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Sample records for african rift zone

  1. East African Rift

    NASA Technical Reports Server (NTRS)

    2008-01-01

    Places where the earth's crust has formed deep fissures and the plates have begun to move apart develop rift structures in which elongate blocks have subsided relative to the blocks on either side. The East African Rift is a world-famous example of such rifting. It is characterized by 1) topographic deep valleys in the rift zone, 2) sheer escarpments along the faulted walls of the rift zone, 3) a chain of lakes within the rift, most of the lakes highly saline due to evaporation in the hot temperatures characteristic of climates near the equator, 4) voluminous amounts of volcanic rocks that have flowed from faults along the sides of the rift, and 5) volcanic cones where magma flow was most intense. This example in Kenya displays most of these features near Lake Begoria.

    The image was acquired December 18, 2002, covers an area of 40.5 x 32 km, and is located at 0.1 degrees north latitude, 36.1 degrees east longitude.

    The U.S. science team is located at NASA's Jet Propulsion Laboratory, Pasadena, Calif. The Terra mission is part of NASA's Science Mission Directorate.

  2. Do melt-rich shear zones lubricate rift flanks? 3-D spatial gradients in anisotropy beneath the East African Rift in Ethiopia

    NASA Astrophysics Data System (ADS)

    Holtzman, B. K.; Gaherty, J. B.; Kendall, J.; Stuart, G.

    2006-12-01

    Melt-enhanced weakening of the mantle may be essential for a continent to break apart and rift. A primary means of understanding this aspect of the dynamics of rifting (and other geodynamic settings) is the interpretation of seismic anisotropy in terms of melt distribution and/or mantle flow direction. In recent rock- deformation experiments, the relationships between flow direction and seismic fast direction are complicated by the presence of water (e.g. Katayama et al., 2004) and segregated melt (Holtzman et al. 2003). In the latter, deviatoric stress drives melt to organize into networks of melt-rich shear zones. Scaling from laboratory to upper mantle conditions predicts characteristic network spacings of <1 km, less than wavelengths of measured seismic waves; thus they should cause significant seismic anisotropy. Measurable gradients in anisotropy may be diagnostic indicators of gradients in melt segregation. Because melt-rich shear zones will weaken the mantle on a large scale, the presence of such anisotropy gradients would map to gradients in viscosity. To map melt distribution beneath the rift and test for the presence or absence of this process, we combine measurements of seismic velocities beneath the East African Rift in Ethiopia with modeling of elastic properties of aligned, segregated melt and olivine fabric. Analysis of SKS phases has shown that fast directions parallel magmatic centers in the rift and splitting magnitudes are largest near the rift flanks, consistent with the hypothesis above (e.g. Kendall et al., 2006). Preliminary analysis of Love-Rayleigh differential times across the rift are consistent with a vertical fast axis, suggesting the presence of a vertically aligned, rift parallel melt phase down to the solidus (Pilidou et al., 2005). We expand on these results by applying a cross-correlation procedure to precisely estimate relative amplitude and phase of surface waves traversing the rift. Data are derived from the EAGLE and Ethiopia

  3. East African Rift Valley, Kenya

    NASA Technical Reports Server (NTRS)

    1990-01-01

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

  4. The Offshore East African Rift System

    NASA Astrophysics Data System (ADS)

    Franke, D.; Klimke, J.; Jokat, W.; Stollhofen, H.; Mahanjane, S.

    2014-12-01

    Numerous studies have addressed various aspects of the East African Rift system but surprisingly few on the offshore continuation of the south-eastern branch of the rift into the Mozambique Channel. The most prominent article has been published almost 30 years ago by Mougenot et al. (1986) and is based on vintage seismic data. Several studies investigating earthquakes and plate motions from GPS measurements reveal recent deformation along the offshore branch of the East African Rift system. Slip vectors from earthquakes data in Mozambique's offshore basins show a consistent NE direction. Fault plane solutions reveal ~ E-W extensional failure with focal depth clustering around 19 km and 40 km, respectively. Here, we present new evidence for neotectonic deformation derived from modern seismic reflection data and supported by additional geophysical data. The modern rift system obviously reactivates structures from the disintegration of eastern Gondwana. During the Jurassic/Cretaceous opening of the Somali and Mozambique Basins, Madagascar moved southwards along a major shear zone, to its present position. Since the Miocene, parts of the shear zone became reactivated and structurally overprinted by the East African rift system. The Kerimbas Graben offshore northern Mozambique is the most prominent manifestation of recent extensional deformation. Bathymetry data shows that it deepens northwards, with approximately 700 m downthrown on the eastern shoulder. The graben can be subdivided into four subbasins by crosscutting structural lineaments with a NW-SE trend. Together with the N-S striking graben-bounding faults, this resembles a conjugate fault system. In seismic reflection data normal faulting is distinct not only at the earthquake epicenters. The faults cut through the sedimentary successions and typically reach the seafloor, indicating ongoing recent deformation. Reference: Mougenot, D., Recq, M., Virlogeux, P., and Lepvrier, C., 1986, Seaward extension of the East

  5. Implications of new gravity data for Baikal Rift zone structure

    NASA Technical Reports Server (NTRS)

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

    1993-01-01

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

  6. Multiple mantle upwellings in the transition zone beneath the northern East-African Rift system from relative P-wave travel-time tomography

    NASA Astrophysics Data System (ADS)

    Civiero, Chiara; Hammond, James O. S.; Goes, Saskia; Fishwick, Stewart; Ahmed, Abdulhakim; Ayele, Atalay; Doubre, Cecile; Goitom, Berhe; Keir, Derek; Kendall, J.-Michael; Leroy, Sylvie; Ogubazghi, Ghebrebrhan; Rümpker, Georg; Stuart, Graham W.

    2015-09-01

    Mantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low-shear-velocity province beneath Southern Africa, the so-called African Superplume, to multiple lower-mantle sources along the rift. We present a new P-wave travel-time tomography model below the northern East-African, Red Sea, and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated data set allows us to image structures of ˜100 km length-scale down to depths of 700-800 km beneath the study region. Our images provide evidence of two clusters of low-velocity structures consisting of features with diameter of 100-200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off-rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100 ± 50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low-velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep-plume material below the transition zone may be spawning these upper mantle upwellings. This article was corrected on 28 SEP 2015. See the end of the full text for details.

  7. Factors controlling depth of continental rift zones

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

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

    NASA Astrophysics Data System (ADS)

    Kebede, Fekadu; Kulhánek, Ota

    1991-09-01

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

  9. Surface deformation in volcanic rift zones

    USGS Publications Warehouse

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

    1983-01-01

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

  10. Magmatic Versus Amagmatic Rifting in the East African Rift System from Pn and Sn Tomography

    NASA Astrophysics Data System (ADS)

    O'Donnell, J. P.; Nyblade, A.

    2014-12-01

    Geodynamic models of rifting currently rely on the mechanism of hot mantle upwelling and decompressional melting to weaken lithospheric rock to the degree that rifting can initiate. However, many rift segments worldwide are apparently amagmatic. The East African Rift System is a prime example, with large sections of the system subaerially amagmatic. We seek to address the question of whether these apparently amagmatic rift segments merely lack a surficial expression of magmatism which exists at depth, or whether rifting is genuinely amagmatic. Based on regional earthquakes recorded by the Tanzania Broadband Seismic Experiment, the Kenya Broadband Seismic Experiment, the AfricaArray East African Seismic Experiment and several permanent GSN stations, we probe for uppermost mantle melt signatures along the East African Rift System using P- and S-wave speed ratios derived from Pn and Sn tomography. Pn- and Sn-velocity models, and their ratio which can be diagnostic of the presence of fluids, will be presented.

  11. Crustal extension in the Baikal rift zone

    USGS Publications Warehouse

    Zorin, Yu; Cordell, L.

    1991-01-01

    Analysis of the gravity field along four profiles crossing the Baikal rift zone permits an estimate of the amount of anomalous mass produced by 1. (1) graben-fill sediments, 2. (2) Moho uplift and intrusion of mantle sills and dikes, 3. (3) an asthenospheric bulge. Crustal extension is evaluated based on the idea of mass and volume balance of material introduced into and removed from the initial volume of the crust. Extension in the Baikal rift increases southwestward from 0.9 km in the Chara depression to 19.3 km in the South Baikal depression. These values generally agree with the position of the Euler pole determined from seismic data (fault plane solutions). Average rotation velocity for the lithospheric plates separated by the rift zone is estimated to be 5.93 ?? 10-4 rad/m.y. over about 30 m.y. ?? 1991.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    NASA Technical Reports Server (NTRS)

    Mohr, P. A.

    1974-01-01

    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.

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

    NASA Astrophysics Data System (ADS)

    Le Gall, B.; VéTel, W.; Morley, C. K.

    2005-04-01

    Remote sensing data and revised seismic reflection profiles provide new insights about the origin of inverted deformation within Miocene-Recent basins of the Turkana rift (northern Kenya) in the eastern branch of the East African rift system. Contractional structures are dominated by weakly inverted sets of fault blocks within <3.7 Myr old synrift series. Most of reverse extensional faults involve components of oblique-slip, whereas associated hanging wall folds are characterized by large wavelength upright folding. The area of basin inversion is restricted to a 40 × 100 km elongated zone overlying a first-order N140°E trending fault zone in the basement, referred to as the N'Doto transverse fault zone (NTFZ). In the proposed kinematic model, inversion tectonics is assigned to permutation of principal stress axes (σ1/σ2) in addition to the clockwise rotation of extension (from nearly N90°E to N130°E) during Pliocene. The transition from pure extension (Miocene) to a wrench faulting regime (Pliocene) first results in the development of T-type fault networks within a dextrally reactivated shear zone (NTFZ). Inversion tectonics occurred later (<3.7 Ma) in response to a still rotated (˜20°) shortening axis (σ1) oriented N40°E that caused the oblique compression of earlier (NS to N20°E) extensional structures within the NTFZ. The origin of basin inversion and strain concentration in the Turkana rift is thus directly linked to a crustal weakness zone, transverse to the rift axis, and involving steep prerift anisotropies.

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

    NASA Astrophysics Data System (ADS)

    Guseva, E. N.

    2016-05-01

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

  16. Environmental health impacts of East African Rift volcanism.

    PubMed

    Davies, T C

    2008-08-01

    The East African Rift Valley (EARV) is a structure of a major order in the Earth's crust. Accompanying volcanic activity has influenced greatly the nature of the soils and the geochemistry of ground and surface waters, an influence that is reflected in water and food quality. Direct volcanic impacts result from the up-welling of volatile, potentially harmful elements (PHE), such as F, As, and Hg, that dissolve directly into groundwaters. Intense tropical weathering results in clear redistribution of all but the most refractory elements to form distinct zones of micronutrient deficiencies and PHE excesses. Of concern, too, is human exposure to volcanic materials such as dust and clay-enriched soil fractions. Further alteration of the landscape geochemistry is being brought about by pollution from human activities, with increasing health concerns in many ecosystems of the region. This review shows that the unique distribution pattern of trace elements, imprinted by the East African Rift volcanism and modified by weathering and anthropogenic factors, correlates with a number of geochemical diseases in man and animals. It is submitted that accurate diagnoses of these diseases and associated health conditions, and prescription of appropriate remedies, must be founded upon a fundamental understanding of how the elements were naturally distributed in the first place. This can only be realised through the construction of complete geochemical databases for the region.

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

    NASA Technical Reports Server (NTRS)

    1983-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  19. Martian canyons and African rifts: Structural comparisons and implications

    NASA Technical Reports Server (NTRS)

    Frey, H. V.

    1978-01-01

    The resistant parts of the canyon walls of the Martian rift complex Valled Marineris were used to infer an earlier, less eroded reconstruction of the major roughs. The individual canyons were then compared with individual rifts of East Africa. When measured in units of planetary radius, Martian canyons show a distribution of lengths nearly identical to those in Africa, both for individual rifts and for compound rift systems. A common mechanism which scales with planetary radius is suggested. Martian canyons are significantly wider than African rifts. The overall pattern of the rift systems of Africa and Mars are quite different in that the African systems are composed of numerous small faults with highly variable trend. On Mars the trends are less variable; individual scarps are straighter for longer than on earth. This is probably due to the difference in tectonic histories of the two planets: the complex history of the earth and the resulting complicated basement structures influence the development of new rifts. The basement and lithosphere of Mars are inferred to be simple, reflecting a relatively inactive tectonic history prior to the formation of the canyonlands.

  20. Martian canyons and African rifts - Structural comparisons and implications

    NASA Technical Reports Server (NTRS)

    Frey, H.

    1979-01-01

    The resistant parts of the canyon walls of the Martian rift complex Valles Marineris have been used to infer an earlier, less eroded reconstruction of the major troughs. The individual canyons are compared with individual rifts of East Africa. When measured in units of planetary radius, Martian canyons show a distribution of lengths nearly identical to those in Africa, both for individual rifts and for compound rift systems. A common mechanism which scales with planetary radius is suggested. Martian canyons are significantly wider than African rifts. This is consistent with the longstanding idea that rift width is related to crustal thickness: most evidence favors a crust on Mars at least 50% thicker than that of Africa. The overall patterns of the rift systems of Africa and Mars are quite different in that the African systems are composed of numerous small faults with highly variable trend. On Mars the trends are less variable; individual scraps are straighter for longer than on earth. The basement and lithosphere of Mars are inferred to be simple, reflecting a relatively inactive tectonic history prior to the formation of the canyonlands.

  1. No thermal anomalies in the mantle transition zone beneath an incipient continental rift: evidence from the first receiver function study across the Okavango Rift Zone, Botswana

    NASA Astrophysics Data System (ADS)

    Yu, Y.; Liu, K. H.; Moidaki, M.; Reed, C. A.; Gao, S. S.

    2015-08-01

    Mechanisms leading to the initiation and early-stage development of continental rifts remain enigmatic, in spite of numerous studies. Among the various rifting models, which were developed mostly based on studies of mature rifts, far-field stresses originating from plate interactions (passive rifting) and nearby active mantle upwelling (active rifting) are commonly used to explain rift dynamics. Situated atop of the hypothesized African Superplume, the incipient Okavango Rift Zone (ORZ) of northern Botswana is ideal to investigate the role of mantle plumes in rift initiation and development, as well as the interaction between the upper and lower mantle. The ORZ developed within the Neoproterozoic Damara belt between the Congo Craton to the northwest and the Kalahari Craton to the southeast. Mantle structure and thermal status beneath the ORZ are poorly known, mostly due to a complete paucity of broad-band seismic stations in the area. As a component of an interdisciplinary project funded by the United States National Science Foundation, a broad-band seismic array was deployed over a 2-yr period between mid-2012 and mid-2014 along a profile 756 km in length. Using P-to-S receiver functions (RFs) recorded by the stations, the 410 and 660 km discontinuities bordering the mantle transition zone (MTZ) are imaged for the first time. When a standard Earth model is used for the stacking of RFs, the apparent depths of both discontinuities beneath the Kalahari Craton are about 15 km shallower than those beneath the Congo Craton. Using teleseismic P- and S-wave traveltime residuals obtained by this study and lithospheric thickness estimated by previous studies, we conclude that the apparent shallowing is the result of a 100-150 km difference in the thickness of the lithosphere between the two cratons. Relative to the adjacent tectonically stable areas, no significant anomalies in the depth of the MTZ discontinuities or in teleseismic P- and S-wave traveltime residuals are

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  3. Innovative tephra studies in the East African Rift System

    NASA Astrophysics Data System (ADS)

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

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

  4. Teleseismic Investigations of the Malawi and Luangwa Rift Zones: Ongoing Observations From the SAFARI Experiment

    NASA Astrophysics Data System (ADS)

    Reed, C. A.; Gao, S. S.; Liu, K. H.; Yu, Y.; Chindandali, P. R. N.; Massinque, B.; Mdala, H. S.; Mutamina, D. M.

    2015-12-01

    In order to evaluate the influence of crustal and mantle heterogeneities upon the initiation of the Malawi rift zone (MRZ) and reactivation of the Zambian Luangwa rift zone (LRZ) subject to Cenozoic plate boundary stress fields and mantle buoyancy forces, we installed and operated 33 Seismic Arrays For African Rift Initiation (SAFARI) three-component broadband seismic stations in Malawi, Mozambique, and Zambia from 2012 to 2014. During the twenty-four month acquisition period, nearly 6200 radial receiver functions (RFs) were recorded. Stations situated within the MRZ, either along the coastal plains or within the Shire Graben toward the south, report an average crustal thickness of 42 km relative to approximately 46 km observed at stations located along the rift flanks. This implies the juvenile MRZ is characterized by a stretching factor not exceeding 1.1. Meanwhile, P-to-S velocity ratios within the MRZ increase from 1.71 to 1.82 in southernmost Malawi, indicating a substantial modification of the crust during Recent rifting. Time-series stacking of approximately 5500 RFs recorded by the SAFARI and 44 neighboring network stations reveals an apparent uplift of 10 to 15 km along both the 410- and 660-km mantle transition zone (MTZ) discontinuities beneath the MRZ and LRZ which, coupled with an apparently normal 250-km MTZ thickness, implies a first-order high-velocity contribution from thickened lithosphere. Preliminary manual checking of SAFARI shear-wave splitting (SWS) measurements provides roughly 650 high-quality XKS phases following a component re-orientation to correct station misalignments. Regional azimuthal variations in SWS fast orientations are observed, from rift-parallel in the vicinity of the LRZ to rift-oblique in the MRZ. A major 60° rotation in the fast orientation occurs at approximately 31°E, possibly resulting from the modulation of mantle flow around a relatively thick lithospheric keel situated between the two rift zones.

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

    NASA Astrophysics Data System (ADS)

    Kinabo, Baraka Damas

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

  6. Surface-wave Tomography of East African Rift System using Ambient Seismic Noise

    NASA Astrophysics Data System (ADS)

    Kim, S.; Kang, T.; Baag, C.; Nyblade, A. A.

    2008-12-01

    The surface-wave tomography technique for the ambient seismic noise is applied to the east African rift system to investigate shallow crustal structures of the region. Even if the technique has been widely used in many regions to investigate crustal structure in the world, there have been difficulties in application of the technique to the east African region because of unstable data conditions of PASSCAL experiments. A meticulous check of record by record enables us of applying the technique to understand the tectonic environment of the region. The long-period data of one month showing good quality in cross-correlation results are used in this study. They are from the 1994-95 Tanzania Passive-Source Seismic Experiment for the Tanzania craton and its surrounding rift zone, and from the 2000-02 Ethiopia/Kenya Broadband Seismic Experiment and the adjacent permanent stations of the African Array for the Ethiopia rift. The Rayleigh- and Love-wave group-speed maps were inverted using LSQR algorithm for several period bands (5 - 50 s). The preliminary group-speed distribution maps yield results roughly consistent with regional geology. The tomographic images of the Tanzania region show a strong high velocity anomaly at the location corresponding to the Tanzania craton and low velocity anomalies at the surrounding rift regions. For the Ethiopia regions, the features of low velocity anomalies roughly agree with the Tertiary volcanic regions. Combining the Tanzania and Ethiopia broadband arrays, the outline of the east African rift system can be identified as the low velocity anomalies in the surface-wave tomographic results. The structural variation with depth and the feature of the regional shear-wave anisotropy of crust will be explored by converting group- speed dispersion curves into shear-wave velocity structure.

  7. Structure and kinematics of the Livingstone Mountains border fault zone, Nyasa (Malawi) Rift, southwestern Tanzania

    NASA Astrophysics Data System (ADS)

    Wheeler, Walter H.; Karson, Jeffrey A.

    Reconnaissance mapping of the Livingstone Mountains border fault zone (LMBFZ) at the northern end of the Nyasa (Malawi) Rift in SW Tanzania constrains the geometry and movement history of this typical rift border fault. The fault is a narrow zone of complex brittle deformation, striking 320°, that overprints and reactivates an older ductile shear zone. Long, straight, NW-trending border fault segments are offset by minor NE-trending faults. These two orthogonal fault sets integrate along strike to produce an overall curved fault trace that is concave towards a major depositional basin in the rift. A typical section through the fault zone shows an E to W progression from gneissic country rock through ductilely deformed country rock, into a zone overprinted by closely spaced fractures and grading into an intensely fractured, massive, flinty, aphanitic mylonite band at the lakeshore. Pseudotachylite veins, probably generated during seismic movement on the border fault, are common within and near the aphanitic mylonite. Slickensides indicate dextral oblique-slip, whereas shear belts and rolled porphyroclasts with complex tails in the older ductile shear zone indicate sub-horizontal sinistral motion. The adjacent rift basin is typical of other East African Rift Basins, and contains at least 4 km of Recent to perhaps Mesozoic sediment. Whereas the minimum net slip on the LMBFZ, in the dominant slickenside direction, is on the order of 10 km, regional geologic considerations suggest that dominantly strike-slip motion preceded the oblique-slip phase that produced the LMBFZ and the adjacent rift basin.

  8. At the tip of a propagating rift - The offshore East African Rift

    NASA Astrophysics Data System (ADS)

    Franke, Dieter; Jokat, Wilfried; Ladage, Stefan; Stollhofen, Harald; Klimke, Jennifer; Lutz, Ruediger; Mahanjane, Stefane; Ehrhardt, Axel; Schreckenberger, Bernd

    2016-04-01

    Numerous studies have addressed various aspects of the East African Rift system (EARS) but surprisingly few the offshore continuation of the south-eastern branch of the rift into the Mozambique Channel. Here, we present new evidence for neotectonic deformation derived from modern seismic reflection data and supported by additional geophysical data. The Kerimbas Graben offshore northern Mozambique is the most prominent manifestation of sub-recent extensional deformation. The seismic reflection data reveals that recent normal faulting often utilizes preexisting, deeply buried half-graben structures which likely are related to the formation of the Somali Basin. The ~30 km wide and ~150 km long symmetric graben is in a stage where the linkage of scattered normal faults already did happen, resulting in increased displacement and accommodation of most of the extension across the basin. However, deep earthquakes below the rift indicate a strong and still preserved lithospheric mantle. Extension is becoming diffuse where an onshore suture, subdividing the northern from the southern metamorphic basement onshore Mozambique, is closest to the offshore rift. It appears likely that this suture is the origin for the variation in rifting style, indicating that mantle fabric resulting from a Cambrian collision has been preserved as mechanical anisotropy of the lithospheric mantle. Further south the rift focuses in an about 30 km wide half-graben. An important finding is that the entire offshore branch of the EARS lacks significant volcanism. Along the offshore EARS there are only negligible indications for recent volcanism in the reflection seismic data such as sills and dikes. Apparently the "Comoros mantle plume" (French and Romanowicz, 2015) has a very minor influence on the progressive extensional deformation along the northern Mozambique continental margin, leading eventually to breakup sometimes in the future. Combining structural with earthquake data reveals that the magma

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

    USGS Publications Warehouse

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

    2003-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2008-07-01

    The Eastern Branch of the East African Rift System is well known in Ethiopia (Main Ethiopian Rift) and Kenya (Kenya or Gregory Rift) and is usually considered to fade away southwards in the North Tanzanian Divergence, where it splits into the Eyasi, Manyara and Pangani segments. Further towards the south, rift structures are more weakly expressed and this area has not attracted much attention since the mapping and exploratory works of the 1950s. In November 4, 2002, an earthquake of magnitude Mb = 5.5 struck Dodoma, the capital city of Tanzania. Analysis of modern digital relief, seismological and geological data reveals that ongoing tectonic deformation is presently affecting a broad N-S trending belt, extending southward from the North Tanzanian Divergence to the region of Dodoma, forming the proposed "Manyara-Dodoma Rift segment". North of Arusha-Ngorongoro line, the rift is confined to a narrow belt (Natron graben in Tanzania) and south of it, it broadens into a wide deformation zone which includes both the Eyasi and Manyara grabens. The two-stage rifting model proposed for Kenya and North Tanzania also applies to the Manyara-Dodoma Rift segment. In a first stage, large, well-expressed topographic and volcanogenic structures were initiated in the Natron, Eyasi and Manyara grabens during the Late Miocene to Pliocene. From the Middle Pleistocene onwards, deformations related to the second rifting stage propagated southwards to the Dodoma region. These young structures have still limited morphological expressions compared to the structures formed during the first stage. However, they appear to be tectonically active as shown by the high concentration of moderate earthquakes into earthquake swarms, the distribution of He-bearing thermal springs, the morphological freshness of the fault scarps, and the presence of open surface fractures. Fault kinematic and paleostress analysis of geological fault data in basement rocks along the active fault lines show that recent

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

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

    NASA Technical Reports Server (NTRS)

    Mohr, P. A. (Principal Investigator)

    1973-01-01

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

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

    USGS Publications Warehouse

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

    2014-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Atekwana, E. A.

    2007-12-01

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

  15. 3-D Evolutionary model of the oblique rift basins-the study of Central African Rifts

    NASA Astrophysics Data System (ADS)

    Cheng, I. W.; Yang, K. M.; Wu, J. C.; Hsiuan, T. H.

    2015-12-01

    Because of their tectonostratigraphically high potentials to yield oil and gas, oblique rift basins are a noteworthy type of global petroleum basins. The main purpose of this study is to investigate characteristics of the structural style and basin evolution of the Central Africa rift subsystem (CAS). Establishing the evolutionary model of CAS by the analyses of tectonostratigraphy, geometry, kinematics of structural features and Particle Flow Code 3-Dimensions (PFC 3D). The results can be references for petroleum basin exploration. The study areas are in the CAS, including the Doba, Doseo and Salamat Basins. Basins are separated by a large-scale strike-slip fault zone (Borogop fault zone) across the CAS. The results showed that the Borogop fault zone pass through the CAS until the Late Cretaceous compression and then became a big-scaled strike-slip fault. The structural geometry of oblique rift is changed with the α angle between the rift trend and trend of tensile stress. PFC-3D indicated that 1) The α of the Doba, Doseo and Salamat Basins are 60°, 60° and 75°, respectively; 2) When the α got close to 45°, the density of the echelon faults alone the basin center decreased gradually; 3) The Doba Basin is wide and shallow which should be affected by meso-scaled stresses. The Doseo and Salamat Basins are narrow and deeper which should be affected by mega-scaled stresses. According to the abovementioned results, we suggest that 1) The Borogop fault zone had less influence on basin architecture of CAS, therefore the basins of CAS are not the pull-apart basins but the extensional basins, and 2) Doba and Doseo-Salamat Basins should probably belong to different type of petroleum systems, which have different scales of stress.

  16. Littoral sedimentation of rift lakes: an illustrated overview from the modern to Pliocene Lake Turkana (East African Rift System, Kenya)

    NASA Astrophysics Data System (ADS)

    Schuster, Mathieu; Nutz, Alexis

    2015-04-01

    Existing depositional models for rift lakes can be summarized as clastics transported by axial and lateral rivers, then distributed by fan-deltas and/or deltas into a standing water body which is dominated by settling of fine particles, and experiencing occasional coarser underflows. Even if known from paleolakes and modern lakes, reworking of clastics by alongshore drift, waves and storms are rarely considered in depositional models. However, if we consider the lake Turkana Basin (East African Rift System, Kenya) it is obvious that this vision is incomplete. Three representative time slices are considered here: the modern Lake Turkana, the Megalake Turkana which developed thanks to the African Humid Period (Holocene), and the Plio-Pleistocene highstand episodes of paleolake Turkana (Nachukui, Shungura and Koobi Fora Formations, Omo Group). First, remarkable clastic morphosedimentary structures such as beach ridges, spits, washover fans, lagoons, or wave-dominated deltas are very well developed along the shoreline of modern lake Turkana, suggesting strong hydrodynamics responsible for a major reworking of the fluvial-derived clastics all along the littoral zone (longshore and cross-shore transport) of the lake. Similarly, past hydrodynamics are recorded from prominent raised beach ridges and spits, well-preserved all around the lake, above its present water-level (~360 m asl) and up to ~455 m. These large-scale clastic morphosedimentary structures also record the maximum extent of Megalake Turkana during the African Humid Period, as well as its subsequent regression forced by the end of the Holocene climatic optimum. Several hundreds of meters of fluvial-deltaic-lacustrine deposits spanning the Pliocene-Pleistocene are exposed in the Turkana basin thanks to tectonic faulting. These deposits are world famous for their paleontological and archeological content that documents the very early story of Mankind. They also preserve several paleolake highstand episodes with

  17. Diachronism in the late Neoproterozoic-Cambrian arc-rift transition of North Gondwana: A comparison of Morocco and the Iberian Ossa-Morena Zone

    NASA Astrophysics Data System (ADS)

    Álvaro, J. Javier; Bellido, Félix; Gasquet, Dominique; Pereira, M. Francisco; Quesada, Cecilio; Sánchez-García, Teresa

    2014-10-01

    In the northwestern border of the West African craton (North Gondwana), a transition from late Neoproterozoic subduction/collision to Cambrian rift processes was recorded in the Anti-Atlas (Morocco) and in the Ossa-Morena Zone (Iberia). Cambrian rifting affected both Pan-African and Cadomian basements in a stepwise and diachronous way. Subsequently, both areas evolved into a syn-rift margin episodically punctuated by uplift and tilting that precluded Furongian sedimentation. A comparison of sedimentary, volcanic and geodynamic evolution is made in the late Neoproterozoic (Pan-African and Cadomian) belts and Cambrian rifts trying to solve the apparent diachronous (SW-NE-trending) propagation of an early Palaeozoic rifting regime that finally led to the opening of the Rheic Ocean.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    In the early stages of continental rifting, East African Rift (EAR) basins are conventionally depicted as asymmetric basins bounded on one side by a ~100 km-long border fault. As rifting progresses, strain concentrates into the rift center, producing intra-rift faults. The timing and nature of the transition from border fault to intra-rift-dominated strain accommodation is unclear. Our study focuses on this transitional phase of continental rifting by exploring the spatial and temporal evolution of faulting in the Natron (border fault initiation at ~3 Ma) and Magadi (~7 Ma) basins of northern Tanzania and southern Kenya, respectively. We compare the morphologies and activity histories of faults in each basin using field observations and remote sensing in order to address the relative contributions of border faults and intra-rift faults to crustal strain accommodation as rifting progresses. The ~500 m-high border fault along the western margin of the Natron basin is steep compared to many border faults in the eastern branch of the EAR, indicating limited scarp degradation by mass wasting. Locally, the escarpment shows open fissures and young scarps 10s of meters high and a few kilometers long, implying ongoing border fault activity in this young rift. However, intra-rift faults within ~1 Ma lavas are greatly eroded and fresh scarps are typically absent, implying long recurrence intervals between slip events. Rift-normal topographic profiles across the Natron basin show the lowest elevations in the lake-filled basin adjacent to the border fault, where a number of hydrothermal springs along the border fault system expel water into the lake. In contrast to Natron, a ~1600 m high, densely vegetated, border fault escarpment along the western edge of the Magadi basin is highly degraded; we were unable to identify evidence of recent rupturing. Rift-normal elevation profiles indicate the focus of strain has migrated away from the border fault into the rift center, where

  19. Present-day kinematics of the East African Rift

    NASA Astrophysics Data System (ADS)

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

    2014-04-01

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

  20. Helium isotopes in fluids of the Baykal rift zone

    SciTech Connect

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

    1993-02-01

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

  1. Exploring Crustal Structure and Mantle Seismic Anisotropy Associated with the Incipient Southern and Southwestern Branches of the East African Rift System

    NASA Astrophysics Data System (ADS)

    Yu, Y.; Reed, C. A.; Gao, S. S.; Liu, K. H.; Massinque, B.; Mdala, H. S.; Chindandali, P. R. N.; Moidaki, M.; Mutamina, D. M.

    2014-12-01

    In spite of numerous geoscientific studies, the mechanisms responsible for the initiation and development of continental rifts are still poorly understood. The key information required to constrain various geodynamic models on rift initiation can be derived from the crust/mantle structure and anisotropy beneath incipient rifts such as the Southern and Southwestern branches of the East African Rift System. As part of a National Science Foundation funded interdisciplinary project, 50 PASSCAL broadband seismic stations were deployed across the Malawi, Luangwa, and Okavango rift zones from the summer of 2012 to the summer of 2014. Preliminary results from these 50 SAFARI (Seismic Arrays for African Rift Initiation) and adjacent stations are presented utilizing shear-wave splitting (SWS) and P-S receiver function techniques. 1109 pairs of high-quality SWS measurements, consisting of fast polarization orientations and splitting times, have been obtained from a total of 361 seismic events. The results demonstrate dominantly NE-SW fast orientations throughout Botswana as well as along the northwestern flank of the Luangwa rift valley. Meanwhile, fast orientations beneath the eastern Luangwa rift flank rotate from NNW to NNE along the western border of the Malawi rift. Stations located alongside the western Malawi rift border faults yield ENE fast orientations, with stations situated in Mozambique exhibiting more E-W orientations. In the northern extent of the study region, fast orientations parallel the trend of the Rukwa and Usangu rift basins. Receiver function results reveal that, relative to the adjacent Pan-African mobile belts, the Luangwa rift zone has a thin (30 to 35 km) crust. The crustal thickness within the Okavango rift basin is highly variable. Preliminary findings indicate a northeastward thinning along the southeast Okavango border fault system congruent with decreasing extension toward the southwest. The Vp/Vs measurements in the Okavango basin are roughly

  2. Multi Scale Imaging of Seismic Structure beneath the Western Branch of the East-African Rift

    NASA Astrophysics Data System (ADS)

    Jakovlev, A.; Rumpker, G.; Koulakov, I.

    2010-12-01

    In this study, we investigate the crustal and upper mantle velocity structure beneath the East African Rift System (EARS) as a whole and beneath the Rwenzori Mountains in western Uganda in particular. The most interesting features here is the mountain range of approximately 50 km-wide, with a length of about 150 km, which is situated within the western branch of the East African rift zone and reaches the altitudes of more than 5000 m. The joint tomographic inversion was used to investigate the velocity structure beneath the Rwenzoris on a local scale down to the depth of about 75 - 80 km. Arrival times from 2053 local and 284 teleseismic earthquakes recorded by a temporary network equipped 35 short period and broad-band stations were used as an input for the joint tomographic inversion. The network is covering an area of 140×90 km2. To estimate resolution ability and stability of the obtained results we performed many different tests. The tests show that the best horizontal resolution is achieved in the northern part of the study area, where the density of the ray coverage is highest. In shallow depths where local earthquake rays dominate, the vertical and horizontal resolution is relatvely high, while in deeper sections covered with teleseismic rays, the anomalies seem to be strongly smeared and can be interpreted only on a qualitative level. Velocity structure in the upper crust agrees with the distribution of the main geological units, such as sedimentary basins, igneous outcrops, thermal fields etc. For the uppermost mantle, our results reveal an inclined boundary between the high-velocity Tanzania craton and low-velocity patterns beneath the rift. The same position of the contact between craton and rift zone was obtained in the regional tomographic study of the EARS as a whole. This regional study was based on the ISC data, which contains information about arrival times of signals from earthquakes at more than 7000 stations of the global seismological network

  3. Combining detrital geochronology and sedimentology to assess basin development in the Rukwa Rift of the East African Rift System

    NASA Astrophysics Data System (ADS)

    Hilbert-Wolf, Hannah; Roberts, Eric; Mtelela, Cassy; Downie, Bob

    2015-04-01

    We have employed a multifaceted approach to sedimentary provenance analysis in order to assess the timing and magnitude of tectonic events, sedimentation, and landscape development in the Western Branch of the East African Rift System. Our approach, termed 'Sedimentary Triple Dating', integrates: (1) U-Pb dating via LA-ICPMS; (2) fission track; and (3) (U-Th)/He thermochronology of detrital zircon and apatite. We integrate geochronology, thermochronology, and provenance analysis to relate the initiation of rifting events to regional dynamic uplift, sedimentation patterns, and interpret the far-reaching climatic and evolutionary effects of fluctuating rift flank topography in the Rukwa Rift, a segment of the Western Branch. This work provides additional data to support the recent concept of synchronous development of the Western and Eastern branches of the East African Rift System ~25 Ma, and better constrains the age, location and provenance of subsequent rifting and sedimentation events in the Rukwa Rift Basin. Investigation of well cuttings and outcrop samples from the Neogene-Recent Lake Beds Succession in the Rukwa Rift Basin revealed a suite of previously unrecognized tuffaceous deposits at the base of the succession. A population of euhedral, magmatic zircons from a basal Lake Beds tuff and Miocene-Pliocene detrital zircons from well cuttings suggest that Neogene rift reactivation and volcanism began ~9-10 Ma. This timing is consistent with demonstrated rifting in Uganda and Malawi, as well as with the initiation of volcanism in the Rungwe Volcanic Province at the southern end of the Rukwa Rift, and the estimated development of Lake Tanganyika to the north. Moreover, there appear to be a suite of unconformity bounded stratigraphic units that make up the Lower Lake Beds succession, and detrital zircon maximum depositional ages from these units suggests episodic sedimentation in the rift, punctuated by long hiatuses or uplift, rather than steady subsidence and

  4. Analogies Between the East African Rift Around the Tanzania Craton and the Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Montesi, L. G.

    2013-12-01

    Continental rifts and oceanic spreading centers both accommodate plate divergence but their morphologies are often quite different. Yet, ultraslow spreading centers, especially the Southwest Indian ridge at the 9 to 16°E area (SWIR), present good analogies for the East African Rift (EAR), including localized volcanism, avolcanic segments, and a continuous but not straight rift axis. The archetypal oceanic spreading center features transform offsets. Volcanism is continuous along the ridge axis and is most vigorous at the center of spreading segments. By contrast, continental rifts do not feature transform offsets. The orientation of the rift can change along strike. Several rift segments are purely tectonic, with relatively isolated volcanic centers. The EAR around the Tanzania Craton clearly shows this kind of morphology. Ultraslow spreading centers share many of these features. The SWIR, in particular, displays dramatic changes in orientation, with volcanism localized at the junction between segments of different obliquity. Melt production and transport are controlled by the effective spreading rate, a combination of plate divergence velocity and rift obliquity. Ultraslow spreading center all have an effective spreading rate less than 13 mm/yr. At that speed the thickness of the thermal boundary layer is similar to the depth from which magma can be effectively extracted, opening the possibility for long-distance transport of magma along axis without extraction. Volcanic centers correspond to the location where the magma transport system first encounters a tectonically damaged zone that enables extraction to the surface. The effective velocity of the EAR in the Kenya dome is less than 4mm/yr firmly on par with ultraslow ridges. In fact, to generate magma by mantle upwelling at such a slow opening rate requires a higher mantle temperature or fertility than in the oceanic domain. Both opening rate and effective velocity increase northward along the Eastern branch

  5. Kinematic problem of oblique rift zones in Iceland

    SciTech Connect

    Jancin, M.

    1985-01-01

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

  6. Ambient noise tomography of the East African Rift in Mozambique

    NASA Astrophysics Data System (ADS)

    Domingues, Ana; Silveira, Graça; Ferreira, Ana M. G.; Chang, Sung-Joon; Custódio, Susana; Fonseca, João F. B. D.

    2016-03-01

    Seismic ambient noise tomography is applied to central and southern Mozambique, located in the tip of the East African Rift (EAR). The deployment of MOZART seismic network, with a total of 30 broad-band stations continuously recording for 26 months, allowed us to carry out the first tomographic study of the crust under this region, which until now remained largely unexplored at this scale. From cross-correlations extracted from coherent noise we obtained Rayleigh wave group velocity dispersion curves for the period range 5-40 s. These dispersion relations were inverted to produce group velocity maps, and 1-D shear wave velocity profiles at selected points. High group velocities are observed at all periods on the eastern edge of the Kaapvaal and Zimbabwe cratons, in agreement with the findings of previous studies. Further east, a pronounced slow anomaly is observed in central and southern Mozambique, where the rifting between southern Africa and Antarctica created a passive margin in the Mesozoic, and further rifting is currently happening as a result of the southward propagation of the EAR. In this study, we also addressed the question concerning the nature of the crust (continental versus oceanic) in the Mozambique Coastal Plains (MCP), still in debate. Our data do not support previous suggestions that the MCP are floored by oceanic crust since a shallow Moho could not be detected, and we discuss an alternative explanation for its ocean-like magnetic signature. Our velocity maps suggest that the crystalline basement of the Zimbabwe craton may extend further east well into Mozambique underneath the sediment cover, contrary to what is usually assumed, while further south the Kaapval craton passes into slow rifted crust at the Lebombo monocline as expected. The sharp passage from fast crust to slow crust on the northern part of the study area coincides with the seismically active NNE-SSW Urema rift, while further south the Mazenga graben adopts an N-S direction

  7. Hydrothermal vents is Lake Tanganyika, East African Rift system

    SciTech Connect

    Tiercelin, J.J.; Pflumio, C.; Castrec, M.

    1993-06-01

    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.

  8. Differential opening of the Central and South Atlantic Oceans and the opening of the West African rift system

    NASA Astrophysics Data System (ADS)

    Fairhead, J. D.; Binks, R. M.

    1991-02-01

    Plate tectonic studies of the development of the Central and South Atlantic Oceans using Seasat and Geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time. Such models enable an initial assessment of the differential opening between these two oceanic basins to be determined. The Equatorial Atlantic is an integral part of this oceanic rifting process, allowing stresses arising from the differential opening to be dissipated into both the Caribbean and Africa along its northern and southern boundaries respectively. The tectonic model for the West African rift system, based on geological and geophysical studies, shows a series of strike-slip fault zones diverging into Africa from the Gulf of Guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones. The development of the West African rift system was contemporaneous with the early opening of the South Atlantic, continued to develop well after the final breakup of South America from Africa and did not cease until the late Cretaceous when there was a major phase of basin inversion and deformation. Santonian ( ~ 80 Ma) deformation across the Benue Trough (Nigeria) is broadly contemporaneous with dextral shear reactivation of the central African fracture system which, in turn resulted in renewed extension in the Sudan basins during the late Cretaceous and early Tertiary. This paper illustrates the close linkage in both time and space between the history of the African rift basins and the opening of the Atlantic. Both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the Central and South Atlantic in order to dissipate stresses generated by this opening. The Mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics.

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

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

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

  10. Multiple mantle upwellings beneath the Northern East-African Rift System from relative P- and S-wave traveltime tomography

    NASA Astrophysics Data System (ADS)

    Civiero, Chiara; Hammond, James; Goes, Saskia; Fishwick, Stewart; Ahmed, Abdulhakim; Ayele, Atalay; Doubre, Cecile; Goitom, Berhe; Keir, Derek; Kendall, Mike; Leroy, Sylvie; Ogubazghi, Ghebrebrhan; Rumpker, Georg; Stuart, Graham

    2015-04-01

    Mantle plumes have been invoked as the likely cause of East African Rift volcanism and extension. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume, the African Superplume, connected to the LLSVP beneath Southern Africa, to one or more distinct lower-mantle sources along the rift. We present a new relative travel-time tomography model that images detailed P- and S- wave velocities from P,S and SKS phases below the northern East-African, Red Sea and Gulf of Aden rift. Data comes from stations that cover the area from Tanzania to Saudi Arabia. The aperture of the integrated dataset allows us to image for the first time structures of ~100 km length scale down to depths of 900 km beneath this region. Our images provide evidence of at least two low-velocity structures with a diameter of ~200 km that continue through the transition zone and into the lower mantle: the first extends to at least 900 km beneath Afar, and a second reaching at least 750 km depth just west of the Main Ethiopian Rift, a region with off-rift volcanism. Taking into account seismic sensitivity to temperature and thermally controlled phase boundary topography, we interpret these features as multiple focused upwellings from below the transition zone with excess temperatures of 100±50 K. The scale of the upwellings is smaller than any of the previously proposed lower mantle plume sources. This suggests the ponding or flow of deep-plume material below the transition zone may be spawning smaller upper-mantle upwellings.

  11. The Kaena Ridge Submarine Rift Zone off Oahu, Hawaii

    NASA Astrophysics Data System (ADS)

    Smith, J. R.

    2002-12-01

    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

  12. Fault Growth and Propagation and its Effect on Surficial Processes within the Incipient Okavango Rift Zone, Northwest Botswana, Africa (Invited)

    NASA Astrophysics Data System (ADS)

    Atekwana, E. A.

    2010-12-01

    The Okavango Rift Zone (ORZ) is suggested to be a zone of incipient continental rifting occuring at the distal end of the southwestern branch of the East African Rift System (EARS), therefore providing a unique opportunity to investigate neotectonic processes during the early stages of rifting. We used geophysical (aeromagnetic, magnetotelluric), Shuttle Radar Tomography Mission, Digital Elevation Model (SRTM-DEM), and sedimentological data to characterize the growth and propagation of faults associated with continental extension in the ORZ, and to elucidate the interplay between neotectonics and surficial processes. The results suggest that: (1) fault growth occurs by along axis linkage of fault segments, (2) an immature border fault is developing through the process of “Fault Piracy” by fault-linkages between major fault systems, (3) significant discrepancies exits between the height of fault scarps and the throws across the faults compared to their lengths in the basement, (4) utilization of preexisting zones of weakness allowed the development of very long faults (> 25-100 km) at a very early stage of continental rifting, explaining the apparent paradox between the fault length versus throw for this young rift, (5) active faults are characterized by conductive anomalies resulting from fluids, whereas, inactive faults show no conductivity anomaly; and 6) sedimentlogical data reveal a major perturbation in lake sedimentation between 41 ka and 27 ka. The sedimentation perturbation is attributed to faulting associated with the rifting and may have resulted in the alteration of hydrology forming the modern day Okavango delta. We infer that this time period may represent the age of the latest rift reactivation and fault growth and propagation within the ORZ.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  14. Climate Change Affects the East African Rift Valley Lakes

    NASA Astrophysics Data System (ADS)

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

    2004-12-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

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

    SciTech Connect

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

    1989-11-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    Late Cenozoic deformation zones in Japan may be divided into two types: (1) arc-arc collision zones like those of Izu and the Hokkaido axial zone, and (2) reactivated back-arc marginal rift (BMR) systems. A BMR develops during a secondary rifting event that follows the opening of a back-arc basin. It forms close to the volcanic front and distant from the spreading center of the basin. In Japan, a BMR system developed along the Sea of Japan coast following the opening of the Japan Sea. The BMR appears to be the weakest, most deformable part of the arc back-arc system. When active rifting in the marginal basins ended, thermal subsidence, and then mechanical subsidence related to the onset of a compressional stress regime, allowed deposition of up to 5 km of post-rift, deep-marine to fluvial sedimentation. Continued compression produced fault-related folds in the post-rift sediments, in thin-skin style deformation. Shortening reached a maximum in the BMR system compared to other parts of the back-arc, suggesting that it is the weakest part of the entire system. We examined the structure of the BMR system using active source seismic investigation and earthquake tomography. The velocity structure beneath the marginal rift basin shows higher P-wave velocity in the upper mantle/lower crust which suggests significant mafic intrusion and thinning of the upper continental crust. The syn-rift mafic intrusive forms a convex shape, and the boundary between the pre-rift crust and the mafic intrusive dips outward. In the post-rift compressional stress regime, the boundary of the mafic body reactivated as a reverse fault, forming a large-scale wedge thrust and causing further subsidence of the rift basin. The driver of the intense shortening event along the Sea of Japan coast in SW Japan was the arrival of a buoyant young (15 Ma) Shikoku basin at the Nankai Trough. Subduction stalled and the backarc was compressed. As the buoyant basin cooled, subduction resumed, and the rate of

  18. Transition from a localized to wide deformation along Eastern branch of Central East African Rift: Insights from 3D numerical models

    NASA Astrophysics Data System (ADS)

    Leroy, S. D.; Koptev, A.; Burov, E. B.; Calais, E.; Gerya, T.

    2015-12-01

    The Central East African Rift (CEAR) bifurcates in two branches (eastern, magma-rich and western, magma-poor) surrounding strong Tanzanian craton. Intensive magmatism and continental flood basalts are largely present in many of the eastern rift segments, but other segments, first of all the western branch, exhibit very small volcanic activity. The Eastern rift is characterized by southward progression of the onset of volcanism, the extensional features and topographic expression of the rift vary significantly north-southward: in northern Kenya the deformation is very wide (some 150-250 km in E-W direction), to the south the rift narrows to 60-70 km, yet further to the south the deformation widens again in the so-called Tanzania divergence zone. Widening of the Eastern branch within its southern part is associated with the impingement of the southward-propagating rift on the strong Masai block situated to east of the Tanzanian craton. To understand the mechanisms behind this complex deformation distribution, we implemented a 3Dl ultra-high resolution visco-plastic thermo-mechanical numerical model accounting for thermo-rheological structure of the lithosphere and hence captures essential features of the CEAR. The preferred model has a plume seeded slightly to the northeast of the craton center, consistent with seismic tomography, and produces surface strain distribution that is in good agreement with observed variation of deformation zone width along eastern side of Tanzanian craton: localized above bulk of mantle material deflected by cratonic keel narrow high strain zone (Kenia Rift) is replaced by wide distributed deformations within areas situated to north (northern Kenya, Turkana Rift) and to south (Tanzania divergence, Masai block) of it. These results demonstrate significant differences in the impact of the rheological profile on rifting style in case of dominant active rifting compared to dominant passive rifting. Narrow rifting, conventionally attributed to

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

    SciTech Connect

    Genik, G.J. )

    1993-08-01

    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.

  20. Tomography of the East African Rift System in Mozambique

    NASA Astrophysics Data System (ADS)

    Domingues, A.; Silveira, G. M.; Custodio, S.; Chamussa, J.; Lebedev, S.; Chang, S. J.; Ferreira, A. M. G.; Fonseca, J. F. B. D.

    2014-12-01

    Unlike the majority of the East African Rift, the Mozambique region has not been deeply studied, not only due to political instabilities but also because of the difficult access to its most interior regions. An earthquake with M7 occurred in Machaze in 2006, which triggered the investigation of this particular region. The MOZART project (funded by FCT, Lisbon) installed a temporary seismic network, with a total of 30 broadband stations from the SEIS-UK pool, from April 2011 to July 2013. Preliminary locations of the seismicity were estimated with the data recorded from April 2011 to July 2012. A total of 307 earthquakes were located, with ML magnitudes ranging from 0.9 to 3.9. We observe a linear northeast-southwest distribution of the seismicity that seems associated to the Inhaminga fault. The seismicity has an extension of ~300km reaching the Machaze earthquake area. The northeast sector of the seismicity shows a good correlation with the topography, tracing the Urema rift valley. In order to obtain an initial velocity model of the region, the ambient noise method is used. This method is applied to the entire data set available and two additional stations of the AfricaARRAY project. Ambient noise surface wave tomography is possible by computing cross-correlations between all pairs of stations and measuring the group velocities for all interstation paths. With this approach we obtain Rayleigh wave group velocity dispersion curves in the period range from 3 to 50 seconds. Group velocity maps are calculated for several periods and allowing a geological and tectonic interpretation. In order to extend the investigation to longer wave periods and thus probe both the crust and upper mantle, we apply a recent implementation of the surface-wave two-station method (teleseismic interferometry - Meier el al 2004) to augment our dataset with Rayleigh wave phase velocities curves in a broad period range. Using this method we expect to be able to explore the lithosphere

  1. The Lake Albert Rift (uganda, East African Rift System): Deformation, Basin and Relief Evolution Since 17 Ma

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    This study is based on a coupled basin infilling study and a landforms analysis of the Lake Albert Rift located at the northern part of the western branch of the East African Rift. The basin infilling study is based on both subsurface data and outcrops analysis. The objective was to (1) obtain an age model based on onshore mammals biozones, (2) to reconstruct the 3D architecture of the rift using sequence stratigraphy correlations and seismic data interpretation, (3) to characterize the deformation and its changes through times and (4) to quantify the accommodation for several time intervals. The infilling essentially consists of isopach fault-bounded units composed of lacustrine deposits wherein were characterized two major unconformities dated at 6.2 Ma (Uppermost Miocene) and 2.7 Ma (Pliocene-Pleistocene boundary), coeval with major subsidence and climatic changes. The landforms analysis is based on the characterization and relative dating (geometrical relationships with volcanism) of Ugandan landforms which consist of stepped planation surfaces (etchplains and peplians) and incised valleys. We here proposed a seven-steps reconstruction of the deformation-erosion-sedimentation relationships of the Lake Albert Basin and its catchments: - 55-45 Ma: formation of laterites corresponding to the African Surface during the very humid period of the Lower-Middle Eocene; - 45-22: stripping of the African Surface in response of the beginning of the East-African Dome uplift and formation of a pediplain which associated base level is the Atlantic Ocean; - 17-2.5 Ma: Initiation of the Lake Albert Basin around 17 Ma and creation of local base levels (Lake Albert, Edward and George) on which three pediplains tend to adapt; - 18 - 16 Ma to 6.2 Ma: "Flexural" stage (subsidence rate: 150-200 m/Ma; sedimentation rate 1.3 km3/Ma between 17 and 12 Ma and 0.6 km3/Ma from 12 to 6 Ma) - depocenters location (southern part of Lake Albert Basin) poorly controlled by fault; - 6.2 Ma to 2

  2. Carbonate-Silicate Association in the Kamafugite of the Toro-Ankole Province (East African Rift)

    NASA Astrophysics Data System (ADS)

    Muravyeva, Natalya; Senin, Valery

    2010-05-01

    Carbonatite melts play an important role in the magmatism of the East African Rift Zone. A tight spatial association of high-Mg ultrapotassic and carbonatite rocks in the Western branch of the East African Rift suggests the genetic relationship of their parental melts. New evidence of such connection of kamafugitic and carbonatite magmas were obtained during study of the volcanic rocks of kamafugitic affinity in Toro-Ankole province. Primary carbonates (calcite and dolomite) were found as inclusions in olivine from ugandite and mafurite of the Bunyaruguru volcanic field. In the ugandite carbonates contain in the crystallized melt inclusions in olivine phenocrysts consist also of kalsilite, clinopyroxene, mica, and titanomagnetite. Some inclusions reach up to 40x75µm, the host olivine is 150x300µm in size. In the mafurite carbonates form rare microlites, microphenocrysts, and lenses with cavities in central parts, and occur as inclusions in olivine phenocrysts and aggregates in the adjacent zones. These aggregates are multiphase and in mineral set resemble carbonatites: they contain kalsilite, clinopyroxene, magnetite, phillipsite, and mica. Also the two-phase carbonate-sulfate inclusions in olivine were found. The most part of carbonates in composition correspond to calcites with low magnesium, iron, sodium, strontium, and barium contents. The carbonates from two-phase inclusions in olivine from the mafurite are significantly higher magnesian, approaching pure dolomite. The different types of carbonate in mafurite vary in Sr, Ba, Na and K. The highest content of these elements is observed in groundmass carbonates, reaching 2.44 wt % SrO, 1.25% BaO, 0.64% Na2O, and 1.23% K2O. Such assemblage in kamafugites have not been described yet. Some olivines contain microinclusions of sulfate (barite), occurs as fine (10-20 µm) rounded inclusions. The presence of barite inclusions in the olivine of the studied mafurite indicates the high sulfur content in the primary melt

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

    NASA Astrophysics Data System (ADS)

    Kluegel, A.; Walter, T. R.

    2003-04-01

    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

  4. The East African rift system in the light of KRISP 90

    USGS Publications Warehouse

    Keller, Gordon R.; Prodehl, C.; Mechie, J.; Fuchs, K.; Khan, M.A.; Maguire, Peter K.H.; Mooney, W.D.; Achauer, U.; Davis, P.M.; Meyer, R.P.; Braile, L.W.; Nyambok, I.O.; Thompson, G.A.

    1994-01-01

    On the basis of a test experiment in 1985 (KRISP 85) an integrated seismic-refraction/teleseismic survey (KRISP 90) was undertaken to study the deep structure beneath the Kenya rift down to depths of 100-150 km. This paper summarizes the highlights of KRISP 90 as reported in this volume and discusses their broad implications as well as the structure of the Kenya rift in the general framework of other continental rifts. Major scientific goals of this phase of KRISP were to reveal the detailed crustal and upper mantle structure under the Kenya rift, to study the relationship between mantle updoming and the development of sedimentary basins and other shallow structures within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system and within a global perspective and to elucidate fundamental questions such as the mode and mechanism of continental rifting. The KRISP results clearly demonstrate that the Kenya rift is associated with sharply defined lithospheric thinning and very low upper mantle velocities down to depths of over 150 km. In the south-central portion of the rift, the lithospheric mantle has been thinned much more than the crust. To the north, high-velocity layers detected in the upper mantle appear to require the presence of anistropy in the form of the alignment of olivine crystals. Major axial variations in structure were also discovered, which correlate very well with variations in the amount of extension, the physiographic width of the rift valley, the regional topography and the regional gravity anomalies. Similar relationships are particularly well documented in the Rio Grande rift. To the extent that truly comparable data sets are available, the Kenya rift shares many features with other rift zones. For example, crustal structure under the Kenya, Rio Grande and Baikal rifts and the Rhine Graben is generally symmetrically centered on the rift valleys. However, the Kenya rift is distinctive, but not unique, in terms of

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

    SciTech Connect

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

    1983-07-01

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

  6. Deformation and seismicity associated with continental rift zones propagating toward continental margins

    NASA Astrophysics Data System (ADS)

    Lyakhovsky, V.; Segev, A.; Schattner, U.; Weinberger, R.

    2012-01-01

    We study the propagation of a continental rift and its interaction with a continental margin utilizing a 3-D lithospheric model with a seismogenic crust governed by a damage rheology. A long-standing problem in rift-mechanics, known as thetectonic force paradox, is that the magnitude of the tectonic forces required for rifting are not large enough in the absence of basaltic magmatism. Our modeling results demonstrate that under moderate rift-driving tectonic forces the rift propagation is feasible even in the absence of magmatism. This is due to gradual weakening and "long-term memory" of fractured rocks that lead to a significantly lower yielding stress than that of the surrounding intact rocks. We show that the style, rate and the associated seismicity pattern of the rift zone formation in the continental lithosphere depend not only on the applied tectonic forces, but also on the rate of healing. Accounting for the memory effect provides a feasible solution for thetectonic force paradox. Our modeling results also demonstrate how the lithosphere structure affects the geometry of the propagating rift system toward a continental margin. Thinning of the crystalline crust leads to a decrease in the propagation rate and possibly to rift termination across the margin. In such a case, a new fault system is created perpendicular to the direction of the rift propagation. These results reveal that the local lithosphere structure is one of the key factors controlling the geometry of the evolving rift system and seismicity pattern.

  7. Structural style of the Turkana Rift, Kenya

    SciTech Connect

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

    1988-03-01

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

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

    Technology Transfer Automated Retrieval System (TEKTRAN)

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

  9. Post-Pan-African tectonic evolution of South Malawi in relation to the Karroo and recent East African rift systems

    NASA Astrophysics Data System (ADS)

    Castaing, C.

    1991-05-01

    Structural studies conducted in the Lengwe and Mwabvi Karroo basins and in the basement in South Malawi, using regional maps and published data extended to cover Southeast Africa, serve to propose a series of geodynamic reconstructions which reveal the persistence of an extensional tectonic regime, the minimum stress σ3 of which has varied through time. The period of Karroo rifting and the tholeiitic and alkaline magmatism which terminated it, were controlled by NW-SE extension, which resulted in the creation of roughly NE-SW troughs articulated by the Tanganyika-Malawi and Zambesi pre-transform systems. These were NW-SE sinistral-slip systems with directions of movement dipping slightly to the Southeast, which enabled the Mwanza fault to play an important role in the evolution of the Karroo basins of the Shire Valley. The Cretaceous was a transition period between the Karroo rifting and the formation of the Recent East African Rift System. Extension was NE-SW, with some evidence for a local compressional episode in the Lengwe basin. Beginning in the Cenozoic, the extension once more became NW-SE and controlled the evolution in transtension of the Recent East African Rift System. This history highlights the major role of transverse faults systems dominated by strike-slip motion in the evolution and perpetuation of the continental rift systems. These faults are of a greater geological persistence than the normal faults bounding the grabens, especially when they are located on major basement anisotropies.

  10. Early growth of Kohala volcano and formation of long Hawaiian rift zones

    USGS Publications Warehouse

    Lipman, P.W.; Calvert, A.T.

    2011-01-01

    Transitional-composition pillow basalts from the toe of the Hilo Ridge, collected from outcrop by submersible, have yielded the oldest ages known from the Island of Hawaii: 1138 ?? 34 to 1159 ?? 33 ka. Hilo Ridge has long been interpreted as a submarine rift zone of Mauna Kea, but the new ages validate proposals that it is the distal east rift zone of Kohala, the oldest subaerial volcano on the island. These ages constrain the inception of tholeiitic volcanism at Kohala, provide the first measured duration of tholeiitic shield building (???870 k.y.) for any Hawaiian volcano, and show that this 125-km-long rift zone developed to near-total length during early growth of Kohala. Long eastern-trending rift zones of Hawaiian volcanoes may follow fractures in oceanic crust activated by arching of the Hawaiian Swell in front of the propagating hotspot. ?? 2011 Geological Society of America.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

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

    NASA Technical Reports Server (NTRS)

    Mohr, P. A. (Principal Investigator)

    1973-01-01

    The author has identified the following significant results. 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.

  14. The seismotectonics of Southeastern Tanzania: Implications for the propagation of the eastern branch of the East African Rift

    NASA Astrophysics Data System (ADS)

    Mulibo, Gabriel D.; Nyblade, Andrew A.

    2016-04-01

    Seismicity patterns and focal mechanisms in southeastern Tanzania, determined from data recorded on temporary and permanent AfricaArray seismic stations, have been used to investigate the propagation direction of the Eastern branch of the East African Rift System southward from the Northern Tanzania Divergence Zone (NTDZ). Within the NTDZ, the rift zone is defined by three segments, the Eyasi segment to the west, the Manyara segment in the middle, and the Pangani segment to the east. Results show that most of the seismicity (~ 75%) extends to the south of the Manyara segment along the eastern margin of the Tanzania Craton, and at ~ 6-7° S latitude trends to the SE along the northern boundary of the Ruvuma microplate, connecting with a N-S zone of seismicity offshore southern Tanzania and Mozambique. A lesser amount of seismicity (~ 25%) is found extending from the SE corner of the Tanzania Craton at ~ 6-7° S latitude southwards towards Lake Nyasa. This finding supports a model of rift propagation via the Manyara segment to the southeast of the Tanzania Craton along the northern boundary of the Ruvuma microplate. However, given the limited duration of the seismic recordings used in this study, the possibility of another zone of extension developing to the south towards Lake Nyasa (Malawi) cannot be ruled out. Focal mechanisms along the boundary between the Victoria and the Ruvuma microplates and offshore southeastern Tanzania show a combination of normal and strike slip faulting indicating mainly extension with some sinistral motion, consistent with the mapped geologic faults and a clockwise rotation of the Ruvuma microplate.

  15. Arshan palaeoseismic feature of the Tunka fault (Baikal rift zone, Russia)

    NASA Astrophysics Data System (ADS)

    Smekalin, Oleg P.; Shchetnikov, Alexander A.; White, Dustin

    2013-01-01

    The traditional concept of the rift development of flank depressions in the Baikal rift zone is now doubted in view of some indicators for compression deformations identified by the seismogeological and geodetic methods. Besides, the paleoseismological investigations revealed seismogenic strike-slips and reverse faults in the Tunka fault zone that is a major structure-controlling element of the Tunka rift depression. However, a detailed study of the upslope-facing scarp in the Arshan paleoseismogenic structure zone has shown that its formation might be due to rift mechanism of basin formation. Age estimation has been made for the previously unknown pre-historic earthquake whose epicentral area coincides with the western flank of the Arshan paleoseismogenic structure. Judging from previously determined ages of paleoearthquakes, the mean recurrence period for faulting events on the central Tunka fault is 2780-3440 years.

  16. Benue trough and the mid-African rift system

    SciTech Connect

    Thomas, D.

    1996-01-29

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

  17. Transfer zones and fault reactivation in inverted rift basins: Insights from physical modelling

    NASA Astrophysics Data System (ADS)

    Konstantinovskaya, Elena A.; Harris, Lyal B.; Poulin, Jimmy; Ivanov, Gennady M.

    2007-08-01

    Lateral transfer zones of deformation and fault reactivation were investigated in multilayered silicone-sand models during extension and subsequent co-axial shortening. Model materials were selected to meet similarity criteria and to be distinguished on CT scans; this approach permitted non-destructive visualisation of the progressive evolution of structures. Transfer zones were initiated by an orthogonal offset in the geometry of a basal mobile aluminium sheet and/or by variations of layer thickness or material rheology in basal layers. Transfer zones affected rift propagation and fault kinematics in models. Propagation and overlapping rift culminations occurred in transfer zones during extension. During shortening, deviation in the orientation of frontal thrusts and fold axes occurred within transfer zones in brittle and ductile layers, respectively. CT scans showed that steep (58-67°) rift-margin normal faults were reactivated as reverse faults. The reactivated faults rotated to shallower dips (19-38°) with continuing shortening after 100% inversion. Rotation of rift phase faults appears to be due to deep level folding and uplift during the inversion phase. New thrust faults with shallow dips (20-34°) formed outside the inverted graben at late stages of shortening. Frontal ramps propagated laterally past the transfer structure during shortening. During inversion, the layers filling the rift structures underwent lateral compression at the depth, the graben fill was pushed up and outwards creating local extension near the surface. Sand marker layers in inverted graben have showed fold-like structures or rotation and tilting in the rifts and on the rift margins. The results of our experiments conform well to natural examples of inverted graben. Inverted rift basins are structurally complex and often difficult to interpret in seismic data. The models may help to unravel the structure and evolution of these systems, leading to improved hydrocarbon exploration

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

    NASA Astrophysics Data System (ADS)

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

    2014-08-01

    We present a detailed study about the initiation and reactivations of Zeit-El Tor transfer zone, south Gulf of Suez rift, and its structural setting and tectonic evolution with respect to the Cretaceous-Cenozoic tectonic movements in North Egyptian margin. NE trending zone of opposed-dipping faults (22 km wide) has transferred the NE and SW rotations of the sub-basins in central and south Gulf of Suez rift, respectively. The evolution of this zone started by reactivation of the NE oriented late Neoproterozoic fractures that controlled the occurrence of Dokhan Volcanics in the rift shoulders. Later, the Syrian Arc contraction reactivated these fractures by a sinistral transpression during the Late Cretaceous-Eocene time. N64°E extension of the Oligo-Miocene rift reactivated the NE fractures by a sinistral transtension. During this rifting, the NE trending faults forming the transfer zone were more active than the rift-bounding faults; the Upper Cretaceous reverse faults in the blocks lying between these NE trending faults were rotated; and drape-related reverse faults and the positive flower structures were formed. Tectonic inversion from contraction to extension controlled the distribution and thickness of the Upper Cretaceous-Miocene rocks.

  19. P-wave tomography reveals a westward dipping low velocity zone beneath the Kenya Rift

    NASA Astrophysics Data System (ADS)

    Park, Yongcheol; Nyblade, Andrew A.

    2006-04-01

    Three teleseismic P-wave travel time data sets (KRISP 1985, 1989-1990 Kenya Broadband Seismic Experiment) have been inverted to obtain a new tomographic model of the upper mantle beneath the Kenya Rift. The model shows a 0.5-1.5% low velocity anomaly below the rift extending to about 150 km depth. Below ~150 km depth, the anomaly broadens to the west toward the Tanzania Craton, suggesting a westward dip to the structure. Tomographic images to the south in Tanzania and to the north in Ethiopia also show westward dipping low velocity anomalies below depths of ~150-200 km. The presence of westward dipping low velocity structures along much of the East African rift (Ethiopia, Kenya and Tanzania) is difficult to explain with a plume model and is consistent with some models of the African Superplume showing anomalous lower and upper mantle structure connecting at mid-mantle depths under the western side of East Africa.

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

    NASA Astrophysics Data System (ADS)

    Alvarez, Román

    2002-10-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  2. Current kinematics and dynamics of Africa and the East African Rift System

    NASA Astrophysics Data System (ADS)

    Stamps, D. S.; Flesch, L. M.; Calais, E.; Ghosh, A.

    2014-06-01

    Although the East African Rift System (EARS) is an archetype continental rift, the forces driving its evolution remain debated. Some contend buoyancy forces arising from gravitational potential energy (GPE) gradients within the lithosphere drive rifting. Others argue for a major role of the diverging mantle flow associated with the African Superplume. Here we quantify the forces driving present-day continental rifting in East Africa by (1) solving the depth averaged 3-D force balance equations for 3-D deviatoric stress associated with GPE, (2) inverting for a stress field boundary condition that we interpret as originating from large-scale mantle tractions, (3) calculating dynamic velocities due to lithospheric buoyancy forces, lateral viscosity variations, and velocity boundary conditions, and (4) calculating dynamic velocities that result from the stress response of horizontal mantle tractions acting on a viscous lithosphere in Africa and surroundings. We find deviatoric stress associated with lithospheric GPE gradients are ˜8-20 MPa in EARS, and the minimum deviatoric stress resulting from basal shear is ˜1.6 MPa along the EARS. Our dynamic velocity calculations confirm that a force contribution from GPE gradients alone is sufficient to drive Nubia-Somalia divergence and that additional forcing from horizontal mantle tractions overestimates surface kinematics. Stresses from GPE gradients appear sufficient to sustain present-day rifting in East Africa; however, they are lower than the vertically integrated strength of the lithosphere along most of the EARS. This indicates additional processes are required to initiate rupture of continental lithosphere, but once it is initiated, lithospheric buoyancy forces are enough to maintain rifting.

  3. Petroleum geology of rift basins in Niger, Chad, and the Central African Republic

    SciTech Connect

    Genik, G.J. )

    1991-03-01

    Ten Cretaceous-Tertiary rift basins in Niger, Chad, and the Central African Republic (C.A.R.) are defined and the petroleum geology is overviewed. This paper is based on proprietary exploration results derived from more than 1 million km{sup 2} of aeromagnetics, 10,520 line km of gravity profiles, 49,721 km of reflection seismic, and 50 exploration wells. The data were acquired by Exxon with partners Shell, Chevron, Elf, Conoco, Texaco, and Amax Oil Gas, Inc., during the years 1969-1989. In Niger and Chad, the West African rift subsystem includes the extensional basins of Termit, Tefidet, Tenere, Grein/Kafra, N'Djel Edji, and Bongor. These rift basins contain up to 15,000 m of Cretaceous to Cenozoic continental and marine clastics. Key exploration elements are Tertiary and Cretaceous fluvial to tidal sandstone reservoirs, Tertiary and Cretaceous marine to lacustrine shale source rocks, and seals, with traps in normal fault blocks and anticlinal closures. There are six oil discoveries in the Termit basin. In Chad and the C.A.R., the Central African rift subsystem incorporates the extensional Doba and transtensional Doseo and Salamat basins flanking the Borogop dextral wrench fault. These basins contain up to 7,500 m of chiefly Cretaceous continental clastics. Key exploration elements are Lower and Upper Cretaceous fluvial to lacustrine sandstone reservoirs, Lower Cretaceous lacustrine shale source rocks, lacustrine to flood plain shale and mudstone seals, with traps in mainly faulted anticlinal closures. There are six oil discoveries in the Doba basin and three in the Doseo basin. The studied petroleum geology in the rifts of Niger, Chad, and the C.A.R. indicates that potentially commercial volumes of oil remain to be discovered.

  4. Petroleum geology of rift basins in Niger, Chad, and Central African Republic

    SciTech Connect

    Genik, G.J. )

    1991-08-01

    Ten Cretaceous-Tertiary rift basins in Niger, Chad and the Central African Republic (C.A.R.) are defined and the petroleum geology is overviewed based on proprietary exploration results derived from more than one million km{sup 2} of aeromagnetics, 10,520 line-km of gravity profiles, 49,721 km of reflection seismic, and 50 exploration wells. The data were acquired by Exxon with partners Shell, Chevron, Elf, Conoco, Texaco, and Amax Oil Gas During 1969-1989. In Niger and Chad, the West African rift subsystem includes the extensional basins of Termit, Tefidet, Tenere, Grein/Kafra, N'Djel Edji, and Bongor. These rift basins contain up to 15,000 m of Cretaceous to Cenozoic continental and marine clastics. Key exploration elements are Tertiary and Cretaceous fluvial to tidal sandstone reservoirs, Tertiary and Cretaceous marine to lacustrine shale source rocks and seals, with traps in normal fault blocks and anticlinal closures. There have been six oil discoveries in the Termit basin. In C.A.R., the Central African rift subsystem incorporates the extensional Doba and transtensional Doseo and Salamat basins flanking the Borogop dextral wrench fault. These basins contain up to 7,500 m of chiefly Cretaceous continental clastics. key exploration elements are Lower and Upper Cretaceous fluvial to lacustrine sandstone reservoirs, Lower Cretaceous lacustrine shale source rocks, lacustrine to flood-plain shale and mudstone seals, with traps in mainly faulted anticlinal closures. There have been six oil discoveries in the Doba basin and three in the Doseo basin. The studied petroleum geology in the rifts of Niger, Chad, and C.A.R. indicates that potentially commercial volumes of oil remain to be discovered.

  5. A new brachypterous scarab species, Orphnus longicornis (Coleoptera: Scarabaeidae: Orphninae), from the East African Rift.

    PubMed

    Frolov, Andrey; Akhmetova, Lilia

    2015-11-05

    The Afrotropical Region is the center of the diversity of the scarab beetle genus Orphnus MacLeay, 1819 (Coleoptera: Scarabaeidae: Orphninae), with 94 species occurring from Sahel in the north to Little Karoo in the south (Paulian, 1948; Petrovitz, 1971; Frolov, 2008). The East African Rift is one of the richest regions of the Afrotropics housing more than 20 species of Orphnus (Paulian, 1948; Frolov, 2013), most of which are endemic to this region. Yet the scarab beetle fauna of the East African Rift, and especially the Eastern Arc Mountains, is still inadequately studied. Examination of the material housed in the Museum of Natural History of Humboldt-Universität, Berlin, Germany (ZMHUB), revealed a series of brachypterous Orphnus beetles belonging to an undescribed species. The new species is described and illustrated below.

  6. A Numerical and Analogue Study of Dike Ascent in Asymmetric Continental Rift Zones

    NASA Astrophysics Data System (ADS)

    Schierjott, J.; Maccaferri, F.; Acocella, V.; Rivalta, E.

    2015-12-01

    In continental rift zones, tectonic extension generates deep topographic depressions, typically graben or half-graben structures, confined by large border faults. Volcanism may be distributed within, at the border and outside of the depressions, and the mechanisms controlling this distribution are debated. Recently, Maccaferri et al. (2014) proposed that the reorientation of the principal stresses linked to crustal thinning and overall crustal mass redistribution in rift zones modifies the expected trajectory of ascending magma pockets and plays a fundamental role in the distribution of volcanism at the surface. However, the model does not explain why volcanism is asymmetric in most continental rift zones. The goal of this study is to investigate the relation between the characteristic distribution of volcanism at the surface, the distribution and geometry of magma storage at depth, and the observed geometric asymmetry of the grabens at most rift zones. By using a boundary element model for dike propagation and analogue laboratory experiments we evaluate the ascent path of magmatic dikes in asymmetric continental rifts.We find that the position of the magma source along the cross section of the rift and its spatial extent and the asymmetry of the graben cross section are the most important factors controlling one-sided volcanic activity at surface. For dikes starting beneath the rift's center, the more asymmetric the rift structure the more likely is asymmetric volcanic activity. Dikes are deflected to the shallow rift side and no volcanism develops on the deep side or only focused in one spot. However, if the position of the magma ponding region is offset towards the deep side of the graben, the dikes tend to emerge on the rift shoulder adjacent to such deep side. To a minor extent, also the starting depth of the dikes, any topographic loading on the graben flanks due to flank uplift and the background tectonic stress impact the surface distribution of volcanism

  7. Quantifying the morphometric variability of monogenetic cones in volcanic fields: the Virunga Volcanic Province, East African Rift

    NASA Astrophysics Data System (ADS)

    Poppe, Sam; Grosse, Pablo; Barette, Florian; Smets, Benoît; Albino, Fabien; Kervyn, François; Kervyn, Matthieu

    2016-04-01

    Volcanic cone fields are generally made up of tens to hundreds of monogenetic cones, sometimes related to larger polygenetic edifices, which can exhibit a wide range of morphologies and degrees of preservation. The Virunga Volcanic Province (VVP) developed itself in a transfer zone which separates two rift segments (i.e. Edward and Kivu rift) within the western branch of the East-African Rift. As the result of volcanic activity related to this tectonic regime of continental extension, the VVP hosts eight large polygenetic volcanoes, surrounded by over 500 monogenetic cones and eruptive fissures, scattered over the vast VVP lava flow fields. Some cones lack any obvious geo-structural link to a specific Virunga volcano. Using recent high-resolution satellite images (SPOT, Pléiades) and a newly created 5-m-resolution digital elevation model (TanDEM-X), we have mapped and classified all monogenetic cones and eruptive fissures of the VVP. We analysed the orientation of all mapped eruptive fissures and, using the MORVOLC program, we calculated a set of morphometric parameters to highlight systematic spatial variations in size or morphometric ratios of the cones. Based upon morphological indicators, we classified the satellite cones into 4 categories: 1. Simple cones with one closed-rim crater; 2. Breached cones with one open-rim crater; 3. Complex cones with two or more interconnected craters and overlapping cones; 4. Other edifices without a distinguishable crater or cone shape (e.g. spatter mounds and levees along eruptive fissures). The results show that cones are distributed in clusters and along alignments, in some cases parallel with the regional tectonic orientations. Contrasts in the volumes of cones positioned on the rift shoulders compared to those located on the rift valley floor can possibly be attributed to contrasts in continental crust thickness. Furthermore, higher average cone slopes in the East-VVP (Bufumbira zone) and central-VVP cone clusters suggest

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

    USGS Publications Warehouse

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

    1990-01-01

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

  9. The role of discrete intrabasement shear zones during multiphase continental rifting

    NASA Astrophysics Data System (ADS)

    Phillips, Thomas B.; Jackson, Christopher A.-L.; Bell, Rebecca E.; Duffy, Oliver B.; Fossen, Haakon

    2016-04-01

    Rift systems form within areas of relatively weak, heterogeneous lithosphere, containing a range of pre-existing structures imparted from previous tectonic events. The extent to which these structures may reactivate during later rift phases, and therefore affect the geometry and evolution of superposed rift systems, is poorly understood. The greatest obstacle to understanding how intrabasement structures influence the overlying rift is obtaining detailed constraints on the origin and 3D geometry of structures within crystalline basement. Such structures are often deeply buried beneath rift systems and therefore rarely sampled directly. In addition, due to relatively low internal acoustic impedance contrasts and large burial depths, crystalline basement typically appears acoustically transparent on seismic reflection data showing no resolvable internal structure. However, offshore SW Norway, beneath the Egersund Basin, intrabasement structures are exceptionally well-imaged due to large impedance contrasts within a highly heterogeneous and shallow basement. We use borehole-constrained 2D and 3D seismic reflection data to constrain the 3D geometry of these intrabasement reflections, and examine their interactions with the overlying rift system. Two types of intrabasement structure are observed: (i) thin (c. 100 m) reflections displaying a characteristic trough-peak-trough wavetrain; and (ii) thick (c. 1 km), sub-parallel reflection packages dipping at c. 30°. Through 1D waveform modelling we show that these reflection patterns arise from a layered sequence as opposed to a single interface. Integrating this with our seismic mapping we correlate these structures to the established onshore geology; specifically layered mylonites associated with the Caledonian thrust belt and cross-cutting extensional Devonian shear zones. We observe multiple phases of reactivation along these structures throughout multiple rift events, in addition to a range of interactions with

  10. Sedimentary budgets of the Tanzania coastal basin and implications for uplift history of the East African rift system

    NASA Astrophysics Data System (ADS)

    Said, Aymen; Moder, Christoph; Clark, Stuart; Abdelmalak, Mohamed Mansour

    2015-11-01

    Data from 23 wells were used to quantify the sedimentary budgets in the Tanzania coastal basin in order to unravel the uplift chronology of the sourcing area located in the East African Rift System. We quantified the siliciclastic sedimentary volumes preserved in the Tanzania coastal basin corrected for compaction and in situ (e.g., carbonates) production. We found that the drainage areas, which supplied sediments to this basin, were eroded in four episodes: (1) during the middle Jurassic, (2) during the Campanian-Palaeocene, (3) during the middle Eocene and (4) during the Miocene. Three of these high erosion and sedimentation periods are more likely related to uplift events in the East African Rift System and earlier rift shoulders and plume uplifts. Indeed, rapid cooling in the rift system and high denudation rates in the sediment source area are coeval with these recorded pulses. However, the middle Eocene pulse was synchronous with a fall in the sea level, a climatic change and slow cooling of the rift flanks and thus seems more likely due to climatic and eustatic variations. We show that the rift shoulders of the East African rift system have inherited their present relief from at least three epeirogenic uplift pulses of middle Jurassic, Campanian-Palaeocene, and Miocene ages.

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

    NASA Astrophysics Data System (ADS)

    Weldesenbet, S. F.; Wohnlich, S.

    2012-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Chenin, Pauline; Beaumont, Christopher

    2013-04-01

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

  13. Images of Kilauea East Rift Zone eruption, 1983-1993

    USGS Publications Warehouse

    Takahashi, Taeko Jane; Abston, C.C.; Heliker, C.C.

    1995-01-01

    This CD-ROM disc contains 475 scanned photographs from the U.S. Geological Survey Hawaii Observatory Library. The collection represents a comprehensive range of the best photographic images of volcanic phenomena for Kilauea's East Rift eruption, which continues as of September 1995. Captions of the images present information on location, geologic feature or process, and date. Short documentations of work by the USGS Hawaiian Volcano Observatory in geology, seismology, ground deformation, geophysics, and geochemistry are also included, along with selected references. The CD-ROM was produced in accordance with the ISO 9660 standard; however, it is intended for use only on DOS-based computer systems.

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

    NASA Technical Reports Server (NTRS)

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

    1994-01-01

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

  15. 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

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

    1998-01-01

    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.

  16. A refinement of the chronology of rift-related faulting in the Broadly Rifted Zone, southern Ethiopia, through apatite fission-track analysis

    NASA Astrophysics Data System (ADS)

    Balestrieri, Maria Laura; Bonini, Marco; Corti, Giacomo; Sani, Federico; Philippon, Melody

    2016-03-01

    To reconstruct the timing of rift inception in the Broadly Rifted Zone in southern Ethiopia, we applied the fission-track method to basement rocks collected along the scarp of the main normal faults bounding (i) the Amaro Horst in the southern Main Ethiopian Rift and (ii) the Beto Basin in the Gofa Province. At the Amaro Horst, a vertical traverse along the major eastern scarp yielded pre-rift ages ranging between 121.4 ± 15.3 Ma and 69.5 ± 7.2 Ma, similarly to two other samples, one from the western scarp and one at the southern termination of the horst (103.4 ± 24.5 Ma and 65.5 ± 4.2 Ma, respectively). More interestingly, a second traverse at the Amaro northeastern terminus released rift-related ages spanning between 12.3 ± 2.7 and 6.8 ± 0.7 Ma. In the Beto Basin, the ages determined along the base of the main (northwestern) fault scarp vary between 22.8 ± 3.3 Ma and 7.0 ± 0.7 Ma. We ascertain through thermal modeling that rift-related exhumation along the northwestern fault scarp of the Beto Basin started at 12 ± 2 Ma while in the eastern margin of the Amaro Horst faulting took place later than 10 Ma, possibly at about 8 Ma. These results suggest a reconsideration of previous models on timing of rift activation in the different sectors of the Ethiopian Rift. Extensional basin formation initiated more or less contemporaneously in the Gofa Province (~ 12 Ma) and Northern Main Ethiopian Rift (~ 10-12 Ma) at the time of a major reorganization of the Nubia-Somalia plate boundary (i.e., 11 ± 2 Ma). Afterwards, rift-related faulting involved the Southern MER (Amaro Horst) at ~ 8 Ma, and only later rifting seemingly affected the Central MER (after ~ 7 Ma).

  17. What the volcanism of the East African Rift tells us on its evolution and dynamics: a reappraisal

    NASA Astrophysics Data System (ADS)

    Michon, Laurent

    2015-04-01

    The East African Rift (EAR) is one of the most studied tectonic structures on Earth. Classically, it is described as extending from Afar in the North to the Malawi rift in the South, along the eastern and western branches, respectively. A widely accepted consensus also exists on two main points: 1- the rift initiated first with plume emplacement below the northern part of the eastern branch and 2- extension and volcanism subsequently migrated southward along the western branch (e.g., Ebinger, 1989). However, an increasing amount of new geochronological data on the volcanic activity in the southern part of the East African Rift tends to weaken these interpretations and imposes a reassessment of the rift dynamics. The volcanic activity being one of the main characteristics of this rift, I use it here to determine the lateral extension of the rift system and to assess the rift activity through times. First, the volcanism unambiguously indicates that the rift is not limited to the African continent but can be traced in the Mozambique Channel and in Madagascar where it is closely related to active tectonics (graben and transfer faults) initiated since at least the Miocene. Moreover, the synthesis of more than 800 published geochronological data of volcanic products distributed over the overal East African Rift allows the distinction of two parts. The first part, the Northern EAR, corresponds to the sole eastern branch, which is characterized by volcanic plateaus resulting from huge magma flux during three main periods (32-28, 18-12, 6-0 Ma). Provinces of the second part, the Southern EAR (the western branch, the Mozambique Channel and Madagascar), share rift-related scattered volcanic centres characterized by coeval periods of activity since the Oligocene (28-24, 20-16 and 12-0 Ma). This synthesis highlights the lack of southward migration of the volcanism during the evolution of the East African Rift and instead reveals the almost synchronous development of the

  18. Sismotectonics in the western branch of the East African Rift System

    NASA Astrophysics Data System (ADS)

    Delvaux, Damien; Kervyn, François; Mulumba, Jean-Luc; Kipata, Louis; Sebagenzi, Stanislas; Mavonga, Georges; Macheyeki, Athanas; Temu, Elly Bryan

    2013-04-01

    The western branch of the East African rift system is known of its particular seismic activity with larger magnitude (up to Ms 7.3) and more frequent destructive earthquakes than in the eastern branch. As a contribution to the IGCP 601 project Seismotectonic Map of Africa, we compiled the known active faults, thermal springs and historical seismicity in Central Africa. Using the rich archives of the Royal Museum for Central Africa, publications and own field observations, we present a compilation of available data relative to the current seismotectonic activity along the western branch of the East African rift system, in DRC, Rwanda, Burundi and Tanzania. Neotectonic activity related to the western rift branch is in general well expressed and relatively well studied in the eastern flank of this rift branch, in Uganda, Rwanda, Burundi and Tanzania. In contrast, the western flank of this rift branch, largely exposed in the DRC, has attracted less attention. However, data collected during the colonial times show significant sismotectonic activity in East DRC, not only in the western flank of the western rift branch, but extending far westwards up to the margin of the Congo basin. In particular, our predecessors paid a special attention to the mapping and description of thermal springs, noticing that they are often controlled by active faults. In addition, the operators of the relatively dense network of meteorological stations installed in the DRC, Rwanda and Burundi also recorded were with variable level of completeness and detail the earthquakes that they could felt. This provides a rich database that is used to complete the existing knowledge on historical seismicity. An important effort has still to be paid to identify and map potentially active fault due to poor field accessibility, tropical climate weathering and vegetation coverage. The main problem in the compilation of active fault data is that very few of them have been investigated by paleoseismic trenching

  19. Multiproxy Evidence for a Positive Hydrological Budget during the Little Ice Age in the East African Rift, Kenya

    NASA Astrophysics Data System (ADS)

    Goman, M.; Ashley, G. M.; Hover, V. C.; Owen, R.

    2011-12-01

    Hominin evolution took place in Africa during the Plio-Pleistocene and climate change is thought to be a factor, with Africa experiencing a general cooling and increasing aridification over the last several million years. Today, the climate of the East African Rift Valley of Kenya is characterized as semi-arid with evapotranspiration four times precipitation. Water resources are a valuable commodity for the many millions of inhabitants of the Valley. The short instrumental record shows precipitation fluctuates at sub-decadal timeframes as a result of the ENSO cycle; while during prehistory variations in monsoonal precipitation occurred on Milankovitch timescales (i.e. African Humid Period). Both timescales exhibit significant impacts on the distribution of surface water. However, little is known regarding precipitation variability over sub-millennial timescales. Emerging paleoclimate data indicates that the near surface presence of water has also varied over century length timescales. We present paleoclimate data from multiple sites along a north-south 600 km transect of the Gregory Rift Valley (Kenya) that indicate the region experienced wetter conditions during the Little Ice Age (A.D. 1400-1850). Our reconstructions of landscape and climate during this time frame rely upon a multiproxy and interdisciplinary approach. We discuss data from a variety of environmental settings (e.g. lakes, wetlands, and springs) that indicate an overall increase in hydrologic balance. Evidence is derived from biologic microfossils such as pollen, diatom and testate amoebae assemblages as well as inorganic components of the sedimentary record and geomorphic changes. The data differs significantly from studies undertaken to the west in Uganda and the Congo, where negative hydrologic balances occurred during the Little Ice Age. While the atmospheric dynamics causing this disparity are not yet recognized, interactions between the Intertropical Convergence Zone and the Congo Air Boundary

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

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

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

  1. A numerical and analogue study of dike ascent in asymmetric continental rift zones

    NASA Astrophysics Data System (ADS)

    Schierjott, Jana; Maccaferri, Francesco; Keir, Derek; Kemna, Andreas; Rivalta, Eleonora

    2015-04-01

    In continental rift zones, tectonic extension is responsible for the creation of deep topographic depressions bordered by large border faults. Volcanism may be distributed within, at the border and outside of the depressions, and the mechanisms controlling this distribution are debated. Recently, Maccaferri et al. (2014) proposed that the reorientation of the principal stresses linked to crustal thinning and overall crustal mass redistribution in rift zones modifies the expected trajectory of ascending magma pockets and plays a fundamental role in the distribution of volcanism at the surface. However, the model does not explain why volcanism is asymmetric in most continental rift zones. The goal of this study is to investigate the relation between the characteristic distribution of volcanism at the surface, the distribution and geometry of magma storage at depth, and the observed geometric asymmetry of the grabens at most rift zones. By using a boundary element model for dike propagation and analogue laboratory experiments we evaluate the ascent path of magmatic dikes in asymmetric continental rifts. We find that introducing asymmetry of various degrees into the models has a huge impact on the modeled location of the surface volcanic activity. In particular, varying model parameters such as the half-graben width and depth and the degree of asymmetry leads to numerous different scenarios, including one-sided volcanic activity when the degree of asymmetry is very high and the half-graben is not too deep. For wider or deeper half-grabens and moderate asymmetry a larger proportion of the magma tends to become arrested as horizontal intrusions at depth.

  2. Fault-related Soil Efflux of Mantle-derived CO2 in the Magadi and Natron Basins, East African Rift

    NASA Astrophysics Data System (ADS)

    Lee, H.; Muirhead, J.; Fischer, T. P.; Kattenhorn, S. A.; Ebinger, C. J.; Thomas, N.; Kianji, G.; Onguso, B.; Maqway, M. D.

    2014-12-01

    The Magadi (Kenya) and Natron (Tanzania) basins of the East African Rift are in an early stage (< 7 Ma) of continental rifting. The many normal faults observed in these areas create sediment-filled basins and a large number of alkaline springs feed water into two major lakes (Lake Magadi and Natron). Earthquake swarms reported in 1998 (Magadi) and 2007 (Natron) were accompanied by surface ruptures. Although CO2 is a major component of magmatic volatiles and fault-related fluids that may facilitate earthquakes and fault weakening, the soil CO2 efflux of continental rifts is poorly known. Here, we report soil CO2 flux rates measured in the Magadi and Natron basins, and carbon isotope values (δ13C, ‰ vs. PDB) to constrain CO2sources. Soil CO2 fluxes were measured at fault zones, horsts, grabens, and surface ruptures by EGM-4 (PP systems) with a gas accumulation chamber. A t-shaped connector with a needle was used for gas sampling into evacuated glass vials with a rubber septum. δ13C values were measured by isotope ratio mass spectrometer with a gas bench at the stable isotope laboratory, University of New Mexico. The fault zones in the Magadi basin have higher maximum CO2 flux rates (< 533.52 g m-2 d-1) and heavier δ13C values (< -3.8 ‰) than the Natron basin (< 147.12 g m-2 d-1 and < -6.2 ‰, respectively). In both areas, soil CO2 efflux is insignificant (< 10 g m-2 d-1) in both horsts and the middle of grabens with lighter δ 13C values (~ -10 ‰) likely resulting from significant air contribution. The highest CO2 flux rates (< 919.44 g m-2 d-1) were measured at recent surface ruptures, but they have lighter δ13C values (-10 to -15 ‰), implying significant air and biogenic C contributions. Our results indicate that (1) normal faults are pathways that deliver mantle-derived CO2 to the surface, (2) the Magadi basin exhibits greater mantle-derived CO2 than the Natron basin, and (3) recent ruptures are zones of shallow CO2 degassing.

  3. Minimal Role of Basal Shear Tractions in Driving Nubia-Somalia Divergence Across the East African Rift System

    NASA Astrophysics Data System (ADS)

    Stamps, D. S.; Calais, E.; Iaffaldano, G.; Flesch, L. M.

    2012-12-01

    The Nubian and Somalian plates actively diverge along the topographically high, ~5000 km long East African Rift System (EARS). As no major subduction zones bound Africa, one can assume that the forces driving the Nubia-Somalia plate system result primarily from mantle buoyancies and lateral variation in lithospheric gravitational potential energy. Images from seismic tomography and convection models suggest active mantle flow beneath Africa. However, the contribution from large-scale convection to the force balance driving plate divergence across the EARS remains in question. In this work we investigate the impact of mantle shear tractions on the dynamics of Nubia-Somalia divergence across the EARS. We compare surface motions inferred from GPS observations with strain rates and velocities predicted from dynamic models where basal shear stresses are (1) derived from forward mantle circulation models and (2) inferred from stress field boundary conditions that balance buoyancy forces in the African lithosphere. Upper mantle anisotropy derived from seismic observations beneath Africa provide independent constraints for the latter. Preliminary results suggest that basal shear tractions play a minor role in the dynamics of Nubia-Somalia divergence along the EARS. This result implies mantle-lithosphere decoupling, possibly promoted by a low viscosity asthenosphere. We corroborate the robustness of our results with estimates of upper mantle viscosity based on local upper mantle temperature estimates and rheological parameters obtained from laboratory experiments.

  4. Parameters influencing the location and characteristics of volcanic eruptions in a youthful extensional setting: Insights from the Virunga Volcanic Province, in the Western Branch of the East African Rift System

    NASA Astrophysics Data System (ADS)

    Smets, Benoît; d'Oreye, Nicolas; Kervyn, Matthieu; Kervyn, François

    2016-04-01

    The East African Rift System (EARS) is often mentioned as the modern archetype for rifting and continental break-up (Calais et al., 2006, GSL Special Publication 259), showing the complex interaction between rift faults, magmatism and pre-existing structures of the basement. Volcanism in the EARS is characterized by very active volcanoes, several of them being among the most active on Earth (Wright et al., 2015, GRL 42). Such intense volcanic activity provides useful information to study the relationship between rifting, magmatism and volcanism. This is the case of the Virunga Volcanic Province (VVP) located in the central part of the Western Branch of the EARS, which hosts two of the most active African volcanoes, namely Nyiragongo and Nyamulagira. Despite the intense eruptive activity in the VVP, the spatial distribution of volcanism and its relationship with the extensional setting remain little known. Here we present a study of the interaction between tectonics, magmatism and volcanism at the scale of the Kivu rift section, where the VVP is located, and at the scale of a volcano, by studying the dense historical eruptive activity of Nyamulagira. Both the complex Precambrian basement and magmatism appear to contribute to the development of the Kivu rift. The presence of transfer zones north and south of the Lake Kivu rift basin favoured the development of volcanic provinces at these locations. Rift faults, including reactivated Precambrian structures influenced the location of volcanism within the volcanic provinces and the rift basin. At a more local scale, the historical eruptive activity of Nyamulagira highlights that, once a composite volcano developed, the gravitational stress field induced by edifice loading becomes the main parameter that influence the location, duration and lava volume of eruptions.

  5. Evolution of the East African rift: Drip magmatism, lithospheric thinning and mafic volcanism

    NASA Astrophysics Data System (ADS)

    Furman, Tanya; Nelson, Wendy R.; Elkins-Tanton, Linda T.

    2016-07-01

    The origin of the Ethiopian-Yemeni Oligocene flood basalt province is widely interpreted as representing mafic volcanism associated with the Afar mantle plume head, with minor contributions from the lithospheric mantle. We reinterpret the geochemical compositions of primitive Oligocene basalts and picrites as requiring a far more significant contribution from the metasomatized subcontinental lithospheric mantle than has been recognized previously. This region displays the fingerprints of mantle plume and lithospheric drip magmatism as predicted from numerical models. Metasomatized mantle lithosphere is not dynamically stable, and heating above the upwelling Afar plume caused metasomatized lithosphere with a significant pyroxenite component to drip into the asthenosphere and melt. This process generated the HT2 lavas observed today in restricted portions of Ethiopia and Yemen now separated by the Red Sea, suggesting a fundamental link between drip magmatism and the onset of rifting. Coeval HT1 and LT lavas, in contrast, were not generated by drip melting but instead originated from shallower, dominantly anhydrous peridotite. Looking more broadly across the East African Rift System in time and space, geochemical data support small volume volcanic events in Turkana (N. Kenya), Chyulu Hills (S. Kenya) and the Virunga province (Western Rift) to be derived ultimately from drip melting. The removal of the gravitationally unstable, metasomatized portion of the subcontinental lithospheric mantle via dripping is correlated in each case with periods of rapid uplift. The combined influence of thermo-mechanically thinned lithosphere and the Afar plume together thus controlled the locus of continental rift initiation between Africa and Arabia and provide dynamic support for the Ethiopian plateau.

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

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

  7. New constraints on the present-day kinematics of the East African Rift from GPS and earthquake slip vector data

    NASA Astrophysics Data System (ADS)

    Hartnady, C.; Calais, E.; Ebinger, C.; Nocquet, J.

    2004-12-01

    The East African Rift (EAR), a major 5,000 km long and up to 1,000 km wide tectonic structure that marks the extensional boundary between the Nubian and Somalian plate, is interpreted either as a wide zone of uniformly distributed, diffuse deformation, or as a mosaic of microplates. Testing these models and quantifying the present-day kinematics of the EAR has so far resited investigation because of a critical lack of geodetic data within the EAR as well as on the surrounding Nubian and Somalian plates. Here, we present an updated GPS velocity field covering the Nubian and Somalian plates and combine it with earthquake slip vectors along the EAR in a joint inversion. Our objectives are to better constrain the Somalia/Nubia plate motion and to try to resolve block motions within the plate boundary zone. We find a Somalia/Nubia angular velocity similar to the one proposed by Fernandes et al. (EPSL, 222, 2004). We show that Tanzanian craton, nested between the western and eastern branches of the EAR and underlained by an upper mantle plume, can be modeled as an independent block, rotating counterclockwise w.r.t. Nubia. We discuss the implications of this kinematic model on the tectonics of the EAR.

  8. Central Washington seismicity; Evidence for a reactivated buried continental rift and northwest-trending structural zones

    SciTech Connect

    Johnson, P.A. )

    1989-11-01

    Analysis of central Washington seismicity of the past two decades reveals some interesting features. Shallow seismicity and deep seismicity occur as different geographic distributions. Concentration of seismicity along north- to northwest-oriented trends appears to be related to a buried continental rift and possible associated fault zones. Hypothesized extensions of the Chiwaukum graben and Straight Creek fault systems are plausible structural controls on the seismicity.

  9. Seismological investigation of the Okavango Rift, Botswana

    NASA Astrophysics Data System (ADS)

    Yu, Youqiang

    The mechanisms of rifting have been intensively investigated using geological and geophysical techniques beneath mature rift zones. However, current understanding on the earliest stages of rifting is seriously limited. Here we employ recently archived data from 17 broadband seismic stations traversing northern Botswana to conduct the first shear wave splitting and mantle transition zone (MTZ) studies within the Okavango Rift Zone (ORZ). The ORZ is an incipient continental rift situated at the terminal of the southwestern branch of the East African Rift System. The resulting normal MTZ thickness and consistently rift-parallel fast polarizations imply an absence of significant thermal anomalies in the upper mantle, ruling out the role of mantle plumes in the initiation of the ORZ. The observed anisotropy beneath the ORZ and adjacent areas is mainly attributed to the relative movement between the lithosphere and asthenosphere with regional contributions from fabrics in the lithosphere and flow deflection by the bottom of the lithosphere. Our observations imply that the initiation and development of the ORZ can be initiated following a passive mode from the consequences of relative movements between the South African block and the rest of the African plate along a zone of lithospheric weakness between the Congo and Kalahari cratons. In addition, an approach was developed to effectively remove the near surface reverberations in the resulting receiver functions, decipher the P-to-S converted phases associated with the Moho discontinuity, and thus resolve sub-sediment crustal structure beneath stations sitting on a low-velocity sedimentary layer.

  10. 3D conductivity image of a young continental rift: Taupo Volcanic Zone, New Zealand

    NASA Astrophysics Data System (ADS)

    Heise, W.; Caldwell, T. G.; Bibby, H. M.; Bennie, S. L.

    2009-04-01

    The Taupo Volcanic Zone (TVZ), in the North Island, New Zealand is a continental back arc rift associated with the subduction of the Pacific Plate under the Australian Plate and is characterised by the eruption of large volumes of rhyolitic magma during the last 1.6 Ma and an exceptionally high present-day heat flow. Data from 220 magnetotelluric soundings covering the central (rhyolitic) part of the TVZ were analysed using 3D inverse resistivity modelling and phase tensor visualisation techniques. Modelling results compare well with the thickness of conductive volcaniclastic material in filling the rift basin and calderas and expected from observed gravity anomalies. Phase tensor ellipticity correlates well with the resistivity gradient in the 3D inversion model showing how the phase data control the inversion and allowing identification of which structures are, or are not, resolved by the data. The inverse modelling results show a zone of high conductivity in the lower crust and upper-mantle along the central rift-axis that correlates with a zone of high phase observed at long periods. An unusual feature of the MT data at periods of 3-30s is the large phase tensor skew angle values that coincide with the margins of a localized gravity high in the centre of the survey area. This feature appears to be caused by the interaction of a thick near surface layer of high conductive volcaniclastic material with conductive structures at greater depth.

  11. The origin of Mauna Loa's Nīnole Hills: Evidence of rift zone reorganization

    NASA Astrophysics Data System (ADS)

    Zurek, Jeffrey; Williams-Jones, Glyn; Trusdell, Frank; Martin, Simon

    2015-10-01

    In order to identify the origin of Mauna Loa volcano's Nīnole Hills, Bouguer gravity was used to delineate density contrasts within the edifice. Our survey identified two residual anomalies beneath the Southwest Rift Zone (SWRZ) and the Nīnole Hills. The Nīnole Hills anomaly is elongated, striking northeast, and in inversions both anomalies merge at approximately -7 km above sea level. The positive anomaly, modeled as a rock volume of ~1200 km3 beneath the Nīnole Hills, is associated with old eruptive vents. Based on the geologic and geophysical data, we propose that the gravity anomaly under the Nīnole Hills records an early SWRZ orientation, now abandoned due to geologically rapid rift-zone reorganization. Catastrophic submarine landslides from Mauna Loa's western flank are the most likely cause for the concurrent abandonment of the Nīnole Hills section of the SWRZ. Rift zone reorganization induced by mass wasting is likely more common than currently recognized.

  12. Latitudinal Hydrologic Variability Along the East African Rift, Over the Past 200 Kyr

    NASA Astrophysics Data System (ADS)

    Scholz, C. A.

    2014-12-01

    Within the deep sediments of the large lakes of Africa's Great Rift Valley are continuous environmental records of remarkable antiquity and fidelity. Not only do stratigraphic sections from these basins extend back millions of years, many of the intervals represented contain high-resolution material of decadal resolution or better. East African lake basins remain sparsely sampled however, with only a few long and continuous records available. Our ability to image the lakes using seismic reflection methods greatly exceeds our opportunities for coring and drilling however; assessing stratal relationships observed in the geophysical data permits powerful inferences about past hydrologic changes. With intensive hydrocarbon exploration work underway in East Africa, industry well data can also help constrain and ground truth basin histories. Substantial spatio-temporal hydrologic variability is observed in East African basins over the past 200 kyr. Paleohydrological changes in the late Pleistocene and early Holocene are now well constrained in the northern hemisphere East African topics, with widespread aridity and in some cases lake desiccation observed during Heinrich Event 1. A climate recovery followed in the northern hemisphere East African tropics, with the early Holocene African Humid Period a time of positive water balance across most of the rift valley. The paleohydrology of southern hemisphere tropical East Africa is more equivocal, for instance with negligible draw-down of Lake Malawi at HE1. Whereas these late Pleistocene events represent substantial climate reorganizations, severe droughts during the middle-late Pleistocene (150-65 kyr BP) were far more intense, and produced much more severe drawdowns of Lakes Malawi and Tanganyika. Scientific drill cores, kullenberg cores, and extensive seismic reflection data sets from Lakes Malawi and Tanganyika provide indisputable evidence for lowstands of -500m and -600 m respectively. Climate changes that lowered the

  13. The Rift Valley of African Plate in Elasto-Plastic Creeping over Magma Motion

    NASA Astrophysics Data System (ADS)

    Nakamura, Shigehisa

    2016-04-01

    This is a brief note to a problem on the Rift Valley in the eastern Africa. It is said that this valley was formed in an age 20,000,000 years before present though the valley is yet continuing to move eastward at an annual rate of about 5 cm/year in a geographical trend. Adding to some of the scientists tell that the separation threat of the easternAfrica from the mother land of the Africa under the effect of African crust motion over the magma. However, it is now geological understanding that the land of the Africa has been kept its basic coastal configulation in geographic pattern since the time more than 20,000,000 years before present. Sothat, it is hard to consider the above noted African land separation by part could be in the next age in a time scale of 20,000,000 years. As far as, we concern the geographic data obtaoned by the ground based survey of the African typical mountain peaks, the highest mountain peak 5885m (in 1980) is for Kilimanjaro, Kibo Peak though one of the scientific almanacs tells us its peak height as 5890m (in 2009). As for the Mount Kenia, the peak height is as 5199m (in 1980) and 5200m(in 2009). At a glance, it looks to be a trend in altimetry of the African typical mountain. Now, what trends are noted for the peak heights could be taken to suggesting the geological activity on the earth surface to maintain in a spherical shape approximately on the orbit around the Sun. In these several ten years, the digitizing of the data has been promoted even for the topographic patterns on the earth though its time scaling is extremely short comparing to the geological time scaling. Now, it should be found what is effective to monitor any trends of the African crust in motion as well as variations of the mountain peaks.

  14. Ambient Noise Tomography of the East African Rift System in Mozambique

    NASA Astrophysics Data System (ADS)

    Domingues, Ana; Custódio, Susana; Chamussa, José; Silveira, Graça; Chang, Sung-Joon; Lebedev, Sergei; Ferreira, Ana; Fonseca, João

    2014-05-01

    Project MOZART - MOZAmbique Rift Tomography (funded by FCT, Lisbon) deployed a total of 30 temporary broadband seismic stations from the SEIS-UK Pool in central and south Mozambique and in NE South Africa. The purpose of this project is the study of the East African Rift System (EARS) in Mozambique. We estimated preliminary locations with the data recorded from April 2011 to July 2012. A total of 307 earthquakes were located, with ML magnitudes ranging from 0.9 to 3.9. We observe a linear northeast-southwest distribution of the seismicity that seems associated to the Inhaminga fault. The seismicity in the northeast sector correlates well with the topography, tracing the Urema rift valley. The seismicity extends to ~300km, reaching the M7 2006 Machaze earthquake area. In order to obtain an initial velocity model of the region, we applied the ambient noise method to the MOZART data and two additional stations from AfricaARRAY. Cross-correlations were computed between all pairs of stations, and we obtained Rayleigh wave group velocity dispersion curves for all interstation paths, in the period range from 3 to 50 seconds. The geographical distribution of the group velocity anomalies is in good agreement with the geology map of Mozambique, having lower group velocities in sedimentary basins areas and higher velocities in cratonic regions. We also observe two main regions with different velocities that may indicate a structure not proposed in previous studies. We perform a three-dimensional inversion to obtain the S-wave velocity of the crust and upper mantle, and in order to extend the investigation to longer periods we apply a recent implementation of the surface-wave two-station method (teleseismic interferometry), while augmenting our dataset with Rayleigh wave phase velocities curves in broad period ranges. In this way we expect to be able to look into the lithosphere-asthenosphere depth range.

  15. Human Dispersals Along the African Rift Valley in the Late Quaternary

    NASA Astrophysics Data System (ADS)

    Tryon, C. A.; Faith, J. T.; Peppe, D. J.

    2014-12-01

    Climate- and tectonic-driven environmental dynamics of the East African Rift System (EARS) during the Quaternary played an important role in the demographic history of early Homo sapiens, including expansions of modern humans across and out of Africa. Human forager population size, geographic range, and behaviors such as hunting strategies and residential mobility likely varied in response to changes in the local and regional environment. Throughout the Quaternary, floral and faunal change was linked at least in part to variations in moisture availability, temperature, and atmospheric CO2, which in addition to uplift and faulting, contributed to the expansion and contraction of a number of large lakes that served as biogeographic barriers to many taxa. This is particularly clear for the Lake Victoria basin, where biogeographic, geological, and paleontological evidence documents repeated expansion and contraction of the ranges of species in response to lake level and vegetation change. Across much of eastern Africa, the topography of the rift facilitated north-south dispersals, the timing of which may have depended in part on the expansion and contraction of the equatorial forest belt. Dispersal potential likely increased during the more arid periods of the late Quaternary, when the roles of lakes and forests as dispersal barriers was reduced and the extent of low net primary productivity dry grasslands increased, the latter requiring large home ranges for human foragers, conditions suitable for range expansions within H. sapiens.

  16. Estimating the age of formation of lakes: An example from Lake Tanganyika, East African Rift system

    SciTech Connect

    Cohen, A.; Soreghan, M.J.; Scholz, C.A.

    1993-06-01

    Age estimates for ancient lakes are important for determining their histories and their rates of biotic and tectonic evolution. In the absence of dated core material from the lake`s sedimentary basement, several techniques have been used to generate such age estimates. The most common of these, herein called the reflection seismic-radiocarbon method (RSRM), combines estimates of short-term sediment-accumulation rates derived from radiocarbon-dated cores and depth-to-basement estimates derived from reflection-seismic data at or near the same locality to estimate an age to basement. Age estimates form the RSRM suggest that the structural basins of central Lake Tanganyika began to form between 9 and 12 Ma. Estimates for the northern and southern basins are younger (7 to 8 Ma and 2 to 4 Ma, respectively). The diachroneity of estimates for different segments of the lake is equivocal, and may be due to erosional loss of record in the northern and southern structural basins or to progressive opening of the rift. The RSRM age estimates for Lake Tanganyika are considerably younger than most prior estimates and clarify the extensional history of the western branch of the East African Rift system. 31 refs., 3 figs., 1 tab.

  17. Rift Valley Fever Outbreak with East-Central African Virus Lineage in Mauritania, 2003

    PubMed Central

    Faye, Ousmane; Diallo, Mawlouth; Diop, Djibril; Bezeid, O. Elmamy; Bâ, Hampathé; Niang, Mbayame; Dia, Ibrahima; Mohamed, Sid Ahmed Ould; Ndiaye, Kader; Diallo, Diawo; Ly, Peinda Ogo; Diallo, Boubacar; Nabeth, Pierre; Simon, François; Lô, Baïdy

    2007-01-01

    In October 2003, 9 human cases of hemorrhagic fever were reported in 3 provinces of Mauritania, West Africa. Test results showed acute Rift Valley fever virus (RVFV) infection, and a field investigation found recent circulation of RVFV with a prevalence rate of 25.5% (25/98) and 4 deaths among the 25 laboratory-confirmed case-patients. Immunoglobulin M against RVFV was found in 46% (25/54) of domestic animals. RVFV was also isolated from the mosquito species Culex poicilipes. Genetic comparison of virion segments indicated little variation among the strains isolated. However, phylogenetic studies clearly demonstrated that these strains belonged to the East-Central African lineage for all segments. To our knowledge, this is the first time viruses of this lineage have been observed in an outbreak in West Africa. Whether these strains were introduced or are endemic in West Africa remains to be determined. PMID:18214173

  18. The 1990 to 1991 Sudan earthquake sequence and the extent of the East african rift system.

    PubMed

    Girdler, R W; McConnell, D A

    1994-04-01

    One of the largest earthquakes ever recorded in Africa (surface wave magnitude M(s) = 7.2) occurred about 50 kilometers east of the Upper River Nile on 20 May 1990. Four days later, two more large earthquakes (M(s) = 6.4 and 7.0) occurred about 50 kilometers to the northwest in the Nile Valley. In the following months, a further 60 events were recorded by seismic stations worldwide. The earthquakes are associated with two fault systems: one east of the Nile with azimuth southeast and one along the Nile Valley with azimuth north-northeast. The activity alternated between the two fault systems and indicates that the northern extremity of the western branch of the East African Rift System extends at least 350 kilometers north of Lake Albert.

  19. Rift Valley fever outbreak with East-Central African virus lineage in Mauritania, 2003.

    PubMed

    Faye, Ousmane; Diallo, Mawlouth; Diop, Djibril; Bezeid, O Elmamy; Bâ, Hampathé; Niang, Mbayame; Dia, Ibrahima; Mohamed, Sid Ahmed Ould; Ndiaye, Kader; Diallo, Diawo; Ly, Peinda Ogo; Diallo, Boubacar; Nabeth, Pierre; Simon, François; Lô, Baïdy; Diop, Ousmane Madiagne

    2007-07-01

    In October 2003, 9 human cases of hemorrhagic fever were reported in 3 provinces of Mauritania, West Africa. Test results showed acute Rift Valley fever virus (RVFV) infection, and a field investigation found recent circulation of RVFV with a prevalence rate of 25.5% (25/98) and 4 deaths among the 25 laboratory-confirmed case-patients. Immunoglobulin M against RVFV was found in 46% (25/54) of domestic animals. RVFV was also isolated from the mosquito species Culex poicilipes. Genetic comparison of virion segments indicated little variation among the strains isolated. However, phylogenetic studies clearly demonstrated that these strains belonged to the East-Central African lineage for all segments. To our knowledge, this is the first time viruses of this lineage have been observed in an outbreak in West Africa. Whether these strains were introduced or are endemic in West Africa remains to be determined.

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

    USGS Publications Warehouse

    Cervelli, Peter; Segall, P.; Amelung, F.; Garbeil, H.; Meertens, C.; Owen, S.; Miklius, Asta; Lisowski, M.

    2002-01-01

    Deformation associated with an earthquake swarm on 12 September 1999 in the Upper East Rift Zone of Kilauea Volcano was recorded by continuous GPS receivers and by borehole tiltmeters. Analyses of campaign GPS, leveling data, and interferometric synthetic aperture radar (InSAR) data from the ERS-2 satellite also reveal significant deformation from the swarm. We interpret the swarm as resulting from a dike intrusion and model the deformation field using a constant pressure dike source. Nonlinear inversion was used to find the model that best fits the data. The optimal dike is located beneath and slightly to the west of Mauna Ulu, dips steeply toward the south, and strikes nearly east-west. It is approximately 3 by 2 km across and was driven by a pressure of ??? 15 MPa. The total volume of the dike was 3.3 x 106 m3. Tilt data indicate a west to east propagation direction. Lack of premonitory inflation of Kilauea's summit suggests a passive intrusion; that is, the immediate cause of the intrusion was probably tensile failure in the shallow crust of the Upper East Rift Zone brought about by persistent deep rifting and by continued seaward sliding of Kilauea's south flank.

  1. The East African Rift System and the impact of orographic changes on regional climate and the resulting aridification

    NASA Astrophysics Data System (ADS)

    Sommerfeld, Anja; Prömmel, Kerstin; Cubasch, Ulrich

    2016-09-01

    Several proxy data indicate an aridification of the East African climate during the Neogene, which might be influenced by the orographic changes of the East African Rift System (EARS) induced by tectonic forcing during the last 20 million years. To investigate the impact of the orography and especially of the rifts, the regional climate model CCLM is used, covering the EARS with Lake Victoria in the centre of the model domain. CCLM is driven by the ERA-Interim reanalysis and applied with a double-nesting method resulting in a very high spatial resolution of 7 km. The resolution clearly shows the shoulders and rifts of the western and eastern branch of the EARS and the Rwenzoris within the western branch. To analyse the orographic influence on climate, a new technique of modifying the orography is used in this sensitivity study. The shoulders of the branches are lowered and the rifts are elevated, resulting in a smoothed orography structure with less altitude difference between the shoulders and rifts. The changes in 2 m-temperature are very local and associated with the changes in the orography. The vertically integrated moisture transport is characterised by less vortices, and its zonal component is increased over the branches. The resulting amount of precipitation is mainly decreased west of the western branch and increased in the rift of the western branch. In the eastern branch, however, the changes in the amount of precipitation are not significant. The changes in the precipitation and temperature patterns lead to a shift of biomes towards a vegetation coverage characterised by more humid conditions in the northern part of the model domain and more arid conditions in the South. Thus, the aridification found in the proxy data can be attributed to the orographic changes of the rifts only in the northern model domain.

  2. Giant seismites and megablock uplift in the East African Rift: evidence for Late Pleistocene large magnitude earthquakes.

    PubMed

    Hilbert-Wolf, Hannah Louise; Roberts, Eric M

    2015-01-01

    In lieu of comprehensive instrumental seismic monitoring, short historical records, and limited fault trench investigations for many seismically active areas, the sedimentary record provides important archives of seismicity in the form of preserved horizons of soft-sediment deformation features, termed seismites. Here we report on extensive seismites in the Late Quaternary-Recent (≤ ~ 28,000 years BP) alluvial and lacustrine strata of the Rukwa Rift Basin, a segment of the Western Branch of the East African Rift System. We document examples of the most highly deformed sediments in shallow, subsurface strata close to the regional capital of Mbeya, Tanzania. This includes a remarkable, clastic 'megablock complex' that preserves remobilized sediment below vertically displaced blocks of intact strata (megablocks), some in excess of 20 m-wide. Documentation of these seismites expands the database of seismogenic sedimentary structures, and attests to large magnitude, Late Pleistocene-Recent earthquakes along the Western Branch of the East African Rift System. Understanding how seismicity deforms near-surface sediments is critical for predicting and preparing for modern seismic hazards, especially along the East African Rift and other tectonically active, developing regions.

  3. Giant Seismites and Megablock Uplift in the East African Rift: Evidence for Late Pleistocene Large Magnitude Earthquakes

    PubMed Central

    Hilbert-Wolf, Hannah Louise; Roberts, Eric M.

    2015-01-01

    In lieu of comprehensive instrumental seismic monitoring, short historical records, and limited fault trench investigations for many seismically active areas, the sedimentary record provides important archives of seismicity in the form of preserved horizons of soft-sediment deformation features, termed seismites. Here we report on extensive seismites in the Late Quaternary-Recent (≤ ~ 28,000 years BP) alluvial and lacustrine strata of the Rukwa Rift Basin, a segment of the Western Branch of the East African Rift System. We document examples of the most highly deformed sediments in shallow, subsurface strata close to the regional capital of Mbeya, Tanzania. This includes a remarkable, clastic ‘megablock complex’ that preserves remobilized sediment below vertically displaced blocks of intact strata (megablocks), some in excess of 20 m-wide. Documentation of these seismites expands the database of seismogenic sedimentary structures, and attests to large magnitude, Late Pleistocene-Recent earthquakes along the Western Branch of the East African Rift System. Understanding how seismicity deforms near-surface sediments is critical for predicting and preparing for modern seismic hazards, especially along the East African Rift and other tectonically active, developing regions. PMID:26042601

  4. Giant seismites and megablock uplift in the East African Rift: evidence for Late Pleistocene large magnitude earthquakes.

    PubMed

    Hilbert-Wolf, Hannah Louise; Roberts, Eric M

    2015-01-01

    In lieu of comprehensive instrumental seismic monitoring, short historical records, and limited fault trench investigations for many seismically active areas, the sedimentary record provides important archives of seismicity in the form of preserved horizons of soft-sediment deformation features, termed seismites. Here we report on extensive seismites in the Late Quaternary-Recent (≤ ~ 28,000 years BP) alluvial and lacustrine strata of the Rukwa Rift Basin, a segment of the Western Branch of the East African Rift System. We document examples of the most highly deformed sediments in shallow, subsurface strata close to the regional capital of Mbeya, Tanzania. This includes a remarkable, clastic 'megablock complex' that preserves remobilized sediment below vertically displaced blocks of intact strata (megablocks), some in excess of 20 m-wide. Documentation of these seismites expands the database of seismogenic sedimentary structures, and attests to large magnitude, Late Pleistocene-Recent earthquakes along the Western Branch of the East African Rift System. Understanding how seismicity deforms near-surface sediments is critical for predicting and preparing for modern seismic hazards, especially along the East African Rift and other tectonically active, developing regions. PMID:26042601

  5. History of the development of the East African Rift System: A series of interpreted maps through time

    NASA Astrophysics Data System (ADS)

    Macgregor, Duncan

    2015-01-01

    This review paper presents a series of time reconstruction maps of the 'East African Rift System' ('EARS'), illustrating the progressive development of fault trends, subsidence, volcanism and topography. These maps build on previous basin specific interpretations and integrate released data from recent petroleum drilling. N-S trending EARS rifting commenced in the petroliferous South Lokichar Basin of northern Kenya in the Late Eocene to Oligocene, though there seem to be few further deep rifts of this age other than those immediately adjoining it. At various times during the Mid-Late Miocene, a series of small rifts and depressions formed between Ethiopia and Malawi, heralding the main regional rift subsidence phase and further rift propagation in the Plio-Pleistocene. A wide variation is thus seen in the ages of initiation of EARS basins, though the majority of fault activity, structural growth, subsidence, and associated uplift of East Africa seem to have occurred in the last 5-9 Ma, and particularly in the last 1-2 Ma. These perceptions are key to our understanding of the influence of the diverse tectonic histories on the petroleum prospectivity of undrilled basins.

  6. Ancient origin and recent divergence of a haplochromine cichlid lineage from isolated water bodies in the East African Rift system.

    PubMed

    Hermann, C M; Sefc, K M; Koblmüller, S

    2011-11-01

    Phylogenetic analysis identified haplochromine cichlids from isolated water bodies in the eastern branch of the East African Rift system as an ancient lineage separated from their western sister group in the course of the South Kenyan-North Tanzanian rift system formation. Within this lineage, the close phylogenetic relatedness among taxa indicates a recent common ancestry and historical connections between now separated water bodies. In connection with a total lack of local genetic diversity attributable to population bottlenecks, the data suggest cycles of extinction and colonization in the unstable habitat provided by the small lakes and rivers in this geologically highly active area.

  7. Assessing Microseismicity of the Northern Mid-Continent Rift Zone and Surrounding Regions

    NASA Astrophysics Data System (ADS)

    Bartz, D.; Wysession, M. E.; Wiens, D. A.; Aleqabi, G. I.; Shore, P.; van der Lee, S.; Jurdy, D. M.; Stein, S. A.; Revenaugh, J.; Wolin, E.; Bollmann, T. A.; Frederiksen, A. W.; Darbyshire, F. A.

    2014-12-01

    SPREE is a flexible array (FA) deployment associated with the EarthScope project with the aim of better understanding the Mid-Continent Rift Zone (MCRZ). We have used data from SPREE FA and TA stations to detect and locate small earthquakes in the vicinity of the northwestern arm of the Mid-Continent Rift Zone, covering Minnesota, Wisconsin, and parts of Iowa, Michigan, and Ontario. This region, now stable craton, is a failed Precambrian rift marked by low levels of recorded seismicity, perhaps resulting in part from a historic dearth of stations deployed in the region. We assessed this possibility by taking advantage of the densest array that has been deployed in the region. Processing has allowed for regional stress constraints and a means of distinguishing earthquakes from mine blasts, and a quantitative method to differentiate natural earthquakes from mine blasting events, frequent here. After automated event identification, a manual review confirmed 14 earthquakes and several hundred blasting events with magnitudes ranging from M1 - M3. Observed seismicity is in reasonable agreement with available historical data. We use these earthquakes in conjunction with historical catalogs to estimate regional recurrence intervals for events of greater magnitudes. While initial earthquake/blast discrimination was done manually, the ratio of Rayleigh to P-wave amplitude appears to be a reliable discriminant for distinguishing between earthquakes and mine blasting in this region in a systematic way, with earthquakes displaying a systematically depressed ratio. Additionally, P-wave first-motion data and S/P amplitude ratios from natural events constrain focal mechanisms and regional stresses. These methods indicate roughly WSW-ENE compression, consistent with other determinations and absolute plate motion. The majority of events detected lie some distance from the actual rift; we thus conclude current seismicity bears no preferred association with the MCR.

  8. Shaded Relief with Height as Color, Virunga and Nyiragongo Volcanoes and the East African Rift

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Volcanic, tectonic, erosional and sedimentary landforms are all evident in this comparison of two elevation models of a region along the East African Rift at Lake Kivu. The area shown covers parts of Congo, Rwanda and Uganda.

    These two images show exactly the same area. The image on the left was created using the best global topographic data set previously available, the U.S. Geological Survey's GTOPO30. In contrast, the much more detailed image on the right was generated with data from the Shuttle Radar Topography Mission, which collected enough measurements to map 80 percent of Earth's landmass at this level of precision. Elevation is color coded, progressing from green at the lower elevations through yellow to brown at the higher elevations. A false sun in the northwest (upper left) creates topographic shading.

    Lake Kivu is shown as black in the Shuttle Radar Topography Mission version (southwest corner). It lies within the East African Rift, an elongated tectonic pull-apart depression in Earth's crust. The rift extends to the northeast as a smooth lava- and sediment-filled trough. Two volcanic complexes are seen in the rift. The one closer to the lake is the Nyiragongo volcano, which erupted in January 2002, sending lava toward the lake shore and through the city of Goma. East of the rift, even more volcanoes are seen. These are the Virunga volcano chain, which is the home of the endangered mountain gorillas. Note that the terrain surrounding the volcanoes is much smoother than the eroding mountains that cover most of this view, such that topography alone is a good indicator of the extent of the lava flows. But this clear only at the higher spatial resolution of the shuttle mission's data set.

    For some parts of the globe, Shuttle Radar Topography Mission measurements are 30 times more precise than previously available topographical information, according to NASA scientists. Mission data will be a welcome resource for national and local governments

  9. Mapping of the major structures of the African rift system using ERTS-1

    NASA Technical Reports Server (NTRS)

    Mohr, P. A. (Principal Investigator)

    1973-01-01

    The author has identified the following significant results. The structural margin of western Afar with the Ethiopian plateau is marked by a rather wide zone of crustal deformation. ERTS-1 imagery has now permitted a more precise mapping of the structures of this marginal zone, and in particular of the discontinuous marginal graben. The tectonic style of the graben is different in the north from the south, and in the latter region the graben is discordant with the regional tectonic trend. The structural margin of the southern Afar with the Somalian plateau is formed, in the western sector, by a remarkable series of fault-zone splays. Afar-plateau boundary fault-zones successively curve northeast and then NNE to become Afar floor fault-zones, with a distance of about 25 km separating successive turnoffs. The transition from Ethiopian rift to Gulf of Aden tread faulting along this margin is fascinatingly complex. A simplistic crustal thinning model is not adequate to explain all observed structural features of the Afar margins.

  10. Mapping Extensional Structures in the Makgadikgadi Pans, Botswana with remote sensing and aeromagnetic data: Implication for the continuation of the East African Rift System in southern Africa

    NASA Astrophysics Data System (ADS)

    Fetkovich, E. J.; Atekwana, E. A.; Abdelsalam, M. G.; Atekwana, E. A.; Katumwehe, A. B.

    2015-12-01

    Okavango Rift Zone to the northwest of the Makgadikgadi Pans. Our results suggest that the southwestern continuation of this rift system might be a 400 km wide and diffused zone of extension that encompasses both the Okavango Rift Zone and the Makgadikgadi Pans. Our study has implication for understanding within-plate kinematics and seismicity of the African continent.

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

    SciTech Connect

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

    1987-04-01

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

  12. Rift Valley Fever Virus Circulating among Ruminants, Mosquitoes and Humans in the Central African Republic

    PubMed Central

    Nakouné, Emmanuel; Kamgang, Basile; Berthet, Nicolas; Manirakiza, Alexandre; Kazanji, Mirdad

    2016-01-01

    Background Rift Valley fever virus (RVFV) causes a viral zoonosis, with discontinuous epizootics and sporadic epidemics, essentially in East Africa. Infection with this virus causes severe illness and abortion in sheep, goats, and cattle as well as other domestic animals. Humans can also be exposed through close contact with infectious tissues or by bites from infected mosquitoes, primarily of the Aedes and Culex genuses. Although the cycle of RVFV infection in savannah regions is well documented, its distribution in forest areas in central Africa has been poorly investigated. Methodology/Principal Findings To evaluate current circulation of RVFV among livestock and humans living in the Central African Republic (CAR), blood samples were collected from sheep, cattle, and goats and from people at risk, such as stock breeders and workers in slaughterhouses and livestock markets. The samples were tested for anti-RVFV immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies. We also sequenced the complete genomes of two local strains, one isolated in 1969 from mosquitoes and one isolated in 1985 from humans living in forested areas. The 1271 animals sampled comprised 727 cattle, 325 sheep, and 219 goats at three sites. The overall seroprevalence of anti-RVFV IgM antibodies was 1.9% and that of IgG antibodies was 8.6%. IgM antibodies were found only during the rainy season, but the frequency of IgG antibodies did not differ significantly by season. No evidence of recent RVFV infection was found in 335 people considered at risk; however, 16.7% had evidence of past infection. Comparison of the nucleotide sequences of the strains isolated in the CAR with those isolated in other African countries showed that they belonged to the East/Central African cluster. Conclusion and significance This study confirms current circulation of RVFV in CAR. Further studies are needed to determine the potential vectors involved and the virus reservoirs. PMID:27760144

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

  14. Early-stage rifting in the southwest East African Rift: Insights from new reflection seismic data from Lakes Tanganyika and Malawi (Nyasa)

    NASA Astrophysics Data System (ADS)

    Scholz, C. A.; Wood, D. A.; Shillington, D. J.; McCartney, T.; Accardo, N. J.

    2015-12-01

    The western branch of the East African Rift is characterized by modest amounts of mainly amagmatic extension; deeply-subsided, fault-controlled basins; and large-magnitude, deep seismicity. Lakes Tanganyika and Malawi are two of the world's largest lakes, with maximum water depths of 1450 and 700 m respectively. Newly acquired seismic reflection data, along with newly reprocessed legacy data reveal thick sedimentary sections, in excess of 5 km in some localities. The 1980's vintage legacy data from Project PROBE have been reprocessed through pre-stack depth migration in Lake Tanganyika, and similar reprocessing of legacy data from Lake Malawi is forthcoming. New high-fold and large-source commercial and academic data have recently been collected in southern Lake Tanganyika, and in the northern and central basins of Lake Malawi as part of the 2015 SEGMeNT project. In the case of Lake Tanganyika, new data indicate the presence of older sediment packages that underlie previously identified "pre-rift" basement (the "Nyanja Event"). These episodes of sedimentation and extension may substantially predate the modern lake. These deep stratal reflections are absent in many localites, possibly on account of attenuation of the acoustic signal. However in one area of southern Lake Tanganyika, the newly-observed deep strata extend axially for ~70 km, likely representing deposits from a discrete paleolake. The high-amplitude Nyanja Event is interpreted as the onset of late-Cenozoic rifting, and the changing character of the overlying depositional sequences reflects increasing relief in the rift valley, as well as the variability of fluvial inputs, and the intermittent connectivity of upstream lake catchments. Earlier Tanganyika sequences are dominated by shallow lake and fluvial-lacustrine facies, whereas later sequences are characterized by extensive gravity flow deposition in deep water, and pronounced erosion and incision in shallow water depths and on littoral platforms. The

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

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

    USGS Publications Warehouse

    Moore, R.B.

    1992-01-01

    Detailed geologic mapping and radiocarbon dating of tholeiitic basalts covering about 275 km2 on the lower east rift zone (LERZ) and adjoining flanks of Kilauea volcano, Hawaii, show that at least 112 separate eruptions have occurred during the past 2360 years. Eruptive products include spatter ramparts and cones, a shield, two extensive lithic-rich tuff deposits, aa and pahoehoe flows, and three littoral cones. Areal coverage, number of eruptions and average dormant interval estimates in years for the five age groups assigned are: (I) historic, i.e. A D 1790 and younger: 25%, 5, 42.75; (II) 200-400 years old: 50%, 15, 14.3: (III) 400-750 years old: 20%, 54, 6.6; (IV) 750-1500 years old: 5%, 37, 20.8; (V) 1500-3000 years old: <1%, 1, unknown. At least 4.5-6 km3 of tholeiitic basalt have been erupted from the LERZ during the past 1500 years. Estimated volumes of the exposed products of individual eruptions range from a few tens of cubic meters for older units in small kipukas to as much as 0.4 km3 for the heiheiahulu shield. The average dormant interval has been about 13.6 years during the past 1500 years. The most recent eruption occurred in 1961, and the area may be overdue for its next eruption. However, eruptive activity will not resume on the LERZ until either the dike feeding the current eruption on the middle east rift zone extends farther down rift, or a new dike, unrelated to the current eruption, extends into the LERZ. ?? 1992 Springer-Verlag.

  17. Molecular genetic analyses of historical lake sediments from the East African Rift Valley

    NASA Astrophysics Data System (ADS)

    Epp, L. S.; Stoof, K.; Trauth, M. H.; Tiedemann, R.

    2009-04-01

    Ancient DNA research, especially that of environmental samples, has to date focussed mainly on samples obtained from colder regions, owing to better DNA preservation. We explored the potential of using ancient DNA from sediments and sediment cores of shallow lakes in Kenya. These lakes, located in the eastern branch of the East African Rift Valley, are in close proximity, yet display strikingly different hydrological and geological features. Present day lakes range in alkalinity from pH 11 (Lake Elmenteita) to pH 8 (Lake Naivasha), and in depth from less than one meter to 15 meters. Historically they have undergone a number of drastic changes in lake level and environmental conditions, both on geological timescales and during the last centuries. Within this setting we employed molecular genetic methods to study DNA from recent and historic lake sediments, focussing on rotifers and diatoms. We analyzed population and species succession in the alkaline-saline crater lake Sonachi since the beginning of the 19th century, as well as distributions in recent and historic sediments of other lakes of the East African Rift System. To specifically detect diatoms, we developed a protocol using taxon-specific polymerase chain reactions and separation of products by denaturing high performance liquid chromatography (DHPLC). Employing this protocol we retrieved "ancient" DNA from a number of taxonomically diverse organisms, but found diatoms only in sediments younger than approximately 90 years. Using higly specific reactions for rotifers of the genus Brachionus, we tracked species and population succession in Lake Sonachi during the last 200 years. Populations were dominated by a single mitochondrial haplotype for a period of 150 years, and two putatively intraspecific turnovers in dominance occurred. They were both correlated to major environmental changes documented by profound visible changes in sediment composition of the core: the deposition of a volcanic ash and a

  18. Reef growth and volcanism on the submarine southwest rift zone of Mauna Loa, Hawaii

    USGS Publications Warehouse

    Moore, J.G.; Normark, W.R.; Szabo, B. J.

    1990-01-01

    A marine sampling program, utilizing the PISCES-5 submersible operated by the Hawaii Undersea Research Laboratory (NOAA), has confirmed the presence of a major submerged coral reef offshore from Ka Lae (South Point), Hawaii. The top of the reef is now 150-160 m below sea level. Radiocarbon and Useries dating indicates that it drowned about 13.9 ka by the combined effects of island subsidence (2.5 mm/year) and the rapid rise of sea level at the end of the last glaciation so that the relative submergence rate of more than 10 mm/year exceeded the upward growth rate of the reef. The submerged reef caps the offshore part of the southwest rift-zone ridge of Mauna Loa, which has apparently undergone little volcanic activity offshore since 170 ka, and possibly since 270 ka. This fact suggests that rift zone activity is becoming increasingly restricted toward the upper part of the volcano, a condition possibly heralding the end of the shield-building stage. ?? 1990 Springer-Verlag.

  19. The Central Metasedimentary Belt (Grenville Province) as a failed back-arc rift zone: Nd isotope evidence

    NASA Astrophysics Data System (ADS)

    Dickin, A. P.; McNutt, R. H.

    2007-07-01

    Nd isotope data are presented for granitoid orthogneisses from the Central Metasedimentary Belt (CMB) of the Grenville Province in order to map the extent of juvenile Grenvillian-age crust within this orogenic belt that is composed mostly of older crustal terranes. The data reveal a 150 km-wide belt of juvenile crust in Ontario, but this belt contains a block of pre-Grenvillian crust (containing the Elzevir pluton) which yields an estimated crustal formation age of 1.5 Ga. The recognition of an older block within the CMB has profound implications for its structure and tectonic evolution, because it implies that juvenile Grenvillian crust, apparently forming a wide NE-SW belt, is in fact distributed in two narrower segments with approximately N-S strike. We suggest that the CMB comprises an en echelon series of ensimatic rift segments, created by back-arc spreading behind a continental margin arc. These rift segments extend southwards (in the subsurface) into the northeastern Unites States. The rift segments contain abundant marble outcrops, consistent with marine incursion into the rift zone, and these deposits also continue northwards into a 'Marble domain' of the CMB in Quebec. However, crustal formation ages in the latter domain are largely pre-Grenvillian, implying that the Quebec rift segment was ensialic. Hence, we interpret the CMB in Ontario and Quebec as the northern termination of a failed back-arc rift zone.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  1. Evolution of the western East African Rift System reflected in provenance changes of Miocene to Pleistocene synrift sediments (Albertine Rift, Uganda)

    NASA Astrophysics Data System (ADS)

    Schneider, Sandra; Hornung, Jens; Hinderer, Matthias

    2016-08-01

    Miocene to Pleistocene synrift sediments in the Albertine Graben reflect the complex geodynamic evolution in the Western branch of the East African Rift System. In this study we focus on the provenance of these siliciclastic deposits to identify sediment sources and supply paths with the ultimate goal to reconstruct the exhumation history of different tectonic blocks during prolonged rifting, with specific focus on the uplift of the Rwenzori Mountains in Uganda. We present framework and heavy mineral petrographic data combined with varietal studies of detrital garnet and rutile, based on logged sediment sections on the Ugandan side of Lake Albert (Kisegi-Nyabusosi area). The analyzed sedimentary units have a feldspatho-quartzose composition and distinct variations in heavy mineral assemblages and mineral chemical composition indicating two provenance changes. The Miocene part of the stratigraphy is dominated by garnet, zircon, tourmaline and rutile, whereas Pliocene to Pleistocene sediment yields high amounts of less stable amphibole and epidote. An abrupt switch in heavy mineral assemblages occurs during the early Pliocene ( 5.5-5.0 Ma) and clearly postdates the formation of Palaeolake Obweruka at 8 Ma. Provenance signatures point to major sediment supply from the northeast and subsequently from the southeast. We interpret this first shift as transition from the pre-rift to the syn-rift stage. In this scenario, formation of Palaeolake Obweruka is due to higher humidity in the upper Miocene, rather than forced rifting. A second change of sediment composition is documented by mineral geochemistry and coincides with fragmentation of Palaeolake Obweruka starting at 2.5 Ma. Detrital garnet in sediment of Miocene to Pliocene age is rich in pyrope and almandine and calculated Zr-in-rutile temperatures range between 550 and 950 °C. In contrast, garnet occurring in Pleistocene sediment (Nyabusosi Formation) has a higher spessartine component and rutile thermometry is

  2. Aerosolized rift valley fever virus causes fatal encephalitis in african green monkeys and common marmosets.

    PubMed

    Hartman, Amy L; Powell, Diana S; Bethel, Laura M; Caroline, Amy L; Schmid, Richard J; Oury, Tim; Reed, Douglas S

    2014-02-01

    Rift Valley fever (RVF) is a veterinary and human disease in Africa and the Middle East. The causative agent, RVF virus (RVFV), can be naturally transmitted by mosquito, direct contact, or aerosol. We sought to develop a nonhuman primate (NHP) model of severe RVF in humans to better understand the pathogenesis of RVF and to use for evaluation of medical countermeasures. NHP from four different species were exposed to aerosols containing RVFV. Both cynomolgus and rhesus macaques developed mild fevers after inhalation of RVFV, but no other clinical signs were noted and no macaque succumbed to RVFV infection. In contrast, both marmosets and African green monkeys (AGM) proved susceptible to aerosolized RVF virus. Fever onset was earlier with the marmosets and had a biphasic pattern similar to what has been reported in humans. Beginning around day 8 to day 10 postexposure, clinical signs consistent with encephalitis were noted in both AGM and marmosets; animals of both species succumbed between days 9 and 11 postexposure. Marmosets were susceptible to lower doses of RVFV than AGM. Histological examination confirmed viral meningoencephalitis in both species. Hematological analyses indicated a drop in platelet counts in both AGM and marmosets suggestive of thrombosis, as well as leukocytosis that consisted mostly of granulocytes. Both AGM and marmosets would serve as useful models of aerosol infection with RVFV.

  3. Rift zones and magma plumbing system of Piton de la Fournaise volcano: How do they differ from Hawaii and Etna?

    NASA Astrophysics Data System (ADS)

    Michon, Laurent; Ferrazzini, Valérie; Di Muro, Andrea; Villeneuve, Nicolas; Famin, Vincent

    2015-09-01

    On ocean basaltic volcanoes, magma transfer to the surface proceeds by subvertical ascent from the mantle lithosphere through the oceanic crust and the volcanic edifice, possibly followed by lateral propagation along rift zones. We use a 19-year-long database of volcano-tectonic seismic events together with detailed mapping of the cinder cones and eruptive fissures 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 deep plumbing system is rooted between Piton des Neiges and Piton de la Fournaise volcanoes and has a N30-40 orientation. Above 20 km below sea level (bsl), the main axis switches to a N120 orientation, which permits magma transfer from the lithospheric mantle to the base of the oceanic crust, below the summit of Piton de la Fournaise. The related NW-SE rift zone is 15 km wide, linear, spotted by small to large pyroclastic cones and related lava flows and emits slightly alkaline magmas resulting from high-pressure fractionation of clinopyroxene ± olivine. This rift zone has low magma production rate of ~ 0.5-3.6 × 10- 3 m3s- 1 and an eruption periodicity of around 200 years over the last 30 ka. Seismic data suggest that the long-lasting activity of this rift zone result from regional NNE-SSW extension, which reactivates inherited lithospheric faults by the effect of underplating and/or thermal erosion of the mantle lithosphere. The shallow plumbing system (< 11 km bsl) connects the base of the crust with the Central Cone. It is separated from the deep plumbing system by a relatively large aseismic zone between 8 and 11 km bsl, which may represent a deep storage level of magma. The shallow plumbing system feeds frequent, short-lived summit and flank (NE and SE flanks) eruptions along summit and outer rift zones, respectively

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

    NASA Astrophysics Data System (ADS)

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

    2003-04-01

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

  5. Deformation Sources in Kīlauea's Southwest Rift Zone Inferred from the Modeling of Geodetic and Seismic Data

    NASA Astrophysics Data System (ADS)

    Wauthier, C.; Roman, D. C.; Poland, M. P.; Miklius, A.; Fukushima, Y.; Hooper, A. J.; Cayol, V.

    2014-12-01

    For much of the first 20 years of Kīlauea's 1983-present ERZ (East Rift Zone) eruption, deformation was characterized by subsidence at the summit and along both rift zones. We speculate that subsidence of the rift zones was caused by deep rift opening and basal fault slip. A 3D Mixed-Boundary Element model including deep rift-zone opening (running from ~3 to 9 km depth beneath Kīlauea's East and Southwest Rift Zones) as well as slip on the décollement fault that underlies the volcano's south flank (at ~9 km depth) can indeed explain most of the deformation imaged by InSAR data from RADARSAT-1 and JERS-1 spanning two distinct background periods: 1993-1997 and 2000-2003, respectively. At the end of 2003, however, Kīlauea's summit began a 4-year-long period of inflation that culminated in an ERZ dike intrusion and small eruption during 17-19 June 2007—the "Father's Day" (FD) event. On the basis of deformation, seismicity, effusion rate, and lava chemistry and temperature, the FD event was interpreted as the result of forcible intrusion of magma driven by high pressure within the summit magma storage area, as opposed to a passive response to deep rift zone opening. This period of summit inflation is particularly interesting in 2006. According to daily GPS data, two distinct periods can be defined, spanning January to March 2006 and March to end of 2006. A major seismic swarm occurred during the first period while the south caldera area was inflating. The beginning of the second period corresponds to a switch from subsidence to inflation of the SWRZ (Southwest Rift Zone). The SWRZ had been subsiding since the last eruptive episode there in 1974, with the exception of a few dike injections in 1981-82. To investigate the magmatic processes which occurred during 2006 and their implications in terms of the magma plumbing system and local stress field, we integrate contemporary geodetic data from InSAR and GPS with seismic and geologic observations of the SWRZ.

  6. Post pan-african denudation history of southwestern Madagascar during the complex rift-drift evolution of the island: new aspects from titanite and apatite fission track analyses

    NASA Astrophysics Data System (ADS)

    Emmel, B.; Jacobs, J.

    2003-04-01

    Titanite and apatite fission track (FT) thermochronology from 53 basement outcrops in southwest Madagascar reveal a protracted post Pan-African history of extensional tectonism, denudation and sedimentation. The titanite FT ages range between 276 ± 14 Ma and 379 ± 38 Ma. Apatite FT ages vary between 117 ± 26 Ma and 379 ± 19 Ma with mean track length scattering between 11.7 ± 0.59 μm and 13.74 ± 0.21 μm. Combined titanite and apatite FT data were used to calculate denudation rates. Samples from the paleo western margin of Madagascar along the N-S striking Pan-African Ejeda shear zone give above-average denudation rates (100-205 mMa-1) during Carboniferous times. The shear zone was probably reactivated during this times. In contrast the calculated denudation rates for samples from the interior of the island are moderate (25-120 mMa-1). Vitrinite reflectance data from the Sakoa coal area as well as titanite and apatite FT data imply that during the Permo-Triassic rifting, the areas along the paleo western margin that previously underwent fast denudation were buried by a sedimentary cover of up to ˜4.5 km. At this time, a graben developed further inland along the NW-SE striking transcontinental Bongolava-Ranotsara shear zone (BRSZ). Modelled time-temperature paths indicate that the area within the BRSZ remained cool and unaffected since Carboniferous times whereas the samples northeast and southwest of the BRSZ suggest phases of differential cooling during Permian-Triassic times. Seismic data from the Morondava basin indicate that during the Middle Jurassic drift between Madagascar and East-Africa a rift jump towards the west occurred. Modelled time-temperature histories of basement units from the paleo western margin, buried during Permo Triassic times, were exhumed during Jurassic times. This is most probably related with the modified rift kinematics and the associated southwest migration of the margin. Modelled time-temperature paths of all samples from

  7. Implications of historical eruptive-vent migration on the northeast rift zone of Mauna Loa Volcano, Hawaii

    SciTech Connect

    Lockwood, J.P. )

    1990-07-01

    Five times within the past 138 yr (1852, 1855-1856, 1880-1881, 1942, and 1984), lava flows from vents on the northeast rift zone of Mauna Loa Volcano have reached within a few kilometres of Hilo (the largest city on the Island of Hawaii). Most lavas erupted on this right zone in historical time have traveled northeastward (toward Hilo), because their eruptive vents have been concentrated north of the rift zone's broad topographic axis. However, with few exceptions each successive historical eruption on the northeast rift zone has occurred farther southeast than the preceding one. Had the 1984 eruptive vents (the most southeasterly yet) opened less than 200 m farther southeast, the bulk of the 1984 lavas would have flowed away from Hilo. If this historical vent-migration pattern continues, the next eruption on the northeast rift zone could send lavas to the southeast, toward less populated areas. The historical Mauna Loa vent-migration patterns mimic southeastern younging of the Hawaiian-Emperor volcanic chain and may be cryptically related to northwestward movement of the Pacific plate. Systematic temporal-spatial vent-migration patterns may characterize eruptive activity at other volcanoes with flank activity and should be considered as an aid to long-term prediction of eruption sites.

  8. Seismic Anisotropy of the Lithosphere/Asthenosphere System Beneath the Rwenzori Region of the East-African Rift

    NASA Astrophysics Data System (ADS)

    Homuth, B.; Löbl, U.; Batte, A.; Link, K.; Kasereka, C.; Rumpker, G.

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-11-01

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

  10. Velocity structure around the Baikal rift zone from teleseismic and local earthquake traveltimes and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Petit, Carole; Koulakov, Ivan; Deverchère, Jacques

    1998-10-01

    We present new results on the velocity structure of the Baikal rift zone, Asia, deduced from a comparative teleseismic and local tomography analysis. The aim of this paper is to better identify the role of deep mantle processes versus that of far-field tectonic effects on the occurrence of extensional tectonics within a continental plate. We use 36000 traveltimes of P-refracted waves from the ISC catalogues and Pg and Pn traveltimes of 578 earthquakes recorded by the Russian regional network to determine a velocity model by the use of local and teleseismic inversion procedures. The models show that some velocity patterns are continuous from the surface down to at least 400 km. Among them, a narrow negative anomaly goes through Mongolia and follows the southern and eastern margins of the Siberian craton: this structure is interpreted as a thin mantle plume rising beneath the rift axis. However, our results do not evidence any wide asthenospheric upwarp at this place. Other velocity anomalies observed near the surface are not deeply rooted. In particular, a negative anomaly is observed at shallow levels (48 km) beneath the northern third of Lake Baikal, which is disconnected from deeper structures. It may be explained by the existence of underplated magmatic material at the bottom of the crust. By comparing the geometry of deep-rooted anomalies to the present-day stress field patterns, we conclude that the sub-lithospheric mantle dynamics is not the main factor controlling extensional processes in the Baikal rift. However, it does contribute to a thermal weakening of the lithosphere along a mechanical discontinuity bounding the Siberian shield. We finally conclude that three favourable conditions are gathered in the Baikal area to generate extension: far-field extensional stress field, mechanical inherited lithospheric weakness and heat supply. Further studies should help to precise the genetic link between these three factors.

  11. Internal structure of Puna Ridge: evolution of the submarine East Rift Zone of Kilauea Volcano, Hawai ̀i

    NASA Astrophysics Data System (ADS)

    Leslie, Stephen C.; Moore, Gregory F.; Morgan, Julia K.

    2004-01-01

    Multichannel seismic reflection, sonobuoy, gravity and magnetics data collected over the submarine length of the 75 km long Puna Ridge, Hawai ̀i, resolve the internal structure of the active rift zone. Laterally continuous reflections are imaged deep beneath the axis of the East Rift Zone (ERZ) of Kilauea Volcano. We interpret these reflections as a layer of abyssal sediments lying beneath the volcanic edifice of Kilauea. Early arrival times or 'pull-up' of sediment reflections on time sections imply a region of high P-wave velocity ( Vp) along the submarine ERZ. Refraction measurements along the axis of the ridge yield Vp values of 2.7-4.85 km/s within the upper 1 km of the volcanic pile and 6.5-7 km/s deeper within the edifice. Few coherent reflections are observed on seismic reflection sections within the high-velocity area, suggesting steeply dipping dikes and/or chaotic and fractured volcanic materials. Southeastward dipping reflections beneath the NW flank of Puna Ridge are interpreted as the buried flank of the older Hilo Ridge, indicating that these two ridges overlap at depth. Gravity measurements define a high-density anomaly coincident with the high-velocity region and support the existence of a complex of intrusive dikes associated with the ERZ. Gravity modeling shows that the intrusive core of the ERZ is offset to the southeast of the topographic axis of the rift zone, and that the surface of the core dips more steeply to the northwest than to the southeast, suggesting that the dike complex has been progressively displaced to the southeast by subsequent intrusions. The gravity signature of the dike complex decreases in width down-rift, and is absent in the distal portion of the rift zone. Based on these observations, and analysis of Puna Ridge bathymetry, we define three morphological and structural regimes of the submarine ERZ, that correlate to down-rift changes in rift zone dynamics and partitioning of intrusive materials. We propose that these

  12. A common mantle plume source beneath the entire East African Rift System revealed by coupled helium-neon systematics

    NASA Astrophysics Data System (ADS)

    Halldórsson, Sæmundur A.; Hilton, David R.; Scarsi, Paolo; Abebe, Tsegaye; Hopp, Jens

    2014-04-01

    We report combined He-Ne-Ar isotope data of mantle-derived xenoliths and/or lavas from all segments of the East Africa Rift System (EARS). Plume-like helium isotope (3He/4He) ratios (i.e., greater than the depleted MORB mantle (DMM) range of 8 ± 1RA) are restricted to the Ethiopia Rift and Rungwe, the southernmost volcanic province of the Western Rift. In contrast, neon isotope trends reveal the presence of an ubiquitous solar (plume-like) Ne component throughout the EARS, with (21Ne/22Ne)EX values (where (21Ne/22Ne)EX is the air-corrected 21Ne/22Ne ratio extrapolated to Ne-B) as low as 0.034, close to that of solar Ne-B (0.031). Coupling (21Ne/22Ne)EX with 4He/3He ratios indicates that all samples can be explained by admixture between a single mantle plume source, common to the entire rift, and either a DMM or subcontinental lithospheric mantle source. Additionally, we show that the entire sample suite is characterized by low 3He/22NeS ratios (mostly < 0.2)—a feature characteristic of oceanic hot spots such as Iceland. We propose that the origin of these unique noble gas signatures is the deeply rooted African Superplume which influences magmatism throughout eastern Africa. We argue that the Ethiopia and Kenya domes represent two different heads of this common mantle plume source.

  13. Tomographic study of the East African Rift in Mozambique - Initial results

    NASA Astrophysics Data System (ADS)

    Domingues, A.; Chamussa, J. R.; Antunes, E.; Custodio, S.; Silveira, M. M.; Helffrich, G. R.; Ferreira, A. M.; Fonseca, J. F.

    2012-12-01

    Extensively investigated from the Red Sea to southern Tanzania, the East African Rift (EAR) structure is still unknown on its southern tip, Mozambique. The M7 Machaze earthquake of 2006, in central Mozambique, shed new light on the location of the rifting activity, motivating the current initiative. During 2011, project MOZART (funded by FCT, Lisbon, PI J. Fonseca) deployed a network of 30 broadband (120s) seismic stations from the SEIS-UK Pool in central Mozambique and NE South Africa, to investigate the structure of the southernmost section of the EAR through a number of techniques. We present preliminary results of data quality control based on full-waveform modeling, hypocentral locations, and regional structure based on the analysis of ambient noise. Data quality control based on full-waveform modeling relies on comparisons between MOZART waveform data and synthetic seismograms computed with a spectral element method (Komatitsch and Tromp, 2002). These comparisons allow an understanding of how well existing global 3D and 1D Earth models, which were built from independent data, explain MOZART data, and how suitable MOZART data are for waveform tomography. The 3D Earth model synthetics explain the surface wave phases well, but are more limited at matching amplitudes, showing that there is still scope for improvement of the Earth model. The 1D Earth PREM synthetics explain surface wave amplitudes broadly as well as the 3D Earth model, but lead to much poorer phase fits. We will also present preliminary results of the analysis of local seismicity. Local hypocenters are determined using the SEISAN software. The hypocenter inversion is a modified version of HYPOCENTER (Lienert et al., 1986, Lienert,1991, Lienert and Havskov, 1995). The current version uses different seismic phases for earthquake location. The hypocentral locations of small to moderate earthquakes help to identify the most seismically active regions. Finally, we present the first results of the

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

    NASA Astrophysics Data System (ADS)

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

    2000-05-01

    The Baringo Trachyte Fault System is located within the central Kenya Rift and forms part of a N-S-trending linked extensional fault network. This fault system bounds to the west the 8 km deep Baringo Basin which itself lies within the axial valley of the central Kenya Rift. It mainly affects a middle Pleistocene trachytic dome (510 ka), the so-called Baringo Trachyte (BT). A morphotectonic study of the 10 km long BT master fault and associated downthrow geometries provides constraints on the evolution of a magma-type rift fault system from an initial stage of crack opening through to propagation. A model of radial fault growth is proposed in order to account for the longitudinal segmentation of the main fault escarpment from the median part to the tips. The small-scale half-graben geometry developed in the median high-strain zone is progressively accommodated laterally by both flexure and related narrow compensation grabens. The resulting crack swarms are well-developed at the free southern tip zone. Both the spatial distribution of rock-breaking products and their relations to the immediate hangingwall provide further evidence for this hypothesis. Well-developed screes and other gravity-driven structures (slumps) preferentially occur along the median part of the Baringo Trachyte Fault Escarpment, probably as earthquake-induced features. The hangingwall fault zone shows an asymmetrical triangular-shape with a maximum width of about half the length of the main scarp. This zone of maximum deformation and subsidence appears to be laterally controlled by two major, conjugate, transverse basement discontinuities lying with a conjugate geometry. Its internal architecture is dominated by antithetic westerly-dipping normal faults bounding discrete half-grabens, locally infilled by syn-tectonic volcaniclastics. Chronological data on hydrothermal silica filling open cracks on the BT footwall suggest that the master fault evolution occurred from 345 to 198 ka, as the

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

    USGS Publications Warehouse

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

    1989-01-01

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

  16. The influence of oceanic fracture zones on the segmentation of continental margins and the evolution of intra-continental rift systems: Case studies from the Atlantic

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    fracture zones do not extend up to the offshore Angolan and conjugate Brazilian margins, we conclude that small offset transform faulting did not influence the evolution of the continental margin as has been previously suggested. On a regional scale, the evolution of the Africa-wide Mesozoic rift system is intimately linked to global plate tectonics and to changes in plate interactions. On a basinal scale, changes in the orientation of the dominant stress field resulting from plate reorganisation have had a clear impact on the deformation history and fault geometries of rift basins. We demonstrate this relationship by correlating the timing of changes in South Atlantic fracture zone geometries and African margin unconformities with major unconformities that are observed in a unified stratigraphy chart for the West and Central African Rift System. We propose a controlling mechanism in which changes in plate stress control the effective elastic strength of a plate, resulting in a focused change in isostatic response over continental margins.

  17. Kinematics and dynamics of Nubia-Somalia divergence along the East African rift

    NASA Astrophysics Data System (ADS)

    Stamps, Dorothy Sarah

    Continental rifting is fundamental to the theory of plate tectonics, yet the force balance driving Earth's largest continental rift system, the East African Rift (EAR), remains debated. The EAR actively diverges the Nubian and Somalian plates spanning ˜5000 km N-S from the Red Sea to the Southwest Indian Ridge and ˜3000 km NW-SE from eastern Congo to eastern Madagascar. Previous studies suggest either lithospheric buoyancy forces or horizontal tractions dominate the force balance acting to rupture East Africa. In this work, we investigate the large-scale dynamics of Nubia-Somalia divergence along the EAR driving present-day kinematics. Because Africa is largely surrounded by spreading ridges, we assume plate-plate interactions are minimal and that the major driving forces are gradients in gravitational potential energy (GPE), which includes the effect of vertical mantle tractions, and horizontal basal tractions arising from viscous coupling to horizontal mantle flow. We quantify a continuous strain rate and velocity field based on kinematic models, an updated GPS velocity solution, and the style of earthquake focal mechanisms, which we use as an observational constraint on surface deformation. We solve the 3D force balance equations and calculate vertically averaged deviatoric stress for a 100 km thick lithosphere constrained by the CRUST2.0 crustal density and thickness model. By comparing vertically integrated deviatoric stress with integrated lithospheric strength we demonstrate forces arising from gradients in gravitational potential energy are insufficient to rupture strong lithosphere, hence weakening mechanisms are required to initiate continental rupture. The next step involves inverting for a stress field boundary condition that is the long-wavelength minimum energy deviatoric stress field required to best-fit the style of our continuous strain rate field in addition to deviatoric stress from gradients in GPE. We infer the stress field boundary condition

  18. Fault segmentation, deep rift earthquakes and crustal rheology: Insights from the 2009 Karonga sequence and seismicity in the Rukwa-Malawi rift zone

    NASA Astrophysics Data System (ADS)

    Fagereng, Å.

    2013-08-01

    The Rukwa-Malawi rift zone has a record of seismic events down to depths in excess of 30 km, deep for a zone of active continental extension. This deep seismicity, as well as the presence of long (~ 100 km) border faults, has previously been explained by the long-term bulk rheology of intact, old, cold, anhydrous strong crust in east Africa, or the presence of mafic material in the lower crust. The Karonga sequence of 2009 showed a style of faulting different from continuous slip along long border faults, and is interpreted as segmented failure of hanging wall faults. Coulomb stress transfer in this sequence is calculated, and found to be consistent with segmented slip on a fault system synthetic to a nearby border fault and restricted to depths < 12 km. The inferred thermal structure of the Malawi rift indicates that slip at depths in excess of 30 km occurs at temperatures greater than the 350-450 °C commonly inferred at the base of the seismogenic zone. Crustal strength calculations indicate that long border faults and deep seismicity require the presence of a weak zone of localized deformation with increased strain rate (or fluid pressure), within a strong lower crust. A hypothesis is proposed where shallow, segmented frictional failure occurs in regions of relatively strong, intact crust (e.g. the Karonga sequence), whereas long border faults and deep earthquakes are representative of zones of weakness within strong crust. This hypothesis, if correct, implies that seismogenic thickness can vary within thick elastic lithosphere, such that localized weak zones of the crust enable nucleation of larger seismic events, whereas strong, intact crust favors smaller, segmented events and a shallower seismogenic zone.

  19. Clinopyroxene-host disequilibrium (Sr-Nd-Pb isotope systematics) in ultra-potassic magmas from East-African Rift: Implications for magma mixing and source heterogeneity

    NASA Astrophysics Data System (ADS)

    Muravyeva, Natalya; Belyatsky, Boris; Senin, Valeriy

    2014-05-01

    Nd, Pb and Sr isotope ratios have been determined for kamafugite lava and clinopyroxene and phlogopite phenocrysts from Toro-Ankole and Virunga volcanic fields of the East African Rift. The whole rock Sr - Nd isotopic signatures of kamafugites (87Sr/86Sr: 0.70463 - 0.70536; 143Nd/144Nd: 0.51249 - 0.51255) suggest derivation from an EM1-type mantle source. In contrast, Pb isotopic compositions of the same samples (206Pb/204Pb: 19.00 - 19.57; 207Pb/204Pb: 15.69 - 15.74; 208Pb/204Pb: 39.30 - 40.26) reveal a similarity to EM2-type mantle. New Nd, Pb and Sr isotopic data for clinopyroxene (87Sr/86Sr: 0.70473 - 0.70503; 143Nd/144Nd: 0.51250 - 0.51254; 206Pb/204Pb: 18.04 - 18.17; 207Pb/204Pb: 15.58 - 15.60; 208Pb/204Pb: 38.09 - 38.23) suggest derivation from an EM1-like source, and indicate Sr and Pb isotope disequilibrium between clinopyroxene and corresponding host rock. Moreover, clinopyroxenes demonstrating a greater degree of isotopic disequilibrium with their host rock are more sodic in composition. The isotopic disequilibrium is corroborated by the presence of chemical zoning within clinopyroxene, which suggests rapid magma ascent rates preventing melt homogenization. The Pb isotopic ratios for both mineral and corresponding whole rock, together with published data on East African rift-related alkaline centers, define a trend interpreted to represent a mixing line for melts derived from sources such as EM1 and as HIMU. The similar isotopic compositions for clinopyroxene from the different volcanic rocks within the East African Rift suggest the existence of a common, older mantle source for their parental melts. The origin of these melts can be attributed to an enrichment event ~ 400-500 Ma, i.e., significantly prior the younger (Quaternary) ultrapotassic magmatism. Our preferred interpretation for the results reported here involves the mixing of the melts derived from EM1- and HIMU-like sources, which were rapidly transported to the Earth's surface. The primary

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  1. Petrogenesis of lamprophyres from Chhota Udepur area, Narmada rift zone, and its relation to Deccan magmatism

    NASA Astrophysics Data System (ADS)

    Chalapathi Rao, N. V.; Dharma Rao, C. V.; Das, Sanjay

    2012-02-01

    Geological setting, petrology and geochemistry of a new find of lamprophyre dykes, intruding the Precambrian basement gneisses from the Chhota Udepur area, Narmada rift zone, Western India, are presented. Of the three lamprophyre dykes, two of them display E-W trend paralleling that of the Narmada rift, extend up to ˜2.5 km in length and constitute the longest yet recorded lamprophyre dykes from the Indian shield. The Chhota Udepur lamprophyres (CUL) show several overlapping mineralogical and geochemical characteristic features displayed by alkaline- and calc-alkaline lamprophyres and hence it is not straight forward to pigeon-hole them in existing lamprophyre classification schemes. They are porphyritic-panidiomorphic with euhedral to sub-hedral phenocrysts and microphenocrysts of olivine (Fo 78.33-78.42 Fa 21.24-21.37), diopsidic augite to titaniferous augite (Wo 54.83-59.03 En 36.75-41.36 Fs 0.1-5.66), biotite and amphibole (kaersutite). Feldspar (orthoclase perthite and plagioclase) is essentially confined to the groundmass along with accessory phases such as apatite and spinel (ti-magnetite). Rare carbonate ocelli are also present. In terms of major element geochemistry the CUL are nepeheline-normative and predominantly sodic (Na 2O > K 2O); yet their K 2O and SiO 2 contents are sufficiently high to signal traits of calc-alkaline lamprophyres. On the other hand, the CUL exclude negative spikes at Nb-Ta which are considered to be a characteristic of subduction-related (calc-alkaline) magmas, are highly evolved (Mg#: 27.82-55.45), enriched in incompatible trace elements (high La/Nb, Zr/Nb) and have fractionated REE distribution patterns (La/Yb N ratios: 35-44). Post-emplacement alteration and crustal contamination have had little influence on their geochemistry. Th/Yb, Ta/Yb, Nb/U and Ce/Pb ratios of the CUL imply an OIB type-enriched mantle source, similar to that displayed by other Deccan-related lamprophyres. A metasomatised (enriched) garnet lherzolite

  2. Seismic hazard assessment of the Kivu rift segment based on a new sismo-tectonic zonation model (Western Branch of the East African Rift system)

    NASA Astrophysics Data System (ADS)

    Havenith, Hans-Balder; Delvaux, Damien

    2015-04-01

    In the frame of the Belgian GeoRisCA multi-risk assessment project focused on the Kivu and Northern Tanganyika Region, a seismic hazard map has been produced for this area. It is based on a on a recently re-compiled catalogue using various local and global earthquake catalogues. The use of macroseismic epicenters determined from felt earthquakes allowed to extend the time-range back to the beginning of the 20th century, thus spanning about 100 years. The magnitudes have been homogenized to Mw and the coherence of the catalogue has been checked and validated. The seismo-tectonic zonation includes 10 seismic source areas that have been defined on the basis of the regional geological structure, neotectonic fault systems, basin architecture and distribution of earthquake epicenters. The seismic catalogue was filtered by removing obvious aftershocks and Gutenberg-Richter Laws were determined for each zone. On the basis of this seismo-tectonic information and existing attenuation laws that had been established by Twesigomwe (1997) and Mavonga et al. (2007) for this area, seismic hazard has been computed with the Crisis 2012 (Ordaz et al., 2012) software. The outputs of this assessment clearly show higher PGA values (for 475 years return period) along the Rift than the previous estimates by Twesigomwe (1997) and Mavonga (2007) while the same attenuation laws had been used. The main reason for these higher PGA values is likely to be related to the more detailed zonation of the Rift structure marked by a strong gradient of the seismicity from outside the rift zone to the inside. Mavonga, T. (2007). An estimate of the attenuation relationship for the strong ground motion in the Kivu Province, Western Rift Valley of Africa. Physics of the Earth and Planetary Interiors 62, 13-21. Ordaz M, Martinelli F, Aguilar A, Arboleda J, Meletti C, D'Amico V. (2012). CRISIS 2012, Program for computing seismic hazard. Instituto de Ingeniería, Universidad Nacional Autónoma de M

  3. Ambient Noise Tomography of the East African Rift System in Mozambique

    NASA Astrophysics Data System (ADS)

    Domingues, A.; Chamussa, J.; Silveira, G. M.; Custodio, S.; Lebedev, S.; Chang, S.; Ferreira, A. M.; Fonseca, J. F.

    2013-12-01

    A wide range of studies has shown that the cross-correlation of ambient noise can provide an estimate of the Greens functions between pairs of stations. Project MOZART (funded by FCT, Lisbon, PI J. Fonseca) deployed 30 broadband (120s) seismic stations from the SEIS-UK Pool in Central Mozambique and NE South Africa, with the purpose of studying the East African Rift System (EARS) in Mozambique. We applied the Ambient Noise Tomography (ANT) method to broadband seismic data recorded from March 2011 until July 2012. Cross-correlations were computed between all pairs of stations, and from these we obtained Rayleigh wave group velocity dispersion curves for all interstation paths, in the period range from 3 to 50 seconds. We tested various approaches for pre-processing the ambient noise data regarding time-domain and spectral normalisation, as well as the use of phase cross-correlations. Moreover, we examined the robustness of our dispersion maps by splitting our dataset into various sub-sets of Green's functions with similar paths and by quantifying the differences between the dispersion maps obtained from the various sub-sets of data. We find that while the geographical distribution of the group velocity anomalies is well constrained, the amplitudes of the anomalies are slightly less robust. We performed a three-dimensional inversion to obtain the S-wave velocity of the crust and upper mantle. In addition, our preliminary results show a good correlation between the Rayleigh wave group velocity and the geology of Mozambique. In order to extend the investigation to longer periods and, thus, to be able to look into the lithosphere-asthenosphere depth range in the upper mantle, we apply a recent implementation of the surface-wave two-station method (teleseismic interferometry) and augment our dataset with Rayleigh wave phase velocities curves in broad period ranges.

  4. Reactivation of a segmented hyper-extended rift system: the example of the Pamplona transfer zone in the western Pyrenees

    NASA Astrophysics Data System (ADS)

    Lescoutre, Rodolphe; Schaeffer, Frédéric; Masini, Emmanuel; Manatschal, Gianreto

    2016-04-01

    Numerous studies have revealed the importance of rift-inheritance on the formation of orogens but little consideration was given to rift segmentation and the role of transfer zones on the architecture of mountain chains. Indeed, structural mapping of passive margins pointed out the occurrence of a strong variability in the rift architecture along the margin when crossing through peculiar features that represent transfer zones. These transfer zones are generally oriented in the extension direction and relay the deformation between rift segments. The aim of this study is twofold: 1) characterize and define the Pamplona fault system as well as the structures and architecture of the basins bounding this major paleo-transfer fault located in the Western Pyrenees, and 2) understand its role during the subsequent Pyrenean convergence. The influence of the Pamplona fault system on the structuration of the Mauléon basin to the northeast and the Basque-Cantabrian basin to the southwest is substantial as expressed by their large offset and the occurrence of exhumed deep crustal and mantle rocks flooring the two basins. On the one hand, field work in the Labourd Massif and the western termination of the Mauléon basin enabled to describe faults and their relations to sedimentary sequences. This work also allowed describing the formation and reactivation of faults according to their orientation and their activity with respect to key markers (pre-Trias and post-Cenomanian). A strong relationship between rift architecture (proximal to distal domains) and structural inheritance is suggested. On the other hand, preliminary results from fieldwork, literature compilation and new tomographic imaging enable to determine the role and the history of the Pamplona fault system during Late Cretaceous compression. A significant work of this starting PhD project will be to determine the rift structures that have been reactivated and to assess their influence on the final architecture of the

  5. Miocene Onset of Extension in the Turkana Depression, Kenya: Implications for the Geodynamic Evolution of the East African Rift System

    NASA Astrophysics Data System (ADS)

    Boone, S.; Gleadow, A. J. W.; Kohn, B. P.; Seiler, C.

    2015-12-01

    The Paleogene-Recent East African Rift System (EARS) is the foremost modern example of continental rifting, providing much of our understanding of the early stages of continental breakup. The EARS traverses two regions of crustal uplift, the Ethiopian and East African Domes, separated by the Turkana Depression. This wide region of subdued topography coincides with the NW-SE trend of the Jurassic-Paleogene Anza Rift. Opinions on the fundamental geodynamic driver for EARS rifting are divided, however, principally between models involving migrating plume(s) and a single elongated 'superplume'. While competing models have similar topographic outcomes, they predict different morphotectonic evolutions for the Turkana Depression. Models inferring southward plume-migration imply that the plume must have passed below the Turkana Depression during the Paleogene, in order to have migrated to the East African Dome by the Miocene. The possible temporal denudational response to such plume activity is testable using low temperature thermochronology. We present apatite fission track (AFT) and (U-Th)/He (AHe), and zircon (U-Th)/He (ZHe) data from the Lapurr Range, an uplifted Precambrian basement block in northern Turkana. Low radiation damage ZHe results displaying an age range of ~70-210 Ma, and combined with stratigraphic evidence, suggest ~4-6 km of Jurassic-Early Cretaceous denudation, probably associated with early Anza Rift tectonism. AFT ages of ~9-15 Ma imply subsequent burial beneath no more than ~4 km of overburden, thus preserving the Jurassic-Cretaceous ZHe ages. Together with AFT results, AHe data (~3-19 Ma) support ~2-4 km of Miocene-Pliocene uplift of the Lapurr Range in the footwall of the E-dipping Lapurr normal fault. Miocene AFT and AHe ages are interpreted to reflect the initiation of the EARS in the Turkana Depression. If extension is associated with plume activity, then upwelling in the Turkana region is unlikely to have started prior to the Miocene, much

  6. Mesozoic and early Tertiary rift tectonics in East Africa

    NASA Astrophysics Data System (ADS)

    Bosworth, William

    1992-08-01

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

  7. The Lithosphere of The East African Rift System: Insights From Three-Dimensional Density Modelling

    NASA Astrophysics Data System (ADS)

    Woldetinsae, G.; Götze, H. J.

    2004-12-01

    We use the gravity data that cover the large part of the Afro-Arabian rift system, the eastern branch (Ethiopia-Afar and northern Kenya), in order to produce a regional density model. In an earlier work the new and old gravity data were compiled, evaluated and homogenised using a consistent data reduction procedures. Three basic constraints widely spaced over a 1500 km rift length have been generated between 1969 and 2003 by an international consortium with information from isostatic models, global tomography, geological, geochemical evidences, and petrological and experimental results. These are integrated and applied to the model to constrain upper and lower crustal structures underneath the Rift and Plateau areas. New crustal thickness estimations (Dugda et al., 2004 in press) and inferences from recent velocity models along the axis of the Main Ethiopian Rift (Keller et al., 2004) are added to the density model. Thirty parallel planes cutting across the entire plateau region and Rift system (Afar-Ethiopia-Kenya) are interactively modelled using a starting geometry that invoke asthenospheric upwelling. Densities for the upper crust are calculated using Nafe Drake method, averaged from earlier interpretation and measured ones from the Geological Survey of Ethiopia database (e.g. Geothermal project, GSE petrophysical laboratory, pers. communication). Densities for lower crust are estimated using the approach by Sobolov and Babyko (1994). We used also lower crustal densities calculated by (Simyu and Keller, 1997) for the northern part of Kenya rift. The preliminary model offers a possibility to quantify depth, thickness and volumes of different geological interfaces and bodies. As for example, the estimation of the volume of volcanic constructs on the western plateau of Ethiopia is relatively larger than the eastern plateau. The load map derived from the model indicated maximum crustal loads at the crust/mantle interface (ca. 40km) on the eastern and western flanks

  8. Geochemical and isotopic investigations of the Cumbre Vieja rift zone, La Palma (Canary Islands)

    NASA Astrophysics Data System (ADS)

    Johansen, T. S.; Hauff, F.; Hoernle, K.; Kokfelt, T. F.

    2003-04-01

    New major and trace element data, and Sr, Nd, Pb, U, Th and Ra isotopes have been generated for historic and prehistoric lavas from the Cumbre Vieja rift zone on La Palma in the Canary Islands. This is the most active area of the Canary Islands having hosted 7 historical eruptions. The prehistoric volcanism, which extends back to 123 ka, has been extensively age-dated making the Cumbre Vieja an ideal place to investigate the compositional evolution of an ocean island rift zone. The N-S elongated rift zone is traced 20 km on land and another ~15-20 km on its submarine extension towards the south. Our data show that the chemistry of the lavas is highly alkaline and forms a differentiation trend from very primitive basanite (mg# 65-74), via tephrite to phonolite. Along-axis variation of Sr-Nd-Pb isotopic compositions is limited indicating that the melts are derived from a single, relatively homogeneous mantle source, providing good opportunities to investigate melting processes, the timing of differentiation and/or assimilation of the volcanic edifice. Of particular interest is the complex 1585 A.D. eruption, where we discovered composite volcanic bombs that consist of basanitic margins and phonolitic cores. In one case, the phonolite was squeezed through a basanitic lid and cooling cracks in the bomb, providing clear evidence that the basanite and phonolite were both erupted as melts. Altered phonolitic blocks were also found that were partially molten along their edges by basanitic lava, suggesting that the phonolite blocks are older than the lava. The Sr-Nd-Pb isotope ratios of the 1585 A.D. samples overlap within analytical error indicating derivation from a common source and possibly from a common parental melt. The (230Th/238U) disequilibria of the basanites and phonolites are also similar within analytical error whereas the phonolites have distinctively higher 238U/232Th ratios. The higher 238U/232Th isotope ratios in the phonolites could be due to the

  9. Lava Flow Morphologies and Structural Features Along the Axis of the South Rift Zone of Loihi Seamount, Hawaii

    NASA Astrophysics Data System (ADS)

    Deemer, J. L.; Kurz, M. D.; Fornari, D. J.

    2009-12-01

    In an effort to document the morphology of the deep South Rift Zone of the Loihi Seamount, we report new observations collected in 2008 using ROV Jason2 on the R/V Thomas G. Thompson (C. Moyer and K. Edwards, chief scientists). The South Rift Zone extends more than 20 kilometers from 4950 meters depth at its base to Loihi’s summit at 980 meters. To date, there are few studies of the deep Loihi South Rift Zone and this work provides important geologic context for ongoing microbiological studies of Loihi (Fe-Oxidizing Microbial Observatory Project). Existing EM300 multibeam bathymetry provides the broader context necessary for interpreting smaller-scale Loihi South Rift features mapped using Jason2. Three Jason2 dives included continuous low-altitude (<~4 m) video recording of seafloor features in conjunction with high-resolution digital still photography, near-bottom multibeam sonar data over small areas, and geologic sampling. Video and image data were used to create a nearly-continuous geologic profile along the axis of the South Rift. The transect area likely consists of young flows, as indicated by the ubiquitous presence of glassy flows and a complete absence of sediment cover, with the exception of microbial mats in the deepest section (FeMO Deep). Flows are predominantly lobate and pillow types. Pyroclastic deposits are expressed as loose volcanic sand in localized depressions, and are found at depths as great as 4909 meters. Distinct meter-scale flow morphologies were identified and constrained, and this information can be used to determine rudimentary stratigraphic relationships of individual flows. The compilation of lava flow morphologies from the ROV data, in conjunction with along-axis structure, bathymetry, and distribution of talus and pyroclastic deposits, will be presented.

  10. Variations of seismic velocities in the Kachchh rift zone, Gujarat, India, during 2001-2013

    NASA Astrophysics Data System (ADS)

    Mandal, Prantik

    2016-03-01

    We herein study variations of seismic velocities in the main rupture zone (MRZ) of the Mw 7.7 2001 Bhuj earthquake for the time periods [2001-05, 2006-08, 2009-10 and 2011-13], by constructing dVp(%), dVs(%) and d(Vp/Vs)(%) tomograms using high-quality arrival times of 28,902 P- and 28,696 S-waves from 4644 precise JHD (joint hypocentral determination) relocations of local events. Differential tomograms for 2001-05 reveal a marked decrease in seismic velocities (low dVp, low dVs and high d(Vp/Vs)) in the MRZ (at 5-35 km depths) during 2001-10, which is attributed to an increase in crack/fracture density (higher pore fluid pressure) resulted from the intense fracturing that occurred during the mainshock and post-seismic periods. While we observe a slight recovery or increase in seismic velocities 2011-13, this could be related to the healing process (lower pore fluid pressure due to sealing of cracks) of the causative fault zone of the 2001 Bhuj mainshock. The temporal reduction in seismic velocities is observed to be higher at deeper levels (more fluid enrichment under near-lithostatic pressure) than that at shallower levels. Fluid source for low velocity zone (LVZ) at 0-10 km depths (with high d(Vp/Vs)) could be attributed to the presence of meteoric water or soft alluvium sediments with higher water content, while fluid source for LVZ at 10-35 km depths could be due to the presence of brine fluids (released from the metamorphic dewatering) and volatile CO2 (emanating from the crystallization of carbonatite melts in the asthenosphere), in fractures and pores. We also imaged two prominent LVZs associated with the Katrol Hill fault zone and Island Belt fault zone, extending from shallow upper-crust to sub-crustal depth, which might be facilitating the deeper circulation of metamorphic fluids/volatile CO2, thereby, the generation of lower crustal earthquakes occurring in the Kachchh rift zone.

  11. An Episode 56 Perspective on Post-2001 Comagmatic Mixing Along Kilauea's East Rift Zone

    NASA Astrophysics Data System (ADS)

    Thornber, C.; Orr, T.; Lowers, H.; Heliker, C.; Hoblitt, R.

    2007-12-01

    O Hamo eruption (episode 56), as with the January 1997 Napua Crater event (episode 54), the summit deflated and Pu`u O`o collapsed as magma was drawn from either end of the active rift conduit toward a zone of extension. In both cases, magma returned to the Pu`u `O`o vent area after the conduit repressurized. However, in contrast to cool and porphyritic hybrid magma erupted through isolated and chemically evolved rift magma reservoirs at Napau Crater, the episode 56 lava is relatively primitive (8.7 wt% MgO) and 30 to 50°C hotter at 1160°C. This is likely to be summit-derived magma from within the active rift conduit beneath Kane Nui o Hamo. The episode 56 lava is ~15°C hotter than the late episode 55 hybrid magmas with consistently low incompatible elements and likely represents the recharge component that maintained a shallow reservoir at near-cotectic conditions beneath the vicinity of the Pu`u `O`o vents for the last several years. Both lava erupted from Pu`u `O`o in early June, 2007(episode 57), and lava the from the July 21-24 sequence of fissure eruptions down-rift of Pu`u `O`o (early episode 58) contain a distinctly hybrid phenocryst and glomerocryst assemblage, suggesting a flushing of cooler crystal-laden magma from the conduit.

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

    NASA Astrophysics Data System (ADS)

    Ring, Uwe

    2008-08-01

    The >5-km-high Rwenzori Mountains in the East African Rift form a promontory on the rift shoulder and are an extreme expression of rift-mountain uplift. This study reports the pattern and the kinematics of major faults in the Rwenzori region. The fault pattern is characterized by a first set of N/NNE-striking normal faults that are offset by mainly NE- to E-striking faults. Fault-slip data indicate a change in the extension direction from ENE/E to SE. The latter direction is in accord with earthquake focal data and borehole breakouts. Major faults with displacements >6 km occur at the boundaries of the Rwenzori Mountains; within the range minor faults with offsets ≤2 km have been mapped. Flexural isostatic rebound of the footwall of large-displacement back-to-back normal faults bounding the Rwenzori horst and additional footwall uplift of a young crosscutting normal fault probably caused cumulative (surface) uplift of ˜3-4 km. I propose that the change in the kinematics of the rift faults and the largely contemporaneous onset of glaciation in the Rwenzori Mountains in the Middle Pleistocene were important for enhanced rock uplift. Glacial erosion rates were 1.5-4 mm/a and eroded 1-2 km of material off the top of the Rwenzoris. Glacial erosion and the retreat of the glaciers during interglacial periods caused removal of loads leading to isostatic rebound, which reduced horizontal stresses promoting normal faulting and enhanced rock uplift, thereby elevating the remaining terrain, transforming it into one with high peaks and deeply incised valleys.

  13. Coulomb Stress Change and Seismic Hazard of Rift Zones in Southern Tibet after the 2015 Mw7.8 Nepal Earthquake and Its Mw7.3 Aftershock

    NASA Astrophysics Data System (ADS)

    Dai, Z.; Zha, X.; Lu, Z.

    2015-12-01

    In southern Tibet (30~34N, 80~95E), many north-trending rifts, such as Yadong-Gulu and Lunggar rifts, are characterized by internally drained graben or half-graben basins bounded by active normal faults. Some developed rifts have become a portion of important transportation lines in Tibet, China. Since 1976, eighty-seven >Mw5.0 earthquakes have happened in the rift regions, and fifty-five events have normal faulting focal mechanisms according to the GCMT catalog. These rifts and normal faults are associated with both the EW-trending extension of the southern Tibet and the convergence between Indian and Tibet. The 2015 Mw7.8 Nepal great earthquake and its Mw7.3 aftershock occurred at the main Himalayan Thrust zone and caused tremendous damages in Kathmandu region. Those earthquakes will lead to significant viscoelastic deformation and stress changes in the southern Tibet in the future. To evaluate the seismic hazard in the active rift regions in southern Tibet, we modeled the slip distribution of the 2015 Nepal great earthquakes using the InSAR displacement field from the ALOS-2 satellite SAR data, and calculated the Coulomb failure stress (CFS) on these active normal faults in the rift zones. Because the estimated CFS depends on the geometrical parameters of receiver faults, it is necessary to get the accurate fault parameters in the rift zones. Some historical earthquakes have been studied using the field data, teleseismic data and InSAR observations, but results are in not agreement with each other. In this study, we revaluated the geometrical parameters of seismogenic faults occurred in the rift zones using some high-quality coseismic InSAR observations and teleseismic body-wave data. Finally, we will evaluate the seismic hazard in the rift zones according to the value of the estimated CFS and aftershock distribution.

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    Three types of hydrotherms (nitric, carbonaceous and methane) are distinguished within the Baikal Rift Zone. The unloading sites of nitric therms are mostly located in the central and north-eastern parts of the Rift. Several chemical types are found among nitric therms (Pinneker, Pisarsky, Lomonosov, 1968; Lomonosov, 1974, etc.). The formation of terms being various in chemical compositions is associated with effect of several factors, i.e. various chemical, mineralogical compositions of rocks, various temperatures, extent of interaction in water-rock system, etc. The ratio data of water oxygen and hydrogen isotopes of the studied thermal springs indicate that water is largely of meteoric origin. All established ratios of oxygen (δ18OSMOW = -19.5‰ - -17.5‰) and hydrogen (δDSMOW = -155‰ - - 130‰) isotopes are along the line of meteoric waters. Oxygen values from -20‰ to -5‰ are characteristic of the current meteoric and surface waters in the region. The average value equals -16.5‰ in Lake Baikal. By our data, a large group with oxygen lighter isotope composition that corresponds to isotope ratio being specific for glaciers is revealed in fissure-vein waters. Significant shift toward the oxygen getting heavier is observed in some springs. It is mostly observed in the springs that form chemical composition within the area of the intrusive and metamorphic rock distribution. As a result of hydrolysis reaction of alumosilicates, heavy isotope passes from rocks into water molecule, whereas oxygen heavy isotope passes from rocks into solutes during decomposition of carbonates. High contents of fluoride and sulfate-ions are specific feature of the Baikal Rift Zone most nitric therms. Water is tapped in one of the drill holes, where fluoride-ion dominates in its anion composition (46.7 mg/dm3) and pH reaches 10, 12. The sulphate sulphur isotope composition studies carried out allow to conclude that its heavy isotope (δ34SCDT = +25‰ - +30‰) prevails in

  15. Surface wave tomography across Afar, Ethiopia: Crustal structure at a rift triple-junction zone

    NASA Astrophysics Data System (ADS)

    Guidarelli, M.; Stuart, G.; Hammond, J. O. S.; Kendall, J. M.; Ayele, A.; Belachew, M.

    2011-12-01

    The Afar Depression in northeast Africa contains the rift triple-junction between the Nubia, Arabia and Somalia plates. We analyze Rayleigh wave group velocity from 250 regional earthquakes recorded by 40 broadband stations to study the crustal structure across Afar and adjacent plateau regions in northern Ethiopia. The dispersion velocities are inverted to obtain surface wave tomographic maps for periods between 5 and 25 seconds, sensitive to approximately the top 30 km of the lithosphere. The tomographic maps show a significant low dispersion velocity anomaly (>20%) within the upper crust, below the site of recent dyke intrusions (2005-present) in the Dabbahu and Manda-Hararo magmatic segments. Similar low velocity regions are imaged where magma intrusion in the Afar crust has been inferred over the last decade from seismicity or volcanic eruptions. We invert two group velocity curves to compare the S-wave velocity structure of the crust within an active magmatic segment with that of adjacent areas; the active region has a low velocity zone (Vs ˜ 3.2 km/s), between about 6-12 km, which we infer to be due to the presence of partial melt within the lower crust.

  16. The post-Mazama northwest rift zone eruption at Newberry Volcano, Oregon

    USGS Publications Warehouse

    McKay, Daniele; Donnelly-Nolan, Julie M.; Jensen, Robert A.; Champion, Duane E.

    2009-01-01

    The northwest rift zone (NWRZ) eruption took place at Newberry Volcano ~7000 years ago after the volcano was mantled by tephra from the catastrophic eruption that destroyed Mount Mazama and produced the Crater Lake caldera. The NWRZ eruption produced multiple lava flows from a variety of vents including cinder cones, spatter vents, and fissures, possibly in more than one episode. Eruptive behaviors ranged from energetic Strombolian, which produced significant tephra plumes, to low-energy Hawaiian-style. This paper summarizes and in part reinterprets what is known about the eruption and presents information from new and ongoing studies. Total distance spanned by the eruption is 32 km north-south. The northernmost flow of the NWRZ blocked the Deschutes River upstream from the city of Bend, Oregon, and changed the course of the river. Renewed mafic activity in the region, particularly eruptions such as the NWRZ with tephra plumes and multiple lava flows from many vents, would have significant impacts for the residents of Bend and other central Oregon communities.

  17. Constraining the African pole of rotation

    NASA Astrophysics Data System (ADS)

    Asfaw, Laike M.

    1992-08-01

    In the absence of well defined transform faults in the East African rift system for constraining the plate kinematic reconstruction, the pole of relative motion for the African (Nubian) and Somalian plates has been determined from residual motion. If Africa and Somalia are to continue to drift apart along the East African rift system (which would then evolve into a series of ridges offset by transform faults) then incipient transform faults that may reflect the direction of relative motion should already be in place along the East African rift system. The incipient transforms along the East African rift system are characterized by shear zones, such as the Zambezi shear zone in the south and the Aswa and Hamer shear zones in the north. Some of these shear zones have been associated with recent strike-slip faulting in the NW-SE direction during periods of earthquakes. Provided that these, consistently NW-SE oriented, strike-slip movements in the shear zones give the direction of relative motion of the adjacent plates, then they can be used to constrain the position of the Africa-Somalia Euler pole. Due to the fact that identifying transform faults in the East African rift system is difficult and because the genesis of transform faults characterizing a plate boundary at an inception stage is not well known, the discussion here is limited to the northern segment of the East African rift system where shear zones are better characterized by the existing geophysical data. The characterizing features vary with latitude, indicating the complexity of the problem of the genesis of transform faults. I believe, however, that the relatively well defined intra-continental transform fault in the northern East African rift system, which is characterized by strike-slip faulting and earthquakes, constrains the pole of relative motion for the African and Somalian plates to a position near 1.5°S and 29.0°E.

  18. Gravity study of the Central African Rift system: a model of continental disruption 2. The Darfur domal uplift and associated Cainozoic volcanism

    NASA Astrophysics Data System (ADS)

    Bermingham, P. M.; Fairhead, J. D.; Stuart, G. W.

    1983-05-01

    Gravity studies of the Darfur uplift, Western Sudan, show it to be associated with a circular negative Bouguer anomaly, 50 mGal in amplitude and 700 km across. A three-dimensional model interpretation of the Darfur anomaly, using constraints deduced from geophysical studies of similar but more evolved Kenya and Ethiopia domes, suggests either a low-density laccolithic body at mid-lithospheric depth (~ 60 km) or a thinned lithosphere with emplacement at high level of low-density asthenospheric material. The regional setting of the Darfur uplift is described in terms of it being an integral part of the Central African Rift System which is shown to be broadly equivalent to the early to middle Miocene stage in the development of the Afro-Arabian Rift System. Comparisons between these rift systems suggest that extensional tectonics and passive rifting, resulting in the subsiding sedimentary rift basins associated with the Ngaoundere, Abu Gabra, Red Sea and Gulf of Aden rifts, are more typical of the early stage development of passive continental margins than the active domal uplift and development of rifted features associated with the Darfur, Kenya and Ethiopia domes.

  19. Is Kīlauea's East Rift Zone eruption running out of gas?

    NASA Astrophysics Data System (ADS)

    Sutton, A. J.; Elias, T.; Orr, T. R.; Patrick, M. R.; Poland, M. P.; Thornber, C. R.

    2015-12-01

    Gases exsolving from magma are a key force that drives eruptive activity, and emissions from Kīlauea's East Rift Zone (ERZ) dominated the volcano's gas release from the beginning of the long-running and voluminous Pu'u 'Ō'ō eruption in 1983, through February 2008. In the months prior to the March 2008 onset of eruptive activity within Halema'uma'u Crater, however, SO2 degassing at the summit climbed substantially, and summit gas release has remained elevated since. These unprecedented emissions associated with the new summit eruption effectively began robbing gas from magma destined for Kīlauea's ERZ. As a result, ERZ SO2discharge, which had averaged 1,700 +-380 t/d for the previous 15 years, declined sharply and steadily beginning in September, 2008, and reached a new steady low of 380 +- 100 t/d by early 2011. This level persisted through mid-2015. In the years since the late 2008 downturn in ERZ SO2 emissions, there has been an overall slowdown in ERZ eruptive activity. Elevated emissions and effusive activity occurred briefly during the 2011 Kamoamoa fissure eruption and two other outbreaks at Pu'u 'Ō'ō , but otherwise ERZ eruptive activity had waned by 2010, when effusion rates were measured at about half of the long-term rate. Also, the sulfur preserved in ERZ olivine melt-inclusions, which provides a record of pre-eruptive SO2degassing, has steadily declined along with equilibration temperatures of host olivine phenocrysts, since 2008. We suggest that the drop in gas content of magma reaching the ERZ, owing to summit pre-eruptive degassing, has contributed significantly to the downturn in ERZ activity. While SO2 emissions from the ERZ have dropped to sustained levels lower than anything seen in the past 20 years, summit emissions have remained some of the highest recorded since regular measurements began at Kīlauea in 1979. Overall, average total SO2 discharge from Kīlauea in 2014, summit and ERZ, is still about 50% higher than for the 15 years prior

  20. Magma Supply Rate Controls Vigor (And Longevity) of Kīlauea's Ongoing East Rift Zone Eruption

    NASA Astrophysics Data System (ADS)

    Poland, M. P.; Anderson, K. R.

    2015-12-01

    Since 1983, Kīlauea Volcano, Hawai'i, has erupted almost continuously from vents on the East Rift Zone—at 32 years and counting, this is the longest-duration eruption in the past 500 years. Although forecasting the onset of eruptive activity using geophysical, geochemical, and geological monitoring has been demonstrated repeatedly at Kīlauea and elsewhere, little progress has been made in forecasting an eruption's waning or end, particularly in the case of long-lived eruptions. This is especially important at Kīlauea for at least two reasons: (1) caldera formation at the end of another decades-long eruption, in the 15th century, raises the possibility of a link between eruption duration and caldera formation; and (2) long-lived eruptions can have an enduring effect on local population and infrastructure, as demonstrated by the repeated destruction of property by Kīlauea's ongoing rift zone eruption. Data from the past 15 years indicate that the magma supply rate to Kīlauea is an important control on eruptive activity. Joint inversions of geophysical, geochemical, and geological observations demonstrate that in 2006 the supply rate was nearly double that of 2000-2001, resulting in an increase in lava discharge, summit inflation, and the formation of new eruptive vents. In contrast, the magma supply during 2012, and likely through 2014, was less than that of 2000-2001. This lower supply rate was associated with a lower lava discharge and may have played a role in the stalling of lava flows above population centers in the Puna District during 2014-2015. Heightened eruptive vigor may be expected if magma supply increases in the future; however, a further decrease in supply rate—which is likely already below the long-term average—may result in cessation of the eruption. Multidisciplinary monitoring, and particularly tracking of CO2 emissions and surface deformation, should be able to detect changes in supply rate before they are strongly manifested at the

  1. Intraplate magmatism related to opening of the southern Iapetus Ocean: Cambrian Wichita igneous province in the Southern Oklahoma rift zone

    NASA Astrophysics Data System (ADS)

    Hanson, Richard E.; Puckett, Robert E.; Keller, G. Randy; Brueseke, Matthew E.; Bulen, Casey L.; Mertzman, Stanley A.; Finegan, Shane A.; McCleery, David A.

    2013-08-01

    Southern Oklahoma and adjacent parts of Texas contain an extensive igneous province emplaced during Early Cambrian rifting within the Southern Oklahoma rift zone. The rift zone was initiated in association with the opening of the southern Iapetus Ocean during Rodinia supercontinent breakup and later became the site of a series of linked uplifts and basins as a result of late Paleozoic inversion. Igneous rocks within the rift are referred to as the Wichita province and are present mostly in the subsurface, although critical exposures occur in the Wichita and Arbuckle Mountains in southwestern and southern Oklahoma. Wells drilled into basement in the region provide a wealth of information on the distribution and relations of the major igneous units in the upper crust, and geophysical data provide important constraints on deeper levels of the rift zone. The upper parts of the igneous rift fill comprise the Carlton Rhyolite Group, which has an estimated subsurface areal extent of ~ 40,000 km2, and the related Wichita sheet granites, which intrude the lower parts of the rhyolite succession. These rocks have A-type characteristics and were emplaced after intrusion, tilting and uplift of a large tholeiitic layered mafic complex; smaller bodies of gabbro also intruded the layered complex after it was tilted. U-Pb zircon and 40Ar/39Ar geochronology indicates the felsic rocks and at least some of the mafic units were emplaced in a relatively narrow time frame at ~ 539-530 Ma. Basalts and intermediate lavas are present only in the subsurface. Our new work shows these lavas to have tholeiitic to mildly alkaline compositions and to be more extensive than previously realized, forming thick successions that both underlie and are intercalated with the rhyolites. Diabase dikes were intruded into older crust during initial opening of the rift, and a suite of late diabase intrusions also cuts the rhyolites and granites, indicating that mafic magma was supplied to the rift throughout

  2. Strain Localisation at Rift Segment Boundaries: An Example from the Bocana Transfer Zone in Central Baja California, Mexico

    NASA Astrophysics Data System (ADS)

    Seiler, C.; Gleadow, A. J.; Kohn, B. P.

    2012-12-01

    Rifts are commonly segmented into several hundred kilometre long zones of opposing upper-plate transport direction with boundaries defined by accommodation and transfer zones. A number of such rift segments have been recognized in the northern Gulf of California, a youthful oceanic basin that is currently undergoing the rift-drift transition. However, detailed field studies have so far failed to identify suitable structures that could accommodate the obvious deformation gradients between different rift segments, and the nature of strain transfer at segment boundaries remains enigmatic. The situation is even less clear in central and southern Baja California, where a number of rift segments have been hypothesized but it is unknown whether the intervening segment boundaries facilitate true reversals in the upper-plate transport direction, or whether they simply accommodate differences in the timing, style or magnitude of deformation. The Bocana transfer zone (BTZ) in central Baja California is a linear, WNW-ESE striking structural discontinuity separating two rift segments with different magnitudes and styles of extensional deformation. North of the BTZ, the Libertad fault is part of the Main Gulf Escarpment, which represents the breakaway fault that separates the Gulf of California rift to the east from the relatively stable western portion of the Baja peninsula. The N-striking Libertad escarpment developed during the Late Miocene (~10-8Ma) and exhibits a topographic relief of ca. 1,000m along a strike-length of ca. 50km. Finite displacement decreases from ~1000m in the central fault segment to ~500m further south, where the fault bends SE and merges with the BTZ. In the hanging wall of the Libertad fault, a series of W-tilted horsts are bound along their eastern margins by two moderate-displacement E-dipping normal faults. South of the BTZ, extension was much less than further north, which explains the comparatively subdued relief and generally shallower tilt of

  3. Seismotectonics of the transitional region from the Baikal Rift Zone to orogenic rise of the Stanovoi range

    NASA Astrophysics Data System (ADS)

    Ovsyuchenko, A. N.; Trofimenko, S. V.; Marakhanov, A. V.; Karasev, P. S.; Rogozhin, E. A.

    2010-01-01

    This paper is based on the data obtained during the field study of active faults carried out in 2005-2006 in the Chita and Amur oblast and South Yakutia in connection with detailed seismic demarcation of the projected East Siberia-Pacific Ocean pipeline route. The comprehensive geomorphic and geophysical fieldwork was focused on paleoseismogeology and accompanied by trenching in the zones of reactivated faults. These works allowed us to specify the available information on the present-day structure, seismotectonic regime, and potential seismic hazard of the conjugation of the Baikal Rift Zone and the arched-block rise of the Stanovoi Ridge.

  4. Rift initiation with volatiles and magma

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  5. U-series Chronology of volcanoes in the Central Kenya Peralkaline Province, East African Rift

    NASA Astrophysics Data System (ADS)

    Negron, L. M.; Ma, L.; Deino, A.; Anthony, E. Y.

    2012-12-01

    We are studying the East African Rift System (EARS) in the Central Kenya Peralkaline Province (CKPP), and specifically the young volcanoes Mt. Suswa, Longonot, and Menengai. Ar dates by Al Deino on K-feldspar phenocrysts show a strong correlation between older Ar ages and decreasing 230Th/232Th, which we interpret to reflect the age of eruption. This system has been the subject of recent research done by several UTEP alumni including Antony Wamalwa using potential field and magnetotelluric (MT) data to identify and characterize fractures and hydrothermal fluids. Also research on geochemical modeling done by John White, Vanessa Espejel and Peter Omenda led to the hypothesis of possible disequilibrium in these young, mainly obsidian samples in their post eruptive history. A pilot study of 8 samples, (also including W-2a USGS standard and a blank) establish the correlation that was seen between the ages found by Deino along with the 230/232Th ratios. All 8 samples from Mt. Suswa showed a 234U/238U ratio of (1) which indicates secular equilibrium or unity and that these are very fresh samples with no post-eruptive decay or leaching of U isotopes. The pilot set was comprised of four samples from the ring-trench group (RTG) with ages ranging from 7ka-present, two samples from the post-caldera stage ranging from 31-10ka, one sample from the syn-caldera stage dated at 41ka, and one sample from the pre-caldera stage dated at 112ka. The young RTG had a 230/232Th fractionation ratio of 0.8 ranging to the older pre-caldera stage with a 230/232Th ratio of 0.6. From this current data and research of 14C ages by Nick Rogers, the data from Longonot volcano was also similar to the 230/232Th ratio we found. Rogers' data places Longonot volcano ages to be no more than 20ka with the youngest samples also roughly around 0.8 disequilibrium. These strong correlations between the pilot study done for Mt. Suswa, 40Ar ages by Deino, along with 14C ages from Rogers have led to the

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

    USGS Publications Warehouse

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

    1983-01-01

    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.

  7. Crustal rheology and depth distribution of earthquakes: Insights from the central and southern East African Rift System

    NASA Astrophysics Data System (ADS)

    Albaric, Julie; Déverchère, Jacques; Petit, Carole; Perrot, Julie; Le Gall, Bernard

    2009-04-01

    The seismicity depth distribution in the central and southern East African Rift System (EARS) is investigated using available catalogs from local, regional and global networks. We select well-determined events and make a re-assessment of these catalogs, including a relocation of 40 events and, where necessary, a declustering. About 560 events are finally used for determining foci depth distribution within 6 areas of the EARS. Assuming that short-term deformation expressed by seismicity reflects the long-term mechanical properties of the lithosphere, we build yield strength envelopes from seismicity depth distribution. Using brittle and ductile laws, we predict the strength percentage spaced every 5 km (or sometimes 2 km) in the crust, for a given composition and a specific geotherm, and constrain it with the relative abundance of seismicity. Results of this modeling indicate significant local and regional variations of the thermo-mechanical properties of the lithosphere which are broadly consistent with previous studies based on independent modelings. In order to explain relatively deep earthquakes, a highly resistant, mafic lower crust is generally required. We also find evidence for changes in the strength magnitude and in the depth of the brittle-ductile transitions which are clearly correlated to tectonic provinces, characterized by contrasted thermal gradients and basement types. A clear N-S increase and deepening of the peak strength level is evidenced along the eastern branch of the EARS, following a consistent southward migration of rifting since ~ 8 Ma. We also detect the presence of a decoupling layer in the Kenya rift, which suggests persisting influences of the deep crustal structures (Archaean and Proterozoic) on the behavior of the extending crust. More generally, our results suggest that seismicity peaks and cut-off depths may provide good proxies for bracketing the brittle-ductile transitions within the continental crust.

  8. He-Ne-Ar isotope studies of mafic volcanic rocks and mantle xenoliths from the East African Rift System - contrasting isotope signals in different rift branches

    NASA Astrophysics Data System (ADS)

    Halldorsson, S. A.; Hilton, D. R.; Scarsi, P.; Abebe, T.; Massi, K. M.; Barry, P. H.; Fischer, T. P.; de Moor, J.; Rudnick, R. L.

    2010-12-01

    Helium isotope studies of the East African Rift System (EARS) suggest the involvement of a deep mantle plume(s) beneath the northern (Ethiopian) segment [1-3]. The highest 3He/4He (RA) signatures found to date show a close association with the greatest magma volumes erupted since the Early Cenozoic in the region. While the helium isotope characteristics are well established in the Ethiopia-Afar region, Ne and Ar systematics remain poorly constrained. Using a combined He-Ne-Ar isotope approach, our aim is to determine the regional extent of the influence of the Afar plume and to distinguish between subcontinental lithospheric mantle (SCLM) and/or a possible second mantle plume sources located to the south of the Turkana Depression. Xenoliths and mafic lavas from N-Tanzania display a limited range in He isotopes (5-7 RA) with exceptions at Arusha (7.8RA) and Labait (8.7RA), through 7.1-8.7 RA in N-Kenya and S-Ethiopia, to 14.3 RA in the Main Ethiopian Rift and Afar, spanning nearly the entire range of previously reported values. The mean 3He/4He ratio from of lavas and xenoliths from N-Tanzania is remarkably close to the global average of 6.1±0.9 (RA) for continental xenoliths and basalts, thought to represent the SCLM [4]. Thus far, only MORB-like values of 7.3-8.3 RA have been found in volcanics of the Western rift. Initial Ne isotope data reveal the presence of a solar-like Ne component in xenoliths from the Ethiopia-Afar region, with extrapolated 21Ne/22Neex ratios of 0.0365 (assuming Ne-B = 12.5). This trend overlaps that of the Loihi-Kilauea line (L-K). Interestingly, a xenolith from N-Tanzania has a 21Ne/22Neex ratio of 0.0415, falling on a trajectory intermediate between MORB and L-K. The highest 40Ar/36Ar ratio obtained on phenocrysts/xenoliths to date is 1510. The generally low 3He/4He ratios of N-Tanzania likely result from different mixing proportions of asthenospheric sources with lithospheric material, the latter having developed lower 3He/4He ratios

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

    SciTech Connect

    Keller, G.R.

    1996-12-31

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

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

    SciTech Connect

    Keller, G.R. )

    1996-01-01

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

  11. Rift Valley fever virus infection in African Buffalo (Syncerus caffer) herds in rural South Africa: Evidence of interepidemic transmission

    USGS Publications Warehouse

    LaBeaud, A.D.; Cross, P.C.; Getz, W.M.; Glinka, A.; King, C.H.

    2011-01-01

    Rift Valley fever virus (RVFV) is an emerging biodefense pathogen that poses significant threats to human and livestock health. To date, the interepidemic reservoirs of RVFV are not well defined. In a longitudinal survey of infectious diseases among African buffalo during 2000-2006, 550 buffalo were tested for antibodies against RVFV in 820 capture events in 302 georeferenced locations in Kruger National Park, South Africa. Overall, 115 buffalo (21%) were seropositive. Seroprevalence of RVFV was highest (32%) in the first study year, and decreased progressively in subsequent years, but had no detectable impact on survival. Nine (7%) of 126 resampled, initially seronegative animals seroconverted during periods outside any reported regional RVFV outbreaks. Seroconversions for RVFV were detected in significant temporal clusters during 2001-2003 and in 2004. These findings highlight the potential importance of wildlife as reservoirs for RVFV and interepidemic RVFV transmission in perpetuating regional RVFV transmission risk. Copyright ?? 2011 by The American Society of Tropical Medicine and Hygiene.

  12. Trace element and isotope geochemistry of geothermal fluids, East Rift Zone, Kilauea, Hawaii

    SciTech Connect

    West, H.B.; Delanoy, G.A.; Thomas, D.M. . Hawaii Inst. of Geophysics); Gerlach, D.C. ); Chen, B.; Takahashi, P.; Thomas, D.M. Evans and Associates, Redwood City, CA )

    1992-01-01

    A research program has been undertaken in an effort to better characterize the composition and the precipitation characteristic of the geothermal fluids produced by the HGP-A geothermal well located on the Kilauea East Rift Zone on the Island of Hawaii. The results of these studies have shown that the chemical composition of the fluids changed over the production life of the well and that the fluids produced were the result of mixing of at least two, and possibly three, source fluids. These source fluids were recognized as: a sea water composition modified by high temperature water-rock reactions; meteoric recharge; and a hydrothermal fluid that had been equilibrated with high temperature reservoir rocks and magmatic volatiles. Although the major alkali and halide elements show clearly increasing trends with time, only a few of the trace transition metals show a similar trend. The rare earth elements, were typically found at low concentrations and appeared to be highly variable with time. Studies of the precipitation characteristics of silica showed that amorphous silica deposition rates were highly sensitive to fluid pH and that increases in fluid pH above about 8.5 could flocculate more than 80% of the suspended colloidal silica in excess of its solubility. Addition of transition metal salts were also found to enhance the recovery fractions of silica from solution. The amorphous silica precipitate was also found to strongly scavenge the alkaline earth and transition metal ions naturally present in the brines; mild acid treatments were shown to be capable of removing substantial fractions of the scavenged metals from the silica flocs yielding a moderately pure gelatinous by-product. Further work on the silica precipitation process is recommended to improve our ability to control silica scaling from high temperature geothermal fluids or to recover a marketable silica by-product from these fluids prior to reinjection.

  13. Puhimau thermal area: a window into the upper east rift zone of Kilauea Volcano, Hawaii?

    USGS Publications Warehouse

    McGee, K.A.; Sutton, A.J.; Elias, T.; Doukas, M.P.; Gerlach, T.M.

    2006-01-01

    We report the results of two soil CO2 efflux surveys by the closed chamber circulation method at the Puhimau thermal area in the upper East Rift Zone (ERZ) of  volcano, Hawaii. The surveys were undertaken in 1996 and 1998 to constrain how much CO2 might be reaching the ERZ after degassing beneath the summit caldera and whether the Puhimau thermal area might be a significant contributor to the overall CO2 budget of  . The area was revisited in 2001 to determine the effects of surface disturbance on efflux values by the collar emplacement technique utilized in the earlier surveys. Utilizing a cutoff value of 50 g m−2 d−1 for the surrounding forest background efflux, the CO2 emission rates for the anomaly at Puhimau thermal area were 27 t d−1 in 1996 and 17 t d−1 in 1998. Water vapor was removed before analysis in all cases in order to obtain CO2 values on a dry air basis and mitigate the effect of water vapor dilution on the measurements. It is clear that Puhimau thermal area is not a significant contributor to  CO2 output and that most of  CO2 (8500 t d−1) is degassed at the summit, leaving only magma with its remaining stored volatiles, such as SO2, for injection down the ERZ. Because of the low CO2emission rate and the presence of a shallow water table in the upper ERZ that effectively scrubs SO2 and other acid gases, Puhimau thermal area currently does not appear to be generally well suited for observing temporal changes in degassing at  .

  14. Trace element and isotope geochemistry of geothermal fluids, East Rift Zone, Kilauea, Hawaii

    NASA Astrophysics Data System (ADS)

    West, H. B.; Delanoy, G. A.; Thomas, D. M.; Gerlach, D. C.; Chen, B.; Takahashi, P.; Thomas, D. M.

    1992-03-01

    A research program has been undertaken in an effort to better characterize the composition and the precipitation characteristic of the geothermal fluids produced by the HGP-A geothermal well located on the Kilauea East Rift Zone on the island of Hawaii. The results of these studies have shown that the chemical composition of the fluids changed over the production life of the well and that the fluids produced were the result of the mixing of at least two, and possibly three, source fluids. These source fluids were recognized as a sea water composition modified by high temperature water-rock reactions; meteoric recharge; and a hydrothermal fluid that had been equilibriated with high temperature reservoir rocks and magmatic volatiles. Although the major alkali and halide elements show clearly increasing trends with time, only a few of the trace transition metals show a similar trend. The rare earth elements were typically found at low concentrations and appeared to be highly variable with time. Studies of the precipitation characteristics of silica showed that amorphous silica deposition rates were highly sensitive to fluid pH and that increases in fluid pH above about 8.5 could flocculate more than 80 percent of the suspended colloidal silica in excess of its solubility. Addition of transition metal salts were also found to enhance the recovery fractions of silica from solution. The amorphous silica precipitate was also found to strongly scavenge the alkaline earth and transition metal ions naturally present in the brines; mild acid treatments were shown to be capable of removing substantial fractions of the scavenged metals from the silica flocs, yielding a moderately pure gelatinous by-product. Further work on the silica precipitation process is recommended to improve our ability to control silica scaling from high temperature geothermal fluids or to recover a marketable silica by-product from these fluids prior to reinjection.

  15. Numerical reconstruction of Late-Cenosoic evolution of normal-fault scarps in Baikal Rift Zone

    NASA Astrophysics Data System (ADS)

    Byzov, Leonid; San'kov, Vladimir

    2014-05-01

    Numerical landscape development modeling has recently become a popular tool in geo-logic and geomorphic investigations. We employed this technique to reconstruct Late-Cenosoic evolution of Baikal Rift Zone mountains. The objects of research were Barguzin Range and Svyatoy Nos Upland. These structures are formed under conditions of crustal extension and bounded by active normal faults. In our experiments we used instruments, engineered by Greg Tucker (University of Colo-rado) - CHILD (Channel-Hillslope Integrated Landscape Development) and 'Bedrock Fault Scarp'. First program allowed constructing the complex landscape model considering tectonic uplift, fluvial and hillslope processes; second program is used for more accurate simulating of triangular facet evolution. In general, our experiments consisted in testing of tectonic parameters, and climatic char-acteristic, erosion and diffusion properties, hydraulic geometry were practically constant except for some special runs. Numerous experiments, with various scenarios of development, showed that Barguzin range and Svyatoy Nos Upland has many common features. These structures characterized by internal differentiation, which appear in height and shape of slopes. At the same time, individual segments of these objects are very similar - this conclusion refers to most developing parts, with pronounced facets and V-shaped valleys. Accordingly modelling, these landscapes are in a steady state and are undergoing a uplift with rate 0,4 mm/yr since Early Pliocene (this solution accords with AFT-dating). Lower segments of Barguzin Range and Svyatoy Nos Upland also have some general fea-tures, but the reasons of such similarity probably are different. In particular, southern segment of Svyatoy Nos Upland, which characterized by relative high slope with very weak incision, may be formed as result very rapid fault movement or catastrophic landslide. On the other hand, a lower segment of Barguzin Range (Ulun segment, for example

  16. Off-axis magmatism along a subaerial back-arc rift: Observations from the Taupo Volcanic Zone, New Zealand.

    PubMed

    Hamling, Ian J; Hreinsdóttir, Sigrun; Bannister, Stephen; Palmer, Neville

    2016-06-01

    Continental rifting and seafloor spreading play a fundamental role in the generation of new crust. However, the distribution of magma and its relationship with tectonics and volcanism remain poorly understood, particularly in back-arc settings. We show evidence for a large, long-lived, off-axis magmatic intrusion located on the margin of the Taupo Volcanic Zone, New Zealand. Geodetic data acquired since the 1950s show evidence for uplift outside of the region of active extension, consistent with the inflation of a magmatic body at a depth of ~9.5 km. Satellite radar interferometry and Global Positioning System data suggest that there was an increase in the inflation rate from 2003 to 2011, which correlates with intense earthquake activity in the region. Our results suggest that the continued growth of a large magmatic body may represent the birth of a new magma chamber on the margins of a back-arc rift system. PMID:27386580

  17. Off-axis magmatism along a subaerial back-arc rift: Observations from the Taupo Volcanic Zone, New Zealand

    PubMed Central

    Hamling, Ian J.; Hreinsdóttir, Sigrun; Bannister, Stephen; Palmer, Neville

    2016-01-01

    Continental rifting and seafloor spreading play a fundamental role in the generation of new crust. However, the distribution of magma and its relationship with tectonics and volcanism remain poorly understood, particularly in back-arc settings. We show evidence for a large, long-lived, off-axis magmatic intrusion located on the margin of the Taupo Volcanic Zone, New Zealand. Geodetic data acquired since the 1950s show evidence for uplift outside of the region of active extension, consistent with the inflation of a magmatic body at a depth of ~9.5 km. Satellite radar interferometry and Global Positioning System data suggest that there was an increase in the inflation rate from 2003 to 2011, which correlates with intense earthquake activity in the region. Our results suggest that the continued growth of a large magmatic body may represent the birth of a new magma chamber on the margins of a back-arc rift system. PMID:27386580

  18. Off-axis magmatism along a subaerial back-arc rift: Observations from the Taupo Volcanic Zone, New Zealand.

    PubMed

    Hamling, Ian J; Hreinsdóttir, Sigrun; Bannister, Stephen; Palmer, Neville

    2016-06-01

    Continental rifting and seafloor spreading play a fundamental role in the generation of new crust. However, the distribution of magma and its relationship with tectonics and volcanism remain poorly understood, particularly in back-arc settings. We show evidence for a large, long-lived, off-axis magmatic intrusion located on the margin of the Taupo Volcanic Zone, New Zealand. Geodetic data acquired since the 1950s show evidence for uplift outside of the region of active extension, consistent with the inflation of a magmatic body at a depth of ~9.5 km. Satellite radar interferometry and Global Positioning System data suggest that there was an increase in the inflation rate from 2003 to 2011, which correlates with intense earthquake activity in the region. Our results suggest that the continued growth of a large magmatic body may represent the birth of a new magma chamber on the margins of a back-arc rift system.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  20. The Ngorongoro Volcanic Highland and its relationships to volcanic deposits at Olduvai Gorge and East African Rift volcanism.

    PubMed

    Mollel, Godwin F; Swisher, Carl C

    2012-08-01

    The Ngorongoro Volcanic Highland (NVH), situated adjacent and to the east of Olduvai Gorge in northern Tanzania, is the source of the immense quantities of lava, ignimbrite, air fall ash, and volcaniclastic debris that occur interbedded in the Plio-Pleistocene sedimentary deposits in the Laetoli and Olduvai areas. These volcanics have proven crucial to unraveling stratigraphic correlations, the age of these successions, the archaeological and paleontological remains, as well as the source materials from which the bulk of the stone tools were manufactured. The NVH towers some 2,000 m above the Olduvai and Laetoli landscapes, affecting local climate, run-off, and providing varying elevation - climate controlled ecosystem, habitats, and riparian corridors extending into the Olduvai and Laetoli lowlands. The NVH also plays a crucial role in addressing the genesis and history of East African Rift (EAR) magmatism in northern Tanzania. In this contribution, we provide age and petrochemical compositions of the major NVH centers: Lemagurut, basalt to benmorite, 2.4-2.2 Ma; Satiman, tephrite to phonolite, 4.6-3.5 Ma; Oldeani, basalt to trachyandesite, 1.6-1.5 Ma; Ngorongoro, basalt to rhyolite, 2.3-2.0 Ma; Olmoti, basalt to trachyte, 2.0-1.8 Ma; Embagai, nephelinite to phonolite, 1.2-0.6 Ma; and Engelosin, phonolite, 3-2.7 Ma. We then discuss how these correlate in time and composition with volcanics preserved at Olduvai Gorge. Finally, we place this into context with our current understanding as to the eruptive history of the NVH and relationship to East African Rift volcanism. PMID:22404967

  1. The Ngorongoro Volcanic Highland and its relationships to volcanic deposits at Olduvai Gorge and East African Rift volcanism.

    PubMed

    Mollel, Godwin F; Swisher, Carl C

    2012-08-01

    The Ngorongoro Volcanic Highland (NVH), situated adjacent and to the east of Olduvai Gorge in northern Tanzania, is the source of the immense quantities of lava, ignimbrite, air fall ash, and volcaniclastic debris that occur interbedded in the Plio-Pleistocene sedimentary deposits in the Laetoli and Olduvai areas. These volcanics have proven crucial to unraveling stratigraphic correlations, the age of these successions, the archaeological and paleontological remains, as well as the source materials from which the bulk of the stone tools were manufactured. The NVH towers some 2,000 m above the Olduvai and Laetoli landscapes, affecting local climate, run-off, and providing varying elevation - climate controlled ecosystem, habitats, and riparian corridors extending into the Olduvai and Laetoli lowlands. The NVH also plays a crucial role in addressing the genesis and history of East African Rift (EAR) magmatism in northern Tanzania. In this contribution, we provide age and petrochemical compositions of the major NVH centers: Lemagurut, basalt to benmorite, 2.4-2.2 Ma; Satiman, tephrite to phonolite, 4.6-3.5 Ma; Oldeani, basalt to trachyandesite, 1.6-1.5 Ma; Ngorongoro, basalt to rhyolite, 2.3-2.0 Ma; Olmoti, basalt to trachyte, 2.0-1.8 Ma; Embagai, nephelinite to phonolite, 1.2-0.6 Ma; and Engelosin, phonolite, 3-2.7 Ma. We then discuss how these correlate in time and composition with volcanics preserved at Olduvai Gorge. Finally, we place this into context with our current understanding as to the eruptive history of the NVH and relationship to East African Rift volcanism.

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

    NASA Astrophysics Data System (ADS)

    Bosworth, William

    1994-12-01

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

  3. Thick lithosphere, deep crustal earthquakes and no melt: a triple challenge to understanding extension in the western branch of the East African Rift

    NASA Astrophysics Data System (ADS)

    O'Donnell, J. P.; Selway, K.; Nyblade, A. A.; Brazier, R. A.; Tahir, N. El; Durrheim, R. J.

    2016-02-01

    Geodynamic models predict that rifting of thick, ancient continental lithosphere should not occur unless it is weakened by heating and magmatic intrusion. Therefore, the processes occurring along sections of the western branch of the East African Rift, where ˜150 km thick, Palaeoproterozoic lithosphere is rifting with no surface expression of magmatism, are a significant challenge to understand. In an attempt to understand the apparently amagmatic extension we probed the regional uppermost mantle for signatures of thermal alteration using compressional (Vp) and shear (Vs) wave speeds derived from Pn and Sn tomography. Pervasive thermal alteration of the uppermost mantle and possibly the presence of melt can be inferred beneath the Rungwe volcanic centre, but no signatures on a similar scale were discerned beneath amagmatic portions of the western rift branch encompassing the southern half of the Lake Tanganyika rift and much of the Rukwa rift. In this region, Vp and Vs wave speeds indicate little, if any, heating of the uppermost mantle and no studies have reported dyking. Vp/Vs ratios are consistent with typical, melt-free, olivine-dominated upper mantle. Although our resolution limit precludes us from imaging potential localised magmatic intrusions with dimensions of tens of kilometres, the absence of surface volcanism, the amagmatic upper crustal rupture known to have occurred at disparate locations on the western branch, the presence of lower crustal seismicity and the low temperatures implied by the fast seismic wave speeds in the lower crust and uppermost mantle in this region suggests possible amagmatic extension. Most dynamic models predict that this should not happen. Indeed even with magmatic intrusion, rifting of continental lithosphere >100 km thick is considered improbable under conditions found on Earth. Yield strength envelopes confirm that currently modelled stresses are insufficient to produce the observed deformation along these portions of the

  4. Marburg, Ebola and Rift Valley Fever virus antibodies in East African primates.

    PubMed

    Johnson, B K; Gitau, L G; Gichogo, A; Tukei, P M; Else, J G; Suleman, M A; Kimani, R; Sayer, P D

    1982-01-01

    Sera from 464 primates held at four institutes in Kenya were tested by indirect immunofluorescence for the presence of antibodies against Marburg, Ebola, Congo haemorrhagic fever, Rift Valley fever and Lassa viruses. Four of 136 vervet monkeys were positive for Marburg virus antibodies and three of 184 baboons had antibodies against Ebola virus. One baboon was positive for Marburg virus antibodies. Two vervet monkeys, three baboons and one grivet monkey (of 56 tested) had antibodies against Rift Valley fever virus. No Congo or Lassa virus antibodies were detected. A sample of 88 sera of more arboreal primates (Sykes, blue and colobus monkeys) were negative against all five antigens, as were sera from 58 staff members of the institutes who worked with or near the animals.

  5. Deriving spatial patterns from a novel database of volcanic rock geochemistry in the Virunga Volcanic Province, East African Rift

    NASA Astrophysics Data System (ADS)

    Poppe, Sam; Barette, Florian; Smets, Benoît; Benbakkar, Mhammed; Kervyn, Matthieu

    2016-04-01

    The Virunga Volcanic Province (VVP) is situated within the western branch of the East-African Rift. The geochemistry and petrology of its' volcanic products has been studied extensively in a fragmented manner. They represent a unique collection of silica-undersaturated, ultra-alkaline and ultra-potassic compositions, displaying marked geochemical variations over the area occupied by the VVP. We present a novel spatially-explicit database of existing whole-rock geochemical analyses of the VVP volcanics, compiled from international publications, (post-)colonial scientific reports and PhD theses. In the database, a total of 703 geochemical analyses of whole-rock samples collected from the 1950s until recently have been characterised with a geographical location, eruption source location, analytical results and uncertainty estimates for each of these categories. Comparative box plots and Kruskal-Wallis H tests on subsets of analyses with contrasting ages or analytical methods suggest that the overall database accuracy is consistent. We demonstrate how statistical techniques such as Principal Component Analysis (PCA) and subsequent cluster analysis allow the identification of clusters of samples with similar major-element compositions. The spatial patterns represented by the contrasting clusters show that both the historically active volcanoes represent compositional clusters which can be identified based on their contrasted silica and alkali contents. Furthermore, two sample clusters are interpreted to represent the most primitive, deep magma source within the VVP, different from the shallow magma reservoirs that feed the eight dominant large volcanoes. The samples from these two clusters systematically originate from locations which 1. are distal compared to the eight large volcanoes and 2. mostly coincide with the surface expressions of rift faults or NE-SW-oriented inherited Precambrian structures which were reactivated during rifting. The lava from the Mugogo

  6. Hydrocarbon accumulation on rifted Continental Margin - examples of oil migration pathways, west African salt basins

    SciTech Connect

    Blackwelder, B.W.

    1989-03-01

    Examination of the oil fields in the Gabon, Lower Congo, and Cuanza basins allows modeling of oil migration and a more accurate ranking of prospects using geologic risk factors. Oil accumulations in these basins are in strata deposited during Cretaceous rift and drift phases, thus providing a diversity of geologic settings to examine. Oil accumulations in rift deposits are located on large faulted anticlines or in truncated units atop horst features. Many of these oil fields were sourced from adjacent organic shales along short direct migration paths. In Areas where source rock is more remote to fields or to prospective structures, faulting and continuity of reservoir rock are important to the migration of hydrocarbons. Because Aptian salts separate rift-related deposits from those of the drift stage, salt evacuation and faulting of the salt residuum are necessary for oil migration from the pre-salt sequences into the post-salt section. Oil migration within post-salt strata is complicated by the presence of salt walls and faulted carbonate platforms. Hydrocarbon shows in wells drilled throughout this area provide critical data for evaluating hydrocarbon migration pathways. Such evaluation in combination with modeling and mapping of the organic-rich units, maturation, reservoir facies, structural configurations, and seals in existing fields allows assessment of different plays. Based on this information, new play types and prospective structures can be ranked with respect to geologic risk.

  7. Geophysical setting of the Reelfoot Rift and relations between rift structures and the New Madrid seismic zone

    USGS Publications Warehouse

    Hildenbrand, T.G.; Hendricks, J.D.

    1995-01-01

    In the winter of 1811-12, three of the largest historic earthquakes in the United States occurred near New Madrid, Missouri. Seismicity continues to the present day throughout a tightly clustered pattern of epicenters centered on the bootheel of Missouri, including parts of northeastern Arkansas, northwestern Tennessee, western Kentucky, and southern Illinois. In 1990, the New Madrid seismic zone/Central United States became the first seismically active region east of the Rocky Mountains to be designated a priority research area within the National Earthquake Hazards Reduction Program (NEHRP). This Professional Paper is a collection of papers, some published separately, presenting results of the newly intensified research program in this area. Major components of this research program include tectonic framework studies, seismicity and deformation monitoring and modeling, improved seismic hazard and risk assessments, and cooperative hazard mitigation studies.

  8. Rift-zone magmatism: Petrology of basaltic rocks transitional from CFB to MORB, southeastern Brazil margin

    NASA Astrophysics Data System (ADS)

    Fodor, R. V.; Vetter, S. K.

    1984-12-01

    Compositions of basaltic samples from the southeastern Brazil passive margin (18° 24° S) depict the change from continental to oceanic lithosphere during the opening of the South Atlantic Ocean. Samples studied range from 138 to 105 m.y. old and are from 12 Petrobrás drill cores recovered from the coastline to about 150 km offshore in the Espirito Santo, Campos, and Santos basins. Compositions vary, ranging, for example, from 49 54 wt.% SiO2, 0.5 3.0 wt.% TiO2, 0.6 5.0 FeO*/MgO, and <1->6 La/ Yb(n), but can be grouped: (i) basalts enriched in incompatible elements, such as K (some K2O>2.0 wt.%), Rb (>18 ppm), Zr (>120 ppm), and LREE (some FeO* 16 wt.%; most with SiO2 51 54 wt.%), and resembling Serra Geral continental flood basalts (SG-CFB) of southern Brazil; (ii) basalts less enriched, or transitional, in incompatible elements, having K2O <0.40 wt.% and flat REE patterns, and resemble N. Atlantic diabases and FAMOUS basalts; and (iii) one depleted sample, Ce/Yb(n)=0.7, where Ce(n)=4. Expressed in oceanic-basalt terminology and Zr-Nb-Y abundances, ‘enriched’ samples are P- and T-type MORB (e.g., Zr/Nb ˜4 25), ‘transitional’ samples are T-type (Zr/ Nb ˜8 27), and the ‘depleted’ sample is N-type MORB (Zr/Nb>30). Trace-element ratios (e.g., Zr/Nb, Zr/Y) link the Brazil margin basalts to a heterogeneous mantle (attributed to metasomatic veining) of variably proportioned mixtures of depleted-mantle (N-MORB) and plume (P-MORB, e.g., Tristan hotspot) materials. The various compositions therefore reflect, in part, different zones of melting during the separation of Gondwanaland, where gradual decompression during rifting enabled concurrent melting of upper, more depleted (non- or sparsely-veined) mantle and enriched (densely-veined) mantle. Within the time represented, melting produced enriched, transitional, and depleted magmas that were emplaced subaerially, hypabyssally, and subaqueously; they mark the transition from CFB before rifting and separation

  9. Structure of backarc inner rifts as a weakest zone of arc-backarc system: a case study of the Sea of Japan

    NASA Astrophysics Data System (ADS)

    Sato, Hiroshi; Ishiyama, Tasuya; Kato, Naoko; Abe, Susumu; Saito, Hideo; Shiraishi, Kazuya; Abe, Shiori; Iwasaki, Takaya; Inaba, Mitsuru; No, Tetsuo; Sato, Takeshi; Kodaira, Shuichi; Takeda, Tetsuya; Matsubara, Makoto; Kodaira, Chihiro

    2015-04-01

    A backarc inner rift is formed after a major opening of backarc basin near a volcanic front away from the spreading center of a major backarc basin. An obvious example is the inner rift along the Izu-Bonin arc. Similar inner rift zones have been developed along the Sea of Japan coast of Honshu island, Japan. NE and SW Japan arcs experienced strong shortening after the Miocene backarc rifting. The amount of shortening shows its maximum along the backarc inner rifts, forming a fold-and-thrust of thick post-rift sediments over all the structure of backarc. The rift structure has been investigated by onshore-offshore deep seismic reflection/wide-angle reflection surveys. We got continuous onshore-offshore image using ocean bottom cable and collected offshore seismic reflection data using two ships to obtain large offset data in the difficult area for towing a long streamer cable. The velocity structure beneath the rift basin was deduced by refraction tomography in the upper curst and earthquake tomography in the deeper part. It demonstrates larger P-wave velocity in upper mantle and lower crust, suggesting a large amount of mafic intrusion and thinning of upper continental crust. The deeper seismicity in the lower crust beneath the rift basin accords well to the mafic intrusive rocks. Syn-rift volcanism was bimodal, comprising a reflective unit of mafic rocks around the rift axis and a non-reflective unit of felsic rocks near the margins of the basins. Once rifting ended, thermal subsidence, and subsequently, mechanical subsidence related to the onset of the compressional regime, allowed deposition of up to 5 km of post-rift, deep marine to fluvial sedimentation. Continued compression produced fault-related folds in the post-rift sediments, characterized by thin-skin style of deformation. The syn-rift mafic intrusion in the crust forms convex shape and the boundary between pre-rift crust and mafic intrusive shows outward dipping surface. Due to the post rift

  10. Planation surfaces as a record of medium to large wavelength deformation: the example of the Lake Albert Rift (Uganda) on the East African Dome

    NASA Astrophysics Data System (ADS)

    Brendan, Simon; François, Guillocheau; Cécile, Robin; Jean, Braun; Olivier, Dauteuil; Massimo, Dall'Asta

    2016-04-01

    African relief is characterized by planation surfaces, some of them of continental scale. These surfaces are slightly deformed according to different wavelengths (x10 km; x100 km, x1000 km) which record both mantle dynamics (very long wavelength, x 1000 km) and lithosphere deformation (long wavelength deformation, x 100 km). Different types of these planation surfaces are recognized: - Etchplains capped by iron-duricrust which correspond to erosional nearly flat weathered surfaces resulting from the growth of laterites under warm and humid conditions. - Pediments which define mechanical erosional surfaces with concave or rectilinear profiles delimited by upslope scarps connected upstream with the upper landforms. We here focused on the Lake Albert Rift at the northern termination of the western branch of the East African Rift System of which the two branches are surimposed on the East-African Dome. Different wavelengths of deformation were characterized based on the 3D mapping of stepped planation surfaces: (1) very long wavelength deformations resulting from the uplift of the East African Dome; (2) long wavelength deformations resulting from the opening of the eastern branch and (3) medium wavelength deformations represented by the uplift of rift shoulders like the Rwenzori Mountains. The paleo-landscape reconstruction of Uganda shows the existence of four generations of landforms dated according to their geometrical relationships with volcanic rocks. A four stepped evolution of the Ugandan landforms is proposed: • 70 - 22 Ma: generation of two weathered planation surfaces (etchplain Uw and Iw). The upper one (Uw) records a very humid period culminating at time of the Early Eocene Climatic Optimum (70-45 Ma). It corresponds to the African Surface. A first uplift of the East African Dome generates a second lower planation surface (Iw) connected to the Atlantic Ocean base level; • 17-2.7 Ma: planation of large pediplains connected to the local base level induced

  11. An inventory survey at the site of the proposed Kilauea Middle East Rift Zone (KMERZ), Well Site No. 2

    SciTech Connect

    Kennedy, Joseph

    1991-03-01

    At the request of True Mid Pacific Geothermal, Archaeological Consultants of Hawaii, Inc. has conducted an inventory survey at the site of the proposed Kilauea Middle East Rift Zone (KMERZ), Well Site No.2, TMK: 1-2-10:3. The Principal Investigator was Joseph Kennedy M.A., assisted by Jacob Kaio, Field Supervisor and field crew Mark Borrello B.A., Michael O'Shaughnessy B.A., and Randy Adric. This report supercedes all previous reports submitted to the Historic Presentation Section of the Department of Land and Natural Resources. In addition to 100% surface coverage of the 400 x 400 foot well pad itself, 100% surface coverage of a substantial buffer zone was also completed. This buffer zone was established by the Department of Land and Natural Resources, Historic Preservation personnel and extends 1000 feet east and west of the well site and 500 feet north and south of the well site.

  12. Seismological Investigations of Crustal and Mantle Structures Beneath the Incipient Okavango Rift

    NASA Astrophysics Data System (ADS)

    Gao, S. S.; Yu, Y.; Liu, K. H.; Reed, C. A.; Moidaki, M.; Mickus, K. L.; Atekwana, E. A.

    2015-12-01

    Rifting plays a significant role in the evolution of sedimentary basins. However, our current understandings on rifting mechanisms are mostly based on studies of mature rifts. Here we report results from the first teleseismic investigations of the incipient Okavango rift zone (ORZ), which is located at the southwestern terminal of the East African Rift System in northern Botswana. Data used in the study were recorded by the 17 broadband seismic stations deployed along a NW-SE profile traversing the ORZ with a recording duration of 2 years starting in the summer of 2012. Receiver function and shear wave splitting techniques have been employed to explore upper mantle thermal anomalies and anisotropy. The resulting dominantly absolute plate motion-parallel fast polarization orientations and normal mantle transition zone thickness ruled out the possible existence of one or more mantle plumes in the upper mantle or mantle transition zone beneath the ORZ. The Moho beneath the Okavango rift zone is uplifted by 4-5 km and is symmetric with regard to the rift axis, favoring a pure shear model of early-stage continental extension. The observations favor a passive model for rift initiation in which rifts develop inside ancient orogenic zones as the result of relative movements between Archean cratonic blocks.

  13. East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift

    NASA Astrophysics Data System (ADS)

    Garcin, Yannick; Melnick, Daniel; Strecker, Manfred R.; Olago, Daniel; Tiercelin, Jean-Jacques

    2012-05-01

    The 'wet' early to mid-Holocene of tropical Africa, with its enhanced monsoon, ended with an abrupt shift toward drier conditions and was ultimately replaced by a drier climate that has persisted until the present day. The forcing mechanisms, the timing, and the spatial extent of this major climatic transition are not well understood and remain the subject of ongoing research. We have used a detailed palaeo-shoreline record from Lake Turkana (Kenya) to decipher and characterise this marked climatic transition in East Africa. We present a high-precision survey of well-preserved palaeo-shorelines, new radiocarbon ages from shoreline deposits, and oxygen-isotope measurements on freshwater mollusk shells to elucidate the Holocene moisture history from former lake water-levels in this climatically sensitive region. In combination with previously published data our study shows that during the early Holocene the water-level in Lake Turkana was high and the lake overflowed temporarily into the White Nile drainage system. During the mid-Holocene (~ 5270 ± 300 cal. yr BP), however, the lake water-level fell by ~ 50 m, coeval with major episodes of aridity on the African continent. A comparison between palaeo-hydrological and archaeological data from the Turkana Basin suggests that the mid-Holocene climatic transition was associated with fundamental changes in prehistoric cultures, highlighting the significance of natural climate variability and associated periods of protracted drought as major environmental stress factors affecting human occupation in the East African Rift System.

  14. Evidences of a lithospheric fault zone in the Sicily Channel continental rift (southern Italy) from instrumental seismicity data

    NASA Astrophysics Data System (ADS)

    Calò, M.; Parisi, L.

    2014-10-01

    Sicily Channel is a portion of Mediterranean Sea, between Sicily (Southern Italy) and Tunisia, representing a part of the foreland Apennine-Maghrebian thrust belt. The seismicity of the region is commonly associated with the normal faulting related to the rifting process and volcanic activity of the region. However, certain seismic patterns suggest the existence of some mechanism coexisting with the rifting process. In this work, we present the results of a statistical analysis of the instrumental seismicity and a reliable relocalization of the events recorded in the last 30 yr in the Sicily Channel and western Sicily using the Double Difference method and 3-D Vp and Vs tomographic models. Our procedure allows us to discern the seismic regime of the Sicily sea from the Tyrrhenian one and to describe the main features of an active fault zone in the study area that could not be related to the rifting process. We report that most of the events are highly clustered in the region between 12.5°-13.5°E and 35.5°-37°N with hypocentral depth of 5-40 km, and reaching 70 km depth in the southernmost sector. The alignment of the seismic clusters, the distribution of volcanic and geothermal regions and the location of some large events occurred in the last century suggest the existence of a subvertical shear zone extending for least 250 km and oriented approximately NNE-SSW. The spatial distribution of the seismic moment suggests that this transfer fault zone is seismically discontinuous showing large seismic gaps in proximity of the Ferdinandea Island, and Graham and Nameless Bank.

  15. Stable isotope-based Plio-Pleistocene ecosystem reconstruction of some of the earliest hominid fossil sites in the East African Rift System (Chiwondo Beds, N Malawi)

    NASA Astrophysics Data System (ADS)

    Lüdecke, Tina; Thiemeyer, Heinrich; Schrenk, Friedemann; Mulch, Andreas

    2014-05-01

    The isotope geochemistry of pedogenic carbonate and fossil herbivore enamel is a powerful tool to reconstruct paleoenvironmental conditions in particular when climate change plays a key role in the evolution of ecosystems. Here, we present the first Plio-Pleistocene long-term carbon (δ13C), oxygen (δ18O) and clumped isotope (Δ47) records from pedogenic carbonate and herbivore teeth in the Malawi Rift. These data represent an important southern hemisphere record in the East African Rift System (EARS), a key region for reconstructing vegetation patterns in today's Zambezian Savanna and correlation with data on the evolution and migration of early hominids across the Inter-Tropical Convergence Zone. As our study site is situated between the well-known hominid-bearing sites of eastern and southern Africa in the Somali-Masai Endemic Zone and Highveld Grassland it fills an important geographical gap for early hominid research. 5.0 to 0.6 Ma fluviatile and lacustrine deposits of the Chiwondo Beds (NE shore of Lake Malawi) comprise abundant pedogenic carbonate and remains of a diverse fauna dominated by large terrestrial mammals. These sediments are also home to two hominid fossil remains, a mandible of Homo rudolfensis and a maxillary fragment of Paranthropus boisei, both dated around 2.4 Ma. The Chiwondo Beds therefore document early co-existence of these two species. We evaluate δ13C data from fossil enamel of different suid, bovid, and equid species and contrast these with δ13C and δ18O values of pedogenic carbonate. We complement the latter with clumped isotope soil temperature data. Results of almost 800 pedogenic carbonate samples from over 20 sections consistently average δ13C = -8.5 ‰ over the past 5 Ma with no significant short-term δ13C excursions or long-term trends. The data from molar tooth enamel of nine individual suids of the genera Metridiochoerus, Notochoerus and Nyanzachoerus support these findings with average δ13C = -10.0 ‰. The absence

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

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

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

  17. Selected time-lapse movies of the east rift zone eruption of KĪlauea Volcano, 2004–2008

    USGS Publications Warehouse

    Orr, Tim R.

    2011-01-01

    Since 2004, the U.S. Geological Survey's Hawaiian Volcano Observatory has used mass-market digital time-lapse cameras and network-enabled Webcams for visual monitoring and research. The 26 time-lapse movies in this report were selected from the vast collection of images acquired by these camera systems during 2004–2008. Chosen for their content and broad aesthetic appeal, these image sequences document a variety of flow-field and vent processes from Kīlauea's east rift zone eruption, which began in 1983 and is still (as of 2011) ongoing.

  18. Lava Flow Hazard Assessment, as of August 2007, for Kilauea East Rift Zone Eruptions, Hawai`i Island

    USGS Publications Warehouse

    Kauahikaua, Jim

    2007-01-01

    The most recent episode in the ongoing Pu'u 'O'o-Kupaianaha eruption of Kilauea Volcano is currently producing lava flows north of the east rift zone. Although they pose no immediate threat to communities, changes in flow behavior could conceivably cause future flows to advance downrift and impact communities thus far unaffected. This report reviews lava flow hazards in the Puna District and discusses the potential hazards posed by the recent change in activity. Members of the public are advised to increase their general awareness of these hazards and stay up-to-date on current conditions.

  19. Basaltic Martian analogues from the Baikal Rift Zone and Mongolian terranes

    NASA Astrophysics Data System (ADS)

    Gurgurewicz, J.; Kostylew, J.

    2007-08-01

    In order to compare the results of studies of the western part of the Valles Marineris canyon on Mars there have been done field works on terrestrial surface areas similar with regard to geological setting and environmental conditions. One of the possible terrestrial analogues of the Valles Marineris canyon is the Baikal Rift Zone [1]. Field investigations have been done on the south end of the Baikal Lake, in the Khamar-Daban massif, where the outcrops of volcanic rocks occur. The second part of the field works has been done in the Mongolian terranes: Mandalovoo, Gobi Altay and Bayanhongor, because of environmental conditions being similar to those on Mars. The Mandalovoo terrane comprises a nearly continuous Paleozoic islandarc sequence [2]. In the Gobi Altay terrane an older sequence is capped by younger Devonian-Triassic volcanic-sedimentary deposits [2]. The Bayanhongor terrane forms a northwest-trending, discontinuous, narrow belt that consists of a large ophiolite allochton [3]. The collected samples of basalts derive from various geologic environments. The CORONA satellite-images have been used for the imaging of the Khamar-Daban massif and the Mandalovoo terrane. These images have the same spatial resolution and range as the Mars Orbiter Camera images of the Mars Global Surveyor mission. In the Mandalovoo terrane these images allowed to find an area with large amounts of tectonic structures, mainly faults (part of the Ongi massif), similar to the studied area on Mars. Microscopic observations in thin sections show diversification of composition and structures of basalts. These rocks have mostly a porphyric structure, rarely aphyric. The main components are plagioclases, pyroxenes and olivines phenocrysts, in different proportions. The groundmass usually consist of plagioclases, pyroxenes and opaques. The most diversified are basalts from the Mandalovoo terrane. Infrared spectroscopy has been used to analyse the composition of the rock material and compare

  20. Hydrothermal petroleum from lacustrine sedimentary organic matter in the East African Rift.

    PubMed

    Simoneit, B R; Aboul-Kassim, T A; Tiercelin, J J

    2000-03-01

    Cape Kalamba oil seeps occur at the south end of the Ubwari Peninsula, at the intersection of faults controlling the morphology of the northern basin of the Tanganyika Rift, East Africa. Oil samples collected at the surface of the lake 3-4 km offshore from Cape Kalamba have been studied. The aliphatic hydrocarbon and biomarker compositions, with the absence of the typical suite of polynuclear aromatic hydrocarbons, indicate an origin from hydrothermal alteration of immature microbial biomass in the sediments. These data show a similarity between a tar sample from the beach and the petroleum from the oil seeps, and confirm that the source of these oils is from organic matter consisting mainly of bacterial and degraded algal biomass, altered by hydrothermal activity. The compositions also demonstrate a < 200 degrees C temperature for formation/generation of this hydrothermal petroleum, similar to the fluid temperature identified for the Pemba hydrothermal site located 150 km north of Cape Kalamba. The 14C age of 25.6 ka B.P. obtained for the tar ball suggests that Pleistocene lake sediments could be the source rock. Hydrothermal generation may have occurred slightly before 25 ka B.P., during a dry climatic environment, when the lake level was lower than today. These results also suggest that the Cape Kalamba hydrothermal activity did not occur in connection with an increased flux of meteoric water, higher water tables and lake levels as demonstrated in the Kenya Rift and for the Pemba site. Hydrothermal petroleum formation is a facile process also in continental rift systems and should be considered in exploration for energy resources in such locales. PMID:17654787

  1. Hydrothermal petroleum from lacustrine sedimentary organic matter in the East African Rift.

    PubMed

    Simoneit, B R; Aboul-Kassim, T A; Tiercelin, J J

    2000-03-01

    Cape Kalamba oil seeps occur at the south end of the Ubwari Peninsula, at the intersection of faults controlling the morphology of the northern basin of the Tanganyika Rift, East Africa. Oil samples collected at the surface of the lake 3-4 km offshore from Cape Kalamba have been studied. The aliphatic hydrocarbon and biomarker compositions, with the absence of the typical suite of polynuclear aromatic hydrocarbons, indicate an origin from hydrothermal alteration of immature microbial biomass in the sediments. These data show a similarity between a tar sample from the beach and the petroleum from the oil seeps, and confirm that the source of these oils is from organic matter consisting mainly of bacterial and degraded algal biomass, altered by hydrothermal activity. The compositions also demonstrate a < 200 degrees C temperature for formation/generation of this hydrothermal petroleum, similar to the fluid temperature identified for the Pemba hydrothermal site located 150 km north of Cape Kalamba. The 14C age of 25.6 ka B.P. obtained for the tar ball suggests that Pleistocene lake sediments could be the source rock. Hydrothermal generation may have occurred slightly before 25 ka B.P., during a dry climatic environment, when the lake level was lower than today. These results also suggest that the Cape Kalamba hydrothermal activity did not occur in connection with an increased flux of meteoric water, higher water tables and lake levels as demonstrated in the Kenya Rift and for the Pemba site. Hydrothermal petroleum formation is a facile process also in continental rift systems and should be considered in exploration for energy resources in such locales.

  2. Neotectonic faults and stress field in the East African Rift System around the Tanzanian Craton - A contribution to the seismotectonic map of Africa

    NASA Astrophysics Data System (ADS)

    Delvaux, Damien; Macheyeki, Athanas Simon; Fernandes, Rui-Manuel; Ayele, Atalay; Meghraoui, Mustapha

    2015-04-01

    As a contribution to the UNESCO-IUGS IGCP 601 project "Seismotectonics and seismic hazards in Africa" and in preparation of the Seismotectonic Map of Africa, we compiled the neotectonic faults related to the East African Rift System around the Tanzanian craton. The initial aim was to identify and map the potentially active faults. Faults are usually defined as active when they show seismogenic displacement during the last 10,000 to 100,000 years, generally on the basis of paleoseismic investigation. In East Africa, however, very few faults have been studied by paleoseismic techniques and even fewer have known historical seismic activation. To address this issue, we mapped faults that show morphological indications of displacement. We used the SRTM DTM (90 and 30 m when available to us), with artificial shading as basis for identify neotectonic faults, in combination with existing data from geological maps, publications and reports, complemented by our own field observations. Thermal springs often occur along tectonically active faults. We use them to distinguish present-day faulting from other mapped faults as they are in most cases structurally controlled. In parallel, we used also the available focal mechanisms and geological fault-slip data to constrain the stress second-order stress field (at the scale of rift segments) and locally also the third-order stress field (at the local scale). All these elements are combined and compared with existing kinematic models for the East African Rift based on earthquake slip vectors, GPS measurements and geologic indicators. The comparison evidences some local discrepancies between the stress field and the direction of opening, probably due to the interactions between different rift segments, as in the Rukwa rift, Mbeya southern junction between the eastern and western rift branches, and in the Manyara-Natron area.

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

    NASA Astrophysics Data System (ADS)

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

    2004-07-01

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

  4. Regional framework, structural and petroleum aspects of rift basins in Niger, Chad and the Central African Republic (C.A.R.)

    NASA Astrophysics Data System (ADS)

    Genik, G. J.

    1992-10-01

    This paper overviews the regional framework, tectonic, structural and petroleum aspects of rifts in Niger, Chad and the C.A.R. The data base is from mainly proprietary exploration work consisting of some 50,000 kilometres of seismic profiles, 50 exploration wells, one million square kilometres of aeromagnetics coverage and extensive gravity surveys. There have been 13 oil and two oil and gas discoveries. A five phased tectonic history dating from the Pan African orogeny (750-550 MY B.P.) to the present suggests that the Western Central African Rift System (WCAS) with its component West African Rift Subsystem (WAS) and Central African Subsystem (CAS) formed mainly by the mechanical separation of African crustal blocks during the Early Cretaceous. Among the resulting rift basins in Niger, Chad and the C.A.R., seven are in the WAS—Grein, Kafra, Tenere. Tefidet, Termit, Bongor, and N'Dgel Edgi and three, Doba, Doseo, and Salamat are in the CAS. The WAS basins in Niger and Chad are all extensional and contain more than 14,000 m of continental to marine Early Cretaceous to Recent clastic sediments and minor amounts of volcanics. Medium to light oil (20° API-46° API) and gas have been discovered in the Termit basin in reservoir, source and seal beds of Late Cretaceous and Palaeogene age. The most common structural styles are extensional normal fault blocks and transtensional synthetic and antithetic normal fault blocks. The CAS Doba, Doseo and Salamat are extensional to transtensional rift basins containing up to 7500 m of terrestrial mainly Early Cretaceous clastics. Heavy to light oil (15°-39° API) and gas have been discovered in Doba and Doseo basins. Source rocks are Early Cretaceous lacustrine shales, whereas reservoirs and seals are both Early and Late Cretaceous. Dominant structural styles are extensional and transtensional fault blocks, transpressional anticlines and flower structures. The existence of a total rift basin sediment volume of more than one

  5. Diverse Eruptions at Approximately 2,200 Years B.P. on the Great Rift, Idaho: Inferences for Magma Dynamics Along Volcanic Rift Zones

    NASA Technical Reports Server (NTRS)

    Hughes, S. S.; Nawotniak, S. E. Kobs; Borg, C.; Mallonee, H. C.; Purcell, S.; Neish, C.; Garry, W. B.; Haberle, C. W.; Lim, D. S. S.; Heldmann, J. L.

    2016-01-01

    Compositionally and morphologically diverse lava flows erupted on the Great Rift of Idaho approximately 2.2 ka (kilo-annum, 1000 years ago) during a volcanic "flare-up" of activity following an approximately 2 ky (kiloyear, 1000 years) hiatus in eruptions. Volcanism at Craters of the Moon (COTM), Wapi and Kings Bowl lava fields around this time included primitive and evolved compositions, separated over 75 kilometers along the approximately 85 kilometers-long rift, with striking variability in lava flow emplacement mechanisms and surface morphologies. Although the temporal associations may be coincidental, the system provides a planetary analog to better understand magma dynamics along rift systems, including that associated with lunar floor-fractured craters. This study aims to help bridge the knowledge gap between ancient rift volcanism evident on the Moon and other terrestrial planets, and active rift volcanism, e.g., at Hawai'i and Iceland.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  7. Overlapping rift zones at the 5.5°S offset of the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Lonsdale, Peter

    1983-11-01

    A Seabeam and magnetometer survey of the Pacific-Nazca plate boundary around 5.5°S mapped a "nontransform offset" whose geology and kinematics seem typical of a whole class of structures formed where fast-spreading rises are laterally offset for distances less than the width of a subaxial magma chamber. Though the spreading center is abruptly displaced by 15 km right laterally, there is no trace of strike-slip faulting. Instead the ends of two 100 km-long axial volcanic ridges veer 15° toward each other and overlap for more than 20 km. In this overlapping region, between 5°22'S and 5°32'S, accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one-sided spreading. A continuous but dog-legged magma chamber is inferred to underlie both rift zones and an intervening 9-km wide volcano-studded basin. A strong magnetic signature from the crust, which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank; in the past 0.5×106 years the southern axial ridge has propagated north at 40 mm/yr, 55% of the spreading half-rate.

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    The East African rift valley is a major tectonic feature that shapes Central Africa and defines linear-shaped lowlands between highland ranges due to the action of geologic faults associated to earthquakes and volcanism. The region of interest, covering the Virunga Volcanic Province in eastern DRC, western Rwanda and Burundi, and southwest Uganda, is threatened by a rare combination of several types of geohazards, while it is also one of the most densely populated region of Africa. These geohazards can globally be classified as seismic, volcanic and landslide hazards. Landslides, include a wide range of ground movements, such as rock falls, deep failure of slopes and shallow debris flows. Landslides are possibly the most important geohazard in terms of recurring impact on the populations, causing fatalities every year and resulting in structural and functional damage to infrastructure and private properties, as well as serious disruptions of the organization of societies. Many landslides are observed each year in the whole region, and their occurrence is clearly linked to complex topographic, lithologic and vegetation signatures coupled with heavy rainfall events, which is the main triggering factor. The source mechanisms underlying landslide triggering and dynamics in the region of interest are still poorly understood, even though in recent years, some progress has been made towards appropriate data collection. Taking into account difficulties of field accessibility, we present a methodology to study landslide processes by multi-scale and multi-sensor remote sensing data from very high to low resolution (Pléiades, TRMM, CosmoSkyMed, Sentinel). The research will address the evolution over time of such data combined with other earth observations (seismic ground based networks, catalogues, rain gauge networks, GPS surveying, field observations) to detect and study landslide occurrence, dynamics and evolution. This research aims to get insights into the rainfall

  9. The palaeo-lake Suguta and its importance for understanding lake level fluctuations in the East African Rift System

    NASA Astrophysics Data System (ADS)

    Junginger, A.; Olago, D. O.; Trauth, M. H.

    2010-12-01

    We studied the most recent dry-wet-dry cycle in the presently arid Suguta Valley in the Northern Kenya Rift where a 300-m-deep lake has formed during the so-called African Humid Period (AHP, 14.8-5.5 ka BP). Hydromodeling suggests that a relatively moderate 25% increase in precipitation was responsible for this dramatic lake level rise, which demonstrates the character of the Suguta Valley as an amplifier lake system. To detect the response of this lake system to climate fluctuations and their possible driving mechanisms with a focus on abrupt vs. gradual changes, we reconstructed a palaeo-lake level record for the time between 14 and 5 ka BP from up to 40 m thick lake-sediment sequences at three locations in the ~2,500 km2 palaeo-lake Suguta area. The sediments have been investigated for sediment characteristics such as grain size distributions, detrital and authigenic mineral phases, geochemical properties and microfossil assemblages. The stratigraphy for the sequences is based on 38 AMS 14C ages of biogenic carbonate and charcoal samples. Parallel dating of charcoal and snail-shell samples show age differences between 1,570-2,240 years suggesting a remarkably high, but well-defined reservoir age for palaeo-Lake Suguta most likely due to aged groundwater or 14C depleted CO2 degassing from active volcanoes. The observed reservoir effect highlights the potential problems while correlating East African lake level records with chronologies based on 14C datings of aquatic materials. The new chronology of water level fluctuations in the amplifier-lake Suguta indicates a general dry-wet-dry cycle synchronous with other lake chronologies during the AHP and multiple short-term fluctuations with abrupt lake level drops between 100 to 300 m within 100 to 200 years at 12.8-11.6 (during Younger Dryas time), 11.1-10.9; 10.4-10.2; 9.5-9.1; 9.0-8.8; 8.5-8.1 (during the 8.2 ka event) cal ka BP that seem to be linked with changes in the coupling between atmosphere and ocean

  10. Mineralogical-geochemical features of travertines of the modern continental hydrotherms: A G-1 well, Tunka depression, Baikal rift zone

    NASA Astrophysics Data System (ADS)

    Soktoev, B. R.; Rikhvanov, L. P.; Ilenok, S. S.; Baranovskaya, N. V.; Taisaev, T. T.

    2015-07-01

    The mineral and chemical composition of travertines is studied in the modern discharge zone of the hydrothermal fluids of the Tunka depression, Baikal rift zone. The matrix of travertines is mostly made up of aragonite and calcite, which host about 20 mineral phases of Ag, Au, Pb, Cu, Sb, Sn, Fe, and other chemical elements. Similar rocks have previously been found in areas of modern submarine ore formation and tectonically active structures of the crust (New Zealand, the Cheleken Peninsula and others). Our materials confirm the opinion of some researchers who study modern hydrothermal ore formation in spreading zones that the formation of hydrothermal deposits requires favorable geochemical barriers rather than significant contents of metals in thermal waters. It is shown that microbial communities, concentrating chemical elements playing an important role in formation of ore mineralization in the discharge zones of thermal waters may be these barriers. According to our data, at the territory of the Tunka depression, thermal carbonic waters with endogenic components are delivered to the upper crustal horizons, involved in the existing hydrogeological systems, mixed with waters of active water exchange, and contribute to their chemical composition. This is manifested in the specific elemental and micromineral (Au, Ag, etc.) composition of the limescale of drinking water. In this local discharge zone, an effect of radioactive orphans has been found, which is similar to that established in barite chimneys from the Juan-de-Fuca Ridge.

  11. Complex Rift-Parallel, Strike-Slip Faulting in Iceland: Kinematic Analysis of the Gljúfurá Fault Zone

    NASA Astrophysics Data System (ADS)

    Nanfito, A.; Karson, J. A.

    2009-12-01

    strike of the fault zone and cut across the deformation fabrics. Although no features could be correlated across the fault zone to constrain the lateral displacement, empirical gouge thickness/displacement scaling relationships suggest at least several kilometers of slip. Previous studies interpreted the Gljúfurá Fault Zone was one of a family of parallel, strike-slip “bookshelf” faults associated with a transform fault zone linking two now-extinct rifts. Unlike the well-known rift-parallel, strike-slip faults of the South Iceland Seismic Zone and the Tjornes Fracture Zone, the Gljúfurá Fault Zone appears to be an isolated structure with substantially larger displacement that is discordant with surrounding basement lineaments. Major strike-slip faults of this kind can provide important clues in the reconstruction of ridge-hot spot interactions in Iceland.

  12. Evolution of LILE-enriched small melt fractions in the lithospheric mantle: a case study from the East African Rift

    NASA Astrophysics Data System (ADS)

    Bedini, R. M.; Bodinier, J.-L.; Dautria, J.-M.; Morten, L.

    1997-12-01

    Spinel-peridotite xenoliths from Mega (East African Rift, Sidamo region, SE Ethiopia) show variable degrees of recrystallization coupled with trace-element variations. The less recrystallized samples (deformed xenoliths) consist of apatite-bearing porphyroclastic peridotites. They are strongly enriched in LILE (Ba, Th, U, Sr and LREE), with negative anomalies of the HFSE (Nb, Ta, Zr, Hf and Ti). The most recrystallized samples (granular xenoliths) consist of apatite-free peridotites with coarse-grained, granular textures. These samples are depleted or only slightly enriched in LILE and display no significant HFSE anomaly. We suggest that the inverse relationship between recrystallization and trace-element enrichment results from km-scale variation in volume and composition of melts pervasively infiltrated in the lithosphere. The deformed xenoliths record interaction with LILE-enriched small melt fractions, at low melt/rock ratio, while the granular xenoliths were extensively re-equilibrated with a higher fraction of basaltic melt, at higher melt/rock ratio. With a numerical simulation of reactive porous flow at the transition between adiabatic and conductive geotherms in the mantle, it is shown that these two processes were possibly coeval and associated with thermo-mechanical erosion of the lower lithosphere above a mantle plume.

  13. The Hominin Sites and Paleolakes Drilling Project: inferring the environmental context of human evolution from eastern African rift lake deposits

    NASA Astrophysics Data System (ADS)

    Cohen, A.; Campisano, C.; Arrowsmith, R.; Asrat, A.; Behrensmeyer, A. K.; Deino, A.; Feibel, C.; Hill, A.; Johnson, R.; Kingston, J.; Lamb, H.; Lowenstein, T.; Noren, A.; Olago, D.; Owen, R. B.; Potts, R.; Reed, K.; Renaut, R.; Schäbitz, F.; Tiercelin, J.-J.; Trauth, M. H.; Wynn, J.; Ivory, S.; Brady, K.; O'Grady, R.; Rodysill, J.; Githiri, J.; Russell, J.; Foerster, V.; Dommain, R.; Rucina, S.; Deocampo, D.; Russell, J.; Billingsley, A.; Beck, C.; Dorenbeck, G.; Dullo, L.; Feary, D.; Garello, D.; Gromig, R.; Johnson, T.; Junginger, A.; Karanja, M.; Kimburi, E.; Mbuthia, A.; McCartney, T.; McNulty, E.; Muiruri, V.; Nambiro, E.; Negash, E. W.; Njagi, D.; Wilson, J. N.; Rabideaux, N.; Raub, T.; Sier, M. J.; Smith, P.; Urban, J.; Warren, M.; Yadeta, M.; Yost, C.; Zinaye, B.

    2016-02-01

    The role that climate and environmental history may have played in influencing human evolution has been the focus of considerable interest and controversy among paleoanthropologists for decades. Prior attempts to understand the environmental history side of this equation have centered around the study of outcrop sediments and fossils adjacent to where fossil hominins (ancestors or close relatives of modern humans) are found, or from the study of deep sea drill cores. However, outcrop sediments are often highly weathered and thus are unsuitable for some types of paleoclimatic records, and deep sea core records come from long distances away from the actual fossil and stone tool remains. The Hominin Sites and Paleolakes Drilling Project (HSPDP) was developed to address these issues. The project has focused its efforts on the eastern African Rift Valley, where much of the evidence for early hominins has been recovered. We have collected about 2 km of sediment drill core from six basins in Kenya and Ethiopia, in lake deposits immediately adjacent to important fossil hominin and archaeological sites. Collectively these cores cover in time many of the key transitions and critical intervals in human evolutionary history over the last 4 Ma, such as the earliest stone tools, the origin of our own genus Homo, and the earliest anatomically modern Homo sapiens. Here we document the initial field, physical property, and core description results of the 2012-2014 HSPDP coring campaign.

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

    NASA Astrophysics Data System (ADS)

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

    2014-10-01

    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.

  15. Paleomagnetism of Miocene East African Rift sediments and the calibration of the geomagnetic reversal time scale

    NASA Astrophysics Data System (ADS)

    Tauxe, L.; Monaghan, M.; Drake, R.; Curtis, G.; Staudigel, H.

    1985-05-01

    Paleomagnetic stratigraphy and K-Ar age determinations are reported for the type section of the middle Miocene Ngorora Formation, found in the Kenya rift valley. The magnetostratigraphy is well correlated to the geomagnetic reversal time scale (GRTS) and spans from the lower part of Chron C5 (9) to Chron C5AB-r (14). K-Ar dates were determined for euhedral sanidines, handpicked from seven tuff horizons within the Ngorora Formation and the underlying Turn phonolite flow. These dates can therefore be tied directly to the GRTS. The eight dates fall into three discrete groups averaging 12.5±0.22 Ma (mean and standard deviation of results from four tuffs), 11.6±0.06 Ma (mean and standard deviation from three tuffs), and 10.16±0.38 (average of three analyses from one tuff). We interpret the age groups as resulting from three successive eruptive episodes, the stratigraphic positions of which are well constrained. In spite of episodic supply at the eruptive source, sediment accumulation is continuous at the resolution of the GRTS. This suggests that accumulation is controlled by basin subsidence rather than sediment supply. Sanidine dates support an age for the older boundary of marine magnetic anomaly 5 of about 10 Ma, as opposed to 11.12 Ma, suggested by the most recent results from Icelandic basaltic lava flows.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

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

    USGS Publications Warehouse

    Prodehl, C.; Mechie, J.; Kaminski, W.; Fuchs, K.; Grosse, C.; Hoffmann, H.; Stangl, R.; Stellrecht, R.; Khan, M.A.; Maguire, Peter K.H.; Kirk, W.; Keller, Gordon 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

    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.

  18. Magma transport and olivine crystallization depths in Kīlauea's east rift zone inferred from experimentally rehomogenized melt inclusions

    NASA Astrophysics Data System (ADS)

    Tuohy, Robin M.; Wallace, Paul J.; Loewen, Matthew W.; Swanson, Donald A.; Kent, Adam J. R.

    2016-07-01

    Concentrations of H2O and CO2 in olivine-hosted melt inclusions can be used to estimate crystallization depths for the olivine host. However, the original dissolved CO2 concentration of melt inclusions at the time of trapping can be difficult to measure directly because in many cases substantial CO2 is transferred to shrinkage bubbles that form during post-entrapment cooling and crystallization. To investigate this problem, we heated olivine from the 1959 Kīlauea Iki and 1960 Kapoho (Hawai'i) eruptions in a 1-atm furnace to temperatures above the melt inclusion trapping temperature to redissolve the CO2 in shrinkage bubbles. The measured CO2 concentrations of the experimentally rehomogenized inclusions (⩽590 ppm for Kīlauea Iki [n = 10]; ⩽880 ppm for Kapoho, with one inclusion at 1863 ppm [n = 38]) overlap with values for naturally quenched inclusions from the same samples, but experimentally rehomogenized inclusions have higher within-sample median CO2 values than naturally quenched inclusions, indicating at least partial dissolution of CO2 from the vapor bubble during heating. Comparison of our data with predictions from modeling of vapor bubble formation and published Raman data on the density of CO2 in the vapor bubbles suggests that 55-85% of the dissolved CO2 in the melt inclusions at the time of trapping was lost to post-entrapment shrinkage bubbles. Our results combined with the Raman data demonstrate that olivine from the early part of the Kīlauea Iki eruption crystallized at <6 km depth, with the majority of olivine in the 1-3 km depth range. These depths are consistent with the interpretation that the Kīlauea Iki magma was supplied from Kīlauea's summit magma reservoir (∼2-5 km depth). In contrast, olivine from Kapoho, which was the rift zone extension of the Kīlauea Iki eruption, crystallized over a much wider range of depths (∼1-16 km). The wider depth range requires magma transport during the Kapoho eruption from deep beneath the summit

  19. Magma transport and olivine crystallization depths in Kīlauea’s East Rift Zone inferred from experimentally rehomogenized melt inclusions

    USGS Publications Warehouse

    Tuohy, Robin M; Wallace, Paul J.; Loewen, Matthew W; Swanson, Don; Kent, Adam J R

    2016-01-01

    Concentrations of H2O and CO2 in olivine-hosted melt inclusions can be used to estimate crystallization depths for the olivine host. However, the original dissolved CO2concentration of melt inclusions at the time of trapping can be difficult to measure directly because in many cases substantial CO2 is transferred to shrinkage bubbles that form during post-entrapment cooling and crystallization. To investigate this problem, we heated olivine from the 1959 Kīlauea Iki and 1960 Kapoho (Hawai‘i) eruptions in a 1-atm furnace to temperatures above the melt inclusion trapping temperature to redissolve the CO2 in shrinkage bubbles. The measured CO2 concentrations of the experimentally rehomogenized inclusions (⩽590 ppm for Kīlauea Iki [n=10]; ⩽880 ppm for Kapoho, with one inclusion at 1863 ppm [n=38]) overlap with values for naturally quenched inclusions from the same samples, but experimentally rehomogenized inclusions have higher within-sample median CO2 values than naturally quenched inclusions, indicating at least partial dissolution of CO2 from the vapor bubble during heating. Comparison of our data with predictions from modeling of vapor bubble formation and published Raman data on the density of CO2 in the vapor bubbles suggests that 55-85% of the dissolved CO2 in the melt inclusions at the time of trapping was lost to post-entrapment shrinkage bubbles. Our results combined with the Raman data demonstrate that olivine from the early part of the Kīlauea Iki eruption crystallized at <6 km depth, with the majority of olivine in the 1-3 km depth range. These depths are consistent with the interpretation that the Kīlauea Iki magma was supplied from Kīlauea’s summit magma reservoir (∼2-5 km depth). In contrast, olivine from Kapoho, which was the rift zone extension of the Kīlauea Iki eruption, crystallized over a much wider range of depths (∼1-16 km). The wider depth range requires magma transport during the Kapoho eruption from deep beneath the

  20. Structure of the active rift zone and margins of the northern Imperial Valley from Salton Seismic Imaging Project (SSIP) data

    NASA Astrophysics Data System (ADS)

    Livers, A.; Han, L.; Delph, J. R.; White-Gaynor, A. L.; Petit, R.; Hole, J. A.; Stock, J. M.; Fuis, G. S.

    2012-12-01

    First-arrival refraction data were used to create a seismic velocity model of the upper crust across the actively rifting northern Imperial Valley and its margins. The densely sampled seismic refraction data were acquired by the Salton Seismic Imaging Project (SSIP) , which is investigating rift processes in the northern-most rift segment of the Gulf of California extensional province and earthquake hazards at the southern end of the San Andreas Fault system. A 95-km long seismic line was acquired across the northern Imperial Valley, through the Salton Sea geothermal field, parallel to the five Salton Butte volcanoes and perpendicular to the Brawley Seismic Zone and major strike-slip faults. Nineteen explosive shots were recorded with 100 m seismometer spacing across the valley and with 300-500 m spacing into the adjacent ranges. First-arrival travel times were picked from shot gathers along this line and a seismic velocity model was produced using tomographic inversion. Sedimentary basement and seismic basement in the valley are interpreted to be sediment metamorphosed by the very high heat flow. The velocity model shows that this basement to the west of the Brawley Seismic Zone is at ~4-km depth. The basement shallows to ~2-km depth in the active geothermal field and Salton Buttes volcanic field which locally coincide with the Brawley Seismic Zone. At the eastern edge of the geothermal field, the basement drops off again to ~3.5-km depth. The eastern edge of the valley appears to be fault bounded by the along-strike extension of the Sand Hills Fault, an inactive strike-slip fault. The seismic velocities to the east of the fault correspond to metamorphic rock of the Chocolate Mountains, different from the metamorphosed basement in the valley. The western edge of the valley appears to be fault bounded by the active Superstition Hills Fault. To the west of the valley, >4-km deep valley basement extends to the active Superstition Hills Fault. Basement then shallows

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

    NASA Astrophysics Data System (ADS)

    Delvaux, Damien; Smets, Benoît

    2015-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2003-04-01

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

  3. Anomalously high b-values in the South Flank of Kilauea volcano, Hawaii: Evidence for the distribution of magma below Kilauea's East rift zone

    USGS Publications Warehouse

    Wyss, M.; Klein, F.; Nagamine, K.; Wiemer, S.

    2001-01-01

    The pattern of b-value of the frequency-magnitude relation, or mean magnitude, varies little in the Kaoiki-Hilea area of Hawaii, and the b-values are normal, with b = 0.8 in the top 10 km and somewhat lower values below that depth. We interpret the Kaoiki-Hilea area as relatively stable, normal Hawaiian crust. In contrast, the b-values beneath Kilauea's South Flank are anomalously high (b = 1.3-1.7) at depths between 4 and 8 km, with the highest values near the East Rift zone, but extending 5-8 km away from the rift. Also, the anomalously high b-values vary along strike, parallel to the rift zone. The highest b-values are observed near Hiiaka and Pauahi craters at the bend in the rift, the next highest are near Makaopuhi and also near Puu Kaliu. The mildest anomalies occur adjacent to the central section of the rift. The locations of the three major and two minor b-value anomalies correspond to places where shallow magma reservoirs have been proposed based on analyses of seismicity, geodetic data and differentiated lava chemistry. The existence of the magma reservoirs is also supported by magnetic anomalies, which may be areas of dike concentration, and self-potential anomalies, which are areas of thermal upwelling above a hot source. The simplest explanation of these anomalously high b-values is that they are due to the presence of active magma bodies beneath the East Rift zone at depths down to 8 km. In other volcanoes, anomalously high b-values correlate with volumes adjacent to active magma chambers. This supports a model of a magma body beneath the East Rift zone, which may widen and thin along strike, and which may reach 8 km depth and extend from Kilauea's summit to a distance of at least 40 km down rift. The anomalously high b-values at the center of the South Flank, several kilometers away from the rift, may be explained by unusually high pore pressure throughout the South Flank, or by anomalously strong heterogeneity due to extensive cracking, or by both

  4. Influence of rheological layering on the formation of offset basins at inherited weak zones during continental rifting: effects of stiff and pliable layers

    NASA Astrophysics Data System (ADS)

    Chenin, Pauline; Beaumont, Christopher

    2013-04-01

    We use numerical modelling to investigate the influence of lithosphere rheological layering on the reactivation of inherited crust and mantle weak zones during continental rifting. Such reactivation often leads to the formation of offset basins, ie. basins whose development is concomitant with the rifting event, but whose location is offset/set off the main rift/locus of the breakup. Offset rift basins are ubiquitous features of rifted continental margins and are often located at inherited sutures and their fold-and-thrust belts. We use the software Sopale nested to test the effects of different lithospheres comprising Stiff and/or Pliable crust and mantle layers. Here Stiff (S) implies a nonlinear flow law with a high stress exponent (n ~> 10,000), a plastic material, and Pliable (P) means a low stress exponent (n~ 2 - 5) as in ductile, power-law creep of rocks. To achieve this rheological change without modifying the thermal structure of the model, we introduce a scaling factor f in the power-law creep parametrization of the viscosity, such that large values of f result in Coulomb frictional-plastic failure of a layer and small values result in power-law creep. One weak (ie. with reduced internal angle of friction, φ = 2°) zone is embedded in the central part of the uppermost mantle lithosphere and two weak zones are embedded in the upper crust, offset on either side of the mantle weak zone by 150 km in most models. During extension of the model lithosphere weak zones embedded in a stiff layer are preferentially and rapidly reactivated, whereas the same zones are either ignored or slowly reactivated when embedded in pliable layers. This is because necking instabilities grow much more rapidly in stiff layers than in pliable ones. Moreover, the intensity of coupling between the crust and the mantle determines which layer controls the morphology of the model continental margin. When the crust is strongly coupled to the underlying mantle, offset basins only form at

  5. Structural controls on the spatial distribution and geochemical composition of volcanism in a continental rift zone; an example from Owens Valley, eastern California

    NASA Astrophysics Data System (ADS)

    Haproff, P. J.; Yin, A.

    2014-12-01

    Bimodal volcanism is common in continental rift zones. Structural controls to the emplacement and compositions of magmas, however, are not well understood. To address this issue, we examine the location, age, and geochemistry of active volcanic centers, and geometry and kinematics of rift-related faults across the active transtensional Owens Valley rift zone. Building on existing studies, we postulate that the spatial distribution and geochemical composition of volcanism are controlled by motion along rift-bounding fault systems. Along-strike variation in fault geometry and characteristics of active volcanism allow us to divide Owens Valley into three segments: southern, northern, and central. The southern segment of Owens Valley is a simple shear, asymmetric rift bounded to the west by the east-dipping Sierra Nevada frontal fault (SNFF). Active vents of Coso volcanic field are distributed along the eastern rift shoulder and characterized by the eruption of bimodal lavas. The SNFF within this segment is low-angle and penetrates through the lithosphere and into the ductile asthenosphere, allowing for mantle-derived magma to migrate across the weakest part of the fault zone beneath the eastern rift shoulder. Magma thermally weakens wall rocks and eventually stalls in the crust where the melt develops a greater felsic component prior to eruption. The northern segment of Owens Valley displays similar structural geometry, as the west-dipping White Mountains fault (WMF) is listric at depth and offsets the crust and mantle lithosphere, allowing for vertical transport of magma and reservoir emplacement within the crust. Bimodal lavas periodically erupted in the Long Valley Caldera region along the western rift shoulder. The central segment of Owens Valley is a pure shear, symmetric graben generated by motion along the SNFF and WMF. The subvertical, right-slip Owens Valley fault (OVF) strikes along the axis of the valley and penetrates through the lithosphere into the

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

    NASA Astrophysics Data System (ADS)

    Carvajal, L. C.; Mann, P.

    2015-12-01

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

  7. Rift Valley fever in a zone potentially occupied by Aedes vexans in Senegal: dynamics and risk mapping

    NASA Astrophysics Data System (ADS)

    Tourre, Y. M.; Vignolles, C.; Lacaux, J.-P.; Bigeard, G.; Ndione, J.-A.; Lafaye, M.

    2009-09-01

    This paper presents an analysis of the interaction between the various variables associated with Rift Valley fever (RVF) such as the mosquito vector, available hosts and rainfall distribution. To that end, the varying zones potentially occupied by mosquitoes (ZPOM), rainfall events and pond dynamics, and the associated exposure of hosts to the RVF virus by Aedes vexans, were analyzed in the Barkedji area of the Ferlo, Senegal, during the 2003 rainy season. Ponds were identified by remote sensing using a high-resolution SPOT-5 satellite image. Additional data on ponds and rainfall events from the Tropical Rainfall Measuring Mission were combined with in-situ entomological and limnimetric measurements, and the localization of vulnerable ruminant hosts (data derived from QuickBird satellite). Since "Ae. vexans productive events” are dependent on the timing of rainfall for their embryogenesis (six days without rain are necessary to trigger hatching), the dynamic spatio-temporal distribution of Ae. vexans density was based on the total rainfall amount and pond dynamics. Detailed ZPOM mapping was obtained on a daily basis and combined with aggressiveness temporal profiles. Risks zones, i.e. zones where hazards and vulnerability are combined, are expressed by the percentages of parks where animals are potentially exposed to mosquito bites. This new approach, simply relying upon rainfall distribution evaluated from space, is meant to contribute to the implementation of a new, operational early warning system for RVF based on environmental risks linked to climatic and environmental conditions.

  8. Sr-Nd-Pb isotope systematics and clinopyroxene-host disequilibrium in ultra-potassic magmas from Toro-Ankole and Virunga, East-African Rift: Implications for magma mixing and source heterogeneity

    NASA Astrophysics Data System (ADS)

    Muravyeva, N. S.; Belyatsky, B. V.; Senin, V. G.; Ivanov, A. V.

    2014-12-01

    Nd, Pb and Sr isotope ratios have been determined for kamafugite lava and clinopyroxene phenocrysts from Bunyaruguru (Toro-Ankole) and Virunga volcanic fields of the East African Rift. The whole rock Sr-Nd isotopic signatures of kamafugites (87Sr/86Sr: 0.70463-0.70536; 143Nd/144Nd: 0.51249-0.51255) suggest derivation from an EM1-type mantle source. In contrast, Pb isotopic compositions of the same samples (206Pb/204Pb: 19.00-19.57; 207Pb/204Pb: 15.69-15.74; 208Pb/204Pb: 39.30-40.26) reveal a similarity to EM2-type mantle. New Nd, Pb and Sr isotopic data for clinopyroxene (87Sr/86Sr: 0.70473-0.70503; 143Nd/144Nd: 0.51250-0.51254; 206Pb/204Pb: 18.04-18.17; 207Pb/204Pb: 15.58-15.60; 208Pb/204Pb: 38.09-38.23) suggest derivation from an EM1-like source, and indicate Sr and Pb isotope disequilibrium between clinopyroxene and corresponding host rock. Moreover, clinopyroxenes exhibiting a greater degree of isotopic disequilibrium with their host rock are more sodic in composition. The isotopic disequilibrium is corroborated by the presence of chemical zoning within clinopyroxene, which suggests rapid magma ascent rates preventing melt homogenization. The Pb isotopic ratios for both mineral and corresponding whole rock, together with published data on East African rift-related alkaline centers, define a trend interpreted to represent a mixing line for melts derived from sources such as EM1 and as HIMU. The similar isotopic compositions for clinopyroxene from the different volcanic rocks within the East African Rift suggest the existence of a common, older mantle source for their parental melts. The origin of these melts can be attributed to an enrichment event ~ 400-500 Ma, i.e., significantly prior the younger ultrapotassic magmatism. Our preferred interpretation for the results reported here involves the mixing of melts derived from EM1- and HIMU-like sources, which were rapidly transported to the Earth's surface. The primary magmas formed as the result of melting of a

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    In the Central Mediterranean, the differential roll-back of the subducting Nubia Plate caused the Neogene-Quaternary extrusion of the Calabrian arc onto the oceanic Ionian slab, and the opening of the oceanic Tyrrhenian Basin, in the overriding Eurasia Plate. The differential motion at the edges of the arc was largely accommodated along transform faults that propagated across the orogenic belt. Since the Late Quaternary, the southern edge of the arc has been replaced by the roughly N-S oriented Siculo-Calabrian Rift Zone (SCRZ) that formed as the NNW-directed normal faults of NE Sicily, crossing the orogenic belt, have linked the NNE-oriented Tyrrhenian margin of southern Calabria with the NNW-trending Africa-Ionian boundary of southeastern Sicily. Our study focused on the Sicily shoulder of the SCRZ, where the transition zone between the extensional belt and the still active Nubia-Eurasia convergent margin is characterized by two distinct mobile crustal wedges, both lying on an upwarped Mantle, where a re-orientations of the σ1 is combined with volcanism (e.g. Etna, Aeolian islands) and a huge tectonic uplift. In southeastern Sicily, the Hyblean-Etnean region evolved, since about 0.85 Ma, as an indipendent crustal wedge, moving towards the NNW and pointing to the active Mt. Etna volcano. A local ENE crustal stretching accompanied the traslation of the block and pre-dated the ESE-oriented extension governing the propagation of the southernmost branch of the SCR, which started at about 330 ka B.P.. Similarly, the Peloritani-Aeolian region, flanked by the 125 ka-old NE-Sicily branch of the rift zone, represents a mostly submerged crustal wedge that migrates towards the NE, diverging from the rest of the Sicily collision zone and pointing to the Stromboli volcano. The Peloritani-Aeolian block is characterized by the occurrence of a wide central NE-oriented collapsed basin contoured by an actively uplifting region, whose tectonic boundaries are evidenced by a sharp

  10. The permeability of fault zones: a case study of the Dead Sea rift (Middle East)

    NASA Astrophysics Data System (ADS)

    Ran, Gabay; Eyal, Shalev; Yoseph, Yechieli; Amir, Sagy; Noam, Weisbrod

    2014-03-01

    Fault zone architecture plays an important role in flow regimes of hydrological systems. Fault zones can act as conduits, barriers, or conduits/barrier systems depending on their spatial architecture. The goal of this study is to determine the fault-zone permeability structure and its effect on the local hydrogeological system in the Dead Sea fault system. Permeability was measured on small-scale outcrop plug samples at four faults along the Dead Sea fault system, and large-scale slug tests in four boreholes, in different parts of the fault, at Yair fault in Israel. The research results show that values in the damage zone are two to five orders of magnitude higher than those of the fault core (~3.5 × 10-10, 1 × 10-15 m2 respectively), resulting in an anisotropic permeability structure for the overall fault zone and preferable flow parallel to the fault. A set of injection tests in the Yair fault damage zone revealed a water-pressure-dependent behavior. The permeability of this zone increases when employing a higher water pressure in the fault fracture-dominated damage zone, due to the reopening of fractures.

  11. Magma genesis of the acidic volcanism in the intra-arc rift zone of the Izu volcanic arc, Japan

    NASA Astrophysics Data System (ADS)

    Haraguchi, S.; Tokuyama, H.; Ishii, T.

    2010-12-01

    The Izu volcanic arc extends over 550 km from the Izu Peninsula, Japan, to the Nishinoshima Trough or Sofugan tectonic line. It is the northernmost segment of the Izu-Bonin-Mariana arc system, which is located at the eastern side of the Philippine Sea Plate. The recent magmatism of the Izu arc is bimodal and characterized by basalt and rhyolite (e.g. Tamura and Tatsumi 2002). In the southern Izu arc, volcanic front from the Aogashima to the Torishima islands is characterized by submarine calderas and acidic volcanisms. The intra-arc rifting, characterized by back-arc depressions, small volcanic knolls and ridges, is active in this region. Volcanic rocks were obtained in 1995 during a research cruise of the R/V MOANA WAVE (Hawaii University, cruise MW9507). Geochemical variation of volcanic rocks and magma genesis was studied by Hochstaedter et al. (2000, 2001), Machida et al (2008), etc. These studies focused magma and mantle dynamics of basaltic volcanism in the wedge mantle. Acidic volcanic rocks were also dredged during the curies MW9507. However, studies of these acidic volcanics were rare. Herein, we present petrographical and chemical analyses of these acidic rocks, and compare these results with those of other acidic rocks in the Izu arc and lab experiments, and propose a model of magma genesis in a context of acidic volcanism. Dredge sites by the cruise MW9507 are 120, and about 50 sites are in the rift zone. Recovered rocks are dominated by the bimodal assemblage of basalt-basaltic andesite and dacite-rhyolite. The most abundant phase is olivine basalt, less than 50 wt% SiO2. Andesites are minor in volume and compositional gap from 56 to 65 wt% SiO2 exists. The across-arc variation of the HFSE contents and ratios, such as Zr/Y and Nb/Zr of rhyolites exhibit depleted in the volcanic front side and enriched in reararc side. This characteristic is similar to basaltic volcanism pointed out by Hochstaedter et al (2000). The petrographical features of rhyolites

  12. Effects of Oblique Extension and Inherited Structure Geometry on Transfer Zone Development in Continental Rifts: A 4D Analogue Modeling Approach

    NASA Astrophysics Data System (ADS)

    Zwaan, Frank; Schreurs, Guido

    2015-04-01

    INTRODUCTION Inherited structures in the crust form weak zones along which deformation will focus during rifting. Along-strike connection of rift segments may occur along transfer zones, as observed in East Africa. Previous studies have focused on numerical and analog modeling of transfer zones (e.g. Acocella et al., 1999, Allken et al., 2012). We elaborate upon those by investigating the effects of 1) oblique extension and 2) the geometry of linked and non-linked inherited structures on the development of transfer zones. A further improvement is the use of X-ray Computer Tomography (CT) for detailed internal analysis. METHODS The experimental set-up (see Schreurs & Colleta, 1998) contains two sidewalls with a base of compressed foam and plexiglass bars stacked in between. Decompressing this base results in distributed deformation of the overlying model materials. Deforming the model laterally with a mobile base plate produces the strike-slip components for oblique extension. Divergence velocities are in the order of 5 mm/h, translating to ca. 5 mm/Ma in nature, and 1 cm represents 10 km. A 2 cm thick layer of viscous silicone represents the ductile lower crust and a 2 cm quartz sand layer the brittle upper crust. Inherited structures are created with thin lines of silicon laid down on top of the basal silicone layer. Several models were run in a CT-scanner to reveal the 3D evolution of internal structures with time, hence 4D. RESULTS Localization of deformation along the pre-defined structures works well. The models show that the structural style changes with extension obliquity, from wide rift structures to narrower rifts with internal oblique-slip and finally strike-slip structures. Furthermore, rift offset is an important parameter influencing the occurrence of linkage: increasing rift offset decreases linkage as previously observed by Allken et al. (2012). However, increasing divergence obliquity promotes transfer zone formation, as does the presence of rift

  13. Tectono-stratigraphic signature of a rapid multistage subsiding rift basin in the Tyrrhenian-Apennine hinge zone (Italy): A possible interaction of upper plate with subducting slab

    NASA Astrophysics Data System (ADS)

    Milia, Alfonsa; Torrente, Maurizio M.

    2015-05-01

    The Campania Plain is a rapidly subsiding Quaternary basin that formed on the eastern margin of the Tyrrhenian Sea in association with the younger phase of Tyrrhenian rifting. It is located in the hinge area between the Apennines fold-thrust belt and the Tyrrhenian extensional backarc basin. By combining original stratigraphic analyses of well logs and seismic profiles we built a basin subsidence curve, mapped the fault pattern of the Campania Plain and analyzed the impact of the block faulting on the sedimentology and stratigraphic architecture of the basin fill. Well data indicate that the Quaternary succession consists of offshore, shoreface and coal-bearing coastal plain deposits arranged to form thick aggradational and retrogradational units. The sequence stratigraphy interpretation of well logs permitted us to recognize thirteen depositional sequences and the stratigraphic signatures of the rift stages. The study area corresponds to a sediment overfilled/balanced infill basin type that resulted from superposition of several rifting events characterized by high rates of basin subsidence. Taking into account the geological data of the adjacent areas, we propose a Pliocene-Quaternary rifting evolution of the upper Tyrrhenian plate consisting of four episodes. Two peculiar features of the Tyrrhenian rifting are a skip of the extensional axial zone eastwards leaving the previous zone of high strain localization (Vavilov basin), followed by a dramatic change (90°) of the direction of extension. Because these Tyrrhenian features cannot be accounted for by the current rifting models we hypothesized a link between the evolution of upper plate and subducting slab. The proposed geodynamic scenario is characterized by a progressive rupture of the subducting plate and formation of extensional basins in the upper plate.

  14. Volatile Chemistry of the 2007 to Present Explosive Eruption of Oldoinyo Lengai Volcano, East African Rift

    NASA Astrophysics Data System (ADS)

    de Moor, J.; Fischer, T. P.; King, P. L.; Sharp, Z.; Shaw, A. M.; Mangasini, F.

    2008-12-01

    We characterize the volatile chemistry of the ongoing explosive eruption at Oldoinyo Lengai (OL) in the Gregory Rift Valley of N Tanzania. Fieldwork was conducted from 4-8 April 2008, during which time OL exhibited Strombolian to ash plume-producing activity. Eight distinct ash lapilli layers were sampled 900m from the crater. Mini-DOAS SO2 flux measurements were conducted on 6, 7, and 8 April. Despite moderate eruptive activity, SO2 concentrations were very low, from ~ 20ppm.m to below detection. A low concentration plume was detected on 7 April, allowing a SO2 flux estimate of 0.2-0.4 tons/day. SIMS analyses of carbonatite lavas erupted in 2005 show very high S concentrations (0.62wt %), suggesting that the low SO2 flux is due to partitioning of S into the melt. Ash leachates were analyzed as a proxy for plume chemistry and to assess health risks associated with mobile elements in the ashes. The solutions had high pH of 10.6 to 11.1. This has implications for pH fluctuations of Lake Natron (pH ~10; located 20km N of the crater), which may correlate with lacustrine ash deposition during passed explosive activity at OL. In the uppermost ash layer (deposited on 4/5/2008; not influenced by rain) dominant mobile ions are Cl (18120mg/kg), SO4 (26616mg/kg), PO4 (2393mg/kg), and F (534mg/kg), Na (101679mg/kg), K (22544mg/kg), Ca (721mg/kg), and Si (189mg/kg). Leachate S/Cl from this pristine ash is 0.49, compared to 0.29 measured by SIMS in lavas from 2005. Using the SO2 flux and the S/Cl in the leachates, the Cl flux was 0.5-0.8 tons/day. High concentrations of leachable ions, particularly F, on ash presents health hazards (F poisoning; water source contamination) to local communities. Concentrations in the underlying ashes are lower (40-129 mg/kg Cl, 965-3223 mg/kg SO4 , 66-104 mg/kg F, 40-335 mg/kg PO4 ) than those in the upper deposit due to leaching by rain prior to deposition of the uppermost ash layer. FTIR spectroscopy of ashes shows at least two carbonate

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

    SciTech Connect

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

    1990-05-01

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

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

    SciTech Connect

    Hermance, J.F.

    1982-01-01

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

  17. Early Cambrian granitoids of North Gondwana margin in the transition from a convergent setting to intra-continental rifting (Ossa-Morena Zone, SW Iberia)

    NASA Astrophysics Data System (ADS)

    Sánchez-García, T.; Pereira, M. F.; Bellido, F.; Chichorro, M.; Silva, J. B.; Valverde-Vaquero, P.; Pin, Ch.; Solá, A. R.

    2014-07-01

    Two distinct Cambrian magmatic pulses are recognized in the Ossa-Morena Zone (SW Iberia): an early rift-(ER) and a main rift-related event. This Cambrian magmatism is related to intra-continental rifting of North Gondwana that is thought to have culminated in the opening of the Rheic Ocean in Lower Ordovician times. New data of whole-rock geochemistry (19 samples), Sm-Nd-Sr isotopes (4 samples) and ID-TIMS U-Pb zircon geochronology (1 sample) of the Early Cambrian ER plutonic rocks of the Ossa-Morena Zone are presented in this contribution. The ER granitoids (Barreiros, Barquete, Calera, Salvatierra de los Barros and Tablada granitoid Massifs) are mostly peraluminous granites. The Sm-Nd isotopic data show moderate negative ɛNdt values ranging from -3.5 to +0.1 and TDM ages greatly in excess of emplacement ages. Most ER granitoids are crustal melts. However, a subset of samples shows a transitional anorogenic alkaline tendency, together with more primitive isotopic signatures, documenting the participation of lower crust or mantle-derived sources and suggesting a local transient advanced stage of rifting. The Barreiros granitoid is intrusive into the Ediacaran basement of the Ossa-Morena Zone (Série Negra succession) and has yielded a crystallization age of 524.7 ± 0.8 Ma consistent with other ages of ER magmatic pulse. This age: (1) constrains the age of the metamorphism developed in the Ediacaran back-arc basins before the intrusion of granites and (2) defines the time of the transition from the Ediacaran convergent setting to the Lower Cambrian intra-continental rifting in North Gondwana.

  18. Aeromagnetic evidence for a major strike-slip fault zone along the boundary between the Weddell Sea Rift and East Antarctica

    NASA Astrophysics Data System (ADS)

    Jordan, T. A.; Ferraccioli, F.; Ross, N.; Siegert, M. J.; Corr, H.; Leat, P. T.; Bingham, R. G.; Rippin, D. M.; le Brocq, A.

    2012-04-01

    The >500 km wide Weddell Sea Rift was a major focus for Jurassic extension and magmatism during the early stages of Gondwana break-up, and underlies the Weddell Sea Embayment, which separates East Antarctica from a collage of crustal blocks in West Antarctica. Here we present new aeromagnetic data combined with airborne radar and gravity data collected during the 2010-11 field season over the Institute and Moeller ice stream in West Antarctica. Our interpretations identify the major tectonic boundaries between the Weddell Sea Rift, the Ellsworth-Whitmore Mountains block and East Antarctica. Digitally enhanced aeromagnetic data and gravity anomalies indicate the extent of Proterozoic basement, Middle Cambrian rift-related volcanic rocks, Jurassic granites, and post Jurassic sedimentary infill. Two new joint magnetic and gravity models were constructed, constrained by 2D and 3D magnetic depth-to-source estimates to assess the extent of Proterozoic basement and the thickness of major Jurassic intrusions and post-Jurassic sedimentary infill. The Jurassic granites are modelled as 5-8 km thick and emplaced at the transition between the thicker crust of the Ellsworth-Whitmore Mountains block and the thinner crust of the Weddell Sea Rift, and within the Pagano Fault Zone, a newly identified ~75 km wide left-lateral strike-slip fault system that we interpret as a major tectonic boundary between East and West Antarctica. We also suggest a possible analogy between the Pagano Fault Zone and the Dead Sea transform. In this scenario the Jurassic Pagano Fault Zone is the kinematic link between extension in the Weddell Sea Rift and convergence across the Pacific margin of West Antarctica, as the Dead Sea transform links Red Sea extension to compression within the Zagros Mountains.

  19. Oligocene Termite Nests with In Situ Fungus Gardens from the Rukwa Rift Basin, Tanzania, Support a Paleogene African Origin for Insect Agriculture.

    PubMed

    Roberts, Eric M; Todd, Christopher N; Aanen, Duur K; Nobre, Tânia; Hilbert-Wolf, Hannah L; O'Connor, Patrick M; Tapanila, Leif; Mtelela, Cassy; Stevens, Nancy J

    2016-01-01

    Based on molecular dating, the origin of insect agriculture is hypothesized to have taken place independently in three clades of fungus-farming insects: the termites, ants or ambrosia beetles during the Paleogene (66-24 Ma). Yet, definitive fossil evidence of fungus-growing behavior has been elusive, with no unequivocal records prior to the late Miocene (7-10 Ma). Here we report fossil evidence of insect agriculture in the form of fossil fungus gardens, preserved within 25 Ma termite nests from southwestern Tanzania. Using these well-dated fossil fungus gardens, we have recalibrated molecular divergence estimates for the origins of termite agriculture to around 31 Ma, lending support to hypotheses suggesting an African Paleogene origin for termite-fungus symbiosis; perhaps coinciding with rift initiation and changes in the African landscape. PMID:27333288

  20. Oligocene Termite Nests with In Situ Fungus Gardens from the Rukwa Rift Basin, Tanzania, Support a Paleogene African Origin for Insect Agriculture

    PubMed Central

    Roberts, Eric M.; Todd, Christopher N.; Aanen, Duur K.; Nobre, Tânia; Hilbert-Wolf, Hannah L.; O’Connor, Patrick M.; Tapanila, Leif; Mtelela, Cassy; Stevens, Nancy J.

    2016-01-01

    Based on molecular dating, the origin of insect agriculture is hypothesized to have taken place independently in three clades of fungus-farming insects: the termites, ants or ambrosia beetles during the Paleogene (66–24 Ma). Yet, definitive fossil evidence of fungus-growing behavior has been elusive, with no unequivocal records prior to the late Miocene (7–10 Ma). Here we report fossil evidence of insect agriculture in the form of fossil fungus gardens, preserved within 25 Ma termite nests from southwestern Tanzania. Using these well-dated fossil fungus gardens, we have recalibrated molecular divergence estimates for the origins of termite agriculture to around 31 Ma, lending support to hypotheses suggesting an African Paleogene origin for termite-fungus symbiosis; perhaps coinciding with rift initiation and changes in the African landscape. PMID:27333288

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

    NASA Astrophysics Data System (ADS)

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

    2014-04-01

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

  2. Initial Results of Scientific Drilling on Lake Malawi, East African Rift

    NASA Astrophysics Data System (ADS)

    Scholz, C. A.; Johnson, T. C.; King, J.; Cohen, A. S.; Lyons, R. P.; Kalindekafe, L.; Forman, S. L.; McHargue, L. R.; Singer, B. S.

    2005-12-01

    Lake Malawi is one of the largest and oldest lakes in the southern hemisphere and in the tropical latitudes, measuring more than 580 km in length, with a maximum depth of 700 m, and with an estimated age of more than 7 Ma. The lake water column is anoxic below 200 m, thus preserving an extensive record of laminated (varved) sediments. The lake's hydrologic budget is hypersensitive to minor changes in precipitation:evaporation, and signals of lake level and limnologic change respond markedly to regional climate variations. During February-March 2005 a scientific drilling program was conducted on the deep, open waters of Lake Malawi, using a converted local fuel barge outfitted with a deep-water-capable geotechnical drilling rig, a portable dynamic positioning system, and a suite of sampling tools designed for deep lake coring. A total of 623 m of core was recovered in seven holes at two sites on the lake. The high-resolution North Basin site (water depth 359 m) was triple-cored, providing redundant sampling of the sedimentary section representing the past ~75-100 kyr; the Central Basin deep site (592 m water depth) was double cored over the period of the past ~200 kyr, and single-cored to 380 m, covering perhaps 1.5 million years. The North Basin high-resolution site targeted a hemipelagic sequence, which in core samples consists of alternating zones of laminated and homogenous silty mud. At the base of these cores we observe well-sorted medium-grained shoreface sand deposits, that we interpret to be a transgressive surface deposited following a severe (500 m) lake lowstand. Density, magnetic susceptibility, TOC, and C/N profiles closely follow lithologic changes, and correspond to periods of enhanced or diminished productivity and dramatically varying lake level. Initial dating of the North Basin site (total core length 38 m) is underway using a combination of Radiocarbon, Tephra Ar-Ar, and Optically-Stimulated Luminescence methods, and initial results indicate a

  3. The Timing of Early Magmatism and Extension in the Southern East African Rift: Tracking Geochemical Source Variability with 40Ar/39Ar Geochronology at the Rungwe Volcanic Province, SW Tanzania

    NASA Astrophysics Data System (ADS)

    Mesko, G. T.; Class, C.; Maqway, M. D.; Boniface, N.; Manya, S.; Hemming, S. R.

    2014-12-01

    The Rungwe Volcanic Province is the southernmost expression of volcanism in the East African Rift System. Rungwe magmatism is focused in a transfer zone between two weakly extended rift segments, unlike more developed rifts where magmatism occurs along segment axes (e.g. mid-ocean ridges). Rungwe was selected as the site of the multinational SEGMeNT project, an integrated geophysical, geochronological and geochemical study to determine the role of magmatism during early stage continental rifting. Argon geochronology is underway for an extensive collection of Rungwe volcanic rocks to date the eruptive sequence with emphasis on the oldest events. The age and location of the earliest events remains contested, but is critical to evaluating the relationship between magmatism and extension. Dated samples are further analyzed to model the geochemistry and isotopic signature of each melt's source and define it as lithospheric, asthenospheric, or plume. Given the goals, the geochronology focuses on mafic lavas most likely to preserve the geochemical signature of the mantle source. Groundmass was prepared and analyzed at the LDEO AGES lab. Twelve preliminary dates yield ages from 8.5 to 5.7Ma, consistent with prior results, supporting an eruptive episode concurrent with tectonic activity on the Malawi and Rukwa border faults (Ebinger et al., JGR 1989; 1993). Three additional samples yield ages from 18.51 to 17.6 Ma, consistent with the 18.6 ±1.0 Ma age obtained by Rasskazov et al. (Russ. Geology & Geophys. 2003). This eruptive episode is spatially limited to phonolite domes in the Usangu Basin and a mafic lava flow on the uplifted Mbeya Block. These eruptions predate the current tectonic extensional structure, suggesting magmatism predates extension, or that the two are not highly interdependent. No Rungwe samples dated yet can be the source of the of 26Ma carbonatitic tuffs in the nearby Songwe River Basin sequence (Roberts et al., Nature Geoscience 2012). Isochron ages

  4. Magma-tectonic interactions in Kīlauea's Southwest Rift Zone in 2006 through coupled geodetic/seismological analysis

    NASA Astrophysics Data System (ADS)

    Wauthier, C.; Roman, D. C.; Poland, M. P.

    2015-12-01

    For much of the first 20 years of Kīlauea's 1983-present Pu'u 'Ō'ō eruption, deformation was characterized by subsidence at the volcano's summit and along both the East Rift Zone (ERZ) and Southwest Rift Zone (SWRZ). At the end of 2003, however, Kīlauea's summit began a 4-year period of inflation due to a surge in magma supply to the volcano. In 2006, the SWRZ also experienced atypical inflation, which was last observed in 1981-82 during a series of dike intrusions. To investigate the active magma sources and their interactions with faulting in the SWRZ during 2006, we integrate contemporary geodetic data from InSAR and GPS with double-couple fault-plane solutions for volcano-tectonic earthquakes and Coulomb stress modeling. According to the rate of deformation measured in daily GPS data, two distinct periods can be defined, spanning January to 15 March 2006 (period 1) and 16 March to 30 September 2006 (period 2). Geodetic models suggest that, during period 1, deformation, due to pressurization of magma in a vertical prolate-spheroidal conduit, in the south caldera area. In addition, a major seismic swarm occurred in both the SWRZ and ERZ. Our preliminary results also suggest that, during period 2, magma was still overpressurizing the same prolate-spheroid but a subhorizontal sill also intruded further to the southwest in the seismic SWRZ (SSWRZ). The beginning of period 2 also corresponds to a switch from subsidence to inflation of the SWRZ. Faulting in the upper ERZ is primarily strike-slip, with no obvious change in FPS orientation between periods 1 and 2. In contrast, faulting in the upper SSWRZ occurs as dip-slip motion on near-vertical faults. SSWRZ FPS show a mix of orientations including NW- and NE-striking faults, which along with relative earthquake locations, suggest a series of right-stepping fault segments, particularly during period 2. Calculated Coulomb stress changes indicate that faulting in the upper SSWRZ may result from stresses produced by

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

    NASA Astrophysics Data System (ADS)

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

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

  6. How Mountains Become Rifts

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  7. Extension and Basin Evolution of the East Kivu Graben, Rwanda, East African Rift: Results of New Multichannel Seismic Reflection Imaging

    NASA Astrophysics Data System (ADS)

    Scholz, C. A.; Zhang, X.; Wood, D.; Mburu, D.

    2012-12-01

    The East Kivu Graben resides within the eastern part of Lake Kivu, the highest Great Lake in the western branch of the East African Rift. The lake is more than 440 m deep in the East Kivu Basin, with a catchment comprised of Precambrian metasedimentary rocks and late-Cenozoic volcanics. Lake Kivu is renowned for its uniquely stratified water column, which is charged with considerable quantities of dissolved CO2 and methane, the former due to magmatic degassing. In February and March 2012 514 km of single- and multi-channel seismic reflection data were acquired in the Rwandan waters of Lake Kivu. The 24-fold multichannel seismic data were acquired aboard a modular research vessel, using a 600 m-long hydrophone streamer and single 40 cubic inch airgun. Extension in the East Kivu basin is largely accommodated along a major N-S striking, east-dipping boundary fault observed along the eastern edge of Iwawa Island, and extending for ~40 km along the length of the basin. Numerous intrabasinal normal faults occur to the east of the boundary fault, commonly displacing the lake floor and controlling the location of modern sublacustrine channels. The deepest sedimentary reflections observed on the new MCS data are 1.2-1.5 km below lake floor, near the center of the basin and boundary fault. Crystalline basement is not observed in these deepest areas however, suggesting the presence of a substantial sedimentary section below the imaged strata. Stratal surfaces dip steeply to the west over large areas of the half-graben basin. An acoustically transparent seismic sequence up to ~25 m thick is observed at the lake floor, which overlies a pronounced erosional unconformity over much of the basin. Some intrabasinal normal faults are draped by and do not penetrate the upper sequences, indicating several generations of fault activity in the basin. The late-Pleistocene exposure surface likely correlates to the previous lake level low stage that persisted prior to volcanic damming by

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

    NASA Astrophysics Data System (ADS)

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

    1996-05-01

    The assessment of water resources in the Rift Valley environment is important for population, agriculture and energy-related issues and depends on a good understanding of the relationship between freshwater lakes and regional groundwater. This can be hampered by the amount of fluid-rock interaction which occurs throughout the rift, obscuring original hydrochemical signatures. However, O and H stable isotope ratios can be used as tracers of infiltration over sometimes considerable distances, while showing that the volcanic edifices of the rift floor have varying effects on groundwater flow patterns. Specific cases from Kenya and Ethiopia are considered, including Lakes Naivasha, Baringo, Awasa and Zwai. In addition to their physical tracing role, stable isotopes can reveal information about processes of fluid-rock interaction. The general lack of O isotope shifting in rift hydrothermal systems suggests a high water:rock ratio, with the implication that these systems are mature. Carbon isotope studies on the predominantly bicarbonate waters of the rift show how they evolve from dilute meteoric recharge to highly alkaline waters, via the widespread silicate hydrolysis promoted by the flux of mantle carbon dioxide which occurs in most parts of the rift. There appears to be only minor differences in the C cycle between Kenya and Ethiopia.

  9. Hydrological constraints of paleo-Lake Suguta in the Northern Kenya Rift during the African Humid Period (15-5 ka BP)

    NASA Astrophysics Data System (ADS)

    Junginger, Annett; Trauth, Martin H.

    2013-12-01

    During the African Humid Period (AHP, 15-5 ka BP) an almost 300 m deep paleo-lake covering 2200 km2 developed in the Suguta Valley, in the Northern Kenya Rift. Data from lacustrine sediments and paleo-shorelines indicate that a large paleo-lake already existed by 13.9 ka BP, and record rapid water level fluctuations of up to 100 m within periods of 100 years or less, and a final lowstand at the end of the AHP (5 ka BP). We used a hydro-balance model to assess the abruptness of these water level fluctuations and identify their causes. We observed that fluctuations within the AHP were caused by abrupt changes in precipitation of 26-40%. Despite the absence of continuous lacustrine data documenting the onset of the AHP in the Suguta Valley, we conclude from the hydro-balance model that only an abrupt onset to the AHP, prior to 14.8 ka BP, could have led to high water levels recorded. The modeling results suggest that the sudden increase in rainfall was the direct consequence of an eastward migration of the Congo Air Boundary (CAB), caused by an enhanced atmospheric pressure gradient between East Africa and southern Asia during a northern hemisphere (NH) summer insolation maximum. In contrast, the end of the AHP must have been gradual despite an abrupt change in the source of precipitation when a decreasing pressure gradient between Asia and Africa prevented the CAB from reaching the study area. This abruptness was probably buffered by a contemporaneous change in precession producing an insolation maximum at the equator during September-October. This change would have meant that the only rain source was the Intertropical Convergence Zone (ITCZ), which would have carried a greater amount of moisture during the short rainy season thus slowing the fall in water level over a period of about 1000 years in association with the reduction in insolation. The results of this study provide an indication of the amount of time available for humans in north-eastern Africa to adapt

  10. Episodic Rifting Events Within the Tjörnes Fracture Zone, an Onshore-Offshore Ridge-Transform in N-Iceland

    NASA Astrophysics Data System (ADS)

    Brandsdottir, B.; Magnusdottir, S.; Karson, J. A.; Detrick, R. S.; Driscoll, N. W.

    2015-12-01

    The multi-branched plate boundary across Iceland is made up of divergent and oblique rifts, and transform zones, characterized by entwined extensional and transform tectonics. The Tjörnes Fracture Zone (TFZ), located on the coast and offshore Northern Iceland, is a complex transform linking the northern rift zone (NVZ) on land with the Kolbeinsey Ridge offshore. Extension across TFZ is partitioned across three N-S trending rift basins; Eyjafjarðaráll, Skjálfandadjúp (SB) and Öxarfjörður and three WNW-NW oriented seismic lineaments; the Grímsey Oblique Rift, Húsavík-Flatey Faults (HFFs) and Dalvík Lineament. We compile the tectonic framework of the TFZ ridge-transform from aerial photos, satellite images, multibeam bathymetry and high-resolution seismic reflection data (Chirp). The rift basins are made up of normal faults with vertical displacements of up to 50-60 m, and post-glacial sediments of variable thickness. The SB comprises N5°W obliquely trending, eastward dipping normal faults as well as N10°E striking, westward dipping faults oriented roughly perpendicular to the N104°E spreading direction, indicative of early stages of rifting. Correlation of Chirp reflection data and tephrachronology from a sediment core within SB reveal major rifting episodes between 10-12.1 kyrs BP activating the whole basin, followed by smaller-scale fault movements throughout Holocene. Onshore faults have the same orientations as those mapped offshore and provide a basis for the interpretation of the kinematics of the faults throughout the region. These include transform parallel right-lateral, strike-slip faults separating domains dominated by spreading parallel left-lateral bookshelf faults. Shearing is most prominent along the HFFs, a system of right-lateral strike-slip faults with vertical displacement up to 15 m. Vertical fault movements reflect increased tectonic activity during early postglacial time coinciding with isostatic rebound enhancing volcanism within

  11. Hierarchical segmentation of the Malawi Rift: The influence of inherited lithospheric heterogeneity and kinematics in the evolution of continental rifts

    NASA Astrophysics Data System (ADS)

    Laó-Dávila, Daniel A.; Al-Salmi, Haifa S.; Abdelsalam, Mohamed G.; Atekwana, Estella A.

    2015-12-01

    We used detailed analysis of Shuttle Radar Topography Mission-digital elevation model and observations from aeromagnetic data to examine the influence of inherited lithospheric heterogeneity and kinematics in the segmentation of largely amagmatic continental rifts. We focused on the Cenozoic Malawi Rift, which represents the southern extension of the Western Branch of the East African Rift System. This north trending rift traverses Precambrian and Paleozoic-Mesozoic structures of different orientations. We found that the rift can be hierarchically divided into first-order and second-order segments. In the first-order segmentation, we divided the rift into Northern, Central, and Southern sections. In its Northern Section, the rift follows Paleoproterozoic and Neoproterozoic terrains with structural grain that favored the localization of extension within well-developed border faults. The Central Section occurs within Mesoproterozoic-Neoproterozoic terrain with regional structures oblique to the rift extent. We propose that the lack of inherited lithospheric heterogeneity favoring extension localization resulted in the development of the rift in this section as a shallow graben with undeveloped border faults. In the Southern Section, Mesoproterozoic-Neoproterozoic rocks were reactivated and developed the border faults. In the second-order segmentation, only observed in the Northern Section, we divided the section into five segments that approximate four half-grabens/asymmetrical grabens with alternating polarities. The change of polarity coincides with flip-over full-grabens occurring within overlap zones associated with ~150 km long alternating border faults segments. The inherited lithospheric heterogeneity played the major role in facilitating the segmentation of the Malawi Rift during its opening resulting from extension.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  13. Geophysical evidence of pre-sag rifting and post-rifting fault reactivation in the Parnaíba basin, Brazil

    NASA Astrophysics Data System (ADS)

    Lopes de Castro, David; Hilário Bezerra, Francisco; Adolfo Fuck, Reinhardt; Vidotti, Roberta Mary

    2016-04-01

    This study investigated the rifting mechanism that preceded the prolonged subsidence of the Paleozoic Parnaíba basin in Brazil and shed light on the tectonic evolution of this large cratonic basin in the South American platform. From the analysis of aeromagnetic, aerogravity, seismic reflection and borehole data, we concluded the following: (1) large pseudo-gravity and gravity lows mimic graben structures but are associated with linear supracrustal strips in the basement. (2) Seismic data indicate that 120-200 km wide and up to 300 km long rift zones occur in other parts of the basins. These rift zones mark the early stage of the 3.5 km thick sag basin. (3) The rifting phase occurred in the early Paleozoic and had a subsidence rate of 47 m Myr-1. (4) This rifting phase was followed by a long period of sag basin subsidence at a rate of 9.5 m Myr-1 between the Silurian and the late Cretaceous, during which rift faults propagated and influenced deposition. These data interpretations support the following succession of events: (1) after the Brasiliano orogeny (740-580 Ma), brittle reactivation of ductile basement shear zones led to normal and dextral oblique-slip faulting concentrated along the Transbrasiliano Lineament, a continental-scale shear zone that marks the boundary between basement crustal blocks. (2) The post-orogenic tectonic brittle reactivation of the ductile basement shear zones led to normal faulting associated with dextral oblique-slip crustal extension. In the west, pure-shear extension induced the formation of rift zones that crosscut metamorphic foliations and shear zones within the Parnaíba block. (3) The rift faults experienced multiple reactivation phases. (4) Similar processes may have occurred in coeval basins in the Laurentia and Central African blocks of Gondwana.

  14. Geochemistry of 24 Ma Basalts from Northeast Egypt: Implications for Small-Scale Convection Beneath the East African Rift System

    NASA Astrophysics Data System (ADS)

    Endress, C. A.; Furman, T.; Ali Abu El-Rus, M.

    2009-12-01

    Basalts ~24 Ma in the Cairo-Suez and Fayyum districts of NE Egypt represent the youngest and northernmost lavas potentially associated with the initiation of rifting of the Red Sea. The age of these basalts corresponds to a time period of significant regional magmatism that occurred subsequent to emplacement of 30 Ma flood basalts attributed to the Afar Plume in Ethiopia and Yemen. Beginning ~28 Ma, widespread magmatism occurred across supra-equatorial Africa in Hoggar (Algeria), Tibesti (Chad), Darfur (Sudan), Turkana (Kenya) and Samalat, Bahariya, Quesir and the Sinai Peninsula (Egypt) (e.g. Allegre et al., 1981; Meneisy, 1990; Baldridge et al., 1991; Wilson and Guiraud, 1992; Furman et al., 2006; Lucassen et al., 2008). Available geochemical and isotopic data indicate that Hoggar and Darfur basalts are similar to Turkana lavas, although no direct link between the N African lavas and the Kenya Plume has been made. New geochemical data on the NE Egyptian basalts provide insight into the thermochemical, isotopic, and mineralogical characteristics of the mantle beneath the region in which they were emplaced. The basalts are subalkaline with OIB-like incompatible trace element abundances and homogeneous major element, trace element and isotopic geochemistry. They display relatively flat ITE patterns, with notable positive Pb and negative P anomalies. Isotopic (143Nd/144Nd = 0.51274-0.51285, 87Sr/86Sr = 0.7049-0.7050) and trace element signatures (Ce/Pb = 16-22, Ba/Nb = 9-14, and La/Nb = 0.9-1.0) are consistent with melting of a sub-lithospheric source that has been slightly contaminated by continental crust during ascent and emplacement. The Pb isotopic ratios (206Pb/204Pb = 18.53-18.62, 207Pb/204Pb = 15.59-15.64, and 208Pb/204Pb = 38.80-39.00) in the Egyptian basalts are close to the range of those found in the 30 Ma Ethiopian flood basalts, which are distinct from the more highly radiogenic, high-μ type signature seen in basalts from Turkana, Darfur, and Hoggar

  15. Nature and evolution of lithospheric mantle beneath the southern Ethiopian rift zone: evidence from petrology and geochemistry of mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Alemayehu, Melesse; Zhang, Hong-Fu; Sakyi, Patrick Asamoah

    2016-06-01

    Mantle xenoliths hosted in Quaternary basaltic lavas from the Dillo and Megado areas of the southern Ethiopian rift are investigated to understand the geochemical composition and associated processes occurring in the lithospheric mantle beneath the region. The xenoliths are comprised of predominantly spinel lherzolite with subordinate harzburgite and clinopyroxenite. Fo content of olivine and Cr# of spinel for peridotites from both localities positively correlate and suggest the occurrence of variable degrees of partial melting and melt extraction. The clinopyroxene from lherzolites is both LREE depleted (La/Sm(N) = 0.11-0.37 × Cl) and LREE enriched (La/Sm(N) = 1.88-15.72 × Cl) with flat HREEs (Dy/Lu(N) = 0.96-1.31 × Cl). All clinopyroxene from the harzburgites and clinopyroxenites exhibits LREE-enriched (La/Sm(N) = 2.92-27.63.1 × Cl and, 0.45 and 1.38 × Cl, respectively) patterns with slight fractionation of HREE. The 143Nd/144Nd and 176Hf/177Hf ratios of clinopyroxene from lherzolite range from 0.51291 to 0.51370 and 0.28289 to 0.28385, respectively. Most of the samples define ages of 900 and 500 Ma on Sm-Nd and Lu-Hf reference isochrons, within the age range of Pan-African crustal formation. The initial Nd and Hf isotopic ratios were calculated at 1, 1.5, 2 and 2.5 Ga plot away from the trends defined by MORB, DMM and E-DMM which were determined from southern Ethiopian peridotites, thus indicating that the Dillo and Megado xenoliths could have been produced by melt extraction from the asthenosphere during the Pan-African orogenic event. There is no significant difference in 87Sr/86Sr ratios between the depleted and enriched clinopyroxene. This suggests that the melts that caused the enrichment of the clinopyroxene are mainly derived from the depleted asthenospheric mantle from which the xenoliths are extracted. Largely, the mineralogical and isotopic compositions of the xenoliths show heterogeneity of the CLM that could have been produced from various

  16. Multispectral thermal infrared mapping of sulfur dioxide plumes: A case study from the East Rift Zone of Kilauea Volcano, Hawaii

    USGS Publications Warehouse

    Realmuto, V.J.; Sutton, A.J.; Elias, T.

    1997-01-01

    The synoptic perspective and rapid mode of data acquisition provided by remote sensing are well suited for the study of volcanic SO2 plumes. In this paper we describe a plume-mapping procedure that is based on image data acquired with NASA's airborne thermal infrared multispectral scanner (TIMS) and apply the procedure to TIMS data collected over the East Rift Zone of Kilauea Volcano, Hawaii, on September 30, 1988. These image data covered the Pu'u 'O'o and Kupaianaha vents and a skylight in the lava tube that was draining the Kupaianaha lava pond. Our estimate of the SO2 emission rate from Pu'u 'O'o (17 - 20 kg s-1) is roughly twice the average of estimates derived from correlation spectrometer (COSPEC) measurements collected 10 days prior to the TIMS overflight (10 kg s-1). The agreement between the TIMS and COSPEC results improves when we compare SO2 burden estimates, which are relatively independent of wind speed. We demonstrate the feasibility of mapping Pu'u 'O'o - scale SO2 plumes from space in anticipation of the 1998 launch of the advanced spaceborne thermal emission and reflectance radiometer (ASTER). Copyright 1997 by the American Geophysical Union.

  17. Reconnaissance gas measurements on the East Rift Zone of Kilauea Volcano, Hawai'i by Fourier transform infrared spectroscopy

    USGS Publications Warehouse

    McGee, Kenneth A.; Elias, Tamar; Sutton, A. Jefferson; Doukas, Michael P.; Zemek, Peter G.; Gerlach, Terrence M.

    2005-01-01

    We report the results of a set of measurements of volcanic gases on two small ground level plumes in the vicinity of Pu`u `O`o cone on the middle East Rift Zone (ERZ) of Kilauea volcano, Hawai`i on 15 June 2001 using open-path Fourier transform infrared (FTIR) spectroscopy. The work was carried out as a reconnaissance survey to assess the monitoring and research value of FTIR measurements at this volcano. Despite representing emissions of residual volatiles from lava that has undergone prior degassing, the plumes contained detectable amounts of CO2, CO, SO2, HCl, HF and SiF4. Various processes, including subsurface cooling, condensation of water in the atmospheric plume, oxidation, dissolution in water, and reactions with wall rocks at plume vents affect the abundance of these gases. Low concentrations of volcanic CO2 measured against a high ambient background are not well constrained by FTIR spectroscopy. Although there appear to be some differences between these gases and Pu`u `O`o source gases, ratios of HCl/SO2, HF/SO2 and CO/SO2 determined by FTIR measurements of these two small plumes compare reasonably well with earlier published analyses of ERZ vent samples. The measurements yielded emission rate estimates of 4, 11 and 4 t d-1

  18. A volcanic province near the western termination of the Charlie-Gibbs Fracture Zone at the rifted margin, offshore northeast Newfoundland

    NASA Astrophysics Data System (ADS)

    Keen, C. E.; Dafoe, L. T.; Dickie, K.

    2014-06-01

    A mid-Cretaceous to Late Cretaceous volcanic province, named here the Charlie-Gibbs Volcanic Province, is described near the western termination of the Charlie-Gibbs Fracture Zone, against the rifted continental margin northeast of Newfoundland. We used seismic data to map 14 volcanic seamounts, now buried below younger sediments. They rise 0.7 to 2 s two-way time (twt) above the surrounding basement level and are about 8-30 km wide. Some are conical while others are more flat-topped. Underlying igneous units resembling flows and sills are also observed. Based on magnetic modeling of the large positive magnetic anomalies associated with the seamounts, the total thickness of igneous rocks can locally reach about 8 km. This magmatism occurred in the vicinity of the Charlie-Gibbs Fracture Zone and extends about 150 km to the north along the rifted continental margin. The volcanic province also forms the northern boundary of the Jurassic-Early Cretaceous Orphan Basin, along a major transform margin there. Truncation of rift-related structures which extend to deep crustal levels is observed at the transform, along trends similar to those of prerift Appalachian terrane boundaries on the adjacent shelf. This suggests the existence of a preexisting weak zone in the continental lithosphere within which a complex strike-slip fault system developed and may have controlled the location of final continental breakup between the Rockall and North American plates in the Late Cretaceous.

  19. Geochemistry and petrology of andesites from the north rift zone of Axial Seamount, Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Smithka, I. N.; Perfit, M. R.; Clague, D. A.; Wanless, V. D.

    2014-12-01

    In 2013, the ROV Doc Ricketts onboard R/V Western Flyer explored ~4 km of an elongate pillow ridge up to ~300 m high along the eastern edge of the north rift zone of Axial Seamount. The steep-sided volcanic ridge is constructed of large pillow lavas up to 2-3 m in diameter and smaller elongated pillow tubes. Of the 27 samples collected during dive D526, all but one are andesites making it one of the largest confirmed high-silica exposures along a mid-ocean ridge (MOR). Based on radiocarbon ages of sediment on top of flows, the mounds are at least ~1390 years old. This minimum age is much younger than the 56 Ka age calculated based on distance from the rift axis, indicating eruption off-axis through older, colder crust and supporting the hypothesis and model calculations that extensive fractional crystallization (>85%) caused the high silica content. The andesitic lavas are primarily glassy, highly vesicular, crusty, and sparsely phyric with small (~1 mm) plagioclase crystals and olivine, clinopyroxene, and Fe-Ti oxide microphenocrysts. Microprobe analyses of glasses are similar to wax-core samples previously collected from this area but are more compositionally variable. Excluding one basalt (7.7 wt% MgO) sampled between mounds, the lavas are basaltic andesites and andesites (53-59 wt% SiO2) with <3 wt% MgO and 12.8-15.7 wt% FeO concentrations. Incompatible trace element abundances are ~4-6 times more enriched than in Axial Seamount T-MORB. Primitive mantle-normalized patterns are similar to those of high-silica lavas from other MORs (southern Juan de Fuca Ridge, 9N East Pacific Rise) with significant positive U anomalies, large negative Sr anomalies, small negative Eu anomalies, and slight positive Zr-Hf anomalies. The andesites are more enriched in light rare earth elements than basalts from Axial Seamount ((La/Yb)N 1.35-1.4 vs. 0.7-1.27) and N-MORB from the southern Juan de Fuca Ridge. The andesites also have high Cl (~0.3-0.6 wt%) and H2O (~1.60-1.71 wt

  20. Inferno Chasm Rift Zone, Idaho: A Terrestrial Analog for Plains-style Volcanism in Southeastern Mare Serenitatis on the Moon

    NASA Astrophysics Data System (ADS)

    Garry, W. B.; Hughes, S. S.; Kobs-Nawotniak, S. E.

    2015-12-01

    Volcanic features aligned along a linear graben in southeastern Mare Serenitatis (19°N, 27.5°E) on the Moon resemble a series of effusive basaltic landforms erupted along the Inferno Chasm rift zone within Craters of the Moon National Monument and Preserve (COTM), Idaho (42°58'00"N, 113°11'25"W). This region in Idaho is the type-locale for terrestrial plains-style volcanism. Examples of lunar plains-style volcanism have previously been described within Orientale Basin at Lacus Veris and Lacus Autumni, but this eruption style has not been used to describe the site in Mare Serenitatis. The SSERVI FINESSE team (Field Investigations to Enable Solar System Science and Exploration) has documented the features along Inferno Chasm rift using a LiDAR, Differential Global Positioning Systems, and Unmanned Aerial Vehicles (UAV) to compare with Lunar Reconnaissance Orbiter Narrow-Angle Camera images and digital terrain models. The region in southeastern Mare Serenitatis provides one of the best concentrations of features representative of lunar plains-style volcanism. On the Moon, these features include a cone (Osiris), a flat-topped dome, a rille-like channel (Isis), a vent, and a possible perched lava pond. In Idaho, the analog features include a dome (Grand View Crater), a rille-like channel (Inferno Chasm), vents (Cottrells Blowout, Horse Butte), and a perched lava pond (Papadakis). Both the scale and morphology of the features on the Moon are similar to the features in Idaho. For example, the channel in Isis is ~3 km long, 283 m-wide, and 25 m deep compared to Inferno Chasm which is ~1.7 km long, 100 m wide, and 20 m deep. The slope of the channel in Isis is -1.2°, while the channel in Inferno Chasm has a slope of -0.33°. The alignment of landforms on the Moon and Idaho are both consistent with dike emplacement. Observations of the flow stratigraphy for features in Idaho will inform the potential eruption conditions of the individual features on the Moon.

  1. An integrated geophysical study of the northern Kenya rift

    NASA Astrophysics Data System (ADS)

    Mariita, Nicolas O.; Keller, G. Randy

    2007-06-01

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

  2. Constraining the Composition of the Subcontinental Lithospheric Mantle Beneath the East African Rift: FTIR Analysis of Water in Spinel Peridotite Mantle Xenoliths

    NASA Technical Reports Server (NTRS)

    Erickson, Stephanie Gwen; Nelson, Wendy R.; Peslier, Anne H.; Snow, Jonathan E.

    2014-01-01

    The East African Rift System was initiated by the impingement of the Afar mantle plume on the base of the non-cratonic continental lithosphere (assembled during the Pan-African Orogeny), producing over 300,000 kmof continental flood basalts approx.30 Ma ago. The contribution of the subcontinental lithospheric mantle (SCLM) to this voluminous period of volcanism is implied based on basaltic geochemical and isotopic data. However, the role of percolating melts on the SCLM composition is less clear. Metasomatism is capable of hybridizing or overprinting the geochemical signature of the SCLM. In addition, models suggest that adding fluids to lithospheric mantle affects its stability. We investigated the nature of the SCLM using Fourier transform infrared spectrometry (FTIR) to measure water content in mantle xenoliths entrained in young (1 Ma) basaltic lavas from the Ethiopian volcanic province. The mantle xenoliths consist dominantly of spinel lherzolites and are composed of nominally anhydrous minerals, which can contain trace water as H in mineral defects. Eleven mantle xenoliths come from the Injibara-Gojam region and two from the Mega-Sidamo region. Water abundances of olivines in six samples are 1-5ppm H2O while the rest are below the limit of detection (<0.5 ppm H2O); orthopyroxene and clinopyroxene contain 80-238 and 111-340 ppm wt H2O, respectively. Two xenoliths have higher water contents - a websterite (470 ppm) and dunite (229 ppm), consistent with involvement of ascending melts. The low water content of the upper SCLM beneath Ethiopia is as dry as the oceanic mantle except for small domains represented by percolating melts. Consequently, rifting of the East African lithosphere may not have been facilitated by a hydrated upper mantle.

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

    NASA Astrophysics Data System (ADS)

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

    2005-08-01

    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 (δVP~-1.5 per cent, δVS~-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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  6. Transient Hydrothermal Alteration in Fault Zones Cutting the Lower Oceanic Crust, Hess Deep Rift

    NASA Astrophysics Data System (ADS)

    McCaig, Andrew; Titarenko, Sofya; Cliff, Robert; Ivan, Savov; Adrian, Boyce

    2015-04-01

    IODP Expedition 345 drilled the first holes in the lower plutonic crust at a fast-spreading ridge, recovering primitive layered gabbros [1]. Alteration occurred as: 1) a largely static pseudomorphic alteration, predominantly in the greenschist and sub-greenschist facies with mainly talc and serpentine replacing olivine, and prehnite replacing plagioclase. Talc sometimes overprints serpentine mesh texture. 2) an overprinting metasomatic alteration, spatially related to cataclastic fault zones and macroscopic veins, dominated by prehnite and chlorite. Secondary clinopyroxene and epidote locally overprint the prehnite-chlorite assemblage, but the last events are veins of prehnite and zeolite. Metamorphosed dykes show chilled margins within the cataclasites, and are themselves affected by cataclastic deformation. Faults, dykes and overprinting alteration are all inferred to be related to the westward propagation of Cocos-Nazca spreading forming Hess Deep. 87Sr/86Sr ratios of small whole rock samples of cataclasites and dyke rocks are in the range 0.7037 - 0.7048, indicating alteration by seawater at moderate integrated fluxes. The highest values were in cataclasites overprinted by prehnite. Sampling of individual minerals has been undertaken using a microscope mounted drill, and shows that alteration is mainly affecting secondary minerals, with late prehnite veins ranging up to Sr isotope ratios of 0.7054. δ18O values range from +1 to + 6 per mil. Combined with metamorphic data this indicates alteration at temperatures between 200 and 400 °C. Secondary clinopyroxene and talc replacing serpentine are interpreted to indicate transient prograde hydrothermal events. Preliminary modelling using Comsol Multiphysics suggests that the temperatures of the overprinting alteration, as well as transient prograde events, could be achieved in a permeable fault slot cutting through crust 0.5 to 1 m.y. old. The prehnite-chlorite assemblage is predicted to be important in off

  7. Transient Hydrothermal Alteration In Fault Zones Cutting The Lower Oceanic Crust, Hess Deep Rift

    NASA Astrophysics Data System (ADS)

    McCaig, A. M.; Titarenko, S.; Cliff, R. A.; Savov, I. P.; Boyce, A.; Dutt, R.

    2014-12-01

    IODP Expedition 345 drilled the first holes in the lower plutonic crust at a fast-spreading ridge, recovering primitive layered gabbros [1]. Alteration occurred as: 1) a largely static pseudomorphic alteration, predominantly in the greenschist and sub-greenschist facies with mainly talc and serpentine replacing olivine, and prehnite replacing plagioclase. Talc sometimes overprints serpentine mesh texture. 2) an overprinting metasomatic alteration, spatially related to cataclastic fault zones and macroscopic veins, dominated by prehnite and chlorite. Secondary clinopyroxene and epidote locally overprint the prehnite-chlorite assemblage, but the last events are veins of prehnite and zeolite. Metamorphosed dykes show chilled margins within the cataclasites, and are themselves affected by cataclastic deformation. Faults, dykes and overprinting alteration are inferred to be related to the westward propagation of Cocos-Nazca spreading forming Hess Deep. 87Sr/86Sr ratios of small whole rock samples of cataclasites and dyke rocks are in the range 0.7037 - 0.7048, indicating alteration by seawater at moderate integrated fluxes. The highest values were in cataclasites overprinted by prehnite. Sampling of individual minerals has been undertaken using a microscope mounted drill, and shows that alteration is mainly affecting secondary minerals, with late prehnite veins ranging up to 0.7054. δ18O values range from +1 to + 6 per mil. Combined with metamorphic data this indicates alteration at temperatures between 200 and 400 °C. Secondary clinopyroxene and talc replacing serpentine are interpreted to indicate transient prograde hydrothermal events. Preliminary modelling using Comsol Multiphysics suggests that the temperatures of the overprinting alteration, as well as transient prograde events, could be achieved in a permeable fault slot cutting through crust 0.5 to 1 m.y. old. The prehnite-chlorite assemblage is predicted to be important in off-axis alteration, common in any

  8. Gravity anomaly and crustal density structure in Jilantai rift zone and its adjacent region

    NASA Astrophysics Data System (ADS)

    Wu, Guiju; Shen, Chongyang; Tan, Hongbo; Yang, Guangliang

    2016-08-01

    This paper deals with the interpretation of Bouguer gravity anomalies measured along a 250 km long Suhaitu-Etuokeqi gravity profile located at the transitional zone of the Alxa and Ordos blocks where geophysical characteristics are very complex. The analysis is carried out in terms of the ratio of elevation and Bouguer gravity anomaly, the normalized full gradient of a section of the Bouguer gravity anomaly ( G h ) and the crustal density structure reveal that (1) the ratio of highs and lows of elevation and Bouguer gravity anomaly is large between Zhengyiguan fault (F4) and Helandonglu fault (F6), which can be explained due to crustal inhomogeneities related to the uplift of the Qinghai-Tibet block in the northeast; (2) the main active faults correspond to the G h contour strip or cut the local region, and generally show strong deformation characteristics, for example the Bayanwulashan mountain front fault ( F1) or the southeast boundary of Alxa block is in accord with the western change belt of G h , a belt about 10 km wide that extends to about 30 km; (3) Yinchuan-Pingluo fault ( F8) is the seismogenic structure of the Pingluo M earthquake, and its focal depth is about 15 km; (4) the Moho depth trend and Bouguer gravity anomaly variation indicates that the regional gravity field is strongly correlated with the Moho discontinuity.

  9. Gravity anomaly and crustal density structure in Jilantai rift zone and its adjacent region

    NASA Astrophysics Data System (ADS)

    Wu, Guiju; Shen, Chongyang; Tan, Hongbo; Yang, Guangliang

    2016-08-01

    This paper deals with the interpretation of Bouguer gravity anomalies measured along a 250 km long Suhaitu-Etuokeqi gravity profile located at the transitional zone of the Alxa and Ordos blocks where geophysical characteristics are very complex. The analysis is carried out in terms of the ratio of elevation and Bouguer gravity anomaly, the normalized full gradient of a section of the Bouguer gravity anomaly (G h ) and the crustal density structure reveal that (1) the ratio of highs and lows of elevation and Bouguer gravity anomaly is large between Zhengyiguan fault (F4) and Helandonglu fault (F6), which can be explained due to crustal inhomogeneities related to the uplift of the Qinghai-Tibet block in the northeast; (2) the main active faults correspond to the G h contour strip or cut the local region, and generally show strong deformation characteristics, for example the Bayanwulashan mountain front fault (F1) or the southeast boundary of Alxa block is in accord with the western change belt of G h , a belt about 10 km wide that extends to about 30 km; (3) Yinchuan-Pingluo fault (F8) is the seismogenic structure of the Pingluo M earthquake, and its focal depth is about 15 km; (4) the Moho depth trend and Bouguer gravity anomaly variation indicates that the regional gravity field is strongly correlated with the Moho discontinuity.

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

    NASA Astrophysics Data System (ADS)

    Neukirchen, Florian; Finkenbein, Thomas; Keller, Jörg

    2010-12-01

    A 500 m sequence of horizontal lava flows forms the Gregory rift escarpment of the western rift shoulder between Lake Natron and Oldoinyo Lengai. A detailed volcanic stratigraphy of this >1.2 Ma evolution of the EAR in Northern Tanzania is presented. The sequence is formed by several distinct rock suites, with increasing alkalinity from base to top. Alkali olivine basalts of the Waterfall Sequence at the base are followed by a basanite series, and by a range of evolved nephelinites forming the upper part of the escarpment. Numerous dykes and Strombolian scoria deposits indicate local fissure eruptions as opposed to or in addition to more distant sources. Primitive compositions within each of the series indicate variable candidates for primary magmas. The composition of the basanite suite ranges from primitive mantle melts (high Mg#, Cr, Ni) to more evolved rocks, in particular hawaiites, generated by fractionation of olivine, pyroxene and magnetite. Inter-bedded within the basanite suite, one single olivine melilitite flow with high Mg# and abundant olivine and pyroxene megacrysts is the only primitive candidate for the nephelinite suite. However, in view of the large compositional gap and marked differences in incompatible element ratios, a relation between this flow and the nephelinites remains hypothetical. The variation within the evolved nephelinite series can be partly explained by fractionation of pyroxene, apatite, perovskite (and some nepheline), while magma mixing is indicated by zonation patterns of pyroxene. The most evolved nephelinite, however, differs significantly from all other nephelinites in major and trace elements. Thus the entire sequence is petrologically not a coherent evolution, rather the result of different mantle melts fractionating under variable conditions. Carved into the rift scarp of the study area west of Engare Sero is a young explosion crater, the Sekenge Crater. Sekenge Tuffs are olivine melilitites, similar to other craters and

  11. The Olorgesailie Drilling Project (ODP): a high-resolution drill core record from a hominin site in the East African Rift Valley

    NASA Astrophysics Data System (ADS)

    Dommain, R.; Potts, R.; Behrensmeyer, A. K.; Deino, A. L.

    2014-12-01

    The East African rift valley contains an outstanding record of hominin fossils that document human evolution over the Plio-Pleistocene when the global and regional climate and the rift valley itself changed markedly. The sediments of fossil localities typically provide, however, only short time windows into past climatic and environmental conditions. Continuous, long-term terrestrial records are now becoming available through core drilling to help elucidate the paleoenvironmental context of human evolution. Here we present a 500,000 year long high-resolution drill core record obtained from a key fossil and archeological site - the Olorgesailie Basin in the southern Kenya Rift Valley, well known for its sequence of archeological and faunal sites for the past 1.2 million years. In 2012 two drill cores (54 and 166 m long) were collected in the Koora Plain just south of Mt. Olorgesailie as part of the Olorgesailie Drilling Project (ODP) to establish a detailed climate and ecological record associated with the last evidence of Homo erectus in Africa, the oldest transition of Acheulean to Middle Stone Age technology, and large mammal species turnover, all of which are documented in the Olorgesailie excavations. The cores were sampled at the National Lacustrine Core Facility. More than 140 samples of tephra and trachytic basement lavas have led to high-precision 40Ar/39Ar dating. The cores are being analyzed for a suite of paleoclimatic and paleoecological proxies such as diatoms, pollen, fungal spores, phytoliths, ostracodes, carbonate isotopes, leaf wax biomarkers, charcoal, and clay mineralogy. Sedimentological analyses, including lithological descriptions, microscopic smear slide analysis (242 samples), and grain-size analysis, reveal a highly variable sedimentary sequence of deep lake phases with laminated sediments, diatomites, shallow lake and near shore phases, fluvial deposits, paleosols, interspersed carbonate layers, and abundant volcanic ash deposits. Magnetic

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

    NASA Astrophysics Data System (ADS)

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

    2015-02-01

    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.

  13. Archaeology in the Kilauea East Rift Zone: Part 1, Land-use model and research design, Kapoho, Kamaili and Kilauea Geothermal Subzones, Puna District, Hawaii Island

    SciTech Connect

    Burtchard, G.C.; Moblo, P.

    1994-07-01

    The Puna Geothermal Resource Subzones (GRS) project area encompasses approximately 22,000 acres centered on the Kilauea East Rift Zone in Puna District, Hawaii Island. The area is divided into three subzones proposed for geothermal power development -- Kilauea Middle East Rift, Kamaili and Kapoho GRS. Throughout the time of human occupation, eruptive episodes along the rift have maintained a dynamic landscape. Periodic volcanic events, for example, have changed the coastline configuration, altered patterns of agriculturally suitable sediments, and created an assortment of periodically active, periodically quiescent, volcanic hazards. Because of the active character of the rift zone, then, the area`s occupants have always been obliged to organize their use of the landscape to accommodate a dynamic mosaic of lava flow types and ages. While the specific configuration of settlements and agricultural areas necessarily changed in response to volcanic events, it is possible to anticipate general patterns in the manner in which populations used the landscape through time. This research design offers a model that predicts the spatial results of long-term land-use patterns and relates them to the character of the archaeological record of that use. In essence, the environmental/land-use model developed here predicts that highest population levels, and hence the greatest abundance and complexity of identifiable prehistoric remains, tended to cluster near the coast at places that maximized access to productive fisheries and agricultural soils. With the possible exception of a few inland settlements, the density of archaeological remains expected to decrease with distance from the coastline. The pattern is generally supported in the regions existing ethnohistoric and archaeological record.

  14. Regional assessment of lake ecological states using Landsat: A classification scheme for alkaline-saline, flamingo lakes in the East African Rift Valley

    NASA Astrophysics Data System (ADS)

    Tebbs, E. J.; Remedios, J. J.; Avery, S. T.; Rowland, C. S.; Harper, D. M.

    2015-08-01

    In situ reflectance measurements and Landsat satellite imagery were combined to develop an optical classification scheme for alkaline-saline lakes in the Eastern Rift Valley. The classification allows the ecological state and consequent value, in this case to Lesser Flamingos, to be determined using Landsat satellite imagery. Lesser Flamingos depend on a network of 15 alkaline-saline lakes in East African Rift Valley, where they feed by filtering cyanobacteria and benthic diatoms from the lakes' waters. The classification developed here was based on a decision tree which used the reflectance in Landsat ETM+ bands 2-4 to assign one of six classes: low phytoplankton biomass; suspended sediment-dominated; microphytobenthos; high cyanobacterial biomass; cyanobacterial scum and bleached cyanobacterial scum. The classification accuracy was 77% when verified against in situ measurements. Classified imagery and timeseries were produced for selected lakes, which show the different ecological behaviours of these complex systems. The results have highlighted the importance to flamingos of the food resources offered by the extremely remote Lake Logipi. This study has demonstrated the potential of high spatial resolution, low spectral resolution sensors for providing ecologically valuable information at a regional scale, for alkaline-saline lakes and similar hypereutrophic inland waters.

  15. Bentho-Pelagic Divergence of Cichlid Feeding Architecture Was Prodigious and Consistent during Multiple Adaptive Radiations within African Rift-Lakes

    PubMed Central

    Cooper, W. James; Parsons, Kevin; McIntyre, Alyssa; Kern, Brittany; McGee-Moore, Alana; Albertson, R. Craig

    2010-01-01

    Background How particular changes in functional morphology can repeatedly promote ecological diversification is an active area of evolutionary investigation. The African rift-lake cichlids offer a calibrated time series of the most dramatic adaptive radiations of vertebrate trophic morphology yet described, and the replicate nature of these events provides a unique opportunity to test whether common changes in functional morphology have repeatedly facilitated their ecological success. Methodology/Principal Findings Specimens from 87 genera of cichlid fishes endemic to Lakes Tanganyka, Malawi and Victoria were dissected in order to examine the functional morphology of cichlid feeding. We quantified shape using geometric morphometrics and compared patterns of morphological diversity using a series of analytical tests. The primary axes of divergence were conserved among all three radiations, and the most prevalent changes involved the size of the preorbital region of the skull. Even the fishes from the youngest of these lakes (Victoria), which exhibit the lowest amount of skull shape disparity, have undergone extensive preorbital evolution relative to other craniofacial traits. Such changes have large effects on feeding biomechanics, and can promote expansion into a wide array of niches along a bentho-pelagic ecomorphological axis. Conclusions/Significance Here we show that specific changes in trophic anatomy have evolved repeatedly in the African rift lakes, and our results suggest that simple morphological alterations that have large ecological consequences are likely to constitute critical components of adaptive radiations in functional morphology. Such shifts may precede more complex shape changes as lineages diversify into unoccupied niches. The data presented here, combined with observations of other fish lineages, suggest that the preorbital region represents an evolutionary module that can respond quickly to natural selection when fishes colonize new lakes

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  17. Orthorhombic fault fracture patterns and non-plane strain in a synthetic transfer zone during rifting: Lennard shelf, Canning basin, Western Australia

    NASA Astrophysics Data System (ADS)

    Miller, John McL.; Nelson, E. P.; Hitzman, M.; Muccilli, P.; Hall, W. D. M.

    2007-06-01

    A complex series of faults occur within transfer zones normal to the WNW-trending rifted northern margin of the Canning basin (Western Australia). These zones controlled basinal fluid flow and the formation of some carbonate-hosted Mississippi Valley-type Zn-Pb deposits along the basin margin during Devonian to Carboniferous rifting. The study area has a regional fault geometry similar to a synthetic overlapping transfer zone. Surface and underground mapping in this transfer zone, combined with 3D modelling, indicate the faults and related extension fractures have an orthorhombic geometry. The orthorhombic fault-fracture mesh developed in response to three-dimensional non-plane strain in which the intermediate finite extension magnitude was non-zero. Pre-mineralisation marine calcite fill in the fault-fracture mesh indicates that it formed early in the deformation history. Later deformation that overprints the Zn-Pb mineralisation and fault-fracture mesh, was associated with a different maximum extension direction and this modified and reactivated the faults with both dip-slip and oblique-slip movement and tilting of earlier structures. The orthorhombic geometry is not observed at a regional scale (>10 × 10 km), indicating probable scale-dependant behaviour. This study indicates that this transfer zone developed either by (1) strain partitioning with synchronous strike-slip structures and adjacent zones of non-plane extension, or (2) by a component of non-plane extension sub-parallel to the basin margin followed by subsequent transtensional overprint of the system (preferred model). Synthetic overlapping transfer zones are inferred to be key regions where orthorhombic fault geometries may develop.

  18. InSAR and GPS measurements along the Kivu segment of the East African Rift System during the 2011-2012 Nyamulagira volcanic eruption.

    NASA Astrophysics Data System (ADS)

    Nobile, Adriano; Geirsson, Halldor; Smets, Benoît; d'Oreye, Nicolas; Kervyn, François

    2016-04-01

    Along the East African Rift System (EARS), magma intrusions represent a major component in continental rifting. When these intrusions reach the surface, they cause volcanic eruptions. This is the case of the last flank eruption of Nyamulagira, which occurred from November 6 2011 to April 2012. Nyamulagira is an active shield volcano with a central caldera, located in the eastern part of the Democratic Republic of Congo, along the Kivu segment of the East African Rift System. From 1948 to 2012, Nyamulagira mostly showed a particular eruptive cycle with 1) classical short-lived (i.e., 20-30 days) flank eruptions, sometimes accompanied with intracrateral activity, which occurred every 1-4 years on average, and 2) less frequent long-lived (i.e., several months) eruptions usually emitting larger volumes of lava that take place at larger distance (>8 km) from the central caldera. The 2011-2012 Nyamulagira eruption is of that second type. Here we used InSAR data from different satellite (Envisat, Cosmo SkyMed, TerraSAR-X and RADARSAT) to measure pre-, co and post-eruptive ground displacement associated with the Nyamulagira 2011-2012 eruption. Results suggest that a magma intrusion preceded by two days the eruption. This intrusion corresponded to the migration of magma from a shallow reservoir (~3km) below the caldera to the two eruptive fissures located ~11 km ENE of the central edifice. Available seismic data are in agreement with InSAR results showing increased seismic activity since November 4 2011, with long- and short-period earthquakes swarms. Using analytical models we invert the measured ground displacements during the first co-eruptive month to evaluate the deformation source parameters and the mechanism of magma emplacement for this eruption. GPS data from permanent stations in the KivuGNet network are used to constrain the temporal evolution of the eruption and evaluate far-field deformation, while the InSAR data is more sensitive to the near-field deformation

  19. Hydrothermal circulation, serpentinization, and degassing at a rift valley-fracture zone intersection: Mid-Atlantic Ridge near 15[degree]N, 45[degree]W

    SciTech Connect

    Rona, P.A.; Nelson, T.A. ); Bougault, H.; Charlou, J.L.; Needham, H.D. ); Appriou, P. ); Trefry, J.H. ); Eberhart, G.L.; Barone, A. )

    1992-09-01

    A hydrothermal system characterized by high ratios of methane to both manganese and suspended particulate matter was detected in seawater sampled at the eastern intersection of the rift valley of the Mid-Atlantic Ridge with the Fifteen-Twenty Fracture Zone. This finding contrasts with low ratios in black smoker-type hydrothermal systems that occur within spreading segments. Near-bottom water sampling coordinated with SeaBeam bathymetry and camera-temperature tows detected the highest concentrations of methane at fault zones in rocks with the appearance of altered ultramafic units in a large dome that forms part of the inside corner high at the intersection. The distinct chemical signatures of the two types of hydrothermal systems are inferred to be controlled by different circulation pathways related to reaction of seawater primarily with ultramafic rocks at intersections of spreading segments with fracture zones but with mafic rocks within spreading segments.

  20. Differentiation and magma mixing on Kilauea's east rift zone: A further look at the eruptions of 1955 and 1960. Part II. The 1960 lavas

    USGS Publications Warehouse

    Wright, T.L.; Helz, R.T.

    1996-01-01

    New and detailed petrographic observations, mineral compositional data, and whole-rock vs glass compositional trends document magma mixing in lavas erupted from Kilauea's lower east rift zone in 1960. Evidence includes the occurrence of heterogeneous phenocryst assemblages, including resorbed and reversely zoned minerals in the lavas inferred to be hybrids. Calculations suggest that this mixing, which is shown to have taken place within magma reservoirs recharged at the end of the 1955 eruption, involved introduction of four different magmas. These magmas originated beneath Kilauea's summit and moved into the rift reservoirs beginning 10 days after the eruption began. We used microprobe analyses of glass to calculate temperatures of liquids erupted in 1955 and 1960. We then used the calculated proportions of stored and recharge components to estimate the temperature of the recharge components, and found those temperatures to be consistent with the temperature of the same magmas as they appeared at Kilauea's summit. Our studies reinforce conclusions reached in previous studies of Kilauea's magmatic plumbing. We infer that magma enters shallow storage beneath Kilauea's summit and also moves laterally into the fluid core of the East rift zone. During this process, if magmas of distinctive chemistry are present, they retain their chemical identity and the amount of cooling is comparable for magma transported either upward or laterally to eruption sites. Intrusions within a few kilometers of the surface cool and crystallize to produce fractionated magma. Magma mixing occurs both within bodies of previously fractionated magma and when new magma intersects a preexisting reservoir. Magma is otherwise prevented from mixing, either by wall-rock septa or by differing thermal and density characteristics of the successive magma batches.

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

    NASA Astrophysics Data System (ADS)

    Zwaan, Frank; Schreurs, Guido

    2016-04-01

    INTRODUCTION During the early stages of rifting, rift segments may form along non-continuous and/or offset pre-existing weaknesses. It is important to understand how these initial rift segments interact and connect to form a continuous rift system. A previous study of ours (Zwaan et al., in prep) investigated the influence of dextral oblique extension and rift offset on rift interaction. Here we elaborate upon our previous work by using analogue models to assess the added effects of 1) sinistral oblique extension as observed along the East African Rift and 2) the geometry of linked and non-linked inherited structures. METHODS Our set-up involves a base of foam and plexiglass that forces distributed extension in the overlying model materials: a sand layer for the brittle upper crust and a viscous sand/silicone mixture for ductile lower crust. A mobile base plate allows lateral motion for oblique extension. We create inherited structures, along which rift segments develop, with right-stepping offset lines of silicone (seeds) on top of the basal viscous layer. These seeds can be connected by an additional weak seed that represents a secondary inherited structural grain (model series 1) or disconnected and laterally discontinuous (over/underlap, model series 2). Selected models are run in an X-ray computer topographer (CT) to reveal the 3D evolution of internal structures with time that can be quantified with particle image velocitmetry (PIV) techniques. RESULTS Models in both series show that rift segments initially form along the main seeds and then generally propagate approximately perpendicular to the extension direction: with orthogonal extension they propagate in a parallel fashion, dextral oblique extension causes them to grow towards each other and connect, while with sinistral oblique extension they grow away from each other. However, sinistral oblique extension can also promote rift linkage through an oblique- or strike-slip zone oriented almost parallel to

  2. Monitoring the NW volcanic rift-zone of Tenerife, Canary Islands, Spain: sixteen years of diffuse CO_{2} degassing surveys

    NASA Astrophysics Data System (ADS)

    Rodríguez, Fátima; Halliwell, Simon; Butters, Damaris; Padilla, Germán; Padrón, Eleazar; Hernández, Pedro A.; Pérez, Nemesio M.

    2016-04-01

    Tenerife is the largest of the Canary Islands and, together with Gran Canaria, is the only one that has developed a central volcanic complex characterized by the eruption of differentiated magmas. At present, one of the most active volcanic structures in Tenerife is the North-West Rift-Zone (NWRZ), which has hosted two historical eruptions: Arenas Negras in 1706 and Chinyero in 1909. Since the year 2000, 47 soil CO2 efflux surveys have been undertaken at the NWRZ of Tenerife Island to evaluate the temporal and spatial variations of CO2 efflux and their relationships with the volcanic-seismic activity. We report herein the last results of diffuse CO2 efflux survey at the NWRZ carried out in July 2015 to constrain the total CO2 output from the studied area. Measurements were performed in accordance with the accumulation chamber method. Spatial distribution maps were constructed following the sequential Gaussian simulation (sGs) procedure. During 2015 survey, soil CO2 efflux values ranged from non-detectable up to 103 g m‑2 d‑1. The total diffuse CO2 output released to atmosphere was estimated at 403 ± 17 t d‑1, values higher than the background CO2 emission estimated on 143 t d‑1. For all campaigns, soil CO2 efflux values ranged from non-detectable up to 141 g m‑2 d‑1, with the highest values measured in May 2005. Total CO2 output from the studied area ranged between 52 and 867 t d‑1. Temporal variations in the total CO2 output showed a temporal correlation with the onsets of seismic activity, supporting unrest of the volcanic system, as is also suggested by anomalous seismic activity recorded in the area during April 22-29, 2004. Spatial distribution of soil CO2 efflux values also showed changes in magnitude and amplitude, with higher CO2 efflux values located along a trending WNW-ESE area. Subsurface magma movement is proposed as a cause for the observed changes in the total output of diffuse CO2 emission, as well as for the spatial distribution of

  3. Enemies and turncoats: bovine tuberculosis exposes pathogenic potential of Rift Valley fever virus in a common host, African buffalo (Syncerus caffer)

    PubMed Central

    Beechler, B. R.; Manore, C. A.; Reininghaus, B.; O'Neal, D.; Gorsich, E. E.; Ezenwa, V. O.; Jolles, A. E.

    2015-01-01

    The ubiquity and importance of parasite co-infections in populations of free-living animals is beginning to be recognized, but few studies have demonstrated differential fitness effects of single infection versus co-infection in free-living populations. We investigated interactions between the emerging bacterial disease bovine tuberculosis (BTB) and the previously existing viral disease Rift Valley fever (RVF) in a competent reservoir host, African buffalo, combining data from a natural outbreak of RVF in captive buffalo at a buffalo breeding facility in 2008 with data collected from a neighbouring free-living herd of African buffalo in Kruger National Park. RVF infection was twice as likely in individual BTB+ buffalo as in BTB− buffalo, which, according to a mathematical model, may increase RVF outbreak size at the population level. In addition, co-infection was associated with a far higher rate of fetal abortion than other infection states. Immune interactions between BTB and RVF may underlie both of these interactions, since animals with BTB had decreased innate immunity and increased pro-inflammatory immune responses. This study is one of the first to demonstrate how the consequences of emerging infections extend beyond direct effects on host health, potentially altering the dynamics and fitness effects of infectious diseases that had previously existed in the ecosystem on free-ranging wildlife populations. PMID:25788592

  4. Inland termination of the Weddell Sea Rift against a major Jurassic strike-slip fault zone between East and West Antarctica

    NASA Astrophysics Data System (ADS)

    Jordan, Tom; Ferraccioli, Fausto; Leat, Phil; Ross, Neil; Bingham, Rob; Rippin, David; LeBrocq, Anne; Corr, Hugh; Siegert, Martin

    2013-04-01

    The Weddell Sea Embayment (WSE) lies in a key position to study the nature of the tectonic boundary between East and West Antarctica and the development of continental rifting processes and magmatism during the early stages of Gondwana break-up. Evidence for continental rifting within the WSE derives from previous reconnaissance geophysical investigations offshore and geological studies of the associated Jurassic magmatism onshore. Seismic data reveal high stretching factors beneath the Weddell Sea Rift (WSR) between 1.5 and 3.0, and gravity data suggest a crustal thickness of ca 27 km and an effective elastic thickness of ~35 km for the rifted region. Geochemical interpretations indicate that a Middle Jurassic LIP, including extensive mafic tholeiites and some Jurassic granitic intrusions may be related to a superplume that impinged beneath the WSE. Here we present results from a recent aerogeophysical investigation that sheds new light into the previously largely unknown inland extent of the WSR beneath the West Antarctic Ice Sheet. This includes new insights into its magmatic patterns, as well as the nature of its tectonic boundaries with the adjacent Ellsworth-Whitmore block (EWM) and the margin of East Antarctica. Aeromagnetic images were interpreted to reveal pre-rift rocks, including Proterozoic basement, Middle Cambrian rift-related volcanics and metasediments and rift-related Jurassic granitoids. Magnetic depth-to-source estimates were calculated and help constrain two joint magnetic and gravity forward models for the study region. These models were used to assess crustal thickness variations, the extent of Proterozoic basement, and the thickness of Jurassic intrusions and inferred post-Jurassic sedimentary infill. The Jurassic granitoids were modelled as 5-8 km thick. These intrusions include roughly circular plutons, emplaced at the transition between the thicker crust of the EWM block and the thinner crust of the WSR, and more elongated bodies emplaced

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  6. Diffuse CO2 emission from the NE volcanic rift-zone of Tenerife (Canary Islands, Spain): a 15 years geochemical monitoring

    NASA Astrophysics Data System (ADS)

    Padilla, Germán; Alonso, Mar; Shoemaker, Trevor; Loisel, Ariane; Padrón, Eleazar; Hernández, Pedro A.; Pérez, Nemesio M.

    2016-04-01

    The North East Rift (NER) volcanic zone of Tenerife Island is one of the three volcanic rift-zones of the island (210 km2). The most recent eruptive activity along the NER volcanic zone took place in the 1704-1705 period with the volcanic eruptions of Siete Fuentes, Fasnia and Arafo volcanoes. The aim of this study was to report the results of a soil CO2 efflux survey undertaken in June 2015, with approximately 580 measuring sites. In-situ measurements of CO2 efflux from the surface environment of NER volcanic zone were performed by means of a portable non-dispersive infrared spectrophotometer (NDIR) model LICOR Li800 following the accumulation chamber method. To quantify the total CO2 emission from NER volcanic zone, soil CO2 efflux contour maps were constructed using sequential Gaussian simulation (sGs) as interpolation method. The total diffuse CO2 emission rate was estimated in 1209 t d‑1, with CO2 efflux values ranging from non-detectable (˜0.5 g m‑2 d‑1) up to 123 g m‑2 d‑1, with an average value of 5.9 g m‑2 d‑1. If we compare these results with those obtained in previous surveys developed in a yearly basis, they reveal slightly variations from 2006 to 2015, with to pulses in the CO2 emission observed in 2007 and 2014. The main temporal variation in the total CO2 output does not seem to be masked by external variations. First peak precedes the anomalous seismicity registered in and around Tenerife Island between 2009 and 2011, suggesting stress-strain changes at depth as a possible cause for the observed changes in the total output of diffuse CO2 emission. Second peak could be related with futures changes in the seismicity. This study demonstrates the importance of performing soil CO2 efflux surveys as an effective surveillance volcanic tool.

  7. Genetic evidence for Rift Valley fever outbreaks in Madagascar resulting from virus introductions from the East African mainland rather than enzootic maintenance.

    PubMed

    Carroll, Serena A; Reynes, Jean-Marc; Khristova, Marina L; Andriamandimby, Soa Fy; Rollin, Pierre E; Nichol, Stuart T

    2011-07-01

    Rift Valley fever virus (RVFV), a mosquito-borne phlebovirus, has been detected in Madagascar since 1979, with occasional outbreaks. In 2008 to 2009, a large RVFV outbreak was detected in Malagasy livestock and humans during two successive rainy seasons. To determine whether cases were due to enzootic maintenance of the virus within Madagascar or to importation from the East African mainland, nine RVFV whole genomic sequences were generated for viruses from the 1991 and 2008 Malagasy outbreaks. Bayesian coalescent analyses of available whole S, M, and L segment sequences were used to estimate the time to the most recent common ancestor for the RVFVs. The 1979 Madagascar isolate shared a common ancestor with strains on the mainland around 1972. The 1991 Madagascar isolates were in a clade distinct from that of the 1979 isolate and shared a common ancestor around 1987. Finally, the 2008 Madagascar viruses were embedded within a large clade of RVFVs from the 2006-2007 outbreak in East Africa and shared a common ancestor around 2003 to 2004. These results suggest that the most recent Madagascar outbreak was caused by a virus likely arriving in the country some time between 2003 and 2008 and that this outbreak may be an extension of the 2006-2007 East African outbreak. Clustering of the Malagasy sequences into subclades indicates that the viruses have continued to evolve during their short-term circulation within the country. These data are consistent with the notion that RVFV outbreaks in Madagascar result not from emergence from enzootic cycles within the country but from recurrent virus introductions from the East African mainland.

  8. The Teisseyre-Tornquist Zone - early Palaeozoic strike-slip plate boundary or Ediacaran rifted margin of Baltica?

    NASA Astrophysics Data System (ADS)

    Mazur, Stanislaw; Krzywiec, Piotr; Malinowski, Michal; Lewandowski, Marek; Buffenmeyer, Vinton; Green, Christopher

    2016-04-01

    The Teisseyre-Tornquist Zone (TTZ) is the longest European tectonic and geophysical lineament extending from the Baltic Sea in the northwest to the Black Sea in the southeast. This tectonic feature defines a transition between the thick crust of the East European Craton (EEC) and the thinner crust of the Palaeozoic Platform to the southwest. Being a profound zone of crustal and lithospheric thickness perturbation, the TTZ has usually been considered a Caledonian tectonic suture formed due to the closure of the Tornquist Ocean. The suture was hypothesised to originate from the collision between Baltica and Avalonia or large-scale strike-slip displacement along strike of the Caledonian Orogen. However, some minority views postulated the continuation of Baltica crystalline basement farther to the southwest up to the Elbe Lineament and the margin of the Variscan Belt. We studied the ION Geophysical PolandSPAN survey that consists of 10 regional, seismic depth profiles covering the SW margin of the EEC and the TTZ in Poland. Since the PolandSPAN profiles image to ~30 km depth their interpretation was integrated with the potential fields data and earlier results of refraction sounding to better image the deep structure of the TTZ. Our data show that the NW and central sections of the TTZ correspond, at the Moho level, to a relatively narrow crustal keel and a significant Moho step at the transition from the EEC to the Palaeozoic Platform. However, top of basement above the TTZ is smooth and moderately sloping towards the southwest. In the central part of the TTZ, top of Precambrian is covered by undisturbed lower Palaeozoic sediments. In contrast, the lower Palaeozoic sediments are involved in a latest Silurian, thin-skinned fold-and-thrust belt along the NW section of the TTZ, where the sharply defined Caledonian Deformation Front adjoins a rigid basement buttress above the TTZ. Finally, the crustal keel is mostly missing from the SE section of the TTZ. Instead, this

  9. Hydrocarbon potential of intracratonic rift basins

    SciTech Connect

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

    1984-09-01

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

  10. Exhumation, cooling and deformation history of the necking zone of the fossil Adriatic rifted margin: the Campo/Grosina section (S-Switzerland and N-Italy)

    NASA Astrophysics Data System (ADS)

    Petri, Benoît; Mohn, Geoffroy; Wijbrans, Jan R.; Manatschal, Gianreto; Beltrando, Marco

    2016-04-01

    The Austroalpine units in SE Switzerland and N-Italy preserve remnants of the fossil Adriatic rifted margin. Notably the Campo-Grosina units represent the necking zone where major crustal thinning was accommodated during the Jurassic rifting. This contribution aims to unravel the complex tectonic evolution recorded in these units from the late Carboniferous - early Permian to the Jurassic rifting. The cooling and exhumation of the Campo and overlying Grosina units, separated by the Eita shear zone are explored by the acquisition of 40Ar/39Ar on hornblende, muscovite and biotite. New geochronological data on the Grosina unit present 40Ar/39Ar ages between 273 and 261 Ma for muscovite and between 248 and 246 Ma for biotite. The Campo unit shows clearly younger ages between 210 and 177 Ma on hornblende, between 186 and 176 Ma on muscovite and between 174 and 171 Ma on biotite. Numerous data were discarded due to frequent excess 40Ar on amphiboles, probably associated to the emplacement of the Sondalo gabbro with a high 40Ar/36Ar ratio in Permian times. These new ages, together with a compilation of existing ages obtained with different chronometers (U-Pb, Sm-Nd, Rb-Sr, K-Ar, 40Ar/39Ar) and performed on different lithologies from both the Campo and the Grosina units allow an estimation of the cooling rates for these units to be done. The new results show that both the Campo and the Grosina units underwent a cooling rate around 10°C/Ma in Permian time. The Grosina unit, being in a shallower crustal level, did not record the Jurassic cooling, reaching up to 50°C/Ma in the Campo unit. The notable difference in cooling rates between the Permian and the Jurassic events attests of a cooling without being associated to an exhumation in Permian times, whereas the Campo unit cooled rapidly in Jurassic times, due to an exhumation and an emplacement in shallow crustal levels. The latter tectonic event was likely caused by shearing along the Eita or other greenschist facies

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

    USGS Publications Warehouse

    Thornber, C.R.

    2001-01-01

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

  12. Contact zone permeability at intrusion boundaries: New results from hydraulic testing and geophysical logging in the Newark Rift Basin, New York, USA

    USGS Publications Warehouse

    Matter, J.M.; Goldberg, D.S.; Morin, R.H.; Stute, M.

    2006-01-01

    Hydraulic tests and geophysical logging performed in the Palisades sill and the underlying sedimentary rocks in the NE part of the Newark Rift Basin, New York, USA, confirm that the particular transmissive zones are localized within the dolerite-sedimentary rock contact zone and within a narrow interval below this contact zone that is characterized by the occurrence of small layers of chilled dolerite. Transmissivity values determined from fluid injection, aquifer testing, and flowmeter measurements generally fall in the range of 8.1E-08 to 9.95E-06 m2/s and correspond to various scales of investigation. The analysis of acoustic and optical BHTV images reveals two primary fracture sets within the dolerite and the sedimentary rocks - subhorizontal fractures, intersected by subvertical ones. Despite being highly fractured either with subhorizontal, subvertical or both fracture populations, the dolerite above and the sedimentary rocks below the contact zone and the zone with the layers of chilled dolerite are significantly less conductive. The distribution of the particular conductive intervals is not a function of the two dominant fracture populations or their density but rather of the intrusion path of the sill. The intrusion caused thermal fracturing and cracking of both formations, resulting in higher permeability along the contact zone. ?? Springer-Verlag 2005.

  13. Understanding Chad Basin Evolution Since Miocene: Climate and Vegetation Simulations, Roles of Orbital Parameters and East African Rift.

    NASA Astrophysics Data System (ADS)

    Sepulchre, P.; Ramstein, G.; Krinner, G.; Schuster, M.; Fluteau, F.; Kageyama, M.; Tiercelin, J.; Vignaud, P.; Brunet, M.

    2004-12-01

    Since the discovery of the earliest hominid known, Chad basin is a major place to study paleoclimates and hominid evolution. This discovery implies to re-evaluate the "East Side Story" paradigm for early hominids. To achieve this goal, we have performed numerical simulations to quantify the climatic and vegetation response of the Rift Uplift. We used a zoomed (144 X 108) AGCM (LMDz from IPSL). On the one hand, offline continental biosphere model (ORCHIDEE) has been used to simulate the vegetation response over western and eastern parts of the rift. On the other hand, since geomorphologic evidences have shown that from Upper Miocene to mid-Holocene Lake Chad had known several level oscillations leading to a huge lake known as Mega Lake Chad (MLC), we also ran atmospheric simulations to demonstrate, with boundary conditions at 6 000 BP, that orbital forcing allowed the existence of a MLC. Volume and surface of the lake have been calculated using an adapted lake model. These simulations have shown that the ITCZ shift induced by the mid-Holocene orbital parameters drives the existence of a MegaChad. Our model result having been tested successfully for the last occurrence of the MLC, we will apply it to Upper Miocene accounting for topographic changes, in order to reconstruct as accurately as possible the first hominids environments.

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

    NASA Astrophysics Data System (ADS)

    Tesfaye, Samson; Ghebreab, Woldai

    2013-11-01

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

  15. Seroprevalence of Rift Valley fever and lumpy skin disease in African buffalo (Syncerus caffer) in the Kruger National Park and Hluhluwe-iMfolozi Park, South Africa.

    PubMed

    Fagbo, Shamsudeen; Coetzer, Jacobus A W; Venter, Estelle H

    2014-01-01

    Rift Valley fever and lumpy skin disease are transboundary viral diseases endemic in Africa and some parts of the Middle East, but with increasing potential for global emergence. Wild ruminants, such as the African buffalo (Syncerus caffer), are thought to play a role in the epidemiology of these diseases. This study sought to expand the understanding of the role of buffalo in the maintenance of Rift Valley fever virus (RVFV) and lumpy skin disease virus (LSDV) by determining seroprevalence to these viruses during an inter-epidemic period. Buffaloes from the Kruger National Park (n = 138) and Hluhluwe-iMfolozi Park (n = 110) in South Africa were sampled and tested for immunoglobulin G (IgG) and neutralising antibodies against LSDV and RVFV using an indirect enzyme-linked immunosorbent assay (I-ELISA) and the serum neutralisation test (SNT). The I-ELISA for LSDV and RVFV detected IgG antibodies in 70 of 248 (28.2%) and 15 of 248 (6.1%) buffaloes, respectively. Using the SNT, LSDV and RVFV neutralising antibodies were found in 5 of 66 (7.6%) and 12 of 57 (21.1%), respectively, of samples tested. The RVFV I-ELISA and SNT results correlated well with previously reported results. Of the 12 SNT RVFV-positive sera, three (25.0%) had very high SNT titres of 1:640. Neutralising antibody titres of more than 1:80 were found in 80.0% of the positive sera tested. The LSDV SNT results did not correlate with results obtained by the I-ELISA and neutralising antibody titres detected were low, with the highest (1:20) recorded in only two buffaloes, whilst 11 buffaloes (4.4%) had evidence of co-infection with both viruses. Results obtained in this study complement other reports suggesting a role for buffaloes in the epidemiology of these diseases during inter-epidemic periods. PMID:25686252

  16. Seroprevalence of Rift Valley fever and lumpy skin disease in African buffalo (Syncerus caffer) in the Kruger National Park and Hluhluwe-iMfolozi Park, South Africa.

    PubMed

    Fagbo, Shamsudeen; Coetzer, Jacobus A W; Venter, Estelle H

    2014-10-16

    Rift Valley fever and lumpy skin disease are transboundary viral diseases endemic in Africa and some parts of the Middle East, but with increasing potential for global emergence. Wild ruminants, such as the African buffalo (Syncerus caffer), are thought to play a role in the epidemiology of these diseases. This study sought to expand the understanding of the role of buffalo in the maintenance of Rift Valley fever virus (RVFV) and lumpy skin disease virus (LSDV) by determining seroprevalence to these viruses during an inter-epidemic period. Buffaloes from the Kruger National Park (n = 138) and Hluhluwe-iMfolozi Park (n = 110) in South Africa were sampled and tested for immunoglobulin G (IgG) and neutralising antibodies against LSDV and RVFV using an indirect enzyme-linked immunosorbent assay (I-ELISA) and the serum neutralisation test (SNT). The I-ELISA for LSDV and RVFV detected IgG antibodies in 70 of 248 (28.2%) and 15 of 248 (6.1%) buffaloes, respectively. Using the SNT, LSDV and RVFV neutralising antibodies were found in 5 of 66 (7.6%) and 12 of 57 (21.1%), respectively, of samples tested. The RVFV I-ELISA and SNT results correlated well with previously reported results. Of the 12 SNT RVFV-positive sera, three (25.0%) had very high SNT titres of 1:640. Neutralising antibody titres of more than 1:80 were found in 80.0% of the positive sera tested. The LSDV SNT results did not correlate with results obtained by the I-ELISA and neutralising antibody titres detected were low, with the highest (1:20) recorded in only two buffaloes, whilst 11 buffaloes (4.4%) had evidence of co-infection with both viruses. Results obtained in this study complement other reports suggesting a role for buffaloes in the epidemiology of these diseases during inter-epidemic periods.

  17. Tectonic stress evolution in the Pan-African Lufilian Arc and its foreland (Katanga, DRC): orogenic bending, late orogenic extensional collapse and transition to rifting

    NASA Astrophysics Data System (ADS)

    Kipata, M. L.; Delvaux, D.; Sebagenzi, M. N.; Cailteux, J.; Sintubin, M.

    2012-04-01

    Between the paroxysm of the Lufilian orogeny at ~ 550 Ma and the late Neogene to Quaternary development of the south-western branch of the East African rift system, the tectonic evolution of the Lufilian Arc and Kundelungu foreland in the Katanga region of the Democratic Republic of Congo remains poorly unknown although it caused important Cu-dominated mineral remobilizations leading to world-class ore deposits. This long period is essentially characterized by brittle tectonic deformations that have been investigated by field studies in open mines spread over the entire arc and foreland. Paleostress tensors were computed for a database of 1450 fault-slip data by interactive stress tensor inversion and data subset separation, and the relative succession of 8 brittle deformation events established. The oldest brittle structures observed are related to the Lufilian brittle compressional climax (stage 1). They have been re-oriented during the orogenic bending that led to the arcuate shape of the belt. Unfolding the stress directions from the first stage allows to reconstruct a consistent NE-SW direction of compression for this stage. Constrictional deformation occurred in the central part of the arc, probably during orogenic bending (Stage 2). After the orogenic bending, a sequence of 3 deformation stages marks the progressive onset of late-orogenic extension: strike-slip deformations (stages 3-4) and late-orogenic arc-parallel extension (stage 5). It is proposed that these 3 stages correspond to orogenic collapse. In early Mesozoic, NW-SE compression was induced by a transpressional inversion, interpreted as induced by far-field stresses generated at the southern active margin of Gondwana (stage 6). Since then, this region was affected by rift-related extension, successively in a NE-SW direction (stage 7, Tanganyika trend) and NW-SE direction (stage 8, Moero trend).

  18. Archaeology in the Kilauea East Rift Zone: Part 2, A preliminary sample survey, Kapoho, Kamaili and Kilauea geothermal subzones, Puna District, Hawaii island

    SciTech Connect

    Sweeney, M.T.K.; Burtchard, G.C.

    1995-05-01

    This report describes a preliminary sample inventory and offers an initial evaluation of settlement and land-use patterns for the Geothermal Resources Subzones (GRS) area, located in Puna District on the island of Hawaii. The report is the second of a two part project dealing with archaeology of the Puna GRS area -- or more generally, the Kilauea East Rift Zone. In the first phase of the project, a long-term land-use model and inventory research design was developed for the GRS area and Puna District generally. That report is available under separate cover as Archaeology in the Kilauea East Rift Zone, Part I: Land-Use Model and Research Design. The present report gives results of a limited cultural resource survey built on research design recommendations. It offers a preliminary evaluation of modeled land-use expectations and offers recommendations for continuing research into Puna`s rich cultural heritage. The present survey was conducted under the auspices of the United States Department of Energy, and subcontracted to International Archaeological Research Institute, Inc. (IARII) by Martin Marietta Energy Systems, Inc. The purpose of the archaeological work is to contribute toward the preparation of an environmental impact statement by identifying cultural materials which could be impacted through completion of the proposed Hawaii Geothermal Project.

  19. Distribution of differentiated tholeiitic basalts on the lower east rift zone of Kilauea Volcano, Hawaii: a possible guide to geothermal exploration.

    USGS Publications Warehouse

    Moore, R.B.

    1983-01-01

    Geological mapping of the lower east rift zone indicates that >100 eruptions have extruded an estimated 10 km3 of basalt during the past 2000 yr; six eruptions in the past 200 yr have extruded approx 1 km3. The eruptive recurrence interval has ranged 1-115 yr since the middle of the 18th century and has averaged 20 yr or less over the past 2000 yr. New chemical analyses (100) indicate that the tholeiites erupted commonly differentiated beyond olivine control or are hybrid mixtures of differentiates with more mafic (olivine-controlled) summit magmas. The distribution of vents for differentiated lavas suggests that several large magma chambers underlie the lower east rift zone. Several workers have recognized that a chamber underlies the area near a producing geothermal well, HGP-A; petrological and 14C data indicate that it has existed for at least 1300 yr. Stratigraphy, petrology and surface-deformation patterns suggest that two other areas, Heiheiahulu and Kaliu, also overlie magma chambers and show favourable geothermal prospects.-A.P.

  20. Factors controlling depth of continental rifts

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  2. Drinking water quality in the Ethiopian section of the East African Rift Valley I--data and health aspects.

    PubMed

    Reimann, Clemens; Bjorvatn, Kjell; Frengstad, Bjørn; Melaku, Zenebe; Tekle-Haimanot, Redda; Siewers, Ulrich

    2003-07-20

    Drinking water samples were collected throughout the Ethiopian part of the Rift Valley, separated into water drawn from deep wells (deeper than 60 m), shallow wells (<60 m deep), hot springs (T>36 degrees C), springs (T<32 degrees C) and rivers. A total of 138 samples were analysed for 70 parameters (Ag, Al, As, B, Ba, Be, Bi, Br, Ca, Cd, Ce, Cl, Co, Cr, Cs, Cu, Dy, Er, Eu, F, Fe, Ga, Gd, Ge, Hf, Hg, Ho, I, In, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, Nd, Ni, NO(2), NO(3), Pb, Pr, Rb, Sb, Se, Si, Sm, Sn, SO(4), Sr, Ta, Tb, Te, Th, Ti, Tl, Tm, U, V, W, Y, Yb, Zn, Zr, temperature, pH, conductivity and alkalinity) with ion chromatography (anions), spectrometry (ICP-OES and ICP-MS, cations) and parameter-specific (e.g. titration) techniques. In terms of European water directives and WHO guidelines, 86% of all wells yield water that fails to pass the quality standards set for drinking water. The most problematic element is fluoride (F), for which 33% of all samples returned values above 1.5 mg/l and up to 11.6 mg/l. The incidence of dental and skeletal fluorosis is well documented in the Rift Valley. Another problematic element may be uranium (U)-47% of all wells yield water with concentrations above the newly suggested WHO maximum acceptable concentration (MAC) of 2 microg/l. Fortunately, only 7% of the collected samples are above the 10 microg/l EU-MAC for As in drinking water.

  3. Spatial and temporal variations of diffuse CO_{2} degassing at the N-S volcanic rift-zone of Tenerife (Canary Islands, Spain) during 2002-2015 period

    NASA Astrophysics Data System (ADS)

    Alonso, Mar; Ingman, Dylan; Alexander, Scott; Barrancos, José; Rodríguez, Fátima; Melián, Gladys; Pérez, Nemesio M.

    2016-04-01

    Tenerife is the largest of the Canary Islands and, together with Gran Canaria Island, is the only one with a central volcanic complex that started to grow at about 3.5 Ma. Nowadays the central complex is formed by Las Cañadas caldera, a volcanic depression measuring 16×9 km that resulted from multiple vertical collapses and was partially filled by post-caldera volcanic products. Up to 297 mafic monogenetic cones have been recognized on Tenerife, and they represent the most common eruptive activity occurring on the island during the last 1 Ma (Dóniz et al., 2008). Most of the monogenetic cones are aligned following a triple junction-shaped rift system, as result of inflation produced by the concentration of emission vents and dykes in bands at 120o to one another as a result of minimum stress fracturing of the crust by a mantle upwelling. The main structural characteristic of the southern volcanic rift (N-S) of the island is an apparent absence of a distinct ridge, and a fan shaped distribution of monogenetic cones. Four main volcanic successions in the southern volcanic rift zone of Tenerife, temporally separated by longer periods (˜70 - 250 ka) without volcanic activity, have been identified (Kröchert and Buchner, 2008). Since there are currently no visible gas emissions at the N-S rift, diffuse degassing surveys have become an important geochemical tool for the surveillance of this volcanic system. We report here the last results of diffuse CO2 efflux survey at the N-S rift of Tenerife, performed using the accumulation chamber method in the summer period of 2015. The objectives of the surveys were: (i) to constrain the total CO2 output from the studied area and (ii) to evaluate occasional CO2 efflux surveys as a volcanic surveillance tool for the N-S rift of Tenerife. Soil CO2 efflux values ranged from non-detectable up to 31.7 g m-2 d-1. A spatial distribution map, constructed following the sequential Gaussian simulation (sGs) procedure, did not show an

  4. Spatial and temporal variations of diffuse CO_{2} degassing at the N-S volcanic rift-zone of Tenerife (Canary Islands, Spain) during 2002-2015 period

    NASA Astrophysics Data System (ADS)

    Alonso, Mar; Ingman, Dylan; Alexander, Scott; Barrancos, José; Rodríguez, Fátima; Melián, Gladys; Pérez, Nemesio M.

    2016-04-01

    Tenerife is the largest of the Canary Islands and, together with Gran Canaria Island, is the only one with a central volcanic complex that started to grow at about 3.5 Ma. Nowadays the central complex is formed by Las Cañadas caldera, a volcanic depression measuring 16×9 km that resulted from multiple vertical collapses and was partially filled by post-caldera volcanic products. Up to 297 mafic monogenetic cones have been recognized on Tenerife, and they represent the most common eruptive activity occurring on the island during the last 1 Ma (Dóniz et al., 2008). Most of the monogenetic cones are aligned following a triple junction-shaped rift system, as result of inflation produced by the concentration of emission vents and dykes in bands at 120o to one another as a result of minimum stress fracturing of the crust by a mantle upwelling. The main structural characteristic of the southern volcanic rift (N-S) of the island is an apparent absence of a distinct ridge, and a fan shaped distribution of monogenetic cones. Four main volcanic successions in the southern volcanic rift zone of Tenerife, temporally separated by longer periods (˜70 - 250 ka) without volcanic activity, have been identified (Kröchert and Buchner, 2008). Since there are currently no visible gas emissions at the N-S rift, diffuse degassing surveys have become an important geochemical tool for the surveillance of this volcanic system. We report here the last results of diffuse CO2 efflux survey at the N-S rift of Tenerife, performed using the accumulation chamber method in the summer period of 2015. The objectives of the surveys were: (i) to constrain the total CO2 output from the studied area and (ii) to evaluate occasional CO2 efflux surveys as a volcanic surveillance tool for the N-S rift of Tenerife. Soil CO2 efflux values ranged from non-detectable up to 31.7 g m‑2 d‑1. A spatial distribution map, constructed following the sequential Gaussian simulation (sGs) procedure, did not show an

  5. 13C/Palynological evidence of differential residence times of organic carbon prior to its sedimentation in East African Rift Lakes and peat bogs

    NASA Astrophysics Data System (ADS)

    Hillaire-Marcel, Claude; Aucour, Anne-Marie; Bonnefille, Raymonde; Riollet, Guy; Vincens, Annie; Williamson, David

    Most terrestrial plants producing large amounts of organic matter in the East African Rift follow the Calvin (C3) photosynthetic pathway. Their end products have δ13C values of ca. -27 ± 2‰ (vs. PDB). On the contrary, most Cyperaceae (notably Cyperus papyrus and C. latifolius) are characterized by higher 13C contents ° 13C = -10.5 ± 1‰ ) in relation to their Hatch and Slack (C4) photosynthetic cycle. In consequence, δ13C values in total organic matter (TOM) from peat bog or lake cores essentially responded to the proportion of detritus from C4-Cyperaceae. Immediate evidence of the development or disappearance of Cyperaceae around lake margins or in peat bogs can be found in pollen assemblages. Lag times between pollen signals and correlative ° 13C shifts in TOM from cores are therefore indicative of the residence time of organic matter prior to its sedimentation. Delayed sedimentation of TOM will result in 14C anomalies which depend on several parameters, most of them being site specific as shown by examples from a peat bog in Burundi and from southern Lake Tanganyika. An independent assessment of the chronology by high resolution paleomagnetic correlations indicates a ca. 1.5 ka apparent 14C age of TOM in Lake Tanganyika at the Pleistocene-Holocene transition.

  6. Causes of unrest at silicic calderas in the East African Rift: New constraints from InSAR and soil-gas chemistry at Aluto volcano, Ethiopia

    NASA Astrophysics Data System (ADS)

    Hutchison, William; Biggs, Juliet; Mather, Tamsin A.; Pyle, David M.; Lewi, Elias; Yirgu, Gezahegn; Caliro, Stefano; Chiodini, Giovanni; Clor, Laura E.; Fischer, Tobias P.

    2016-08-01

    Restless silicic calderas present major geological hazards, and yet many also host significant untapped geothermal resources. In East Africa, this poses a major challenge, although the calderas are largely unmonitored their geothermal resources could provide substantial economic benefits to the region. Understanding what causes unrest at these volcanoes is vital for weighing up the opportunities against the potential risks. Here we bring together new field and remote sensing observations to evaluate causes of ground deformation at Aluto, a restless silicic volcano located in the Main Ethiopian Rift (MER). Interferometric Synthetic Aperture Radar (InSAR) data reveal the temporal and spatial characteristics of a ground deformation episode that took place between 2008 and 2010. Deformation time series reveal pulses of accelerating uplift that transition to gradual long-term subsidence, and analytical models support inflation source depths of ˜5 km. Gases escaping along the major fault zone of Aluto show high CO2 flux, and a clear magmatic carbon signature (CO2-δ13C of -4.2‰ to -4.5‰). This provides compelling evidence that the magmatic and hydrothermal reservoirs of the complex are physically connected. We suggest that a coupled magmatic-hydrothermal system can explain the uplift-subsidence signals. We hypothesize that magmatic fluid injection and/or intrusion in the cap of the magmatic reservoir drives edifice-wide inflation while subsequent deflation is related to magmatic degassing and depressurization of the hydrothermal system. These new constraints on the plumbing of Aluto yield important insights into the behavior of rift volcanic systems and will be crucial for interpreting future patterns of unrest.

  7. Pre-existing discontinuities and oblique rifting in the Kenya Rift: Comparisons with analogue models.

    NASA Astrophysics Data System (ADS)

    Rolet, J.; Gloaguen, R.; Gloaguen, R.; Dooley, T.; McClay, K.

    2001-12-01

    Oblique rift structures such as the SSE-trending Aswa Transverse Zone in the Kenya rift are poorly understood and are rarely taken into account in geometric and kinematic models for the origin of this rift zone. However, remote sensing demonstrates that transverse structures are quite numerous and have a significant influence on the geometry and segmentation of the rift and the development of faults within or at the boundaries of the oblique zones. The importance of these transverse zones varies depending on their orientation and position with respect to the main rift. The origin of these oblique zones can be directly related to pre-existing fabrics and shear zones in the Precambrian basement and thus act as mechanically distinct structural domains during later extensional events. In order to assess the importance and role of these oblique structures we used optical (SPOT, LANDSAT) and microwave (RADARSAT, ERS) data combined with field observations and measurements. Collected structural data were then compared with scaled physical models of orthogonal and oblique rifting in order to refine the rift model. The data and comparison with physical models suggest that these transverse zones are best described as oblique rift zones where the rift border faults are parallel to the basement grain whereas intra-rift fault systems form orthogonal to the extension direction. This model also implies that the present day extension direction in Kenya is oriented E-W.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  9. SO2 from episode 48A eruption, Hawaii: Sulfur dioxide emissions from the episode 48A East Rift Zone eruption of Kilauea volcano, Hawaii

    USGS Publications Warehouse

    Andres, R.J.; Kyle, P.R.; Stokes, J.B.; Rose, William I.

    1989-01-01

    An SO2 flux of 1170??400 (1??) tonnes per day was measured with a correlation spectrometer (COSPEC) in October and November 1986 from the continuous, nonfountaining, basaltic East Rift Zone eruption (episode 48A) of Kilauea volcano. This flux is 5-27 times less than those of highfountaining episodes, 3-5 times greater than those of contemporaneous summit emissions or interphase Pu'u O'o emissions, and 1.3-2 times the emissions from Pu'u O'o alone during 48A. Calculations based on the SO2 emission rate resulted in a magma supply rate of 0.44 million m3 per day and a 0.042 wt% sulfur loss from the magma upon eruption. Both of these calculated parameters agree with determinations made previously by other methods. ?? 1989 Springer-Verlag.

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

    SciTech Connect

    Larchenkov, E. )

    1994-07-01

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

  11. Commerce geophysical lineament - Its source, geometry, and relation to the Reelfoot rift and New Madrid seismic zone

    USGS Publications Warehouse

    Langenheim, V.E.; Hildenbrand, T.G.

    1997-01-01

    The Commerce geophysical lineament is a northeast-trending magnetic and gravity feature that extends from central Arkansas to southern Illinois over a distance of ???400 km. It is parallel to the trend of the Reelfoot graben, but offset ???40 km to the northwest of the western margin of the rift floor. Modeling indicates that the source of the aeromagnetic and gravity anomalies is probably a mafic dike swarm. The age of the source of the Commerce geophysical lineament is not known, but the linearity and trend of the anomalies suggest a relationship with the Reelfoot rift, which has undergone episodic igneous activity. The Commerce geophysical lineament coincides with several topographic lineaments, movement on associated faults at least as young as Quaternary, and intrusions of various ages. Several earthquakes (Mb > 3) coincide with the Commerce geophysical lineament, but the diversity of associated focal mechanisms and the variety of surface structural features along the length of the Commerce geophysical lineament obscure its relation to the release of present-day strain. With the available seismicity data, it is difficult to attribute individual earthquakes to a specific structural lineament such as the Commerce geophysical lineament. However, the close correspondence between Quaternary faulting and present-day seismicity along the Commerce geophysical lineament is intriguing and warrants further study.

  12. The 1170 and 1202 CE Dead Sea Rift earthquakes and long-term magnitude distribution of the Dead Sea Fault zone

    USGS Publications Warehouse

    Hough, S.E.; Avni, R.

    2009-01-01

    In combination with the historical record, paleoseismic investigations have provided a record of large earthquakes in the Dead Sea Rift that extends back over 1500 years. Analysis of macroseismic effects can help refine magnitude estimates for large historical events. In this study we consider the detailed intensity distributions for two large events, in 1170 CE and 1202 CE, as determined from careful reinterpretation of available historical accounts, using the 1927 Jericho earthquake as a guide in their interpretation. In the absence of an intensity attenuation relationship for the Dead Sea region, we use the 1927 Jericho earthquake to develop a preliminary relationship based on a modification of the relationships developed in other regions. Using this relation, we estimate M7.6 for the 1202 earthquake and M6.6 for the 1170 earthquake. The uncertainties for both estimates are large and difficult to quantify with precision. The large uncertainties illustrate the critical need to develop a regional intensity attenuation relation. We further consider the distribution of magnitudes in the historic record and show that it is consistent with a b-value distribution with a b-value of 1. Considering the entire Dead Sea Rift zone, we show that the seismic moment release rate over the past 1500 years is sufficient, within the uncertainties of the data, to account for the plate tectonic strain rate along the plate boundary. The results reveal that an earthquake of M7.8 is expected within the zone on average every 1000 years. ?? 2011 Science From Israel/LPPLtd.

  13. Statistical Modeling of the Abundance of Vectors of West African Rift Valley Fever in Barkédji, Senegal

    PubMed Central

    Talla, Cheikh; Diallo, Diawo; Dia, Ibrahima; Ba, Yamar; Ndione, Jacques-André; Sall, Amadou Alpha; Morse, Andy; Diop, Aliou; Diallo, Mawlouth

    2014-01-01

    Rift Valley fever is an emerging mosquito-borne disease that represents a threat to human and animal health. The exophilic and exophagic behavior of the two main vector in West Africa (Aedes vexans and Culex poicilipes), adverse events post-vaccination, and lack of treatment, render ineffective the disease control. Therefore it is essential to develop an information system that facilitates decision-making and the implementation of adaptation strategies. In East Africa, RVF outbreaks are linked with abnormally high rainfall, and can be predicted up to 5 months in advance by modeling approaches using climatic and environmental parameters. However, the application of these models in West Africa remains unsatisfactory due to a lack of data for animal and human cases and differences in the dynamics of the disease emergence and the vector species involved in transmission. Models have been proposed for West Africa but they were restricted to rainfall impact analysis without a spatial dimension. In this study, we developed a mixed Bayesian statistical model to evaluate the effects of climatic and ecological determinants on the spatiotemporal dynamics of the two main vectors. Adult mosquito abundance data were generated from July to December every fortnight in 2005–2006 at 79 sites, including temporary ponds, bare soils, shrubby savannah, wooded savannah, steppes, and villages in the Barkédji area. The results demonstrate the importance of environmental factors and weather conditions for predicting mosquito abundance. The rainfall and minimum temperature were positively correlated with the abundance of Cx. poicilipes, whereas the maximum temperature had negative effects. The rainfall was negatively correlated with the abundance of Ae. vexans. After combining land cover classes, weather conditions, and vector abundance, our model was used to predict the areas and periods with the highest risks of vector pressure. This information could support decision-making to improve

  14. Statistical modeling of the abundance of vectors of West African Rift Valley fever in Barkédji, Senegal.

    PubMed

    Talla, Cheikh; Diallo, Diawo; Dia, Ibrahima; Ba, Yamar; Ndione, Jacques-André; Sall, Amadou Alpha; Morse, Andy; Diop, Aliou; Diallo, Mawlouth

    2014-01-01

    Rift Valley fever is an emerging mosquito-borne disease that represents a threat to human and animal health. The exophilic and exophagic behavior of the two main vector in West Africa (Aedes vexans and Culex poicilipes), adverse events post-vaccination, and lack of treatment, render ineffective the disease control. Therefore it is essential to develop an information system that facilitates decision-making and the implementation of adaptation strategies. In East Africa, RVF outbreaks are linked with abnormally high rainfall, and can be predicted up to 5 months in advance by modeling approaches using climatic and environmental parameters. However, the application of these models in West Africa remains unsatisfactory due to a lack of data for animal and human cases and differences in the dynamics of the disease emergence and the vector species involved in transmission. Models have been proposed for West Africa but they were restricted to rainfall impact analysis without a spatial dimension. In this study, we developed a mixed Bayesian statistical model to evaluate the effects of climatic and ecological determinants on the spatiotemporal dynamics of the two main vectors. Adult mosquito abundance data were generated from July to December every fortnight in 2005-2006 at 79 sites, including temporary ponds, bare soils, shrubby savannah, wooded savannah, steppes, and villages in the Barkédji area. The results demonstrate the importance of environmental factors and weather conditions for predicting mosquito abundance. The rainfall and minimum temperature were positively correlated with the abundance of Cx. poicilipes, whereas the maximum temperature had negative effects. The rainfall was negatively correlated with the abundance of Ae. vexans. After combining land cover classes, weather conditions, and vector abundance, our model was used to predict the areas and periods with the highest risks of vector pressure. This information could support decision-making to improve RVF

  15. Serological Evidence of Rift Valley Fever Virus Circulation in Domestic Cattle and African Buffalo in Northern Botswana (2010–2011)

    PubMed Central

    Jori, Ferran; Alexander, Kathleen A.; Mokopasetso, Mokganedi; Munstermann, Suzanne; Moagabo, Keabetswe; Paweska, Janusz T.

    2015-01-01

    Rift Valley fever (RVF) is endemic in many countries in Sub-Saharan Africa and is responsible for severe outbreaks in livestock characterized by a sudden onset of abortions and high neonatal mortality. During the last decade, several outbreaks have occurred in Southern Africa, with a very limited number of cases reported in Botswana. To date, published information on the occurrence of RVF in wild and domestic animals from Botswana is very scarce and outdated, despite being critical to national and regional disease control. To address this gap, 863 cattle and 150 buffalo sampled at the interface between livestock areas and the Chobe National Park (CNP) and the Okavango Delta (OD) were screened for the presence of RVF virus (RVFV) neutralizing antibodies. Antibodies were detected in 5.7% (n = 863), 95% confidence intervals (CI) (4.3–7.5%) of cattle and 12.7% (n = 150), 95% CI (7.8–19.5%) of buffalo samples. The overall prevalence was significantly higher (p = 0.0016) for buffalo [12.7%] than for cattle [5.7%]. Equally, when comparing RVF seroprevalence in both wildlife areas for all pooled bovid species, it was significantly higher in CNP than in OD (9.5 vs. 4%, respectively; p = 0.0004). Our data provide the first evidence of wide circulation of RVFV in both buffalo and cattle populations in Northern Botswana and highlight the need for further epidemiological and ecological investigations on RVF at the wildlife–livestock–human interface in this region. PMID:26664990

  16. Serological Evidence of Rift Valley Fever Virus Circulation in Domestic Cattle and African Buffalo in Northern Botswana (2010-2011).

    PubMed

    Jori, Ferran; Alexander, Kathleen A; Mokopasetso, Mokganedi; Munstermann, Suzanne; Moagabo, Keabetswe; Paweska, Janusz T

    2015-01-01

    Rift Valley fever (RVF) is endemic in many countries in Sub-Saharan Africa and is responsible for severe outbreaks in livestock characterized by a sudden onset of abortions and high neonatal mortality. During the last decade, several outbreaks have occurred in Southern Africa, with a very limited number of cases reported in Botswana. To date, published information on the occurrence of RVF in wild and domestic animals from Botswana is very scarce and outdated, despite being critical to national and regional disease control. To address this gap, 863 cattle and 150 buffalo sampled at the interface between livestock areas and the Chobe National Park (CNP) and the Okavango Delta (OD) were screened for the presence of RVF virus (RVFV) neutralizing antibodies. Antibodies were detected in 5.7% (n = 863), 95% confidence intervals (CI) (4.3-7.5%) of cattle and 12.7% (n = 150), 95% CI (7.8-19.5%) of buffalo samples. The overall prevalence was significantly higher (p = 0.0016) for buffalo [12.7%] than for cattle [5.7%]. Equally, when comparing RVF seroprevalence in both wildlife areas for all pooled bovid species, it was significantly higher in CNP than in OD (9.5 vs. 4%, respectively; p = 0.0004). Our data provide the first evidence of wide circulation of RVFV in both buffalo and cattle populations in Northern Botswana and highlight the need for further epidemiological and ecological investigations on RVF at the wildlife-livestock-human interface in this region. PMID:26664990

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

    NASA Astrophysics Data System (ADS)

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

    2014-09-01

    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.

  18. Two-step magma flooding of the upper crust during rifting: The Early Paleozoic of the Ossa Morena Zone (SW Iberia)

    NASA Astrophysics Data System (ADS)

    Sánchez-García, T.; Quesada, C.; Bellido, F.; Dunning, G. R.; González del Tánago, J.

    2008-12-01

    The Ossa Morena Zone of SW Iberia represents a continental arc accreted to the Iberian Autochthon during the Late Proterozoic-Early Cambrian Cadomian orogeny. A subsequent Cambrian-Ordovician rifting event is recorded in this zone, which was accompanied by intrusion/eruption of large volumes of igneous rocks. Exposed crustal segments show both volcanic and shallow plutonic rocks that according to their relationship with coeval sedimentary successions can be assigned to one of two periods of magma emplacement: i) an Early Igneous Event, exclusively comprised of acid peraluminous rocks associated with migmatite formation during development of core-complex structures in mid-upper crust environments; and ii) a Main Igneous Event, which produced predominantly basaltic and acid (rhyolite) rocks and minor amounts of intermediate (trachyte) rocks. Tholeiites and alkaline rocks predominate in this suite but minor calcalkaline peraluminous compositions are also present. Besides, a volumetrically unimportant but petrologically significant group of Mg-rich rocks also occurs within the Main Igneous Event. These latter rocks are interpreted to reflect high partial melting rates of a protolith similar to the primitive mantle. All the outlined characteristics provide evidence for large heterogeneity within the rift-related association that may be due to several causes, such as the involvement of various magma sources (asthenospheric, lithospheric, crustal) and/or involvement of various petrogenetic processes in their generation and evolution. Radiometric (U-Pb zircon) dating yielded c. 530 ± 5 Ma ages for the Early Igneous Event and a longer duration, 517-502 ± 2 Ma, for the Main Igneous Event. The large volume of magma emplaced into upper crustal environments, along with the presence of abundant dikes, suggest that magma ascent benefited from coeval extensional tectonism. It is suggested that they represent the igneous expression of rifting in connection with a severe thermal

  19. Post-rift uplift, paleorelief and sedimentary fluxes: the case example of the African margin of the South Atlantic

    NASA Astrophysics Data System (ADS)

    Guillocheau, F.; Dauteuil, O.

    2012-04-01

    Several attempts have been made to identify different paleosurfaces since the classical works of Lester King (1942, 1949) at the scale of Africa. Thermochronologists and river geomorphologists criticized this approach. This criticism mainly concerned the age of the surfaces, that were (1) poorly constraints and (2) a king of catechism on which all studies must refer. Nevertheless, those planation surfaces exist and are key features of the present-day morphology of Africa. In details, real planation surfaces are (1) no more than two or three and (2) can be deformed and then merged together. Those surfaces are incised by large smooth valleys, called pediments or glacis (with some semantic differences between English and French-speaking geomorphologists). Those pediments formed a pre-network of rivers, later re-incised by the present-day incised narrow valleys. Those different morphological structures can be dated using (1) their merge with sedimentary basins, (2) their relationship with the different types of dated weathering periods and (3) their relationships with volcanism. They also can be used as a proxy of the deformation based on the differences of elevation of the planations surfaces or on the shape of the pediments. From the Orange River to the Cameroon Volcanic Line, including the Congo Cuvette, two planations surfaces were identified (the Bauxitic or African surface, the intermediate surface), at least two generations of pediment valleys and the present-day incised valley network. The African surface is of Late Paleocene to Middle Eocene age with a climax during this last period and two major periods of uplift can be identified and mapped (1) Late Eocene-Early Oligocene and (2) Lower Miocene. Most of the relief is fossil since that period, excepted in the Angola Mountains were deformations are active during Plio-Pleistocene times. Those uplifts of smoother, most of the time weathered, relief than today, had important consequences on the petrology and the

  20. Minerals as mantle fingerprints: Sr-Nd-Pb-Hf in clinopyroxene and He in olivine distinguish an unusual ancient mantle lithosphere beneath the East African Rift System

    NASA Astrophysics Data System (ADS)

    Nelson, W. R.; Shirey, S. B.; Graham, D. W.

    2011-12-01

    The East African Rift System is a complex region that holds keys to understanding the fundamental geodynamics of continental break-up. In this region, the volcanic record preserves over 30 Myrs of geochemical variability associated with the interplay between shallow and deep asthenospheric sources, continental lithospheric mantle, and continental crust. One fundamental question that is still subject to debate concerns the relationship between the lithospheric mantle and the voluminous flood basalt province that erupted at ~30 Ma in Ethiopia and Yemen. Whole-rock Re-Os isotopic data demonstrate the high-Ti (HT2) flood basalts (187Os/188Ost = 0.1247-0.1329) and peridotite xenoliths (187Os/188Ost = 0.1235-0.1377) from NW Ethiopia have similar isotopic compositions. However, Sr-Nd-Pb-Hf isotopic signatures from peridotite clinopyroxene grains are different from those of the flood basalts. The peridotite clinopyroxene separates bear isotopic affinities to anciently depleted mantle (87Sr/86Sr = 0.7019-0.7029; ɛNd = 12.6-18.5; ɛHf = 13.8-27.6) - more depleted than the MORB source - rather than to the OIB-like 30 Ma flood basalts (87Sr/86Sr ~ 0.704; ɛNd = 4.7-6.7; ɛHf = 12.1-13.5). Peridotite clinopyroxenes display two groups of 206Pb/204Pb compositions: the higher 206Pb/204Pb group (18.7-19.3) is compositionally similar to the flood basalts (206Pb/204Pb = 18.97-19.02) whereas the lower 206Pb/204Pb group (17.1-17.9) overlaps with depleted mantle. This suggests that the Pb isotope systematics in some of the peridotites have been metasomatically perturbed. Helium isotopes were analyzed by crushing olivine separated from the peridotites and the flood basalts. Olivine in the peridotites has low He concentrations (0.78-4.7 ncc/g) and low 3He/4He (4.6-6.6 RA), demonstrating that they cannot be the petrogenetic precursor to the high 3He/4He (>12 RA) flood basalts. Notably, these peridotites have 3He/4He signatures consistent with a lithospheric mantle source. Therefore

  1. Geochemistry of basement rocks from SE Kenya and NE Tanzania: indications for rifting and early Pan-African subduction

    NASA Astrophysics Data System (ADS)

    Bauernhofer, A. H.; Hauzenberger, C. A.; Wallbrecher, E.; Muhongo, S.; Hoinkes, G.; Mogessie, A.; Opiyo-Akech, N.; Tenczer, V.

    2009-12-01

    Amphibolites and orthogneisses from the Taita Hills-Galana River area (SE Kenya) indicate their broad geological-tectonic setting. There are groups of subduction-related rocks which show characteristic REE (rare earth element) patterns and enrichment or varying concentrations of HFS (high field strength) elements. The groups can be assigned to tectonostratigraphic domains marked by different structural styles (e.g., thrust- or strike slip dominated). Tholeiitic gneisses, often emerging as folded and isolated (ridge-shaped) leucocratic bodies, belong to a group of rocks located between the thrust- and strike-slip domain. Compared to calc-alkaline gneisses of the area they contain more mafic inclusions and have lower LIL (large ionic lithophile), HFS and light REE values. These gneisses have chemical characteristics of M-type granitoids of oceanic island arc signature. Intrusion ages of ~955-845 Ma determined for these rocks suggest early Pan-African subduction. Mafic to ultramafic rocks from the Pare mountains of NE Tanzania show evidence of ophiolitic cumulates, subduction settings were also observed for the granulite areas in central and southern Tanzania. Together with the widespread arc settings documented in the Arabian-Nubian Shield, the presented data supports the continuation of an island-continental arc range across Kenya-Tanzania to Mozambique.

  2. Hawaii Rifts

    SciTech Connect

    Nicole Lautze

    2015-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  4. Late Miocene to Recent inversion of the Aure rift, eastern Papua New Guinea: A consequence of transpression along the Owen-Stanley fault zone

    NASA Astrophysics Data System (ADS)

    Ott, B.; Mann, P.; Campos Aguiniga, H.

    2009-12-01

    The north-trending Aure trough formed as a Paleocene failed rift during the opening of the northeast-trending Coral Sea between continental crust in northeastern Australia and Papua New Guinea. This study describes 1,600 km of onshore and offshore seismic data tied to six offshore wells in order to understand the tectonic controls on the style and chronology of deformation. Eocene-middle Miocene fill in the rifted basin includes a 7-km-thick section of marine, fine-grained sandstone and shale. Inversion of the basin fill starting in late Miocene time has created a SW-propagating fold-thrustbelt with the oldest folds and thrusts forming in the late Miocene, and the youngest still forming today. Seismic data allow for the southern extent of the deformed Aure belt to be continued 75 km offshore in the eastern Gulf of Papua where total shortening is estimated to be 18 km for late Miocene strata, 6km for early Pliocene strata, and less than 2km for Plio-Pleistocene and Tertiary strata. The trend of folds and thrusts on the south coast of the Papuan Peninsula closely parallels the strike of the transpressional segment of the left-lateral Owens-Stanley fault zone (OSFZ) along the center of the peninsula. As the OSFZ becomes more transtensional in the eastern part of the Papuan peninsula, folds on the south coast become less prominent. For this reason, it is proposed that the late Miocene age for initial inversion of the Aure trough matches the initiation of left-lateral motion along the OSFZ.

  5. Serological survey of African horse sickness in selected districts of Jimma zone, Southwestern Ethiopia.

    PubMed

    Bitew, Molalegne; Andargie, Ashenafi; Bekele, Mihreteab; Jenberie, Shiferaw; Ayelet, Gelagay; Gelaye, Esayas

    2011-12-01

    A cross-sectional serological survey was undertaken in selected districts of different agro-ecology of Jimma zone (Dedo, Yebu, Seka, Serbo, and Jimma town) from November 2009 to February 2010 to determine the seroprevalence of African horse sickness virus and associated risk factors of the disease. Two hundred seventy-four equids (189 horses, 43 mules, and 47 donkeys) with a history of non-vaccination for at least 2 years were selected randomly from the above areas. Sera samples were collected and assayed for the presence of specific antibody against African horse sickness virus using blocking ELISA. An overall seroprevalence of 89 (32.5%) was found and it was 24 (51.1%) for donkeys, 13 (30.2%) for mules, and 52(28.3%) for horses. Seroprevalence was significantly (X(2) = 11.05, P < 0.05) different among the different species of equids. Seroprevalence was also significantly (X(2) = 11.43, P < 0.05) different among the different agro-ecological areas being higher in highlands 47 (40.5%) followed by midland 30 (34.5%) and lowland 12 (16.9%). Age and sex were not significantly (X(2) = 3.15, P > 0.05 and X(2) = 3.38, P > 0.05, respectively) associated with seroprevalence of AHSV. The present study showed that African horse sickness (AHS) is highly prevalent disease for the horses followed by mules and then donkeys in Jimma zone explained by lower seroconversion rate. Therefore, control strategy against AHS should target at high risk species of all age and sex in their locality in the initial stage for better containment of the disease.

  6. Magmatism in rifting and basin formation

    NASA Astrophysics Data System (ADS)

    Thybo, H.

    2008-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    1992-10-01

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

  9. Continental breakup in Africa: From superplume to rifting

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    The low seismic velocities that underlie the East African continent are a ubiquitous feature of global tomographic images, and can readily explain both observed plateau uplift and the volcanic Cenozoic geological record in the region. However, knowledge of the morphology of the African Superplume, and the nature of the mantle flow-field remain incomplete. Over the last decade East Africa has seen many deployments of seismic stations, with the aim of understanding continental breakup in the region. We have combined data from 5 of these experiments, to enable us to resolve high resolution models of upper mantle P- and S- wave velocities, and seismic anisotropy extending from the Red Sea to Kenya. The tomographic inversions highlight a sheet like upwelling beneath this whole region. It is oriented SW-NE and extends from at least the transition zone to the crust. This is most likely associated with upwelling material associated with the African superplume. In the uppermost 100 km, strong P- and S- wave low velocity anomalies underlie the most recent rift related volcanism and are likely associated with high temperatures and partial melt. High quality SKS splitting results, from a variety of back azimuths, reveal depth variations in anisotropy beneath large parts of Ethiopia. The lower layer parallels the SW-NE trend of the low velocity anomaly, suggesting an LPO fabric due to mantle flow. The upper layer parallels structural features at the surface, including aligned melt in the crust/lithosphere at the rift axis, and Pan-African fabrics in regions not characterised by Quaternary volcanism. These results suggest that thermal instabilities arising from upwelling material provides heat for melting and uplift, but rifting may follow pre-existing weaknesses in the lithosphere.

  10. Facies architecture of a Triassic rift-related Silicic Volcano-Sedimentary succession in the Tethyan realm, Peonias subzone, Vardar (Axios) Zone, northern Greece; Regional implications

    NASA Astrophysics Data System (ADS)

    Asvesta, Argyro; Dimitriadis, Sarantis

    2010-06-01

    In northern Greece, along the western edge of the Paleozoic Vertiscos terrane (Serbomacedonian massif) and within the Peonias subzone - the eastern part of the Vardar (Axios) Zone - a Silicic Volcano-Sedimentary (SVS) succession of Permo(?)-Skythian to Mid Triassic age records the development of a faulted continental margin and the formation of rhyolitic volcanoes along a continental shelf fringed by neritic carbonate accumulations. It represents the early rifting extensional stages that eventually led to the opening of the main oceanic basin in the western part of the Vardar (Axios) Zone (the Almopias Oceanic Basin). Even though the SVS succession is deformed, altered, extensively silicified and metamorphosed in the low greenschist facies, primary textures, original contacts and facies relationships are recognized in some places allowing clues for the facies architecture and the depositional environment. Volcanic and sedimentary facies analysis has been carried out at Nea Santa and Kolchida rhyolitic volcanic centres. Pyroclastic facies, mostly composed of gas-supported lapilli tuffs and locally intercalated accretionary lapilli tuffs, built the early cones which were then overridden by rhyolitic aphyric and minor K-feldspar-phyric lava flows. The characteristics of facies, especially the presence of accretionary lapilli, imply subaerial to coastal emplacement at this early stage. The mature and final stages of volcanism are mostly represented by quartz-feldspar porphyry intrusions that probably occupied the vents. At Nea Santa area, the presence of resedimented hyaloclastite facies indicates subaqueous emplacement of rhyolitic lavas and/or lobes. Moreover, quartz-feldspar-phyric sills and a partly extrusive dome featuring peperites at their margins are inferred to have intruded unconsolidated, wet carbonate sediments of the overlying Triassic Neritic Carbonate Formation, in a shallow submarine environment. The dome had probably reached above wave-base as is

  11. Geomorphometric reconstruction of post-eruptive surfaces of the Virunga Volcanic Province (East African Rift), constraint of erosion ratio and relative chronology

    NASA Astrophysics Data System (ADS)

    Lahitte, Pierre; Poppe, Sam; Kervyn, Matthieu

    2016-04-01

    Quaternary volcanic landforms result from a complex evolution, involving volcanic constructional events and destructive ones by collapses and long-term erosion. Quantification, by morphometric approaches, of the evolution through time of the volcano shape allows the estimation of relative ages between volcanoes sharing the same climate and eruptive conditions. We apply such method to six volcanoes of the Virunga Volcanic Province in the western branch of the East African Rift Valley that still has rare geochronological constraints. As they have comparable sizes, volcanic history and erupted products, these edifices may have undergone comparable conditions of erosion which justify the deduction of relative chronology from their erosion pattern. Our GIS-based geomorphometric approach, the SHAPEVOLC algorithm, quantifies erupted or dismantled volumes by numerically modeling topographies resulting from the eruptive construction of each volcano. Constraining points are selected by analyses of morphometric properties of each cell of the current DEM, as the loci where the altitude is still representative of the un-eroded volcanic surfaces. A primary elevation surface is firstly adjusted to these constraining points by modeling a first-order pseudo-radial surface defined by: 1. the curve best fitting the concave-upwards volcano profile; 2. the location and elevation of the volcano summit; and 3. the possible eccentricity and azimuth parameters that allow to stretch and contract contours to adjust the shape of the model to the elliptically-shaped surface of the volcano. A second-order surface is next computed by local adjustment of the first-order surface to the constraining points to obtain the definitive primary elevation surface of the considered volcanic construct. Amount of erosion is obtained by summing the difference in elevation between reconstructed surfaces and current ones that allows to establish relative ages of volcanoes. For the 6 studied Virunga volcanoes

  12. East African coast overlooked. [Oil and gas potential of the east African coast

    SciTech Connect

    Not Available

    1994-09-01

    This paper reviews the petroleum and gas potential of the Tanzania-Mozambique basinal areas. It discusses the locations of the various sedimentary basins in the onshore and near offshore areas, including the central African rift zone. The paper describes the structure, stratigraphy, and petroleum geology of these basins. Finally the paper reviews the exploration history and the outlook for the future of these basins.

  13. Lower Crustal Seismicity, Volatiles, and Evolving Strain Fields During the Initial Stages of Cratonic Rifting

    NASA Astrophysics Data System (ADS)

    Lambert, C.; Muirhead, J.; Ebinger, C. J.; Tiberi, C.; Roecker, S. W.; Ferdinand-Wambura, R.; Kianji, G.; Mulibo, G. D.

    2014-12-01

    The volcanically active East African rift system in southern Kenya and northern Tanzania transects thick cratonic lithosphere, and comprises several basins characterized by deep crustal seismicity. The US-French-Tanzania-Kenya CRAFTI project aims to understand the role of magma and volatile movement during the initiation and evolution of rifting in cratonic lithosphere. Our 38-station broadband network spans all or parts of fault-bounded rift segments, enabling comparison of lithospheric structure, fault kinematics, and seismogenic layer thickness with age and proximity to the deeply rooted Archaen craton. Seismicity levels are high in all basins, but we find profound differences in seismogenic layer thickness along the length of the rift. Seismicity in the Manyara basin occurs almost exclusively within the lower crust, and in spatial clusters that have been active since 1990. In contrast, seismicity in the ~ 5 My older Magadi basin is localized in the upper crust, and the long border fault bounding the west side of the basin is seismically inactive. Between these two basins lies the Natron rift segment, which shows seismicity between ~ 20 and ~2 km depth, and high concentrations at Oldoinyo Lengai and Gelai volcanoes. Older volcanoes on the uplifted western flank (e.g., Ngorongoro) experience swarms of activity, suggesting that active magmatism and degassing are widespread. Focal mechanisms of the frequent earthquakes recorded across the array are spatially variable, and indicate a stress field strongly influenced by (1) Holocene volcanoes, (2) mechanical interactions between adjacent rift basins, and (3) a far-field ESE-WNW extensional stress regime. We explore the spatial correlation between zones of intense degassing along fault systems and seismicity, and examine the influence of high gas pressures on lower and upper crustal seismicity in this youthful cratonic rift zone.

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

    NASA Astrophysics Data System (ADS)

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

    2013-02-01

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

  15. Cambrian rift-related magmatism in the Ossa-Morena Zone (Iberian Massif): Geochemical and geophysical evidence of Gondwana break-up

    NASA Astrophysics Data System (ADS)

    Sarrionandia, F.; Carracedo Sánchez, M.; Eguiluz, L.; Ábalos, B.; Rodríguez, J.; Pin, C.; Gil Ibarguchi, J. I.

    2012-10-01

    Volcanic rocks of Cambrian age from Zafra (Ossa-Morena Zone, Iberian Massif) are the result of rift processes that affected Cadomian arc units accreted to the NW edge of Gondwana during the Neoproterozoic-Early Cambrian transition. Tephrite to rhyolite volcanics define an alkaline transitional association (Coombs type). Basic-ultrabasic rocks exhibit typical alkaline REE-patterns, strongly enriched in LREE with respect to HREE. Two parental magmas are identified, one with a mantle signature, lack of Nb negative anomaly and εNd500Ma + 3.8 to + 4.2; another with crustal contribution, minor Nb negative anomaly and εNd500Ma + 0.8 to + 1.8. Intermediate-acid rocks show variable REE fractionation and share geochemical characteristics of both basic-ultrabasic groups with restricted εNd500Ma + 2.2 to 3.1 and general absence of Nb negative anomaly. Basic-ultrabasic melts resulted from different amounts of partial melting of a homogeneous source and segregation at the garnet-spinel transition zone. We argue that the "Hales transition" recently recognized in reflection seismic experiments of SW Iberia might image such a source region. Mantle-derived magmas ponded at the base of the crust and weakly interacted with crustal rocks/melts, whilst intermediate-acid rocks were generated by plagioclase ± clinopyroxene ± amphibole fractionation. Melt ascent took place through fractures, with limited crustal interaction. Based upon the new geochemical results and complementary cartographic and geophysical data, a model is presented for the Cambrian break-up of North Gondwana due to magma ascent from the mantle.

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

    NASA Astrophysics Data System (ADS)

    Martone, Alexis; Montesi, Laurent

    2015-11-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  18. The inverted Triassic rift of the Marrakech High Atlas: A reappraisal of basin geometries and faulting histories

    NASA Astrophysics Data System (ADS)

    Domènech, Mireia; Teixell, Antonio; Babault, Julien; Arboleya, Maria-Luisa

    2015-11-01

    The High Atlas of Morocco is an aborted rift developed during the Triassic-Jurassic and moderately inverted during the Cenozoic. The Marrakech High Atlas, with large exposures of basement and Triassic early syn-rift deposits, is ideal to investigate the geometries of the deepest parts of a rift, constituting a good analogue for pre-salt domains. It allows unraveling geometries and kinematics of the extensional and compressional structures and the influence that they exert over one another. A detailed structural study of the main Triassic basins and basin-margin faults of the Marrakech High Atlas shows that only a few rift faults were reactivated during the Cenozoic compressional stage in contrast to previous interpretations, and emphasizes that fault reactivation cannot be taken for granted in inverted rift systems. Preserved extensional features demonstrate a dominant dip-slip opening kinematics with strike-slip playing a minor role, at variance to models proposing a major strike-slip component along the main basin-bounding faults, including faults belonging to the Tizi n'Test fault zone. A new Middle Triassic paleogeographic reconstruction shows that the Marrakech High Atlas was a narrow and segmented orthogonal rift (sub-perpendicular to the main regional extension direction which was ~ NW-SE), in contrast to the central and eastern segments of the Atlas rift which developed obliquely. This difference in orientation is attributed to the indented Ouzellarh Precambrian salient, part of the West African Craton, which deflected the general rift trend in the area evidencing the major role of inherited lithospheric anisotropies in rift direction and evolution. As for the Cenozoic inversion, total orogenic shortening is moderate (~ 16%) and appears accommodated by basement-involved large-scale folding, and by newly formed shortcut and by-pass thrusting, with rare left-lateral strike-slip indicators. Triassic faults commonly acted as buttresses.

  19. Parga Chasma: Coronae and Rifting on Venus

    NASA Technical Reports Server (NTRS)

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

    2005-01-01

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

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

    SciTech Connect

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

    1995-12-01

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

  1. Mid-Continent rift system: a frontier hydrocarbon province

    SciTech Connect

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

    1984-04-01

    The Mid-Continent rift system can be traced by the Mid-Continent geophysical anomaly (MGA) from the surface exposure of the Keweenawan Supergroup in the Lake Superior basin southwest in the subsurface through Wisconsin, Minnesota, Iowa, Nebraska, and Kansas. Outcrop and well penetrations of the late rift Keweenawan sedimentary rocks reveal sediments reflecting a characteristic early continental rift clastic sequence, including alluvial fans, deep organic-rich basins, and prograding fluvial plains. Sedimentary basins where these early rift sediments are preserved can be located by upward continuation of the aeromagnetic profiles across the rift trend and by gravity models. Studies of analog continental rifts and aulacogens show that these gravity models should incorporate (1) a deep mafic rift pillow body to create the narrow gravity high of the MGA, and (2) anomalously thick crust to account for the more regional gravity low. Preserved accumulations of rift clastics in central rift positions can then be modeled to explain the small scale notches which are found within the narrow gravity high. Indigenous oil in Keweenawan sediments in the outcrop area and coaly partings in the subsurface penetrations of the Keweenawan clastics support the analogy between these rift sediments and the exceptionally organic-rich sediments of the East African rift. COCORP data across the rift trend in Kansas show layered deep reflectors and large structures. There is demonstrable source, reservoir, and trap potential within the Keweenawan trend, making the Mid-Continent rift system a frontier hydrocarbon province.

  2. Mid-continent rift system: a frontier hydrocarbon province

    SciTech Connect

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

    1984-04-01

    The Mid-continent rift system can be traced by the Mid-continent geophysical anomaly (MGA) from the surface exposure of the Keweenawan Supergroup in the Lake Superior basin southwest in the subsurface through Wisconsin, Minnesota, Iowa, Nebraska, and Kansas. Outcrop and well penetrations of the late rift Keweenawan sedimentary rocks reveal sediments reflecting a characteristic early continental rift clastic sequence, including alluvial fans, deep organic-rich basins, and prograding fluvial plains. Sedimentary basins where these early rift sediments are preserved can be located by upward continuation of the aeromagnetic profiles across the rift trend and by gravity models. Studies of analog continental rifts and aulacogens show that these gravity models should incorporate (1) a deep mafic rift pillow body to create the narrow gravity high of the MGA, and (2) anomalously thick crust to account for the more regional gravity low. Preserved accumulations of rift clastics in central rift positions can then be modeled to explain the small scale notches which are found within the narrow gravity high. Indigenous oil in Keweenawan sediments in the outcrop area and coaly partings in the subsurface penetrations of the Keweenawan clastics support the analogy between these rift sediments and the exceptionally organic-rich sediments of the East African rift. COCORP data across the rift trend in Kansas show layered deep reflectors and large structures. There is demonstrable source, reservoir, and trap potential within the Keweenawan trend, making the Mid-Continent rift system a frontier hydrocarbon province.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    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

  4. Eruption dynamics and degassing histories of high fountaining episodes of the Pu`u `O`o eruption in the East Rift Zone of Kilauea, 1983-1986

    NASA Astrophysics Data System (ADS)

    Holt, S. J.; Carey, R.; Houghton, B. F.; Swanson, D. A.; Orr, T. R.; Patrick, M. R.

    2013-12-01

    Between January 1983 and July 1986, activity at Kilauea was localised at Pu`u `O`o in the East Rift Zone. During this time activity was characterised by short (0.2 to 16 days), high intensity fountaining episodes of varying heights, which were separated by lengthy repose periods (10 to 50 days). Microtextural analysis of juvenile pyroclasts produced by eruptions such as these can be highly valuable in providing information on the nature of the evolution of the magma's volatiles in the shallow conduit. The growing literature on microtextural analysis of the vesicles in basaltic pyroclasts suggests that variations in eruption form, duration and intensity is controlled by the degassing and outgassing of the magma, and in variations in the timing and extent of the nucleation, growth and coalescence of bubbles in the magma and the degree in which they are mechanically coupled to the melt (Stovall et al. 2011, Parcheta et al. 2013). We have collected samples from three high fountaining episodes that were well characterised by Hawaiian Volcano Observatory staff at the time of the eruption. These episodes produced fountains with heights between 391-445 m, and volumes of 1.1x107 m3 and 1.7x107 m3 of lava (bulk volume). We describe here microtextural characteristics and matrix glass volatile contents of pyroclasts erupted from the three episodes and we couple this data together with multidisciplinary geophysical and visual datasets collected by HVO staff. The combination of quantitative vesicle microtextures and syn-eruptive volatiles within groundmass glass permit us to quantify the degassing histories of magma erupted in each eruptive episode. This data will be combined with visual observations of fountain height and geophysical data (geodetic and seismic) in order to better define the factors that govern the variations in the eruption intensity of Hawaiian volcanism.

  5. Colorado Basin Structure and Rifting, Argentine passive margin

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    The Argentine margin presents a strong segmentation with considerable strike-slip movements along the fracture zones. We focus on the volcanic segment (between the Salado and Colorado transfer zones), which is characterized by seaward dipping reflectors (SDR) all along the ocean-continent transition [e.g. Franke et al., 2006; Gladczenko et al., 1997; Hinz et al., 1999]. The segment is structured by E-W trending basins, which differs from the South African margin basins and cannot be explained by classical models of rifting. Thus the study of the relationship between the basins and the Argentine margin itself will allow the understanding of their contemporary development. Moreover the comparison of the conjugate margins suggests a particular evolution of rifting and break-up. We firstly focus on the Colorado Basin, which is thought to be the conjugate of the well studied Orange Basin [Hirsch et al., 2009] at the South African margin [e.g. Franke et al., 2006]. This work presents results of a combined approach using seismic interpretation and structural, isostatic and thermal modelling highlighting the structure of the crust. The seismic interpretation shows two rift-related discordances: one intra syn-rift and the break-up unconformity. The overlying sediments of the sag phase are less deformed (no sedimentary wedges) and accumulated before the generation of oceanic crust. The axis of the Colorado Basin trends E-W in the western part, where the deepest pre-rift series are preserved. In contrast, the basin axis turns to a NW-SE direction in its eastern part, where mainly post-rift sediments accumulated. The most distal part reaches the margin slope and opens into the oceanic basin. The general basin direction is almost orthogonal to the present-day margin trend. The most frequent hypothesis explaining this geometry is that the Colorado Basin is an aborted rift resulting from a previous RRR triple junction [e.g. Franke et al., 2002]. The structural interpretation

  6. Differentiation and magma mixing on Kilauea's east rift zone - A further look at the eruptions of 1955 and 1960. Part I. The late 1955 lavas

    USGS Publications Warehouse

    Helz, R.T.; Wright, T.L.

    1992-01-01

    The lavas of the 1955 east rift eruption of Kilauea Volcano have been the object of considerable petrologic interest for two reasons. First, the early 1955 lavas are among the most differentiated ever erupted at Kilauea, and second, the petrographic character and chemical composition of the lava being erupted changed significantly during the eruption. This shift, from more differentiated (MgO=5.0-5.7%) to more magnesian (MgO=6.2-6.8%) lava, has been variously interpreted, as either due to systematic excavation of a zoned, differentiated magma body, or to invasion of the differentiated magma by more primitive magma, followed by rapid mixing and eruption of the resulting hybrid magmas. Petrologic examination of several nearvent spatter samples of the late 1955 lavas shows abundant evidence for magma mixing, including resorbed and/or reversely zoned crystals of olivine, augite and plagioclase. In addition, the compositional ranges of olivine, plagioclase and groundmass sulfide are very large, implying that the assemblages are hybrid. Core compositions of olivine phenocrysts range from Fo85 to Fo77. The most magnesian olivines in these samples must have originally crystallized from a melt containing 8.0-8.5% MgO, which is distinctly more magnesian than the bulk composition of the late 1955 lavas. The majorelement and trace-element data are either permissive or supportive of a hybrid origin for the late 1955 lavas. In particular, the compositional trends of the 1955 lavas on plots of CaO vs MgO, and the virtual invariance of Al2O3 and Sr in these plagioclase-phyric lavas are more easily explained by magma mixing than by fractionation. The pattern of internal disequilibrium/re-equilibration in the late 1955 spatter samples is consistent with reintrusion and mixing having occurred at least twice, during the latter part of the 1955 eruption. Plagioclase zonation preserves possible evidence for additional, earlier reintrusion events. Least-squares modelling the mixing of

  7. Subcontinental rift initiation and ocean-continent transitional setting of the Dinarides and Vardar zone: Evidence from the Krivaja-Konjuh Massif, Bosnia and Herzegovina

    NASA Astrophysics Data System (ADS)

    Faul, Ulrich H.; Garapić, Gordana; Lugović, Boško

    2014-08-01

    The Dinaride and Vardar zone ophiolite belts extend from the south-eastern margins of the Alps to the Albanian and Greek ophiolites. Detailed sampling of the Krivaja-Konjuh massif, one of the largest massifs in the Dinaride belt, reveals fertile compositions and an extensive record of deformation at spinel peridotite facies conditions. High Na2O clinopyroxene and spinel-orthopyroxene symplectites after garnet indicate a relatively high pressure, subcontinental origin of the southern and western part of Krivaja, similar to orogenic massifs such as Lherz, Ronda and the Eastern Central Alpine peridotites. Clinopyroxene and spinel compositions from Konjuh show similarities with fertile abyssal peridotite. In the central parts of the massif the spinel lherzolites contain locally abundant patches of plagioclase, indicating impregnation by melt. The migrating melt was orthopyroxene undersaturated, locally converting the peridotites to massive olivine-rich troctolites. Massive gabbros and more evolved gabbro veins cross-cutting peridotites indicate continued melt production at depth. Overall we infer that the massif represents the onset of rifting and early stages of formation of a new ocean basin. In the south of Krivaja very localized chromitite occurrences indicate that much more depleted melts with supra-subduction affinity traversed the massif that have no genetic relationship with the peridotites. This indicates that volcanics with supra-subduction affinity at the margins of the Krivaja-Konjuh massif record separate processes during closure of the ocean basin. Comparison with published compositional data from other Balkan massifs shows that the range of compositions within the Krivaja-Konjuh massif is similar to the compositional range of the western massifs of the Dinarides. The compositions of the Balkan massifs show a west to east gradient, ranging from subcontinental on the western side of the Dinarides to depleted mid-ocean ridge/arc compositions in the Vardar

  8. Impact of Baltimore Healthy Eating Zones: an environmental intervention to improve diet among African American youth.

    PubMed

    Shin, Ahyoung; Surkan, Pamela J; Coutinho, Anastasia J; Suratkar, Sonali R; Campbell, Rebecca K; Rowan, Megan; Sharma, Sangita; Dennisuk, Lauren A; Karlsen, Micaela; Gass, Anthony; Gittelsohn, Joel

    2015-04-01

    This study assessed the impact of a youth-targeted multilevel nutrition intervention in Baltimore City. The study used a clustered randomized design in which 7 recreation centers and 21 corner stores received interventions and 7 additional recreation centers served as comparison. The 8-month intervention aimed to increase availability and selection of healthful foods through nutrition promotion and education using point-of purchase materials such as posters and flyers in stores and interactive sessions such as taste test and cooking demonstrations. Two hundred forty-two youth-caregiver dyads residing in low-income areas of Baltimore City recruited from recreation centers were surveyed at baseline using detailed instruments that contained questions about food-related psychosocial indicators (behavioral intentions, self-efficacy, outcome expectancies, and knowledge), healthful food purchasing and preparation methods, and anthropometric measures (height and weight). The Baltimore Healthy Eating Zones intervention was associated with reductions in youth body mass index percentile (p = .04). In subgroup analyses among overweight and obese girls, body mass index for age percentile decreased significantly in girls assigned to the intervention group (p = .03) and in girls with high exposure to the intervention (p = .013), as opposed to those in comparison or lower exposure groups. Intervention youth significantly improved food-related outcome expectancies (p = .02) and knowledge (p < .001). The study results suggest that the Baltimore Healthy Eating Zones multilevel intervention had a modest impact in reducing overweight or obesity among already overweight low-income African American youth living in an environment where healthful foods are less available. Additional studies are needed to determine the relative impact of health communications and environmental interventions in this population, both alone and in combination.

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

    SciTech Connect

    Bosworth, W.

    1987-05-01

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

  10. Characterising East Antarctic Lithosphere and its Rift Systems using Gravity Inversion

    NASA Astrophysics Data System (ADS)

    Vaughan, Alan P. M.; Kusznir, Nick J.; Ferraccioli, Fausto; Leat, Phil T.; Jordan, Tom A. R. M.; Purucker, Michael E.; Golynsky, A. V. Sasha; Rogozhina, Irina

    2013-04-01

    Since the International Geophysical Year (1957), a view has prevailed that East Antarctica has a relatively homogeneous lithospheric structure, consisting of a craton-like mosaic of Precambrian terranes, stable since the Pan-African orogeny ~500 million years ago (e.g. Ferracioli et al. 2011). Recent recognition of a continental-scale rift system cutting the East Antarctic interior has crystallised an alternative view of much more recent geological activity with important implications. The newly defined East Antarctic Rift System (EARS) (Ferraccioli et al. 2011) appears to extend from at least the South Pole to the continental margin at the Lambert Rift, a distance of 2500 km. This is comparable in scale to the well-studied East African rift system. New analysis of RadarSat data by Golynsky & Golynsky (2009) indicates that further rift zones may form widely distributed extension zones within the continent. A pilot study (Vaughan et al. 2012), using a newly developed gravity inversion technique (Chappell & Kusznir 2008) with existing public domain satellite data, shows distinct crustal thickness provinces with overall high average thickness separated by thinner, possibly rifted, crust. Understanding the nature of crustal thickness in East Antarctica is critical because: 1) this is poorly known along the ocean-continent transition, but is necessary to improve the plate reconstruction fit between Antarctica, Australia and India in Gondwana, which will also better define how and when these continents separated; 2) lateral variation in crustal thickness can be used to test supercontinent reconstructions and assess the effects of crystalline basement architecture and mechanical properties on rifting; 3) rift zone trajectories through East Antarctica will define the geometry of zones of crustal and lithospheric thinning at plate-scale; 4) it is not clear why or when the crust of East Antarctica became so thick and elevated, but knowing this can be used to test models of

  11. Mapping Precambrian Basement Fabric with Magnetic Data in the Karonga Basin Area and its Control on the Development of the Malawi Rift.

    NASA Astrophysics Data System (ADS)

    Johnson, T.; Abdelsalam, M. G.; Atekwana, E. A.; Chindandali, P. R. N.; Clappe, B.; Laó-Dávila, D. A.; Dawson, S.; Hull, C. D.; Nyalugwe, V.; Salima, J.

    2015-12-01

    The Malawi Rift forms the southern termination of the western branch of the East African Rift System. It is suggested that it propagates from the Rungwe Volcanic Province in the north for ~700 km into Mozambique in the south. The northern portion of the Malawi Rift is dominated by the Mesoproterozoic basement rocks of the Ubendian-Usagaran belts to the north and west and the Irumide Belt in the south. The Mugese shear zone (MSZ) forms the boundary between the Ubendian-Usagaran and Irumide Belts. We used magnetic data to determine the relationship between the geology of the nascent Malawi Rift and the strong magnetic fabric observed in the Mugese shear zone from aeromagnetic maps. We integrated the aeromagnetic data with ground magnetic data acquired along two W-E transects using a cesium vapor magnetometer at a nominal station spacing of 500 m. We also acquired kinematic data (strike and dip) on exposed basement geology and Karoo sediments. Both transects extend from the uplifted basement areas cutting across the MSZ into the rift floor sediments. Our results show that the MSZ is characterized by a prominent WNW-ESE magnetic anomaly that is parallel to the basement fabric north of the town of Karonga but changes orientation to NNW-SSE south of Karonga. This shear zone is composed of gneisses in amphibolite to granulite facies that are steeply dipping (50-80°) to the west. The strong magnetization and magnetic lineation of the MSZ results from alternating light and dark colored gneissic bands. This magnetization is strongest in unweathered basement rocks and lowest in weathered basement rocks and Karoo sediments. The orientation of the strong magnetic fabric of the Mugese shear zone may play an important role on the accommodation of strain within the rift basin. Detailed mapping of the magnetic fabric can improve our understanding of the formation of faults in the nascent Malawi Rift.

  12. Evidence of Ancient Rifts Beneath Texas

    NASA Astrophysics Data System (ADS)

    Irie, K.; Velasco, A. A.

    2011-12-01

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

  13. [Rift Valley fever].

    PubMed

    Pépin, M

    2011-06-01

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

  14. Recognition of hyper-extended rifted margin remnants in the internal zone of the Alpine belt: A tribute to Marco Beltrando

    NASA Astrophysics Data System (ADS)

    Mohn, Geoffroy; Manatschal, Gianreto

    2016-04-01

    Marco Beltrando was part of the young generation of Alpine geologists who challenged the interpretation of the Western Alps by combining a classical field approach and modern techniques (e.g. 40Ar/39Ar and (U-Th)/He thermochronology). His work provides the foundation to re-interpret some of the classical sections through the Alpine belt and may impact the way of thinking about the nature and structure of internal parts of collisional orogens. This contribution will present the main outcomes of the work of Marco Beltrando and their implications for the understanding of Alpine type orogens. Since his PhD, Marco Beltrando focused most of his work on the study of the internal parts of the Western Alps. He investigated in great details the complex, multiphase structural and metamorphic evolution of the Penninic units in the Western Alps. He concluded that these units went through several cycles of shortening and extension during the Alpine orogeny, with major implications for the Alps but also other orogenic belts. After his PhD, he focused his research on the pre-orogenic evolution of the Alpine belt. He first worked on the Petit St. Bernard area, where he identified relics of the former hyper-extended Tethyan rifted margin. Thanks to his work and his amazing knowledge of the Western Alps, he understood the potential importance of rift-inheritance in controlling the architecture and evolution of the Alpine belt. In parallel to the study of the orogenic evolution, he developed a new methodology to recognize rift-related lithostratigraphic units in highly deformed and metamorphosed parts of the Alps. His innovative work allowed a re-assessment of several areas in the Western Alps and demonstrates the importance of rift inheritance. Recently, he started a new research project on the evolution of the Southern Alps highlighting the importance of heating and cooling cycles resulting from complex successions of rifting events. In spite of his young age, Marco Beltrando was at

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

    USGS Publications Warehouse

    Hall, Jerome; Wilson, Terry; Henrys, Stuart

    2007-01-01

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

  16. Influence of lower crustal rheology on onset and distribution of melting and serpentinisation during rifting: comparison with the Brazilian/African conjugate margins

    NASA Astrophysics Data System (ADS)

    Perez-Gussinye, M.; Araujo, M. N.; Romeiro, M. T.; Martinez, M. A.; Morgan, J. P.; Ros, E.

    2014-12-01

    The onset and distribution of melting and serpentinisation during rifting determine the continent-ocean transition width and composition and have been shown to depend on extension velocity. Conductive cooling during slow rifting favors serpentinisation and inhibits melting (Perez-Gussinye et al., 2006). Here we use numerical modeling to show that, additionally, lower crustal rheology, which also controls margin symmetry and width (Brune et al. 2014), strongly influences the onset and distribution of melting and serpentinisation. We find that strong lower crust rheologies effectively couple deformation in upper crust and mantle and lead to rapid crustal break-up through crust-cutting faults (see Brune et al., 2014), allowing serpentinisation to start relatively early and producing narrow, symmetric margins. Coupling of lithospheric layers leads to rapid asthenospheric uplift and the onset of melting at a relatively early stage during extension. For slow velocities, serpentinisation starts before melting, and the little magma produced probably ponds under the serpentinite layer exhumed after crustal break-up, generating a wide continent-ocean transition. For the same extension velocities, relatively weak lower crust shows a long initial phase of distributed faulting, with moderate lithospheric thinning, followed by a long phase of sequential, oceanward younging faults, producing wider, asymmetric margins. Serpentinisation is insignificant because lower crustal flow towards the tip of the active fault inhibits the formation of crust cutting faults. Asthenospheric upwelling is less pronounced, and the onset and amount of melting is delayed with respect to the stronger lower crust case. When crustal break-up occurs magma rises to form oceanic crust and hence a narrow continent-ocean transition. Along Brazil and Africa the margin's symmetry, width and continent-ocean transition type change as the onshore terranes in which they developed go from cratons to mobile belts

  17. Archean inheritance in zircon from late Paleozoic granites from the Avalon zone of southeastern New England: an African connection

    USGS Publications Warehouse

    Zartman, R.E.; Don, Hermes O.

    1987-01-01

    In southeastern New England the Narragansett Pier Granite locally intrudes Carboniferous metasedimentary rocks of the Narragansett basin, and yields a monazite UPb Permian emplacement age of 273 ?? 2 Ma. Zircon from the Narragansett Pier Granite contains a minor but detectable amount of an older, inherited component, and shows modern loss of lead. Zircon from the late-stage, aplitic Westerly Granite exhibits a more pronounced lead inheritance -permitting the inherited component to be identified as Late Archean. Such old relict zircon has not been previously recognized in Proterozoic to Paleozoic igneous rocks in New England, and may be restricted to late Paleozoic rocks of the Avalon zone. We suggest that the Archean crustal component reflects an African connection, in which old Archean crust was underplated to the Avalon zone microplate in the late Paleozoic during collision of Gondwanaland with Avalonia. ?? 1987.

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

    NASA Astrophysics Data System (ADS)

    Corti, Giacomo

    2009-09-01

    The Main Ethiopian Rift is a key sector of the East African Rift System that connects the Afar depression, at Red Sea-Gulf of Aden junction, with the Turkana depression and Kenya Rift to the South. It is a magmatic rift that records all the different stages of rift evolution from rift initiation to break-up and incipient oceanic spreading: it is thus an ideal place to analyse the evolution of continental extension, the rupture of lithospheric plates and the dynamics by which distributed continental deformation is progressively focused at oceanic spreading centres. The first tectono-magmatic event related to the Tertiary rifting was the eruption of voluminous flood basalts that apparently occurred in a rather short time interval at around 30 Ma; strong plateau uplift, which resulted in the development of the Ethiopian and Somalian plateaus now surrounding the rift valley, has been suggested to have initiated contemporaneously or shortly after the extensive flood-basalt volcanism, although its exact timing remains controversial. Voluminous volcanism and uplift started prior to the main rifting phases, suggesting a mantle plume influence on the Tertiary deformation in East Africa. Different plume hypothesis have been suggested, with recent models indicating the existence of deep superplume originating at the core-mantle boundary beneath southern Africa, rising in a north-northeastward direction toward eastern Africa, and feeding multiple plume stems in the upper mantle. However, the existence of this whole-mantle feature and its possible connection with Tertiary rifting are highly debated. The main rifting phases started diachronously along the MER in the Mio-Pliocene; rift propagation was not a smooth process but rather a process with punctuated episodes of extension and relative quiescence. Rift location was most probably controlled by the reactivation of a lithospheric-scale pre-Cambrian weakness; the orientation of this weakness (roughly NE-SW) and the Late

  19. Torque exerted on the side of crustal blocks controls the kinematics of Ethiopian Rift

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    Plate tectonic stress at active plate boundary can arises from 1) a torque applied on the side of lithospheric blocks and 2) a torque at the base of the lithosphere due to the flow of the underlying mantle. In this paper we use a simple force balance analysis to compare side and basal shear stresses and their contribution in driving kinematics and deformation in the Ethiopian Rift (ER), in the northern part of the East African Rift System (EARS). Assuming the constraints of the ER given by the dimension of the lithospheric blocks, the strain rate, the viscosity of the low velocity zone (LVZ) and the depth of the brittle-ductile transition zone, the lateral torque is several orders of magnitude higher than the basal torque. The minor contribution of basal torque might be due to low viscosity in the LVZ. Both Africa and Somalia plates are moving to the "west" relative to the mantle and there are not slabs that can justify this pull and consequent motion. Therefore, we invoke that westerly oriented tidal torque on Africa and Somalia plates in providing the necessary side torque in the region. This plate motion predicts significant sinistral transtension along the ER and rift parallel strike-slip faulting similar to the estimated angular velocity vector for tectonic blocks and GPS observations. Vertical axis block rotations are observed in areas where the lithospheric mantle is removed and strain is widely distributed.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

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

    SciTech Connect

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

    1995-03-01

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

  2. High and low pressure phase equilibria of a mildly alkalic lava from the 1965 Surtsey eruption: Implications for the evolution of mildly alkalic and transitional basalts in the south-eastern propagating rift zone of Iceland.

    NASA Astrophysics Data System (ADS)

    Thy, P.

    1991-01-01

    The south-eastern propagating rift zone of Iceland shows a progression from tholeiitic, to transitional, and mildly alkalic basalts going toward the front of the propagator. A petrogenetic model has been formulated based on low and high pressure melting experiments. The evolution of the magmas behind the front of the propagating rift is dominated by near surface processes (e.g., Katla and Hekla volcanic systems). Compared with the one atmosphere liquid line of descent, the Vestmannaeyjar lavas, erupted at the front of the propagating rift, have systematically higher Al 2O 3 and Na 2O contents consistent with the experimentally determined effects of high pressure, Labradoritic plagioclase megacrysts, which occur in the early phase of the Surtsey eruption at the front of the propagator, are consistent with the predicted effect of high pressure on plagioclase composition. On the other hand, augites similar to those of the high pressure experiments are unknown among the phenocryst and xenolith assemblages of the Vestmannaeyjar lavas (and other Icelandic lavas). Petrographic evidence points toward a high water content in the evolved lavas of the Vestmannaeyjar. A relatively high water activity and high pressure in the magma chambers at the front of the propagator could have caused a significant suppression of the liquids temperature, in particular for plagioclase. Seismic and magnetotelluric evidence suggest that magma chambers behind the propagating front occur at the depth equivalent to 2-3 kbar of pressure. At the front, magma chambers have been located by geophysical evidence at significantly greater depths equivalent to 3-8 kbar. The lavas erupted at the front of the propagator are located to the alkalic side of the thermal divide and, therefore, can be expected to evolve toward nepheline saturation under slightly hydrous conditions. The most evolved of these lavas are of ferrobasaltic compositions and may be saturated with augite. The lavas erupted behind the

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

    NASA Astrophysics Data System (ADS)

    Park, Munjae; Jung, Haemyeong; Kil, Youngwoo

    2015-04-01

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

  4. The Role of Rift Obliquity During Pangea Fragmentation

    NASA Astrophysics Data System (ADS)

    Brune, S.; Butterworth, N. P.; Williams, S.; Müller, D.

    2014-12-01

    Does supercontinent break-up follow specific laws? What parameters control the success and the failure of rift systems? Recent analytical and geodynamic modeling suggests that oblique rifting is energetically preferred over orthogonal rifting. This implies that during rift competition, highly oblique branches proceed to break-up while less oblique ones become inactive. These models predict that the relative motion of Earth's continents during supercontinent break-up is affected by the orientation and shape of individual rift systems. Here, we test this hypothesis based on latest plate tectonic reconstructions. Using PyGPlates, a recently developed Python library that allows script-based access to the plate reconstruction software GPlates, we quantify rift obliquity, extension velocity and their temporal evolution for continent-scale rift systems of the past 200 Myr. Indeed we find that many rift systems contributing to Pangea fragmentation involved strong rift obliquity. East and West Gondwana for instance split along the East African coast with a mean obliquity of 55° (measured as the angle between local rift trend normal and extension direction). While formation of the central and southern South Atlantic segment involved a low obliquity of 10°, the Equatorial Atlantic opened under a high angle of 60°. Rifting between Australia and Antarctica involved two stages with 25° prior to 100 Ma followed by 50° obliquity and distinct increase of extension velocity. Analyzing the entire passive margin system that formed during Pangea breakup, we find a mean obliquity of 40°, with a standard deviation of 20°. Hence 50% of these margins formed with an angle of 40° or more. Considering that many conceptual models of rifting and passive margin formation assume 2D deformation, our study quantifies the degree to which such 2D models are globally applicable, and highlights the importance of 3D models where oblique rifting is the dominant mode of deformation.

  5. Structure and stratigraphy of the Rukwa rift

    NASA Astrophysics Data System (ADS)

    Kilembe, Elias A.; Rosendahl, Bruce R.

    1992-08-01

    Combining recently acquired multifold seismic data with well and gravity information and field mapping, a comprehensive picture of the structure and stratigraphy of the Rukwa rift has emerged. The Rukwa rift lies between the Tanganyika and Nyasa (Malawi) rifts in the western branch of the East African rift system in southwest Tanzania. The Rukwa rift is a NW-trending half-graben basin that is 350 km long and 40 km wide. Unlike the neighboring Tanganyika and Nyasa rifts, there is no evidence of half-graben polarity reversals in the Rukwa rift. The NW-trending boundary fault system lies on the northeastern side of the basin and comprises a series of listric faults. Most internal faults also show listric forms and trend N-S, oblique to the boundary fault. The basal sedimentary section is the Permo-Triassic (Karroo) Sequence. This is overlain by the Red Bed Sandstone Sequence, in which both Mesozoic and Tertiary fauna have been found. The Cenozoic Lake Bed Sequence is the highest unit and covers nearly all of the present basin. Sediment thicknesses commonly reach 7 km and attain a maximum of 12 km at the southeastern end of the basin. The Lake Bed Sequence is the thickest unit in the main depocentre, but the Karroo Sequence is often the thickest unit on the shoaling side of the half-graben. The Rukwa rift is here interpreted to have evolved as a strike- to oblique-slip pull-apart basin, based on numerous indications of NW-trending strike-slip faulting.

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

    NASA Astrophysics Data System (ADS)

    Shchetnikov, Alexander

    2016-11-01

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

  7. Crustal Strain Patterns in Magmatic and Amagmatic Early Stage Rifts: Border Faults, Magma Intrusion, and Volatiles

    NASA Astrophysics Data System (ADS)

    Ebinger, C. J.; Keir, D.; Roecker, S. W.; Tiberi, C.; Aman, M.; Weinstein, A.; Lambert, C.; Drooff, C.; Oliva, S. J. C.; Peterson, K.; Bourke, J. R.; Rodzianko, A.; Gallacher, R. J.; Lavayssiere, A.; Shillington, D. J.; Khalfan, M.; Mulibo, G. D.; Ferdinand-Wambura, R.; Palardy, A.; Albaric, J.; Gautier, S.; Muirhead, J.; Lee, H.

    2015-12-01

    Rift initiation in thick, strong continental lithosphere challenges current models of continental lithospheric deformation, in part owing to gaps in our knowledge of strain patterns in the lower crust. New geophysical, geochemical, and structural data sets from youthful magmatic (Magadi-Natron, Kivu), weakly magmatic (Malawi, Manyara), and amagmatic (Tanganyika) sectors of the cratonic East African rift system provide new insights into the distribution of brittle strain, magma intrusion and storage, and time-averaged deformation. We compare and contrast time-space relations, seismogenic layer thickness variations, and fault kinematics using earthquakes recorded on local arrays and teleseisms in sectors of the Western and Eastern rifts, including the Natron-Manyara basins that developed in Archaean lithosphere. Lower crustal seismicity occurs in both the Western and Eastern rifts, including sectors on and off craton, and those with and without central rift volcanoes. In amagmatic sectors, lower crustal strain is accommodated by slip along relatively steep border faults, with oblique-slip faults linking opposing border faults that penetrate to different crustal levels. In magmatic sectors, seismicity spans surface to lower crust beneath both border faults and eruptive centers, with earthquake swarms around magma bodies. Our focal mechanisms and Global CMTs from a 2007 fault-dike episode show a local rotation from ~E-W extension to NE-SE extension in this linkage zone, consistent with time-averaged strain recorded in vent and eruptive chain alignments. These patterns suggest that strain localization via widespread magma intrusion can occur during the first 5 My of rifting in originally thick lithosphere. Lower crustal seismicity in magmatic sectors may be caused by high gas pressures and volatile migration from active metasomatism and magma degassing, consistent with high CO2 flux along fault zones, and widespread metasomatism of xenoliths. Volatile release and

  8. Kinematics and Dynamics of the Main Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Jay, C.; Flesch, L. M.; Bendick, R. O.

    2015-12-01

    Although the East African Rift System (EARS) is often cited as a type example for "narrow" rifting (where strain is localized along the rift axis), the true extent of rift-related deformation remains largely unknown due to sparse geophysical observations outside of the main rift valley. Our study, which takes this large scale approach, investigates the distribution of deformation in the Main Ethiopian Rift (MER) and surrounding regions, including the Ethiopian Highlands to the west of the rift valley, Somali Platform to the east, and Afar Triple Junction. We first construct kinematic, self-consistent strain rate and velocity fields on a 1° by 1° grid using continuous spline interpolations of strain rate observations (earthquake and fault data, plate rotations, and GPS velocities). Next, we calculate the deviatoric stress field associated with gravitational potential energy (GPE) by integrating density as a function of depth using published crustal density structures (CRUST1.0) and newly obtained receiver functions. We then directly solve for the deviatoric stress field associated with the lateral density variations by assuming a minimum energy stress field (e.g. Flesch et al. [2001]). Finally, we look for symmetries and asymmetries in both the strain rate and GPE deviatoric stress fields to assess the source of observed, off-rift deformation. We compare our results to published global and regional models that include the East African Rift and Iceland. Results suggest that the MER is not an end-member, "narrow" type rift, and that heterogeneities in lithospheric strength likely play an important role in governing the kinematics of rifting in Ethiopia.

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

    NASA Astrophysics Data System (ADS)

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

    2011-02-01

    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.

  10. Land - Ocean Climate Linkages and the Human Evolution - New ICDP and IODP Drilling Initiatives in the East African Rift Valley and SW Indian Ocean

    NASA Astrophysics Data System (ADS)

    Zahn, R.; Feibel, C.; Co-Pis, Icdp/Iodp

    2009-04-01

    The past 5 Ma were marked by systematic shifts towards colder climates and concomitant reorganizations in ocean circulation and marine heat transports. Some of the changes involved plate-tectonic shifts such as the closure of the Panamanian Isthmus and restructuring of the Indonesian archipelago that affected inter-ocean communications and altered the world ocean circulation. These changes induced ocean-atmosphere feedbacks with consequences for climates globally and locally. Two new ICDP and IODP drilling initiatives target these developments from the perspectives of marine and terrestrial palaeoclimatology and the human evolution. The ICDP drilling initiative HSPDP ("Hominid Sites and Paleolakes Drilling Project"; ICDP ref. no. 10/07) targets lacustrine depocentres in Ethiopia (Hadar) and Kenya (West Turkana, Olorgesailie, Magadi) to retrieve sedimentary sequences close to the places and times where various species of hominins lived over currently available outcrop records. The records will provide a spatially resolved record of the East African environmental history in conjunction with climate variability at orbital (Milankovitch) and sub-orbital (ENSO decadal) time scales. HSPDP specifically aims at (1) compiling master chronologies for outcrops around each of the depocentres; (2) assessing which aspects of the paleoenvironmental records are a function of local origin (hydrology, hydrogeology) and which are linked with regional or larger-scale signals; (3) correlating broad-scale patterns of hominin phylogeny with the global beat of climate variability and (4) correlating regional shifts in the hominin fossil and archaeological record with more local patterns of paleoenvironmental change. Ultimately the aim is to test hypotheses that link physical and cultural adaptations in the course of the hominin evolution to local environmental change and variability. The IODP initiative SAFARI ("Southern African Climates, Agulhas Warm Water Transports and Retroflection

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

    USGS Publications Warehouse

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

    1992-01-01

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

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

    USGS Publications Warehouse

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

    2005-01-01

    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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  14. Impact of rheological layering on rift asymmetry

    NASA Astrophysics Data System (ADS)

    Jaquet, Yoann; Schmalholz, Stefan M.; Duretz, Thibault

    2015-04-01

    Although numerous models of rift formation have been proposed, what triggers asymmetry of rifted margins remains unclear. Parametrized material softening is often employed to induce asymmetric fault patterns in numerical models. Here, we use thermo-mechanical finite element models that allow softening via thermal weakening. We investigate the importance of lithosphere rheology and mechanical layering on rift morphology. The numerical code is based on the MILAMIN solver and uses the Triangle mesh generator. Our model configuration consists of a visco-elasto-platic layered lithosphere comprising either (1) only one brittle-ductile transition (in the mantle) or (2) three brittle-ductile transitions (one in the upper crust, one in the lower crust and one in the mantle). We perform then two sets of simulations characterized by low and high extensional strain rates (5*10-15 s-1, 2*10-14 s-1). The results show that the extension of a lithosphere comprising only one brittle-ductile transition produces a symmetric 'neck' type rift. The upper and lower crusts are thinned until the lithospheric mantle is exhumed to the seafloor. A lithosphere containing three brittle-ductile transitions favors strain localization. Shear zones at different horizontal locations and generated in the brittle levels of the lithosphere get connected by the weak ductile layers. The results suggest that rheological layering of the lithosphere can be a reason for the generation of asymmetric rifting and subsequent rift morphology.

  15. The 1974 Ethiopian rift geodimeter survey

    NASA Technical Reports Server (NTRS)

    Mohr, P.

    1977-01-01

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

  16. Evolution of a magma-driven earthquake swarm and triggering of the nearby Oldoinyo Lengai eruption, as resolved by InSAR, ground observations and elastic modeling, East African Rift, 2007

    NASA Astrophysics Data System (ADS)

    Baer, G.; Hamiel, Y.; Shamir, G.; Nof, R.

    2008-07-01

    An earthquake swarm struck the North Tanzania Divergence, East African Rift over a 2 month period between July and September 2007. It produced approximately 70 M > 4 earthquakes (peak magnitude Mw 5.9), and extensive surface deformation, concurrent with eruptions at the nearby Oldoinyo Lengai volcano. The spatial and temporal evolution of the entire deformation event was resolved by Interferometric Synthetic Aperture Radar (InSAR) observations, owing to a particularly favorable acquisition programming of the Envisat and ALOS satellites, and was verified by detailed ground observations. Elastic modeling based on the InSAR measurements clearly distinguishes between normal faulting, which dominated during the first week of the event, and intermittent episodes of dike propagation, oblique dike opening and dike-induced faulting during the following month. A gradual decline in the intensity of deformation occurred over the final weeks. Our observations and modeling suggest that the sequence of events was initiated by pressurization of a deep-seated magma chamber below Oldoinyo Lengai which opened the way to lateral dike injection, and dike-induced faulting and seismicity. As dike intrusion terminated, silicate magma ascended the volcano conduit, reacted with the carbonatitic magma, and set off a major episode of explosive ash eruptions producing mixed silicate-carbonatitic ejecta. The rise of the silicate magma within the volcano conduit is attributed to bubble growth and buoyancy increase in the magma chamber either due to a temporary pressure drop after the termination of the diking event, or due to the dynamic effects of seismic wave passage from the earthquake swarm. Similar temporal associations between earthquake swarms and major explosive ash eruptions were observed at Oldoinyo Lengai over the past half century.

  17. Melt inclusion evidence for CO2-rich melts beneath the western branch of the East African Rift: implications for long-term storage of volatiles in the deep lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Hudgins, T. R.; Mukasa, S. B.; Simon, A. C.; Moore, G.; Barifaijo, E.

    2015-05-01

    We present new major element, trace element, and volatile (H2O, CO2, S, F, and Cl) concentrations of olivine-hosted melt inclusions from five high-K, low-silica basanites from the western branch of the East African Rift System and use these data to investigate the generation of H2O- and CO2-rich melts at up to ~150 km depth. Measured H2O and CO2 concentrations reach ~2.5 and ~1 wt%, respectively, representing some of the highest CO2 concentrations measured in a melt inclusion to date. These measurements represent direct evidence of the high CO2 and H2O concentrations required to generate high-K alkaline lavas, and the CO2 that has been previously inferred to be necessary for the low mantle potential temperatures in the area. Ratios of CO2/Nb, CO2/Ba, and CO2/Cl are used to estimate an initial melt CO2 concentration of 5-12 wt%. The measured CO2 concentrations are consistent with CO2 solubilities determined by molecular dynamics calculations and high-pressure experiments for melt generation at 3-6 GPa; the depth of melting suggested by previous studies in the area. These melt inclusions measurements represent direct evidence for the presence of H2O- and CO2-rich melts in the deep upper mantle that have been proposed based on experimental and seismic evidence. Primitive-mantle normalized trace element patterns more closely resemble those found in subduction settings rather than ocean island basalt, and ratios of slab fluid tracers such as Li/Dy and B/Be indicate that the measured volatile abundances may be related to Neoproterozoic subduction during the assembly of Gondwana, implying the storage of volatiles in the mantle by subduction-related metasomatism.

  18. The role of tephra studies in African paleoanthropology as exemplified by the Sidi Hakoma Tuff

    NASA Astrophysics Data System (ADS)

    WoldeGabriel, Giday; Endale, Tamrat; White, Tim D.; Thouveny, Nicolas; Hart, William K.; Renne, Paul R.; Asfaw, Berhane

    2013-01-01

    Beginning in the 1960s, geological and paleoanthropological exploration of the Ethiopian rift system's basins have led to the discovery and assembly of the most comprehensive record of human biological and technological change during the last 6 million years. The hominid fossils, including partial skeletons, were primarily discovered in the Afar Rift, the Main Ethiopian Rift, and in the Omo Basin of the broadly rifted zone of SW Ethiopia. The paleoanthropological research areas within the SW Afar Rift that have yielded many diverse hominid species and the oldest stone tools are, from north to south, Woranso-Mille (aff. Ardipithecus and Au. afarensis), Hadar (Au. afarensis, Homo sp.), Dikika (Au. afarensis), Gona (Ar. kadabba, Ar. ramidus, H. erectus, and oldest stone tools), Middle Awash (Ar. kadabba, Ar. ramidus, Au. anamensis, Au. afarensis, Au. garhi, H. erectus, H. rhodesiensis, H. sapiens idaltu, and the oldest paleo-butchery locality), and Galili (Au. afarensis). Additional hominid remains were discovered at Melka Kunture on the banks of the Awash River near its source along the western margin of the central part of the Main Ethiopian Rift (H. erectus), at Konso (H. erectus and A. boisei), and at the southern end of the MER, and in the Omo Basin (Au. anamensis, Au. afarensis, Au. aethiopicus, Au. boisei, H. habilis, and H. erectus). Distal and sometimes proximal tephra units interbedded within fossilifeous sedimentary deposits have become key elements in this work by providing chronological and correlative control and depositional contexts. Several regional tephra markers have been identified within the northern half of the eastern African rift valley in Ethiopia and Kenya, and in marine sediments of the Gulf of Aden Rift and the NW Indian Ocean. Out of the many regional tephra stratigraphic markers that range in age from the early Pliocene (3.97 Ma) to the late Pleistocene (0.16 Ma), the Sidi Hakoma Tuff (SHT) has been more widely identified and thoroughly

  19. Fluid pressure and flow at great depth in the continental crust. A discussion in relation to topography, temperature and salinity distribution using as an example the KTB Fault Zones in connection with the Eger Rift Hot Spot.

    NASA Astrophysics Data System (ADS)

    Kessels, W.; Kuhlmann, S.; Li, X.

    2006-12-01

    Hydraulic investigations in and between the two KTB boreholes have shown that groundwater flow is possible at great depth in the crystalline crust. Remarkable permeability was found particularly in the SE1 and SE2 fault zones. The results from a long term pump and injection test, and the related three-dimensional groundwater modelling (Graesle et al., 2006), document the existence of a large-scale (more than 10 km) hydraulic reservoir in the crystalline crust. According to this calculation, an overpressure of 0.4 MPa can be still be expected in KTB-HB in 2009, 4 years after the end of the injection. The good match with the measurement data confirms groundwater pathways at a scale of more than 10 km. The isotopic water composition recovered from the KTB pilot hole indicates a downward water flow along the SE2 fault zone, which is in contact with the Franconian Line. Moreover, there is a deep upward groundwater flow 60 km away in the western Eger Rift Valley as indicated e.g. by the temperature signature and gas flow observations. Therefore, the demand for fluid mass continuity means that water is being supplied by a downstream groundwater flow, probably from the Franconian Line. The question of potential driving processes must be answered to understand and quantify the flow in the deeper crust at a scale of 10 km to 100 km. The processes must result in a sufficient horizontal pressure gradient to allow groundwater flow at great depth. The density variations of groundwater with depth are highly relevant for the calculation of horizontal pressure differences. The two independent potential fields of gravity and pressure have to be considered. Differentiation into 4 relevant driving processes is required: \\bullet The groundwater surface topography related to the groundwater recharge and mean regional distance between neighbouring valleys \\bullet Geothermal gradient and water density depending on temperature and pressure \\bullet Different salt contents in adjacent

  20. The Middle Neoproterozoic Sidi Flah Group (Anti-Atlas, Morocco): synrift deposition in a Pan-African continent/ocean transition zone

    NASA Astrophysics Data System (ADS)

    Fekkak, A.; Pouclet, A.; Benharref, M.

    2003-08-01

    The Middle Neoproterozoic (Cryogenian) Sidi Flah Group rocks are located in the Saghro inlier of the Eastern Anti-Atlas and consists of siliciclastic detrital sediment, interbedded basaltic lavas and small ultramafic bodies. Sediment deposition occurred in three turbiditic formations of a deep-sea fan environment and was controlled by synsedimentary collapses. The composition of sandstones and typological study of zircons indicate that detrital material came from the gneisses and granites of a proximal craton. The lavas are synsedimentary subaqueous flows. They show chemical signatures of initial rift tholeiites and of plume-related alkaline intraplate basalts. The ultramafic rocks are serpentinized peridotites that were emplaced along N160° synsedimentary faults as numerous bodies 20-50 m in size. Their petrographical (Cr-spinel signature) and chemical features correspond to intracontinental ultramafic cumulates. The emplacement of the ultramafic rocks was associated with hydrothermal activity that generated calcareous and siliceous rocks such as ophicalcites and jaspers. All the features of the sediments, the lavas and the ultramafic bodies strongly suggest a continent-ocean transition geotectonic context, in an advanced stage of continental rifting that we attribute to the pre-Pan-African ocean passive margin extension.

  1. Tectonics of the South Georgia Rift

    NASA Astrophysics Data System (ADS)

    Heffner, David M.

    Triassic rifting of the supercontinent Pangea left behind numerous basins on what is now the eastern North American margin. The South Georgia Rift (SGR) was thought to be the best preserved of these basins having been capped by thick basalt flows of the Central Atlantic Magmatic Province (CAMP) and later buried beneath the Cretaceous and younger Coastal Plain. Because it is buried beneath the Coastal Plain, the SGR is only known through sparse drilling and geophysical methods. Despite this limited dataset, the SGR is the only one of the eastern North American Triassic basins known to overlie the ancient Alleghanian suture between Laurentia and Gondwana, although it isn't clear what influence this lithospheric weakness played in formation of the rift. The SGR has been variably interpreted as a singular large basin or as isolated sub-basins separated by transfer zones. Transfer zones are rift-transverse structural features that link major faults of rift sub-basins and accommodate differences in extensional strain. Transfer zones have been previously hypothesized to be present in the SGR based on onshore projections of Central Atlantic fracture zones, but observations confirming their existence, such as reversal in sub-basin polarity, have been lacking. Three separate hypotheses are tested related to the SGR: 1) the J-Horizon corresponds everywhere with basalt; 2) transfer zones are an important structural component of the SGR; 3) structural features of the Central Atlantic Ocean are related to transfer zones of the SGR. Reanalysis of existing well and seismic data shows that the extent of the flood basalt in the SGR is restricted and that the J-Horizon coincides with the base of the Coastal Plain. Subsurface mapping reveals reversals in sub-basin polarity, confirming the existence of previously hypothesized transfer zones. Small circle projections of the transfer zones correlate with oceanic features, and Central Atlantic fracture zones project onshore into inferred

  2. Description of Events Where African Buffaloes (Syncerus caffer) Strayed from the Endemic Foot-and-Mouth Disease Zone in South Africa, 1998-2008.

    PubMed

    van Schalkwyk, O L; Knobel, D L; De Clercq, E M; De Pus, C; Hendrickx, G; Van den Bossche, P

    2016-06-01

    African buffaloes (Syncerus caffer) are reservoir hosts of Southern African Territories (SAT) foot-and-mouth disease (FMD) virus strains. In South Africa, infected buffaloes are found in the FMD-infected zone comprising the Kruger National Park (KNP) and its adjoining reserves. When these buffaloes stray into livestock areas, they pose a risk of FMD transmission to livestock. We assessed 645 records of stray buffalo events (3124 animals) from the FMD infected zone during 1998-2008 for (i) their temporal distribution, (ii) group size, (iii) age and gender composition, (iv) distance from the infected zone fence and (v) outcome reported for each event. A maximum entropy model was developed to evaluate spatial predictors of stray buffalo events and assess current disease control zones. Out of all buffaloes recorded straying, 38.5% escaped from the FMD infected zone during 2000/2001, following floods that caused extensive damage to wildlife fences. Escape patterns were not apparently influenced by season. The median size of stray groups was a single animal (IQR [1-2]). Adult animals predominated, comprising 90.4% (620/686) of the animals for which age was recorded. Of the 315 events with accurate spatial information, 204 (64.8%) were recorded within 1 km from the FMD infected zone. During late winter/spring (June-October), stray buffaloes were found significantly closer to the FMD infected zone (median = 0.3 km, IQR [0.1-0.6]). Less than 13% (40/315) of stray groups reached the FMD protection zone without vaccination, posing a higher risk of spreading FMD to these more susceptible livestock. Model outputs suggest that distance from the FMD infected zone, urban areas and permanent water sources contributed almost 85% to the spatial probability of stray buffalo events. Areas with a high probability for stray buffalo events were well covered by current disease control zones, although FMD risk mitigation could be improved by expanding the vaccination zone in certain areas.

  3. Halomonas magadii sp. nov., a new member of the genus Halomonas, isolated from a soda lake of the East African Rift Valley.

    PubMed

    Duckworth, A W; Grant, W D; Jones, B E; Meijer, D; Márquez, M C; Ventosa, A

    2000-02-01

    A number of novel alkaliphilic organotrophic bacteria have been isolated from several saline and alkaline East African soda lakes. The new isolates grow at pH values between 7.0 and 11.0, with pH optima for growth between 9.0 and 10.0. Growth occurs at total salts concentration between 0% and 20% (w/v) with optimum at 0%-7% (w/v). Phylogenetic analyses based on 16S rDNA sequence comparison indicate that these isolates are related (>96% similarity) to members of the Halomonadaceae within the gamma-3 subdivision of the Proteobacteria. These analyses indicate that existing species within the Halomonadaceae fell within three main groups, one group comprising the type species of Halomonas, Halomonas elongata, and a number of other known species including one soda lake isolate. A second group constituting most of the remaining known species of Halomonas and related Chromohalobacter spp. includes 3 soda lake isolates with high DNA-DNA homologies. The third group included Halomonas halodenitrificans, Halomonas desiderata, Halomonas cupida, and 13 soda lake isolates. Phenotypic comparisons indicated that the majority of soda lake strains shared similar morphological, phenotypic, and chemotaxonomic properties to known strains of Halomonas but grew under alkaline conditions. The 3 soda lake isolates with high DNA-DNA homologies were, however, significantly different in antibiotic sensitivity pattern and in the utilization of several substrates, were unable to reduce nitrite, and showed low DNA-DNA homologies with known halomonads in the same group. We propose that these isolates comprise a new species of the genus Halomonas that we name Halomonas magadii sp. nov. The type strain is strain 21 MI (NCIMB 13595).

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

    SciTech Connect

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

    1993-11-01

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

  5. Oldest Homo and Pliocene biogeography of the Malawi Rift.

    PubMed

    Schrenk, F; Bromage, T G; Betzler, C G; Ring, U; Juwayeyi, Y M

    1993-10-28

    The Malawi Rift and Pliocene palaeofaunas, which include a hominid mandible attributed to Homo rudolfensis, provide a biogeographical link between the better known Plio-Pleistocene faunal records of East and Southern Africa. The Malawi Rift is in a latitudinal position suitable for recording any hominid and faunal dispersion towards the Equator that was brought on by increased aridity of the Late Pliocene African landscape. The evidence suggests that Pliocene hominids originated in the eastern African tropical domain and dispersed to southern Africa only during more favourable ecological circumstances.

  6. From rifting to passive margin: the examples of the Red Sea, Central Atlantic and Alpine Tethys

    NASA Astrophysics Data System (ADS)

    Favre, P.; Stampfli, G. M.

    1992-12-01

    Evolution of the Red Sea/Gulf of Suez and the Central Atlantic rift systems shows that an initial, transtensive rifting phase, affecting a broad area around the future zone of crustal separation, was followed by a pre-oceanic rifting phase during which extensional strain was concentrated on the axial rift zone. This caused lateral graben systems to become inactive and they evolved into rift-rim basins. The transtensive phase of diffuse crustal extension is recognized in many intra-continental rifts. If controlling stress systems relax, these rifts abort and develop into palaeorifts. If controlling stress systems persist, transtensive rift systems can enter the pre-oceanic rifting stage, during which the rift zone narrows and becomes asymmetric as a consequence of simple-shear deformation at shallow crustal levels and pure shear deformation at lower crustal and mantle-lithospheric levels. Preceding crustal separation, extensional denudation of the lithospheric mantle is possible. Progressive lithospheric attenuation entails updoming of the asthenosphere and thermal doming of the rift shoulders. Their uplift provides a major clastic source for the rift basins and the lateral rift-rim basins. Their stratigraphic record provides a sensitive tool for dating the rift shoulder uplift. Asymmetric rifting leads to the formation of asymmetric continental margins, corresponding in a simple-shear model to an upper plate and a conjugate lower plate margin, as seen in the Central Atlantic passive margins of the United States and Morocco. This rifting model can be successfully applied to the analysis of the Alpine Tethys palaeo-margins (such as Rif and the Western Alps).

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

    NASA Astrophysics Data System (ADS)

    Swanberg, Chandler A.

    1983-05-01

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

  8. Rift Valley Fever Virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

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

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

    NASA Astrophysics Data System (ADS)

    Gawthorpe, Robert; Ford, Mary

    2015-04-01

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