whose most recent events where detected in 2004 and 2007. The lateral movements are themselves triggered by shallow sill intrusions below the east flank. We propose that the sub-vertical magma intrusions along the perpendicular summit rift zones, sill intrusions and subsequent magma injections along the outer rift zones are controlled by cycles of stress permutations. Recurrent dyke injections along the summit rift zone in an extensional stress field reduce the deviatoric stress until a switch of the axes of principal stresses and a sill intrusion. The related flank lateral destabilization restores the extensional stress field and initiates a new cycle of stress permutations. To sum up, rift zones of Piton de la Fournaise present strong geometrical and dynamical differences. On the one hand, the lower plumbing system feeds rift zones showing striking similarities to those developed in Hawaii during the alkaline postshield stage. On the other hand, the rift zones connected to upper plumbing system and the related volcano flank movement can be compared to the eruptive and east flank dynamics of Mount Etna.

Michon, Laurent; Ferrazzini, Valérie; Di Muro, Andrea; Chaput, Marie; Famin, Vincent

2014-05-01

347

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

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

348

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

349

Tectonic and eustatic controls on carbonate platforms of the Jurassic High Atlas rift of Morocco  

SciTech Connect

Stratigraphic studies of Lower and Middle Jurassic carbonates along the southern margin of the High Atlas rift document five major stages of platform development a response to rift tectonics and eustasy. The five stages are as follows. (1) Early Sinemurian marine transgression over Triassic-Liassic continental red beds and basalts lead to the development of regionally extensive, cyclic carbonate platforms. (2) Middle to late Sinemurian marked the initiation of Liassic rifting, with synrift platforms restricted to the rift margin and to localized horsts within the rift axis. Synrift platforms developed diverse depositional systems, with a marked change to rimmed margins flanked by steep slopes and deep (400-500 m) marine basins. Two orders of cyclicity shallowing-upward cycles and bundles of cycles dominated the platform tops. (3) Late Pliensbachian subaerial exposure, resulting in termination of Liassic platform development, was recorded by regressive seaward shifts in facies belts, microkarstification, and deposition of continental red beds across the platforms. (4) Early to middle Toarcian transgression, yielding landward shifts in facies belts on platform tops, was signified by deposition of noncyclic, skeletal carbonate sequences. (5) Middle Toarcian platform drowning was followed by deposition of Toarcian-Aalenian amonite-bearing marine shales (100 m thick) blanketing (downlapping, width source from the south) the southern platform, whereas only a condensed sequence (2-3 m thick) of glauconitic, ammonite-skeletal carbonate strata covered the submerged, isolated, axial rift platform of Jebel Bou Dahar. Prolonged sediment starvation on Bou Dahar combined with depositional onlap of its slope by basin-filling shales (Toarcian to Bajocian) and distal carbonate turbidites (Aalenian), also sourced from the southern margin of the rift, produccd a drowning onlap unconformity.

Crevello, P. (Marathon Oil Co., Littleton, CO (USA))

1990-05-01

350

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

NASA Astrophysics Data System (ADS)

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

Bottenberg, Helen Carrie

351

Climate Change Affects the East African Rift Valley Lakes  

NASA Astrophysics Data System (ADS)

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

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

2004-12-01

352

Advances in Rift Valley Fever Research: Insights for Disease Prevention  

PubMed Central

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

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

2011-01-01

353

Fluid flow in fault zones from an active rift  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

354

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

355

Probable rift origin of Canada Basin, Arctic Ocean  

USGS Publications Warehouse

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

Tailleur, Irvin L.

1973-01-01

356

Probable rift origin of the Canada basin, Arctic Ocean  

USGS Publications Warehouse

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

Tailleur, Irvin L.

1973-01-01

357

Rift Valley Fever Virus Infection in Golden Syrian Hamsters  

PubMed Central

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

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

2015-01-01

358

Observations on rift valley fever virus and vaccines in Egypt  

PubMed Central

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

2011-01-01

359

Persistence of Rift Valley fever virus in East Africa  

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

360

Climate-disease connections: Rift Valley Fever in Kenya  

NASA Technical Reports Server (NTRS)

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 Nino/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.

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

2001-01-01

361

[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

362

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

NASA Astrophysics Data System (ADS)

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

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

2013-11-01

363

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

364

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

365

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

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

366

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

NASA Astrophysics Data System (ADS)

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

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

2013-02-01

367

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

NASA Astrophysics Data System (ADS)

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

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

368

Geochemical signals of progressive continental rupture in the Main Ethiopian Rift  

NASA Astrophysics Data System (ADS)

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

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

2003-04-01

369

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

E-print Network

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

MacPhee, Daniel

2006-01-01

370

Stress changes associated with volcanic sources: constraints on Kilauea rift dynamics  

NASA Astrophysics Data System (ADS)

This paper studies the effects of strain sources at depth, simulating volcanic source activation, on the changes of Coulomb failure stress in the surrounding volume. Such simulations are aimed at clarifying the relationship between volcano dynamics and seismicity generated by stress changes. The effect of isotropic overpressure or dike opening on the Coulomb stress patterns is shown here, in presence of a background tectonic stress. Seismicity patterns resulting from such stress changes can thus be used to shed light on the sources involved. Simulations of Coulomb stress changes associated to the opening of the Northeast rifts at Kilauea volcano (Hawaii) have been performed, showing that seismicity at this area is well explained by stress changes produced by rift opening (i.e. without any contribution out of local sources), both in terms of location patterns and focal mechanisms. In particular, the widely observed focusing of seismicity in the southern flank can be explained in terms of Coulomb stress changes generated by rift opening, once the different opposing masses in the northern and southern flank is taken into account. In addition, Coulomb stress changes due to rift opening are shown to explain both the occurrence of large earthquakes and of the seaward sliding of the Southern flank, on the low-angle reverse faults hypothesized at the base of the rift zone.

Troise, C.

2001-08-01

371

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

NASA Technical Reports Server (NTRS)

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

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

1986-01-01

372

Lower crustal bodies in the Møre volcanic rifted margin: Geophysical determination and geological implications  

NASA Astrophysics Data System (ADS)

Understanding nature, structure and age of Lower Crustal Bodies (LCBs) and their relation to the crustal structure of the Møre margin (Mid-Norwegian margin), and in a more general way, of magma-rich rifted margins, is a key issue to decipher the tectono-magmatic processes found in volcanic rifted margins. In light of 2D potential field modelling combined with reflection and refraction seismic data, we reinvestigated the crustal nature of the Møre margin and adjacent Jan Mayen corridor. In the proximal domain, our study shows that the LCBs most likely represent inherited crustal bodies and not necessarily rift-related serpentinised mantle as previously proposed. To fit all geophysical observations, both lower and middle crustal layers need to be preserved over a large part of the Møre Basin. For the distal margin, the interpretation of the LCBs is more difficult. Our preferred interpretation is that they are mainly made of boudins of hyper-extended, pre-rift lower continental crustal rocks more or less intruded by Early Tertiary magmatic material. Our seismic, magnetic and gravity data does not easily support large scale exhumation of serpentinised mantle in the inner and is unlikely in the outer parts of the Møre Basin. The deep structures of the Mid-Norwegian magma rich rifted margin result from the poly-phase stretching and thinning of complex inherited crustal structures, locally intruded by Early Tertiary magmatic material.

Nirrengarten, M.; Gernigon, L.; Manatschal, G.

2014-12-01

373

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

SciTech Connect

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

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

1995-11-01

374

Geodyamic Models of the Tectonomagmatic Evolution of the West Antarctic Rift System  

NASA Astrophysics Data System (ADS)

Finite element geodynamic models reproduce the general aspects of the style and timing of extension in the West Antarctic Rift System (WARS). In particular, the models simulate a prolonged period of diffuse extension throughout the WARS during the Cretaceous and early Cenozoic followed by later focused extension in the Victoria Land Basin during the middle Paleogene. Two families of models are identified that are consistent with the Paleogene onset of magmatism in the West Antarctic Rift System under both normal and warm (70-120º hotter than global average) mantle potential temperatures. The two families of models produce distinctly different melt distribution patterns across the WARS. In models invoking normal mantle temperatures the greatest thickness of melt is confined to the Victoria Land Basin region. In models invoking warmer mantle temperatures, the greatest thickness of melt is widely distributed in the region currently underlaying the West Antarctic Ice Sheet. The models indicate that 1) the onset of Cenozoic magmatism in the Victoria Land Basin can be explained as rift induced without requiring the impingement of a plume or a change in plate motion, 2) if rifting is associated with a plume then the magmatic rocks under the West Antarctic Ice Sheet should be late Cretaceous to early Cenozoic in age, and 3) if the magmatic rocks under the West Antarctic Ice Sheet are older or younger than the main stage of rifting (ca. 105 Ma to 35 Ma), a syn-extensional plume could not have been present.

Anoka, J. L.; Harry, D. L.

2007-12-01

375

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

PubMed

Because of its subaerial exposure the Asal rift segment provides an exceptional opportunity to quantify the deformation field of an active rift and assess the contribution of tectonics and volcanism to rifting processes. The present topography of the Asal rift results from the tectonic dismemberment during the last 100,000 years of a large central volcanic edifice that formed astride the rift zone 300,000 to 100,000 years ago. Three-dimensional deformation of this volcano has been quantified from the combined analysis of the topography and geology. The analysis indicates that spreading at 17 to 29 millimeters per year in a N40 degrees +/- 5 degrees E direction accounts for most of the separation between Arabia and Somalia. The small topographic subsidence relative to extension suggests that tectonic thinning of the crust has been balanced by injection and underplating of magmatic material of near crustal density. The methodology developed in this study could also be applied to quantify deformation in relatively inaccessible areas where the main available information is topography or bathymetry. PMID:17770894

De Chabalier, J B; Avouac, J P

1994-09-16

376

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

377

Rift migration explains continental margin asymmetry and crustal hyper-extension  

PubMed Central

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

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

2014-01-01

378

Rift migration explains continental margin asymmetry and crustal hyper-extension.  

PubMed

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

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

2014-01-01