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Sample records for fluids east rift

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  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. 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

  10. 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.

  11. 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

  12. East Antarctic rifting triggers uplift of the Gamburtsev Mountains.

    PubMed

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

    2011-11-16

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

  13. East Antarctic rifting triggers uplift of the Gamburtsev Mountains.

    PubMed

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

    2011-11-17

    The Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 1958. The preservation of Alpine topography in the Gamburtsevs may reflect extremely low long-term erosion rates beneath the ice sheet, but the mountains' origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long rift system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1 Gyr ago), was preserved as in some old orogens and was rejuvenated during much later Permian (roughly 250 Myr ago) and Cretaceous (roughly 100 Myr ago) rifting. Much like East Africa, the interior of East Antarctica is a mosaic of Precambrian provinces affected by rifting processes. Our models show that the combination of rift-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that rifting and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere. PMID:22094700

  14. 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.

  15. 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

  16. East Antarctic rifting triggers uplift of the Gamburtsev Mountains

    USGS Publications Warehouse

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

    2011-01-01

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

  17. 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.

  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. 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.

  20. 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

  1. 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.

  2. 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

  3. 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

  4. 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.

  5. 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

  6. 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.

  7. 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.

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

    SciTech Connect

    Wickstrom, L.H. )

    1992-01-01

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

  9. 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.

  10. 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

  11. 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.

  12. 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.

  13. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    The East African Rift (EAR) is an active continental rift and ideal to investigate the processes of rift initiation and the breaking apart of continental lithosphere. Mantle and crust-derived fluids may play a pivotal role in both magmatism and faulting in the EAR. For instance, large quantities of mantle-derived volatiles are emitted at Oldoinyo Lengai volcano [1, 2]. Throughout the EAR, CO2-dominated volatile fluxes are prevalent [3, 4] and often associated with faults (i.e. Rungwe area, Tanzania, [5, 6]). The purpose of this study is to examine the relationship between volcanism, faulting and the volatile compositions, focusing on the central and southern Kenyan and northern Tanzanian section of the EAR. We report our analysis results for samples obtained during a 2013 field season in Kenya. Gases were sampled at fumaroles and geothermal plants in caldera volcanoes (T=83.1-120.2°C) and springs (T=40-79.6°C and pH 8.5-10) located near volcanoes, intra-rift faults, and a transverse fault (the Kordjya fault, a key fluid source in the Magadi rift) by 4N-NaOH solution-filled and empty Giggenbach bottles. Headspace gases were analyzed by a Gas Chromatograph and a Quadrupole Mass Spectrometer at the University of New Mexico. Both N2/Ar and N2/He ratios of all gases (35.38-205.31 and 142.92-564,272, respectively) range between air saturated water (ASW, 40 and ≥150,000) and MORB (100-200 and 40-50). In addition, an N2-Ar-He ternary diagram supports that the gases are produced by two component (mantle and air) mixing. Gases in the empty bottles from volcanoes and springs have N2 (90.88-895.99 mmom/mol), CO2 (2.47-681.21 mmom/mol), CH4 (0-214.78 mmom/mol), O2 (4.47-131.12 mmom/mol), H2 (0-35.78 mmom/mol), Ar (0.15-10.65 mmom/mol), He (0-2.21 mmom/mol), and CO (0-0.08 mmom/mol). Although some of the samples show an atmospheric component, CO2 is a major component in most samples, indicating both volcanoes and springs are emitting CO2. Gases from volcanoes are enriched in

  15. 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.

  16. 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.

  17. Persistence of Rift Valley fever virus in East Africa

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  18. Rift Valley fever on the east coast of Madagascar.

    PubMed

    Morvan, J; Saluzzo, J F; Fontenille, D; Rollin, P E; Coulanges, P

    1991-01-01

    In March 1990, a Rift Valley fever virus (RVFV) outbreak was suspected in the district of Fenerive on the east coast of Madagascar after an abnormally high incidence of abortions and disease in livestock. Sera from humans and cattle were tested for RVFV antibodies by immunofluorescence assay (IFA) and ELISA-IgM capture. Sera and mosquitoes collected in the same area were tested for virus isolation by tissue culture and suckling mouse intracerebral inoculation, and for antigen detection by an ELISA antigen capture assay. Among cattle from the area, RVFV antibody prevalence was 58.6% by IFA and 29.6% by ELISA-IgM. In contrast, human populations in the same area had a lower RVFV antibody prevalence, with 8.01% IFA and 5.4% IgM-positive sera. No RVFV antigen was detected and virus isolation was unsuccessful from the sera and mosquito pools tested. Different hypotheses concerning the emergence and diffusion of RVFV in this area and the occurrence of the outbreak are discussed.

  19. 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

  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. 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.

  3. 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

  4. 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

  5. The origin and geologic evolution of the East Continent Rift Basin

    SciTech Connect

    Drahovzal, J.A. . Kentucky Geological Survey)

    1992-01-01

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

  6. Seismic Migration Imaging of the Lithosphere beneath the Afar Rift System, East Africa

    NASA Astrophysics Data System (ADS)

    Lee, T. T. Y.; Chen, C. W.; Rychert, C.; Harmon, N.

    2015-12-01

    The Afar Rift system in east Africa is an ideal natural laboratory for investigating the incipient continental rifting, an essential component of plate tectonics. The Afar Rift is situated at the triple junction of three rifts, namely the southern Red Sea Rift, Gulf of Aden Rift and Main Ethiopian Rift (MER). The ongoing continental rifting at Afar transitions to seafloor spreading toward the southern Red Sea. The tectonic evolution of Afar is thought to be influenced by a mantle plume, but how the plume affects and interacts with the Afar lithosphere remains elusive. In this study, we use array seismic data to produce high-resolution migration images of the Afar lithosphere from scattered teleseismic wavefields to shed light on the lithospheric structure and associated tectonic processes. Our preliminary results indicate the presence of lithospheric seismic discontinuities with depth variation across the Afar region. Beneath the MER axis, we detect a pronounced discontinuity at 55 km depth, characterized by downward fast-to-slow velocity contrast, which appears to abruptly deepen to 75 km depth to the northern flank of MER. This discontinuity may be interpreted as the lithosphere-asthenosphere boundary. Beneath the Ethiopian Plateau, on the other hand, a dipping structure with velocity increase is identified at 70-90 km depth. Further synthesis of observations from seismic tomography, receiver functions, and seismic anisotropy in the Afar region will offer better understanding of tectonic significance of the lithospheric discontinuities.

  7. Evidence From Adakitic Rocks For Lithosphere Recycling At The U.S. East Coast Volcanic Rifted Margin

    NASA Astrophysics Data System (ADS)

    Meyer, R.; Van Wijk, J. W.

    2014-12-01

    Magmatism at magmatic rifted margins is dominated by asthenospheric melts generated during decompression melting from the upwelling asthenosphere. However reported magma compositions are diverse, giving clues about additional processes during rifting and continental rupture. We report here on adakites from the U.S. East Coast that are related to breakup of the North Atlantic. The sampled adakites are bi-modal, and show a clear high-Si and low-Si major and trace element chemistry. During the closure of the proto-Atlantic and its related back-arc basins, the Appalachian lithosphere sampled an ophiolitic mélange within major sutures. Subsequent metamorphic events in the Blue Ridge province of the U.S. East Coast increased the rock density of the ophiolites (eclogite) and partly hydrated these mafic to ultramafic rocks. Such a preconditioned lithosphere will likely develop gravitational instabilities at the asthenosphere-lithosphere boundary during rifting. We use geodynamic models to show when gravitational instabilities form below the East Coast margin, and how they eventually delaminate mantle lithosphere and lower crust material. After foundering, the delaminated lithosphere undergoes metamorphism, heats up, and interacts with fluids and melts in the surrounding convecting mantle. Partial melting of this metamorphic lithology produces high-Si adakitic melts. These melts percolated upwards through the mantle and were partially injected into the Valley and Ridge province. Where melt-to-peridotite (asthenosphere) ratios were small, the melts and fluids leaving the sinking lithospheric block became fixed within the peridotites. Geochemical modeling suggests, that 5 to 10% melting degrees of such a metasomatically overprinted and fertile asthenosphere produced the low-Si adakitic rocks of the Virginia adakites.

  8. 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

  9. 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

  10. 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.

  11. Tectonic and climatic control on evolution of rift lakes in the Central Kenya Rift, East Africa

    NASA Astrophysics Data System (ADS)

    Bergner, A. G. N.; Strecker, M. R.; Trauth, M. H.; Deino, A.; Gasse, F.; Blisniuk, P.; Dühnforth, M.

    2009-12-01

    The long-term histories of the neighboring Nakuru-Elmenteita and Naivasha lake basins in the Central Kenya Rift illustrate the relative importance of tectonic versus climatic effects on rift-lake evolution and the formation of disparate sedimentary environments. Although modern climate conditions in the Central Kenya Rift are very similar for these basins, hydrology and hydrochemistry of present-day lakes Nakuru, Elmenteita and Naivasha contrast dramatically due to tectonically controlled differences in basin geometries, catchment size, and fluvial processes. In this study, we use eighteen 14C and 40Ar/ 39Ar dated fluvio-lacustrine sedimentary sections to unravel the spatiotemporal evolution of the lake basins in response to tectonic and climatic influences. We reconstruct paleoclimatic and ecological trends recorded in these basins based on fossil diatom assemblages and geologic field mapping. Our study shows a tendency towards increasing alkalinity and shrinkage of water bodies in both lake basins during the last million years. Ongoing volcano-tectonic segmentation of the lake basins, as well as reorganization of upstream drainage networks have led to contrasting hydrologic regimes with adjacent alkaline and freshwater conditions. During extreme wet periods in the past, such as during the early Holocene climate optimum, lake levels were high and all basins evolved toward freshwater systems. During drier periods some of these lakes revert back to alkaline conditions, while others maintain freshwater characteristics. Our results have important implications for the use and interpretation of lake sediment as climate archives in tectonically active regions and emphasize the need to deconvolve lacustrine records with respect to tectonics versus climatic forcing mechanisms.

  12. Recent Outbreaks of Rift Valley Fever in East Africa and the Middle East

    PubMed Central

    Himeidan, Yousif E.; Kweka, Eliningaya J.; Mahgoub, Mostafa M.; El Rayah, El Amin; Ouma, Johnson O.

    2014-01-01

    Rift Valley fever (RVF) is an important neglected, emerging, mosquito-borne disease with severe negative impact on human and animal health. Mosquitoes in the Aedes genus have been considered as the reservoir, as well as vectors, since their transovarially infected eggs withstand desiccation and larvae hatch when in contact with water. However, different mosquito species serve as epizootic/epidemic vectors of RVF, creating a complex epidemiologic pattern in East Africa. The recent RVF outbreaks in Somalia (2006–2007), Kenya (2006–2007), Tanzania (2007), and Sudan (2007–2008) showed extension to districts, which were not involved before. These outbreaks also demonstrated the changing epidemiology of the disease from being originally associated with livestock, to a seemingly highly virulent form infecting humans and causing considerably high-fatality rates. The amount of rainfall is considered to be the main factor initiating RVF outbreaks. The interaction between rainfall and local environment, i.e., type of soil, livestock, and human determine the space-time clustering of RVF outbreaks. Contact with animals or their products was the most dominant risk factor to transfer the infection to humans. Uncontrolled movement of livestock during an outbreak is responsible for introducing RVF to new areas. For example, the virus that caused the Saudi Arabia outbreak in 2000 was found to be the same strain that caused the 1997–98 outbreaks in East Africa. A strategy that involves active surveillance with effective case management and diagnosis for humans and identifying target areas for animal vaccination, restriction on animal movements outside the affected areas, identifying breeding sites, and targeted intensive mosquito control programs has been shown to succeed in limiting the effect of RVF outbreak and curb the spread of the disease from the onset. PMID:25340047

  13. Recent outbreaks of rift valley Fever in East Africa and the middle East.

    PubMed

    Himeidan, Yousif E; Kweka, Eliningaya J; Mahgoub, Mostafa M; El Rayah, El Amin; Ouma, Johnson O

    2014-01-01

    Rift Valley fever (RVF) is an important neglected, emerging, mosquito-borne disease with severe negative impact on human and animal health. Mosquitoes in the Aedes genus have been considered as the reservoir, as well as vectors, since their transovarially infected eggs withstand desiccation and larvae hatch when in contact with water. However, different mosquito species serve as epizootic/epidemic vectors of RVF, creating a complex epidemiologic pattern in East Africa. The recent RVF outbreaks in Somalia (2006-2007), Kenya (2006-2007), Tanzania (2007), and Sudan (2007-2008) showed extension to districts, which were not involved before. These outbreaks also demonstrated the changing epidemiology of the disease from being originally associated with livestock, to a seemingly highly virulent form infecting humans and causing considerably high-fatality rates. The amount of rainfall is considered to be the main factor initiating RVF outbreaks. The interaction between rainfall and local environment, i.e., type of soil, livestock, and human determine the space-time clustering of RVF outbreaks. Contact with animals or their products was the most dominant risk factor to transfer the infection to humans. Uncontrolled movement of livestock during an outbreak is responsible for introducing RVF to new areas. For example, the virus that caused the Saudi Arabia outbreak in 2000 was found to be the same strain that caused the 1997-98 outbreaks in East Africa. A strategy that involves active surveillance with effective case management and diagnosis for humans and identifying target areas for animal vaccination, restriction on animal movements outside the affected areas, identifying breeding sites, and targeted intensive mosquito control programs has been shown to succeed in limiting the effect of RVF outbreak and curb the spread of the disease from the onset. PMID:25340047

  14. 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.

  15. Chronology of volcanism and rift basin propagation - Rungwe volcanic province, East Africa

    NASA Technical Reports Server (NTRS)

    Ebinger, C. J.; Deino, A. L.; Drake, R. E.; Tesha, A. L.

    1989-01-01

    The spatial and temporal development of along-axis segmentation in youthful continental rifts was investigated using the results of field, remote sensing, and K-Ar geochronology studies conducted in four (Rukwa, Songwe, Usangu, and Karonga) rift basins within the Rungwe volcanic province in East Africa. Results indicated that the Rukwa and Karonga border fault segments formed between 7.25 and 5 m.y. ago, the Usangu border fault segment developed between 3 and 2 m.y. ago, and subsidence along the Songwe border fault segment had occurred by 0.5 Ma. It is shown that individual basins developed diachronously, each following a similar sequence: (1) initial border fault development; (2) asymmetric basin subsidence/flank uplift and the development of monoclines opposite the border faults; and (3) continued subsidence and tilting along intrabasinal faults with flexural upwarping of the rift flanks, enhancing basinal asymmetries.

  16. 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.

  17. 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.

  18. Finite-element modelling of the structure and evolution of the South Kenya Rift, East Africa

    NASA Astrophysics Data System (ADS)

    Gölke, Matthias; Mechie, James

    1994-09-01

    A finite-element (FE) model was used in this study to investigate the intraplate rifting process in south Kenya, East Africa. The rifting model shows that the important factors influencing the amount of shoulder uplift and rift subsidence include the horizontal deviatoric extensional stresses, the viscosity of the lower crust, and the dimension and density contrast of the low-velocity upper-mantle anomaly. Thus, for instance, a reduced lower-crust viscosity as well as an increased extensional far-field stress favours the subsidence of the rift basin. In turn, for a reduced lower-crust viscosity, the magnitude of the applied extensional stress should be reduced in order to end up with the same topography of the rifted area as can be obtained with a more viscous lower crust combined with a relatively high extensional far-field stress. In addition, it also became obvious that the uplift of the crust-mantle boundary beneath the rift depends predominantly on the size and density contrast of the upper-mantle anomaly. An increased density contrast between the upper-mantle anomaly and normal mantle results in increased buoyancy forces thus producing an increased uplift of the crust-mantle boundary. In turn, in order to avoid too large values for the surface topography, a reduced lower-crust viscosity in the order of η = 10 21 Pa s is required. Models without an upper-mantle low-density anomaly have resulted in crustal thickening beneath the rift and not in crustal thinning. In accordance with the geological field evidence, we have modelled the evolution of the South Kenya Rift in two stages. In the first stage, from 16 to 10 Ma ago, a broad regional uplift of a few hundred metres produced by a relatively small but broad low-density upper-mantle anomaly was accompanied by an insignificant amount of faulting. In the second stage, from 10 Ma ago to the present day, significant faulting accompanied by the buoyancy effects of a larger, low-density, low-velocity upper

  19. Seismic evidence for a crustal magma reservoir beneath the upper east rift zoneof Kilauea volcano, Hawaii

    USGS Publications Warehouse

    Lin, Guoqing; Amelung, Falk; Lavallee, Yan; Okubo, Paul G.

    2014-01-01

    An anomalous body with low Vp (compressional wave velocity), low Vs (shear wave velocity), and high Vp/Vs anomalies is observed at 8–11 km depth beneath the upper east rift zone of Kilauea volcano in Hawaii by simultaneous inversion of seismic velocity structure and earthquake locations. We interpret this body to be a crustal magma reservoir beneath the volcanic pile, similar to those widely recognized beneath mid-ocean ridge volcanoes. Combined seismic velocity and petrophysical models suggest the presence of 10% melt in a cumulate magma mush. This reservoir could have supplied the magma that intruded into the deep section of the east rift zone and caused its rapid expansion following the 1975 M7.2 Kalapana earthquake.

  20. 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.

  1. Cenozoic extension in the Kenya Rift from low-temperature thermochronology: Links to diachronous spatiotemporal evolution of rifting in East Africa

    NASA Astrophysics Data System (ADS)

    Torres Acosta, Verónica; Bande, Alejandro; Sobel, Edward R.; Parra, Mauricio; Schildgen, Taylor F.; Stuart, Finlay; Strecker, Manfred R.

    2015-12-01

    The cooling history of rift shoulders and the subsidence history of rift basins are cornerstones for reconstructing the morphotectonic evolution of extensional geodynamic provinces, assessing their role in paleoenvironmental changes and evaluating the resource potential of their basin fills. Our apatite fission track and zircon (U-Th)/He data from the Samburu Hills and the Elgeyo Escarpment in the northern and central sectors of the Kenya Rift indicate a broadly consistent thermal evolution of both regions. Results of thermal modeling support a three-phased thermal history since the early Paleocene. The first phase (~65-50 Ma) was characterized by rapid cooling of the rift shoulders and may be coeval with faulting and sedimentation in the Anza Rift basin, now located in the subsurface of the Turkana depression and areas to the east in northern Kenya. In the second phase, very slow cooling or slight reheating occurred between ~45 and 15 Ma as a result of either stable surface conditions, very slow exhumation, or subsidence. The third phase comprised renewed rapid cooling starting at ~15 Ma. This final cooling represents the most recent stage of rifting, which followed widespread flood-phonolite emplacement and has shaped the present-day landscape through rift shoulder uplift, faulting, basin filling, protracted volcanism, and erosion. When compared with thermochronologic and geologic data from other sectors of the East African Rift System, extension appears to be diachronous, spatially disparate, and partly overlapping, likely driven by interactions between mantle-driven processes and crustal heterogeneities, rather than the previously suggested north-south migrating influence of a mantle plume.

  2. Role of Faults in Controlling Hydrothermal Fluid Flow, Salinity, Helium, and CO2 Transport along the Rio Grande Rift

    NASA Astrophysics Data System (ADS)

    Woolsey, E. E.; Person, M. A.; Crossey, L. J.; Karlstrom, K. E.

    2011-12-01

    Groundwater 3He/4He anomalies ( ≤ 0.65 Ra) reported in the Rio Grande Rift are probable indications of complex mixing of mantle, heat, and crustal fluids. A suite of new noble gas and water samples were recently collected across the Albuquerque and Socorro Basins of the Rio Grande Rift including one from the recently completed geothermal slim hole near the NMT campus to help determine the flux of mantle derived fluids through the crust. High CO2 levels measured at the travertine depositing springs at Tierra Amarilla near San Yisidro on the Nacimiento fault provide further indication of mantle degassing. The water discharged at the Tierra Amarilla springs carries mantle volatiles and contains high salinity and elevated trace metals such as arsenic. A better understanding of the hydrologic controls is necessary to assess the degradation of water quality. A series of east-west and north-south basin-scale, cross-sectional hydrologic models were constructed along the Rio Grande Rift in the Albuquerque and Socorro Basins to assess the relative importance of faults as conduits for meteoric and deep endogenic fluids. These models extend to a depth of 15 km to incorporate deeply derived inputs. The north-south cross-section was developed from geologic maps, well bore lithologic logs, as well as gravity and seismic-surveys. Separate east-west models were developed for the lystric and sub-vertical normal faults in the southern Albuquerque Basin to quantify how differently CO2 and helium transport responds in each fault system using the same hydrologic parameters. New and existing groundwater salinity, temperature, 3He/4He, and 14C data provide the ground truth for model calibration. The cross-sectional models used in this study illustrate the importance of deeply penetrating, moderately permeable fault zones (10-14 to 10-16 m2) in advective transport of groundwater and mantle volatiles to shallow crustal levels. Rio Grande surface water increases in salinity from north to

  3. 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

  4. Crustal heterogeneity and basement influence on the development of the Kenya Rift, East Africa

    NASA Astrophysics Data System (ADS)

    Smith, Martin; Mosley, Peter

    1993-01-01

    Structural and metamorphic studies within the exposed Precambrian basement in Kenya indicate that a number of heterogeneities exist within the lithosphere beneath the line of the Kenya Rift. Of these, the most important is the mechanical and thermal contrast between thick, cold, and rigid Archean lithosphere and thinner anisotropic Proterozoic mobile belt crust/lithosphere. Structural, geophysical, and heat flow data indicate that during a late Proterozoic collisional event the margin of the Tanzanian Archean craton was reworked, overthrust, and effectively buried by tectonically emplaced "Mozambique Belt" rocks. Its position now lies some 100 km east and northeast of the exposed outcrop. These variations in crust/lithosphere type may be directly correlated with the morphotectonic and structural framework of the Kenya Rift and in particular are reflected in the spatial patterns of Cenozoic volcanism within the Gregory Rift zone. Along the craton margin and within the mobile belt a series of late Proterozoic continental-scale NW-SE and N-S trending ductile/brittle shear zones are fundamental. The reactivation of these shear zones under varying stress field conditions is presented as a model to account for the patterns of rift subsidence, including the location and geometry of graben structures, and the intrusion of magmas since early Miocene times.

  5. 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

  6. A Small Ocean Rift Leads to a New View of the Galapagos Microplate: the Incipient Rift at 2 Degrees North, East of the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Klein, E. M.; Smith, D. K.; Williams, C. M.; Schouten, H.

    2004-12-01

    The Galapagos microplate (GMP) shares a complex plate boundary configuration with the surrounding Cocos, Nazca and Pacific plates. While the configuration of the microplate's southern boundary with the Nazca plate is relatively well understood, the nature of its northern boundary with the Cocos plate has remained more elusive. Work by Lonsdale and co-workers (1988; 1992) identified an "incipient" spreading center east of and orthogonal to the East Pacific Rise (EPR) at 2deg40minN, forming a portion of the northern boundary of the GMP. They described this spreading center as a slowly diverging, westward propagating rift, verging toward the EPR. In 2002, we mapped and sampled a broad region centered on the IR, extending ~140 km east of the EPR. Various geological, geophysical and rock sampling tools were used, including Seabeam2000 bathymetry and side-scan (amplitude); towed magnetometer; bottom photography (14 camera tows using the WHOI camera system); water column hydrothermal surveying and rock sampling. Our recent analysis of the bathymetric, side-scan and magnetic data suggests that the IR forms a lozenge-shaped feature, which tapers both westward toward the EPR and eastward, away from the EPR. Within the eastern half of the lozenge, there is a sinuous trough that trends ~100° to the southeast, along which reflection amplitudes indicate high-reflectivity consistent with relatively sparse sediment cover. The eastern end of this trough clearly cuts north-south-oriented abyssal hills and then dies out in the vicinity of 101deg29minW. The photographic results show sparsely sedimented lavas, often with delicate ornamentation and basaltic glass, with local in-filling of sediment between pillows. In a number of photographs, particularly along the eastern portion of the IR, lavas appear to emanate from local fissures with southeast-northwest orientations. Taken together, these data suggest that the IR is magmatically active along its length and has propagated eastward

  7. 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

  8. 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.

  9. 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

  10. 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.

  11. 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

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

    SciTech Connect

    Tiercelin, J.J.; Lezzar, K.E. ); Richert, J.P. )

    1994-07-01

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

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

    NASA Astrophysics Data System (ADS)

    Ricketts, Jason William

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

  14. Topography, river network and recent fault activity at the margins of the Central Main Ethiopian Rift (East Africa)

    NASA Astrophysics Data System (ADS)

    Molin, Paola; Corti, Giacomo; Sembroni, Andrea

    2016-04-01

    Along its length, the Main Ethiopian Rift (MER) in East Africa records a transition from early fault-dominated morphology in the South to axial magma assisted-rifting typical of continental break-up in the North. It is one of the few locations on Earth offering a complete picture of the evolution of continental rifting and thus provides a unique opportunity to directly analyze how the drainage network reorganize under extensional tectonic forcing. In this paper we present a new analysis of the river network and relative landforms - complemented with a summary of recent geological data - at both rift margins of the Central MER, a key sector of the rift capturing the phase of drainage reorganization between incipient and mature rifting. This analysis shows that hydrography is strongly influenced by recent tectonics. Rectangular drainage patterns, windgaps, and lacustrine/swampy areas formed by structural dams document that the rivers are in continuous competition with fault activity. The irregular longitudinal profiles (with knickpoints/knickzones in correspondence with faults) also suggest that rivers are in a transient state of disequilibrium related to recent tectonic activity at rift margins, in agreement with previous geological and seismological data. A more regional analysis extended to the adjoining Northern and Southern MER indicates that rifting evolves from initial stages characterized by margins poorly incised by rivers with gentle channel gradients (except in correspondence with faults), to mature phases in which rift margins are highly incised by a well organized fluvial network composed by concave and steep rivers. Our regional analysis also indicates a stronger and/or more recent tectonic activity at the rift margins proceeding to the south, in line with previous models of rift development.

  15. Topography, river network and recent fault activity at the margins of the Central Main Ethiopian Rift (East Africa)

    NASA Astrophysics Data System (ADS)

    Molin, Paola; Corti, Giacomo

    2015-11-01

    Along its length, the Main Ethiopian Rift (MER) in East Africa records a transition from early fault-dominated morphology in the South to axial magma assisted-rifting typical of continental break-up in the North. It is one of the few locations on Earth offering a complete picture of the evolution of continental rifting and thus provides a unique opportunity to directly analyze how the drainage network reorganizes under extensional tectonic forcing. In this paper we present a new analysis of the river network and relative landforms-complemented with a summary of recent geological data-at both rift margins of the Central MER, a key sector of the rift capturing the phase of drainage reorganization between incipient and mature rifting. This analysis shows that hydrography is strongly influenced by recent tectonics. Rectangular drainage patterns, windgaps, and lacustrine/swampy areas formed by structural dams document that the rivers are in continuous competition with fault activity. The irregular longitudinal profiles (with knickpoints/knickzones in correspondence with faults) also suggest that rivers are in a transient state of disequilibrium related to recent tectonic activity at rift margins, in agreement with previous geological and seismological data. A more regional analysis extended to the adjoining Northern and Southern MER indicates that rifting evolves from initial stages characterized by margins poorly incised by rivers with gentle channel gradients (except in correspondence with faults), to mature phases in which rift margins are highly incised by a well organized fluvial network composed by concave and steep rivers. Our regional analysis also indicates a stronger and/or more recent tectonic activity at the rift margins proceeding to the south, in line with previous models of rift development.

  16. 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

  17. 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

  18. Kanda fault: A major seismogenic element west of the Rukwa Rift (Tanzania, East Africa)

    NASA Astrophysics Data System (ADS)

    Vittori, Eutizio; Delvaux, Damien; Kervyn, François

    1997-09-01

    The NW-SE trending Rukwa Rift, part of the East African Rift System, links the approximately N-S oriented Tanganyika and Nyassa (Malawi) depressions. The rift has a complex half-graben structure, generally interpreted as the result of normal and strike-slip faulting. Morphological and structural data (e.g. fault scarps, faceted spurs, tilting of Quaternary continental deposits, volcanism, seismicity) indicate Late Quaternary activity within the rift. In 1910 an earthquake of M = 7.4 (historically the largest felt in Africa) struck the Rukwa region. The epicentre was located near the Kanda fault, which affects the Ufipa plateau, separating the Rukwa depression from the south-Tanganyika basin. The geomorphic expression of the Kanda fault is a prominent fresh-looking scarp more than 180 km long, from Tunduma to north of Sumbawanga, that strikes roughly NW-SE, and dips constantly northeast. No evidence for horizontal slip was observed. Generally, the active faulting affects a very narrow zone, and is only locally distributed over several subparallel scarps. The height of the scarp progressively decreases towards the northwest, from about 40-50 m to a few metres north of Sumbawanga. Faulted lacustrine deposits exposed in a road cut near Kaengesa were dated as 8340 ± 700 and 13 600 ± 1240 radiocarbon years. These low-energy deposits now hang more than 15 m above the present-day valley floor, suggesting rapid uplift during the Holocene. Due to its high rate of activity in very recent times, the Kanda Fault could have produced the 1910 earthquake. Detailed paleoseismological studies are used to characterize its recent history. In addition, the seismic hazard posed by this fault, which crosses the fast growing town of Sumbawanga, must be seriously considered in urban planning.

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. 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

  4. Diachronous Growth of Normal Fault Systems in Multiphase Rift Basins: Structural Evolution of the East Shetland Basin, Northern North Sea

    NASA Astrophysics Data System (ADS)

    Claringbould, Johan S.; Bell, Rebecca E.; A-L. Jackson, Christopher; Gawthorpe, Robert L.; Odinsen, Tore

    2015-04-01

    Our ability to determine the structural evolution and interaction of fault systems (kinematically linked group of faults that are in the km to 10s of km scale) within a rift basin is typically limited by the spatial extent and temporal resolution of the available data and methods used. Physical and numerical models provide predictions on how fault systems nucleate, grow and interact, but these models need to be tested with natural examples. Although field studies and individual 3D seismic surveys can provide a detailed structural evolution of individual fault systems, they are often spatially limited and cannot be used examine the interaction of fault systems throughout the entire basin. In contrast, regional subsurface studies, commonly conducted on widely spaced 2D seismic surveys, are able to capture the general structural evolution of a rift basin, but lack the spatial and temporal detail. Moreover, these studies typically describe the structural evolution of rifts as comprising multiple discrete tectonic stages (i.e. pre-, syn- and post-rift). This simplified approach does not, however, consider that the timing of activity can be strongly diachronous along and between faults that form part of a kinematically linked system within a rift basin. This study focuses on the East Shetland Basin (ESB), a multiphase rift basin located on the western margin of the North Viking Graben, northern North Sea. Most previous studies suggest the basin evolved in response to two discrete phases of extension in the Permian-Triassic and Middle-Late Jurassic, with the overall geometry of the latter rift to be the result of selective reactivation of faults associated with the former rift. Gradually eastwards thickening intra-rift strata (deposited between two rift phases) that form wedges between and within fault blocks have led to two strongly contrasting tectonic interpretations: (i) Early-Middle Jurassic differential thermal subsidence after Permian-Triassic rifting; or (ii

  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. 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.

  7. 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

  8. 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

  9. 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.

  10. Sedimentation History and Provenance Analysis of a Late Mesozoic Rifting Event at Tavan Har, East Gobi, Mongolia

    ERIC Educational Resources Information Center

    Davidson, Sarah Cain

    2005-01-01

    The East Gobi Basin (EGB), which covers over 1.5 million square kilometers in southeastern Mongolia, is one of several basins in eastern China and Mongolia that was formed by extension and intracontinental rifting during the late Mesozoic. For reasons that are poorly understood, the continental lithosphere covering areas that are now known as…

  11. Geologic Map of the Middle East Rift Geothermal Subzone, Kilauea Volcano, Hawaii

    USGS Publications Warehouse

    Trusdell, Frank A.; Moore, Richard B.

    2006-01-01

    K'lauea is an active shield volcano in the southeastern part of the Island of Hawai'i. The middle east rift zone (MERZ) map includes about 27 square kilometers of the MERZ and shows the distribution of the products of 37 separate eruptions during late Holocene time. Lava flows erupted during 1983-96 have reached the mapped area. The subaerial part of the MERZ is 3-4 km wide and about 18 km long. It is a constructional ridge, 50-150 m above the adjoining terrain, marked by low spatter ramparts and cones as high as 60 m. Lava typically flowed either northeast or southeast, depending on vent location relative to the topographic crest of the rift zone. The MERZ receives more than 100 in. of rainfall annually and is covered by tropical rain forest. Vegetation begins to grow on lava a few months after its eruption. Relative heights of trees can be a guide to relative ages of underlying lava flows, but proximity to faults, presence of easily weathered cinders, and human activity also affect the rate of growth. The rocks have been grouped into five basic age groups. The framework for the ages assigned is provided by eight radiocarbon ages from previous mapping by the authors and a single date from the current mapping effort. The numerical ages are supplemented by observations of stratigraphic relations, degree of weathering, soil development, and vegetative cover.

  12. 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.

  13. Deltas of the Lake Malawi rift, east Africa: Seismic expression and exploration implications

    SciTech Connect

    Scholz, C.A.

    1995-11-01

    High-resolution, air-gun-sourced seismic reflection surveys over the offshore regions of five river deltas in Lake Malawi in the East African rift system reveal considerable variability in acoustic facies and stratigraphic architecture. This variability can largely be attributed to the influences of different structural settings, and to a lesser degree to high-amplitude (100-400 m) and high-frequency (1000 to 100,000 yr) fluctuations in lake level. Deltas on flexural and axial margins in the rift lake show well-developed progradational geometries. In contrast, a delta on a steep, accommodation zone margin distributes coarse sediments over a broad depositional apron, rather than concentrating sediment in discrete progradational lobes as on the other deltas. A large border fault margin river delta displays the most complex tectonic and stratigraphic architecture of all the deltas studied. It contains several delta-associated facies, including prograding clinoform packages, fan deltas stacked against a boundary fault, and extensive subaqueous fans. Flexural margin lowstand deltas may be the most prospective for hydrocarbon exploration due to their large, internally well-organized, progradational lobes and their close proximity to deep-water, high total organic carbon lacustrine source facies.

  14. Basement interpretations from airborne magnetic and gravity data over the Lambert Rift region of East Antarctica

    NASA Astrophysics Data System (ADS)

    McLean, M. A.; Wilson, C. J. L.; Boger, S. D.; Betts, P. G.; Rawling, T. J.; Damaske, D.

    2009-06-01

    Geological exposures in the Lambert Rift region of East Antarctica comprise scattered coastal outcrops and inland nunataks sporadically protruding through the Antarctic ice sheet from Prydz Bay to the southernmost end of the Prince Charles Mountains. This study utilized airborne magnetic, gravity, and ice radar data to interpret the distribution and architecture of tectonic terranes that are largely buried beneath the thick ice sheet. Free-air and Bouguer gravity data are highly influenced by the subice and mantle topography, respectively. Gravity stripping facilitated the removal of the effect of ice and Moho, and the residual gravity data set thus obtained for the intermediate crustal level allowed a direct comparison with magnetic data. Interpretation of geophysical data also provided insight into the distribution and geometry of four tectonic blocks: namely, the Vestfold, Beaver, Mawson, and Gamburtsev domains. These tectonic domains are supported by surface observations such as rock descriptions, isotopic data sets, and structural mapping.

  15. 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

  16. 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.

  17. 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

  18. 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.

  19. Discussion on final rifting evolution and breakup : insights from the Mid Norwegian - North East Greenland rifted system

    NASA Astrophysics Data System (ADS)

    Peron-Pinvidic, Gwenn; Terje Osmundsen, Per

    2016-04-01

    In terms of rifted margin studies, the characteristics of the distal and outer domains are among the today's most debated questions. The architecture and composition of deep margins are rarely well constrained and hence little understood. Except from in a handful number of cases (eg. Iberia-Newfoundland, Southern Australia, Red Sea), basement samples are not available to decipher between the various interpretations allowed by geophysical models. No consensus has been reached on the basement composition, tectonic structures, sedimentary geometries or magmatic content. The result is that non-unique end-member interpretations and models are still proposed in the literature. So, although these domains mark the connection between continents and oceans, and thus correspond to unique stages in the Earth's lithospheric life cycle, their spatial and temporal evolution are still unresolved. The Norwegian-Greenland Sea rift system represents an exceptional laboratory to work on questions related to rifting, rifted margin formation and sedimentary basin evolution. It has been extensively studied for decades by both the academic and the industry communities. The proven and expected oil and gas potentials led to the methodical acquisition of world-class geophysical datasets, which permit the detailed research and thorough testing of concepts at local and regional scales. This contribution is issued from a three years project funded by ExxonMobil aiming at better understanding the crustal-scale nature and evolution of the Norwegian-Greenland Sea. The idea was to take advantage of the data availability on this specific rift system to investigate further the full crustal conjugate scale history of rifting, confronting the various available datasets. In this contribution, we will review the possible structural and sedimentary geometries of the distal margin, and their connection to the oceanic domain. We will discuss the definition of 'breakup' and introduce a first order conceptual

  20. 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.

  1. 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

  2. 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.

  3. 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

  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.

  5. 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

  6. 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

  7. 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.

  8. East Mariana Basin tholeiites: Cretaceous intraplate basalts or rift basalts related to the Ontong Java plume?

    USGS Publications Warehouse

    Castillo, P.R.; Pringle, M.S.; Carlson, R.W.

    1994-01-01

    basement in the Nauru and East Mariana Basins is Jurassic in age, the geochemical and chronological results discussed here suggest that the basement formed during a Cretaceous rifting event within the Jurassic crust. This magmatic and tectonic event was created by the widespread volcanism responsible for the genesis of the large oceanic plateaus of the western Pacific. ?? 1994.

  9. Significant cenozoic faulting, east margin of the Espanola basin, Rio Grande Rift, New Mexico

    SciTech Connect

    Vernon, J.H. ); Riecker, R.E.

    1989-03-01

    Tectonic interpretation of the east margin of the Espanola Basin, Rio Grande rift, New Mexico, has been controversial. Previous authors have disagreed as to whether significant faulting defines the boundary between the basin and the Sangre de Cristo Mountains. A more recent geophysical basin transect that suggests no significant faulting and field observation of faceted spurs along the western Sangre de Cristo Mountain front indicating a faulted margin motivate our study. The east margin of the Espanola Basin for about 37 km north of Santa Fe, New Mexico, is marked by a complex array of significant, late Cenozoic high-angle faults. Locally, three parallel, north-trending, high-angle faults cut Precambrian basement and Tertiary basin-full rocks along the basin margin. Elsewhere along the margin, tilted fault blocks and intersecting faults occur. Fault area, fault attitude with depth, magnitude of fault motion, and timing of fault motion remain uncertain. However, faults studied in detail are 1-2 km long, have minimum dip-slip motion of 33-100 m, and underwent movement during the late Cenozoic. Potentially significant tectonic and seismic hazard implications arise from the possibility of post-150 ka fault motion.

  10. 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

  11. Analogy between natural gas found in lakes of rift valley system of east Africa and its allied gas in Japan

    SciTech Connect

    Fukuta, O.

    1984-09-01

    The Afar triangle in northeastern Ethiopia is where the Red Sea rift, the Carlsberg Ridge of the Indian Ocean, and the Rift Valley system of east Africa meet. In 1979, J. Welhan and H. Craig reported that hydrothermal vents at 21/sup 0/N, on the East Pacific Rise, are discharging turbid waters. Mixtures of the plumes with ambient seawater contain significant amounts of dissolved H/sub 2/ and CH/sub 4/ as well as mantel-derived /sup 3/He-rich helium. The /sup 3/He//sup 4/He ratios of rock samples obtained earlier by J. Lupton and H. Craig from the Mid-Oceanic Ridge, including the Mid-Atlantic Ridge and the east Pacific Rise, are extremely high at an almost constant value of (1.3 +/- 0.2) x 10/sup -5/, which they defined as the MOR-type helium. However, the deep brines of the Red Sea contain about 1,000 times more methane than normal seawater does, according to Gold and Soter in 1980. Much evidence leads us to believe that large amounts of /sup 3/He-rich helium-bearing natural gas have been gushing out in many places of the Rift Valley of east Africa for a long time. In 1980, Gold and Soter stated that Lake Kivu, which occupies part of the East African rift valley, contains 50 million tons of dissolved methane for which there is no adequate microbial source. The Japanese Islands began to separate from the Asian continent during the early Miocene. The early Miocene was characterized by intensive volcanic activity that produced large amounts of pyroclastics and other volcanic rocks, generally called green tuff in Japan. It has been suggested that oil and gas in green tuff is derived from the upper mantle.

  12. Syn-Rift Systems of East Godavari Sub Basin: Its Evolution and Hydrocarbon Prospectivity

    NASA Astrophysics Data System (ADS)

    Dash, J., Jr.; Zaman, B.

    2014-12-01

    Krishna Godavari (K.G.) basin is a passive margin basin developed along the Eastern coast of India. This basin has a polyhistoric evolution with multiple rift systems. Rift basin exploration has provided the oil and gas industry with almost one third of discovered global hydrocarbon resources. Understanding synrift sequences, their evolution, depositional styles and hydrocarbon prospectivity has become important with recent discovery of the wells, G-4-6,YS-AF and KG-8 in the K.G. offshore basin. The East Godavari subbasin is a hydrocarbon producing basin from synrift and pre-rift sediments, and hence this was selected as the study area for this research. The study has been carried out by utilizing data of around 58 wells (w1-w58) drilled in the study area 25 of which are hydrocarbon bearing with organic thickness varying from 200 m to 600 m. Age data generated by palaentology and palynology studies have been utilized for calibration of key well logs to differentiate between formations within prerift and synrift sediments. The electrologs of wells like resistivity, gamma ray, neutron, density and sonic logs have been utilized for correlation of different formations in all the drilled wells. The individual thicknesses of sand, shale and coal in the formations have been calculated and tabulated. For Golapalli formation, the isopach and isolith maps were generated which revealed that there were four depocentres with input from the north direction. Schematic geological cross sections were prepared using the well data and seismic data to understand the facies variation across the basin. The sedimentological and petrophysical analysis reports and electro log suites were referred to decipher the environment of deposition, the reservoir characteristics, and play types. The geochemical reports [w4 (Tmax)= 455-468 °C; w1 (Tmax) = 467-514 °C; w4(VRO)= 0.65-0.85; w1(VRO)= 0.83-1.13] revealed the source facies, its maturation and migration timings i.e. the petroleum systems

  13. Magma Rich Events at Magma-Poor Rifted Margins: A South-East Indian Example

    NASA Astrophysics Data System (ADS)

    Harkin, Caroline; Kusznir, Nick; Tugend, Julie; Manatschal, Gianreto; Horn, Brian

    2016-04-01

    The south-east Indian continental rifted margin, as imaged by the INE1-1000 deep long-offset seismic reflection section by ION Geophysical, is a classic example of a magma-poor rifted margin, showing highly thinned continental crust, or possibly exhumed mantle, within the ocean-continent transition (OCT). Outboard, the steady-state oceanic crust is between 4 and 5 km thickness, consistent with magma-poor continental breakup and sea-floor spreading. It is therefore surprising that between the hyper-extended crust showing thin or absent continental crust (of approximately 75 km width) and the anomalously thin steady-state oceanic crust, there appears to be a region of thicker magmatic crust of approximately 11 km thickness and 100 km width. Magmatic events, at or just after continental breakup, have also been observed at other magma-poor rifted margins (e.g. NE Brazil). This interpretation of magma-poor OCT structure and thinner than global average oceanic crust separated by thicker magmatic crust on the SE Indian margin is supported by gravity inversion; which uses a 3D spectral technique and includes a lithosphere thermal gravity anomaly correction. Residual depth anomaly (RDA) analysis corrected for sediment loading using flexural backstripping, gives a small negative value (approximately -0.1 km) over the steady-state oceanic crust compared with a positive value (approximately +0.3 km) over the thicker magmatic crust. This RDA difference is consistent with the variation in crustal thickness seen by the seismic reflection interpretation and gravity inversion. We use joint inversion of the time domain seismic reflection and gravity data to investigate the average basement density and seismic velocity of the anomalously thick magmatic crust. An initial comparison of Moho depth from deep long-offset seismic reflection data and gravity inversion suggests that its basement density and seismic velocity are slightly less than that of the outboard steady-state oceanic

  14. Riphean rifting and major Palaeoproterozoic crustal boundaries in the basement of the East European Craton: geology and geophysics

    NASA Astrophysics Data System (ADS)

    Bogdanova, S. V.; Pashkevich, I. K.; Gorbatschev, R.; Orlyuk, M. I.

    1996-12-01

    The East European Craton, which occupies the northeastern half of the European continent, is characterized by the presence of giant, even transcratonic systems of Riphean (Meso- to Neoproterozoic) rifts (aulacogens). The largest of these Riphean rift systems tend to follow the course of Palaeoproterozoic sutures and junction zones that subdivide the craton into three crustal segments with autonomous development histories. These segments, named Fennoscandia, Volgo-Uralia and Sarmatia, are marked by distinct T-MAGSAT anomalies. Genetically, the intersegment junction zones represent different types of collisional and accretional interaction between the component crustal segments of the craton. In accordance with this, the Riphean rift systems vary in width, the presence or absence of Moho uplifts, and other properties like, for instance, heat flow values. However, all of them follow belts of weakly magnetized crust studded with local magnetic maxima due to the presence of mafic rocks associated with the rifting. The rift system-intersegment junction zone relationships, in consequence, similarly represent a range of variation and cannot be referred to one single model.

  15. Diachronous fault array growth within continental rift basins: Quantitative analyses from the East Shetland Basin, northern North Sea

    NASA Astrophysics Data System (ADS)

    Claringbould, Johan; Bell, Rebecca; Jackson, Christopher; Gawthorpe, Robert; Odinsen, Tore

    2016-04-01

    The evolution of rift basins has been the subject of many studies, however, these studies have been mainly restricted to investigating the geometry of rift-related fault arrays. The relative timing of development of individual faults that make up the fault array is not yet well constrained. First-order tectono-stratigraphic models for rifts predict that normal faults develop broadly synchronously throughout the basin during a temporally distinct 'syn-rift' episode. However, largely due to the mechanical interaction between adjacent structures, distinctly diachronous activity is known to occur on the scale of individual fault segments and systems. Our limited understanding of how individual segments and systems contribute to array-scale strain largely reflects the limited dimension and resolution of the data available and methods applied. Here we utilize a regional extensive subsurface dataset comprising multiple 3D seismic MegaSurveys (10,000 km2), long (>75km) 2D seismic profiles, and exploration wells, to investigate the evolution of the fault array in the East Shetland Basin, North Viking Graben, northern North Sea. Previous studies propose this basin formed in response to multiphase rifting during two temporally distinct extensional phases in the Permian-Triassic and Middle-to-Late Jurassic, separated by a period of tectonic quiescence and thermal subsidence in the Early Jurassic. We document the timing of growth of individual structures within the rift-related fault array across the East Shetland Basin, constraining the progressive migration of strain from pre-Triassic-to-Late Jurassic. The methods used include (i) qualitative isochron map analysis, (ii) quantitative syn-kinematic deposit thickness difference across fault & expansion index calculations, and (iii) along fault throw-depth & backstripped displacement-length analyses. In contrast to established models, we demonstrate that the initiation, growth, and cessation of individual fault segments and

  16. 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

  17. 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.

  18. 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

  19. 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

  20. 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

  1. 13. Detail of hydraulic fluid piping, east wall of bay ...

    Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

    13. Detail of hydraulic fluid piping, east wall of bay - Mississippi River 9-Foot Channel, Lock & Dam No. 1, In Mississippi River at Mississippi Boulevard, below Ford Parkway Bridge, Saint Paul, Ramsey County, MN

  2. 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.

  3. The Proto-Indian Ocean and a probable paleozoic/mesozoic triradial rift system in East Africa

    NASA Astrophysics Data System (ADS)

    Cannon, R. T.; Simiyu Siambi, W. M. N.; Karanja, F. M.

    1981-02-01

    established until the displacement of Madagascar away from Africa, probably in the Cretaceous. Accepting the above northern position of Madagascar, the writers also postulate that in East Africa the fault-bounded Karroo depositional basins (troughs) were located within a major triradial rift system extending from Lake Malawi at least as far as eastern Kenya (some 1600 km). This rift system, if valid, was established within Gondwanaland over a period ˜100 m.y. in the Paleozoic/Mesozoic (pre-breakup) in marked contrast to the East African Rift System (classical rift valleys) which is mainly a Cainozoic phenomenon (post-breakup). It is, therefore, considered that there is a fundamental difference in origin between the two rift systems.

  4. 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

  5. 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

  6. 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

  7. 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.

  8. 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

  9. Tectonic Evolution of the Northern Malawi rift, East Africa: Structural Controls on Sediment Dispersal in a Large Lake Basin.

    NASA Astrophysics Data System (ADS)

    Mortimer, E.; Paton, D.; Scholz, C.; Strecker, M.

    2005-12-01

    The Malawi Rift is an integral part of the East African Rift system (EARs), the type example of a rift system in its youthful stages of development. Understanding the spatio-temporal evolution of this rift system provides insights into the past histories and controls on more ancient rift basins. Fundamental questions remain regarding the structural styles, partitioning, and sediment dispersal patterns within large lacustrine basins that are common to young rifts. Such basins are particularly useful recorders of climatic fluctuations. In areas with pronounced climatic variability and high evaporation/precipitation ratios this may lead to, often transient, basin isolation that may dramatically affect sediment and facies distribution. We investigate the structural evolution and sedimentation patterns of the North basin, Malawi Rift. We utilise two seismic reflection data sets: the first, collected by project PROBE, records the entire basin-fill (Sequences 1-3) at medium resolution; the second provides a closely-spaced high-resolution survey of the past ~1 Ma (Sequence 3). These data document the development of the basin-bounding and intra-basin faults. Throughout its history, the basin-bounding fault has accommodated the greatest strain and therefore exerted the fundamental control on the distribution of sediment within the rift. Present-day sediments enter the basin axially, although there is evidence that transverse supply was also important in the past. We identify 11 intra-basin structures that strike parallel or sub-parallel to this border fault. These intra-basin faults, active from the earliest stages, have initial segment lengths of ~30 km, but rapidly reach lengths in excess of 80 km in the north of the basin, and 60 km in the south. Much of the sedimentation along the faults occurred during Sequence 2 (~2.7-1.6 Ma), a period of relative over-filling of the basin. While the basin-bounding fault controls the overall sediment architecture, all 11 intra

  10. 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

  11. 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  .

  12. 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.

  13. The crust and upper mantle of central East Greenland - implications for continental accretion and rift evolution

    NASA Astrophysics Data System (ADS)

    Schiffer, Christian; Balling, Niels; Ebbing, Jörg; Holm Jacobsen, Bo; Bom Nielsen, Søren

    2016-04-01

    The geological evolution of the North Atlantic Realm during the past 450 Myr, which has shaped the present-day topographic, crustal and upper mantle features, was dominated by the Caledonian orogeny and the formation of the North Atlantic and associated igneous activity. The distinct high altitude-low relief landscapes that accompany the North Atlantic rifted passive margins are the focus of a discussion of whether they are remnant and modified Caledonian features or, alternatively, recently uplifted peneplains. Teleseismic receiver function analysis of 11 broadband seismometers in the Central Fjord Region in East Greenland indicates the presence of a fossil subduction complex, including a slab of eclogitised mafic crust and an overlying wedge of hydrated mantle peridotite. This model is generally consistent with gravity and topography. It is shown that the entire structure including crustal thickness variations and sub-Moho heterogeneity gives a superior gravity and isostatic topographic fit compared to a model with a homogeneous lithospheric layer (1). The high topography of >1000 m in the western part of the area is supported by the c. 40 km thick crust. The eastern part requires buoyancy from the low velocity/low density mantle wedge. The geometry, velocities and densities are consistent with structures associated with a fossil subduction zone. The spatial relations with Caledonian structures suggest a Caledonian origin. The results indicate that topography is isostatically compensated by density variations within the lithosphere and that significant present-day dynamic topography seems not to be required. Further, this structure is suggested to be geophysically very similar to the Flannan reflector imaged north of Scotland, and that these are the remnants of the same fossil subduction zone, broken apart and separated during the formation of the North Atlantic in the early Cenozoic (2). 1) Schiffer, C., Jacobsen, B.H., Balling, N., Ebbing, J. and Nielsen, S

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

    NASA Astrophysics Data System (ADS)

    Baqués, Vinyet; Travé, Anna; Cantarero, Irene

    2013-04-01

    The Penedès half-graben, located in the north-western part of the Mediterranean, is a NE-SW oriented basin generated during the Miocene rifting. This graben is bounded to the northwest by the SE-dipping Vallès-Penedès fault, which places the Mesozoic rocks in contact with the Miocene basin-fill. The basin is filled with an up to 4 km thick succession of sediments divided into three lithostratigraphic units. From base to top: (1) a lower continental complex, (2) a continental to marine complex and (3) an upper continental complex. These units are covered by Pliocene deposits which onlap a Messinian regional erosive surface. The structural features within the Penedès half-graben allow defining three deformational phases during the Miocene rifting. The first, during the syn-rift, two successive stages of NE-SW normal faults were formed. The second, during the early post-rift, one stage of NE-SW normal faults and one minor compression phase with a dextral directional developed. The third, during the late post-rift, two successive stages of N-S trending extensional fractures (faults and joints) and one minor compression with a sinistral component developed. The fractures related to the syn-rift stage acted as conduits for meteoric fluids both, in the phreatic and in the vadose zone. During the early post-rift, Fe2+- rich fluids precipitated oxides along the NE-SW fault planes. The dextral directional faults served as conduits for meteoric fluids which reequilibrated totally the marine Miocene host rocks under the phreatic environment. The late post-rift stage was characterized by marine fluids upflowing through the N-S fractures, probably derived from the Miocene marine interval, which mixed with meteoric fluids producing dolomitization. The second set of N-S fractures served as conduits for meteoric fluids characterised by δ13C-depleted soil-derived CO2 attributed to precipitation in the vadose zone. The change from phreatic to vadose meteoric environment and the

  15. 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

  16. 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

  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. 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.

  20. Structural comparison of archetypal Atlantic rifted margins (Angola - Esperito Santo, Iberia - Newfoundland, mid.Norway - East Greenland)

    NASA Astrophysics Data System (ADS)

    Peron-Pinvidic, Gwenn; Manatschal, Gianreto; Terje Osmundsen, Per

    2013-04-01

    In the last decade, a number of new geological and numerical models have been proposed to explain intriguing observations from deep margin settings that were previously not well understood. These new models, together with the increasing amount of high-quality geophysical data, now allow to compare observations from different margins. Key areas are the Iberia-Newfoundland conjugates, the North-East and South Atlantic systems. A first-order structural similarity appears between the architectures of these rifted margins, including magma-poor as well as magma-rich ones. Typical is the seawards arrangement of characteristic entities such as platforms, necking zones, ocean-continent transitions and marginal/outer highs. The arrangement appears to reflect a commonality with respect to the tectonic processes involved in rifted margin formation. The study of magma-poor and magma-rich margins notably suggests that hyper-extension does not preclude a magmatic breakup. We propose to clarify the definition of a number of terms typically used in rifted margin studies. Then we will present a review of available information from the Angola-Gabon, Iberia-Newfoundland and Norway-Greenland margins, usually referred to as the archetypes of hyper-extended, magma-poor or volcanic margins. We will discuss their similarities and differences and review the related deformation modes.

  1. Lithoprobe east: marine deep seismic reflection results across the Appalachians and the rifted continental margin northeast of Newfoundland

    SciTech Connect

    Keen, C.E.; Stockmal, G.S.; O'Brien, S.J.; Quinlan, G.

    1985-01-01

    Marine deep seismic reflection data have been collected across the Appalachian Orogen and the rifted continental margin northeast of Newfoundland. Results across the Appalachian Orogen show the extent of the ancient Grenvillian passive margin beneath the terranes to the east, the nature of terrane boundaries at depth, and the relationships between surface geological features and the seismically defined crustal geometry. The results across the rifted margin define a decollement zone below the faulted continental basement. These basement fault blocks do not exhibit a listric geometry. The continental crust thins beneath the sedimentary basins occupying the rifted margin, although there does not appear to be a simple relationship between crustal thickness and basin subsidence. The ocean-continent transition is marked by a landward dip of the oceanic crust which disappears below the thinned continental crust near the transition. The ocean-continent boundary is not a vertical boundary between crustal types, rather there appears to be continuity of oceanic-type crust below the continent, perhaps forming a high velocity lower crustal layer across the continental margin.

  2. 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

  3. 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.

  4. 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.

  5. 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

  6. Fluids in mantle xenoliths related to multiple metasomatism from the Rio Grande Rift, New Mexico

    NASA Astrophysics Data System (ADS)

    Park, M.; Berkesi, M.; Jung, H.; Kil, Y.; Szabo, C.

    2012-12-01

    Mantle-derived volatile-rich fluid inclusions can give important information on chemical features and physical condition on fluid regimes in the upper mantle. These volatiles may also play important role in understanding the fluid/mantle rock interaction in the lithospheric mantle causing mantle metasomatism associated with shallow subduction process. Spinel peridotite xenoliths, hosted in alkali basalts (~15 Ma age), were collected from Adam's Diggings in the Rio Grande Rift (RGR), New Mexico, USA. We selected five representative spinel peridotite xenoliths which are abundant in fluid inclusions (FIs). Based on fluid inclusion petrography, three kinds of orthopyroxene-hosted FIs were identified: Type IA (healed fracture-related, large, negative crystal shape; 10 - 25 μm), Type IB (containing opaque mineral, small, negative crystal shape; 5 - 10 μm), and Type IC (exsolved spinel-related spherical shape; 5 - 10 μm). We studied the FIs system by using heating-freezing stage (microthermometry), high resolution Raman spectroscopy and FIB-SEM (Focused Ion Beam-Scanning Electron Microscopy) techniques. These FIs are characterized as CO2-dominated with other minor components (visible melting occurred at -58.0 ~ -56.8 ± 0.2 °C). The calculated CO2 density for Type IC, IB and IA turned out to be 1.05 - 1.12 g/cm3, 0.98 - 1.08 g/cm3, and 0.69 - 0.86 g/cm3, respectively. In addition to the CO2-rich liquid, Type IA, IB and IC FIs contain magnesite as step-daughter phase proved by Raman spectroscopy. Raman spectroscopy also provided clear evidence the presence of H2O in Type IA fluid inclusions and of N2 both in Type IB and Type IC FIs. The former FIs have hydrous solid (amphibole?) and the latter ones contain Fe-rich sulfide minerals which were confirmed by FIB-SEM technique. A thin glass film with vesicles on the wall of the FIs generally occurs in Type IA. Furthermore, in Type IA FIs anhydrite as step-daughter mineral were also identified by FIB-SEM technique. Based on

  7. 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.

  8. Multi-year monitoring of rift propagation on the Amery Ice Shelf, East Antarctica

    NASA Astrophysics Data System (ADS)

    Fricker, H. A.; Young, N. W.; Coleman, R.; Bassis, J. N.; Minster, J.-B.

    2005-01-01

    We use satellite imagery from four sensors (Multi-angle Imaging SpectroRadiometer (MISR), Enhanced Thematic Mapper (ETM), and RADARSAT and ERS Synthetic Aperture Radar (SAR) to monitor the lengths of two rifts on the Amery Ice Shelf, from 1996 to 2004. We find that the rifts have each been propagating at a steady annual rate for the past 5 years. Superimposed on this steady rate is a seasonal signal, where propagation rates are significantly higher in the summer period (i.e., September-April) than in the winter period (i.e., April-September). Possible causes of this summer-winter effect are changing properties of the ice mélange, which fills the rifts, and seasonal changes in ocean circulation beneath the ice shelf.

  9. The effect of an East Pacific Rise offset on the formation of secondary cracks ahead of the Cocos-Nazca Rift at the Galapagos Triple Junction

    NASA Astrophysics Data System (ADS)

    Smith, D. K.; Montesi, L. G.; Schouten, H.; Zhu, W.

    2011-12-01

    A succession of short-lived, E-W trending cracks at the Galapagos Triple Junction north and south of the Cocos-Nazca (C-N) Rift, has been explained by a simple crack interaction model. The locations of where the cracks initiate are controlled by tensile stresses generated at the East Pacific Rise (EPR) by two interacting cracks: One representing the north-south trending EPR, and the other the large, westward propagating C-N Rift, whose tip is separated from the EPR by a distance D. The model predicts symmetric cracking at the EPR north and south of the C-N Rift tip. Symmetry in the distribution of cracks north and south of the C-N Rift is observed and especially remarkable between 2.5 and 1.5 Ma when the rapid jumping of cracks toward the C-N Rift appears synchronous. The rapid jumping can be explained by decreasing D, which means that the tip of the C-N Rift was moving closer to the EPR. Symmetry of cracking breaks down at 1.5 Ma, however, with the establishment of the Dietz Deep Rift, the southern boundary of the Galapagos microplate. Symmetry of cracking also breaks down on older crust to the east between about 100 35'W and 100 45'W (about 2.6 Ma) where a rapid jumping of cracks toward the C-N Rift is observed in the south cracking region. There is no evidence of similar rapid jumping in the north cracking region. It could be simply that the response to changing the value of D is not always as predicted. It could also be that the shape of the EPR has not always been symmetric about the C-N Rift, as assumed in the model. Currently, an overlapping spreading center with a 15 km east-west offset between the limbs of the EPR has formed at 1 50'N. We assess the importance of the geometry of the EPR on the crack interaction model. The model has been modified to include a ridge offset similar to what is observed today. We find that the region of stress enhancement at the EPR (where cracks initiate) is subdued south of the C-N Rift tip because of the EPR offset. It is

  10. 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

  11. 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.

  12. 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.

  13. Images of the East Africa Rift System from the Joint Inversion of Body Waves, Surface Waves, and Gravity: Investigating the Role of Magma in Early-Stage Continental Rifting

    NASA Astrophysics Data System (ADS)

    Roecker, S. W.; Ebinger, C. J.; Tiberi, C.; Mulibo, G. D.; Ferdinand-Wambura, R.; Muzuka, A.; Khalfan, M.; Kianji, G.; Gautier, S.; Albaric, J.; Peyrat, S.

    2015-12-01

    With several rift segments at different stages of the rifting cycle, and the last orogenic episode more than 500 Mya, the young (<7 My) Eastern rift system in northern Tanzania and southern Kenya offers an ideal venue to study the role of magma and other fluids in continental rifting. To estimate both the location and volume of magma beneath the rift system, we generated 3D elastic wave images of the crust and uppermost mantle of this region by analyzing data recorded by a local deployment of 40 broad band seismic stations over a period of two years. We jointly inverted P and S wave arrival times from locally recorded earthquakes with Rayleigh wave dispersion curves derived from cross correlating ambient noise. These results were combined with Bouguer gravity anomalies to increase resolution and add constraints. The ambient noise signal appears to be channeled along the axis of the rift system, suggesting a waveguide effect. Tests with synthetic data estimate a spatial resolution in our images on the order of a few km. Our results demonstrate fundamental modifications of continental crustal structure by magmatic processes during the first few My of rift basin development. To first order, our models are dominated by regions of exceptionally low (by 10-20%) shear wavespeed relative to that of average continental crust. To a large extent the wavespeeds mimic the topography, with the slowest shear wave speeds corresponding to the lowest elevations, and tracing out a NE-SW striking region about 20 km wide from the Natron basin in the north to a NW-SE region of similar width beneath the Manyara basin in the south. These low wavespeeds are likely to be a consequence of the presence of magma and other fluids from at least 30 km depth, the limit of depth resolution for this dataset and near the base of the crust (~35 km), and extending to upper crustal levels in some areas. Somewhat surprisingly, a second region of significant low wavespeed beneath the Ngorongoro caldera

  14. 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.

  15. 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

  16. 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.

  17. 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.

  18. 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

  19. 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.

  20. Forecast and Validation of the Rift Valley fever outbreak in East Africa: 2006-2007

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Background The instantaneous occurrence of El Nino / Southern Oscillation (ENSO) warm events and anomalous warming of the equatorial western Indian Ocean (WIO) are associated with elevated and widespread rainfall over East Africa. Such, sustained, heavy rainfall in East is associated with the emerg...

  1. 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.

  2. 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

  3. 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).

  4. 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.

  5. 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.

  6. 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

  7. 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

    region and/or from deep beneath Kīlauea's east rift zone. The deeply derived olivine crystals and their host magma mixed with stored, more evolved magma in the rift zone, and the mixture was later erupted at Kapoho.

  8. 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

    summit region and/or from deep beneath Kīlauea’s east rift zone. The deeply derived olivine crystals and their host magma mixed with stored, more evolved magma in the rift zone, and the mixture was later erupted at Kapoho.

  9. Fluid inclusion and stable isotopes studies of epithermal gold-bearing veins in the SE Afar Rift (Djibouti)

    NASA Astrophysics Data System (ADS)

    Moussa, N.; Boiron, M. C.; Grassineau, N.; Fouquet, Y.; Le Gall, B.; Mohamed, J.

    2015-12-01

    The Afar rift results from the interaction of a number of actively-propagating tectono-magmatic axes. Recent field investigations in the SE Afar rift have emphasized the importance of hydrothermal system in rift-related volcanic complexes. Mineralization occur as gold-silver bearing veins and are associated with felsic volcanism. Late carbonate veins barren of sulfides and gold are common. The morphologies and textures of quartz show crustiform colloform banding, massive and breccias. Microthermometric measurements were made on quartz-hosted two phases (liquid + vapor) inclusions; mean homogenization temperature range from 150°C to 340°C and ice-melting temperatures range from -0.2° to 1.6°C indicating that inclusion solutions are dilute and contain 0.35 to 2.7 equivalent wt. % NaCl. Furthermore, δ18O and δ13C values from calcite range from 3.7 to 26.6 ‰ and -7.5 to 0.3‰, respectively. The presence of platy calcite and adularia indicate that boiling condition existed. This study shows that precious-metal deposition mainly occurred from hydrothermal fluids at 200°C at around 300 and 450 m below the present-day surface in a typical low-sulphidation epithermal environment.

  10. 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.

  11. 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

  12. 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

  13. 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

  14. Has Rift Valley fever virus evolved with increasing severity in human populations in East Africa?

    PubMed

    Baba, Marycelin; Masiga, Daniel K; Sang, Rosemary; Villinger, Jandouwe

    2016-01-01

    Rift Valley fever (RVF) outbreaks have occurred across eastern Africa from 1912 to 2010 approximately every 4-15 years, most of which have not been accompanied by significant epidemics in human populations. However, human epidemics during RVF outbreaks in eastern Africa have involved 478 deaths in 1998, 1107 reported cases with 350 deaths from 2006 to 2007 and 1174 cases with 241 deaths in 2008. We review the history of RVF outbreaks in eastern Africa to identify the epidemiological factors that could have influenced its increasing severity in humans. Diverse ecological factors influence outbreak frequency, whereas virus evolution has a greater impact on its virulence in hosts. Several factors could have influenced the lack of information on RVF in humans during earlier outbreaks, but the explosive nature of human RVF epidemics in recent years mirrors the evolutionary trend of the virus. Comparisons between isolates from different outbreaks have revealed an accumulation of genetic mutations and genomic reassortments that have diversified RVF virus genomes over several decades. The threat to humans posed by the diversified RVF virus strains increases the potential public health and socioeconomic impacts of future outbreaks. Understanding the shifting RVF epidemiology as determined by its evolution is key to developing new strategies for outbreak mitigation and prevention of future human RVF casualties. PMID:27329846

  15. Has Rift Valley fever virus evolved with increasing severity in human populations in East Africa?

    PubMed Central

    Baba, Marycelin; Masiga, Daniel K; Sang, Rosemary; Villinger, Jandouwe

    2016-01-01

    Rift Valley fever (RVF) outbreaks have occurred across eastern Africa from 1912 to 2010 approximately every 4–15 years, most of which have not been accompanied by significant epidemics in human populations. However, human epidemics during RVF outbreaks in eastern Africa have involved 478 deaths in 1998, 1107 reported cases with 350 deaths from 2006 to 2007 and 1174 cases with 241 deaths in 2008. We review the history of RVF outbreaks in eastern Africa to identify the epidemiological factors that could have influenced its increasing severity in humans. Diverse ecological factors influence outbreak frequency, whereas virus evolution has a greater impact on its virulence in hosts. Several factors could have influenced the lack of information on RVF in humans during earlier outbreaks, but the explosive nature of human RVF epidemics in recent years mirrors the evolutionary trend of the virus. Comparisons between isolates from different outbreaks have revealed an accumulation of genetic mutations and genomic reassortments that have diversified RVF virus genomes over several decades. The threat to humans posed by the diversified RVF virus strains increases the potential public health and socioeconomic impacts of future outbreaks. Understanding the shifting RVF epidemiology as determined by its evolution is key to developing new strategies for outbreak mitigation and prevention of future human RVF casualties. PMID:27329846

  16. 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.

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

    PubMed

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

    2003-01-01

    Total mercury (THg) concentrations were measured for various fish species from Lakes Turkana, Naivasha and Baringo in the rift valley of Kenya. The highest THg concentration (636 ng g(-1) wet weight) was measured for a piscivorous tigerfish Hydrocynus forskahlii from Lake Turkana. THg concentrations for the Perciformes species, the Nile perch Lates niloticus from Lake Turkana and the largemouth bass Micropterus salmoides from Lake Naivasha ranged between 4 and 95 ng g(-1). The tilapiine species in all lakes, including the Nile tilapia Oreochromis niloticus, had consistently low THg concentrations ranging between 2 and 25 ng g(-1). In Lake Naivasha, the crayfish species, Procambrus clarkii, had THg concentrations similar to those for the tilapiine species from the same lake, which is consistent with their shared detritivore diet. THg concentrations in all fish species were usually consistent with their known trophic position, with highest concentrations in piscivores and declining in omnivores, insectivores and detritivores. One exception is the detritivore Labeo cylindricus from Lake Baringo, which had surprisingly elevated THg concentrations (mean=75 ng g(-1)), which was similar to those for the top trophic species (Clarias and Protopterus) in the same lake. Except for two Hydrocynus forskahlii individuals from Lake Turkana, which had THg concentrations near or above the international marketing limit of 500 ng g(-1), THg concentrations in the fish were generally below those of World Health Organization's recommended limit of 200 ng g(-1) for at-risk groups.

  18. 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

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. 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

  4. 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.

  5. Origin and Role of Recycled Crust in Flood Basalt Magmatism: Case Study of the Central East Greenland Rifted Margin

    NASA Astrophysics Data System (ADS)

    Brown, E.; Lesher, C. E.

    2015-12-01

    Continental flood basalts (CFB) are extreme manifestations of mantle melting derived from chemically/isotopically heterogeneous mantle. Much of this heterogeneity comes from lithospheric material recycled into the convecting mantle by a range of mechanisms (e.g. subduction, delamination). The abundance and petrogenetic origins of these lithologies thus provide important constraints on the geodynamical origins of CFB magmatism, and the timescales of lithospheric recycling in the mantle. Basalt geochemistry has long been used to constrain the compositions and mean ages of recycled lithologies in the mantle. Typically, this work assumes the isotopic compositions of the basalts are the same as their mantle source(s). However, because basalts are mixtures of melts derived from different sources (having different fusibilities) generated over ranges of P and T, their isotopic compositions only indirectly represent the isotopic compositions of their mantle sources[1]. Thus, relating basalts compositions to mantle source compositions requires information about the melting process itself. To investigate the nature of lithologic source heterogeneity while accounting for the effects of melting during CFB magmatism, we utilize the REEBOX PRO forward melting model[2], which simulates adiabatic decompression melting in lithologically heterogeneous mantle. We apply the model to constrain the origins and abundance of mantle heterogeneity associated with Paleogene flood basalts erupted during the rift-to-drift transition of Pangea breakup along the Central East Greenland rifted margin of the North Atlantic igneous province. We show that these basalts were derived by melting of a hot, lithologically heterogeneous source containing depleted, subduction-modified lithospheric mantle, and <10% recycled oceanic crust. The Paleozoic mean age we calculate for this recycled crust is consistent with an origin in the region's prior subduction history, and with estimates for the mean age of

  6. 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

  7. 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

  8. Evolution of the Lake Victoria basin in the context of coeval rift initiation in East Africa: a 3D numerical model approach

    NASA Astrophysics Data System (ADS)

    Wichura, Henry; Quinteros, Javier; Melnick, Daniel; Brune, Sascha; Schwanghart, Wolfgang; Strecker, Manfred R.

    2015-04-01

    Over the last four years sedimentologic and thermochronologic studies in the western and eastern branches of the Cenozoic East African Rift System (EARS) have supported the notion of a broadly contemporaneous onset of normal faulting and rift-basin formation in both segments. These studies support previous interpretations based on geophysical investigations from which an onset of rifting during the Paleogene had been postulated. In light of these studies we explore the evolution of the Lake Victoria basin, a shallow, unfaulted sedimentary basin centered between both branches of the EARS and located in the interior of the East African Plateau (EAP). We quantify the fluvial catchment evolution of the Lake Victoria basin and assess the topographic response of African crust to the onset of rifting in both branches. Furthermore, we evaluate and localize the nature of strain and flexural rift-flank uplift in both branches. We use a 3D numerical forward model that includes nonlinear temperature- and stress-dependent elasto-visco-plastic rheology. The model is able to reproduce the flexural response of variably thick lithosphere to rift-related deformation processes such as lithospheric thinning and asthenospheric upwelling. The model domain covers the entire EAP and integrates extensional processes in a heterogeneous, yet cold and thick cratonic block (Archean Tanzania craton), which is surrounded by mechanically weaker Proterozoic mobile belts, which are characterized by thinner lithosphere ("thin spots"). The lower limits of the craton (170 km) and the mobile belts (120 km) are simulated by different depths of the 1300 °C lithosphere-asthenosphere boundary. We assume a constant extension rate of 4 mm/a throughout the entire simulation of 30 Ma and neglect the effect of dynamic topography and magmatism. Even though the model setup is very simple and the resolution is not high enough to calculate realistic rift-flank uplift, it intriguingly reveals important topographic

  9. 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

  10. 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

  11. Lake level change and total water discharge in East Africa Rift Valley from satellite-based observations

    NASA Astrophysics Data System (ADS)

    Hassan, Ayman A.; Jin, Shuanggen

    2014-06-01

    The measurement of total basin water discharge is important for understanding the hydrological and climatologic issues related to the water and energy cycles. Climatic extreme events are normal climatic occurrences in Africa. For example, extensive droughts are regular features in the last few decades in parts of East Africa, which suffers from a lack of in situ observations as well as a lack of regional hydrological models. In this study, multi-disciplinary different types of space-borne observations and global hydrological models are used to study total water discharge in the Great Rift Valley of East Africa (i.e. Lakes Victoria, Tanganyika, and Malawi) from January 2003 to December 2012. The data include the following: (1) total water storage (TWS) variations from Gravity Recovery and Climate Experiment (GRACE), (2) the lake level variations from Satellite Alimetric data, (3) rainfall from Tropical Rainfall Measurement Mission (TRMM) products, (4) soil moisture from WaterGAP Global Hydrology Model (WGHM), and (5) water fluxes from Global Land Data Assimilation System (GLDAS). Results show that a significant decline in the average lake level is found for all of the three lakes between 2003 and 2006. GRACE TWS variations of the whole basin area show the same pattern of variation as the average lake level variations estimated from Altimetric data. The TWS in the basin area of Lakes Victoria and Malawi is governed by the surface water stored in each lake itself, while for Lake Tanganyika, it is governed by both surface water and the soil moisture content in the basin area. Furthermore, the effect of rainfall on TWS is also studied. A phase lag of ~ 2 months is found between TRMM rainfall and GRACE TWS (generally, rainfall precedes the GRACE TWS) for the three lakes. In addition, the regional evapotranspiration ET is estimated from the water balance equation using GRACE land-water solutions, rainfall data from TRMM and runoff values obtained as a fraction of rainfall

  12. 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.

  13. 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

  14. Post-rift uplift and focused fluid flow in the passive margin of northern South China Sea

    NASA Astrophysics Data System (ADS)

    Wu, Shiguo; Gao, Jinwei; Zhao, Shujuan; Lüdmann, Thomas; Chen, Duanxin; Spence, George

    2014-03-01

    An important post-rifting tectonic event with extensive uplift and fluid flow took place at the northern margin of the South China Sea (SCS) during the late Miocene. Based on an analysis of two- and three-dimensional seismic data plus drilling well data, the tectonic event mainly affected the Dongsha Rise and the adjacent depressions. Tectonic deformation likely occurred in the Dongsha Rise, the eastern part of the Panyu Swell and several depressions of the Zhu I Sag. The post-rifting deformation in the study area was characterized by faulting, erosion, igneous activity and hydrothermal fluid flow. The tectonic movement caused abundant secondary NW and WNW tensional and transtensional faults, with lengths of 1 to 10 km. It also created a clear angular unconformity which marks the time of the tectonic movement. Late Neogene igneous rocks are found in the strata in the Dongsha Rise only close to the continent-ocean transition zone. Geothermal activity occurred in the Liuhua carbonate platform and adjacent depressions. Deformation is more intense in the eastern area than in the western area. According to sequence stratigraphy, we infer that the tectonic event mainly occurred in the late period of Late Miocene and ended at the Mio/Pliocene boundary (5.5 Ma). The dynamic mechanism for tectonic movement could be associated with subduction of the South China Sea slab beneath the Philippine Sea Plate at the Manila trench. When subduction of lower density crust within the continental-ocean transition zone was initiated, large resistance stresses may have led to lithosphere bending and crustal uplift in the Dongsha area.

  15. The Lower Triassic Sorkh Shale Formation of the Tabas Block, east central Iran: Succesion of a failed-rift basin at the Paleotethys margin

    USGS Publications Warehouse

    Lasemi, Y.; Ghomashi, M.; Amin-Rasouli, H.; Kheradmand, A.

    2008-01-01

    The Lower Triassic Sorkh Shale Formation is a dominantly red colored marginal marine succession deposited in the north-south trending Tabas Basin of east central Iran. It is correlated with the unconformity-bounded lower limestone member of the Elika Formation of the Alborz Mountains of northern Iran. The Sorkh Shale is bounded by the pre-Triassic and post-Lower Triassic interregional unconformities and consists mainly of carbonates, sandstones, and evaporites with shale being a minor constituent. Detailed facies analysis of the Sorkh Shale Formation resulted in recognition of several genetically linked peritidal facies that are grouped into restricted subtidal, carbonate tidal flat, siliciclastic tidal flat, coastal plain and continental evaporite facies associations. These were deposited in a low energy, storm-dominated inner-ramp setting with a very gentle slope that fringed the Tabas Block of east central Iran and passed northward (present-day coordinates) into deeper water facies of the Paleotethys passive margin of northern Cimmerian Continent. Numerous carbonate storm beds containing well-rounded intraclasts, ooids and bioclasts of mixed fauna are present in the Sorkh Shale Formation of the northern Tabas Basin. The constituents of the storm beds are absent in the fair weather peritidal facies of the Sorkh Shale Formation, but are present throughout the lower limestone member of the Elika Formation. The Tabas Block, a part of the Cimmerian continent in east central Iran, is a rift basin that developed during Early Ordovician-Silurian Paleotethys rifting. Facies and sequence stratigraphic analyses of the Sorkh Shale Formation has revealed additional evidence supporting the Tabas Block as a failed rift basin related to the Paleotethys passive margin. Absence of constituents of the storm beds in the fair weather peritidal facies of the Sorkh Shale Formation, presence of the constituents of the storm beds in the fair weather facies of the Elika Formation (the

  16. 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.

  17. The genus Atheris (Serpentes: Viperidae) in East Africa: phylogeny and the role of rifting and climate in shaping the current pattern of species diversity.

    PubMed

    Menegon, M; Loader, S P; Marsden, S J; Branch, W R; Davenport, T R B; Ursenbacher, S

    2014-10-01

    Past climatic and tectonic events are believed to have strongly influenced species diversity in the Eastern Afromontane Biodiversity Hotspot. We investigated the phylogenetic relationships and historical biogeography of the East African genus Atheris (Serpentes: Viperidae), and explored temporal and spatial relationships between Atheris species across Africa, and the impact of palaeoclimatic fluctuations and tectonic movements on cladogenesis of the genus. Using mitochondrial sequence data, the phylogeny of East African species of Atheris shows congruent temporal patterns that link diversification to major tectonic and aridification events within East Africa over the last 15million years (my). Our results are consistent with a scenario of a delayed direct west-east colonisation of the Eastern Arc Mountains of Atheris by the formation of the western rift. Based on the phylogenetic patterns, this terrestrial, forest-associated genus has dispersed into East Africa across a divided route, on both west-southeasterly and west-northeasterly directions (a C-shaped route). Cladogenesis in the Eastern Arc Mountains and Southern Highlands of Tanzania corresponds to late Miocene and Plio-Pleistocene climatic shifts. Taxonomically, our data confirmed the monophyly of Atheris as currently defined, and reveal four major East African clades, three of which occur in discrete mountain ranges. Possible cryptic taxa are identified in the Atheris rungweensis and A. ceratophora clades.

  18. Aeromagnetic signatures reveal a back-arc basin imposed upon the inherited rifted margin of the East Antarctic craton

    NASA Astrophysics Data System (ADS)

    Armadillo, E.; Ferraccioli, F.; Jordan, T. A.; Bozzo, E.

    2009-12-01

    The Wilkes Subglacial Basin (WSB) represents a largely unexplored, approximately 1400 km-long and up to 600 km-wide subglacial depression, buried beneath the over 3 km-thick East Antarctic Ice Sheet. During the 2005-06 austral summer an extensive aerogeophysical survey was flown to investigate the WSB adjacent to northern Victoria Land (NVL), and included the acquisition of new airborne radar, aeromagnetic and aerogravity data. Several contrasting models for the origin of the basin have been previously proposed, and are based primarily on relatively sparse gravity data. These range from Cenozoic flexure, to distributed crustal extension of unknown age (possibly Mesozoic to Cenozoic), and even compression along the margin of craton. Our recent aeromagnetic data reveal that the basin is structurally controlled and has a tectonic origin, at least adjacent to NVL. The eastern margin of the basin is imposed upon an Early Paleozoic thrust fault belt, which can be traced under the ice using aeromagnetic signatures from exposures in Oates Land and the Ross Sea coast. Aeromagnetic patterns reveal that the western margin of the basin is imposed upon a Proterozoic-age shear zone mapped in the Mertz Glacier, and that is interpreted from geological studies to represent the continuation of a coeval shear zone in Australia. The broad aeromagnetic and satellite magnetic low over the WSB contrasts with the high over the un-reworked Proterozoic craton to the west of the basin, and is interpreted to reflect Neoproterozoic-age sediments deposited along the rifted margin of the craton. Magnetic intrusions within the WSB are interpreted as back-arc plutons that formed later in response to Cambrian-Ordovician age subduction along the paleo-Pacific margin of Gondwana. The aeromagnetic interpretation for a former broad back-arc basin in the WSB is supported by the occurrence of low-grade metasedimentary rocks of back-arc affinity in Oates Land, and also by the similarity in long

  19. Thermo-rheological aspects of crustal evolution during continental breakup and melt intrusion: The Main Ethiopian Rift, East Africa

    NASA Astrophysics Data System (ADS)

    Lavecchia, Alessio; Beekman, Fred; Clark, Stuart R.; Cloetingh, Sierd A. P. L.

    2016-08-01

    The Cenozoic-Quaternary Main Ethiopian Rift (MER) is characterized by extended magmatic activity. Although magmatism has been recognized as a key element in the process of continental breakup, the interaction between melts and intruded lithosphere is still poorly understood. We have performed a 2D thermo-rheological modeling study of continental crust incorporating rheological variations due to melt intrusion-related thermal perturbation. The model is calibrated based on the characteristics of lithologies occurring in the MER and its extensional history, and includes the effect of metamorphism and anatexis on crustal strength and rheological features. During Miocene early rift phases strain in the MER was mainly accommodated through rift border faults, whereas Pliocene-to-recent extension history is characterized by magma assisted rifting with most strain accommodated across magmatic segments in the rift axis. Consequently, very little strain is distributed in the old Pan-African to Paleogene crust during Pliocene to Holocene times. The magmatic activity along the rift axis created ≈ 20 km thick magmatic segments, with growth rate estimated to range from ≈ 3.5 mm yr- 1 to ≈ 6 mm yr- 1. Our model suggests that the strain transfer from Miocene rift border faults to magmatic segments was favored by a moderate increase in crustal strength, due to prograde metamorphism subsequent to the melt-induced thermal perturbation. Under such conditions, crustal stretching may not constitute an effective extension mechanism, thus strain may be preferentially accommodated by melt injection along hot, partially molten magmatic segments. Anatexis has been detected in our simulations, with melt fractions sufficient to break-up the crust solid framework and migrate. This determines local variations of rheological behavior and may induce seismicity. However, resulting melt percentages are not sufficient to induce widespread, crust-derived volcanic activity. Subsequently, volcanism

  20. 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.

  1. 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.

  2. 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.

  3. Rift propagation at craton margin.: Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times

    NASA Astrophysics Data System (ADS)

    Le Gall, B.; Nonnotte, P.; Rolet, J.; Benoit, M.; Guillou, H.; Mousseau-Nonnotte, M.; Albaric, J.; Deverchère, J.

    2008-02-01

    A revised kinematic model is proposed for the Neogene tectono-magmatic development of the North Tanzanian Divergence where the axial valley in S Kenya splits southwards into a wide diverging pattern of block faulting in association with the disappearance of volcanism. Propagation of rifting along the S Kenya proto-rift during the last 8 Ma is first assumed to have operated by linkage of discrete magmatic cells as far S as the Ngorongoro-Kilimanjaro transverse volcanic belt that follows the margin of cratonic blocks in N Tanzania. Strain is believed to have nucleated throughout the thermally-weakened lithosphere in the transverse volcanic belt that might have later linked the S Kenya and N Tanzania rift segments with marked structural changes along-strike. The North Tanzanian Divergence is now regarded as a two-armed rift pattern involving: (1) a wide domain of tilted fault blocks to the W (Mbulu) that encompasses the Eyasi and Manyara fault systems, in direct continuation with the Natron northern trough. The reactivation of basement fabrics in the cold and intact Precambrian lithosphere in the Mbulu domain resulted in an oblique rift pattern that contrasts with the orthogonal extension that prevailed in the Magadi-Natron trough above a more attenuated lithosphere. (2) To the E, the Pangani horst-like range is thought to be a younger (< 1 Ma) structure that formed in response to the relocation of extension S of the Kilimanjaro magmatic center. A significant contrast in the mechanical behaviour of the stretched lithosphere in the North Tanzanian diverging rift is assumed to have occurred on both sides of the Masai cratonic block with a mid-crustal decoupling level to the W where asymmetrical fault-basin patterns are dominant (Magadi-Natron and Mbulu), whereas a component of dynamical uplift is suspected to have caused the topographic elevation of the Pangani range in relation with possible far-travelled mantle melts produced at depth further N.

  4. The Eagle and East Eagle sulfide ore-bearing mafic-ultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution

    NASA Astrophysics Data System (ADS)

    Ding, Xin; Li, Chusi; Ripley, Edward M.; Rossell, Dean; Kamo, Sandra

    2010-03-01

    The Eagle and East Eagle intrusions are small, subvertical dike-like mafic-ultramafic bodies that cut Proterozoic sedimentary strata in the Baraga Basin in northern Michigan. The Eagle intrusion hosts a newly discovered magmatic Ni-Cu-PGE deposit. The nearby East Eagle intrusion also contains sulfide mineralization, but the extent of this mineralization has yet to be determined by further drilling. Both intrusions contain olivine-bearing rocks such as feldspathic peridotite, melatroctolite, and olivine melagabbro. Sulfide accumulations range from disseminated at both Eagle and East Eagle to semimassive and massive at Eagle. U-Pb baddeleyite dating gives a crystallization age of 1107.2 ± 5.7 Ma for the Eagle intrusion, coeval with eruption of picritic basalts at the base of the volcanic succession in the Midcontinent Rift System (MRS). The Fo contents of olivine cores in the Eagle and East Eagle intrusions vary between 75 and 85 mol %, higher than those of olivine in larger layered intrusions in the MRS such as the Duluth Complex. The FeO/MgO ratios and Al2O3 contents of the parental magmas for the Eagle and East Eagle intrusions inferred from olivine and spinel compositions are similar to those of picritic basalts in the base of the MRS volcanic succession. These petrochemical data suggest that the Eagle and East Eagle intrusions are the intrusive equivalents of high-MgO basalts that erupted in the early stages of continental magmatism associated with the development of the rift. Variations in mineral compositions and incompatible trace element ratios suggest that at least three major pulses of magmas were involved in the formation of low-sulfide rocks in the Eagle intrusion. Lower Fo contents of olivine associated with semimassive sulfides as compared to that of olivine in low-sulfide rocks suggest that the magma associated with the semimassive sulfide was more fractionated than the parental magmas of the low-sulfide rocks in the Eagle intrusion. Accumulation of

  5. Crustal faults exposed in the Pito Deep Rift: Conduits for hydrothermal fluids on the southeast Pacific Rise

    NASA Astrophysics Data System (ADS)

    Hayman, Nicholas W.; Karson, Jeffrey A.

    2009-02-01

    The escarpments that bound the Pito Deep Rift (northeastern Easter microplate) expose in situ upper oceanic crust that was accreted ˜3 Ma ago at the superfast spreading (˜142 mm/a, full rate) southeast Pacific Rise (SEPR). Samples and images of these escarpments were taken during transects utilizing the human-occupied vehicle Alvin and remotely operated vehicle Jason II. The dive areas were mapped with a "deformation intensity scale" revealing that the sheeted dike complex and the base of the lavas contain approximately meter-wide fault zones surrounded by fractured "damage zones." Fault zones are spaced several hundred meters apart, in places offset the base of the lavas, separate areas with differently oriented dikes, and are locally crosscut by (younger) dikes. Fault rocks are rich in interstitial amphibole, matrix and vein chlorite, prominent veins of quartz, and accessory grains of sulfides, oxides, and sphene. These phases form the fine-grained matrix materials for cataclasites and cements for breccias where they completely surround angular to subangular clasts of variably altered and deformed basalt. Bulk rock geochemical compositions of the fault rocks are largely governed by the abundance of quartz veins. When compositions are normalized to compensate for the excess silica, the fault rocks exhibit evidence for additional geochemical changes via hydrothermal alteration, including the loss of mobile elements and gain of some trace metals and magnesium. Microstructures and compositions suggest that the fault rocks developed over multiple increments of deformation and hydrothermal fluid flow in the subaxial environment of the SEPR; faults related to the opening of the Pito Deep Rift can be distinguished by their orientation and fault rock microstructure. Some subaxial deformation increments were likely linked with violent discharge events associated with fluid pressure fluctuations and mineral sealing within the fault zones. Other increments were linked with

  6. Prediction, Assessment of the Rift Valley Fever Activity in East and Southern Africa 2006 - 2008 and Possible Vector Control Strategies

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Historical episodic outbreaks of Rift Valley fever (RVF) since the early 1950s have been associated with cyclical patterns (El Niño and La Niña) of El Niño Southern Oscillation (ENSO) phenomenon which results in elevated and widespread rainfall over the RVF endemic areas of Africa. Using satellite ...

  7. Prediction, Assessment of the Rift Valley fever Activity in East and Southern Africa 2006 - 2008 and Possible Vector Control Strategies

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Historical outbreaks of Rift Valley fever (RVF) since the early 1950s have been associated with cyclical patterns of the El Nino/Southern Oscillation (ENSO) phenomenon which results in elevated and widespread rainfall over the RVF endemic areas of Africa. Using satellite measurements of global and ...

  8. Kinematics of the oblique faults in the east central Gulf of Suez Rift, Wadi Araba, Sinai Peninsula, Egypt

    NASA Astrophysics Data System (ADS)

    Abdeen, Mamdouh; Abdelmaksoud, Ashraf

    2014-05-01

    The Oligo-Miocene Gulf of Suez rift is characterized by four fault trends; a rift-parallel trend, two trends oblique to the rift trend and a cross trend. The rift-parallel trend strikes 310o to 340o and is referred to as the Clysmic trend. The two trends, which are oblique to the Clysmic trend, strike 350o to 030o and 280o to 310o; the first has been referred to as the north-oblique (N-oblique), and the second as the northwest-oblique (NW-oblique). The cross trend includes faults nearly orthogonal to the Clysmic trend i.e. they strike between 050o and 075o. Image interpretation and detailed field mapping and structural studies at a scale of 1: 20,000 of the Wadi Araba area in southwest Sinai Peninsula indicate e Clysmic faults are mostly normal showing major dip-slip movements. The oblique faults were found to be younger than the Clysmic faults and that the N-oblique faults are characterized by major sinistral strike-slip movement, while the NW-oblique faults are characterized by major dextral strike-slip movement. Cross cutting relationship, geometry and palaeostress analysis indicate that the oblique faults are conjugate Riedel shears originated due to NE to NNE extension related to the Aqaba-Levant transform that has been active since the Middle Miocene.

  9. 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

  10. 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

  11. 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.

  12. 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.

  13. 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

  14. 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

  15. 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.

  16. 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.

  17. 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.

  18. Geochemical characterization of fluids along the Dead Sea Rift: implications for fluids sources and regional geodynamic setting

    NASA Astrophysics Data System (ADS)

    Inguaggiato, Claudio; Censi, Paolo; D'Alessandro, Walter; Zuddas, Pierpaolo

    2016-04-01

    The Dead Sea Fault where a lateral displacement between the African and Arabian plates occurs is characterized by anomalous heat flux in the northern Israel area close to the border with Syria and Jordan (Shalev et al., 2012). The concentrations of He and CO2, and isotopic composition of He and total dissolved inorganic carbon were studied in cold and thermal waters collected along the Dead Sea Fault, in order to investigate the source of volatiles and their relationship with the tectonic framework of the Dead Sea Fault. The waters with higher temperature (up to 57.2 ° C) are characterized by higher amounts of CO2and helium (up to 55.72 and 1.91*10-2 cc l-1, respectively). Helium isotopic data (R/Ra from 0.11 to 2.14) and 4He/20Ne ratios (0.41 - 106.86) show the presence of deep-deriving fluids consisting of a variable mixture of mantle and crust end-members, with the former reaching up to 35%. Carbon isotope signature of total dissolved carbon from hot waters falls within the range of magmatic values, suggesting the delivery of deep-seated CO2. The geographical distribution of helium isotopic data and isotopic carbon (CO2) values coupled with (CO2/3He ratios) indicate a larger contribution of mantle-derived fluids affecting the northern part of the investigated area, where the waters reach the highest temperature and anomalous heat flux was recognized by Shalev et al. (2012). Such occurrence is probably favoured by the peculiar tectonic framework recognized in the northern part of Israel (Segev et al., 2006), including a Moho discontinuity up-rise and/or the presence of a deep fault system coupled with the recent magmatic activity. References: Segev, A., Rybakov, M., Lyakhovsky, V, Hofstetter, A, Tibor, G., Goldshmidt, V., 2006. The structure, isostasy and gravity field of the Levant continental margin and the southeast Mediterranean area. Tectonophysics 425, 137-157. Shalev, E., Lyakhosky, V., Weinstein, Y., Ben-Avraham, Z., 2013. The thermal structure of Israel

  19. How Closed are Closed Lakes in Rifts? Significance of Hydraulic Gradients for the Budgets of Paleo-Lakes in East Africa

    NASA Astrophysics Data System (ADS)

    Olaka, L. A.

    2015-12-01

    The hydrogeology of rift lakes is complex due to the potential influence of faults and porous volcanic and volcanoclastic media on groundwater flow. We conducted a comprehensive study that integrated geological and hydrogeological data as well as the application of a linear decay model to estimate the groundwater flow between the two of the best-studied lake systems in East Africa, the adjacent Lakes Naivasha and Nakuru- Elmenteita in the Central Kenya Rift. Whereas both lake basins host relatively shallow lakes today, paleo-shorelines and sediments suggest >100 m deep lakes during a wetter climate during the Early Holocene during the so-called African Humid Period. Stable isotope data show variations form highly depleted to more enriched d18O waters. The linear-reservoir depletion model simulates the decline of the Early Holocene lakes in both basins to the modern levels. The altitude difference of ca. 100 m of both paleo-lake levels enables us to estimate the duration of the groundwater decline and the connectivity of the two basins via the Eburru/Gilgil barrier. The results suggest a decline of the groundwater levels during ca. 5 kyrs if there is no recharge, and between 2-2.7 kyrs based on the modern recharge of 0.52 m/yr as the end members of the delay time introduced by subsurface water flow to the hydrology of the lake system. The latter value suggests that ca. 40.95 cubic kilometres of water flowed from Lake Naivasha to Nakuru- Elmenteita at maximum lake level in the Early Holocene following the hydraulic gradient concurrent to the topographic slope. The unexpectedly large volume, more than half of the volume of the paleo-Lake Naivasha during the Early Holocene, emphasizes the importance of groundwater in hydrological modelling of paleo-lakes in rifts. Moreover, the subsurface connectivity of rift lakes also causes a significant lag time to the system introducing a nonlinear component to the system that has to be considered while interpreting paleo

  20. 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

  1. 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

    We used Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) and aeromagnetic data to map extensional structures in the Makgadikgadi Pans in northeastern Botswana. These pans are a major morphological feature in Southern Africa characterized by the presence of low lying and flat topography with the highest elevation of 945 m. This topography was a result of multiple filling and desiccation of paleo-lakes that accompanied alternation of wetter and dryer climate during the Late Quaternary period. The objective of our study was to map the extent and distribution of normal faults using their morphological expression and magnetic signature, and examine their relationship with paleo-shorelines of the pans. We: (1) Created a hill shade relief map from the SRTM DEM; (2) Extracted regional NW-SE trending topographic profiles across the pans; (3) Constructed displacement profiles for major normal faults; and (4) Created tilt derivative images from the aeromagnetic data. We found that: (1) The northeastern part of the pan is dissected by three morphologically-defined NE-trending normal faults. The along strike continuity of these faults is in the range of 75 and 170 km and they are spaced at ~30 km apart from each other. (2) The topographic profiles suggest that the exposed minimum vertical displacement (EMVD), defined by poorly developed escarpments, is in the range of 0 m and 49 m. (3) The displacement profiles of the faults is characterized by maximum EMVD in the middle of the faults and that it decays towards the fault tips. These faults are also apparent in the aeromagnetic maps where they seem to displace E-W trending Karoo-age dikes. (4) At least the outer paleo-shoreline of the pans is modified by the NE-trending faults. This suggests that the faults are younger than the paleo-shorelines, which is suggested to have been developed between 500 and 100 ka. Traditionally, the southwestern extension of the East African Rift System has been assigned to the

  2. 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.

  3. 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

  4. Rift basins - Origin, history, and distribution

    NASA Technical Reports Server (NTRS)

    Burke, K. C.

    1985-01-01

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

  5. 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.

  6. 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.

  7. Infiltration of late Palaeozoic evaporative brines in the reelfoot rift: A possible salt source for Illinois Basin formation waters and MVT mineralizing fluids

    USGS Publications Warehouse

    Rowan, E.L.; De Marsily, G.

    2001-01-01

    Salinities and homogenization temperatures of fluid inclusions in Mississippi Valley-type (MVT) deposits provide important insights into the regional hydrology of the Illinois basin/Reelfoot rift system in late Palaeozoic time. Although the thermal regime of this basin system has been plausibly explained, the origin of high salinities in the basin fluids remains enigmatic. Topographically driven flow appears to have been essential in forming these MVT districts, as well as many other districts worldwide. However, this type of flow is recharged by fresh water making it difficult to account for the high salinities of the mineralizing fluids over extended time periods. Results of numerical experiments carried out in this study provide a possible solution to the salinity problem presented by the MVT zinc-lead and fluorite districts at the margins of the basin system. Evaporative concentration of surface water and subsequent infiltration into the subsurface are proposed to account for large volumes of brine that are ultimately responsible for mineralization of these districts. This study demonstrates that under a range of geologically reasonable conditions, brine infiltration into an aquifer in the deep subsurface can coexist with topographically driven flow. Infiltration combined with regional flow and local magmatic heat sources in the Reelfoot rift explain the brine concentrations as well as the temperatures observed in the Southern Illinois and Upper Mississippi Valley districts.

  8. Expected fluid residence times, thermal breakthrough, and tracer test design for characterizing a hydrothermal system in the Upper Rhine Rift Valley

    NASA Astrophysics Data System (ADS)

    Ghergut, I.; Meixner, J.; Rettenmaier, D.; Maier, F.; Nottebohm, M.; Ptak, T.; Sauter, M.

    2012-04-01

    Relying on the structural-hydrogeological model proposed by J. Meixner (2009) for a particular hydrothermal system in South-West Germany (on the East side of the Upper Rhine Rift, this reservoir being used to demonstrate electricity production by means of a well doublet), we set up a distributed-parameter model (using Feflow) enabling to numerically simulate fluid ages, temperature evolutions and tracer test signals for a number of contrasting assumptions w. r. to (a) the nature of boundary conditions and hydrogeological characteristics of remotely situated, large-scale natural faults, (b) the degree of permeability contrast between different system compartments, (c) the hydrogeological characteristics of a naturally-occurring fault, located between injection and production wells. It appears that a spike dimensioning allowing for tracer signals to become detectable during the first three years after tracer injection in all of the contrasting a/b/c scenarios is not feasible in practice. In some of the a/b/c cases considered, the system will act like a very large reservoir, with fluid residence times in the order of decades, and extreme dilution of injected tracers. Even using preparative-scale cleaning of samples, brine separation, sample enrichment by solid phase extraction, evaporative concentrating etc. followed by state-of-the-art chromatography techniques to separate between tracer and natural background, it will not be possible to lower tracer detection limits below a certain threshold, which is mainly dictated by the amount of certain naturally-occurring aromatics in the reservoir fluids. On practical reasons, the spike dimensioning will be limited to some hundred kilogram of one or two organic tracers. This implies that part of the above-mentioned, contrasting a/b/c scenarios will remain indistinguishable during the first three years after tracer injection. However, for this reservoir structure, there is not a bijective correspondence between early

  9. 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.

  10. Composition of the Southeast Mariana Forearc Rift pillow lavas : interaction between adiabatic decompression mantle melting and ultra-shallow slab-derived fluids

    NASA Astrophysics Data System (ADS)

    Ribeiro, J. M.; Stern, R. J.; Kelley, K. A.; Ishizuka, O.; Ren, M.; Ohara, Y.; Reagan, M. K.; Bloomer, S.; Anthony, E.

    2009-12-01

    The Mariana intraoceanic arc system is related to the subduction of the Pacific plate beneath the Philippine Sea plate. A northward-propagating forearc rift extending from the southernmost backarc basin spreading ridge to the trench, called the SE Mariana Forearc Rift (SEMFR), has been inferred by regional HMR-1 bathymetric and backscatter mapping south of Guam (see Martinez & Stern abstract). SEMFR formed by lateral stretching as a result of slab rollback and collision of the Caroline ridge with the IBM arc. This forearc rift provides an unusual opportunity to study melts generated very shallow (~ 40 km deep) above a subduction zone. Forearc rifts are extensional zones occurring in subduction settings providing the opportunity to sample rocks produced by adiabatic asthenosphere decompression and fluid-metasomatized mantle melting. During YK08-08 in July 2008, manned submersible Shinkai 6500 Dive 1096 dived in SEMFR and sampled a 663 m thick section of fresh tholeiitic pillow lavas. These lavas are composed of upper primitive basalts (Mg# = 61 - 67) and lower basaltic andesites (Mg# = 49 - 51). The upper series phenocrysts are olivine (Fo85-87), diopside, and plagioclase (An80-83) with scattered olivine xenocrysts (3 mm, Fo92) hosting Cr-spinel (Cr# = 68 - 69). Lower series lavas contain phenocrysts of olivine (Fo76-79), augite and two plagioclases (An66-71 and An80-86), perhaps indicating magma mixing. Flat REE patterns, and high Mg# ( > 60) of the upper basalts indicate generation by fractional melting of spinel peridotite. LA-ICP-MS analysis show that lower series clinopyroxenes formed in equilibrium with more REE-enriched melts than their host rock, also supporting an interpretation of magma mixing. Discrimination diagrams using Th-Ba-Nb-Yb systematics (Pearce, 2008, Lithos, v. 100; Pearce and Stern, 2006, Geophysical Monograph Series 166, AGU) show that both series lavas have composition similar to that of Mariana backarc basin but with higher Ba/Nb content

  11. Composition and spatial evolution of mantle and fluids released beneath the active Southeast Mariana Forearc Rift: do they have arc or backarc basin signatures?

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    Fluids of progressively changing composition are released from the subducting slab. Whereas the composition and effects of deep fluids are understood from studying arcs and backarc basin (BAB) lavas, those released at shallower depths beneath forearcs are less well known. Forearc rifts give us a unique opportunity to study the composition of ultra-shallow subduction-related fluids. At the southern end of the Mariana arc, the S.E. Mariana Forearc Rift (SEMFR), was discovered by HMR-1 sonar swath mapping (Martinez et al. 2000, JGR), and investigated in July 2008 by the manned submersible Shinkai 6500. The rift extends from the trench to the BAB spreading axis, where a magma chamber was recently documented (Becker et al., 2010, G-cubed). SEMFR is opening due to continued widening of the Mariana Trough BAB. Two suites of tholeiitic pillow lavas were recovered from the N.E. flank of the rift (dive 1096; slab depth ~ 30 ± 5 km), indicating recent magmatic activity. Dive 1096 lavas consist of upper primitive basalts (Mg# ≥ 60) and lower fractionated, basaltic andesites (Mg# < 60), separated by a thin sediment layer. Geochemical and isotopic studies show that these lavas were produced by extensive hydrous melting (≥ 15%) of a common depleted MORB-like mantle (Nb/Yb ~ 1, ɛNd ~ 9.3), likely S. Mariana BAB mantle, that interacted with < 3% metasomatic fluids. Thermobarometry constraints (Lee et al., 2009, EPSL) suggest that the primary melts equilibrated with the mantle at ~ 28 km, just above the slab, with a mean temperature ~1230°C. The fluid was enriched in fluid-mobile elements (Rb, Ba, K, U, Sr, Pb, Cs), mobilized from the ultra-shallow slab at low temperature, as well as melt-mobile elements (e.g. Th, LREE), released deeper and hotter. These fluids contribute 100% Cs, 97% Rb, 99% Ba, 69% Th, 74% U, 80% K, 83% Pb, 71% Sr, 45% La, 33% Ce, 20% Nd and 11% Sm to the magma. SEMFR lavas acquired BAB-like deep subduction component as well as arc-like ultra

  12. 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

  13. Analysis of Marine Gravity Anomalies in the Ulleung Basin (East Sea/Sea of Japan) and Its Implications for the Architecture of Rift-Dominated Backarc Basin

    NASA Astrophysics Data System (ADS)

    Lee, Sang-Mook; Kim, Yoon-Mi

    2016-04-01

    Marginal basins locate between the continent and arc islands often exhibit diverse style of opening, from regions that appear to have formed by well-defined and localized spreading center (manifested by the presence of distinct seafloor magnetic anomaly patterns) to those with less obvious zones of extension and a broad magmatic emplacement most likely in the lower crust. Such difference in the style of back-arc basin formation may lead to marked difference in crustal structure in terms of its overall thickness and spatial variations. The Ulleung Basin, one of three major basins in the East Sea/Sea of Japan, is considered to represent a continental rifting end-member of back-arc opening. Although a great deal of work has been conducted on the sedimentary sections in the last several decades, the deep crustal sections have not been systematically investigated for long time, and thus the structure and characteristics of the crust remain poorly understood. This study examines the marine gravity anomalies of the Ulleung Basin in order to understand the crustal structure using crucial sediment-thickness information. Our analysis shows that the Moho depth in general varies from 16 km at the basin center to 22 km at the margins. However, within the basin center, the inferred thickness of the crust is more or less the same (10-12 km), thus by varying only about 10-20% of the total thickness, contrary to the previous impression. The almost-uniformly-thick crust that is thicker than a normal oceanic crust (~ 7 km) is consistent with previous observations using ocean bottom seismometers and recent deep seismic results from the nearby Yamato Basin. Another important finding is that small residual mantle gravity anomaly highs exist in the northern part of the basin. These highs are aligned in the NNE-SSW direction which correspond to the orientation of the major tectonic structures on the Korean Peninsula, raising the possibility that, though by a small degree, they are a

  14. 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.

  15. Geothermal fluids to irrigate energy crops on Imperial East Mesa Desert, California

    SciTech Connect

    Robinson, F.E.; Thomas, T.R.; Singh, K.

    1981-10-01

    Geothermal fluid at 2200 parts per million(ppM) total dissolved solids from the East Mesa, California was compared to 1400 ppM ground water from a 100 m well as irrigation sources for Beta vulgaris (L.) USH-11 sugar beet and Tamarix aphylla (L.) athel, a biomass and windbreak tree. In 1980 there was no significant difference between the yields from the two waters. In 1981, there was no significant difference between biomass production from the waters on the athel. The geothermal water produced a larger yield of sucrose than the ground water. The data suggest that raising sugar beets with either water is feasible on the East Mesa.

  16. 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.

  17. 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

  18. 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.

  19. 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

  20. Cenozoic rift formation in the northern Caribbean

    NASA Technical Reports Server (NTRS)

    Mann, P.; Burke, K.

    1984-01-01

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

  1. Extraordinary phase separation and segregation in vent fluids from the southern East Pacific Rise

    USGS Publications Warehouse

    Von Damm, K. L.; Lilley, M.D.; Shanks, Wayne C.; Brockington, M.; Bray, A.M.; O'Grady, K. M.; Olson, E.; Graham, A.; Proskurowski, G.

    2003-01-01

    The discovery of Brandon vent on the southern East Pacific Rise is providing new insights into the controls on midocean ridge hydrothermal vent fluid chemistry. The physical conditions at the time ofsampling (287 bar and 405??C) place the Brandon fluids very close to the critical point of seawater (298 bar and 407??C). This permits in situ study of the effects of near criticalphenomena, which are interpreted to be the primary cause of enhanced transition metal transport in these fluids. Of the five orifices on Brandon sampled, three were venting fluids with less than seawater chlorinity, and two were venting fluids with greater than seawater chlorinity. The liquid phase orifices contain 1.6-1.9 times the chloride content of the vapors. Most other elements, excluding the gases, have this same ratio demonstrating the conservative nature of phase separation and the lack of subsequent water-rock interaction. The vapor and liquid phases vent at the same time from orifices within meters of each other on the Brandon structure. Variations in fluid compositions occur on a time scale of minutes. Our interpretation is that phase separation and segregation must be occurring 'real time' within the sulfide structure itself. Fluids from Brandon therefore provide an unique opportunity to understand in situ phase separation without the overprinting of continued water-rock interaction with the oceanic crust, as well as critical phenomena. ?? 2002 Elsevier Science B.V. All rights reserved.

  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. 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.

  4. Evolution along the Great Rift Valley: phenotypic and genetic differentiation of East African white-eyes (Aves, Zosteropidae).

    PubMed

    Habel, Jan Christian; Borghesio, Luca; Newmark, William D; Day, Julia J; Lens, Luc; Husemann, Martin; Ulrich, Werner

    2015-11-01

    The moist and cool cloud forests of East Africa represent a network of isolated habitats that are separated by dry and warm lowland savannah, offering an opportunity to investigate how strikingly different selective regimes affect species diversification. Here, we used the passerine genus Zosterops (white-eyes) from this region as our model system. Species of the genus occur in contrasting distribution settings, with geographical mountain isolation driving diversification, and savannah interconnectivity preventing differentiation. We analyze (1) patterns of phenotypic and genetic differentiation in high- and lowland species (different distribution settings), (2) investigate the potential effects of natural selection and temporal and spatial isolation (evolutionary drivers), and (3) critically review the taxonomy of this species complex. We found strong phenotypic and genetic differentiation among and within the three focal species, both in the highland species complex and in the lowland taxa. Altitude was a stronger predictor of phenotypic patterns than the current taxonomic classification. We found longitudinal and latitudinal phenotypic gradients for all three species. Furthermore, wing length and body weight were significantly correlated with altitude and habitat type in the highland species Z. poliogaster. Genetic and phenotypic divergence showed contrasting inter- and intraspecific structures. We suggest that the evolution of phenotypic characters is mainly driven by natural selection due to differences in the two macro-habitats, cloud forest and savannah. In contrast, patterns of neutral genetic variation appear to be rather driven by geographical isolation of the respective mountain massifs. Populations of the Z. poliogaster complex, as well as Z. senegalensis and Z. abyssinicus, are not monophyletic based on microsatellite data and have higher levels of intraspecific differentiation compared to the currently accepted species. PMID:26640665

  5. Evolution along the Great Rift Valley: phenotypic and genetic differentiation of East African white-eyes (Aves, Zosteropidae).

    PubMed

    Habel, Jan Christian; Borghesio, Luca; Newmark, William D; Day, Julia J; Lens, Luc; Husemann, Martin; Ulrich, Werner

    2015-11-01

    The moist and cool cloud forests of East Africa represent a network of isolated habitats that are separated by dry and warm lowland savannah, offering an opportunity to investigate how strikingly different selective regimes affect species diversification. Here, we used the passerine genus Zosterops (white-eyes) from this region as our model system. Species of the genus occur in contrasting distribution settings, with geographical mountain isolation driving diversification, and savannah interconnectivity preventing differentiation. We analyze (1) patterns of phenotypic and genetic differentiation in high- and lowland species (different distribution settings), (2) investigate the potential effects of natural selection and temporal and spatial isolation (evolutionary drivers), and (3) critically review the taxonomy of this species complex. We found strong phenotypic and genetic differentiation among and within the three focal species, both in the highland species complex and in the lowland taxa. Altitude was a stronger predictor of phenotypic patterns than the current taxonomic classification. We found longitudinal and latitudinal phenotypic gradients for all three species. Furthermore, wing length and body weight were significantly correlated with altitude and habitat type in the highland species Z. poliogaster. Genetic and phenotypic divergence showed contrasting inter- and intraspecific structures. We suggest that the evolution of phenotypic characters is mainly driven by natural selection due to differences in the two macro-habitats, cloud forest and savannah. In contrast, patterns of neutral genetic variation appear to be rather driven by geographical isolation of the respective mountain massifs. Populations of the Z. poliogaster complex, as well as Z. senegalensis and Z. abyssinicus, are not monophyletic based on microsatellite data and have higher levels of intraspecific differentiation compared to the currently accepted species.

  6. Detection and Response for Rift Valley fever

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever is a viral disease that impacts domestic livestock and humans in Africa and the Middle East, and poses a threat to military operations in these areas. We describe a Rift Valley fever Risk Monitoring website, and its ability to predict risk of disease temporally and spatially. We al...

  7. 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.

  8. Strontium isotope constraints on fluid flow in the upper oceanic crust at the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Gillis, Kathryn M.; Coogan, Laurence A.; Pedersen, Rolf

    2005-03-01

    Strontium isotopes are useful tracers of fluid-rock interaction in marine hydrothermal systems and provide a potential way to quantify the amount of seawater that passes through these systems. We have determined the whole-rock Sr-isotopic compositions of a section of upper oceanic crust that formed at the fast-spreading East Pacific Rise, now exposed at Hess Deep. This dataset provides the first detailed comparison for the much-studied Ocean Drilling Program (ODP) drill core from Site 504B. Whole-rock and mineral Sr concentrations indicate that Sr-exchange between hydrothermal fluids and the oceanic crust is complex, being dependent on the mineralogical reactions occurring; in particular, epidote formation takes up Sr from the fluid increasing the 87Sr/ 86Sr of the bulk-rock. Calculating the fluid-flux required to shift the Sr-isotopic composition of the Hess Deep sheeted-dike complex, using the approach of Bickle and Teagle [1] [M.J. Bickle, D.A.H. Teagle, Strontium alteration in the Troodos ophiolite: implications for fluid fluxes and geochemical transport in mid-ocean ridge hydrothermal systems. Earth Planet. Sci. Lett. 113 (1992) 219-237] gives a fluid-flux similar to that determined for ODP Hole 504B. This suggests that the level of isotopic exchange observed in these two regions is probably typical for modern oceanic crust. Unfortunately, uncertainties in the modeling approach do not allow us to determine a fluid-flux that is directly comparable to fluxes calculated by other methods.

  9. 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

  10. 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

  11. 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

  12. Geochemistry of Fast-Spreading Lower Oceanic Crust: Results from Drilling at the Hess Deep Rift (ODP Leg 147 and IODP Expedition 345; East Pacific Rise)

    NASA Astrophysics Data System (ADS)

    Godard, M.; Falloon, T.; Gillis, K. M.; Akizawa, N.; de Brito Adriao, A.; Koepke, J.; Marks, N.; Meyer, R.; Saha, A.; Garbe-Schoenberg, C. D.

    2014-12-01

    The Hess Deep Rift, where the Cocos Nazca Ridge propagates into the young, fast-spread East Pacific Rise crust, exposes a dismembered, but nearly complete, lower crustal section. The extensive exposures of the plutonic crust were drilled at 3 sites during ODP Leg 147 (Nov. 1992-Jan. 1993) and IODP Expedition 345 (Dec. 2012-Feb. 2013). We report preliminary results of a bulk rock geochemical study (major and trace elements) carried out on 109 samples representative of the different drilled lithologies. The shallowest gabbroic rocks were sampled at ODP Site 894. They comprise gabbronorite, gabbro, olivine gabbro and gabbronorite. They have evolved compositions with Mg# 39-55, Yb 4-8 x chondrite and Eu/Eu* 1-1.6. Olivine gabbro and troctolite were dominant at IODP Site U1415, with minor gabbro, gabbronorite and clinopyroxene oikocryst-bearing troctolite and gabbro. All U1415 gabbroic rocks have primitive compositions except for one gabbronorite rubble that is similar in composition to the shallow gabbros. Olivine gabbro, gabbro and gabbronorite overlap in composition: they have high Mg# (79-87) and Ni (130-570 ppm), low TiO2 (0.1-0.3 wt.%) and Yb (1.3-2.3 x chondrite) and positive Eu anomaly (Eu/Eu*=1.9-2.7). Troctolite has high Mg# (81-89), Ni (260-1500 ppm) and low TiO2 (<0.1 wt.%) and Yb (~0.5xchondrite) and large Eu/Eu* (>4). ODP Site 895 recovered sequences of highly depleted harzburgite, dunite and troctolite (Yb down to <0.1xchondrite) that are interpreted as a mantle-crust transition zone. Basalts were recovered at Sites 894 and U1415: they have low Yb (0.5-0.9xN6MORB) and are depleted in the most incompatible elements (Ce/Yb=0.6-0.9xN-MORB). The main geochemical characteristics of Site U1415 and 894 gabbroic rocks are consistent with formation as a cumulate sequence from a common parental MORB melt; troctolites are the most primitive end-member of this sequence. They overlap in composition with the most primitive of slow and fast spread crust gabbroic rocks.

  13. Occurrences of Orthopyroxene in the "Multi-textured" Layered Gabbros from the Hess Deep Rift, East Pacific Rise (the Site U1415P, IODP Expedition 345)

    NASA Astrophysics Data System (ADS)

    Hoshide, T.; Machi, S.; Maeda, J. I.

    2015-12-01

    IODP Exp.345 drilled three main holes (Holes U1415 I, J & P) from the lowermost plutonic crust exposed at the Hess Deep Rift, East Pacific Rise and primitive layered gabbroic rocks were newly discovered from these holes (Gillis et al., 2014). One of the mysteries about the layered gabbros is the fact that Opx, which is considered to appear in the late stage of crystallization on the basis of crystallization experiments of MORB, occurs as a dominant phase in many of the layered gabbros. In this presentation, we report the occurrence of Opx from the Hole U1415P and consider the significance of Opx in the origin of the layered gabbros. Hole U1415P (about 100m in thickness) is divided into two units, the upper Multi-textured Layered Gabbro Series (MLGS) and lower Troctolite Series (TS). Gabbroic rocks from the MLGS contain Opx (< 4 vol%) and are macroscopically classified into Opx-bearing olivine gabbro. However, these rocks are mesoscopically (on cm scale) inhomogeneous and have a great variation of mode, grain size and texture. On the other hand, TS consists of homogeneous troctolites and Opx rarely occurs in the series. The occurrences of Opx from the MLGS are as follows: (i) coarse-grained Opx+Cpx+Pl vein parallel to the layered structure of the surrounding troctolite (ii) undeformed Opx+Pl veinlets in kinked Ol (iii) Opx in the concave of anhedral Ol (iv) Opx rimming Cr-spl crystals in contact with Ol. The occurrence of Opx like (ii) and (iii) resembles the texture which is considered to be formed by the reaction between mantle peridotite and a SiO2-saturated melt (e.g, Piccardo et al., 2007). The facts that Opx is often found in association with Cr-spl and Cr-spl lamellae occur in pyroxenes of the Opx+Cpx+Pl vein suggest that the SiO2-saturated melt which reacted with Ol was rich in chromium. In addition, Cr-spl crystals rimmed by Opx contain multiphase-solid inclusions. The inclusions should be key in understanding the chemical composition of the reacted melt.

  14. Characterization of Microbial Communities Associated With Deep-Sea Hydrothermal Vent Animals of the East Pacific Rise and the Galápagos Rift

    NASA Astrophysics Data System (ADS)

    Ward, N.; Page, S.; Heidelberg, J.; Eisen, J. A.; Fraser, C. M.

    2002-12-01

    The composition of microbial communities associated with deep-sea hydrothermal vent animals is of interest because of the key role of bacterial symbionts in driving the chemosynthetic food chain of the vent system, and also because bacterial biofilms attached to animal exterior surfaces may play a part in settlement of larval forms. Sequence analysis of 16S ribosomal RNA (rRNA) genes from such communities provides a snapshot of community structure, as this gene is present in all Bacteria and Archaea, and a useful phylogenetic marker for both cultivated microbial species, and uncultivated species such as many of those found in the deep-sea environment. Specimens of giant tube worms (Riftia pachyptila), mussels (Bathymodiolus thermophilus), and clams (Calyptogena magnifica) were collected during the 2002 R/V Atlantis research cruises to the East Pacific Rise (9N) and Galápagos Rift. Microbial biofilms attached to the exterior surfaces of individual animals were sampled, as were tissues known to harbor chemosynthetic bacterial endosymbionts. Genomic DNA was extracted from the samples using a commercially available kit, and 16S rRNA genes amplified from the mixed bacterial communities using the polymerase chain reaction (PCR) and oligonucleotide primers targeting conserved terminal regions of the 16S rRNA gene. The PCR products obtained were cloned into a plasmid vector and the recombinant plasmids transformed into cells of Escherichia coli. Individual cloned 16S rRNA genes were sequenced at the 5' end of the gene (the most phylogenetically informative region in most taxa) and the sequence data compared to publicly available gene sequence databases, to allow a preliminary assignment of clones to taxonomic groups within the Bacteria and Archaea, and to determine the overall composition and phylogenetic diversity of the animal-associated microbial communities. Analysis of Riftia pachyptila exterior biofilm samples revealed the presence of members of the delta and

  15. Geology and ore fluid geochemistry of the Jinduicheng porphyry molybdenum deposit, East Qinling, China

    NASA Astrophysics Data System (ADS)

    Li, Hongying; Ye, Huishou; Wang, Xiaoxia; Yang, Lei; Wang, Xiuyuan

    2014-01-01

    Jinduicheng deposit is a giant Mesozoic porphyry Mo system deposit in the East Qinling molybdenum belt, Shaanxi Province, China. The mineralization is associated with the I-type Jinduicheng granite porphyry. Both the porphyry stock and country rocks underwent intense hydrothermal alteration. The alteration, with increasing distance from the parent intrusion, changes from silicification, through potassic and phyllic assemblages, carbonation, to propylitic assemblages. Molybdenite, the dominant ore mineral, occurs in veinlets, most of which are hosted by the altered country rocks, with less than 25% of the ore in the porphyry body. The hydrothermal system comprises four stages, including pre-ore quartz and K-feldspar; two ore stages of quartz, K-feldspar, molybdenite, and Pb- And Zn-bearing sulfides; and post-ore quartz and carbonate. Six main types of primary fluid inclusions are present in hydrothermal quartz, including two-phase aqueous, one-phase aqueous, three-phase CO2-bearing, CO2-dominated fluid inclusions, gas inclusions, and melt inclusions. The homogenization temperatures of fluid inclusions range from 210 to 290 °C in the pre-ore stage, 150-310 °C in ore stage I, 150-360 °C in the ore stage II, and 195-325 °C in the post-ore stage quartz. Estimated salinities of the ore-forming fluids range from 6.9 to 13.5, 4.3 to 12.3, 6.2 to 12.4, and 3.4 to 9.9 wt.% NaCl equiv. in stages 1-4, respectively. The δ34S values of pyrite in the two ore stages range from 2.8‰ to 4.3‰, whereas the δ34S values of molybdenite range from 2.9‰ to 6.2‰. The data suggest both magmatic and crustal sources of sulfur. The δD and δ18O values for the hydrothermal fluids are -57.2‰ to -84.4‰ and 8.0‰ to -3.2‰, respectively. The fluid inclusion and stable data indicate that the pre-ore hydrothermal fluids were mostly of magmatic origin, but the fluids responsible for ore deposition were mixed magmatic and meteoric, and eventually meteoric water dominated the system

  16. 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.

  17. 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

  18. Abundance and isotope systematics of carbon in subglacial basalts, geothermal gases and fluids from Iceland's rift zones

    NASA Astrophysics Data System (ADS)

    Barry, P. H.; Hilton, D. R.; Fueri, E.; Halldorsson, S. A.; Fischer, T. P.; Gronvold, K.

    2010-12-01

    P. H. BARRY1*, D. R. HILTON1, E. FÜRI1, S.A. HALLDÓRSON1, T.P. FISCHER2, K. GRONVOLD3 1 Scripps Institution of Oceanography, UCSD, La Jolla, California 92093, USA (*Correspondence: pbarry@ucsd.edu). 2University of New Mexico, Albuquerque, NM 87131, USA. 3University of Iceland, Askja, Sturlugata 7, IS-101, Reykjavik, Iceland Carbon dioxide (CO2) is the dominant non-aqueous volatile species found in oceanic basalts and geothermal fluids and serves as the carrier gas for trace volatiles such as He and other noble gases. The aim of this study is to identify the superimposed effects of degassing and crustal contamination on the CO2 systematics of the Icelandic hotspot in order to reveal and characterize the carbon abundance and isotopic features of the underlying mantle source. Our approach involves coupling CO2 with He, utilizing the sensitivity of 3He/4He ratios to reveal mantle and crustal inputs. We report new C-isotope (δ13C) and abundance characteristics for a suite of 47 subglacial basalts and 50 geothermal gases and fluids from Iceland. CO2 contents in hyaloclastite glasses are extremely low (10-100 ppm) and likely residual following extensive degassing whereas geothermal fluids are dominated by CO2 (>90 %). C-isotopes range from -27.2 to -3.6 ‰ (vs. PDB) for basalts and from -18.8 to 2.86 ‰ (vs. PDB) for geothermal samples (mean = -4.2 ± 3.6 ‰). CO2/3He ratios range from 108 to 1012 for basalts and from 105 to 1012 for geothermal samples: In both cases, our results extend He-CO2 relationships over a much broader range than reported previously [1]. Taken together, these data suggest that several processes including mixing, degassing, and/or syn- or post-eruptive crustal contamination may act to modify CO2 source characteristics. Equilibrium degassing models are compatible with ~75 % of the basalt data, and preliminary results indicate that initial Icelandic source characteristics are ~500 ppm CO2 and δ13C ~ -5 ‰ (vs. PDB). These values are high

  19. 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.

  20. 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.

  1. 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.

  2. 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

  3. 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.

  4. Prediction, Assessment of the Rift Valley Fever Activity in East and Southern Africa 2006–2008 and Possible Vector Control Strategies

    PubMed Central

    Anyamba, Assaf; Linthicum, Kenneth J.; Small, Jennifer; Britch, Seth C.; Pak, Edwin; de La Rocque, Stephane; Formenty, Pierre; Hightower, Allen W.; Breiman, Robert F.; Chretien, Jean-Paul; Tucker, Compton J.; Schnabel, David; Sang, Rosemary; Haagsma, Karl; Latham, Mark; Lewandowski, Henry B.; Magdi, Salih Osman; Mohamed, Mohamed Ally; Nguku, Patrick M.; Reynes, Jean-Marc; Swanepoel, Robert

    2010-01-01

    Historical outbreaks of Rift Valley fever (RVF) since the early 1950s have been associated with cyclical patterns of the El Niño/Southern Oscillation (ENSO) phenomenon, which results in elevated and widespread rainfall over the RVF endemic areas of Africa. Using satellite measurements of global and regional elevated sea surface temperatures, elevated rainfall, and satellite derived-normalized difference vegetation index data, we predicted with lead times of 2–4 months areas where outbreaks of RVF in humans and animals were expected and occurred in the Horn of Africa, Sudan, and Southern Africa at different time periods from September 2006 to March 2008. Predictions were confirmed by entomological field investigations of virus activity and by reported cases of RVF in human and livestock populations. This represents the first series of prospective predictions of RVF outbreaks and provides a baseline for improved early warning, control, response planning, and mitigation into the future. PMID:20682905

  5. Prediction, assessment of the Rift Valley fever activity in East and Southern Africa 2006-2008 and possible vector control strategies.

    PubMed

    Anyamba, Assaf; Linthicum, Kenneth J; Small, Jennifer; Britch, Seth C; Pak, Edwin; de La Rocque, Stephane; Formenty, Pierre; Hightower, Allen W; Breiman, Robert F; Chretien, Jean-Paul; Tucker, Compton J; Schnabel, David; Sang, Rosemary; Haagsma, Karl; Latham, Mark; Lewandowski, Henry B; Magdi, Salih Osman; Mohamed, Mohamed Ally; Nguku, Patrick M; Reynes, Jean-Marc; Swanepoel, Robert

    2010-08-01

    Historical outbreaks of Rift Valley fever (RVF) since the early 1950s have been associated with cyclical patterns of the El Niño/Southern Oscillation (ENSO) phenomenon, which results in elevated and widespread rainfall over the RVF endemic areas of Africa. Using satellite measurements of global and regional elevated sea surface temperatures, elevated rainfall, and satellite derived-normalized difference vegetation index data, we predicted with lead times of 2-4 months areas where outbreaks of RVF in humans and animals were expected and occurred in the Horn of Africa, Sudan, and Southern Africa at different time periods from September 2006 to March 2008. Predictions were confirmed by entomological field investigations of virus activity and by reported cases of RVF in human and livestock populations. This represents the first series of prospective predictions of RVF outbreaks and provides a baseline for improved early warning, control, response planning, and mitigation into the future. PMID:20682905

  6. 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.

  7. Fluid flow modeling at the Lusi mud eruption, East java, Indonesia.

    NASA Astrophysics Data System (ADS)

    Collignon, Marine; Schmid, Daniel; Mazzini, Adriano

    2016-04-01

    The 29th of may 2006, gas water and mud breccia started to erupt at several localities along the Watukosek fault system, in the Sidoarjo Regency in East java, Indonesia. The most prominent eruption, named Lusi, is still active and covering a surface of nearly 7 km2, resulting in the displacement of ~ 30 000 people. Although the origin and the chemical composition of the erupted fluids have been documented, the mechanical and physical properties of the mud are poorly constrained, and many aspects still remain not understood. Very little is known about the internal dynamics of the Lusi conduit(s). In this study, conducted in the framework of the Lusi Lab project (ERC grant n°308126) we use both analytical and numerical methods to better understand the flow dynamics within the main conduit and to try to explain the longevity of the edifice. The 2D numerical model considers a vertical conduit with a reservoir at its base and solves the stokes equations, discretized on a finite element mesh. Although, three phases (solid, liquid and gas) are present in nature, we only consider the liquid phase. The solid phase is treated as rigid particles in suspension in the liquid. The gaseous phase (methane and carbon dioxide) is treated in an analytical manner using the equations of state of the H2O-CO2 and H2O-CH4 systems. Here, we discuss the effects of density, viscosity, gas concentration and clasts concentration and size on the dynamics of the flow in the conduit as well as implications of the conduit stability.

  8. 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.

  9. Geothermal system at 21°N, East Pacific Rise: physical limits on geothermal fluid and role of adiabatic expansion

    USGS Publications Warehouse

    Bischoff, J.L.

    1980-01-01

    Pressure-volume-temperature relations for water at the depth of the magma chamber at 21°N on the East Pacific Rise suggest that the maximum subsurface temperature of the geothermal fluid is about 420°C. Both the chemistry of the discharging fluid and thermal balance considerations indicate that the effective water/rock ratios in the geothermal system are between 7 and 16. Such low ratios preclude effective metal transport at temperatures below 350°C, but metal solubilization at 400°C and above is effective even at such low ratios. It is proposed that the 420°C fluid ascends essentially adiabatically and in the process expands, cools, and precipitates metal sulfides within the upper few hundred meters of the sea floor and on the sea floor itself.

  10. Rift Valley Fever, Mayotte, 2007–2008

    PubMed Central

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

    2009-01-01

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

  11. 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.

  12. Nature of local magma storage zones and geometry of conduit systems below balsatic eruption sites - Pu'u 'O'o, Kilauea East Rift, Hawaii, example

    NASA Technical Reports Server (NTRS)

    Wilson, Lionel; Head, James W., III

    1988-01-01

    The fluid dynamics of the well-documented eruptive episodes at Pu'u 'O'o, Kilauea are used to investigate quantitatively the size and shape of the shallow conduit system beneath the vent. The possible geometry of this region is considered. The dynamics of the eruptive episodes is used to place restrictions on the size and shape of the region and thermal calculations are used to show that the geometry is consistent with the region being the fluid residue of the partially cooled, major preepisode 1 dike. The Pu'u 'O'o example is used to illustrate some general properties of shallow magma storage zones.

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

    NASA Astrophysics Data System (ADS)

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

    2014-04-01

    Increased displacement rates have been observed following manylarge earthquakes and magmatic events. Although an order of magnitude smaller than the displacements associated with the main event, the post-seismic or post-rifting deformation may continue for years to decades after the initial earthquake or dyke intrusion. Due to the rare occurrence of subaerial rifting events, there are very few observations to constrain models of post-rifting deformation. In 2005 September, a 60-km-long dyke was intruded along the Dabbahu segment of the Nubia-Arabia Plate boundary (Afar, Ethiopia), marking the beginning of an ongoing rifting episode. Continued activity has been monitored using satellite radar interferometry and data from global positioning system instruments deployed around the rift in response to the initial intrusion. Using multiple satellite passes, we are able to separate the rift perpendicular and vertical displacement fields around the Dabbahu segment. Rift perpendicular and vertical rates of up to 180 and 240 mm yr-1, respectively. Here, we show that models of viscoelastic relaxation alone are insufficient to reproduce the observed deformation field and that a large portion of the observed signal is related to the movement of magma within the rift segment. Our models suggest upper mantle viscosities of 1018-19 Pa s overlain by an elastic crust of between 15 and 30 km. To fit the observations, inflation and deflation of magma chambers in the centre of the rift and to the south east of the rift axis is required at rates of ˜0.13 and -0.08 km3 yr-1.

  14. Flexural modeling of the midcontinent rift

    SciTech Connect

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

    1988-08-10

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

  15. 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

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. [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.

  1. 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

  2. U-Pb isotopic behaviour of zircon during upper-amphibolite facies fluid infiltration in the Napier Complex, east Antarctica

    NASA Astrophysics Data System (ADS)

    Carson, Christopher J.; Ague, Jay J.; Grove, Marty; Coath, Christopher D.; Harrison, T. Mark

    2002-06-01

    Understanding the factors that contribute to U-Pb discordance in zircon is essential for interpreting isotopic data and for assessing the validity of concordia intercept ages. Modification caused by interaction with metamorphic fluids is often cited as a primary means by which significant or even complete isotopic resetting of U-Pb systematics in zircon might be achieved under subsolidus conditions. We present a field example from the Napier Complex, east Antarctica, in which a Palaeoproterozoic (2450-70 Ma) zircon population interacted locally with an Early Palaeozoic (498±1.7 Ma) aqueous fluid at upper-amphibolite facies conditions. Conventional ion microprobe analysis of sectioned and polished grain surfaces indicates that fluid interaction resulted in minor disturbance of U and Pb in zircons (both normal and reverse discordance) with limited displacement along a chord with a lower intercept that coincides with the timing of fluid infiltration. In contrast, ion probe 'drilling' or depth profiling into unpolished natural zircon crystal surfaces revealed extensive disturbance of U-Pb systematics, to depths of ˜0.2 μm, with near-surface ages consistent with the timing of fluid influx at ˜498 Ma. Although zircon underwent some radiogenic Pb redistribution during fluid interaction, infiltrating fluids resulted in minimal grain-scale isotopic modification of zircon. Based on ion probe depth profiling results, we propose that limited normal discordance observed in the conventional ion microprobe zircon analyses, in this case, is controlled by an analytical mixture of reset and/or recrystallised zircon along penetrative micro-fracture networks with that of adjacent unaffected zircon. We also suggest that the observed reverse discordance is genuine, resulting from localised intra-grain net accumulations of radiogenic Pb. We conclude that the isotopic response of zircon, in this case, is controlled by the interaction of an aqueous metamorphic fluid, of low to moderate

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

    NASA Astrophysics Data System (ADS)

    Guo, L.; Keller, G. R.

    2010-12-01

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

  5. Composition of hydrothermal fluids and mineralogy of associated chimney material on the East Scotia Ridge back-arc spreading centre

    NASA Astrophysics Data System (ADS)

    James, Rachael H.; Green, Darryl R. H.; Stock, Michael J.; Alker, Belinda J.; Banerjee, Neil R.; Cole, Catherine; German, Christopher R.; Huvenne, Veerle A. I.; Powell, Alexandra M.; Connelly, Douglas P.

    2014-08-01

    The East Scotia Ridge is an active back-arc spreading centre located to the west of the South Sandwich island arc in the Southern Ocean. Initial exploration of the ridge by deep-tow surveys provided the first evidence for hydrothermal activity in a back-arc setting outside of the western Pacific, and we returned in 2010 with a remotely operated vehicle to precisely locate and sample hydrothermal sites along ridge segments E2 and E9. Here we report the chemical and isotopic composition of high- and low-temperature vent fluids, and the mineralogy of associated high-temperature chimney material, for two sites at E2 (Dog’s Head and Sepia), and four sites at E9 (Black & White, Ivory Tower, Pagoda and Launch Pad). The chemistry of the fluids is highly variable between the ridge segments. Fluid temperatures were ∼350 °C at all vent sites except Black & White, which was significantly hotter (383 °C). End-member chloride concentrations in E2 fluids (532-536 mM) were close to background seawater (540 mM), whereas Cl in E9 fluids was much lower (98-220 mM) indicating that these fluids are affected by phase separation. Concentrations of the alkali elements (Na, Li, K and Cs) and the alkaline earth elements (Ca, Sr and Ba) co-vary with Cl, due to charge balance constraints. Similarly, concentrations of Mn and Zn are highest in the high Cl fluids but, by contrast, Fe/Cl ratios are higher in E9 fluids (3.8-8.1 × 10-3) than they are in E2 fluids (1.5-2.4 × 10-3) and fluids with lowest Cl have highest Cu. Although both ridge segments are magmatically inflated, there is no compelling evidence for input of magmatic gases to the vent fluids. Fluid δD values range from 0.2‰ to 1.5‰, pH values (3.02-3.42) are not especially low, and F concentrations (34.6-54.4 μM) are lower than bottom seawater (62.8 μM). The uppermost sections of conjugate chimney material from E2, and from Ivory Tower and Pagoda at E9, typically exhibit inner zones of massive chalcopyrite enclosed

  6. 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.

  7. 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).

  8. 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

  9. Chemical and isotopic characteristics of the coso east flankhydrothermal fluids: implications for the location and nature of the heatsource

    SciTech Connect

    Christenson, B.W.; Kennedy, B.M.; Adams, M.C.; Bjornstad, S.C.; Buck, C.

    2007-01-08

    Fluids have been sampled from 9 wells and 2 fumaroles fromthe East Flank of the Coso hydrothermal system with a view toidentifying, if possible, the location and characteristics of the heatsource inflows into this portion of the geothermal field. Preliminaryresults show that there has been extensive vapor loss in the system, mostprobably in response to production. Wells 38A-9, 51-16 and 83A-16 showthe highest CO2-CO-CH4-H2 chemical equilibration temperatures, rangingbetween 300-340oC, and apart from 38A-9, the values are generally inaccordance with the measured temperatures in the wells. Calculatedtemperatures for the fractionation of 13C between CO2 and CH4 are inexcess of 400oC in fluids from wells 38A-9, 64-16-RD2 and 51A-16,obviously pointing to equilibrium conditions from deeper portions of thereservoir. Given that the predominant reservoir rock lithologies in theCoso system are relatively silicic (granitic to dioritic), the isotopicsignatures appear to reflect convective circulation and equilibrationwithin rocks close to the plastic-brittle transition. 3He/4He signatures,in conjunction with relative volatile abundances in the Coso fluids,point to a possibly altered mantle source for the heat sourcefluids.

  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. 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...

  12. 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

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  14. 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

  15. Fault Orientations at Obliquely Rifted Margins: Where? When? Why?

    NASA Astrophysics Data System (ADS)

    Brune, Sascha

    2015-04-01

    Present-day knowledge of rifted margin formation is largely based on 2D seismic lines, 2D conceptual models, and corroborated by 2D numerical experiments. However, the 2D assumption that the extension direction is perpendicular to the rift trend is often invalid. In fact, worldwide more than 75% of all rifted margin segments have been formed under significant obliquity exceeding 20° (angle measured between extension direction and rift trend normal): During formation of the Atlantic Ocean, oblique rifting dominated at the sheared margins of South Africa and Patagonia, the Equatorial Atlantic margins, separation of Greenland and North America, and it played a major role in the protracted rift history of the North East Atlantic. Outside the Atlantic Ocean, oblique rifting occurred during the split between East and West Gondwana, the separation of India and Australia, India and Madagascar, Australia and Antarctica, as well as Arabia and Africa. It is presently observed in the Gulf of California, the Aegean and in the East African Rift. Despite its significance, the degree to which oblique lithospheric extension affects first-order rift and passive margin properties like surface stress pattern, fault azimuths, and basin geometry, is still not entirely clear. This contribution provides insight in crustal stress patterns and fault orientations by applying a 3D numerical rift model to oblique extensional settings. The presented forward experiments cover the whole spectrum of oblique extension (i.e. rift-orthogonal extension, low obliquity, high obliquity, strike-slip deformation) from initial deformation to breakup. They are conducted using an elasto-visco-plastic finite element model and involve crustal and mantle layers accounting for self-consistent necking of the lithosphere. Results are thoroughly compared to previous analogue experiments, which yields many similarities but also distinct differences for late rift stages and for high obliquity. Even though the model

  16. The mesoproterozoic midcontinent rift system, Lake Superior region, USA

    USGS Publications Warehouse

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

    2001-01-01

    Exposures in the Lake Superior region, and associated geophysical evidence, show that a 2000 km-long rift system developed within the North American craton ??? 1109-1087 Ma, the age span of the most of the volcanic rocks. This system is characterized by immense volumes of mafic igneous rocks, mostly subaerial plateau basalts, generated in two major pulses largely by a hot mantle plume. A new ocean basin was nearly formed before rifting ceased, perhaps due to the remote effect of the Grenville continental collision to the east. Broad sagging/subsidence, combined with a system of axial half-grabens separated along the length of the rift by accommodation zones, provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post-rift clastic sediments, both along the rift axis and in basins flanking a central, post-volcanic horst. Pre-rift mature, quartzose sandstones imply little or no uplift prior to the onset of rift volcanism. Early post-rift red-bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings; the exception is one gray to black carbon-bearing lacustrine(?) unit. This early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red-bed, fluvial sediments with an extra-basinal provenance. ?? 2001 Elsevier Science B.V. All rights reserved.

  17. The Mesoproterozoic Midcontinent Rift System, Lake Superior Region, USA

    NASA Astrophysics Data System (ADS)

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

    2001-06-01

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

  18. [The Great Rift Valley. Parasitological results].

    PubMed

    Nozais, J P

    1985-01-01

    East Africa is separated from the continent by the Great Rift Valley which was created at the end of the secondary era limiting then the East Africa under-continent with peculiar fauna and flora features. A several million years long isolation, during the tertiary era, seems to explain that a certain number of protozoan and helminthic diseases present peculiar clinical, epidemiological, therapeutical and parasitological features. The occurrence of those peculiar strains tends to indicate that in this region, for example, the resistance of P. falciparum to amino-4-quinolines is a regional feature which should not largely expand to the rest of the African continent.

  19. Volcanism at rifts

    SciTech Connect

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

    1989-07-01

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

  20. DSL-120A High-Resolution, Near-Bottom Side Scan Sonar Imaging of Mid-Ocean Ridge Crests: East Pacific Rise, Galapagos Rift and Mid-Atlantic Ridge

    NASA Astrophysics Data System (ADS)

    Fornari, D.

    2003-04-01

    The DSL-120A 120kHz side-looking sonar system is one of the survey vehicles in the US National Deep Submergence Facility (NDSF) operated by the Woods Hole Oceanographic Institution for the academic community. Survey speeds for the DSL-120A sonar range from 1.0-1.5 knots depending on terrain roughness. The sonar fish is towed approx. 100 m above the seafloor yielding a backscatter imagery swath of 1.2 km and phase-bathymetry swath of approximately 500-600 m. Data resolution is 1 m for side scan imagery and 4 m for phase-bathymetry. DSL-120A data can resolve bathymetric features with linear dimensions more than 25x smaller than those detectable by hull-mounted multibeam sonars at ridge crest seafloor depths. Data resulting from the DSL-120A sonar system have provided the requisite base maps to understand the volcanic and tectonic evolution of seafloor in many geographic and tectonic settings including several segments of the mid-ocean ridge. These maps have provided site-specific information for planning and carrying out detailed photographic imaging, sampling, and other in situ investigations, and ocean crustal drilling. Backscatter images reveal variations in volcanic emplacement style at different spreading rate and tectonic settings along the MOR crest. DSL-120 sonar data have been collected at the Mid-Atlantic Ridge - 37N, the East Pacific Rise (between 9-10N, an overlapping spreading center at 3 20'N, and at 1 45'N) and within the tectonically dominated Galapagos Rift at 97.5W. Despite the 'fast' spreading nature of crust at the EPR between 1-10N, the sonar images show that volcanic emplacement changes dramatically along-strike depending on the local tectonic setting, and on more regional variations that are likely related to magmatic supply along and across adjacent ridge segments boundaries of various scales. Representative side scan images from these areas will be presented to illustrate different modes of volcanic eruption and emplacement styles

  1. The South China sea margins: Implications for rifting contrasts

    USGS Publications Warehouse

    Hayes, D.E.; Nissen, S.S.

    2005-01-01

    Implications regarding spatially complex continental rifting, crustal extension, and the subsequent evolution to seafloor spreading are re-examined for the northern and southern-rifted margins of the South China Sea. Previous seismic studies have shown dramatic differences in the present-day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south China. Although the total crustal extension is presumed to be the same along the margin and adjacent ocean basin, the amount of continental crustal extension that occurred is much less along the east and central segments of the margin than along the western segment. This difference was accommodated by the early formation of oceanic crust (creating the present-day South China Sea basin) adjacent to the eastern margin segment while continued extension of continental crust was sustained to the west. Using the observed cross-sectional areas of extended continental crust derived from deep penetration seismics, two end-member models of varying rift zone widths and varying initial crustal thicknesses are qualitatively examined for three transects. Each model implies a time difference in the initiation of seafloor spreading inferred for different segments along the margin. The two models examined predict that the oceanic crust of the South China Sea basin toward the west did not begin forming until sometime between 6-12 my after its initial formation (???32 Ma) toward the east. These results are compatible with crustal age interpretations of marine magnetic anomalies. Assuming rifting symmetry with conjugate margin segments now residing along the southern portions of the South China Sea basin implies that the total width of the zone of rifting in the west was greater than in the east by about a factor of two. We suggest the most likely causes of the rifting differences were east-west variations in the rheology of the pre-rift crust and associated east-west variations in the

  2. 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

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

    USGS Publications Warehouse

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

    2009-01-01

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

  4. Rift Valley Fever (RVF)

    MedlinePlus

    ... Outbreak resources, VHF information for specific groups, virus ecology, references... RVF Distribution Map Rift Valley Fever Transmission ... Outbreaks Outbreak Summaries RVF Distribution Map Resources Virus Ecology File Formats Help: How do I view different ...

  5. Methane-bearing fluid inclusions as evidence of the collisional nature of high-alumina granites, northern Sikhote-Alin, Russian Far East

    SciTech Connect

    Berdnikov, N.V.; Karsakov, L.P.

    1999-07-01

    The Khungariisk high-alumina S-type granites (northern Sikhote-Alin fold system, Russian Far East) have been formed as a result of anatectic melting during collision of the Anyuy sialic block with the continental margin in the Early Cretaceous. Methane-rich fluid inclusions are characteristic of these granites. The CH{sub 4}-H{sub 2}O fluid composition is explained by the release of water and methane from the sedimentary units residing on the oceanic plate, as it was subducted beneath sialic block. This fluid seeped into the heated and deformed metamorphic rocks of the overlying block and initiated partial fusion of the granites. A comparison of the new data with the results of fluid-inclusion studies of high-alumina collisional granites from the Pamir Mountains and Japan reveals similarities. Thus, methane-bearing fluid is probably one of the general features of this granite type, reflecting its anatectic origin.

  6. Mid-Continent rift system - a frontier hydrocarbon province

    SciTech Connect

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

    1983-08-01

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

  7. The 1973 Ethiopian-Rift geodimeter survey

    NASA Technical Reports Server (NTRS)

    Mohr, P. A.

    1974-01-01

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

  8. 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.

  9. Evidence of contemporary and ancient excess fluid pressure in the New Madrid seismic zone of the Reelfoot Rift, central United States

    USGS Publications Warehouse

    McKeown, F.A.; Diehl, S.

    1994-01-01

    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.

  10. 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.

  11. 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.

  12. 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.

  13. Feedbacks between deformation and reactive melt transport in the mantle lithosphere during rifting

    NASA Astrophysics Data System (ADS)

    Tommasi, A.; Baptiste, V.; Vauchez, A. R.; Fort, A.

    2014-12-01

    The East-African rift associates lithospheric thinning with extensive volcanism. Melts, even at low fractions, reduce the mantle viscosity. They also carry and exchange heat, mainly via reactions (latent heat), modifying the temperature and the rheology, which in turn controls their transport through the lithospheric mantle. Analysis of microstructures and crystal preferred orientations of mantle xenoliths from different localities along the East-African rift system highlights strong feedbacks between deformation, melt transport, and thermal evolution in the lithospheric mantle. Microstructures change markedly from south (young) to north (mature rift). In Tanzania, mylonitic to porphyroclastic peridotites predominate in on-axis localities, while off-axis ones are coarse-granular to porphyroclastic, pointing to heterogeneous deformation and variable annealing due to local interaction with fluids or to different time lags between deformation and extraction. Mylonites point to strain localization but there is no evidence for dominant grain boundary sliding: ubiquituous intracrystalline deformation in olivine and orthopyroxene and strong CPO record dislocation creep with dominant [100] glide in olivine. Synkinematic replacement of opx by olivine in both mylonitic and porphyroclastic peridotites suggests that deformation continued in the presence of melt under near-solidus conditions. This heating was transient: exsolutions in opx record cooling before extraction. Mega peridotites, which sample the southern border of the Ethiopian plateau, are coarse-porphyroclastic and show widespread metasomatism by basalts or by evolved volatile-rich low melt fractions. The former predated or was coeval to deformation, since olivine and pyroxene CPO are coherent. Exsolutions in opx imply that the high primary equilibration temperatures, which are consistent with the coarse-grained microstructures, are linked to transient heating. Finally, the fine-grained polygonal microstructures

  14. Continental rifting: a planetary perspective

    SciTech Connect

    Muehlberger, W.R.

    1985-01-01

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

  15. 3D Chirp Sonar Images on Fluid Migration Pathways and Their Implications on Seafloor Stability East of the Fangliao Submarine Canyon Offshore SW Taiwan

    NASA Astrophysics Data System (ADS)

    Lu, Y. W.; Liu, C. S.; Su, C. C.; Hsu, H. H.; Chen, Y. H.

    2015-12-01

    This study utilizes both chirp sonar images and coring results to investigate the unstable seafloor strata east of the Fangliao Submarine Canyon offshore southwestern Taiwan. We have constructed 3D chirp sonar images from a densely surveyed block to trace the attitude of an acoustic transparent layer and features caused by fluid activities. Based on the distribution of this transparent layer and fluid-related features, we suggest that this transparent layer forms a pathway for fluid migration which induces fluid-related characters such as acoustic blanking and fluid chimneys in the 3D chirp sonar images. Cored seafloor samples are used in this study to investigate the sediment compositions. The 210Pb activity profiles of the cores show oscillating and unsteady values at about 20~25 cm from core top. The bulk densities of the core samples in the same section (about 20~25 cm from core top) give values lower than those at deeper parts of the cores. These results indicate that the water content is much higher in the shallow sediments than in the deeper strata. From core sample analyses, we deduce that the local sediments are disturbed by liquefaction. From the analyses of 3D chirp sonar images and core data, we suggest that the seafloor east of the Fangliao Submarine Canyon is in an unstable condition, if disturbed by earthquakes, submarine landslides and gravity flows could be easily triggered and cause some geohazards, like breaking submarine cables during the 2006 Pingtung earthquake event.

  16. 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

  17. Timing the Initiation of Extension Along the Shanxi Rift, China, With Implications for the Cenozoic Tectonics of Asia

    NASA Astrophysics Data System (ADS)

    Webb, A.; Yin, A.; Burnard, P.; Blythe, A.

    2002-12-01

    The Shanxi rift accommodates east-west extension along the eastern margin of the Ordos block in the North China craton. Previous workers have assumed a Paleogene initiation age for the southern part of the Shanxi rift, because Paleogene sediments occur in the graben basin. However, these sediments also occur along the rift shoulder in the footwalls of major rift-bounding faults, implying Paleogene sediments may be pre-rift. The central and northern parts of the Shanxi rift contain only Neogene sediments, suggesting a Neogene initiation age. Three hypotheses for the timing of rift initiation are proposed: (1) the Shanxi rift initiated in the Paleogene, (2) the rift initiated in the Neogene, and (3) the southern part of the Shanxi rift initiated first in the Paleogene, and subsequent rifting propagated to the north in the Neogene. These hypotheses are being tested by (i) (U-Th) He dating of apatite and (ii) fission track dating of apatite and zircon from east-west transects across graben-bounding normal faults along the Shanxi rift. East-west extension is active across east Asia, occurring in the Himalaya, Tibet, along the northwest and east margins of the Ordos Block, the Hangay extensional system, and in the Lake Baikal region. These systems may be induced by a common dynamic cause because they share the same extension direction. Genetic relations among these systems can be tested by determining (1) kinematic relationships and (2) timing of rifting. To the south the Shanxi rift terminates at the left-slip Qinling fault, which may connect with the left-slip Kunlun fault to the west via a large contractional step-over structure. Both faults are important structures accommodating the Indo-Asian collision in the past 10 m.y. The Kunlun fault marks the northern boundary of active north-trending rifts in Tibet. If the Kunlun fault does kinematically connect with the Qinling fault, these relationships suggest a dynamic link between active east-west extension in Tibet and

  18. Rift Valley Fever Review

    Technology Transfer Automated Retrieval System (TEKTRAN)

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

  19. Continental rifting - Progress and outlook

    NASA Technical Reports Server (NTRS)

    Baker, B. H.; Morgan, P.

    1981-01-01

    It is noted that in spite of the flood of new data on continental rifts in the last 15 years, there is little consensus about the basic mechanisms and causes of rifting. The remarkable similarities in rift cross sections (shown in a figure), are considered to suggest that the anomalous lithospheric structure of rifts is more dependent on lithosphere properties than the mode of rifting. It is thought that there is a spectrum of rifting processes for which two fundamental mechanisms can be postulated: an active mechanism, whereby thermal energy is transmitted into the lithosphere from the underlying asthenosphere, and a passive mechanism by which mechanical energy is transmitted laterally through the lithosphere as a consequence of plate interactions at a distance. In order to permit the concept of the two fundamentally different mechanisms to be tested, a tentative classification is proposed that divides rifts into two basic categories: active rifting and passive rifting. Here, the magnitude of active rifting will depend on the rate at which lithosphere moves over the thermal source, with rifts being restricted to stationary or slow-moving plates.

  20. 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

  1. P Wave Velocity Structure Beneath the Baikal Rift Axis

    NASA Astrophysics Data System (ADS)

    Brazier, R. A.; Nyblade, A. A.; Boman, E. C.

    2001-12-01

    Over 100 p wave travel times from the 1500 km en echelon Baikal Rift system are used in this study.The events range 3 to 13 degrees from Talaya, Russia (TLY) along the axis of southwest northeast trending rift in East Siberia. A Herglotz Wiechert inversion of these events resolved a crust of 6.4 km/s and a gradient in the mantle starting at 35 km depth and 7.7 km/s down to 200 km depth and 8.2 km/s. This is compatible with Gao et al,1994 cross sectional structure which cuts the rift at about 400km from TLY. The Baikal Rift hosts the deepest lake and is the most seismically active rift in the world. It is one of the few continental rifts, it separates the Siberian craton and the Syan-Baikal mobile fold belt. Two events, the March 21 1999 magnitude 5.7 earthquake 638 km from TLY and the November 13th 1995 magnitude 5.9 earthquake 863 km from TLY were modeled for there PnL wave structure using the discrete wavenumber method and the Harvard CMT solutions with adjusted depths from p-pP times. The PnL signals match well. A genetic algorithm will used to perturb the velocity structure and compare to a selection of the events between 3 and 13 degrees many will require moment tensor solutions.

  2. Exploring the contrasts between fast and slow rifting

    NASA Astrophysics Data System (ADS)

    Morgan, Jason P.; de Monserrat, Albert; White, Lloyd; Hall, Robert

    2016-04-01

    Researchers are now finding that extension sometimes occurs at rates much faster than the mean rates observed in the development of passive margins. Examples of rapid and ultra-rapid extension are found in several locations in Eastern Indonesia. This includes in northern and central Sulawesi as well as in eastern- and westernmost New Guinea. The periods of extension are associated with sedimentary basin growth as well as phases of crustal melting and rapid uplift. This is recorded through seismic imagery of basins offshore Sulawesi and New Guinea as well as through new field studies of the onshore geology in these regions. A growing body of new geochronological and biostratigraphic data provide some control on the rates of processes, indicating that rates of extension are typically at least twice as fast and potentially an order of magnitude faster than the fastest rates applied for more commonly studied rift settings (e.g. Atlantic opening, East African Rift, Australia-Antarctica opening). Here we explore a suite of experiments more appropriate for rifting episodes in Eastern Indonesia, and compare the evolution of these 'fast' (20-100 mm/year full rate) rifting models to experiments with the same crustal geometries rifting at ~5-20 mm/year. In particular, we explore to what depths hot lower crust and mantle can be exhumed by fast rifting, and whether we can produce the p-T-t paths implied by recent onshore geological studies.

  3. Intracontinental rift comparisons: Baikal and Rio Grande Rift Systems

    NASA Astrophysics Data System (ADS)

    Lipman, P. W.; Logatchev, N. A.; Zorin, Y. A.; Chapman, C. E.; Kovalenko, V.; Morgan, P.

    Both the Baikal rift in Siberia and the Rio Grande rift in New Mexico, Colorado and Texas are major intracontinental extensional structures of Cenozoic age that affect regions about 1500 km long and several hundred km wide (Figures 1, 2). In the summer of 1988 these rifts were visited by study groups of U.S. and Soviet geoscientists during cooperative field workshops sponsored by the Soviet Academy of Sciences, U.S. National Academy of Sciences, and U.S. Geological Survey.In the Rio Grande region, we spent 2 weeks examining rift features between El Paso, Tex., and Denver, Colo. Particular emphasis was on the sedimentary record of rift evolution, widespread volcanic activity from inception of rifting to the present, geophysical expression of rift features, and relations between rifting and the larger-scale evolution of the North American Cordillera. In the Baikal region, which presents formidable logistic problems for a workshop, we travelled by bus, truck, helicopter, and ship to examine young seismotectonic features, rift-related basalt, and bounding structures of the Siberian craton that influenced rift development (Figure 3).

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

    NASA Astrophysics Data System (ADS)

    Bellahsen, N.; Husson, L.; Autin, J.; Leroy, S.; d'Acremont, E.

    2013-11-01

    On the basis of field and geophysical data, analog and numerical models, we here discuss the role of buoyancy forces arising from thickness variations in the lithosphere during rifting. In the Gulf of Aden, an oceanized Tertiary oblique rift, several successive directions of extension and associated normal faults suggest that transient stress rotations occurred during rifting. Especially, rift-parallel faults (070°E) overprinted the early divergence-perpendicular normal faults (110°E). Moreover, some first-order differences are noticeable between the western part of the Gulf, which deformed under the Afar hot spot influence, and the eastern part. In the western Gulf of Aden, the ocean-continent transition (OCT) and the oceanic ridge have cut obliquely through the inherited and reactivated Mesozoic basins (100°E to 140°E). The OCT trend is parallel to the overall Gulf trend (070°E). In the eastern part, the oceanization occurred within few syn-rift 110°E-trending basins and the OCT trends mostly perpendicular to the divergence direction. Here, we propose that this contrast is strongly controlled by the Afar hot spot: during rifting times, the hot spot likely induced a hot thermal anomaly in the western asthenosphere. This may have triggered both thermal buoyancy forces and thermal weakening of the lithosphere that helped localizing the rift obliquely. In such localized rift, rift-perpendicular trending crustal buoyancy forces (i.e. around 160°E) have enhanced rift-parallel normal faults (070°E) during final rift localization into a narrow zone strongly oblique to the early syn-rift basins. As a consequence of the Afar hot spot, in the west, the ridge is long and straight; in the east, the ridge segments are rather long too (although less than in the west) as the ridge initiated parallel to the OCT; in between, the ridge is more segmented as both the hot spot influence gradually decreases eastward and the ridge initiated obliquely to the OCT.

  5. North America's Midcontinent Rift: when Rift MET Lip

    NASA Astrophysics Data System (ADS)

    Stein, C. A.; Stein, S. A.; Kley, J.; Keller, G. R., Jr.; Bollmann, T. A.; Wolin, E.; Zhang, H.; Frederiksen, A. W.; Ola, K.; Wysession, M. E.; Wiens, D.; Alequabi, G.; Waite, G. P.; Blavascunas, E.; Engelmann, C. A.; Flesch, L. M.; Rooney, T. O.; Moucha, R.; Brown, E.

    2015-12-01

    Rifts are segmented linear depressions, filled with sedimentary and igneous rocks, that form by extension and often evolve into plate boundaries. Flood basalts, a class of Large Igneous Provinces (LIPs), are broad regions of extensive volcanism due to sublithospheric processes. Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. North America's Midcontinent Rift (MCR) is an unusual combination. Its 3000-km length formed as part of the 1.1 Ga rifting of Amazonia (Precambrian NE South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established, but contains an enormous volume of igneous rocks. MCR volcanics are significantly thicker than other flood basalts, due to deposition in a narrow rift rather than a broad region, giving a rift geometry but a LIP's magma volume. Structural modeling of seismic reflection data shows an initial rift phase where flood basalts filled a fault-controlled extending basin, and a postrift phase where volcanics and sediments were deposited in a thermally subsiding basin without associated faulting. The crust thinned during rifting and rethickened during the postrift phase and later compression, yielding the present thicker crust. The coincidence of a rift and LIP yielded the world's largest deposit of native copper. This combination arose when a new rift associated with continental breakup interacted with a mantle plume or anomalously hot or fertile upper mantle. Integration of diverse data types and models will give insight into questions including how the magma source was related to the rifting, how their interaction operated over a long period of rapid plate motion, why the lithospheric mantle below the MCR differs only slightly from its surroundings, how and why extension, volcanism, and compression varied along the rift arms, and how successful seafloor spreading ended the rift phase. Papers

  6. Westward drift, rift asymmetry and continental uplift

    NASA Astrophysics Data System (ADS)

    Doglioni, C.; Carminati, E.; Bonatti, E.

    2003-04-01

    Although not predicted by classic plate tectonics theory, the topography of ocean ridges and rifts show a distinct asymmetry, when depth is plotted both vs. distance from the ridge and square root of the age of the oceanic crust. The eastern sides of the East Pacific Rise, of the mid Atlantic ridge, of the NW Indian ridge are in average more elevated than the conjugate flank to the west and eastern sides show slower subsidence rates. A similar asymmetry can be observed across the Red Sea and Baikal rifts. We suggest that depleted and lighter asthenosphere generated by partial melting below the ocean ridges shifts 'eastward' relative to the lithosphere, determining a density deficit below the eastern flank. The 'eastward' migration of the lighter Atlantic asthenosphere under the African continent, could eventually have contributed to the anomalous post-rift uplift of Africa and explain the anomalously higher topography of Africa with respect to other continents. This model suggests that the 'westward' drift of the lithosphere relative to the underlying mantle might be a global phenomenon and not just a mean delay.

  7. New vectors of Rift Valley fever in West Africa.

    PubMed Central

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

    1998-01-01

    After an outbreak of Rift Valley fever in Southern Mauritania in 1987, entomologic studies were conducted in a bordering region in Sénégal from 1991 to 1996 to identify the sylvatic vectors of Rift Valley fever virus. The virus was isolated from the floodwater mosquitoes Aedes vexans and Ae. ochraceus. In 1974 and 1983, the virus had been isolated from Ae. dalzieli. Although these vectors differ from the main vectors in East and South Africa, they use the same type of breeding sites and also feed on cattle and sheep. Although enzootic vectors have now been identified in West Africa, the factors causing outbreaks remain unclear. PMID:9621201

  8. Continental rifting and the origin of Beta Regio, Venus

    NASA Technical Reports Server (NTRS)

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

    1981-01-01

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

  9. 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.

  10. 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.

  11. The Midcontinent rift system in Kansas

    SciTech Connect

    Berendsen, P. . Kansas Geological Survey)

    1993-03-01

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

  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. Metallogeny of the midcontinent rift system of North America

    USGS Publications Warehouse

    Nicholson, S.W.; Cannon, W.F.; Schulz, K.J.

    1992-01-01

    country rocks; the interaction between magma and country rocks was important in generation of the magmatic CuNi sulfide deposits. A mantle plume origin has been proposed for the formation of the Midcontinent rift. More than 1 million km3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift, providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines. These brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume. Hydrothermal deposits were emplaced for at least 30-40 m.y. after rift magmatism and extension ceased. This time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift, or may be due to the timing of post-rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep portions of the rift. ?? 1992.

  14. The Thinning of the lithosphere before Magmatic Spreading is Established at the Western End of the Cocos-Nazca Rift

    NASA Astrophysics Data System (ADS)

    Smith, D. K.; Schouten, H.

    2015-12-01

    The transition from rifting of oceanic lithosphere to full magmatic spreading is examined at the Galapagos triple junction (GTJ) where the tip of the Cocos-Nazca spreading center (called C-N Rift) is propagating westward and breaking apart 0.5 Ma lithosphere formed at the East Pacific Rise near 2 15'N. Bathymetric mapping of the western section of the C-N Rift is limited, but sufficient to obtain a first-order understanding of how seafloor spreading is established. An initial rifting stage is followed by rifting with magma supply and lastly, full magmatic spreading is established. The flexural rotation of normal faults that border the rift basins is used to document thinning of the effective elastic thickness of the lithosphere before magmatic spreading begins. The earliest faults show small outward rotation (1-5 degrees) for their offset suggesting that they cut thick lithosphere. Subsequent faults closer to the axis have larger outward rotations (up to 35-40 degrees) with larger offset indicating that the lithosphere was much thinner at the time of faulting and that low-angle detachment faults are forming. It is during late stage rifting and prior to full magmatic spreading that detachment faults such as the Intrarift ridge along Hess Deep rift are observed. Studies of low-angle detachment faulting during continental breakup at the Woodlark Basin suggest that their formation signals the input of magma beneath the rift. If this also is the case at the C-N Rift then magma is being supplied beneath Hess Deep rift. The axis of the segment immediately east of Hess Deep rift is characterized by a shallow graben with small seamounts scattered along it, typical of segments farther to the east, and we infer that full magmatic seafloor spreading has been established here. Our results provide new information on the formation of divergent boundaries in oceanic lithosphere, and place constraints on the supply of magma to a newly developing plate boundary.

  15. The role of inheritance in structuring hyperextended rift systems

    NASA Astrophysics Data System (ADS)

    Manatschal, Gianreto; Lavier, Luc; Chenin, Pauline

    2015-04-01

    -related mantle processes may control the rheology of the mantle, the magmatic budget, the thermal structure and the localization of final rifting Conversely, the deformation in hyperextended domains is strongly controlled by weak hydrated minerals (e.g. clay, serpentinite) that result form the breakdown of feldspar and olivine due to fluid and reaction assisted deformation and is consequently not inherited but the result of rift induced processes. These key observations show that both inheritance and rift-induced processes play a significant role in the development of magma-poor rift systems and that the role of inheritance may change as the physical conditions vary during the evolving rifting and as rift-induced processes (serpentinization; magma) become more important. Thus, it is not only important to determine the "genetic code" of a rift system, but also to understand how it interacts and evolves during rifting. Understand how far these new ideas and concepts derived from the southern North Atlantic and Alpine Tethys can be translated to other less explored hyperextended rift systems will be one of the challenges of the future research in rifted margins.

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

    USGS Publications Warehouse

    Burton, William C.; Southworth, Scott

    2010-01-01

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

  17. Mesozoic Rifting in the German North Sea

    NASA Astrophysics Data System (ADS)

    Lutz, R.; Jähne, F.; Arfai, J.

    2013-12-01

    The Central Graben is the southernmost expressions of the Mesozoic North Sea rift system that includes the Viking Graben, Moray Firth-Witch Ground grabens and the Horda-Egersund half graben. In the southern North Sea the Central Graben extends across the Dutch and the German exclusive economic zones. The structure of the Central Graben in German territorial waters was mapped in great detail in 2D and 3D seismic data and the stratigraphy has been constraint by borehole data. We provide a detailed review of the rifting activity in the German North Sea sector both in time and space and the link between rifting and salt movement. Major rifting activity started in the Central Graben during the Late Triassic and peaked during the Late Jurassic when extensive rift grabens formed, further influenced by halokinetic movements. First subsidence in the Central Graben area appears in the Early Triassic. This is documented by thickness variations in the sedimentary strata from the Triassic to the Jurassic. Remarkably thick sediments were deposited during the Late Triassic along the eastern border fault of the Central Graben and in the Late Jurassic sediments accumulated along graben-wide extensional faults and in rim-synclines of salt-structures. A basin inversion commenced in the Late Cretaceous resulting in an erosion of wide portions of Lower Cretaceous rocks or even complete removal in some parts. The area to the east of the Central Graben faced a completely different evolution. In this area major rifting activity initiated already in the Early to Middle Triassic. This is evident from huge packages of Middle Buntsandstein to Muschelkalk (Middle Triassic) sediments in the Horn Graben. Jurassic doming, forming the Mid-North Sea High, resulted in almost complete erosion of Lower and Middle Jurassic sediments in the central German North Sea. Sedimentation continued during the Early and Late Cretaceous. The Glückstadt Graben, which is a structure located farther east has a

  18. 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

  19. Speculations on the origin of the North American Midcontinent rift

    USGS Publications Warehouse

    Cannon, W.F.; Hinze, W. J.

    1992-01-01

    The Midcontinent rift is an example of lithospheric extension and flood basalt volcanism induced when a new mantle plume arrived near the base of the lithosphere. Very large volumes of basaltic magma were generated and partly erupted before substantial lithospheric extension began. Volcanism continued, along with extension and deep rift subsidence, for the ensuing 15 m.y. Much of the basaltic magma, including some of the earliest flows, was formed by partial melting of isotopically primitive asthenosphere contained in the plume head. The intense but relatively short duration of rifting and magmatism is a result of the dissipation of thermal and mechanical energy in the plume head. As the plume head spread beneath the lithosphere, it stretched the overlying lithosphere radially away from the Lake Superior region, the triple junction of the rift system, and partially melted to form the great volume of basalt and related intrusive rocks of the region. The plume arrived beneath a continent that was under compression as a result of the ongoing Grenville orogeny that affected a large region east of the rift. That compression prevented full continental separation and eventually returned the region to compressional tectonics as the energy of the plume head waned. ?? 1992.

  20. Masirah Graben, Oman: A hidden Cretaceous rift basin

    SciTech Connect

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

    1995-06-01

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

  1. 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.

  2. 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.

  3. The MOZART Project - MOZAmbique Rift Tomography

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  4. 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

  5. Stable isotopic evidence for fluid flow and fluid/rock interaction during thrust faulting in Pumpkin Valley shale and Rome Formation, east Tennessee

    SciTech Connect

    Butler, B.K.; Haase, C.S. )

    1989-08-01

    The Pumpkin Valley Shale and the underlying Rome Formation form the lower portions of the Copper Creek and White Oak Mountain thrust sheets in east Tennessee. The Pumpkin Valley Shale consists of shale and mudstone with subordinate amounts of interbedded siltstone. The Rome Formation is composed predominantly of sandstone with interbedded shale and siltstone toward the base of the formation. The percentage of illite increases from 20% to over 80% of the bulk clay mineralogy toward the base of the section. Porosity is occluded by quartz, phyllosilicate, and calcite cements. Both formations contain calcite-filled and, less commonly, quartz-filled Alleghenian fractures and joints.

  6. 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.

  7. Temporal trends in vent fluid iron and sulfide chemistry following the 2005/2006 eruption at East Pacific Rise, 9°50'N

    NASA Astrophysics Data System (ADS)

    Yücel, Mustafa; Luther, George W.

    2013-04-01

    The chemistry of vent fluids that emanate to the seafloor undergoes dramatic changes after volcanic eruptions. Data on these changes are still limited, but the best studied example is the East Pacific Rise (EPR) at 9°50'N, where the temporal evolution of the vent fluid chemistry after the 1991/1992 eruption was documented. The area underwent another eruption sequence during late 2005/early 2006, and here we show that a similar evolution is recurring in the iron and sulfide contents of the high-temperature fluids sampled in June 2006, January 2007, and June 2008. The vents have had increasing dissolved iron and decreasing acid-volatile sulfide (free sulfide plus FeS) concentrations with 1 order of magnitude variation. In addition, chromium reducible sulfide (mainly pyrite) also had fivefold decreasing concentrations over the 3 years. Our results confirm a pattern that was noted only once before for 9°50'N EPR and emphasize the dramatic yearly variability in the concentrations of iron-sulfur species emanating from vents.

  8. 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

  9. 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.

  10. Neogene Development of the Terror Rift, western Ross Sea, Antarctica

    NASA Astrophysics Data System (ADS)

    Sauli, C.; Sorlien, C. C.; Busetti, M.; De Santis, L.; Wardell, N.; Henrys, S. A.; Geletti, R.; Wilson, T. J.; Luyendyk, B. P.

    2015-12-01

    Terror Rift is a >300 km-long, 50-70 km-wide, 14 km-deep sedimentary basin at the edge of the West Antarctic Rift System, adjacent to the Transantarctic Mountains. It is cut into the broader Victoria Land Basin (VLB). The VLB experienced 100 km of mid-Cenozoic extension associated with larger sea floor spreading farther north. The post-spreading (Neogene) development of Terror Rift is not well understood, in part because of past use of different stratigraphic age models. We use the new Rossmap seismic stratigraphy correlated to Cape Roberts and Andrill cores in the west and to DSDP cores in the distant East. This stratigraphy, and new fault interpretations, was developed using different resolutions of seismic reflection data included those available from the Seismic Data Library System. Depth conversion used a new 3D velocity model. A 29 Ma horizon is as deep as 8 km in the south, and a 19 Ma horizon is >5 km deep there and 4 km-deep 100 km farther north. There is a shallower northern part of Terror Rift misaligned with the southern basin across a 50 km right double bend. It is bounded by steep N-S faults down-dropping towards the basin axis. Between Cape Roberts and Ross Island, the Oligocene section is also progressively-tilted. This Oligocene section is not imaged within northern Terror Rift, but the simplest hypothesis is that some of the Terror Rift-bounding faults were active at least during Oligocene through Quaternary time. Many faults are normal separation, but some are locally vertical or even reverse-separation in the upper couple of km. However, much of the vertical relief of the strata is due to progressive tilting (horizontal axis rotation) and not by shallow faulting. Along the trend of the basin, the relief alternates between tilting and faulting, with a tilting margin facing a faulted margin across the Rift, forming asymmetric basins. Connecting faults across the basin form an accommodation zone similar to other oblique rifts. The Neogene basin is

  11. Magmatic nature of Sn-bearing fluids from isotopic (H,O) data of tourmalines (Solnechnoye deposit, Far East of Russia)

    NASA Astrophysics Data System (ADS)

    Sushchevskaya, Tatiana; Ignatiev, Alexander; Velivetskaya, Tatiana

    2010-05-01

    It was shown, that the main factors, controlled Sn-W ore deposition in hydrothermal systems in connection with granites were: a) cooling, b) mixing of fluids of different composition and genesis, c) heterogenization (boiling) of mineral forming fluids (Heinrich, 1990; et al). The results of physical chemical modelling of water-rock interaction pointed to magmatic nature of tin-bearing fluids in the hydrothermal systems, connected with granites. Tin concentration in model magmatogeneous fluid was calculated to be three times higher than in the model exogenic fluid (Sushchevskaya, Ryzhenko, 1998). New data on the source of ore forming fluids, which had formed the large economic Solnechnoye Sn deposit (Far East of Russia), were obtained from hydrogen and oxygen isotopic composition of tourmalines from successive mineral associations. Hydrogen position in the tourmaline crystal structure allows to suppose, that postproductive processes did not change the initial δD values, in contrast to phyllosilicates, often used for this purpose. During mineral formation at the Solnechnoye deposit tourmaline was formed practically without interruption. Its composition was gradually changed from schorl to dravite, when temperature decreased. The ores of cassiterite-tourmaline type were formed in a vertically dipping fracture zones, extended in a metamorphosed sandstone-shale rocks. Hydrogene and oxygen isotopic composition of fluids were calculated from tourmaline data with the help of equations, proposed in (Kotzer et al., 1993). The temperature intervals for these calculations were accepted as: 1- 450-400o C for quartz-tourmaline preore stage, 2 - 400-350o C -for early productive quartz-cassiterite stage, 3 - 350-300o C - for late quartz- cassiterite stage. The obtained results corresponded to magmatic character of the fluids of productive stage of the Solnechnoye deposit, because their isotopic composition lied in the interval: δ18О (3.2-9.3)‰, δD -(31.7-76.6)‰. The

  12. 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.

  13. Rift to drift transition in Siberian Arctic and its impact on continental margin architecture

    NASA Astrophysics Data System (ADS)

    Drachev, S. S.

    2003-04-01

    The East Siberian Arctic Continental Margin (ESAM) represents a rare case of rifting to spreading transition. Present-day geodynamics of this plate tectonic interplay is characterized by a very slow plate divergence in the Laptev Sea as this regions is located just landward of the slowest spreading center worldwide (the Gakkel Ridge), close to the pole of North American/Eurasian plate rotation. However the existing geological and geophysical data, mainly seismic reflection and potential field data, allow conclusion that this situation has been far different in the past. Just after its formation at the end of Late Cretaceous through a series of plate convergence and folding episodes the crust of the ESAM has been strongly modified by an intense rifting. The earliest rift episode took place eastward of the present Laptev Sea, in the East Siberian Sea and probably Chukchi seas, where presently abandoned rifts are stretched landward along the principal weakened zones in the ESAM basement. This rifting might have been related to a spreading episode in the Amerasia Basin and perhaps was triggered by a mantle plume ca. 120 mln. yr. ago (De Long and Franz Joseph Land basalts). Outer parts of the ERAM might have also been rifted away to create marginal blocks, as the Arlis and Chukchi plateau. Second rift event was clearly related to the opening of the Eurasia Basin, preceding it and remaining active through the Cenozoic. The rift to drift transition has been taking place in a huge, “dry” and still active Laptev Rift System, which is a landward projection of the Gakkel Ridge spreading axis. This extension had a major effect on the western ERAM causing strong normal faulting and crustal thinning, up to 70% in some places. However, total crustal extension in the Laptev Rift System is considerably smaller than a value of total opening of the Eurasia Basin, so the spreading is not completely accommodated by the rifting. It may be speculated that a major portion of this

  14. Diking-induced moderate-magnitude earthquakes on a youthful rift border fault: The 2002 Nyiragongo-Kalehe sequence, D.R. Congo

    NASA Astrophysics Data System (ADS)

    Wauthier, C.; Smets, B.; Keir, D.

    2015-12-01

    On 24 October 2002, Mw 6.2 earthquake occurred in the central part of the Lake Kivu basin, Western Branch of the East African Rift. This is the largest event recorded in the Lake Kivu area since 1900. An integrated analysis of radar interferometry (InSAR), seismic and geological data, demonstrates that the earthquake occurred due to normal-slip motion on a major preexisting east-dipping rift border fault. A Coulomb stress analysis suggests that diking events, such as the January 2002 dike intrusion, could promote faulting on the western border faults of the rift in the central part of Lake Kivu. We thus interpret that dike-induced stress changes can cause moderate to large-magnitude earthquakes on major border faults during continental rifting. Continental extension processes appear complex in the Lake Kivu basin, requiring the use of a hybrid model of strain accommodation and partitioning in the East African Rift.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  16. Kinematics and Dynamics of the Kivu Rift System from Seismic Anisotropy, Seismicity, and Structural Analyses

    NASA Astrophysics Data System (ADS)

    Zal, H. J.; Wood, D. A.; Ebinger, C. J.; Scholz, C. A.; d'Oreye, N.; Carn, S. A.; Rutagarama, U.

    2014-12-01

    The westward-tilted Kivu rift in East Africa is bounded by the ~100 km-long, seismically active West Kivu border fault, and dammed at its northern end by flows from the Virunga Volcanic Province. Earlier work delineated faults along the basin margins, but little was known of active faults beneath Lake Kivu, and the lithospheric structure was unexplored. The aims of this study are to determine the kinematics of normal faults and their relation to pre-existing basement structures; to examine the locations of earthquakes with respect to faults in order to delineate zones of active faulting; to evaluate models for the modification of lithosphere by extension and mantle plume processes using seismic shear wave splitting measurements; and to evaluate the role of volcanic loading within the Virunga volcanic province on the evolution of the Kivu basin. We determine rift fault and volcanic fissure locations and orientations using merged high-resolution CHIRP bathymetric and Space Radar Topography Mission data. The majority of faults in the northern sector strike NNE, whereas NE faults are equally important in the southern basin, marking the Kivu-Rusizi accommodation zone. Seismic data was acquired from an 8-station array deployed between March 2012 and April 2013. Although the majority of earthquakes beneath the rift (excluding the active volcanoes) occur at depths of 8-20 km, unusually shallow earthquakes (2-4 km) are located along submerged faults within the East Kivu basin and suggest high pore pressures within the upper crust. Using simple elastic plate flexure model calculations we estimate the maximum deflection of the plate to be ~7 km, using an effective elastic thickness of ~7.5 km. We propose that the rapid subsidence of the ~400 m deep northern Kivu basin occurred in response to volcanic construction. We evaluate models for the modification of lithosphere using shear wave splitting measurements. Splitting results with backazimuths ranging from 88˚ - 98˚ and 240

  17. 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.

  18. Active fault segments as potential earthquake sources: Inferences from integrated geophysical mapping of the Magadi fault system, southern Kenya Rift

    NASA Astrophysics Data System (ADS)

    Kuria, Z. N.; Woldai, T.; van der Meer, F. D.; Barongo, J. O.

    2010-06-01

    Southern Kenya Rift has been known as a region of high geodynamic activity expressed by recent volcanism, geothermal activity and high rate of seismicity. The active faults that host these activities have not been investigated to determine their subsurface geometry, faulting intensity and constituents (fluids, sediments) for proper characterization of tectonic rift extension. Two different models of extension direction (E-W to ESE-WNW and NW-SE) have been proposed. However, they were based on limited field data and lacked subsurface investigations. In this research, we delineated active fault zones from ASTER image draped on ASTER DEM, together with relocated earthquakes. Subsequently, we combined field geologic mapping, electrical resistivity, ground magnetic traverses and aeromagnetic data to investigate the subsurface character of the active faults. Our results from structural studies identified four fault sets of different age and deformational styles, namely: normal N-S; dextral NW-SE; strike slip ENE-WSW; and sinistral NE-SW. The previous studies did not recognize the existence of the sinistral oblique slip NE-SW trending faults which were created under an E-W extension to counterbalance the NW-SE faults. The E-W extension has also been confirmed from focal mechanism solutions of the swarm earthquakes, which are located where all the four fault sets intersect. Our findings therefore, bridge the existing gap in opinion on neo-tectonic extension of the rift suggested by the earlier authors. Our results from resistivity survey show that the southern faults are in filled with fluid (0.05 and 0.2 Ωm), whereas fault zones to the north contain high resistivity (55-75 Ωm) material. The ground magnetic survey results have revealed faulting activity within active fault zones that do not contain fluids. In addition, the 2D inversion of the four aero-magnetic profiles (209 km long) revealed: major vertical to sub vertical faults (dipping 75-85° east or west); an

  19. Seismic monitoring of the Olkaria Geothermal area, Kenya Rift valley

    NASA Astrophysics Data System (ADS)

    Simiyu, Silas M.; Keller, G. Randy

    2000-01-01

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

  20. Rifting and Post-Rift Reactivation of The Eastern Sardinian Margin (Western Tyrrhenian Back-Arc Basin) Evidenced by the Messinian Salinity Crisis Markers and Salt Tectonics

    NASA Astrophysics Data System (ADS)

    Gaullier, V.; Chanier, F.; Vendeville, B.; Lymer, G.; Lofi, J.; Sage, F.; Maillard, A.; Thinon, I.

    2014-12-01

    The Eastern Sardinian margin formed during the opening of the Tyrrhenian Sea, a back-arc basin created by continental rifting and oceanic spreading related to the eastward migrating Apennine subduction system from middle Miocene to Pliocene times. We carried out the "METYSS" project aiming at better understanding the Miocene-Pliocene relationships between crustal tectonics and salt tectonics in this key-area, where rifting is pro parte coeval with the Messinian Salinity Crisis (MSC, 5.96-5.33 Ma) and Messinian salt décollement creates thin-skinned tectonics. Thereby, we use the MSC seismic markers and the deformation of viscous salt and its brittle overburden as proxies to better delineate the timing of rifting and post-rift reactivation, and especially to quantifying vertical and horizontal movements. Our mapping of the Messinian Erosion Surface and of Messinian Upper and Mobile Units shows that a rifted basin already existed by the Messinian times, revealing a major pre-MSC rifting episode across the entire domain. Because salt tectonics can create fan-shaped geometries in sediments, syn-rift deposits have to be carefully re-examined in order to decipher the effects of crustal tectonics (rifting) and salt tectonics. Our data surprisingly showed that there are no clues for Messinian syn-rift sediments along the East-Sardinia Basin and Cornaglia Terrace, hence no evidence for rifting after Late Tortonian times. Nevertheless, widespread deformation occurred during the Pliocene and is attributed to post-rift reactivation. Some Pliocene vertical movements have been evidenced by discovering localized gravity gliding of the salt and its Late Messinian (UU) and Early Pliocene overburden. To the South, crustal-scale southward tilting triggered along-strike gravity gliding of salt and cover recorded by upslope extension and downslope shortening. To the North, East of the Baronie Ridge, there was some post-salt crustal activity along a narrow N-S basement trough, bounded

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  2. Massive and prolonged deep carbon emissions associated with continental rifting

    NASA Astrophysics Data System (ADS)

    Lee, Hyunwoo; Muirhead, James D.; Fischer, Tobias P.; Ebinger, Cynthia J.; Kattenhorn, Simon A.; Sharp, Zachary D.; Kianji, Gladys

    2016-02-01

    Carbon from Earth’s interior is thought to be released to the atmosphere mostly via degassing of CO2 from active volcanoes. CO2 can also escape along faults away from active volcanic centres, but such tectonic degassing is poorly constrained. Here we use measurements of diffuse soil CO2, combined with carbon isotopic analyses to quantify the flux of CO2 through fault systems away from active volcanoes in the East African Rift system. We find that about 4 Mt yr-1 of mantle-derived CO2 is released in the Magadi-Natron Basin, at the border between Kenya and Tanzania. Seismicity at depths of 15-30 km implies that extensional faults in this region may penetrate the lower crust. We therefore suggest that CO2 is transferred from upper-mantle or lower-crustal magma bodies along these deep faults. Extrapolation of our measurements to the entire Eastern rift of the rift system implies a CO2 flux on the order of tens of megatonnes per year, comparable to emissions from the entire mid-ocean ridge system of 53-97 Mt yr-1. We conclude that widespread continental rifting and super-continent breakup could produce massive, long-term CO2 emissions and contribute to prolonged greenhouse conditions like those of the Cretaceous.

  3. Sulfur isotope study of chimney minerals and vent fluids from 21°N, East Pacific Rise: Hydrothermal sulfur sources and disequilibrium sulfate reduction

    NASA Astrophysics Data System (ADS)

    Woodruff, Laurel G.; Shanks, Wayne C., III

    1988-05-01

    Sulfur isotopic analyses of hydrothermal chimneys and coexisting fluids collected from hot vent areas at 21°N, East Pacific Rise, indicate δ34S values for sulfide minerals of 0.9-6.2‰ and for aqueous H2S of 1.3-5.5‰. The chemical processes resulting in mineral zoning of chimney structures are a controlling influence on 34S variation within individual chimneys. In general, coarsely crystalline chalcopyrite, which lines chimney interiors, is depleted in δ34S relative to exterior ZnS (either sphalerite or wurtzite). Chalcopyrite which formed by replacement of other sulfides has higher sulfur isotope values than chalcopyrite which precipitated directly from hydrothermal fluid. Bornite and other Cu-rich sulfides, which occur as thin layers replacing the outer margins of massive chalcopyrite zones, have the highest δ34S values of any 21°N samples, ranging from 5.2‰ to 6.2‰. The significant 34S enrichment in bornite requires reduction of isotopically heavy seawater-derived sulfate in the chimney walls. Vent fluid δ34SH2S values are similar to δ34S values of inner wall chimney sulfides for seven of 11 samples. In other samples, aqueous H2S is enriched isotopically in δ34S compared with coexisting sulfides, suggesting admixture of H2S derived from reduced seawater sulfate. The δ34S value of end-member hydrothermal fluid at 21°N is about 1.0-1.5‰, which is somewhat higher than basaltic sulfide, the main source of sulfur in the hydrothermal system. This 34S enrichment apparently is due to sulfate reduction in the deep hydrothermal system during conversion of basaltic pyrrhotite to pyrite. The South West (SW) vent field has significantly higher average δ34S values (3.1‰) compared with the other 21°N vent fields. The SW field is situated on the edge of a lava collapse structure, promoting development of an extensive feeder zone which is the site of near-surface sulfate reduction during redissolution of previously deposited anhydrite. The δ34S values for

  4. 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.

  5. Rift-stage evolution of northern Gulf of Mexico

    SciTech Connect

    Watkins, J.S.

    1989-03-01

    Distribution of salt, location of major growth-fault trends, gravity and magnetic anomalies, and deep seismic reflections appear to be related to relict rift-stage basement structures in the northern Gulf of Mexico and Gulf Coast. The basement contains a number of discrete salt basins, probably half grabens, formed during initial rifting. Transform faults trending north-northwest-south-southeast form the sides of the basins. Salt thickness is variable but increases in basins to the east and south. The Corsair fault appears to root in the northern edge of a salt basin, but the surface of the fault crops out over an interbasin high. East-west dislocations of the gravity and magnetic fields coincide with the inferred transform faults; in some cases, linear magnetic anomalies are associated with interbasin highs. Strong deep reflectors are locally associated with basement highs. Overall, the picture appears to be one of north-south rifting, broken by north-northwest-south-southeast-trending transform faults. The overall pattern suggests left-lateral shear.

  6. A continental rift model for the La Grande greenstone belt

    NASA Technical Reports Server (NTRS)

    Skulski, T.; Hynes, A.; Liu, M.; Francis, D.; Rivard, B.; Stamatelopoulou-Seymour, K.

    1986-01-01

    Stratigraphic relationships and the geochemistry of volcanic rocks contrain the nature and timing of the tectonic and magmatic processes in the pre-deformational history of the La Grande greenstone belt in the Superior Province of north-central Quebec. The lowermost supracrustals in this belt are obscured by syntectonic granitoid intrusives. The supracrustal succession in the western part of the belt consists of a lower sequence of immature clastic sediments and mafic volcanoclastics, overlain by pillowed and massive basalts. Further east, along tectonic strike, a lower sequence of mafic volcanoclastics and immature clastic sediments is overlain by a thick sequence of pillowed and massive basalts, and resedimented coarse clastic sediments and banded iron formation. These are overlain by assive basaltic andesites, andesites and intermediate volcanoclastics intercalated with immature clastic sediments. In contrast, in the eastern part of the belt lenses of felsic volcanics and volcanoclastics occur at the base of the succession and pillowed and massive basalts are overlain by komatiites at the top. The La Grande greenstone belt can be explained as the product of continental rifting. The restricted occurence of komatiites, and eastwardly directed paleocurrents in clastic sediments in the central part of the belt are consistent with rifting commencing in the east and propagating westward with time. The increase in depth of emplacement and deposition with time of the lower three units in the central part of the belt reflects deposition in a subsiding basin. These supracrustal rocks are believed to represent the initial rift succession.

  7. 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

  8. 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.

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

    NASA Astrophysics Data System (ADS)

    Nelson, W. R.; Furman, T.

    2013-12-01

    The configuration of continents we know today is the result of several billion years of active Wilson Cycle tectonics. The rifting of continents and subsequent development of ocean basins is an integral part of long-term planetary-scale recycling processes. The products of this process can be seen globally, and the East African Rift System (EARS) provides a unique view of extensional processes that actively divide a continent. Taken together with the adjoining Red Sea and Gulf of Aden, the EARS has experienced over 40 Ma of volcanism and ~30 Ma of extension. While early (pre-rift) volcanism in the region is attributed to mantle plume activity, much of the subsequent volcanism occurs synchronously with continental rifting. Numerous studies indicate that extension and magmatism are correlated: extension leads to decompression melting while magmatism accommodates further extension (e.g. Stein et al., 1997; Buck 2004; Corti 2012). Evaluation of the entire EARS reveals significant geochemical patterns - both spatial and temporal - in the volcanic products. Compositional variations are tied directly to the melt source(s), which changes over time. These variations can be characterized broadly by region: the Ethiopian plateau and Turkana Depression, the Kenya Rift, and the Western Rift. In the Ethiopian plateau, early flood basalt volcanism is dominated by mantle plume contributions with variable input from lherzolitic mantle lithosphere. Subsequent alkaline shield volcanism flanking the juvenile Main Ethiopian Rift records the same plume component as well as contributions from a hydrous peridotitic lithosphere. The hydrous lithosphere does not contribute indefinitely. Instead, young (< 2 Ma) volcanism taps a combination of the mantle plume and anhydrous depleted lithospheric mantle. In contrast, volcanism in the Kenya Rift and the Western Rift are derived dominantly from metasomatized lithospheric mantle rather than mantle plume material. These rifts lie in the mobile

  10. 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.

  11. Rifting on Venus: Implications for lithospheric structure

    NASA Technical Reports Server (NTRS)

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

    1985-01-01

    Lithospheric strength envelopes on Venus are reviewed and their implications for large scale rifting are discussed. Their relationship to crustal thicnesses and thermal gradients are explored. Also considered are the implications of a theory for rift formation.

  12. Geodynamic models of the Wilson Cycle: From rifts to mountains to rifts

    NASA Astrophysics Data System (ADS)

    Buiter, Susanne; Tetreault, Joya; Torsvik, Trond

    2015-04-01

    The Wilson Cycle theory that oceans close and reopen along the former suture is a fundamental concept in plate tectonics. The theory suggests that subduction initiates at a passive margin, closing the ocean, and that future continental extension localises at the ensuing collision zone. Each stage of the Wilson Cycle will therefore be characterised by inherited structural and thermal heterogeneities. Here we investigate the role of Wilson Cycle inheritance by considering the influence of (1) passive margin structure on continental collision and (2) collision zones on passive margin formation. Passive margins may be preferred locations for subduction initiation because inherited faults and areas of exhumed serpentinized mantle may weaken a margin enough to localise shortening. If subduction initiates at a passive margin, the shape and structure of the passive margins will affect future continental collision. Our review of present-day passive margins along the Atlantic and Indian Oceans reveals that most passive margins are located on former collision zones. Continental break-up occurs on relatively young sutures, such as Morocco-Nova Scotia, and on very old sutures, such as the Greenland-Labrador and East Antarctica-Australia systems. This implies that it is not always post-collisional collapse that initiates the extensional phase of a Wilson Cycle. We highlight the impact of collision zone inheritance on continental extension and rifted margin architecture. We show numerical experiments of one Wilson Cycle of subduction, collision, and extension. Subduction initiates at a tapered passive margin. Closure of a 60 Ma ocean leads to continental collision and slab break-off, followed by some tens of kilometres of slab eduction. Mantle flow above the sinking detached slab enhances deformation in the rift area. The resulting rift exposes not only continental crust, but also subduction-related sediments and oceanic crust remnants. Renewed subduction in the post

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

    NASA Astrophysics Data System (ADS)

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

    2011-11-01

    The Galapagos triple junction is not a simple ridge-ridge-ridge (RRR) triple junction. The Cocos-Nazca Rift (C-N Rift) tip does not meet the East Pacific Rise (EPR). Instead, two secondary rifts form the link: Incipient Rift at 2°40‧N and Dietz Deep volcanic ridge, the southern boundary of the Galapagos microplate (GMP), at 1°10‧N. Recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the GMP (∼1.5 Ma). South of C-N Rift a band of northeast-trending cracks cuts EPR-generated abyssal hills. It is a mirror image of a band of cracks previously identified north of C-N Rift on the same age crust. In both areas, the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the EPR spreading center and cut eastward into pre-existing topography. Each crack formed a short-lived triple junction until it was abandoned and a new crack and triple junction initiated nearby. Between 2.5 and 1.5 Ma, the pattern of cracking is remarkably symmetric about C-N Rift providing support for a crack interaction model in which crack initiation at the EPR axis is controlled by stresses associated with the tip of the westward-propagating C-N Rift. The model also shows that offsets of the EPR axis may explain times when cracking is not symmetric. South of C-N Rift, cracks are observed on seafloor as old as 10.5 Ma suggesting that this triple junction has not been a simple RRR triple junction during that time.

  14. Evaluation of lamb and calf responses to Rift Valley fever MP-12 vaccination

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is an important viral disease of animals and humans in Africa and the Middle East that is transmitted by mosquitoes. The disease is of concern to international agricultural and public health communities. The RVF MP-12 strain has been the most safety tested attenuated vaccine ...

  15. Structural inheritance during normal fault growth in multi-phase rifts; a case study from the Northern North Sea

    NASA Astrophysics Data System (ADS)

    Fazli Khani, Hamed; Bell, Rebecca E.; Fossen, Haakon; A-L. Jackson, Christopher; Rotevatn, Atle; Gawthorpe, Robert L.

    2015-04-01

    In multi-phase rift systems such as the northern North Sea rift, pre-existing basement structures influence the nucleation, growth and linkage of rift-related normal faults. However, our understanding of the degree of physical and kinematic linkage between basement and cover structures is limited, since deep structures are generally poorly imaged on seismic reflection data. In the North Sea Rift, two main phases of rifting are recognized in the Permian-Triassic and Middle Jurassic-to-Early Cretaceous. Moreover, prior to rifting, the area underwent multiple episodes of deformation during the Ordovician-Devonian Caledonian orogeny and Devonian extension. In this study we investigate the influence of pre-existing structures on the i) evolution of Permian-Triassic and Middle Jurassic-Early Cretaceous normal fault systems and ii) distribution of strain during reactivation of older structures in the northern North Sea rift. For this purpose we utilize 2D (-9 s TWT) and 3D seismic reflection and borehole data from the North Viking Graben, covering the Horda Platform in the east and the East Shetland Basin in the west. We show that low-angle (< 30°) intrabasement reflections extend, in some areas, upward into the Triassic section. West-dipping and east-dipping intrabasement structures are identified in the Horda Platform and East Shetland Basin respectively, while in the Northern Viking Graben area both west and east-dipping structures are mapped. At depth, some of intrabasement structures terminate against high-amplitude reflections in the lower-crust. This study documents dissimilar development of Intrabasement structures in the Horda Platform, Viking Graben and East Shetland Basin. In the Viking Graben and Horda Platform these structures are more developed and in some places cross-cut each other, while in the East Shetland Basin, only two sets of structures have been mapped. We also show that intrabasement structures in the Horda Platform are generally lower angle than

  16. 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

  17. 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

  18. Chemical controls on the composition of vent fluids at 13°-11°N and 21°N, East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Bowers, Teresa Suter; Campbell, Andrew C.; Measures, Chris I.; Spivack, Arthur J.; Khadem, Mitra; Edmond, John M.

    1988-05-01

    Six vent fields sampled at 13°-11°N, East Pacific Rise (EPR) in May 1984 exhibit large interfield variations and a much wider range of chemical compositions than previously observed at 21°N. Measured pH at 25°C are acidic, ranging from 3.1 to 3.7. Sodium and chloride vary from 40% lower to 30% higher than seawater. Iron concentrations range from 2 to 10 mmol/kg, compared with 0.7-2.5 mmol/kg at 21°N. Other sulfide-forming metals (Cu, Zn, Cd, and Pb) are generally lower at 11°-13°N than at 21°N. Reliable temperature measurements were obtained at only two of the six vents and both were 350° ± 5°C. The vent fields at 21°N, EPR were resampled in August 1985, thus extending to almost 6 years the time period over which they have been monitored (previous expeditions were made in November 1979 and November 1981). Campbell et al. (this issue) have shown that the chemistry of the hydrothermal fluids from these fields has been very stable over the period of repeated observation. Equilibrium calculations for the fluids from the fields at 13°N and 21°N, using a greatly improved thermodynamic data base, are described in this report. They indicate that the chemistry is rock buffered and that the stability of these systems over time is a result of equilibrium control with respect to a greenschist-type mineral assemblage at depth. Calculated high-temperature pH of the fluids range from 4.1 to 4.7 with those from 13°-11°N at the more acidic end of the range. Calculated affinities show that the fluids are close to, or at, saturation with respect to quartz, albite, muscovite, smectite/chlorite, epidote, and pyrrhotite. The computations imply that lower-temperature vents such as National Geographic Smoker (NGS) (273°C) may have their silica concentrations controlled by equilibrium with respect to a phase other than quartz. A comparison of fluid chemistry between NGS and vent 5 at 11°N suggests that the latter vent may also have a temperature < 300°C. While the fluid

  19. Geochronology and fluid inclusion study of the Yinjiagou porphyry-skarn Mo-Cu-pyrite deposit in the East Qinling orogenic belt, China

    NASA Astrophysics Data System (ADS)

    Wu, Guang; Chen, Yuchuan; Li, Zongyan; Liu, Jun; Yang, Xinsheng; Qiao, Cuijie

    2014-01-01

    The Yinjiagou Mo-Cu-pyrite deposit of Henan Province is located in the Huaxiong block on the southern margin of the North China craton. It differs from other Mo deposits in the East Qingling area because of its large pyrite resource and complex associated elements. The deposit's mineralization process can be divided into skarn, sulfide, and supergene episodes with five stages, marking formation of magnetite in the skarn episode, quartz-molybdenite, quartz-calcite-pyrite-chalcopyrite-bornite-sphalerite, and calcite-galena-sphalerite in the sulfide episode, and chalcedony-limonite in the supergene episode. Re-Os and 40Ar-39Ar dating indicates that both the skarn-type and porphyry-type orebodies of the Yinjiagou deposit formed approximately 143 Ma ago during the Early Cretaceous. Four types of fluid inclusions (FIs) have been distinguished in quartz phenocryst, various quartz veins, and calcite vein. Based on petrographic observations and microthermometric criteria the FIs include liquid-rich, gas-rich, H2O-CO2, and daughter mineral-bearing inclusions. The homogenization temperature of FIs in quartz phenocrysts of K-feldspar granite porphyry ranges from 341 °C to >550 °C, and the salinity is 0.4-44.0 wt% NaCl eqv. The homogenization temperature of FIs in quartz-molybdenite veins is 382-416 °C, and the salinity is 3.6-40.8 wt% NaCl eqv. The homogenization temperature of FIs in quartz-calcite-pyrite-chalcopyrite-bornite-sphalerite ranges from 318 °C to 436 °C, and the salinity is 5.6-42.4 wt% NaCl eqv. The homogenization temperature of FIs in quartz-molybdenite stockworks is in a range of 321-411 °C, and the salinity is 6.3-16.4 wt% NaCl eqv. The homogenization temperature of FIs in quartz-sericite-pyrite is in a range of 326-419 °C, and the salinity is 4.7-49.4 wt% NaCl eqv. The ore-forming fluids of the Yinjiagou deposit are mainly high-temperature, high-salinity fluids, generally with affinities to an H2O-NaCl-KCl ± CO2 system. The δ18OH2O values of ore

  20. 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.

  1. Diversity of Meiofauna from the 9°50′N East Pacific Rise across a Gradient of Hydrothermal Fluid Emissions

    PubMed Central

    Gollner, Sabine; Riemer, Barbara; Martínez Arbizu, Pedro; Le Bris, Nadine; Bright, Monika

    2010-01-01

    Background We studied the meiofauna community at deep-sea hydrothermal vents along a gradient of vent fluid emissions in the axial summit trought (AST) of the East Pacific Rise 9°50′N region. The gradient ranged from extreme high temperatures, high sulfide concentrations, and low pH at sulfide chimneys to ambient deep-sea water conditions on bare basalt. We explore meiofauna diversity and abundance, and discuss its possible underlying ecological and evolutionary processes. Methodology/Principal Findings After sampling in five physico-chemically different habitats, the meiofauna was sorted, counted and classified. Abundances were low at all sites. A total of 52 species were identified at vent habitats. The vent community was dominated by hard substrate generalists that also lived on bare basalt at ambient deep-sea temperature in the axial summit trough (AST generalists). Some vent species were restricted to a specific vent habitat (vent specialists), but others occurred over a wide range of physico-chemical conditions (vent generalists). Additionally, 35 species were only found on cold bare basalt (basalt specialists). At vent sites, species richness and diversity clearly increased with decreasing influence of vent fluid emissions from extreme flow sulfide chimney (no fauna), high flow pompei worm (S: 4–7, H'loge: 0.11–0.45), vigorous flow tubeworm (S: 8–23; H'loge: 0.44–2.00) to low flow mussel habitats (S: 28–31; H'loge: 2.34–2.60). Conclusions/Significance Our data suggest that with increasing temperature and toxic hydrogen sulfide concentrations and increasing amplitude of variation of these factors, fewer species are able to cope with these extreme conditions. This results in less diverse communities in more extreme habitats. The finding of many species being present at sites with and without vent fluid emissions points to a non endemic deep-sea hydrothermal vent meiofaunal community. This is in contrast to a mostly endemic macrofauna but similar

  2. Inflation rates, rifts, and bands in a pāhoehoe sheet flow

    USGS Publications Warehouse

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

    2012-01-01

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

  3. North Sinai-Levant rift-transform continental margin

    SciTech Connect

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

    1985-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-02-01

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

  5. 3-D magnetotelluric image of offshore magmatism at the Walvis Ridge and rift basin

    NASA Astrophysics Data System (ADS)

    Jegen, Marion; Avdeeva, Anna; Berndt, Christian; Franz, Gesa; Heincke, Björn; Hölz, Sebastian; Neska, Anne; Marti, Anna; Planert, Lars; Chen, J.; Kopp, Heidrun; Baba, Kiyoshi; Ritter, Oliver; Weckmann, Ute; Meqbel, Naser; Behrmann, Jan

    2016-06-01

    The Namibian continental margin marks the starting point of the Tristan da Cunha hotspot trail, the Walvis Ridge. This section of the volcanic southwestern African margin is therefore ideal to study the interaction of hotspot volcanism and rifting, which occurred in the late Jurassic/early Cretaceous. Offshore magnetotelluric data image electromagnetically the landfall of Walvis Ridge. Two large-scale high resistivity anomalies in the 3-D resistivity model indicate old magmatic intrusions related to hot-spot volcanism and rifting. The large-scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2-D offshore seismic profiles. One of the high resistivity anomalies (above 500 Ωm) has three arms of approximately 100 km width and 300 km to 400 km length at 120° angles in the lower crust. One of the arms stretches underneath Walvis Ridge. The shape is suggestive of crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting. A second, smaller anomaly of 50 km width underneath the continent ocean boundary may be attributed to magma ascent during rifting. We attribute a low resistivity anomaly east of the continent ocean boundary and south of Walvis Ridge to the presence of a rift basin that formed prior to the rifting.

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

    SciTech Connect

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

    1989-06-10

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

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  8. Rapid spatiotemporal variations in rift structure during development of the Corinth Rift, central Greece

    NASA Astrophysics Data System (ADS)

    Nixon, Casey W.; McNeill, Lisa C.; Bull, Jonathan M.; Bell, Rebecca E.; Gawthorpe, Robert L.; Henstock, Timothy J.; Christodoulou, Dimitris; Ford, Mary; Taylor, Brian; Sakellariou, Dimitris; Ferentinos, George; Papatheodorou, George; Leeder, Mike R.; Collier, Richard E. LI.; Goodliffe, Andrew M.; Sachpazi, Maria; Kranis, Haralambos

    2016-05-01

    The Corinth Rift, central Greece, enables analysis of early rift development as it is young (<5 Ma) and highly active and its full history is recorded at high resolution by sedimentary systems. A complete compilation of marine geophysical data, complemented by onshore data, is used to develop a high-resolution chronostratigraphy and detailed fault history for the offshore Corinth Rift, integrating interpretations and reconciling previous discrepancies. Rift migration and localization of deformation have been significant within the rift since inception. Over the last circa 2 Myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift, but this transition did not occur synchronously along strike. Isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620-340 ka, characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at ~30 m/kyr. Since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin, demonstrated by lateral growth of discrete depocenters at ~40 m/kyr. A single central depocenter formed by circa 130 ka, indicating full fault linkage. These results indicate that rift localization is progressive (not instantaneous) and can be synchronous once a rift border fault system is established. This study illustrates that development processes within young rifts occur at 100 kyr timescales, including rapid changes in rift symmetry and growth and linkage of major rift faults.

  9. 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.

  10. Middle Jurassic - Early Cretaceous rifting on the Chortis Block in Honduras: Implications for proto-Caribbean opening (Invited)

    NASA Astrophysics Data System (ADS)

    Rogers, R. D.; Emmet, P. A.

    2009-12-01

    Regional mapping integrated with facies analysis, age constraints and airborne geophysical data reveal WNW and NE trends of Middle Jurassic to Early Cretaceous basins which intersect in southeast Honduras that we interpret as the result of rifting associated with the breakup of the Americas and opening of the proto-Caribbean seaway. The WNW-trending rift is 250 km long by 90 km wide and defined by a basal 200 to 800 m thick sequence of Middle to Late Jurassic fluvial channel and overbank deposits overlain by transgressive clastic shelf strata. At least three sub-basins are apparent. Flanking the WNW trending rift basins are fault bounded exposures of the pre-Jurassic continental basement of the Chortis block which is the source of the conglomeratic channel facies that delineate the axes of the rifts. Cretaceous terrigenous strata mantle the exposed basement-cored rift flanks. Lower Cretaceous clastic strata and shallow marine limestone strata are dominant along this trend indicating that post-rift related subsidence continued through the Early Cretaceous. The rifts coincide with a regional high in the total magnetic intensity data. We interpret these trends to reflect NNE-WSW extension active from the Middle Jurassic through Early Cretaceous. These rifts were inverted during Late Cretaceous shortening oriented normal to the rift axes. To the east and at a 120 degree angle to the WNW trending rift is the 300 km long NE trending Guayape fault system that forms the western shoulder of the Late Jurassic Agua Fria rift basin filled by > 2 km thickness of clastic marine shelf and slope strata. This NE trending basin coincides with the eastern extent of the surface exposure of continental basement rocks and a northeast-trending fabric of the Jurassic (?) metasedimentary basement rocks. We have previously interpreted the eastern basin to be the Jurassic rifted margin of the Chortis block with the Guayape originating as a normal fault system. These two rifts basin intersect

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

    NASA Astrophysics Data System (ADS)

    Hollingsworth, James; Leprince, SéBastien; Ayoub, FrançOis; Avouac, Jean-Philippe

    2012-11-01

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

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

    NASA Technical Reports Server (NTRS)

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

    1976-01-01

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

  13. 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].

  14. 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.

  15. Albertine Rift, Uganda: Deformation-Sedimentation-Erosion relationships

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  16. 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.

  17. The Transition from Initial Rifting to Ultra-Slow Seafloor Spreading within Endeavor Deep

    NASA Astrophysics Data System (ADS)

    Pockalny, R. A.; Larson, R. L.; Popham, C. T.; Natland, J. H.

    2004-12-01

    Endeavor Deep is a NW-SE trending, 3 km-deep rift basin located along the divergent portion of the Nazca/Juan Fernandez plate boundary. The rift basin is the result of the propagation of the East Ridge toward the northwest with relative motion across the ridge defined by a rapidly rotating (5.5 degrees/myr) Euler Pole located ~100 km to the northwest. The close proximity of Endeavor Deep to this Euler Pole results in a rapidly varying velocity field along the length of the deep and represents a unique location to study the effect of varying divergence rates on initial crustal extension. Recently collected EM300 bathymetry, DSL120 sidescan, surface-towed magnetics and JASON II observations have documented 4 distinct stages of rifting along the 70 km length of Endeavor Deep. These stages include (from NW to SE): amagmatic rifting, distributed initial volcanism, centralized waxing volcanism, and crustal formation by ultra-slow seafloor spreading. Amagmatic extension, evolving to rifting, occurs at spreading rates less than 13 km/myr and is characterized by rapidly deepening rift depths from NW to SE with an overall increase in depth of about 2.5 km. Extension is accommodated over a width of about 10-15 km and some flexural uplift of the defining scarps is observed. Distributed initial volcanism occurs at spreading rates from 13-14 km/myr and is characterized by coalesced volcanic constructs (100-200 m-high, 1-2 km-wide) across the width of the rift floor. The depth of the rift basin becomes fairly constant, but the cross-sectional area of the deep continues to increase. Centralized waxing volcanism occurs at spreading rates from 14-17 km/myr and is characterized by pillow ridges and tectonic lineations along the central portion of the rift floor which are oriented parallel to the long axis of the rift basin (orthogonal to the direction of extension). The floor of the rift basin begins to shoal and the cross-sectional area of the deep decreases initially and then

  18. New data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system

    NASA Astrophysics Data System (ADS)

    Dobrynina, A. A.; Sankov, V. A.; Chechelnitsky, V. V.

    2016-05-01

    The investigation data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system obtained with a seismic coda envelope and sliding window are considered. Eleven local districts were described by one-dimensional attenuation models characterized by alternation of high and low attenuation layers, which are consistent with the results obtained previously by Yu.F. Kopnichev for the southwestern flank of the Baikal rift system [9]. The subcrust of the lithosphere contains a thin layer with high attenuation of seismic waves likely related to higher heterogeneity (fragmentation) and occurrence of fluids. The lithosphere basement depth varies from 100-120 km in the west within the Baikal folded area to 120-140 km in the east within the Siberian Platform. It is concluded that there are two asthenosphere layers. Based on specific features of the lithosphere and upper mantle structure, it can be assumed that they were subject to gradual modification involving fluidization processes and partial melting in the Late Cenozoic extension under the influence of distant tectogenesis sources.

  19. Interaction between an incipient rift and a cratonic lithosphere : The North Tanzania Rift seen from some seismic tools

    NASA Astrophysics Data System (ADS)

    Gautier, Stéphanie; Plasman, Matthieu; Tiberi, Christel; Lopez, Marie; Peyrat, Sophie; Perrot, Julie; Albaric, Julie; Déverchère, Jacques; Deschamps, Anne; Ebinger, Cindy; Roecker, Steven; Mulibo, Gabriel; Wambura, Richard Ferdinand; Muzuka, Alfred; Msabi, Michael; Gama, Remigius

    2016-04-01

    The North Tanzania part of the East African Rift is the place of an incipient break up of the lithosphere. This continental rifting happens on the edge of the Tanzanian craton, and their interaction leads to major changes in the surface deformation. The evolution of the rift and its morphology is strongly linked to the inherited structures, particularly the Proterozoic belts and the craton itself. It is thus of prime interest to image the structure of the craton edges to fully understand its impact on the localisation of the current deformation at the surface. Since 2007 different multidisciplinary projects have taken place in this area to address this question. We present here a work based on a collaborative work between French, American and Tanzanian institutes that started in 2013. About 35 seismological stations were installed for 2 years in the Natron lake region, and 10 short period instruments were added for 9 months in the Manyara area to record local and telesismic events. We have analysed more than a hundred teleseismic events to compute the receiver function, and we finally obtain a Moho map of the region as well as azimuthal distribution of converted phases. The stations located on the edge of the rift and near the craton present a continuous evolution of their crustal pattern in the RF signal. Especially, we identify a clear phase at about 7s for those stations that corresponds to an interface separating a normal upper mantle from a very slow mantle at about 70 km depth. We first model those receiver functions to perfectly fit the signal, and more precisely the transverse component, which shows a strong and coherent pattern. Second, the local seismic network we have installed for 9 months in Manyara region advantageously completed the 2007 SEISMOTANZ network. In this part of the rift the seismicity is deep (20-30 km) and clustered without any magmatism record at the surface, opposite to Natron area. We could then relocalize the deep seismicity observed

  20. Identifying deformation styles and causes at two deforming volcanoes of the Central Main Ethiopian Rift with seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Nowacki, Andy; Wilks, Matthew; Kendall, J.-Michael; Biggs, Juliet; Ayele, Atalay; Tulu, Beshahe; James, Wookey

    2016-04-01

    shown. Little evidence for a 'mushy', aseismic zone is found where geodetic studies have suggested a magma chamber is present. Hydrothermal processes may be responsible for much of the edifice loading, and we observe a positive correlation between rainfall and seismicity. At Corbetti, a completely different pattern emerges. Anisotropy is largest (up to 0.3 s) within the caldera, and weak outside. Fast shear waves are oriented northwest (NW), strongly oblique to Wonji or border faults, but parallel to a cross-rift structure, the Wendo Genet scarp, whose surface expression ends east of the caldera. Deep (20 km) earthquakes are located on this feature using the Corbetti and Aluto seismic arrays alongside Addis Ababa University stations. Intriguingly, shear wave splitting patterns are totally different for a few ray paths which avoid the Wendo Genet fault, indicating that away from this zone of deformation, the usual, rift-parallel faulting behaviour again holds sway. In this instance, the presence of anisotropy strong enough to overprint the background trend may require the alignment of fluids, and possibly melt. We suggest that this is evidence of a nascent transform zone within the rift.

  1. 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.

  2. Mechanical response of the south flank of kilauea volcano, hawaii, to intrusive events along the rift systems

    USGS Publications Warehouse

    Dvorak, J.J.; Okamura, A.T.; English, T.T.; Koyanagi, R.Y.; Nakata, J.S.; Sako, M.K.; Tanigawa, W.T.; Yamashita, K.M.

    1986-01-01

    Increased earthquake activity and compression of the south flank of Kilauea volcano, Hawaii, have been recognized by previous investigators to accompany rift intrusions. We further detail the temporal and spatial changes in earthquake rates and ground strain along the south flank induced by six major rift intrusions which occurred between December 1971 and January 1981. The seismic response of the south flank to individual rift intrusions is immediate; the increased rate of earthquake activity lasts from 1 to 4 weeks. Horizontal strain measurements indicate that compression of the south flank usually accompanies rift intrusions and eruptions. Emplacement of an intrusion at a depth greater than about 4 km, such as the June 1982 southwest rift intrusion, however, results in a slight extension of the subaerial portion of the south flank. Horizontal strain measurements along the south flank are used to locate the January 1983 east-rift intrusion, which resulted in eruptive activity. The intrusion is modeled as a vertical rectangular sheet with constant displacement perpendicular to the plane of the sheet. This model suggests that the intrusive body that compressed the south flank in January 1983 extended from the surface to about 2.4 km depth, and was aligned along a strike of N66??E. The intrusion is approximately 11 km in length, extended beyond the January 1983 eruptive fissures, which are 8 km in length and is contained within the 14-km-long region of shallow rift earthquakes. ?? 1986.

  3. Geophysical investigations of the tectonic boundary between East and west antarctica.

    PubMed

    Brink, U S; Bannister, S; Beaudoin, B C; Stern, T A

    1993-07-01

    The Transantarctic Mountains (TAM), which separate the West Antarctic rift system from the stable shield of East Antarctica, are the largest mountains developed adjacent to a rift. The cause of uplift of mountains bordering rifts is poorly understood. One notion based on observations of troughs next to many uplifted blocks is that isostatic rebound produces a coeval uplift and subsidence. The results of an over-snow seismic experiment in Antarctica do not show evidence for a trough next to the TAM but indicate the extension of rifted mantle lithosphere under the TAM. Furthermore, stretching preceded the initiation of uplift, which suggests thermal buoyancy as the cause for uplift.

  4. Geophysical investigations of the tectonic boundary between East and West Antarctica

    USGS Publications Warehouse

    ten Brink, U.S.; Bannister, S.; Beaudoin, B.C.; Stern, T.A.

    1993-01-01

    The Transantarctic Mountains (TAM), which separate the West Antarctic rift system from the stable shield of East Antarctica, are the largest mountains developed adjacent to a rift. The cause of uplift of mountains bordering rifts is poorly understood. One notion based on observations of troughs next to many uplifted blocks is that isostatic rebound produces a coeval uplift and subsidence. The results of an over-snow seismic experiment in Antarctica do not show evidence for a trough next to the TAM but indicate the extension of rifted mantle lithosphere under the TAM. Furthermore, stretching preceded the initiation of uplift, which suggests thermal buoyancy as the cause for uplift.

  5. Abrupt plate accelerations shape rifted continental margins

    NASA Astrophysics Data System (ADS)

    Brune, Sascha; Williams, Simon E.; Butterworth, Nathaniel P.; Müller, R. Dietmar

    2016-08-01

    Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth’s major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength–velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.

  6. Abrupt plate accelerations shape rifted continental margins.

    PubMed

    Brune, Sascha; Williams, Simon E; Butterworth, Nathaniel P; Müller, R Dietmar

    2016-08-11

    Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth's major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength--velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.

  7. Buried troughs, grabens and rifts in Sudan

    NASA Astrophysics Data System (ADS)

    Salama, R. B.

    The deep lineaments and shear patterns of Sudan follow two main directions :NNW (Red Sea trend) and ENE (Gulf of Aden trend). Precambrian mobile belts trend NE and NW. Palaezoic(?) sediments occupy NE-SW aligned grabens. Mesozoic continental sediments with NW paleotrends were deposited in two major depressions also aligned NW. Cainozoic uplift and volcanism was associated with domal uplifts along NE and SE axes. Fracturing and faulting in NW and NE directions resulted in the formation of NW-SE fault bounded grabens ranging in depth from 1 to 11 km. Extending from the western boundaries of Sudan to the eastern borders with Ethiopia, the Sudanese Cainozoic rift systemforms the largest rift system in Africa and includes: (a) Bahr E1 Arab Rift, (b) White Nile Rift, (c) Blue Nile Rift, (d) River Atbara Rift and (e) Wadi El Kuu Rift. The grabens and trouhs within the rift system form the main groundwater basins of Sudan. The discovery of oil in three of these rifts will encourage the exploration for oil in the others and a search for similar structures in the northern areas of Sudan.

  8. Abrupt plate accelerations shape rifted continental margins.

    PubMed

    Brune, Sascha; Williams, Simon E; Butterworth, Nathaniel P; Müller, R Dietmar

    2016-08-11

    Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth's major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength--velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time. PMID:27437571

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

    SciTech Connect

    Pyron, A.

    1991-08-01

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

  10. Regional 3D Numerical Modeling of the Lithosphere-Mantle System: Implications for Continental Rift-Parallel Surface Velocities

    NASA Astrophysics Data System (ADS)

    Stamps, S.; Bangerth, W.; Hager, B. H.

    2014-12-01

    The East African Rift System (EARS) is an active divergent plate boundary with slow, approximately E-W extension rates ranging from <1-6 mm/yr. Previous work using thin-sheet modeling indicates lithospheric buoyancy dominates the force balance driving large-scale Nubia-Somalia divergence, however GPS observations within the Western Branch of the EARS show along-rift motions that contradict this simple model. Here, we test the role of mantle flow at the rift-scale using our new, regional 3D numerical model based on the open-source code ASPECT. We define a thermal lithosphere with thicknesses that are systematically changed for generic models or based on geophysical constraints in the Western branch (e.g. melting depths, xenoliths, seismic tomography). Preliminary results suggest existing variations in lithospheric thicknesses along-rift in the Western Branch can drive upper mantle flow that is consistent with geodetic observations.

  11. 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

  12. 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

  13. Investigating the Influence of Pre-Existing Basement Structures on the Propagation of the Malawi Rift using SRTM, RADARSAT, and Aeromagnetic Data

    NASA Astrophysics Data System (ADS)

    Robertson, K.; Atekwana, E. A.; Abdelsalam, M. G.; Laó-Dávila, D. A.

    2015-12-01

    The Malawi rift is a Neogene, amagmatic rift located where the Western Branch of the East Africa Rift System (EARS) terminates. In more mature rifts, magmatism is frequently recognized as a driving factor in rift propagation; however, the amagmatic nature of the Malawi rift permits investigation into the relationship between pre-existing structures and current rift propagation, without the influence of magmatism. To map surface structures, we used Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) and RADARSAT imagery over southern Malawi. To process the SRTM data, we applied edge enhancing filters and derivatives, and extracted topographic profiles to examine scarp height and minimum vertical exposed displacement. We mapped morphologically-defined structures by filtering the RADARSAT imagery using an enhanced lee filter to reduce noise and a Laplacian filter for edge enhancement. To examine Precambrian basement structures, we filtered aeromagnetic data using vertical and horizontal derivatives, tilt derivative, and analytic signal to create magnetic anomaly maps. Surface mapping indicated three primary trends in the southern Malawi rift: NW-SE (dominant), NE-SW, both of which are most likely the remnants of Mesozoic Karoo rifting, and a NNE-SSW trend seen in Neogene rifting. The Precambrian basement structural mapping also reveals three primary trends: WNW-ESE, NE-SW, and NW-SE. Ductile deformation causes the dominant basement fabric to change, switching polarity as the rift propagated southward from NE-SW orientation to NW-SE and WNW-ESE orientations, and back to a NE-SW orientation. In general, the surficial structures follow this trend. In some areas, however, the more recent rifting cut across pre-existing basement structures, possibly due to rheological heterogeneities or selective strain partitioning. Nonetheless, pre-existing basement structures played a critical role in strain localization and fault propagation in Malawi. However

  14. Crustal and lithospheric structure of the west Antarctic Rift System from geophysical investigations: a review

    USGS Publications Warehouse

    Behrendt, John C.

    1999-01-01

    The active West Antarctic Rift System, which extends from the continental shelf of the Ross Sea, beneath the Ross Ice Shelf and the West Antarctic Ice Sheet, is comparable in size to the Basin and Range in North America, or the East African rift systems. Geophysical surveys (primarily marine seismic and aeromagnetic combined with radar ice sounding) have extended the information provided by sparse geologic exposures and a few drill holes over the ice and sea covered area. Rift basins developed in the early Cretaceous accompanied by the major extension of the region. Tectonic activity has continued episodically in the Cenozoic to the present, including major uplift of the Transantarctic Mountains. The West Antarctic ice sheet, and the late Cenozoic volcanic activity in the West Antarctic Rift System, through which it flows, have been coeval since at least Miocene time. The rift is characterized by sparse exposures of late Cenozoic alkaline volcanic rocks extending from northern Victoria Land throughout Marie Byrd Land. The aeromagnetic interpretations indicate the presence of > 5 x 105 km2 (> 106 km3) of probable late Cenozoic volcanic rocks (and associated subvolcanic intrusions) in the West Antarctic rift. This great volume with such limited exposures is explained by glacial removal of the associated late Cenozoic volcanic edifices (probably hyaloclastite debris) concomitantly with their subglacial eruption. Large offset seismic investigations in the Ross Sea and on the Ross Ice Shelf indicate a ~ 17-24-km-thick, extended continental crust. Gravity data suggest that this extended crust of similar thickness probably underlies the Ross Ice Shelf and Byrd Subglacial Basin. Various authors have estimated maximum late Cretaceous-present crustal extension in the West Antarctic rift area from 255-350 km based on balancing crustal thickness. Plate reconstruction allowed < 50 km of Tertiary extension. However, paleomagnetic measurements suggested about 1000 km of post

  15. Hydrothermal mineral deposits and fossil biota from a young (0.1 Ma) abyssal hill on the flank of the fast spreading East Pacific Rise: Evidence for pulsed hydrothermal flow and tectonic tapping of axial heat and fluids

    NASA Astrophysics Data System (ADS)

    Benjamin, Sara B.; Haymon, Rachel M.

    2006-05-01

    Heat flow data indicate that most hydrothermal heat loss from ocean lithosphere occurs on the flanks of the mid-ocean ridge, but few ridge flank hydrothermal sites are known. We describe the first nonseamount, abyssal hill hydrothermal mineral deposits to be recovered from the fast spreading East Pacific Rise (EPR) flanks. Deposits were sampled at two sites on an abyssal hill ˜5 km east of the EPR axis, just north of Clipperton Fracture Zone at 10°20'N, on ˜0.1 Ma lithosphere. "Tevnia Site" is on the axis-facing fault scarp of the hill, and "Ochre Site" is located ˜950 m farther east near the base of the outward-facing slope. Clusters of fragile, biodegradable Tevnia worm tubes at both sites indicate that hydrothermal fluids carried sufficient H2S to sustain Tevnia worms, and that fluid flow waned too recently to allow time for tube destruction. Presence of microbial mats and other biota also are consistent with recent waning of flow. The deposits are mineralogically zoned, from nontronite-celadonite to hydrous Fe-oxide+opaline silica to Mn-oxide (birnessite and todorokite). This places them into a distinctive class of Fe-Si-Mn hydrothermal deposits found along tectonic cracks and faults in young oceanic crust, and suggests that (1) deposits precipitated along an O2 gradient between ambient seawater and hydrothermal fluid; (2) fluid temperatures were <150°C and (3) undiluted fluids were Mg-depleted, and Fe-, K-, Si- and Mn-enriched. These fluids may derive from high temperature seawater-basalt interaction ± phase separation proximal to the axial melt zone, and lose Cu and Zn before venting due to conductive cooling and/or pH increase. Ochre Site samples are purely hydrothermal; however, Tevnia Site samples incorporate volcanic, sedimentary, and fossil components, and exhibit at least three generations of fracturing and hydrothermal cementation. The Tevnia Site breccias accumulated on the exposed fault scarp, possibly during multiple slip events and

  16. 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.

  17. Geomorphology in a Rifting Environment. Can Channel Profiles from Arid Sinai, Egypt, Provide Constraints on Tectonics?

    NASA Astrophysics Data System (ADS)

    Oswald, A.; Stüwe, K.

    2012-04-01

    Significant surface uplift along rift flanks results in the formation of rift-parallel mountain belts. The combination of uplift and erosion processes characterizes fluvial systems in the vicinity of the rift. Geometry of the drainage network and channel profiles may provide information on timing and geometry of rifting. However it is not clear if traditional approaches to fluvial erosion (e.g. the stream power approach) can be used to infer aspects of landscape evolution to areas, which show fluvial geomorphology, but are arid at present, like Sinai, Egypt. In this project we set out to test this and provide a description of channels as a database for our considerations. Sinai is located at a major rift system, the Red Sea, which branches into the Gulf of Suez and the Gulf of Aqaba bordering Sinai from West and South to South East, respectively. Uplift associated with the rifting exposed bedrock in Southern Sinai and topography of up to more than 2.5km, which also acts as a drainage divide between East and West. Despite Sinai's present day aridity, the large scale geomorphology appears to be characterized by fluvial features. In contrast to channels along the northern Gulf of Aqaba, channels draining to the Gulf of Suez show a more classical profile with clear knickpoints that seem however not to have mirgrated significantly from the basin-mountain transition. As the change to arid climate may have preserved to some state a previous configuration, the knickpoints may not be used to put a time frame on the uplift, but could in combination with paleoclimate data indicate changes in the relative importance of uplift versus erosion.

  18. Seismic Analysis of Magmatism in the Galapagos Archipelago and East Africa

    NASA Astrophysics Data System (ADS)

    Tepp, Gabrielle

    Magmatism and deformation are consequences of fundamental processes shaping Earth's ˜150 km-thick continental and <125 km-thick oceanic plates. Earthquake seismology encompasses many methods to detect compositional and thermal boundaries from Earth's surface to the dynamic mantle driving plate tectonics. This work uses three different seismic methods to probe magma migration and storage and tectonism in two intraplate hotspot provinces: the Galapagos and East Africa. First, seismic body-wave tomography is used to image magma within oceanic crust of the largest Galapagos volcano, Sierra Negra. A laterally large, low-velocity region with many smaller, high-magnitude velocity anomalies is imaged at 8--15.5 km depths. No sharp seismic velocity increase is imaged within the resolvable depths, indicating that the thickened crust is at least 16 km deep. The second study involves a spectral analysis of earthquakes induced by the intrusion of thin sheets of magma rising beneath the Afar rift, East Africa. Earthquakes have varying spectral content, some with unusually large amplitude low-frequency content and enhanced surface waves. The analysis showed no clear boundaries between spectral types, suggesting that they are all primarily the result of brittle failure. Deep dike segments (tops > 3 km) induce only high-frequency volcano-tectonic earthquakes, while shallower dike segments induce the full range of spectral types. This suggests that low-frequency content is a result of shallow hypocenters, with path and site effects, surface ruptures, and dike fluid interactions all possible secondary causes. In the final study, shear-wave splitting analysis of teleseismic body-wave phases is conducted to evaluate strain and crack fabrics at the base of the continental plate as a consequence of magmatism, mantle flow, and plate stretching in the Western rift, East Africa. On average, fast directions are northeast, consistent with geodynamic models of mantle flow from the African

  19. Petroleum habitat of east Siberia, Russia

    USGS Publications Warehouse

    Clarke, J.W.

    1994-01-01

    East Siberia comprises three petroleum provinces - Lena-Tunguska, Lena-Vilyuy, and Yenisey-Anabar - that occupy the area of the Siberian craton. Petroleum has been generated and has accumulated in Precambrian rifts beneath the sedimentary basins and, more importantly, within the section of the basin itself. The platformal deposits of the basins extend beneath overthrusts on the east and south and are covered by sedimentary rocks of the West Siberian province on the west. Permafrost and gas hydrate deposits are present throughout most of East Siberia. The hydrocarbon habitat of the three provinces is described. -from Author

  20. Hydrologic Windows and the Formation of Low-Temperature Geothermal Anomalies along the Rio Grande Rift, New Mexico

    NASA Astrophysics Data System (ADS)

    Pepin, J.; Person, M. A.; Kelley, S.; Timmons, S.; Owens, L.; Witcher, J. C.; Phillips, F. M.; Gable, C. W.; Coblentz, D. D.; Campbell, A.

    2013-12-01

    Within the Rio Grande Rift in New Mexico, gaps in Mesozoic and Tertiary confining units are common geologic features. They are created as a result of fault block rotation, erosion, lithological variations and emplacement of magmatic intrusions. These hydrologic windows were first proposed by Witcher (1988, Geothermal resources of southwestern New Mexico and southeastern Arizona: New Mexico Geological Society 39th Field Conference Guidebook, p. 191-197) as a mechanism to permit relatively hot geothermal fluids to discharge at the surface within the Rio Grande Rift. To explore the role of hydrologic windows in these occurrences, we have developed two-dimensional and three-dimensional hydrothermal models of both the Socorro and the Truth or Consequences geothermal resource areas. These finite-element models simulate groundwater flow, heat transfer, solute transport, and residence times. The 2D cross-sectional models help establish the depth of geothermal fluid circulation and crystalline-basement permeability structure required to account for hot-spring temperature conditions near the surface. The three-dimensional models help to assess the effects of water-table configuration and east-west oriented accommodation zones on shallow heat-flow patterns. We utilized carbon-14 groundwater age dating, salinity, and silica concentrations collected from wells and warm springs to calibrate these models. Apparent carbon-14 ages of groundwater samples collected from the 300-meter deep Woods Tunnel geothermal slim hole near Socorro and a 15-meter deep alluvial well from the Riverbend Spa in Truth or Consequences were 20,000 and 6,000 years old, respectively. Maximum geothermal temperatures based on silica concentrations at these two sites are estimated to range from 60 to 87 degrees Celsius. In order to reproduce observed temperature anomalies and groundwater residence times, groundwater circulation must have been within the crystalline basement, two to six kilometers beneath

  1. Seismic anisotropy of the lithosphere/asthenosphere system beneath the Rwenzori region of the Albertine Rift

    NASA Astrophysics Data System (ADS)

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

    2016-09-01

    Shear-wave splitting measurements from local and teleseismic earthquakes are used to investigate the seismic anisotropy in the upper mantle beneath the Rwenzori region of the East African Rift system. 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. Shear phases from local events within the crust are characterized by an average delay time of 0.04 s. Delay times from local mantle earthquakes are in the range of 0.2 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 horizontal transverse isotropy (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.

  2. 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.