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Sample records for active continental rift

  1. Continental Rifts

    NASA Astrophysics Data System (ADS)

    Rosendahl, B. R.

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

  2. Continental rift jumps

    NASA Astrophysics Data System (ADS)

    Wood, Charles A.

    1983-05-01

    Continental rift jumps, analogous to jumps of oceanic spreading ridges, are here proposed to be common. Good examples exist in Iceland and Afar (both transitional from ridge to rift jumps), West Africa (Benue Trough and Cameroon Volcanic Line), and Kenya. Indeed, the Kenya rift appears to have jumped c. 100 km eastward c. 10 m.y. ago and is currently jumping further to the east. Possible jumps exist in the Baikal rift, the Limagne-Bresse rift pair, and parallel to ancient continental margins (e.g., the Triassic basins of the eastern U.S. to Baltimore Canyon and Georges Bank). Continental rifts jump distances that are approximately equal to local lithosphere thickness, suggesting that jumped rifts are controlled by lithosphere fracturing, but there appears to be no reason for the fracturing except migration of hot spots.

  3. 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.

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

    NASA Astrophysics Data System (ADS)

    Weldesenbet, S. F.; Wohnlich, S.

    2012-12-01

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

  5. Fluoride and Geothermal Activities In Continental Rift Zones, Ethiopia

    NASA Astrophysics Data System (ADS)

    Weldesenbet, S. F.

    2012-12-01

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

  6. 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.

  7. Hydrothermal Petroleum in Active Continental Rift: Lake Chapala, Western Mexico, Initial Results.

    NASA Astrophysics Data System (ADS)

    Zarate-del Valle, P. F.; Simoneit, B. R.; Ramirez-Sanchez, H. U.

    2003-12-01

    Lake Chapala in western Mexico is located partially in the Citala Rift, which belongs to the well-known neotectonic Jalisco continental triple junction. The region is characterized by active volcanism (Ceboruco, Volcan de Fuego), tectonic (1995 earthquake, M=8, 40-50 mm to SW) and hydrothermal (San Juan Cosala & Villa Corona spas and La Calera sinter deposit) activities. Hydrothermal petroleum has been described in active continental rift (East African Rift) and marine spreading zones (Guaymas Basin, Gulf of California). In 1868 the Mexican local press reported that manifestations of bitumen were appearing in front of the Columba Cap on the mid south shore of Lake Chapala. This bitumen is linked to the lake bottom and when the water level decreases sufficiently it is possible to access these tar bodies as islands. Because of these manifestations the Mexican oil company (PEMEX) drilled an exploration well (2,348m) at Tizapan El Alto without success. Hydrothermal activity is evident in the tar island zone as three in-shore thermal springs (26.8 m depth, 48.5° C, pH 7.8 and oriented N-S). The preliminary analyses by GC-MS of the tar from these islands indicate hydrothermal petroleum derived from lake sedimentary organic matter, generated at low temperatures (150° -200° C). The tars contain no n-alkanes, no PAH or other aromatics, but a major UCM of branched and cyclic hydrocarbons and mature biomarkers derived from lacustrine biota. The biomarkers consist of mainly 17α (H),21β (H)-hopanes ranging from C27 to C34 (no C28), gammacerane, tricyclic terpanes (C20-C26), carotane and its cracking products, and drimanes (C14-C16). The biomarker composition indicates an organic matter source from bacteria and algae, typical of lacustrine ecosystems. 14C dating of samples from two tar islands yielded ages exceeding 40 kyrs, i.e., old carbon from hydrothermal/tectonic remobilization of bitumen from deeper horizons to the surface. The occurrence of hydrothermal petroleum in

  8. The Late Paleozoic Southern Margin of the Siberian paleocontinent: transformation from an active continental margin to intracontinental rifting

    NASA Astrophysics Data System (ADS)

    Kozlovsky, A. M.; Yarmolyuk, V. V.; Sal'Nikova, E. B.

    2009-04-01

    The large volcanoplutonic belt was formed on the southern margin of Siberian paleocontinent in the Early Carboniferous-Early Permian. Now it's stretched through whole Mongolia and the adjacent region of China. In the belt structure there are defined the successive rock complexes: the older one represented by differentiated basalt-andesite-rhyodacite series and younger bimodal complex of basalt-comendite-trachyrhyolite composition. The granodiorite-plagiogranite and diorite-monzonite-granodiorite plutonic massifs are associated with the former, while peralkaline granite massifs are characteristic of the latter. Geochronological results and geological relations between rocks of the bimodal and differentiated complexes showed first that rocks of the differentiated complex originated 350 to 330 Ma ago at the initial stage of forming of the marginal continental belt, linked with development active continental margin. This is evident from geochronological dates obtained for the Adzh-Bogd and Edrengiyn-Nuruu massifs and for volcanic associations of the complex. The dates are consistent with paleontological data. The bimodal association was formed later, 320 to 290 Ma ago. The time span separating formation of two igneous complexes ranges from several to 20-30 m.y. in different areas of the marginal belt. The bimodal magmatism was interrelated with rifting responsible for development of the Gobi-Tien Shan rift zone in the belt axial part and the Main Mongolian lineament along the belt northern boundary. Loci of bimodal rift magmatism likely migrated with time: the respective magmatic activity first initiated on the west of the rift system and then advanced gradually eastward with development of rift structures. Normal granitoids untypical but occurring nevertheless among the products of rift magmatism in addition to peralkaline massifs are assumed to have been formed, when the basic magmatism associated with rifting stimulated crustal anatexis and generation of crustal

  9. Geothermal activity at continental rift Citala, Western Mexico, where Lake Chapala is emplaced: past and present

    NASA Astrophysics Data System (ADS)

    Zárate-del Valle, P. F.

    2003-04-01

    Lake Chapala is a tectonic lake developed on a continental rift named Citala (CRC) which belongs to a tectonically active zone in Western Mexico: the so-called Jalisco continental triple junction. Fossil sinter deposit, thermal spring, hydrothermal (hy) petroleum manifestation and hy alteration halo characterized the Lake Chapala basin. On the SE shore, outcrops a carbonate deposit named ``La Calera" (LC) which consists of a carbonate fossil sinter that measures 2 km in E-W direction and 600 m in N-S direction and overlays andesitic rock. With a thickness of approximately 5 m and a roughly horizontal attitude, the LC is characterized by a two-fold structure: when massive, it is colored in yellow brownish and grey and elsewhere it shows a pseudo-brecciated structure and when banded, yellow and dark millimetre alternated bands can be seen. The LC is marked by vuggy porosity and silica (quartz and chalcedony) vein lets. Under microscope a pseudo-micritic texture is observed; vugs coated by iron oxides, are filled with calcite, and/or quartz, chalcedony and clay minerals. Six samples of LC were analysed (LODC-UParis VI) for their stable isotopes (δ18O and δ13C): From δ13C{PDB} values we have two sets of data: -8.03 to -8.69 ppm that means a no contribution of organic carbon (oc) and -0.35 to -0.75 ppm meaning an important contribution of oc; from δ18O{PDB} values: -8.5 to -10.27 ppm we deduced a deposit in meteoric water with a temperature deposition higher than the surface. The CRC is characterized also by the presence of hydrothermal petroleum (hp): Inside the Chapala and ˜2 km from SE shore (Los Arcos) there are some small spots made of hp which look like islands (<3-4 m^2) linked to the bottom of the lake which consist of solid bitumen. Thermal springs (ths) occur both inside and outside the lake Chapala: the water in out-shore ths is of carbonate type (69^oC; ˜ 240 mg L-1 [HCO_3]^-; with one exception: the ths at the San Juan Cosalá spa (N shore), which is

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

    NASA Astrophysics Data System (ADS)

    Kervyn, F.

    The Rukwa rift is a deep sedimentary basin that is considered as a tectonic trans- fer zone between the Tanganyika and the Malawi troughs. The tectonic evolution of the depression is controlled by the reactivation of proterozoic structures and started with the deposition of the permo-triasic Karoo sediments. In the southeast, the rift is divided into two facing half graben separated by a Precambrian horst, whereas its northwestern part has a more symmetrical graben structure. Although most of the vertical displacement is accommodated by the Lupa eastern boundary fault, onshore shallow seismic profiles have confirmed the co-occurrence of intrabasin synthetic- and strike-slip faults within the sub surface sediments. Both normal and dextral strike-slip movement are indeed observed in the basin in response to the E-W to WNW-SSE ex- tension. The region has a moderate seismic activity and the earthquakes magnitude is generally below M 6.5. However, a M 7.4 earthquake occurred in the Rukwa region in 1910 but its exact location remains uncertain. The current research aimed at the identi- fication of active faults within the recent deposits of the basin by the combination in a GIS of radar interferometric data with topographical and geological maps, geophysical data, and field observations. Radar interferometry (InSAR) was found to be especially suitable for DEM computation in low relief areas where available topographic data are limited in accuracy. Numerous topographic lineaments were observed on InSAR DEM, and follow two main directions, both oblique to the main NW-SE trend of the rift. On the one hand, the GIS analysis confirms that the observed lineaments corre- spond to real natural alignment such like the drainage for example, and are therefore not related to atmospheric artefacts. On the other hand, the field observations revealed that in most cases, the topographic lineaments are very subtle and difficult to identify. However, direct correlations with tectonic

  11. 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

  12. Lena Trough (Arctic Ocean): Active mantle exhumation on a continental rifted margin

    NASA Astrophysics Data System (ADS)

    Snow, J. E.; Hellebrand, E.; von der Handt, A.; Nauret, F.

    2004-12-01

    Lena Trough is the northern continuation of the Mid-Atlantic Ridge through Fram Strait and into the Arctic Ocean. The rifting of Lena Trough began in the Miocene, and significantly, is the final and the most recent event in the separation of the North American from the Eurasian continent. Lena Trough was mapped in 1999, 2001 and 2004 by PFS Polarstern (Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany), revealing sea floor structures that are inconsistent with any normally conceived mid-ocean ridge spreading, and instead indicative of late continental rifting. Lena Trough is shown to be a deep, fault-bounded basin with depths of 3800-4200m, and irregular, steep valley sides that are oblique to the spreading direction. Basement horst structures that outcrop as sigmoidal ridges with steeply dipping sides project out of the valley floor. These basement ridges are roughly parallel along flow lines to the valley walls on either side. Ridge-orthogonal topography is simply absent (ie no segments trending parallel nor fracture zones perpendicular to Gakkel Ridge). Most faults trend approximately SSE-NNW, an obliquity with respect to Gakkel Ridge (SW-NE) of about 55°. The basement ridges are composed nearly entirely of fertile mantle peridotite, as are the valley walls. Only at the northern and southern extremities of Lena Trough do basalts appear at all. The peridotites compositions are consistent with either continental or oceanic (asthenospheric) mantle. They show evidence of low-degree mantle melting, followed by high-level stagnation in a thick lithosphere. This evidence (veining, impregnation) is more evident where little or no basaltic cover is present, while peridotites dredged in the vicinity of basalts tend to be more residual. This may indicate some degree of magmatic focusing in the absence of a basaltic crust per se. Lena Trough contains rare, highly alkaline basalts that are unlike any compositions dredged from mid-ocean ridges

  13. Lena Trough (Arctic Ocean): Active mantle exhumation on a continental rifted margin

    NASA Astrophysics Data System (ADS)

    Snow, J. E.; Hellebrand, E.; von der Handt, A.; Nauret, F.

    2007-12-01

    Lena Trough is the northern continuation of the Mid-Atlantic Ridge through Fram Strait and into the Arctic Ocean. The rifting of Lena Trough began in the Miocene, and significantly, is the final and the most recent event in the separation of the North American from the Eurasian continent. Lena Trough was mapped in 1999, 2001 and 2004 by PFS Polarstern (Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany), revealing sea floor structures that are inconsistent with any normally conceived mid-ocean ridge spreading, and instead indicative of late continental rifting. Lena Trough is shown to be a deep, fault-bounded basin with depths of 3800-4200m, and irregular, steep valley sides that are oblique to the spreading direction. Basement horst structures that outcrop as sigmoidal ridges with steeply dipping sides project out of the valley floor. These basement ridges are roughly parallel along flow lines to the valley walls on either side. Ridge-orthogonal topography is simply absent (ie no segments trending parallel nor fracture zones perpendicular to Gakkel Ridge). Most faults trend approximately SSE-NNW, an obliquity with respect to Gakkel Ridge (SW-NE) of about 55°. The basement ridges are composed nearly entirely of fertile mantle peridotite, as are the valley walls. Only at the northern and southern extremities of Lena Trough do basalts appear at all. The peridotites compositions are consistent with either continental or oceanic (asthenospheric) mantle. They show evidence of low-degree mantle melting, followed by high-level stagnation in a thick lithosphere. This evidence (veining, impregnation) is more evident where little or no basaltic cover is present, while peridotites dredged in the vicinity of basalts tend to be more residual. This may indicate some degree of magmatic focusing in the absence of a basaltic crust per se. Lena Trough contains rare, highly alkaline basalts that are unlike any compositions dredged from mid-ocean ridges

  14. Introduction - Processes of continental rifting

    NASA Technical Reports Server (NTRS)

    Morgan, P.; Baker, B. H.

    1983-01-01

    It is thought likely that thermal thinning and/or diapirism can cause the extensional stress required for rifting. The rifting, however, will not occur unless the regional tectonic regime permits the sides of the rift to diverge. Whereas passive plate extension could cause rifting in isolation, the extension and rifting are likely to be localized where the lithosphere is weakest over an existing thermal anomaly. In those cases where asthenospheric diapirism occurs, which is essentially a response to thinning of the lithosphere by thermal thinning or plate extension, the effects of diapirism may completely mask the initiating mechanism. It is believed that anomalous heat transfer into the lithosphere, diapirism, and magmatism must all figure in rifting, along with a deviatoric stress field that will permit extension in a developing rift. Even though the models are useful in permitting idealized processes to be quantified and tested, better knowledge of lithosphere properties is considered necessary, in particular knowledge of mantle viscosity and its temperature dependence.

  15. Three-dimensional electrical resistivity image of magma beneath an active continental rift, Taupo Volcanic Zone, New Zealand

    NASA Astrophysics Data System (ADS)

    Heise, Wiebke; Caldwell, T. Grant; Bibby, Hugh M.; Bennie, Stewart L.

    2010-05-01

    Magmatic activity in regions of continental extension may result in huge (>400 km3) explosive eruptions of viscous, gas-rich silicic-magma. Geochemical and geological data suggest that the large volumes of magma erupted are produced by extracting interstitial liquid from a long-lived ‘mush zone’ (a mixture of solid crystals and liquid melt) that accumulates in liquid-dominated lenses at the top of a much thicker region of lower melt-fraction mush. Such lenses will be highly electrically conductive compared with normal mid-crustal rocks. Here we use results of 220 magnetotelluric (MT) soundings to construct a 3-D electrical resistivity image of the northern (silicic) part of New Zealand's Taupo Volcanic Zone, a young continental rift associated with very high heat flow and intense silicic volcanism. The electrical resistivity image shows a plume-like structure of high conductivity, interpreted to be a zone of interconnected melt, rising from depths >35 km beneath the axis of extension.

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

    NASA Astrophysics Data System (ADS)

    Kinabo, Baraka Damas

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

  17. 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

    Pliocene (post 3.2 Ma)-recent extensional stress field generated by relative motion between Nubia and Somalia plates (roughly ESE-WNW) suggest that oblique rifting conditions have controlled rift evolution. However, it is still unclear if these kinematical boundary conditions have remained steady since the initial stages of rifting or the kinematics has changed during the Late Pliocene or at the Pliocene-Pleistocene boundary. Analysis of geological-geophysical data suggests that continental rifting in the MER evolved in two different phases. An early (Mio-Pliocene) continental rifting stage was characterised by displacement along large boundary faults, subsidence of rift depression with local development of deep (up to 5 km) asymmetric basins and diffuse magmatic activity. In this initial phase, magmatism encompassed the whole rift, with volcanic activity affecting the rift depression, the major boundary faults and limited portions of the rift shoulders (off-axis volcanism). Progressive extension led to the second (Pleistocene) rifting stage, characterised by a riftward narrowing of the volcano-tectonic activity. In this phase, the main boundary faults were deactivated and extensional deformation was accommodated by dense swarms of faults (Wonji segments) in the thinned rift depression. The progressive thinning of the continental lithosphere under constant, prolonged oblique rifting conditions controlled this migration of deformation, possibly in tandem with the weakening related to magmatic processes and/or a change in rift kinematics. Owing to the oblique rifting conditions, the fault swarms obliquely cut the rift floor and were characterised by a typical right-stepping arrangement. Ascending magmas were focused by the Wonji segments, with eruption of magmas at surface preferentially occurring along the oblique faults. As soon as the volcano-tectonic activity was localised within Wonji segments, a strong feedback between deformation and magmatism developed: the thinned

  18. Discussion of Continental Rifts and Their Structure

    NASA Astrophysics Data System (ADS)

    Gilbert, M. C.

    2011-12-01

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

  19. Buried Mesozoic rift basins of Moroccan Atlantic continental margin

    SciTech Connect

    Mohamed, N.; Jabour, H.; El Mostaine, M.

    1995-08-01

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

  20. 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

  1. Regional magnetic anomaly constraints on continental rifting

    NASA Technical Reports Server (NTRS)

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

    1985-01-01

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

  2. Volcanic field elongation, vent distribution and tectonic evolution of continental rift: The Main Ethiopian Rift example

    NASA Astrophysics Data System (ADS)

    Mazzarini, Francesco; Le Corvec, Nicolas; Isola, Ilaria; Favalli, Massimiliano

    2015-04-01

    Magmatism and faulting operate in continental rifts and interact at a variety of scales, however their relationship is complex. The African rift, being the best example for both active continental rifting and magmatism, provides the ideal location to study the interplay between the two mechanisms. The Main Ethiopian Rift (MER), which connects the Afar depression in the north with the Turkana depression and Kenya Rift to the south, consists of two distinct systems of normal faults and its floor is scattered with volcanic fields formed by tens to several hundreds monogenetic, generally basaltic, small volcanoes and composite volcanoes and small calderas. The distribution of vents defines the overall shape of the volcanic field. Previous work has shown that the distribution of volcanic vents and the shape of a field are linked to its tectonic environment and its magmatic system. In order to distinguish the impact of each mechanism, we analyzed four volcanic fields located at the boundary between the central and northern MER, three of them (Debre Zeyit, Wonji and Kone) grew in the rift valley and one (Akaki) on the western rift shoulder. The elongation and shape of the fields were analyzed based on their vent distribution using the Principal Component Analysis (PCA), the Vent-to-Vent Distance (VVD), and the two dimensional symmetric Gaussian kernel density estimate methods. We extracted from these methods several parameters characterizing the spatial distribution of points (e.g., eccentricity (e), eigenvector index (evi), angular dispersion (Da)). These parameters allow to define at least three types of shape for volcanic fields: strong elongate (line and ellipse), bimodal/medium elongate (ellipse) and dispersed (circle) shapes. Applied to the natural example, these methods well differentiate each volcanic field. For example, the elongation of the field increases from shoulder to rift axis inversely to the angular dispersion. In addition, the results show that none of

  3. Dynamics of continental rift propagation: the end-member modes

    NASA Astrophysics Data System (ADS)

    Van Wijk, J. W.; Blackman, D. K.

    2005-01-01

    An important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation. In regions of rift propagation, specifically transition zones from continental rifting to seafloor spreading, it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones. The focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere. More specifically, this study explores how modeled rift behavior depends on the value of rheological parameters of the crust. A three-dimensional finite element model is used to simulate lithosphere deformation in an extensional regime. The chosen approach emphasizes understanding the tectonic forces involved in rift propagation. Dependent on plate strength, two end-member modes are distinguished. The stalled rift phase is characterized by absence of rift propagation for a certain amount of time. Extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone, which requires a buildup of shear stresses near the rift tip and significant intra-plate deformation. This stage represents a situation in which a rift meets a locked zone. Localized deformation changes to distributed deformation in the locked zone, and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend. In the alternative rift propagation mode, rift propagation is a continuous process when the initial crust is weak. The extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses. Model predictions address aspects of previously unexplained rift evolution in the Laptev Sea, and its contrast with the tectonic evolution of, for example, the Gulf of Aden and Woodlark Basin.

  4. Gravity study of the Central African Rift system: A model of continental disruption 1. The Ngaoundere and Abu Gabra Rifts

    NASA Astrophysics Data System (ADS)

    Browne, S. E.; Fairhead, J. D.

    1983-05-01

    A regional compilation of published and unpublished gravity data for Central Africa is presented and reveals the presence of a major rift system, called here, the Central African Rift System. It is proposed that the junction area between the Ngaoundere and Abu Gabra rift arms in Western Sudan forms an incipient intraplate, triple-junction with the as yet unfractured, but domally uplifted and volcanically active, Darfur swell. It is only the Darfur swell that shows any similarities to the uplift and rift history of East Africa. The other two rifts arms are considered to be structurally similar to the early stages of passive margin development and thus reflect more closely the initial processes of continental fragmentation than the structures associated with rifting in East Africa.

  5. 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.

  6. Seismic tomography of continental rifts revisited: from relative to absolute heterogeneities

    NASA Astrophysics Data System (ADS)

    Achauer, Ulrich; Masson, Frédéric

    2002-11-01

    Tomographic images for four major continental rift zones, namely the southern Rhine Graben (SRG, Germany/France), the Gregory rift (Kenya) which is the central part of the East African rift system, the Rio Grande rift (RGR) in the United States and the Lake Baikal rift zone (LBR) in Russia have been revisited by calculating and comparing absolute velocity models. The four rifts exhibit strong structural differences in the uppermost mantle down to more than 300-km depth, suggesting major differences in their geodynamic evolution albeit their similarity in age and similar surface expression. The comparative analysis suggests that tomographic images of rift zones can be used to characterize continental rifts, once the corrections to obtain absolute velocities have been carried out. Our results suggest that while the Kenya and the Rio Grande rift may be considered active with large upwelling plumes being the main controlling factor in the evolution, the southern Rhine Graben and the Lake Baikal rift are more likely passive rifts, where complex regional stress fields and inherited structures play the governing role in the evolution.

  7. Extension across the Laptev Sea continental rifts constrained by gravity modeling

    NASA Astrophysics Data System (ADS)

    Mazur, S.; Campbell, S.; Green, C.; Bouatmani, R.

    2015-03-01

    The Laptev Shelf is the area where the Gakkel Ridge, an active oceanic spreading axis, approaches a continental edge, causing a specific structural style dominated by extensive rift structures. From the latest Cretaceous to the Pliocene, extension exerted on the Laptev Shelf created there several deep subsided rifts and high-standing basement blocks. To understand syn-rift basin geometries and sediment supply relationships across the Laptev Shelf, accurate extension estimates are essential. Therefore, we used 2-D gravity modeling and 3-D gravity inversion to constrain the amount of crustal stretching across the North America-Eurasia plate boundary in the Laptev Shelf. The latest Cretaceous-Cenozoic extension in that area is partitioned among two rift zones, the Laptev Rift System and the New Siberian Rift. These rifts were both overprinted on the Eurasian margin that had been stretched by 190-250 km before the Late Cretaceous. While the Laptev Rift System, connected to the Gakkel Ridge, reveals increasing extension toward the shelf edge (190-380 km), the New Siberian Rift is characterized by approximately uniform stretching along strike (110-125 km). The architecture of the Laptev Rift System shows that the finite extension of about 500 km is sufficient to entirely eliminate crystalline continental crust. In the most stretched rift segment, continental mantle is exhumed at the base of the Late Mesozoic basement. The example of the Laptev Rift System shows that extension driven by divergent plate movement is a sufficient cause to produce almost complete continental breakup without an increased heat input from the asthenospheric mantle.

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

    NASA Astrophysics Data System (ADS)

    Stuart, G.

    2003-12-01

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

  9. Thermal Evolution of Continental Rifting in Corsica (France)

    NASA Astrophysics Data System (ADS)

    Seymour, N. M.; Stockli, D. F.; Beltrando, M.; Smye, A.

    2014-12-01

    Present thermal evolution models for continental rifting are based on pure-shear extension (McKenzie 1978), in which crustal and mantle strain is co-located and all rocks cool throughout rifting. However, the multi-phase rift model of Lavier and Manatschal (2006) accommodates lithospheric extension via spatially offset crustal and mantle strains, producing depth-dependent thinning and exhumation of lithospheric mantle. Significant reheating of the upper plate is a natural consequence of this model. We seek to constrain the temperature-time history of the upper-plate Tethyan margin preserved in Corsica to discriminate between the two thermal models. A record of the conditions and timing of reheating is preserved in the age and trace element compositions of metamorphic zircon overgrowths. Zircon from the hanging wall and footwall of the Jurassic-age Belli Piani shear zone (Beltrando et al 2013) were depth-profiled for both U-Pb and trace element concentrations via LA-ICP-MS split streaming. Across both sides of the shear zone, U-Pb ages show a strong population of 275-300 Ma grains. However, a subset of footwall grains show 165-210 Ma overgrowths. These ages indicate that the margin reached temperature conditions sufficient for zircon saturation and subsequent zircon growth. These lower crustal findings are consistent with prior observations made within the sedimentary succession, which records rapid thermal uplift, karstification, and subsequent drowning of Triassic dolostones contemporaneous with the opening of the Alpine Tethys (Decarlis and Lualdis 2008). Ti-in-zircon thermometry yields temperatures of ~720°C in the hanging wall and ~830°C in the footwall. This is consistent with the appearance of overgrowths, and provides further support that the Belli Piani shear zone was active during Jurassic rifting. Collectively, these data point directly to a rift-coeval reheating event that affected the entire crustal pile and lend support to the multi-stage Lavier and

  10. The Porcupine Basin: from rifting to continental breakup

    NASA Astrophysics Data System (ADS)

    Reston, Timothy; Gaw, Viola; Klaeschen, Dirk; McDermott, Ken

    2015-04-01

    Southwest of Ireland, the Porcupine Basin is characterized by axial stretching factors that increase southward to values greater than six and typical of rifted margins. As such, the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup, and in particular the processes of mantle serpentinisation, and the onset of detachment faulting. We have processed through to prestack depth migration a series of E-W profiles crossing the basin at different axial stretching factors and linked by a N-S profile running close to the rift axis. Our results constrain the structure of the basin and have implications for the evolution of rifted margins. In the north at a latitude of 52.25N, no clear detachment is imaged, although faults do appear to cut down into the mantle, so that serpentinisation may have started. Further south (51.75N), a bright reflection (here named P) cuts down to the west from the base of the sedimentary section, is overlain by small fault blocks and appears to represent a detachment fault. P may in part follow the top of partially serpentinized mantle: this interpretation is consistent with gravity modelling, with numerical models of crustal embrittlement and mantle serpentinization during extension and with wide-angle data (see posters of Prada and of Watremez). Furthermore, P closely resembles the S reflection west of Iberia, where such serpentinites are well documented. P develops where the crust was thinned to less than 3 km during rifting, again similar to S. Although overall the basin remains symmetrical, the consistent westward structural dip of the detachment implies that, at high stretching factors, extension became asymmetric. Analysis of the depth sections suggests that the detachment may have been active as a rolling hinge rooting at low-angle beneath the Porcupine Bank, consistent with the presence of a footwall of serpentinites. This requires very weak

  11. 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.

  12. 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.

  13. Generation of Continental Rifts, Basins and Swells by Lithosphere Instabilities

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    Domal uplifts, volcanism, basin formation and rifting have often struck the same continent in different areas at the same time. Their characteristics and orientations are difficult to reconcile with mantle convection or tectonic forces and suggest a driving mechanism that is intrinsic to the continent. The rifts seem to develop preferentially at high angles to the edge of the continent whereas swells and basins seem confined to the interior. Another intriguing geometrical feature is that the rifts often branch out in complicated patterns at their landward end. In Western Africa, for example, magmatic activity currently occurs in a number of uplifted areas including the peculiar Cameroon Volcanic Line that stretches away from the continental margin over about 1000 km. Magmatic and volcanic activity has been sustained along this line for 70 My with no age progression. The mantle upwelling that feeds the volcanoes is not affected by absolute plate motions and hence is attached to the continent. The Cameroon Volcanic Line extends to the Biu swell to the North and the Jos plateau to the West defining a striking Y-shaped pattern. This structure segues into several volcanic domes including the Air, the Hoggar, the Darfur, the Tibesti and the Haruj domes towards the Mediterranean coast. Another example is provided by North America, where the late Proterozoic-early Ordovician saw the formation of four major basins, the Michigan, Illinois, Williston and Hudson Bay, as well as of major rifts in southern Oklahoma and the Mississipi Valley within a short time interval. At the same time, a series of uplifts developed, such as the Ozark and Nashville domes. Motivated by these observations, we have sought an explanation in the continental lithosphere itself. We describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent. We carried out fluid mechanics laboratory experiments on buoyant

  14. Magma-compensated crustal thinning in continental rift zones.

    PubMed

    Thybo, H; Nielsen, C A

    2009-02-12

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

  15. Rifting of Continental Interiors: Some New Geophysical Data and Interpretations

    NASA Astrophysics Data System (ADS)

    Keller, G. R.

    2005-12-01

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

  16. 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.

  17. Magmatic expression of lithospheric thinning across continental rifts

    NASA Astrophysics Data System (ADS)

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

    1994-05-01

    Studies of magmatism associated with continental rifting have traditionally focused only on volcanism within the downfaulted axial zone and along its immediate flanks. Teleseismic travel-time delay studies during the last decade have confirmed the results of earlier gravity surveys of rifted areas, showing that thinning at the base of the continental lithosphere occurs throughout a zone up to about 10 times wider than the physiographic expression of the rift. It is, therefore, logical to consider rifting-related magmatism on the same scale. Potential sources of mafic magmas in rift zones are the thinned subcontinental lithospheric mantle (SCLM), the convecting mantle beneath the continental plate and mixtures of the two. Detailed elemental and radiogenic isotope geochemical studies show that, during the initial extension of continental rifts, the associated mafic magmatism tends to be: (1) relatively sodic and from predominantly convecting mantle sources at the rift axis; (2) relatively potassic and from predominantly lithospheric mantle sources at the margins of the thinned-plate zone. This underlying geochemical pattern is obscured in many instances by such processes as crustal contamination and magma mixing within open-system reservoirs. The mafic ultrapotassic component that provides a distinctive input to SCLM-source magmas appears to be largely fusible at temperatures well below the dry solidus of SCLM; so that, in some cases, prolonged magmatism at a site causes removal of most or all of the potassic lithosphere-source melt (as mafic ultrapotassic magmas or as a contribution to mixed-source melts) without destruction of that lithosphere segment as a geophysically defined unit. Such a zone of refractory lithosphere permits subsequent, recognisable, convecting mantle source melts to penetrate it and reach the surface. These principles are illustrated by discussion of the Neogene-Quaternary magmatism of the Rio Grande, East African, Rhine and Baikal rifts, in

  18. Stress perturbation associated with the Amazonas and other ancient continental rifts

    USGS Publications Warehouse

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

    1996-01-01

    rift case, because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift-normal compression to be between 0.25 and 1.0, our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced (for ratios >0.5) or enhanced (for ratios <0.5). Additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust-faulting stress regime in the vicinity of the Amazonas rift. A rift-normal stress associated with the seismically active New Madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike-slip faults parallel to the axis of the rift, although this results in a 20-40% reduction in the local horizontal shear stress within the seismic zone. Sparse stress data in the vicinity of the seismically quiescent Midcontinent rift of the central United States suggest a stress state similar to that of New Madrid, with the local horizontal shear stress potentially reduced by as much as 60%. Thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows. The modeling and analysis here demonstrate that rift-normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension.

  19. 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.

  20. Synchronous oceanic spreading and continental rifting in West Antarctica

    NASA Astrophysics Data System (ADS)

    Davey, F. J.; Granot, R.; Cande, S. C.; Stock, J. M.; Selvans, M.; Ferraccioli, F.

    2016-06-01

    Magnetic anomalies associated with new ocean crust formation in the Adare Basin off north-western Ross Sea (43-26 Ma) can be traced directly into the Northern Basin that underlies the adjacent morphological continental shelf, implying a continuity in the emplacement of oceanic crust. Steep gravity gradients along the margins of the Northern Basin, particularly in the east, suggest that little extension and thinning of continental crust occurred before it ruptured and the new oceanic crust formed, unlike most other continental rifts and the Victoria Land Basin further south. A preexisting weak crust and localization of strain by strike-slip faulting are proposed as the factors allowing the rapid rupture of continental crust.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  2. Early Continental Rifting of the South China Sea

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  3. 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.

  4. An updated global earthquake catalogue for stable continental regions: reassessing the correlation with ancient rifts

    NASA Astrophysics Data System (ADS)

    Schulte, Saskia M.; Mooney, Walter D.

    2005-06-01

    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 presence

  5. 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

  6. Rifted Continental Margins: The Case for Depth-Dependent Extension

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.; Beaumont, Christopher

    2015-04-01

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

  7. Fault-Controlled Fluid Migration during Early-Stage Continental Rifting in the Magadi Basin, Kenya

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    The mechanisms controlling the migration of mantle-derived, CO­2-rich fluids in early-stage continental rifts are poorly constrained, yet have important implications for processes occurring during the initiation of continental breakup. Within the East African Rift specifically, the role of normal fault structures in transporting fluids, and the role these fluids play in driving deformation, is yet to be addressed. The 7 Ma Magadi Basin of the EAR exhibits active hydrothermal fluid flow amongst an excellently exposed array of normal faults, providing a unique opportunity to test the mechanics of fault-controlled fluid migration at an early-stage continental rift setting. We present a study utilizing both geochemical and structural data collected from active and fossilized fluid systems observed along faults in the Magadi Basin. The distribution and orientation of veins and systematic fracture sets around fault zones were recorded in the field, and fault throws were measured using a Trimble GPS. Larger faults were analyzed remotely using aerial imagery and the Aster GDEM v.2. Fault data were then compared with CO2 flux measured on soil and from gas-emitting fractures in and around fault zones using an accumulation chamber. Our data reveal that CO2-rich fluids travel along fault-parallel fractures within fault zones, and fault-oblique fracture sets in the accommodation zones between fault segments. Fluids rising through faults may additionally be diverted along lithologic boundaries in fault grabens, such as the contact between lavas and overlying sedimentary fill. The highest CO2 flux observed in the Magadi region occurs in the central axis of the rift, along faults with the highest observable throws (>150 m) as well as the 1998 earthquake rupture. This study illustrates a direct link between fluid flow and faulting during the earliest stages of continental rifting. High CO2 soil flux and active hydrothermal fluid flow is, therefore, a potential indicator of faults

  8. Signs of continental rifting in the southwestern Japanese Island Arc

    NASA Astrophysics Data System (ADS)

    Chernysheva, E. A.; Eroshenko, D. V.

    2016-03-01

    The southwestern margin of the Japan Arc evolved in the geodynamic regime of continental rifting during the Miocene-Pleistocene. This has been verified by broad manifestations of metasomatosis of mantle peridotites that underlie the lithosphere of the Japan Islands and by episodes of deep magmatism (kimberlites and melilitites) in the region. The high enrichment of deep melts in incompatible rare and rare earth elements is partially preserved in melts of regional basalts from smaller depths. In contrast, spreading basalts of the Sea of Japan and subduction basalts from the Nankai trench at the boundary with the Philippine Plate are extremely depleted in rare elements.

  9. 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.

  10. The Chukchi Borderland: a Sediment-starved Rifted Continental Margin

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  12. 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

  13. Geochemical signals of progressive continental rupture in the Main Ethiopian Rift

    NASA Astrophysics Data System (ADS)

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

    2003-04-01

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

  14. 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.

  15. 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

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

    PubMed

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

    2008-12-11

    Continental rifts begin and develop through repeated episodes of faulting and magmatism, but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented. In highly evolved rifts, tensile stresses from far-field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions. These rifting crises are rarely observed in thick lithosphere during the initial stages of rifting. Here we show that most of the strain during the July-August 2007 seismic crisis in the weakly extended Natron rift, Tanzania, was released aseismically. Deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping. This event provides compelling evidence for strain accommodation by magma intrusion, in addition to slip along normal faults, during the initial stages of continental rifting and before significant crustal thinning. PMID:19079058

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

    NASA Astrophysics Data System (ADS)

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

    2004-11-01

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

  18. The role of fluids in lower-crustal earthquakes near continental rifts.

    PubMed

    Reyners, Martin; Eberhart-Phillips, Donna; Stuart, Graham

    2007-04-26

    The occurrence of earthquakes in the lower crust near continental rifts has long been puzzling, as the lower crust is generally thought to be too hot for brittle failure to occur. Such anomalous events have usually been explained in terms of the lower crust being cooler than normal. But if the lower crust is indeed cold enough to produce earthquakes, then the uppermost mantle beneath it should also be cold enough, and yet uppermost mantle earthquakes are not observed. Numerous lower-crustal earthquakes occur near the southwestern termination of the Taupo Volcanic Zone (TVZ), an active continental rift in New Zealand. Here we present three-dimensional tomographic imaging of seismic velocities and seismic attenuation in this region using data from a dense seismograph deployment. We find that crustal earthquakes accurately relocated with our three-dimensional seismic velocity model form a continuous band along the rift, deepening from mostly less than 10 km in the central TVZ to depths of 30-40 km in the lower crust, 30 km southwest of the termination of the volcanic zone. These earthquakes often occur in swarms, suggesting fluid movement in critically loaded fault zones. Seismic velocities within the band are also consistent with the presence of fluids, and the deepening seismicity parallels the boundary between high seismic attenuation (interpreted as partial melt) within the central TVZ and low seismic attenuation in the crust to the southwest. This linking of upper and lower-crustal seismicity and crustal structure allows us to propose a common explanation for all the seismicity, involving the weakening of faults on the periphery of an otherwise dry, mafic crust by hot fluids, including those exsolved from underlying melt. Such fluids may generally be an important driver of lower-crustal seismicity near continental rifts. PMID:17460671

  19. Crustal Structure at a Young Continental Rift: A Receiver Function Study from Lake Tanganyika

    NASA Astrophysics Data System (ADS)

    Hodgson, I. D. S.; Illsley-Kemp, F.; Gallacher, R. J.; Keir, D.; Ebinger, C. J.; Drooff, C.; Khalfan, M.

    2015-12-01

    Lake Tanganyika, in western Tanzania, spans a large section of the Western rift yet there are very few constraints on bulk crustal and upper mantle structure. The Western rift system has no surface expression of magmatism, which is in stark contrast to the Eastern branch. This observation is difficult to reconcile with the approximately coeval initiation of rifting of the two branches. The variation in the nature of rifting provides a perfect setting to test current hypotheses for the initiation of continental breakup and early-stage development of continental rifts. The deployment of a seismic network of 13 broadband instruments on the south eastern shore of Lake Tanganyika, for 16 months, between 2014 and 2015 provides a unique opportunity to investigate extensional processes in thick continental lithosphere. We present here results from a P to S receiver function study that provides information on bulk crustal Vp/Vs ratio along the rift; a property that is sensitive to the presence of magmatic intrusions in the lower crust. Additionally this method allows us to map variations in crustal thickness both parallel and perpendicular to the rift axis. These results thus provide unprecedented insight into the large-scale mechanics of early-stage continental rifting along the non-volcanic Western rift.

  20. The Formation of Non-Volcanic Rifted Margins by the Progressive Extension of the Continental Lithosphere

    NASA Astrophysics Data System (ADS)

    Reston, T. J.; Perez-Gussinye, M.; Gaw, V.; Phipps Morgan, J.

    2003-12-01

    Rifted margins include two main end-members: those termed "Volcanic Rifted Margins - VRMs" where magmatism is much more voluminous than predicted by passive asthenospheric upwelling (e.g. White et al., 1989), and those where magmatism is consistent or even less than the same predictions. The latter are termed "Non-Volcanic Rifted Margins - NVRMs" to emphasise the contrast with the VRMs: the name does not exclude the presence of minor amounts of magmatic activity. The NVRMs are typified by the North Biscay, south Australian, SW Greenland, and the West Iberian margins, which share a number of common characteristics: - extreme crustal thinning, increasing towards the ocean; - presence of well-defined rotated fault blocks. However at the feather edge of the continent there is an extension discrepancy: the amount that can be inferred from the geometry of these faults is far less than that indicated by the crustal thinning observed; - presence in places of a detachment fault at the base of the fault blocks; - little evidence for synrift magmatism; - the presence of a broad zone of partially serpentinised mantle (Boillot et al., 1988; Whitmarsh et al., 1996; Krawczyk et al., 1996; Pickup et al., 1996), both occurring beneath the highly thinned and faulted continental crust, and as a zone of exhumed continental mantle, now largely buried by postrift sediments. We show that such margins are the logical result of progressive extension of continental lithosphere above cool sub-lithospheric mantle. The key factors controlling the development of the margin are the rheological evolution of the crust (explaining the serpentinisation of the mantle), the occurrence of multiple phases of faulting (explaining the apparent extension discrepancy), and the temperature structure of the sub-continental mantle (explaining the lack of magmatism).

  1. The role of rifting in the evolution of the continental margin of Eastern Asia: Geophysical evidence

    NASA Astrophysics Data System (ADS)

    Rodnikov, A. G.; Rodnikova, R. D.; Zorina, Yu. G.

    1992-08-01

    The role of rift processes is analysed in the structural evolution of the continental margins of Eastern Asia including the Indo-China Peninsula and North China plain. Paleoreconstructions were made for the Indo-China Peninsula to characterize individual stages of rifting covering the Late Cretaceous-Eocene, Oligocene-Middle Miocene and Late Pliocene-Early Quaternary epochs. The rifting of continental margins occurred synchronously with spreading processes in marginal seas, whereas the formation of rift structures in the North China plain was concurrent with the formation of a deep-water basin of the Philippine Sea. The development of asthenospheric diapire led to crustal extension and was responsible for the formation of rift structures in marginal seas and continental margins.

  2. Prolonged post-rift magmatism on highly extended crust of divergent continental margins (Baiyun Sag, South China Sea)

    NASA Astrophysics Data System (ADS)

    Zhao, Fang; Alves, Tiago M.; Wu, Shiguo; Li, Wei; Huuse, Mads; Mi, Lijun; Sun, Qiliang; Ma, Benjun

    2016-07-01

    Three-dimensional (3D) seismic, borehole and geochemical data reveal a prolonged phase of post-rift magmatism on highly extended crust of the Baiyun Sag, South China Sea. Two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the South China Sea. Biostratigraphic data from exploration wells BY7-1 and BY2, complemented by K-Ar datings from core samples, confirm that magmatic activity in the Baiyun Sag occurred in two main stages: (1) a first episode at the base of the Miocene (23.8 Ma); and (2) a second episode occurring at the end of the Early Miocene (17.6 Ma). The relative location of volcanic complexes in the Baiyun Sag, and their stratigraphic position, reveals prolonged magmatism inboard of the ocean-continent transition zone during continental breakup. We suggest that magmatism in the Baiyun Sag reflects progressive continental breakup in the South China Sea, with the last volcanic episode marking the end of a breakup sequence representing the early post-rift tectonic events associated with the continental breakup process. Seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup.

  3. 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.

  4. 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

  5. Misho mafic complex - A part of paleotethyan oceanic crust or a magmatism in continental rift?

    NASA Astrophysics Data System (ADS)

    Azimzadeh, Zohreh; Jahangiri, Ahmad; Saccani, Emilio; Dilek, Yildirim

    2013-04-01

    Misho Mafic Complex (NW Iran) represents a significant component of the West Cimmerian domain in Paleo-Tethys. The Misho Mafic Complex (MMC) consists of gabbro (mainly) and norıte,olivine gabbro, anorthosite and diorite with the east- west sereight. MMC has ıntrussıved ın Kahar sedımrtery Infta- Cambrıan rocks, crosscut by abundant basaltic dykes and the overlying basaltic sheeted dyke complex. Kahar sedimentary rocks are representing the northern margin of Gondwana. Misho mafic complex are covered by Permian sedimentary rocks. The gabbros and basaltic dykes have MORB affinities. MMC formed as a product of interactions between a depleted MORB-type asthenosphere and plume-type material. Mafic rocks represent an early Carboniferous magmatic event developed during the continental break-up of the northern edge of Gondwanaland that led to the opening of Paleotethys. Alternatively, these magmas may have been emplaced into the continental crust at the continental margin soon after the oceanic crust was formed (that is the oceanic crust was still narrow). There is no data for discriminating between these two hypotheses. In first hypothesis MMC is a part of ophiolites related to paleotethyan oceanic crust and the rocks that were above this crustal level should have necessarily been eroded. In another hypothesis Misho complex represents an aborted rift in a triple junction. Above a mantle plume, the continental crust breaks along three directions at 120 degrees. But, soon after, the extension proceeds along two of these three direction. Between them is formed the oceanic crust. The continental extension along the third direction is aborted. Here no oceanic crust if formed and there is only rifted, thinned continental crust. But, also in the aborted branch MORB magmatism can occur for short time. In this hypothesis, the Misho complex was never associated with oceanic crust, but was anyway associated with the opening of the Paleotethys. This magmatism was originally

  6. Continental breakup and UHP rock exhumation in action: GPS results from the Woodlark Rift, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Wallace, Laura M.; Ellis, Susan; Little, Tim; Tregoning, Paul; Palmer, Neville; Rosa, Robert; Stanaway, Richard; Oa, John; Nidkombu, Edwin; Kwazi, John

    2014-11-01

    show results from a network of campaign Global Positioning System (GPS) sites in the Woodlark Rift, southeastern Papua New Guinea, in a transition from seafloor spreading to continental rifting. GPS velocities indicate anticlockwise rotation (at 2-2.7°/Myr, relative to Australia) of crustal blocks north of the rift, producing 10-15 mm/yr of extension in the continental rift, increasing to 20-40 mm/yr of seafloor spreading at the Woodlark Spreading Center. Extension in the continental rift is distributed among multiple structures. These data demonstrate that low-angle normal faults in the continents, such as the Mai'iu Fault, can slip at high rates nearing 10 mm/yr. Extensional deformation observed in the D'Entrecasteaux Islands, the site of the world's only actively exhuming Ultra-High Pressure (UHP) rock terrane, supports the idea that extensional processes play a critical role in UHP rock exhumation. GPS data do not require significant interseismic coupling on faults in the region, suggesting that much of the deformation may be aseismic. Westward transfer of deformation from the Woodlark Spreading Center to the main plate boundary fault in the continental rift (the Mai'iu fault) is accommodated by clockwise rotation of a tectonic block beneath Goodenough Bay, and by dextral strike slip on transfer faults within (and surrounding) Normanby Island. Contemporary extension rates in the Woodlark Spreading Center are 30-50% slower than those from seafloor spreading-derived magnetic anomalies. The 0.5 Ma to present seafloor spreading estimates for the Woodlark Basin may be overestimated, and a reevaluation of these data in the context of the GPS rates is warranted.

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

    PubMed

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

    2001-09-13

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

  8. Glacial erosion, transport, and deposition on the rifted continental margin of Dronning Maud Land

    NASA Astrophysics Data System (ADS)

    Huang, Xiaoxia; Jokat, Wilfried

    2015-04-01

    The seismic refraction data suggest that the continental margin off Dronning Maud Land had complex and long-lived rift history, which were formed during break-up of Gondwana. The relief of the Dronning Maud Land by middle and late Mesozoic tectonic activity had a strong spatial control on both early fluvial and subsequent glacial erosion. In the later, ice streams have formed along pre-existing tectonic grabens and fluvial valleys and played significant role in transporting sediments. The existing topography determines where ice grows, flows, and erodes as well as how sediment has been deposited. The East Antarctic Ice sheet is moist erosive near its margins, where high driving forces, flow velocities and basal pressure gradients combine to create distinct glacial geomorphology. However, Our multichannel seismic reflection data and marine records demonstrate lack of glacial sediments and severe glacial erosions on the Dronning Maud Land continental margin. We proposed that the continuous uplift of Dronning Maud Land had acted as barrier, which blocked the sediments to be transported further to the continental margin and deep sea since the Middle and Late Mesozoic. A series of glacial erosional and sedimentary features will be reported based on seismic reflection data from several surveys to understand the sedimentation process and geomorphology of the glaciated Dronning Maud Land continental margin.

  9. Modelling of sea floor spreading initiation and rifted continental margin formation

    NASA Astrophysics Data System (ADS)

    Tymms, V. J.; Isimm Team

    2003-04-01

    Recent observations of depth dependent (heterogeneous) stretching where upper crustal extension is much less than that of the lower crust and lithospheric mantle at both non-volcanic and volcanic margins plus the discovery of broad domains of exhumed continental mantle at non-volcanic rifted margins are not predicted by existing quantitative models of rifted margin formation which are usually based on intra-continental rift models subjected to very large stretching factors. New conceptual and quantitative models of rifted margin formation are required. Observations and continuum mechanics suggest that the dominant process responsible for rifted continental margin formation is sea-floor spreading of the young ocean ridge, rather than pre-breakup intra-continental rifting. Simple fluid flow models of ocean ridge processes using analytical iso-viscous corner-flow demonstrate that the divergent motion of the upwelling mantle beneath the ocean ridge, when viewed in the reference frame of the young continental margin, shows oceanward flow of the lower continental crust and lithospheric mantle of the young rifted margin giving rise to depth dependent stretching as observed. Single-phase fluid-models have been developed to model the initiation of sea-floor spreading and the thermal, stretching and thinning evolution of the young rifted continental margin. Finite element fluid-flow modelling incorporating the evolving temperature dependent viscosity field on the fluid flow also show depth dependent stretching of the young continental margin. Two-phase flow models of ocean ridges incorporating the transport of both solid matrix and melt fluid (Spiegelman &Reynolds 1999) predict the divergent motion of the asthenosphere and lithosphere matrix, and the focusing of basaltic melt into the narrow axial zone spreading centre at ocean ridges. We are adapting two-phase flow models for application to the initiation of sea-floor spreading and rifted continental margin formation. i

  10. Final Rifting and Continental Breakup in the South China Sea

    NASA Astrophysics Data System (ADS)

    Franke, D.; Savva, D.; Pubellier, M. F.; Steuer, S.; Mouly, B.; Auxietre, J. L.; Meresse, F.; Chamot-Rooke, N. R. A.

    2014-12-01

    The magma-poor or intermediate magmatic South China Sea basin shows a triangular shape with a SW pointing apex, which manifests a preceding propagating rift. The earliest phase of rifting started in the Early Paleocene when a Mesozoic convergent margin changed to extension. After about 30 Myrs of rifting, breakup in the major eastern subbasin of the SCS occurred in the Early Oligocene and subsequent breakup of the southwest subbasin took place in the Late Oligocene. Seismic reflection data imaging conjugate crustal sections result in a conceptual model for rift-evolution at conjugate margins in time and space. Distinct are regular undulations in the crust-mantle boundary. Individual rift basins are bounded to crustal blocks by listric normal faults on either side. Moho uplifts are distinct beneath major rift basins, while the Moho is downbended beneath crustal blocks. Most of the basin-bounding faults sole out within the middle crust. At the distal margins, detachment faults are located at a mid-crustal level where a weak zone decouples crust and mantle lithosphere during rifting. The lower crust in contrast is interpreted as being strong. Only in the region within about 50 km from the oceanic domain we suggest that normal faults reach the mantle, enabling potentially a coupling between the crust and the mantle. Here, at the proximal margins detachment fault dip either seaward or landward. Largely symmetric structures result from the initial rifting stage. At the future breakup position either of the rift basin bounding faults subsequently penetrates the entire crust, resulting in asymmetry at this location. However, asymmetric deformation which is controlled by large scale detachment faulting is confined to narrow areas and does not result in a margin-wide simple-shear model. Rather considerable along-margin variations are suggested resulting in alternating "upper and lower plate" margins.

  11. 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

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

    USGS Publications Warehouse

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

    1983-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Updated models for continental rift zones need to address the role and development of low-angle normal fault networks, episodicity of extension, and interaction of 'active and passive' driving mechanisms. In the Rio Grande rift, USA, low-angle normal faults are found throughout the entire length of the rift, but make up a small percentage of the total fault population. The low-angle Jeter and Knife Edge faults, for example, crop out along the SW and NE margins of the Albuquerque basin, respectively. Apatite fission track (AFT) age-elevation data and apatite (U-Th)/He (AHe) ages from these rift flank uplifts record cooling between ~21 - 16 Ma in the NE rift flank and ~20 - 10 Ma in the SW, which coincides with times of rapid extension and voluminous syntectonic sedimentation. The timing of exhumation is also similar to rift flanks farther north in active margins based on AFT data alone. In addition, synthetic faults in the hanging wall of each low-angle fault become progressively steeper and younger basinward, and footwall blocks are the highest elevation along the rift flanks. These observations are consistent with a model where initially high-angle faults are shallowed in regions of maximum extension. As they rotate, new intrabasinal faults emerge which also can be rotated if extension continues. These relationships are similarly described in mature core complexes, and if these processes continued in the Rio Grande rift, it could eventually result in mid-crustal ductily deformed rocks in the footwall placed against surficial deposits in the hanging wall across faults that have been isostatically rotated to shallow dips. Although existing data are consistent with highest strain rates during a pulse of extension along the entire length of the rift 20-10 Ma., GPS-constrained measurements suggest that the rift is still actively-extending at 1.23-1.39 nstr/yr (Berglund et al., 2012). Additional evidence for Quaternary extension comes from travertine deposits that are

  14. Extension velocity partitioning, rheological crust-mantle and intra-crustal decoupling and tectonically inherited structures: consequences for continental rifting dynamics.

    NASA Astrophysics Data System (ADS)

    Wang, Kun; Mezri, Leila; Burov, Evgueni; Le Pourhiet, Laetitia

    2015-04-01

    We implemented series of systematic thermo-mechanical numerical models testing the importance of the rheological structure and extension rate partitioning for continental rift evolution. It is generally assumed that styles of continental rifting are mainly conditioned by the initial integrated strength of the lithosphere. For example, strong plates are expected to undergo extension in narrow rifting mode, while weak lithospheres would stretch in wide rifting mode. However, we show that this classification is largely insufficient because the notion of the integrated strength ignores the internal rheological structure of the lithosphere that may include several zones of crust-mantle or upper-crust-intermediate (etc) crust decoupling. As well, orogenic crusts characterizing most common sites of continental extension may exhibit inverted lithological sequences, with stronger and denser formerly lower crustal units on top of weaker and lighter upper crustal units. This all may result in the appearance of sharp rheological strength gradients and presence of decoupling zones, which may lead to substantially different evolution of the rift system. Indeed, strong jump-like contrasts in the mechanical properties result in mechanical instabilities while mechanical decoupling between the competent layers results in overall drop of the flexural strength of the system and may also lead to important horizontal flow of the ductile material. In particular, the commonly inferred concept of level of necking (that assumes the existence of a stationary horizontal stretching level during rifting) looses its sense if necking occurs at several distinct levels. In this case, due to different mechanical strength of the rheological layers, several necking levels develop and switch from one depth to another resulting in step-like variations of rifting style and accelerations/decelerations of subsidence during the active phase of rifting. During the post-rifting phase, initially decoupled

  15. The Ratio Between Magma Supply and Lithospheric Stretching Rates Controls the Architecture of Continental and Oceanic Rifts

    NASA Astrophysics Data System (ADS)

    Bourgeois, O.; Dauteuil, O.

    2010-12-01

    Magma-poor rifts (e.g. Rhine Graben, North Sea), non-volcanic passive continental margins (e.g. Galicia) and slow-spreading oceanic ridges (e.g. Mid-Atlantic Ridge), are composed of faulted crustal blocks that dip generally away from the rift axis. By contrast, magma-rich rifts (e.g. Afars), volcanic passive margins (e.g. Norway, Greenland, Namibia) and hotspot-influenced slow-spreading oceanic ridges (e.g. Iceland), are composed of faulted crustal blocks that dip generally towards the rift axis. On the basis of a detailed structural study of Iceland (Bourgeois et al. 2005, Geodinamica Acta 18:59-80), we demonstrate that, in magma-rich rifts, lithospheric stretching is accomodated in a long-term deformation strip, n x 100 km wide, by the development of successive roll-over structures controlled by growth-faults and underlain by shallow magma chambers. As a given roll-over structure progressively develops and tilts in response to lithospheric stretching, it is continuously covered by lavas erupted from the associated magma chamber and reaching the surface through dike swarms dominantly located along the growth fault. After a lifetime of a few My, this roll-over structure dies at the expense of the activation of a new, laterally offset, one. Correspondingly, such roll-over structures form successively at different places within a diffuse plate boundary n x 100 km wide. After several roll-over structures have developed and died, the overall structure of the long-term deformation strip is composed of faulted crustal blocks that generally dip towards the rift axis. This architecture differs from that of magma-poor rifts, where lithospheric strectching is accomodated in a fixed and narrow (n x 10 km) strip, by the developpement of outward-tilted blocks. Physical laboratory experiments conducted with analogue materials demonstrate that this difference in rift architectures is controlled by the ratio between the rate of lithospheric stretching and the rate of magma supply

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

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

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

  17. Thermo-mechanical modeling of continental rift evolution over mantle upwelling in presence of far-field stresses

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    We conducted fully-coupled high resolution rheologically consistent 3D thermo-mechanical numerical models to investigate the processes of mantle-lithosphere interaction (MLI) in presence of preexisting far-field tectonic stresses. MLI-induced topography exhibits strongly asymmetric small-scale 3D features, such as rifts, flexural flank uplifts and complex faults structures. This suggests a dominant role of continental rheological structure and intra-plate stresses in controlling continental rifting and break-up processes above mantle upwelling while reconciling the passive (far-field tectonic stresses) versus active (plume-activated) rift concepts as our experiments show both processes in action. We tested different experiments by varying two principal controlling parameters: 1) horizontal extension velocity and 2) Moho temperature used as simplified indicator of the thermal and rheological lithosphere layering. An increase in the applied extension expectedly gives less localized deformation at lithospheric scale: the growth of external velocity from 1.5 mm/years to 6 mm/years leads to enlargement of the rift zones from 75-175 km to 150-425 km width. On the contrary, increasing of the lithospheric geotherm has an opposite effect leading to narrowing of the rift zone: the change of the Moho isotherm from 600°C to 800°C causes diminution of the rift width from 175-425 km to 75-150 km. Some of these finding are contra-intuitive in terms of usual assumptions. The models refer to strongly non-linear impact of far-field extension rates on timing of break-up processes. Experiments with relatively fast far-field extension (6 mm/years) show intensive normal fault localization in crust and uppermost mantle above the plume head at 15-20 Myrs after the onset of the experiment. When plume head material reaches the bottom of the continental crust (at 25 Myrs), the latter is rapidly ruptured (<1 Myrs) and several steady oceanic floor spreading centers develop. Slower (3 mm

  18. Basalt volatile fluctuations during continental rifting: An example from the Rio Grande Rift, USA

    NASA Astrophysics Data System (ADS)

    Rowe, Michael C.; Lassiter, John C.; Goff, Kathleen

    2015-05-01

    Hydration and metasomatism of the lithospheric mantle potentially influences both the magmatic and tectonic evolution of southwestern North America. Prior studies have suggested that volatile enrichments to the mantle underlying western North America resulted from shallow subduction of the Farallon Plate during the Laramide (˜74-40 Ma). This study examines temporal and spatial variations in volatile elements (H2O, Cl, F, and S) determined from olivine and orthopyroxene-hosted melt inclusions along and across the Rio Grande Rift, the easternmost extent of Laramide shallow subduction. Maximum chlorine enrichments are observed in the southern rift with a Cl/Nb of ˜210 and reduce with time to MORB-OIB levels (˜5-17). Measured water abundances are <0.8 wt % in rehomogenized inclusions; however, calculated H2O, based on Cl/Nb systematics, primarily varies from 0.5 to 2 wt % H2O. Sulfur abundances (<0.61 wt %), and calculated sulfide saturation, indicate magmas with high Cl/Nb also contain oxidized sulfur. The abundance of fluorine in melt inclusions (up to 0.2 wt %) is not correlated to other volatile elements. Temporal variations in melt inclusion volatile abundances coupled with varying isotopic (Sr-Nd-Pb) whole-rock systematics suggest a transition from lithospheric to asthenospheric melt generation in the southern RGR and potential lithosphere-asthenosphere melt mixing in the central RGR. East to west decrease in volatile enrichment likely reflects a combination of varying mantle sources and early removal of metasomatized lithospheric mantle underlying regional extension. Results indicate, from multiple causes, subduction-related volatile enrichment to the lithospheric mantle is ephemeral, and strong enrichments in volatiles are not preserved in active magmatic-tectonic provenances.

  19. The Effect of Temperature Dependent Rheology on a Kinematic Model of Continental Breakup and Rifted Continental Margin Formation

    NASA Astrophysics Data System (ADS)

    Tymms, V. J.; Kusznir, N. J.

    2004-12-01

    The effect of temperature dependent rheology has been examined for a model of continental lithosphere thinning by an upwelling divergent flow field within continental lithosphere and asthenosphere leading to continental breakup and rifted continental margin formation. The model uses a coupled FE fluid flow and thermal solution and is kinematically driven using a half divergence rate Vx and upwelling velocity Vz. Viscosity structure is modified by the evolving temperature field of the model through the temperature dependent Newtonian rheology. Continental lithosphere and asthenosphere material are advected by the fluid-flow field in order to predict crustal and mantle lithosphere thinning leading to rifted continental margin formation. The results of the temperature dependent rheology model are compared with those of a simple isoviscous model. The temperature dependent rheology model predicts continental lithosphere thinning and depth dependent stretching, similar to that predicted by the uniform viscosity model. However compared with the uniform viscosity model the temperature dependent rheology predicts greater amounts of thinning of the continental crust and lithospheric mantle than the isoviscous solutions. An important parameter within the kinematic model of continental lithosphere breakup and rifted continental margin development is the velocity ratio Vz/Vx. For non-volcanic margins, Vz/Vx is thought to be around unity. Applying a velocity ratio Vz/Vx of unity gives a diffuse ocean-continent transition and exhumation of continental lithospheric mantle. For volcanic margins, Vz/Vx is of order 10, falling to unity with a half-life of order 10 Ma, leading to a more sharply defined ocean-continent transition. While Vx during continental breakup may be estimated, Vz can only be inferred. FE fluid flow solutions, in which Vz is not imposed and without an initial buoyancy driven flow component, predict a velocity ratio Vz/Vx of around unity for both temperature

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

    NASA Astrophysics Data System (ADS)

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

    2010-10-01

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

  1. Retrodeformation of the southern Upper Rhine Graben: new insights on continental oblique rifting

    NASA Astrophysics Data System (ADS)

    Bertrand, Guillaume; Horstmann, Mathias; Hermann, Oliver; Behrmann, Jan H.

    2005-02-01

    Surface flattening retrodeformations of the southern Upper Rhine Graben (URG) are presented in this paper. The two models presented cover the southwestern and southeastern parts of the URG (Colmar, France, and Freiburg, Germany, areas respectively). Results from these models indicate that the URG resulted from a sinistrally oblique extension for a significant part of the rifting. Early extension was along a nearly E-W (i.e. orthogonal) direction and concentrated on the main border faults. Deformation then propagated toward the graben interior as stretching direction rotated counter clockwise of 20-40°. Systematic along-strike variation of cumulated heave, throw and slip is also shown. It is suggested that this along-strike variation of fault displacement could be a characteristic feature of oblique continental rifting in general. Offset analyses of selected geological horizons indicate that some fault segments (Vosgian fault, near Selestat, France) were possibly active prior to deposition of early Tertiary sediments. Finally, close spatial coincidence between major fault segments and recent earthquakes hypocenters indicates that part of the fault system, at least in the Freiburg area, is still active.

  2. Fault-controlled hydration of the upper mantle during continental rifting

    NASA Astrophysics Data System (ADS)

    Bayrakci, G.; Minshull, T. A.; Sawyer, D. S.; Reston, T. J.; Klaeschen, D.; Papenberg, C.; Ranero, C.; Bull, J. M.; Davy, R. G.; Shillington, D. J.; Perez-Gussinye, M.; Morgan, J. K.

    2016-05-01

    Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems. Serpentinized mantle rocks are found beneath the sea floor at slow- to ultraslow-spreading mid-ocean ridges and are thought to be present at about half the world’s rifted margins. Serpentinite is also inferred to exist in the downgoing plate at subduction zones, where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults. Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that sea water reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth.

  3. Buried Mesozoic rift basins of the U. S. middle Atlantic continental margin

    SciTech Connect

    Benson, R.N. )

    1991-08-01

    The Atlantic continental margin is one of the frontier areas for oil and gas exploration in the US. Most the activity has been offshore where Upper Jurassic-Lower Cretaceous siliciclastic and carbonate rocks have been the drilling objectives, with only one significant but noncommercial gas discover. Onshore, recent exploration activities have focused on early Mesozoic rift basins buried beneath the postrift sediments of the middle Atlantic coastal plain. Many of the basins are of interest because they contain fine-grained lacustrine rocks that have sufficient organic richness, if not lost through hydrocarbon generation, to be classified as source beds for oil or gas. Locations of inferred rift basins beneath the middle Atlantic coastal plain were determined by analysis of drill-hole data in combination with gravity anomaly and aeromagnetic maps. Two basins in Delaware and the Queen Anne basin of Maryland are imaged on a regional Vibroseis profile. Areas enclosing inferred rift basins in the offshore region were mapped from interpretation of seismic reflection profiles. Assuming that petroleum source beds are present in the basin (synrift) rocks, hydrocarbon-generation models (Lopatin method) indicate that for a basin just offshore Delaware that is buried by 7 km of postrift sediments, only dry gas would be present in reservoir rocks; for the Norfolk basin of the Virginia coast buried by only 3 km of postrift rocks, the upper few hundred meters of synrift rocks are still within the oil-generation window. The less deeply buried basins beneath the coastal plain likely are still within the oil window.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    The Magadi rift in southern Kenya formed at ~7 Ma within Proterozoic rocks of the Mozambique orogenic belt, parallel to its contact with the Archean Tanzania craton. The rift is bounded to the west by the ~1600-m-high Nguruman border fault. The rift center is intensely dissected by normal faults, most of which offset ~1.4-0.8 Ma lavas. Current E-W extensional velocities are ~2-4 mm/yr. Published crustal tomography models from the rift center show narrow high velocity zones in the upper crust, interpreted as cooled magma intrusions. Local, surface-wave, and SKS-splitting measurements show a rift-parallel anisotropy interpreted to be the result of aligned melt zones in the lithosphere. Our field observations suggest that recent fault activity is concentrated at the rift center, consistent with the location of the 1998 seismic swarm that was associated with an inferred diking event. Fault zones are pervasively mineralized by calcite, likely from CO2-rich fluids. A system of fault-fed springs provides the sole fluid input for Lake Magadi in the deepest part of the basin. Many of these springs emanate from the Kordjya fault, a 50-km-long, NW-SE striking, transverse structure connecting a portion of the border fault system (the NW-oriented Lengitoto fault) to the current locus of strain and magmatism at the rift center. Sampled springs are warm (44.4°C) and alkaline (pH=10). Dissolved gas data (mainly N2-Ar-He) suggests two-component mixing (mantle and air), possibly indicating that fluids are delivered into the fault zone from deep sources, consistent with a dominant role of magmatism to the focusing of strain at the rift center. The Kordjya fault has developed prominent fault scarps (~150 m high) despite being oblique to the dominant ~N-S fault fabric, and has utilized an en echelon alignment of N-S faults to accommodate its motion. These N-S faults show evidence of sinistral-oblique motion and imply a bookshelf style of faulting to accommodate dextral-oblique motion

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

    NASA Astrophysics Data System (ADS)

    Cowie, Leanne; Kusznir, Nick; Leroy, Sylvie

    2013-04-01

    It has been proposed that some rifted continental margins have anomalous subsidence and that at break-up they were elevated at shallower bathymetries than the isostatic response predicted by classical rift models (McKenzie, 1978). The existence of anomalous syn- or early-post break-up subsidence of this form would have important implications for our understanding of the geodynamics of continental break-up and sea-floor spreading initiation. We have investigated subsidence of the young rifted continental margin of the eastern Gulf of Aden, focussing on the western Oman margin (break-up age 17.6 Ma). Lucazeau et al. (2008) have found that the observed bathymetry here is approximately 1 km shallower than the predicted bathymetry. In order to examine the proposition of an anomalous early post break-up subsidence history of the Omani Gulf of Aden rifted continental margin, we have determined the subsidence of the oldest oceanic crust adjacent to the continent-ocean boundary (COB) using residual depth anomaly (RDA) analysis corrected for sediment loading and oceanic crustal thickness variation. RDAs corrected for sediment loading using flexural backstripping and decompaction have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous subsidence of the Gulf of Aden rifted continental margin. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions of Crosby and McKenzie (2009). Non-zero RDAs at the Omani Gulf of Aden rifted continental margin can be the result of non standard oceanic crustal thickness or the effect of mantle dynamic topography or a non-classical rift and break-up model. Oceanic crustal basement thicknesses from gravity inversion together with Airy isostasy have been used to predict a "synthetic" gravity RDA, in order to determine the RDA contribution from non-standard oceanic crustal thickness. Gravity inversion, used to determine crustal basement thickness

  6. Fault growth and propagation during incipient continental rifting: Insights from a combined aeromagnetic and Shuttle Radar Topography Mission digital elevation model investigation of the Okavango Rift Zone, northwest Botswana

    NASA Astrophysics Data System (ADS)

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

    2008-06-01

    Digital Elevation Models (DEM) extracted from the Shuttle Radar Topography Mission (SRTM) data and high-resolution aeromagnetic data are used to characterize the growth and propagation of faults associated with the early stages of continental extension in the Okavango Rift Zone (ORZ), northwest Botswana. Significant differences in the height of fault scarps and the throws across the faults in the basement indicate extended fault histories accompanied by sediment accumulation within the rift graben. Faults in the center of the rift either lack topographic expressions or are interpreted to have become inactive, or have large throws and small scarp heights indicating waning activity. Faults on the outer margins of the rift exhibit either (1) large throws or significant scarp heights and are considered older and active or (2) throws and scarp heights that are in closer agreement and are considered young and active. Fault linkages between major fault systems through a process of "fault piracy" have combined to establish an immature border fault for the ORZ. Thus, in addition to growing in length (by along-axis linkage of segments), the rift is also growing in width (by transferring motion to younger faults along the outer margins while abandoning older faults in the middle). Finally, utilization of preexisting zones of weakness allowed the development of very long faults (>100 km) at a very early stage of continental rifting, explaining the apparent paradox between the fault length versus throw for this young rift. This study clearly demonstrates that the integration of the SRTM DEM and aeromagnetic data provides a 3-D view of the faults and fault systems, providing new insight into fault growth and propagation during the nascent stages of continental rifting.

  7. Tectonic structure of the continental Rifting of the northern margin of the South China Sea

    NASA Astrophysics Data System (ADS)

    Cameselle, A. L.; Ranero, C. R.; Franke, D.; Barckhausen, U.

    2012-12-01

    We present multichannel seismic reflection images of about 2250 km of reprocessed seismic records collected during Sonne cruise 49 across the northern margin of the South China Sea. The data were collected in four seismic lines striking perpendicular to the strike of the margin. The lines cross the outer continental shelf and slope, and two of them continue across the central region of the basin. The four lines have been carefully reprocessed to improve signal to noise ratio including pre-stack statistical predictive deconvolution, pre-stack multiple attenuation by radon filtering and by FK filtering. Semblance-based velocity picking was before and after processing with analyses every 5 km or closer in areas of rough basement topography. Constant velocity stacks were also used in areas of interest. The data were stacked and subsequently post-stack time migrated using an FD algorithm and time and space variant smooth velocity models. The resulting sections display in detail the structure of post-rift and syn-rift sediment, the basement structure of fault-bounded blocks, often fault reflections, and very often clear and fairly continuous reflections from crust - mantle boundary. The four seismic images show the tectonic structure formed during -now inactive- rifting. From west to east the tectonic structure changes considerably as the amount of extension attained increases towards the east. In the western region the lines show the structure of the Xisha Trough, from the region where continental rifting stopped before continental-crust break up and separation to the east where a extension may have led to break up and subsequent to seafloor spreading. The images show clearly how the continental crust-mantle boundary progressively shallows to reach close to the top of the basement. The conjugate continental flanks of the rift display rather different styles of faulting producing a very asymmetric structure. The conjugate flanks display rather different amounts of fault

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

    PubMed

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

    2014-01-01

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

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

    PubMed Central

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

    2014-01-01

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

  10. The continent-ocean transition of the rifted South China continental margin

    NASA Astrophysics Data System (ADS)

    Cameselle, Alejandra L.; Ranero, César R.; Franke, Dieter; Barckhausen, Udo

    2014-05-01

    The continent to ocean transition (COT) architecture of rifted margins represents a key aspect in the study of the variability of different rifting systems and thus, to understand lithospheric extension and final break-up processes. We used 2250 km of reprocessed multichannel seismic data along 4 regional lines and magnetic data acquired across the NW South China continental margin to investigate a previously poorly defined COT. The along-strike structure of the NW subbasin of the South China Sea presents different amounts of extension allowing the study of conjugate pairs of continental margins and their COT in a relative small region. The time-migrated seismic sections allow us to interpreted clear continental and oceanic domains from differences in internal reflectivity, faulting style, fault-block geometry, the seismic character of the top of the basement, the geometry of sediment deposits, and Moho reflections. The continental domain is characterized by arrays of normal faults and associated tilted blocks overlaid by syn-rift sedimentary units. The Moho is imaged as sub-horizontal reflections that define a fairly continuous boundary typically at 8-10 s TWT. Estimation of the thickness of the continental crust using 6 km/s average velocity indicates a ~22 km-thick continental crust under the uppermost slope thinning abruptly to ~9-6 km under the lower slope. The oceanic crust has a comparatively highly reflective top of basement, little-faulting, not discernible syn-tectonic strata, and fairly constant thickness (4-8 km) over tens of km distance defined by usually clear Moho reflections. The COT can be very well defined based on MSC images and occurs across a ~5-10 km narrow zone. Rifting in the NW subbasin resulted in asymmetric conjugate margins. Arrays of tilted fault blocks covered by abundant syn-rift sediment are displayed across the northwestern South China continental margin, whereas the conjugate Macclesfield Bank margin shows abrupt thinning and

  11. Rifted continental margins: geometric control on crustal architecture and melting

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    A new model is provided for the distribution of magma-poor and magma-rich rifted margins. The South Atlantic, Central Atlantic, North Atlantic - Arctic (Eurasia Basin), and Red Sea all are magma-rich at their distal ends and magma-poor at their proximal ends (with respect to their poles of rotation). The well-known architectural zonation across fully developed magma-poor margins (limited crustal stretching, hyperextension, exhumed mantle, oceanic crust) is also observed along the lengths of many margins at the super-regional scale. Zones of exhumed mantle, marking magma-poor margin, can be mapped for thousands of kilometers. Likewise can zones of seaward dipping reflectors (SDR) marking magma-rich margins. At this scale, the age of the oceanic crust becomes younger in the direction of the rotation pole, implying that the continents ruptured by rift tip propagation (and rotation pole propagation). Propagation is also manifested by the age of pre-break-up magmatism, break-up unconformity, and margin uplift. Hence, the classic cross-sectional depiction of margin evolution has a third dimension. The degree of melting follows the same pattern. At the distal end of e.g. the South Atlantic, SDR zones are wide and gradually thin toward the rotation pole. Eventually exhumed mantle takes over, marking the transition to the magma-poor margins, which remain to the proximal end of rifting. SDR zones also thin laterally from ca 10-15 km thickness at the continent-ocean boundary (COB) to ca 7 km thick oceanic crust beyond the SDRs. Outcrop data demonstrate that also exhumed mantle contains up to ca 12% melt, infiltrated in the peridotites. Thus, melting is largest at the distal ends near the COB, and decreases both laterally toward the evolving ocean and along strike toward the rift tip. Accepting that continents are rigid to a first order, the linear rate of extension at any given location along an evolving rift and ocean, is governed by the angular rate of opening, the distance

  12. Extension on rifted continental margins: Observations vs. models.

    NASA Astrophysics Data System (ADS)

    Skogseid, Jakob

    2014-05-01

    Mapping the signature of extensional deformation on rifted margins is often hampered by thick sedimentary or volcanic successions, or because salt tectonics makes sub-salt seismic imaging challenging. Over the past 20 years the literature is witnessing that lack of mapable faults have resulted in a variety of numerical models based on the assumption that the upper crust takes little or no extensional thinning, while the observed reduction of crustal thickness is taken up in the middle and lower crust, as well as in the mantle. In this presentation two case studies are used to highlight the difference that 3D seismic data may have on our understanding. The small patches of 3D resolution data allow us to get a glance of the 'real' signature of extensional faulting, which by analogy can be extrapolate from one margin segment to the next. In the South Atlantic salt tectonics represents a major problem for sub-salt imaging. The conjugate margins of Brazil and Angola are, however, characterized by pronounced crustal thinning as documented by crustal scale 2D reflection and refraction data. Off Angola the 3D 'reality' demonstrates that upper crustal extension by faulting is comparable to the full crustal, as well as lithospheric thinning as derived from refraction data and basin subsidence analysis. The mapped faults are listric low angle faults that seem to detach at mid crustal levels. 2D seismic has in the past been interpreted to indicate that almost no extensional faulting can be mapped towards the base of the so-called 'sag basin'. The whole concept of the 'sag basin', often ascribed to as crustal thinning without upper crustal deformation, is in fact related to this 'lack of observation', and furthermore, have caused the making of different types of dynamic models attempting to account for this. In the NE Atlantic significant Paleocene extensional faulting is locally seen adjacent to the 50 to more than 200 km wide volcanic cover on each side of the breakup axis

  13. 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.

  14. Imaging continental breakup using teleseismic body waves: The Woodlark Rift, Papua New Guinea

    NASA Astrophysics Data System (ADS)

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

    2015-09-01

    This study images the upper mantle beneath the D'Entrecasteax Islands, Papua New Guinea, providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers. Differential travel times from P and S-wave teleseisms recorded during the 2010-2011 CDPapua passive seismic experiment are used to invert for separate VP and VS velocity models of the continental rift. A low-velocity structure marks the E-W axis of the rift, correlating with the thinnest crust, high heat flow, and a linear trend of volcanoes. This slow region extends 250 km along strike from the oceanic spreading centers, demonstrating significant mantle extension ahead of seafloor breakup. The rift remains narrow to depth indicating localization of extension, perhaps as a result of mantle hydration. A high-VP structure at depths of 90-120 km beneath the north of the array is more than 6.5% faster than the rift axis and contains well-located intermediate depth earthquakes. These independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction, although the polarity of subduction cannot be resolved. This geometry is gravitationally unstable; downwelling or small-scale convection could have facilitated rifting and rapid lithospheric removal, although this may require a wet mantle to be realistic on the required time scales. The high-V structure agrees with the maximum P,T conditions recorded by young ultra-high pressure rocks exposed on the rift axis and may be implicated in their genesis.

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

    NASA Astrophysics Data System (ADS)

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

    2010-10-01

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

  16. Flow of material under compression in weak lower continental crust can cause post-rift uplift of passive continental margins

    NASA Astrophysics Data System (ADS)

    Chalmers, James

    2014-05-01

    There are mountain ranges up to more than 2 km high along many passive continental margins (e.g. Norway, eastern Australia, eastern Brazil, SE and SW Africa, east and west Greenland etc.), dubbed Elevated Passive Continental Margins (EPCMs). EPCMs contain several features in common and observations indicate that uplift of these margins took place after continental break-up. There are many explanations for their formation but none that satisfy all the observations. Lack of a geodynamical mechanism has meant that there has been difficulty in getting the community to accept the observational evidence. Formation of a passive continental margin must take place under conditions of tension. After rifting ceases, however, the margin can come under compression from forces originating elsewhere on or below its plate, e.g. orogeny elsewhere in the plate or sub-lithospheric drag. The World Stress Map (www.world-stress-mp.org) shows that, where data exists, all EPCMs are currently under compression. Under sufficient compression, crust and/or lithosphere can fold, and Cloetingh & Burov (2010) showed that many continental areas may have folded in this way. The wavelengths of folding observed by Cloetingh & Burov (2010) imply that the lower crust is likely to be of intermediate composition; granitic lower crust would fold with a shorter wavelength and basic lower crust would mean that the whole lithosphere would have to fold as a unit resulting in a much longer wavelength. Continental crust more than 20 km thick would be separated from the mantle by a weak layer. However, crust less thick than that would contain no weak layers would become effectively annealed to the underlying strong mantle. Under sufficient horizontal compression stress, material can flow in the lower weak layer towards a continental margin from the continental side. The annealed extended crust and mantle under the rift means, however, that flow cannot continue towards the ocean. Mid- and lower crustal material

  17. Continental rifting in the Woodlark Basin, Papua New Guinea: A comparison of different estimates of extension at the rifting-spreading transition.

    NASA Astrophysics Data System (ADS)

    Partlow, J.; Goodliffe, A. M.

    2014-12-01

    The Woodlark Basin is one of few places where it is possible to investigate an active transition from continental rifting to seafloor spreading. The Papuan Peninsula began N-S extension at 8.4 Ma, followed by seafloor spreading at 6 Ma. To date, seafloor spreading has propagated west 500 km. In the proximity of the modern rifting to spreading transition the northern margin has subsided 2-3 km with minor brittle faulting. The southern margin has subsided a similar amount but is characterized by large faults. Previous work shows that the observed continental extension is half the amount resolved by seafloor-spreading kinematics. It has been proposed that this discrepancy is due to mid-crustal decoupling, where the mantle lithosphere and lower crust are detached. The N-S profile across the current rifting to spreading transition is a natural laboratory for extensional environments. The work herein presented is a continuation of prior studies, but incorporates a new approach to extensional modeling, specifically the use of the Move software package. The profile presented includes ODP Leg 180 wells. Structural and stratigraphic interpretations originate from nearby seismic lines. Biostratigraphy and paleomagnetism data are the basis for age-depth relationships. Interpreted sedimentary packages permit backstripping and decompaction models that assume Airy Isostasy. Extension is estimated through the restoration of fault heaves and back rotation of fault blocks. From previous studies we know the width of the Papuan Peninsula to be 320 km in the vicinity of the profile presented. Furthermore, those studies estimate 220 km of extension across the margin based on Euler pole kinematics. This gives an original margin width of about 100 km, and Beta greater than 3. We present herein an extension estimate based on 2-D kinematic modeling, and contrast this with prior extension estimates of 111-115 km.

  18. Fission track analysis, rift shoulder uplift, and tectonic modeling of the Norwegian Continental Margin

    SciTech Connect

    Andriessen, P.; Van Der Beek, P.; Cloetingh, S.; Rohrman, M. )

    1993-09-01

    Apatite fission track analysis from southern Norway and Sweden, across the Permian Carboniferous Oslo rift, are presented and discussed in relation to different rifting scenarios. Vertical and horizontal apatite fission tack profiles in middle and southern Norway unravel the post-Carboniferous history of the Fennoscandian shield. Fission track apatite ages range from 240 Ma in the south to 160 Ma in the north, and according to spontaneous fission track length measurements, they must be interpreted as mixed ages, indicating minor amounts of Paleozoic-Mesozoic sedimentary cover. Apatite fission track length and age modeling suggest rapid cooling and uplift in the Tertiary for the southernmost part of Norway, suggesting a differential uplift of the basement. the obtained data are important for the reconstruction of burial and thermal histories of Cenozoic sedimentary basins of the Norwegian continental margin in the northern North Sea, where diverse rifting events, intraplate stress regimes, and inversion tectonics are involved. Fission track analysis puts constraints on tectonic modeling of uplift of rift flanks and the Norwegian continental margin and yields information for these assessment of hydrocarbon potentials of the sedimentary basins.

  19. Impact of lithosphere rheology on 3D continental rift evolution in presence of mantle plumes: insights from numerical models

    NASA Astrophysics Data System (ADS)

    Koptev, Alexander; Burov, Evgueni; Gerya, Taras

    2015-04-01

    We implement fully-coupled high resolution 3D thermo-mechanical numerical models to investigate the impact of the laterally heterogeneous structure and rheological stratification of the continental lithosphere on the plume-activated rifting and continental break-up processes in presence of preexisting far-field tectonic stresses. In our experiments, the "plumes" represent short-lived diapiric upwellings that have no continuous feeding from the depth. Such upwellings may be associated with "true" plumes but also with various instabilities in the convective mantle. The models demonstrate that the prerequisite of strongly anisotropic strain localization during plume-lithosphere interaction (linear rift structures instead of axisymmetric radial faulting) refers to simultaneous presence of a mantle upwelling and of (even extremely weak) directional stress field produced by far-field tectonic forces (i.e. ultra-slow far field extension at < 3 mm/y). Although in all experiments the new-formed spreading centers have similar orientations perpendicular to the direction of the main far-field axis, the models with homogeneous lithosphere show that their number and spatial location is different for various extension rates and thermo-rheological structures of the lithosphere: relatively slow extension (3 mm/year) and colder isotherm (600-700°C at Moho depth) at the crustal bottom lead to the development of single rifts, whereas "faster" external velocities (6 mm/year) and "hotter" crustal geotherm (800°C at Moho depth) result in dual (sometimes asymmetric) rift evolution. On the contrary, the models with heterogeneous lithosphere (thick cratonic block with cold and thick depleted mantle embedded into «normal» lithosphere) and the plume centered below the craton, systematically show similar behaviors: two symmetrical and coeval rifting zones embrace the cratonic micro-plate along its long sides. The experiments where the initial plume position has been laterally shifted with

  20. 3D thermo-mechanical models of continental breakup and transition from rifting to continental break-up and spreading

    NASA Astrophysics Data System (ADS)

    Koptev, Alexander; Burov, Evgueni; Gerya, Taras

    2014-05-01

    We conducted high-resolution 3D thermo-mechanical numerical modeling experiments to explore evolution and styles of plume-activated rifting in presence of preexisting far-field tectonic stress/strain field and tectonic heritage (in form of cratonic blocks embedded in «normal lithosphere»). The experiments demonstrate strong dependence of rifting style on preexisting far-field tectonic stress/strain field and initial thermo-rheological profile, as well as on the tectonic heritage. The models with homogeneous lithosphere demonstrate strongly non-linear impact of far-field extension rates on timing of break-up processes. Experiments with relatively fast far-field extension (6 mm/y) show intensive normal fault localization in crust and uppermost mantle above the zones of plume-head emplacement some 15-20 Myrs after the onset of the experiment. When plume head material reaches the bottom of the continental crust (at ~25 Myrs), the latter is rapidly ruptured (<1 Myrs) and several steady oceanic floor spreading centers develop. Slower (3 mm/y) far-field velocities result in disproportionally longer break-up time (from 60 to 70 Myrs depending on initial isoterm at the crust bottom). Although in all experiments with homogeneous lithosphere spreading centers have similar orientation perpendicular to the direction of far-field extension, their number and spatial location are different for different extension rates and thermo-rheological structures of the lithosphere. On the contrary, in case of normal lithosphere containing embedded cratonic block, spreading zones develop symmetrically, embracing cratonic micro-plate along its long sides. Presence of cratonic blocks leads to splitting of the plume head onto initially nearly symmetrical parts, each of which flows towards beneath the craton borders. This craton-controlled distribution of plume material causes the crustal strain localization and uprise of plume material along the craton boundaries. Though there is a net

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

    The production, storage, and eruption of magma within continental and oceanic rift zones shapes the structure and morphology of the plate boundary, and the buoyancy force of the magma adds to the tectonic forces driving plate divergence. Dikes transport magma from crustal and sub-crustal magma reservoirs, accommodate extensional strain, and impart a length scale to rift deformation. From seismic and geodetic detection of rare ridge intrusion events, dikes of length 10-30 km are emplaced over time periods of hours to days. Both the temporal and spatial patterns of faulting and magmatism help to initiate and maintain the ~50 km along-axis segmentation of mid-ocean ridges and highly evolved continental rifts, and are fundamental to our understanding of lithospheric deformation within rift zones. Four years of seismicity data and field observations from the ongoing rifting episode in the subaerial southern Red Sea rift provide fundamental constraints on the time and length scales of magma transport in this incipient seafloor spreading center. Migrating swarms of tectonic, volcano-tectonic, and very low frequency earthquakes mark the along-axis propagation of 10-20 km-long dikes that initiate in the aseismic axial zone near the mid-segment (Belachew et al., 2009). Post-dike seismicity occurs along the length of the dike zones for months to years after large volume dike intrusions, illuminating differences in the dike height (Cote et al., 2009), as well as syn- and post-dike faulting. Episodic volcano-tectonic and low-frequency earthquakes near the centers of the 3 adjoining tectono-magmatic segments in the southern Red Sea rift suggest that their segment centers are underlain by pressurized magma chambers. For example, moderate magnitude, volcano-tectonic earthquakes occurred at the centers of 4 tectono-magmatic segments during the 5-day earthquake swarm and eruption on the Erta’Ale magmatic segment in 2008. Ongoing studies examine the distribution of melt zones

  2. Influence of the mechanical coupling and inherited strength variations on the geometry of continental rifts.

    NASA Astrophysics Data System (ADS)

    Philippon, Melody; van Delft, Pim; van Winden, Matthijs; Zamuroviç, Dejan; Sokoutis, Dimitrios; Willingshofer, Ernst; Cloetingh, Sierd

    2013-04-01

    The geometry of continental rifts is strongly controlled by the rheology of the lithosphere at the onset of rifting. This initial geometry will further control the development of ocean spreading centers and the structure of adjacent passive margins. Therefore, understanding the influence of coupling between the different layers of the lithosphere with and without laterally variable strength in the crust is key when investigating continental rifts. In this study we infer the influence of coupling in the crust on the rift geometry by means of crustal scale analogue experiments, where we characterize the response of the crust to deformation in terms of the strength ratio between brittle and ductile crust. The degree of coupling has been varied for setups containing or not a pre-existing weak zone. To allow a better description of the geometry obtained in our models, some key observations such as: a) the degree of tilting of the blocks, b) the total width of the graben, c) the displacement along the main fault and d) the distribution of thinning in the lower crust are monitored. Models containing a weak zone are compared to natural examples of the inherited Mozambique Ocean suture zones (MOSZ) in the Red Sea rift. The modelling results suggest that deformation is not a-priori localized within pre-existing weak zones unless the coupling between the brittle and the ductile crust is high. With respect to the MOSZ, we infer that: (1) Jurassic NW-SE trending grabens developed parallel to but not within the MOSZ and hence reflect a low degree of coupling whereas (2) Eocene rifting in the Red Sea occurred under coupled conditions as deformation strongly focused within the MOSZ. Models without weak zone shows that large-scale detachment faults can also form within a highly coupled crust, which is at variance to the common perception that detachment faulting demands strong decoupling. Our findings shed light on natural rift systems, which show a wide range of geometries that

  3. Rifted Structure of the Vietnam Continental Margin Near the South China Sea Spreading Center

    NASA Astrophysics Data System (ADS)

    Reid, I. D.; Fyhn, M. B.; Boldreel, L. O.; Nielsen, L. H.; Duc, N. A.; Huyen, N. T.; Thang, L. D.

    2007-12-01

    The extinct spreading center of the South China Sea intersects the continental margin off Vietnam, providing an excellent opportunity to study the interaction of these two features. As part of a collaborative project between the Geological Survey of Denmark and Greenland, the University of Copenhagen and the Vietnam Petroleum Institute, the crustal structure of this area has been investigated by the use of seismic reflection profiles, to provide control on the sedimentary and basement structure, combined with modelling of gravity data from global satellite altimetry, to constrain the crustal thickness. A complex pattern of rifting is seen, which may be ascribed to the complex stress fields of the propagating rift axis, together with an apparent progression in structure. In the more oceanic area, the rifting is relatively sharp, with fairly rapid crustal thnning of about 10 km. Towards the continent, in the region of the tip of the rift axis, the crustal thinning is less, around 5-7 km, and takes place over a greater distance. In the absence of data on the deep crustal structure it is not possible to determine the absolute crustal thickness with certainty, but the gravity modelling suggests that the pre-existing crust was no more than 20 km thick, having been thinned in earlier stages of formation of the South China Sea. A preliminary analysis of the isostatic balance along the various transects was inconclusive but suggests that the sedimentary sequences are largely isostatically compensated, rather than being supported by lithospheric rigidity. Detailed modelling of the rifting and subsidence may provide further insight into the processes that occur when an oceanic spreading center intersects and propagates into a continental margin.

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

    NASA Astrophysics Data System (ADS)

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

    1983-05-01

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

  5. The rifting of continental and oceanic lithosphere: Observations from the Woodlark Basin

    NASA Astrophysics Data System (ADS)

    Goodliffe, Andrew Mark

    A detailed marine geophysical survey of the Woodlark Basin has given us a high resolution picture of the evolution of the Woodlark Basin. An algorithm developed for this study, which reconstructs bathymetry and magnetization grids to selected ages, has revealed many of the details of the evolution of this young ocean basin. The Woodlark Basin formed by the nucleation of spreading segments in sites of focused continental rifting. These segments, which are on the order of 100 km long, subsequently grew by propagation. Segments form in an overlapping configuration, resulting in the deformation and rotation of intervening continental lithosphere. Transform faults form some time later, cutting through continental lithosphere to join the tips of the spreading segments. Continental margins formed by nucleation of a spreading segment are distinct from those formed by propagation. Nucleation margins have concordant abyssal hill fabric, continent/ocean boundary and continental rift fabric. The continent/ocean boundary (COB) of propagation margins is discordant with abyssal hill fabric, but may be either concordant or discordant with continental rift fabric. A third type of COB, formed when there is no propagation, results in abyssal hill fabric perpendicular to the COB. Similar geometries result from a COB formed on a transform fault. Seismicity on the margins after the initiation of sea-floor spreading, and the inward curvature of abyssal fabric formed on spreading centers propagating into the continental margin, demonstrate that extension continues on the margins for up to 1 Ma. Large reorientations of the spreading center take place by propagation or synchronous reorientation. The present-day sea-floor reveals that its 500-km-long spreading center reoriented synchronously, without propagation, about 80 ka. There is no evidence of the V-shaped pseudofault geometry typical of spreading center propagation, nor of the progressive fanning of sea-floor fabric characteristic of

  6. Impact of gravity processes on the initial post-rift stages of construction and evolution of a continental margin: Insights from the eastern Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Baurion, Celine; Gorini, Christian; Leroy, Sylvie; Migeon, Sebastien; Lucazeau, Francis; Bache, Francois; Zaragosi, Sebastien; Smit, Jeroen; Al-Toubi, Khalfan; dos Reis, Antonio

    2013-04-01

    The study of the post-rift sediment architecture and continental slope morphology leads to a reconstruction of the initial stages of formation and evolution of gravity-driven processes on the northern margin of the eastern Gulf of Aden. The slope-related features and associated deposits in the deep basin along this young passive margin are investigated through the analysis of a set of seismic-reflection and multibeam bathymetry data. This study demonstrates how preconditioning and triggering factors (tectonics, climate and eustatic variations) can interact and control the margin morphology and post-rift sediment architecture in a source-to-sink perspective. The combined geomorphological and stratigraphic study of this margin allows us to identify three morphological domains inherited from the structural segmentation. The monsoon climate combined with a major eustatic lowstand is proposed as the most likely set of factors preconditioning slope destabilisation on the whole margin. These factors also enhance the effect of the late post-rift uplift of the eastern morphological domain of the studied margin. The formation and distribution of the slope-related features are thus mainly controlled by active faults on the continental slope and the potential effect of bottom currents at the base of the continental slope. The oversteepening of the continental slope in the eastern domain of the studied margin is probably the main triggering factor controlling the generation of failure processes and subsequent canyon formation by upslope erosion. The analysis of canyon location and morphology along the uplifted part of the continental slope reveals the long-term influence of secondary slope-related features, contour currents and turbidite flows on the development of canyons. As a consequence of the late post-rift uplift that only affected the eastern part of the studied margin, huge volumes of sediment were accumulated in mass-transport complexes at the foot of numerous slope

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

    PubMed

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

    2006-07-20

    Seafloor spreading centres show a regular along-axis segmentation thought to be produced by a segmented magma supply in the passively upwelling mantle. On the other hand, continental rifts are segmented by large offset normal faults, and many lack magmatism. It is unclear how, when and where the ubiquitous segmented melt zones are emplaced during the continental rupture process. Between 14 September and 4 October 2005, 163 earthquakes (magnitudes greater than 3.9) and a volcanic eruption occurred within the approximately 60-km-long Dabbahu magmatic segment of the Afar rift, a nascent seafloor spreading centre in stretched continental lithosphere. Here we present a three-dimensional deformation field for the Dabbahu rifting episode derived from satellite radar data, which shows that the entire segment ruptured, making it the largest to have occurred on land in the era of satellite geodesy. Simple elastic modelling shows that the magmatic segment opened by up to 8 m, yet seismic rupture can account for only 8 per cent of the observed deformation. Magma was injected along a dyke between depths of 2 and 9 km, corresponding to a total intrusion volume of approximately 2.5 km3. Much of the magma appears to have originated from shallow chambers beneath Dabbahu and Gabho volcanoes at the northern end of the segment, where an explosive fissural eruption occurred on 26 September 2005. Although comparable in magnitude to the ten year (1975-84) Krafla events in Iceland, seismic data suggest that most of the Dabbahu dyke intrusion occurred in less than a week. Thus, magma intrusion via dyking, rather than segmented normal faulting, maintains and probably initiated the along-axis segmentation along this sector of the Nubia-Arabia plate boundary. PMID:16855588

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

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

  9. The Cryogenian intra-continental rifting of Rodinia: Evidence from the Laurentian margin in eastern North America

    NASA Astrophysics Data System (ADS)

    McClellan, Elizabeth; Gazel, Esteban

    2014-10-01

    The geologic history of the eastern North American (Laurentian) margin encompasses two complete Wilson cycles that brought about the assembly and subsequent disaggregation of two supercontinents, Rodinia and Pangea. In the southern and central Appalachian region, basement rocks were affected by two episodes of crustal extension separated by > 100 m.y.; a Cryogenian phase spanning the interval 765-700 Ma and an Ediacaran event at ~ 565 Ma. During the Cryogenian phase, the Mesoproterozoic continental crust was intruded by numerous A-type felsic plutons and extensional mafic dikes. At ~ 760-750 Ma a bimodal volcanic sequence erupted onto the uplifted and eroded basement. This sequence, known as the Mount Rogers Formation (MRF), comprises a bimodal basalt-rhyolite lower section and an upper section of dominantly peralkaline rhyolitic sheets. Here, we provide new geochemical evidence from the well-preserved volcanic rocks of the Cryogenian lower MRF, with the goal of elucidating the process that induced the initial stage of the break-up of Rodinia and how this affected the evolution of the eastern Laurentian margin. The geochemical compositions of the Cryogenian lavas are remarkably similar to modern continental intra-plate settings (e.g., East African Rift, Yellowstone-Snake River Plain). Geochemical, geophysical and tectonic evidence suggests that the common denominator controlling the melting processes in these settings is deep mantle plume activity. Thus, evidence from the MRF suggests that the initial phase of extension of the Laurentian margin at ~ 760-750 Ma was possibly triggered by mantle plume activity. It is possible that lithospheric weakness caused by a mantle plume that impacted Rodinia triggered the regional extension and produced the intra-continental rifting that preceded the breakup of the Laurentian margin.

  10. Hydrogeological structure of a seafloor hydrothermal system related to backarc rifting in a continental margin setting

    NASA Astrophysics Data System (ADS)

    Ishibashi, Jun-ichiro

    2016-04-01

    Seafloor hydrothermal systems in the Okinawa Trough backarc basin are considered as related to backarc rifting in a continental margin setting. Since the seafloor is dominantly covered with felsic volcaniclastic material and/or terrigenous sediment, hydrothermal circulation is expected to be distributed within sediment layers of significantly high porosity. Deep drilling through an active hydrothermal field at the Iheya North Knoll in the middle Okinawa Trough during IODP Expedition 331 provided a unique opportunity to directly access the subseafloor. While sedimentation along the slopes of the knoll was dominated by volcanic clasts of tubular pumice, intense hydrothermal alteration was recognized in the vicinity of the hydrothermal center even at very shallow depths. Detailed mineralogical and geochemical studies of hydrothermal clay minerals in the altered sediment suggest that the prevalent alteration is attributed to laterally extensive fluid intrusion and occupation within the sediment layer. Onboard measurements of physical properties of the obtained sediment revealed drastic changes of the porosity caused by hydrothermal interactions. While unaltered sediment showed porosity higher than 70%, the porosity drastically decreased in the layer of anhydrite formation. On the other hand, the porosity remained high (~50%) in the layer of only chlorite alteration. Cap rock formation caused by anhydrite precipitation would inhibit the ascent of high temperature fluids to the seafloor. Moreover, an interbedded nature of pelagic mud units and matrix-free pumice deposits may prompt formation of a tightly layered architecture of aquifers and aquicludes. This sediment architecture should be highly conducive to lateral flow pseudo-parallel to the surface topography. Occurrence of sphalerite-rich sulfides was recognized as associated with detrital and altered sediment, suggesting mineralization related to subsurface chemical processes. Moreover, the vertical profiles of

  11. Continental rifting and metamorphic core complex formation ahead of the Woodlark spreading ridge, D'Entrecasteaux Islands, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Little, Timothy A.; Baldwin, S. L.; Fitzgerald, P. G.; Monteleone, B.

    2007-02-01

    We evaluate the role of a metamorphic core complex (MCC) on Normanby Island in the Woodlark rift. Located <30 km from an active mid-ocean ridge (MOR), a >1 km thickness of blueschist-derived mylonites formed in a midcrustal shear zone during the Pliocene at ˜400-500°C. This top-to-the-north zone appears to have reactivated the gently dipping base of the Papuan ophiolite (Papuan Ultramafic Body, PUB), and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the MCC and rapid subsidence of the Woodlark Rise. Mylonites in the MCC's lower plate have been exhumed along a detachment as a result of >50 km of slip at rates of >12 mm/yr. The inactive, back-tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the Plio-Pleistocene plate motion. A second SE vergent detachment has been established on the opposite flank of this rolling-hinge style MCC, probably since <0.5 Ma. Centimeters per year slip rates on these two faults can account for most of the Pleistocene plate motion in this eastern sector of the Woodlark rift, and confirm the important role of MCCs in exhuming very young HP rocks in this rift. Paleopiezometry of mylonites using recrystallized quartz grain size indicates flow stresses of ˜30 MPa before the rocks were overprinted by extension fractures. These results imply high pore fluid pressures (λ > 0.8) at depth, and provide a sufficient mechanism for activating low-angle normal faults in the rift. MCC inception was not localized to the tip of the Woodlark MOR. Instead, extreme crustal thinning near the MCC preconditioned later continental breakup. The lower crust appears to be weak, thickening beneath unloaded footwalls to uplift MCCs above sea level, and flowing laterally to even out regional crustal thickness contrasts on a 1-6 m.y. timescale. Deep-seated transforms separate rheologically distinct domains in which extension has been localized along the weak PUB

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

    NASA Astrophysics Data System (ADS)

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

    2014-08-01

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

  13. Dynamic magmatic processes at a continental rift caldera, observed using satellite geodesy

    NASA Astrophysics Data System (ADS)

    Lloyd, Ryan; Biggs, Juliet; Birhanu, Yelebe; Wilks, Matt; Gottsmann, Jo; Kendall, Mike; Lewi, Elias

    2016-04-01

    Large silicic calderas are a key feature of developing continental rifts, such as the Main Ethiopian Rift (MER), and are often observed to be deforming. Corbetti is one such example of a Holocene caldera in the MER that is undergoing deformation. However, the cause of the unrest, and the relationship to rift processes such as magma storage, transport and extension remain poorly understood. To investigate, we use InSAR (ascending and descending Cosmo-SkyMed data) and continuous GPS to observe the temporal and spatial evolution of sustained uplift at the Corbetti Caldera. Within the caldera, which was thought to have formed ~200 ka, there is evidence for numerous periods of resurgent volcanism in the form of plinian eruptions as well as effusive obsidian flows. How the sources of these varying styles of volcanism are reconciled at depth and in time is currently poorly constrained. Previous research has shown that pre-rift structures have a significant influence on the strain field, and hence on the magmatic and hydrothermal processes which drive it. The Cosmo-SkyMed data used in this study was specifically chosen such that each ascending image has a corresponding descending image acquired as contemporaneously as possible. This is necessary, given the rate of uplift, so as to reduce the number of assumptions when constructing time-series from multiple look directions, and when incorporating GPS data. We decompose the ascending and descending line-of-site deformation signals into vertical and east-west components and use finite source modeling to constrain the depth and geometry of the source of deformation. These results are then compared to available seismic, dynamic microgravity and magnetotelluric data to better understand this system, and how it is related to the volcanic hazard and local geothermal resources.

  14. Magmatism at passive margins: Effect of depth-dependent rifting and depleted continental lithospheric counterflow

    NASA Astrophysics Data System (ADS)

    Lu, Gang; Huismans, Ritske

    2016-04-01

    Rifted continental margins may have a variety of structural and magmatic styles, resulting in narrow or wide, magma-dominated or magma-poor conjugate margins. Some magma-poor margins differ from the classical uniform extension (McKenzie) model in that continental crust breaks up significantly earlier or later than continental mantle lithosphere and establishment of mature mid-ocean ridge is significantly delayed. The best-known examples are observed at: 1) the Iberia-Newfoundland conjugate margins (Type I) with a narrow transition between oceanic and continental crust; and 2) ultra-wide central South Atlantic margins (Type II) where the continental crust spans wide regions while the mantle lithosphere beneath has been removed. These margins are explained by depth-dependent extension. In this study, we perform 2D thermo-mechanical finite element numerical experiments to investigate magmatism at passive margins with depth-dependent extension. A melting prediction model is coupled with the thermo-mechanical model, in which temperature, density and viscosity feedbacks are considered. For the standard models, the crust is either strong and coupled (Type I-A models), or weak and decoupled (Type II-A models) with mantle lithosphere. In addition, models with a buoyant, depleted (cratonic) lower mantle lithosphere (referred as C models) are also investigated. We illustrate that Type I-A/C models develop Type I narrow margins, whereas Type II-A/C models develop Type II wide margins. In the C models, the buoyant lower mantle lithosphere flows laterally towards the ridge (i.e. the counterflow), resulting in the exhumation (in Type I-C models) or underplating (in Type II-C models) of the continental mantle lithosphere. Magmatic productivity is strongly prohibited when counterflow is developed. We argue that Type I-A and I-C models are comparable with the Aden Gulf rifted margins and the Iberia-Newfoundland conjugate margins, respectively. The Type II-A/C models are consistent

  15. The Eastern Sardinian Margin (Tyrrhenian Sea, Western Mediterranean) : a key area to study the rifting and post-breakup evolution of a back-arc passive continental margin

    NASA Astrophysics Data System (ADS)

    Gaullier, Virginie; Chanier, Frank; Vendeville, Bruno; Maillard, Agnès; Thinon, Isabelle; Graveleau, Fabien; Lofi, Johanna; Sage, Françoise

    2016-04-01

    The Eastern Sardinian passive continental margin formed during the opening of the Tyrrhenian Sea, which is a back-arc basin created by continental rifting and oceanic spreading related to the eastward migrating Apennine subduction system (middle Miocene to Pliocene). Up to now, rifting in this key area was considered to be pro parte coeval with the Messinian Salinity Crisis (MSC, 5.96-5.32 Ma). 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 quantify vertical and horizontal movements. On this young, highly-segmented margin, the Messinian Erosion Surface and the Upper and Mobile Units are systematically associated, respectively, to basement highs and deeper basins, showing that a rifted deep-sea domain already existed by Messinian times, therefore a major pre-MSC rifting episode occurred across the entire domain. Data show that there are no signs of Messinian syn-rift sediments, hence no evidence for rifting after Late Tortonian times. Moreover, because salt tectonics creates fan-shaped geometries in sediments, syn-rift deposits have to be carefully re-examined to distinguish the effects of crustal tectonics (rifting) and salt tectonics. We also precise that rifting is clearly diachronous from the upper margin (East-Sardinia Basin) to the lower margin (Cornaglia Terrace) with two unconformities, attributed respectively to the necking and to the lithospheric breakup unconformities. The onshore part of the upper margin has been recently investigated in order to characterize the large crustal faults affecting the Mesozoic series (geometry, kinematics and chronology) and to decipher the role of the structural inheritance and of the early rifting. Seaward, we also try to constrain the architecture and timing of the continent-ocean transition, between the hyper-extended continental crust and the first oceanic crust. Widespread

  16. Constraining lithosphere deformation modes during continental breakup for the Iberia-Newfoundland conjugate rifted margins

    NASA Astrophysics Data System (ADS)

    Jeanniot, Ludovic; Kusznir, Nick; Mohn, Geoffroy; Manatschal, Gianreto; Cowie, Leanne

    2016-06-01

    A kinematic model of lithosphere and asthenosphere deformation has been used to investigate lithosphere stretching and thinning modes during continental rifting leading to breakup and seafloor spreading. The model has been applied to two conjugate profiles across the Iberia-Newfoundland rifted margins and quantitatively calibrated using observed present-day water loaded subsidence and crustal thickness, together with observed mantle exhumation, subsidence and melting generation histories. The kinematic model uses an evolving prescribed flow-field to deform the lithosphere and asthenosphere leading to lithospheric breakup from which continental crustal thinning, lithosphere thermal evolution, decompression melt initiation and subsidence are predicted. We explore the sensitivity of model predictions to extension rate history, deformation migration and buoyancy induced upwelling. The best fit calibrated models of lithosphere deformation evolution for the Iberia-Newfoundland conjugate margins require; (1) an initial broad region of lithosphere deformation with passive upwelling, (2) lateral migration of deformation, (3) an increase in extension rate with time, (4) focussing of the deformation and (5) buoyancy induced upwelling. The model prediction of exhumed mantle at the Iberia-Newfoundland margins, as observed, requires a critical threshold of melting to be exceeded before melt extraction. The preferred calibrated models predict faster extension rates and earlier continental crustal separation and mantle exhumation for the Iberia Abyssal Plain-Flemish Pass conjugate margin profile than for the Galicia Bank-Flemish Cap profile to the north. The predicted N-S differences in the deformation evolution give insights into the 3D evolution of Iberia-Newfoundland margin crustal separation.

  17. A Spatially Explicit Degree-day Model of Rift Valley Fever Transmission Risk in the Continental United States

    Technology Transfer Automated Retrieval System (TEKTRAN)

    A degree-day model was used to assess the risk of Rift Valley Fever (RVF) transmission within five target states in the continental United States: California, Minnesota, Nebraska, New York, and Texas. Each state was evaluated on a 10-km grid using the average of historical daily temperature extreme...

  18. Crustal and uppermost mantle structure beneath the continental rifting area of the Gulf of Suez from earthquake tomography

    NASA Astrophysics Data System (ADS)

    El Khrepy, Sami; Koulakov, Ivan; Al-Arifi, Nassir

    2016-02-01

    Suez rift is one of the active seismic zones in the northward continuation of the Red Sea, as indicated by recent earthquake records from the Egyptian National Seismological Network (ENSN). We present a new model of P and S wave velocities in the crust and uppermost mantle beneath the Gulf of Suez and surrounding areas, including the northern portion of the Red Sea. Using the records from 94 seismic stations, we analyzed ~ 66,000 P and ~ 17,000 S wave arrival times from 9700 events. The travel time tomography inversion was performed using the iterative LOTOS code. The spatial resolutions of the derived models were assessed using several synthetic tests. The most prominent anomaly is a sharp high-velocity anomaly beneath the Red Sea, which is observed in both the P and S models at all depth intervals. We interpret this anomaly to be oceanic crust that was formed through extension associated with a dispersed system of spreading centers. Beneath the Gulf of Suez, the upper and middle crusts appear to be strongly heterogeneous and are dominated by low-velocity anomalies, indicative of the continental nature of the crusts. In contrast, at a depth of 30 km, we observe a prominent high-velocity anomaly along Gulf of Suez, which is interpreted to be the result of crustal thinning associated with extension between the Sinai block and the African Plate. The thickness of the crust beneath the rift is estimated to be approximately 25 km, whereas that in the surrounding areas appears to be 30-35 km. In the northwestern part of the area, we observe a low-velocity zone in the middle and lower crusts that coincide with intense seismicity and a well-developed system of recent faults on the surface. This region may mark a possible area of northward propagation of the Suez Rift zone.

  19. Short-term forecasting of aftershock sequences, microseismicity and swarms inside the Corinth Gulf continental rift

    NASA Astrophysics Data System (ADS)

    Segou, Margarita

    2014-05-01

    Corinth Gulf (Central Greece) is the fastest continental rift in the world with extension rates 11-15 mm/yr with diverse seismic deformation including earthquakes with M greater than 6.0, several periods of increased microseismic activity, usually lasting few months and possibly related with fluid diffusion, and swarm episodes lasting few days. In this study I perform a retrospective forecast experiment between 1995-2012, focusing on the comparison between physics-based and statistical models for short term time classes. Even though Corinth gulf has been studied extensively in the past there is still today a debate whether earthquake activity is related with the existence of either a shallow dipping structure or steeply dipping normal faults. In the light of the above statement, two CRS realization are based on resolving Coulomb stress changes on specified receiver faults, expressing the aforementioned structural models, whereas the third CRS model uses optimally-oriented for failure planes. The CRS implementation accounts for stress changes following all major ruptures with M greater than 4.5 within the testing phase. I also estimate fault constitutive parameters from modeling the response to major earthquakes at the vicinity of the gulf (Aσ=0.2, stressing rate app. 0.02 bar/yr). The generic ETAS parameters are taken as the maximum likelihood estimates derived from the stochastic declustering of the modern seismicity catalog (1995-2012) with minimum triggering magnitude M2.5. I test whether the generic ETAS can efficiently describe the aftershock spatio-temporal clustering but also the evolution of swarm episodes and microseismicity. For the reason above, I implement likelihood tests to evaluate the forecasts for their spatial consistency and for the total amount of predicted versus observed events with M greater than 3.0 in 10-day time windows during three distinct evaluation phases; the first evaluation phase focuses on the Aigio 1995 aftershock sequence (15

  20. Anomalous Subsidence at Rifted Continental Margins: Distinguishing Mantle Dynamic Topography from Anomalous Oceanic Crustal Thickness

    NASA Astrophysics Data System (ADS)

    Cowie, L.; Kusznir, N. J.

    2012-12-01

    It has been proposed that some continental rifted margins have anomalous subsidence histories and that at breakup they were elevated at shallower bathymetries than the isostatic response of classical rift models (McKenzie 1978) would predict. The existence of anomalous syn or post breakup subsidence of this form would have important implications for our understanding of the geodynamics of continental breakup and rifted continental margin formation, margin subsidence history and the evolution of syn and post breakup depositional systems. We have investigated three rifted continental margins; the Gulf of Aden, Galicia Bank and the Gulf of Lions, to determine whether the oceanic crust in the ocean-continent transition of these margins has present day anomalous subsidence and if so, whether it is caused by mantle dynamic topography or anomalous oceanic crustal thickness. Residual depth anomalies (RDA) corrected for sediment loading, using flexural backstripping and decompaction, have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous oceanic bathymetry and subsidence at these margins. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions from Crosby & McKenzie (2009). Non-zero sediment corrected RDAs may result from anomalous oceanic crustal thickness with respect to the global average, or from mantle dynamic uplift. Positive RDAs may result from thicker than average oceanic crust or mantle dynamic uplift; negative RDAs may result from thinner than average oceanic crust or mantle dynamic subsidence. Gravity inversion incorporating a lithosphere thermal gravity anomaly correction and sediment thickness from 2D seismic data has been used to determine Moho depth and oceanic crustal basement thickness. The reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. The gravity inversion crustal basement thicknesses

  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. Which mantle below the active rift segments in Afar?

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

    Haproff, P. J.; Yin, A.

    2014-12-01

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

  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. Relief evolution of the Continental Rift of Southeast Brazil revealed by in situ-produced 10Be concentrations in river-borne sediments

    NASA Astrophysics Data System (ADS)

    Salgado, André Augusto Rodrigues; Rezende, Eric de Andrade; Bourlès, Didier; Braucher, Régis; da Silva, Juliana Rodrigues; Garcia, Ricardo Alexandrino

    2016-04-01

    This study aims to quantify the denudation dynamics of the Brazilian passive margin along a segment of the Continental Rift of Southeast Brazil. The denudation rates of 30 basins that drain both horsts of the continental rift, including the mountain ranges of the Serra do Mar (seaside horst); and the Serra da Mantiqueira (continental horst); were derived from 10Be concentrations measured in sand-sized river sediment. The mean denudation rate ranges from 9.2 m Ma-1 on the plateau of the Serra do Mar to 37.1 m Ma-1 along the oceanic escarpment of the Serra do Mar. The seaward-facing scarps of both mountain ranges exhibit mean denudation rates that are approximately 1.5 times those of the inland-facing scarps. The escarpments of the horst nearer to the ocean (Serra do Mar) exhibit higher denudation rates (mean 30.2 m Ma-1) than the escarpments of the continental horst (Serra da Mantiqueira) (mean 16.5 m Ma-1). The parameters that impact these denudation rates include the catchment relief, the slope gradient, the rock and the climate. The incongruent combination of a mountainous landscape and moderate to low 10Be-based denudation rates averaging at ∼20 m Ma-1 suggests a reduction in intraplate tectonic activity beginning in the Middle Quaternary or earlier.

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

    SciTech Connect

    Martinez, F.; Cochran, J.R. )

    1989-09-10

    The northern Red Sea is a continental rift in the process of transition from continental to oceanic rifting. We present 191 new heat flow measurements from the northern Red Sea forming three traverses across the water covered portion of the rift. The heat flow across the rift systematically increases from values of about 125 mW/m{sup 2} seaward of the coasts to average values greater than 250 mW/m{sup 2} in the axial depression. The heat flow measurements are evaluated for environmental disturbances. These are found to be generally small. The largest estimated disturbance results from the relief of the seafloor and of the top of a subbottom evaporite layer. The relief on these surfaces can account for the 20% point to point scatter typically observed in the heat flow measurements. Limits are placed on systematic disturbances to the heat flow pattern across the rift. The estimated largest systematic disturbance results from sediment blanketing which may cause a reduction in the heat flow on the order of 10%.

  7. Evidences of a Lithospheric Fault Zone in the Sicily Channel Continental Rift (Southern Italy) from Instrumental Seismicity Data

    NASA Astrophysics Data System (ADS)

    Parisi, L.; Calo, M.

    2013-12-01

    The Sicily Channel continental rift is located in the African Plate and is submerged by a shallow sea extending from the northern coast of Africa to the southern coast of Sicily (southern Italy). The area is affected by an extensional regime since early Pliocene, which thins the continental crust and produces NW-SE oriented Pantelleria, Linosa and Malta grabens. The rift-related volcanic activity is represented by Pantelleria and Linosa Islands and a series of magmatic manifestations roughly NNE-SSW aligned, from Linosa Island to the Nameless Bank, in proximity of the Sicilian coast. Recent rapid magmatic ascents occurred along the strip near to the Sicilian coast in a region named Graham Bank. The NNE-SSW strip has already been recognised as a separation belt between the western sector of the rift (Pantelleria graben) and the eastern one (Linosa and Malta grabens). Seismic profiles suggest the presence of near vertical structures associated with strike slip fault zones. Bathymetric data show a 15-20 km wide zone characterised by several shallow basins irregularly alternated by topographic highs. However, evidences of a N-S or NNE-SSW orientated faults have not been found. In this work we re-localised the instrumental seismicity recorded between 1981 and 2012 in the Sicily Channel and western Sicily using the Double Difference method (Waldhauser, 2001, 2012) and 3D Vp and Vs models (Calò et al., 2013). The statistical analysis of the relocated seismicity together with the study of seismic energy release distribution allows us to describe the main patterns associated with the active faults in the western Sicily Straits. Here we find that most of the events in the Sicily Channel are highly clustered between 12.5°- 13.5°E and 35.5°-37°N with hypocentral depth between 5-40 km, reaching in some cases 70 km of depth. Seismic events seem to be aligned along a sub-vertical shear zone that is long at least 250 km and oriented approximately NNE-SSW. The spatial

  8. Early Cretaceous rifting and exposure of periodotite on the Galicia continental margin: preliminary results of ocean drilling program Leg 103

    SciTech Connect

    Winterer, E.; Boillot, G.; Meyer, A.; Applegate, J.; Baltuck, M.; Bergen, J.; Comas, M.; Davies, T.; Dunham, K.; Evans, C.; Girardeau, J.

    1985-01-01

    Results of drilling near the ocean-continent boundary on the Galicia margin of Iberia shed new light on the timing of rifting and demonstrate the presence at the foot of the margin of a ridge of foliated, lineated, sheared and serpentinized harzburgite, probably representing oceanic mantle. Fifty km east of the periodotite ridge, on a continental fault block, the stratigraphic section sampled during Leg 103 above Hercynian basement comprises: (1) at least 250m of Upper Jurassic and possibly lowest Cretaceous limestone, dolomite and minor sandstone and claystone deposited in relatively shallow water before rifting began; (2) about 20m of Valanginian calpionellid marlstone, probably deposited in moderate depths at the onset of rifting; (3) from about 500 to 1500m of Valanginian and Hauterivian turbidite sandstone rich in terrestrial plant debris, and Barremian and Aptian( ) claystone and marlstone deposited in deeper water during rifting; and (4) about 700m of sediments deposited after Aptian time, when rifting ceased and oceanic spreading between Iberia and Newfoundland began. The lithology and seismic stratigraphy of the wedges of clastic sediments laid down during rifting show the progressive filling of basins that formed by episodic listric faulting that began very early in the Cretaceous and continued for about 25 my. The Lower Cretaceous turbidite sandstone cored on the Galicia margin correlates with thick Lower Cretaceous turbidites cored off Morocco during DSDP Leg 50, and with Wealden deltaic and fluviatile deposits on both sides of the Atlantic.

  9. Oblique continental rifting revealed by 3D retro-deformation : example of the Upper Rhine Graben

    NASA Astrophysics Data System (ADS)

    Bertrand, G.; Horstmann, M.; Herrmann, O.; Behrmann, J.

    2003-04-01

    Our work has been done within the EU funded ENTEC network, which goal is to study the environmental impact of tectonics in the Upper Rhine Graben (URG). The URG is a NNE-trending crustal-scale small-displacement segment of the European Cenozoic rift system. Subsidence and syn-rift sedimentation started in the late Eocene and reached their maximum during Oligocene and lower Miocene. We present two 3D tectonic models that cover the SE and SW borders of the URG (Freiburg area, SW Germany, and Colmar area, NE France, respectively). As the URG is an asymmetric structure, it was crucial to model both sides. Our goal was to infer the movement history of the fault system, to identify areas of strain concentrations that could help locating possible active movements. Our models include several pre-Tertiary geological horizons, that were retrodeformed as passive objects along the faults. Assuming that "pre-rift" sediments were horizontal, our objective was to obtain this geometry by retrodeforming the models. The best quality of restoration was obtained for displacement directions of N80E to N90E on the main border faults, and N50E to N60E on inner faults. Best results also were obtained with sequences of retrodeformation from the graben center toward its borders. It suggests that faulting migrated toward the graben interior. Our study also shows considerable along-strike variations of cumulated slip on both sides of the graben, with amplitudes up to 2.5 km. This caused warping of the basement with a 30--35 km wavelength. Moreover, analyses of displacement reveal that offset of the base Tertiary is locally smaller than of older horizons, suggesting that segments of the W border fault were active prior to deposition of early Tertiary sediments. Finally, the seismicity in the Freiburg model reveals close coincidence between depth projection of faults and hypocenters of recent earthquakes. This suggests on-going activity of part, at least, of the fault system. Our two models are

  10. Timing and Magnitude of Depth-dependent Lithosphere Stretching on the Lofoten Segment of the Norwegian Rifted Continental Margin

    NASA Astrophysics Data System (ADS)

    Kusznir, N.; Roberts, A.; Hunsdale, R.

    2002-12-01

    Flexural backstripping and forward structural-and-stratigraphic modelling show that depth-dependent lithosphere stretching occurs on the outer part of the Norwegian rifted margin. Subsidence analysis on the Lofoten segment of the margin shows substantial thinning of the continental lithosphere within 100 km of the COB at continental breakup time (at approx. 54 Ma), while the upper crust shows no significant faulting and extension at breakup or immediately preceding breakup in the Palaeocene. For the Lofoten Margin beta stretching-factors approaching infinity are required at 54 Ma west of the Utroest Ridge to restore Top Basalt and the Top Taare to presumed sub-aerial depositional environments. Breakup age beta stretching-factors are predicted to rapidly reduce towards the east of the Utroest Ridge. For the mid-Lofoten margin, an additional Eocene crustal thinning event younger than 54 Ma is required to explain observed margin subsidence; post-breakup subsidence with a beta stretching-factor of infinity is insufficient to generate observed post-breakup subsidence. The absence of significant Palaeocene extension on the Lofoten margin, and the additional Eocene subsidence and faulting, implies that depth-dependent stretching of the Norwegian rifted margin occurred during early sea-floor spreading rather than during pre-breakup intra-continental rifting. For the Voering segment of the Norwegian rifted margin, south of the Bivroest Transform and Lineament System, smaller b stretching-factors of ~ 1.8 to 2.5 are needed to restore Top Basalt and Top Taare to sea level. No similar magnitude of extension by faulting is observed in the upper crust (Roberts et al.1997). Depth dependent stretching of margin lithosphere is also observed in the northern Moere Basin. Depth-dependent stretching has been observed at other rifted continental margins including the Galicia, Goban Spur, NW Australian and South China Sea rifted margins (Driscoll and Karner 1998, Davis and Kusznir 2002

  11. Lithosphere continental rifting and necking in 3D analogue experiments: role of plate divergence rate.

    NASA Astrophysics Data System (ADS)

    Nestola, Y.; Storti, F.; Cavozzi, C.

    2014-12-01

    The evolution of lithosphere necking is a fundamental parameter controlling the structural architecture and thermal state of rifted margins. Despite a large number of analogue and numerical modelling studies on lithosphere extension are available in the literature, a quantitative experimental description of lithosphere necking evolution is still lacking. Extensional strain rate and thermal layering of the lithosphere exert a fundamental control on necking shape and evolution. We focused our experimental work on the former parameter and simulated the progression of lithosphere thinning and necking during asymmetric orthogonal rifting at different plate divergence rates. Our models involve a 4-layer mechanical continental lithosphere, which rests on a glucose syrup asthenosphere. Both the topography and the base of the lithosphere were monitored by time-lapse laser scanning. This technical approach allowed us to quantify the evolution in space and time of the thinning factors for the crust, mantle, and lithosphere as a whole. Laser-scanning monitoring provided also a detailed picture of the evolving neck shape, which shows a strong dependency on the strain-rate. At low strain-rates, necking is "boxed" with steep flanks and a flat-lying roof, and few deep basins develop at surface. At high strain-rates, more distributed thinning occurs and isolates portions of less deformed mantle. More distributed deformation affects the model topography. Despite large differences in shape, the aspect ratio (amplitude/wavelength) of the cross-sectional neck shapes converges towards very similar values at the end of the experiments.The significant differences and evolutionary pathways produced by the plate divergence rate on the lithosphere necking profile, suggest that this parameter exert a fundamental control on localization vs. distribution of deformation in the crust as in the whole mechanical lithosphere. Furthermore, it can exert a fundamental control on the time and space

  12. Post-rift unroofing of the NW Africa passive continental margin during the Central Atlantic opening

    NASA Astrophysics Data System (ADS)

    Ghorbal, B.; Bertotti, G.; Andriessen, P. A. M.

    2009-04-01

    Many passive margins considered as being stable for long times, show however late uplift and exhumation at regional scale as assessed by low temperature geochronometry. A large amount of Lower Cretaceous terrigeneous sediments laid down in most of basins along the NW Africa continental margins indicate that a major episode of erosion occurred during early post-rift period in the Central Atlantic. AFT and (U-Th)/He dating performed, along a roughly >500 km N-S transect, on pre-Mesozoic basement rocks from Western Meseta to the Anti-Atlas (Morocco, NW Africa) document a fully unexpected widespread unroofing during the Middle-Late Jurassic to early Late Cretaceous, with AFT and (U-Th)/He ages ranging respectively between 120-170Ma and 115-165Ma. A well documented age cluster of 140±20Ma measured for the Moroccan Meseta, Atlas domains and Anti-Atlas belt designates those domains as potentially being the source areas of the detritic sediments considering the proximity of the depositional basins. Absence of major fault separating the Anti-Atlas from the rest of the Western African Craton during the Mesozoic suggests the unroofing region to extend further in Morocco, as far south as the Reguibat (Mauritania) or even New Guinea, documented by our investigation, and perhaps even further when confirmed by additional AFT and AHe data.

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

    USGS Publications Warehouse

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

    1991-01-01

    The region of the present Rio Grande rift and southeastern Colorado Plateau underwent lithospheric extension during middle to late Cenozoic deformation affecting the entire southwestern U.S. Lithospheric mantle was disrupted, and in many regions displaced or replaced by asthenospheric mantle at depths from which basaltic magmas were derived and erupted to the surface. Study of the igneous rocks erupted or intruded during this deformation yields insights into processes of magmatism associated with extension of continental lithosphere. Magmatic rocks associated with an early (late Oligocene-early Miocene) ductile phase of extension are dominantly basaltic andesites and related, calc-alkaline intermediate to silicic derivative rocks. Mafic magmas were probably derived from isotopically "enriched" lithospheric mantle. Igneous rocks associated with a later (middle Miocene-Holocene), more brittle phase of extension include widespread basaltic rocks and localized central volcanoes of intermediate to silicic composition. Isotopic compositions of mafic rocks, which include both tholeiitic and alkalic basalts, correlate strongly with tectonic setting and lithospheric structure. Basalts erupted in areas of greatest crustal extension, such as the central and southern rift and Basin and Range province, were derived from isotopically "depleted" (correlated with "asthenospheric") mantle. Also, isotopic compositions of Pliocene to Holocene basalts are slightly more depleted than those of Miocene basalts, suggesting that subcrustal lithospheric mantle was thinned during late Miocene extension. Intermediate rocks of the central volcanoes formed by a complex combination of processes, probably dominated by fractional crystallization and by assimilation of upper and lower crust in isolated, small magma chambers. The petrologic, geochemical, and isotopic data are compatible with a model, derived first from geophysical data, whereby lithosphere is thinned beneath the central rift and

  14. From continental platform towards rifting of the Tisza Unit in the Late Triassic to Early Cretaceous

    NASA Astrophysics Data System (ADS)

    Császár, Géza; Szinger, Balázs; Piros, Olga

    2013-08-01

    The Upper Triassic-Lower Cretaceous successions of the Transdanubian part of the Mecsek and Villány- Bihor Zones of the Tisza Unit have been studied from the lithological, lithostratigraphical, sedimentological, microfossil and microfacies points of view in order to correlate and interpret the significant differences between them and to draw a conclusion about their geological and paleogeographical history. After an overview of the paleogeographical reconstructions of the broader area, the succession of the Mecsek and Villány-Bihor Zones and the debated Máriakéménd-Bár Range are introduced. Until the end of the Middle Triassic the study area acted as an entity. The first fundamental difference between the two zones can be recognized in the Late Triassic when marine carbonates were replaced by thick fluvial siliciclastics in the Mecsek Zone, while it is represented only by small, local lenses with a few and thin dolostone intercalations in the Villány Zone. The Mecsek Zone is bordered southward by one of the large listric faults to the north of which very thick siliciclastics developed in the Early to Middle Jurassic, whereas it is highly lacunose in the larger western part of the Villány-Bihor Zone. The break at the base is subaerial, higher in the succession it is shallow submarine. The sediment is silty, occasionally sandy crinoidal limestone of late Early Jurassic or even Middle Jurassic in age. The Upper Jurassic in the Mecsek Zone is composed of deep-water cherty limestone while in the Villány Zone it became a thick, shallowing pelagic limestone with reworked patch reef fragments. It is clear evidence that the Mecsek Zone had a thinned continental crust thanks to the nearby rift zone while in the Villány Zone the crust remained thick. The actualized version of the Plašienka's paleogeographical model (Plašienka 2000) is introduced

  15. Thin viscous middle-crust and evolving fault distribution during continental rifting: Insights from analog modeling experiments

    NASA Astrophysics Data System (ADS)

    Keppler, R.; Rosas, F. M.; Nagel, T. J.

    2013-11-01

    Analog modeling of continental rifting, assuming a crustal scale “jelly sandwich”-like rheology, was carried out to test the mechanical effect of varying the absolute thickness of a weak (viscous) middle crust (silicone layer), interbedded between a brittle upper crust (sand layer) and a strong lower crust. Results consistently show a delocalization of the brittle deformation (i.e. a uniform scattering of the faults) throughout the upper brittle layer. This effect is interpreted to be associated with pressure driven flow in the viscous layer, caused by the tectonic collapse of upper brittle fault blocks into the viscous substratum. A reduction of the overall viscous layer thickness increases its resistance to accommodate internal thickness variations, which promotes delocalization of the fault pattern in the upper brittle layer. Our results contribute to the understanding of the mechanics of the so-called “upper plate paradox”, a large-scale structure often recognized at non-volcanic rifted margins. A thin viscous middle crust provides means of decoupling the deformation affecting upper and lower crust during rifting. On one hand this promotes a uniform scattering of faults throughout the upper brittle crust, on the other hand it allows for a strong localization of the deformation in lower crust and upper mantle expressed by the lithospheric necking in the rift center.

  16. Anomalous Subsidence of the Ocean Continent Transition at Rifted Continental Margins: Observations from the Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Cowie, L.; Kusznir, N. J.

    2011-12-01

    It has been proposed that some continental rifted margins have anomalous early subsidence histories and that at break-up they were elevated at shallower bathymetries than the isostatic response of classical rift models (McKenzie 1978) would predict. The existence of anomalous syn- or early post-breakup subsidence, of this form, would have important implications for our understanding of the geodynamics of continental breakup and sea-floor spreading initiation and important consequences for syn- and post-breakup depositional systems. Possible explanations for anomalous subsidence during continental breakup could include transient effects as the continental geotherm evolves towards an oceanic form, or small scale convection. Lucazeau et al. (2008) have reported anomalously high heat-flows in the ocean continent transition (OCT) of the young rifted margin of the Eastern Gulf of Aden which would have implications for its subsidence history. In order to verify (or otherwise) the proposition of an anomalous early post-breakup subsidence history in the Eastern Gulf of Aden, we have determined anomalous oceanic subsidence using residual depth anomaly (RDA) analysis and have compared lithosphere thinning across the OCT measured using subsidence analysis with continental crustal basement thinning from gravity inversion. Both 3D regional and localised 2D analyses have been carried out. The localised studies focus on published seismic reflection lines (Autin et al, 2010; D'Acremont et al, 2005; Fournier et al, 2007; Leroy et al, 2004; Leroy et al, 2010; Lucazeau et al 2008; Lucazeau et al 2010). RDAs have been calculated by comparing observed and predicted oceanic bathymetries. Regional 3D RDAs for the Gulf of Aden, without a sediment correction, show positive RDAs between 3km and 4km at the rifted margins decreasing to 0.5km at the ocean ridge axis. Localised 2D sediment corrected RDA profiles determined within and adjacent to the OCT of the Eastern Gulf of Aden are also

  17. Continental subduction induced tremor activity?

    NASA Astrophysics Data System (ADS)

    Tai, H. J.; Chen, K. H.; Ide, S.; Mouyen, M.; Byrne, T. B.

    2015-12-01

    Southern Central Range of Taiwan, a place where deep-seated tectonic tremors (a proxy of slow slip) and earthquake swarms are closely located in space and highly correlated in time, provides rare opportunity towards the understanding of physical mechanisms governing different style of slip. To identify tremor events, we used the identification scheme similar to Ide et al. (2015) but applied slightly different techniques: (1) Higher waveform cross-correlation coefficient (>0.6) (2) careful visual inspection for excluding local earthquakes and short-lasted event (duration < 60 s) (3) Signal to noise ratio higher than 1.2 and lower than 30 (4) No spatio-temporal clustering technique used. During the study period of 2007-2012, we identified 2320 tremor events with duration ranging from 60 s to 1550 s. They are located underneath southern Central Range, forming a NS-striking and SE-dipping pipe-like structure at a depth of 20-40 km. The up-dip extension of this tremor structure reaches an aseismic zone under the western flank of Central Range at shallow depths, where is an area characterized by high heat flow, low Vp and Vs anomaly. Such seismic gap was explained by the buoyancy induced crust detachment during continental subduction of Eurasian Plate. This detachment may open a new channel for hot and ductile material ascending to shallow depth, producing high temperatures along the way. This provides a common mechanism for down-dip tremor and up-dip shallow seismic gap along the same eastern dipping channel. In addition, the tremor events are found to be mostly occurred in high tides and exhibit higher correlation with tide data from west coast of Taiwan. This may again imply the association between tremor activity and subduction of Eurasian Plate.

  18. Predicting Rifted Continental Margin Subsidence History From Satellite Gravity Derived Crustal Thinning: Application to North Atlantic Margins

    NASA Astrophysics Data System (ADS)

    Hurst, N. W.; Kusznir, N. J.; Roberts, A. M.; White, R. S.

    2004-05-01

    3D spectral inversion of satellite derived gravity anomaly data (Smith and Sandwell 1997) and bathymetry data (Gebco 2003) has been used to determine oceanic and continental margin crustal thickness for the North Atlantic between 50 and 70 degrees N. The inverse technique incorporates a correction for the large negative thermal gravity anomaly present in the oceanic and stretched continental lithosphere. This correction can be determined using ocean isochron data for oceanic lithosphere, and margin rift age and beta stretching estimates derived iteratively from crustal basement thickness determined from the gravity inversion for the stretched continental lithosphere. A correction for the gravity anomaly contribution from sediments may be determined using thickness estimates derived from seismic reflection MCS data. Density depth variation within sediments is predicted assuming compaction. Crustal thicknesses determined using a thermal gravity correction derived from ocean isochron data give crustal thicknesses that are consistent with seismic observations. The resulting basement thickness determined from gravity inversion for the thinned continental margin lithosphere may be used to produce estimates of crustal thinning and stretching. Flexural backstripping and reverse post-breakup thermal subsidence modelling may be used to restore present 2D (or 3D) stratigraphic cross sections to earlier post-breakup times. Thermal subsidence arises from the cooling of stretched continental lithosphere and the recently formed oceanic lithosphere, and may be predicted from beta stretching factor (McKenzie 1978) and rift age. Beta stretching factors derived from gravity anomaly inversion have been used to predict reverse thermal subsidence for N Atlantic rifted margins. The resulting palaeo-bathymetric restorations show emergence of the Hatton Bank and NE Faroes rifted margins in early post-breakup times. The predicted palaeo-bathymetries are consistent with palaeo

  19. Is the Ventersdorp rift system of southern Africa related to a continental collision between the Kaapvaal and Zimbabwe Cratons at 2.64 Ga AGO?

    NASA Technical Reports Server (NTRS)

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

    1985-01-01

    Rocks of the Ventersdorp Supergroup were deposited in a system of northeast trending grabens on the Kaapvaal Craton approximately 2.64 Ga ago contemporary with a continental collision between the Kaapvaal and Zimbabwe Cratons. It is suggested that it was this collision that initiated the Ventersdorp rifting. Individual grabens strike at high angles toward the continental collision zone now exposed in the Limpopo Province where late orogenic left-lateral strike-slip faulting and anatectic granites are recognized. The Ventersdorp rift province is related to extension in the Kaapvaal Craton associated with the collision, and some analogy is seen with such rifts as the Shansi and Baikal Systems associated with the current India-Asia continental collision.

  20. Transfer/transform relationships in continental rifts and margins and their control on syn- and post-rift denudation: the case of the southeastern Gulf of Aden, Socotra Island, Yemen

    NASA Astrophysics Data System (ADS)

    Pik, Raphael; Bellahsen, Nicolas; Leroy, Sylvie; Denele, Yoann; Razin, Philippe; Ahmed, Abdulhakim; Khanbari, Khaled

    2013-04-01

    Transfer zones are ubiquist features in continental rifts and margins, as well as transform faults in oceanic lithosphere. Here, we present the structural study of such a structure (the Hadibo Transfer Zone, HTZ) from the southeastern Gulf of Aden, in Socotra Island, Yemen. There, from field data, the HTZ is interpreted as being reactivated, obliquely to divergence, since early rifting stages. Then, from a short review of transfer/transform fault zone geometries worldwide, we derive a classification in terms of relative importance (1st, 2nd, 3rd order), geometry, and location. We suggest that the HTZ is a 1st order transfer fault zone as it controls the initiation of a 1st order oceanic transform fault zone. We then investigate the denudation history of the region surrounding the HTZ in order to highlight the interplay of normal and transfer/transform tectonic structures in the course of rift evolution. Samples belong from two distinct East and West domains of the Socotra Island, separated by the (HTZ). Tectonic denudation started during the Priabonian-Rupelian along flat normal faults and removed all the overlying sedimentary formations, allowing basement exhumation up to the surface (~ 1.2 - 1.6 km of exhumation). Forward t-T modelling of the data requires a slightly earlier date and shorter period for development of rifting in the E-Socotra domain (38 - 34 Ma), compared to the W-Socotra domain (34 - 25 Ma), which suggests that the HTZ was already active at that time. A second major event of basement cooling and exhumation (additional ~ 0.7 - 1 km), starting at about ~ 20 Ma, has only been recorded on the E-Socotra domain. This second denudation phase significantly post-dates local rifting period but appears synchronous with Ocean Continent Transition (OCT: 20 - 17.6 Ma). This late syn-OCT uplift is maximum close to the HTZ, in the wedge of hangingwall delimited by this transfer system and the steep north-dipping normal faults that accommodated the vertical

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  2. Spatial and temporal variations in fault activity during early development of rift polarity within the offshore Corinth rift, central Greece

    NASA Astrophysics Data System (ADS)

    Nixon, C. W.; Moyle, A.; McNeill, L. C.; Bell, R. E.; Bull, J. M.; Henstock, T.

    2014-12-01

    The Corinth rift, Greece, is a young, highly active rift. A combined dense network of marine geophysical data and onshore exposure makes Corinth a natural laboratory for investigating early rift and fault formation. Rifts commonly develop a primary polarity during their formation resulting from a dominant fault set. However, how this occurs and develops is less clear. Here we characterise this process by establishing how a dominant fault set develops within the Corinth rift. Using a high spatio-temporal resolution chronostratigraphic and rift fault model, we investigate the variations in the distribution of displacement and faulting along and across the rift axis; focussing on the partitioning of deformation between N- and S-dipping faults, at a temporal resolution of ca. 100 kyr or less. Along-strike cumulative fault displacement profiles indicate overall equal distribution of strain between major S- and N-dipping faults over the last ca. 1.5 Myr. In detail, two peaks in cumulative displacement coincide with the early development of two discrete depocentres before ca. 600 ka. Since this time, displacement has become focussed on N-dipping faults with S-dipping faults becoming less active. Syn-rift isochore maps illuminate this change: a switch in rift polarity from a dominant N-thickening depocentre to a dominant S-thickening depocentre between ca. 530-420 kyr (a rapid change in rift structure and strain distribution). This change is accommodated by transfer of activity between major faults but also by formation of numerous non-basement cutting small faults. As major S-dipping faults decrease in slip rate from ca. 600 ka, they become segmented into smaller faults with variable slip rates. In contrast, N-dipping faults on the rift's southern margin, with increased activity post ~0.5-0.4 Ma, become kinematically and geometrically linked with almost equal slip rates along strike by ca. 130 kyr, controlling the single major depocentre of the present day. Our results

  3. Continental underthrusting and obduction during the Cretaceous closure of the Rocas Verdes rift basin, Cordillera Darwin, Patagonian Andes

    NASA Astrophysics Data System (ADS)

    Klepeis, Keith; Betka, Paul; Clarke, Geoffrey; Fanning, Mark; Hervé, Francisco; Rojas, Lisandro; Mpodozis, Constantino; Thomson, Stuart

    2010-06-01

    The Patagonian Andes record a period of Cretaceous-Neogene orogenesis that began with the compressional inversion of a Late Jurassic rift called the Rocas Verdes basin. Detrital zircon ages from sediment that filled the southern part of the basin provide a maximum depositional age of ˜148 Ma, suggesting that the basin opened approximately simultaneously along its length during the Late Jurassic. Structural data and U-Pb isotopic ages on zircon from granite plutons near the Beagle Channel (55°S) show that basin inversion involved two stages of shortening separated by tens of millions of years. An initial stage created a small (˜60 km wide) thrust wedge that placed the basaltic floor of the Rocas Verdes basin on top of adjacent continental crust prior to ˜86 Ma. Structures and metamorphic mineral assemblages preserved in an exhumed middle to lower crustal shear zone in Cordillera Darwin suggest that this obduction was accompanied by south directed subduction of the basaltic crust and underthrusting of continental crust to depths of ˜35 km beneath a coeval volcanic arc. A subsequent stage of out-of-sequence thrusting, culminating in the Paleogene, shortened basement and Upper Jurassic igneous rock in the internal part of the belt by at least ˜50 km, forming a bivergent thrust wedge. This latter period coincided with the exhumation of rocks in Cordillera Darwin and expansion of the fold-thrust belt into the Magallanes foreland basin. This orogen provides an important example of how orogenesis initiated and led to continental underthrusting and obduction of basaltic crust during closure of a quasi-oceanic rift basin.

  4. 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

  5. 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

  6. Groundwater fluoride enrichment in an active rift setting: Central Kenya Rift case study.

    PubMed

    Olaka, Lydia A; Wilke, Franziska D H; Olago, Daniel O; Odada, Eric O; Mulch, Andreas; Musolff, Andreas

    2016-03-01

    Groundwater is used extensively in the Central Kenya Rift for domestic and agricultural demands. In these active rift settings groundwater can exhibit high fluoride levels. In order to address water security and reduce human exposure to high fluoride in drinking water, knowledge of the source and geochemical processes of enrichment are required. A study was therefore carried out within the Naivasha catchment (Kenya) to understand the genesis, enrichment and seasonal variations of fluoride in the groundwater. Rocks, rain, surface and groundwater sources were sampled for hydrogeochemical and isotopic investigations, the data was statistically and geospatially analyzed. Water sources have variable fluoride concentrations between 0.02-75 mg/L. 73% exceed the health limit (1.5mg/L) in both dry and wet seasons. F(-) concentrations in rivers are lower (0.2-9.2mg/L) than groundwater (0.09 to 43.6 mg/L) while saline lake waters have the highest concentrations (0.27-75 mg/L). The higher values are confined to elevations below 2000 masl. Oxygen (δ(18)O) and hydrogen (δD) isotopic values range from -6.2 to +5.8‰ and -31.3 to +33.3‰, respectively, they are also highly variable in the rift floor where they attain maximum values. Fluoride base levels in the precursor vitreous volcanic rocks are higher (between 3750-6000 ppm) in minerals such as cordierite and muscovite while secondary minerals like illite and kaolinite have lower remnant fluoride (<1000 ppm). Thus, geochemical F(-) enrichment in regional groundwater is mainly due to a) rock alteration, i.e. through long residence times and natural discharge and/or enhanced leakages of deep seated geothermal water reservoirs, b) secondary concentration fortification of natural reservoirs through evaporation, through reduced recharge and/or enhanced abstraction and c) through additional enrichment of fluoride after volcanic emissions. The findings are useful to help improve water management in Naivasha as well as similar

  7. Drilling to Resolve the Evolution of the Corinth Rift

    NASA Astrophysics Data System (ADS)

    McNeill, Lisa; Sakellariou, Dimitris; Nixon, Casey

    2014-05-01

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

  8. Teleseismic wave front anomalies at a Continental Rift: no mantle anomaly below the central Upper Rhine Graben

    NASA Astrophysics Data System (ADS)

    Kirschner, Stephanie; Ritter, Joachim; Wawerzinek, Britta

    2011-08-01

    The deep structure of the Upper Rhine Graben (URG), a continental rift in SW Germany and E France, is still poorly known. This deficit impedes a full understanding of the geodynamic evolution of this prominent rift. We study the lithosphere-asthenosphere structure using teleseismic waveforms obtained from the passive broad-band TIMO project across the central URG. The recovered, crust-corrected traveltime residuals relative to the iasp91 earth model are tiny (mostly less than 0.2-0.3 s). The average measured slowness (<1 s deg-1) and backazimuth (<5°) deviations are also very small and do not show any systematic wave front anomalies. These observed perturbation values are smaller than expected ones from synthetic 3-D ray tracing modelling with anomalies exceeding 2-3 per cent seismic velocity in the mantle. Thus there is no significant hint for any deep-seated anomaly such as a mantle cushion, etc. This result means that the rifting process did not leave behind a lower lithospheric signature, which could be clearly verified with high-resolution teleseismic experiments. The only significant traveltime perturbation at the central URG is located at its western side in the upper crust around a known geothermal anomaly. The upper crustal seismic anomaly with traveltime delays of 0.2-0.3 s cannot be explained with increased temperature alone. It is possibly related to a zone of highly altered granite. In the west of our network a traveltime anomaly (0.6-0.7 s delay) related with the Eifel plume is confirmed by the TIMO data set.

  9. Upper Devonian depositional system of Bel'kov Island (New Siberian Islands): An intracontinental rift or a continental margin?

    NASA Astrophysics Data System (ADS)

    Danukalova, M. K.; Kuzmichev, A. B.; Aristov, V. A.

    2014-09-01

    The archipelago of New Siberian Islands situated on the northeastern continental shelf of Eurasia is considered a part of an exotic terrane that collided with Siberia in the Early Cretaceous. Bel'kov Island is located close to the inferred western boundary of this terrane and thus should demonstrate attributes of its localization at the margin of the Paleozoic oceanic basin. The Upper Devonian section on Bel'kov Island is a continuous sequence of deepwater terrigenous rocks, which indicates a tendency toward deepening of the basin previously revealed on adjacent Kotel'ny Island. The lowermost Upper Devonian unit on Bel'kov Island is represented by thin Domanik-like strata resting on the Middle Devonian carbonate platform. The main body of the Upper Devonian sequence, more than 4 km in total thickness, is made up of gravity-flow sediments including turbidites, clay and block diamictites, and olistostromes in the upper part of the section, which accumulated at the slope of the basin or its rise. At many levels, these sediments have been redeposited by along-slope currents. The uppermost unit of organogenic limestone is evidence for compensation of the trough. According to conodont assemblages, the deepwater terrigenous rocks were deposited from the early Frasnian to the early Tournaisian. This time is known for extensive rifting in the eastern Siberian Platform. The data obtained allowed us to reconstruct a NNW-trending Late Devonian rift basin on the Laptev Sea shelf similar to other rifts at the eastern margin of the Siberian Platform.

  10. Faulting and hydration of the upper crust of the SW Okinawa Trough during continental rifting: Evidence from seafloor compliance inversion

    NASA Astrophysics Data System (ADS)

    Kuo, Ban-Yuan; Crawford, Wayne C.; Webb, Spahr C.; Lin, Ching-Ren; Yu, Tai-Chieh; Chen, Liwen

    2015-06-01

    The elastic response of seafloor to ocean gravity wave loading, or seafloor compliance, provides a constraint on the elastic properties of the crust. We measured seafloor compliance at three ocean bottom seismometer (OBS) sites around Taiwan—two in the southwestern (SW) Okinawa Trough and one on the Ryukyu arc—and performed inversion for crustal structures beneath them. Models best fitting the data demonstrate a decrease in upper crustal shear velocity and an increase in the compressional/shear velocity ratio from the arc site to the trough sites with increasing amount of back-arc extension. This variation suggests that the upper continental crust is highly faulted and hydrated during rifting of the Eurasian lithosphere.

  11. 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

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

    SciTech Connect

    Blackwelder, B.W.

    1989-03-01

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

  13. Late Paleozoic crustal history of central coastal Queensland interpreted from geochemistry of Mesozoic plutons: The effects of continental rifting

    USGS Publications Warehouse

    Allen, C.M.; Wooden, J.L.; Chappell, B.W.

    1997-01-01

    The eastern margin of Australia is understood to be the result of continental rifting during the Cretaceous and Tertiary. Consistent with this model, Cretaceous igneous rocks (granites to basalts) in a continental marginal setting near Bowen, Queensland are isotonically retarded, having isotopic ratios similar to those of most island arcs (Sri = 0.7030-0.7039, ??Nd = +6.46 to +3.00 and 206Pb/204Pb = 18.44-18.77, 207Pb/204Pb = 15.552-15.623, and 208Pb/204Pb = 37.90-38.52). These isotopic signatures are much less evolved than the Late Carboniferous-Permian batholith that many Cretaceous plutons intrude. As rocks ranging in age from about 300-100 Ma are well exposed near Bowen, we can track magma evolution through time. The significant change of magma source occurred much earlier than the Cretaceous based on the fact that Triassic granites in the same area are also isotonically primitive. We attribute the changes of magma composition to crustal rifting during the Late Permian and earliest Triassic. The Cretaceous rocks (actually latest Jurassic to Cretaceous, 145-98 Ma) themselves show compositional trends with time. Rocks of appropriate mineralogy for Al-in-hornblende geobarometry yield pressures ranging from 250 to 80 MPa for rocks ranging in age from 145 to 125 Ma, respectively. More significantly, this older group is relatively compositionally restricted, and is Sr-rich, and Y- and Zr-poor compared to 120-98 Ma rocks. This younger groups is bimodal, being comprised principally of basalts and rhyolites (granites). REE patterns for a given rock type, however, do not differ with age tribute these relatively subtle trace element differences to small differences in conditions (T, aH2O) at the site of melting. Cretaceous crustal rifting can explain the range of rock types and the spatial distribution of rocks < 120 Ma in a longitudinal strip between and overlapping with provinces of older Cretaceous intrusions. A subduction-related setting is assigned to the 145-125 Ma

  14. Microstructural and seismic properties of the upper mantle underneath a rifted continental terrane (Baja California): An example of sub-crustal mechanical asthenosphere?

    NASA Astrophysics Data System (ADS)

    Palasse, L. N.; Vissers, R. L. M.; Paulssen, H.; Basu, A. R.; Drury, M. R.

    2012-09-01

    The Gulf of California rift is a young and active plate boundary that links the San Andreas strike-slip fault system in California to the oceanic spreading system of the East Pacific Rise. The xenolith bearing lavas of the San Quintin volcanic area provide lower crust and upper mantle samples from beneath Baja California peninsula. The microstructures, crystallographic preferred orientations (CPO) and petrology of the San Quintin xenoliths suggest that the continental lithosphere in this region has undergone several stages of deformation, recrystallisation and melt-rock interaction. Melt-rock interactions have led to enrichment in olivine while fine-grained microstructures suggest intense deformation in an active shear zone in the shallow upper mantle. In this study we highlight the effect of the fine-grained mylonitic shear zone development in the upper mantle as an important process of weakening of continental lithosphere. The results of the microstructural study show a reduction in CPO strength with increasing grain size reduction. Most CPOs are consistent with dominant slip on the {0kl}[100] system. As a consequence, corresponding seismic anisotropies decrease for both P- and S-waves with increasing grain size reduction. The shallow crystallographic fabric can be related to active shear zones, which accommodate the relative motion between the Northern Baja terrane and the Pacific plate. Estimates of the strain rate, stress and viscosity indicate that the shallow mantle beneath Northern Baja is thermally and chemically lithospheric but mechanically has similar viscosity as the asthenosphere. The Northern Baja terrane is an interesting case of continental crust lying directly on low viscosity upper mantle.

  15. 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.

  16. Rheological variations across an active rift system -- results from lithosphere-scale 3D gravity and thermal models of the Kenya Rift

    NASA Astrophysics Data System (ADS)

    Meeßen, Christian; Sippel, Judith; Cacace, Mauro; Scheck-Wenderoth, Magdalena; Fishwick, Stewart; Heine, Christian; Strecker, Manfred R.

    2015-04-01

    Due to its tectono-volcanic activity and economic (geothermal and petroleum) potential, the eastern branch of the East African Rift System (EARS) is one of the best studied extensional systems worldwide and an important natural laboratory for the development of geodynamic concepts on rifting and nascent continental break-up. The Kenya Rift, an integral part of the eastern branch of the EARS, has formed in the area of weak Proterozoic crust of the Mozambique mobile belt adjacent to the rheologically stronger Archean Tanzania craton. To assess the variations in lithospheric strength between different tectonic domains and their influence on the tectonic evolution of the region, we developed a set of structural, density, thermal and rheological 3D models. For these models we integrated multi-disciplinary information, such as published geological field data, sediment thicknesses, well information, existing structural models, seismic refraction and reflection data, seismic tomography, gravity and heat-flow data. Our main approach focused on combined 3D isostatic and gravity modelling. The resulting lithosphere-scale 3D density model provides new insights into the depth distribution of the crust-mantle boundary and thickness variations of different crustal density domains. The latter further facilitate interpretations of variations of lithologies and related physical rock properties. By considering lithology-dependent heat production and thermal conductivity, we calculate the conductive thermal field across the region of the greater Kenya Rift. Finally, the assessed variations in lithology and temperature allow deriving differences in the integrated strength of the lithosphere across the different tectonic domains.

  17. Understanding Along-strike Variations in Extension and Magmatism in Active Rifts: Discontinuous Structure Along the Main Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Keranen, K. M.; Klemperer, S. L.

    2006-12-01

    A compilation of recent geophysical and geological data reveals a discontinuity in the structure of the Main Ethiopian Rift (MER) at ~8.5°N. Recent wide-angle seismic data (from the 2003 EAGLE project) recorded along the axis of the MER show a rapid increase of crustal thickness from c. 26 km in the NE to c. 40 km in the SW at this latitude, and receiver functions recorded on the northwestern plateau show a change in crustal thickness from over 40 km in the NE to c. 33 km in the SW. The thin crust (c. 26 km) in the NE segment of the rift is markedly thinner than the adjacent rift shoulders (over 40 km), as expected for an active rift. In contrast, the thick rift crust to the SW (c. 40 km) is apparently *thicker* than the crust of the adjacent northern rift shoulder. We consider two hypotheses to explain these observations: 1. The crust within the rift valley in the SW has been thickened by magmatic processes, i.e. a high degree of magmatism (underplating) resulting from the modest extension of unusually hot mantle has led to rift-crust thickening rather than thinning; or 2. The thick crust along the active-source profile in the SW represents pre-rift crustal thickness, which the active MER has as yet barely modified. The former hypothesis is unlikely because crustal structure in the SW appears relatively unmodified by magmatic processes, e.g. there is no observed 7.x km/s layer at the base of the crust and only very slightly elevated velocities are present in the lower or upper crust. In the latter hypothesis, extension of the MER may have hardly affected the location of the wide-angle profile SW of 8.5°N; rather, this latitude represents a discontinuity between the northern MER and a distinct rift segment south of 8.5°N. Seismic tomography from EAGLE active-source and broadband data supports this hypothesis, showing crustal and mantle segmentation (between NE and SW) at this location. Along with surface geological data, these data indicate that the northern MER

  18. Transient groundwater-lake interactions in a continental rift: Sea of Galilee, Israel

    USGS Publications Warehouse

    Hurwitz, S.; Stanislavsky, E.; Lyakhovsky, V.; Gvirtzman, H.

    2000-01-01

    The Sea of Galilee, located in the northern part of the Dead Sea rift, is currently an intermediate fresh-water lake. It is postulated that during a short highstand phase of former Lake Lisan in the late Pleistocene, saline water percolated into the subsurface. Since its recession from the Kinarot basin and the instantaneous formation of the fresh-water lake (the Sea of Galilee), the previously intruded brine has been flushed backward toward the lake. Numerical simulations solving the coupled equations of fluid flow and of solute and heat transport are applied to examine the feasibility of this hypothesis. A sensitivity analysis shows that the major parameters controlling basin hydrodynamics are lake-water salinity, aquifer permeability, and aquifer anisotropy. Results show that a highstand period of 3000 yr in Lake Lisan was sufficient for saline water to percolate deep into the subsurface. Because of different aquifer permeabilities on both sides of the rift, brine percolated into a aquifers on the western margin, whereas percolation was negligible on the eastern side. In the simulation, after the occupation of the basin by the Sea of Galilee, the invading saline water was leached backward by a topography-driven flow. It is suggested that the percolating brine on the western side reacted with limestone at depth to form epigenetic dolomite at elevated temperatures. Therefore, groundwater discharging along the western shores of the Sea of Galilee has a higher calcium to magnesium ratio than groundwater on the eastern side.

  19. Seismic Evidence for an Active Southern Rio Grande Rift

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  20. Non-extensivity and complexity in the earthquake activity at the West Corinth rift (Greece)

    NASA Astrophysics Data System (ADS)

    Michas, Georgios; Vallianatos, Filippos; Sammonds, Peter

    2013-04-01

    Earthquakes exhibit complex phenomenology that is revealed from the fractal structure in space, time and magnitude. For that reason other tools rather than the simple Poissonian statistics seem more appropriate to describe the statistical properties of the phenomenon. Here we use Non-Extensive Statistical Physics [NESP] to investigate the inter-event time distribution of the earthquake activity at the west Corinth rift (central Greece). This area is one of the most seismotectonically active areas in Europe, with an important continental N-S extension and high seismicity rates. NESP concept refers to the non-additive Tsallis entropy Sq that includes Boltzmann-Gibbs entropy as a particular case. This concept has been successfully used for the analysis of a variety of complex dynamic systems including earthquakes, where fractality and long-range interactions are important. The analysis indicates that the cumulative inter-event time distribution can be successfully described with NESP, implying the complexity that characterizes the temporal occurrences of earthquakes. Further on, we use the Tsallis entropy (Sq) and the Fischer Information Measure (FIM) to investigate the complexity that characterizes the inter-event time distribution through different time windows along the evolution of the seismic activity at the West Corinth rift. The results of this analysis reveal a different level of organization and clusterization of the seismic activity in time. Acknowledgments. GM wish to acknowledge the partial support of the Greek State Scholarships Foundation (IKY).

  1. Evolution history of the Heuksan Basin, a continental rift basin in the Yellow Sea

    NASA Astrophysics Data System (ADS)

    Yoon, S.; Kim, G.; Park, M.

    2011-12-01

    This study focuses on the establishment of evolutionary framework of the Heuksan Basin regions located on the southwestern part of the concession block III of the Korean offshore in the Yellow Sea. Three fault-bounded sub-basins (SB-1, SB-2, and SB-3) are recognized on the basis of topographic map of acoustic basement, which are generally in asymmetric half-graben geometry trending WNW-ESE. The axial lengths of the sub-basins are generally less than 80 km in axial length and 20 km in width. The acoustic basement occurs at around 0.6 to 0.8 s twt bsl in generally, locally much deeper at about 2.4 s twt bsl in the deepest part of the sub-basins, and it is correlated with the Upper Paleozoic to Lower Mesozoic sequence of thick micritic limestones overlying major clastic cycles with thin coals. In the sedimentary succession, two erosional surfaces (ES-1 and ES-2) are identified based on the interpretation of truncated geometries and lap-out patterns, which provide three stratigraphic units (SU-1, SU-2, and SU-3). During the Late Cretaceous to Oligocene the basin had opened by NE-SW extension or transtension. By this tectonic movement, WNW-ESE trending sub-basins with an array of listric normal faults were formed. Until the cessation of the opening, the wedge-shaped syn-rift unit (SU-1) infilled the depressions of the rift basins. In the Late Oligocene to Early Miocene time the extensional or transtensional tectonic movement of the Heuksan Basin region was aborted or partly inverted into compressional or transpressional regime. By this tectonic movement, acoustic basement and syn-rift unit (SU-1) might have uplifted, and have experienced a regional planation process. Topographic irregularities were removed by erosion and infilling process. SU-1 might have formed during this period in the localized topographic lows of the basins. Subaerial to shallow-marine planation process on the slightly folded and uplifted SU-1during subsequent tectonic inversion in the Early Miocene

  2. Style of rifting and the stages of Pangea breakup

    NASA Astrophysics Data System (ADS)

    Frizon de Lamotte, Dominique; Fourdan, Brendan; Leleu, Sophie; Leparmentier, François; Clarens, Philippe

    2015-05-01

    Pangea results from the progressive amalgamation of continental blocks achieved at 320 Ma. Assuming that the ancient concept of "active" versus "passive" rifting remains pertinent as end-members of more complex processes, we show that the progressive Pangea breakup occurred through a succession of rifting episodes characterized by different tectonic evolutions. A first episode of passive continental rifting during the Upper Carboniferous and Permian led to the formation of the Neo-Tethys Ocean. Then at the beginning of Triassic times, two short episodes of active rifting associated to the Siberian and Emeishan large igneous provinces (LIPs) failed. The true disintegration of Pangea resulted from (1) a Triassic passive rifting leading to the emplacement of the central Atlantic magmatic province (200 Ma) LIP and the subsequent opening of the central Atlantic Ocean during the lowermost Jurassic and from (2) a Lower Jurassic active rifting triggered by the Karoo-Ferrar LIP (183 Ma), which led to the opening of the West Indian Ocean. The same sequence of passive then active rifting is observed during the Lower Cretaceous with, in between, the Parana-Etendeka LIP at 135 Ma. We show that the relationships between the style of rifts and their breakdown or with the type of resulting margins (as magma poor or magma dominated) are not straightforward. Finally, we discuss the respective role of mantle global warming promoted by continental agglomeration and mantle plumes in the weakening of the continental lithosphere and their roles as rifting triggers.

  3. Continental Break-up and the dynamics of rifting in backarc basins : the Gulf of Lions margin

    NASA Astrophysics Data System (ADS)

    Jolivet, L.; Gorini, C.; Bache, F.; Smit, J.; Leroy, S.

    2012-04-01

    Deep seismic profiles and subsidence history of the Gulf of Lions margin reveal a non-classical evolution with intense stretching of the distal margin and delayed subsidence, despite a rather weak extension of the onshore and shallow offshore portion of the margin. The interpretation of an unpublished MCS profile (TGS-NOPEC) and published geophysical data leads us to revisit this evolution. The 70 km-long domain of extremely thinned continental crust, the GoL MCC, has been extracted from below the margin by the south-eastward flow of hot asthenosphere in the backarc region during rollback of the Apennines slab. The combination of Eocene crustal thickening related to formation of the Pyrenees and the nearby volcanic arc and associated hot asthenosphere makes the upper mantle and the lower crust weak enough to flow south-eastward entrained by the underlying asthenospheric flow due to slab retreat. The upper crust, more resistant, has been left behind and was only moderately thinned. The overall hot geodynamic environment also explains the subaerial conditions during most of the rifting stage and the delayed subsidence after the breakup. The efficiency of such a basal drag is not ascertained and it should certainly be further tested but, in the Mediterranean backarc regions, the coupling between asthenospheric and lower crustal deformation seems quite strong as suggested by the comparison of stretching directions in MCCs and seismic anisotropy of SKS waves which suggests that shear stresses due to asthenospheric flow toward retreating subduction zones can be transmitted up to the lower crust. This model cannot be simply used for Atlantic-type passive margins because they usually do not show exhumed lower crust within the continent-ocean transition but the role that an astheospheric flow could play during rifting should be looked at.

  4. Barite-forming environments along a rifted continental margin, Southern California Borderland

    USGS Publications Warehouse

    Hein, James R.; Zierenberg, Robert A.; Maynard, J. Barry; Hannington, Mark D.

    2007-01-01

    The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults. Barite samples from two sea-cliff sites and four offshore sites in the SCCB were analyzed for mineralogy, chemical (54 elements) and isotopic (S, Sr) compositions, and petrography. Barite from Palos Verdes (PV) Peninsula sea-cliff outcrops is hosted by the Miocene Monterey Formation and underlying basalt; carbonate rocks from those outcrops were analyzed for C, O, and Sr isotopes and the basalt for S isotopes. Cold-seep barite from Monterey Bay, California was analyzed for comparison. SCCB offshore samples occur at water depths from about 500 to 1800 m. Those barites vary significantly in texture and occurrence, from friable, highly porous actively growing seafloor mounds to dense, brecciated, vein barite. This latter type of barite contrasts with cold-seep barite in being much more coarse grained, forms thick veins in places, and completely replaced rock clasts in breccia. The barite samples range from 94 to 99 wt% BaSO4, with low trace-element contents, except for high Sr, Zr, Br, U, and Hg concentrations compared to their crustal abundances. δ34S for SCCB offshore barites range from 21.6‰ to 67.4‰, and for PV barite from 62‰ to 70‰. Pyrite from PV sea-cliff basalt and sedimentary rocks that host the barites averages 7.8‰ and 2.2‰, respectively. Two offshore barite samples have δ34S values (21.6‰, 22.1‰) close to that of modern seawater sulfate, whereas all other samples are enriched to strongly enriched in 34S. 87Sr/86Sr ratios for the barites vary over a narrow range of 0.70830–0.70856 and are much lower than that of modern seawater and also lower than the middle Miocene seawater ratio, the time

  5. Barite-forming environments along a rifted continental margin, Southern California Borderland

    NASA Astrophysics Data System (ADS)

    Hein, James R.; Zierenberg, Robert A.; Maynard, J. Barry; Hannington, Mark D.

    2007-06-01

    The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults. Barite samples from two sea-cliff sites and four offshore sites in the SCCB were analyzed for mineralogy, chemical (54 elements) and isotopic (S, Sr) compositions, and petrography. Barite from Palos Verdes (PV) Peninsula sea-cliff outcrops is hosted by the Miocene Monterey Formation and underlying basalt; carbonate rocks from those outcrops were analyzed for C, O, and Sr isotopes and the basalt for S isotopes. Cold-seep barite from Monterey Bay, California was analyzed for comparison. SCCB offshore samples occur at water depths from about 500 to 1800 m. Those barites vary significantly in texture and occurrence, from friable, highly porous actively growing seafloor mounds to dense, brecciated, vein barite. This latter type of barite contrasts with cold-seep barite in being much more coarse grained, forms thick veins in places, and completely replaced rock clasts in breccia. The barite samples range from 94 to 99 wt% BaSO 4, with low trace-element contents, except for high Sr, Zr, Br, U, and Hg concentrations compared to their crustal abundances. δ34S for SCCB offshore barites range from 21.6‰ to 67.4‰, and for PV barite from 62‰ to 70‰. Pyrite from PV sea-cliff basalt and sedimentary rocks that host the barites averages 7.8‰ and 2.2‰, respectively. Two offshore barite samples have δ34S values (21.6‰, 22.1‰) close to that of modern seawater sulfate, whereas all other samples are enriched to strongly enriched in 34S. 87Sr/ 86Sr ratios for the barites vary over a narrow range of 0.70830-0.70856 and are much lower than that of modern seawater and also lower than the middle Miocene seawater ratio, the time

  6. Continental breakup and the dynamics of rifting in back-arc basins: The Gulf of Lion margin

    NASA Astrophysics Data System (ADS)

    Jolivet, Laurent; Gorini, Christian; Smit, Jeroen; Leroy, Sylvie

    2015-04-01

    Deep seismic profiles and subsidence history of the Gulf of Lion margin reveal an intense stretching of the distal margin and strong postrift subsidence, despite weak extension of the onshore and shallow offshore portions of the margin. We revisit this evolution from the geological interpretation of an unpublished multichannel seismic profile and other published geophysical data. We show that an 80 km wide domain of thin lower continental crust, the "Gulf of Lion metamorphic core complex," is present in the ocean-continent transition zone and exhumed mantle makes the transition with oceanic crust. The exhumed lower continental crust is bounded upward and downward by shallow north dipping detachments. The presence of exhumed lower crust in the deep margin explains the discrepancy between the amount of extension deduced from normal faults in the upper crust and total extension. We discuss the mechanism responsible for exhumation and present two scenarios: the first one involving a simple coupling between mantle extension due to slab retreat and crustal extension and the second one involving extraction of the lower crust and mantle from below the margin by the southeastward flow of hot asthenosphere in the back-arc region during slab rollback. In both scenarios, the combination of Eocene crustal thickening related to the Pyrenees, the nearby volcanic arc, and a shallow lithosphere-asthenosphere boundary weakened the upper mantle and lower crust enough to make them flow southeastward. The overall hot geodynamic environment also explains the subaerial conditions during most of the rifting stage and the delayed subsidence after breakup.

  7. Physics-based and statistical earthquake forecasting in a continental rift zone: the case study of Corinth Gulf (Greece)

    NASA Astrophysics Data System (ADS)

    Segou, Margarita

    2016-01-01

    I perform a retrospective forecast experiment in the most rapid extensive continental rift worldwide, the western Corinth Gulf (wCG, Greece), aiming to predict shallow seismicity (depth <15 km) with magnitude M ≥ 3.0 for the time period between 1995 and 2013. I compare two short-term earthquake clustering models, based on epidemic-type aftershock sequence (ETAS) statistics, four physics-based (CRS) models, combining static stress change estimations and the rate-and-state laboratory law and one hybrid model. For the latter models, I incorporate the stress changes imparted from 31 earthquakes with magnitude M ≥ 4.5 at the extended area of wCG. Special attention is given on the 3-D representation of active faults, acting as potential receiver planes for the estimation of static stress changes. I use reference seismicity between 1990 and 1995, corresponding to the learning phase of physics-based models, and I evaluate the forecasts for six months following the 1995 M = 6.4 Aigio earthquake using log-likelihood performance metrics. For the ETAS realizations, I use seismic events with magnitude M ≥ 2.5 within daily update intervals to enhance their predictive power. For assessing the role of background seismicity, I implement a stochastic reconstruction (aka declustering) aiming to answer whether M > 4.5 earthquakes correspond to spontaneous events and identify, if possible, different triggering characteristics between aftershock sequences and swarm-type seismicity periods. I find that: (1) ETAS models outperform CRS models in most time intervals achieving very low rejection ratio RN = 6 per cent, when I test their efficiency to forecast the total number of events inside the study area, (2) the best rejection ratio for CRS models reaches RN = 17 per cent, when I use varying target depths and receiver plane geometry, (3) 75 per cent of the 1995 Aigio aftershocks that occurred within the first month can be explained by static stress changes, (4) highly variable

  8. Time-calibrated models support congruency between Cretaceous continental rifting and titanosaurian evolutionary history.

    PubMed

    Gorscak, Eric; O'Connor, Patrick M

    2016-04-01

    Recent model-based phylogenetic approaches have expanded upon the incorporation of extinct lineages and their respective temporal information for calibrating divergence date estimates. Here, model-based methods are explored to estimate divergence dates and ancestral ranges for titanosaurian sauropod dinosaurs, an extinct and globally distributed terrestrial clade that existed during the extensive Cretaceous supercontinental break-up. Our models estimate an Early Cretaceous (approx. 135 Ma) South American origin for Titanosauria. The estimated divergence dates are broadly congruent with Cretaceous geophysical models of supercontinental separation and subsequent continental isolation while obviating the invocation of continuous Late Cretaceous continental connections (e.g. ephemeral land bridges). Divergence dates for mid-Cretaceous African and South American sister lineages support semi-isolated subequatorial African faunas in concordance with the gradual northward separation between South America and Africa. Finally, Late Cretaceous Africa may have linked Laurasian lineages with their sister South American lineages, though the current Late Cretaceous African terrestrial fossil record remains meagre. PMID:27048465

  9. Mantle compensation of active metamorphic core complexes at Woodlark rift in Papua New Guinea.

    PubMed

    Abers, Geoffrey A; Ferris, Aaron; Craig, Mitchell; Davies, Hugh; Lerner-Lam, Arthur L; Mutter, John C; Taylor, Brian

    2002-08-22

    In many highly extended rifts on the Earth, tectonic removal of the upper crust exhumes mid-crustal rocks, producing metamorphic core complexes. These structures allow the upper continental crust to accommodate tens of kilometres of extension, but it is not clear how the lower crust and underlying mantle respond. Also, despite removal of the upper crust, such core complexes remain both topographically high and in isostatic equilibrium. Because many core complexes in the western United States are underlain by a flat Moho discontinuity, it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating. These processes should decouple upper-crust extension from that in the mantle. In contrast, here we present seismic observations of metamorphic core complexes of the western Woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension. These core complexes are actively being exhumed at a rate of 5-10 km Myr(-1), and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density, as indicated by low seismic velocities. We conclude that, at least in this case, the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon, but must involve mantle extension. PMID:12192406

  10. Isotopic characterisation of the sub-continental lithospheric mantle beneath Zealandia, a rifted fragment of Gondwana

    NASA Astrophysics Data System (ADS)

    Waight, Tod E.; Scott, James M.; van der Meer, Quinten H. A.

    2013-04-01

    The greater New Zealand region, known as Zealandia, represents an amalgamation of crustal fragments accreted to the paleo-Pacific Gondwana margin and which underwent significant thinning during the subsequent split from Australia and Antarctica in the mid-Cretaceous following opening of the Tasman Sea and the Southern Ocean. We present Sr, Nd and Pb isotopes and laser ablation trace element data for a comprehensive suite of clinopyroxene separates from spinel peridotite xenoliths (lherzolite to harzburgite) from the sub-continental lithospheric mantle across southern New Zealand. These xenoliths were transported to the surface in intra-plate alkaline volcanics that erupted across the region in the Eocene and Miocene (33-10 m.y.a.). Most of the volcanic suites have similar geochemical and isotopic properties that indicate melting of an OIB-like mantle source in the garnet stability zone and that contained a HIMU component. The volcanics have tapped two adjacent but chemically contrasting upper mantle domains: a fertile eastern domain and an extremely depleted western domain. Both domains underlie Mesozoic metasedimentary crust. Radiogenic isotope compositions of the clinopyroxene have 87Sr/86Sr between 0.7023 to 0.7035, 143Nd/144Nd between 0.5128 and 0.5132 (corresponding to ?Nd between +3 and +13) with a few samples extending to even more depleted compositions, 206Pb/204 Pb between ca. 19.5 to 21.5 and 208Pb/204 Pb between ca. 38.5 to 40.5. No correlations are observed between isotopic composition, age or geographical separation. These isotopic compositions indicate that the sub-continental lithospheric mantle under southern New Zealand has a regionally distinct and pervasive FOZO to HIMU - like signature. The isotopic signatures are also similar to those of the alkaline magmas that transported the xenoliths and suggest that most of the HIMU signature observed in the volcanics could be derived from a major source component in the sub-continental lithospheric mantle

  11. Geochemical and geochronological constraints on the origin and evolution of rocks in the active Woodlark Rift of Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Zirakparvar, Nasser Alexander

    Tectonically active regions provide important natural laboratories to glean information that is applicable to developing a better understanding of the geologic record. One such area of the World is Papua New Guinea, much of which is situated in an active and transient plate boundary zone. The focus of this PhD research is to develop a better understanding of rocks in the active Woodlark Rift, situated in Papua New Guinea's southernmost reaches. In this region, rifting and lithospheric rupture is occurring within a former subduction complex where there is a history of continental subduction and (U)HP metamorphism. The lithostratigraphic units exposed in the Woodlark Rift provide an opportunity to better understand the records of plate boundary processes at many scales from micron-sized domains within individual minerals to regional geological relationships. This thesis is composed of three chapters that are independent of one another but are all related to the overall goal of developing a better understanding of the record of plate boundary processes in the rocks currently exposed in the Woodlark Rift. The first chapter, published in its entirety in Earth and Planetary Science Letters (2011 v. 309, p. 56 - 66), is entitled 'Lu-Hf garnet geochronology applied to plate boundary zones: Insights from the (U)HP terrane exhumed within the Woodlark Rift'. This chapter focuses on the use of the Lu-Hf isotopic system to date garnets in the Woodlark Rift. Major findings of this study are that some of the rocks in the Woodlark Rift preserve a Lu-Hf garnet isotopic record of initial metamorphism and continental subduction occurring in the Late Mesozoic, whereas others only preserve a record of tectonic processes related to lithospheric rupture during the initiation of rifting in the Late Cenozoic. The second chapter is entitled 'Geochemical and geochronological constraints on the origin of rocks in the active Woodlark Rift of Papua New Guinea: Recognizing the dispersed

  12. Non-depleted sub-continental mantle beneath the Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element evidence from Midcontinent Rift basalts

    SciTech Connect

    Paces, J.B. ); Bell, K. )

    1989-08-01

    Midcontinent Rift flood basalts represent a sample of the relatively shallow, sub-continental upper mantle beneath the Canadian Shield at 1.1 Ga. A thick sequence of olivine tholeiite lavas, including minor intermediate to rhyolitic lavas, from the Portage Lake Volcanics (PLV) in northern Michigan have initial Nd and Sr isotopic compositions which cluster near Bulk Earth values. The effects of assimilation of old LREE-enriched continental crust into mantle-derived fractionating liquids are isotopically discernible in evolved lavas as well as in olivine tholeiites from the lowest portion of the volcanic pile. However, the effects of crustal contamination decrease with stratigraphic height and are absent in more primitive lavas in the upper half of the section. The source for PLV tholeiites is substantially less depleted than previously reported mantle values from the Superior Province. An origin for the PLV source is compatible with either of several mantle evolution models. The PLV source may have been associated with upwelling of a LIL element-enriched, asthenospheric plume which emplaced non-depleted material from deeper sources into the shallow sub-continental mantle beneath the Midcontinent Rift during continental break-up. Alternatively, the PLV source may have originated by enrichment of refractory sub-continental lithospheric mantle which was previously depleted in incompatible trace elements during Archean-aged melt extraction and continental crust formation. Concurrent generation of carbonatite magmas in other areas beneath the Superior Province indicates the widespread presence of sub-continental mantle with substantially higher {epsilon}{sub Nd}(T) and lower {epsilon}{sub Sr}(T) than the PLV source.

  13. Rio Grande rift: An overview

    NASA Astrophysics Data System (ADS)

    Olsen, Kenneth H.; Scott Baldridge, W.; Callender, Jonathan F.

    1987-11-01

    low-angle normal faulting throughout the rift region which was subsequently offset by high-angle normal faulting during the later deformational event. Volcanism of the Rio Grande rift is minor compared to some other continental rifts. Most of the volcanism is basaltic and occurred less than about 5 m.y. ago. Compositions range from alkalic to tholeiitic, with no unique spatial or temporal pattern. Magmas were probably derived from a variety of depths, indicating an unintegrated heat source with only local melting. Basaltic andesites and related calc-alkaline rocks erupted in the southern rift between about 30 and 18 m.y. ago were not uniquely related to the rifting process. Rather, the thermal pulse which generated these magmas was part of the previous, subduction-related event. Our interpretation of existing data concerning the evolution of the Rio Grande rift does not fit either simple active or passive "end-member" models. In particular, there is no compelling evidence for a major thermal event in the mantle uniquely associated with rifting. Yet heat—inherited from the immediately-preceding deformational regime—was certainly a critical factor in, and was probably a necessary condition for, rifting.

  14. Crustal structure and rift tectonics across the Cauvery-Palar basin, Eastern Continental Margin of India based on seismic and potential field modelling

    NASA Astrophysics Data System (ADS)

    Twinkle, D.; Rao, G. Srinivasa; Radhakrishna, M.; Murthy, K. S. R.

    2016-03-01

    The Cauvery-Palar basin is a major peri-cratonic rift basin located along the Eastern Continental Margin of India (ECMI) that had formed during the rift-drift events associated with the breakup of eastern Gondwanaland (mainly India-Sri Lanka-East Antarctica). In the present study, we carry out an integrated analysis of the potential field data across the basin to understand the crustal structure and the associated rift tectonics. The composite-magnetic anomaly map of the basin clearly shows the onshore-to-offshore structural continuity, and presence of several high-low trends related to either intrusive rocks or the faults. The Curie depth estimated from the spectral analysis of offshore magnetic anomaly data gave rise to 23 km in the offshore Cauvery-Palar basin. The 2D gravity and magnetic crustal models indicate several crustal blocks separated by major structures or faults, and the rift-related volcanic intrusive rocks that characterize the basin. The crustal models further reveal that the crust below southeast Indian shield margin is ˜36 km thick and thins down to as much as 13-16 km in the Ocean Continent Transition (OCT) region and increases to around 19-21 km towards deep oceanic areas of the basin. The faulted Moho geometry with maximum stretching in the Cauvery basin indicates shearing or low angle rifting at the time of breakup between India-Sri Lanka and the East Antarctica. However, the additional stretching observed in the Cauvery basin region could be ascribed to the subsequent rifting of Sri Lanka from India. The abnormal thinning of crust at the OCT is interpreted as the probable zone of emplaced Proto-Oceanic Crust (POC) rocks during the breakup. The derived crustal structure along with other geophysical data further reiterates sheared nature of the southern part of the ECMI.

  15. Role of magmatism in continental lithosphere extension: an introduction to tectnophysics special issue

    SciTech Connect

    Van Wijk, Jolante W

    2008-01-01

    The dynamics and evolution of rifts and continental rifted margins have been the subject of intense study and debate for many years and still remain the focus of active investigation. The 2006 AGU Fall Meeting session 'Extensional Processes Leading to the Formation of Basins and Rifted Margins, From Volcanic to Magma-Limited' included several contributions that illustrated recent advances in our understanding of rifting processes, from the early stages of extension to breakup and incipient seafloor spreading. Following this session, we aimed to assemble a multi-disciplinary collection of papers focussing on the architecture, formation and evolution of continental rift zones and rifted margins. This Tectonophysics Special Issue 'Role of magmatism in continental lithosphere extension' comprises 14 papers that present some of the recent insights on rift and rifted margins dynamics, emphasising the role of magmatism in extensional processes. The purpose of this contribution is to introduce these papers.

  16. Shield volcanoes of Marie Byrd Land, West Antarctic rift: oceanic island similarities, continental signature, and tectonic controls

    NASA Astrophysics Data System (ADS)

    LeMasurier, Wesley

    2013-06-01

    The Marie Byrd Land volcanic province is largely defined by 18 large (up to ~1,800 km3) alkaline shield volcanoes, each surmounted by a summit section of varied felsic rocks dominated by trachytic flows. They are distributed over a 500 × 800-km block-faulted dome within the West Antarctic rift. The basement contact of volcanic sections is ~500 masl at one site and 3,000 mbsl at another, 70 km away, which illustrates the scale of block faulting but complicates an understanding of volcanic structure. Furthermore, the continental ice sheet buries 16 volcanoes to progressively greater heights inland. However, five are sufficiently exposed to allow meaningful comparisons with alkaline oceanic island volcanoes; these comparisons are used as a guide to estimate the structure of Marie Byrd Land volcanoes. The type example for this study is Mt. Murphy, the most completely exposed volcano. It consists of a 1,400-m section of alkaline basalt overlain by trachyte and benmoreite flows that make up ~7-13 % of the volcano volume. In gross structure and composition, Mt. Murphy is similar to Gran Canaria volcano, Canary Islands, but the percent of felsic rock may be three times that of Gran Canaria, if the estimate is approximately correct. Departures from the oceanic island example are believed to represent the imprint of the Marie Byrd Land lithosphere and tectonic environment on volcano evolution. These include a lack of order in the sequence of felsic rock types, lack of progression toward more silica undersaturated compositions with time, absence of a highly undersaturated mafic resurgent stage, and perhaps, a relatively large volume of felsic rock.

  17. Petrology and Sr-Nd-Pb isotope geochemistry of Late Cretaceous continental rift ignimbrites, Kap Washington peninsula, North Greenland

    NASA Astrophysics Data System (ADS)

    Thorarinsson, Sigurjon B.; Holm, Paul M.; Duprat, Helene I.; Tegner, Christian

    2012-03-01

    The Late Cretaceous-Palaeocene (71-61 Ma) Kap Washington Group (KWG) volcanic sequence is exposed at the north coast of Greenland. The sequence is bimodal and was erupted in a continental rift setting during the opening of the Arctic Ocean. The succession exposed on the Kap Washington peninsula, which forms the bulk of the KWG sequence (> 5 km thick), has been sampled along four traverses with a combined stratigraphic thickness of ca. 1500 m. The sampled sequence is dominated by silicic ignimbrites (69-79 wt.% SiO2) showing geochemical features typical of ferroan, A-type granitoids. The ignimbrites range from sparsely phyric, mildly peraluminous compositions [ASI = Al2O3/(CaO + Na2O + K2O) = 1.05-1.20] to feldspar + quartz ± sodic amphibole ± Fe-Ti oxide phyric peralkaline compositions [PI = (Na2O + K2O)/Al2O3 = 1.00-1.40]. The peraluminous ignimbrites appear to overlie the peralkaline ignimbrites, although stratigraphy is complicated by faulting. Fiamme imbrication indicates that both types were erupted from a vent area located north of the Kap Washington peninsula. The peralkaline ignimbrites have Sr-Nd-Pb isotopic compositions which overlap with the compositions of KWG basalts, indicating a dominantly basaltic source. The more peralkaline compositions were generated by up to ca. 50% fractional crystallisation of alkali feldspar-quartz-dominated assemblages from mildly peralkaline parental magmas, themselves probably derived by fractionation of trachytic magmas. The peraluminous ignimbrites have slightly negative ɛNd(i) and more radiogenic 207Pb/204Pbi and 208Pb/204Pbi. Modelling indicates that they are not cogenetic with the peralkaline ignimbrites and they are inferred to have originated by partial melting of hybridised mafic crust. Petrographic evidence suggests that magma mixing was an important process and variations in Nd-Pb isotopes and trace element ratios indicate mixing between peralkaline and peraluminous magma batches.

  18. Paleomagnetism and paleointensity of Mid-Continental Rift System basalts at Silver Mountain and Sturgeon River Falls (Upper Michigan)

    NASA Astrophysics Data System (ADS)

    Kulakov, E.; Piispa, E. J.; Laird, M. S.; Smirnov, A. V.; Diehl, J. F.

    2009-12-01

    Paleomagnetic and paleointensity data from Precambrian rocks are of great importance for understanding the early geodynamo and tectonic evolution of the Earth. We will present results from a rock magnetic and paleomagnetic investigation of basaltic lava flow sequences at Silver Mountain and Sturgeon River Falls in Upper Michigan. While the Silver Mountain and Sturgeon River Falls lava flows have not been radiometrically dated, these rocks have been assigned to the Siemens Creek Volcanics, the lowermost member of ~1.1 Ga Powder Mill Group (PMG). The PMG represents one of the oldest volcanic units associated with the Mid-Continental Rift System (MCRS). We sampled 13 lava flows from the Silver Mountain and two lava flows from the Sturgeon River Falls exposures (a minimum of 15 cores per flow were taken). Paleomagnetic directions were determined from detailed thermal and/or alternating field demagnetization preceded by an initial low-temperature (liquid nitrogen) demagnetization. Most specimens revealed a single- or a two-component remanent magnetization. At both locations, the characteristic remanent magnetization (ChRM) has a reversed direction with very steep inclination similar to that found in other rocks representing the early stages of MCRS. Our magnetic hysteresis measurements, unblocking temperature spectra, and scanning electron microscopy analyses suggest low-Ti, pseudosingle-domain titanomagnetite as the principal magnetic carrier in these rocks. For paleointensity determinations, we applied the multispecimen parallel differential pTRM method. These data add to the Precambrian paleointensity database which otherwise remains limited because of alteration and other factors hampering the applicability of conventional Thellier double-heating method.

  19. U-Pb geochronology of the Kap Washington Volcanic Province, North Greenland: Constraints on the timing of continental rifting and implications for the development of the Arctic Basin

    NASA Astrophysics Data System (ADS)

    Thorarinsson, S. B.; Holm, P. M.; Tappe, S.; Heaman, L.; Tegner, C.

    2009-12-01

    The Kap Washington volcanic sequence at the north coast of Greenland is bimodal with alkaline basalts, trachytic to rhyolitic lavas, tuffs and ignimbrites predominating. In terms of geochemistry and distribution of rock types, the sequence bears resemblance to presently active continental rift systems, e.g. the Main Ethiopian Rift. Associated with the volcanics is a swarm of coast-normal alkaline basaltic dykes which intensifies towards the outer coast. The volcanics are believed to be linked to rifting in the Arctic Basin and have featured prominently in geotectonic reconstructions of the Arctic region (e.g. Batten et al. 1981). Here we report the first U-Pb zircon ages from silicic lavas and intrusions of the Kap Washington sequence. A total of ten samples have been dated and the duration of magmatism is constrained at present to ca. 10 million years - from 71 to 61 Ma (based on 206Pb/238U ages of concordant analyses). Three age ‘groups’ have been identified: 71-69 Ma (n = 6); 68-65 Ma (n = 2); and 64-61 Ma (n = 2). The oldest group comprises trachytic and rhyolitic lava flows from Kap Kane and a rhyolitic sill from the Kap Washington peninsula. These ages agree well with new 40Ar/39Ar ages obtained on amphiboles from benmoreitic tuffs exposed on Kap Kane (Holm et al., this session) and suggest that most of the ~1.5 km thick Kap Kane sequence was extruded within a period of 1-2 million years. The two younger groups comprise silicic lavas exposed on Lockwood Island. The exposed sequence on Lockwood Island is estimated to be 3-4 km thick and was previously thought to be the oldest part of the succession (Brown et al. 1987). The large scatter in ages on Lockwood Island indicates that magmatism was episodic rather than continuous. The new age data from the Kap Washington volcanics together with 40Ar/39Ar ages for the associated dyke swarm (Kontak et al. 2001) suggest that continental extension and magmatism occurred in the area between ca. 82 and 61 Ma. This age

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

    SciTech Connect

    Sanford, A.; Balch, R.; House, L.; Hartse, H.

    1995-12-01

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

  1. The Effects of Far-Field Boundary Conditions on 2D Numerical Solutions for Continental Rifting: Tests and Recipes for Improved Treatment of Asthenosphere Flow and Melting

    NASA Astrophysics Data System (ADS)

    Morgan, J. P.; de Monserrat, A.; Hall, R.; Taramon, J. M.; Perez-Gussinye, M.

    2015-12-01

    This work focuses on improving current 2D numerical approaches to modeling the boundary conditions associated with computing accurate deformation and melting associated with continental rifting. Recent models primarily use far-field boundary conditions that have been used for decades with little assessment of their effects on asthenospheric flow beneath the rifting region. All are clearly extremely oversimplified — Huismans and Buiter assume there is no vertical flow into the rifting region, with the asthenosphere flowing uniformly into the rifting region from the sides beneath lithosphere moving in the opposing direction, Armitage et al. and van Wijk use divergent velocities on the upper boundary to impose break-up within a Cartesian box, while other studies generally assume there is uniform horizontal flow away from the center of rifting, with uniform vertical flow replenishing the material pulled out of the sides of the computational region. All are likely to significantly shape the pattern of asthenospheric flow beneath the stretching lithosphere that is associated with pressure-release melting and rift volcanism. Thus while ALL may lead to similar predictions of the effects of crustal stretching and thinning, NONE may lead to accurate determination of the the asthenospheric flow and melting associated with lithospheric stretching and breakup. Here we discuss a suite of numerical experiments that compare these choices to likely more realistic boundary condition choices like the analytical solution for flow associated with two diverging plates stretching over a finite-width region, and a high-resolution 2-D region embedded within a cylindrical annulus 'whole mantle cross-section' at 5% extra numerical problem size. Our initial results imply that the choice of far-field boundary conditions does indeed significantly influence predicted melting distributions and melt volumes associated with continental breakup. For calculations including asthenospheric melting

  2. Basement controls on rifting and the associated formation of ocean transform faults—Cretaceous continental extension of the southern margin of Australia

    NASA Astrophysics Data System (ADS)

    Miller, John McL.; Norvick, Martin S.; Wilson, Christopher J. L.

    2002-11-01

    The initial stage of continental extension between Australia and Antarctica was associated with lateral changes in extension direction along the margin that reflects the three-dimensional nature of strain during continental rifting. In the Cretaceous Otway Basin, this change in extension direction was related to substantial rheological differences in the lithosphere across the boundary between two Paleozoic fold belts, the Lachlan and Delamerian, with the net extension direction at a high angle to this boundary. The initial Early Cretaceous rifting preserved within the onshore Otway Basin has two main structural subdomains in the eastern and western Otway Basins distinguished by different structural trends of Early Cretaceous normal faulting. This is not controlled by a variation in preexisting structural weaknesses within the underlying Paleozoic basement because the same geometry of extensional structures also occurs within the basement to the north irrespective of the preexisting structural grain. The eastern Otway Basin is dominated by NE-striking NW-dipping normal faults. In the western Otway Basin, the faults define arrays of predominantly NE-dipping or SW-dipping faults separated by wide accommodation zones defined by folding and variably striking and dipping faults. The partitioning of strain along the boundary between the eastern and western Otway Basins is accommodated by a progressive change in strike of faults and not via a transfer fault. Younger rifting in the Late Cretaceous had a similar extension direction in the western Otway basin, but had a dominant seaward dip, extension appears to have been hindered in the eastern Otway basin by a Proterozoic/Paleozoic basement feature. These factors produced a region of diverging extension along the lithospheric boundary between the Delamerian and Lachlan Fold Belts that lead to failure along this boundary and the formation of a localized sinistral trans-tensional graben, the Shipwreck trough, in the early

  3. Modelling of Continental Lithosphere Breakup and Rifted Margin Formation in Response to an Upwelling Divergent Flow Field Incorporating a Temperature Dependent Rheology

    NASA Astrophysics Data System (ADS)

    Tymms, V. J.; Kusznir, N. J.

    2005-05-01

    We numerically model continental lithosphere deformation leading to breakup and sea floor spreading initiation in response to an imposed upwelling and divergent flow field applied to continental lithosphere and asthenosphere. The model is used to predict rifted continental margin lithosphere thinning and temperature structure. Model predictions are compared with observed rifted margin structure for four diverse case studies. Prior to application of the upwelling divergent flow field the continental lithosphere is undeformed with a uniform temperature gradient. The upwelling divergent flow field is defined kinematically using boundary conditions consisting of the upwelling velocity Vz at the divergence axis and the half divergence rate Vx . The resultant velocity field throughout the continuum is computed using finite element (FE) code incorporating a Newtonian temperature dependent rheology. The flow field is used to advect the continental lithosphere material and lithospheric and asthenospheric temperatures. Viscosity structure is hence modified and the velocities change correspondingly in a feedback loop. We find the kinematic boundary conditions Vz and Vx to be of first order importance. A high Vz/Vx (greater than10), corresponding to buoyancy assisted flow, leads to minimal mantle exhumation and a well defined continent ocean transition consistent with observations at volcanic margins. For Vz/Vx near unity, corresponding to plate boundary driven divergence, mantle exhumation over widths of up to 100 km is predicted which is consistent with observations at non-volcanic margins. The FE method allows the upwelling velocity Vz to be propagated upwards from the top of the asthenosphere to the Earth's surface without the requirement of imposing Vx. When continental breakup is achieved the half divergence velocity Vx can be applied at the lithosphere surface and the upwelling velocity Vz left free. We find this time and space dependent set of boundary conditions is

  4. ALVIN investigation of an active propagating rift system, Galapagos 95.5° W

    USGS Publications Warehouse

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

    1992-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Zwaan, Frank; Schreurs, Guido

    2015-04-01

    -connecting inherited zones, whose strike is at an angle of >15° with respect to the divergence direction. CT-analysis indicates that faulting initiated shortly after the start of the experiments, while structures become only clearly visible at the surface only after 1:30h (4% extension). Rift boundary fault angles tend to decrease from an initial 70° to ca. 55° after 4:00h (10% extension). Further CT-analysis will reveal the 3D evolution of the transform zones in more detail. REFERENCES Acocella, V., Faccenna, C., Funiciello, R., Rossetti, F., 1999. Sand-box modelling of basement-controlled transfer zones in extensional domains. Terra Nova, Vol. 11, No. 4, pp 149-156 Allken, V., Huismans, R. S., Thieulot, C., 2012. Factors controlling the mode of rift interaction in brittle-ductile coupled systems: A 3D numerical study, Geochem. Geophys. Geosyst. Vol. 13, Q05010 Schreurs, G., Colletta, B. (1998) Analogue modelling of faulting in zones of continental transpression and transtension. In: Holdsworth, R. E., Strachan R. A., Dewey, J. F., (eds.) 1998. Continental Transpressional and Transtensional Tectonics. Geological Society, London, Special Publications. No. 135, pp 59-79

  6. A multidisciplinary study in the geodynamic active western Eger rift (Central Europe): The Quaternary volcanic complex Mytina and the recent CO2-degassing zone Hartousov

    NASA Astrophysics Data System (ADS)

    Flechsig, C.; Heinicke, J.; Kaempf, H. W.; Nickschick, T.; Mrlina, J.

    2013-12-01

    The Eger rift (Central Europe) belongs to the European Cenozoic rift system and represents an approximately 50 km wide and 300 km long ENE-WSW striking continental rift that formed during the Upper Cretaceous-Tertiary transition. This rift zone is one of the most active seismic regions in Central Europe. Especially, the western part of the Eger rift area is dominated by ongoing hidden magmatic processes in the intra-continental lithospheric mantle. Besides of known quaternary volcanoes, these processes take place in absence of any presently active volcanism at the surface. However, they are expressed by a series of phenomena distributed over a relatively large area, like occurrence of repeated earthquake swarms, surface exhalation of mantle-derived and CO2-enriched fluids at mofettes and mineral springs, and enhanced heat flow. At present this is the only known intra-continental region where such deep-seated, active lithospheric processes currently occur. The aim of the project is to investigate the tectonic/geologic near surface structure and the degassing processes of the mofette field of Hartousov, where soil gas measurements (concentration and flux rate) in an area of appr. 3x2 km traced a permeable NS extended segment of a fault zone and revealed highly permeable Diffuse Degassing Structures (DDS). The second target is volcanic environment of the Quaternary volcanic complex Mytina maar and the cinder cone Zelezna hurka/Eisenbühl. The investigations are intended to clarify: a) the spatio-temporal reconstruction of the maar complex, and the palaeo volcanic scenario (geological model, tectonic settings, distribution of pyroclastica, b) the geological structure and the tectonic control of the recent degassing zone, and c) the comperative interpretation of both regions in the consideration of potential future volcanic risk assessment in sub-regions of the western Eger Rift. To investigate both regions the following methods are used: geoelectrics, geomagnetics

  7. Measurement of sediment and crustal thickness corrected RDA for 2D profiles at rifted continental margins: Applications to the Iberian, Gulf of Aden and S Angolan margins

    NASA Astrophysics Data System (ADS)

    Cowie, Leanne; Kusznir, Nick

    2014-05-01

    Subsidence analysis of sedimentary basins and rifted continental margins requires a correction for the anomalous uplift or subsidence arising from mantle dynamic topography. Whilst different global model predictions of mantle dynamic topography may give a broadly similar pattern at long wavelengths, they differ substantially in the predicted amplitude and at shorter wavelengths. As a consequence the accuracy of predicted mantle dynamic topography is not sufficiently good to provide corrections for subsidence analysis. Measurements of present day anomalous subsidence, which we attribute to mantle dynamic topography, have been made for three rifted continental margins; offshore Iberia, the Gulf of Aden and southern Angola. We determine residual depth anomaly (RDA), corrected for sediment loading and crustal thickness variation for 2D profiles running from unequivocal oceanic crust across the continental ocean boundary onto thinned continental crust. Residual depth anomalies (RDA), corrected for sediment loading using flexural backstripping and decompaction, have been calculated by comparing observed and age predicted oceanic bathymetries at these margins. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions from Crosby & McKenzie (2009). Non-zero sediment corrected RDAs may result from anomalous oceanic crustal thickness with respect to the global average or from anomalous uplift or subsidence. Gravity anomaly inversion incorporating a lithosphere thermal gravity anomaly correction and sediment thickness from 2D seismic reflection data has been used to determine Moho depth, calibrated using seismic refraction, and oceanic crustal basement thickness. Crustal basement thicknesses derived from gravity inversion together with Airy isostasy have been used to correct for variations of crustal thickness from a standard oceanic thickness of 7km. The 2D profiles of RDA corrected for both sediment loading and non-standard crustal

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

    SciTech Connect

    Sanford, A.; Balch, R.; Hartse, H.; House, L.

    1995-03-01

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

  9. Cenozoic rifting in the West Antarctic Rift System

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

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

  10. What role does crustal heterogeneity play on continental break-up; the interplay of a foldbelt, rift system and ocean basin in the South Atlantic

    NASA Astrophysics Data System (ADS)

    Paton, Douglas; Mortimer, Estelle; Hodgson, Neil

    2015-04-01

    Although extensively studied, two key questions remain unanswered regarding the evolution of the southern South Atlantic. Firstly, where is the Cape Foldbelt (CFB) in offshore South Africa? The CFB is part of the broader Gonwanian Orogeny that prior to South Atlantic rifting continued into the Ventana Foldbelt of Argentina but to date its location in the offshore part of South Africa remains enigmatic. Secondly, the conjugate rift basin to South Africa is the Colorado Basin in Argentina but why does it trend east-west despite its perpendicular orientation to the Atlantic spreading ridge? Current plate models and structural understands cannot explain these fundamental questions. We use newly acquired deep reflection seismic data in the Orange Basin, South Africa, to develop a new structural model for the southern South Atlantic. We characterise the geometry of the Cape Foldbelt onshore and for the first time correlate it into the offshore. We show that it has a north-south trend immediately to the north of the Cape Peninsula but then has a syntaxis (Garies syntaxis) that results in a change to an east-west orientation. This forms the missing jigsaw piece of the Atlantic reconstruction as this is directly beside the restored Colorado Basin. When considered within the pre-break up structural configuration our observations imply that prior to the main phase of Atlantic rifting in the Mezosoic there was significant variation in crustal geometry incorporating the Orange Basin of South Africa, the Colorado Basin and the Gariep Belt of Namibia. These faults were active during Gondwana rifting, but the Colorado rift failed resulting in the present day location of the South Atlantic. Not only do our results improve our understanding of the evolution of the South Atlantic ocean, they highlight the importance of differentiating between early rift evolution and strain localisation during the subsequent rift phase prior to seafloor spreading.

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

    NASA Astrophysics Data System (ADS)

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

    2015-07-01

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

  12. Crustal structure during active rifting in the central Salton Trough, California, constrained by the Salton Seismic Imaging Project (SSIP)

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Seismic refraction and reflection travel times from the Salton Seismic Imaging Project (SSIP) were used to constrain crustal structure during active continental rifting in the central Salton Trough, California. SSIP, funded by NSF and USGS, acquired seismic data in and across the Salton Trough in 2011 to investigate rifting processes at the northern end of the Gulf of California extensional province and earthquake hazards at the southern end of the San Andreas Fault system. Seven lines of refraction and low-fold reflection data were acquired onshore, two lines and a grid of airgun and OBS data were acquired in the Salton Sea, and onshore-offshore data were recorded. North American lithosphere in the central Salton Trough appears to have been rifted apart and replaced by new crust added by magmatism from below and sedimentation from above. Ongoing active rifting of this new crust is manifested by shallow (<10km depth) seismicity in the oblique Brawley Seismic Zone (connecting the Imperial and San Andreas transform faults), the small Salton Buttes volcanoes, and very high heat flow that enables geothermal energy production. Analyses of the onshore-offshore seismic line that extends along the axis of the Salton Trough, parallel to the direction of plate motion, constrains rifted crustal structure. Crystalline basement (~5 km/s) generally occurs at ~4 km depth, but is at 2-3 km depth in a localized region beneath the Salton Buttes and Salton Sea geothermal field. This crystalline rock is interpreted to be late Pliocene to Quaternary Colorado River sediment that has been metamorphosed by high heat flow to a depth of at least 10km. The shallower basement under the volcanic and geothermal field is due to more intense metamorphism and hydrothermal alteration in this region of extreme heat flow. Faster velocity (6.2-6.4 km/s) observed at 10-13 km depth might be the remains of ruptured pre-existing crust or might be produced by deeper magmatism. Seismic travel times indicate

  13. Characterising Seismicity at Alutu, an Actively Deforming Volcano in the Main Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Wilks, M.; Nowacki, A.; Kendall, J. M.; Wookey, J. M.; Biggs, J.; Bastow, I. D.; Ayele, A.; Bedada, T.

    2013-12-01

    The Main Ethiopian Rift (MER) provides a unique example of the tectonic and volcanic processes occuring during the transition from continental rifting to oceanic spreading. Situated 100 km south of Addis Ababa along the eastern rift margin, Alutu is a silicic stratovolcano that geodetic measurements (InSAR and GPS) have shown is actively deforming. Though the volcano has received relatively little scientific attention it is also a site of economic significance as a geothermal power plant resides within the caldera. As part of ARGOS (Alutu Research Geophysical ObservationS), a multi-disciplinary project aiming to investigate the magmatic and hydrothermal processes occuring at Alutu, a seismic network of 12 broadband seismometers was deployed in January 2012. Other components of ARGOS include InSAR, GPS, geologic mapping and magnetotellurics. From the seismic dataset, P- and S-wave arrivals across the array were manually picked and used to locate events using a non-linear earthquake location algorithm (NonLinLoc) and a predefined 1D velocity model. Perturbations were later applied to this velocity model to investigate the sensitivity of the locations and evaluate the true uncertainties of the solutions. Over 1000 events were successfully located during 2012, where picks were possible at 4 or more stations. Seismicity clusters at both shallow depths (z<2 km) beneath the caldera and at deeper depths of 5-15 km. There is a significant increase in seismicity during the rainy months, suggesting the shallow events may be related to the hydrothermal system. We interpret the deeper events as being magmatic in origin. Events are also located along the eastern border faults that bound the outer edges of the MER and highlights that seismicity arises concurrently via tectonic processes. An adapted version of Richter's original local magnitude scale (ML) to account for attenuation within the MER (Keir et al., 2006) was then used to compute magnitudes for the best located events

  14. Variation in styles of rifting in the Gulf of California.

    PubMed

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

    2007-07-26

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  16. 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

  17. Deep seismic reflection data of EDGE U.S. mid-Atlantic continental-margin experiment: Implications for Appalachian sutures and Mesozoic rifting and magmatic underplating

    NASA Astrophysics Data System (ADS)

    Sheridan, Robert E.; Musser, Douglas L.; Glover, Lynn, III; Talwani, Manik; Ewing, John I.; Holbrook, W. Steven; Purdy, G. Michael; Hawman, Robert; Smithson, Scott

    1993-06-01

    The EDGE seismic experiment across the Virginia continental margin delineated a Paleozoic suture, buried Appalachian terranes, and Mesozoic rifting and magmatic events. The seismic grid revealed that the Mesozoic Norfolk rift basin exists only in the northern one-third of the previously mapped area. The north-striking listric border fault of the Norfolk basin half-graben parallels seismic laminations in the basement. The Jurassic volcanic wedge pinches out just landward of the Baltimore Canyon trough hinge zone and downlaps on the hummocky oceanic basement under the continental rise. Under the continental slope, the volcanic wedgereaches depths >9 s (20 km). Two distinct intracrustal reflections at 4.0-5.0 s and at 7.0 s TWIT (two-way traveltime) dip southeastward at low angles (˜15°). The Moho reflection is disrupted where it is intersected by the 7.0 s reflection. Northwest of this point the Moho dips landward; seaward it is horizontal. Seaward of this point, the lower-crustal boundary laminations exist in a narrow interval (10.5-11.0 s) and are of strong amplitude. These changes in the Moho and lower crust represent the seaward edge of the Grenville-age North American crust and the landward edge of Jurassic magmatic underplating. A northwest-dipping reflection observed for the first time on the U.S. Atlantic margin may be the top of the Jurassic magmatic- underplating layer; the northwest-dipping reflection truncates the southeast- dipping 7.0 s TWTT reflection. Landward projection of the 7.0 s reflection yields a north-south trace on the postrift unconformity under the center of lower Chesapeake Bay. This trace is near a basement fault between low-grade metamorphic rocks (Carolina slate-Avalonia) on the east and high-grade rocks (Goochland terrane) on the west. This fault boundary and the southeastdipping 7.0 s reflection probably represent the Taconic suture.

  18. 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

  19. The connection between iron ore formations and "mud-shrimp" colonizations around sunken wood debris and hydrothermal sediments in a Lower Cretaceous continental rift basin, Mecsek Mts., Hungary

    NASA Astrophysics Data System (ADS)

    Jáger, Viktor; Molnár, Ferenc; Buchs, David; Koděra, Peter

    2012-09-01

    In the Early Cretaceous, the continental rift basin of the Mecsek Mts. (Hungary), was situated on the southern edge of the European plate. The opening of the North Atlantic Ocean created a dilatational regime that expanded to the southern edge of the European plate, where several extensional basins and submarine volcanoes were formed during the Early Cretaceous epoch. Permanent seaquake activity caused high swell events during which a large amount of terrestrial wood fragments entered into submarine canyons from rivers or suspended woods which had sunk into the deep seafloor. These fragments created extended wood-fall deposits which contributed large-scale flourishing of numerous burrowing thalassinid crustaceans. Twelve different thalassinid coprolite ichnospecies can be found in the Berriasian-Hauterivian volcano-sedimentary formations. According to the seladonitic crustacean burrows which associated with framboidal pyrite containing Zoophycos and Chondrites ichnofossils (i.e. a "fodinichnia" trace fossil association), the bottom water was aerobic and the pore water was anaerobic; in the latter sulfate reduction occurred. The preservation of wood fragments around thalassinid burrows can be explained by rapid sedimentation related to turbidity currents. Due to the low temperature hydrothermal circulations of seawater, large amounts of iron were released from intrusive, pillowed basaltic sills; these sills intruded into soft, water-saturated sediments containing large amounts of thalassinid excrement. In the coprolites can be found idiomorphic mineral particles originating from the basalts, and coprolites can often be found in peperitic interpillow sediments. This indicates that the life-activity of the decapoda crustaceans in many Lower Cretaceous occurrences initially preceded the first magmatic eruptions. The paroxysm of the rift volcanism took place during the Valanginian age, when some submarine volcanoes emerged above sea level, reaching a maximum height of

  20. 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

  1. Pliocene granodioritic knoll with continental crust affinities discovered in the intra-oceanic Izu-Bonin-Mariana Arc: Syntectonic granitic crust formation during back-arc rifting

    NASA Astrophysics Data System (ADS)

    Tani, Kenichiro; Dunkley, Daniel J.; Chang, Qing; Nichols, Alexander R. L.; Shukuno, Hiroshi; Hirahara, Yuka; Ishizuka, Osamu; Arima, Makoto; Tatsumi, Yoshiyuki

    2015-08-01

    A widely held hypothesis is that modern continental crust of an intermediate (i.e. andesitic) bulk composition forms at intra-oceanic arcs through subduction zone magmatism. However, there is a critical paradox in this hypothesis: to date, the dominant granitic rocks discovered in these arcs are tonalite, rocks that are significantly depleted in incompatible (i.e. magma-preferred) elements and do not geochemically and petrographically represent those of the continents. Here we describe the discovery of a submarine knoll, the Daisan-West Sumisu Knoll, situated in the rear-arc region of the intra-oceanic Izu-Bonin-Mariana Arc. Remotely-operated vehicle surveys reveal that this knoll is made up entirely of a 2.6 million year old porphyritic to equigranular granodiorite intrusion with a geochemical signature typical of continental crust. We present a model of granodiorite magma formation that involves partial remelting of enriched mafic rear-arc crust during the initial phase of back-arc rifting, which is supported by the preservation of relic cores inherited from initial rear-arc source rocks within magmatic zircon crystals. The strong extensional tectonic regime at the time of intrusion may have allowed the granodioritic magma to be emplaced at an extremely shallow level, with later erosion of sediment and volcanic covers exposing the internal plutonic body. These findings suggest that rear-arc regions could be the potential sites of continental crust formation in intra-oceanic convergent margins.

  2. 3D Dynamics of Oblique Rift Systems: Fault Evolution from Rift to Break-up

    NASA Astrophysics Data System (ADS)

    Brune, S.

    2014-12-01

    Rift evolution and passive margin formation has been thoroughly investigated using conceptual and numerical models in two dimensions. However, the 2D assumption that the extension direction is perpendicular to the rift trend is often invalid. In fact, the majority of rift systems that lead to continental break-up during the last 150 My involved moderate to high rift obliquity. Yet, 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. Even though the model setup is very simple (horizontally layered, no inherited faults), its evolution exhibits a variety of fault orientations that are solely caused by the interaction of far-field stresses with rift-intrinsic buoyancy and strength. Depending on rift obliquity, these orientations involve rift-parallel, extension-orthogonal, and intermediate normal fault directions as well as strike-slip faults. Allowing new insights on fault patterns of the proximal and distal margins, the model shows that individual fault populations are activated in a characteristic multi-phase evolution driven by lateral density variations of the evolving rift system. Model results are in very good agreement with inferences from the well-studied Gulf of Aden and provide testable predictions for other rifts and passive margins worldwide.

  3. Physical Processes Contributing To Small-scale Vertical Movements During Changing Inplane Stresses In Rift Basins and At Passive Continental Margins

    NASA Astrophysics Data System (ADS)

    Paulsen, G. E.; Nielsen, S. B.; Hansen, D. L.

    The vertical movements during a regional stress reversal in a rifted basin or on a passive continental margin are examined using a numerical 2D thermo-mechanical finite element model with a visco-elastic-plastic rheology. Three different physical mechanisms are recognized in small-scale vertical movements at small inplane force variations: elastic dilatation, elastic flexure, and permanent deformation. Their rela- tive importance depend on the applied force, the duration of the force, and the thermal structure of the lithosphere. Elastic material dilatation occurs whenever the stress state changes. A reversal from extension to compression therefore immediately leads to elastic dilatation, and re- sults in an overall subsidence of the entire profile. Simultaneously with dilatation the lithosphere reacts with flexure. The significance of the flexural component strongly depends on the thermal structure of the lithosphere. The polarity and amplitude of the flexure depends on the initial (before compression) loading of the lithosphere. Gener- ally, the flexural effects lead to subsidence of the overdeep in the landward part of the basin and a small amount of uplift at the basin flanks. The amplitudes of the flexural response are small and comparable with the amplitudes of the elastic dilatation. With continuing compression permanent deformation and lithospheric thickening becomes increasingly important. Ultimately, the thickened part of the lithosphere stands out as an inverted zone. The amount of permanent deformation is directly connected with the size and duration of the applied force, but even a relatively small force leads to inversion tectonics in the landward part of the basin. The conclusions are: 1) small stress induced vertical movements in rift basins and at passive continental margins are the result of a complex interaction of at least three different processes, 2) the total sediment loaded amplitudes resulting from these pro- cesses are small (2-300 m) for

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

    USGS Publications Warehouse

    Robbins, E.I.

    1983-01-01

    A study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of processes that form rift valleys with those that take place in and around rift lakes. The deposition of the precursors of petroleum, gas, oil shale, coal, phosphate, barite, Cu-Pb-Zn sulfides, and uranium begins with erosion of uplifted areas, and the consequent input of abundant nutrients and solute loads into swamps and tectonic lakes. Hot springs and volcanism add other nutrients and solutes. The resulting high biological productivity creates oxidized/reduced interfaces, and anoxic and H2S-rich bottom waters which preserves metal-bearing organic tissues and horizons. In the depositional phases, the fine-grained lake deposits are in contact with coarse-grained beach, delta, river, talus, and alluvial fan deposits. Earthquake-induced turbidites also are common coarse-grained deposits of rift lakes. Postdepositional processes in rifts include high heat flow and a resulting concentration of the organic and metallic components that were dispersed throughout the lakebeds. Postdepositional faulting brings organic- and metal-rich sourcebeds in contact with coarse-grained host and reservoir rocks. A suite of potentially economic deposits is therefore a characteristic of rift valleys. ?? 1983.

  5. Strain distribution in the East African Rift from GPS measurements

    NASA Astrophysics Data System (ADS)

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

    2008-12-01

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

  6. Tectonics and stratigraphic development of a rifted continental margin: An example from the Eocene-middle Miocene, Taishi Basin, central Taiwan

    NASA Astrophysics Data System (ADS)

    Lee, Yan-Ching; Lin, Andrew. T.

    2015-04-01

    The rifting and forming age of South China Sea crust is about 58~37 Ma, and the shallow marine sequences of South China Sea were uplifted and exposed in Taiwan mountain belt. While most strata of Backbone Range and Hsueshan Range are metamorphosed, Western Foothills are the remaining strata. As to central Taiwan, those sequences are the critical place to explore the Cenozoic history of South China Sea rifting, since the stratigraphy record includes syn-rift to post-breakup strata. This study synthesizes field survey and borehole data to draft the tectonic and geological background of northern margin of the South China Sea, and thereby establish an evolutionary model of the target basin, Taishi Basin, from late Eocene to middle Miocene. Itemized stratigraphy strata examined from field can be nicely correlated to those of wells, and the result can be used to outline Taishi Basin. The trend shows the succession thickening toward the west and north. Most of well data shows pyroclastic deposits at bottom, succession covered on top are all sedimentary deposits. The lithology transfers from mud-dominated to sand-occupied for three times, which indicates converting of sequence. Twenty onshore and offshore exploration wells in the western Taiwan were incorporated. We identify eight types of electro-facies, which can be concluded into depositional environments. The vertical change of paleo-environments indicates different types of parasequences. By stacking individual parasequence, twelve sequences were recognized. In Western Foothills, central Taiwan, strata of more than one kilometer thickness was examined by this study, twenty-four lithofacies were discriminated, including five mudstones, three Sand-Mud laminations, seven sandstones, one conglomerate and seven types of pyroclastic deposits. Depositional environments were delivered, including (1) wave-dominated and tidal-influenced coasts, (2) wave-dominated estuary, (3) offshore continental shelf and (4) volcano apron

  7. The occurrence of a complete continental rift type of volcanic rocks suite along the Yerer-Tullu Wellel Volcano Tectonic Lineament, Central Ethiopia

    NASA Astrophysics Data System (ADS)

    Abebe Adhana, Tsegaye

    2014-11-01

    The Yerer-Tullu Wellel Volcano-tectonic Lineament (YTVL) is an E-W trending fault system or aborted rift that intercepts the Main Ethiopian Rift (MER) at Debre Zeyt (Bishoftu)/Yerer, in the eastern periphery of Addis Ababa. The structure is in correspondence with the westward extension of the southern margin of the Gulf of Aden rift. The YTVL extends for more than 500 km with a very clear northern fault margin, between Addis Ababa and Ambo known as the “Ambo Fault”. The southern margin is indicated by an E-W trending segmented lineaments at the latitude of about N 8°30‧, the Bedele-Metu being the most clear segment. In between these limits there are several evolved central volcanoes and cinder cones. The central volcanoes range in age from 12 to 7 Ma in the western most (Tullu Wellel) and gradually the upper limit get younger towards East to less than 1 Ma in the Wenchi and Debre Zeyt (Bishoftu) areas. These volcanic products cover the whole spectrum of a continental rift volcanic rocks suite: (1) in the eastern zone (Yerer-Bishoftu) the suite is silica over-saturated, ranging in composition from transitional basalt to peralkaline rhyolite, (2) moving westwards, between Wechacha and Wenchi, the rocks suite is silica saturated ranging in composition from alkali basalt to trachyte, (3) further West between Ijaji-Konchi and Nekemt the rocks suite is silica under-saturated ranging in composition from basanite to phonolite. Crossing the Dedessa lineament, the Tullu Wellel rocks appear to be silica saturated. Within a single suite fractional crystallization is the predominant evolutional process even in the silica over-saturated suite. The westwards progressive silica under-saturation and increase in alkalinity (except for the Tullu Wellel volcanic centers) is interpreted by the gradual deepening of an anomalous mantle where partial fusion took place. Therefore, as distance increases from the MER junction to the West, the amount of melt on the upper mantle was

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    the 2005 rifting episode. This second magmatic centre supplies magma to the remaining 2/3 of the segment, but scarcely impacts its Northern termination (where the Dabbahu activity predominates) - except during extraordinary events when dykes are long enough to reach those parts, as in 2005. The eruption ages of the different lava units correlates with their degrees of differentiation, allowing different magmatic cycles of about a few tens of years each to be distinguished. During the first recorded magmatic cycle (~70 ka to ~55 ka), Dabbahu is built of wide-spreading pāhoehoe flows around localised eruptive centres. The resulting topography of the volcanic edifice remains low, and is only slightly affected by rift-related fault activity, with the development of minor scarps. The second recorded magmatic cycle (~50 ka to ~20 ka) coincides with a strong development of Dabbahu topography - underlined by the change in lava morphology with well channelized 'a'ā flows since 50 ka. Tectonic activity also clearly increases over this period, with the initiation of the major fault scarps of the rift, which have been dated at around 35 ka. Our study underlines the role of the magma supply and availability beneath Dabbahu in the evolution both topographies of Dabbahu volcano and of the rift depression morphology.

  9. The Cenozoic volcanism in the Kivu rift: Assessment of the tectonic setting, geochemistry, and geochronology of the volcanic activity in the South-Kivu and Virunga regions

    NASA Astrophysics Data System (ADS)

    Pouclet, A.; Bellon, H.; Bram, K.

    2016-09-01

    The Kivu rift is part of the western branch of the East African Rift system. From Lake Tanganyika to Lake Albert, the Kivu rift is set in a succession of Precambrian zones of weakness trending NW-SE, NNE-SSW and NE-SW. At the NW to NNE turn of the rift direction in the Lake Kivu area, the inherited faults are crosscut by newly born N-S fractures which developed during the late Cenozoic rifting and controlled the volcanic activity. From Lake Kivu to Lake Edward, the N-S faults show a right-lateral en echelon pattern. Development of tension gashes in the Virunga area indicates a clockwise rotation of the constraint linked to dextral oblique motion of crustal blocks. The extensional direction was W-E in the Mio-Pliocene and ENE-WSW in the Pleistocene to present time. The volcanic rocks are assigned to three groups: (1) tholeiites and sodic alkali basalts in the South-Kivu, (2) sodic basalts and nephelinites in the northern Lake Kivu and western Virunga, and (3) potassic basanites and potassic nephelinites in the Virunga area. South-Kivu magmas were generated by melting of spinel + garnet lherzolite from two sources: an enriched lithospheric source and a less enriched mixed lithospheric and asthenospheric source. The latter source was implied in the genesis of the tholeiitic lavas at the beginning of the South-Kivu tectono-volcanic activity, in relationships with asthenosphere upwelling. The ensuing outpouring of alkaline basaltic lavas from the lithospheric source attests for the abortion of the asthenospheric contribution and a change of the rifting process. The sodic nephelinites of the northern Lake Kivu originated from low partial melting of garnet peridotite of the sub-continental mantle due to pressure release during swell initiation. The Virunga potassic magmas resulted from the melting of garnet peridotite with an increasing degree of melting from nephelinite to basanite. They originated from a lithospheric source enriched in both K and Rb, suggesting the

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

    PubMed

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

    2013-07-01

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

  11. Multichannel seismic depth sections and interval velocities over outer continental shelf and upper continental slope between Cape Hatteras and Cape Cod: rifted margins

    USGS Publications Warehouse

    Grow, John A.; Mattick, Robert E.; Schlee, John S.

    1979-01-01

    Six computer-generated seismic depth sections over the outer continental shelf and upper slope reveal that subhorizontal Lower Cretaceous reflectors continue 20 to 30 km seaward of the present shelf edge. Extensive erosion on the continental slope has occurred primarily during the Tertiary, causing major unconformities and retreat of the shelf edge to its present position. The precise age and number of erosional events is not established, but at least one major erosional event is thought to be Oligocene and related to a marine regression in response to a worldwide eustatic lowering of sea level. Velocities derived from the multichannel data reveal distinctive ranges and lateral trends as functions of sediment age, depth of burial, and distance from the coastline. Seismic units beneath the shelf and slope of inferred Tertiary age range from 1.7 to 2.7 km/sec, increasing with age and depth of burial. Units interpreted as Upper Cretaceous rocks beneath the shelf range from 2.3 to 3.6 km/sec and show a distinct lateral increase across the shelf followed by a decrease beneath the present continental slope. Inferred Lower Cretaceous and Upper Jurassic rocks beneath the shelf increase from 3.7 to 4.8 km/sec from nearshore to offshore and indicate a change in facies from clastic units below the inner shelf to carbonate units beneath the outer shelf and upper continental slope. Both reflection and refraction data suggest that thin, high-velocity limestone units (5.0 km/sec) are present within the Lower Cretaceous and Upper Jurassic units beneath the outermost shelf edge, but that these change lithology or pinch out before reaching the middle shelf. Although lateral changes in velocity across the shelf and local velocity inversions appear, the interval velocities along the length of the margin show excellent continuity between Cape Hatteras and Cape Cod. The high-velocity horizons within the Lower Cretaceous and Upper Jurassic shelf-edge complex indicate the presence of a

  12. Closing of the Midcontinent-Rift - a far-field effect on Grenvillian compression

    USGS Publications Warehouse

    Cannon, W.F.

    1994-01-01

    The Midcontinent rift formed in the Laurentian supercontinent between 1109 and 1094 Ma. Soon after rifting, stresses changed from extensional to compressional, and the central graben of the rift was partly inverted by thrusting on original extensional faults. Thrusting culminated at about 1060 Ma but may have begun as early as 1080 Ma. On the southwest-trending arm of the rift, the crust was shortened about 30km; on the southeast-trending arm, strike-slip motion was dominant. The rift developed adjacent to the tectonically active Grenville province, and its rapid evolution from an extensional to a compressional feature at c1080 Ma was coincident with renewal of northwest-directed thrusting in the Grenville, probably caused by continent-continent collision. A zone of weak lithosphere created by rifting became the locus for deformation within the otherwise strong continental lithosphere. Stresses transmitted from the Grenville province utilized this weak zone to close and invert the rift. -Author

  13. Receiver function and magnetotelluric analysis to understand the first stage of a continental lithospheric break-up : case of the North Tanzanian Rift

    NASA Astrophysics Data System (ADS)

    Plasman, M.; Tiberi, C.; Tarits, P.; Hautot, S.; Gautier, S.; Ebinger, C. J.; Mulibo, G. D.; Khalfan, M.

    2015-12-01

    First stage of continental break-up, though intensively studied, is yet poorly understood. This is partly because actual rifting areas are either too mature (more than 10 My) or not easily accessible (thick sediment cover or under water). The North Tanzania part of the East African Rift is the place of a lithosphere's early break-up (less than 5My) in response to a combination of regional pulling forces and mantle upwelling. Deformation there results from complex interactions between magmatic intrusions, faulting, asthenospheric dynamics and far field stresses. CoLiBrEA (ANR) and CRAFTI (NSF) are two multidisciplinary projects which collaboratively focus on this area to understand the interactions between faults and magma, the role of inherited structures and rheological heterogeneities of the lithosphere. For that purpose, we deployed 38 broadband seismic stations in the Natron and Ngorongoro areas from January 2013 to December 2014 and carried out a 120 km East-West magnetotelluric (MT) profile to image the crustal and mantle structures. The 3D resistivity model, obtained from the inversion of the MT data along the profile, shows an highly heterogeneous crust with three-dimensional structures over a more homogeneous upper mantle. The first inversion result from the receiver function (RF) by the Zhu and Kanamori's inversion method show a thick crust (~35 km) with important variations (maximum 15km) especially in the Ngorongoro area, and an average Vp/Vs ratio of 1.75. We then completed this study by combining the MT data and the RF at the 11 sites of the EW profile. For each site, we built a 1D velocity model (Vs and VpVs) obtained by combining the Sambridge forward solution with a non linear descent research algorithm and constrained by the resistivity structure. The inversion shows an heterogeneous crust obviously dominated by the Moho interface at different depths, with low velocity layers mainly corresponding to low resistivity features.

  14. En echelon Miocene rifting in the southwestern United States and model for vertical-axis rotation in continental extension

    SciTech Connect

    Bartley, J.M. ); Glazner, A.F. )

    1991-12-01

    Two areas of intense early Miocene crustal extension in the southwestern United States, the Colorado River trough and the central Mojave Desert, are separated by a weakly deformed area in the eastern Mojave Desert. The authors propose that these areas form a left-stepping en echelon rift system linked by a ductile detachment at depth. The en echelon geometry explains the southward loss of displacement in the central Mojave Desert and northward loss of coeval displacement in the Colorado River trough, and it incorporates seismic reflection evidence that mid-crustal Tertiary extensional mylonites continue beneath the weakly deformed area. This geometry also explains clockwise paleomagnetic declination anomalies from lower Miocene rocks as recording thin-skinned, detached rotations; large-scale block rotations are not required. Obliquity of the northeast-trending crustal-extension vector to the east-west-trending early Miocene synextensional volcanic belt may have caused the en echelon pattern to develop.

  15. The connection between iron ore formations and "mud-shrimp" colonizations around sunken wood debris and hydrothermal sediments in a Lower Cretaceous continental rift basin, Mecsek Mts., Hungary

    NASA Astrophysics Data System (ADS)

    Jáger, Viktor; Molnár, Ferenc; Buchs, David; Koděra, Peter

    2012-09-01

    In the Early Cretaceous, the continental rift basin of the Mecsek Mts. (Hungary), was situated on the southern edge of the European plate. The opening of the North Atlantic Ocean created a dilatational regime that expanded to the southern edge of the European plate, where several extensional basins and submarine volcanoes were formed during the Early Cretaceous epoch. Permanent seaquake activity caused high swell events during which a large amount of terrestrial wood fragments entered into submarine canyons from rivers or suspended woods which had sunk into the deep seafloor. These fragments created extended wood-fall deposits which contributed large-scale flourishing of numerous burrowing thalassinid crustaceans. Twelve different thalassinid coprolite ichnospecies can be found in the Berriasian-Hauterivian volcano-sedimentary formations. According to the seladonitic crustacean burrows which associated with framboidal pyrite containing Zoophycos and Chondrites ichnofossils (i.e. a "fodinichnia" trace fossil association), the bottom water was aerobic and the pore water was anaerobic; in the latter sulfate reduction occurred. The preservation of wood fragments around thalassinid burrows can be explained by rapid sedimentation related to turbidity currents. Due to the low temperature hydrothermal circulations of seawater, large amounts of iron were released from intrusive, pillowed basaltic sills; these sills intruded into soft, water-saturated sediments containing large amounts of thalassinid excrement. In the coprolites can be found idiomorphic mineral particles originating from the basalts, and coprolites can often be found in peperitic interpillow sediments. This indicates that the life-activity of the decapoda crustaceans in many Lower Cretaceous occurrences initially preceded the first magmatic eruptions. The paroxysm of the rift volcanism took place during the Valanginian age, when some submarine volcanoes emerged above sea level, reaching a maximum height of

  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. Morpho-structural evolution of a volcanic island developed inside an active oceanic rift: S. Miguel Island (Terceira Rift, Azores)

    NASA Astrophysics Data System (ADS)

    Sibrant, A. L. R.; Hildenbrand, A.; Marques, F. O.; Weiss, B.; Boulesteix, T.; Hübscher, C.; Lüdmann, T.; Costa, A. C. G.; Catalão, J. C.

    2015-08-01

    The evolution of volcanic islands is generally marked by fast construction phases alternating with destruction by a variety of mass-wasting processes. More specifically, volcanic islands located in areas of intense regional deformation can be particularly prone to gravitational destabilisation. The island of S. Miguel (Azores) has developed during the last 1 Myr inside the active Terceira Rift, a major tectonic structure materializing the present boundary between the Eurasian and Nubian lithospheric plates. In this work, we depict the evolution of the island, based on high-resolution DEM data, stratigraphic and structural analyses, high-precision K-Ar dating on separated mineral phases, and offshore data (bathymetry and seismic profiles). The new results indicate that: (1) the oldest volcanic complex (Nordeste), composing the easternmost part of the island, was dominantly active between ca. 850 and 750 ka, and was subsequently affected by a major south-directed flank collapse. (2) Between at least 500 ka and 250 ka, the landslide depression was massively filled by a thick lava succession erupted from volcanic cones and domes distributed along the main E-W collapse scar. (3) Since 250 kyr, the western part of this succession (Furnas area) was affected by multiple vertical collapses; associated plinian eruptions produced large pyroclastic deposits, here dated at ca. 60 ka and less than 25 ka. (4) During the same period, the eastern part of the landslide scar was enlarged by retrogressive erosion, producing the large Povoação valley, which was gradually filled by sediments and young volcanic products. (5) The Fogo volcano, in the middle of S. Miguel, is here dated between ca. 270 and 17 ka, and was affected by, at least, one southwards flank collapse. (6) The Sete Cidades volcano, in the western end of the island, is here dated between ca. 91 and 13 ka, and experienced mutliple caldera collapses; a landslide to the North is also suspected from the presence of a

  18. Rheological implications of sediment transport for continental rifting and its impact in margin geometry and major unconformities

    NASA Astrophysics Data System (ADS)

    Andres-Martinez, Miguel; Perez-Gussinye, Marta; Armitage, John; Morgan, Jason

    2016-04-01

    The inner dynamics of the Earth such as mantle convection, geochemical reactions and isostasy have been typically interpreted as the main engine of plate tectonics and crustal deformation. However, nowadays it is well established that processes transporting material along the surface of the Earth influence the inner dynamics. Surface processes play a key role particularly during rifting, where great subsidence rates occur at synrift basins while shoulder uplift provides rock to be eroded for later infilling of these basins. Erosion implies unloading of the crust which favours uplift, and sedimentation at basins results in loading which favours subsidence. Consequently, erosion and sedimentation amplify stresses and the flexural response of the lithosphere in situations with extensive faulting. These changes to the stress field may be large enough to result in changes in the evolution of rifting and its modes of extension. Additionally, higher subsidence rates and thermal blanketing due to sediments may result in higher geotherms and consequently, a weaker/more-viscous behaviour of the crustal rocks. This would also have a large impact on the deformation style during extension. Here, we explore the interactions between surface processes and tectonics using numerical modelling. Experiments are run with the absence of sediment transport and with different sediment transport regimes for 35 and 40 km crustal thicknesses. Tests with higher transport coefficient show more effective localization of deformation into upper crustal faults which results in effective crustal thinning, larger blocks and longer-lived faults. Our experiments also prove that more effective surface processes reduce the length of margins generated by sequential faulting. For our end member situations, high sedimentation rates lead to pure shear extension of the crust induced by high temperatures, which finally results in broad extension and symmetric margins. Furthermore, our model allows for the

  19. Tectonic evolution and setting of the Sa'al Complex, southern Sinai, Egypt: A Proterozoic continental back-arc rift model

    NASA Astrophysics Data System (ADS)

    Fowler, A.; Hassen, I.; Hassan, M.

    2015-04-01

    The Sa'al Complex is a mainly low grade metamorphosed polydeformed volcanosedimentary sequence exposed in the northern central Sinai basement, Egypt. Details of the stratigraphy, sedimentology and petrography of the three formations: Agramiya, Ra'ayan and Zaghara Formations are described. The earliest deformation (D1) is related to extensional tectonism and HT-LP regional metamorphism. The main D1 structure is a bedding-parallel S1 foliation with at least 50% vertical shortening in the well-foliated Ra'ayan phyllites. Earlier models that explained S1 by bedding-parallel shearing are rejected. The Sa'al volcanism, D1 extension and HT metamorphism were probably associated with back-arc rifting in a continental arc setting, similar to the modern Taupo Zone of New Zealand. Later deformations, D2 and D3, involved folding about NE-trending and NW-trending axial planes, respectively. D2 was probably a result of compressional stresses typical of continental back-arc regions, and resulted in development of steep NW-vergent imbricate thrusts and NE-trending F2 meso- and macrofolds. The Firinga gabbro and the Wadi Murad foliated diorite intruded along D2 backthrusts, while the main diorite intrusion dominating the centre of the complex intruded along D2 steepened imbricate thrusts. F3 deformation may be related to the latest convergence of the east and west Gondwana, and has correlatives in the Kid and Feiran Complexes. A final deformation D4 that generated the main strike-slip faults in the area correlates with NE-SW trending σ1, inconsistent with a Najd origin. Recent geochronological results from U/Pb zircon studies are difficult to reconcile with stratigraphic and intrusion field evidence, and apparently require very tight time constraints on the main metamorphism, D1 and D2 deformations of the complex.

  20. The East African rift system

    NASA Astrophysics Data System (ADS)

    Chorowicz, Jean

    2005-10-01

    This overview paper considers the East African rift system (EARS) as an intra-continental ridge system, comprising an axial rift. It describes the structural organization in three branches, the overall morphology, lithospheric cross-sections, the morphotectonics, the main tectonic features—with emphasis on the tension fractures—and volcanism in its relationships with the tectonics. The most characteristic features in the EARS are narrow elongate zones of thinned continental lithosphere related to asthenospheric intrusions in the upper mantle. This hidden part of the rift structure is expressed on the surface by thermal uplift of the rift shoulders. The graben valleys and basins are organized over a major failure in the lithospheric mantle, and in the crust comprise a major border fault, linked in depth to a low angle detachment fault, inducing asymmetric roll-over pattern, eventually accompanied by smaller normal faulting and tilted blocks. Considering the kinematics, divergent movements caused the continent to split along lines of preexisting lithospheric weaknesses marked by ancient tectonic patterns that focus the extensional strain. The hypothesis favored here is SE-ward relative divergent drifting of a not yet well individualized Somalian plate, a model in agreement with the existence of NW-striking transform and transfer zones. The East African rift system comprises a unique succession of graben basins linked and segmented by intracontinental transform, transfer and accommodation zones. In an attempt to make a point on the rift system evolution through time and space, it is clear that the role of plume impacts is determinant. The main phenomenon is formation of domes related to plume effect, weakening the lithosphere and, long after, failure inducing focused upper mantle thinning, asthenospheric intrusion and related thermal uplift of shoulders. The plume that had formed first at around 30 Ma was not in the Afar but likely in Lake Tana region (Ethiopia

  1. Coastal and submarine instabilities distribution in the tectonically active SW margin of the Corinth Rift (Psathopyrgos, Achaia, Greece)

    NASA Astrophysics Data System (ADS)

    Simou, Eirini; Papanikolaou, Dimitrios; Lykousis, Vasilios; Nomikou, Paraskevi; Vassilakis, Emmanuel

    2014-05-01

    The Corinth Rift, one of the most active rifts in the world as local extension trending NE-SW reaches the amount of 14±2 mm/yr, corresponds to one of the largest zones of seismically active normal faulting. The formation, growth and migration southwards of the prevailing fault systems, which evolve simultaneously with the intense morphogenetic processes, are overprinted in the age, facies and thickness of the Plio-Pleistocene sequences constructing the south margin of the western Gulf of Corinth. The dominant fault blocks, defined by east-west trending, north dipping normal faults, are accompanied by several morphological features and anomalies, noticed in both the terrestrial and the marine environment. Our main aim has been to examine how the tectonic evolution, in combination with the attendant fierce erosional and sedimentary processes, has affected the morphology through geodynamic processes expressed as failures in the wider coastal area. High resolution multibeam bathymetry in combination with the available land surface data have contributed to submarine and subaerial morphological mapping. These have been used as a basis for the detection of all those geomorphic features that indicate instabilities probably triggered, directly or indirectly, by the ongoing active tectonic deformation. The interpretation of the combined datasets shows that the southwestern margin of the Corinth Rift towards Psathopyrgos fault zone is characterized by intense coastal relief and a narrow, almost absent, continental shelf, which passes abruptly to steep submarine slopes. These steep slope values denote the effects of the most recent brittle deformation and are related to coastal and submarine instabilities and failures. High uplift rates and rapid sedimentation, indicative of the regional high-energy terrestrial and submarine environment, are subsequently balanced by the transportation of the seafloor currents, especially where slope gradients decrease, disintegrating the

  2. Structural, sedimentary and igneous evidence for the genesis and emplacement of the rifted continental margin of the Southern Neotethys, SE Turkey

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair; Parlak, Osman; Dumitrica, Paulian; Tasli, Kemal; Yıldırım, Nail

    2014-05-01

    Evidence of the rift, spreading and closure history of the Southern Neotethys is revealed by allochthonous continental margin and ocean-derived units that were emplaced onto the Arabian foreland during latest Cretaceous (Adıyaman area). The structurally lower Karadut Complex is a broken formation, mainly composed of a fragmented sequence of pelagic/hemipelagic carbonates, radiolarites and redeposited limestones. Sedimentary structures and petrographic work suggest that detrital material was mostly derived from the Arabian margin in the form of gravity flows rich in shallow-water carbonate material. Interbedded siliceous sediments are dated as Early Toarcian and Late Albian using radiolarians, whereas hemipelagic carbonates are dated as Turonian-Santonian using planktic foraminifera. The outcrops of the Karadut Complex are restored as Late Cretaceous slope, to base-of-slope deposits of the Arabian continental margin. The more widely exposed, generally structurally higher, Koçali Complex comprises variably disrupted thrust sheets that are in places folded on a kilometric scale. Intact successions were measured in several of the volcanic-sedimentary thrust sheets. The successions begin with basaltic volcanic rocks that are interbedded with volcaniclastic, radiolarian and carbonate sediments, and then pass upwards into thin-bedded non-calcareous ribbon radiolarites, shales and thin to medium-bedded redeposited limestones. Previous work documented a relatively intact sequence of ocean island basalt (OIB)-type (intra-plate) basaltic lavas and volcaniclastic sediments, associated with Middle Carnian-Rhaetian radiolarites. Study of several different thrust sheets during this work indicates the presence of widespread OIB and also enriched mid-ocean ridge-type basalt (E-MORB). Associated radiolarites are dated as Early Norian, Early Pliensbachian and Bajocian, extending the known age range of the Koçali Complex succession. Variably dismembered ophiolitic rocks, mostly

  3. Geochemistry and zircon ages of mafic dikes in the South Qinling, central China: evidence for late Neoproterozoic continental rifting in the northern Yangtze block

    NASA Astrophysics Data System (ADS)

    Zhu, Xiyan; Chen, Fukun; Liu, Bingxiang; Zhang, He; Zhai, Mingguo

    2015-01-01

    Neoproterozoic volcanic-sedimentary sequences of the southern Qinling belt, central China, were intruded by voluminous mafic dikes. secondary ion mass spectrometry zircon U-Pb dating indicates that these dikes were emplaced at 650.8 ± 5.2 Ma, coeval with mafic rocks occurring at the northern margin of the Yangtze block. The dikes are characterized by enrichment of large ion lithophile elements, high Ti contents (up to 3.73 wt%) and Nb/Ta ratios between 14.5 and 19.6, suggesting a mantle source of oceanic island basalt affinity. Initial 87Sr/86Sr ratios show positive correlation with SiO2 contents and negative correlation with Zr/Nb ratios, implying that these rocks were affected by crustal contamination during the magma ascend and emplacement process. The dikes have initial ɛ Nd values of +0.2 to +3.3, low 206Pb/204Pb ratios of 16.96-17.45, and moderate 87Sr/86Sr ratios of 0.7043-0.7076, likely pointing to the involvement of an enriched mantle source. The mafic dikes and coeval mafic volcanic equivalents in the South Qinling and the northern Yangtze are hypothesized to be related with the prolonged breakup of the supercontinent Rodinia, suggesting that continental rifting lasted until ca. 650 Ma.

  4. Varying styles of magmatic strain accommodation across the East African Rift

    NASA Astrophysics Data System (ADS)

    Muirhead, James D.; Kattenhorn, Simon A.; Le Corvec, Nicolas

    2015-09-01

    Observations of active dike intrusions provide present day snapshots of the magmatic contribution to continental rifting. However, unravelling the contributions of upper crustal dikes over the timescale of continental rift evolution is a significant challenge. To address this issue, we analyzed the morphologies and alignments of >1500 volcanic cones to infer the distribution and trends of upper crustal dikes in various rift basins across the East African Rift (EAR). Cone lineament data reveal along-axis variations in the distribution and geometries of dike intrusions as a result of changing tectonomagmatic conditions. In younger (<10 Ma) basins of the North Tanzanian Divergence, dikes are largely restricted to zones of rift-oblique faulting between major rift segments, referred to here as transfer zones. Cone lineament trends are highly variable, resulting from the interplay between (1) the regional stress field, (2) local magma-induced stress fields, and (3) stress rotations related to mechanical interactions between rift segments. We find similar cone lineament trends in transfer zones in the western branch of the EAR, such as the Virunga Province, Democratic Republic of the Congo. The distributions and orientations of upper crustal dikes in the eastern branch of the EAR vary during continental rift evolution. In early-stage rifts (<10 Ma), upper crustal dikes play a limited role in accommodating extension, as they are confined to areas in and around transfer zones. In evolved rift basins (>10 Ma) in Ethiopia and the Kenya Rift, rift-parallel dikes accommodate upper crustal extension along the full length of the basin.

  5. Seismic imaging of the geodynamic activity at the western Eger rift in central Europe

    NASA Astrophysics Data System (ADS)

    Mullick, N.; Buske, S.; Hrubcova, P.; Ruzek, B.; Shapiro, S.; Wigger, P.; Fischer, T.

    2015-04-01

    The western Eger rift at the Czech-German border in central Europe is an important geodynamically active area within the European Cenzoic rift system (ECRS) in the forelands of the Alps. Along with two other active areas of the ECRS, the French Massif Central and the east and west Eifel volcanic fields, it is characterized by numerous CO2-rich fluid emission points and frequent micro-seismicity. Existence of a plume(s) is indicated in the upper mantle which may be responsible for these observations. Here we reprocess a pre-existing deep seismic reflection profile '9HR' and interpret the subsurface structures as mapped by seismic reflectivity with previous findings, mainly from seismological and geochemical studies, to investigate the geodynamic activity in the subsurface. We find prominent hints of pathways which may allow magmatic fluids originating in the upper mantle to rise through the crust and cause the observed fluid emanations and earthquake activity.

  6. Probing depth dependencies of melt emplacement on time dependent quantities in a continental rift scenario with melting and melt extraction

    NASA Astrophysics Data System (ADS)

    Wallner, Herbert; Schmeling, Harro

    2014-05-01

    intruded or infiltrated, solidified melt modifies composition and physical properties of the affected lithosphere. Above a critical fraction limit melt is extracted and intruded above. The uppermost front of extraction, petrophysically seen as LAB, defines the lower boundary of the emplacement zone. The upper boundary is related to various quantities, particularly temperature, melt curve, melt front, stress, dynamic pressure and more. Changes of intrusion level imply different convection patterns affecting intensity of erosion of the lower lithosphere, doming rate of asthenosphere and melt-induced weakening. Thus, the shape and location and therefore its dependence influences intensively the dynamics of rifting.

  7. Structural and stratigraphic evolution of the Anza rift, Kenya

    NASA Astrophysics Data System (ADS)

    Bosworth, William; Morley, Chris K.

    1994-09-01

    The Anza rift is a large, multi-phase continental rift basin that links the Lamu embayment of southern Kenya with the South Sudan rifts. Extension and deposition of syn-rift sediments are known to have commenced by the Neocomian. Aptian-Albian strata have, thus far, not been encountered during limited drilling campaigns and, in at least one well, are replaced by a significant unconformity. Widespread rifting occurred during the Cenomanian to Maastrichtian, and continued into the Early Tertiary. Marine waters appear to have reached the central Anza rift in the Cenomanian, and a second marine incursion may have occurred during the Campanian. As no wells have yet reached basement in the basinal deeps, the possibility exists that the Anza rift may have initiated in the Late Jurassic, in conjunction with extension to the south in the Lamu embayment and to the north in the Blue Nile rift of Sudan. Structural and stratigraphic evolution in the Anza rift followed a pattern that has now been inferred in several rift settings. Early phases of extension were accommodated by moderately dipping faults that produced large stratal rotations. Sedimentary environments were dominantly fluvial, with associated small lakes and dune fields. Volcanic activity is documented for the early Neocomian, but its extent is unknown. This initial style of deformation and sedimentation may have continued through several of the earliest pulses of rifting. By the Late Cretaceous, a new system of steeply dipping faults was established, that produced a deep basin without significant rotation of strata in the north, and only minor rotation in the south. This basin geometry favored the establishment of large, deep lakes, which occasionally were connected to the sea. The older basins were partly cannibalized during the sedimentary in-filling of these successor basins. Early Senonian volcanism was encountered in one well, and reflection seismic evidence suggests that one or more thick, regionally

  8. Boundary separating the seismically active reelfoot rift from the sparsely seismic Rough Creek graben, Kentucky and Illinois

    USGS Publications Warehouse

    Wheeler, R.L.

    1997-01-01

    The Reelfoot rift is the most active of six Iapetan rifts and grabens in central and eastern North America. In contrast, the Rough Creek graben is one of the least active, being seismically indistinguishable from the central craton of North America. Yet the rift and graben adjoin. Hazard assessment in the rift and graben would be aided by identification of a boundary between them. Changes in the strikes of single large faults, the location of a Cambrian transfer zone, and the geographic extent of alkaline igneous rocks provide three independent estimates of the location of a structural boundary between the rift and the graben. The boundary trends north-northwest through the northeastern part of the Fluorspar Area Fault Complex of Kentucky and Illinois, and has no obvious surface expression. The boundary involves the largest faults, which are the most likely to penetrate to hypocentral depths, and the boundary coincides with the geographic change from abundant seismicity in the rift to sparse seismicity in the graben. Because the structural boundary was defined by geologic variables that are expected to be causally associated with seismicity, it may continue to bound the Reelfoot rift seismicity in the future.

  9. Geochemistry of the metavolcanic rocks in the vicinity of the MacLellan Au-Ag deposit and an evaluation of the tectonic setting of the Lynn Lake greenstone belt, Canada: Evidence for a Paleoproterozoic-aged rifted continental margin

    NASA Astrophysics Data System (ADS)

    Glendenning, Michael W. P.; Gagnon, Joel E.; Polat, Ali

    2015-09-01

    The Paleoproterozoic (ca. 1900 Ma) Lynn Lake greenstone belt of northern Manitoba, Canada, has been previously characterized as comprising a series of tectonically juxtaposed intra-oceanic-derived metavolcanic rocks. The results of more recent local and regional studies, however, support a significant contribution of continental crust during formation of the metasedimentary, metavolcanic, and intrusive igneous rocks that comprise the majority of the Lynn Lake greenstone belt. The tectonic model previously proposed for the Lynn Lake greenstone belt, however, did not consider the geodynamics of the Lynn Lake greenstone belt in the context of all available data. In this study, we report the results of outcrop mapping and petrographic analysis, as well as major, minor, and trace element geochemical analyses for 54 samples from the Northern terrane, and integrate and compare the results with data from previously published studies. These data are used to recharacterize the metavolcanic rocks and to develop a new geodynamic model for the formation of the Lynn Lake greenstone belt. Ultramafic to intermediate rocks in the vicinity of the MacLellan Au-Ag deposit are characterized primarily by E-MORB-like trace element characteristics and Th-Nb-La systematics, which are interpreted to be the result of a primary, plume-derived melt interacting with continental lithosphere at a thinned (i.e., rifted) continental margin. Similarly, the majority of the mafic to intermediate rocks that comprise the Lynn Lake greenstone belt are characterized by flat to E-MORB-like trace element patterns and Th-Nb-La systematics, which are consistent with mantle plume-derived, contaminated, oceanic continental rift or rifted margin setting rocks. This study suggests that the metavolcanic rocks of the Lynn Lake greenstone belt were derived via rifting between the Superior and Hearne Cratons, which resulted in the formation and growth of the Manikewan Ocean. Alternatively, the metavolcanic rocks

  10. Salt tectonics in the SW Alps (Italy-France): From rifting to the inversion of the European continental margin in a context of oblique convergence

    NASA Astrophysics Data System (ADS)

    Decarlis, A.; Maino, M.; Dallagiovanna, G.; Lualdi, A.; Masini, E.; Seno, S.; Toscani, G.

    2014-12-01

    The SW Alps result from the inversion of the European continental margin during the oblique convergence between Europe and Adria since the Cretaceous. The orogenic deformation is controlled by two factors: the inherited sedimentary and structural record and the geodynamic interaction between the two plates. In this paper we present a stratigraphic and structural analysis of the external SW Alps (Ventimiglia-Menton area) in order to define the sedimentary and deformational geometries of the chain and to reconstruct the evolutionary history. The field-data highlight the preeminent role played by inherited salt-structures, which derive from the depositional history experienced by the European margin since the Mesozoic onwards. From Late Triassic to Jurassic, evaporites and carbonates deposited as a response to the Thetyan rifting. The following emplacement of the Cretaceous flysch and of the Eocene foreland basin succession was strongly influenced by the extensionally-triggered salt diapirism and by the interactions with deformations connected to the Pyrenees dynamics. The resulting geologic discontinuities (i.e. diapir-induced highs and basins, inherited normal and trasform faults) strongly influenced the successive Oligo-Miocene evolution of the belt in the study area. Observed changes in the thrusts and folds kinematics are considered as the results of rotation during their approaching to inherited highs. Furthermore, the overturning of thrusts and folds in the front of the diapiric flanks are associated with the progressively salt squeezing into the anticlines cores promoted by ongoing Alpine compression. Finally, the kinematic data from the study area show radical differences in the tectonic transport direction with respect to the rest of the SW Alps (NW- to W-ward in the Ventimiglia-Menton area, S- to SW-ward in Provence and Ligurian Alps). This difference is interpreted to be caused by the relative motions of crustal blocks dominated by transpressive tectonics

  11. Initiation of Extension in South China Continental Margin during the Active-Passive Margin Transition: Thermochronological and Kinematic Constraints

    NASA Astrophysics Data System (ADS)

    Zuo, X.; Chan, L. S.

    2015-12-01

    The South China continental margin is characterized by a widespread magmatic belt, prominent NE-striking faults and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from active to passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in this area. Our zircon fission-track data in this region show two episodes of exhumation: The first episode, occurring during 170-120Ma, affected local parts of the Nanling Range. The second episode, a more regional exhumation event, occurred during 115-70Ma, including the Yunkai Terrane and the Nanling Range. Numerical geodynamic modeling was conducted to simulate the subduction between the paleo-Pacific plate and the South China Block. The modeling results could explain the fact that exhumation of the granite-dominant Nanling Range occurred earlier than that of the gneiss-dominant Yunkai Terrane. In addition to the difference in rock types, the heat from Jurassic-Early Cretaceous magmatism in Nanling may have softened the upper crust, causing the area to exhume more readily than Yunkai. Numerical modeling results also indicate that (1) high lithospheric geothermal gradient, high slab dip angle and low convergence velocity favor the reversal of crustal stress state from compression to extension in the upper continental plate; (2) late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The inversion of stress regime in the continental crust from compression to crustal extension imply that the Late Cretaceous-early Paleogene red-bed basins in South China could have formed during the late stage of the subduction, accounting for the occurrence of volcanic events in some sedimentary basins. We propose that the rifting started as early as Late Cretaceous, probably before the cessation of subduction process.

  12. 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

  13. 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

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

    USGS Publications Warehouse

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

    1995-01-01

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

  15. Geophysical studies of the West Antarctic Rift System

    NASA Astrophysics Data System (ADS)

    Behrendt, J. C.; Lemasurier, W. E.; Cooper, A. K.; Tessensohn, F.; TréHu, A.; Damaske, D.

    1991-12-01

    and the Byrd Subglacial Basin areas). The near absence of earthquakes in the West Antarctic rift system probably results from a combination of primarily sparse seismograph coverage and, secondarily, suppression of earthquakes by the ice sheet (e.g., Johnston, 1987) and very high seismicity shortly after deglaciation in the Ross Embayment followed by abnormally low seismicity at present (e.g., Muir Wood, 1989). The evidence of high temperatures at shallow depth beneath the Ross Sea continental shelf and adjacent Transantarctic Mountains is supportive of thermal uplift of the mountains associated with lateral heat conduction from the rift and can possibly also explain the volcanism, rifting, and high elevation of the entire rift shoulder to the Ellsworth-Horlick-Whitmore Mountains. We infer that the Gondwana breakup and the West Antarctic rift are part of a continuously propagating rift that started in the Jurassic when Africa separated from East Antarctica (including the failed Jurassic Transantarctic rift). Rifting proceeded clockwise around East Antarctica to the separation of New Zealand and the Campbell Plateau about 85-95 Ma and has continued (with a spreading center jump) to its present location in the Ross Embayment and West Antarctica. The Cenozoic activity of the West Antarctic rift system appears to be continuous in time with rifting in the same area that began only in the late Mesozoic. Although the mechanism for rifting is not completely explained, we suggest a combination of the flexural rigidity model (Stem and ten Brink, 1989) proposed for the Ross Embayment and the thermal plume or hot spot concepts. The propagating rift may have been "captured" by the thermal plume.

  16. Thermomechanical models of the Rio Grande rift

    SciTech Connect

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

    1980-01-01

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

  17. The geology and geophysics of the Oslo rift

    NASA Technical Reports Server (NTRS)

    Ruder, M. E.

    1981-01-01

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

  18. Distribution of deformation on an active normal fault network, NW Corinth Rift

    NASA Astrophysics Data System (ADS)

    Ford, Mary; Meyer, Nicolas; Boiselet, Aurélien; Lambotte, Sophie; Scotti, Oona; Lyon-Caen, Hélène; Briole, Pierre; Caumon, Guillaume; Bernard, Pascal

    2013-04-01

    Over the last 20-25 years, geodetic measurements across the Gulf of Corinth have recorded high extension rates varying from 1.1 cm/a in the east to a maximum of 1.6 cm/a in the west. Geodetic studies also show that current deformation is confined between two relatively rigid blocks defined as Central Greece (to the north) and the Peloponnesus to the south. Active north dipping faults (<1 Ma) define the south coast of the subsiding Gulf, while high seismicity (major earthquakes and micro-seismicity) is concentrated at depth below and to the north of the westernmost Gulf. How is this intense deformation distributed in the upper crust? Our objectives here are (1) to propose two models for the distribution of deformation in the upper crust in the westernmost rift since 1 Ma, and (2) to place the tectonic behaviour of the western Gulf in the context of longer term rift evolution. Over 20 major active normal faults have been identified in the CRL area based specific characteristics (capable of generating earthquakes M> 5.5, active in the last 1 M yrs, slip rate >0.5 mm/a). Because of the uncertainty related to fault geometry at depth two models for 3D fault network geometry in the western rift down to 10 km were constructed using all available geophysical and geological data. The first model assumes planar fault geometries while the second uses listric geometries for major faults. A model for the distribution of geodetically-defined extension on faults is constructed along five NNE-SSW cross sections using a variety of data and timescales. We assume that the role of smaller faults in accommodating deformation is negligible so that extension is fully accommodated on the identified major faults. Uncertainties and implications are discussed. These models provide estimates of slip rate for each fault that can be used in seismic hazard models. A compilation of onshore and offshore data shows that the western Gulf is the youngest part of the Corinth rift having initiated

  19. Detection of Rift Valley fever viral activity in Kenya by satellite remote sensing imagery

    NASA Technical Reports Server (NTRS)

    Linthicum, Kenneth J.; Bailey, Charles L.; Davies, F. Glyn; Tucker, Compton J.

    1987-01-01

    Data from the advanced very high resolution radiometer on board the National Oceanic and Atmospheric Administration's polar-orbiting meteorological satellites have been used to infer ecological parameters associated with Rift Valley fever (RVF) viral activity in Kenya. An indicator of potential viral activity was produced from satellite data for two different ecological regions in Kenya, where RVF is enzootic. The correlation between the satellite-derived green vegetation index and the ecological parameters associated with RVF virus suggested that satellite data may become a forecasting tool for RVF in Kenya and, perhaps, in other areas of sub-Saharan Africa.

  20. Detection of Rift Valley fever viral activity in Kenya by satellite remote sensing imagery.

    PubMed

    Linthicum, K J; Bailey, C L; Davies, F G; Tucker, C J

    1987-03-27

    Data from the advanced very high resolution radiometer on board the National Oceanic and Atmospheric Administration's polar-orbiting meteorological satellites have been used to infer ecological parameters associated with Rift Valley fever (RVF) viral activity in Kenya. An indicator of potential viral activity was produced from satellite data for two different ecological regions in Kenya, where RVF is enzootic. The correlation between the satellite-derived green vegetation index and the ecological parameters associated with RVF virus suggested that satellite data may become a forecasting tool for RVF in Kenya and, perhaps, in other areas of sub-Saharan Africa. PMID:3823909

  1. Rift Valley fever virus infection induces activation of the NLRP3 inflammasome

    PubMed Central

    Ermler, Megan E.; Traylor, Zachary; Patel, Krupen; Schattgen, Stefan A.; Vanaja, Sivapriya K.; Fitzgerald, Katherine A.; Hise, Amy G.

    2014-01-01

    Inflammasome activation is gaining recognition as an important mechanism for protection during viral infection. Here, we investigate whether Rift Valley fever virus, a negative-strand RNA virus, can induce inflammasome responses and IL-1β processing in immune cells. We have determined that RVFV induces NLRP3 inflammasome activation in murine dendritic cells, and that this process is dependent upon ASC and caspase-1. Furthermore, absence of the cellular RNA helicase adaptor protein MAVS/IPS-1 significantly reduces extracellular IL-1β during infection. Finally, direct imaging using confocal microscopy shows that the MAVS protein co-localizes with NLRP3 in the cytoplasm of RVFV infected cells. PMID:24418550

  2. 76 FR 58273 - Agency Information Collection Activities; Proposed Collection; Comment Request; Outer Continental...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2011-09-20

    ... Collection Activities; Proposed Collection; Comment Request; Outer Continental Shelf Air Regulations; EPA ICR... all outer continental shelf (OCS) sources except those located in the Gulf of Mexico west of 87.5... boundary extends three leagues (about nine miles) from the coastline. Title: Outer Continental Shelf...

  3. 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

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

    NASA Astrophysics Data System (ADS)

    Tesfaye, Samson; Ghebreab, Woldai

    2013-11-01

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

  5. Active Extensional Faulting at the Southern Half-Graben Belt of the Tepic-Zacoalco Rift, Western Mexico

    NASA Astrophysics Data System (ADS)

    Rosas-Elguera, J.; Ferrari, L.; Delgado, M.; Uribe, A.; Valdivia, L.; Castillo, R.

    2003-12-01

    In the past decade much debate has centered upon the kinematics and the mechanism of continental deformation in western Mexico and the motion of the Jalisco block relative to North America. Two distinct models have been proposed. The first one suggest a NW-motion of the Jalisco block that would implies a right-lateral faulting along the Tepic-Zacoalco rift (TZR). More recently others authors have documented a N-NE extensional tectonics active since late Miocene and suggested that the continental boundaries of the Jalisco block, are older structures reactivated by plate boundary forces. Studies on the crustal seismicity and the kinematics of Quaternary faults provide another constraint on the direction of motion between the Jalisco block and North America. On November 4, 5, 6, and 7, 1995, one month after the October 09, 1995, Manzanillo earthquake (Mw = 8.0), a swarm of small events was felt in the Amatlan de Ca¤as half-graben and recorded by the regional seismic network of Comision Federal de Electricidad. The coda magnitude of the largest event was Mc = 2.5-3.6 and the events were located depth ranging from 6 to 10 km. This seismic activity provoked that people from Pie de la Cuesta and Yerbabuena villages were evacuated. After that a seismic station equipped with an analogic seismograph MEQ-800 at Pie de la Cuesta was installed for three months. During the same time, October, 1995, some houses distributed along a WNW trend in Ameca city underwent severe damages, they are. The digital elevations model of the Ameca city suggest that several structures tectonics are shorter than 2 km are present in the area. The present direction of motion of the Rivera plate relative to North America plate along Middle America Trench has been estimated between N19° E to N48° E (e.g. Bandy et al., 1996). During the October 09, 1995, subduction-related earthquake (Mw = 8.0) a GPS network recorded a SW motion of the Jalisco block which could be associated to an elastic deformation

  6. Mantle thermal structure and active upwelling during continental breakup in the North Atlantic

    NASA Astrophysics Data System (ADS)

    Holbrook, W. Steven; Larsen, H. C.; Korenaga, J.; Dahl-Jensen, T.; Reid, I. D.; Kelemen, P. B.; Hopper, J. R.; Kent, G. M.; Lizarralde, D.; Bernstein, S.; Detrick, R. S.

    2001-08-01

    Seismic reflection and refraction data acquired on four transects spanning the Southeast Greenland rifted margin and Greenland-Iceland Ridge (GIR) provide new constraints on mantle thermal structure and melting processes during continental breakup in the North Atlantic. Maximum igneous crustal thickness varies along the margin from >30 km in the near-hotspot zone (<500 km from the hotspot track) to ˜18 km in the distal zone (500-1100 km). Magmatic productivity on summed conjugate margins of the North Atlantic decreases through time from 1800±300 to 600±50 km 3/km/Ma in the near-hotspot zone and from 700±200 to 300±50 km 3/km/Ma in the distal zone. Comparison of our data with the British/Faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist. Joint consideration of crustal thickness and mean crustal seismic velocity suggests that along-margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature. The thermal anomaly (Δ T) at breakup was modest (˜100-125°C), varied little along the margin, and transient. Data along the GIR indicate that the potential temperature anomaly (125±50°C) and upwelling ratio (˜4 times passive) of the Iceland hotspot have remained roughly constant since 56 Ma. Our results are consistent with a plume-impact model, in which (1) a plume of radius ˜300 km and Δ T of ˜125°C impacted the margin around 61 Ma and delivered warm material to distal portions of the margin; (2) at breakup (56 Ma), the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin; and (3) by 45 Ma, both the remaining plume head and the distal warm layer were exhausted, with excess magmatism thereafter largely confined to a narrow (<200 km radius) zone immediately above the Iceland plume stem. Alternatively, the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been

  7. 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

  8. Active fault systems of the Kivu rift and Virunga volcanic province, and implications for geohazards

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    H Zal, C Ebinger, D. Wood, C. Scholz, N. d'Oreye, S. Carn, U. Rutagarama The weakly magmatic Western rift system, East Africa, is marked by fault-bounded basins filled by freshwater lakes that record tectonic and climatic signals. One of the smallest of the African Great Lakes, Lake Kivu, represents a unique geohazard owing to the warm, saline bottom waters that are saturated in methane, as well as two of the most active volcanoes in Africa that effectively dam the northern end of the lake. Yet, the dynamics of the basin system and the role of magmatism were only loosely constrained prior to new field and laboratory studies in Rwanda. In this work, we curated, merged, and analyzed historical and digital data sets, including spectral analyses of merged Shuttle Radar Topography Mission topography and high resolution CHIRP bathymetry calibrated by previously mapped fault locations along the margins and beneath the lake. We quantitatively compare these fault maps with the time-space distribution of earthquakes located using data from a temporary array along the northern sector of Lake Kivu, as well as space-based geodetic data. During 2012, seismicity rates were highest beneath Nyiragongo volcano, where a range of low frequency (1-3 s peak frequency) to tectonic earthquakes were located. Swarms of low-frequency earthquakes correspond to periods of elevated gas emissions, as detected by Ozone Monitoring Instrument (OMI). Earthquake swarms also occur beneath Karisimbi and Nyamuragira volcanoes. A migrating swarm of earthquakes in May 2012 suggests a sill intrusion at the DR Congo-Rwanda border. We delineate two fault sets: SW-NE, and sub-N-S. Excluding the volcano-tectonic earthquakes, most of the earthquakes are located along subsurface projections of steep border faults, and intrabasinal faults calibrated by seismic reflection data. Small magnitude earthquakes also occur beneath the uplifted rift flanks. Time-space variations in seismicity patterns provide a baseline

  9. Basin evolution, organization of faulting and the distribution of displacement within the Gulf of Corinth rift

    NASA Astrophysics Data System (ADS)

    Nixon, C. W.; McNeill, L. C.; Henstock, T.; Bull, J. M.; Bell, R. E.; Christodoulou, D.; Papatheodorou, G.; Taylor, B.; Ferentinos, G.; Sakellariou, D.; Lykousis, V.; Sachpazi, M.; Ford, M.; Goodliffe, A. M.; Leeder, M.; Gawthorpe, R. L.; Collier, R. E.; Clements, B.

    2013-12-01

    The Gulf of Corinth is a rare example of continental rifting in its initial stages of development, with an extremely dense network of marine geophysical data collected over the past two decades, making it an ideal case study for investigating early rift evolution. Through the integration of numerous seismic reflection surveys, totalling ~3930 km of seismic profiles and covering a range of frequencies (both high resolution seismic and multi-channel seismic, analogue and digital), we present: 1. a refined chronostratigraphic model for the syn-rift sediments that have been deposited in the developing offshore Corinth basin over the past ~1-2 Ma and 2. a detailed rift fault network with confirmed locations, lengths, fault interactions and development, and details of recent displacement. Our results show that chronostratigraphic models from the West Eratini basin are coherent with models from the central part of the rift. We divide the rift stratigraphy into two sequences: a late rift sequence comprising recent interbedded marine-lacustrine sediments deposited over the last ~600 kyr, and a thick early rift sequence with deposits up to ~1-2 Ma of contrasting seismic and sedimentological character. The late rift sequence is divided into six packages and can be correlated with 100 kyr glacio-eustatic cycles. We identify multiple unconformities, including a basin wide unconformity that separates the early and late rift sequences. The unconformities are attributed to differences in fault development and basin subsidence pattern along the rift. Combining the refined chronostratigraphic model with the detailed fault network allows us to: a) determine relative timings of fault activity and basin development; b) estimate absolute fault displacements both spatially and temporally at high resolution (e.g. for each interpreted 100 kyr package); c) calculate sediment flux into the basin during each stratigraphic time interval and spatial distribution of syn-rift sediment through

  10. Investigating Continental Margins: An Activity to Help Students Better Understand the Continental Margins of North America

    ERIC Educational Resources Information Center

    Poli, Maria-Serena; Capodivacca, Marco

    2011-01-01

    Continental margins are an important part of the ocean floor. They separate the land above sea level from the deep ocean basins below and occupy about 11% of Earth's surface. They are also economically important, as they harbor both mineral resources and some of the most valuable fisheries in the world. In this article students investigate North…

  11. Estimation of age of Dali-Ganis rifting and associated volcanic activity, Venus

    NASA Technical Reports Server (NTRS)

    Basilevsky, A. T.

    1993-01-01

    This paper deals with the estimation of age for the Dali and Ganis Chasma rift zones and their associated volcanism based on photogeologic analysis of stratigraphic relations of rift-associated features with impact craters which have associated features indicative of their age. The features are radar-dark and parabolic, and they are believed to be mantles of debris derived from fallout of the craters' ejecta. They are thought to be among the youngest features on the Venusian surface, so their 'parent' craters must also be very young, evidently among the youngest 10 percent of Venus' crater population. Dali Chasma and Ganis Chasma are a part of a system of rift zones contained within eastern Aphrodite and Atla Regio which is a significant component of Venus tectonics. The rifts of this system are fracture belts which dissect typical Venusian plains with rare islands of tessera terrain. The rift zone system consists of several segments following each other (Diane, Dali, Ganis) and forming the major rift zone line, about 10,000 km long, which has junctions with several other rift zones, including Parga Chasma Rift. The junctions are usually locations of rift-associated volcanism in the form of volcanic edifices (Maat and Ozza Montes) or plain-forming flows flooding some areas within the rift zones and the adjacent plains.

  12. Ambient noise tomography of the western Corinth Rift, Greece

    NASA Astrophysics Data System (ADS)

    Giannopoulos, Dimitrios; Rivet, Diane; Sokos, Efthimios; Deschamps, Anne; Paraskevopoulos, Paraskevas; Lyon-Caen, Hélène; Pascal, Bernard; Tselentis, G.-Akis

    2016-04-01

    The Corinth Rift separates Peloponnesus to the south from main-land Greece to the north. It is one of the most active extensional intra-continental rifts in the world, with geodetically measured rates of extension varying from ~5 mm/yr at the eastern part to ~15 mm/yr at the western part. This work presents a first attempt to study the crustal velocity structure of the western Corinth Rift using ambient noise recordings. We used 3 yrs (01/2012-12/2014) of continuous waveform data recorded at 24 stations from the Corinth Rift Laboratory (CRL) and the Hellenic Unified Seismological Network (HUSN). All available vertical component time-series were cross-correlated to extract Rayleigh wave Green's functions. Group velocity dispersion curves between 0.5 and 7 s period were measured for each station pair by applying frequency-time analysis and then inverted to build group velocity maps of the study area. At the studied periods, the northern coast of the Corinth Rift is generally imaged as a region of elevated seismic velocities compared to the southern coast. More specifically, low velocities are observed in areas of Plio-Quaternary syn-rift sediments such as off-shore regions of the rift, the Mornos delta and a large part of the southern coast. Higher velocities are observed in pre-rift basement structures which are dominated mostly by carbonates. The preliminary results demonstrate good agreement with the major geological features of the area and agree relatively well with previous local earthquake tomography studies. This work will be the base for further investigations towards the study of the Corinth Rift structure using long time-series of ambient noise data.

  13. Magma storage depths beneath an active rift volcano in Afar (Dabbahu), constrained by melt inclusion analyses, seismicity and Interferometric Synthetic Aperture Radar (INSAR)

    NASA Astrophysics Data System (ADS)

    Field, L.; Blundy, J.; Wright, T. J.; Yirgu, G.; Afar Consortium

    2010-12-01

    Dabbahu volcano is located at the northern end of the active Manda Hararo rift segment in western Afar, Ethiopia. In 2005 a major rifting episode began in the segment, which has been modelled as basalt dyke injections (1). Seismic activity, inflation and deflation have been recorded at the volcano. The aim of this research is to provide an insight into the history and evolution of a silicic magmatic centre in the rift, and to contribute to the wider aims of the NERC Afar Consortium to track the creation, migration, evolution and emplacement of magma from the asthenosphere to the crust. The volatile contents of rare melt inclusions trapped within phenocrysts of alkali feldspar, clinopyroxene and olivine from Dabbahu have been studied using secondary ion mass spectrometry. The host lavas are mildly peralkaline obsidians, which, based on field evidence and preliminary results from 40Ar-39Ar dating, represent the youngest samples on the volcano (<4 ka). Whilst the obsidian and pumice groundmass glasses are largely degassed, the H2O contents of the analysed inclusions are up to 5.8 wt%. CO2 contents are generally low; <462 ppm in the alkali feldspar-hosted inclusions, but higher values (up to 1457 ppm) have been found in the clinopyroxene-hosted inclusions. The pressure (and depth) of pre-eruptive magma storage beneath Dabbahu has been constrained using H2O and CO2 data, which suggest shallow magma storage at depths of ~1 - 5 km below the surface. These depths are consistent with observations from recorded seismicity and InSAR at Dabbahu. Seismicity has been recorded from deformation caused by deflation of the magma chamber following the 2005 dyke emplacement event (Oct 2005 - Apr 2006)(2) and InSAR has monitored deflation and subsequent steady inflation after this event. We show that melt inclusions accurately record a stable, shallow magma chamber as corroborated by remote sensing and geophysical observations at Dabbahu volcano. 1 Ayele et al. 2009 ‘September 2005

  14. Late Mesozoic and Cenozoic rifting and its dynamic setting in Eastern China and adjacent areas

    NASA Astrophysics Data System (ADS)

    Ren, Jianye; Tamaki, Kensaku; Li, Sitian; Junxia, Zhang

    2002-02-01

    During the Late Mesozoic and Cenozoic, extension was widespread in Eastern China and adjacent areas. The first rifting stage spanned in the Late Jurassic-Early Cretaceous times and covered an area of more than 2 million km 2 of NE Asia from the Lake Baikal to the Sikhot-Alin in EW direction and from the Mongol-Okhotsk fold belt to North China in NS direction. This rifting was characterized by intracontinental rifts, volcanic eruptions and transform extension along large-scale strike-slip faults. Based on the magmatic activity, filling sequence of basins, tectonic framework and subsidence analysis of basins, the evolution of this area can be divided into three main developmental phases. The first phase, calc-alkaline volcanics erupted intensely along NNE-trending faults, forming Daxing'anling volcanic belt, NE China. The second phase, Basin and Range type fault basin system bearing coal and oil developed in NE Asia. During the third phase, which was marked by the change from synrifting to thermal subsidence, very thick postrift deposits developed in the Songliao basin (the largest oil basin in NE China). Following uplift and denudation, caused by compressional tectonism in the near end of Cretaceous, a Paleogene rifting stage produced widespread continental rift systems and continental margin basins in Eastern China. These rifted basins were usually filled with several kilometers of alluvial and lacustrine deposits and contain a large amount of fossil fuel resources. Integrated research in most of these rifting basins has shown that the basins are characterized by rapid subsidence, relative high paleo-geothermal history and thinned crust. It is now accepted that the formation of most of these basins was related to a lithospheric extensional regime or dextral transtensional regime. During Neogene time, early Tertiary basins in Eastern China entered a postrifting phase, forming regional downwarping. Basin fills formed in a thermal subsidence period onlapped the fault

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  16. Rift-drift evolution of the outer Norwegian margin

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

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

  17. Seismic Imaging of a Continental Intraplate: Long-Term Persistence of Fossil Rifts and Hot Spots in the Central and Eastern United States

    NASA Astrophysics Data System (ADS)

    Pollitz, F. F.; Mooney, W. D.

    2015-12-01

    Seismic surface waves from the Transportable Array of Earthscope's USArray are used to estimate phase velocity structure of 18 to 125s Rayleigh waves, then corrected for lateral crustal thickness variations (with CRUST1.0) and inverted to obtain three-dimensional crust and upper mantle structure of the Central and Eastern United States (CEUS) down to ~200 km. The obtained lithosphere structure confirms previously imaged features in the CEUS, e.g., the low seismic velocity signature of Proterozoic to Cambrian fossil rifts, the very low velocity at >150 km depth below an Eocene volcanic center in northwestern Virginia, and the very low velocity along a corridor stretching from eastern New York to New Hampshire. The model also reveals new features. The high-velocity Granite-Rhyolite Province sharply bounds the Grenville front at mid-lithosphere depth, suggesting that it acted as a backstop during the Grenville orogeny ca. 1.2 - 1.0 Ga. High-velocity mantle extending ˜ 200 km deep stretches from the Archean Superior Craton well into the Proterozoic terrains (Granite-Rhyolite, Mazatzal and Yavapai provinces). This is consistent with independent seismic velocity images and suggests that the thickness of Proterozoic lithosphere is generally ˜ 200 km. A deep low-velocity zone in central Texas is associated with the late Cretaceous Travis and Uvalde volcanic fields, and a similar deep low-velocity zone is located beneath the South Georgia Rift, which contains Jurassic basalts associated with the Central Atlantic magmatic province. Hotspot tracks may be associated with several of the low-velocity zones, and the central Texas, New York-New Hampshire, and southern Georgia zones may also be associated with the former rifted Laurentia margin. This suggests a systematic pattern whereby transient mantle thermal perturbations are accentuated near former failed rifts or rift margins.

  18. 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.

  19. Modelling Rift Valley fever (RVF) disease vector habitats using active and passive remote sensing systems

    NASA Technical Reports Server (NTRS)

    Ambrosia, Vincent G.; Linthicum, K. G.; Bailey, C. L.; Sebesta, P.

    1989-01-01

    The NASA Ames Ecosystem Science and Technology Branch and the U.S. Army Medical Research Institute of Infectious Diseases are conducting research to detect Rift Valley fever (RVF) vector habitats in eastern Africa using active and passive remote-sensing. The normalized difference vegetation index (NDVI) calculated from Landsat TM and SPOT data is used to characterize the vegetation common to the Aedes mosquito. Relationships have been found between the highest NDVI and the 'dambo' habitat areas near Riuru, Kenya on both wet and dry data. High NDVI values, when combined with the vegetation classifications, are clearly related to the areas of vector habitats. SAR data have been proposed for use during the rainy season when optical systems are of minimal use and the short frequency and duration of the optimum RVF mosquito habitat conditions necessitate rapid evaluation of the vegetation/moisture conditions; only then can disease potential be stemmed and eradication efforts initiated.

  20. Broad Spectrum Antiviral Activity of Favipiravir (T-705): Protection from Highly Lethal Inhalational Rift Valley Fever

    PubMed Central

    Caroline, Amy L.; Powell, Diana S.; Bethel, Laura M.; Oury, Tim D.; Reed, Douglas S.; Hartman, Amy L.

    2014-01-01

    Background Development of antiviral drugs that have broad-spectrum activity against a number of viral infections would be of significant benefit. Due to the evolution of resistance to currently licensed antiviral drugs, development of novel anti-influenza drugs is in progress, including Favipiravir (T-705), which is currently in human clinical trials. T-705 displays broad-spectrum in vitro activity against a number of viruses, including Rift Valley Fever virus (RVFV). RVF is an important neglected tropical disease that causes human, agricultural, and economic losses in endemic regions. RVF has the capacity to emerge in new locations and also presents a potential bioterrorism threat. In the current study, the in vivo efficacy of T-705 was evaluated in Wistar-Furth rats infected with the virulent ZH501 strain of RVFV by the aerosol route. Methodology/Principal Findings Wistar-Furth rats are highly susceptible to a rapidly lethal disease after parenteral or inhalational exposure to the pathogenic ZH501 strain of RVFV. In the current study, two experiments were performed: a dose-determination study and a delayed-treatment study. In both experiments, all untreated control rats succumbed to disease. Out of 72 total rats infected with RVFV and treated with T-705, only 6 succumbed to disease. The remaining 66 rats (92%) survived lethal infection with no significant weight loss or fever. The 6 treated rats that succumbed survived significantly longer before succumbing to encephalitic disease. Conclusions/Significance Currently, there are no licensed antiviral drugs for treating RVF. Here, T-705 showed remarkable efficacy in a highly lethal rat model of Rift Valley Fever, even when given up to 48 hours post-infection. This is the first study to show protection of rats infected with the pathogenic ZH501 strain of RVFV. Our data suggest that T-705 has potential to be a broad-spectrum antiviral drug. PMID:24722586

  1. Tectonic Controls on the Volumes and Petrologic Evolution of Pantellerite-Trachyte-Phonolite Volcanoes in a Continental Rift Setting, Marie Byrd Land, Antarctica

    NASA Astrophysics Data System (ADS)

    Lemasurier, W. E.

    2010-12-01

    The 18 alkaline volcanoes in Marie Byrd Land (MBL) are characterized by large volumes of felsic rock and a large range in composition, from trachyte to pantellerite, comendite, and phonolite. These characteristics are controlled largely by mantle plume activity, a stationary plate environment, and lithospheric structure of the West Antarctic rift system in coastal MBL. Felsic rocks occur either as summit sections of basalt volcanoes, or comprising all the rock exposed above ice level. Their exposed volumes range from ~30-780 km3 in individual volcanoes. Seven fall between 200-780 km3. Field observations and one seismic traverse show that felsic sections are underlain by thick (1-5 km) sequences of basalt, dominated by basanite. Thus, in spite of their large volumes, felsic rocks appear to make up only ~10% of all the volcanic rock in the province. In four of these volcanoes, pantellerite, trachyte, and phonolite eruptions alternated with each other, and/or took place coevally from the same edifice, indicating that these magmas were available throughout the ~15 m.y. histories of these volcanoes from isolated, but closely adjacent upper crustal reservoirs. Isotope data record crustal contamination in some felsic rocks, but constrain it to <3%, at most. This, plus the results of major and trace element modeling, imply that pantellerites, trachytes and phonolites all evolved from basanite magma by fractional crystallization (FC). Phonolites could have evolved largely by low-pressure FC of basanite in the upper crust. However, modeling and experimental data suggest that 90-95% of pantellerite evolution took place below the crust, where inclusion of kaersutite among fractionated phases, in a low fO2 environment, were key to developing an FeO-rich and SiO2-rich pantellerite lineage from basanite. The complexity of the felsic suite seems related to the presence of mechanical boundaries at the base of the lithosphere (~50 km) and base of the crust (~25 km), that trapped

  2. Temporal evolution of continental lithospheric strength in actively deforming regions

    USGS Publications Warehouse

    Thatcher, W.; Pollitz, F.F.

    2008-01-01

    It has been agreed for nearly a century that a strong, load-bearing outer layer of earth is required to support mountain ranges, transmit stresses to deform active regions and store elastic strain to generate earthquakes. However the dept and extent of this strong layer remain controversial. Here we use a variety of observations to infer the distribution of lithospheric strength in the active western United States from seismic to steady-state time scales. We use evidence from post-seismic transient and earthquake cycle deformation reservoir loading glacio-isostatic adjustment, and lithosphere isostatic adjustment to large surface and subsurface loads. The nearly perfectly elastic behavior of Earth's crust and mantle at the time scale of seismic wave propagation evolves to that of a strong, elastic crust and weak, ductile upper mantle lithosphere at both earthquake cycle (EC, ???10?? to 103 yr) and glacio-isostatic adjustment (GIA, ???103 to 104 yr) time scales. Topography and gravity field correlations indicate that lithosphere isostatic adjustment (LIA) on ???106-107 yr time scales occurs with most lithospheric stress supported by an upper crust overlying a much weaker ductile subtrate. These comparisons suggest that the upper mantle lithosphere is weaker than the crust at all time scales longer than seismic. In contrast, the lower crust has a chameleon-like behavior, strong at EC and GIA time scales and weak for LIA and steady-state deformation processes. The lower crust might even take on a third identity in regions of rapid crustal extension or continental collision, where anomalously high temperatures may lead to large-scale ductile flow in a lower crustal layer that is locally weaker than the upper mantle. Modeling of lithospheric processes in active regions thus cannot use a one-size-fits-all prescription of rheological layering (relation between applied stress and deformation as a function of depth) but must be tailored to the time scale and tectonic

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

    NASA Astrophysics Data System (ADS)

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

    2004-07-01

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

  4. 78 FR 27427 - Outer Continental Shelf (OCS) Geological and Geophysical Exploration Activities in the Gulf of...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2013-05-10

    ... Bureau of Ocean Energy Management Outer Continental Shelf (OCS) Geological and Geophysical Exploration... ] Geological and Geophysical Exploration for Mineral Resources on the Gulf of Mexico Outer Continental Shelf... activities in GOM waters. It will also provide information for future decisions regarding Outer...

  5. 78 FR 33859 - Outer Continental Shelf (OCS) Geological and Geophysical Exploration Activities in the Gulf of...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2013-06-05

    ... Bureau of Ocean Energy Management Outer Continental Shelf (OCS) Geological and Geophysical Exploration.... SUMMARY: On May 10, 2013, BOEM published a document in the Federal Register (78 FR 27427) entitled ``Outer Continental Shelf Geological and Geophysical Exploration Activities in the Gulf of Mexico.'' This...

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

    USGS Publications Warehouse

    Behrendt, John C.

    1994-01-01

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

  7. The role of Variscan to pre-Jurassic active extension in controlling the architecture of the rifted passive margin of Adria: the example of the Canavese Zone (Western Southern Alps, Italy)

    NASA Astrophysics Data System (ADS)

    Succo, Andrea; De Caroli, Sara; Centelli, Arianna; Barbero, Edoardo; Balestro, Gianni; Festa, Andrea

    2016-04-01

    The Canavese Zone, in the Italian Western Southern Alps, represents the remnant of the Jurassic syn-rift stretching, thinning and dismemberment of the distal passive margin of Adria during the opening of the Penninic Ocean (i.e., Northern Alpine Tethys). Our findings, based on detailed geological mapping, structural analysis and stratigraphic and petrographic observations, document however that the inferred hyper-extensional dismemberment of this distal part of the passive margin of Adria, up to seafloor spreading, was favored by the inherited Variscan geometry and crustal architecture of the rifted margin, and by the subsequent Alpine-related strike-slip deformation. The new field data document, in fact, that the limited vertical displacement of syn-extensional (syn-rift) Jurassic faults was ineffective in producing and justifying the crustal thinning observed in the Canavese Zone. The deformation and thinning of the continental basement of Adria are constrained to the late Variscan time by the unconformable overlying of Late Permian deposits. Late Cretaceous-Early Paleocene and Late Cenozoic strike-slip faulting (i.e., Alpine and Insubric tectonic stages) reactivated previously formed faults, leading to the formation of a complex tectonic jigsaw which only partially coincides with the direct product of the Jurassic syn-rift dismemberment of the distal part of the passive margin of Adria. Our new findings document that this dismemberment of the rifted continental margin of Adria did not simply result from the syn-rift Jurassic extension, but was strongly favored by the inheritance of older (Variscan and post-Variscan) tectonic stages, which controlled earlier lithospheric weakness. The formation of rifted continental margins by extension of continental lithosphere leading to seafloor spreading is a complex and still poorly understood component of the plate tectonic cycle. Geological mapping of rifted continental margins may thus provide significant information to

  8. Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

    NASA Astrophysics Data System (ADS)

    Becker, K.; Franke, D.; Trumbull, R.; Schnabel, M.; Heyde, I.; Schreckenberger, B.; Koopmann, H.; Bauer, K.; Jokat, W.; Krawczyk, C. M.

    2014-10-01

    High-velocity lower crust (HVLC) and seaward-dipping reflector (SDR) sequences are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow for a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal cross-sectional areas of HVLC 4 times larger than at the South American margin, a finding that is opposite to the asymmetric distribution of the flood basalts in the Paraná-Etendeka Large Igneous Province. Also, the position of the HVLC with respect to the SDR sequences varies consistently along both margins. Close to the Falkland-Agulhas Fracture Zone in the south, a small body of HVLC is not accompanied by SDRs. In the central portion of both margins, the HVLC is below the inner SDR wedges while in the northern area, closer to the Rio Grande Rise-Walvis Ridge, large volumes of HVLC extend far seaward of the inner SDRs. This challenges the concept of a simple extrusive/intrusive relationship between SDR sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and breakup process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple-shear-dominated extension.

  9. Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental break-up

    NASA Astrophysics Data System (ADS)

    Becker, K.; Franke, D.; Trumbull, R. B.; Schnabel, M.; Heyde, I.; Schreckenberger, B.; Koopmann, H.; Bauer, K.; Jokat, W.; Krawczyk, C. M.

    2014-06-01

    High-velocity lower crust (HVLC) and seaward dipping reflector sequences (SDRs) are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal four times larger cross sectional areas of HVLC than at the South American margin, a finding that is in sharp contrast to the distribution of the flood basalts in the Paraná-Etendeka Large Igneous Provinces (LIP). Also, the position of the HVLC with respect to the seaward dipping reflector sequences varies consistently along both margins. Close to the Falkland-Agulhas Fracture Zone a small body of HVLC is not accompanied by seaward dipping reflectors. In the central portion of both margins, the HVLC is below the inner seaward dipping reflector wedges while in the northern area, closer to the Rio Grande Rise/Walvis Ridge, large volumes of HVLC extend far seawards of the inner seaward dipping reflectors. This challenges the concept of a simple extrusive/intrusive relationship between seaward dipping reflector sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and break-up process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple shear dominated extension.

  10. High-Ti continental tholeiites from the Aznam trough, northwestern Saudi Arabia: evidence of ``abortive'' rifting in the ``embryonic'' stage of Red Sea opening

    NASA Astrophysics Data System (ADS)

    Suayah, Ismail B.; Rogers, John J. W.; Dabbagh, Mohammad E.

    1991-05-01

    The Aznam trough is a NW-trending, asymmetric graben on the northern Saudi Arabian coast of the Red Sea. Miocene or younger, high-Ti, tholeiitic basalts are exposed In the northern part of the trough. These (Aznam) basalts contain microphenocrysts of altered olivine, calcic plagioclase and clinopyroxene with subordinate amounts of ilmenite and orthopyroxene in a cryptocrystalline groundmass. A minor xenocryst/xenolith assemblage consists of quartz, sieve-textured plagioclase and a variety of crustal rock fragments. The Aznam basalts have TiO 2 contents > 3.0%, elevated 87Sr/ 86Sr ratios (0.70486-0.70546), FeO∗/MgO ratios > 2.0, and Mg-numbers between 47 and 50. They are compositionally and isotopically more evolved than tholeiites (MORB) from the Red Sea trough. Minor crustal contamination of the initial basaltic magmas is shown by the elevated Sr isotopic composition and enrichment in some LIL components. Major-element modeling indicates that the Aznam basalts can be generated by fractionation of the major phenocryst phases out of typical Red Sea MORB. Trace-element distributions are mostly consistent with this conclusion. With the exception of minor crustal contamination at Aznam, the Aznam basalts are compositionally almost identical with high-Ti basalts formed in "abortive," volcanically dormant, rifts in Iceland. Apparently the same process of magmatism and rift jumping that occurred in Iceland also occurred (and may still be occurring) in the northern Red Sea region as a short-lived embryonic rifting that is a precursor to sea-floor spreading in the Red Sea trough.

  11. Alkaline series related to Early-Middle Miocene intra-continental rifting in a collision zone: An example from Polatlı, Central Anatolia, Turkey

    NASA Astrophysics Data System (ADS)

    Temel, Abidin; Yürür, Tekin; Alıcı, Pınar; Varol, Elif; Gourgaud, Alain; Bellon, Hervé; Demirbağ, Hünkar

    2010-06-01

    A large volcanic area (˜7600 km 2), the Galatean Volcanic Province (GVP), developed in northwest Central Anatolia during the Miocene along the Neo-Tethys Ocean suture zone possibly by post-collisional processes. The GVP mainly comprises 20-14 My old acid to intermediate volcanites with a geochemical signature indicating a mantle source modified by earlier (Late Cretaceous) subduction-related events. 100 km south of the GVP, near Polatlı, Ankara, basaltic rocks that cover large areas are intercalated with the Miocene deposits of the Beypazarı basin, an intra-continental subsidence zone at the southwest of the GVP. Field observations, geochemistry and K-Ar age dating of the Polatlı volcanites show that they are Early (19.9 Ma) to mid (14.1 Ma) Miocene in age, covering an area as large as 215 km 2. Variations in lava thickness and the thickness of the underlying silicified/baked zones suggest that the basaltic lavas erupted from a southern source, possibly from the Eskişehir fault zone, and flowed northwards. Most Polatlı samples have chemical compositions that indicate derivation from a mantle source with crustal contamination during ascent. They do not display any characteristic to suggest a subductional component. Although the GVP and Polatlı lavas formed close in time and space, they were derived from different mantle sources. Considering the positions of these two magmatic regions with regard to the Tethyan suture zone, we propose that the mantle beneath the GVP and near the suture zone memorised the earlier subduction while the mantle beneath Polatlı that is located about 100 km further from the suture zone remained apparently unchanged. After a significant volume of magma was consumed in the GVP, a later (˜10 My) and last activity (Güvem activity) has produced quantitatively much less basaltic rocks where this subductional signature seems to completely disappear. Considering that the western Anatolian crust is proposed to undergo extension since the

  12. Large historical earthquakes and tsunamis in a very active tectonic rift: the Gulf of Corinth, Greece

    NASA Astrophysics Data System (ADS)

    Triantafyllou, Ioanna; Papadopoulos, Gerassimos

    2014-05-01

    The Gulf of Corinth is an active tectonic rift controlled by E-W trending normal faults with an uplifted footwall in the south and a subsiding hangingwall with antithetic faulting in the north. Regional geodetic extension rates up to about 1.5 cm/yr have been measured, which is one of the highest for tectonic rifts in the entire Earth, while seismic slip rates up to about 1 cm/yr were estimated. Large earthquakes with magnitudes, M, up to about 7 were historically documented and instrumentally recorded. In this paper we have compiled historical documentation of earthquake and tsunami events occurring in the Corinth Gulf from the antiquity up to the present. The completeness of the events reported improves with time particularly after the 15th century. The majority of tsunamis were caused by earthquake activity although the aseismic landsliding is a relatively frequent agent for tsunami generation in Corinth Gulf. We focus to better understand the process of tsunami generation from earthquakes. To this aim we have considered the elliptical rupture zones of all the strong (M≥ 6.0) historical and instrumental earthquakes known in the Corinth Gulf. We have taken into account rupture zones determined by previous authors. However, magnitudes, M, of historical earthquakes were recalculated from a set of empirical relationships between M and seismic intensity established for earthquakes occurring in Greece during the instrumental era of seismicity. For this application the macroseismic field of each one of the earthquakes was identified and seismic intensities were assigned. Another set of empirical relationships M/L and M/W for instrumentally recorded earthquakes in the Mediterranean region was applied to calculate rupture zone dimensions; where L=rupture zone length, W=rupture zone width. The rupture zones positions were decided on the basis of the localities of the highest seismic intensities and co-seismic ground failures, if any, while the orientation of the maximum

  13. Rates of volcanic activity along the southwest rift zone of Mauna Loa volcano, Hawaii.

    USGS Publications Warehouse

    Lipman, P.W.

    1981-01-01

    Flow-by-flow mapping of the 65 km long subaerial part of the southwest rift zone and adjacent flanks of Mauna Loa Volcano, Hawaii, and about 50 new 14C dates on charcoal from beneath these flows permit estimates of rates of lava accumulation and volcanic growth over the past 10 000 years. The sequence of historic eruptions along the southwest rift zone, beginning in 1868, shows a general pattern of uprift migration and increasing eruptive volume, culminating in the great 1950 eruption. No event comparable to 1950, in terms of volume or vent length, is evident for at least the previous 1000 years. Rates of lava accumulation along the zone have been subequal to those of Kilauea Volcano during the historic period but they were much lower in late prehistoric time (unpubl. Kilauea data by R. T. Holcomb). Rates of surface covering and volcanic growth have been markedly asymmetric along Mauna Loa's southwest rift zone. Accumulation rates have been about half again as great on the northwest side of the rift zone in comparison with the southeast side. The difference apparently reflects a westward lateral shift of the rift zone of Mauna Loa away from Kilauea Volcano, which may have acted as a barrier to symmetrical growth of the rift zone. -Author

  14. 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).

  15. Rift basins of ocean-continent convergent margins

    SciTech Connect

    Forsythe, R.D.; Newcomb, K.R.

    1986-05-01

    Modern and ancient circum-Pacific convergent margins contain many examples of forearc basins where subsidence, occurring simultaneously with subduction of oceanic lithosphere, is controlled by rifting transverse to the margin. The elongate axes of these deep and narrow basins jut obliquely from the plate margin into the interior of the forearc. Similar to aulacogens, faulting and related subsidence appear greatest at their seaward limits and decreases inland. Examples from eastern Pacific forearcs suggest that localized rifting accommodates margin-parallel extension of forearc blocks that are kinetically linked to motions along major margin-parallel strike-slip fault systems. The most prominent examples of modern forearc rift basins are the Sanak and East Sanak basins of the western Alaska Peninsula subduction zone. In this region, the continental shelf is being rifted apart by a series of northwest- and northeast-trending faults. Basement-activated normal faults bounding the basins have listric geometries. Seismostratigraphic relationships within the basins indicate the protracted, synsedimentary, and active nature of faulting and basin subsidence. Along the Peru-Chile trench, two prominent rifted basins also occur: the Gulf of Guayaquil and the Gulf of Penas-Taitao basin of southern Chile. There, margin-parallel rifting controls subsidence in localized basins at the southern terminus to margin-parallel dextral fault systems. These and other examples suggest that strike-slip motion and transverse rifting of forearcs is a common phenomenon inadequately described by existing two-dimensional models of forearcs. Margin-parallel motions of forearc blocks can be related not only to oblique plate convergence, but also to the geometric and compositional nature of the overriding and subducted plates.

  16. Quantifying the Temporal and Spatial Response of Channel Steepness to Changes in Rift Basin Architecture

    NASA Astrophysics Data System (ADS)

    Robinson, Scott M.

    Quantifying the temporal and spatial evolution of active continental rifts contributes to our understanding of fault system evolution and seismic hazards. Rift systems also preserve robust paleoenvironmental records and are often characterized by strong climatic gradients that can be used to examine feedbacks between climate and tectonics. In this thesis, I quantify the spatial and temporal history of rift flank uplift by analyzing bedrock river channel profiles along footwall escarpments in the Malawi segment of the East Africa Rift. This work addresses questions that are widely applicable to continental rift settings: (1) Is rift-flank uplift sufficiently described by theoretical elliptical along-fault displacement patterns? (2) Do orographic climate patterns induced by rift topography affect rift-flank uplift or morphology? (3) How do uplift patterns along rift flanks vary over geologic timescales? In Malawi, 100-km-long border faults of alternating polarity bound half-graben sedimentary basins containing up to 4km of basin fill and water depths up to 700m. Orographically driven precipitation produces climatic gradients along footwall escarpments resulting in mean annual rainfall that varies spatially from 800 to 2500 mm. Temporal oscillations in climate have also resulted in lake lowstands 500 m below the modern shoreline. I examine bedrock river profiles crossing the Livingstone and Usisya Border Faults in northern Malawi using the channel steepness index (Ksn) to assess importance of these conditions on rift flank evolution. River profiles reveal a consistent transient pattern that likely preserves a temporal record of slip and erosion along the entire border fault system. These profiles and other topographic observations, along with known modern and paleoenvironmental conditions, can be used to interpret a complete history of rift flank development from the onset of rifting to present. I interpret the morphology of the upland landscape to preserve the onset

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

    NASA Astrophysics Data System (ADS)

    Tiercelin, Jean-Jacques; Thouin, Catherine; Kalala, Tchibangu; Mondeguer, André

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  19. The role of signal transducer and activator of transcription 3 in Rift Valley fever virus infection.

    PubMed

    Pinkham, Chelsea; An, Soyeon; Lundberg, Lindsay; Bansal, Neha; Benedict, Ashwini; Narayanan, Aarthi; Kehn-Hall, Kylene

    2016-09-01

    Rift Valley fever (RVF) is a zoonotic disease that can cause severe illness in humans and livestock, triggering spontaneous abortion in almost 100% of pregnant ruminants. In this study, we demonstrate that signal transducer and activator of transcription 3 (STAT3) is phosphorylated on its conserved tyrosine residue (Y705) following RVFV infection. This phosphorylation was dependent on a major virulence factor, the viral nonstructural protein NSs. Loss of STAT3 had little effect on viral replication, but rather resulted in cells being more susceptible to RVFV-induced cell death. Phosphorylated STAT3 translocated to the nucleus, coinciding with inhibition of fos, jun, and nr4a2 gene expression, and the presence of STAT3 and NSs at the nr4a2 promoter. NSs was found predominantly in the cytoplasm of STAT3 null cells, indicating that STAT3 influences NSs nuclear localization. Collectively, these data demonstrate that STAT3 functions in a pro-survival capacity through modulation of NSs localization. PMID:27318793

  20. Landform development in a zone of active Gedi Fault, Eastern Kachchh rift basin, India

    NASA Astrophysics Data System (ADS)

    Kothyari, Girish Ch.; Rastogi, B. K.; Morthekai, P.; Dumka, Rakesh K.

    2016-02-01

    An earthquake of 2006 Mw 5.7 occurred along east-west trending Gedi Fault (GF) to the north of the Kachchh rift basin in western India which had the epicenter in the Wagad upland, which is approximately 60 km northeast of the 2001 Mw 7.7 earthquake site (or epicenter). Development of an active fault scarp, shifting of a river channel, offsetting of streams and uplift of the ground indicate that the terrain is undergoing active deformation. Based on detailed field investigations, three major faults that control uplifts have been identified in the GF zone. These uplifts were developed in a step-over zone of the GF, and formed due to compressive force generated by left-lateral motion within the segmented blocks. In the present research, a terrace sequence along the north flowing Karaswali river in a tectonically active GF zone has been investigated. Reconstructions based on geomorphology and terrace stratigraphy supported by optical chronology suggest that the fluvial aggradation in the Wagad area was initiated during the strengthening (at ~ 8 ka) and declining (~ 4 ka) of the Indian Summer Monsoon (ISM). The presence of younger valley fill sediments which are dated ~ 1 ka is ascribed to a short lived phase of renewed strengthening of ISM before present day aridity. Based on terrace morphology two major phases of enhanced uplift have been estimated. The older uplift event dated to 8 ka is represented by the Tertiary bedrock surfaces which accommodated the onset of valley-fill aggradation. The younger event of enhanced uplift dated to 4 ka was responsible for the incision of the older valley fill sediments and the Tertiary bedrock. These ages suggest that the average rate of uplift ranges from 0.3 to 1.1 mm/yr during the last 9 ka implying active nature of the area.

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

    USGS Publications Warehouse

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

    1991-01-01

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

  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. Time evolution of a rifted continental arc: Integrated ID-TIMS and LA-ICPMS study of magmatic zircons from the Eastern Srednogorie, Bulgaria

    NASA Astrophysics Data System (ADS)

    Georgiev, S.; von Quadt, A.; Heinrich, C. A.; Peytcheva, I.; Marchev, P.

    2012-12-01

    Eastern Srednogorie in Bulgaria is the widest segment of an extensive magmatic arc that formed by convergence of Africa and Europe during Mesozoic to Tertiary times. Northward subduction of the Tethys Ocean beneath Europe in the Late Cretaceous gave rise to a broad range of basaltic to more evolved magmas with locally associated Cu-Au mineralization along this arc. We used U-Pb geochronology of single zircons to constrain the temporal evolution of the Upper Cretaceous magmatism and the age of basement rocks through which the magmas were emplaced in this arc segment. High precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) was combined with laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) for spatial resolution within single zircon grains. Three tectono-magmatic regions are distinguished from north to south within Eastern Srednogorie: East Balkan, Yambol-Burgas and Strandzha. Late Cretaceous magmatic activity started at ~ 92 Ma in the northernmost East Balkan region, based on stratigraphic evidence and limited geochronology, with the emplacement of minor shallow intrusions and volcanic rocks onto pre-Cretaceous basement. In the southernmost Strandzha region, magmatism was initiated at ~ 86 Ma with emplacement of gabbroic to dioritic intrusions and related dikes into metamorphic basement rocks that have previously been overprinted by Jurassic-Lower Cretaceous metamorphism. The Yambol-Burgas region is an extensional basin between the East Balkan and the Strandzha regions, which broadens and deepens toward the Black Sea further east and is filled with a thick pile of marine sediments and submarine extrusive volcanic rocks accompanied by coeval intrusions. This dominantly mafic magmatism in the intermediate Yambol-Burgas region commenced at ~ 81 Ma and produced large volumes of potassium-rich magma until ~ 78 Ma. These shoshonitic to ultrapotassic basaltic to intermediate magmas formed by differentiation of ankaramitic (high

  4. Rift propagation

    NASA Technical Reports Server (NTRS)

    Parmentier, E. M.; Schubert, G.

    1989-01-01

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

  5. Research activities on submarine landslides in gentle continental slope

    NASA Astrophysics Data System (ADS)

    Morita, S.; Goto, S.; Miyata, Y.; Nakamura, Y.; Kitahara, Y.; Yamada, Y.

    2013-12-01

    In the north Sanrikuoki Basin off Shimokita Peninsula, NE Japan, a great number of buried large slump deposits have been identified in the Pliocene and younger formations. The basin has formed in a very gentle continental slope of less than one degree in gradient and is composed of well-stratified formations which basically parallel to the present seafloor. This indicates that the slumping have also occurred in such very gentle slope angle. The slump units and their slip surfaces have very simple and clear characteristics, such as layer-parallel slip on the gentle slope, regularly imbricated internal structure, block-supported with little matrix structure, widespread dewatering structure, and low-amplitude slip surface layer. We recognize that the large slump deposits group of layer-parallel slip in this area is an appropriate target to determine 'mechanism of submarine landslides', that is one of the subjects on the new IODP science plan for 2013 and beyond. So, we started some research activities to examine the feasibility of the future scientific drilling. The slump deposits were recognized basically by 3D seismic analysis. Further detailed seismic analysis using 2D seismic data in wider area of the basin is being performed for better understanding of geologic structure of the sedimentary basin and the slump deposits. This will be good source to extract suitable locations for drill sites. Typical seismic features and some other previous studies imply that the formation fluid in this study area is strongly related to natural gas, of which condition is strongly affected by temperature. So, detailed heat flow measurements was performed in the study area in 2013. For that purpose, a long-term water temperature monitoring system was deployed on the seafloor in October, 2012. The collected water temperature variation is applied to precise correction of heat flow values. Vitrinite reflectance analysis is also being carried out using sediments samples recovered by IODP

  6. The importance of rift history for volcanic margin formation.

    PubMed

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

    2010-06-17

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

  7. From rifting to continental collision, new insights on the Atlas Mountains building using low thermal chronometries (High Atlas of Marrakech, Morocco)

    NASA Astrophysics Data System (ADS)

    Ghorbal, B.; Stuart, F.; Bertotti, G.; Andriessen, P. A. M.

    2009-04-01

    We present apatite (U-Th)/He and fission track results in order to constrain the vertical movement's history of the western and central High Atlas Mountains of Morocco. Samples were collected along a 200 km long transect stretching from the Jebilet Massif in the North to the northern border of the Central Anti-Atlas chain in the south thereby traversing the Old Massif of Marrakech and the Siroua Plateau. Fission track and (U-Th)/He ages range from 10 to 163 Ma and from 8 to 152 Ma, respectively. Thermal modeling using this data as input resulted in five heating and/or cooling phases in the Late Jurassic-Early Cretaceous, Late Cretaceous, Eocene, Miocene and post-Miocene. Ages generally display an overall trend of rejuvenated ages from both margins (13-162 Ma) towards the axial zone (8-73 Ma) of the orogenic belt. Following the end of rift-related subsidence in the Liassic, extension stopped in the north margin but continued until the Dogger to Late Jurassic in the southern edges of the belts. Thermal modeling of samples collected from the northern external zones of the High Atlas suggests an unexpected phase of Middle-Late Jurassic exhumation (with a rate of 150-300m/Ma), which is generalized to the whole Atlas system from Late Jurassic to Early Cretaceous, bringing rocks at the Earth's surface before the Late Cretaceous. Exhumation that brought rocks at the Earth's surface before the Late Cretaceous, is followed by a quiet tectonic period with little vertical movements (110 to 90Ma). After a quiet tectonic period in the Cenomanian-Turonian time, vertical movements renewed with subsidence (~120m/Myr) throughout the entire Atlas domains before the Senonian. From the end of the Late Cretaceous onwards, inversion take place, signaling the onset of a final exhumation phase. Exhumation began in the external domains (Jebilet, Northern Sub-Atlas zone and Siroua) at rates of 33-130m/Ma affecting the Axial zone of the belt somewhat later, where higher amounts and rates of

  8. 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.

  9. Southeast Papuan crustal tectonics: Imaging extension and buoyancy of an active rift

    NASA Astrophysics Data System (ADS)

    Abers, G. A.; Eilon, Z.; Gaherty, J. B.; Jin, G.; Kim, YH.; Obrebski, M.; Dieck, C.

    2016-02-01

    Southeast Papua hosts the world's youngest ultra-high-pressure (UHP) metamorphic rocks. These rocks are found in an extensional setting in metamorphic core complexes. Competing theories of extensional shear zones or diapiric upwelling have been suggested as driving their exhumation. To test these theories, we analyze the CDPAPUA temporary array of 31 land and 8 seafloor broadband seismographs. Seismicity shows that deformation is being actively accommodated on the core complex bounding faults, offset by transfer structures in a manner consistent with overall north-south extension rather than radial deformation. Rayleigh wave dispersion curves are jointly inverted with receiver functions for crustal velocity structure. They show crustal thinning beneath the core complexes of 30-50% and very low shear velocities at all depths beneath the core complexes. On the rift flanks velocities resemble those of normal continents and increase steadily with depth. There is no evidence for velocity inversions that would indicate that a major density inversion exists to drive crustal diapirs. Also, low-density melt seems minor within the crust. Together with the extension patterns apparent in seismicity, these data favor an extensional origin for the core complexes and limit the role of diapirism as a secondary exhumation mechanism, although deeper mantle diapirs may be undetected. A small number of intermediate-depth earthquakes, up to 120 km deep, are identified for the first time just northeast of the D'Entrecasteaux Islands. They occur at depths similar to those recorded by UHP rocks and similar temperatures, indicating that the modern seismicity occurs at the setting that generates UHP metamorphism.

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  11. Rifting to spreading in the Gulf of Aden

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    graben). About 20 Ma ago, the emplacement of the OCT started in the east with exhumation of the subcontinental mantle. Farther west, the system was heated up by the strong influence of the Afar hot-spot, which led to breakup with much less extension. In the Gulf of Aden (s.str), up to the Shukra El Sheik fracture zone, oceanic spreading started 17.6 Ma ago. West of this fracture zone, oceanic accretion started 10 Ma ago, and 2 Ma ago in the Gulf of Tadjoura. Post-rift deformation of the eastern margins of the Gulf of Aden can be seen in the distal and proximal domains. Indeed, the substantial post-rift uplift of these margins could be associated with either the continental break-up, or activity of the Afar hotspot and related volcanic/magmatic activity. The Afar plume is therefore important for several reasons. It allows the localization of deformation along the Red Sea/Aden system and the rapid opening of the Gulf after the continental break-up.

  12. Mosquitoes and the environment in Nile Delta villages with previous rift valley fever activity

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Egypt is affected by serious human and animal mosquito-borne diseases such as Rift Valley fever (RVF). We investigated how potential RVF virus mosquito vector populations are affected by environmental conditions in the Nile Delta region of Egypt by collecting mosquitoes and environmental data from t...

  13. 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

  14. Fractal nature and scaling of normal faults, Rio Grande rift, NM: Implications for growth and strain

    SciTech Connect

    Carter, K.E.

    1994-09-01

    In this paper I introduce a suite of Quaternary normal faults from within an active continental rift and characterize the nature of the relationship between fault dimensions. I address the statistical and geological significance of the fractal analysis used in that characterization and discuss the tectonic implications. Specifically, I suggest (1) scaling laws for a previously unanalyzed population of young normal faults in rift environment; (2) implications for fault growth models in this area, in particular, addressing self-similar growth implied from the population; and (3) estimates for the total strain in this part of the basin, considering the contribution of small to unobserved faults.

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

    NASA Astrophysics Data System (ADS)

    Bennett, Scott Edmund Kelsey

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

  16. A temperature-limited assessment of the risk of Rift Valley fever transmission and establishment in the continental United States of America.

    PubMed

    Konrad, Sarah K; Miller, Scott N

    2012-05-01

    The rapid spread of West Nile virus across North America after its introduction in 1999 highlights the potential for foreign arboviruses to become established in the United States of America. Of particular concern is Rift Valley fever virus (RVFV), which has been responsible for multiple African epidemics resulting in death of both humans and livestock, as well as major economic disruption due to livestock loss and trade restrictions. Modern globalization, travel, and commerce allow viruses to easily jump from one continent to another; and it is likely only a matter of time before RVFV reaches North American shores. We used a degree-day model in combination with livestock population data and a pathways analysis to identify regions and times where RVFV is most likely to enter and become established in the United States of America. Transmission risk of the disease varies across the country from 325 annual risk days in parts of Florida to zero risk days in the far North and in high mountain regions. Areas of particular concern are where there are a high number of possible tranmission days, a large livestock population, and proximity to likely locations for the disease to enter the country via mosquito vector or human host. These areas should be monitored closely during transmission "risk seasons" so that if the virus does enter the country and begins to become established, it can be quickly controlled and eliminated before spreading further. Areas most at risk include the Baltimore and New York City metro areas as well as much of the region between these urban centers; most of Texas, especially around Houston; Florida; Atlanta; southwest Nebraska; southern California and Arizona; and the central valley of California. PMID:22639118

  17. 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.

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

    NASA Astrophysics Data System (ADS)

    Bennett, Scott Edmund Kelsey

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

  19. Fault-slip accumulation in an active rift over thousands to millions of years and the importance of paleoearthquake sampling

    NASA Astrophysics Data System (ADS)

    Mouslopoulou, Vasiliki; Nicol, Andrew; Walsh, John; Begg, John; Townsend, Dougal; Hristopulos, Dionissios

    2013-04-01

    The catastrophic earthquakes that recently (September 4th, 2010 and February 22nd, 2011) hit Christchurch, New Zealand, show that active faults, capable of generating large-magnitude earthquakes, can be hidden beneath the Earth's surface. In this study we combine near-surface paleoseismic data with deep (<5 km) onshore seismic-reflection lines to explore the growth of normal faults over short (<27 kyr) and long (>1 Ma) timescales in the Taranaki Rift, New Zealand. Our analysis shows that the integration of different timescale datasets provides a basis for identifying active faults not observed at the ground surface, estimating maximum fault-rupture lengths, inferring maximum short-term displacement rates and improving earthquake hazard assessment. We find that fault displacement rates become increasingly irregular (both faster and slower) on shorter timescales, leading to incomplete sampling of the active-fault population. Surface traces have been recognised for <50% of the active faults and along ∼50% of their lengths. The similarity of along-strike displacement profiles for short and long time intervals suggests that fault lengths and maximum single-event displacements have not changed over the last 3.6 Ma. Therefore, rate changes are likely to reflect temporal adjustments in earthquake recurrence intervals due to fault interactions and associated migration of earthquake activity within the rift.

  20. Fault-slip accumulation in an active rift over thousands to millions of years and the importance of paleoearthquake sampling

    NASA Astrophysics Data System (ADS)

    Mouslopoulou, Vasiliki; Nicol, Andrew; Walsh, John J.; Begg, John G.; Townsend, Dougal B.; Hristopulos, Dionissios T.

    2012-03-01

    The catastrophic earthquakes that recently (September 4th, 2010 and February 22nd, 2011) hit Christchurch, New Zealand, show that active faults, capable of generating large-magnitude earthquakes, can be hidden beneath the Earth's surface. In this article we combine near-surface paleoseismic data with deep (<5 km) onshore seismic-reflection lines to explore the growth of normal faults over short (<27 kyr) and long (>1 Ma) timescales in the Taranaki Rift, New Zealand. Our analysis shows that the integration of different timescale datasets provides a basis for identifying active faults not observed at the ground surface, estimating maximum fault-rupture lengths, inferring maximum short-term displacement rates and improving earthquake hazard assessment. We find that fault displacement rates become increasingly irregular (both faster and slower) on shorter timescales, leading to incomplete sampling of the active-fault population. Surface traces have been recognised for <50% of the active faults and along ≤50% of their lengths. The similarity of along-strike displacement profiles for short and long time intervals suggests that fault lengths and maximum single-event displacements have not changed over the last 3.6 Ma. Therefore, rate changes are likely to reflect temporal adjustments in earthquake recurrence intervals due to fault interactions and associated migration of earthquake activity within the rift.

  1. Non-extensivity and long-range correlations in the earthquake activity at the West Corinth rift (Greece)

    NASA Astrophysics Data System (ADS)

    Michas, G.; Vallianatos, F.; Sammonds, P.

    2013-09-01

    In the present work the statistical properties of the earthquake activity in a highly seismic region, the West Corinth rift (Central Greece), are being studied by means of generalized statistical physics. By using a dataset that covers the period 2001-2008, we investigate the earthquake energy distribution and the distribution of the time intervals (interevent times) between the successive events. As has been reported previously, these distributions exhibit complex statistical properties and fractality. By using detrended fluctuation analysis (DFA), a well-established method for detection of long-range correlations in non-stationary signals, it is shown that long-range correlations are also present in the earthquake activity. The existence of these properties motivates us to use non-extensive statistical physics (NESP) to investigate the statistical properties of the frequency-magnitude and the interevent time distributions, along with other well-known relations in seismology, such as the gamma distribution for interevent times. The results of the analysis indicate that the statistical properties of the earthquake activity can be successfully reproduced by means of NESP and that the earthquake activity at the West Corinth rift is correlated at all-time scales.

  2. Geochemistry of the Neoproterozoic Johnnie Formation and Stirling Quartzite, southern Nopah Range, California: Deciphering the roles of climate, tectonics, and sedimentary process in reconstructing the early evolution of a rifted continental margin

    NASA Astrophysics Data System (ADS)

    Schoenborn, William A.

    sediments. A total of 104 detrital zircon grains from two stratigraphically distinct samples of the Neoproterozoic Johnnie Formation in southeastern California were analyzed by SHRIMP. Samples were taken from quartz arenites in the lower and middle Johnnie Formation, which overlay sediments of rift-basin origin in the Kingston Peak Formation, to ascertain the position within the succession of the rift-to-drift transition. A 207Pb/206Pb age profile of detrital zircons from the lower Johnnie Formation has major peaks at 1749 Ma, and 1658 Ma, a subordinate peak at 1461 Ma, a lesser peak at 1239 Ma, and a few older Paleoproterozoic and Archean grains. A sample from the middle member has peaks at 1428 Ma, 1319 Ma, and 1074 Ma; a number of Paleoproterozoic peaks, and a number of peaks of Archean age similar to the Stirling Quartzite. The middle Johnnie Formation has a greater proportion of late Grenville age detritus, lesser amounts of older ˜1400 Ma Mesoproterozoic grains than the either the lower Johnnie Formation or Stirling Quartzite. When combined with detrital zircon data from the overlying strata, these data indicate a general increase upsection of late Grenville age detritus from ˜30% in the middle Johnnie Formation to ˜5% in the upper Stirling Quartzite to ˜60% in the Wood Canyon Formation in response to erosion of source areas, which temporarily shifted the influx of sediments from distal to local sources. These data support a Laurentian provenance for Johnnie Formation sediments consistent with contributions from both distal sources in the cratonic interior and local basement sources in the rift shoulder. Thus the Johnnie Formation-middle Wood Canyon Formation succession formed an early passive margin partly constrained by the continental edge consistent with recent sequence stratigraphic interpretations. A statistical comparison of Johnnie Formation and Stirling Quartzite detrital age distributions to those from Mesoproterozoic successions in the western United

  3. Ocean basin structure offshore the Southeastern United States: Is it the rift's fault?

    NASA Astrophysics Data System (ADS)

    Heffner, D. M.; Knapp, J. H.

    2012-12-01

    Continental rifts typically exhibit an asymmetrical geometry where major normal faults bound one side of a rotated block, and the rift basins thin toward the opposite hinge side. Differential extension on major faults is accommodated by transverse structures referred to as transfer zones, across which the asymmetric geometry may reverse polarity. It was proposed several decades ago that oceanic transforms between mid-ocean ridge spreading centers are inherited features of these rift-related transfer zones, and that these intra-plate structures are tectonically active. However, preserved evidence of onshore transfer zones is often lacking, particularly along the rifted margin of Eastern North America, and recent studies have suggested that oceanic transforms are not inherited structures. Observations of seismic reflection data integrated with well data show that adjacent basin bounding faults of the South Georgia Rift, a Triassic rift in the southeastern United States, occur on opposite sides of the rift flanks. The Walterboro and Warner Robins Transfer Zones identified in this study project along the small circles of Schettino and Turco (2009) into the Jacksonville and Bahamas Fracture Zones respectively. This projection is particularly interesting as it suggests the correlation of continental to oceanic features is one step south compared to previous studies which projected the Blake Spur Fracture Zone through the Charleston, SC region, and the Jacksonville Fracture Zone through Georgia. Using the same small circle solution, the Blake Spur Fracture Zone projects through the Blake Outer Ridge, a sedimentary drift which shows up prominently as a gravity anomaly the core of which has been identified as a tectonic feature reminiscent of a fracture zone on the basis of gravity modeling (Dove et al., 2007). Although no transfer zone has been identified where this small circle project onshore, it is approximately coincident with axis of the Cape Fear Arch which has been

  4. Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study

    NASA Astrophysics Data System (ADS)

    Beltrando, Marco; Manatschal, Gianreto; Mohn, Geoffroy; Dal Piaz, Giorgio Vittorio; Vitale Brovarone, Alberto; Masini, Emmanuel

    2014-04-01

    lithostratigraphic architecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/exhumation, (2) the presence of clasts of basement rocks in the neighboring meta-sediments, indicating the original proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramafic rocks pre-dating Alpine metamorphism, indicating that they were juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continental basement and meta-sediments.

  5. Along-rift Variations in Deformation and Magmatism in the Ethiopian and Afar Rift Systems

    NASA Astrophysics Data System (ADS)

    Keir, D.; Bastow, I. D.; Corti, G.; Mazzarini, F.; Rooney, T. O.

    2015-12-01

    The geological record at rifts and margins worldwide often reveals along-strike variations in volumes of extruded and intruded igneous rocks. These variations may be the result of asthenospheric heterogeneity, variations in rate, and timing of extension; alternatively, preexisting plate architecture and/or the evolving kinematics of extension during breakup may exert first-order control on magmatism. The Ethiopian and Afar Rift systems provide an excellent opportunity to address this since it exposes, along strike, several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north. Here we perform studies of distribution and style of volcanism and faulting along strike in the MER and Afar. We also incorporate synthesis of geophysical, geochemical, and petrological constraints on magma generation and emplacement in order to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rift. Along-rift changes in extension by magma intrusion and plate stretching, and the three-dimensional focusing of melt where the rift dramatically narrows each influence igneous intrusion, volcanism and subsidence history. In addition, rift obliquity plays an important role in localizing intrusion into the crust beneath en echelon volcanic segments. Along-strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting, as well as the physical state of the convecting mantle at the time of breakup.

  6. Edaphics, active tectonics and animal movements in the Kenyan Rift - implications for early human evolution and dispersal

    NASA Astrophysics Data System (ADS)

    Kübler, Simon; Owenga, Peter; Rucina, Stephen; King, Geoffrey C. P.

    2014-05-01

    The quality of soils (edaphics) and the associated vegetation strongly controls the health of grazing animals. Until now, this has hardly been appreciated by paleo-anthropologists who only take into account the availability of water and vegetation in landscape reconstruction attempts. A lack of understanding the importance of the edaphics of a region greatly limits interpretations of the relation between our ancestors and animals over the last few million years. If a region lacks vital trace elements then wild grazing and browsing animals will avoid it and go to considerable length and take major risks to seek out better pasture. As a consequence animals must move around the landscape at different times of the year. In complex landscapes, such as tectonically active rifts, hominins can use advanced group behaviour to gain strategic advantage for hunting. Our study in the southern Kenya rift in the Lake Magadi region shows that the edaphics and active rift structures play a key role in present day animal movements as well as the for the location of an early hominin site at Mt. Olorgesailie. We carried out field analysis based on studying the relationship between the geology and soil development as well as the tectonic geomorphology to identify 'good' and 'bad' regions both in terms of edaphics and accessibility for grazing animals. We further sampled different soils that developed on the volcanic bedrock and sediment sources of the region and interviewed the local Maasai shepherds to learn about present-day good and bad grazing sites. At the Olorgesailie site the rift valley floor is covered with flood trachytes; basalts only occur at Mt. Olorgesailie and farther east up the rift flank. The hominin site is located in lacustrine sediments at the southern edge of a playa that extends north and northwest of Mt. Olorgesailie. The lakebeds are now tilted and eroded by motion on two north-south striking faults. The lake was trapped by basalt flows from Mt. Olorgesailie

  7. How Complex is Orogeny? the Role of Rift Inheritance in the Evolution of the Western Alps

    NASA Astrophysics Data System (ADS)

    Beltrando, M.; Mohn, G.; Manatschal, G.

    2012-12-01

    Numerical and conceptual models of the evolution of convergent plate margins normally rely on paleogeographic reconstructions consisting exclusively of (1) oceanic lithosphere, made of mantle peridotites, mafic crust and post-rift sediments and (2) continental lithosphere, with subcontinental mantle and a 20-30 km thick crust, overlain by pre-, syn- and post- rift sediments. However, lithological associations characteristic of the high-pressure part of Alpine-type orogenic belts often fail to match either end members. As a result, 'anomalous' lithological associations, including Paleozoic continental basement directly in contact with Jurassic ophiolites and/or post-rift sediments, have generally been attributed to complex subduction/orogenic dynamics, responsible for chaotic mixing of continental and oceanic lithologies. In contrast to this commonly held view, recent studies have shown that a large part of the apparent complexity of the axial zone of the Western Alps is inherited from Jurassic rifting, since most of the subducted Tethyan lithosphere originally consisted of variably serpentinized subcontinental mantle locally overlain by slivers of continental basement. This conclusion is based on a number of observations, including (1) the consistency of the lithostratigraphic architecture over large areas, despite Alpine deformation, excluding chaotic mixing during subduction/exhumation, (2) widespread pre-metamorphic brittle deformation in continental basement and ultramafics, indicating that they were juxtaposed by fault activity prior to the deposition of post-rift sediments, (3) the presence of clasts of basement rocks in the neighboring sediments, indicating the original proximity of the different lithologies, (4) the common Alpine tectonometamorphic evolution of the different lithologies from the oldest preserved deformation/metamorphic stages. The basement-cover relationships documented in the Western Alps are typical of present day magma-poor rifted

  8. Radial Anisotropy beneath the Main Ethiopian Rift and Afar Depression

    NASA Astrophysics Data System (ADS)

    Accardo, N. J.; Gaherty, J. B.; Jin, G.; Shillington, D. J.

    2014-12-01

    The Main Ethiopian Rift (MER) and Afar uniquely capture the final stages of transition from continental rifting in the broader East African Rift System to incipient seafloor spreading above a mantle hotspot. Studies of the region increasingly point to magmatism as a controlling factor on continental extension. However, the character and depth extent of these melt products remain contentious. Radial anisotropy derived from surface waves provides a unique diagnostic constraint on the presence of oriented melt pockets versus broader oriented anisotropic fabrics. This study investigates the thermal and radially anisotropic structure beneath the broader MER and Afar to resolve the magmatic character of the region and ultimately to understand the role of magmatism in present day rift development. We utilize 104 stations from 4 collocated arrays in the MER/Afar region to constrain radial anisotropy within the upper mantle via the inversion of Love- and Rayleigh-wave observations between 25 and 100 s period. We employ a multi-channel cross-correlation algorithm to obtain inter-station phase and amplitude information. The multi-channel phase observations are inverted for dynamic phase velocity across the array, which are then corrected for focusing and multipathing using the amplitude observations via Helmholtz tomography. We jointly invert Love- and Rayleigh-wave structural phase velocity measurements employing crustal constraints from co-located active source experiments to obtain estimates of Vsv and Vsh between 50 - 170 km depth. Preliminary results readily reveal the distinct shear velocity structure beneath the MER and Afar. Within the MER, shear velocity structure suggests pronounced low velocities accompanied by strong anisotropy between 80 - 140 km depth beneath the western Ethiopian plateau and rift valley. Within Afar, shear velocity structure is more varied with the slowest velocities found at shallow depths (less than 70 km depth), accompanied by weak

  9. Early Jurassic paleopoles from the Hartford continental rift basin (eastern North America): Was an abrupt change in polar wander associated with the Central Atlantic Magmatic Province?

    NASA Astrophysics Data System (ADS)

    Kent, D. V.; Olsen, P. E.

    2007-12-01

    The recent recognition of what may be the largest igneous province on Earth, the ~200 Ma Central Atlantic magmatic province (CAMP), with its close temporal proximity to major biotic turnover at the Triassic/Jurassic boundary, adds impetus for seeking confirmation of possibly related geodynamic phenomena. For example, CAMP emplacement seems to coincide temporally with an abrupt change in North American apparent polar wander at the so-called J1 cusp, which has been suggested to reflect a major plate reorganization or an episode of true polar wander. However, early Jurassic paleopoles from the Moenave and Wingate Formations from the Colorado Plateau that virtually define the J1 cusp have few reliable counterparts from elsewhere in North America. The thick section of cyclical Lower Jurassic continental sediments with interbedded CAMP lava flows in the Hartford basin of Connecticut and Massachusetts provides an opportunity to test the reality of the J1 cusp. We collected about 400 oriented samples distributed over 80 outcrop sites that represent a ~2500 meter-thick composite section of the Shuttle Meadow and East Berlin sedimentary formations, which are interbedded with CAMP lava units, and the lower Portland Formation, which consists of cyclical lacustrine to fluvial sediments of Early Jurassic age that conformably overlie the CAMP extrusive zone in the Hartford basin. Normal and reverse polarity ChRM directions define a coherent magnetostratigraphy and are supported by a reversal test and a positive fold test. The distribution of ChRM direction from the sediments is flattened and the mean is significantly shallower than from the coeval CAMP lavas. E/I analysis of the Hartford sedimentary ChRM data produces a result consistent with the geomagnetic field model at a mean flattening factor of 0.54; the corrected mean direction is steeper and not significantly different from the mean inclination of the Newark and Hartford CAMP volcanic units.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  11. 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

    geological formations \\bullet Gas content in the water and gas dissolution The interpretation of these processes for the Eger Rift Franconian Line area results in horizontal pressure gradients up to 0.5 MPa/km. With these pressure gradients in deep fault zones similar to the KTB fault zones SE1 and SE2, a remarkable groundwater flow is also possible in the deep crystalline crust. For only a 1 MPa pressure difference between the Franconian Line and the Eger Rift Valley, which lie nearly 60 km apart, we get a tracer velocity of 1.0 to 5.0 m/a (using the Darcy relation and porosities for the hydraulic KTB data). The flow system at great depth is determined mainly by the counteractive forces of salinity and temperature with a nonlinear relation to the water density. References GRAESLE, W., KESSELS, W., KUEMPEL, H.-J., LI, XUAN (2006): HYDRAULIC OBSERVATIONS FROM A ONE YEAR FLUID PRODUCTION TEST IN THE 4000 M DEEP KTB PILOT BOREHOLE. GEOFLUIDS, 6, 8 23 KESSELS, W., KUECK, J. (1995): HYDRAULIC COMMUNICATION IN CRYSTALLINE ROCK BETWEEN THE TWO BOREHOLES OF THE CONTINENTAL DEEP DRILLING PROJECT IN GERMANY. INT. J. ROCK MECH. MIN. SCI. &GEOMECH. ABSTR., 32, 37 47

  12. Evolutionary model of the oblique rift basins- Central African Rifts

    NASA Astrophysics Data System (ADS)

    Yang, Kenn-Ming; Cheng, I.-Wen; Wu, Jong-Chang

    2016-04-01

    The geometry of oblique-rifting basin is strongly related with the angle (α) between the trend of rift and that of regional major extensional stress. The main purpose of this study is to investigate characteristics of geometry and kinematics of structure and tectono-stratigraphy during basin evolution of Central African Rifts (CAS). In this study, we simulated the formation of oblique-rifting basin with Particle Flow Code 3-Dimensions-(PFC 3D) and compared the simulation results with the tectonic settings of a series of basin in CAS. CAS started to develop in Early Cretaceous (130Ma) and lasted until the Late Cretaceous (85Ma-80Ma). The following collision between the African and Eurasian plates imposed compressional stress on CAS and folded the strata in the rift basins. Although the characteristics of rift basin formation remain controversial, palinspastic sections constructed in this study show that, in the Early Cretaceous, the rift basins are mainly characterized by normal faults and half-grabens. In the Late Cretaceous, the morphology of the rift basins was altered by large-scaled tectonic compression with the active Borogop Fault of regional scale. Also, en echelon trend of normal faults in the basins were measured and the angles between the trend with that of the rift axes of each basin were demonstrated, indicating that the development of CAS was affected by the regional extensional stress with a dextral component during the rifting process and, therefore, the rift basins were formed by oblique-rifting. In this study, we simulated the oblique-rifting basin model of various α with Particle Flow Code 3-Dimensions-(PFC 3D). The main theory of PFC 3D is based on the Discrete Element Method (DEM), in which parameters are applied to every particle in the models. We applied forces acting on both sides of rift axis, which α are 45°, 60°, 75° and 90° respectively, to simulate basin formation under oblique-rifting process. The study results of simulation

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  14. 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

    the extension direction. This occurs when rifts are laterally sufficiently far apart and local effects probably overrule the far-field stresses. Our CT- and PIV-analyses will reveal this surprising effect in more detail. The influence of rift-connecting seeds (model series 1) on rift interaction is limited. Only when they are oriented some 30° or more oblique to the extension direction, can they be activated. In most of these cases oblique-slip fault zones (transfer zones) form along the rift-connecting weak zone, linking the rift segments. Transfer zone structures depend on the angle between the seed orientation and extension direction: the higher the angle, the wider the fault zone. However, these observations are only valid under dextral oblique extension conditions; none of our rift-connecting weak zones (connecting right-stepping rift segments) are activated when sinistral oblique extension is applied. Still our models show how structural inheritance can control the orientation and structuration of transfer zones between rift segments that later on might evolve into oceanic transform faults. REFERENCE Zwaan, F., Schreurs, G., Naliboff, J., Buiter, S.J.H. (in revision) Insights into the effects of oblique extension on continental rift interaction from 3D analogue and numerical models.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  16. Late Quaternary high resolution sequence stratigraphy of an active rift, the Sperchios Basin, Greece: An analogue for subtle stratigraphic plays

    SciTech Connect

    Eliet, P.P.; Gawthorpe, R.L.

    1996-12-31

    The Sperchios Basin is an active asymmetric graben, bounded to the south by a major border fault system with major fault segments typically 20-30 km long. The basin is dominated by a major axial fluvio-deltaic system which enters the partially enclosed Maliakos Gulf to the east. Lateral sourced depositional systems within the basin comprise hanging-wall and footwall-derived alluvial fans and a narrow coastal plain along the footwall scarp bordering the Maliakos Gulf. High resolution seismic data from the Maliakos Gulf reveals three late Quaternary progradational parasequences sourced from axial and lateral depositional systems, with a regional late-Pleistocene transgressive surface dated at circa. 10 ka BP within the Maliakos Gulf. Differential subsidence of the late Pleistocene transgressive surface indicates marked variation in subsidence from 2.4 m ka{sup -1} at fault segment centers to 0.8 m ka{sup -1} at segment boundaries. The geometry and internal variability of each parasequence is controlled by the interplay of the local accommodation development and fluctuations in sediment supply and climatic conditions. The Sperchios Rift provides a modem analogue for subtle stratigraphic plays within ancient extensional basins. The study of controls on sediment source and transport patterns within active rifts has refined our appreciation of the controls on potential reservoir distribution and geometries.

  17. Late Quaternary high resolution sequence stratigraphy of an active rift, the Sperchios Basin, Greece: An analogue for subtle stratigraphic plays

    SciTech Connect

    Eliet, P.P. ); Gawthorpe, R.L. )

    1996-01-01

    The Sperchios Basin is an active asymmetric graben, bounded to the south by a major border fault system with major fault segments typically 20-30 km long. The basin is dominated by a major axial fluvio-deltaic system which enters the partially enclosed Maliakos Gulf to the east. Lateral sourced depositional systems within the basin comprise hanging-wall and footwall-derived alluvial fans and a narrow coastal plain along the footwall scarp bordering the Maliakos Gulf. High resolution seismic data from the Maliakos Gulf reveals three late Quaternary progradational parasequences sourced from axial and lateral depositional systems, with a regional late-Pleistocene transgressive surface dated at circa. 10 ka BP within the Maliakos Gulf. Differential subsidence of the late Pleistocene transgressive surface indicates marked variation in subsidence from 2.4 m ka[sup -1] at fault segment centers to 0.8 m ka[sup -1] at segment boundaries. The geometry and internal variability of each parasequence is controlled by the interplay of the local accommodation development and fluctuations in sediment supply and climatic conditions. The Sperchios Rift provides a modem analogue for subtle stratigraphic plays within ancient extensional basins. The study of controls on sediment source and transport patterns within active rifts has refined our appreciation of the controls on potential reservoir distribution and geometries.

  18. Rocas Verdes Ophiolite Complexes in the Southernmost Andes: Remnants of the Mafic Igneous Floor of a Back-arc Basin that Rifted the South American Continental Crust in the Late Jurrassic and Early Cretaceous

    NASA Astrophysics Data System (ADS)

    Stern, C. R.

    2001-12-01

    The Rocas Verdes are an en echelon group of late Jurassic and early Cretaceous igneous complexes in the southernmost Andes. They consist of mafic pillow lavas, dikes and gabbros interpreted as the upper portions of ophiolite complexes formed along mid-ocean-ridge-type spreading centers. When secondary metamorphic affects are accounted for, the geochemistry of mafic Rocas Verdes rocks are similar to ocean-ridge basalts (MORB). The spreading centers that generated the Rocas Verdes rifted the southwestern margin of the Gondwana continental crust, during the start of break-up in the southern Atlantic, to form the igneous floor of a back-arc basin behind a contemporaneous convergent plate boundary magmatic arc. Late Jurassic and early Cretaceous sediments from both the magmatic arc on the southwest and the continental platform on the northeast of the basin were deposited in the Rocas Verdes basin, and these sediments are interbedded with mafic pillow lavas along the margins of the Rocas Verdes mafic complexes. Also, mafic dikes and gabbros intrude older pre-Andean and Andean lithologies along both flanks of the Rocas Verdes, and leucocratic country rocks are engulfed in the Rocas Verdes mafic complexes. These relations indicate that the Rocas Verdes complexes formed in place and are autochthonous, having been uplifted but not obducted, which may explain the lack of exposure of the deeper ultramafic units. Zircon U/Pb ages of 150+/-1 Ma for the Larsen Harbour Formation, a southern extension of the Rocas Verdes belt on South Georgia Island, and 138+/-2 Ma for the Sarmiento complex, the northernmost in the Rocas Verdes belt, indicate that this basin may have formed by "unzipping" from the south to the north, with the southern portion beginning to form earlier and developing more extensively than the northern portion of the basin. Paleomagnetic data suggest that the Rocas Verdes basin developed in conjunction with the displacement of the Antarctic Peninsula and opening of

  19. Serpentized mantle at rifted margins: The Goban Spur example

    NASA Astrophysics Data System (ADS)

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

    2002-12-01

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

  20. The evolution of passive rifting: contributions from field and laboratory studies to the interpretation of modelling results

    NASA Astrophysics Data System (ADS)

    Piccardo, Giovanni; Ranalli, Giorgio

    2015-04-01

    Direct field/laboratory, structural/petrologic investigations of mantle lithosphere from orogenic peridotites in Alpine-Apennine ophiolites provide significant constraints to the rift evolution of the Jurassic Ligurian Tethys ocean (Piccardo et al., 2014, and references therein). These studies have shown that continental extension and passive rifting were characterized by an important syn-rift "hidden" magmatic event, pre-dating continental break-up and sea-floor spreading. Occurrence of km-scale bodies of reactive spinel-harzburgites and impregnated plagioclase-peridotites, formed by melt/peridotite interaction, and the lack of any extrusive counterpart, show that the percolating magmas remained stored inside the mantle lithosphere. Petrologic-geochemical data/modelling and mineral Sm/Nd age constraints evidence that the syn-rift melt infiltration and reactive porous-flow percolation through the lithosphere were induced by MORB-type parental liquids formed by decompression melting of the passively upwelling asthenosphere. Melt thermal advection through, and melt stagnation within the lithosphere, heated the mantle column to temperatures close to the dry peridotite solidus ("asthenospherization" of mantle lithosphere). Experimental results of numerical/analogue modelling of the Ligurian rifting, based on field/laboratory constraints, show that: (1) porous flow percolation of asthenospheric melts resulted in considerable softening of the mantle lithosphere, decreasing total strength TLS from 10 to 1 TN m-1 as orders of magnitude (Ranalli et al. 2007), and (2) the formation of an axial lithospheric mantle column, with softened rheological characteristics (Weakened Lithospheric Mantle - WLM), induced necking instability in the extending lithosphere and subsequent active upwelling of the asthenosphere inside the WLM zone (Corti et al., 2007). Therefore, the syn-rift hidden magmatism (melt thermo-chemical-mechanical erosion, melt thermal advection and melt storage

  1. 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

  2. 78 FR 48180 - Consolidation of Officer in Charge, Marine Inspection For Outer Continental Shelf Activities...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2013-08-07

    ... SECURITY Coast Guard Consolidation of Officer in Charge, Marine Inspection For Outer Continental Shelf Activities; Eighth Coast Guard District AGENCY: Coast Guard, DHS. ACTION: Notice and request for comments. SUMMARY: The Coast Guard is considering establishing a single Officer in Charge, Marine Inspection...

  3. 78 FR 5836 - Adjustment of Service Fees for Outer Continental Shelf Activities

    Federal Register 2010, 2011, 2012, 2013, 2014

    2013-01-28

    ... From the Federal Register Online via the Government Publishing Office DEPARTMENT OF THE INTERIOR Bureau of Ocean Energy Management Adjustment of Service Fees for Outer Continental Shelf Activities... updated in 2008, with Federal Register Notice 73 FR 49943. BOEM is now adjusting various cost...

  4. Atlantic update, July 1986--June 1990: Outer Continental Shelf oil and gas activities

    SciTech Connect

    Karpas, R.M.; Gould, G.J.

    1990-10-01

    This report describes outer continental shelf oil and gas activities in the Atlantic Region. This edition of the Atlantic Update includes an overview of the Mid-Atlantic Planning Area and a summary of the Manteo Prospect off-shore North Carolina. 6 figs., 8 tabs.

  5. Rift initiation in cratonic lithosphere: Seismicity patterns in the Manyara-Natron-Magadi basins and Oldoinyo Lengai volcano

    NASA Astrophysics Data System (ADS)

    Lambert, C.; Rodzianko, A.; Rasendra, N.; Msabi, M.; Muirhead, J.; Ebinger, C. J.; Tiberi, C.; Roecker, S. W.; Ferdinand, R. W.; Mtelela, K.; Muzuka, A.

    2013-12-01

    The CRAFTI project consists of a 2-year seismic acquisition program to quantify the partitioning of strain between faulting and magmatism during the early stages of continental rifting in Archaean and Proterozoic lithosphere. The <7 My Eastern Rift System in northern Tanzania and southern Kenya provides an ideal study system, as it comprises several rift segments at different stages of the rifting cycle. We present preliminary results from 38 broadband seismometers deployed in Tanzania in January 2013, and 4 stations in Kenya deployed in July 2013. The network includes a rift-perpendicular transect, and spans parts of 3 discrete rift basins in different stages of development: Manyara, Natron, Magadi. Initial analyses indicate relatively low noise levels at all stations deployed in Maasai bomas and rural schools, and good to excellent transmission, except near Quaternary Gelai, Oldoinyo Lengai, and Kitumbeine volcanoes. We present time-space relations of seismicity for the first 6 months' of data, and focal mechanisms for the largest events during that time period. Hypocentral locations are compared with the locations of eruptive centers, dike intrusions, and sub-surface projections of faults mapped in a complementary part of the CRAFTI project. The spatial and temporal distribution of earthquake activity will help identify the contributions of faulting and magmatism in each basin, and in the identification of subsurface magma reservoirs in this youthful rift system.

  6. Active diapirism and slope steepening, northern Gulf of Mexico continental slope.

    USGS Publications Warehouse

    Martin, R.G.; Bouma, A.H.

    1982-01-01

    Large diapiric and nondiapiric masses of Jurassic salt and Tertiary shale underlie the northern Gulf of Mexico continental slope and adjacent outer continental shelf. Local steepening of the sea floor in response to the vertical growth of these structures is a serious concern to those involved in the site selection and the construction of future oil and gas production and transportation facilities in this frontier petroleum province. The evidence given in this paper supports the conclusion that the present continental slope region of the northern Gulf of Mexico is undergoing active diapirism and consequent slope steepening. Because most of the sediment on the flanks of diapiric structures consists of underconsolidated muds, slumping will take place regularly in response to further diapiric movement.-from Authors

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

    NASA Astrophysics Data System (ADS)

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

    2013-02-01

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

  8. Mosquitoes and the Environment in Nile Delta Villages with Previous Rift Valley Fever Activity.

    PubMed

    Zayed, Abdelbaset B; Britch, Seth C; Soliman, Mohamed I; Linthicum, Kenneth J

    2015-06-01

    Egypt is affected by serious human and animal mosquito-borne diseases such as Rift Valley fever (RVF). We investigated how potential RVF virus mosquito vector populations are affected by environmental conditions in the Nile Delta region of Egypt by collecting mosquitoes and environmental data from 3 key governorates before and after 2012 seasonal flooding. We found that environmental effects varied among species, life stages, pre- and postflood groupings, and geographic populations of the same species, and that mosquito community composition could change after flooding. Our study provides preliminary data for modeling mosquitoes and mosquito-borne diseases in the Nile Delta region. PMID:26181689

  9. The evolution of fault geometry and lithosphere mechanical response to faulting during lithosphere hyper-extension at magma-poor rifted margins

    NASA Astrophysics Data System (ADS)

    Gómez Romeu, Júlia; Kusznir, Nick; Manatschal, Gianreto; Roberts, Alan

    2016-04-01

    The geometry of upper lithosphere extensional faulting and the mechanical response of the lithosphere during continental breakup are controversial. The lithosphere response to extensional faulting at magma-poor rifted margins controls the distribution of thinned continental crust, exhumed mantle, continental allochthons and syn-tectonic sediments leading to the complexity of heterogeneous structure of hyper-extended domain at these margins. In order to better understand the evolving fault geometry and lithosphere mechanics during magma-poor rifted margin formation, we investigate extensional faulting for the tectonic end-members of continental rifting and slow sea-floor spreading. We presume that these end-members faulting styles both contribute to lithosphere thinning during rifted margin evolution as continental rifting evolves into sea-floor spreading. For continental rifting, large extensional faults that rupture the seismogenic brittle upper lithosphere have been shown to be planar and steeply dipping by earthquake seismology and geodesy (Stein and Barrientos 1985; Jackson 1987). These results are supported by seismic reflection imaging and structural modelling of rift basins (Kusznir et al., 1991, 1995). Individual fault heaves for continental rifting seldom exceeds approximately 10 km. The effective elastic thickness, used to parameterize lithosphere flexural strength for syn-tectonic response to extensional faulting during continental rifting, are typically between 1.5 and 3 km. For slow-spreading ocean ridges we examine extensional fault geometry and lithosphere flexural response to cumulative faulting. We focus on the TAG area (deMartin et al., 2007) and the 15°N area (Schroeder et al., 2007) of the Mid-Atlantic Ridge using a flexural isostatic extensional faulting model (Buck 1988; Kusznir et al., 1991). Modelling of fault controlled bathymetry at slow-spreading ocean ridges shows that active extensional faults at depth have a steep dip (50° - 70

  10. Understanding Tensions: Activity Systems Analysis of Cross-Continental Collaboration

    ERIC Educational Resources Information Center

    Ryder, LanHui Zhang; Yamagata-Lynch, Lisa

    2014-01-01

    Using the lens of Vygotsky's sociocultural theory, activity theory, and Engeström's activity systems analysis, this qualitative study explores students' experiences in the context of a sixteen-week transpacific collaboration between seven students at Northern Illinois University (NIU) and seven students from Shandong Normal…

  11. 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.

  12. Rift Valley fever dynamics in Senegal: a project for pro-active adaptation and improvement of livestock raising management.

    PubMed

    Lafaye, Murielle; Sall, Baba; Ndiaye, Youssou; Vignolles, Cecile; Tourre, Yves M; Borchi, Franc Ois; Soubeyroux, Jean-Michel; Diallo, Mawlouth; Dia, Ibrahima; Ba, Yamar; Faye, Abdoulaye; Ba, Taibou; Ka, Alioune; Ndione, Jacques-André; Gauthier, Hélène; Lacaux, Jean-Pierre

    2013-11-01

    The multi-disciplinary French project "Adaptation à la Fiévre de la Vallée du Rift" (AdaptFVR) has concluded a 10-year constructive interaction between many scientists/partners involved with the Rift Valley fever (RVF) dynamics in Senegal. The three targeted objectives reached were (i) to produce--in near real-time--validated risk maps for parked livestock exposed to RVF mosquitoes/vectors bites; (ii) to assess the impacts on RVF vectors from climate variability at different time-scales including climate change; and (iii) to isolate processes improving local livestock management and animal health. Based on these results, concrete, pro-active adaptive actions were taken on site, which led to the establishment of a RVF early warning system (RVFews). Bulletins were released in a timely fashion during the project, tested and validated in close collaboration with the local populations, i.e. the primary users. Among the strategic, adaptive methods developed, conducted and evaluated in terms of cost/benefit analyses are the larvicide campaigns and the coupled bio-mathematical (hydrological and entomological) model technologies, which are being transferred to the staff of the "Centre de Suivi Ecologique" (CSE) in Dakar during 2013. Based on the results from the AdaptFVR project, other projects with similar conceptual and modelling approaches are currently being implemented, e.g. for urban and rural malaria and dengue in the French Antilles. PMID:24258902

  13. 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

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  15. The speciation of marine particulate iron adjacent to active and passive continental margins

    NASA Astrophysics Data System (ADS)

    Lam, Phoebe J.; Ohnemus, Daniel C.; Marcus, Matthew A.

    2012-03-01

    We use synchrotron-based chemical-species mapping techniques to compare the speciation of suspended (1-51 μm) marine particulate iron collected in two open ocean environments adjacent to active and passive continental margins. Chemical-species mapping provides speciation information for heterogeneous environmental samples, and is especially good for detecting spectroscopically distinct trace minerals and species that could not be detectable by other methods. The average oxidation state of marine particulate iron determined by chemical-species mapping is comparable to that determined by standard bulk X-ray Absorption Near Edge Structure spectroscopy. Using chemical-species mapping, we find that up to 43% of particulate Fe in the Northwest Pacific at the depth of the adjacent active continental margin is in the Fe(II) state, with the balance Fe(III). In contrast, particulate iron in the eastern tropical North Atlantic, which receives the highest dust deposition on Earth and is adjacent to a passive margin, is dominated by weathered and oxidized Fe compounds, with Fe(III) contributing 90% of total iron. The balance is composed primarily of Fe(II)-containing species, but we detected individual pyrite particles in some samples within an oxygen minimum zone in the upper thermocline. Several lines of evidence point to the adjacent Mauritanian continental shelf as the source of pyrite to the water column. The speciation of suspended marine particulate iron reflects the mineralogy of iron from the adjacent continental margins. Since the solubility of particulate iron has been shown to be a function of its speciation, this may have implications for the bioavailability of particulate iron adjacent to passive compared to active continental margins.

  16. Petrological and Geochemical characterization of central Chihuahua basalts: a possible local sign of rifting activity

    NASA Astrophysics Data System (ADS)

    Espejel-Garcia, V. V.; Garcia-Rascon, M.; Villalobos-Aragon, A.; Morton-Bermea, O.

    2012-12-01

    The central part of the mexican state, Chihuahua, is the oriental border of the Sierra Madre Occidental (silicic large igneous province), which consist of series of ignimbrites divided into two volcanic groups of andesites and rhyolites. In the central region of Chihuahua, the volcanic rocks are now part of the Basin and Range, allowing the presence of mafic rocks in the lower areas. The study area is located approximately 200 km to the NW of Chihuahua city near to La Guajolota town, in the Namiquipa County. There are at least 5 outcrops of basalts to the west of the road, named Puerto de Lopez, Malpaises, El Tascate, Quebrada Honda, and Carrizalio, respectively. These outcrops have only been previously described by the Mexican Geologic Survey (SGM) as thin basaltic flows, with vesicles filled with quartz, and phenocrystals of labradorite, andesine, oligoclase and olivine. Petrologically, the basalts present different textures, from small phenocrysts of plagioclase in a very fine matrix to large, zoned and sometimes broken phenocrysts of plagioclase in a coarser matrix. All samples have olivine in an advanced state of alteration, iddingsite. The geochemical analyses report that these basaltic flows contain characteristics of rift basalts. The rocks have a normative olivine values from 5.78 to 27.26 and nepheline values from 0 to 2.34. In the TAS diagram the samples straddle the join between basalt and trachy-basalt, reflecting a high K2O content. The Mg# average is 0.297, a value that suggests that the basalts do not come from a primitive magma. The basalts have high values of Ba (945-1334 ppm), Cu (54-147 ppm), and Zn (123-615 ppm). The contents of Rb (23-57 ppm), Sr (659-810 ppm), Y (26-33 ppm), Zr (148-217 ppm) and Cr (79-98 ppm) are characteristics of rift basalts. Using discrimination diagrams, the basalts plot in the field of within plate, supporting the rifting origin. Outcrops of other basalts, at about 80 to 100 km to the east of the study area, Lomas El

  17. Heat flow and continental breakup: The Gulf of Elat (Aqaba)

    NASA Technical Reports Server (NTRS)

    Ben-Avraham, Z.; Vonherzen, R. P.

    1985-01-01

    Heat flow measurements were made in the major basins of the Gulf of Elat (Aqaba), northern Red Sea. The gulf is located at the southern portion of the Dead Sea rift which is a transform plate boundary. Gradient measurements at each site were made with a probe which allows multiple penetration of the bottom during a single deployment of the instrument. Thermal conductivity was determined by needle probe measurements on sedimentary cores. The mean heat flux, about 80 mWm(-2), is significantly above the continental mean, and probably also above that from the adjacent Sinai and Arabian continental blocks. The heat flow appears to increase from north to south. Such an increase may be related to the more advanced rifting stage of the Red Sea immediately to the south, which presently includes creation of an oceanic crust. This trend also corresponds to the general trend of the deep crustal structure in the gulf. Evidence from various geophysical fields suggest a gradual thinning of the crust towards the direction of the Red Sea where a normal oceanic crust exists. The heat flow data, together with other geophysical data, indicate a propagation of mature rifting activity from the Red Sea into the Gulf of Elat. This process is acting simultaneously with the transform motion along the Dead Sea rift.

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

    The Gulf of Corinth is a classic young active continental rift initiating <5 Ma and with current extension rates up to 15 mm/yr. The modern rift (ca. 1-2 Myr old) has been studied extensively both onshore and offshore. In this paper we bring together the results of study of the offshore rift with existing onshore data to generate a model for how the modern rift has tectonically evolved, how strain is distributed across and along the rift, how slip on individual major faults controlling rift basin subsidence has changed over relatively short timescales (e.g. <0.5 Myr) and how extension in the upper crust through fault displacement compares with whole crustal extension over the history of the rift. The results indicate that the rift stratigraphy is divided into two units (pre- and post- ca. 0.5Ma). The two units indicate markedly different rift basin geometry during these two time periods. Two separated depocentres 20-50 km long were created controlled by N- and S-dipping faults before ca. 0.5 Ma, while since ca. 0.5 Ma a single depocentre (80 km long) has been controlled by several connected N-dipping faults, with maximum subsidence focused between the two older depocentres. Thus isolated but nearby faults can persist for timescales ca. 1 Ma and form major basins before becoming linked. There is a general evolution towards a dominance of N-dipping faults; however, in the western Gulf strain is distributed across several active N- and S-dipping faults throughout rift history, producing a more complex basin geometry. Examination of extension at a larger spatial and temporal scale suggests that uniform pure shear extension without the need for a significant N-S dipping detachment fault is a viable extension mechanism for at least the western rift where constraints are greater. These results also indicate that the present day strain distribution indicated by GPS data cannot have persisted over the lifetime of the modern rift. We are now building on these studies by

  19. Faulting in a propagating continental rift: Insight from the late Miocene structural development of the Abert Rim fault, southern Oregon, USA

    NASA Astrophysics Data System (ADS)

    Scarberry, Kaleb C.; Meigs, Andrew J.; Grunder, Anita L.

    2010-06-01

    New geological mapping and 40Ar- 39Ar ages reveal a temporal progression of faulting along a major extensional fault (the Abert Rim fault) at the active margin of the Basin and Range Province in the western U.S. The onset of extensional deformation near Lake Abert coincided with widespread basaltic volcanism. Fault cross-cutting relationships and tilt of volcanic layers demonstrate that NW-striking faults formed between ˜ 8.9 and 7.5 Ma and were subsequently cut by the NNE-striking Abert Rim fault. Sequential restoration of cross-sections indicate that the Abert Rim fault south of 42°40' N latitude had > 250 m of stratigraphic separation prior to deposition of the Rattlesnake Tuff at 7 Ma. Low values of net extension (˜ 4%) and long-term rates of geologic deformation (≪ 1 mm/yr) suggest that regional extension since ˜ 10 Ma has occurred primarily by diking. We model province wide along- and across-strike expansion within the margin of the Basin and Range Province as regional dilation accompanying northward propagation of the Walker Lane transform. The growth of this extensional province is superimposed on a previously unrecognized early Miocene volcanic landscape marked by volcanism between ˜ 21.4 and 22.3 Ma. Dike orientations within one of these early Miocene volcanoes and evidence for the existence of as much as 500 m of paleo-topography prior to widespread deposition of the Steens basalts at ˜ 16 Ma suggests that late Miocene faults near Lake Abert may reactivate an older structural fabric.

  20. Tectonic Framework of the Kachchh Rift Basin

    NASA Astrophysics Data System (ADS)

    Talwani, P.; Gangopadhyay, A. K.

    2001-05-01

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

  1. Pre-existing oblique transfer zones and transfer/transform relationships in continental margins: New insights from the southeastern Gulf of Aden, Socotra Island, Yemen

    NASA Astrophysics Data System (ADS)

    Bellahsen, N.; Leroy, S.; Autin, J.; Razin, P.; d'Acremont, E.; Sloan, H.; Pik, R.; Ahmed, A.; Khanbari, K.

    2013-11-01

    Transfer zones are ubiquitous features in continental rifts and margins, as are transform faults in oceanic lithosphere. Here, we present a structural study of the Hadibo Transfer Zone (HTZ), located in Socotra Island (Yemen) in the southeastern Gulf of Aden. There, we interpret this continental transfer fault zone to represent a reactivated pre-existing structure. Its trend is oblique to the direction of divergence and it has been active from the early up to the latest stages of rifting. One of the main oceanic fracture zones (FZ), the Hadibo-Sharbithat FZ, is aligned with and appears to be an extension of the HTZ and is probably genetically linked to it. Comparing this setting with observations from other Afro-Arabian rifts as well as with passive margins worldwide, it appears that many continental transfer zones are reactivated pre-existing structures, oblique to divergence. We therefore establish a classification system for oceanic FZ based upon their relationship with syn-rift structures. Type 1 FZ form at syn-rift structures and are late syn-rift to early syn-OCT. Type 2 FZ form during the OCT formation and Type 3 FZ form within the oceanic domain, after the oceanic spreading onset. The latter are controlled by far-field forces, magmatic processes, spreading rates, and oceanic crust rheology.

  2. 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.

  3. Lithology and temperature: How key mantle variables control rift volcanism

    NASA Astrophysics Data System (ADS)

    Shorttle, O.; Hoggard, M.; Matthews, S.; Maclennan, J.

    2015-12-01

    Continental rifting is often associated with extensive magmatic activity, emplacing millions of cubic kilometres of basalt and triggering environmental change. The lasting geological record of this volcanic catastrophism are the large igneous provinces found at the margins of many continents and abrupt extinctions in the fossil record, most strikingly that found at the Permo-Triassic boundary. Rather than being considered purely a passive plate tectonic phenomenon, these episodes are frequently explained by the involvement of mantle plumes, upwellings of mantle rock made buoyant by their high temperatures. However, there has been debate over the relative role of the mantle's temperature and composition in generating the large volumes of magma involved in rift and intra-plate volcanism, and even when the mantle is inferred to be hot, this has been variously attributed to mantle plumes or continental insulation effects. To help resolve these uncertainties we have combined geochemical, geophysical and modelling results in a two stage approach: Firstly, we have investigated how mantle composition and temperature contribute to melting beneath Iceland, the present day manifestation of the mantle plume implicated in the 54Ma break up of the North Atlantic. By considering both the igneous crustal production on Iceland and the chemistry of its basalts we have been able to place stringent constraints on the viable temperature and lithology of the Icelandic mantle. Although a >100°C excess temperature is required to generate Iceland's thick igneous crust, geochemistry also indicates that pyroxenite comprises 10% of its source. Therefore, the dynamics of rifting on Iceland are modulated both by thermal and compositional mantle anomalies. Secondly, we have performed a global assessment of the mantle's post break-up thermal history to determine the amplitude and longevity of continental insulation in driving excess volcanism. Using seismically constrained igneous crustal

  4. Crustal Rheology and Rifted Margin Architecture: Comparing Iberia-Newfoundland, Central South Atlantic, and South China Sea

    NASA Astrophysics Data System (ADS)

    Brune, Sascha

    2015-04-01

    migration of the rift centre, which generates sequential fault activity within the brittle crust. Rift migration results from two processes: (i) Strain hardening takes place in the rift centre due to cooling of upwelling mantle material. (ii) The formation of a low viscosity crustal pocket adjacent to the rift centre is caused by heat transfer from the mantle and viscous strain softening of the lower crust. These mechanisms generate a lateral strength contrast that promotes rift migration in a steady-state manner forming a wide sliver of hyper-extended crust on one margins side, while the conjugate margin becomes narrow. In contrast to these Atlantic examples where wide margins are formed diachronously, the South China Sea evolved in wide rift mode. Here, several hundred kilometres of highly attenuated continental crust are deformed simultaneously during ~40 My of extension. Numerical modelling suggests that the presence of weak, ductile crust enabled the formation of two wide and symmetric margins. Independent indicators for a weak crust come from super-deep basins on the northern margin. These basins appear to be created after the end of active extension and with a significant deficit in brittle faulting, which suggests that subsidence was controlled by sediment loading and accommodated by lower crustal flow, a style of basin formation that is only possible in the presence of low crustal viscosity.

  5. The rift to drift evolution of the South China Sea

    NASA Astrophysics Data System (ADS)

    Ranero, Cesar R.; Cameselle, Alejandra; Franke, Dieter; Barckhausen, Udo

    2016-04-01

    Re-processing with modern algorithms of multichannel seismic reflection records from the South China Sea provide novel images on the crustal structure of the continental margin and its boundary zone with the oceanic crust (COB). The selected re-processed seismic lines strike perpendicular to the margins' trend and cross the entire basin, providing complementary images of conjugated rift segments of the NW, SW, and E sub-basins. Re-processed sections image the post-rift and syn-rift sediment, and fault-bounded basement blocks, often also intra-crustal fault reflections that together provide detailed information of the tectonic structural style during rifting. Further, the largest imaging improvement has been obtained in the delineation of -very often- clear fairly continuous reflections from the crust-mantle boundary across the continental margin into the oceanic crust. The images show how crustal thickness and structure change in parallel to changes in the tectonic style of the deformation during the evolution of the rift. The interpreted COB occurs in regions where the tectonic style displays the most noticeable changes from segments where extension is dominated by normal faulting to segments where faulting is comparatively minor and the crust shows fairly gentle lateral thickness variations; these latter segments are interpreted as oceanic crust. The identification of the continental and oceanic tectonic domains permits to study the along-strike evolution in rifting processes and rift segmentation. Also, the comparison of the tectonic structure of the conjugated flanks of the continental rift across the ocean basins is used to understand the last stages of rifting and the relative importance of tectonic extension and magmatism in final break up and spreading initiation. Although there is ample evidence of important volcanism in the images, with some spectacular large conical volcanoes formed over continental crust and numerous sill-like reflections in the

  6. 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].

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

    NASA Astrophysics Data System (ADS)

    Meyer, R.; Hertogen, J.

    2008-12-01

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

  8. Sedimentary sequences of the Pacific-Indian Ocean sector of the Antarctic continental margin

    SciTech Connect

    Cooper, A.; Eittreim, S. ); Anderson, J. ); Stagg, H. )

    1990-06-01

    Seismic-reflection data across the Pacific-Indian Ocean sector of the Antarctic continental margin commonly reveal preglacial and glacial sedimentary sections up to 14 km thick. In this sector, diverse tectonic regimes have controlled the locations of preglacial rift deposits as well as glacial-till deltas. These regimes include major rift embayments, passive margins, formerly active and presently active margins, and active rifts. The sedimentary sections are principally of Mesozoic and Cenozoic age, although Paleozoic strata may exist at great depth. The upper parts of these sections commonly comprise prograding and aggrading sigmoidal sequences that are separated by unconformities and are up to 6 km thick. Where drilled in Prydz Bay and the Ross Sea, these upper sequences are solely glacial marine rocks of early Oligocene and younger age. The lower portions of the sections are commonly well-layered sequences that infill structural basins. The evolution of these sedimentary sequences is strongly controlled by extensional tectonic processes. Depocenters are located primarily within rift structures that formed initially during Gondwana breakup and later during magmatic-arc development. Rift-related deposits fill the basement grabens and are unconformably covered by glacial-till deltas. The till deltas apparently have been deposited beneath and at the front of former grounded ice sheets that selectively moved through rift embayments and over thermally subsiding margins. Since initial Cenozoic glaciation, these thick till deltas have prograded the continental shelf edge up to 70 km seaward to its present location. The sedimentary sequences underlying the Antarctic margin hold a record of Antarctic (Gondwana) rifting and glaciation - a record that would, if drilled, greatly improve their understanding of global climate and sea-level changes.

  9. The subsurface structure and stratigraphic architecture of rift-related units in the Lishu Depression of the Songliao Basin, China

    NASA Astrophysics Data System (ADS)

    Wang, Hongyu; Fan, Tailiang; Wu, Yue

    2015-03-01

    This contribution reports the basin configuration feature, stratigraphy and sedimentary architecture of the Lishu Depression in the Songliao Basin, China. The activity rate, distribution and style of local faulting demonstrate the timing and extent of regional rifting. Distinct episodes of compressional tectonic activity caused uplift and exposure of strata evident as the traditional syn- and post-rift stages of basin evolution. These episodes led to the sequential denudation of the Upper Jurassic Huoshiling Formation, Lower Cretaceous Yingcheng and Denglouku Formations, and corresponding regional unconformities. Acting in tandem with regional compression, activity along the major boundary faults influenced the evolving basin configuration, as well as seismic sequences and sedimentary patterns. Seismic, well log and drill core data described here show subdivision sections of the Lishu Depression strata according to discrete phases of the traditional syn-rift stage of deposition. We refer to these sub-stages as the initial rifting, the intensive rifting and the recession phases. The basin configuration shifted from a graben/half-graben configuration during the initial rifting phase, to a dustpan-shaped half-graben pattern during the subsequent phase of intensive rifting, and finally into a gentle sedimentary basin during the final recession phase. The early seismic sequence divides into a lowstand systems tract (LST), transgressive systems tract (TST) and highstand systems tract (HST). Evidence of the LST within the seismic sequence becomes less apparent with the intensive rifting phase, while the HST occupied an increasing proportion of the section. The shallow water depositional fill formed during the final recession phase consists only of TST and HST components. Depositional environment then shifts from alluvial fan and shallow lacustrine systems to fan delta, braided delta - lake, and finally to a braided fluvial setting. The vertical stacking pattern shifts

  10. The offshore East African Rift System: Structural framework at the toe of a juvenile rift

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

    The Cenozoic East African Rift System (EARS) extends from the Red Sea to Mozambique. Here we use seismic reflection and bathymetric data to investigate the tectonic evolution of the offshore branch of the EARS. The data indicate multiple and time transgressive neotectonic deformations along ~800 km of the continental margin of northern Mozambique. We observe a transition from a mature rift basin in the north to a juvenile fault zone in the south. The respective timing of deformation is derived from detailed seismic stratigraphy. In the north, a ~30 km wide and more than 150 km long, N-S striking symmetric graben initiated as half-graben in the late Miocene. Extension accelerated in the Pliocene, causing a continuous conjugate border fault and symmetric rift graben. Coevally, the rift started to propagate southward, which resulted in a present-day ~30 km wide half-graben, approximately 200 km farther south. Since the Pleistocene, the rift has continued to propagate another ~300 km, where the incipient rift is reflected by subrecent small-scale normal faulting. Estimates of the overall brittle extension of the matured rift range between 5 and 12 km, with an along-strike southward decrease of the extension rate. The offshore portion of the EARS evolves magma poor, similar to the onshore western branch. The structural evolution of the offshore EARS is suggested to be related to and controlled by differing inherited lithospheric fabrics. Preexisting fabrics may not only guide and focus extension but also control rift architecture.

  11. Active mud volcanoes on the continental slope of the Canadian Beaufort Sea

    NASA Astrophysics Data System (ADS)

    Paull, C. K.; Dallimore, S. R.; Caress, D. W.; Gwiazda, R.; Melling, H.; Riedel, M.; Jin, Y. K.; Hong, J. K.; Kim, Y.-G.; Graves, D.; Sherman, A.; Lundsten, E.; Anderson, K.; Lundsten, L.; Villinger, H.; Kopf, A.; Johnson, S. B.; Hughes Clarke, J.; Blasco, S.; Conway, K.; Neelands, P.; Thomas, H.; Côté, M.

    2015-09-01

    Morphologic features, 600-1100 m across and elevated up to 30 m above the surrounding seafloor, interpreted to be mud volcanoes were investigated on the continental slope in the Beaufort Sea in the Canadian Arctic. Sediment cores, detailed mapping with an autonomous underwater vehicle, and exploration with a remotely operated vehicle show that these are young and actively forming features experiencing ongoing eruptions. Biogenic methane and low-chloride, sodium-bicarbonate-rich waters are extruded with warm sediment that accumulates to form cones and low-relief circular plateaus. The chemical and isotopic compositions of the ascending water indicate that a mixture of meteoric water, seawater, and water from clay dehydration has played a significant role in the evolution of these fluids. The venting methane supports extensive siboglinid tubeworms communities and forms some gas hydrates within the near seafloor. We believe that these are the first documented living chemosynthetic biological communities in the continental slope of the western Arctic Ocean.

  12. Where is the West Antarctic Rift System in the Amundsen Sea and Bellingshausen Sea sectors?

    NASA Astrophysics Data System (ADS)

    Gohl, Karsten; Kalberg, Thomas; Eagles, Graeme; Dziadek, Ricarda; Kaul, Norbert; Spiegel, Cornelia; Lindow, Julia

    2015-04-01

    The West Antarctic Rift System (WARS) is one of the largest continental rifts globally, but its lateral extent, distribution of local rifts, timing of rifting phases, and mantle processes are still largely enigmatic. It has been presumed that the rift and its crustal extensional processes have widely controlled the history and development of West Antarctic glaciation with an ice sheet of which most is presently based at sub-marine level and which is, therefore, likely to be highly sensitive to ocean warming. While the western domain of the WARS in the Ross Sea has been studied in some detail, only recently have various geophysical and geochemical/thermochronological analyses revealed indications for its eastern extent in the Amundsen Sea and Bellingshausen Sea sectors of the South Pacific realm. The current model, based on these studies and additional data, suggests that the WARS activity included tectonic translateral, transtensional and extensional processes from the Amundsen Sea Embayment to the Bellingshausen Sea region of the southern Antarctic Peninsula. We present the range of existing hypotheses regarding the extent of the eastern WARS as well as published and yet unpublished data that support a conceptual WARS model for the eastern West Antarctica with implications for glacial onset and developments.

  13. Late Miocene termination of tectonic activity on the detachment in the Alaşehir Rift, Western Anatolia: Depositional records of the Göbekli Formation and high-angle cross-cutting faults

    NASA Astrophysics Data System (ADS)

    Sen, Fatih

    2016-04-01

    Western Anatolia is a well-known province of continental extension in the world. Most distinctive structural elements of the region are E-W trending grabens. The Alaşehir Rift/Graben is an asymmetric rift/graben trending E-W between Ahmetli and Turgutlu in its western part and continues eastwardly in a NW-SE direction to Alaşehir (Philadelphia in ancient Greek). The stratigraphy of the region consists of metamorphic rocks of the Menderes Massif (Paleozoic-lower Cenozoic) and the syn-extensional Salihli granitoid (middle Miocene) forming the basement unit and overlying sedimentary cover rocks of Neogene-Quaternary. These rocks are cut and deformed by the Karadut detachment fault and various low-angle normal faults (antithetic and synthetic faults of the Karadut detachment fault), which are also cut by various younger high-angle normal faults. It is possible to observe two continuous sequences of different time intervals in that Miocene deposits of the first rifting phase are covered by Plio-Quaternary sediments of second rifting phase with a "break-up" unconformity. In lower levels of a measured stratigraphic section (583 m) of the Göbekli formation which has lower age of late Miocene and upper age of early Pliocene, the presence of angular to sub-angular clasts of the blocks and conglomerates suggests alluvial-fun origin during an initial stage of deposition. Existence of normal-reverse graded, cross-bedding, pebble imbrications in layers of the pebbly sandstone demonstrates fluvial environment in following levels of the sequence. Existence of lenses and normal graded conglomerates in pebbly sandstones and fine grained sandstones strata evidences a low energy environment. Observed siltstone-claystone intercalations on the middle levels of the sequence indicate an environment with low dipping morphology to be formed as flat plains during this period. In the uppermost levels of the sequence, existence of the pebble imbrications inside pebbly sandstones overlying

  14. Initiation of extension in South China continental margin during the active-passive margin transition: kinematic and thermochronological constraints

    NASA Astrophysics Data System (ADS)

    ZUO, Xuran; CHAN, Lung

    2015-04-01

    The southern South China Block is characterized by a widespread magmatic belt, prominent NE-striking fault zones and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from an active to a passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in this area. In this study, we used zircon fission-track dating (ZFT) and numerical modeling to examine the timing and kinematics of the active-passive margin transition. Our ZFT results on granitic plutons in the SW Cathaysia Block show two episodes of exhumation of the granitic plutons. The first episode, occurring during 170 Ma - 120 Ma, affected local parts of the Nanling Range. The second episode, a more regional exhumation event, occurred during 115 Ma - 70 Ma. Numerical geodynamic modeling was conducted to simulate the subduction between the paleo-Pacific plate and the South China Block. The modeling results could explain the observation based on ZFT data that exhumation of the granite-dominant Nanling Range occurred at an earlier time than the gneiss-dominant Yunkai Terrane. In addition to the difference in geology between Yunkai and Nanling, the heating from Jurassic-Early Cretaceous magmatism in the Nanling Range may have softened the upper crust, causing the area to exhume more readily. Numerical modeling results also indicate that (1) high slab dip angle, high geothermal gradient of lithosphere and low convergence velocity favor the subduction process and the reversal of crustal stress state from compression to extension in the upper plate; (2) the late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The inversion of stress regime in the continental crust from compression to crustal extension has shed light on the geological condition producing the red bed basins during Late Cretaceous

  15. Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins

    NASA Astrophysics Data System (ADS)

    Gerya, Taras; Stöckhert, Bernhard

    2006-04-01

    The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each model, representative P T paths are displayed for selected markers. Both the intensity of subduction erosion and the size of the frontal accretionary wedge are strongly dependent on the rheology chosen for the overriding continental crust. Tectonically eroded upper and lower continental crust is carried down to form a broad orogenic wedge, intermingling with detached oceanic crust and sediments from the subducted plate and hydrated mantle material from the overriding plate. A small portion of the continental crust and trench sediments is carried further down into a narrow subduction channel, intermingling with oceanic crust and hydrated mantle material, and to some extent extruded to the rear of the orogenic wedge underplating the overriding continental crust. The exhumation rates for (ultra)high pressure rocks can exceed subduction and burial rates by a factor of 1.5 3, when forced return flow in the hanging wall portion of the self-organizing subduction channel is focused. The simulations suggest that a minimum rate of subduction is required for the formation of a subduction channel, because buoyancy forces may outweigh drag forces for slow subduction. For a weak upper continental crust, simulated by a high pore pressure coefficient in the brittle regime, the orogenic wedge and megascale melange reach a mid- to upper-crustal position within 10 20 Myr (after 400 600 km of subduction). For a strong upper crust, a continental lid persists over the entire time span covered by the simulation. The structural pattern is similar in all cases, with four zones from trench toward arc

  16. The influence of pre-existing structures on the evolution of the southern Kenya Rift Valley — evidence from seismic and gravity studies

    NASA Astrophysics Data System (ADS)

    Birt, C. S.; Maguire, P. K. H.; Khan, M. A.; Thybo, H.; Keller, G. R.; Patel, J.

    1997-09-01

    The Kenya Rift is an active continental rift that has developed since the Late Oligocene. Although a thermal origin for the rifting episode is indicated by the scale of volcanism and its relative timing with uplift and faulting, the influence of pre-existing lithospheric structural controls is poorly understood. The interpretation of a 430-km-long seismic refraction and gravity line across the southern part of the Kenya Rift shows that the rift is developed across a transition zone, thought to represent the sheared Proterozoic boundary between the Archaean Nyanza Craton and the mobile Mozambique Belt. This zone of weakness has been exploited by the recent thermal rifting event. The Moho is at a depth of 33 km beneath the Archaean craton in the western part of the profile, and 40 km beneath the Mozambique Belt in the east. A few kilometres of localised crustal thinning has developed across the transition from thin to thick crust. At the surface, brittle faulting has formed an asymmetric rift basin 3.6 km deep, filled with low-velocity volcanic rocks. Basement velocities show a transition across the same area from low velocities (6.0 km s -1) in the Archaean, to high velocities (6.35 km s -1) in the Proterozoic. Mid-crustal layers show no deformation that can be attributed to the rifting event. Poorly constrained upper mantle velocities of 7.8 km s -1 beneath the southern rift confirm the continuation of the axial low-velocity zone imaged in previous seismic experiments. This is interpreted as the effect of small degrees of partial melt caused by elevated mantle temperatures. Gravity modelling suggests a contribution to the Bouguer anomaly from below the Moho, invoking the need for deep density contrasts. The regional gravity gradient necessary to model the Bouguer anomaly is used as supporting evidence for mantle-plume type circulation beneath the uplifted East African Plateau to the west of the Kenya Rift.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  18. Volcanic history of the Colorado River extensional corridor: Active or passive rifting

    SciTech Connect

    Howard, K.A. )

    1993-04-01

    Magmatism and extension began nearly simultaneously in the Colorado River extensional corridor (CREC) between 34 and 35[degree] N. Initial eruptions of basanite at 23--19.5 Ma were low-volume but spanned a region now twice as wide as the 100-km-wide corridor. Extensional tilting of this age was local. A large flux of calc-alkaline basalt, andesite, dacite, and rhyolite was erupted at 22--18.5 Ma. They accumulated to average thicknesses of [approximately]1 km in the early CREC basin, and were accompanied by extensional tilting. Dike swarms, necks, and plutons represent intrusive equivalents. Plutons concentrate in the central belt of metamorphic core complexes, the most highly extended areas. Massive eruption at 18.5 Ma of the rhyolitic Peach Springs Tuff marked an ensuing lowered rate of volcanic output, a change to bimodal volcanism, much tilting and extension, and deposition of thick (to [approximately]2 km) synextensional clastic sediments 18--14 Ms. By 14--12 Ma, extensional tilting had largely ceased, and eruptions were sparse and basaltic only, as they have been since. Basalt compositions reveal changing patterns of trace-element composition that bear on sources. The early basanites have OIB-like compositions on spidergram plots, suggesting origin from the asthenosphere as would be expected from initiation of rifting driven by hot mantle upwelling. Basalts 20--12 Ma show low concentrations of Nb and Ta as in subduction-related arc magmas. Post-extensional basalts erupted 15--10 Ma exhibit a transition back toward primitive compositions seen in Quaternary alkalic basalts.

  19. Morphology of turbidite systems within an active continental margin (the Palomares Margin, western Mediterranean)

    NASA Astrophysics Data System (ADS)

    Perez-Hernandez, S.; Comas, M. C.; Escutia, C.

    2014-08-01

    The Palomares Margin, an NNE-SSW segment of the South Iberian Margin located between the Alboran and the Algerian-Balearic basins, is dissected by two major submarine canyon systems: the Gata (in the South) and the Alías-Almanzora (in the North). New swath bathymetry, side-scan sonar images, accompanied by 5 kHz and TOPAS subbottom profiles, allow us to recognize these canyons as Mediterranean examples of medium-sized turbidite systems developed in a tectonically active margin. The Gata Turbidite System is confined between residual basement seamounts and exhibits incised braided channels that feed a discrete deep-sea fan, which points to a dominantly coarse-grained turbiditic system. The Alías-Almanzora Turbidite System, larger and less confined, is a good example of nested turbiditic system within the canyon. Concentric sediment waves characterize the Alías-Almanzora deep-sea fan, and the size and acoustic character of these bedforms suggest a fine-grained turbidite system. Both canyons are deeply entrenched on a narrow continental shelf and terminate at the base of the continental slope as channels that feed deep sea fans. While the Alías-Almanzora Turbidite System is the offshore continuation of seasonal rivers, the Gata Turbidite System is exclusively formed by headward erosion along the continental slope. In both cases, left-lateral transpressive deformation influences their location, longitudinal profiles, incision at the upper sections, and canyon bending associated with specific fault segments.

  20. Relative earthquake location for remote offshore and tectonically active continental regions using surface waves

    NASA Astrophysics Data System (ADS)

    Cleveland, M.; Ammon, C. J.; Vandemark, T. F.

    2015-12-01

    Earthquake locations are a fundamental parameter necessary for reliable seismic monitoring and seismic event characterization. Within dense continental seismic networks, event locations can be accurately and precisely estimated. However, for many regions of interest, existing catalog data and traditional location methods provide neither accurate nor precise hypocenters. In particular, for isolated continental and offshore areas, seismic event locations are estimated primarily using distant observations, often resulting in inaccurate and imprecise locations. The use of larger, moderate-size events is critical to the construction of useful travel-time corrections in regions of strong geologic heterogeneity. Double difference methods applied to cross-correlation measured Rayleigh and Love wave time shifts are an effective tool at providing improved epicentroid locations and relative origin-time shifts in these regions. Previous studies have applied correlation of R1 and G1 waveforms to moderate-magnitude vertical strike-slip transform-fault and normal faulting earthquakes from nearby ridges. In this study, we explore the utility of phase-match filtering techniques applied to surface waves to improve cross-correlation measurements, particularly for smaller magnitude seismic events. We also investigate the challenges associated with applying surface-wave location methods to shallow earthquakes in tectonically active continental regions.

  1. Comparison of marine gas hydrates in sediments of an active and passive continental margin

    USGS Publications Warehouse

    Kvenvolden, K.A.

    1985-01-01

    Two sites of the Deep Sea Drilling Project in contrasting geologic settings provide a basis for comparison of the geochemical conditions associated with marine gas hydrates in continental margin sediments. Site 533 is located at 3191 m water depth on a spit-like extension of the continental rise on a passive margin in the Atlantic Ocean. Site 568, at 2031 m water depth, is in upper slope sediment of an active accretionary margin in the Pacific Ocean. Both sites are characterized by high rates of sedimentation, and the organic carbon contents of these sediments generally exceed 0.5%. Anomalous seismic reflections that transgress sedimentary structures and parallel the seafloor, suggested the presence of gas hydrates at both sites, and, during coring, small samples of gas hydrate were recovered at subbottom depths of 238m (Site 533) and 404 m (Site 568). The principal gaseous components of the gas hydrates wer methane, ethane, and CO2. Residual methane in sediments at both sites usually exceeded 10 mll-1 of wet sediment. Carbon isotopic compositions of methane, CO2, and ??CO2 followed parallel trends with depth, suggesting that methane formed mainly as a result of biological reduction of oxidized carbon. Salinity of pore waters decreased with depth, a likely result of gas hydrate formation. These geochemical characteristics define some of the conditions associated with the occurrence of gas hydrates formed by in situ processes in continental margin sediments. ?? 1984.

  2. Anatomy of an earthquake multiplet active over several years in the western part of the Corinth rift

    NASA Astrophysics Data System (ADS)

    Godano, Maxime; Bernard, Pascal; Marsan, David; Dublanchet, Pierre

    2014-05-01

    The Corinth rift is one of the most seismically zones in Europe. The seismic activity is characterized by numerous multiplets. A multiplet is a group of earthquakes with similar waveforms resulting from close location and focal mechanism. Multiplets are often associated with small asperities and can be seen as repeated ruptures due to transient forcing as silent creep or pore pressure front diffusion. Detailed analysis of the multiplets in the Corinth rift is an opportunity to better understand fault dynamics, small earthquake rupture mechanics and coupling with aseismic processes. We focus on a large multiplet (500 x 500 m) located under the northern coast of the Corinth gulf at 8 km depth. This multiplet was more or less regularly active between 2000 and 2007. During this period, 56 events were recorded. The most observed recurrence time is of 23 days but can vary between 1 and 115 days. We estimate the source parameters of the 56 earthquakes by following a two-step approach based on the analysis of the displacement seismic spectrum. First, the scalar seismic moment and the magnitude are computed from the amplitude of the low frequency part (plateau) of the P and S spectrum. Second the source size is calculated from the P and S corner frequencies. Corner frequencies are determined by inverting spectral ratio (i.e. the ratio between the spectra of two collocated earthquakes). The advantage of working with spectral ratio is to eliminate the trade-off between corner frequency and anelastic attenuation if Q factor is poorly known. Spectral ratio inversion is performed following a Bayesian formalism. The magnitudes scale between 1.20 and 2.76. The seismic activity is characterized by relatively high magnitude events (b-value = 0.82) until the mainshock (mid-2003) and low magnitude events after (b-value = 1.21). The source radii globally range between 50 and 200 m. The source overlapping is strong; some fault patches have ruptured up to19 times which has produced a

  3. Colorado Basin Structure and Rifting, Argentine passive margin

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    partly supports this hypothesis and shows two main directions of faulting: margin-parallel faults (~N30°) and rift-parallel faults (~N125°). A specific distribution of the two fault sets is observed: margin-parallel faults are restrained to the most distal part of the margin. Starting with a 3D structural model of the basin fill based on seismic and well data the deeper structure of the crust beneath the Colorado Basin can be evaluate using isostatic and thermal modelling. Franke, D., et al. (2002), Deep Crustal Structure Of The Argentine Continental Margin From Seismic Wide-Angle And Multichannel Reflection Seismic Data, paper presented at AAPG Hedberg Conference "Hydrocarbon Habitat of Volcanic Rifted Passive Margins", Stavanger, Norway Franke, D., et al. (2006), Crustal structure across the Colorado Basin, offshore Argentina Geophysical Journal International 165, 850-864. Gladczenko, T. P., et al. (1997), South Atlantic volcanic margins Journal of the Geological Society, London 154, 465-470. Hinz, K., et al. (1999), The Argentine continental margin north of 48°S: sedimentary successions, volcanic activity during breakup Marine and Petroleum Geology 16(1-25). Hirsch, K. K., et al. (2009), Tectonic subsidence history and thermal evolution of the Orange Basin, Marine and Petroleum Geology, in press, doi:10.1016/j.marpetgeo.2009.1006.1009

  4. Magma-assisted rifting in Ethiopia.

    PubMed

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

    2005-01-13

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

  5. Mantle support of the East African Rift System

    NASA Astrophysics Data System (ADS)

    Lin, S.; van Keken, P. E.; Brandenburg, J. P.; Furman, T.; Bryce, J.

    2007-12-01

    The African Superplume is a region of slow seismic wave velocities in the lower mantle under southern Africa. The uplift, volcanism and rifting that defines the much of eastern and southern Africa suggest a dynamic link between lower mantle dynamics and near-surface processes affecting the African plate. The dynamic link between the lower mantle and the surface, and the structure and dynamics of the upper mantle below the East African Rift System (EARS) remain unclear. As part of a comprehensive geochemical and numerical investigation of basaltic magmatism in the EARS we have modeled the interaction between putative upper mantle plumes and the rifting continental lithosphere. The modeling provides dynamically tested scenarios that explain the observed episodes of Cenozoic volcanism. Results from recent models that provided an explanation for the present day distribution of volcanism (Lin et al., EPSL, 237, 2005) suggest two plumes below Afar and Tanzania whose uplift is influenced by lithospheric topography. In new 3D modeling we provide improved quantification of the mantle involvement in generating EARS volcanism as constrained by the timing of uplift and regional volcanism. The time scales of episodicity of the volcanism observed at Turkana (related to the Tanzania-Kenya plume) since 45 Ma can be explained by deep- seated time-dependent plume activity. We suggest that this time-dependence is due to thermochemical interactions of dense recycled oceanic crust in the thermally hot regions in the African superplume region (Lin and Van Keken, Nature, 436, 2005).

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

    NASA Technical Reports Server (NTRS)

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

    1985-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Gilbert, M. C.

    2004-12-01

    The Southern Oklahoma Aulacogen (SOA), at the southern end of Laurentia (present coordinates), if behaving as neo-rifts, such as the Rio Grande Rift, presumably possessed a rift structure in the Cambrian with a continental thickness of about 28km. Seismic data, though sparse, suggest a present thickness of the SOA is about 45km, indistinguishable from adjacent rifted Proterozoic crust. By what process do we add 15km to the original SOA crust: underplating, eclogite-gabbro transformation, or deformation? This question has bearing on how we understand and interpret all paleo-rifts now a part of continental cores. Geology of the southern Midcontinent of North America does not show evidence of significant thermal events in the Phanerozoic. This effectively rules out underplating and phase transformation as a cause of change in M-discontinuity depth. Present SOA outcrops are in the Wichita Mountains of southwestern Oklahoma, part of the easternmost Ancestral Rockies. These outcrops are in the Wichita-Amarillo crustal block uplifted about 7km in the Pennsylvanian. The Anadarko Basin to the north went down about 7km. Large Pennsylvanian thrust faults in the upper brittle crust are documented. Thus it appears that compressive deformation may be able to account for the change in crustal thickness from neo-rift type to paleo-rift and craton type. However, the accommodation made in the lower crust may be more dramatic than deformation in the upper crust because shortening, and thickening of the order of 2X, is probably required. Comparisons with other paleo-rifts in North America, such as the Middle Proterozoic Midcontinent Rift and the NeoProterozoic Reelfoot Rift, show that their crustal thicknesses now also match their previously rifted margins. Can the same sequence, as seems to be the case with the SOA, apply to other paleo-rifts?

  8. 40Ar- 39Ar ages of intrusions in East Greenland: Rift-to-drift transition over the Iceland hotspot

    NASA Astrophysics Data System (ADS)

    Tegner, C.; Brooks, C. K.; Duncan, R. A.; Heister, L. E.; Bernstein, S.

    2008-03-01

    Sixteen 40Ar- 39Ar ages are presented for alkaline intrusions to appraise prolonged post-breakup magmatism of the central East Greenland rifted margin, the chronology of rift-to-drift transition, and the asymmetry of magmatic activity in the Northeast Atlantic Igneous Province. The alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland. The area south of the Kangerlussuaq Fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56-54 Ma, 50-47 Ma and 37-35 Ma. In the Kangerlussuaq Fjord, which coincides with a major tectonic lineament possibly the failed arm of a triple junction, the alkaline plutons span from 56 to 40 Ma. To the north and within the continental flood basalt succession, alkaline intrusions of the north-south trending Wiedemann Fjord-Kronborg Gletscher lineament range from 52 to 36 Ma. We show that post-breakup magmatism of the East Greenland rifted margin can be linked to reconfiguration of spreading ridges in the Northeast Atlantic. Northwards propagation of the proto-Kolbeinsey ridge rifted the Jan Mayen micro-continent away from central East Greenland and resulted in protracted rift-to-drift transition. The intrusions of the Wiedemann Fjord-Kronborg Gletscher lineament are interpreted as a failed continental rift system and the intrusions of the Kangerlussuaq Fjord as off-axis magmatism. The post-breakup intrusions south of Kangerlussuaq Fjord occur landward of the Greenland-Iceland Rise and are explained by mantle melting caused first by the crossing of the central East Greenland rifted margin over the axis of the Iceland mantle plume (50-47 Ma) and later by uplift associated with regional plate-tectonic reorganization (37-35 Ma). The Iceland mantle plume was instrumental in causing protracted rift-to-drift transition and post-breakup tholeiitic and alkaline magmatism on the East Greenland rifted margin, and asymmetry

  9. 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.

  10. Phanerozoic Rifting Phases And Mineral Deposits

    NASA Astrophysics Data System (ADS)

    Hassaan, Mahmoud

    2016-04-01

    connected with NW,WNW and N-S faults genetically related to volcano-hydrothermal activity associated the Red Sea rifting. At Sherm EL-Sheikh hydrothermal manganese deposit occurs in Oligocene clastics within fault zone. Four iron-manganese-barite mineralization in Esh-Elmellaha plateau are controlled by faults trending NW,NE and nearly E-W intersecting Miocene carbonate rocks. Barite exists disseminated in the ores and as a vein in NW fault. In Shalatee - Halaib district 24 manganese deposits and barite veins with sulphide patches occur within Miocene carbonates distributed along two NW fault planes,trending 240°and 310° and occur in granite and basalt . Uranium -lead-zinc sulfide mineralization occur in Late Proterozoic granite, Late Cretaceous sandstones, and chiefly in Miocene clastic-carbonate-evaporate rocks. The occurrences of uranium- lead-zinc and iron-manganese-barite mineralization have the characteristic features of hypogene cavity filling and replacement deposits correlated with Miocene- Recent Aden volcanic rocks rifting. In western Saudi Arabia barite-lead-zinc mineralization occurs at Lat. 25° 45' and 25° 50'N hosted by Tertiary sediments in limestone nearby basaltic flows and NE-SW fault system. The mineralized hot brines in the Red Sea deeps considered by the author a part of this province. The author considers the constant rifting phases of Pangea and then progressive fragmentation of Western Gondwana during the Late Carboniferous-Lias, Late Jurassic-Early Aptian, Late Aptian - Albian and Late Eocene-Early Miocene and Oligocene-Miocene, responsible for formation of the mineral deposits constituting the M provinces. During these events, rifting, magmatism and hydrothermal activities took place in different peri-continental margins.

  11. The rift to break-up evolution of the Gulf of Aden: Insights from 3D numerical lithospheric-scale modelling

    NASA Astrophysics Data System (ADS)

    Brune, Sascha; Autin, Julia

    2013-11-01

    The Gulf of Aden provides an ideal setting to study oblique rifting since numerous structural data are available onshore and offshore. Recent surveys showed that the spatio-temporal evolution of the Gulf of Aden rift system is dominated by three fault orientations: displacement-orthogonal (WSW), rift-parallel (WNW) and an intermediate E-W trend. The oldest parts of the rift that are exposed onshore feature displacement-orthogonal and intermediate directions, whereas the subsequently active necking zone involves mainly rift-parallel faults. The final rift phase recorded at the distal margin is characterised by displacement-orthogonal and intermediate fault orientations. We investigate the evolution of the Gulf of Aden from rift initiation to break-up by means of 3D numerical experiments on lithospheric scale. We apply the finite element model SLIM3D which includes realistic, elasto-visco-plastic rheology and a free surface. Despite recent advances, 3D numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved. We address this issue by proposing a simple post-processing method that uses the surface stress-tensor to evaluate stress regime (extensional, strike-slip, compressional) and preferred fault azimuth. The described method is applicable to any geodynamic model and easy to introduce. Our model reproduces the observed fault pattern of the Gulf of Aden and illustrates how multiple fault directions arise from the interaction of local and far-field tectonic stresses in an evolving rift system. The numerical simulations robustly feature intermediate faults during the initial rift phase, followed by rift-parallel normal faulting at the rift flanks and strike-slip faults in the central part of the rift system. Upon break-up, displacement-orthogonal as well as intermediate faults occur. This study corroborates and extends findings from previous analogue experiments of oblique rifting on lithospheric scale and allows new

  12. Rifting and breakup in the South China Sea

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    The magma-poor or intermediate magmatic South China Sea is a natural laboratory for studying rifting and breakup. The basin shows an irregular triangular shape with a SW pointing apex, which manifests a preceding propagating rift. The earliest phase of rifting started in the Early Paleocene when a Mesozoic convergent margin changed to extension. After about 30 Million years of rifting, breakup in the major eastern subbasin of the SCS occurred in the Early Oligocene but rifting continued and subsequent breakup of the southwest subbasin took place in the Late Oligocene. The wide Early Cenozoic South China Sea rift preserves the initial rift architecture at the distal margins. Seismic reflection data imaging conjugate crustal sections at the South China Sea margins result in a conceptual model for rift-evolution at conjugate magma-poor margins in time and space. Most distinct are regular undulations in the crust-mantle boundary. Individual rift basins are bounded to crustal blocks by listric normal faults on either side. Moho uplifts are distinct beneath major rift basins, while the Moho is downbended beneath crustal blocks, with a wavelength of undulations in the crust-mantle boundary that approximately equals the thickness of the continental crust. Most of the basin-bounding faults sole out within the middle crust. At the distal margins, detachment faults are located at a mid-crustal level where a weak zone decouples crust and mantle lithosphere during rifting. The lower crust in contrast is interpreted as being strong. Only in the region within about 50 km from the continent-ocean transition (COT) we suggest that normal faults reach the mantle, enabling potentially a coupling between the crust and the mantle. Here, at the proximal margins detachment fault dip either seaward or landward. This may indicate the presence of exhumed mantle bordering the continental margins. Post-rift shallow-water platform carbonates indicate a delay in subsidence during rifting in the

  13. 76 FR 79705 - Information Collection Activities: Operations in the Outer Continental Shelf for Minerals Other...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2011-12-22

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  14. 77 FR 15118 - Information Collection Activities: Operations in the Outer Continental Shelf for Minerals Other...

    Federal Register 2010, 2011, 2012, 2013, 2014

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  15. Lower-crustal intrusion on the North Atlantic continental margin.

    PubMed

    White, R S; Smith, L K; Roberts, A W; Christie, P A F; Kusznir, N J; Roberts, A M; Healy, D; Spitzer, R; Chappell, A; Eccles, J D; Fletcher, R; Hurst, N; Lunnon, Z; Parkin, C J; Tymms, V J

    2008-03-27

    When continents break apart, the rifting is sometimes accompanied by the production of large volumes of molten rock. The total melt volume, however, is uncertain, because only part of it has erupted at the surface. Furthermore, the cause of the magmatism is still disputed-specifically, whether or not it is due to increased mantle temperatures. We recorded deep-penetration normal-incidence and wide-angle seismic profiles across the Faroe and Hatton Bank volcanic margins in the northeast Atlantic. Here we show that near the Faroe Islands, for every 1 km along strike, 360-400 km(3) of basalt is extruded, while 540-600 km(3) is intruded into the continent-ocean transition. We find that lower-crustal intrusions are focused mainly into a narrow zone approximately 50 km wide on the transition, although extruded basalts flow more than 100 km from the rift. Seismic profiles show that the melt is intruded into the lower crust as sills, which cross-cut the continental fabric, rather than as an 'underplate' of 100 per cent melt, as has often been assumed. Evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures, with no requirement for either significant active small-scale mantle convection under the rift or the presence of fertile mantle at the time of continental break-up, as has previously been suggested for the North Atlantic Ocean. PMID:18368115

  16. 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

  17. 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

  18. Post-rift tectonic reactivation and its effect on deep-water deposits in the Qiongdongnan Basin, northwestern South China Sea

    NASA Astrophysics Data System (ADS)

    Mao, Kainan; Xie, Xinong; Xie, Yuhong; Ren, Jianye; Chen, Hui

    2015-09-01

    The post-rift evolution of extensional basins is traditionally thought to be dominated by thermal subsidence due to cessation of the major fault activity during the post-rift stage. The Qiongdongnan Basin, which is located in the northwestern continental margins of the South China Sea, has exhibited significant deviations from typical post-rift characteristics. In the basin, a distinct tectonic reactivation occurred since the Late Miocene (11.6 Ma). Three notable aspects of the observed tectonic reactivation during the post-rift stage include, (1) pre-existing fault reactivation, (2) multiple large-scale magmatic intrusions, and (3) rapid post-rift subsidence. During this period the basin infill significantly changed in depositional environments shifting rapidly from littoral-neritic to bathyal-abyssal environments since Late Miocene. The pre-existing fault activity along the No. 2 fault of the basin resulted in the formation of initial shelf breaks and led to the development of continental slope. In addition, the pre-existing faults along the Central Depression zone created a small sub-basin with distinctive axial negative topography characteristics formed between structural highs. These geomorphological changes led to the formation of the Central Canyon. Large-scale magmatic intrusions occurred along the fault zone in the Central Depression of the basin during the post-rift stage. Those deviations, as evidenced from pre-existing fault reactivation, magmatic intrusions, and rapid post-rift subsidence in the Qiongdongnan Basin is believed to be related to the Hainan Plume event.

  19. 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.

  20. A quantitative geomorphological approach to constraining the volcanic and tectonic evolution of the active Dabbahu rift segment, Afar, Ethiopia.

    NASA Astrophysics Data System (ADS)

    Medynski, Sarah; Pik, Raphaël; Burnard, Peter; Vye-Brown, Charlotte; Blard, Pierre-Henri; France, Lydéric; Dumont, Stéphanie; Grandin, Raphaël; Schimmelpfennig, Irene; Benedetti, Lucilla; Ayalew, Dereje; Yirgu, Gezahegn

    2013-04-01

    In the Afar depression (Ethiopia), extension is organised along rift segments that morphologically resemble oceanic rifts. Segmentation results from interactions between dyke injection and volcanism, as observed during the well-documented 2005 rifting event on the Dabbahu rift segment. This tectono-volcanic crisis was observed in detail via remote sensing techniques, providing invaluable information on the present-day tectonic - magmatic interplay during a sequence of dyke intrusions. However, lack of data remains on timescales of 1 to 100 kyr, the period over which the main morphology of the rift is acquired. The Dabbahu rift segment represents an ideal natural laboratory to study the evolution of rift morphology as a response to volcanic and tectonic influences. We use cosmogenic nuclides (3He and 36Cl) to determine the ages of young (<100 kyr) lava flows and to date the initiation and movement of fault scarps, which cut the lavas. Where possible, we analysed vertical profiles along fault scarps, in an attempt to distinguish individual tectonic events that offset the scarp, estimate their amplitudes and date the recurrence intervals. These geochronological constraints, combined with major & trace element compositions, field mapping and digital mapping (Landsat, ASTER and SPOT imagery), provide valuable insights on the magmatic and tectonic history of the segment. The results show that over the last 100 ka, the northern part of the Dabbahu segment was supplied by at least two different magma reservoirs, which can be identified from their distinctive chemistries. The main reservoir is located beneath Dabbahu volcano at the northern tip of the rift segment, and has been supplied with magma for at least 72 ka. The second reservoir is located further south on the rift axis and corresponds to the current mid-segment magma chamber, which was responsible for the 2005 rifting episode. Two magmatic cycles linked to the Dabbahu magma chamber were recorded, lasting 20-30 kyr

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  2. Mapping the evolving strain field during continental breakup from crustal anisotropy in the Afar Depression.

    PubMed

    Keir, Derek; Belachew, M; Ebinger, C J; Kendall, J-M; Hammond, J O S; Stuart, G W; Ayele, A; Rowland, J V

    2011-01-01

    Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magma intrusion, yet the spatial and temporal scales over which these alternate mechanisms localize extensional strain remain controversial. Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup. The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres. The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture. PMID:21505441

  3. Mapping the evolving strain field during continental breakup from crustal anisotropy in the Afar Depression

    PubMed Central

    Keir, Derek; Belachew, M.; Ebinger, C.J.; Kendall, J.-M.; Hammond, J.O.S.; Stuart, G.W.; Ayele, A.; Rowland, J.V.

    2011-01-01

    Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magma intrusion, yet the spatial and temporal scales over which these alternate mechanisms localize extensional strain remain controversial. Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup. The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres. The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture. PMID:21505441

  4. Investigation of shallow gas hydrate occurrence and gas seep activity on the Sakhalin continental slope, Russia

    NASA Astrophysics Data System (ADS)

    Jin, Young Keun; Baranov, Boris; Obzhirov, Anatoly; Salomatin, Alexander; Derkachev, Alexander; Hachikubo, Akihiro; Minami, Hrotsugu; Kuk Hong, Jong

    2016-04-01

    The Sakhalin continental slope has been a well-known gas hydrate area since the first finding of gas hydrate in 1980's. This area belongs to the southernmost glacial sea in the northern hemisphere where most of the area sea is covered by sea ice the winter season. Very high organic carbon content in the sediment, cold sea environment, and active tectonic regime in the Sakhalin slope provide a very favorable condition for occurring shallow gas hydrate accumulation and gas emission phenomena. Research expeditions under the framework of a Korean-Russian-Japanese long-term international collaboration projects (CHAOS, SSGH-I, SSGH-II projects) have been conducted to investigate gas hydrate occurrence and gas seepage activities on the Sakhalin continental slope, Russia from 2003 to 2015. During the expeditions, near-surface gas hydrate samples at more than 30 sites have been retrieved and hundreds of active gas seepage structures on the seafloor were newly registered by multidisciplinary surveys. The gas hydrates occurrence at the various water depths from about 300 m to 1000 m in the study area were accompanied by active gas seepage-related phenomena in the sub-bottom, on the seafloor, and in the water column: well-defined upward gas migration structures (gas chimney) imaged by high-resolution seismic, hydroacoustic anomalies of gas emissions (gas flares) detected by echosounders, seafloor high backscatter intensities (seepage structures) imaged by side-scan sonar and bathymetric structures (pockmarks and mounds) mapped by single/multi-beam surveys, and very shallow SMTZ (sulphate-methane transition zone) depths, strong microbial activities and high methane concentrations measured in sediment/seawater samples. The highlights of the expeditions are shallow gas hydrate occurrences around 300 m in the water depth which is nearly closed to the upper boundary of gas hydrate stability zone in the area and a 2,000 m-high gas flare emitted from the deep seafloor.

  5. Rapid spatio-temporal variations in rift zone deformation, Corinth rift, Greece

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    The Gulf of Corinth is a young and highly active rift (<5 Ma) in its initial stages of development. An abundance of marine geophysical data and onshore exposures makes it an ideal case study for investigating early rift and fault development. Using a high resolution chronstratigraphic and rift fault model we investigate along strike variations in the basin development within the rift over the past 1-2 Myr and establishing a history of fault activity on major basin controlling faults, at temporal resolutions of ca. 100 kyr or less. We focus on variations in depocentre development and the distribution of displacement and faulting along and across the rift axis; focussing on the partitioning of deformation between N-dipping and S-dipping faults. The rift basin geometry has a complex history and varies spatially along strike of the rift. We highlight a major change in rift structure ca. 600 ka, changing from a complex rift zone to a uniform asymmetric graben. Syn-rift isochore maps identify two stages that accommodate this change: 1. a switch in rift polarity from a dominant N-thickening depocentre to a dominant S-thickening depocentre between ca. 620-420 ka (a rapid change in rift structure and strain distribution). This change is accommodated by transfer of activity between major faults but also by formation of numerous non-basement cutting small faults. 2. Progressive localization of deformation onto major N-dipping faults on the rift's southern margin. This is characterised by depocentre growth and linkage and increased activity on major N-dipping faults since ~340 ka, with faults becoming kinematically and geometrically linked with almost equal slip rates along strike by ca. 130 ka. Ultimately our results show that the early evolution of a rift fault network can be complex but that a dominant fault set eventually forms even in the earliest stages of rifting. Furthermore a switch in rift polarity is a progressive process with deformation becoming distributed before

  6. Very early rift sedimentation in the Turkana depression (EARS, Kenya): example of the Topernawi Formation

    NASA Astrophysics Data System (ADS)

    Nutz, A.; Ragon, T.; Schuster, M.; Ghienne, J. F.

    2015-12-01

    Sedimentation associated with very early phase of continental rifting remains poorly understood as related deposits lie at deepest part of basins and rarely outcrop at the surface. However, understanding of these sediments are essential first to better-constrain early extensional phase and second in term of potential resources. The Turkana depression is a rift system active since the Paleogene, which makes the connection between Kenyan and Ethiopian domes. The southern area consists of four asymmetrical and juxtaposed grabens: the Lokichar, Turkwell, Kerio and South Lake Basin, which have been intensively documented through oil exploration. The northern part is structured into a single asymmetrical graben, the North Lake basin, less-known even oil exploration started. In this contribution, a sedimentary system preserved on the rift shoulder of the North Lake Basin is presented. Referred to as the Topernawi Fm, it is interpreted as recording the earliest phase of Cenozoic rifting in the area. The Topernawi Fm delineates a relic sedimentary basin of limited extension (3 - 5 km). Boundaries of the basin are inherited from basement structures, more precisely from the reactivation during Late Oligocene to Early Miocene of a previous transfer zone producing N40-50° border faults. Basin fills is up to 80 m thick and includes first alluvial fan associated with the reactivation of these faults and then braided fluvial deposits from axial system. Above, several volcanic events recurrently emplaced pyroclastic deposits, repeatedly reworked by fluvial channels. Subsequently, N-S trend faulting cut the Topernawi system during the development of the North Lake Basin and led to its partial preservation over the present-day rift shoulder. Geological maps, structural sections across Topernawi basin and sedimentary facies are presented. An integrated model is proposed to illustrate the basin evolution. Implications for rifting in Northern Turkana depression are discussed.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    Magmatism and faulting are preponderant processes involved in continental rifting. Their interaction, relative importance, and dependence to the rheological properties of the lithosphere and to the timing of rifting, remain poorly known. To address this question, we have used the results from a seismological experiment, called SEISMO-TANZ (35 stations, broadband and enlarged-band), launched in the North Tanzanian Divergence (NTD) for 6 months in 2007. The region encompasses one of the youngest parts of the East African rift (EAR) and is characterized by the development of the rift into the Tanzanian craton. The NTD is often considered as non-volcanic compared to other places in EAR and the lithosphere is highly resistant. More than 2000 local earthquakes were recorded, highlighting active faults and one magmatic intrusion. Inherited structures play a key role as guides for dykes and slips. 26 Focal mechanisms (double-couple hypothesis) were obtained from P-wave polarities and indicate a transtensive deformation in the southern part of the region (Manyara rift). The stress inversion performed indicates a stable, well-determined σ3 axis striking ESE-WNW. From 25 teleseismic events recorded during the experiment, we have measured seismic anisotropy (SKS splitting) and present here our last results. Fast polarization directions are quite homogeneously NE-SW and delays times increase from the craton (W) to the Mozambique belt (E). Fossilized anisotropy and dykes or melt-filled lenses alignments would both explain the majority of these observations. We finally compare these results with other seismic anisotropy measurements made in EAR and with geodetic and seismotectonic analyses in order to better assess the origin of the strain pattern in this part of the rift, and to discuss the respective role of magmatism, faulting and fabrics in the extending lithosphere.

  8. The Midcontinent Rift and Grenville connection

    SciTech Connect

    Cambray, F.W.; Fujita, K. . Dept. of Geological Sciences)

    1994-04-01

    The Mid-Proterozoic, Midcontinent Rift System (MRS) is delineated by an inverted U shaped gravity and magnetic anomaly. It terminates in southeast Michigan but a less continuous series of anomalies and sediments, the Eastcontinent Rift occur on a north-south line through Ohio and Kentucky. The geometry allows for a north-south opening, the Lake Superior section being orthogonal to opening, the western arm transtensional and the north-south trending eastern arm a transform boundary offset by pull-apart basins. The opening and closing of the rift overlaps in time with the Grenville Orogeny. Grenville age rocks can also be found in the Llano uplift of Texas. The authors propose a model to explain the temporal and geographic association of the opening and closing of the MRS with the Grenville Orogeny that involves irregular suturing between two continental masses. Initiation of Grenville suturing, associated with south dipping subduction, in the northeast and in the Llano area of Texas would leave portion of unclosed ocean in between. Tensional stresses in the continental crust adjacent to the oceanic remnant could lead to its fragmentation and the formation of the MRS. The remaining oceanic lithosphere would eventually subduct, limiting the opening of the MRS. Continued convergence of the plates would induce compressional stresses thus accounting for the deformation of the MRS. An analogy is made with more recent opening of the Red Sea, Gulf of Aden Rift System in association with irregular collision along the Zagros-Bitlis Sutures.

  9. Polyphased rifting to post-breakup evolution of the Coral Sea region, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Bulois, Cédric; Pubellier, Manuel; Chamot-Rooke, Nicolas; Delescluse, Matthias

    2016-04-01

    The Coral Sea Basin, offshore Papua New Guinea, is generally described as a rift propagator that opened through the Australian craton during the Late Cretaceous. Rifting was later followed by spreading activity during Palaeocene to lowermost Eocene times and basin inversion during the Cenozoic. Herein, we specifically describe the extensional structures and show that the area has actually a much longer history that dates back from the Late Palaeozoic. A special focus is made on the northern margin of the Coral Sea Basin along which subsurface and HD topographic data were recently acquired. Extension took place discontinuously from the Late Palaeozoic to the Lower Cenozoic through several rift megacycles that include extensional pulses and relaxation episodes. The first rift megacycle (R1), poorly documented, occurred during the Triassic along an old Permo-Triassic, NS-trending structural fabric. Evidence of Permo-Triassic features is principally observed in the western part of the Coral Sea near the Tasman Line, a major lithospheric discontinuity that marks the eastern limit of the underlying Australian craton in Papua New Guinea. This early Triassic framework was reactivated during a Jurassic rifting stage (R2), resulting in small (~10/20km) tilted basins bounded by major NS, NE-SW and EW normal faults. Extension formed a large basin, floored by oceanic crust that might have connected with the Tethys Ocean. The Owen Stanley Oceanic Basin containing deep-marine sediments now obducted in the Ocean Stanley Thrust Belt are likely to represent this oceanic terrane. Both R1 and R2 megacycles shaped the geometry of the Jurassic Australian margin. A third Cretaceous extensional megacycle (R3) only reactivated the largest faults, cutting through the midst of this early stretched continental margin. It formed wider, poorly tilted basins and terminated with the onset of the Coral Sea seafloor spreading from Danian to Ypresian times (61.8 to 53.4 Myr). Then, the overall

  10. 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 ...

  11. Assessment and recommendations for two sites with active and potential aquaculture production in Rift Valley and Coast Provinces, Kenya

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Kenya has a long history of local fish consumption. The population in the Lake Victoria area (Rift Valley Province) Northwest of Nairobi and coastal communities (Coast Province) have historically included fish in their diet. Migration from villages to urban areas and increasing commerce has created ...

  12. The Example of Eastern Africa: the dynamic of Rift Valley fever and tools for monitoring virus activity

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever is a mosquito-borne viral zoonosis that primarily affects animals but also has the capacity to infect humans. Outbreaks of this disease in eastern Africa are closely associated with periods of heavy rainfall and forecasting models and early warning systems have been developed to en...

  13. 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 ...

  14. Evolution of post-rift sediment transport on a young rifted margin : Insights from the eastern part of the Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Baurion, C.; Gorini, C.; Leroy, S.; Lucazeau, F.; Migeon, S.

    2012-04-01

    The formation of gravity-driven sedimentary systems on continental rifted margins results from the interaction between climate, ocean currents and tectonic activity. During the early stages of margin evolution, the tectonic processes are probably as important as climate for the sedimentary architecture. Therefore, the young margins (ca. 35 Ma) of the Gulf of Aden provide the opportunity to evaluate the respective roles of monsoon and tectonic uplift in the formation and evolution through the post-rift period of gravity-driven deposits (Mass Transport Complexes (MTCs) and deep-sea systems) on the continental slopes and in the oceanic basin respectively. Here we present a combined geomorphologic and stratigraphic study of the northern margin (Oman and Yemen) and the southern margin (Socotra island), in which we classified and interpreted the gravity-driven processes, their formation and their evolution during the post-rift period. The interpretation of seismic lines reveals the presence of bottom currents since the drift phase, suggesting that the Gulf of Aden was connected to the world oceans at that time. An abrupt depositional change affected the eastern basin of the Gulf of Aden around 10 Ma or thereafter (Chron 5), characterised by the first occurrence of deep sea fans and an increase in the number of MTCs. The first occurrence of MTCs may be explained by the combined 2nd-3rd order fall of the relative sea-level (Serravalian/Tortonian transition). This variation of relative sea level combined with a climatic switch (Asian monsoon onset around 15 Ma and its intensification around 7-8 Ma) control the sediment flux. The youngest unit of the post-rift supersequence is characterised by a second important MTC occurrence that is restricted to the eastern part of the deep basin. This is caused by a late uplift of the northern and southern margins witnessed onshore by the presence of young stepped marine terraces.

  15. Magmatism During Rifting Controls the Polarity of Tilted Blocks

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  16. Chapter 34: Geology and petroleum potential of the rifted margins of the Canada Basin

    USGS Publications Warehouse

    Houseknecht, D.W.; Bird, K.J.

    2011-01-01

    Three sides of the Canada Basin are bordered by high-standing, conjugate rift shoulders of the Chukchi Borderland, Alaska and Canada. The Alaska and Canada margins are mantled with thick, growth-faulted sediment prisms, and the Chukchi Borderland contains only a thin veneer of sediment. The rift-margin strata of Alaska and Canada reflect the tectonics and sediment dispersal systems of adjacent continental regions whereas the Chukchi Borderland was tectonically isolated from these sediment dispersal systems. Along the eastern Alaska-southern Canada margin, termed herein the 'Canning-Mackenzie deformed margin', the rifted margin is deformed by ongoing Brooks Range tectonism. Additional contractional structures occur in a gravity fold belt that may be present along the entire Alaska and Canada margins of the Canada Basin. Source-rock data inboard of the rift shoulders and regional palaeogeographic reconstructions suggest three potential source-rock intervals: Lower Cretaceous (Hauterivian-Albian), Upper Cretaceous (mostly Turonian) and Lower Palaeogene. Burial history modelling indicates favourable timing for generation from all three intervals beneath the Alaska and Canada passive margins, and an active petroleum system has been documented in the Canning-Mackenzie deformed margin. Assessment of undiscovered petroleum resources indicates the greatest potential in the Canning-Mackenzie deformed margin and significant potential in the Canada and Alaska passive margins. ?? 2011 The Geological Society of London.

  17. Imaging the lithosphere of rifted passive margins using waveform tomography: North Atlantic, South Atlantic and beyond

    NASA Astrophysics Data System (ADS)

    Lebedev, Sergei; Schaeffer, Andrew; Celli, Nicolas Luca

    2016-04-01

    Lateral variations in seismic velocities in the upper mantle reflect variations in the temperature of the rocks at depth. Seismic tomography thus provides a proxy for lateral changes in the temperature and thickness of the lithosphere. It can map the deep boundaries between tectonic blocks with different properties and age of the lithosphere. Our 3D tomographic models of the upper mantle and the crust at the Atlantic and global scales are constrained by an unprecedentedly large global dataset of broadband waveform fits (over one million seismograms) and provide improved resolution of the lithosphere, compared to other available models. The most prominent high-velocity anomalies, seen down to 150-200 km depths, indicate the cold, thick, stable mantle lithosphere beneath Precambrian cratons, including those in North America, Greenland, northern and eastern Europe, Africa and South America. The dominant, large-scale, low-velocity feature is the global system of mid-ocean ridges, with broader low-velocity regions near hotspots, including Iceland. Currently active continental rifts show highly variable expression in the upper mantle, from pronounced low velocities to weak anomalies; this correlates with the amount of magmatism within the rift zone. Rifted passive margins have typically undergone cooling since the rifting and show more subtle variations in their seismic-velocity structure. Their thermal structure and evolution, however, are also shaped by 3D geodynamic processes since their formation, including cooling by the adjacent cratonic blocks inland and heating by warm oceanic asthenosphere.

  18. Seismically Articulating Kilauea Volcano's Active Conduits, Rift Zones, and Faults through HVO's Second Fifty Years

    NASA Astrophysics Data System (ADS)

    Okubo, P.; Nakata, J.; Klein, F.; Koyanagi, R.; Thelen, W.

    2011-12-01

    While seismic monitoring of active Hawaiian volcanoes began 100 years ago, the build-up of the U. S. Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO) seismographic network to its current configuration began in 1955, when Jerry Eaton established remote stations that telemetered data via landline to recorders at HVO. With network expansion through the 1960's, earthquake location and cataloging capabilities have evolved to afford a computer processed seismic catalog now spanning fifty years. Location accuracy and catalog completeness to smaller magnitudes have increased. Research and insights developed using HVO's seismic record have exploited the ability to seismically monitor volcanic activity at depth, to identify active regions within the volcanoes on the basis of computed hypocentral locations, to infer regions of magma storage by recognizing different families of volcanic earthquakes, and to forecast volcanic activity in both short and longer term from seismicity patterns. HVO's seismicity catalog was central to calculations of probabilistic seismic hazards. The ability to develop and implement additional analytical and interpretive capabilities has kept pace with improvements in both field and laboratory hardware and software. While the basic capabilities continue as part of HVO's core monitoring, additional interpretive capabilities now include adding details of volcanic and earthquake source regions, and viewing seismic data in juxtaposition with other observatory data streams. As HVO looks to its next century of volcano studies, research and development continue to shape the future. Broadband seismic recording at HVO has enabled extensive study by Chouet, Dawson, and co-workers of the relationship of very-long-period seismic sources beneath Kilauea's summit caldera to magma supply and transport. Recent upgrades have improved the ability to use these data in seismic cataloging and research. Data processing upgrades have bolstered the ability to

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

    progradation of a conglomeratic fan delta system testifies the decrease of the accommodation rate and a strong basinward system shift controlled by a general uplift of the margin. This surrection phase leading to a subaerial exposure is interpreted as the consequence of the continental breakup at the Burdigalian time (17.6 Ma) and the set up of Ocean-Continent Transition (OCT). At the middle Miocene time, new subsidence phase is associated with a partial marine incursion and to the set up of proximal shallow marine carbonate deposits (Adawnib Fm.) and the lateral equivalent conglomeratic alluvial fan deposits (Nar Fm.). This post-rift unit records a progressive decrease of the tectonic activity, which may be related to the migration of the deformation towards the distal margin up to the oceanic spreading in the Gulf of Aden. Late deformation phases (erosive paleo-surface at the top the post-rift conglomerates, preservation of uplifted paleo-beach deposits) may imply a large-scale geodynamic processes.

  20. Insights into extensional processes during magma assisted rifting: Evidence from aligned scoria cones

    NASA Astrophysics Data System (ADS)

    Rooney, Tyrone O.; Bastow, Ian D.; Keir, Derek

    2011-04-01

    Mechanical and magmatic processes exert first-order control on the architecture and evolution of rifts. As a continental rift develops towards a new oceanic spreading centre, extension that is initially accommodated in a broad zone of faulting and ductile stretching must transition towards a narrow zone of focused magmatic intrusion. The Main Ethiopian Rift (MER), part of the East African Rift System, is an ideal location to study this transition because it captures rifting processes during continental breakup. In this contribution we synthesise geochemical data from scoria cones in the Wonji Fault Belt (WFB) and Silti-Debre Zeyit Fault Zone (SDFZ) in the MER to provide new constraints on the development of mantle melting columns and magmatic plumbing systems since the onset of rifting. We utilize the extensive geophysical and geochemical databases, collected in the Ethiopian Rift, to show that geochemical evidence of heterogeneity in the depth of the mantle melting column which produced Quaternary rift basalts correlates with lithospheric structure. When combined with existing observations of asymmetry across the rift in terms of depth of melting column and magmatic plumbing systems, it is evident that the mechanical structure of the rift, defined during the initial stages of breakup, has played a dominant role in the initial development of magma assisted rifting in the MER. Surface structures and crustal-scale geophysical studies have suggested the WFB is analogous to a sea-floor spreading centre. However, the geochemical characteristics of rift basalts are consistent with mantle tomography that shows no evidence beneath the MER for passive magmatic upwelling beneath discrete rift segments as is observed in the ocean basins. Collectively, the Ethiopian data show that the distribution of mantle melts during the initiation of magma assisted rifting is fundamentally influenced by lithospheric structures formed during earlier syn-rift stretching.

  1. 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

  2. The Role of the Strain History on the Modes of Continental Extension

    NASA Astrophysics Data System (ADS)

    Capitanio, F. A.; Salerno, V. M.

    2015-12-01

    Continental rifts evolve through deformation episodes into a variety of diverse basins with complex structural styles, subsidence and thermal histories, which may include melting. Existing models have probed the role of initial lithospheric rheological layering, geothermal gradients and stretching rates on continental rifting, however continental rifts evolution often includes several extensional phases, which are not easily explained by a single rifting approach. Here, we address the role of episodic stretching on the long-term evolution of continental rifts. By means of numerical modelling we investigate the development of rifting patterns as a consequence of distinct phases of lithospheric extension under different stretching rates, modifying the lithosphere's rheological layering, and intervening cooling, during which the lithosphere regains its original thickness, and their effect on subsequent rifting. The models show that the time-dependent boundary conditions have a fundamental control on lithospheric-scale strain localization/delocalization and, consequently, on the tectonic rifting style and its evolution, resulting in a variety of basins ranging from narrow to wide rifts to hyper-thinned and with diverse melt yield in time, reproducing the first-order features of major basins on Earth. On the base of the rifting history we propose a classification that support the interpretation of rift basins and passive margins.

  3. The effects of thick sediment upon continental breakup: seismic imaging and thermal modeling of the Salton Trough, southern California

    NASA Astrophysics Data System (ADS)

    Han, L.; Hole, J. A.; Lowell, R. P.; Stock, J. M.; Fuis, G. S.; Driscoll, N. W.; Kell, A. M.; Kent, G. M.; Harding, A. J.; Gonzalez-Fernandez, A.; Lázaro-Mancilla, O.

    2015-12-01

    Continental rifting ultimately creates a deep accommodation space for sediment. When a major river flows into a late-stage rift, thick deltaic sediment can change the thermal regime and alter the mechanisms of extension and continental breakup. The Salton Trough, the northernmost rift segment of the Gulf of California plate boundary, has experienced the same extension as the rest of the Gulf, but is filled to sea level by sediment from the Colorado River. Unlike the southern Gulf, seafloor spreading has not initiated. Instead, seismicity, high heat flow, and minor volcanoes attest to ongoing rifting of thin, transitional crust. Recently acquired controlled-source seismic refraction and wide-angle reflection data in the Salton Trough provide constraints upon crustal architecture and active rift processes. The crust in the central Salton Trough is only 17-18 km thick, with a strongly layered but relatively one-dimensional structure for ~100 km in the direction of plate motion. The upper crust includes 2-4 km of Colorado River sediment. Crystalline rock below the sediment is interpreted to be similar sediment metamorphosed by the high heat flow and geothermal activity. Meta-sediment extends to at least 9 km depth. A 4-5 km thick layer in the middle crust is either additional meta-sediment or stretched pre-existing continental crust. The lowermost 4-5 km of the crust is rift-related mafic magmatic intrusion or underplating from partial melting in the hot upper mantle. North American lithosphere in the Salton Trough has been almost or completely rifted apart. The gap has been filled by ~100 km of new transitional crust created by magmatism from below and sedimentation from above. These processes create strong lithologic, thermal, and rheologic layering. While heat flow in the rift is very high, rapid sedimentation cools the upper crust as compared to a linear geotherm. Brittle extension occurs within new meta-sedimentary rock. The lower crust, in comparison, is

  4. New Isotopic Constraints on the Sources of Methane at Sites of Active Continental Serpentinization

    NASA Astrophysics Data System (ADS)

    Wang, D. T.; Gruen, D.; Morrill, P. L.; Rietze, A.; Nealson, K. H.; Kubo, M. D.; Cardace, D.; Schrenk, M. O.; Hoehler, T. M.; McCollom, T. M.; Etiope, G.; Hosgormez, H.; Schoell, M.; Ono, S.

    2014-12-01

    of methane, and the flow of energy and carbon, in areas of active continental serpentinization. [1] Ono et al. (2014) Anal. Chem. 86, 6487. [2] Morrill et al. (2013) Geochim. Cosmochim. Acta 109, 222. [3] Cardace et al. (2013) Sci. Dril. 16, 45. [4] Etiope et al. (2011) Earth Planet. Sci. Lett. 310, 96.

  5. Trace element characteristics of lithospheric and asthenospheric mantle in the Rio Grande rift region

    SciTech Connect

    Perry, F.V.

    1994-06-01

    Trace element analyses of 10 mafic volcanic rocks from the Colorado Plateau transition zone, Colorado Plateau, Rio Grande rift, and Great Plains were obtained to characterize the trace element characteristics of asthenospheric and lithospheric mantle beneath these regions. Characterization of these mantle reservoirs using the trace element contents of basalts allows one to track the response of the lithosphere to continental rifting and extension.

  6. Active faulting in apparently stable peninsular India: Rift inversion and a Holocene-age great earthquake on the Tapti Fault

    NASA Astrophysics Data System (ADS)

    Copley, Alex; Mitra, Supriyo; Sloan, R. Alastair; Gaonkar, Sharad; Reynolds, Kirsty

    2014-08-01

    We present observations of active faulting within peninsular India, far from the surrounding plate boundaries. Offset alluvial fan surfaces indicate one or more magnitude 7.6-8.4 thrust-faulting earthquakes on the Tapti Fault (Maharashtra, western India) during the Holocene. The high ratio of fault displacement to length on the alluvial fan offsets implies high stress-drop faulting, as has been observed elsewhere in the peninsula. The along-strike extent of the fan offsets is similar to the thickness of the seismogenic layer, suggesting a roughly equidimensional fault rupture. The subsiding footwall of the fault is likely to have been responsible for altering the continental-scale d