Science.gov

Sample records for eger rift central

  1. Prolonged mantle residence of zircon xenocrysts from the western Eger rift

    NASA Astrophysics Data System (ADS)

    Siebel, Wolfgang; Schmitt, Axel K.; Danišík, Martin; Chen, Fukun; Meier, Stefan; Weiß, Stefan; Eroǧlu, Sümeyya

    2009-12-01

    Zircon is a common mineral in continental crustal rocks. As it is not easily altered in processes such as erosion or transport, this mineral is often used in the reconstruction of geological processes such as the formation and evolution of the continents. Zircon can also survive under conditions of the Earth's mantle, and rare cases of zircons crystallizing in the mantle significantly before their entrainment into magma and eruption to the surface have been reported. Here we analyse the isotopic and trace element compositions of large zircons of gem quality from the Eger rift, Bohemian massif, and find that they are derived from the mantle. (U-Th)/He analyses suggest that the zircons as well as their host basalts erupted between 29 and 24million years ago, but fragments from the same xenocrysts reveal U-Pb ages between 51 and 83million years. We note a lack of older volcanism and of fragments from the lower crust, which suggests that crustal residence time before eruption is negligible and that most rock fragments found in similar basalts from adjacent volcanic fields equilibrated under mantle conditions. We conclude that a specific chemical environment in this part of the Earth's upper mantle allowed the zircons to remain intact for about 20-60million years.

  2. CO2 degassing in the Hartoušov mofette area, western Eger Rift, imaged by CO2 mapping and geoelectrical and gravity surveys

    NASA Astrophysics Data System (ADS)

    Nickschick, Tobias; Kämpf, Horst; Flechsig, Christina; Mrlina, Jan; Heinicke, Jens

    2015-11-01

    Strong, subcontinental mantle-dominated CO2 degassing occurs in the Hartoušov and Bublák mofette fields in the western Eger Rift. The combination of CO2 gas flux and soil gas measurements as well as gravity and geoelectric surveys provides insight into the surface and subsurface of this unique mofette area. CO2 soil gas and gas flux measurements reveal that large amounts of carbon dioxide are released via channels with diameters below 1 m. Carbon dioxide emissions of several tens and up to more than 100 kg day-1 are ejected via these small seeps. Measurements with small spacings are necessary to account for the point like, focused gas discharge in the lesser degassing surrounding. We estimate that between 23 and 97 tons of CO2 are released over an area of about 350,000 m2 each day in the Hartoušov mofette field. The application of widely used geostatistical tools leads to estimations of the CO2 discharge with very high standard deviations due to the strong positive skewness of the data distribution. Geophysical investigations via electrical resistivity tomography and gravity measurements were carried out over areas of strong seepage and reveal distinct anomalies in the subsurface below mofettes, indicating rock and sediment alterations and/or sediment transport by pressurised, ascending CO2 and water mobilised by it. This study reveals that the gas emanations only occur west of a morphological step which is related to a N-S-oriented fault zone, the Počatky-Plesná fault zone. The results of CO2 mapping and the geophysical studies can track the course of this fault zone in this area. Our results fit into a tectonic model in which the mofette fields are in the centres of two independent pull-apart basin-like structures. We hypothesise that the sinistral strike-slip movement of the Počatky-Plesná fault zone leads to a pull-apart basin-like opening, at which the strong, mantle-derived CO2 degassing occurs nowadays. Since the Hartoušov and Bublák mofette fields

  3. 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

  4. Fluid pressure and flow at great depth in the continental crust. A discussion in relation to topography, temperature and salinity distribution using as an example the KTB Fault Zones in connection with the Eger Rift Hot Spot.

    NASA Astrophysics Data System (ADS)

    Kessels, W.; Kuhlmann, S.; Li, X.

    2006-12-01

    Hydraulic investigations in and between the two KTB boreholes have shown that groundwater flow is possible at great depth in the crystalline crust. Remarkable permeability was found particularly in the SE1 and SE2 fault zones. The results from a long term pump and injection test, and the related three-dimensional groundwater modelling (Graesle et al., 2006), document the existence of a large-scale (more than 10 km) hydraulic reservoir in the crystalline crust. According to this calculation, an overpressure of 0.4 MPa can be still be expected in KTB-HB in 2009, 4 years after the end of the injection. The good match with the measurement data confirms groundwater pathways at a scale of more than 10 km. The isotopic water composition recovered from the KTB pilot hole indicates a downward water flow along the SE2 fault zone, which is in contact with the Franconian Line. Moreover, there is a deep upward groundwater flow 60 km away in the western Eger Rift Valley as indicated e.g. by the temperature signature and gas flow observations. Therefore, the demand for fluid mass continuity means that water is being supplied by a downstream groundwater flow, probably from the Franconian Line. The question of potential driving processes must be answered to understand and quantify the flow in the deeper crust at a scale of 10 km to 100 km. The processes must result in a sufficient horizontal pressure gradient to allow groundwater flow at great depth. The density variations of groundwater with depth are highly relevant for the calculation of horizontal pressure differences. The two independent potential fields of gravity and pressure have to be considered. Differentiation into 4 relevant driving processes is required: \\bullet The groundwater surface topography related to the groundwater recharge and mean regional distance between neighbouring valleys \\bullet Geothermal gradient and water density depending on temperature and pressure \\bullet Different salt contents in adjacent

  5. Crustal structure of central Lake Baikal: Insights into intracontinental rifting

    USGS Publications Warehouse

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

    2002-01-01

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

  6. Thermochronometric evidence for diffuse extension and two-phase rifting within the Central Arabian Margin of the Red Sea Rift

    NASA Astrophysics Data System (ADS)

    Szymanski, E.; Stockli, D. F.; Johnson, P. R.; Hager, C.

    2016-12-01

    Numerical time-temperature models derived from a 2-D network of apatite and zircon (U-Th)/He ages reveal a three-stage thermotectonic history for the central Arabian rift flank (CARF) of the Red Sea Rift (RSR) system. The pre-rift Arabian-Nubian Shield existed as part of a passive Paleo-Tethyan margin until a widespread tectonic event at 350 Ma exhumed the proto-CARF to mid-to-upper crustal structural levels. After remaining thermally stable through the Mesozoic, the first phase of RSR extension began with a distinct rift pulse at 23 Ma when fault blocks across a 150 km wide area were exhumed along a diffuse set of rift-parallel faults from an average pre-rift flank depth of 1.7 ± 0.8 km. This rift onset age is mirrored in thermochronometric and sequence stratigraphic analyses elsewhere along the Red Sea Nubian and Arabian margins, confirming that rifting occurred concomitantly along the full Red Sea-Gulf of Suez rift system. Diffuse lithospheric extension lasted for 8 Myr before a second rift pulse at 15 Ma, coincident with regional stress realignment, transferred active faulting basinward toward the modern RSR axial trough. CARF time-temperature models indicate that the prevalent rift style during both RSR extensional phases was one of localized, structurally controlled block faulting and contemporaneous dike injection, not wholesale rift flank uplift.

  7. The Central Lake Malawi (Nyasa) Rift: single or multiple rift segments?

    NASA Astrophysics Data System (ADS)

    McCartney, T.; Scholz, C. A.; Shillington, D. J.; Accardo, N. J.; Chindandali, P. R. N.; Kamihanda, G.

    2015-12-01

    Accommodation zones connect rift segments, which are fundamental elements of continental rift architecture. The sedimentary record aids our assessment of the evolution of this linkage. The central basin of Lake Malawi is one of the most structurally complex regions of the Malawi Rift. Border fault margins have been interpreted on both shorelines; three structures within the basin have been interpreted as segments of a corresponding accommodation zone. We investigate these structures by integrating single- and multi-channel reflection seismic data, including new MCS acquired in 2015 for the SEGMeNT project. The stratigraphic record in the central basin, inferred from seismic reflection profiles, provides compelling evidence that most fault-related subsidence is accommodated by the western border fault. Strata on both sides of all three structures dip to the west. The pre-rift basement in the sub-basin west of the central structure is considerably deeper (~ 4 s TWTT sub-bottom) than that in the broader eastern sub-basin (~ 2.5 s TWTT sub-bottom). A syncline in the eastern sub-basin shows little variation in seismic facies, particularly over the last 1.3 m.y. In contrast, the western sub-basin exhibits seismic facies indicative of fluvial input from two major rivers, siliciclastic input from the border fault footwall rising > 1000 m above lake level, and mud diapirs in the deepest part of the sub-basin. Horizons pierced by these diapirs onlap the central structure, suggesting diapir rise postdates relative uplift of the structure. Correlations with the age model from a 2005 scientific drilling project will better constrain this timing. The structural high helps focus siliciclastic sediments into the sub-basin, resulting in the overpressure conditions required for mud diapirism. We hypothesize that the diapirs are the result of sediment loading in the deep main depocenter of the central basin rather than fault mechanisms. The basement highs in the central basin control

  8. Geomorphology of the central Red Sea Rift: Determining spreading processes

    NASA Astrophysics Data System (ADS)

    Augustin, Nico; van der Zwan, Froukje M.; Devey, Colin W.; Ligi, Marco; Kwasnitschka, Tom; Feldens, Peter; Bantan, Rashad A.; Basaham, Ali S.

    2016-12-01

    Continental rifting and ocean basin formation is occurring today in the Red Sea, providing a possible modern analogue for the creation of mid-ocean ridges. Yet many of the seafloor features observed along the axis of the Red Sea appear anomalous compared to ancient and modern examples of mid-ocean ridges in other parts of the world, making it unclear, until recently, whether the Red Sea is truly analogous. Recent work suggests that the main morphological differences between the Red Sea Rift (RSR) and other mid-ocean ridges are due to the presence and movement of giant, submarine salt flows, which blanket large portions of the rift valley and thereby the oceanic crust. Using ship-based, high-resolution multibeam bathymetry of the central RSR between 16.5°N and 23°N we focus here on the RSR volcanic terrains not covered by salt and sediments and compare their morphologies to those observed along slow and ultra-slow spreading ridges elsewhere. Regional variations in style and intensity of volcanism can be related to variations in volcanic activity and mantle heat flow. The Red Sea oceanic seafloor shows typical features of mature (ultra)slow-spreading mid-ocean ridges, such as 2nd order discontinuities (overlapping spreading centres) and magma focussing in the segment centres (forming spreading-perpendicular volcanic ridges of thick oceanic crust). The occurrence of melt-salt interaction at locations where salt glaciers blanket the neovolcanic zone, and the absence of large detachment faults are unique features of the central RSR. These features can be related to the young character of the Red Sea and may be applicable to all young oceanic rifts, associated with plumes and/or evaporites. Thus, the RSR falls in line with (ultra)slow-spreading mid-ocean ridges globally, which makes the Red Sea a unique but highly important type example for initiation of slow rifting and seafloor spreading and one of the most interesting targets for future ocean research.

  9. Structure of the central Terror Rift, western Ross Sea, Antarctica

    USGS Publications Warehouse

    Hall, Jerome; Wilson, Terry; Henrys, Stuart

    2007-01-01

    The Terror Rift is a zone of post-middle Miocene faulting and volcanism along the western margin of the West Antarctic Rift System. A new seismic data set from NSF geophysical cruise NBP04-01, integrated with the previous dataset to provide higher spatial resolution, has been interpreted in this study in order to improve understanding of the architecture and history of the Terror Rift. The Terror Rift contains two components, a structurally-controlled rollover anticlinal arch intruded by younger volcanic bodies and an associated synclinal basin. Offsets and trend changes in fault patterns have been identified, coincident with shifts in the location of depocenters that define rift sub-basins, indicating that the Terror Rift is segmented by transverse structures. Multiple phases of faulting all post-date 17 Ma, including faults cutting the seafloor surface, indicating Neogene rifting and possible modern activity.

  10. Flexural uplift of rift flanks in central Greece

    NASA Astrophysics Data System (ADS)

    Poulimenos, George; Doutsos, Theodor

    1997-12-01

    Uplifts, with elevations of up to 2000 m and short wavelengths (30-35 km), flank three major grabens in central Greece: the Rio, Sparta, and Atalanti grabens. They are bordered on their landward sides by narrow basins oriented parallel to the graben axes: the Manesi and Trichonis basins at the Rio graben and Copais and Istiea basins at the Atalanti graben. The flexural origin of these uplift profiles is investigated here by using thin, broken plate models. It is demonstrated that the observed topography of the graben flanks is consistent with the upward deflection of elastic and viscoelastic plates in response to upward directed forces applied at the graben flanks. In order to evaluate to applicability of each model, their predictions are checked against the observations. The elastic predictive modeling fits well with the observed flexural wavelengths and the flanking seismicity. However, it fails to predict the graben widths and the inferred elastic layer thickness. In contrast, the viscoelastic model successfully explains the graben widths and the "back" basins as flexural hinterland basins and matches the seismological data and the time constraints of rifting. It is therefore suggested that flexural uplift with viscoelastic relaxation accounts for the building of the graben flanks. The invoked viscoelastic models constrain the effective elastic thickness of the plates at 10 km for the Rio graben and 15 km for the Sparta and Atalanti grabens, suggesting a low-viscosity lower crust. Furthermore, they predict low rates of tectonic uplift of the order of 0.1 mm a-1 for the Atalanti graben, intermediate rates of 0.24-0.37 mm a-1 for the Rio graben, and high rates of 0.7-0.9 mm a-1 for the Sparta graben. The latter are quite possibly overestimated since napping events, capable of producing high local relief, are traced normal to the modeled profiles.

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

    SciTech Connect

    Genik, G.J. )

    1993-08-01

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

  12. Mode of rifting in magmatic-rich setting: Tectono-magmatic evolution of the Central Afar rift system

    NASA Astrophysics Data System (ADS)

    Stab, Martin; Bellahsen, Nicolas; Pik, Raphaël; Leroy, Sylvie; Ayalew, Dereje

    2014-05-01

    Observation of deep structures related to break-up processes at volcanic passive margins (VPM) is often a troublesome exercise: thick pre- to syn-breakup seaward-dipping reflectors (SDR) usually mask the continent-ocean boundary and hide the syn-rift tectonic structures that accommodate crustal stretching and thinning. Some of the current challenges are about clarifying 1) if tectonic stretching fits the observed thinning and 2) what is the effect of continuous magma supply and re-thickening of the crust during extension from a rheological point of view? The Afar region in Ethiopia is an ideal natural laboratory to address those questions, as it is a highly magmatic rift that is probably close enough to breakup to present some characteristics of VPM. Moreover, the structures related to rifting since Oligocene are out-cropping, onshore and well preserved. In this contribution, we present new structural field data and lavas (U-Th/He) datings along a cross-section from the Ethiopian Plateau, through the marginal graben down to the Manda-Hararo active rift axis. We mapped continent-ward normal fault array affecting highly tilted trapp series unconformably overlain by tilted Miocene (25-7 Ma) acid series. The main extensional and necking/thinning event took place during the end of this Miocene magmatic episode. It is itself overlain by flat lying Pliocene series, including the Stratoid. Balanced cross-sections of those areas allow us to constrain a surface stretching factor of about 2.1-2.9. Those findings have the following implications: - High beta factor constrained from field observations is at odd with thinning factor of ~1.3 predicted by seismic and gravimetric studies. We propose that the continental crust in Central Afar has been re-thickened by the emplacement of underplated magma and SDR. - The deformation in Central Afar appears to be largely distributed through space and time. It has been accommodated in a 200-300 km wide strip being a diffuse incipient

  13. Magmatic history of Red Sea rifting: perspective from the central Saudi Arabian coastal plain.

    USGS Publications Warehouse

    Pallister, J.S.

    1987-01-01

    An early stage of magmatism related to Red Sea rifting is recorded by a Tertiary dyke complex and comagmatic volcanic rocks exposed on the central Saudi Arabian coastal plain. Field relations and new K/Ar dates indicate episodic magmatism from approx 30 m.y. to the present day and rift-related magmatism as early as 50 m.y. Localized volcanism and sheeted dyke injection ceased at approx 20 m.y. and were replaced by the intrusion of thick gabbro dykes, marking the onset of sea-floor spreading in the central Red Sea. Differences in the depths and dynamics of mantle-melt extraction and transport may account for the transition from mixed alkaline-subalkaline bimodal magmatism of the pre-20 m.y. rift basin to exclusively subalkaline (tholeiitic) magmatism of the Red Sea spreading axis and the alkali basalt volcanism inland.-L.C.H.

  14. Evolution of the central Rio Grande rift, New Mexico: New potassium-argon ages

    NASA Astrophysics Data System (ADS)

    Baldridge, W. S.; Damon, P. E.; Shafiqullah, M.; Bridwell, R. J.

    1980-12-01

    New K sbnd Ar age determinations on mid-Oligocene to Pleistocene volcanic and shallow intrusive rocks from the central Rio Grande rift permit a more detailed understanding of the tectonic and magmatic history of the rift. Initial extension in the region of the central rift may have begun prior to 27 m.y. ago. By 25 m.y. ago broad basins existed and were filling with volcaniclastic sediments derived mainly from volcanic centers in the San Juan and Questa areas. Continued tectonic activity narrowed these basins by 21-19 m.y. ago, indicated in the Santa Fe area by tilting and faulting that immediately postdate 20-m.y.-old latite. Uplift of the Sangre de Cristo, Sandia, and Nacimiento Mountains shed clastic debris of the Santa Fe Group into these basins. Early rift magmatism is characterized by an overlap of mid-Tertiary intermediate intrusive and extrusive activity, extending to 20 m.y. ago, with mafic and ultramafic volcanism, ranging from 25 to 19 m.y. Both volcanism and tectonic activity were minimal during the middle Miocene. About 13 m.y. ago renewed volcanic activity began. Tectonism commenced in the late Miocene, resulting in the present, narrow grabens. The term "Rio Grande rift" should be restricted to these grabens formed during post-mid-Miocene deformation. Widespread eruption of tholeiitic and alkali olivine basalts occurred 3-2 m.y. ago. The Rio Grande drainage system was integrated 4.5-3 m.y. ago, leading to the present erosional regime. These intervals of deformation and magmatism correspond generally with a similar sequence of events in the Basin and Range province south of the Colorado Plateau. This similarity indicates that the Rio Grande rift is not a unique structure in the southwestern U.S., and must be related to the larger context of the entire Basin and Range province.

  15. Lithological Influences on Occurrence of High-Fluoride Waters in The Central Kenya Rift

    NASA Astrophysics Data System (ADS)

    Olaka, L. A.; Musolff, A.; Mulch, A.; Olago, D.; Odada, E. O.

    2013-12-01

    Within the East African rift, groundwater recharge results from the complex interplay of geology, land cover, geomorphology, climate and on going volcano-tectonic processes across a broad range of spatial and temporal scales. The interrelationships between these factors create complex patterns of water availability, reliability and quality. The hydrochemical evolution of the waters is further complex due to the different climatic regimes and geothermal processes going on in this area. High fluoridic waters within the rift have been reported by few studies, while dental fluorosis is high among the inhabitants of the rift. The natural sources of fluoride in waters can be from weathering of fluorine bearing minerals in rocks, volcanic or fumarolic activities. Fluoride concentration in water depends on a number of factors including pH, temperature, time of water-rock formation contact and geochemical processes. Knowledge of the sources and dispersion of fluoride in both surface and groundwaters within the central Kenya rift and seasonal variations between wet and dry seasons is still poor. The Central Kenya rift is marked by active tectonics, volcanic activity and fumarolic activity, the rocks are majorly volcanics: rhyolites, tuffs, basalts, phonolites, ashes and agglomerates some are highly fractured. Major NW-SE faults bound the rift escarpment while the rift floor is marked by N-S striking faults We combine petrographic, hydrochemistry and structural information to determine the sources and enrichment pathways of high fluoridic waters within the Naivasha catchment. A total of 120 water samples for both the dry season (January-February2012) and after wet season (June-July 2013) from springs, rivers, lakes, hand dug wells, fumaroles and boreholes within the Naivasha catchment are collected and analysed for fluoride, physicochemical parameters and stable isotopes (δ2 H, δ18 O) in order to determine the origin and evolution of the waters. Additionally, 30 soil and

  16. Modes of rifting in magma-rich settings: Tectono-magmatic evolution of Central Afar

    NASA Astrophysics Data System (ADS)

    Stab, Martin; Bellahsen, Nicolas; Pik, Raphaël.; Quidelleur, Xavier; Ayalew, Dereje; Leroy, Sylvie

    2016-01-01

    Recent research in Afar (northern Ethiopia) has largely focused on the formation of the present-day ocean-continent transition at active segments (e.g., Manda Hararo). However, the Oligo-Miocene history of extension, from the onset of rifting at ~25 Ma to the eruption of the massive Stratoïd flood basalts at ~4 Ma, remains poorly constrained. Here we present new structural data and radiometric dating from Central Afar, obtained along a zone stretching from the undeformed Oligocene Ethiopian plateau to the Manda Hararo and Tat'Ale active volcanic segments. Basaltic and rhyolitic formations were mapped in two key areas corresponding to the proximal and distal parts of a half-rift. We present a balanced composite cross section of Central Afar, reconstructed using our new data and previously published geophysical data on the crustal structure. Our main findings are as follows: (1) Extension during the Mio-Pliocene corresponds to a "wide rift" style of rifting. (2) The lower crust has been underplated/intruded and rethickened during rifting by magmatic injection. (3) Our restoration points to the existence of midcrustal shear zones that have helped to distribute extension in the upper crust and to localize extension at depth in a necking zone. Moreover, we suggest that there is a close relationship between the location of a shear zone and the underplated/intruded material. In magma-rich environments such as Central Afar, breakup should be achieved once the initial continental crust has been completely replaced by the newly, magmatically accreted crust. Consequently, and particularly in Afar, crustal thickness is not necessarily indicative of breakup but instead reflects differences in tectono-magmatic regimes.

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

    NASA Astrophysics Data System (ADS)

    Delvaux, Damien; Smets, Benoît

    2015-04-01

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

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

    SciTech Connect

    Genik, G.J. )

    1991-03-01

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

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

    SciTech Connect

    Genik, G.J. )

    1991-08-01

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

  20. Evidence from mantle xenoliths for lithosphere removal beneath the central Rio Grande Rift

    NASA Astrophysics Data System (ADS)

    Byerly, Benjamin L.; Lassiter, John C.

    2012-11-01

    Seismic tomography beneath the Central Rio Grande Rift (RGR) at ˜34°N shows a low P and S wave velocity zone in the mantle that extends up the base of the Moho. This low-velocity region has been interpreted by (Gao et al., 2004) to be the result of convective removal of a portion of the once >100 km thick Proterozoic lithosphere. The amount of extension in the central RGR is thought to be low (˜25%) and thus cannot account for the amount of lithosphere thinning suggested by seismic tomography. We measured whole rock and mineral major element, trace element, and isotopic compositions of spinel-peridotite xenoliths erupted along the central axis of the rift (Elephant Butte) and the eastern margin of the Colorado Plateau (Cerro Chato) to determine their depth of origin and mantle provenance and to test the delamination hypothesis. If lithosphere removal has not occurred and the low P and S wave velocities are instead the result of hydration or melt infiltration in the lithosphere, then xenoliths erupted on the rift axis should have geochemical compositions similar to Proterozoic sub-continental lithospheric mantle (SCLM). At Cerro Chato, on the margin of the Colorado Plateau, xenoliths were derived from ˜60 km depth and have geochemical signatures similar to Proterozoic sub-continental lithospheric mantle (e.g. refractory major element compositions, LREE-enrichment, enriched Sr and Nd isotopes, unradiogenic Os isotopes). At Elephant Butte, along the central rift axis, two distinct groups of xenoliths are present. The majority of xenoliths from Elephant Butte are LREE-depleted and have fertile major element compositions. Additionally, these xenoliths have isotopic signatures similar to the range for DMM (e.g. 87Sr/86Sr ranging from 0.7018 to 0.7023, ɛNd ranging from 7 to 21, and 187Os/188Os ranging from 0.122 to 0.130). We interpret this group of xenoliths to be derived from asthenospheric mantle. A less-abundant group of xenoliths at Elephant Butte are LREE

  1. 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

  2. Rift architecture and sedimentology of the Phetchabun Intermontane Basin, central Thailand

    NASA Astrophysics Data System (ADS)

    Remus, David; Webster, Mark; Keawkan, Kanjana

    The Phetchabun Basin, located onshore in central Thailand is one of at least 30 Tertiary intermontane basins identified in Thailand. The basin is a composite of several north-south trending half and full graben, which formed through transtensional dextral shear along the Mae Ping fault zone. Several conjugate strike-slip faults cut the basin and may have acted as the mechanism for formation of the individual graben. The grabens vary in depth from 2500 to 1100 m with the deepest being in the south and progressively shallowing northwards. Seismic cross sections across the basin reveal typical rift structuring. Central graben horsts, hanging wall anticlines and rotated fault blocks predominate within the graben, while wrench style structures are evident along inferred transfer zones. Sedimentation commenced in the Oligocene with syn-rift fluvial deposits and associated rift volcanics. An Oligocene to Mid-Miocene sedimentary sequence (Wichian Buri Group) of fluvial and lacustrine deposits followed. Diorite and diabase intrusives reflect periods of igneous activity during the Early and Late Middle Miocene. After the Mid-Miocene tectonic episode, lacustrine conditions were re-established over much of the basin and predominantly fine-grained sediments were deposited (Chaliang Lab Formation). Plio-Pleistocene sediments comprising lithic sands and shales were deposited in an oxidizing alluvial environment. Waxy oils and dry gas have been recovered from thin bedded sandstone reservoirs and igneous sills. Initial tests at Wichian Buri-1 indicated flow rates up to 500 BOPD from thin sands, while Bo Rang-1 tested gas at rates up to 5.5 MMCFG/D from an altered sill.

  3. 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

    Crustal rheology controls the style of rifting and ultimately the architecture of rifted margins: Hot, weak, or thick continental crust is dominated by ductile deformation and extends symmetrically into a wide rift system. Extension in cold, strong, or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries. This recipe provides the standard framework to understand 2D rift geometry, however, a variety of processes exert significant control on subsequent rift evolution and ultimately on the architecture of rifted margins: inherited structures, melting and volcanism, 3D effects, extension rate, and weakening mechanisms. Numerical forward modelling studies have the opportunity to evaluate the influence of these processes on rift evolution in order to understand the complex interaction between rheology and tectonic history of specific margins. Here I compare the formation of three different magma-poor margin pairs, Iberia-Newfoundland, the Central South Atlantic Rift Segment, and the South China Sea margins within a numerical forward modelling framework. I apply a 2D version of the finite element code SLIM3D, which includes nonlinear temperature- and stress-dependent elasto-visco-plastic rheology and is able to reproduces a wide range of rift-related deformation processes such as flexure, lower crustal flow, and faulting. The Iberia-Newfoundland rifted margins are marked by moderate crustal asymmetry, with ~70 km of hyper-extended crust (less than 10 km thick) on the Iberian side and a very narrow margin on the Newfoundland counterpart. Similar to the Iberia-Newfoundland conjugates, the Central South Atlantic margins are predominantly asymmetric, however involve a much stronger degree of asymmetry with more than 200 km of hyper-extended crust offshore Angola, but only few tens of km at the Brazilian side. Kinematic and numerical modelling suggests that the asymmetry is caused by lateral

  4. Palaeoecological evidence for Holocene environmental change from the Virunga volcanoes in the Albertine Rift, central Africa

    NASA Astrophysics Data System (ADS)

    McGlynn, Gayle; Mooney, Scott; Taylor, David

    2013-02-01

    This study presents two new, well-dated sedimentary pollen and charcoal records from high-altitude crater sites in the Virunga volcanoes, located in the Albertine Rift, central Africa, currently part of one of the world's most biodiverse areas. Here we argue that Holocene vegetation changes in a ca 8000-year palaeoenvironmental record from a crater swamp at an altitude of 3474 m and a ca 2800-year record from a crater lake at an altitude of 4127 m are linked to variations in both climate and human activity. Climatic changes during the mid- to late Holocene are reflected in the high-altitude sites and more widely in adjacent parts of the Albertine Rift. Vegetation changes, comprising a decline in Ericaceous vegetation at ca 5000 cal yrs BP and subsequent expansion of Afroalpine vegetation, together with a later increase in taxa associated with lower montane forest (particularly Podocarpus), reflect increasing aridity during the mid- to late Holocene. Human-induced environmental change in the Virunga volcanoes is apparent only within the last millennium, despite the long history of human occupation of the area. Both study sites record significant forest clearance at ca 900 cal yrs BP, involving a reduction in lower montane forest taxa and increases in disturbance indicators. Changes in the composition of upper montane forest, and particularly the expansion of Hagenia, are possibly linked to anthropogenic-induced changes in the fire regime, and are apparent from ca 900 cal yrs BP. Human-induced environmental modification from the early part of the last millennium, likely associated with onset of the Late Iron Age, appears to have extended to high altitudes. The importance of natural, long-term climate change as a major cause of environmental change in the Albertine Rift has been eclipsed within the last millennium by human-induced environmental effects.

  5. Red Sea rift-related Quseir basalts, central Eastern Desert, Egypt: Petrogenesis and tectonic processes

    NASA Astrophysics Data System (ADS)

    Farahat, Esam S.; Ali, Shehata; Hauzenberger, Christoph

    2017-01-01

    Mineral and whole-rock chemistry of Red Sea rift-related Tertiary basalts from south Quseir city, central Eastern Desert of Egypt is presented to investigate their petrogenesis and relationship to tectonic processes. The south Quseir basalts (SQB) are classified as high-Ti (TiO2 >2 wt.%) subalkaline transitional lava emplaced in an anorogenic tectonic setting. Their Mg# varies from 48 to 53 indicating the evolved nature of the SQB. Pearce element ratios suggest that the SQB magmas evolved via fractional crystallization of olivine + clinopyroxene ± plagioclase, but the absence of Eu anomalies argues against significant plagioclase fractionation. Clinopyroxene compositions provide evidence for polybaric fractionation of the parental mafic magmas. Estimated temperatures of crystallization are 1015 to 1207 °C for clinopyroxene and 1076 to 1155 °C for plagioclase. These values are interpreted to result from early stage crystallization of clinopyroxene followed by concurrent crystallization of clinopyroxene and plagioclase. The incompatible trace element signatures of the SQB (La/Ba = 0.08-0.10 and La/Nb = 0.89-1.04) are comparable to those of ocean island basalts (OIB) generated from an asthenospheric mantle source unaffected by subduction components. Modeling calculations indicate that the SQB primary magmas were derived from 4-5% partial melting of a garnet-bearing lherzolite mantle source. The NE Egyptian basaltic volcanism is spatially and temporally related to Red Sea rifting and to the local E-W striking faults, confirming a relationship to tectonic activity. Our results suggest that the extensional regime associated with Red Sea rifting controlled the generation of the Egyptian basalts, likely as a result of passive upwelling of asthenospheric mantle.

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

    NASA Astrophysics Data System (ADS)

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

    2007-07-01

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

  7. 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

  8. Influence of rifted geometry on the variations in style of tectonic thickening between southern Quebec and central Vermont

    SciTech Connect

    Colpron, M.; Warren, M.J. . Dept. of Geological Sciences)

    1993-03-01

    The style of Taconian deformation and metamorphism varies significantly along strike from the Quebec reentrant toward the New York promontory. Comparison of Upper Proterozoic/Lower Cambrian pre-shelf sedimentary sequences between southern Quebec and central Vermont suggests that, despite important similarities, the configurations of the rifted continental crust differed along strike. In southern Quebec, the base of the rift sequence is characterized by extensive flood basalt volcanism (Tibbit Hill Fm.). The basaltic rocks were subsequently deformed by block-faulting, which acted as basement to overlying rift-clastic sediments. In central Vermont, mafic dikes cut the Middle Proterozoic Lincoln massif and are unconformably overlain by the basal rift sequence. In both areas, a hinge zone, marking the transition between relatively unsubsided and significantly attenuated continental crust, developed early in the sedimentation history. West of the hinge zone, the basal rift-clastic rocks are characterized by a relatively thin sequence indicative of shallow subaqueous to subaerial deposition (Pinnacle Fm.), capped by a dolomitic horizon (White Brook Fm. and Forestdale Mb.) and a disconformity marking renewed extension. East of the hinge zone, rift-clastic rocks are thicker, generally finer-grained, and commonly contain mafic rocks of tholeiitic affinity. In southern Quebec and northern Vermont, significant subsidence accompanied the post-White Brook/Forestdale episode of extension. In contrast, the basement high of the Lincoln massif remained largely unsubsided until the onset of thermal subsidence in the Early Cambrian. Both in southern Quebec and central Vermont, early Taconian deformation was characterized by sub-horizontal westward transport of distal clastic sedimentary rocks.

  9. Mineralization potential along the trend of the Keweenawan- age Central North American Rift System in Iowa, Nebraska, and Kansas

    USGS Publications Warehouse

    Berendsen, P.

    1989-01-01

    The tectonic and sedimentary environment of the Central North American Rift System (CNARS) provides an excellent setting for major mineral deposits. Major north-northeast-trending high-angle normal or reverse faults and northwest-trending transcurrent fault systems may exercise control over ore forming processes. Gabbro and basalt are the dominant igneous rock types. Carbonatite and kimberlite occur in Nebraska and Kansas. Concentrations of Cu, Ni, Co, Ti, Au, Ag and PG minerals are known to occur in this setting. Arkosic sandstone, siltstone, shale, and minor carbonate units occur on top of the rift basalts and in flanking basins where they may reach thicknesses of 10 km (6 miles). The potential for stratiform or unconformity-related metalliferous deposits should be considered. The rift as a whole remains largely unexplored.

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

    NASA Astrophysics Data System (ADS)

    Pollitz, Fred F.; Mooney, Walter D.

    2014-09-01

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

  11. Oblique rift opening revealed by reoccurring magma injection in central Iceland

    PubMed Central

    Ruch, Joël; Wang, Teng; Xu, Wenbin; Hensch, Martin; Jónsson, Sigurjón

    2016-01-01

    Extension deficit builds up over centuries at divergent plate boundaries and is recurrently removed during rifting events, accompanied by magma intrusions and transient metre-scale deformation. However, information on transient near-field deformation has rarely been captured, hindering progress in understanding rifting mechanisms and evolution. Here we show new evidence of oblique rift opening during a rifting event influenced by pre-existing fractures and two centuries of extension deficit accumulation. This event originated from the Bárðarbunga caldera and led to the largest basaltic eruption in Iceland in >200 years. The results show that the opening was initially accompanied by left-lateral shear that ceased with increasing opening. Our results imply that pre-existing fractures play a key role in controlling oblique rift opening at divergent plate boundaries. PMID:27492709

  12. Oblique rift opening revealed by reoccurring magma injection in central Iceland

    NASA Astrophysics Data System (ADS)

    Ruch, Joël; Wang, Teng; Xu, Wenbin; Hensch, Martin; Jónsson, Sigurjón

    2016-08-01

    Extension deficit builds up over centuries at divergent plate boundaries and is recurrently removed during rifting events, accompanied by magma intrusions and transient metre-scale deformation. However, information on transient near-field deformation has rarely been captured, hindering progress in understanding rifting mechanisms and evolution. Here we show new evidence of oblique rift opening during a rifting event influenced by pre-existing fractures and two centuries of extension deficit accumulation. This event originated from the Bárðarbunga caldera and led to the largest basaltic eruption in Iceland in >200 years. The results show that the opening was initially accompanied by left-lateral shear that ceased with increasing opening. Our results imply that pre-existing fractures play a key role in controlling oblique rift opening at divergent plate boundaries.

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

    PubMed Central

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

    2016-01-01

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

  14. Multiple episodes of rifting in Central and East Africa: A re-evaluation of gravity data

    NASA Astrophysics Data System (ADS)

    Ebinger, C. J.; Ibrahim, A.

    1994-12-01

    A compilation of new and existing gravity data, as well as geophysical and geological data, is used to assess the cumulative effects of multiple rifting episodes on crustal and upper mantle density structures beneath the Uganda-Kenya-Ethiopia-Sudan border region. This compilation includes new gravity and geological data collected in 1990 in south-western Ethiopia. Variations in the trends and amplitudes of Bouguer gravity anomalies reveal three overlapping rift systems: Mesozoic, Paleogene and Miocene-Recent. Each of these rift systems is a number of 40 100 km long sedimentary basins, and each system is approximately 1000 km long. The Bouguer anomaly patterns indicate that the Ethiopian and East African plateaux and corresponding gravity anomalies are discrete tectonic features. Models of structural and gravity profiles of two basins (Omo and Chew Bahir basins) suggest that pre-Oligocene (Cretaceous?) strata underlie 3 km or more of Neogene-Recent strata within the northern Kenya rift, and that more than 2 km of Neogene-Recent strata underlie parts of the southern Main Ethiopian rift. The superposition of perhaps three rifting episodes in the Lake Turkana (Omo) region has led to 90% crustal thinning (β ≈ 2).

  15. Ethiopian Central Rift Valley basin hydrologic modelling using HEC-HMS and ArcSWAT

    NASA Astrophysics Data System (ADS)

    Pascual-Ferrer, Jordi; Candela, Lucila; Pérez-Foguet, Agustí

    2013-04-01

    An Integrated Water Resources Management (IWRM) shall be applied to achieve a sustainable development, to increase population incomes without affecting lives of those who are highly dependent on the environment. First step should be to understand water dynamics at basin level, starting by modeling the basin water resources. For model implementation, a large number of data and parameters are required, but those are not always available, especially in some developing countries where different sources may have different data, there is lack of information on data collection, etc. The Ethiopian Central Rift Valley (CRV) is an endorheic basin covering an area of approximately 10,000 km2. For the period 1996-2005, the average annual volume of rainfall accounted for 9.1 Mm3, and evapotranspiration for 8 Mm3 (Jansen et al., 2007). From the environmental point of view, basin ecosystems are endangered due to human activities. Also, poverty is widespread all over the basin, with population mainly living from agriculture on a subsistence economy. Hence, there is an urgent need to set an IWRM, but datasets required for water dynamics simulation are not too reliable. In order to reduce uncertainty of numerical simulation, two semi-distributed open software hydrologic models were implemented: HEC-HMS and ArcSWAT. HEC-HMS was developed by the United States Army Corps of Engineers (USACoE) Hydrologic Engineering Center (HEC) to run precipitation-runoff simulations for a variety of applications in dendritic watershed systems. ArcSWAT includes the SWAT (Soil and Water Assessment Tool, Arnold et al., 1998) model developed for the USDA Agricultural Research Service into ArcGIS (ESRI®). SWAT was developed to assess the impact of land management practices on large complex watersheds with varying soils, land use and management conditions over long periods of time (Neitsch et al., 2005). According to this, ArcSWAT would be the best option for IWRM implementation in the basin. However

  16. Searching for evidence of changes in extreme rainfall indices in the Central Rift Valley of Ethiopia

    NASA Astrophysics Data System (ADS)

    Muluneh, Alemayehu; Bewket, Woldeamlak; Keesstra, Saskia; Stroosnijder, Leo

    2016-02-01

    Extreme rainfall events have serious implications for economic sectors with a close link to climate such as agriculture and food security. This holds true in the Central Rift Valley (CRV) of Ethiopia where communities rely on highly climate-sensitive rainfed subsistence farming for livelihoods. This study investigates changes in ten extreme rainfall indices over a period of 40 years (1970-2009) using 14 meteorological stations located in the CRV. The CRV consists of three landscape units: the valley floor, the escarpments, and the highlands all of which are considered in our data analysis. The Belg (March-May) and Kiremt (June-September) seasons are also considered in the analysis. The Mann-Kendall test was used to detect trends of the rainfall indices. The results indicated that at the annual time scale, more than half (57 %) of the stations showed significant trends in total wet-day precipitation (PRCPTOT) and heavy precipitation days (R10mm). Only 7-35 % of stations showed significant trends, for the other rainfall indices. Spatially, the valley floor received increasing annual rainfall while the escarpments and the highlands received decreasing annual rainfall over the last 40 years. During Belg, 50 % of the stations showed significant increases in the maximum number of consecutive dry days (CDD) in all parts of the CRV. However, most other rainfall indices during Belg showed no significant changes. During Kiremt, considering both significant and non-significant trends, almost all rainfall indices showed an increasing trend in the valley floor and a decreasing trend in the escarpment and highlands. During Belg and Kiremt, the CDD generally showed increasing tendency in the CRV.

  17. Rainwater harvesting for small-scale irrigation of maize in the Central Rift Valley, Ethiopia

    NASA Astrophysics Data System (ADS)

    Keesstra, Saskia; Hartog, Maaike; Muluneh, Alemayehu; Stroosnijder, Leo

    2013-04-01

    In the Central Rift Valley of Ethiopia, small scale farmers mostly rely on rainfall for crop production. The erratic nature of rainfall causes frequent crop failures and makes the region structurally dependent on food aid. Rainwater Harvesting (RWH) is a technique to collect and store runoff that could provide water for livestock, domestic use or small scale irrigation. Usually, such irrigation is promoted for high value crops, but in the light of regional food security it may become interesting to invest in irrigation of maize. In this research, two cemented RWH cisterns were investigated to determine their economic and social potential for supplemental irrigation of maize using drip irrigation. For this, data from test fields with irrigated maize and monitoring of water levels of the cisterns were used, as well as a survey under 30 farmers living close to the experimental site. The results show that catchment size and management should be in balance with the designed RWH system, to prevent too little runoff or flooding. An analysis with Cropwat 8.0 was used to investigate the possibility of irrigating maize with the observed amounts of water in the RWH cisterns. This would suffice for 0.3-0.8 ha of maize. For a RWH cistern with a drip irrigation system to be economically viable, the production on this acreage should become 3-4 ton/ha; 2.5 times higher than the current yield. But the biggest challenge would be to change the perception of respondents, who don't find it logical to spend precious water on a common crop like maize. Therefore, if the Ethiopian government considers the irrigation of maize to be important for regional food security, it is recommended to either subsidize the construction of RWH cisterns or provide credit on favourable terms.

  18. Evaluating spatial and temporal variations of rainfall erosivity, case of Central Rift Valley of Ethiopia

    NASA Astrophysics Data System (ADS)

    Meshesha, Derege Tsegaye; Tsunekawa, Atsushi; Tsubo, Mitsuru; Haregeweyn, Nigussie; Adgo, Enyew

    2015-02-01

    Land degradation in many Ethiopian highlands occurs mainly due to high rainfall erosivity and poor soil conservation practices. Rainfall erosivity is an indicator of the precipitation energy and ability to cause soil erosion. In Central Rift Valley (CRV) of Ethiopia, where the climate is characterized as arid and semiarid, rainfall is the main driver of soil erosion that in turn causes a serious expansion in land degradation. In order to evaluate the spatial and temporal variability of rainfall erosivity and its impact on soil erosion, long-term rainfall data (1980-2010) was used, and the monthly Fournier index (FI) and the annual modified Fournier index (MFI) were applied. Student's t test analysis was performed particularly to examine statistical significances of differences in average monthly and annual erosivity values. The result indicated that, in a similar spatial pattern with elevation and rainfall amount, average annual erosivity is also found being higher in western highlands of the valley and gradually decreased towards the east. The long-term average annual erosivity (MFI) showed a general decreasing trend in recent 10 years (2000-2010) as compared to previous 20 years (1980-1999). In most of the stations, average erosivity of main rainy months (May, June, July, and August) showed a decreasing trend, whereby some of them (about 33.3 %) are statically significant at 90 and 95 % confidence intervals but with high variation in spatial pattern of changes. The overall result of the study showed that rainfall aggression (erosivity) in the region has a general decreasing trend in the recent decade as compared to previous decades, especially in the western highlands of the valley. Hence, it implies that anthropogenic factors such as land use change being coupled with topography (steep slope) have largely contributed to increased soil erosion rate in the region.

  19. Bridging dry spells for maize cropping through supplemental irrigation in the Central Rift Valley of Ethiopia

    NASA Astrophysics Data System (ADS)

    Muluneh Bitew, Alemayehu; Keesstra, Saskia; Stroosnijder, Leo

    2015-04-01

    Maize yield in the Central Rift Valley of Ethiopia (CRV) suffers from dry spells at sensitive growth stages. Risk of crop failure makes farmers reluctant to invest in fertilizer. This makes the CRV food insecure. There are farms with well-maintained terraces and Rain Water Harvesting (RWH) systems using concrete farms ponds. We tested the hypothesis that in these farms supplemental irrigation with simultaneous crop intensification might boost production of a small maize area sufficient to improve food security. Intensification includes a higher plant density of a hybrid variety under optimum fertilization. First we assessed the probability of occurrence of dry spells. Then we estimated the availability of sufficient runoff in the ponds in dry years. During 2012 (dry) and 2013 (wet) on-farm field research was conducted with 10 combinations of supplemental irrigation and plant density. The simplest was rainfed farming with 30,000 plants ha-1. The most advanced was no water stress and 75,000 plants ha-1. Finally we compared our on-farm yield with that of neighbouring farmers. Because 2013 was a wet year no irrigation was needed. Our long term daily rainfall (1970-2011) analysis proves the occurrence of dry spells during the onset of the maize (Belg months March and April). In March there is hardly enough water in the ponds. So, we advise later sowing. Starting from April available water (runoff from a 2.2 ha catchment) matches crop water requirement (for 0.5 ha maize). Significant differences between grain and total biomass yield were observed between rainfed and other irrigation levels. However, since the largest difference is only 12%, the investment in irrigation non-critical drought years is not worth the effort. There was also a limited effect (18-22%) of increasing plant density. So, we advise not to use more than 45,000 plants ha-1. The grain yield and total biomass difference between farmers own practice and our on-farm research was 101% and 84% respectively

  20. 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

  1. Global Climate Change and Sedimentation Patterns in the Neogene Baringo Basin, Central Kenya Rift

    NASA Astrophysics Data System (ADS)

    Deino, A. L.; Kingston, J. D.; Wilson, K. E.; Hill, A.

    2010-12-01

    The Tugen Hills are part of a ~100 km N-S tilted fault block, just west of Lake Baringo within the Central Kenyan Rift Valley. Sediments exposed in this block span the last 16 Ma and have yielded abundant and diverse fossil assemblages including a number of hominoid and hominid specimens. Much research has also focused on documenting the paleoecology of the succession through analyses of fossil floral, faunal, and biogeochemical proxies. Data from the Tugen Hills have revealed a complex evolutionary history of ecosystems characterized by spatial and temporal heterogeneity with no clear evidence of any long-term trends. While these studies suggest that the patterns of heterogeneity may be shifting at short time-scales (104-105 ka), limited temporal resolution has until now generally precluded assessments of environmental change at these scales. Recently published investigations in the Baringo Basin have provided evidence of orbitally mediated environmental change over periods which include hominid fossil localities (Deino et al., 2006; Kingston et al., 2007). The Baringo data represent the only empirical evidence for significant local environmental shifts that can directly be correlated with insolation patterns in equatorial Africa. Sedimentation patterns in the Baringo Basin between ca. 2.70 and 2.55 Ma, controlled by climatic factors, provide a detailed paleoenvironmental record including a sequence of diatomites that record rhythmic cycling of major freshwater lake systems consistent with ~23 kyr Milankovitch precessional periodicity modulated by eccentricity. The timing of the paleolakes most closely approximates insolation maximum for the June/July 30○N insolation curve, suggesting that precipitation patterns in the region are controlled by the African monsoon system. More recent fieldwork has identified older sequences that similarly demonstrate rhythmic cycling of freshwater lake systems. Preliminary 40Ar/39Ar dating of intercalated tephra reveals that

  2. Spatial and Temporal Strain Distribution Along the Central Red Sea Rift - A Study of the Hamd-Jizil Basin in Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Szymanski, E.; Stockli, D.; Johnson, P.; Kattan, F. H.; Al Shamari, A.

    2006-12-01

    Numerous models exploring the rupturing modes and mechanisms of continental lithosphere are based on geological evidence from the Red Sea/Gulf of Suez rift system. Individually, the Red Sea basin is the prototype for many models of orthogonal continental rifting. Despite being a classic example of continental extension, many temporal and spatial strain distribution aspects, as well as the dynamic evolution of the rift architecture of the Red Sea, remain poorly constrained. Critical data come mostly from the Gulf of Suez and the Egyptian and Yemeni margins of the Red Sea; the rift flanks in Sudan and Saudi Arabia have remained largely unstudied, leaving a large information gap along the central portions of the rift system. Improving continental lithosphere rupture models requires an absolute understanding of the timing and magnitude of strain partitioning along the full rift flank. This study focuses on the development of extensional structures, syn- extensional sedimentary deposits, and rift-related Tertiary basaltic volcanism along the central flank of the rift system in Saudi Arabia. Geo- and thermochronometric techniques are used to elucidate the evolution of inboard and outboard strain markers manifested by structurally-controlled extensional basins that parallel the trend of the main Red Sea rift. Constraints on the dynamics of rift flank deformation are achieved through the collection of thermochronometric transects that traverse both the entire Arabian shield and individual normal faults that bound inland basins. Preliminary results show inland basins as asymmetric half-grabens filled by tilted Cenozoic sedimentary strata and separated by exhumed basement fault blocks. The most prominent extensional basin is the NW-trending Hamd-Jizil basin, located north of Madinah, measuring ~200 km along strike and up to 20 km in width. The Hamd-Jizil basin is structurally characterized by two half-grabens exposing a series of syn-rift siliciclastic sedimentary sections

  3. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  5. 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

  6. Extensional salt tectonics in the partially inverted Cotiella post-rift basin (south-central Pyrenees): structure and evolution

    NASA Astrophysics Data System (ADS)

    López-Mir, Berta; Muñoz, Josep Anton; García-Senz, Jesús

    2015-03-01

    The Cotiella Massif in the south-central Pyrenees hosts upper Cretaceous gravity-driven extensional faults which were developed in the Bay of Biscay-Pyrenean paleorift margin of the Atlantic Ocean. They accommodate up to 6 km of post-rift carbonates above relict upper Triassic salt. Subsequent Pyrenean contractional deformation preserved the main extensional features, but most of the upper Triassic salt was expulsed and then dissolved, leaving little indications of the original salt volume. Nonetheless, several distinctive salt-related features are still recognizable both at outcrop and at basin scale, providing an exposed analogue for salt-floored extensional basins developed on passive margins. Based on field research, we re-interpret the tectonic evolution of the area and suggest that passive diapirs were coeval with gravity-driven extension during the development of the Cotiella basin. The given interpretations are supported with detailed geological maps, original structural data, cross sections and outcrop photographs. The discovery of previously unknown post-rift salt structures in the Cotiella Massif is an extra element to consider in the paleogeographic reconstructions of the upper Cretaceous passive margin of the Bay of Biscay-Pyrenean realm and consequently helps in our understanding of the evolution of current Atlantic-type margins.

  7. P wave velocity of the uppermost mantle of the Rio Grande rift region of North Central New Mexico

    SciTech Connect

    Murdock, J.N.; Jaksha, L.H.

    1981-08-10

    A network of seismograph stations has operated in north-central New Mexico since 1975. The network is approximtely 200 by 300 km in size and encompasses the Rio Grande rift there. Several seismic refraction experiments have been reported in the literature for the region of the network and adjacent areas. Because all of the seismic refraction lines are unreversed, P/sub n/ velocities reported were mainly of the inverse travel time slope for the direction of the corresponding line. The values of the inverse slope for those studies range from 7.6 to 8.2 km/s. The purpose of our study is to estimate the P wave velocity of the uppermost mantle by using the time term method. First, we timed the P/sub n/ waves of strong signals from five explosions and eight shallow earthquakes recorded by the network. The main data set, which contains 87 time-distance pairs, was processed by using the time term method. The P/sub n/ velocity estimated by this method is 8.0 +- 0.1 km/s. To corroborate this estimate, we then processed 10 subsets of the main data set in the same way. Almost allof the solutions show velocities 7.9--8.1 km/s, in agreement with the velocity determined for the main data set. The station time terms of the main data set also are substantied, and they suggest that the base of the crust dips northward by a few degrees in the region of the survey. The smallest value reported by other investigators for the inverse slope (7.6 km/s) appears to be related to the dip. The normal P wave velocity of the uppermost mantle of north-central New Mexico places restrictions on thermal models of the rift. For instance, the results exlude the likelihood of a wide zone of asthenosphere at the base of the crust beneath the rift, but they do not exclude a narrow such zone.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    The Main Ethiopian Rift (MER) has undergone extension since ˜8 Ma, and whilst large border faults were active until later stages, since then (2 Ma) seemingly most extension has been via the Wonji Fault Belt (WFB), a series of en-echelon faults perpendicular to current spreading, which possibly focus around magmatic centres. Two such centres are Corbetti and Aluto volcanoes in the central MER. They have shown significant (>5 cm) uplift and subsidence for at least five years, probably erupted in the Holocene, and are geothermal sites. They are presumed therefore to play an active rôle in present-day extension along the rift, via magma injection and brittle deformation; yet a detailed physical explanation of their behaviour remains elusive. We report results from a recent combined seismic-geodetic study (ARGOS) of these areas, focussing on the seismic anisotropy revealed. We confirm that both volcanoes are seismically active, with events located beneath the edifice having mean local magnitude mL = 1.0. Beneath Aluto, there are two main clusters of activity: (1) at depths 5-10 km below sea level (bsl), and (2) between -2 and 0 km bsl. Focal mechanisms show predominantly normal faulting on fault planes striking north-northeast (NNE), and event locations cluster along a similar trend. The identification of the WFB in this region is debated, but we show that only the deepest (5-15 km) events occur along the northeast-trending faults with outcropping to the east. Shear wave splitting of over 5 % is present, and appears to be confined to the top 5 km, since little depth dependence is shown. Fast shear wave orientations have again a NNE trend. These lines of evidence indicate that current seismic deformation, and aligned structures in the top few km, act in response to the current stress field, and not pre-existing features. Any magmatic emplacement occurring above 15 km is likely not as dykes, as these would create large seismic anisotropy at these depths which is not

  9. European Cenozoic rift system

    NASA Astrophysics Data System (ADS)

    Ziegler, Peter A.

    1992-07-01

    The European Cenozoic rift system extends from the coast of the North Sea to the Mediterranean over a distance of some 1100 km; it finds its southern prolongation in the Valencia Trough and a Plio-Pleistocene volcanic chain crossing the Atlas ranges. Development of this mega-rift was paralleled by orogenic activity in the Alps and Pyrenees. Major rift domes, accompanied by subsidence reversal of their axial grabens, developed 20-40 Ma after beginning of rifting. Uplift of the Rhenish Shield is related to progressive thermal lithospheric thinning; the Vosges-Black Forest and the Massif Central domes are probably underlain by asthenoliths emplaced at the crust/mantle boundary. Evolution of this rift system, is thought to be governed by the interaction of the Eurasian and African plates and by early phases of a plate-boundary reorganization that may lead to the break-up of the present continent assembly.

  10. A Crustal Cross Section over the Central North Iberian Margin: New Insights into the Bay of Biscay Inverted Hyperextended Rift

    NASA Astrophysics Data System (ADS)

    Cadenas Martínez, P.; Fernandez Viejo, G.; Pulgar, J. A.; Minshull, T. A.

    2015-12-01

    The Bay of Biscay is a V-shape failed arm of the Atlantic rift which was opened during the Mesozoic and partially closed during the Alpine orogeny in the Cenozoic, when the convergence of the Iberian and European Plates drove to the formation of the Pyrenean-Cantabrian realm in the North Iberian peninsula. A complete crustal cross section through the central part of the North Iberian Margin, representing the southern margin of the Bay of Biscay, is presented here from the interpretation of a high quality deep seismic reflection profile together with boreholes and well logs, acquired for oil and gas exploration purposes. The studied segment of this margin includes a basement high so called Le Danois Bank, and the Asturian basin, one of the sedimentary basins developed during the Mesozoic extensional processes, which was subsequently inverted during the Alpine orogeny. Most of the compression seems to have taken place through uplift of the continental platform and slope and the formation of an accretionary wedge at the bottom of the slope, so it is still possible to elucidate both extensional and compressional features. The basin appears as an asymmetric bowl bounded by synsedimentary normal faults with a maximum thickness of about 6 s TWT, which has been estimated to be equivalent to about 7 km. Depth migration of the seismic profile has revealed the presence of a deeper trough, with a maximum thickness of 13. 5 km at its main depocenter, which closely resembles the sedimentary thickness proposed for other contemporaneous proximal basins. These results support the high degree of extension and the exhumation processes proposed for this margin, deduced from refraction velocities and from the upper crustal and mantle rocks dredged at the slopes of Le Danois High. They will bring new insights to, and further constraints on, geodynamical models for this margin, where the amount of shortening linked with Cenozoic compression and the role of the rift structure during the

  11. Geophysical evidence of Cretaceous volcanics in Logone Birni Basin (Northern Cameroon), Central Africa, and consequences for the West and Central African Rift System

    NASA Astrophysics Data System (ADS)

    Loule, Jean-Pierre; Pospisil, Lubomil

    2013-01-01

    Detailed analyses and interpretation realized by combining existing 2D reflection seismic and Gravity/Magnetic data of the Logone Birni Basin (LBB) in the West and Central African Rift System (WCAS) have revealed the distribution of the main buried volcanic bodies as well as their relationships with the structural and tectonic evolution of this basin. The volcanic activity in the LBB is restricted to the Cretaceous period. Three main volcanic episodes are identified and are associated to the Neocomian, Late Albian and Cenomanian-Turonian rifting phases respectively. The volcanic bodies within the Lower Cretaceous are either lying directly on basement or are mainly interbedded with the contemporaneous sediments whereas the Upper Cretaceous bodies are morphologically expressed in the forms of dykes and sills. The volcanic activity was more intense in the western region of the central LBB (Zina sub-basin) along the Cameroon-Nigeria border whereas it was scanty and scattered in the other parts of the basin. The main volcanic dykes are found on the flanks of the major faults bounding basement horsts or in crestal positions in association with syndepositional faults. Although WCAS is associated with large amount of crustal extension and minor volcanism, the intense volcanic activity observed in LBB during the Cretaceous suggests that the intrusive zone during this period was confined to the basement beneath the study area flanked respectively to the north, south and southwest by the Lake Chad, Poli and Chum triple junctions. Tensional stresses generated by this localized domal uplift accounts for most of the observed tectonic structures where major faults transected the entire lithosphere, thus providing conduits for magma migration.

  12. Palaeostress reconstructions and geodynamics of the Baikal region, Central Asia, Part I. Palaeozoic and Mesozoic pre-rift evolution

    NASA Astrophysics Data System (ADS)

    Delvaux, D.; Moeys, R.; Stapel, G.; Melnikov, A.; Ermikov, V.

    1995-12-01

    This paper presents the first palaeostress results obtained for the basement of the Baikal rift system, in southern Siberia (Russia). Large-scale structural analysis and palaeostress reconstructions show that the Palaeozoic-Mesozoic kinematic history, precursor of the Baikal Cenozoic rifting, is characterized by the succession of six regional palaeostress stages. Stress inversion of fault-slip data and earthquake focal mechanisms is performed using an improved right-dieder method, followed by rotational optimization (D. Delvaux, TENSOR program). The results are interpreted in the light of recent developments in the investigation of regional intraplate stress field, and used as additional constraints for palaeogeodynamic reconstruction of Central Asia. After the final Palaeozoic closure of the Palaeo-Asian ocean on the southern margin of the Siberian platform, the marginal suture with the Sayan-Baikal Caledonian belt was repeatedly and preferentially reactivated during the subsequent Palaeozoic and Mesozoic history. This suture zone also controlled the opening of the Baikal rift system in the Cenozoic. The progressive closure of the Palaeo-Asian and Mongol-Okhotsk oceans generated successive continental collisions, which were recorded in the Baikal area by brittle-ductile and brittle deformations. The first two palaeostress stages correspond to the successive collage of Precambrian microcontinents and Caledonian terranes along the southern margin of the Siberian platform: (1) Late Cambrian-Early Ordovician N-S compression; and (2) Late Silurian-Early Devonian NW-SE compression. The next two stages are related to the remote effects of the complex evolution of the western Palaeo-Asian ocean, southwest of the Siberian continent: (3) Late Devonian-Early Carboniferous N-S compression, recorded only in the Altai region; and (4) Late Carboniferous-Early Permian E-W compression, recorded both in the Altai and Baikal regions. The last stages are the consequences of the

  13. The 1.1-Ga Midcontinent Rift System, central North America: sedimentology of two deep boreholes, Lake Superior region

    NASA Astrophysics Data System (ADS)

    Ojakangas, Richard W.; Dickas, Albert B.

    2002-03-01

    The Midcontinent Rift System (MRS) of central North America is a 1.1-Ga, 2500-km long structural feature that has been interpreted as a triple-junction rift developed over a mantle plume. As much as 20 km of subaerial lava flows, mainly flood basalts, are overlain by as much as 10 km of sedimentary rocks that are mostly continental fluvial red beds. This rock sequence, known as the Keweenawan Supergroup, has been penetrated by a few deep boreholes in the search for petroleum. In this paper, two deep boreholes in the Upper Peninsula of Michigan are described in detail for the first time. Both the Amoco Production #1-29R test, herein referred to as the St. Amour well, and the nearby Hickey Creek well drilled by Cleveland Cliffs Mining Services, were 100% cored. The former is 7238 ft (2410 m) deep and the latter is 5345 ft (1780 m) deep. The entirety of the stratigraphic succession of the Hickey Creek core correlates very well with the upper portion of the St. Amour core, as determined by core description and point-counting of 43 thin sections selected out of 100 studied thin sections. Two Lower Paleozoic units and two Keweenawan red bed units—the Jacobsville Sandstone and the underlying Freda Sandstone—are described. The Jacobsville is largely a feldspatholithic sandstone and the Freda is largely a lithofeldspathic sandstone. Below the Freda, the remaining footage of the St. Amour core consists of a thick quartzose sandstone unit that overlies a heterogenous unit of intercalated red bed units of conglomerate, sandstone, siltstone, and shale; black shale; individual basalt flows; and a basal ignimbritic rhyolite. This lower portion of the St. Amour core presents an enigma, as it correlates very poorly with other key boreholes located to the west and southwest. While a black shale sequence is similar to the petroleum-bearing Nonesuch Formation farther west, there is no conglomerate unit to correlate with the Copper Harbor Conglomerate. Other key boreholes are

  14. Miocene sediment dispersal in the central Española basin, Rio Grande rift, New Mexico, U.S.A.

    NASA Astrophysics Data System (ADS)

    Cavazza, William

    1986-12-01

    The central Española basin in north-central New Mexico represents one of the best opportunities to examine deposits related to the development of the Rio Grande rift. The Miocene Tesuque Formation represents the bulk of the Española basin fill. It is composed of a 2000-2500 m thick sequence of alluvial-fluvial and, subordinately, lacustrine deposits with numerous interbedded ash-fall tuff layers. The overall detrital composition of the Tesuque Formation is very similar throughout the central Española basin. Provenance is primarily from basement uplifts composed of Precambrian igneous and high-grade metamorphic rocks. Nevertheless, the combined use of paleocurrent analysis, and sandstone and conglomerate petrology allows a detailed reconstruction of the sediment paleodispersal system. Two sedimentary provinces are present within the Tesuque Formation: Province A, present in the eastern, central and southern portions of the study area, has a predominantly plutoniclastic and metamorphiclastic composition, and shows systematic paleocurrents toward the west. The sediments were derived from the Precambrian-cored Santa Fe block of the Sangre de Cristo Mountains, located directly to the east of the study area. Province B, present only in the northwestern portion of the study area, is characterized by a minor but significant amount of volcaniclastic and sedimentaclastic detritus, and shows consistent SSW-directed paleocurrents. The source area was possibly located in the area of the Taos Plateau and Latir volcanic fields. An intermediate narrow and discontinuous belt (province A + B) represents a hybrid province, where directional and compositional parameters are gradational.

  15. Post-caldera faulting of the Late Quaternary Menengai caldera, Central Kenya Rift (0.20°S, 36.07°E)

    NASA Astrophysics Data System (ADS)

    Riedl, Simon; Melnick, Daniel; Mibei, Geoffrey K.; Njue, Lucy; Strecker, Manfred R.

    2015-04-01

    A structural geological analysis of young caldera volcanoes is necessary to characterize their volcanic activity, assess their geothermal potential, and decipher the spatio-temporal relationships of faults on a larger tectonic scale. Menengai caldera is one of several major Quaternary trachytic caldera volcanoes that are aligned along the volcano-tectonic axis of the Kenya Rift, the archetypal active magmatic rift and nascent plate boundary between the Nubia and Somalia plates. The caldera covers an area of approximately 80 km² and is among the youngest and also largest calderas in the East African Rift, situated close to Nakuru - a densely populated urban area. There is an increasing interest in caldera volcanoes in the Kenya Rift, because these are sites of relatively young volcanic and tectonic activity, and they are considered important sites for geothermal exploration and future use for the generation of geothermal power. Previous studies of Menengai showed that the caldera collapsed in a multi-event, multiple-block style, possibly as early as 29 ka. In an attempt to characterize the youngest tectonic activity along the volcano-tectonic axis in the transition between the Central and Northern Kenya rifts we first used a high-resolution digital surface model, which we derived by structure-from-motion from an unmanned aerial vehicle campaign. This enabled us to identify previously unrecognized normal faults, associated dyke intrusions and volcanic eruptive centers, and transfer faults with strike-slip kinematics in the caldera interior and its vicinity. In a second step we verified these structures at outcrop scale, assessed their relationship with known stratigraphic horizons and dated units, and performed detailed fault measurements, which we subsequently used for fault-kinematic analysis. The most important structures that we mapped are a series of north-northeast striking normal faults, which cross-cut both the caldera walls and early Holocene lake

  16. 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

  17. SHRIMP U-Pb dating of recurrent Cryogenian and Late Cambrian-Early Ordovician alkalic magmatism in central Idaho: Implications for Rodinian rift tectonics

    USGS Publications Warehouse

    Lund, K.; Aleinikoff, J.N.; Evans, K.V.; duBray, E.A.; deWitt, E.H.; Unruh, D.M.

    2010-01-01

    Composite alkalic plutonic suites and tuffaceous diamictite, although discontinuously exposed across central Idaho in roof pendants and inliers within the Idaho batholith and Challis volcanic-plutonic complex, define the >200-km-long northwest-aligned Big Creek-Beaverhead belt. Sensitive highresolution ion microprobe (SHRIMP) U-Pb zircon dates on these igneous rocks provide direct evidence for the orientation and location of the Neoproterozoic-Paleozoic western Laurentian rift margin in the northern U.S. Cordillera. Dating delimits two discrete magmatic pulses at ca. 665-650 Ma and 500-485 Ma at the western and eastern ends, respectively, of this belt. Together with the nearby 685 Ma volcanic rocks of the Edwardsburg Formation, there is a 200 Ma history of recurrent extensional magmatic pulses along the belt. A similar history of recurrent uplift is reflected in the stratigraphic record of the associated miogeoclinal and cratonal platform basins, suggesting that the Big Creek-Beaverhead belt originated as a border fault during continental rift events. The magmatic belt is paired with the recurrently emergent Lemhi Arch and narrow miogeoclinal facies belts and it lies inboard of a northwest-striking narrow zone of thinned continental crust. These features define a northeast-extending upper-plate extensional system between southeast Washington and southeast Idaho that formed a segment of the Neoproterozoic-Paleozoic miogeocline. This segment was flanked on the north by the St. Mary-Moyie transform zone (south of a narrow southern Canadian upper-plate margin) and on the south by the Snake River transfer zone (north of a broad Great Basin lower-plate margin). These are the central segments of a zigzagshaped Cordilleran rift system of alternating northwest-striking extensional zones offset by northeast-striking transfers and transforms. The data substantiate polyphase rift and continental separation events that included (1) pre-and syn-Windermere rifting, (2) Windermere

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

    NASA Astrophysics Data System (ADS)

    Brown, E.; Lesher, C. E.

    2015-12-01

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

  19. Erosion rates along fault scarps and rift-shoulder environments in central and northern Kenya: Insights from new 10Be-derived basin-wide erosion rates

    NASA Astrophysics Data System (ADS)

    Torres-Acosta, V.; Strecker, M. R.; Schildgen, T. F.; Wittmann, H.; Scherler, D.; Bookhagen, B.

    2011-12-01

    The Kenya Rift is typical example of an active continental rift zone and is a fundamental part of the East African Rift system. The rift valley plays a central role in archiving the relationships between sedimentation, erosion, and climate in the region. However, the links between surface processes (i.e., erosion, sedimentation) and tectonic setting are currently poorly understood. In this study we analyze to what degree tectono-geomorphic setting and/or climatic characteristics control erosion rates in the region. We extract morphometric characteristics of the rift flanks and the plateau surface from SRTM 90-m resolution digital elevation data. We rely on calibrated, satellite-derived Tropical Rainfall Measurement Mission (TRMM 2B31) rainfall to characterize the different climatic compartments throughout the study region. We calculate specific stream power amounts using integrated rainfall as discharge amounts. Next, we analyze the relation between cosmogenic radionuclide (CRN) basin-wide erosion rates and climatic and geomorphic parameters. We determined erosion rates from twenty-six river sand samples acquired from along the flanks of the Elgeyo Escarpment (northern section of western rift flank), the Nguruman Escarpment (southern section of western flank), the Tirr Tirr Plateau (north), the Kapute Plains, and the Suguta Valley. Catchment-wide erosion rates range from 0.001 to 0.1 mm/y across the different climatic compartments. Comparisons to catchment climate and topographic characteristics suggest that more than 60% of variation in erosion can be explain by specific stream power amounts using rainfall as discharge component. The catchment-averaged normalized channel steepness index, which doesn't take into account variations in precipitation, explains only 42% of the variation in erosion rates. These observations demonstrate that the strong spatial variations in erosion rates are largely controlled by both catchment morphology and climatic gradients. In

  20. The distribution and hydrogeological controls of fluoride in the groundwater of central Ethiopian rift and adjacent highlands

    NASA Astrophysics Data System (ADS)

    Ayenew, Tenalem

    2008-05-01

    Occurrence of fluoride (F) in groundwater has drawn worldwide attention, since it has considerable impact on human health. In Ethiopia high concentrations of F in groundwaters used for community water supply have resulted in extensive dental and skeletal fluorosis. As a part of a broader study, the distribution of F in groundwater has been investigated, and compared with bedrock geology and pertinent hydrochemical variables. The result indicates extreme spatial variations. High F concentration is often associated with active and sub-active regional thermal fields and acidic volcanics within high temperature rift floor. Variations in F can also be related to changes in calcium concentration resulting from dissolution of calcium minerals and mixing with waters of different chemical composition originated from variable hydrogeological environment across the rift valley. The concentration of F dramatically declines from the rift towards the highlands with the exception of scattered points associated with thermal springs confined in local volcanic centers. There are also interactions of F-rich alkaline lakes and the surrounding groundwater. Meteoric waters recharging volcanic aquifers become enriched with respect to F along the groundwater flow path from highland recharge areas to rift discharge areas. Locally wells drilled along large rift faults acting as conduits of fresh highland waters show relatively lower F. These areas are likely to be possible sources of better quality waters within the rift. The result of this study has important implications on site selection for water well drilling.

  1. U-Pb ages of metarhyolites of the Catoctin and Mount Rogers formations, central and southern Appalachians: evidence for two pulses of Iapetan rifting

    USGS Publications Warehouse

    Aleinikoff, J.N.; Zartman, R.E.; Walter, M.; Rankin, D.W.; Lyttle, P.T.; Burton, W.C.

    1995-01-01

    U-Pb ages of zircon from rhyolites of the Catoctin and Mount Rogers Formations demonstrate that rifting of the Laurentian continent to form the Iapetus Ocean was a prolonged event spanning 200 m.y. involving two important pulses of extrusive igneous activity. Rhyolitic flows of the non-fossiliferous Catoctin and Mount Rogers Formations, long correlated with one another on the basis of similar stratigraphic constraints, are dated at 564 ?? 9 Ma and 758 ?? 12 Ma, respectively. The data suggest a history of rifting in the central and southern Appalachians spanning 200 m.y. near the end of the Late Proterozoic. The earliest pulses did not proceed to continental separation and are not recorded north of the Potomac River. The later pulse or pulses affected the area from Newfoundland (ages of 617-590 Ma) to North Carolina and resulted in the opening of the Iapetus Ocean. -from Authors

  2. Superimposed positive and negative inversion of the syn-rift fault network preserved in the Montagna dei Fiori Anticline, Central Apennines, Italy

    NASA Astrophysics Data System (ADS)

    Storti, Fabrizio; Balsamo, Fabrizio; Koopman, Anton; Mozafari, Mahtab; Solum, John; Swennen, Rudy; Taberner, Conxita

    2016-04-01

    Syn-rift tectono-sedimentary inheritance is common in thrust-related anticlines exposed in most foreland thrust-fold belts worldwide. Inherited extensional faults provide mechanical weakness zones that typically undergo positive inversion during contraction. This unavoidably has an impact on the evolution of contractional folds. Moreover, duplexing and imbrication of thrust sheets typically produce gravitational instability of inherited fault patterns and negative inversion can be triggered in the late stages of fault-fold interaction. Such polyphase evolutionary histories can deeply influence deformation and fluid flow patterns in fault-related folds and therefore can strongly influence the distribution of structurally controlled processes such as dolomitization. In this contribution we present the results obtained from a multidisciplinary study of the tectono-sedimentary pattern and paleofluid history in carbonates exposed in the Montagna dei Fiori Anticline, at the mountain front of the Central Apennines (central Italy), where the occurrence of both syn-rift fault zones and related sediments has been previously described. Detailed mapping of the central part of the anticline, bed-perpendicular logging of syn-rift and post-rift strata, structural, petrographical, geochemical, microthermometrical, and petrophysical analyses were used to reconstruct the evolution of this anticline, starting from the pre-orogenic architecture up to its subsequent orogenic reworking. These data reveal: (1) the pre-orogenic tectono-sedimentary architecture of a folded Jurassic fault network; (2) multiple superimposition of extensional and contractional episodes of deformation on the same fault zones; (3) the presence of at least one main dolomitization episode, the timing of which is still being deciphered; (4) demonstrate the causal link between faulting and dolomitization, which favoured formation of dolostones along fault zones, particularly in the intersection/abutting areas

  3. Chemistry and chronology of magmatic processes, Central Kenya Peralkaline province, East African Rift

    NASA Astrophysics Data System (ADS)

    Anthony, E.; Deino, A. L.; White, J. C.; Omenda, P. A.

    2014-12-01

    We report here a synthesis of the geochemistry of magma evolution correlated with 40Ar/39Ar, 14 C, and U-series chronology for volcanoes in the Central Kenya Peralkaline Province (CKPP). The volcanic centers - Menengai, Eburru, Olkaria, Longonot, and Suswa - are at the apex of the Kenya Dome, and consist of trachyte, phonolite, comendite, and pantellerite. These volcanic centers are within the graben of the EARS and are characterized by a shield-building phase followed by caldera collapse and subsequent post-caldera eruptions. Geochemical modeling demonstrates that the magmas are the result of fractional crystallization of alkali basaltic magmas and magma mixing. Longonot and Suswa have the most chronologic data -14 C, Ar/Ar and U-series - and they show that the youngest eruptions have 230 Th/232Th of 0.8, which was inherited from the magma system prior to eruption. Subsequent changes in 230 Th/232 Th are due to post-eruptive decay of 230 Th and correlate well with 14 C and Ar/Ar.

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

    USGS Publications Warehouse

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

    2008-01-01

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

  5. Deep-currents along a rift valley of Central Indian Ridge observed by AUV "r2D4"

    NASA Astrophysics Data System (ADS)

    Komaki, K.; Ura, T.; Nagahashi, K.; Tamaki, K.

    2008-12-01

    In December 2006, the Institute of Industrial Science made geological and chemical investigation along rift valleys off the Rodriguez Island in the Central Indian Ridge with an AUV "gr2D4"h and found hydrothermal activities in one of the valleys, named the Great Dodo Lava Plain (water depth of approximately 2700m). In this present study, based on the AUV"fs navigation data taken from the investigation, we figured out the distribution of deep-currents in the Great Dodo Lava Plain and analyzed those currents and their relations with the tidal current and water-mass property. For current velocity, we used water-tracking velocity data measured by the Doppler Velocity Log (DVL) at a layer under the AUV. When the AUV was close to the sea bed, the velocity was adjusted to the absolute current velocity with bottom-tracking velocity measured by the DVL. When the bottom-tracking velocity was not available, the velocity was adjusted with AUV"fs velocity measured by the Inertial Navigation System. The adjusted current velocity data in the Great Dodo Lava Plain were observed for 6.5-hours in the range of 18.32°S-18.45°S, 65.28°E-65.37°E, and most of them reached 20 cm s-1 or more. The direction of the currents is between northwest and northeast, which almost corresponds to the direction of the valley"fs extension (north-northwest). The observed current speed is much higher than the tidal current speed predicted from a barotropic tidal model (< 3 cm s-1), but its meridional velocity component is stronger than the zonal component as well as the observed velocity. The predicted tidal current velocity shows semi-diurnal period, and that is also the case in the observed meridional velocity component. Thus, it is expected that the deep-currents and tidal currents are influenced by the bottom topography extending north-northwestward. For error consideration, we compared near-surface current velocities measured by the AUV and by the shipboard acoustic Doppler velocity profiler of

  6. Adapting to Climate Variability and Change: Experiences from Cereal-Based Farming in the Central Rift and Kobo Valleys, Ethiopia

    NASA Astrophysics Data System (ADS)

    Kassie, Belay Tseganeh; Hengsdijk, Huib; Rötter, Reimund; Kahiluoto, Helena; Asseng, Senthold; Van Ittersum, Martin

    2013-11-01

    Small-holder farmers in Ethiopia are facing several climate related hazards, in particular highly variable rainfall with severe droughts which can have devastating effects on their livelihoods. Projected changes in climate are expected to aggravate the existing challenges. This study examines farmer perceptions on current climate variability and long-term changes, current adaptive strategies, and potential barriers for successful further adaptation in two case study regions—the Central Rift Valley (CRV) and Kobo Valley. The study was based on a household questionnaire, interviews with key stakeholders, and focus group discussions. The result revealed that about 99 % of the respondents at the CRV and 96 % at the Kobo Valley perceived an increase in temperature and 94 % at CRV and 91 % at the Kobo Valley perceived a decrease in rainfall over the last 20-30 years. Inter-annual and intraseasonal rainfall variability also has increased according to the farmers. The observed climate data (1977-2009) also showed an increasing trend in temperature and high inter-annual and intra-seasonal rainfall variability. In contrast to farmers’ perceptions of a decrease in rainfall totals, observed rainfall data showed no statistically significant decline. The interaction among various bio-physical and socio-economic factors, changes in rainfall intensity and reduced water available to crops due to increased hot spells, may have influenced the perception of farmers with respect to rainfall trends. In recent decades, farmers in both the CRV and Kobo have changed farming practices to adapt to perceived climate change and variability, for example, through crop and variety choice, adjustment of cropping calendar, and in situ moisture conservation. These relatively low-cost changes in farm practices were within the limited adaptation capacity of farmers, which may be insufficient to deal with the impacts of future climate change. Anticipated climate change is expected to impose new

  7. Hawaii Rifts

    DOE Data Explorer

    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.

  8. Late Proterozoic diabase dikes of the New Jersey Highlands; a remnant of Iapetan rifting in the north-central Appalachians

    USGS Publications Warehouse

    Volkert, R.A.; Puffer, J.H.

    1995-01-01

    Diabase dikes of widespread occurrence intrude only middle Proterozoic rocks in the New Jersey Highlands. These dikes are enriched in TiO2, P2O5, Zr, and light rare earth elements, and have compositions that range from tholeiitic to alkalic. Dike descriptions, field relations, petrography, geochemistry, petrogenesis, and tectonic setting are discussed. The data are consistent with emplacement in a rift-related, within-plate environment and suggest a correlation with other occurrences of late Proterozoic Appalachian basaltic magmatism.

  9. 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

  10. Evidence for alkaline igneous activity and associated metasomatism in the Reelfoot rift, south-central Midcontinent, U. S. A

    SciTech Connect

    Goldhaber, M.B.; Diehl, S.F.; Sutley, S.J. ); Flohr, M.J.K. )

    1993-03-01

    Alkaline igneous magmatism is commonly associated with intracontinental rifts such as the Reelfoot rift (RR). Direct evidence for alkaline magmatism in the area of the RR occurs as lamprophyre and syenite encountered in deep wells. The authors' new studies of lamprophyres and sedimentary rocks from wells in the region provide additional examples of alkaline magmatism and emphasize the effects of related metasomatism. Sedimentary rocks in the Dow Chemical No. 1 Garrigan well, which is not known to contain lamprophyre dikes, probably also were metasomatically altered, as they contain authigenic fluorapatite, Ce-phosphates, and other REE-rich minerals. Enrichments of incompatible and large ion lithophile elements commonly associated with alkaline magmatism occur in the New Madrid test well, near the crest of the Pascola Arch. The carbonate-free fraction of Paleozoic rocks in this well is highly enriched in Nb (500 ppm), Ba (> 5,000 ppm), La (500 ppm), Th (1,000 ppm), and F (2,400 ppm). Abundant inclusion-rich potassium-feldspar cement in a nearby well may also be the result of alkaline metasomatism. Fluorite and elevated F concentrations are found in several wells in the RR, and contrast with stratigraphically correlative platform carbonates of the Ozark uplift, which lack F enrichment. Well and spring water samples above the RR are enriched in fluorine (as much as 5,000 ppb) compared to samples away from the rift which typically have concentrations two orders of magnitude smaller. The data and observations are consistent with relatively widespread alkaline metasomatism, which was associated with the intrusion of alkaline magmas in the RR.

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

    NASA Astrophysics Data System (ADS)

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

    1997-12-01

    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 (Sr1 = 0.7030-0.7039, ɛNd = +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

  12. 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

  13. Evolution of an Interbasin Mountain-Block Extensional Accommodation Zone Within the Central Colorado Rio Grande Rift, USA

    NASA Astrophysics Data System (ADS)

    Minor, S. A.; Caine, J. S.; Fridrich, C.; Hudson, M. R.

    2015-12-01

    Our understanding of extensional strain transfer and accommodation in continental rifts has grown considerably, but few studied transfer zones exhibit high internal topographic and structural relief. In the Rio Grande rift of Colorado the WNW-trending northern tip of the Sangre de Cristo Range separates the opposite-tilted Upper Arkansas River (UAR) and San Luis half grabens. We have investigated the development and role of faults flanking this "Poncha" intrarift mountain block in transferring extension between rift basins, mountain block surface uplift, and landscape evolution. The topographically rugged Poncha block consists of Proterozoic metamorphic and plutonic rocks overlain on its west and southwest flanks by 34.5-33-Ma volcanic rocks and alluvial deposits of the Mio-Pliocene Dry Union Formation. Similar Dry Union sediments underlie a moderately elevated, strongly dissected older piedmont along the northern front of the mountain block. All of these units are tilted 10-35º to the W and SW. A WNW-trending, right-stepping fault system > 25 km in length separates the piedmont and UAR basin from the steep northern Poncha mountain front. Slip measurements along this fault system, cutting deposits as young as ~200 ka, indicate dextral-normal oblique movement. The NNW-striking, down-to-E southern Sawatch range-front fault system forms the western terminus of the Poncha block where it juxtaposes Dry Union deposits against Sawatch Proterozoic basement rocks. Gently tilted proximal diamicton and alluvial deposits on the downthrown blocks of both range-front faults likely mark Plio-Pleistocene(?) mountain block uplift. Arrays of NNW- to WNW-striking faults cutting volcanic and Dry Union units on the flanks of the Poncha block commonly have normal-oblique slip, with greater tendency for dextral strike-slip components on WNW-striking faults. Preliminary paleomagnetic data from the volcanic rocks detect no significant vertical-axis rotation that accompanied oblique

  14. Turning soil survey data into digital soil maps in the Energy Region Eger Research Model Area

    NASA Astrophysics Data System (ADS)

    Pásztor, László; Dobos, Anna; Kürti, Lívia; Takács, Katalin; Laborczi, Annamária

    2015-04-01

    Agria-Innoregion Knowledge Centre of the Eszterházy Károly College has carried out targeted basic researches in the field of renewable energy sources and climate change in the framework of TÁMOP-4.2.2.A-11/1/KONV project. The project has covered certain issues, which require the specific knowledge of the soil cover; for example: (i) investigation of quantitative and qualitative characteristics of natural and landscape resources; (ii) determination of local amount and characteristics of renewable energy sources; (iii) natural/environmental risk analysis by surveying the risk factors. The Energy Region Eger Research Model Area consists of 23 villages and is located in North-Hungary, at the Western part of Bükkalja. Bükkalja is a pediment surface with erosional valleys and dense river network. The diverse morphology of this area results diversity in soil types and soil properties as well. There was large-scale (1:10,000 and 1:25,000 scale) soil mappings in this area in the 1960's and 1970's which provided soil maps, but with reduced spatial coverage and not with fully functional thematics. To achive the recent tasks (like planning suitable/optimal land-use system, estimating biomass production and development of agricultural and ecomonic systems in terms of sustainable regional development) new survey was planned and carried out by the staff of the College. To map the soils in the study area 10 to 22 soil profiles were uncovered per settlement in 2013 and 2014. Field work was carried out according to the FAO Guidelines for Soil Description and WRB soil classification system was used for naming soils. According to the general goal of soil mapping the survey data had to be spatially extended to regionalize the collected thematic local knowledge related to soil cover. Firstly three thematic maps were compiled by digital soil mapping methods: thickness of topsoil, genetic soil type and rate of surface erosion. High resolution digital elevation model, Earth

  15. Neoproterozoic A-type granitoids of the central and southern Appalachians: Intraplate magmatism associated with episodic rifting of the Rodinian supercontinent

    USGS Publications Warehouse

    Tollo, R.P.; Aleinikoff, J.N.; Bartholomew, M.J.; Rankin, D.W.

    2004-01-01

    Emplacement of compositionally distinctive granitic plutons accompanied two pulses (765-680 and 620-550Ma) of crustal extension that affected the Rodinian craton at the present location of the central Appalachians during the Neoproterozoic. The dominantly metaluminous plutons display mineralogical and geochemical characteristics of A-type granites including high FeO t/MgO ratios, high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu. These dike-like, sheet complexes occur throughout the Blue Ridge province of Virginia and North Carolina, and were emplaced at shallow levels in continental crust during active extension, forming locally multiple-intrusive plutons elongated perpendicular to the axis of extension. New U-Pb zircon ages obtained from the Polly Wright Cove (706??4Ma) and Suck Mountain (680??4Ma) plutons indicate that metaluminous magmas continued to be replenished near the end of the first pulse of rifting. The Suck Mountain body is presently the youngest known igneous body associated with earlier rifting. U-Pb zircon ages for the Pound Ridge Granite Gneiss (562??5Ma) and Yonkers Gneiss (563??2Ma) in the Manhattan prong of southeastern New York constitute the first evidence of plutonic felsic activity associated with the later period of rifting in the U.S. Appalachians, and suggest that similar melt-generation processes were operative during both intervals of crustal extension. Fractionation processes involving primary minerals were responsible for much of the compositional variation within individual plutons. Compositions of mapped lithologic units in a subset of plutons studied in detail define overlapping data arrays, indicating that, throughout the province, similar petrologic processes operated locally on magmas that became successively more chemically evolved. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with results of melting experiments and indicates that metaluminous

  16. Geochemical and Sr-Nd isotopic constraints on the mantle source of Neoproterozoic mafic dikes of the rifted eastern Laurentian margin, north-central Appalachians, USA

    NASA Astrophysics Data System (ADS)

    Volkert, Richard A.; Feigenson, Mark D.; Mana, Sara; Bolge, Louise

    2015-01-01

    fractures likely formed through a combination of southeast-directed extension and strike-slip shear stresses. Geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event. They, along with other mafic dikes in the north-central Appalachians, were emplaced in a within-plate tectonic setting along the rifted margin of eastern Laurentia, prior to opening of the Iapetus Ocean.

  17. Provenance of bentonite layers in the Palaeocene strata of the Central Basin, Svalbard: implications for magmatism and rifting events around the onset of the North Atlantic Igneous Province

    NASA Astrophysics Data System (ADS)

    Jones, Morgan T.; Eliassen, Gauti T.; Shephard, Grace E.; Svensen, Henrik H.; Jochmann, Malte; Friis, Bjarki; Augland, Lars E.; Jerram, Dougal A.; Planke, Sverre

    2016-11-01

    A fold-and-thrust belt developed between Greenland and Svalbard during the Palaeogene, with an associated foreland basin forming in what is now Spitsbergen. This Central Basin is comprised of the Van Mijenfjorden Group, a 2.3 km thick sandstone-shale dominated succession that contains prominent and laterally continuous bentonite layers in the lower formations. These altered tephra layers can be used as stratigraphic markers that connect the basin development with regional explosive volcanism and changes to relative plate motions. We sampled and analysed bentonites from nine borehole cores across the Central Basin. Each layer shows evidence of alteration, with mobile elements such as alkali and alkali earth metals particularly disrupted. However, immobile elements including rare earth elements (REE) and preserved igneous minerals retain a magmatic signature, allowing for comparisons with potential volcanic sources to be made. The majority of bentonites are both evolved and strongly alkaline, with chemical signatures that are much closer to the continental rift events around Ellesmere Island and North Greenland than to the early activity of the North Atlantic Igneous Province (NAIP). There is a clear difference between tephra layers in the mid Palaeocene versus late Palaeocene strata. The early bentonites have a REE signature comparable to the volcanics of the Kap Washington Group exposed in North Greenland. The later bentonites have likely come from volcanic centres in the Nares Strait that are also the source of abundant volcaniclastic sediments in the Judge Daly Promontory, Ellesmere Island. These findings suggest that a mid to late Palaeocene change in locus of volcanic provenance may reflect changes in relative plate motions related to the formation of the West Spitsbergen fold-and-thrust belt and the emplacement of the NAIP. However, the lack of bentonites matching NAIP sources suggests that explosive volcanism was of insufficient magnitude to lead to

  18. Temporal constraints on landscape evolution in response to rifting along the western margin of the Gulf of California, central Baja California Sur, Mexico

    NASA Astrophysics Data System (ADS)

    Mark, C.; Gupta, S.; Carter, A.; Mark, D. F.; Gautheron, C.; Martin Barajas, A.

    2011-12-01

    Rift escarpments and the high-elevation rift flanks associated with them are prominent topographic features at many developing and established passive margins. Numerical models and field investigations typically indicate that there are two principal mechanisms of rift escarpment evolution, commonly termed plateau downwearing and scarp retreat; however, many commonly studied passive margins are ancient, and have thus experienced significant post-rift modification, resulting in controversy as to which model is applicable at any particular margin. Rift flank uplift and the associated landscape changes have been less well studied; the erosional response to such uplift is often sufficient to bevel or erase the pre-rift landscape but insufficient to reset low-temperature thermochronometers, hindering field investigation. We present observations on rift landscape evolution from the Gulf of California: a ~1700 km long, highly oblique rift system. Although the precise timing of rifting remains controversial, it is known to have begun no earlier than the Mid-Miocene, and the incipient passive margins bounding the Gulf therefore retain many youthful features and provide an excellent natural laboratory for investigating landscape responses to rifting. This study examines the exhumation history of the escarpment and the development of the rift flank drainage network in the Loreto area of the Baja California Peninsula, which forms the western rift margin. Apatite (U-Th)/He (AHe) and apatite fission track (AFT) ages obtained from two escarpment-perpendicular transects from the footwall of the Loreto fault indicate that footwall denudation in response to rift flank uplift occurred at ~5.5 Ma. This age is younger than ages reported for other fundamental rift structures along the Baja California margin, and may indicate either a diachronous onset of rifting or a westward migration of extension after rifting began. A closely overlapping 40Ar/39Ar age from a lava situated near the

  19. 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.

  20. Farmers' perceptions of land degradation and their investments in land management: a case study in the Central Rift Valley of Ethiopia.

    PubMed

    Adimassu, Zenebe; Kessler, Aad; Yirga, Chilot; Stroosnijder, Leo

    2013-05-01

    To combat land degradation in the Central Rift Valley (CRV) of Ethiopia, farmers are of crucial importance. If farmers perceive land degradation as a problem, the chance that they invest in land management measures will be enhanced. This study presents farmers' perceptions of land degradation and their investments in land management, and to what extent the latter are influenced by these perceptions. Water erosion and fertility depletion are taken as main indicators of land degradation, and the results show that farmers perceive an increase in both indicators over the last decade. They are aware of it and consider it as a problem. Nevertheless, farmers' investments to control water erosion and soil fertility depletion are very limited in the CRV. Results also show that farmers' awareness of both water erosion and soil fertility decline as a problem is not significantly associated with their investments in land management. Hence, even farmers who perceive land degradation on their fields and are concerned about its increase over the last decade do not significantly invest more in water erosion and soil fertility control measures than farmers who do not perceive these phenomena. Further research is needed to assess which other factors might influence farmers' investments in land management, especially factors related to socioeconomic characteristics of farm households and plot characteristics which were not addressed by this study.

  1. Inheritance of Jurassic rifted margin architecture into the Apennines Neogene mountain building: a case history from the Lucretili Mts. (Latium, Central Italy)

    NASA Astrophysics Data System (ADS)

    Bollati, Andrea; Corrado, Sveva; Marino, Maurizio

    2012-06-01

    The western Lucretili Mts. in the central Apennines (Latium, Italy) have been recently re-mapped in great detail and are the subject of combined stratigraphic, sedimentological and structural investigations. In this paper, we present a new stratigraphic interpretation of the Jurassic paleogeography of western Lucretili Mts., where a rift-derived intrabasinal paleo-high of the Alpine Tethys has been identified for the first time by means of facies analysis and biostratigraphic dating. Recognised facies associations, combined with dated stratigraphic sections, allow to define the morphology of the structural paleo-high and to identify the associated gravity-driven deposits (olistoliths) accumulated in the surrounding basin. Furthermore, we investigated the modes of interaction between Jurassic extensional structures and the subsequent contractional patterns developed during the Tertiary mountain building. In detail, the role played during Apennines tectonics by the paleo-escarpments bounding the paleo-high and by the surrounding olistoliths has been analysed. The paleo-escarpments either acted as focussing features for ENE-directed frontal thrust ramp localisation and were offset with small shortening amounts or reactivated as NNE striking high angle transpressional faults or preserved the original geometries as a result of variable orientation of paleo-escarpments with respect to the Neogene compressive stress field (with ENE oriented sigma1). Newly formed ENE striking tear faults connect these either inherited or neo-formed discontinuities. This complex stratigraphic and structural pattern is substantially different from the previous interpretations of this portion of the central Apennines based on a hypothesised layer-cake stratigraphy deformed by neo-formed Neogene thrusts. This contribution strengthens the importance of integrating facies analyses and structural investigations to detect the influence of pre-orogenic structures on compressive structural patterns

  2. Examination of the Reelfoot Rift Petroleum System, south-central United States, and the elements that remain for potential exploration and development

    USGS Publications Warehouse

    Coleman, James; Pratt, Thomas L.

    2016-01-01

    No production has been established in the Reel-foot rift. However, at least nine of 22 exploratory wells have reported petroleum shows, mainly gas shows with some asphalt or solid hydrocarbon residue. Regional seismic profiling shows the presence of two large inversion structures (Blytheville arch and Pascola arch). The Blytheville arch is marked by a core of structurally thickened Elvins Shale, whereas the Pascola arch reflects the structural uplift of a portion of the entire rift basin. Structural uplift and faulting within the Reelfoot rift since the late Paleozoic appear to have disrupted older conventional hydrocarbon traps and likely spilled any potential conventional petroleum accumulations. The remaining potential resources within the Reelfoot rift are likely shale gas accumulations within the Elvins Shale; however, reservoir continuity and porosity as well as pervasive faulting appear to be significant future challenges for explorers and drillers.

  3. 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.

  4. Precessional forcing of lacustrine sedimentation in the late Cenozoic Chemeron Basin, Central Kenya Rift, and calibration of the Gauss/Matuyama boundary

    USGS Publications Warehouse

    Deino, A.L.; Kingston, J.D.; Glen, J.M.; Edgar, R.K.; Hill, A.

    2006-01-01

    The fluviolacustrine sedimentary sequence of the Chemeron Formation exposed in the Barsemoi River drainage, Tugen Hills, Kenya, contains a package of five successive diatomite/fluvial cycles that record the periodic development of freshwater lakes within the axial portion of the Central Kenya Rift. The overwhelming abundance in the diatomite of planktonic species of the genera Aulacoseira and Stephanodiscus, and the virtual absence of benthic littoral diatoms and detrital material indicate areally extensive, deep lake systems. A paleomagnetic reversal stratigraphy has been determined and chronostratigraphic tie points established by 40Ar/39Ar dating of intercalated tuffs. The sequence spans the interval 3.1-2.35??Ma and bears a detailed record of the Gauss/Matuyama paleomagnetic transition. The 40Ar/39Ar age for this boundary of 2.589 ?? 0.003??Ma can be adjusted to concordance with the Astronomical Polarity Time Scale (APTS) on the basis of an independent calibration to 2.610??Ma, 29??kyr older than the previous APTS age. The diatomites recur at an orbital precessional interval of 23??kyr and are centered on a 400-kyr eccentricity maximum. It is concluded that these diatomite/fluvial cycles reflect a narrow interval of orbitally forced wet/dry climatic conditions that may be expressed regionally across East Africa. The timing of the lacustrine pulses relative to predicted insolation models favors origination of moisture from the northern Africa monsoon, rather than local circulation driven by direct equatorial insolation. This moisture event at 2.7-2.55??Ma, and later East African episodes at 1.9-1.7 and 1.1-0.9??Ma, are approximately coincident with major global climatic and oceanographic events. ?? 2006 Elsevier B.V. All rights reserved.

  5. Subaerial exposure and drowning processes in a carbonate platform during the Mesozoic Tethyan rifting: The case of the Jurassic succession of Western Sicily (central Mediterranean)

    NASA Astrophysics Data System (ADS)

    Sulli, A.; Interbartolo, F.

    2016-01-01

    The Liassic carbonate platform succession outcropping at Monte Maranfusa (central Western Sicily) consists of a shallowing-upward sequence of peritidal carbonates, with Jurassic to Paleogene pelagic limestone and siliciclastic Tertiary covers above. The cyclic sequences of subtidal wackestones/packstones, intertidal microcrystalline carbonates with bird's-eye pores, and supratidal bioclastic grainstones are interbedded with dark layers of the following composition: 1) dark-gray, compact, and well-cemented limestone with blackish clasts, interpreted as calcretes (a type of carbonate soil) and 2) reddish calcite laminae, deformed by elongated cavities, filled with vadose silt, interpreted as paleokarst. This succession is crossed by almost vertical faults, of the Late Liassic to Miocene, which often coincides with neptunian dykes, filled by several generations of Toarcian-Early Miocene pelagic sediments. Another system of dykes, known as neptunian sills, filled by injected Upper Lias-Dogger pelagic sediments, lies parallel to the stratification. The parallel dykes were caused by the flexure of the platform during the Jurassic and presumably by a planar slip in the carbonate rocks, whereas neptunian dykes are caused by faulting episodes. Here, we present evidence that the dark layers in the Liassic succession of Monte Maranfusa, previously described by many authors only as parallel dykes, can actually be interpreted as a) neptunian sills, b) pedogenic calcretes, and c) paleospeleothems. Therefore, we found evidence of exposure/flooding intervals in the evolution of the carbonate platform during the Liassic, linked to different pulses in both the subsidence/tectonic activity and the sea-level oscillations. At the top, Fe-Mn crusts (hardgrounds) seal the carbonate platform succession, which is in turn overlain by condensed pelagic deposits, confirming its drowning during rifting processes.

  6. Oblique rifting at Tempe Fossae, Mars

    NASA Astrophysics Data System (ADS)

    Fernández, Carlos; Anguita, Francisco

    2007-09-01

    This work shows the results of a structural study of the faults observed at the Tempe Rift (northeastern Tharsis region), Mars. A new, detailed map of faults and fault systems was used to geometrically characterize the fracture architecture of the Tempe Rift and to measure fault length, displacement, and spacing data, to analyze the spatial distribution of fault centroids, and to investigate the fractal nature of fault trace maps. A comparison with analog models and the use of conventional techniques of fault population analysis show that the Tempe Rift was most probably generated under sinistral oblique-rifting processes, which highlights the importance of the presence of inherited fractures in the tectonic evolution of the Noachian crust. The angle between the extension direction and the rift axis varies along the Tempe Rift, ranging from 50°-60° at its central southern part to 66°-88° to the southwest. Fault scaling relationships are similar to those found at mid-ocean ridges on Earth with exponential fault length-frequency distributions. Localized, inhomogeneous deformation generated weakly interacting faults, spanning the entire thickness of the mechanical layer. This thickness decreased from southwest to northeast along the rift, along with distance from the central part of the Tharsis dome.

  7. Analysis of the rotation period of asteroids (1865) Cerberus, (2100) Ra-Shalom, and (3103) Eger - search for the YORP effect

    NASA Astrophysics Data System (ADS)

    Ďurech, J.; Vokrouhlický, D.; Baransky, A. R.; Breiter, S.; Burkhonov, O. A.; Cooney, W.; Fuller, V.; Gaftonyuk, N. M.; Gross, J.; Inasaridze, R. Ya.; Kaasalainen, M.; Krugly, Yu. N.; Kvaratshelia, O. I.; Litvinenko, E. A.; Macomber, B.; Marchis, F.; Molotov, I. E.; Oey, J.; Polishook, D.; Pollock, J.; Pravec, P.; Sárneczky, K.; Shevchenko, V. G.; Slyusarev, I.; Stephens, R.; Szabó, Gy.; Terrell, D.; Vachier, F.; Vanderplate, Z.; Viikinkoski, M.; Warner, B. D.

    2012-11-01

    Context. The spin state of small asteroids can change on a long timescale by the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, the net torque that arises from anisotropically scattered sunlight and proper thermal radiation from an irregularly-shaped asteroid. The secular change in the rotation period caused by the YORP effect can be detected by analysis of asteroid photometric lightcurves. Aims: We analyzed photometric lightcurves of near-Earth asteroids (1865) Cerberus, (2100) Ra-Shalom, and (3103) Eger with the aim to detect possible deviations from the constant rotation caused by the YORP effect. Methods: We carried out new photometric observations of the three asteroids, combined the new lightcurves with archived data, and used the lightcurve inversion method to model the asteroid shape, pole direction, and rotation rate. The YORP effect was modeled as a linear change in the rotation rate in time dω/dt. Values of dω/dt derived from observations were compared with the values predicted by theory. Results: We derived physical models for all three asteroids. We had to model Eger as a nonconvex body because the convex model failed to fit the lightcurves observed at high phase angles. We probably detected the acceleration of the rotation rate of Eger dω/dt = (1.4 ± 0.6) × 10-8 rad d-2 (3σ error), which corresponds to a decrease in the rotation period by 4.2 ms yr-1. The photometry of Cerberus and Ra-Shalom was consistent with a constant-period model, and no secular change in the spin rate was detected. We could only constrain maximum values of |dω/dt| < 8 × 10-9 rad d-2 for Cerberus, and |dω/dt| < 3 × 10-8 rad d-2 for Ra-Shalom. Tables 1-3 are available in electronic form at http://www.aanda.org

  8. The upper mantle structure of the central Rio Grande rift region from teleseismic P and S wave travel time delays and attenuation

    USGS Publications Warehouse

    Slack, P.D.; Davis, P.M.; Baldridge, W.S.; Olsen, K.H.; Glahn, A.; Achauer, U.; Spence, W.

    1996-01-01

    The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experiments spanning the Rio Grande rift. Two tomographic inversions of the P wave travel time data are given: an Aki-Christofferson-Husebye (ACH) block model inversion and a downward projection inversion. The tomographic inversions reveal a NE-SW to NNE-SSW trending feature at depths of 35 to 145 km with a velocity reduction of 7 to 8% relative to mantle velocities beneath the Great Plains. This region correlates with the transition zone between the Colorado Plateau and the Rio Grande rift and is bounded on the NW by the Jemez lineament, a N52??E trending zone of late Miocene to Holocene volcanism. S wave delays plotted against P wave delays are fit with a straight line giving a slope of 3.0??0.4. This correlation and the absolute velocity reduction imply that temperatures in the lithosphere are close to the solidus, consistent with, but not requiring, the presence of partial melt in the mantle beneath the Rio Grande rift. The attenuation data could imply the presence of partial melt. We compare our results with other geophysical and geologic data. We propose that any north-south trending thermal (velocity) anomaly that may have existed in the upper mantle during earlier (Oligocene to late Miocene) phases of rifting and that may have correlated with the axis of the rift has diminished with time and has been overprinted with more recent structure. The anomalously low-velocity body presently underlying the transition zone between the core of the Colorado Plateau and the rift may reflect processes resulting from the modern (Pliocene to present) regional stress field (oriented WNW-ESE), possibly heralding future extension across the Jemez lineament and transition zone.

  9. The Haselgebirge evaporitic mélange in central Northern Calcareous Alps (Austria): Part of the Permian to Lower Triassic rift of the Meliata ocean?

    PubMed Central

    Schorn, Anja; Neubauer, Franz; Genser, Johann; Bernroider, Manfred

    2013-01-01

    For the reconstruction of Alpine tectonics of the Eastern Alps, the evaporitic Permian to Lower Triassic Haselgebirge Formation plays a key role in (1) the origin of Haselgebirge bearing nappes, (2) the inclusion of magmatic and metamorphic rocks revealing tectonic processes not preserved in other units, and (3) the debated mode of emplacement of the nappes, namely gravity-driven or tectonic. Within the Moosegg quarry of the central Northern Calcareous Alps gypsum/anhydrite bodies are tectonically mixed with lenses of sedimentary rocks and decimeter- to meter-sized tectonic clasts of plutonic and subvolcanic rocks and rare metamorphics. We examined various types of (1) widespread biotite–diorite, meta-syenite, (2) meta-dolerite and rare ultramafic rocks (serpentinite, pyroxenite) as well as (3) rare metamorphic banded meta-psammitic schists and meta-doleritic blueschists. The apparent 40Ar/39Ar biotite ages from three biotite–diorite, meta-dolerite and meta-doleritic blueschist samples with variable composition and fabrics range from 248 to 270 Ma (e.g., 251.2 ± 1.1 Ma) indicating a Permian age of cooling after magma crystallisation or metamorphism. The chemical composition of biotite–diorite and meta-syenite indicates an alkaline trend interpreted to represent a rift-related magmatic suite. These, as well as Permian to Jurassic sedimentary rocks, were incorporated during Cretaceous nappe emplacement forming the sulphatic Haselgebirge mélange. The scattered 40Ar/39Ar white mica ages of a meta-doleritic blueschist (of N-MORB origin) and banded meta-psammitic schist are ca. 349 and 378 Ma, respectively, proving the Variscan age of pressure-dominated metamorphism. These ages are similar to detrital white mica ages reported from the underlying Rossfeld Formations, indicating a close source–sink relationship. According to our new data, the Haselgebirge bearing nappe was transported over the Lower Cretaceous Rossfeld Formations, which include many clasts

  10. Evidence for dextral transtensional development of the Rio Grande rift, from the Bear Mountains and the Lucero uplift, central New Mexico

    SciTech Connect

    Hayden, S.N. . Dept. of Geology)

    1993-04-01

    Dextral faulting of late Eocene age associated with latest Laramide deformation has been noted for some time along the margins of, and adjacent to, the Rio Grande rift (RGr) in central NM. Recent mapping of the Hell's Mesa (HM) fault zone in the Bear Mountains and of the Comanche/Santa Fe fault zone along the western margin of the RGr at the Lucero uplift has constrained dextral deformation to extend into the Miocene, at least, and possibly into the Pliocene. The HM fault zone forms the eastern margin of the Mulligan Gulch graben between the Bear and Gallinas Mountains to the west. The main ridge of the Bear Mountains is structurally down-dropped by the HM fault and is part of the graben. This ridge is a topographic high, composed of interbedded rhyolitic ash-flow tuffs and basaltic andesite flows of the Oligocene to early Miocene Mogollon-Datil volcanic field, faulted against Eocene sediments of the Baca Formation, and Eocene to early Oligocene volcanic and volcaniclastic rocks of the Datil Group. The western margin of the RGr in the Albuquerque basin along the Lucero uplift, defined by the Comanche fault zone is structurally continuous with portions of the HM fault zone that make a right overstep through the down-dropped Navajo gap area at the southwest corner of the basin. The Comanche fault zone shows dextral-oblique shear geometry along an anastomosing zone of faulting up to 1 km wide. This zone has been intruded by hypabyssal basaltic rocks that have yielded a whole-rock K-Ar date of 27.1 Ma. In the Carrizo Arroyo (CA) area, slickenside lineations on these dikes show a strong dextral component of movement similar to that of the HM zone. The Santa Fe fault is a reverse fault for some distance north of CA. Motion on this fault has disrupted coarse sediments that contain clasts of the ca. 4.0 Ma Carrizo Mesa basalt. These observations are interpreted to indicate that dextral deformation has persisted at least through the earliest stage of extension.

  11. The Haselgebirge evaporitic mélange in central Northern Calcareous Alps (Austria): Part of the Permian to Lower Triassic rift of the Meliata ocean?

    PubMed

    Schorn, Anja; Neubauer, Franz; Genser, Johann; Bernroider, Manfred

    2013-01-11

    For the reconstruction of Alpine tectonics of the Eastern Alps, the evaporitic Permian to Lower Triassic Haselgebirge Formation plays a key role in (1) the origin of Haselgebirge bearing nappes, (2) the inclusion of magmatic and metamorphic rocks revealing tectonic processes not preserved in other units, and (3) the debated mode of emplacement of the nappes, namely gravity-driven or tectonic. Within the Moosegg quarry of the central Northern Calcareous Alps gypsum/anhydrite bodies are tectonically mixed with lenses of sedimentary rocks and decimeter- to meter-sized tectonic clasts of plutonic and subvolcanic rocks and rare metamorphics. We examined various types of (1) widespread biotite-diorite, meta-syenite, (2) meta-dolerite and rare ultramafic rocks (serpentinite, pyroxenite) as well as (3) rare metamorphic banded meta-psammitic schists and meta-doleritic blueschists. The apparent (40)Ar/(39)Ar biotite ages from three biotite-diorite, meta-dolerite and meta-doleritic blueschist samples with variable composition and fabrics range from 248 to 270 Ma (e.g., 251.2 ± 1.1 Ma) indicating a Permian age of cooling after magma crystallisation or metamorphism. The chemical composition of biotite-diorite and meta-syenite indicates an alkaline trend interpreted to represent a rift-related magmatic suite. These, as well as Permian to Jurassic sedimentary rocks, were incorporated during Cretaceous nappe emplacement forming the sulphatic Haselgebirge mélange. The scattered (40)Ar/(39)Ar white mica ages of a meta-doleritic blueschist (of N-MORB origin) and banded meta-psammitic schist are ca. 349 and 378 Ma, respectively, proving the Variscan age of pressure-dominated metamorphism. These ages are similar to detrital white mica ages reported from the underlying Rossfeld Formations, indicating a close source-sink relationship. According to our new data, the Haselgebirge bearing nappe was transported over the Lower Cretaceous Rossfeld Formations, which include many clasts

  12. Off-axis volcanism in the Gregory rift, east Africa: implications for models of continental rifting

    SciTech Connect

    Bosworth, W.

    1987-05-01

    The largest volcanic centers of the Gregory rift occur in two belts located 100 to 150 km east and west of the axis of the rift valley. These off-axis volcanic belts include the highest peaks on the continent of Africa and are interpreted to lie above the intersection of low-angle detachment systems with the base of a regionally thinned lithosphere. These detachment systems are manifested at the surface as a series of breakaway zones and regional bounding faults that produce subbasins with half-graben form. The asymmetry of subbasins alternates along the rift axis, indicating that the polarity of the underlying active detachment systems also reverses. The detachments are separated laterally by regional oblique-slip accommodation zones typified by wrench-style tectonism. Off-axis from the rift, the detachments are inferred to merge along strike as they cut to the base of the lithosphere. This results in irregular but persistent paired zones of volcanism and lithospheric thinning off-axis from the rift proper. The development of major volcanic cones such as Mount Kilimanjaro may be controlled by the interaction of leaky accommodation zones with the regions of structurally thinned lithosphere. The central Kenya hot spot has produced the anomalous quantities of volcanic material that fills the Gregory rift and probably enhances the off-axis volcanism but does not directly control its location. The model proposed here for tectonic controls of volcanism in the Gregory rift may be applicable to Phanerozoic continental rifts in general.

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

    NASA Astrophysics Data System (ADS)

    Bosworth, W.; Stockli, D. F.

    2006-12-01

    The Tertiary Gulf of Suez rift system is one of the best-studied continental rift systems and has inspired many fundamental geodynamic models for continental rifting. However, our limited knowledge of how extensional strain is spatially and temporally distributed has made it difficult to adequately evaluate models for the dynamic evolution of this rift. A critical aspect of constraining the evolution of rifting and rift segmentation in the Gulf of Suez involves acquiring reliable geochronological constraints on extensional faulting. This study has commenced a systematic investigation of the timing and spatial distribution of rifting, lateral rift segmentation, and rift localization within the Gulf of Suez, Egypt, employing apatite and zircon (U-Th)/He thermochronometry. (U-Th)/He thermochronometric analysis of sample transects from exhumed fault blocks within the rift integrated with structural data will allow us to directly determine the timing, distribution, and magnitude of extension. The onset of major rifting (~24-19 Ma) in the Gulf of Suez was marked by the development of crustal domino-style tilt blocks and syn-rift deposition of the late Oligocene non-marine Abu Zenima Fm and non-marine to restricted marine Nukhul Fm. Development of the Gulf of Aqaba-Dead Sea transform cut off the rift from the Red Sea rift at an early extensional stage. Apatite (AHe) and zircon (ZHe) (U- Th)/He data were collected from basement and pre-rift sedimentary sample transects from the central and southern Sinai Peninsula portion and the Gebel El Zeit area in the southern Gulf of Suez as well as from basement samples from selected drill cores off Gebel El Zeit. Preliminary data exhibit partially reset ages trending as old as ~70 Ma (AHe) and ~450 Ma (ZHe) from shallower structural levels (Proterozoic basement and Phanerozoic cover sequence). Structurally deeper samples yield abundant AHe ages of ~22-24 Ma, indicative of rapid cooling and exhumation during the early Miocene. More

  14. Morphotectonics of the Tunka rift and its bordering mountains in the Baikal rift system, Russia

    NASA Astrophysics Data System (ADS)

    Shchetnikov, Alexander

    2016-11-01

    The Tunka section of the Baikal rift system presents a uniform alternation of the following neostructural forms: tilted horsts and asymmetrical block uplifts on the northern flank; the central system of the rift valleys; and the arched uplift of the southern flank. This is a standard set of morphostructural elements for the Baikal rift system. The main morphological feature of the Tunka rift is the strong inclination of its floor, ranging from 900 m to 200 km in general elevation above Lake Baikal. Such traits of recent geodynamics as volcanism, thermal activity, and seismicity are also different from other parts of the rift zone. All of these features of the Tunka rift are related to the deep structure of the rift zone. The peculiarities of the neotectonic structure of the Tunka rift, which are clearly expressed morphologically as is typical of the Baikal rift system, as well as its unique features are in accordance with deep geodynamic processes of the region. On the other hand, the development of the rift basin structures of the southwestern area near Baikal is complicated by inversion deformations. Local uplifts followed by deformations of the basin sedimentary cover and inverted morphostructures expressed in relief are fixed against the background of the general subsidence of blocks of the pre-Cenozoic basement grabens. The Tunka rift has repeatedly experienced inversion deformations throughout its history. The last wave of such deformations involved the southwestern region near Baikal in the second half of the late Pleistocene. During the Quaternary, the positive component prevailed in the whole range of vertical movements of the inter-rift and interbasin blocks; since the late Neogene, these structures have experienced a slow but steady uplift, accompanied by their extension at the expense of the bordering basins. The remote influence of the India-Asia collision on the formation of the southwestern section of the Baikal rift system is very significant and

  15. Multiple provenance of rift sediments in the composite basin-mountain system: Constraints from detrital zircon U-Pb geochronology and heavy minerals of the early Eocene Jianghan Basin, central China

    NASA Astrophysics Data System (ADS)

    Wu, Lulu; Mei, Lianfu; Liu, Yunsheng; Luo, Jin; Min, Caizheng; Lu, Shengli; Li, Minghua; Guo, Libin

    2017-03-01

    Zircon U-Pb geochronology and heavy minerals are used in combination to provide valuable insights into the provenance of the early Eocene Jianghan Basin, central China. Five samples for zircon U-Pb dating and eighty-five samples for heavy mineral analysis were collected from drill cores or cuttings of the Xingouzui Formation. Most analyzed zircons are of magmatic origin, with oscillatory zoning. Detrital zircons from sample M96 located on eastern basin have two dominant age groups of 113-158 Ma and 400-500 Ma, and the other samples located on southern basin have three prominent age populations at 113-158 Ma, 400-500 Ma and 700-1000 Ma. Samples on different parts of the basin show distinct differences in heavy mineral compositions and they apparently divide into two groups according to the content of rutile (higher or lower than 4%). The spatial variations of zircon-tourmaline-rutile (ZTR) indices are marked by some noticeable increasing trends from basin margins to the inner part of the basin. Compared with the potential source areas, this study clarifies the multiple source characteristics of the Jianghan basin in the composite basin-mountain system. The majority of clastic material was supplied from the north source area through rift-trough sediment-transport pathways, and the eastern, southern and northwestern source areas also contributed detritus to the basin. This clastic material is broadly dispersed in the basin. The early Eocene paleogeography implies that rift architecture and rifting process had an important influence on sediment dispersal. This study shows that integrated zircon U-Pb geochronology and heavy mineral analysis is a useful and powerful method to identify sediment provenance.

  16. The 1974 Ethiopian rift geodimeter survey

    NASA Technical Reports Server (NTRS)

    Mohr, P.

    1977-01-01

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

  17. Surface deformation in volcanic rift zones

    USGS Publications Warehouse

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

    1983-01-01

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

  18. The Role of Rift Obliquity During Pangea Fragmentation

    NASA Astrophysics Data System (ADS)

    Brune, S.; Butterworth, N. P.; Williams, S.; Müller, D.

    2014-12-01

    Does supercontinent break-up follow specific laws? What parameters control the success and the failure of rift systems? Recent analytical and geodynamic modeling suggests that oblique rifting is energetically preferred over orthogonal rifting. This implies that during rift competition, highly oblique branches proceed to break-up while less oblique ones become inactive. These models predict that the relative motion of Earth's continents during supercontinent break-up is affected by the orientation and shape of individual rift systems. Here, we test this hypothesis based on latest plate tectonic reconstructions. Using PyGPlates, a recently developed Python library that allows script-based access to the plate reconstruction software GPlates, we quantify rift obliquity, extension velocity and their temporal evolution for continent-scale rift systems of the past 200 Myr. Indeed we find that many rift systems contributing to Pangea fragmentation involved strong rift obliquity. East and West Gondwana for instance split along the East African coast with a mean obliquity of 55° (measured as the angle between local rift trend normal and extension direction). While formation of the central and southern South Atlantic segment involved a low obliquity of 10°, the Equatorial Atlantic opened under a high angle of 60°. Rifting between Australia and Antarctica involved two stages with 25° prior to 100 Ma followed by 50° obliquity and distinct increase of extension velocity. Analyzing the entire passive margin system that formed during Pangea breakup, we find a mean obliquity of 40°, with a standard deviation of 20°. Hence 50% of these margins formed with an angle of 40° or more. Considering that many conceptual models of rifting and passive margin formation assume 2D deformation, our study quantifies the degree to which such 2D models are globally applicable, and highlights the importance of 3D models where oblique rifting is the dominant mode of deformation.

  19. Rift Valley Fever Virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever virus (RVFV) is a mosquito-transmitted virus or arbovirus that is endemic in sub-Saharan Africa. In the last decade, Rift Valley fever (RVF) outbreaks have resulted in loss of human and animal life, as well as had significant economic impact. The disease in livestock is primarily a...

  20. Diapiric origin of the Blytheville and Pascola arches in the Reelfoot rift, east-central United States: Relation to New Madrid seismicity

    SciTech Connect

    McKeown, F.A.; Diehl, S.F.; Glick, E.E. ); Hamilton, R.M. )

    1990-11-01

    Most of the earthquakes in the New Madrid seismic zone correlate spatially with the Blytheville arch and part of the Pascoal arch, which are interpreted to be the same structure. Both arches may have formed by diapirism along the axis of the Reelfoot rift. Seismic, geophysical, and drill-hole data indicate that the rocks in the arches are highly deformed and fractured and have gross lithologic properties that make them weaker than rocks adjacent to the arches. The weaker rocks are inferred to fail seismically more readily than the stronger rocks adjacent to the arches.

  1. How Mountains Become Rifts

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  2. Left-lateral shear inside the North Gulf of Evia Rift, Central Greece, evidenced by relocated earthquake sequences and moment tensor inversion

    NASA Astrophysics Data System (ADS)

    Ganas, Athanassios; Mouzakiotis, Evangelos; Moshou, Alexandra; Karastathis, Vassilios

    2016-07-01

    The use of local velocity model in the analysis of seismicity recorded by the Hellenic Unified Seismological Network (HUSN), provides the opportunity to determine accurate hypocentral solutions using the weighted P- and S-wave arrival times for the November 2013, November 2014 and June 2015 North Gulf of Evia (Euboea) sequences. The hypocentral locations, including the determination of the location uncertainties, are obtained applying the non-linear probabilistic analysis. We also calculated the moment tensor solutions for the main events as well as for the strongest aftershocks. The accurate determination of seismicity showed the activation of two left-lateral, NW-SE striking, near-vertical faults, one onshore near village Taxiarchis (2013 sequence) and one offshore (offshore Malessina Peninsula, 2015 sequence). The 2014 sequence, also offshore Malessina Peninsula ruptured an oblique-slip, north-dipping normal fault with a strike of N280-290°E. All three faults occur at depths 10-16 km, with rupture zone dimensions 5-6 km along strike and 3-4 km along dip. These aftershock depths indicate a seismogenic (brittle) zone of about 15 km in depth for this rift. The left-lateral kinematics indicates strain partitioning inside the rift because of E-W horizontal compression, also evidenced by GPS data. The moderate magnitude of earthquakes (M5.2 ± 0.1) indicates that strike-slip events have a minor contribution to the crustal deformation and to active tectonics of the Gulf.

  3. Phylogeography of the reed frog Hyperolius castaneus (Anura: Hyperoliidae) from the Albertine Rift of Central Africa: implications for taxonomy, biogeography and conservation.

    PubMed

    Greenbaum, Eli; Sinsch, Ulrich; Lehr, Edgar; Valdez, Federico; Kusamba, Chifundera

    2013-11-01

    We examine the systematics of multiple populations of the Albertine Rift endemic amphibian Hyperolius castaneus, which currently incorporates four subspecies. Standard morphometric data were analyzed with principal components analyses and analyses of covariance. Phylogenetic analyses of two mitochondrial (16S, cyt b) and one nuclear (RAG1) genes were analyzed from 41 samples representing three subspecies. Results indicated some significant morphometric differences between the nominate subspecies H. c. castaneus and the Itombwe Plateau subspecies H. c. constellatus, and phylogenetic analyses of molecular data recovered these taxa as reciprocally monophyletic groups. We recognize these two allopatric populations as recently diverged, but distinct species, H. castaneus and H. constellatus. The subspecies H. c. submarginatus from the Kabobo Plateau is transferred to the synonymy of H. constellatus, but the status of the unsampled subspecies H. c. rhodogaster, described from mid-elevations of the western Itombwe Plateau, remains problematic. The phylogeographic pattern of our study resembles some, but not all, Albertine Rift vertebrates that have been examined with molecular data. Hyperolius constellatus is restricted to the Itombwe and Kabobo plateaus, which are of special conservation concern because of high levels of amphibian diversity and endemism, and multiple threats from deforestation, mining activities and road construction.

  4. Geophysical study of the crust and upper mantle beneath the central Rio Grande rift and adjacent Great Plains and Colorado Plateau

    SciTech Connect

    Ander, M.E.

    1981-03-01

    As part of the national hot dry rock (HDR) geothermal program conducted by Los Alamos Scientific Laboratory, a regional deep magnetotelluric (MT) survey of Arizona and New Mexico was performed. The main objective of the MT project was to produce a regional geoelectric contour map of the pervasive deep electrical conductor within the crust and/or upper mantle beneath the Colorado Plateau, Basin and Range Province, and Rio Grande rift. Three MT profiles cross the Jemez lineament. Preliminary one-dimensional analysis of the data suggest the lineament is associated with anomalously high electrical conductivity very shallow in the crust. An MT/audiomagnetotelluric (AMT) study of a 161 km/sup 2/ HDR prospect was performed on the Zuni Indian Reservation, New Mexico. Two-dimensional gravity modeling of a 700-km gravity profile at 34/sup 0/30'N latitude was used to study the crust and upper mantle beneath the Rio Grande rift. Several models of each of three consecutive layers were produced using all available geologic and geophysical constraints. Two short-wavelength anomalies along the gravity profile were analyzed using linear optimization techniques.

  5. Tectono-Sedimentary Analysis of Rift Basins: Insights from the Corinth Rift, Greece

    NASA Astrophysics Data System (ADS)

    Gawthorpe, Robert; Ford, Mary

    2015-04-01

    Existing models for the tectono-sedimentary evolution of rift basins are strongly linked the growth and linkage of normal fault segments and localization of fault activity. Early stages of faulting (rift initiation phase) are characterized by distributed, short, low displacement fault segments, subdued fault-related topography and small depocentres within which sedimentation keeps pace with subsidence. Following linkage and displacement localization (rift climax phase), deformation if focused onto major, crustal-scale fault zones with kilometre-scale displacement. These major faults generate pronounced tilted fault-block topography, with subsidence rates that outpace sedimentation causing a pronounced change to deep-water deposition. Such models have been successful in helping to understand the gross structural and sedimentary evolution of rift basins, but recent work has suggested that pre-existing structures, normal fault interaction with pre-rift salt and antecedent drainage systems significantly alter this initiation-to-climax perspective of rift basin development. The E-W-striking, Pliocene-Pleistocene Corinth rift, central Greece, is an excellent natural laboratory for studying the tectono-sedimentary evolution of rift basins due to its young age, excellent onshore exposure of syn-rift structure and stratigraphy and extensive offshore seismic data. The rift cuts across the NW-SE-striking Hellenide mountain belt and has migrated northward and westward during its evolution. The Hellenide mountain belt significantly influences topography and drainage in the west of the rift. High topography and large antecedent drainage systems, focused along palaeovalleys, provided high sediment flux to NE-flowing alluvial systems that overfilled early-rift depocentres. Further east, away from the main antecedent drainage networks, contemporaneous deposits comprise deep-lacustrine turbidite channel and lobe complexes and basinal marls. Thus the stratigraphic expression within

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

    NASA Astrophysics Data System (ADS)

    Klyuchevskii, Anatoly V.

    2014-07-01

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

  7. 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.

  8. Volcanism at rifts

    SciTech Connect

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

    1989-07-01

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

  9. Identification of central Kenyan Rift Valley Fever virus vector habitats with Landsat TM and evaluation of their flooding status with airborne imaging radar

    NASA Technical Reports Server (NTRS)

    Pope, K. O.; Sheffner, E. J.; Linthicum, K. J.; Bailey, C. L.; Logan, T. M.; Kasischke, E. S.; Birney, K.; Njogu, A. R.; Roberts, C. R.

    1992-01-01

    Rift Valley Fever (RVF) is a mosquito-borne virus that affects livestock and humans in Africa. Landsat TM data are shown to be effective in identifying dambos, intermittently flooded areas that are potential mosquite breeding sites, in an area north of Nairobi, Kenya. Positive results were obtained from a limited test of flood detection in dambos with airborne high resolution L, C, and X band multipolarization SAR imagery. L and C bands were effective in detecting flooded dambos, but LHH was by far the best channel for discrimination between flooded and nonflooded sites in both sedge and short-grass environments. This study demonstrates the feasibility of a combined passive and active remote sensing program for monitoring the location and condition of RVF vector habitats, thus making future control of the disease more promising.

  10. Anisotropic zonation in the lithosphere of Central North America: Influence of a strong cratonic lithosphere on the Mid-Continent Rift

    NASA Astrophysics Data System (ADS)

    Ola, O.; Frederiksen, A. W.; Bollmann, T.; van der Lee, S.; Darbyshire, F.; Wolin, E.; Revenaugh, J.; Stein, C.; Stein, S.; Wysession, M.

    2016-06-01

    We present shear-wave splitting analyses of SKS and SKKS waves recorded at sixteen Superior Province Rifting Earthscope Experiment (SPREE) seismic stations on the north shore of Lake Superior, as well as fifteen selected Earthscope Transportable Array instruments south of the lake. These instruments bracket the Mid-Continent Rift (MCR) and sample the Superior, Penokean, Yavapai and Mazatzal tectonic provinces. The data set can be explained by a single layer of anisotropic fabric, which we interpret to be dominated by a lithospheric contribution. The fast S polarization directions are consistently ENE-WSW, but the split time varies greatly across the study area, showing strong anisotropy (up to 1.48 s) in the western Superior, moderate anisotropy in the eastern Superior, and moderate to low anisotropy in the terranes south of Lake Superior. We locate two localized zones of very low split time (< 0.6 s) adjacent to the MCR: one in the Nipigon Embayment, an MCR-related magmatic feature immediately north of Lake Superior, and the other adjacent to the eastern end of the lake, at the southern end of the Kapuskasing Structural Zone (KSZ). Both low-splitting zones are adjacent to sharp bends in the MCR axis. We interpret these two zones, along with a low-velocity linear feature imaged by a previous tomographic study beneath Minnesota and the Dakotas, as failed lithospheric branches of the MCR. Given that all three of these branches failed to propagate into the Superior Province lithosphere, we propose that the sharp bend of the MCR through Lake Superior is a consequence of the high mechanical strength of the Superior lithosphere ca. 1.1 Ga.

  11. Helium isotope ratios in Ethiopian Rift basalts

    NASA Astrophysics Data System (ADS)

    Scarsi, P.; Craig, H.

    1996-11-01

    Helium isotope ratios were measured in olivine and pyroxene phenocrysts from basalts of the Ethiopian Rift Valley and Afar Depression between 6° and 15°N and 37° and 43°E. 3He/4He ratios range from 6 to 17 times the atmospheric value (RA = 1.4 × 10-6), that is, from ratios less than typical MORB (depleted mantle) helium (R/RA= 8 ± 1) to ratios similar to high-3He hotspots and to the Yellowstone hotspot (R/RA= 16.5). The high 3He/4He ratios occur all along the Ethiopian Rift and well up into the Afar Depression, with a maximum value of 17.0 RA at 8°N in the Rift Axis and a high value of 14.2 RA in the central Tat'Ali sector of the Afar Depression. The ratios decrease to MORB-like values near the edge of the Red Sea, and to sub-MORB ratios (5-6 RA) at the northern end of the Rift (Zula Peninsula) and at the southern end, at lakes Abaya and Chamo. The Ethiopian Rift provides the only continental hotspot terrain in which helium isotope ratios can be compared in detail between volcanic lavas and associated geothermal and volcanic gases, a primary motivation for this work. Comparison with our previously measured ratios in fluids and gases (range 2-15 RA) shows excellent agreement in the areas sampled for both lavas and fluids, and indicates that high-temperature volcanic fluids can be used for establishing helium isotope signatures in such terrains. The high-3He values in both fluids and basalts show that a Primitive Mantle (PM) component is required and that a Lower Mantle High-3He plume is strongly involved as a driving force in the rifting process of the East African Rift System.

  12. 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

  13. 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

  14. Identifying buried segments of active faults in the northern Rio Grande Rift using aeromagnetic, LiDAR,and gravity data, south-central Colorado, USA

    USGS Publications Warehouse

    Ruleman, Cal; Grauch, V. J.

    2013-01-01

    Combined interpretation of aeromagnetic and LiDAR data builds on the strength of the aeromagnetic method to locate normal faults with significant offset under cover and the strength of LiDAR interpretation to identify the age and sense of motion of faults. Each data set helps resolve ambiguities in interpreting the other. In addition, gravity data can be used to infer the sense of motion for totally buried faults inferred solely from aeromagnetic data. Combined interpretation to identify active faults at the northern end of the San Luis Basin of the northern Rio Grande rift has confirmed general aspects of previous geologic mapping but has also provided significant improvements. The interpretation revises and extends mapped fault traces, confirms tectonic versus fluvial origins of steep stream banks, and gains additional information on the nature of active and potentially active partially and totally buried faults. Detailed morphology of surfaces mapped from the LiDAR data helps constrain ages of the faults that displace the deposits. The aeromagnetic data provide additional information about their extents in between discontinuous scarps and suggest that several totally buried, potentially active faults are present on both sides of the valley.

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

    USGS Publications Warehouse

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

    2001-01-01

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

  16. 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.

  17. Guidebook to Rio Grande rift in New Mexico

    USGS Publications Warehouse

    Hawley, J. W.

    1978-01-01

    Discusses the details of geologic features along the rift zone. Included are short papers on topics relative to the overall region. These papers and the road logs are of special interest to any one pursuing further study of the rift. This book is a comprehensive guide to the middle and late Cenozoic geology of the Rio Grande region of Colorado and New Mexico. Though initially used on field trips for the International Symposium on Tectonics and Magmatism of the Rio Grande rift, the guidebook will be useful to anyone interested in the Cenozoic history of the 600-mi-long area extending from central Colorado to El Paso, Texas.

  18. Rift Valley fever vaccines

    PubMed Central

    Ikegami, Tetsuro; Makino, Shinji

    2009-01-01

    Rift Valley fever virus (RVFV), which belongs to the genus Phlebovirus, family Bunyaviridae, is a negative-stranded RNA virus carrying a tripartite RNA genome. RVFV is transmitted by mosquitoes and causes large outbreaks among ruminants and humans in Africa and the Arabian Peninsula. Human patients develop an acute febrile illness, followed by a fatal hemorrhagic fever, encephalitis or ocular diseases, whereas ruminants experience abortions during outbreak. Effective vaccination of both humans and ruminants is the best approach to control Rift Valley fever. This article summarizes the development of inactivated RVFV vaccine, live attenuated vaccine, and other new generation vaccines. PMID:19837291

  19. 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

  20. Central Appalachian Valley and Ridge Province Cenozoic igneous activity and its relation in space and time with the Late Jurassic rift-to-drift-related alkalic dikes

    NASA Astrophysics Data System (ADS)

    Meyer, R.; Schultz, L.; Hendriks, B. W.; Harbor, D. J.; Connors, C. D.

    2011-12-01

    and particularly K-rich, and thus have all the characteristics of delamination magmas. This confirms that delamination seems a substantial process during the rift to drift transition. After Jurassic delamination of lithosphere below Virginia hot geochemically depleted asthenosphere was transformed into lithosphere by lithospherization. This newly formed lithosphere has later been the mantle source of the Cenozoic volcanic activity. As a result, the suggested geodynamic model is not only important to the petrology community but also to understand the local geomorphology, seismicity and presence of hot springs.

  1. Volcanism at Rifts.

    ERIC Educational Resources Information Center

    White, Robert S.; McKenzie, Dan P.

    1989-01-01

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

  2. Tectonics of the South Georgia Rift

    NASA Astrophysics Data System (ADS)

    Heffner, David M.

    Triassic rifting of the supercontinent Pangea left behind numerous basins on what is now the eastern North American margin. The South Georgia Rift (SGR) was thought to be the best preserved of these basins having been capped by thick basalt flows of the Central Atlantic Magmatic Province (CAMP) and later buried beneath the Cretaceous and younger Coastal Plain. Because it is buried beneath the Coastal Plain, the SGR is only known through sparse drilling and geophysical methods. Despite this limited dataset, the SGR is the only one of the eastern North American Triassic basins known to overlie the ancient Alleghanian suture between Laurentia and Gondwana, although it isn't clear what influence this lithospheric weakness played in formation of the rift. The SGR has been variably interpreted as a singular large basin or as isolated sub-basins separated by transfer zones. Transfer zones are rift-transverse structural features that link major faults of rift sub-basins and accommodate differences in extensional strain. Transfer zones have been previously hypothesized to be present in the SGR based on onshore projections of Central Atlantic fracture zones, but observations confirming their existence, such as reversal in sub-basin polarity, have been lacking. Three separate hypotheses are tested related to the SGR: 1) the J-Horizon corresponds everywhere with basalt; 2) transfer zones are an important structural component of the SGR; 3) structural features of the Central Atlantic Ocean are related to transfer zones of the SGR. Reanalysis of existing well and seismic data shows that the extent of the flood basalt in the SGR is restricted and that the J-Horizon coincides with the base of the Coastal Plain. Subsurface mapping reveals reversals in sub-basin polarity, confirming the existence of previously hypothesized transfer zones. Small circle projections of the transfer zones correlate with oceanic features, and Central Atlantic fracture zones project onshore into inferred

  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. Continental rifting - Progress and outlook

    SciTech Connect

    Baker, B.H.; Morgan, P.

    1981-07-21

    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.

  5. Halocinèse précoce associée au rifting jurassique dans l'Atlas central de Tunisie (région de Majoura El Hfay)

    NASA Astrophysics Data System (ADS)

    Tanfous Amri, Dorra; Bédir, Mourad; Soussi, Mohamed; Azaiez, Hajer; Zitouni, Lahoussine; Hédi Inoubli, M.; Ben Boubaker, Kamel

    2005-05-01

    Seismic and sequence stratigraphy analyses, petroleum-well control and surface data studies of the Majoura-El Hfay region in the Central Atlas of Tunisia had led to identify and calibrate Jurassic seismic horizons. Seismic stratigraphic sections, seismic tectonics analyses, isochron and isopach mapping of Jurassic sequences show a differentiated structuring of platform and depocentre blocks limited by deep-seated NE-SW, north-south east-west and NW-SE faults intruded by Upper Triassic salt. The early salt migration seems to have started by the platform fracturing during the Lower Liassic rifting event. These movements are fossilized by thickness variations of Jurassic horizons, aggrading and retrograding onlap and toplap structures between subsiding rim-syncline gutters and high platform flanks intruded by salt pillows and domes. The salt migration is also attested by Middle and Upper Jurassic space depocentre migrations. Around the Majoura-El Hfay study blocks bounded by master faults, Triassic salt have pierced the Cretaceous and Tertiary sedimentary cover in a salt diapir extrusion and salt wall structures. To cite this article: D. Tanfous Amri et al., C. R. Geoscience 337 (2005).

  6. Continental rifts and mineral resources

    SciTech Connect

    Burke, K. . Geosciences Dept.)

    1992-01-01

    Continental rifts are widespread and range in age from the present to 3 b.y. Individual rifts may form parts of complex systems as in E. Africa and the Basin and Range. Rifts have originated in diverse environments such as arc-crests, sites of continental collision, collapsing mountain belts and on continents at rest over the mantle circulation pattern. Continental rift resources can be classified by depth of origin: For example, in the Great Dike, Norilsk and Mwadui magma from the mantle is the host. At shallower depths continental crust partly melted above mafic magma hosts ore (Climax, Henderson). Rift volcanics are linked to local hydrothermal systems and to extensive zeolite deposits (Basin and Range, East Africa). Copper (Zambia, Belt), zinc (Red Dog) and lead ores (Benue) are related to hydrothermal systems which involve hot rock and water flow through both pre-rift basement and sedimentary and volcanic rift fill. Economically significant sediments in rifts include coals (the Gondwana of Inida), marine evaporites (Lou Ann of the Gulf of Mexico) and non-marine evaporites (East Africa). Oil and gas in rifts relate to a variety of source, reservoir and trap relations (North Sea, Libya), but rift-lake sediment sources are important (Sung Liao, Bo Hai, Mina, Cabinda). Some ancient iron ores (Hammersley) may have formed in rift lakes but Algoman ores and greenstone belt mineral deposits in general are linked to oceanic and island arc environments. To the extent that continental environments are represented in such areas as the Archean of the Superior and Slave they are Andean Arc environments which today have locally rifted crests (Ecuador, N. Peru). The Pongola, on Kaapvaal craton may, on the other hand represent the world's oldest preserved, little deformed, continental rift.

  7. 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

  8. [Rift valley fever].

    PubMed

    Markin, V A; Pantiukhov, V B; Markov, V I; Bondarev, V P

    2012-01-01

    In the last quarter of century virus of Rift valley fever (RVF) sharply extended its distribution by moving from Africa to Asia and evolving from low- to high pathogenic for humans causing severe hemorrhagic disease, practically equaling in this respect with some members ofa group of extremely dangerous pathogens. Morbidity and epidemics of RVF are analyzed. Evolution of epidemic development of the infection is examined. Necessity of development of means and methods for diagnostics, prophylaxis and therapy of RVF is underlined.

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

    USGS Publications Warehouse

    McKeown, F.A.; Diehl, S.

    1994-01-01

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

  10. East African Rift Valley, Kenya

    NASA Technical Reports Server (NTRS)

    1990-01-01

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

  11. (40)Ar/(39)Ar dating, paleomagnetism, and tephrochemistry of Pliocene strata of the hominid-bearing Woranso-Mille area, west-central Afar Rift, Ethiopia.

    PubMed

    Deino, Alan L; Scott, Gary R; Saylor, Beverly; Alene, Mulugeta; Angelini, Joshua D; Haile-Selassie, Yohannes

    2010-02-01

    (40)Ar/(39)Ar dating of tuffs and mafic lavas, tephra geochemistry, and paleomagnetic reversal stratigraphy have been used to establish the chronostratigraphy of the Pliocene hominid-bearing fossiliferous succession at Woranso-Mille, a paleontological study area in the western part of the central Afar region of Ethiopia. The succession in the northwestern part of the study area ranges in (40)Ar/(39)Ar age from 3.82-3.570 Ma, encompassed by paleomagnetic subchron C2Ar (4.187-3.596 Ma). One of the major tuff units, locally named the Kilaytoli tuff, is correlative on the basis of age and geochemistry to the Lokochot Tuff of the Turkana Basin. A hominid partial skeleton (KSD-VP-1) was found in strata whose precise stratigraphic position and age is still under investigation, but is believed to correspond to the later part of this interval. Woranso-Mille fills a significant gap in the fossil record of northeastern Africa at the time of the lower to middle Pliocene transition, when many extant species lineages of African fauna were established.

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

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

    SciTech Connect

    Drahovzal, J.A. . Kentucky Geological Survey)

    1992-01-01

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

  14. The relationships between soft-sediment deformation structures and synsedimentary extensional tectonics in Upper Triassic deep-water carbonate succession (Southern Tethyan rifted continental margin - Central Sicily)

    NASA Astrophysics Data System (ADS)

    Basilone, Luca; Sulli, Attilio; Gasparo Morticelli, Maurizio

    2016-10-01

    We describe soft-sediment deformation structures into the Upper Triassic cherty limestone outcropping in the Pizzo Lupo section (Central Sicily, Italy), pertaining to the deep-water palaeodomain of the Southern Tethyan margin. In the study section, mainly consisting of thin-bedded mudstone/marl alternations with bedded chert intercalations, some lithofacies have been separated on the basis of the abundance of the calcium carbonate/clay content and the overall textural features. The deformational structures, displaying different deformational styles as folded and faulted beds, disturbed layers, clastic dikes, and slumps occur mainly in the deformed horizons that involve marl-dominated lithofacies. Small-scale water-escape structures involve beds with nodular fabric. Synsedimentary faults affect the mud-limestone dominated lithofacies, which are characterized by fault-rotating blocks producing lateral thinning. These bodies appear to have moved coherently along an overall planar surface. We relate these soft-sediment deformations to slump sheets, associated with down-slope sliding of sedimentary masses. The deformation mechanism and driving force for these soft-sediment deformations are due essentially to gravitational instability and dewatering. Detailing, rotational (slump) and translational (glide) slides and water-escape are the main processes causing the distinguished deformational styles. The synsedimentary extensional tectonics that affected the Upper Triassic pelagic deposits was the triggering process responsible for the instability of the seafloor inducing loss of coherence of the unconsolidated sediments on the sea bottom, developing a large number of gravity-driven slides. The analysis of both of these SSDSs and their relationships with the structural scenario allow us to hypothesise that they are seismically-induced.

  15. 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.

  16. Exploring the contrasts between fast and slow rifting

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    USGS Publications Warehouse

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

    2005-01-01

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

  19. Application of magnetotelluric in the modeling of underlying structure of Gour Oumelalen (Egere-Aleksod terrane, Central Hoggar, South of Algeria)

    NASA Astrophysics Data System (ADS)

    Boukhalfa, Zakaria; Abderrezak, Bouzid; Khadidja, Ouzegane; Abderrahmane, Bendaoud; Mohamed, Hamoudi; Abdeslam, Abtout; Abdelhamid, Bendekken; Sofiane Said, Bougchiche; Walid, Boukhlouf; Abdelgharfour, Boukar; Aboubakr, Deramchi; Mohamed, Bendali; Abdenaceur, Lemgharbi; Mohammed, Djeddi

    2016-04-01

    The results of a magnetotelluric experiment crossing Ounane granodiorite to the east until the Amadror Wadi to the West, passing through Adrar Ounane in our study area are presented. The magnetotelluric field survey was carried out in the Gour Oumelalen (GO) area during March 2015. We deployed 34 magnetotelluric sites along two parallel EW profiles of a hundred km long. Time series were collected using a V5 system 2000® of Phoenix Geophysics. The first profile located to the north is composed of 18 braodband measurement sites obtained from merging magnetotelluri with audio-magnetotelluric (AMT) data. The second one located 10 km south of the first, is composed of 15 MT sites. An inter-station distance of ~5 km provides good lateral resolution. The MT time series were recorded during about 20 hours which allows to reach a depth of 100 km or more and the AMT data 30 minutes. This allows to get broadband magnetotelluric soundings with good quality data in period range from 0.001 s to 3000 s. In this study we will use the south profile data for modeling the underlying structure of GO. The crustal part of the model shows a resistance bloc, divided by conductive parts which can be interpreted as faults, as regards the lithospheric part it less resistant the upper part, the transition crust / mantle corresponding to MOHO is estimated at more or less 35 km.

  20. Rifts in the tectonic structure of East Antarctica

    NASA Astrophysics Data System (ADS)

    Golynsky, Dmitry; Golynsky, Alexander

    2010-05-01

    It was established that riftogenic and/or large linear tectonic structures in East Antarctica are distributed with a steady regularity with average distance between them about 650 km. All these structures (13) represent objects of undoubted scientific and practical interest and might be considered as immediate objects for conducting integrated geological and geophysical investigations. Analysis and generalization of the RADARSAT satellite system imagery and radio-echosounding survey data collected in the eastern part of Princess Elizabeth Land allow us to distinguish spatial boundaries of previously unknown continental rift system that was proposed to name Gaussberg (Golynsky & Golynsky, 2007). The rift is about 500 km long, and taking into consideration its western continuation in the form of short (fragmented) faults, may exceed 700 km. The elevation difference between depressions and horsts reaches 3 km. The rift structure consists of two sub-parallel depressions separated by segmented horst-like rises (escarpments). Deep depressions within the rift reach more than 800 m bsl near the West Ice Shelf and within the central graben occupied by the Phillipi Glacier. The width of the Gaussberg Rift system varies from 60 km in the south-western area to 150 km near the West Ice Shelf. The Gaussberg rift is considered as a part of the Lambert rift system, which has a complicated structure clearly recognized over both the continent and also its margin. The Gaussberg rift probably exploited a weak zone between the Proterozoic mobile belt and the Archaean Vestfold-Rauer cratonic block. Supposedly it initiated at the turn of Jurassic and Permian epoch or a little bit earlier as in case of the Lambert rift where the Permian graben formation with coal-bearing deposits predetermined the subsequent development of submeridional rift zone. The Gaussberg and also the Scott rift developed in the Queen Marie Land, may be considered as continuations of the Mahanadi Valley rift and

  1. The Midcontinent rift system in Kansas

    SciTech Connect

    Berendsen, P. . Kansas Geological Survey)

    1993-03-01

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

  2. The MOZART Project - MOZAmbique Rift Tomography

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

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

  4. 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

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

    USGS Publications Warehouse

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

    1994-01-01

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

  6. Propagating rifts on midocean ridges

    NASA Astrophysics Data System (ADS)

    Hey, Richard; Duennebier, Frederick K.; Morgan, W. Jason

    1980-07-01

    Spreading center jumps identified west of the Galapagos Islands near 95°W occur in a pattern consistent with the propagating rift hypothesis. A new rift is gradually breaking through the Cocos plate. Each successive jump is slightly longer than the preceding jump. The new spreading center grows at a new azimuth toward the west as the old one dies. The jumps are a manifestation of rift propagation. We extend the analysis of propagating rifts to the case of continuous propagation and predict patterns of magnetic anomalies and bathymetry consistent with the observed patterns. In particular, we correctly predict the trends of fossil spreading centers and V patterns of magnetic anomaly offsets required by the propagating rift hypothesis. Similar V patterns have been observed on many other spreading centers and have been interpreted in various ways. The propagating rift hypothesis appears to offer a simple explanation, consistent with rigid plate tectonics, for each of these patterns. This hypothesis may also have important implications for continental rifting.

  7. 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.

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

    USGS Publications Warehouse

    Burton, William C.; Southworth, Scott

    2010-01-01

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

  9. Rift Valley fever.

    PubMed

    Paweska, J T

    2015-08-01

    Rift Valley fever (RVF) is a mosquito-borne zoonotic viral disease affecting domestic and wild ruminants, camels and humans. The causative agent of RVF, the RVF virus (RVFV), has the capacity to cause large and severe outbreaks in animal and human populations and to cross significant natural geographic barriers. Rift Valley fever is usually inapparent in non-pregnant adult animals, but pregnant animals and newborns can be severely affected; outbreaks are characterised by a sudden onset of abortions and high neonatal mortality. The majority of human infections are subclinical or associated with moderate to severe, non-fatal, febrile illness, but some patients may develop a haemorrhagic syndrome and/or ocular and neurological lesions. In both animals and humans, the primary site of RVFV replication and tissue pathology is the liver. Outbreaks of RVF are associated with persistent high rainfalls leading to massive flooding and the emergence of large numbers of competent mosquito vectors that transmit the virus to a wide range of susceptible vertebrate species. Outbreaks of RVF have devastating economic effects on countries for which animal trade constitutes the main source of national revenue. The propensity of the virus to spread into new territories and re-emerge in traditionally endemic regions, where it causes large outbreaks in human and animal populations, presents a formidable challenge for public and veterinary health authorities. The presence of competent mosquito vectors in RVF-free countries, the wide range of mammals susceptible to the virus, altering land use, the global changes in climate, and increased animal trade and travel are some of the factors which might contribute to international spread of RVF.

  10. Chow Bahir rift: A “failed” rift in southern Ethiopia

    NASA Astrophysics Data System (ADS)

    W-Gabriel, Giday; Aronson, James L.

    1987-05-01

    The Chow Bahir rift system is a major graben in a 300-km-broad rift zone recognized in southern Ethiopia between the Kenyan and Ethiopian domes where the East African rift is not well defined. An extinct (failed) rift discovered along the Omo Canyon to the north and on strike with the Chow Bahir rift ceased activity about 4 m.y. ago. Chow Bahir is in a younger stage of abandonment as the main Ethiopian rift propagates south into this region.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2004-12-01

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

  13. UHP-UHT peak conditions and near-adiabatic exhumation path of diamond-bearing garnet-clinopyroxene rocks from the Eger Crystalline Complex, North Bohemian Massif

    NASA Astrophysics Data System (ADS)

    Haifler, Jakub; Kotková, Jana

    2016-04-01

    Intermediate garnet-clinopyroxene rocks from the Eger Crystalline Complex, North Bohemian Massif, contain microdiamonds enclosed in garnet and zircon. The variable mineral assemblage of these rocks allows for an evaluation of the P-T evolution using numerous univariant equilibria and thermodynamic modelling, in addition to the ternary feldspar solvus, Ti-in-garnet, Zr-in-rutile and Ti-in-zircon thermometry. Zircon mantle domains with diamond inclusions contain 111-189 ppm Ti, reflecting temperatures of 1037-1117 °C. The peak pressure consistent with diamond stability corresponds to c. 4.5-5.0 GPa. Ti-in-garnet thermometry using the Ti content of diamond-bearing garnet core yielded temperatures of 993-1039 °C at c. 5.0 GPa. An omphacite inclusion in garnet (reflecting c. 2.3-2.4 GPa at c. 1050 °C) and metastably preserved kyanite represent relics of eclogite-facies conditions. The dominant high-pressure granulite-facies mineral assemblage of low-Ca garnet, diopsidic clinopyroxene, antiperthitic feldspar and quartz equilibrated at 1.8-2.1 GPa and c. 1050 °C, based on the XGrs isopleth of the garnet mantle, garnet-feldspar-kyanite-quartz univariant equilibria and ternary feldspar solvus. Our thermodynamic modelling shows that a steep decrease of XGrs from a maximum core value of 0.32 to 0.17 at the rim as well as a rimward XMg increase (from 0.42 to 0.50) are consistent with significant decompression without heating. The latter is related to omphacite and kyanite breakdown reactions producing garnet and plagioclase. The Ti content in the rim zone of zircon (13-42 ppm), exsolved plagioclase and K-feldspar associated with matrix diopside and garnet rim, and late biotite reflect temperatures of c. 830-900 °C at c. 1.4 GPa. A similar temperature is recorded by matrix rutile grains, containing 2028-4390 ppm Zr and representing a relatively homogeneous population in contrast to rutile enclosed in garnet with variable Zr content. Our results show that the garnet

  14. 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).

  15. [Rift Valley fever].

    PubMed

    Pépin, M

    2011-06-01

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

  16. 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.

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

    NASA Astrophysics Data System (ADS)

    Zoback, Mary Lou; Richardson, Randall M.

    1996-03-01

    The state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts (specifically a dense rift pillow in the lower crust) may modify substantially the regional stress field. Both shallow (2.0-2.6 km depth) breakout data and deep (20-45 km depth) crustal earthquake focal mechanisms indicate a N to NNE maximum horizontal compression in the vicinity of the Paleozoic Amazonas rift in central Brazil. This compressive stress direction is nearly perpendicular to the rift structure and represents a ˜75° rotation relative to a regional E-W compressive stress direction in the South American plate. Elastic two-dimensional finite element models of the density structure associated with the Amazonas rift (as inferred from independent gravity modeling) indicate that elastic support of this dense feature would generate horizontal rift-normal compressional stresses between 60 and 120 MPa, with values of 80-100 MPa probably most representative of the overall structure. The observed ˜75° stress rotation constrains the ratio of the regional horizontal stress difference to the rift-normal compressive stress to be between 0.25 and 1.0, suggesting that this rift-normal stress may be from 1 to 4 times larger than the regional horizontal stress difference. A general expression for the modification of the normalized local horizontal shear stress (relative to the regional horizontal shear stress) shows that the same ratio of the rift-normal compression relative to the regional horizontal stress difference, which controls the amount of stress rotation, also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress. The potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike-slip stress regimes. In the Amazonas

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

    USGS Publications Warehouse

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

    1996-01-01

    The state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts (specifically a dense rift pillow in the lower crust) may modify substantially the regional stress field. Both shallow (2.0-2.6 km depth) breakout data and deep (20-45 km depth) crustal earthquake focal mechanisms indicate a N to NNE maximum horizontal compression in the vicinity of the Paleozoic Amazonas rift in central Brazil. This compressive stress direction is nearly perpendicular to the rift structure and represents a ???75?? rotation relative to a regional E-W compressive stress direction in the South American plate. Elastic two-dimensional finite element models of the density structure associated with the Amazonas rift (as inferred from independent gravity modeling) indicate that elastic support of this dense feature would generate horizontal rift-normal compressional stresses between 60 and 120 MPa, with values of 80-100 MPa probably most representative of the overall structure. The observed ???75?? stress rotation constrains the ratio of the regional horizontal stress difference to the rift-normal compressive stress to be between 0.25 and 1.0, suggesting that this rift-normal stress may be from 1 to 4 times larger than the regional horizontal stress difference. A general expression for the modification of the normalized local horizontal shear stress (relative to the regional horizontal shear stress) shows that the same ratio of the rift-normal compression relative to the regional horizontal stress difference, which controls the amount of stress rotation, also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress. The potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike-slip stress regimes. In the Amazonas

  19. The geometry of propagating rifts

    NASA Astrophysics Data System (ADS)

    McKenzie, Dan

    1986-03-01

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

  20. Venus: Geology of Beta Regio rift system

    NASA Technical Reports Server (NTRS)

    Nikishin, A. M.; Borozdin, V. K.; Bobina, N. N.

    1992-01-01

    Beta Regio is characterized by the existence of rift structures. We compiled new geologic maps of Beta Regio according to Magellan data. There are many large uplifted tesserae on beta upland. These tesserae are partly buried by younger volcanic cover. We can conclude, using these observations, that Beta upland formed mainly due to lithospheric tectonic uplifting and was only partly constructed by volcanism. Theia Mons is the center of the Beta rift system. Many rift belts are distributed radially to Theia Mons. Typical widths of rifts are 40-160 km. Rift valleys are structurally represented by crustal grabens or half-grabens. There are symmetrical and asymmetrical rifts. Many rifts have shoulder uplifts up to 0.5-1 km high and 40-60 km wide. Preliminary analysis for rift valley structural cross sections lead to the conclusion that rifts originated due to 5-10 percent crustal extension. Many rifts traverse Beta upland and spread to the surrounding lowlands. We can assume because of these data that Beta rift system has an active-passive origin. It formed due to regional tectonic lithospheric extension. Rifting was accelerated by upper-mantle hot spot origination under the center of passive extension (under the Beta Regio).

  1. Rift strength controls rapid plate accelerations: A global analysis of Pangea fragmentation

    NASA Astrophysics Data System (ADS)

    Brune, S.; Williams, S.; Butterworth, N. P.; Müller, D.

    2015-12-01

    Motions of Earth's plates are thought to be driven by slab pull, basal drag, and ridge push. Here we propose that plate motions during supercontinental fragmentation are decisively controlled by the non-linear decay of a resistive force: rift strength. We use state-of-the-art global tectonic reconstructions and the new geotectonic analysis tool pyGPlates to analyze the transition from rifting to sea-floor spreading of well-studied post-Pangea rift systems (Central Atlantic, South Atlantic, Iberia/Newfoundland, Australia/Antarctica, North Atlantic, South China Sea, Gulf of California). In all cases, continental extension starts with a slow phase (< 10 mm/yr, full extension velocity) followed by a rapid acceleration over periods of a few My that introduces a fast rift phase (> 10 mm/yr). The transition from slow to fast extension takes place long before crustal break-up. In fact, we find that approximately half of the present day rifted margin area was created during the slow, and the other half during the fast phase. We reproduce the transition from slow to fast rifting using numerical forward models with force boundary conditions, such that rift velocities are not imposed but instead evolve naturally in response to changing strength of the rift. These models show that the two-phase velocity behavior during rifting and the rapid speed-up are intrinsic features of continental rupture that can be robustly inferred for different crust and mantle rheologies.It has been proposed that abrupt plate accelerations can be caused by plume-lithosphere interaction, subduction initiation, and slab detachment. However, none of these mechanisms explains our result that plate speed-up systematically precedes continental break-up. We therefore propose dynamic rift weakening as a new mechanism for rapid plate motion changes.

  2. 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.

  3. Crustal and sub-continental lithospheric mantle decoupling beneath the Malawi Rift

    NASA Astrophysics Data System (ADS)

    Njinju, Emmanuel Atem

    We analyzed satellite gravity and aeromagnetic data using the two-dimensional (2D) power-density spectrum technique to investigate the lithospheric and thermal structure beneath the magma-starved Malawi Rift, which forms the southern extension of the Western Branch of the East African Rift System. We observed: (1) lack of consistent pattern of crustal thinning and elevated heat flow along the surface expression of the rift. Beneath the Rungwe Volcanic Province (RVP) in the north, the crustal thickness ranges between 40 and 45 km and varies between 35 and 40 km along the entire length of the rift. (2) shallow lithosphere-asthenosphere boundary (LAB) elevated to ˜64 km beneath the entire length of the rift and deeper than 100 km beneath the surrounding Precambrian terranes reaching in places ˜124 km. (3) localized zones of high heat flow (70-75 mWm-2) beneath the RVP, and the central and southern parts of the rift. The central and southern thermal anomalies are due to the presence of uranium deposits in the Karoo sedimentary rocks. We interpret the crustal thickness heterogeneity to have been inherited from pre-existing lithospheric stretching, while strain during the extension of the Malawi Rift is preferentially localized in the sub-continental lithospheric mantle (SCLM). Our interpretation is supported by 2D forward modeling of the gravity data showing uniform stretching of the SCLM by a factor of 1.5 to 1.8 beneath the entire length of the rift. Our results indicate decoupling of the crust from the SCLM during the early stages of the development of the Malawi Rift.

  4. Clastic rocks associated with the Midcontinent rift system in Iowa

    USGS Publications Warehouse

    Anderson, Raymond R.; McKay, Robert M.

    1997-01-01

    The Middle Proterozoic Midcontinent Rift System (MRS) of North America is a failed rift that formed in response to region-wide stresses about 1,100 Ma. In Iowa, the MRS is buried beneath 2,200?3,500 ft of Paleozoic and Mesozoic sedimentary rocks and Quaternary glaciogenic deposits. An extremely large volume of sediments was deposited within basins associated with the rift at several stages during its development. Although the uplift of a rift-axial horst resulted in the erosional removal of most of these clastic rocks from the central region of the MRS in Iowa, thick sequences are preserved in a series of horst-bounding basins. Recent studies incorporating petrographic analysis, geophysical modeling, and other analytical procedures have led to the establishment of a preliminary stratigraphy for these clastic rocks and interpretations of basin geometries. This information has allowed the refinement of existing theories and history of MRS formation in Iowa. Additionally, drill samples previously interpreted as indicating the existence of early Paleozoic basins overlying the Proterozoic MRS basins were re-examined. Samples previously interpreted as deep-lying Paleozoic rocks are now known to have caved from upper levels of the drillhole and were out of stratigraphic position. No deep Paleozoic basins exist in this area. These investigations led to the development of petrographic parameters useful in differentiating the Proterozoic MRS Red clastics from Paleozoic clastic rocks having similar lithologies.

  5. 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.

  6. The Midcontinent rift in the Lake Superior region with emphasis on its geodynamic evolution

    USGS Publications Warehouse

    Cannon, W.F.

    1992-01-01

    The Midcontinent rift is a Middle Proterozoic continental rift which records about 15 m.y. of extension, subsidence, and voluminous volcanism in the period 1109-1094 Ma in the central part of North America. During that time the crust was nearly totally separated and as much as 25 km of subaerial basalts accumulated in a deep central depression. Following extension and volcanism, a longer period of subsidence resulted in development of a post-rift sedimentary basin in which as much a 8 km of fluvial and lacustrine clastic rocks were deposited. Partial inversion of the central depression occurred about 30-50 m.y. after extension to produce the current configuration of a central horst, composed mostly of thick volcanic accumulations, between shallower flanking basins. ?? 1992.

  7. The northern Nevada rift: regional tectono-magmatic relations and Middle Miocene stress direction

    USGS Publications Warehouse

    Zoback, M.L.; McKee, E.H.; Blakely, R.J.; Thompson, G.A.

    1994-01-01

    As defined by the most recent aeromagnetic surveys, the north-northwest-trending northern Nevada rift zone extends for at least 500km from southern Nevada to the Oregon-Nevada border. At several places along the rift, the magnetic anomaly is clearly related to north-northwest-trending dikes and flows that erupted between 17 and 14 Ma. The tectonic significance of the rift is dramatized by its length, its coincidence in time and space with the oldest silicic caldera complex along the Yellowstone hotspot trend, and its parallelism with the subduction zone along the North American coast prior to the establishment of the San Andreas fault. The northern Nevada rift is also equivalent in age, trend, and composition to feeder dikes that fed the main eruptive pulse of the Columbia River flood basalts in northern Oregon ~15.6-16.5 Ma. Because of these similarities, both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south-southeast and north-northwest, respectively, from a central mantle plume. The present north-northwest trend of the rift reflects the state of stress in the Basin and Range during middle Miocene time and is consistent with stress indicators of similar age throughout the Basin and Range Rio Grande rift provinces. -from Authors

  8. 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.

  9. Magnetic signature of North-East Greenland, the Morris Jesup Rise, the Yermak Plateau, the central Fram Strait: Constraints for the rift/drift history between Greenland and Svalbard since the Eocene

    NASA Astrophysics Data System (ADS)

    Jokat, Wilfried; Lehmann, Paul; Damaske, Detlef; Bradley Nelson, J.

    2016-11-01

    A compilation of several aeromagnetic surveys across North Greenland and the Fram Strait area is presented. Across North Greenland, the data are related to the known onshore geology. The correlation between the geological structure and the large-scale magnetic anomalies is excellent. Along the North Greenland coast, the data indicate that the Late Mesozoic Kap Washington volcanics might have a larger areal extent than known from onshore geological mapping. The submarine Morris Jesup Rise is characterized by strong positive magnetic anomalies, and the anomalies across the plateau indicate the presence of distinct volcanic centers. This massive magmatism has occurred latest in conjunction with the Oligocene initial stages of plate divergence between Svalbard and North Greenland that led to the development of Fram Strait. Both plateaux, the Morris Jesup Rise and the Yermak Plateau, are interpreted to be of continental origin and, in the initial rift stage of the Eurasia Basin around 56 Ma, to have been contiguous with the Lomonosov Ridge. More toward the east in the Fram Strait interpretations of seismic and bathymetric data indicate the presence of an active segment of mid-ocean ridge, the Lena Trough. This approximately 280-km-long segment terminates in the south at the NW-SE striking Spitsbergen Fracture Zone, while in the north it bends to connect with the Gakkel Ridge. A shallow water pathway along the line of the trough might have existed between those times and 21 Ma, above the propagating rift system in the north and, further south, over extending and subsiding continental crust of the Svalbard and North-East Greenland margins. Magnetic anomaly identifications indicate that the Lena Trough has remained an ultraslow spreading system, or possibly even all of the magnetic lineations may be signals of serpentinized exhumed mantle rocks rather than of basaltic crust.

  10. ALVIN-SeaBeam studies of the Sumisu Rift, Izu-Bonin arc

    NASA Astrophysics Data System (ADS)

    Taylor, B.; Brown, G.; Fryer, P.; Gill, J. B.; Hochstaedter, A. G.; Hotta, H.; Langmuir, C. H.; Leinen, M.; Nishimura, A.; Urabe, T.

    1990-10-01

    Bimodal volcanism, normal faulting, rapid sedimentation, and hydrothermal circulation characterize the rifting of the Izu-Bonin arc at 31°N. Analysis of the zigzag pattern, in plan view, of the normal faults that bound Sumisu Rift indicates that the extension direction (080° ± 10°) is orthogonal to the regional trend of the volcanic front. Normal faults divide the rift into an inner rift on the arc side, which is the locus for maximum subsidence and sedimentation, and an outer rift further west. Transfer zones that link opposing master faults and/or rift flank uplifts further subdivide the rift into three segments along strike. Volcanism is concentrated along the ENE-trending transfer zone which separates the northern and central rift segments. The differential motion across the zone is accommodated by interdigitating north-trending normal faults rather than by ENE-trending oblique-slip faults. Volcanism in the outer rift has built 50-700 m high edifices without summit craters whereas in the inner rift it has formed two multi-vent en echelon ridges (the largest is 600 m high and 16 km long). The volcanism is dominantly basaltic, with compositions reflecting mantle sources little influenced by arc components. An elongate rhyolite dome and low-temperature hydrothermal deposits occur at the en echelon step in the larger ridge, which is located at the intersection of the transfer zone with the inner rift. The chimneys, veins, and crusts are composed of silica, barite and iron oxide, and are of similar composition to the ferruginous chert that mantles the Kuroko deposits. A 1.2-km transect of seven ALVIN heat flow measurements at 30°48.5'N showed that the inner-rift-bounding faults may serve as water recharge zones, but that they are not necessarily areas of focussed hydrothermal outflow, which instead occurs through the thick basin sediments. The rift basin and arc margin sediments are probably dominated by permeable rhyolitic pumice and ash erupted from submarine

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

    NASA Astrophysics Data System (ADS)

    Hauber, E.; Kronberg, P.

    2003-04-01

    We describe the morphology of a large and complex graben structure in western Thaumasia which was often ascribed to rifting by previous authors (the Thaumasia "rift" or TR). We consider possible fault geometries, determine extension, and discuss shortly possible models for its origin. The TR is characterized by a strong (half)graben asymmetry. The master fault system changes from the western border in the northern part to the eastern border in the southern part (at ˜21^oS). Several profiles across the TR display features that might indicate a listric master fault, including an overall halfgraben geometry, tilted blocks, and a curvature of the hanging wall which is characteristic of a rollover. For a listric fault, the depth D to a detachment can be determined from the surficial fault dip (α), the tilt of the graben floor (θ), and the vertical offset (d). We measure a scarp height d of ˜2000 m and floor tilts θ between 0.9^o and 2.7^o. For α = 60^o, we obtain values of D between ˜33 km and ˜67 km (θ = 2.0^o and 1.0^o). Interestingly, these values correspond very well with recent estimations of the thickness of the elastic lithosphere T_e in S-Tharsis, as given by Zuber et al. (2000): Valles Marineris ˜60 km, Solis Planum ˜35 km. A listric W-dipping master fault in the middle and southern part of the TR might indicate gravitational gliding of an unstable part of the outward verging fold-and-thrust plateau margin towards W, i.e., toward the foreland of Thaumasia. However, slip along planar faults can also produce tilted graben floors and hanging wall flexure, so the observed morphology does not allow any firm statement about the fault geometry. Extension (assuming planar fault planes) was determined using the vertical displacement at faults. In the N, most of the extension occurred along a few major faults. In the S, it has been distributed among many smaller faults. Extension is 0.5 to 4.5 km (strain 1 to 3%). This is much less than 10 km, as previously

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  14. 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].

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

    USGS Publications Warehouse

    Cannon, W.F.

    1989-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-11-01

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

  17. 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.

  18. Rift flank uplift and thermal evolution of an intracratonic rift basin (eastern Canada) determined by combined apatite and zircon (U-Th)/He thermochronology

    NASA Astrophysics Data System (ADS)

    Hardie, Rebecca; Schneider, David; Metcalf, James; Flowers, Rebecca

    2015-04-01

    As a significant portion of the world's oil reserves are retrieved from rift systems, a better understanding of the timing of thermal evolution and burial history of these systems will increase the potential for the discovery of hydrocarbon-bearing rifts. The Ottawa Embayment of the St. Lawrence Platform of eastern Canada is a reactivated intracratonic rift basin related to the opening of the Iapetus Ocean at ca. 620-570 Ma, followed by the formation of the well-developed continental passive margin. Siliciclastic sediments derived from the adjacent uplifted Neoproterozoic Grenville basement provide the basin fill material. Apatite and zircon (U-Th)/He thermochronology allows for low-temperature analysis across the exposed crystalline rift flank into the synrift sedimentary sequence to resolve the unroofing, burial and subsidence history of the region. Samples were collected along a ~250 km NE-SW transect, oblique to the axis of the rift, from Mont-Tremblant, Québec (~900 m) to the central axis of the Paleozoic rift in the Southern Ontario Lowlands (~300 m). Targets included Neoproterozoic metamorphic rocks of the Grenville Province along the rift flank and basinal Cambro-Ordovician Potsdam Group. Samples from the rift flank yield zircon ages from ca. 650 Ma to ca. 560 Ma and apatite ages from ca. 290 Ma to ca. 190 Ma, with a weak positive correlation between age and grain size. Zircon ages demonstrate a strong negative correlation with radiation damage: as eU increases, age decreases. By incorporating (U-Th)/He ages with regional constraints in the thermal modelling program HeFTy, viable temperature time paths for the region can be determined. Through inverse and forward modeling, preliminary rift flank (U-Th)/He ages correspond to post-Grenville cooling with <4 km of post-Carboniferous burial. The data define slow and long episodes of syn- to post-rift cooling with rates between 0.4 and 0.1 °C/Ma. (U-Th)/He dating of samples along the full-length of the transect

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

    NASA Astrophysics Data System (ADS)

    Bottenberg, Helen Carrie

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-09-01

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

  1. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    .5 Ma: Rift stage 1 (subsidence rate: > 500m/Ma up to 600-800 m/Ma; sedimentation rate: 2.4 km3/Ma) - Rifting climax; - 2.5-0.4 Ma: uplift of the Ruwenzori Mountains and shifting from an alluvial system to a network of bedrock river incision - Rift Stage 2 (subsidence rate: 450 to 250 m/Ma; sedimentation rate: 1.5 km3/Ma); - 0.4-0 Ma: long wavelength downwarping of the Tanzanian Craton, initiation of the Lake Victoria trough, drainage network inversion and uplift of the present-day Ugandan escarpment (normal faulting motion of the border faults) with formation of perched valleys associated to the Lower Pleistocene (2.5-0.4 Ma) rivers network. At larger scale, comparison of the Lake Albert Rift evolution with the data available in the basins of both eastern and western branches of the East African Rift System shows that most of the sedimentary basins experienced the same geometrical evolution from large basins with limited fault controls during Late Miocene to narrow true rift in Late Pleistocene (e.g. Northern and Central Kenyan Basins), in agreement with the volcanism distribution, large (width >100 km) during the Miocene times, narrower (width x10 km) from Late Pliocene to Pleistocene times and today limited to narrow rifts.

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

    at near 120 degree angle in southeastern Honduras. We suggest that the intersection of these two trends represents part of a R-R-R triple junction during the breakup of the Americas. The WNW trending rift produced the WNW trending fabric of the central Chortis block and failed in the Early Cretaceous while the NE trending rift continued opening to form the south-facing passive margin of the northern proto-Caribbean basin.

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

    NASA Astrophysics Data System (ADS)

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

    2014-08-01

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

  5. 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.

  6. Controls on the development and evolution of transfer zones: the influence of basement structure and sedimentary thickness in the Suez rift and Red Sea

    NASA Astrophysics Data System (ADS)

    Moustafa, Adel R.

    1997-06-01

    Detailed field mapping of the northern part of the Gebel Um Hammad-Gebel Duwi area on the western margin of the Red Sea indicates oppositely dipping rift blocks separated by a 60-km long, WNW-ESE-oriented, reactivated pre-rift fault of Late Precambrian age parallel to the Najd fault system of the Arabian-Nubian Shield. This fault forms the Sudmain transfer zone between the oppositely tilted half-grabens in the northwestern Red Sea region and is associated by a SE-plunging syncline. This pre-rift fault was reactivated by dextral transtension during the Late Oligocene rift opening. Compared to the transfer zones of the Suez rift, the Sudmain transfer zone is narrower. The Gebel Sufr El Dara transfer zone (between the southern and central half-grabens of the Suez rift) is 20 km wide and is also controlled by pre-rift faults oriented ENE-WSW. The latter were reactivated by left-lateral slip during the rift opening. On the other hand, the Gharandal transfer zone (northern part of the Suez rift) is 40-60 km wide and is not affected by the pre-rift faults in the Precambrian basement, perhaps owing to the large thickness of pre-rift sedimentary rocks in this area. The location of the Gharandal transfer zone was controlled by a NE-SW-oriented 'Syrian arc' fold. This study suggests that the northward increase in the width of transfer zones as well as the northward decrease in the length of half-grabens in the Suez-northern Red Sea rift system are related to the corresponding increase in the thickness of pre-rift Phanerozoic sedimentary section from about 400 m in the south to about 1800 m in the north.

  7. Volcanism and sedimentation along the western margin of the Rio Grande rift between caldera-forming eruptions of the Jemez Mountains volcanic field, north-central New Mexico, USA

    NASA Astrophysics Data System (ADS)

    Jacobs, Elaine P.; WoldeGabriel, Giday; Kelley, Shari A.; Broxton, David; Ridley, John

    2016-11-01

    The Cerro Toledo Formation (CTF), a series of intracaldera rhyolitic dome complexes and their associated extracaldera tephras and epiclastic sedimentary deposits, records the dynamic interplay between volcanic, tectonic, and geomorphic processes that were occurring along the western margin of the Rio Grande rift between major caldera-forming eruptions of the Bandelier Tuff 1.65-1.26 Ma. The Alamo Canyon and Pueblo Canyon Members differ significantly despite deposition within a few kilometers of each other on the Pajarito Plateau. These differences highlight spatial distinctions in vent sources, eruptive styles, and depositional environments along the eastern side of the Jemez Mountains volcanic field during this ca. 400,000 year interval. Intercalated pyroclastic fall deposits and sandstones of the Pueblo Canyon Member reflect deposition with a basin. Thick Alamo Canyon Member deposits of block-and-ash-flow tuff and pyroclastic fall deposits fill a paleovalley carved into coarse grained sedimentary units reflecting deposition along the mountain front. Chemistry and ages of glass from fall deposits together with clast lithologies of sedimentary units, allow correlation of outcrops, subsurface units, and sources. Dates on pyroclastic fall deposits from Alamo Canyon record deep incision into the underlying Otowi Member in the southern part of the Pajarito Plateau within 100 k.y. of the Toledo caldera-forming eruption. Reconstruction of the CTF surface shows that this period of rapid incision was followed by aggradation where sediments largely filled pre-existing paleocanyons. Complex sequences within the upper portion of the Otowi Member in outcrop and in the subsurface record changes in the style of eruptive activity during the waning stages of the Toledo caldera-forming eruption.

  8. The origin of along-rift variations in faulting and magmatism in the Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Keir, Derek; Bastow, Ian D.; Corti, Giacomo; Mazzarini, Francesco; Rooney, Tyrone O.

    2015-03-01

    The geological record at rifts and margins worldwide often reveals considerable 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 Main Ethiopian Rift (MER) in East Africa provides an excellent opportunity to address this dichotomy: 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 volcanic cone density and rift obliquity along strike in the MER. By synthesizing these new data in light of existing geophysical, geochemical, and petrological constraints on magma generation and emplacement, we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin. The timing of rift sector development, the three-dimensional focusing of melt, and the ponding of plume material where the rift dramatically narrows each influence igneous intrusion and volcanism along the MER. However, rifting 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.

  9. Inter-rifting Deformation in an Extensional Rift Segment; the Northern Volcanic Zone, Iceland

    NASA Astrophysics Data System (ADS)

    Pedersen, R.; Masterlark, T.; Sigmundsson, F.; Arnadottir, T.; Feigl, K. L.

    2006-12-01

    The Northern Volcanic Zone (NVZ) in Iceland is an extensional rift segment, forming a sub-aerial exposure of a part of the Mid-Atlantic ridge. The NVZ is bounded to the south by the Icelandic mantle plume, currently beneath the Vatnajökull ice cap, and to the north by the Tjörnes Fracture zone, a transform zone connecting the offset on- and offshore rift segments of the Mid-Atlantic ridge. Based on geologic and tectonic mapping, the NVZ has been divided into five partly overlapping en-echelon fissure swarms, each with a central main volcanic production area. The two fissure swarms with known activity in historic time are, based on geodetic and seismic data, interpreted to have associated shallow crustal magma chambers. These central volcanoes are furthermore the only with caldera collapses associated, reflecting on the maturity of the systems. A series of newly formed InSAR images of the NVZ, spanning the interval from 1993-2006, have been formed, revealing a complex interplay of several tectonic and magmatic processes. Deformation from two subsiding shallow sources appear at the sites of the known crustal magma chambers. Furthermore, subsidence is occurring at varying degrees within the associated relatively narrow fissure swarms (15-20 km). However, the horizontal plate spreading signal is not confined to the fissure systems, and appears to be distributed over a much wider zone (about 100 km). This wide zone of horizontal spreading has previously been measured with campaign GPS surveys. A broad area of uplift situated about 18 km to the north of one of the subsidence centres (Krafla) suggests a deep seated pressurization source near the crust mantle boundary. Movements on previously unrecognized faults are apparent in the data, correlating well with the location of earthquake epicentres from minor seismic activity. Finally, utilization of geothermal resources in the Krafla area affects the deformation fields created by magmatic and tectonic processes, further

  10. Rift Valley Fever in Namibia, 2010

    PubMed Central

    Monaco, Federica; Pinoni, Chiara; Khaiseb, Siegfried; Calistri, Paolo; Molini, Umberto; Bishi, Alec; Conte, Annamaria; Scacchia, Massimo; Lelli, Rossella

    2013-01-01

    During May–July 2010 in Namibia, outbreaks of Rift Valley fever were reported to the National Veterinary Service. Analysis of animal specimens confirmed virus circulation on 7 farms. Molecular characterization showed that all outbreaks were caused by a strain of Rift Valley fever virus closely related to virus strains responsible for outbreaks in South Africa during 2009–2010. PMID:24274469

  11. Rio Grande rift: problems and perspectives

    SciTech Connect

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

    1984-01-01

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

  12. Rift Valley fever outbreak, southern Mauritania, 2012.

    PubMed

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

    2014-02-01

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

  13. Rift Valley fever in Namibia, 2010.

    PubMed

    Monaco, Federica; Pinoni, Chiara; Cosseddu, Gian Mario; Khaiseb, Siegfried; Calistri, Paolo; Molini, Umberto; Bishi, Alec; Conte, Annamaria; Scacchia, Massimo; Lelli, Rossella

    2013-12-01

    During May-July 2010 in Namibia, outbreaks of Rift Valley fever were reported to the National Veterinary Service. Analysis of animal specimens confirmed virus circulation on 7 farms. Molecular characterization showed that all outbreaks were caused by a strain of Rift Valley fever virus closely related to virus strains responsible for outbreaks in South Africa during 2009-2010.

  14. Detection and Response for Rift Valley fever

    Technology Transfer Automated Retrieval System (TEKTRAN)

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

  15. Magmato-tectonic Evolution of Asal Rift, Afar Depression

    NASA Astrophysics Data System (ADS)

    Pinzuti, P.; Manighetti, I.; Humler, E.

    2001-12-01

    faults started to initiate and accommodate part of the extension. Between ~300 and 90 ka, the magmatic activity seems to be mainly governed by a central volcano (Fieale). From ~90 ka on, it localizes within the present inner floor and becomes fissural. The volumes of lava which we estimate to have emplaced during these two periods of time ( ~20 and 10 km3, respectively) imply a mean magmatic flux rate at the surface of ~105m3/yr, similar to the current rate (1978 Ardoukoba eruption: ~10-20.106 m3, recurrence time ~120-230 yr), but ~10 times smaller than the flux rate inferred at depth. Magmatic activity would therefore have also kept constant overall during rifting, although occurring through successive pulses, the major ones being supplied from at least ~10 km3 reservoirs.

  16. Architecture of the intracontinental Jaibaras Rift, Brazil, based on geophysical data

    NASA Astrophysics Data System (ADS)

    Pedrosa, Nilo C.; Vidotti, Roberta M.; Fuck, Reinhardt A.; Castelo Branco, R. M. G.; Almeida, Afonso R. de; Silva, Nilton C. Vieira; Braga, Luiz R. C.

    2017-03-01

    Qualitative and quantitative integration and interpretation of magnetic, gravity and magnetotelluric data help to determine the internal architecture of the Jaibaras rift, and allow assessing the evolution of the Jaibaras Rift within the Precambrian crystalline basement of Borborema Province, NE Brazil. This was achieved by 2D joint modeling of magnetic and gravity data in five sections across the main axis of the Jaibaras Rift. Surface data, rock density measurements, depth constraints from 2D Euler deconvolution and geophysical information from previous work in the area were integrated to constrain the modeling. The magnetic and gravity profiles of the Jaibaras Rift indicate estimated source bodies at depths up to 2.5 km, showing complex configuration for the structural framework, with a set of asymmetric grabens and horsts. The 2D magnetotelluric inversion shows that the Jaibaras Rift is marked by low resistivity values, and maximum thickness of the sedimentary package up to approximately 3 km. Shallow dipping conductive material may represent either a suture zone between the Ceará Central and Médio Coreaú domains or a set of fractures due to horizontal σ1 stress in the Ceará Central Domain. The Jaibaras Rift displays a very complex internal structure, with discontinuous sequences of grabens and horsts, and a significant volume of surface and subsurface volcanic rocks. The sedimentary packages with volcanic rift sequences have variable thicknesses, from 1 to 3 km. These rock units are controlled by normal faults that developed from older discontinuities, such as the Transbrasiliano lineament.

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

    NASA Astrophysics Data System (ADS)

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

    2004-04-01

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

  18. The development of the Midcontinent Rift in the context of rapid paleogeographic change

    NASA Astrophysics Data System (ADS)

    Swanson-Hysell, N.; Vaughn, A. A.; Mustain, M. R.; Feinberg, J. M.

    2012-12-01

    Despite being active for >20 million years and resulting in the thinning of pre-rift crust by a factor of 3 or more, the 1.1 Ga Midcontinent Rift failed to dismember the Laurentian craton. This failure resulted in the preservation of a thick record of rift-related volcanic and sedimentary rocks that give geoscientists a powerful window into the development of this ancient rift. Most models for the development of the Midcontinent Rift attribute its origin to the upwelling and decompression melting of a mantle plume. On the basis of the great volume of generated magma and interpretation of geochemical data, it is argued that the early stage plateau flood basalts of the rift (~1110-1105 Ma) and the main stage volcanics that erupted into the central basin (~1100-1095 Ma) were both dominated by plume-sourced melts. However, this model needs to be reconciled with paleomagnetic data from rift volcanics that reveal a significant decrease in inclination between the early and main stage volcanics. New data we have developed from 90+ flows of the early stage Osler Volcanic Group bolster evidence from the succession at Mamainse Point that this change in inclination is the result of fast equatorward plate motion during the early stage and into the main stage of rift volcanism. Even with >20° of latitudinal motion from the time of initial volcanism to eruption of the thick main stage volcanics, magmatism was largely confined to the same geographic region in a relatively narrow central basin. If a long-lived plume was in a fixed position relative to Earth's spin axis, the large relative motion of Laurentia would make it unable to continue to be a source of melt to the rift. Two possible explanations to reconcile a plume-contribution in the main stage with this latitudinal change are: (1) That the active contribution from an underlying plume was limited to the early stage of volcanism, but substantial volume of material accreted to the lithosphere that was subsequently sampled

  19. Deep structure of the northern Rio Grande rift beneath the San Luis basin (Colorado) from a seismic reflection survey: implications for rift evolution

    NASA Astrophysics Data System (ADS)

    Tandon, Kush; Brown, Larry; Hearn, Thomas

    1999-02-01

    A seismic reflection survey by Chevron across the San Luis basin (northern Rio Grande rift) and San Juan volcanic field of southern Colorado is reprocessed with extended correlation to search for basement structure. The trace of the main bounding fault of the basin, a high-angle normal fault against the Sangre de Cristo Range, can be correlated to a wide zone of dipping reflection fabric and soles out at lower crustal depths (26-28 km). The deeper reflection fabric represent either broad extensional strain or pre-existing structure, such as a Laramide thrust system. The Sangre de Cristo bounding fault in San Luis basin does not sole out at mid-crustal depths but continues into the lower crust with a shallower dip. The basin architecture in the northern Rio Grande rift (San Luis basin) provides little if any evidence that the Sangre de Cristo bounding fault should flatten in a shallow listric fashion. This fault geometry is quite similar to the high-angle bounding fault in the Espanola basin but contrasts with less deeply-rooted faults in the Albuquerque basin in the central Rio Grande rift. Deeper soling out of the Sangre de Cristo bounding fault could be due to less extension in the northern Rio Grande rift and/or greater strength of the lithosphere compared to the central Rio Grande rift. Unequivocal Moho reflections beneath the San Luis basin cannot be identified, probably due to limited signal penetration or a gradational nature of the Moho. The majority of rift-related movement observed on the Sangre de Cristo bounding fault is post-Eocene. Either the western margin of the basin is marked by a tight monocline or a low-angle normal fault.

  20. Rifted continental margins: The case for depth-dependent extension

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.; Beaumont, Christopher

    2014-12-01

    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 Type I, II, and III margin styles are controlled by the strength of the mid/lower crust, which determines the amount of decoupling between upper and lower lithosphere during extension and the excision of crust or mantle. We also predict the styles of sedimentary basins that form on these margins as a test of the concepts presented.

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

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.; Beaumont, Christopher

    2016-04-01

    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

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

    NASA Astrophysics Data System (ADS)

    Huismans, R. S.; Beaumont, C.

    2015-12-01

    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.

  3. Cenozoic thermal, mechanical and tectonic evolution of the Rio Grande rift

    NASA Technical Reports Server (NTRS)

    Morgan, P.; Seager, W. R.; Golombek, M. P.

    1986-01-01

    Two areas of New Mexico which exhibit complex but similar Cenozoic histories of extensional tectonism are analyzed. The first study area is the Basin and Range province and southern Rio Gande rift in southern New Mexico; the second study area is the central Rio Grande rift in central and northern New Mexico, the southern San Luis basin, the Espanola basin, and the Albuquerque basin. Two phases of extension were identified: the first phase which began in mid-Oligocene was characterized by local high-strain extension events, low-angle faulting, and the development of broad, shallow basins, all indicating an approximately NE-SW + or - 25 deg extension direction; the later phase which occurred primarily in the late Miocene, was characterized by synchronous, high-angle faulting, resulting in large vertical strains which produced the modern Rio Grande rift morphology. Extension direction was approximately E-W. Geotherms were estimated and lithospheric strength curves were calculated for these two phases of extension. A high geotherm was deduced for the early phase resulting in a shallow crustal brittle transition, and insignificant mantle strength. The lithosphere subsequently cooled, resulting in a significant zone of mantle strength beneath the Moho. It is concluded that the interrelationship among regional and local prerifting, synrifting, and postrifting events in the Rio Grande rift attests to the fact that the rifting (in the region studied) should be considered in the context of other geologic events.

  4. 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.

  5. Deformation in a hyperslow oceanic rift: Insights from the tectonics of the São Miguel Island (Terceira Rift, Azores)

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

    The evolution of hyperslow oceanic rifts, like the Terceira Rift (TR) in the Azores, is still poorly understood. Here we examine the distribution of strain and magmatism in the portion of the TR making up the Nubia-Eurasia plate boundary. We use São Miguel Island because it stretches most of the TR width, which allows to investigate the TR's architecture and shedding light on TR's age and mode of deformation. From topography and structural analysis, and new measurements of 380 faults and dikes, we show that (1) São Miguel has two main structural directions, N150 and N110, mostly concentrated in the eastern part of the island as an onshore continuation of the faults observed offshore in the NE (N110 faults) and SW (N140) TR walls; (2) a new N50-N80 fault system is identified in São Miguel; (3) fault and dike geometries indicate that eastern São Miguel comprises the TR's northern boundary, and the lack of major faults in central and western São Miguel indicates that rifting is mostly concentrated at master faults bounding the TR. Based on TR's geometry, structural observations and plate kinematics, we estimate that the TR initiated between 1.4 and 2.7 Ma ago and that there is no appreciable seafloor spreading associated with rifting. Based on plate kinematics, on the new structural data, and on São Miguel's structural and volcanic trends, we propose that the eastern two thirds of São Miguel lie along a main TR-related transform fault striking N70-N80, which connects two widely separated N130-N150 TR-trending segments.

  6. The structures, stratigraphy and evolution of the Gulf of Corinth rift, Greece

    NASA Astrophysics Data System (ADS)

    Taylor, Brian; Weiss, Jonathan R.; Goodliffe, Andrew M.; Sachpazi, Maria; Laigle, Mireille; Hirn, Alfred

    2011-06-01

    A multichannel seismic and bathymetry survey of the central and eastern Gulf of Corinth (GoC), Greece, reveals the offshore fault geometry, seismic stratigraphy and basin evolution of one of Earths most active continental rift systems. Active, right-stepping, en-echelon, north-dipping border faults trend ESE along the southern Gulf margin, significantly overlapping along strike. The basement offsets of three (Akrata-Derveni, Sithas and Xylocastro) are linked. The faults are biplanar to listric: typically intermediate angle (˜35° in the centre and 45-48° in the east) near the surface but decreasing in dip and/or intersecting a low- or shallow-angle (15-20° in the centre and 19-30° in the east) curvi-planar reflector in the basement. Major S-dipping border faults were active along the northern margin of the central Gulf early in the rift history, and remain active in the western Gulf and in the subsidiary Gulf of Lechaio, but unlike the southern border faults, are without major footwall uplift. Much of the eastern rift has a classic half-graben architecture whereas the central rift has a more symmetric w- or u-shape. The narrower and shallower western Gulf that transects the >40-km-thick crust of the Hellenides is associated with a wider distribution of overlapping high-angle normal faults that were formerly active on the Peloponnesus Peninsula. The easternmost sector includes the subsidiary Gulfs of Lechaio and Alkyonides, with major faults and basement structures trending NE, E-W and NW. The basement faults that control the rift architecture formed early in the rift history, with little evidence (other than the Vrachonisida fault along the northern margin) in the marine data for plan view evolution by subsequent fault linkage. Several have maximum offsets near one end. Crestal collapse graben formed where the hanging wall has pulled off the steeper onto the shallower downdip segment of the Derveni Fault. The dominant strikes of the Corinth rift faults

  7. Seismological investigation of the Okavango Rift, Botswana

    NASA Astrophysics Data System (ADS)

    Yu, Youqiang

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

  8. Cenozoic rift tectonics of the Japan Sea

    SciTech Connect

    Kimura, K.

    1988-08-01

    The Japan Sea is one of the back-arc basins in trench-arc systems bordering the western Pacific. Recent paleomagnetic works suggest the Japan Sea opened during early to middle Miocene. Radiometric and microfossil ages of the Cenozoic onland sequences in the Japanese Islands elucidate the rift tectonics of the Japan Sea. The rifting history is summarized as follows: nonmarine volcanic formations of prerift stage before 50 Ma, rift-onset unconformity at 40 Ma, nonmarine volcanic formations of synrift stage 20-33 Ma, breakup unconformity 19 Ma showing the opening of the Japan Sea, marine volcanic and sedimentary formations of synrift stage 14.5-18 Ma, beginning of regional subsidence 14.5 Ma corresponding to the end of the Japan Sea opening, marine sedimentary formations of postdrift stage after 14.5 Ma. Rifting is not limited to the synrift stage but is continued to the syndrift stage. Rifting led to a horst-and-graben structure. Thus, the Cenozoic onland sequences in the Japanese Islands are suited for a study of rift tectonics because the sequences were subaerially exposed by the late Miocene-Holocene island-arc tectonics. Rift tectonics cannot be studied as easily in most Atlantic-type passive margins.

  9. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    -extended rift system. In our presentation, we discuss the compressional reactivation of the rift structures by the study of dip sections across the basin, from weakly reactivated sections in the west to strongly reactivated sections in the east. Comparing the sections, it results that the compression reactivated the rift structures (mainly the detachment faults) and that this reactivation occurred in 2 steps. It corresponds to the reactivation through time of the NMB before the SMB. This evolution is in line with an early proto-subduction of the hyper-extended domain beneath the European plate whereas the NMB sediments are wedged, folded and thrust onto the Iberia and Europe margins ("thin-skin" tectonics). The second step occurs when the deformation started to migrate southward resulting in the formation of the axial Pyrenees nappe stack (thick-skin tectonics). These results suggest that the inherited rift structures strongly controlled the initial convergence. Future work will revisit the more reactivated Albian basins throughout the chain to investigate how far the results from western Pyrenees can be used to understand the Central and Eastern Pyrenees. Moreover, this field-oriented study can serve as an example of how rift structures may control style and timing of orogenic processes.

  11. 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

  12. Lithospheric structural controls on magma composition: the Kenya Rift

    NASA Astrophysics Data System (ADS)

    Omenda, P. A.; Simiyu, S.; Anthony, E. Y.; Keller, G. R.; Dean, R. L.

    2001-12-01

    Lithospheric structure, as delineated by geophysics, plays a fundamental role in both felsic and mafic magmatic compositions in the Kenya Rift. With respect to the mafic rocks, there are, first, silica-undersaturated basanites of the Chyulu Hills. This location is off-axis to the rift, where the lithosphere is thick. The lavas have been modeled as high-pressure, small degree partial melts. This origin contrasts to that for the silica-saturated transitional basalts, basaltic trachy-andesites, and andesites in the axis of the rift. These magmas were generated by higher degrees of partial melt and are also much more evolved, with Mg numbers approximately 40 to 50. The lavas have seen substantial crystal fractionation prior to eruption. An important component of lithospheric structure within the rift axis is the Kenya Dome: it is an area of thick crust and high elevation and heat flow. The crust is made thicker by a 6.8 km/sec lower crustal layer. Immediately below this crust is a very slow upper mantle. Velocities become more lithospheric to the south of the Kenya Dome in the vicinity of Suswa. This lithosphere then thickens southward into Tanzania. The felsic central volcanoes of the rift, which are significant geothermal targets, reflect these lithospheric variations. Eburru and Olkaria are both centered on the Kenya Dome. Eburru is pantellerite and can be modeled as resulting from crystallization of silica-saturated basalt. Olkaria is comendite and resulted from fusion of lower crustal syenite. That we find such distinct petrogenesis for two closely spaced volcanoes indicates that this area of very warm mantle has the temperatures necessary to generate high degree partial melt magmas, which evolve into pantellerites, and also fuse the lower crust. Suswa, which is the southernmost volcano and in the area where lithosphere thickens, is composed on phonolites, which can be modeled as resulting from crystallization of silica-undersaturated mafic parents. Presumably

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

    SciTech Connect

    Wickstrom, L.H. )

    1992-01-01

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

  14. The synrift subsidence deficit at rifted margins

    NASA Astrophysics Data System (ADS)

    Reston, T.

    2009-04-01

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

  15. The inverted Triassic rift of the Marrakech High Atlas: A reappraisal of basin geometries and faulting histories

    NASA Astrophysics Data System (ADS)

    Domènech, Mireia; Teixell, Antonio; Babault, Julien; Arboleya, Maria-Luisa

    2015-11-01

    The High Atlas of Morocco is an aborted rift developed during the Triassic-Jurassic and moderately inverted during the Cenozoic. The Marrakech High Atlas, with large exposures of basement and Triassic early syn-rift deposits, is ideal to investigate the geometries of the deepest parts of a rift, constituting a good analogue for pre-salt domains. It allows unraveling geometries and kinematics of the extensional and compressional structures and the influence that they exert over one another. A detailed structural study of the main Triassic basins and basin-margin faults of the Marrakech High Atlas shows that only a few rift faults were reactivated during the Cenozoic compressional stage in contrast to previous interpretations, and emphasizes that fault reactivation cannot be taken for granted in inverted rift systems. Preserved extensional features demonstrate a dominant dip-slip opening kinematics with strike-slip playing a minor role, at variance to models proposing a major strike-slip component along the main basin-bounding faults, including faults belonging to the Tizi n'Test fault zone. A new Middle Triassic paleogeographic reconstruction shows that the Marrakech High Atlas was a narrow and segmented orthogonal rift (sub-perpendicular to the main regional extension direction which was ~ NW-SE), in contrast to the central and eastern segments of the Atlas rift which developed obliquely. This difference in orientation is attributed to the indented Ouzellarh Precambrian salient, part of the West African Craton, which deflected the general rift trend in the area evidencing the major role of inherited lithospheric anisotropies in rift direction and evolution. As for the Cenozoic inversion, total orogenic shortening is moderate (~ 16%) and appears accommodated by basement-involved large-scale folding, and by newly formed shortcut and by-pass thrusting, with rare left-lateral strike-slip indicators. Triassic faults commonly acted as buttresses.

  16. From Rifting of a volcanic province to Oceanic Spreading in the Andaman Sea, South-East Asia.

    NASA Astrophysics Data System (ADS)

    Jourdain, A.; Singh, S. C.; Klinger, Y.

    2015-12-01

    The Andaman Sea is an enigmatic feature in the Indian Ocean region. It contains the volcanic provinces of Alcock and Sewell Rises and an active spreading center. The recent rifting in the Andaman Sea initiated 4.5 Ma ago, rifting the Alcock and Seawell Rises that were formed by extensive volcanism between 23-16 Ma. The spreading started with a full spreading rate of 1.6 cm/yr and increased to 3.8 cm/yr in the last 2.5 Ma. We have access to high-resolution deep seismic reflection data crossing the whole spreading center from the rifted volcanic provinces to the spreading center. The data show the whole oceanic crust up to the Moho. The Andaman Sea is covered with a thick pile of sediments that record the tectonic history of the rift system up to the spreading axis, allowing to decipher the whole process from rifting to spreading for the first time. We see a very rapid phase of transition from the rifting of the rises to the spreading in less than 20 km. Then a succession of at least 7 half-grabens is well recorded by the sediments on both sides of the spreading center. These half grabens are separated by outward tilted low angle detachment faults, which form the base of steeply dipping normal faults due to stretching. These low angle faults seem to connect with axial magma chambers that control evolution of the rift valley. We find that new detachment faults develop every 350,000 years, forming a new rift valley. The images of active normal faults within the central rift valley allow us to quantify the formation of oceanic crust by tectonic versus magmatic processes.

  17. GPS Constraints on the Spatial Distribution of Extension in the Ethiopian Highlands and Main Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Amere, Y. B.; Bendick, R. O.; Fisseha, S.; Lewi, E.; Reilinger, R. E.; King, R. W.; Kianji, G.

    2014-12-01

    27 campaign and 17 continuous GPS sites spanning the Ethiopian Highlands, Main Ethiopian Rift (MER), and Somali Platform in Ethiopia and Eritrea were measured for varying durations between 1995 and 2014. Velocities at these sites show that present day strain in NE Africa is not localized only in the Afar depression and MER system. Rather, velocities as high as 6 mm/yr relative to stable Nubia occur in the central Ethiopian highlands west of the rift bounding faults; the northern and southern Ethiopian highlands host velocities as high as 3 mm/yr. These approach the magnitude of Nubia-Somalia spreading accommodated within the rift itself of 6 + 1 mm/yr with an azimuth of N770E. The combination of distributed low strain rate deformation contiguous with higher strain rate plate boundary deformation is similar to that expressed in other tectonically active continental settings like Basin and Range and Tibetan Plateau.Keywords: deformation, localized, distributed, strain, stable Nubia.

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

    NASA Technical Reports Server (NTRS)

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

    1986-01-01

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

  19. Deepening, and repairing, the metabolic rift.

    PubMed

    Schneider, Mindi; McMichael, Philip

    2010-01-01

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

  20. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    On ocean basaltic volcanoes, magma transfer to the surface occurs along sub-vertical ascent from the mantle lithosphere through the oceanic crust and the volcanic edifice, eventually followed by lateral propagation along rift zones. We use a 17-years-long database of volcano-tectonic seismic events and a detailed mapping of the pyroclastic cones to determine the geometry and the dynamics of the magma paths intersecting the edifice of Piton de la Fournaise volcano. We show that the overall plumbing system, from about 30 km depth to the surface, is composed of two structural levels that feed distinct types of rift zones. The lower plumbing system has a southeastward (N120) orientation and permits magma transfer from the lithospheric mantle to the base of the La Réunion edifice (5 km bsl). The related rift zone is wide, linear, spotted by small to large pyroclastic cones and related lava flows and involving magma resulting from high-pressure fractionation of ol ± cpx and presents an eruption periodicity of around 200 years over the last 30 kyrs. Seismic data suggest that the long-lasting activity of this rift zone result from a regional NNE-SSW extension reactivating inherited lithospheric faults by the effect of underplating and/or thermal erosion of the mantle lithosphere. The upper plumbing system originates at the base of the edifice in the vertical continuity of the lower plumbing system. It feeds frequent (1 eruption every 9 months on average), short-lived summit and distal (flank) eruptions along summit and outer rift zones, respectively. Summit rift zones are short and present an orthogonal pattern restricted to the central active cone of Piton de la Fournaise whereas outer rift zones extend from inside the Enclos Fouqué caldera to the NE and SE volcano flanks. We show that the outer rift zones are genetically linked to the east flank seaward displacements, whose most recent events where detected in 2004 and 2007. The lateral movements are themselves

  2. Crustal Properties Across the Mid-Continent Rift via Transfer Function Analysis

    NASA Astrophysics Data System (ADS)

    Frederiksen, A. W.; Tyomkin, Y.; Campbell, R.; van der Lee, S.; Zhang, H.

    2015-12-01

    The Mid-Continent Rift (MCR), a failed Proterozoic rift structure in central North America, is a dominant feature of North American gravity maps. The rift underwent a combination of extension, magmatism, and later compression, and it is difficult to predict how these events affected the overall crustal thickness and bulk composition in the vicinity of the rift axis, though the associated gravity high indicates that large-volume mafic magmatism took place. The Superior Province Rifting Earthscope Experiment (SPREE) project instrumented the MCR with Flexible Array broadband seismographs from 2011 through 2013 in Minnesota and Wisconsin, along two lines crossing the rift axis as well as a line following the axis. We examine teleseismic P-coda data from SPREE and nearby Transportable Array instruments using a new technique: transfer-function analysis. In this approach, possible models of crustal structure are used to generate a predicted transfer function relating the radial and vertical components of the P coda at a particular site. The transfer function then allows generation of a misfit (between the true radial component and a synthetic radial component predicted from the vertical trace) without the need to perform receiver-function deconvolution, thus avoiding the deconvolution problems encountered with receiver functions in sedimentary basins. We use the transfer-function approach to perform a grid search over three crustal properties: crustal thickness, crustal P/S velocity ratio, and the thickness of an overlying sedimentary basin. Results for our SPREE/TA data set indicate that the crust is significantly thickened along the rift axis, with maximum thicknesses approaching 50 km; the crust is thinner (ca. 40 km) outside of the rift zone. The crustal thickness structure is particularly complex beneath southeastern Minnesota, where very strong Moho topography is present, as well as up to 2 km of sediment; further north, the Moho is smoother and the basin is not

  3. Transient deformation in the Asal-Ghoubbet Rift (Djibouti) since the 1978 diking event: Is deformation controlled by magma supply rates?

    NASA Astrophysics Data System (ADS)

    Smittarello, D.; Grandin, R.; De Chabalier, J.-B.; Doubre, C.; Deprez, A.; Masson, F.; Socquet, A.; Saad, I. A.

    2016-08-01

    The Asal-Ghoubbet Rift (AG Rift) in Djibouti lies in the subaerial continuation of the Aden ridge system, thereby constituting a unique location to study rifting processes and mechanisms involved in continental breakup and oceanic spreading. Continually upgraded and expanded geodetic technology has been used to record the 1978 Asal rifting event and postdiking deformation. In light of recent results obtained for the Manda Hararo-Dabbahu rifting event (2005-2010), we propose that the horizontal and vertical geodetic data can be modeled with a double source, involving a dike-like inflation component aligned along the rift axis and a spherical pressure source located at midsegment below the Fieale caldera. By revisiting the codiking data, we propose that the reservoir below Fieale could have fed, at least partially, the 1978 injection and the contemporaneous Ardoukôba eruption and potentially induced local subsidence due to magma draining out of the central reservoir. As an alternative to previously proposed viscoelastic relaxation models, we reinterpret postdiking observations using a purely elastic rheology. We determine the relative contribution of a midsegment reservoir inflation and a dike-like opening component, together with their respective time evolutions. Our results suggest that interactions between steadily accumulating tectonic strain and temporal variations in melt supply to the shallow magma plumbing system below the AG Rift may entirely explain the geodetic observations and that viscoelastic deformation processes played a minor role in the 30 years following the 1978 rifting event.

  4. Early Jurassic magnetostratigraphy and paleolatitudes from the Hartford continental rift basin (eastern North America): Testing for polarity bias and abrupt polar wander in association with the central Atlantic magmatic province

    NASA Astrophysics Data System (ADS)

    Kent, Dennis V.; Olsen, Paul E.

    2008-06-01

    To determine whether the ˜200 Ma central Atlantic magmatic province (CAMP) coincides with a normal polarity bias and a purported abrupt change in polar wander at the J1 cusp, we collected samples for paleomagnetic study from 80 sites distributed over a ˜2500-m-thick section of sedimentary units that are interbedded with and overlie CAMP lavas in the Hartford basin, which together represent the initial 2.4 Ma of the Jurassic according to cycle stratigraphic analysis. Characteristic directions carried by hematite were isolated by thermal demagnetization in 71 sites and define a coherent magnetostratigraphy supported by a positive reversal test and an interbasin fold test. Despite a pronounced overall normal polarity bias (only three relatively short reverse polarity intervals could be confirmed in the sampled section), normal polarity Chron H24n that encompasses the CAMP extrusive zone is no more than 1.6 Ma in duration. Elongation/inclination analysis of the 315 characteristic directions, which have a flattened distribution, produces a result in agreement with a published mean direction for the CAMP volcanic units as well as published results similarly corrected for inclination error from the Newark basin. The three data sets (CAMP volcanics, Newark corrected sediments, and Hartford corrected sediments) provide a 201 Ma reference pole for eastern North America at 67.0°N, 93.8°E, A95 = 3.2°. Paleopoles from the Moenave and Wingate formations from the Colorado Plateau that virtually define the J1 cusp can be brought into agreement with the 201 Ma reference pole with corrections for net clockwise rotation of the plateau relative to eastern North America and presumed sedimentary inclination error. The corrected data show that apparent polar wander for North America proceeds directly toward higher latitudes over the Late Triassic and Early Jurassic with no obvious change that can be associated with CAMP.

  5. 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.

  6. Rifting Thick Lithosphere - Canning Basin, Western Australia

    NASA Astrophysics Data System (ADS)

    Czarnota, Karol; White, Nicky

    2016-04-01

    The subsidence histories and architecture of most, but not all, rift basins are elegantly explained by extension of ~120 km thick lithosphere followed by thermal re-thickening of the lithospheric mantle to its pre-rift thickness. Although this well-established model underpins most basin analysis, it is unclear whether the model explains the subsidence of rift basins developed over substantially thick lithosphere (as imaged by seismic tomography beneath substantial portions of the continents). The Canning Basin of Western Australia is an example where a rift basin putatively overlies lithosphere ≥180 km thick, imaged using shear wave tomography. Subsidence modelling in this study shows that the entire subsidence history of the <300 km wide and <6 km thick western Canning Basin is adequately explained by mild Ordovician extension (β≈1.2) of ~120 km thick lithosphere followed by post-rift thermal subsidence. This is consistent with the established model, described above, albeit with perturbations due to transient dynamic topography support which are expressed as basin-wide unconformities. In contrast the <150 km wide and ~15 km thick Fitzroy Trough of the eastern Canning Basin reveals an almost continuous period of normal faulting between the Ordovician and Carboniferous (β<2.0) followed by negligible post-rift thermal subsidence. These features cannot be readily explained by the established model of rift basin development. We attribute the difference in basin architecture between the western and eastern Canning Basin to rifting of thick lithosphere beneath the eastern part, verified by the presence of ~20 Ma diamond-bearing lamproites intruded into the basin depocentre. In order to account for the observed subsidence, at standard crustal densities, the lithospheric mantle is required to be depleted in density by 50-70 kg m-3, which is in line with estimates derived from modelling rare-earth element concentrations of the ~20 Ma lamproites and global isostatic

  7. Complexities in rift initiation and development within the Iceland Plateau, North-Atlantic.

    NASA Astrophysics Data System (ADS)

    Brandsdóttir, Bryndís; Hooft, Emilie; Mjelde, Rolf; Murai, Yoshio

    2015-04-01

    Spreading north of Iceland has been complex since the break-up of the North Atlantic in late Paleocene-early Eocene. Magnetic anomalies within the Iceland Plateau, west of the Ægir Ridge and along the Greenland-Iceland-Faeroe Ridge are irregular, formed by plate boundary complexities at breakup, branched accretion zones, westward rifting relocations, and large overlapping rifts. The 700 km long KRISE7 seismic refraction/reflection and gravity profile, straddles 66.5°N, between the Kolbeinsey and Ægir Ridges, crossing the three physiographic provinces that characterize this region: the Iceland Shelf, Iceland Plateau and Norway Basin. On the basis of crustal thickness and velocity structure, combined with older seismic reflection profiles and drill cores, these provinces correspond to three individual spreading rifts that were active at different time periods. The deep, fan shaped Norway basin was formed during the initial opening of the Atlantic by spreading at the now extinct Ægir Ridge. The oldest crust in the western Norway Basin has thickness 8-10 km and this thins to 4-5 km at the Ægir Ridge reflecting the progressive abandonment of spreading at this rift axis. The eastern Iceland Plateau was the locus of an extinct spreading center, which was segmented and overlapped the Ægir Ridge by 300 km. Spreading on the Iceland Plateau rift occurred simultaneously with that on the Ægir Ridge prior to 26 Ma, when the Kolbeinsey Ridge was initiated by a westward rift jump. The Iceland Plateau rift formed by rifting along the continent-ocean transition at the former central E-Greenland margin, associated with the formation of the Jan Mayen Ridge. Lower crustal domes and corresponding gravity highs across the Iceland Plateau mark the location of the extinct rift axis. The crust at the Iceland Plateau rift is thicker (12-15 km) that that at the conjugate Ægir Ridge and formed under active upwelling conditions (normal lower crustal velocities), which we attribute to

  8. Reappraisal of the relationship between the northern Nevada rift and Miocene extension in the northern Basin and Range Province

    USGS Publications Warehouse

    Colgan, Joseph P.

    2013-01-01

    The northern Nevada rift is a prominent mafic dike swarm and magnetic anomaly in north-central Nevada inferred to record the Middle Miocene (16.5-15.0 Ma) extension direction in the northern Basin and Range province in the western United States. From the 245°-250° rift direction, Basin and Range extension is inferred to have shifted 45° clockwise to a modern direction of 290°-300° during the late Miocene. The region surrounding the northern Nevada rift was actively extending while the rift formed, and these domains are all characterized by extension oriented 280°-300°. This direction is distinctly different from the rift direction and nearly identical to the modern Basin and Range direction. Although the rate, structural style, and distribution of Basin and Range extension appear to have undergone a significant change in the late Miocene (ca. 10 Ma), the overall spreading direction does not. Middle Miocene extension was directed perpendicular to the axis of the thickest crust formed during Mesozoic shortening and this orientation may reflect gravitational collapse of this thick crust. Orientation of northern Nevada rift dikes may reflect a short-lived regional stress field related to the onset of Yellowstone hotspot volcanism.

  9. Sublacustrine precipitation of hydrothermal silica in rift lakes: evidence from Lake Baringo, central Kenya Rift Valley

    NASA Astrophysics Data System (ADS)

    Renaut, R. W.; Jones, B.; Tiercelin, J.-J.; Tarits, C.

    2002-04-01

    Many lakes in volcanic regions are fed by hot springs that, in some basins, can contribute a large percentage of the annual recharge, especially during times of aridity. It is important to recognize any contemporary hydrothermal contribution in paleoenvironmental reconstruction of lake basins because recharge from thermal waters can potentially confuse paleoclimatic signals preserved in the lacustrine sedimentary record. Hot spring deposits (travertine, sinter) provide the most tangible evidence for thermal recharge to lakes. Although subaerial spring deposits have been widely studied, lacustrine thermal spring deposits, especially sublacustrine siliceous sinters, remain poorly known. Detailed field, petrographic and scanning electron microscope (SEM) studies have been made of fossil sublacustrine sinter exposed at Soro hot springs along the northeastern shoreline of Ol Kokwe, a volcanic island in Lake Baringo, Kenya. Modern hot springs at Soro, which discharge Na-HCO 3-Cl waters from a deep reservoir (˜180 °C ), have thin (1-10 mm), friable microbial silica crusts around their subaerial vents, but thicker (>1 cm) sinter deposits are not forming. The fossil sinter, which is present as intergranular cements and crusts in littoral conglomerates and sandstones, is composed mainly of opaline silica (opal-A). Three types of fossil sinter are recognized: (1) massive structureless silica, which fills intergranular pores and forms crusts up to 5 cm thick; (2) pore-lining silica, some of which is isopachous, and (3) laminated silica crusts, which formed mainly on the upper surfaces of detrital particles. All three types contain well-preserved diatoms including lacustrine planktonic forms. Microbial remains, mainly filamentous and coccoid bacteria (including cyanobacteria) and extracellular polymeric gels, are locally abundant in the opaline silica, together with detrital clays and thin laminae composed of authigenic chlorite (?). Most of the hydrothermal silica precipitated when the thermal springs were submerged by fresh lake water. Silica precipitated upon rapid cooling of thermal (˜90 °C ) waters at and just below their interface with the overlying cooler (˜25 °C ) lake waters. Microbial mats locally acted as a filter that limited mixing and rapid dilution of the thermal fluids. Some of the silica originally may have been soft and partly gelatinous. Planktonic diatoms and detrital clay rained down, then became incorporated in the amorphous silica. Following a fall in lake level, the opal-A lithified and partly altered to cristobalite (opal-C) and chalcedony. The lowest fossil sinters were later encrusted by calcite stromatolites, with calcite and quartz forming late pore-filling cements. The age of the sublacustrine sinters is unknown, but some of the deposits could date back to the late Pleistocene. Similar conglomerates cemented by hydrothermal silica are present along fault lines at neighbouring Lake Bogoria. Such rocks may provide evidence for deep, hot fluid recharge to lakes when encountered in the geological record.

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

    NASA Astrophysics Data System (ADS)

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

    2011-02-01

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

  11. Continental Rifting and Transform Faulting Along the Jurassic Transantarctic Rift, Antarctica

    NASA Astrophysics Data System (ADS)

    Schmidt, Dwight L.; Rowley, Peter D.

    1986-04-01

    The Transantarctic rift, an extensional continental rift valley, formed between East and West Antarctica during latest Early and Middle Jurassic time and is represented today by the high Transantarctic Mountains, which contain large volumes of continental flood basalt, diabase, and gabbro. Transantarctic rifting marked the beginning of the breakup of Gondwanaland; it was contiguous and synchronous with continental rifting between East Antarctica-India and Africa as represented by the continental basalt and diabase of Queen Maud Land and the Karroo of southern Africa. During Late Jurassic time, about 150 Ma or slightly earlier, East and West Gondwanaland separated and new oceanic crust of the earliest Indian Ocean formed between East Antarctica-India and Africa. If, as assumed, West Antarctica and South America remained fixed through a tip-to-tip join between the Antarctic Peninsula and Tierra del Fuego, then this seafloor spreading required major right-lateral transform faulting of 500 to 1000 km on the Transantarctic rift system between East and West Antarctica. The Transantarctic Mountains were elevated at about the same time in Late Jurassic; such uplifts are characteristic of active rift margins worldwide. During Cenozoic time, extensional block faulting, independent of the Jurassic rifting, further disrupted large areas of West Antarctica. During the same time, the Transantarctic Mountains were further uplifted.

  12. The Midcontinent rift system and the Precambrian basement in southern Michigan

    SciTech Connect

    Smith, W.A. . Dept. of Geology)

    1994-04-01

    The Precambrian basement within Michigan consists of at least three provinces, each characterized by distinctive potential field anomalies: (1) the Eastern Granite-Rhyolite Province (EGRP) in the south, (2) the Grenville Province in the southeast and (3) the Penokean Province to the north. Also located within the basement is the Mid-Michigan rift (MMR), which is the eastern arm of the Midcontinent rift system (MRS). Southwest and parallel to the MMR is a series of linear positive gravity anomalies which has been referred to as the Ft. Wayne rift (FWR) and the Southwest Michigan Anomaly (SWMA). The EGRP, which is characterized by undeformed and unmetamorphosed rhyolite to dacite and epizonal granites, was emplaced ca. 1510--1450 Ma. However, the EGRP may be comprised of several terranes of varying extent and origin based on analysis of potential field data and rock and mineral ages. The MMR and the FWR/SWMA are characterized by linear arrays of positive magnetic and gravity anomalies, which are probably due to thick accumulations of mafic igneous rocks within the rifts. The extent and trends of the FWR/SWMA have been largely inferred from geophysical data with a presumption of the age of about 1,100 Ma. The continuation of the MMR southward into Ohio and Kentucky as a sequence of gravity highs is questionable and needs further resolution. The FWR/SWMA may be part of the East Continent Rift Basin (ECRB). The ECRB, which is a large complex of related rift basins of Keweenawan age (1300 --1100 Ma), may be an extension of the MRS but it is not physically continuous with it. The ECRB lies to the west of the Grenville Front and extends at least from northwest Ohio to central Kentucky. Extensions of the ECRB north and south are speculative.

  13. Structural framework of the Jaibaras Rift, Brazil, based on geophysical data

    NASA Astrophysics Data System (ADS)

    Pedrosa, Nilo Costa, Jr.; Vidotti, Roberta M.; Fuck, Reinhardt A.; Leopoldino Oliveira, Karen M.; Castelo Branco, R. Mariano G.

    2015-03-01

    The Cambro-Ordovician Jaibaras Rift is a NE-SW trending elongated feature, controlled by the Transbrasiliano lineament, locally known as Sobral-Pedro II shear zone (SPIISZ). An integrated study of geophysical data (gammaspectrometry, magnetometry and gravimetry) was undertaken in the Jaibaras Rift area, between Ceará Central (CCD) and Médio Coreaú domains (MCD), northwest Borborema Province. Geophysical data were interpreted qualitatively and quantitatively in order to understand the tectono-magmatic relations and rift formation based on the main geophysical lineaments, source geometry and depth, and separation of geophysical domains. In addition, a 2D gravity model was generated. The results show a structural partition characterized by NE-SW lineaments and E-W inflexions, where CCD presents a relatively mild magnetic field, whilst the MCD field is more disturbed. The Jaibaras Rift is characterized by positive magnetic and gravity anomalies. The SPIISZ, which corresponds to the SE fault edge of the Jaibaras Rift, is marked by strong magnetic dipoles and strong gravity gradients in the profile, showing the deep character of the Transbrasiliano lineament in the region. The Café-Ipueiras fault, at the NW edge of the rift, is well marked in gravity profiles, but displays low contrast of the magnetic field. Interpretation of the gravimetric anomaly map allowed to recognizing the main NE-SW axis, with alternation of maxima and minima in MCD. A regional gravity gradient reveals significant lateral density variation between the MCD and CCD perpendicular to the SPIISZ, emphasizing it as a main continental suture zone between crustal blocks.

  14. Crustal Structure beneath the Rwenzori Region of the Albertine Rift using Ambient-Noise Tomography

    NASA Astrophysics Data System (ADS)

    Kaviani, A.; Paul, A.; Rumpker, G.

    2015-12-01

    In this study we investigate the crustal structure beneath the Rwenzori region by analyzing a 1-year ambient-noise data set recorded by a network of 33 broadband seismic stations that have operated between September 2009 and August 2011. The Rwenzori region, located between the Democratic Republic of Congo and Uganda, is part of the western (Albertine) branch of the East African Rift System (EARS). The region of study is situated between the Albert Rift and the Edward Rift segments and covers an area of approximately 120 km by 50 km. The main objective of the seismological experiment was to address the questions of the uplift of the Rwenzori Mountains in an extensional regime and the absence of a crustal root beneath the mountain range. Any model proposed to address these questions requires the knowledge of the structure of the Rwenzori horst and surrounding rift shoulders. Previous results from local travel-time tomography revealed the presence of low-velocity anomalies in the upper crust beneath the mountain range relative to higher velocities in the surrounding shoulders. However, since the stations used in the previous study only covered the northern part of the region, the resolution of the models proposed by the body-wave tomography was very low beneath the Rwenzori Mountains. Hence, the limits of the Rwenzori horst at depth relative to the rift shoulders are still poorly known. The main objective of our ambient-noise tomography (ANT) is to provide an explanation for the building of Rwenzori Mountains. Due to the small aperture of the seismological network, we are mainly interested in the shallow crustal structure including the boundaries between the central Rwenzori horst and the surrounding rift shoulders as well as the variations in the thickness of the sedimentary basins. We expect that the ANT images will be able to delineate the boundaries between the main tectonic features including the limits of the Rwenzori horst at depth.

  15. Structure and geochronology of the southern Xainza-Dinggye rift and its relationship to the south Tibetan detachment system

    NASA Astrophysics Data System (ADS)

    Zhang, Jinjiang; Guo, Lei

    2007-03-01

    The Xainza-Dinggye rift is one of several north-south trending rifts in central and southern Tibet created by Cenozoic east-west extension during Indo-Asian convergence. The southern part of the rift cuts through the Tethyan and High Himalayas. In the Tethyan Himalaya, this rift consists of an early domal structure and a late normal fault developed during the progressive deformation. The dome is cored by leucogranitic plutons that intruded during extension. Muscovite 40Ar/ 39Ar ages of the mylonitic leucogranite indicate that extension in the Tethyan Himalaya began at ˜8 Ma or before. In the High Himalaya, the rift is controlled by a normal fault dipping to the southeast. This fault has a structural constitution similar to a detachment fault. Its lower block is made up of mylonitic High Himalayan gneiss, intruded by early mylonitic leucogranite sills and late less-deformed biotite-bearing leucogranite dikes. Mica 40Ar/ 39Ar ages of these leucogranites and the retrograded metamorphosed gneiss of the lower block range from ˜13 to ˜10 Ma. In the study area, the south Tibetan detachment system (STDS) is a ductile shear zone composed of mylonitic leucogranite that is intruded by less-deformed leucogranite and overlain by low grade metamorphic rocks. Mica 40Ar/ 39Ar ages of leucogranites in the shear zone and schist from the detachment hanging wall indicate a protracted deformation history of the STDS from ˜19 to ˜13 Ma. The Xainza-Dinggye rift is younger than the STDS because it offsets the STDS; this north-south trending rift belongs to a different tectonic system from the east-west striking STDS, and may be caused by geological process related to India-Asia convergence. This temporal and spatial relationship of the STDS to the rift may indicate an important change in tectonic regime at ˜13 Ma in the building of the plateau.

  16. Kinematics and Dynamics of Observed Along-Rift Surface Motions in the East African Rift System

    NASA Astrophysics Data System (ADS)

    Stamps, D. S.; Bangerth, W.; Hager, B. H.; Kreemer, C.; Saria, E.

    2015-12-01

    Geodetic observations of Nubian and Somalian plate interiors measure ~E-W divergence across the East African Rift System (EARS), which, in the absence of slab pull forces, is driven by shallow, lithospheric buoyancy and mantle shear tractions. Previous studies indicate the former drives E-W divergence a with minimal role of basal shear. In addition to E-W extension, an increasing number of Global Navigation Satellite System (GNSS) stations within the deforming zones of the EARS detect an along-rift component of motion that is inconsistent with our current understanding of the EARS. In this work we investigate the kinematics and dynamics of these along-rift motions. We first calculate a strain rate and velocity field by fitting bi-cubic Bessel splines to new and existing GNSS observations. We resolve regions of localized compression and transtension within individual rifts that are corroborated by independent seismic and geologic observations. In a second step we test the competing roles of shallow topographic stresses and sub-lithospheric basal shear stresses acting beneath individual rifts where we observe along-rift surface motions using the finite element code ASPECT to solve for Stokes flow in a 3D regional geodynamic model. We compare predicted surface motions and mantle flow directions from our geodynamic simulations with our new continuous deformation model based on GNSS observations. Our work indicates topside driven upper mantle flow directions correspond with anomalous along-rift surface motions in several key locations, but our modeled rheological structure impedes basal shear stresses (<1-3 MPa) from driving surface deformation where we observe along-rift surface motions. This work suggests along-rift surface motions are decoupled from asthenospheric flow.

  17. Submarine thermal springs on the Galapagos Rift

    USGS Publications Warehouse

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

    1979-01-01

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

  18. Early structural development of the Okavango rift zone, NW Botswana

    NASA Astrophysics Data System (ADS)

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

    2007-06-01

    Aeromagnetic and gravity data collected across the Okavango rift zone, northwest Botswana are used to map the distribution of faults, provide insights into the two-dimensional shallow subsurface geometry of the rift, and evaluate models for basin formation as well as the role of pre-existing basement fabric on the development of this nascent continental rift. The structural fabric (fold axes and foliation) of the Proterozoic basement terrane is clearly imaged on both gravity and magnetic maps. The strike of rift-related faults (030-050° in the north and 060-070° in the south) parallels fold axes and the prominent foliation directions of the basement rocks. These pre-existing fabrics and structures represent a significant strength anisotropy that controlled the orientation of younger brittle faults within the stress regime present during initiation of this rift. Northwest dipping faults consistently exhibit greater displacements than southeast dipping faults, suggesting a developing half-graben geometry for this rift zone. However, the absence of fully developed half-grabens along this rift zone suggests that the border fault system is not fully developed consistent with the infancy of rifting. Three en-echelon northeast trending depocenters coincide with structural grabens that define the Okavango rift zone. Along the southeastern boundary of the rift, developing border faults define a 50 km wide zone of subsidence within a larger 150 km wide zone of extension forming a rift-in-rift structure. We infer from this observation that the localization of strain resulting from extension is occurring mostly along the southeastern boundary where the border fault system is being initiated, underscoring the important role of border faults in accommodating strain even during this early stage of rift development. We conclude that incipient rift zones may provide critical insights into the development of rift basins during the earliest stages of continental rifting.

  19. 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.

  20. Upwarp of anomalous asthenosphere beneath the Rio Grande rift

    USGS Publications Warehouse

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

    1984-01-01

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

  1. Oil exploration in nonmarine rift basins of interior Sudan

    SciTech Connect

    Schull, T.J.

    1984-04-01

    In early 1975 Chevron Overseas Petroleum Inc. commenced a major petroleum exploration effort in previously unexplored interior Sudan. With the complete cooperation of the Sudanese Government, Chevron has acquired a vast amount of geologic and geophysical data during the past 9 years. These data include extensive aeromagnetic and gravity surveys, 25,000 mi (40,200 km) of seismic data, and the results of 66 wells. This information has defined several large rift basins which are now recognized as a major part of the Central African rift system. The sedimentary basins of interior Sudan are characterized by thick Cretaceous and Tertiary nonmarine clastic sequences. Over 35,000 ft (10,600 m) of sediment have been deposited in the deepest trough, and extensive basinal areas are underlain by more than 20,000 ft (6100 m) of sediment. The depositional sequence includes thick lacustrine shales and claystones, flood plain claystones, and lacustrine, fluvial, and alluvial sandstones and conglomerates. Those lacustrine claystones which were deposited in an anoxic environment provide oil-prone source rocks. Reservoir sandstones have been found in a wide variety of nonmarine sandstone facies. The extensional tectonism which formed these basins began in the Early Cretaceous. Movement along major fault trends continued intermittently into the Miocene. This deformation resulted in a complex structural history which led to the formation of several deep fault-bounded troughs, major interbasin high trends, and complex basin flanks. This tectonism has created a wide variety of structures, many of which have become effective hydrocarbon traps.

  2. 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.

  3. 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

  4. Molecular epidemiology of Rift Valley fever virus.

    PubMed

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

    2011-12-01

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

  5. New perspectives on the geometry of the Albuquerque Basin, Rio Grande rift, New Mexico: Insights from geophysical models of rift-fill thickness

    USGS Publications Warehouse

    Grauch, V. J.; Connell, Sean D.

    2013-01-01

    central part of the Belen subbasin suggests a possible path for the ancestral Rio Grande during late Miocene or early Pliocene time. Variations in rift-fill thickness correspond to pre-rift structures in several places, suggesting that a better understanding of pre-rift history may shed light on debates about structural inheritance within the rift.

  6. Paleomagnetic evidence of oblique rift localization in the Gulf of California

    NASA Astrophysics Data System (ADS)

    Bennett, S. E.; Oskin, M. E.

    2012-12-01

    Paleomagnetic analysis of extensive Miocene ignimbrites in northwest Mexico reveals the timing and distribution of dextral shear associated with the inception of the Gulf of California rift. We drilled new high-precision paleomagnetic reference sites for the regionally extensive 12.5 Ma Tuff of San Felipe (SF) and the 6.4 Ma Tuffs of Mesa Cuadrada (MC) in undeformed, mesa-top exposures in north-central Baja California, west of the rift-bounding San Pedro Martír fault system. The paleomagnetic remanence directions determined at these tectonically stable sites in central Baja California (DSF=212.4°, ISF=-3.0°, nSF=48; DMC=15.6°, IMC=56.2°, nMC=92) are consistent over tens of kilometers. These directions are counter-clockwise from directions measured at most previously published paleomagnetic sites within the rift, an indication that fault blocks within the rift have experienced variable clockwise vertical-axis rotation. Additionally, α95 confidence cones at these new sites (SF=1.3°, MC=1.0°) are smaller than at the previously defined reference site at Mesa Cuadrada (SF=4.1°, MC=8.9°), where few cores were drilled (nSF=6, nMC=3). Comparisons of new paleomagnetic remanence directions from central Baja California with directions from previous intra-rift studies indicate clockwise vertical-axis rotations for SF and MC up to 76 ± 11° and 40 ± 3°, respectively. An important exception is the easternmost SF drill site in central Sonora, where no clockwise rotation has occurred since 12.5 Ma. Although SF is tilted gently to the west at this site, it appears to lie beyond the limit of clockwise vertical-axis rotation related to oblique rifting. The mean magnitudes of the rotation errors (bar∆RSF= 4.3°, bar∆RMC= 6.3°) using these new paleomagnetic sites are lower than using the previous reference sites at Mesa Cuadrada (bar∆RSF=5.6°, bar∆RMC=13.2°), owing mostly to the larger number of cores collected at the new reference sites. At 10 of 11 paired

  7. Rift kinematics during the incipient stages of continental extension: Evidence from the nascent Okavango rift basin, northwest Botswana

    NASA Astrophysics Data System (ADS)

    Modisi, M. P.; Atekwana, E. A.; Kampunzu, A. B.; Ngwisanyi, T. H.

    2000-10-01

    High-resolution aeromagnetic data from the nascent Okavango rift in northwest Botswana provide an unprecedented view of rift kinematics during the incipient stages of continental extension. Crosscutting relationships between west-northwest trending 180 Ma Karoo dikes and reactivated northeast-trending Proterozoic basement faults are used to document the kinematics of Cenozoic faulting during the initial stages of rifting. Depth estimates to the top of the dikes using three-dimensional Euler deconvolution solutions have produced the following interpretations. (1) The Okavango rift is a half graben with a downthrow of ˜200 300 m. (2) The width of the Okavango rift (100 ± 20 km) is similar to that of more mature continental rifts such as the Tanganyika and Baikal rifts. This suggests that the width of continental rifts is acquired early in their evolution and reflects neither the age and maturity of the rift basin, nor the amount of extension. It is suggested that the cumulative downthrow (sediment infill included) and subsidence may be a better indicator of the relative maturity of rift basins. (3) Preexisting basement faults exert a major control during rifting, and reactivation processes do not occur synchronously along the entire length of preexisting faults. (4) The Okavango rift is defined by normal faults; there is no evidence of major strike-slip faults, thus excluding a pull-apart tectonic model for this nascent continental rift stage. (5) The preexisting Sekaka shear zone terminates the Okavango rift to the south, suggesting that such shear zones represent major barriers during longitudinal propagation of rifts. This probably explains why such shear zones commonly evolve into accommodation or transfer zones during further evolution of continental rifts.

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

    NASA Astrophysics Data System (ADS)

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

    2014-09-01

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

  9. 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

  10. Tectonic Geomorphology and Geodynamics of Rifting: Goodenough Basin, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Collier, R. E.

    2006-12-01

    limited in the last few decades. But the tectonic geomorphology and implied uplift rates on its southern margin imply a significant seismic hazard, if controlled by seismogenic displacement events. Continental lithospheric extension west of the propagating tip of the Woodlark Basin has two distinct behaviours: To the north of the Goodenough Basin lie the documented metamorphic core complexes, with evidence for shallow-dipping, seismogenic normal faults. The southern continental rift margin and the Goodenough Basin are characterized by high angle faults and tectonic geomorphologies analogous to rift margins in central Greece or the Gulf of Suez, suggesting different lithospheric conditions. The region thus offers an ideal locale for studying the crustal conditions that determine extensional geodynamic states.

  11. 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

    vary from grabens bounded by high angle normal faults (analogue to the geometry of the Upper Rhine Graben & North Sea Central graben) to listric faults rooting on a basal detachment defining a more asymmetric system (similar to the geometry of the golf of Corinth rift).

  12. Historical volcanism and the state of stress in the East African Rift System

    NASA Astrophysics Data System (ADS)

    Wadge, Geoffrey; Biggs, Juliet; Lloyd, Ryan; Kendall, Michael

    2016-09-01

    Crustal extension at the East African Rift System (EARS) should, as a tectonic ideal, involve a stress field in which the direction of minimum horizontal stress is perpendicular to the rift. A volcano in such a setting should produce dykes and fissures parallel to the rift. How closely do the volcanoes of the EARS follow this? We answer this question by studying the 21 volcanoes that have erupted historically (since about 1800) and find that 7 match the (approximate) geometrical ideal. At the other 14 volcanoes the orientation of the eruptive fissures/dykes and/or the axes of the host rift segments are oblique to the ideal values. To explain the eruptions at these volcanoes we invoke local (non-plate tectonic) variations of the stress field caused by: crustal heterogeneities and anisotropies (dominated by NW structures in the Protoerozoic basement), transfer zone tectonics at the ends of offset rift segments, gravitational loading by the volcanic edifice (typically those with 1-2 km relief) and magmatic pressure in central reservoirs. We find that the more oblique volcanoes tend to have large edifices, large eruptive volumes and evolved and mixed magmas capable of explosive behaviour. Nine of the volcanoes have calderas of varying ellipticity, 6 of which are large, reservoir-collapse types mainly elongated across rift (e.g. Kone) and 3 are smaller, elongated parallel to the rift and contain active lava lakes (e.g. Erta Ale), suggesting different mechanisms of formation and stress fields. Nyamuragira is the only EARS volcano with enough sufficiently well-documented eruptions to infer its long-term dynamic behaviour. Eruptions within 7 km of the volcano are of relatively short duration (<100 days), but eruptions with more distal fissures tend to have greater obliquity and longer durations, indicating a changing stress field away from the volcano. There were major changes in long-term magma extrusion rates in 1977 (and perhaps in 2002) due to major along-rift dyking

  13. Kinematics of the Ethiopian Rift and Absolute motion of Africa and Somalia Plates

    NASA Astrophysics Data System (ADS)

    Muluneh, A. A.; Cuffaro, M.; Doglioni, C.

    2013-12-01

    The Ethiopian Rift (ER), in the northern part of East African Rift System (EARS), forms a boundary zone accommodating differential motion between Africa and Somalia Plates. Its orientation was influenced by the inherited Pan-African collisional system and related lithospheric fabric. We present the kinematics of ER derived from compilation of geodetic velocities, focal mechanism inversions, structural data analysis, and construction of geological profiles. GPS velocity field shows a systematic eastward magnitude increase in NE direction in the central ER. In the same region, incremental extensional strain axes recorded by earthquake focal mechanism and fault slip inversion show ≈N1000E orientation. This deviation between GPS velocity trajectories and orientation of incremental extensional strain is developed due to left lateral transtensional deformation. This interpretation is consistent with the en-échelon pattern of tensional and transtensional faults, the distribution of the volcanic centers, and the asymmetry of the rift itself. Small amount of vertical axis blocks rotation, sinistral strike slip faults and dyke intrusions in the rift accommodate the transtensional deformation. We analyzed the kinematics of ER relative to Deep and Shallow Hot Spot Reference Frames (HSRF). Comparison between the two reference frames shows different kinematics in ER and also Africa and Somalia plate motion both in magnitude and direction. Plate spreading direction in shallow HSRF (i.e. the source of the plumes locates in the asthenosphere) and the trend of ER deviate by about 27°. Shearing and extension across the plate boundary zone contribute both to the style of deformation and overall kinematics in the rift. We conclude that the observed long wavelength kinematics and tectonics are consequences of faster SW ward motion of Africa than Somalia in the shallow HSRF. This reference frame seems more consistent with the geophysical and geological constraints in the Rift. The

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  15. Stratigraphy and rifting history of the Mesozoic-Cenozoic Anza rift, Kenya

    SciTech Connect

    Winn, R.D. Jr.; Steinmetz, J.C. ); Kerekgyarto, W.L. )

    1993-11-01

    Lithological and compositional relationships, thicknesses, and palynological data from drilling cuttings from five wells in the Anza rift, Kenya, indicate active rifting during the Late Cretaceous and Eocene-Oligocene. The earlier rifting possibly started in the Santonian-Coniacian, primarily occurred in the Campanian, and probably extended into the Maastrichtian. Anza rift sedimentation was in lacustrine, lacustrine-deltaic, fluvial, and flood-basin environments. Inferred synrift intervals in wells are shalier, thicker, more compositionally immature, and more poorly sorted than Lower Cretaceous ( )-lower Upper Cretaceous and upper Oligocene( )-Miocene interrift deposits. Synrift sandstone is mostly feldspathic or arkosic wacke. Sandstone deposited in the Anza basin during nonrift periods is mostly quartz arenite, and is coarser and has a high proportion of probable fluvial deposits relative to other facies. Volcanic debris is absent in sedimentary strata older than Pliocene-Holocene, although small Cretaceous intrusions are present in the basin. Cretaceous sandstone is cemented in places by laumontite, possibly recording Campanian extension. Early Cretaceous history of the Anza basin is poorly known because of the limited strata sampled; Jurassic units were not reached. Cretaceous rifting in the Anza basin was synchronous with rifting in Sudan and with the breakup and separation of South America and Africa; these events likely were related. Eocene-Oligocene extension in the Anza basin reflects different stresses. The transition from active rifting to passive subsidence in the Anza basin at the end of the Neogene, in turn, records a reconfigured response of east African plates to stresses and is correlated with formation of the East Africa rift.

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

    NASA Technical Reports Server (NTRS)

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

    1993-01-01

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

  17. Patterns of Quaternary uplift of the Corinth rift southern border (N Peloponnese, Greece) revealed by fluvial landscape morphometry

    NASA Astrophysics Data System (ADS)

    Demoulin, A.; Beckers, A.; Hubert-Ferrari, A.

    2015-10-01

    The Rift of Corinth is a world-class example of young active rifting and, as such, is an ideal natural laboratory of continental extension. However, though much investigated for two decades, several aspects of the mechanisms at work are still poorly understood. The aim of this paper is a detailed morphometric study of the fluvial landscape response to the tectonic uplift of the rift southern shoulder in order to reconstruct the rift's Quaternary evolution, with special attention to timing, location, and intensity of uplift episodes. Based on the use of a large set of catchment and long profile metrics complemented by the new R/SR integrative approach of the regional drainage network, we identified three distinct episodes of uplift of the northern Peloponnese coastal tract, of which the intermediate one, dated around 0.35-0.4 Ma, is only recorded in the topography of the central part of the rift shoulder, and the youngest one appears to have propagated from east to west over the last 10-20 ka. While net uplift remained minimum in the eastern part of the study area during the whole Quaternary, it shows a clear maximum in the central part of the rift shoulder since 0.4 Ma and an eastward shift of this maximum in recent times. Maximum uplift rates calculated from the morphometric data are of > 1.05 and 2-5 mm year- 1 for, the mid-Middle Pleistocene and Holocene uplift episodes, respectively. The morphometric evidence reveals an onshore uplift history remarkably consistent with the rift evolution reconstructed from other data sets. In the long term, it shows a stable pattern of maximum activity in the central part of the rift, confirming previous conclusions about the absence of rift propagation. In the short term, it sheds light on a possible E-W migration of the zone of recent uplift, suggesting that in the near future fault activity and seismic hazard might concentrate in the Heliki-Aegion area, at the western tip of this uplift wave.

  18. The 1973 Ethiopian-Rift geodimeter survey

    NASA Technical Reports Server (NTRS)

    Mohr, P. A.

    1974-01-01

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

  19. Molecular Rift: Virtual Reality for Drug Designers.

    PubMed

    Norrby, Magnus; Grebner, Christoph; Eriksson, Joakim; Boström, Jonas

    2015-11-23

    Recent advances in interaction design have created new ways to use computers. One example is the ability to create enhanced 3D environments that simulate physical presence in the real world--a virtual reality. This is relevant to drug discovery since molecular models are frequently used to obtain deeper understandings of, say, ligand-protein complexes. We have developed a tool (Molecular Rift), which creates a virtual reality environment steered with hand movements. Oculus Rift, a head-mounted display, is used to create the virtual settings. The program is controlled by gesture-recognition, using the gaming sensor MS Kinect v2, eliminating the need for standard input devices. The Open Babel toolkit was integrated to provide access to powerful cheminformatics functions. Molecular Rift was developed with a focus on usability, including iterative test-group evaluations. We conclude with reflections on virtual reality's future capabilities in chemistry and education. Molecular Rift is open source and can be downloaded from GitHub.

  20. Rift Valley fever: A neglected zoonotic disease?

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a serious viral disease of animals and humans in Africa and the Middle East that is transmitted by mosquitoes. First isolated in Kenya during an outbreak in 1930, subsequent outbreaks have had a significant impact on animal and human health, as well as national economies. ...

  1. Reemergence of Rift Valley fever, Mauritania, 2010.

    PubMed

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

    2014-02-01

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

  2. Prediction of a Rift Valley fever Outbreak

    Technology Transfer Automated Retrieval System (TEKTRAN)

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

  3. Diagnostic approaches for Rift Valley Fever

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Disease outbreaks caused by arthropod-borne animal viruses (arboviruses) resulting in significant livestock and economic losses world-wide appear to be increasing. Rift Valley fever (RVF) virus (RVFV) is an important arbovirus that causes lethal disease in cattle, camels, sheep and goats in Sub-Saha...

  4. 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.

  5. Mesozoic-Cenozoic rift-drift sequence of Asian fragments from Gondwanaland

    NASA Astrophysics Data System (ADS)

    Audley-Charles, M. G.; Ballantyne, P. D.; Hall, R.

    1988-12-01

    The University of Cambridge Atlas map-plotting computer program has been used to plot a reconstruction of the dispersal of continental blocks from eastern Gondwanaland from the Late Jurassic (160 Ma) until the Late Miocene (10 Ma). The geological basis for the reconstruction rests on the concept of South Tibet, Burma, western Thailand, Malaya and Sumatra having been part of the eastern Gondwana continent during the late Palaeozoic and early Mesozoic. The reconstruction is also based on the view that these Asian continental blocks were rifted from eastern Gondwanaland in the Late Jurassic. This rifting episode gave rise to the continental margin of central New Guinea and northern Australia. It was associated with the sea-floor spreading anomalies preserved in the floor of the northeastern Indian Ocean. The model considers the indication of crustal uplift in the Middle Jurassic followed by profound subsidence in the Cretaceous and Palaeogene, exposed in the islands of the Outer Banda Arc from Seram to Timor, and the similar Jurassic-Cretaceous facies in eastern Sulawesi implies that these areas formed part of this rifted northern margin of Australia in the Jurassic. Subsequent tectonic collision and associated major strike-slip movements have deformed and disrupted these rock sequences during the Cainozoic. The date of the rifting that removed South Tibet, Burma, western Thailand and Malaya from Gondwanaland is controversial. The indications in favour of the rifting episode having occurred in the Jurassic rather than in the Permo-Triassic are stratigraphie-structural observations on the northwestern Australian shelf, in the Banda Arc and in central New Guinea. The identification of the continental blocks removed by this rifting is based partly on stratigraphie correlation with the Banda Arc and New Guinea and partly on indications that the age of collision of Tibet, Burma, western Thailand and Malaya with Asia was a Late Cretaceous event and not Triassic-Jurassic as

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

    NASA Astrophysics Data System (ADS)

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

    2014-08-01

    Factors controlling the origin of silicic magmas on Iceland are poorly constrained. Here we present new data on H2O content, pressure, temperature, oxygen fugacity, and oxygen isotope composition of rhyolites from Askja, Öræfajökull, and Hekla volcanoes. All these parameters correlate with tectonic (rift and off-rift) setting of the volcanoes. Askja rift rhyolites originate through extensive assimilation of high-temperature hydrothermally altered crust (δ18O < 2‰) at shallow depths (≥1.8 km). These rhyolites are hot (935-1008°C), relatively dry (H2O < 2.7 wt%), and oxidized (QFM = +1.4). Cooler (874-902°C), wet (H2O = 4-6.3 wt%), and non-oxidized (~QFM to QFM-1) off-rift rhyolites (Öræfajökull, Hekla) originate through differentiation deeper in the crust (≥4 km) with almost no or little assimilation of high-T, altered crust, as reflected by slightly lower to normal δ18O values (5.2-6‰). Although off-rift rhyolites predominate during the Holocene, older silicic rocks on Iceland primarily formed in a rift setting possibly analogous to the oldest continental crust on Earth.

  7. Seismic anisotropy beneath the incipient Okavango rift: Implications for rifting initiation

    NASA Astrophysics Data System (ADS)

    Yu, Youqiang; Gao, Stephen S.; Moidaki, Moikwathai; Reed, Cory A.; Liu, Kelly H.

    2015-11-01

    This study represents the first shear-wave splitting investigation of the Okavango rift zone (ORZ), an incipient continental rift belonging to the East African rift system in northern Botswana. Analysis of broadband seismic data recorded along a 750 km long profile of 22 stations traversing the ORZ and adjacent Congo and Kalahari cratons and several Precambrian orogenic zones reveals dominantly NE-SW fast orientations, which are parallel to both the absolute plate motion direction (based on the NNR-NUVEL-1A model) and the trend of most tectonic boundaries, including that of the ORZ. Spatial coherence analysis of the splitting parameters and correspondence between the observed fast orientations and the trend of tectonic features indicate that the main source of observed anisotropy is most likely in the upper asthenosphere, probably due to simple shear associated with the relative movement of the lithosphere against the asthenosphere. The presence of consistently rift-parallel fast orientations and normal splitting times in the ORZ and most parts of southern Africa implies that neither an upper mantle plume nor small-scale convection is the dominant source for rift initiation and development. The first shear-wave splitting measurements in the vicinity of the ORZ favor a model in which continental rifting develops in response to intra-plate relative movement of continental blocks along zones of weakness produced by ancient tectonic events.

  8. Rift Valley fever during rainy seasons, Madagascar, 2008 and 2009.

    PubMed

    Andriamandimby, Soa Fy; Randrianarivo-Solofoniaina, Armand Eugène; Jeanmaire, Elisabeth M; Ravololomanana, Lisette; Razafimanantsoa, Lanto Tiana; Rakotojoelinandrasana, Tsanta; Razainirina, Josette; Hoffmann, Jonathan; Ravalohery, Jean-Pierre; Rafisandratantsoa, Jean-Théophile; Rollin, Pierre E; Reynes, Jean-Marc

    2010-06-01

    During 2 successive rainy seasons, January 2008 through May 2008 and November 2008 through March 2009, Rift Valley fever virus (RVFV) caused outbreaks in Madagascar. Human and animal infections were confirmed on the northern and southern coasts and in the central highlands. Analysis of partial sequences from RVFV strains showed that all were similar to the strains circulating in Kenya during 2006-2007. A national cross-sectional serologic survey among slaughterhouse workers at high risk showed that RVFV circulation during the 2008 outbreaks included all of the Malagasy regions and that the virus has circulated in at least 92 of Madagascar's 111 districts. To better predict and respond to RVF outbreaks in Madagascar, further epidemiologic studies are needed, such as RVFV complete genome analysis, ruminant movement mapping, and surveillance implementation.

  9. Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia

    USGS Publications Warehouse

    Pallister, J.S.; McCausland, W.A.; Jonsson, Sigurjon; Lu, Zhiming; Zahran, H.M.; El, Hadidy S.; Aburukbah, A.; Stewart, I.C.F.; Lundgren, P.R.; White, R.A.; Moufti, M.R.H.

    2010-01-01

    The extensive harrat lava province of Arabia formed during the past 30 million years in response to Red Sea rifting and mantle upwelling. The area was regarded as seismically quiet, but between April and June 2009 a swarm of more than 30,000 earthquakes struck one of the lava fields in the province, Harrat Lunayyir, northwest Saudi Arabia. Concerned that larger damaging earthquakes might occur, the Saudi Arabian government evacuated 40,000 people from the region. Here we use geologic, geodetic and seismic data to show that the earthquake swarm resulted from magmatic dyke intrusion. We document a surface fault rupture that is 8 km long with 91 cm of offset. Surface deformation is best modelled by the shallow intrusion of a north-west trending dyke that is about 10 km long. Seismic waves generated during the earthquakes exhibit overlapping very low- and high-frequency components. We interpret the low frequencies to represent intrusion of magma and the high frequencies to represent fracturing of the crystalline basement rocks. Rather than extension being accommodated entirely by the central Red Sea rift axis, we suggest that the broad deformation observed in Harrat Lunayyir indicates that rift margins can remain as active sites of extension throughout rifting. Our analyses allowed us to forecast the likelihood of a future eruption or large earthquake in the region and informed the decisions made by the Saudi Arabian government to return the evacuees.

  10. Strain distribution across magmatic margins during the breakup stage: Seismicity patterns in the Afar rift zone

    NASA Astrophysics Data System (ADS)

    Brown, C.; Ebinger, C. J.; Belachew, M.; Gregg, T.; Keir, D.; Ayele, A.; Aronovitz, A.; Campbell, E.

    2008-12-01

    Fault patterns record the strain history along passive continental margins, but geochronological constraints are, in general, too sparse to evaluate these patterns in 3D. The Afar depression in Ethiopia provides a unique setting to evaluate the time and space relations between faulting and magmatism across an incipient passive margin that formed above a mantle plume. The margin comprises a high elevation flood basalt province with thick, underplated continental crust, a narrow fault-line escarpment underlain by stretched and intruded crust, and a broad zone of highly intruded, mafic crust lying near sealevel. We analyze fault and seismicity patterns across and along the length of the Afar rift zone to determine the spatial distribution of strain during the final stages of continental breakup, and its relation to active magmatism and dike intrusions. Seismicity data include historic data and 2005-2007 data from the collaborative US-UK-Ethiopia Afar Geodynamics Project that includes the 2005-present Dabbahu rift episode. Earthquake epicenters cluster within discrete, 50 km-long magmatic segments that lack any fault linkage. Swarms also cluster along the fault-line scarp between the unstretched and highly stretched Afar rift zone; these earthquakes may signal release of stresses generated by large lateral density contrasts. We compare Coulomb static stress models with focal mechanisms and fault kinematics to discriminate between segmented magma intrusion and crank- arm models for the central Afar rift zone.

  11. Mesozoic rift basins in western desert of Egypt, their southern extension and impact on future exploration

    SciTech Connect

    Taha, M.A. )

    1988-08-01

    Rift basins are a primary target of exploration in east, central, and west Africa. These intracratonic rift basins range in age from the Triassic to the Neogene and are filled with lagoonal-lacustrine sand-shale sequences. Several rift basins may be present in the Western Desert of Egypt. In the northeastern African platform, the Mesozoic Tethyan strand lines were previously interpreted to have limited southern extension onto the continent. This concept, based upon a relatively limited amount of subsurface data, has directed and focused the exploration for oil and gas to the northernmost 120 km of the Western Desert of Egypt. Recent well and geophysical data indicate a southerly extension of mesozoic rift basins several hundred kilometers inland from the Mediterranean Sea. Shushan/Faghur and Abu Gharadig/Bahrein basins may represent subparallel Mesozoic basins, trending northeast-southwest. Marine Oxfordian-Kimmeridgian sediments were recently reported from wells drilled approximately 500 km south of the present-day Mediterranean shoreline. The link of these basins with the Sirte basin to the southwest in Libya is not well understood. Exploration is needed to evaluate the hydrocarbon potential of such basins.

  12. Revised Eocene-Oligocene kinematics for the West Antarctic rift system

    NASA Astrophysics Data System (ADS)

    Granot, R.; Cande, S. C.; Stock, J. M.; Damaske, D.

    2013-01-01

    Abstract<p label="1">Past plate motion between East and West Antarctica along the West Antarctic <span class="hlt">rift</span> system had important regional and global implications. Although extensively studied, the kinematics of the <span class="hlt">rift</span> during Eocene-Oligocene time still remains elusive. Based on a recent detailed aeromagnetic survey from the Adare and Northern Basins, located in the northwestern Ross Sea, we present the first well-constrained kinematic model with four rotations for Anomalies 12o, 13o, 16y, and 18o (26.5-40.13 Ma). These rotation poles form a cluster suggesting a stable sense of motion during that period of time. The poles are located close to the <span class="hlt">central</span> part of the <span class="hlt">rift</span> implying that the local motion varied from extension in the western Ross Sea sector (Adare Basin, Northern Basin, and Victoria Land Basin) to dextral transcurrent motion in the Ross Ice Shelf and to oblique convergence in the eastern end of the <span class="hlt">rift</span> zone. The results confirm previous estimates of 95 km of extension in the Victoria Land Basin.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2003AGUFM.P31C..06N','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2003AGUFM.P31C..06N"><span id="translatedtitle"><span class="hlt">Rifting</span> and Faulting on icy Satellites</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Nimmo, F.</p> <p>2003-12-01</p> <p>Two kinds of <span class="hlt">rifting</span> have been identified on the icy Galilean satellites [1,2]. Europa possesses ˜10~km wide extensional bands, characterized by very high degrees of local extension, internal deformation on a lengthscale of ˜~100~m, and a general resemblance to mid-ocean ridges on Earth [3]. Ganymede has ˜100~km wide areas of grooved terrain, characterized by km-scale tilted fault blocks [4,5], lower degrees of local extension (stretching factor <1.6 [5]) and a general resemblance to continental <span class="hlt">rifts</span> on Earth [1]. The characteristic spacing of faults on Europa and Ganymede has been used to infer the depth to the brittle-ductile transition (BDT), which depends on the strain rate and the shell thickness [4,6]. Here I present another constraint on these quantities, obtained by considering the circumstances under which narrow (Europa-style) or wide (Ganymede-style) <span class="hlt">rifts</span> may form. The model is based on an analysis of terrestrial continent <span class="hlt">rifting</span> [7]. When an ice shell is extended, the thermal gradient increases and it becomes weaker, favouring further extension. The extension also gives rise to lateral shell thickness variations, which oppose further extension. However, these lateral thickness variations may be removed if the base of the ice shell can flow rapidly. If lateral flow is rapid, narrow zones of extension and high stretching factors are generated. If lateral flow is slow, wider <span class="hlt">rifts</span> and lower stretching factors are favoured. Thick ice shells or high strain rates favour narrow <span class="hlt">rifts</span>; thin ice shells or low strain rates favour wide <span class="hlt">rifts</span>. The existence of wide <span class="hlt">rifts</span> on Ganymede is consistent with a conductive shell thickness of 4-8~km at the time of <span class="hlt">rifting</span>, and agrees with previous estimates of strain rates [8]. To produce narrow <span class="hlt">rifting</span> and the inferred BDT depth on Europa requires a larger shell thickness (8-20~km) and a strain rate >= 10-15~s-1. Based on the likely shell thicknesses, the inferred strain rates for Europa and Ganymede can be explained</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/1986JGR....91.6142K','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/1986JGR....91.6142K"><span id="translatedtitle">Introduction to Special Section on the Rio Grande <span class="hlt">Rift</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Keller, G. R.</p> <p>1986-05-01</p> <p>With the aid of a Penrose Conference in 1974 and an international <span class="hlt">rift</span> conference held in 1978 in Santa Fe, New Mexico, the Rio Grande <span class="hlt">rift</span> has become widely recognized as a major Cenozoic continental <span class="hlt">rift</span> zone. As a result of the 1978 Santa Fe meeting, the American Geophysical Union published a special volume of papers concerned with the Rio Grande <span class="hlt">rift</span> [Riecker, 1979], and the New Mexico Geological Society recently published another volume focused on this <span class="hlt">rift</span> [Baldridge et al., 1984]. These volumes are a manifestation of the research activity which lead to the formation of the Rio Grande <span class="hlt">rift</span> consortium whose purpose is to foster <span class="hlt">rift</span>-related research and communication. This organization has sponsored several special sessions at geological and geophysical meetings and has generally increased the awareness of this important feature.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2009AGUFM.V13E2071A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2009AGUFM.V13E2071A"><span id="translatedtitle">Dike injection and magma mixing in Kenya <span class="hlt">rift</span> volcanoes</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Anthony, E. Y.; Espejel, V.; Biggs, J.</p> <p>2009-12-01</p> <p>A nexus of volcanoes in the <span class="hlt">rift</span> graben at approximately the latitude of Nairobi consist of <span class="hlt">central</span> vent trachyte, phonolite, and peralkaline rhyolite and cinder cone and fissure-fed flows of basalt to benmoreite. The volcanoes are referred to as the <span class="hlt">Central</span> Kenya Peralkaline Province (CKPP, Macdonald and Scaillet, 2006, Lithos 91, 59-73) and formed by a combination of processes including fractional crystallization, magma mixing, and volatile transport (Ren et al., 2006, Lithos 91, 109-124; Macdonald et al., 2008, JPet 49, 1515-1547). This presentation focuses on magma mixing for trachytes and phonolites for Suswa rocks, which are the southernmost part of the CKPP. We also explore the contribution of magma process studies to the interpretation of recent geodetic data, which indicate inflation/deflation of up to 21 cm for Kenyan volcanoes from 1997 to present (Biggs et al., 2009, Geology, in press). Incontrovertible evidence for magma mixing is found in field evidence, where a basaltic trachyandesite ash horizon is found interbedded with syncaldera trachyte (Skilling, 1993, J. Geol. Society London 150, 885-896), hand-specimen and thin-section petrography, and disequilibrium mineral chemistry. Precaldera lavas contain a homogeneous group of anorthoclase crystals with An content 6% or less. Syncaldera samples contain this same group and two other populations: polysynthetic twinned labradorite and andesine and anorthoclase with An content of 17%. Textures for all three groups indicate disequilibrium. Postcaldera flows contain the high and low An anorthoclase populations but lack the polysynthetic twinned labradorite and andesine. These observations suggest a model of injection of mafic magmas via diking into shallow trachtytic magma systems. Recent geodetic studies of dike injection and subsequent seismic/volcanic activity in both Ethiopia and Lengai point to the ongoing importance of these processes to <span class="hlt">rift</span> evolution in East Africa.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014GGG....15.4157E','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014GGG....15.4157E"><span id="translatedtitle">Lucky Strike seamount: Implications for the emplacement and <span class="hlt">rifting</span> of segment-centered volcanoes at slow spreading mid-ocean ridges</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Escartín, J.; Soule, S. A.; Cannat, M.; Fornari, D. J.; Düşünür, D.; Garcia, R.</p> <p>2014-11-01</p> <p>history of emplacement, tectonic evolution, and dismemberment of a <span class="hlt">central</span> volcano within the <span class="hlt">rift</span> valley of the slow spreading Mid-Atlantic Ridge at the Lucky Strike Segment is deduced using near-bottom sidescan sonar imagery and visual observations. Volcano emplacement is rapid (<1 Myr), associated with focused eruptions, and with effusion rates feeding lava flows that bury tectonic features developed prior to and during volcano construction. This volcanic phase likely requires efficient melt pooling and a long-lived crustal magma chamber as a melt source. A reduction in melt supply triggers formation of an axial graben <span class="hlt">rifting</span> the <span class="hlt">central</span> volcano, and the onset of seafloor spreading may eventually split it. At Lucky Strike, this results in two modes of crustal construction. Eruptions and tectonic activity focus at a narrow graben that bisects the <span class="hlt">central</span> volcano and contains the youngest lava flows, accumulating a thick layer of extrusives. Away from the volcano summit, deformation and volcanic emplacement is distributed throughout the <span class="hlt">rift</span> valley floor, lacking a clear locus of accretion and deformation. Volcanic emplacement on the <span class="hlt">rift</span> floor is characterized by axial volcanic ridges fed by dikes that propagate from the <span class="hlt">central</span> axial magma chamber. The mode of rapid volcano construction and subsequent <span class="hlt">rifting</span> observed at the Lucky Strike seamount is common at other <span class="hlt">central</span> volcanoes along the global mid-ocean ridge system.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013EGUGA..15.8575B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013EGUGA..15.8575B"><span id="translatedtitle">Structural inheritance, segmentation, and <span class="hlt">rift</span> localization in the Gulf of Aden oblique <span class="hlt">rift</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>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</p> <p>2013-04-01</p> <p>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 <span class="hlt">rifting</span> 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 <span class="hlt">rift</span> parallel normal faults initiated in the (future) distal margins, after a first stage of distributed <span class="hlt">rifting</span>, and witness the <span class="hlt">rift</span> 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 <span class="hlt">rifting</span> until post <span class="hlt">rift</span> times. During the early <span class="hlt">rifting</span>, they are inherited structures reactivated as oblique normal faults before accommodating strike-slip motion. During the Ocean-Continent Transition (OCT) formation ("post syn-<span class="hlt">rift</span>" 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 <span class="hlt">rift</span> localization strongly oblique to the divergence and, as a consequence, several second order transform F.Z. formed (as</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70001198','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70001198"><span id="translatedtitle">Volcanic rocks and processes of the Mid-Atlantic Ridge <span class="hlt">rift</span> valley near 36 ° 49′ N</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Hekinian, R.; Moore, J.G.; Bryan, W.B.</p> <p>1976-01-01</p> <p>The above relations indicate that the diverse lava types were erupted from a shallow, zoned magma chamber from fissures distributed over the width of the inner <span class="hlt">rift</span> valley and elongate parallel to it. Differentiation was accomplished by cooling and crystallization of plagioclase, olivine, and clinopyroxene toward the margins of the chamber. The <span class="hlt">centrally</span> located hills were built by the piling up of frequent eruption of mainly primitive lavas which also are the youngest flows. In contrast smaller and less frequent eruptions of more differentiated lavas were exposed on both sides of the <span class="hlt">rift</span> valley axis.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016IJEaS.105.1387S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016IJEaS.105.1387S"><span id="translatedtitle">A new tectono-sedimentary model for Cretaceous mixed nonmarine-marine oil-prone Komombo <span class="hlt">Rift</span>, South Egypt</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Selim, S. S.</p> <p>2016-07-01</p> <p>The Komombo Basin is a recently discovered mixed nonmarine-marine, petroliferous basin of Cretaceous age in South Egypt. It is an asymmetrical half graben, synchronous with the Neothys opening and filled with up to 4 km of continental to open marine strata ranging from Early to Late Cretaceous. Despite its great relevance, no detailed sedimentological study concerning this basin has been carried out to date. Here, we present an integrated approach to the borehole and core data, as well as unique outcrop sections to construct a new detailed sedimentological interpretation on depositional systems, controls on basin evolution, basin configuration and regional tectonic setting. Seven depositional systems were recognized: (I) a fluvial fan system, (II) a braidplain system, (III) a siliciclastic lacustrine system, (IV) a lacustrine/lagoonal system, (V) a fluvial-estuarine system, (VI) a tidally affected delta, and (VII) an open marine system. The Komombo Basin evolution can be compartmentalized into three main <span class="hlt">rifting</span> phases: the Berriasian-Early Barremian, Late Barremian, and Aptian-Albian. The first and third <span class="hlt">rifting</span> phases are comparable with the <span class="hlt">rifting</span> phases reported for several basins in North and <span class="hlt">Central</span> Africa. The second <span class="hlt">rifting</span> phase represents a transitional event between the other two phases. The first three depositional systems consist mainly of continental siliciclastics and are dominant in the Berriasian-Early Barremian and Late Barremian <span class="hlt">rifting</span> phases. The lacustrine/lagoon and fluvial-estuarine systems correspond to the Aptian-Albian <span class="hlt">rifting</span> phase, while the Campanian-Maastrichtian open-shelf deposits represents the post-<span class="hlt">rift</span> stage.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.2456F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.2456F"><span id="translatedtitle">At the tip of a propagating <span class="hlt">rift</span> - The offshore East African <span class="hlt">Rift</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Franke, Dieter; Jokat, Wilfried; Ladage, Stefan; Stollhofen, Harald; Klimke, Jennifer; Lutz, Ruediger; Mahanjane, Stefane; Ehrhardt, Axel; Schreckenberger, Bernd</p> <p>2016-04-01</p> <p>Numerous studies have addressed various aspects of the East African <span class="hlt">Rift</span> system (EARS) but surprisingly few the offshore continuation of the south-eastern branch of the <span class="hlt">rift</span> into the Mozambique Channel. Here, we present new evidence for neotectonic deformation derived from modern seismic reflection data and supported by additional geophysical data. The Kerimbas Graben offshore northern Mozambique is the most prominent manifestation of sub-recent extensional deformation. The seismic reflection data reveals that recent normal faulting often utilizes preexisting, deeply buried half-graben structures which likely are related to the formation of the Somali Basin. The ~30 km wide and ~150 km long symmetric graben is in a stage where the linkage of scattered normal faults already did happen, resulting in increased displacement and accommodation of most of the extension across the basin. However, deep earthquakes below the <span class="hlt">rift</span> indicate a strong and still preserved lithospheric mantle. Extension is becoming diffuse where an onshore suture, subdividing the northern from the southern metamorphic basement onshore Mozambique, is closest to the offshore <span class="hlt">rift</span>. It appears likely that this suture is the origin for the variation in <span class="hlt">rifting</span> style, indicating that mantle fabric resulting from a Cambrian collision has been preserved as mechanical anisotropy of the lithospheric mantle. Further south the <span class="hlt">rift</span> focuses in an about 30 km wide half-graben. An important finding is that the entire offshore branch of the EARS lacks significant volcanism. Along the offshore EARS there are only negligible indications for recent volcanism in the reflection seismic data such as sills and dikes. Apparently the "Comoros mantle plume" (French and Romanowicz, 2015) has a very minor influence on the progressive extensional deformation along the northern Mozambique continental margin, leading eventually to breakup sometimes in the future. Combining structural with earthquake data reveals that the magma</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_12");'>12</a></li> <li><a href="#" onclick='return showDiv("page_13");'>13</a></li> <li class="active"><span>14</span></li> <li><a href="#" onclick='return showDiv("page_15");'>15</a></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_14 --> <div id="page_15" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_13");'>13</a></li> <li><a href="#" onclick='return showDiv("page_14");'>14</a></li> <li class="active"><span>15</span></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="281"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2008AGUFM.T43A1976M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2008AGUFM.T43A1976M"><span id="translatedtitle">Forensic investigation of <span class="hlt">rift</span>-to-drift transitions and volcanic <span class="hlt">rifted</span> margins birth</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Meyer, R.; Hertogen, J.</p> <p>2008-12-01</p> <p>Volcanic <span class="hlt">rifted</span> margins (VRM) reflect excess magmatism generated during the <span class="hlt">rift</span>-to-drift transition of a continental <span class="hlt">rift</span> 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) <span class="hlt">Rift</span> 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 <span class="hlt">rift</span>-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 <span class="hlt">rift</span> system is probably the key process generating excess magmatism in LIP related to <span class="hlt">rift</span>-to-drift transitions.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015EGUGA..17.5750M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015EGUGA..17.5750M"><span id="translatedtitle">Volcanic <span class="hlt">rifts</span> bracketing volcanoes: an analogue answer to an old unsolved problem</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Mussetti, Giulio; van Wyk de Vries, Benjamin; Corti, Giacomo; Hagos, Miruts</p> <p>2015-04-01</p> <p>It has been observed in <span class="hlt">Central</span> America that many volcanoes have volcanic alignments and faults at their east and west feet. A quick look at many <span class="hlt">rifts</span> indicates that this also occurs elsewhere. While this feature has been noted for at least 30 years, no explanation has ever really been convincingly put forward. During analogue experiments on <span class="hlt">rifting</span> volcanoes we have mixed the presence of a volcanic edifice with an underlying intrusive complex. The models use a rubber sheet that is extended and provides a broad area of extension (in contrast to many moving plate models that have one localised velocity discontinuity). This well suits the situation in many <span class="hlt">rifts</span> and diffuse strike-slip zones (i.e. <span class="hlt">Central</span> America and the East African <span class="hlt">Rift</span>). We have noted the formation of localised extension bracketing the volcano, the location of which depends on the position of the analogue intrusion. Thus, we think we have found the answer to this long standing puzzle. We propose that diffuse extension of a volcano and intrusive complex generates two zones of faulting at the edge of the intrusion along the axis of greatest extensional strain. These serve to create surface faulting and preferential pathways for dykes. This positioning may also create craters aligned along the axis of extension, which is another notable feature of volcanoes in <span class="hlt">Central</span> America. Paired volcanoes and volcanic uplifts in the Danakil region of Ethiopia may also be a consequence of such a process and lead us to draw some new preliminary cross sections of the Erta Ale volcanic range.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/26710251','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/26710251"><span id="translatedtitle">Assessing the Threat of Amphibian Chytrid Fungus in the Albertine <span class="hlt">Rift</span>: Past, Present and Future.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Seimon, Tracie A; Ayebare, Samuel; Sekisambu, Robert; Muhindo, Emmanuel; Mitamba, Guillain; Greenbaum, Eli; Menegon, Michele; Pupin, Fabio; McAloose, Denise; Ammazzalorso, Alyssa; Meirte, Danny; Lukwago, Wilbur; Behangana, Mathias; Seimon, Anton; Plumptre, Andrew J</p> <p>2015-01-01</p> <p>Batrachochytrium dendrobatidis (Bd), the cause of chytridiomycosis, is a pathogenic fungus that is found worldwide and is a major contributor to amphibian declines and extinctions. We report results of a comprehensive effort to assess the distribution and threat of Bd in one of the Earth's most important biodiversity hotspots, the Albertine <span class="hlt">Rift</span> in <span class="hlt">central</span> Africa. In herpetological surveys conducted between 2010 and 2014, 1018 skin swabs from 17 amphibian genera in 39 sites across the Albertine <span class="hlt">Rift</span> were tested for Bd by PCR. Overall, 19.5% of amphibians tested positive from all sites combined. Skin tissue samples from 163 amphibians were examined histologically; of these two had superficial epidermal intracorneal fungal colonization and lesions consistent with the disease chytridiomycosis. One amphibian was found dead during the surveys, and all others encountered appeared healthy. We found no evidence for Bd-induced mortality events, a finding consistent with other studies. To gain a historical perspective about Bd in the Albertine <span class="hlt">Rift</span>, skin swabs from 232 museum-archived amphibians collected as voucher specimens from 1925-1994 were tested for Bd. Of these, one sample was positive; an Itombwe River frog (Phrynobatrachus asper) collected in 1950 in the Itombwe highlands. This finding represents the earliest record of Bd in the Democratic Republic of Congo. We modeled the distribution of Bd in the Albertine <span class="hlt">Rift</span> using MaxEnt software, and trained our model for improved predictability. Our model predicts that Bd is currently widespread across the Albertine <span class="hlt">Rift</span>, with moderate habitat suitability extending into the lowlands. Under climatic modeling scenarios our model predicts that optimal habitat suitability of Bd will decrease causing a major range contraction of the fungus by 2080. Our baseline data and modeling predictions are important for comparative studies, especially if significant changes in amphibian health status or climactic conditions are encountered</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/22837002','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/22837002"><span id="translatedtitle">Inland thinning of West Antarctic Ice Sheet steered along subglacial <span class="hlt">rifts</span>.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Bingham, Robert G; Ferraccioli, Fausto; King, Edward C; Larter, Robert D; Pritchard, Hamish D; Smith, Andrew M; Vaughan, David G</p> <p>2012-07-25</p> <p>Current ice loss from the West Antarctic Ice Sheet (WAIS) accounts for about ten per cent of observed global sea-level rise. Losses are dominated by dynamic thinning, in which forcings by oceanic or atmospheric perturbations to the ice margin lead to an accelerated thinning of ice along the coastline. Although <span class="hlt">central</span> to improving projections of future ice-sheet contributions to global sea-level rise, the incorporation of dynamic thinning into models has been restricted by lack of knowledge of basal topography and subglacial geology so that the rate and ultimate extent of potential WAIS retreat remains difficult to quantify. Here we report the discovery of a subglacial basin under Ferrigno Ice Stream up to 1.5 kilometres deep that connects the ice-sheet interior to the Bellingshausen Sea margin, and whose existence profoundly affects ice loss. We use a suite of ice-penetrating radar, magnetic and gravity measurements to propose a <span class="hlt">rift</span> origin for the basin in association with the wider development of the West Antarctic <span class="hlt">rift</span> system. The Ferrigno <span class="hlt">rift</span>, overdeepened by glacial erosion, is a conduit which fed a major palaeo-ice stream on the adjacent continental shelf during glacial maxima. The palaeo-ice stream, in turn, eroded the 'Belgica' trough, which today routes warm open-ocean water back to the ice front to reinforce dynamic thinning. We show that dynamic thinning from both the Bellingshausen and Amundsen Sea region is being steered back to the ice-sheet interior along <span class="hlt">rift</span> basins. We conclude that <span class="hlt">rift</span> basins that cut across the WAIS margin can rapidly transmit coastally perturbed change inland, thereby promoting ice-sheet instability.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=4692535','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=4692535"><span id="translatedtitle">Assessing the Threat of Amphibian Chytrid Fungus in the Albertine <span class="hlt">Rift</span>: Past, Present and Future</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Seimon, Tracie A.; Ayebare, Samuel; Sekisambu, Robert; Muhindo, Emmanuel; Mitamba, Guillain; Greenbaum, Eli; Menegon, Michele; Pupin, Fabio; McAloose, Denise; Ammazzalorso, Alyssa; Meirte, Danny; Lukwago, Wilbur; Behangana, Mathias; Seimon, Anton; Plumptre, Andrew J.</p> <p>2015-01-01</p> <p>Batrachochytrium dendrobatidis (Bd), the cause of chytridiomycosis, is a pathogenic fungus that is found worldwide and is a major contributor to amphibian declines and extinctions. We report results of a comprehensive effort to assess the distribution and threat of Bd in one of the Earth’s most important biodiversity hotspots, the Albertine <span class="hlt">Rift</span> in <span class="hlt">central</span> Africa. In herpetological surveys conducted between 2010 and 2014, 1018 skin swabs from 17 amphibian genera in 39 sites across the Albertine <span class="hlt">Rift</span> were tested for Bd by PCR. Overall, 19.5% of amphibians tested positive from all sites combined. Skin tissue samples from 163 amphibians were examined histologically; of these two had superficial epidermal intracorneal fungal colonization and lesions consistent with the disease chytridiomycosis. One amphibian was found dead during the surveys, and all others encountered appeared healthy. We found no evidence for Bd-induced mortality events, a finding consistent with other studies. To gain a historical perspective about Bd in the Albertine <span class="hlt">Rift</span>, skin swabs from 232 museum-archived amphibians collected as voucher specimens from 1925–1994 were tested for Bd. Of these, one sample was positive; an Itombwe River frog (Phrynobatrachus asper) collected in 1950 in the Itombwe highlands. This finding represents the earliest record of Bd in the Democratic Republic of Congo. We modeled the distribution of Bd in the Albertine <span class="hlt">Rift</span> using MaxEnt software, and trained our model for improved predictability. Our model predicts that Bd is currently widespread across the Albertine <span class="hlt">Rift</span>, with moderate habitat suitability extending into the lowlands. Under climatic modeling scenarios our model predicts that optimal habitat suitability of Bd will decrease causing a major range contraction of the fungus by 2080. Our baseline data and modeling predictions are important for comparative studies, especially if significant changes in amphibian health status or climactic conditions are</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2006AGUFM.T33E..06D','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2006AGUFM.T33E..06D"><span id="translatedtitle">Geophysical Survey of the 1978 Seismo-volcanic Crisis in the Asal-Ghoubbet <span class="hlt">Rift</span> (Afar Depression, Djibouti) and the Post-<span class="hlt">rifting</span> Deformations</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Doubre, C.; Ruegg, J.; de Chabalier, J.; Vigny, C.; Jacques, E.</p> <p>2006-12-01</p> <p>In November 1978, a seismo-volcanic crisis occurred in the Asal-Ghoubbet <span class="hlt">Rift</span>, which is located at the western termination of the oceanic Aden Ridge propagating inland into the Afar Depression and accommodates a large part of the divergent motion of the Arabia and Somalia plates. This episode offered the opportunity to study the <span class="hlt">rifting</span> process controlling the evolution of a sub-aerial opening segment at the transition from continental break-up to oceanic spreading. This major crustal spreading episode started with two major earthquakes in the subaerial part of the <span class="hlt">rift</span> (mb=5.3 and 5.0) and was followed by the week-long, basaltic fissure eruption of the Ardukoba at the western tip of the <span class="hlt">central</span> volcanic chain. The geophysical survey carried out for the crisis was possible by means of the Arta Observatory in Djibouti within the framework of field surveys financed by the French agency CNRS-INSU. This allowed the measurements of the surface breaks (dry open fissures up to 100 m, normal fault throws up to 80 cm), the crustal deformation by geodetic networks and leveling (up to 2m of horizontal widening, 70 cm of inner-floor subsidence), and the evolution of the seismic activity (eastward migration along the Aden Ridge) associated with this <span class="hlt">rifting</span> event. Elastic modeling shows that both the deformation pattern and the seismic activity can be explained by the aseismic intrusion of two dykes below the <span class="hlt">rift</span> inner-floor. Subsequently, a continuous geodetic and seismic monitoring has been maintained and shows that the post-dyke injection evolution of the <span class="hlt">rift</span> is dominated by two distinct periods. During the six first years (1979-1986), high rates of horizontal opening and slip of creeping normal faults accommodate the subsidence of the inner-floor surrimposed to the development of a 25 km-wide uplift. Since 1986-87, the strain rates have decreased and currently reach values consistent with long-term velocities deduced from morpho-tectonic studies. The evolution of the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFM.T43H..04E','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFM.T43H..04E"><span id="translatedtitle">Crustal Strain Patterns in Magmatic and Amagmatic Early Stage <span class="hlt">Rifts</span>: Border Faults, Magma Intrusion, and Volatiles</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Ebinger, C. J.; Keir, D.; Roecker, S. W.; Tiberi, C.; Aman, M.; Weinstein, A.; Lambert, C.; Drooff, C.; Oliva, S. J. C.; Peterson, K.; Bourke, J. R.; Rodzianko, A.; Gallacher, R. J.; Lavayssiere, A.; Shillington, D. J.; Khalfan, M.; Mulibo, G. D.; Ferdinand-Wambura, R.; Palardy, A.; Albaric, J.; Gautier, S.; Muirhead, J.; Lee, H.</p> <p>2015-12-01</p> <p><span class="hlt">Rift</span> initiation in thick, strong continental lithosphere challenges current models of continental lithospheric deformation, in part owing to gaps in our knowledge of strain patterns in the lower crust. New geophysical, geochemical, and structural data sets from youthful magmatic (Magadi-Natron, Kivu), weakly magmatic (Malawi, Manyara), and amagmatic (Tanganyika) sectors of the cratonic East African <span class="hlt">rift</span> system provide new insights into the distribution of brittle strain, magma intrusion and storage, and time-averaged deformation. We compare and contrast time-space relations, seismogenic layer thickness variations, and fault kinematics using earthquakes recorded on local arrays and teleseisms in sectors of the Western and Eastern <span class="hlt">rifts</span>, including the Natron-Manyara basins that developed in Archaean lithosphere. Lower crustal seismicity occurs in both the Western and Eastern <span class="hlt">rifts</span>, including sectors on and off craton, and those with and without <span class="hlt">central</span> <span class="hlt">rift</span> volcanoes. In amagmatic sectors, lower crustal strain is accommodated by slip along relatively steep border faults, with oblique-slip faults linking opposing border faults that penetrate to different crustal levels. In magmatic sectors, seismicity spans surface to lower crust beneath both border faults and eruptive centers, with earthquake swarms around magma bodies. Our focal mechanisms and Global CMTs from a 2007 fault-dike episode show a local rotation from ~E-W extension to NE-SE extension in this linkage zone, consistent with time-averaged strain recorded in vent and eruptive chain alignments. These patterns suggest that strain localization via widespread magma intrusion can occur during the first 5 My of <span class="hlt">rifting</span> in originally thick lithosphere. Lower crustal seismicity in magmatic sectors may be caused by high gas pressures and volatile migration from active metasomatism and magma degassing, consistent with high CO2 flux along fault zones, and widespread metasomatism of xenoliths. Volatile release and</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFM.T51F2969S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFM.T51F2969S"><span id="translatedtitle">A Late Cretaceous Orogen Triggering the Tertiary <span class="hlt">Rifting</span> of the West Sunda Plate; Andaman Sea Region</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Sautter, B.; Pubellier, M. F.; Menier, D.</p> <p>2015-12-01</p> <p><span class="hlt">Rifted</span> Basins often develop in internal zones of orogenic belts, although the latter may not be easy to unravel. We chose the example of the super-stretched Andaman sea region affected by several stages of <span class="hlt">rifting</span> in the internal zone of a composite collage of allochthonous terranes. We made use of a set of geophysical, geochronological and structural data to analyze the <span class="hlt">rifting</span> evolution and reconstruct the previous compressional structures. - Starting in the late Oligocene the East Andaman Basin opened as a back arc in a right-lateral pull- apart. The <span class="hlt">rifting</span> propagated Westward to the <span class="hlt">central</span> Andaman basin in the Middle Miocene, and to the oceanic spreading stage in the Pliocene. - An early extension occurred in the Paleogene, marked by widespread opening of isolated continental basins onshore Malay Peninsula and offshore Andaman Shelf and Malacca Straits. The <span class="hlt">rifting</span> was accommodated by LANF's along preexisting weakness zones such as hinges of folds and granitic batholiths. Continuous extension connected the isolated basins offshore, whereas onshore, the grabens remained confined. There, AFT data show an uplift phase around 30Ma. In the Late Cretaceous, a major deformation occurred oblique to the pre-existing Indosinian basement fabrics. The convergence was partitioned into thrusting and uplift of the Cretaceous volcanic arc in Thailand and Myanmar, inversion of Mesozoic basins, and coeval wrenching responsible for large phacoid-shaped crustal slivers bounded by wide strike slip fault zones. The slivers share similar characteristics: a thick continental core of lower Paleozoic sedimentary basins units surrounded by Late Cretaceous granitoids. Radiometric data and fission tracks indicate a widespread thermal anomaly in all West Sunda Plate synchronous to a strong uplift. In the Latest Mesozoic, the Western Margin of Sunda plate was subjected to a major E-W compression, accommodated by oblique conjugate strike slip faults, leading to the formation of a large</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/1991Tectp.199..375K','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/1991Tectp.199..375K"><span id="translatedtitle">The position of continental flood basalts in <span class="hlt">rift</span> zones and its bearing on models of <span class="hlt">rifting</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Kazmin, V. G.</p> <p>1991-12-01</p> <p>Two models of rifting—"pure shear" and "simple shear"—are at present being widely discussed. In this paper attention is drawn to the fact that continental flood basalts (CFB), associated with <span class="hlt">rifts</span>, usually display a strong asymmetry relative to the <span class="hlt">rift</span> axis. The Karroo, the Parana, and the Deccan basalts, as well as the Ethiopian flood basalts and the basalts of the Arabian Shield are concentrated mainly on one side of the respective <span class="hlt">rifts</span>. Structural data indicate that the eruptions occurred on the "upper plates", if the low-angle, simple-shear model of Wernicke (1985) is accepted. It is suggested that the asymmetry of eruptions is caused by the asymmetric position of the asthenopheic rise predicted by this model. The low-angle detachment may serve as a conduit for magmatic fluid. This fits well with data on the existence of large magmatic reservoirs of CFB at subcrustal or crustal levels and explains why eruptions are concentrated in the <span class="hlt">rift</span> and on one of its shoulders, while being restricted on the other. This asymmetry of volcanism supports models of continental <span class="hlt">rifting</span> associated with low-angle simple-shear detachment.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=3111045','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=3111045"><span id="translatedtitle">The Pathogenesis of <span class="hlt">Rift</span> Valley Fever</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Ikegami, Tetsuro; Makino, Shinji</p> <p>2011-01-01</p> <p><span class="hlt">Rift</span> Valley fever (RVF) is an emerging zoonotic disease distributed in sub-Saharan African countries and the Arabian Peninsula. The disease is caused by the <span class="hlt">Rift</span> Valley fever virus (RVFV) of the family Bunyaviridae and the genus Phlebovirus. The virus is transmitted by mosquitoes, and virus replication in domestic ruminant results in high rates of mortality and abortion. RVFV infection in humans usually causes a self-limiting, acute and febrile illness; however, a small number of cases progress to neurological disorders, partial or complete blindness, hemorrhagic fever, or thrombosis. This review describes the pathology of RVF in human patients and several animal models, and summarizes the role of viral virulence factors and host factors that affect RVFV pathogenesis. PMID:21666766</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/21666766','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/21666766"><span id="translatedtitle">The pathogenesis of <span class="hlt">Rift</span> Valley fever.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Ikegami, Tetsuro; Makino, Shinji</p> <p>2011-05-01</p> <p><span class="hlt">Rift</span> Valley fever (RVF) is an emerging zoonotic disease distributed in sub-Saharan African countries and the Arabian Peninsula. The disease is caused by the <span class="hlt">Rift</span> Valley fever virus (RVFV) of the family Bunyaviridae and the genus Phlebovirus. The virus is transmitted by mosquitoes, and virus replication in domestic ruminant results in high rates of mortality and abortion. RVFV infection in humans usually causes a self-limiting, acute and febrile illness; however, a small number of cases progress to neurological disorders, partial or complete blindness, hemorrhagic fever, or thrombosis. This review describes the pathology of RVF in human patients and several animal models, and summarizes the role of viral virulence factors and host factors that affect RVFV pathogenesis.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/1984E%26PSL..67..219F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/1984E%26PSL..67..219F"><span id="translatedtitle">How many <span class="hlt">rifts</span> are there in West Africa?</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Freeth, S. J.</p> <p>1984-02-01</p> <p>The West African <span class="hlt">Rift</span> System has, for the last ten years, been thought to consist of five interconnected <span class="hlt">rifts</span> extending from the Gulf of Guinea deep into the heart of Africa. Careful re-examination of the geophysical evidence makes it quite clear that there are only three interconnected <span class="hlt">rifts</span> in West Africa; the Lower Benue <span class="hlt">Rift</span> which extends to the northeast from the Gulf of Guinea to a triple junction near Chum, and the Gongola and Yola <span class="hlt">Rifts</span> which extend to the north and east, respectively, from the Chum triple junction. These three <span class="hlt">rifts</span> opened during the earlier part of the Mesozoic and were subsequently filled with Cretaceous sediments. The evidence for two further <span class="hlt">rifts</span>, the Ati <span class="hlt">Rift</span> and the Fort Archambault <span class="hlt">Rift</span> which were thought to extend to the northeast and southeast, respectively, from a triple junction at the eastern end of the Yola <span class="hlt">Rift</span>, does not stand up to re-examination. The "Ati <span class="hlt">Rift</span>" was thought to follow a major linear positive gravity anomaly which had been mapped beneath the Quaternary sediments of the Chad Basin. The main gravity anomaly is separated from the Yola <span class="hlt">Rift</span> by over 300 km and is probably due to a linear body of basic volcanic or volcano-clastic rocks associated with a suture of Pan-African age. Within the gap, between the main anomaly and the Yola <span class="hlt">Rift</span>, there are three localised positive anomalies which relate to a gabbro of Precambrian age, a band of dense meta-sediments within the Basement Complex and an acid igneous complex of Palaeogene age. The anomaly as a whole is therefore a sequence of unrelated anomalies, none of which are due to features of Mesozoic age. The "Fort Archambault <span class="hlt">Rift</span>" was thought to follow a major linear negative gravity anomaly which has been mapped beneath the Quaternary sediments of the Chad Basin. To a large extent the negative anomaly overlies the fosse de Baké-Birao (Baké-Birao Basin) which is itself part of a far larger structure that extends, parallel to the southern margin of the West African</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/5011696','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/5011696"><span id="translatedtitle">Seismotectonics of Reelfoot <span class="hlt">rift</span> basement structures</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Dart, R.L.; Swolfs, H.S. )</p> <p>1993-03-01</p> <p>Contour maps of the Precambrian basement surface show major northwest-trending structural features within the boundaries of the northeast-oriented Reelfoot <span class="hlt">rift</span>. These northwest-trending features, southeast of New Madrid, Missouri, consist of a trough flanked on the northeast by a 2-km-high ridge. These features correlate with similar features on an updated depth-to-magnetic basement map. The boundary between the trough and the ridge slopes gently to the southwest. The upward projection of this boundary into the overlying Paleozoic strata may be expressed on a structure-contour map of the Cambrian rocks. The vertical relief of this boundary on the younger datum is inferred to be about 1 km. This Precambrian trough-ridge structure may correlate with a southwest dipping, west-northwest-striking normal fault inferred by Schwalb (1982) to offset rocks of the Cambrian-Ordovician Knox Megagroup that subcrop at the Paleozoic surface. Schwalb (1982) inferred 1.22 km of vertical relief on this fault near the bootheel of Missouri. The nature and significance of this tectonic-structural boundary is unclear, but at the top of the Precambrian basement rocks, it coincides with the southwestern terminus of the New Madrid seismic zone (NMSZ) near the end of the Blytheville arch in northeastern Arkansas. Since the mid-1970's, when instrumental recording began, some of the earthquakes in the NMSZ having the largest magnitudes occurred in this area. The authors working hypothesis is that this trough-ridge structural boundary may concentrate stress and/or may be a barrier that defines the southwestern limit of the seismically active axial fault zone in the <span class="hlt">rift</span>. Future study will concentrate on improving the understanding of the influence of <span class="hlt">rift</span>-bounding faults on the lateral extent of this structure, as well as constructing a tectonic stress model of seismically active <span class="hlt">rift</span> faults and this trough-ridge structure.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/6535806','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/6535806"><span id="translatedtitle">Early Paleozoic sedimentation in Reelfoot <span class="hlt">rift</span></span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Houseknecht, D.W.; Weaverling, P.H.</p> <p>1983-09-01</p> <p>Analysis of subsurface data from deep tests drilled in the northern Mississippi embayment and southern Mid-Continent suggests that earliest Paleozoic sedimentation was dominated by the tectonic evolution of the Reelfoot <span class="hlt">rift</span>. Throughout most of the Mid-Continent, the Upper Cambrian Lamotte (Mt. Simon) sandstone rests nonconformably on Precambrian basement and is overlain by the Bonneterre (Eau Claire) Formation. However, in the area of the Reelfoot <span class="hlt">rift</span>, both the Lamotte and Bonneterre grade into thick, basinal shales that locally display evidence of episodic deposition of coarse clastics, perhaps on submarine fans. Moreover, two major sedimentary units are present beneath the Lamotte-Bonneterre basinal facies within the Reelfoot <span class="hlt">rift</span>. Immediately underlying the Lamotte-Bonneterre shale is a carbonate stratum (probably dolomite) that thickens to more than 1,000 ft (300 m) along the axis of the basin in eastern Arkansas. Underlying this carbonate is a detrital unit that grades from arkosic sandstone near the northern terminus of the basin to a basinal shale southward. This basinal shale is at least several hundred feet thick near the axis of the basin. These two strata occupy the stratigraphic position of the Conasauga (Middle Cambrian) and Rome (Lower Cambrian) Formations of the southern Appalachians. The axial and transverse distribution of these strata suggests that the Reelfoot evolved as paired grabens or half grabens during the Early and Middle Cambrian. Subsequently, the Reelfoot remained the axis for more widespread subsidence and sedimentation throughout much of the Paleozoic.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015EGUGA..17.5705S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015EGUGA..17.5705S"><span id="translatedtitle">the role of magmatism and segmentation in the structural evolution of the Afar <span class="hlt">Rift</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Stab, Martin; Bellahsen, Nicolas; Pik, Raphaël; Quidelleur, Xavier; Ayalew, Dereje; Leroy, Sylvie</p> <p>2015-04-01</p> <p>A common issue at volcanic passive margins (VPM) is the lack of observation of the structures that accommodate stretching and thinning. Indeed, the most distal parts and the Ocean-Continent Transition is often masked by thick seaward-dipping reflectors (SDR) sequences. Some current challenges are then to know if the observed thinning fit the divergence (thinning vs dyking); and what is the rheological effect of magma supply that re-thickens the crust during extension? In the <span class="hlt">Central</span> Afar magmatic <span class="hlt">rift</span> (Ethiopia), the structures related to <span class="hlt">rifting</span> since Oligocene are cropping out onshore and are well preserved. We present here a new structural model based on field data and lavas (U-Th/He and K/Ar) datings along a balanced cross-section of the <span class="hlt">Central</span> Afar Western Margin. We mapped continent-ward normal fault array affecting highly tilted trapp series (29-30 Ma) unconformably overlain by tilted Oligo-Miocene (25-7 Ma) acid series. The main extensional and necking/thinning event took place during the end of this Miocene magmatic episode. The Pliocene flood basalt (Stratoid series) is erupted over an already thinned crust. The bulk extension for the Afar Western Margin is ß ~ 2.50. Our main findings are: - Oligo-Miocene deformation in <span class="hlt">Central</span> Afar appears to be largely distributed through space and time ("magmatic wide <span class="hlt">rift</span>"). It has been accommodated in a 200-300 km wide strip being a diffuse incipient plate boundary during the whole <span class="hlt">rifting</span> history until the formation of present-day magmatic segments. There is a period of tectonic quiescence accompanied with few magma erupted at the surface between 25 Ma and 7 Ma. We suggest that tectonic and magmatic activity was focused at that time on the highly faulted Danakil block and Southern Red Sea, away from our study zone. - ß ~ 2.50 is higher than the thinning factor of ~1.30 observed in geophysical studies. We propose that the continental crust in <span class="hlt">Central</span> Afar has been re-thickened during extension by the syn-<span class="hlt">rift</span></p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012AGUFM.V53C2855V','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012AGUFM.V53C2855V"><span id="translatedtitle">Structural control of the Limagne <span class="hlt">Rift</span> Fault on the Chaîne des Puys</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Van Wyk de Vries, B.; Tiu, G.; Mossoux, S.; Kervyn, M.; Lagmay, A. A.</p> <p>2012-12-01</p> <p>The close proximity of the Limagne Fault to the Chaîne des Puys (Massif <span class="hlt">Central</span>, France) has been noted for a long time. In fact, if the Limagne fault had not raised the Chaîne relative to the Limagne <span class="hlt">Rift</span>, to create a spectacular skyline of volcanoes, there would probably not have been so much interest in them. Early geologists such as Elie de Beaumont, Davy, Lyell, Murchison, and Von Humbolt, were as much concerned with explaining the escarpment by catastrophism or uniformitarianism, as they were with battling over neptunism and plutonism, or craters of elevation. The questions still stands as to what relationship the monogenetic volcanoes have to the fault, and perhaps: are the volcanoes monogenetic because of the fault? - seeing as just to the south of the <span class="hlt">rift</span>, stand the Mont Dore-Sancy and Cantal stratovolcanoes. Considering the spatial-temporal relationship, it becomes clear that there are volcanoes on the fault, in the <span class="hlt">rift</span>, and behind the footwall, but with different ages and configurations. Early volcanoes are cut by the faulting, showing a potential linkage, later volcanoes, as a whole, follow generally the main structural trend NS of the <span class="hlt">rift</span>, but in detail group around a more NE-SW trend, that can be related to Hercynian structures, and to a major transfer fault linking the Limagne <span class="hlt">Rift</span> to the Rhine Graben. These trends host some of the youngest volcanoes, and the fault appears to have been more recently active in this area, and the area of some damaging historic earthquakes. Considering the proximity of the fault as an influence in the monogenetic nature of the volcanism, more needs to be known about the state of faulting in the Tertiary in the Massif <span class="hlt">Central</span>. However, the area of the Chaîne de Puys is highly heterogeneous, with complex associations of Hercynian and Tertiary structures, and thus the crustal structure may be compatible with multiple pathways, and thus a monogenetic tendency. In conclusion, there is a probably spatial and temporal</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013EGUGA..15.2307D','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013EGUGA..15.2307D"><span id="translatedtitle">Sismotectonics in the western branch of the East African <span class="hlt">Rift</span> System</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Delvaux, Damien; Kervyn, François; Mulumba, Jean-Luc; Kipata, Louis; Sebagenzi, Stanislas; Mavonga, Georges; Macheyeki, Athanas; Temu, Elly Bryan</p> <p>2013-04-01</p> <p>The western branch of the East African <span class="hlt">rift</span> system is known of its particular seismic activity with larger magnitude (up to Ms 7.3) and more frequent destructive earthquakes than in the eastern branch. As a contribution to the IGCP 601 project Seismotectonic Map of Africa, we compiled the known active faults, thermal springs and historical seismicity in <span class="hlt">Central</span> Africa. Using the rich archives of the Royal Museum for <span class="hlt">Central</span> Africa, publications and own field observations, we present a compilation of available data relative to the current seismotectonic activity along the western branch of the East African <span class="hlt">rift</span> system, in DRC, Rwanda, Burundi and Tanzania. Neotectonic activity related to the western <span class="hlt">rift</span> branch is in general well expressed and relatively well studied in the eastern flank of this <span class="hlt">rift</span> branch, in Uganda, Rwanda, Burundi and Tanzania. In contrast, the western flank of this <span class="hlt">rift</span> branch, largely exposed in the DRC, has attracted less attention. However, data collected during the colonial times show significant sismotectonic activity in East DRC, not only in the western flank of the western <span class="hlt">rift</span> branch, but extending far westwards up to the margin of the Congo basin. In particular, our predecessors paid a special attention to the mapping and description of thermal springs, noticing that they are often controlled by active faults. In addition, the operators of the relatively dense network of meteorological stations installed in the DRC, Rwanda and Burundi also recorded were with variable level of completeness and detail the earthquakes that they could felt. This provides a rich database that is used to complete the existing knowledge on historical seismicity. An important effort has still to be paid to identify and map potentially active fault due to poor field accessibility, tropical climate weathering and vegetation coverage. The main problem in the compilation of active fault data is that very few of them have been investigated by paleoseismic trenching</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70021976','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70021976"><span id="translatedtitle">Transect across the West Antarctic <span class="hlt">rift</span> system in the Ross Sea, Antarctica</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Trey, H.; Cooper, A. K.; Pellis, G.; Della, Vedova B.; Cochrane, G.; Brancolini, Giuliano; Makris, J.</p> <p>1999-01-01</p> <p>In 1994, the ACRUP (Antarctic Crustal Profile) project recorded a 670-km-long geophysical transect across the southern Ross Sea to study the velocity and density structure of the crust and uppermost mantle of the West Antarctic <span class="hlt">rift</span> system. Ray-trace modeling of P- and S-waves recorded on 47 ocean bottom seismograph (OBS) records, with strong seismic arrivals from airgun shots to distances of up to 120 km, show that crustal velocities and geometries vary significantly along the transect. The three major sedimentary basins (early-<span class="hlt">rift</span> grabens), the Victoria Land Basin, the <span class="hlt">Central</span> Trough and the Eastern Basin are underlain by highly extended crust and shallow mantle (minimum depth of about 16 km). Beneath the adjacent basement highs, Coulman High and <span class="hlt">Central</span> High, Moho deepens, and lies at a depth of 21 and 24 km, respectively. Crustal layers have P-wave velocities that range from 5.8 to 7.0 km/s and S-wave velocities from 3.6 to 4.2 km/s. A distinct reflection (PiP) is observed on numerous OBS from an intra-crustal boundary between the upper and lower crust at a depth of about 10 to 12 km. Local zones of high velocities and inferred high densities are observed and modeled in the crust under the axes of the three major sedimentary basins. These zones, which are also marked by positive gravity anomalies, may be places where mafic dikes and sills pervade the crust. We postulate that there has been differential crustal extension across the West Antarctic <span class="hlt">rift</span> system, with greatest extension beneath the early-<span class="hlt">rift</span> grabens. The large amount of crustal stretching below the major <span class="hlt">rift</span> basins may reflect the existence of deep crustal suture zones which initiated in an early stage of the <span class="hlt">rifting</span>, defined areas of crustal weakness and thereby enhanced stress focussing followed by intense crustal thinning in these areas. The ACRUP data are consistent with the prior concept that most extension and basin down-faulting occurred in the Ross Sea during late Mesozoic time, with</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2010EGUGA..12..170B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2010EGUGA..12..170B"><span id="translatedtitle">Development of the Barents Sea <span class="hlt">rift</span> and its influence on sedimentation and hydrocarbon formation</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Balanyuk, Inna; Dmitrievsky, Anatoly; Shapovalov, Sergey; Chaikina, Olga</p> <p>2010-05-01</p> <p> to the greatest deposits of this depression. An extended field acoustic data collected in the Barents Sea led to understanding of general fundamental problems for all Arctic Seas and, first of all, the problem of Quaternary glaciations. An analysis of Eurasian-Arctic continental margin shows correspondence between the <span class="hlt">rift</span> systems of the shelf with those of the ocean. This relation can be observed by an example of the <span class="hlt">central</span> Arctic region. All the <span class="hlt">rift</span> systems underlying the sediment basin are expressed in the sea bed relief as spacious and extensive graben valleys burnished by lobes. The <span class="hlt">rift</span> structures and the sediment cover are connected by the systems of transversal (or oblique) faults. A study of the regional geologic structure of basins affords believing in the very complicated inner riftogenic structure of the base and the lower part of the sediment cover, which is represented by a combination of grabens and horsts bound by the same tectonic border in the form of high amplitude fault zones, and have a very high density of fault-fractured tectonics. In the same time, a branching of the <span class="hlt">rift</span> structures related to rounding of large solid blocks of the crust, represented by Archaean or Baikal base prominences, takes place. Their frequent structural connection with the continental paleo-<span class="hlt">rift</span> structures is ascertained, but the shelf <span class="hlt">rift</span> system are not their straight prolongation and separated by base thresholds concealed under the sediment cover or by bulkheads expressed even in the modern relief. The information on geodynamic development of the region, thermal convection and modern sedimentation enable assessment of the real potentiality for underwater works in the region, potential gas hydrates resources, and will help to assign a strategy of prospecting work, to range the known fields, to carry out a regional survey for engineering work.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2009EGUGA..11.7856B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2009EGUGA..11.7856B"><span id="translatedtitle">Impact of geodynamic development of the Barents Sea deep <span class="hlt">rift</span> on evolving petroleum systems</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Balanyuk, I.; Dmitrievsky, A.</p> <p>2009-04-01</p> <p>All the Barents Sea deposits are situated in the epicenter on active geodynamic development of the Barents Sea <span class="hlt">rift</span> and, most important, over the zone of listric faults intersection, which consist a knot system over the mantle diapir. This is confirmed by prospecting seismology. Intrusion of hot mantle matter with further cooling down of abnormal lense might be a possible cause of appearance and evolution of ultradeep depressions. A high "seismic stratification" of the lower crust (nearly reaching the basement surface) at time scale about 8 sec. is typical for the inner, the deepest part of the depression. Supposing the "seismic stratified" lower crust correspond to "basalt" layer, this area is nearly upper crust ("granitic-gneiss") free. This fact confurmes conception on development of "granite free gaps" in the depression basement. Thick blocks of "seismically transparent" upper crust corresponding to the "granitic-gneiss" layer are marked out within Kolsk-Kanin monocline. An abrupt thickness decrease and appearance of "stratified" areas takes place at the southern edge of the depression. A filling of the over-<span class="hlt">rift</span> sag with sediments, revival of the faults and their effect on the filtration processes and gas hydrates formation took place in the South Barents Sea depression. Repeating activation of the fault blocks in the basement, especially during late Jurassic - early Cretaceous period contributed to formation of the structures related to the greatest deposits of the South Barents Sea depression. An extended field acoustic data collected in the Barents Sea led to understanding of general fundamental problems for all Arctic Seas and, first of all, the problem of Quaternary glaciations. An analysis of Eurasian-Arctic continental margin shows correspondence between the <span class="hlt">rift</span> systems of the shelf with those of the ocean. This relation can be observed by an example of the <span class="hlt">central</span> Arctic region. All the <span class="hlt">rift</span> systems underlying the sediment basin are expressed in the</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_13");'>13</a></li> <li><a href="#" onclick='return showDiv("page_14");'>14</a></li> <li class="active"><span>15</span></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_15 --> <div id="page_16" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_14");'>14</a></li> <li><a href="#" onclick='return showDiv("page_15");'>15</a></li> <li class="active"><span>16</span></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="301"> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/8153992','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/8153992"><span id="translatedtitle">A zoonotic focus of cutaneous leishmaniasis due to Leishmania tropica at Utut, <span class="hlt">Rift</span> Valley Province, Kenya.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Sang, D K; Njeru, W K; Ashford, R W</p> <p>1994-01-01</p> <p>Several foci of cutaneous leishmaniasis have been identified in <span class="hlt">central</span> Kenya and the <span class="hlt">Rift</span> Valley. One of these was the Utut focus of Leishmania tropica on the floor of the <span class="hlt">Rift</span> Valley between Gilgil and Elementaita, where intense transmission was detected. High infection and scar rates were detected among illegal charcoal burners in a previously uninhabited forest reserve on a lava flow containing numerous caves and rock crevices inhabited by sandflies and mammals which included hyraxes. Multiple lesions, predominantly in the head region, were common. 33% of cases occurred in people who had been less than one year in the area. Although most lesions healed within one or 2 years, some large, recrudescing lesions lasted several years.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013EGUGA..1510403P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013EGUGA..1510403P"><span id="translatedtitle">From surface observations to in depth structures and plate kinematics in oblique <span class="hlt">rifts</span>. Insights from the Main Ethiopian <span class="hlt">Rift</span>.</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Philippon, Melody; Sokoutis, Dimitrios; Willingshofer, Ernst; Corti, Giacomo; Bonini, Marco; Sani, Federico; Cloetingh, Sierd</p> <p>2013-04-01</p> <p>The Main Ethiopian <span class="hlt">Rift</span> (MER) is the oblique NE-SW trending plate boundary between Nubia and Somalia that developed above an inherited lithospheric-scale weak zone, the Mozambique Ocean Suture Zone (MOSZ). In this area, plate's kinematics has been estimated from various sources including: GPS and seismic data, spreading rate estimates, magnetic anomaly & paleostress reconstruction from field and seismic data. These various data sets give a range for the orientation of the stretching direction between N105° and 115°. However, in the MER, it is shown that strain is strongly partitioned between boundary and internal faults. Far field stress is re-oriented along the boundary faults that are activated in pure extension. This observation questions the use of paleostress reconstructions based on fault slip data and focal mechanisms to estimate the direction of plate motion. Detailed analysis of fault orientation and fault kinematics in analogue model illustrates that strain partitioning is triggered by the geometry of the deep-seated weak zone and that fault orientations give a better insight on the direction of stretching than paleostress tensors. Moreover, our model allows the recovering of the far field stress direction and the orientation of the weak zone in depth from surface observation of fault trend data. Applying this model to surface data of the MER give a different stretching direction for the formation of the boundary and the internal fault suggesting a clockwise rotation of Somalia. Also the model gives major constraints on the direction of the deep-seated weak zone. Its orientation is evolving from N62° in the Northern MER, to N18° in the Wide <span class="hlt">Rifted</span> Zone (passing from N35° and N25° for the <span class="hlt">Central</span> MER and the Southern MER).</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/17460671','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/17460671"><span id="translatedtitle">The role of fluids in lower-crustal earthquakes near continental <span class="hlt">rifts</span>.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Reyners, Martin; Eberhart-Phillips, Donna; Stuart, Graham</p> <p>2007-04-26</p> <p>The occurrence of earthquakes in the lower crust near continental <span class="hlt">rifts</span> 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 <span class="hlt">rift</span> 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 <span class="hlt">rift</span>, deepening from mostly less than 10 km in the <span class="hlt">central</span> 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 <span class="hlt">central</span> 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 <span class="hlt">rifts</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/6064904','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/6064904"><span id="translatedtitle">Pre-breakup geology of the Gulf of Mexico-Caribbean: Its relation to Triassic and Jurassic <span class="hlt">rift</span> systems of the region</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Bartok, P. )</p> <p>1993-02-01</p> <p>A review of the pre-breakup geology of west-<span class="hlt">central</span> Pangea, comprised of northern South America, Gulf of Mexico and West Africa, combined with a study of the Mesozoic <span class="hlt">rift</span> trends of the region confirms a relation between the <span class="hlt">rift</span> systems and the underlying older grain of deformation. The pre-breakup analysis focuses attention on the Precambrian, Early Paleozoic and Late Paleozoic tectonic events affecting the region and assumes a Pindell fit. Two Late Precambrian orogenic belts are observed in the west <span class="hlt">central</span> Pangea. Along the northern South American margin and Yucatan a paleo northeast trending Pan-African aged fold belt is documented. A second system is observed along West Africa extending from the High Atlas to the Mauritanides and Rockelides. During the Late Paleozoic, renewed orogenic activity, associated with the Gondwana/Laurentia suture, affected large segments of west <span class="hlt">central</span> Pangea. The general trend of the system is northeast-southwest and essentially parallels the Gyayana Shield, West African, and eastern North American cratons. Mesozoic <span class="hlt">rifting</span> closely followed either the Precambrian trends or the Late Paleozoic orogenic belt. The Triassic component focuses along the western portions of the Gulf of Mexico continuing into eastern Mexico and western South America. The Jurassic <span class="hlt">rift</span> trend followed along the separation between Yucatan and northern South America. At Lake Maracaibo the Jurassic <span class="hlt">rift</span> system eventually overlaps the Triassic <span class="hlt">rifts</span>. The Jurassic <span class="hlt">rift</span> resulted in the [open quotes]Hispanic Corridor[close quotes] that permitted Tethyan and Pacific marine faunas to mix at a time when the Gulf of Mexico underwent continental sedimentation.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2017JSG....96..192D','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2017JSG....96..192D"><span id="translatedtitle">The topology of evolving <span class="hlt">rift</span> fault networks: Single-phase vs multi-phase <span class="hlt">rifts</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Duffy, Oliver B.; Nixon, Casey W.; Bell, Rebecca E.; Jackson, Christopher A.-L.; Gawthorpe, Rob L.; Sanderson, David J.; Whipp, Paul S.</p> <p>2017-03-01</p> <p><span class="hlt">Rift</span> fault networks can be complex, particularly those developed by multiple periods of non-coaxial extension, comprising non-colinear faults with many interactions. Thus, topology, rather than simple geometry, is required to characterise such networks, as it provides a way to describe the arrangement of individual faults in the network. Topology is analysed here in terms of nodes (isolated I nodes or connected Y or X nodes) and branches (I-I, I-C, C-C branches). In map view, the relative proportions of these parameters vary in natural single- and multi-phase <span class="hlt">rift</span> fault networks and in scaled physical models at different stages of development and strain. Interactions in single-phase <span class="hlt">rifting</span> are limited to fault splays and along-strike fault linkage (I node and I-I or I-C branch dominated networks), whereas in multi-phase <span class="hlt">rifting</span> the topology evolves towards Y node and C-C branch dominated networks, with the degree of connectivity increasing with greater strain. The changes in topology and network connectivity have significant implications for fluid flow and reservoir compartmentalisation studies. Furthermore, topology helps to distinguish single and multiple phase extension (i.e. tectonic histories), and thus provide constraints on the geodynamic context of sedimentary basins.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.3304H','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.3304H"><span id="translatedtitle">Phanerozoic <span class="hlt">Rifting</span> Phases And Mineral Deposits</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Hassaan, Mahmoud</p> <p>2016-04-01</p> <p>In North Africa occur Mediterranean and Red Sea metallogenic provinces. In each province distribute 47 iron- manganese- barite and lead-zinc deposits with tectonic-structural control. The author presents in this paper aspects of position of these deposits in the two provinces with Phanerozoic <span class="hlt">rifting</span> . The Mediterranean Province belongs to two epochs, Hercynian and Alpine. The Hercynian Epoch manganese deposits in only Moroccoa- Algeria belong to Paleozoic tectonic zones and Proterozoic volcanics. The Alpine Epoch iron-manganese deposits are of post-orogenic exhalative-sedimentary origin. Manganese deposits in southern Morocco occur in Kabil-Rief quartz-chalcedony veins controlled by faults in andesitic sheets and in bedded pelitic tuffs, strata-form lenses and ore veins, in Precambrian schist and in Triassic and Cretaceous dolomites. Disseminated manganese with quartz and barite and effusive hydrothermal veins are hosted in Paleocene volcanics. Manganese deposits in Algeria are limited and unrecorded in Tunisia. Strata-form iron deposits in Atlas Heights are widespread in sub-<span class="hlt">rift</span> zone among Jurassic sediments inter-bedding volcanic rocks. In Algeria, Group Beni-Saf iron deposits are localized along the Mediterranean coast in terrigenous and carbonate rocks of Jurassic, Cretaceous and Eocene age within faults and bedding planes. In Morocco strata-form hydrothermal lead-zinc deposits occur in contact zone of Tertiary andesite inter-bedding Cambrian shale, Lias dolomites and Eocene andesite. In both Algeria and Tunisia metasomatic Pb-Zn veins occur in Campanian - Maastrichtian carbonates, Triassic breccia, Jurassic limestone, Paleocene sandstones and limestone and Neogene conglomerates and sandstones. The Red Sea metallogenic province belongs to the Late Tertiary-Miocene times. In Wadi Araba hydrothermal iron-manganese deposits occur in Cretaceous sediments within 320°and 310 NW faults related to Tertiary basalt. Um-Bogma iron-manganese deposits are closely</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFM.T44C..04N','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFM.T44C..04N"><span id="translatedtitle">Geochemical evidence for pre- and syn-<span class="hlt">rifting</span> lithospheric foundering in the East African <span class="hlt">Rift</span> System</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Nelson, W. R.; Furman, T.; Elkins-Tanton, L. T.</p> <p>2015-12-01</p> <p>The East African <span class="hlt">Rift</span> System (EARS) is the archetypal active continental <span class="hlt">rift</span>. The <span class="hlt">rift</span> branches cut through the elevated Ethiopian and Kenyan domes and are accompanied by a >40 Myr volcanic record. This record is often used to understand changing mantle dynamics, but this approach is complicated by the diversity of spatio-temporally constrained, geochemically unique volcanic provinces. Various sources have been invoked to explain the geochemical variability across the EARS (e.g. mantle plume(s), both enriched and depleted mantle, metasomatized or pyroxenitic lithosphere, continental crust). Mantle contributions are often assessed assuming adiabatic melting of mostly peridotitic material due to extension or an upwelling thermal plume. However, metasomatized lithospheric mantle does not behave like fertile or depleted peridotite mantle, so this model must be modified. Metasomatic lithologies (e.g. pyroxenite) are unstable compared to neighboring peridotite and can founder into the underlying asthenosphere via ductile dripping. As such a drip descends, the easily fusible metasomatized lithospheric mantle heats conductively and melts at increasing T and P; the subsequent volcanic products in turn record this drip magmatism. We re-evaluated existing data of major mafic volcanic episodes throughout the EARS to investigate potential evidence for lithospheric drip foundering that may be an essential part of the <span class="hlt">rifting</span> process. The data demonstrate clearly that lithospheric drip melting played an important role in pre-flood basalt volcanism in Turkana (>35 Ma), high-Ti "mantle plume-derived" flood basalts and picrites (HT2) from NW Ethiopia (~30 Ma), Miocene shield volcanism on the E Ethiopian Plateau and in Turkana (22-26 Ma), and Quaternary volcanism in Virunga (Western <span class="hlt">Rift</span>) and Chyulu Hills (Eastern <span class="hlt">Rift</span>). In contrast, there is no evidence for drip melting in "lithosphere-derived" flood basalts (LT) from NW Ethiopia, Miocene volcanism in S Ethiopia, or Quaternary</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014EGUGA..16.8722B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014EGUGA..16.8722B"><span id="translatedtitle">Geophysical glimpses into the Ferrigno <span class="hlt">Rift</span> at the northwestern tip of the West Antarctic <span class="hlt">Rift</span> System</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Bingham, Robert; Ferraccioli, Fausto</p> <p>2014-05-01</p> <p>The West Antarctic <span class="hlt">Rift</span> System (WARS) forms one of the largest continental <span class="hlt">rift</span> systems on Earth. The WARS is of major significance as it forms the lithospheric cradle for the marine-based and potentially unstable West Antarctic Ice Sheet (WAIS). Seismic refraction, reflection, aeromagnetic, gravity and drilling in the Ross Sea have revealed most of what we know about its structure, tectonic and magmatic patterns and sedimentary basins. Aerogeophysical research and passive seismic networks have considerably extended our knowledge of the WARS and its influence on the overlying WAIS in the Siple Coast and Amundsen Sea Embayment (ASE) regions. The Bellingshausen Sea Embayment region has however remained largely unexplored, and hence the possible extent of the WARS in this sector has remained poorly constrained. Here we use a combination of reconnaissance ground-based and airborne radar observations, airborne gravity, satellite gravity and aeromagnetic data to investigate the WARS in the Bellingshausen Sea Embayment, in the area of the Ferrigno Ice Stream (Bingham et al., 2012, Nature). This region is of high significance, as it one of the main sectors of the WAIS that is currently exhibiting rapid ice loss, thought to be driven primarily by oceanic warming. Assessing geological controls on subice topography and ice dynamics is therefore of prime importance in this part of the WAIS. Ground-based and airborne radar image a subglacial basin beneath the Ferrigno Ice Stream that is up to 1.5 kilometres deep and that connects the ice-sheet interior to the Bellingshausen Sea margin. We interpret this basin as a narrow, glacially overdeepened <span class="hlt">rift</span> basin that formed at the northwestern tip of the WARS. Satellite gravity data cannot resolve such a narrow <span class="hlt">rift</span> basin but indicate that the crust beneath the region is likely thinned, lending support to the hypothesis that this area is indeed part of the WARS. Widely-spaced aeromagnetic data image a linear low along the inferred</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2006DPS....38.1804H','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2006DPS....38.1804H"><span id="translatedtitle">South Polar Cycloidal <span class="hlt">Rift</span> on Enceladus</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Hurford, Terry A.; Greenberg, R.; Hoppa, G. V.</p> <p>2006-09-01</p> <p>Large <span class="hlt">rifts</span> near the south pole of Enceladus have been observed to be significantly warmer than the surrounding surface (Spencer et al. 2006 Science 311) and are most likely the source of jets of icy particles observed by Cassini (Porco et al. 2006 Science 311). The observed tectonics within the region have been proposed to have formed in response to stress from a diapir-induced reorientation of Encleadus (Nimmo and Pappalardo 2006 Nature 441) which should have dominated the formation of tectonics in the region. One prominent <span class="hlt">rift</span> on Enceladus’ south pole resembles the shape of cycloids on Europa. We explore the possibility that tidal tectonic processes may have formed this <span class="hlt">rift</span> at its current latitude in a similar manner to cycloids on Europa (i.e. that its formation reflects a crack's response to a diurnally varying stress field). The ability of Saturn to raise a significant tide on Enceladus is enhanced, since Enceladus’ surface gravity is small. Moreover, Enceladus’ eccentricity causes the tide to oscillate in magnitude by 1% as it completes an orbit. Not only does the size of the tide on Enceladus change daily, but it oscillates 0.5o eastward and westward. The daily change in position and magnitude of the tidal bulge produces stress on the surface of Enceladus that may play a significant role in the formation of observed surface tectonics. On Europa cycloidal cracks form as a result of tensile cracking in response to this type of diurnally varying stress (Hoppa et al. 1999 Science 285). As the cracks form and propagate across the surface, their paths are affected as the stresses rotate in the region. Modeling observed features allows for their formation locations (relative to Jupiter) to be determined (Hoppa et al. 2001 Icarus 153, Hurford 2005 Thesis).</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFM.T41F..02S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFM.T41F..02S"><span id="translatedtitle">Early-stage <span class="hlt">rifting</span> in the southwest East African <span class="hlt">Rift</span>: Insights from new reflection seismic data from Lakes Tanganyika and Malawi (Nyasa)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Scholz, C. A.; Wood, D. A.; Shillington, D. J.; McCartney, T.; Accardo, N. J.</p> <p>2015-12-01</p> <p>The western branch of the East African <span class="hlt">Rift</span> is characterized by modest amounts of mainly amagmatic extension; deeply-subsided, fault-controlled basins; and large-magnitude, deep seismicity. Lakes Tanganyika and Malawi are two of the world's largest lakes, with maximum water depths of 1450 and 700 m respectively. Newly acquired seismic reflection data, along with newly reprocessed legacy data reveal thick sedimentary sections, in excess of 5 km in some localities. The 1980's vintage legacy data from Project PROBE have been reprocessed through pre-stack depth migration in Lake Tanganyika, and similar reprocessing of legacy data from Lake Malawi is forthcoming. New high-fold and large-source commercial and academic data have recently been collected in southern Lake Tanganyika, and in the northern and <span class="hlt">central</span> basins of Lake Malawi as part of the 2015 SEGMeNT project. In the case of Lake Tanganyika, new data indicate the presence of older sediment packages that underlie previously identified "pre-<span class="hlt">rift</span>" basement (the "Nyanja Event"). These episodes of sedimentation and extension may substantially predate the modern lake. These deep stratal reflections are absent in many localites, possibly on account of attenuation of the acoustic signal. However in one area of southern Lake Tanganyika, the newly-observed deep strata extend axially for ~70 km, likely representing deposits from a discrete paleolake. The high-amplitude Nyanja Event is interpreted as the onset of late-Cenozoic <span class="hlt">rifting</span>, and the changing character of the overlying depositional sequences reflects increasing relief in the <span class="hlt">rift</span> valley, as well as the variability of fluvial inputs, and the intermittent connectivity of upstream lake catchments. Earlier Tanganyika sequences are dominated by shallow lake and fluvial-lacustrine facies, whereas later sequences are characterized by extensive gravity flow deposition in deep water, and pronounced erosion and incision in shallow water depths and on littoral platforms. The</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70033627','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70033627"><span id="translatedtitle">Geometry of the neoproterozoic and paleozoic <span class="hlt">rift</span> margin of western Laurentia: Implications for mineral deposit settings</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Lund, K.</p> <p>2008-01-01</p> <p>The U.S. and Canadian Cordilleran miogeocline evolved during several phases of Cryogenian-Devonian intracontinental <span class="hlt">rifting</span> that formed the western mangin of Laurentia. Recent field and dating studies across <span class="hlt">central</span> Idaho and northern Nevada result in identification of two segments of the <span class="hlt">rift</span> margin. Resulting interpretations of <span class="hlt">rift</span> geometry in the northern U.S. Cordillera are compatible with interpretations of northwest- striking asymmetric extensional segments subdivided by northeast-striking transform and transfer segments. The new interpretation permits integration of miogeoclinal segments along the length of the western North American Cordillera. For the U.S. Cordillera, miogeoclinal segments include the St. Mary-Moyie transform, eastern Washington- eastern Idaho upper-plate margin, Snake River transfer, Nevada-Utah lower-plate margin, and Mina transfer. The <span class="hlt">rift</span> is orthogonal to most older basement domains, but the location of the transform-transfer zones suggests control of them by basement domain boundaries. The zigzag geometry of reentrants and promontories along the <span class="hlt">rift</span> is paralleled by salients and recesses in younger thrust belts and by segmentation of younger extensional domains. Likewise, transform transfer zones localized subsequent transcurrent structures and igneous activity. Sediment-hosted mineral deposits trace the same zigzag geometry along the margin. Sedimentary exhalative (sedex) Zn-Pb-Ag ??Au and barite mineral deposits formed in continental-slope rocks during the Late Devonian-Mississippian and to a lesser degree, during the Cambrian-Early Ordovician. Such deposits formed during episodes of renewed extension along miogeoclinal segments. Carbonate-hosted Mississippi Valley- type (MVT) Zn-Pb deposits formed in structurally reactivated continental shelf rocks during the Late Devonian-Mississippian and Mesozoic due to reactivation of preexisting structures. The distribution and abundance of sedex and MVT deposits are controlled by the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.5624T','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.5624T"><span id="translatedtitle">The deep structure of Alpine-type orogens: how important is <span class="hlt">rift</span>-inheritance?</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Tugend, Julie; Manatschal, Gianreto; Mohn, Geoffroy</p> <p>2016-04-01</p> <p>Collisional belts are commonly thought to result from the closure of oceanic basins and subsequent inversion of former <span class="hlt">rifted</span> margins. The formation and evolution of collisional belts should therefore be closely interlinked with the initial architecture of former <span class="hlt">rifted</span> margins. Reflection and refraction seismic data from present-day magma-poor <span class="hlt">rifted</span> margins show the omnipresence of hyperextended domains (severely thinned continental crust (<10 km) and/or exhumed serpentinized mantle with minor magmatic additions) between unequivocal continental and oceanic domains. Integrating these new observations and exploring their impact on mountain building processes may result in alternative interpretations of the lithospheric structure of collisional orogens. We focus on the Pyrenees and Western to <span class="hlt">Central</span> Alps, respectively resulting from the inversion of a Late Jurassic to Mid Cretaceous and an Early to Mid Jurassic <span class="hlt">rift</span> system eventually floored by hyperextended crust, exhumed mantle or proto-oceanic crust. The <span class="hlt">rift</span>-related pre-collisional architecture of the Pyrenees shows many similarities with that proposed for the Alps; although the width of the hyperextended and in particular of the proto-oceanic domains is little constrained. Contrasting with the Pyrenees, remnants of these domains are largely affected by orogeny-related deformation and show a HP-LT to HT-MP metamorphic overprint in the Alps. Nevertheless, in spite of the occurrence of these highly deformed and metamorphosed rocks constituting the internal parts of the Alps, the overall crustal and lithospheric structure looks surprisingly comparable. High resolution tomographic images across both orogens unravel the occurrence of a velocity anomaly dipping underneath the internal domains and progressively attenuated at depth that we interpret as former hyperextended domains subducted/underthrusted during collision. This interpretation contrasts with the classical assumption that the subducted material is made of</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=5189134','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=5189134"><span id="translatedtitle"><span class="hlt">Rift</span> Valley Fever Outbreak in Livestock, Mozambique, 2014</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Coetzee, Peter; Mubemba, Benjamin; Nhambirre, Ofélia; Neves, Luis; Coetzer, J.A.W.; Venter, Estelle H.</p> <p>2016-01-01</p> <p>In early 2014, abortions and death of ruminants were reported on farms in Maputo and Gaza Provinces, Mozambique. Serologic analysis and quantitative and conventional reverse transcription PCR confirmed the presence of <span class="hlt">Rift</span> Valley fever virus. The viruses belonged to lineage C, which is prevalent among <span class="hlt">Rift</span> Valley fever viruses in southern Africa. PMID:27869589</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/27869589','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/27869589"><span id="translatedtitle"><span class="hlt">Rift</span> Valley Fever Outbreak in Livestock, Mozambique, 2014.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Fafetine, José M; Coetzee, Peter; Mubemba, Benjamin; Nhambirre, Ofélia; Neves, Luis; Coetzer, J A W; Venter, Estelle H</p> <p>2016-12-01</p> <p>In early 2014, abortions and death of ruminants were reported on farms in Maputo and Gaza Provinces, Mozambique. Serologic analysis and quantitative and conventional reverse transcription PCR confirmed the presence of <span class="hlt">Rift</span> Valley fever virus. The viruses belonged to lineage C, which is prevalent among <span class="hlt">Rift</span> Valley fever viruses in southern Africa.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015EGUGA..17.4444B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015EGUGA..17.4444B"><span id="translatedtitle">Fault Orientations at Obliquely <span class="hlt">Rifted</span> Margins: Where? When? Why?</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Brune, Sascha</p> <p>2015-04-01</p> <p>Present-day knowledge of <span class="hlt">rifted</span> margin formation is largely based on 2D seismic lines, 2D conceptual models, and corroborated by 2D numerical experiments. However, the 2D assumption that the extension direction is perpendicular to the <span class="hlt">rift</span> trend is often invalid. In fact, worldwide more than 75% of all <span class="hlt">rifted</span> margin segments have been formed under significant obliquity exceeding 20° (angle measured between extension direction and <span class="hlt">rift</span> trend normal): During formation of the Atlantic Ocean, oblique <span class="hlt">rifting</span> dominated at the sheared margins of South Africa and Patagonia, the Equatorial Atlantic margins, separation of Greenland and North America, and it played a major role in the protracted <span class="hlt">rift</span> history of the North East Atlantic. Outside the Atlantic Ocean, oblique <span class="hlt">rifting</span> occurred during the split between East and West Gondwana, the separation of India and Australia, India and Madagascar, Australia and Antarctica, as well as Arabia and Africa. It is presently observed in the Gulf of California, the Aegean and in the East African <span class="hlt">Rift</span>. Despite its significance, the degree to which oblique lithospheric extension affects first-order <span class="hlt">rift</span> 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 <span class="hlt">rift</span> model to oblique extensional settings. The presented forward experiments cover the whole spectrum of oblique extension (i.e. <span class="hlt">rift</span>-orthogonal extension, low obliquity, high obliquity, strike-slip deformation) from initial deformation to breakup. They are conducted using an elasto-visco-plastic finite element model and involve crustal and mantle layers accounting for self-consistent necking of the lithosphere. Results are thoroughly compared to previous analogue experiments, which yields many similarities but also distinct differences for late <span class="hlt">rift</span> stages and for high obliquity. Even though the model</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/7477344','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/7477344"><span id="translatedtitle">The first australopithecine 2,500 kilometres west of the <span class="hlt">Rift</span> Valley (Chad)</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Brunet, M; Beauvilain, A; Coppens, Y; Heintz, E; Moutaye, A H; Pilbeam, D</p> <p>1995-11-16</p> <p>The first sites with Pliocene and Pleistocene mammals west of the <span class="hlt">Rift</span> Valley in <span class="hlt">Central</span> Africa in northern Chad were reported in 1959 (ref. 1), and documented the presence of mixed savannah and woodland habitats. Further sites and a probable Homo erectus cranio-facial fragment were subsequently discovered. In 1993 a survey of Pliocene and Pleistocene formations in the Borkou-Ennedi-Tibesti Province of Chad (B.E.T.) led to the discovery of 17 new sites in the region of Bahr el Ghazal (classical Arabic for River of the Gazelles) near Koro Toro. One site, KT 12 (15 degrees 58'10"N, 18 degrees 52'46"E) yielded an australopithecine mandible associated with a fauna biochronologically estimated to be 3.0-3.5 Myr old. Australopithecine species described since 1925 are known from southern Africa and from sites spread along the eastern <span class="hlt">Rift</span> Valley from Tanzania to Ethiopia (Fig. 1). This new find from Chad, which is most similar in morphology to Australopithecus afarensis, documents the presence of an early hominid a considerable distance, 2,500 km, west of the <span class="hlt">Rift</span> Valley.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2010AGUFM.T31C2193P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2010AGUFM.T31C2193P"><span id="translatedtitle">Petrological Constraints on Melt Generation Beneath the Asal <span class="hlt">Rift</span> (Djibouti)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Pinzuti, P.; Humler, E.; Manighetti, I.; Gaudemer, Y.; Bézos, A.</p> <p>2010-12-01</p> <p>The temporal evolution of the mantle melting processes in the Asal <span class="hlt">Rift</span> is evaluated from the chemical composition of 95 lava flows sampled along 10 km of the <span class="hlt">rift</span> axis and 8 km off-axis (that is for the last 650 ky). The major element composition and the trace element ratios of aphyric basalts across the Asal <span class="hlt">Rift</span> show a symmetric pattern relative to the <span class="hlt">rift</span> axis and preserved a clear signal of mantle melting depth variations. FeO, Fe8.0, Sm/YbN and Zr/Y increase, whereas SiO2 and Lu/HfN decrease from the <span class="hlt">rift</span> axis to the <span class="hlt">rift</span> shoulders. These variations are qualitatively consistent with a shallower melting beneath the <span class="hlt">rift</span> axis than off-axis and the data show that the melting regime is inconsistent with a passive upwelling model. In order to quantify the depth range and extent of melting, we invert Na8.0 and Fe8.0 contents of basalts based on a pure active upwelling model. Beneath the <span class="hlt">rift</span> axis, melting paths are shallow, from 60 to 30 km. These melting paths are consistent with adiabatic melting in normal-temperature asthenosphere, beneath an extensively thinned mantle lithosphere. In contrast, melting on the <span class="hlt">rift</span> shoulders occurred beneath a thick mantle lithosphere and required mantle solidus temperature 180°C hotter than normal (melting paths from 110 to 75 km). The calculated rate of lithospheric thinning is high (6.0 cm yr-1) and could explain the survival of a metastable garnet within the mantle at depth shallower than 90 km beneath the modern Asal <span class="hlt">Rift</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/5461480','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/5461480"><span id="translatedtitle">Humility and opportunity in Rio Grande <span class="hlt">rift</span></span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Black, B.A. )</p> <p>1989-09-01</p> <p>One of the hardest things to do is to admit a mistake. But we can often learn valuable lessons when we analyze why we made our mistakes and when we admit our vulnerability to the unknown. In 1984, the authors published an article that described what they referred to privately as a geologic Moebius loop where a seismic grid appeared to grossly mistie. They though they stayed in the same Paleozoic carbonate formation all the way around the loop. Drilling subsequent to that article has deepened the mystery of the correlations by proving that the target formations were indeed limestones and showing they may not be the Paleozoic limestones they though they started in but are perhaps a new, previously undescribed sequence of thick Tertiary lacustrine limestone. The Yates La Mesa 2 well in Sec. 24, T17N, R8E, has spawned more questions than it has answered, but it has paradoxically also pointed out the potential for new and exciting plays in the Rio Grande <span class="hlt">rift</span> basins and has given them an opportunity to show how previous misinterpreted work can teach new lessons. Seismic lines in the <span class="hlt">rift</span> and in the Yates well illustrate how they can both deceive and inspire themselves in the search for hydrocarbons.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2010Tectp.488....7A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2010Tectp.488....7A"><span id="translatedtitle">On the geodynamics of the Aegean <span class="hlt">rift</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Agostini, Samuele; Doglioni, Carlo; Innocenti, Fabrizio; Manetti, Piero; Tonarini, Sonia</p> <p>2010-06-01</p> <p>The Aegean <span class="hlt">rift</span> is considered to be either a classic backarc basin, or the result of the westward escape of Anatolia, or the effect of a gravitational collapse of an over-thickened lithosphere. Here these models are questioned. We alternatively present a number of geodynamic and magmatic constraints suggesting a simple model for the genesis of the extension as being related to the differential advancement of the upper lithosphere over a heterogeneous lower African plate. The Greek microplate overrides the Ionian oceanic segment of the African plate faster than the Anatolian microplate over the thicker Levantine more continental segment. This setting is evidenced by GPS-velocity gradient in the hangingwall of the Hellenic-Cyprus subduction system and requires a zone of <span class="hlt">rifting</span> splitting the hangingwall into two microplates. This mechanism is unrelated to the replacement of retreated slab by the asthenosphere as typically occurs in the backarc of west-directed subduction zones. The supposed greater dehydration of the Ionian segment of the slab is providing a larger amount of fluids into the low velocity channel at the top of the asthenosphere, allowing a faster decoupling between the Greek microplate and the underlying mantle with respect to the Anatolian microplate. Slab ruptures associated with the differential retreat controlled by the inherited lithospheric heterogeneities in the lower plate and the proposed upwelling of the mantle suggested by global circulation models would explain the occurrence and coexistence of slab-related and slab-unrelated magmatism.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70011482','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70011482"><span id="translatedtitle">Extension in the Rio Grande <span class="hlt">rift</span>.</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Cordell, L.</p> <p>1982-01-01</p> <p>A positive gravity anomaly along the axis of the Rio Grande <span class="hlt">rift</span> reflects a volume of anomalous mass added at the base of the crust and intruded into the crust. Part of this volume can be associated with vertical uplift of the crust. The remainder of this anomalous volume, plus the volume of surficial graben fill, can be associated with horizontal crustal extension. The volume of crustal uplift in the Rio Grande <span class="hlt">rift</span> is unknown, but this term can be eliminated by means of an independent equation provided by assumption of generalized isostatic equilibrium. The volume and mass equations combined provide a solution for extension of the crust in terms of the following parameters: total anomalous mass deficiency in the mantle lithosphere, total anomalous mass excess in the crust and its density contrast, total anomalous mass deficiency of surficial graben fill and its density contrast, and the volume of material eroded from the uplift. Using standard density estimates and masses determined by equivalent-source modeling of gravity profiles, I obtained 1-km extension at 37oN (Colorado-New Mexico border), 13- km extension at 35oN (Albuquerque, New Mexico), and 24-km extension at 33oN in S New Mexico.-Author</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_14");'>14</a></li> <li><a href="#" onclick='return showDiv("page_15");'>15</a></li> <li class="active"><span>16</span></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_16 --> <div id="page_17" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_15");'>15</a></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li class="active"><span>17</span></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="321"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016E%26PSL.440...43A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016E%26PSL.440...43A"><span id="translatedtitle">Tectonic focusing of voluminous basaltic eruptions in magma-deficient backarc <span class="hlt">rifts</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Anderson, Melissa O.; Hannington, Mark D.; Haase, Karsten; Schwarz-Schampera, Ulrich; Augustin, Nico; McConachy, Timothy F.; Allen, Katie</p> <p>2016-04-01</p> <p>The Coriolis Troughs of the New Hebrides subduction zone are among the youngest backarc <span class="hlt">rifts</span> in the world. They reach depths of >3 km, despite their small size (<100 km in length and only 25-45 km wide) and their proximity to the arc front (˜50 km). The narrow, deep graben morphology is characteristic of magma-deficient arc <span class="hlt">rifts</span> in the early stages of backarc extension, where the rate of extension and subsidence exceeds the magmatic input. Unexpectedly, the youngest graben, the Vate Trough, contains a <span class="hlt">centrally</span>-located 1000-m tall and 14-km wide shield volcano with a large, 5 × 8 km breached summit caldera. The Nifonea axial volcano has a volume of ˜126 km3, reflecting unusually high extrusion rates, given its young age (<3 Ma), and the summit caldera hosts the remnants of a large lava lake, the first described from a submarine backarc setting. Extensive diffuse hydrothermal venting and several clusters of black smoker chimneys, with the highest recorded fluid temperatures (368 °C) in the SW Pacific, occur on the youngest lava flows. Comparison with similar axial volcanoes on the mid-ocean ridges suggests that the 46 ×106 m3 of sheet flows in the caldera could have been erupted in <30 hours. The focusing of voluminous basaltic eruptions into an otherwise magma-deficient backarc has been linked to strong left-lateral transtension caused by clockwise rotation and segmentation of the southern portion of the arc after collision with d'Entrecasteaux ridge. This study shows that the upper plate stresses can result in dramatic variability in magma supply and hydrothermal activity at the earliest stages of arc <span class="hlt">rifting</span> and could explain the wide range of melt compositions, volcanic styles and mineral deposit types found in nascent backarc <span class="hlt">rifts</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016E%26PSL.446...77M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016E%26PSL.446...77M"><span id="translatedtitle">Magmatic cycles pace tectonic and morphological expression of <span class="hlt">rifting</span> (Afar depression, Ethiopia)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Medynski, S.; Pik, R.; Burnard, P.; Dumont, S.; Grandin, R.; Williams, A.; Blard, P.-H.; Schimmelpfennig, I.; Vye-Brown, C.; France, L.; Ayalew, D.; Benedetti, L.; Yirgu, G.</p> <p>2016-07-01</p> <p>The existence of narrow axial volcanic zones of mid-oceanic ridges testifies of the underlying concentration of both melt distribution and tectonic strain. As a result of repeated diking and faulting, axial volcanic zones therefore represent a spectacular topographic expression of plate divergence. However, the submarine location of oceanic ridges makes it difficult to constrain the interplay between tectonic and magmatic processes in time and space. In this study, we use the Dabbahu-Manda Hararo (DMH) magmatic <span class="hlt">rift</span> segment (Afar, Ethiopia) to provide quantitative constraints on the response of tectonic processes to variations in magma supply at divergent plate boundaries. The DMH magmatic <span class="hlt">rift</span> segment is considered an analogue of an oceanic ridge, exhibiting a fault pattern, extension rate and topographic relief comparable to intermediate- to slow-spreading ridges. Here, we focus on the northern and <span class="hlt">central</span> parts of DMH <span class="hlt">rift</span>, where we present quantitative slip rates for the past 40 kyr for major and minor normal fault scarps in the vicinity of a recent (September 2005) dike intrusion. The data obtained show that the axial valley topography has been created by enhanced slip rates that occurred during periods of limited volcanism, suggestive of reduced magmatic activity, probably in association with changes in strain distribution in the crust. Our results indicate that the development of the axial valley topography has been regulated by the lifetimes of the magma reservoirs and their spatial distribution along the segment, and thus to the magmatic cycles of replenishment/differentiation (<100 kyr). Our findings are also consistent with magma-induced deformation in magma-rich <span class="hlt">rift</span> segments. The record of two tectonic events of metric vertical amplitude on the fault that accommodated the most part of surface displacement during the 2005 dike intrusion suggests that the latter type of intrusion occurs roughly every 10 kyr in the northern part of the DMH segment.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012GGG....13.8001S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012GGG....13.8001S"><span id="translatedtitle">Magmatic lithospheric heating and weakening during continental <span class="hlt">rifting</span>: A simple scaling law, a 2-D thermomechanical <span class="hlt">rifting</span> model and the East African <span class="hlt">Rift</span> System</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Schmeling, Harro; Wallner, Herbert</p> <p>2012-08-01</p> <p>Continental <span class="hlt">rifting</span> is accompanied by lithospheric thinning and decompressional melting. After extraction, melt is intruded at shallower depth thereby heating and weakening the lithosphere. In a feedback mechanism this weakening may assist <span class="hlt">rifting</span> and melt production. A one-dimensional kinematic lithospheric thinning model is developed including decompressional melting and intrusional magma deposition. The intrusional heating effect is determined as a function of thinning rate and amount, melting parameters, potential temperature, and the depth range of emplacement. The temperature increases approximately proportionally to the square root of the thinning rate and to the square of the supersolidus potential temperature. Simple scaling laws are derived allowing predicting these effects and the surface heat flux for arbitrary scenarios. Two-dimensional thermomechanical extension models are carried out for a multicomponent (crust-mantle) two-phase (melt-matrix) system with a rheology based on laboratory data including magmatic weakening. In good agreement with the 1-D kinematic models it is found that the lithosphere may heat up by several 100 K. This heating enhances viscous weakening by one order of magnitude or more. In a feedback mechanism <span class="hlt">rifting</span> is dynamically enforced, leading to a significant increase of <span class="hlt">rift</span> induced melt generation. Including the effect of lateral focusing of magma toward the <span class="hlt">rift</span> axis the laws are applied to different segments of the East African <span class="hlt">Rift</span> System. The amount of intrusional heating increases with maturity of the <span class="hlt">rift</span> from O(10 K) to up to 200 K or 400 K at the Afar <span class="hlt">Rift</span> depending on the depth range of the magmatic emplacement.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/19198772','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/19198772"><span id="translatedtitle">Tectonics of the baikal <span class="hlt">rift</span> deduced from volcanism and sedimentation: a review oriented to the Baikal and Hovsgol lake systems.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Ivanov, Alexei V; Demonterova, Elena I</p> <p>2009-01-01</p> <p>As known from inland sedimentary records, boreholes, and geophysical data, the initiation of the Baikal <span class="hlt">rift</span> basins began as early as the Eocene. Dating of volcanic rocks on the <span class="hlt">rift</span> shoulders indicates that volcanism started later, in the Early Miocene or probably in the Late Oligocene. Prominent tectonic uplift took place at about 20 Ma, but information (from both sediments and volcanics) on the initial stage of the <span class="hlt">rifting</span> is scarce and incomplete. A comprehensive record of sedimentation derived from two stacked boreholes drilled at the submerged Akademichesky ridge indicates that the deep freshwater Lake Baikal existed for at least 8.4 Ma, while the exact formation of the lake in its roughly present-day shape and volume is unknown. Four important events of tectonic/environmental changes at about approximately 7, approximately 5, approximately 2.5, and approximately 0.1 Ma are seen in that record. The first event probably corresponds to a stage of <span class="hlt">rift</span> propagation from the historical center towards the wings of the <span class="hlt">rift</span> system. <span class="hlt">Rifting</span> in the Hovsgol area was initiated at about this time. The event of ~5 Ma is a likely candidate for the boundary between slow and fast stages of <span class="hlt">rifting</span>. It is reflected in a drastic change of sedimentation rate due to isolation of the Akademichesky ridge from the <span class="hlt">central</span> and northern Lake Baikal basins. The youngest event of 0.1 Ma is reflected by the (87)0Sr/ (86)Sr ratio increase in Lake Baikal waters and probably related to an increasing rate of mountain growth (and hence erosion) resulting from glacial rebounding. The latter is responsible for the reorganization of the outflow pattern with the termination of the paleo-Manzurka outlet and the formation of the Angara outlet. The event of approximately 2.5 Ma is reflected in the decrease of the (87)Sr/(86)Sr and Na/Al ratios in Lake Baikal waters. We suggest that it is associated with a decrease of the dust load due to a reorganization of the atmospheric circulations in Mainland</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2010EGUGA..12.2376L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2010EGUGA..12.2376L"><span id="translatedtitle">New insights into continental <span class="hlt">rifting</span> from a damage rheology modeling</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Lyakhovsky, Vladimir; Segev, Amit; Weinberger, Ram; Schattner, Uri</p> <p>2010-05-01</p> <p>Previous studies have discussed how tectonic processes could produce relative tension to initiate and propagate <span class="hlt">rift</span> zones and estimated the magnitude of the <span class="hlt">rift</span>-driving forces. Both analytic and semi-analytic models as well as numerical simulations assume that the tectonic force required to initiate <span class="hlt">rifting</span> is available. However, Buck (2004, 2006) estimated the minimum tectonic force to allow passive <span class="hlt">rifting</span> and concluded that the available forces are probably not large enough for <span class="hlt">rifting</span> of thick and strong lithosphere in the absence of basaltic magmatism (the "Tectonic Force" Paradox). The integral of the yielding stress needed for <span class="hlt">rifting</span> over the thickness of the normal or thicker continental lithosphere are well above the available tectonic forces and tectonic <span class="hlt">rifting</span> cannot happen (Buck, 2006). This conclusion is based on the assumption that the tectonic stress has to overcome simultaneously the yielding stress over the whole lithosphere thickness and ignore gradual weakening of the brittle rocks under long-term loading. In this study we demonstrate that the <span class="hlt">rifting</span> process under moderate tectonic stretching is feasible due to gradual weakening and "long-term memory" of the heavily fractured brittle rocks, which makes it significantly weaker than the surrounding intact rock. This process provides a possible solution for the tectonic force paradox. We address these questions utilizing 3-D lithosphere-scale numerical simulations of the plate motion and faulting process base on the damage mechanics. The 3-D modeled volume consists of three main lithospheric layers: an upper layer of weak sediments, middle layer of crystalline crust and lower layer of the lithosphere mantle. Results of the modeling demonstrate gradual formation of the <span class="hlt">rift</span> zone in the continental lithosphere with the flat layered structure. Successive formation of the <span class="hlt">rift</span> system and associated seismicity pattern strongly depend not only on the applied tectonic force, but also on the healing</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015JVGR..303..112M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015JVGR..303..112M"><span id="translatedtitle"><span class="hlt">Rift</span> zones and magma plumbing system of Piton de la Fournaise volcano: How do they differ from Hawaii and Etna?</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Michon, Laurent; Ferrazzini, Valérie; Di Muro, Andrea; Villeneuve, Nicolas; Famin, Vincent</p> <p>2015-09-01</p> <p>On ocean basaltic volcanoes, magma transfer to the surface proceeds by subvertical ascent from the mantle lithosphere through the oceanic crust and the volcanic edifice, possibly followed by lateral propagation along <span class="hlt">rift</span> zones. We use a 19-year-long database of volcano-tectonic seismic events together with detailed mapping of the cinder cones and eruptive fissures to determine the geometry and the dynamics of the magma paths intersecting the edifice of Piton de la Fournaise volcano. We show that the overall plumbing system, from about 30 km depth to the surface, is composed of two structural levels that feed distinct types of <span class="hlt">rift</span> zones. The deep plumbing system is rooted between Piton des Neiges and Piton de la Fournaise volcanoes and has a N30-40 orientation. Above 20 km below sea level (bsl), the main axis switches to a N120 orientation, which permits magma transfer from the lithospheric mantle to the base of the oceanic crust, below the summit of Piton de la Fournaise. The related NW-SE <span class="hlt">rift</span> zone is 15 km wide, linear, spotted by small to large pyroclastic cones and related lava flows and emits slightly alkaline magmas resulting from high-pressure fractionation of clinopyroxene ± olivine. This <span class="hlt">rift</span> zone has low magma production rate of ~ 0.5-3.6 × 10- 3 m3s- 1 and an eruption periodicity of around 200 years over the last 30 ka. Seismic data suggest that the long-lasting activity of this <span class="hlt">rift</span> zone result from regional NNE-SSW extension, which reactivates inherited lithospheric faults by the effect of underplating and/or thermal erosion of the mantle lithosphere. The shallow plumbing system (< 11 km bsl) connects the base of the crust with the <span class="hlt">Central</span> Cone. It is separated from the deep plumbing system by a relatively large aseismic zone between 8 and 11 km bsl, which may represent a deep storage level of magma. The shallow plumbing system feeds frequent, short-lived summit and flank (NE and SE flanks) eruptions along summit and outer <span class="hlt">rift</span> zones, respectively</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2005AGUSM.T43B..07A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2005AGUSM.T43B..07A"><span id="translatedtitle">Structural Evolution of the Incipient Okavango <span class="hlt">Rift</span> Zone, NW Botswana</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Atekwana, E. A.; Kinabo, B. D.; Modisi, M. P.; Hogan, J. P.; Wheaton, D. D.</p> <p>2005-05-01</p> <p>Studies of the East African <span class="hlt">Rift</span> System (EARS) and other continental <span class="hlt">rifts</span> have significantly improved our understanding of <span class="hlt">rifting</span> processes; however, we particularly lack studies of the embryonic stages of <span class="hlt">rift</span> creation. The Okavango <span class="hlt">Rift</span> Zone (ORZ), NW Botswana is one of few places worldwide where one can study the early stages of continental extension prior to the accumulation of significant amounts of sediments, volcanism, and multiphase deformation that obscure the investigation of these early time processes in more evolved continental <span class="hlt">rift</span> zones. In this study, gravity and aeromagnetic data have been used to examine the initiation and development of the nascent ORZ. The Okavango basin in NW Botswana is located at the southern tip of the southwestern branch of the EARS. The <span class="hlt">rift</span> is hosted within the Proterozoic fold and thrust belt of the Ghanzi-Chobe formation. Our objectives include (1) assessing the role of pre-existing structures on the development of <span class="hlt">rift</span> faults and basin architecture, (2) Examining fault linkage patterns and boarder fault development, and (3) determining the shallow subsurface basin geometry. Aeromagnetic data from the ORZ suggest two main structural trends: 1) northeast-southwest (030- 070o) and 2) northwest - southeast (290 - 320o). The 030- 070o structures occur within the <span class="hlt">rift</span> zone and throughout the surrounding basement. They form the main bounding fault system of this incipient <span class="hlt">rift</span>. The NE - SW orientations of <span class="hlt">rift</span> faults mirror the fold axes and foliation of the basement rocks, suggesting that the basement fabric played an important role in localizing the development of faults within the stress regime present during the initiation of this <span class="hlt">rift</span>. Additionally, the greatest throw (~400- ~700 m) occurs along the Kunyere (NW dipping) and Tsau faults (SE dipping), defining a full graben as observed on gravity models. This differs from the half-graben model typical of most continental <span class="hlt">rift</span> zones. Thus, it appears the basin geometry was</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..1813335R','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..1813335R"><span id="translatedtitle">The <span class="hlt">rift</span> to drift evolution of the South China Sea</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Ranero, Cesar R.; Cameselle, Alejandra; Franke, Dieter; Barckhausen, Udo</p> <p>2016-04-01</p> <p>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 <span class="hlt">rift</span> segments of the NW, SW, and E sub-basins. Re-processed sections image the post-<span class="hlt">rift</span> and syn-<span class="hlt">rift</span> sediment, and fault-bounded basement blocks, often also intra-crustal fault reflections that together provide detailed information of the tectonic structural style during <span class="hlt">rifting</span>. 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 <span class="hlt">rift</span>. 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 <span class="hlt">rifting</span> processes and <span class="hlt">rift</span> segmentation. Also, the comparison of the tectonic structure of the conjugated flanks of the continental <span class="hlt">rift</span> across the ocean basins is used to understand the last stages of <span class="hlt">rifting</span> 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</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2006EP%26S...58.1303U','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2006EP%26S...58.1303U"><span id="translatedtitle">Structural pattern at the northwestern sector of the Tepic-Zacoalco <span class="hlt">rift</span> and tectonic implications for the Jalisco block, western Mexico</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Urrutia-Fucugauchi, Jaime; González-Morán, Tomás</p> <p>2006-10-01</p> <p>Analysis of the aeromagnetic anomalies over the northwestern sector of the Tepic-Zacoalco <span class="hlt">rift</span> documents a NE-SW pattern of lineaments that are perpendicular to the inferred NW-SE boundary between the Jalisco block and the Sierra Madre Occidental. The boundary lies within the <span class="hlt">central</span> sector of the Tepic-Zacoalco <span class="hlt">rift</span> immediately north of the Ceboruco and Tepetiltic stratovolcanoes and extends up to the San Juan stratovolcano, where it intersects the NE-SW magnetic anomaly lineament that runs toward the Pacific coast (which intersects two volcanic centers). This N35°E lineament separates the <span class="hlt">central</span> <span class="hlt">rift</span> zone of low amplitude mainly negative anomalies (except those positive anomalies over the stratovolcanoes) from the zone to the north and west characterized by high amplitude positive long wavelength anomalies. The NE-SW lineament is parallel to the western sector of the Ameca graben and the offshore Bahia de Banderas graben and to the structural features of the Punta Mita peninsula at the Pacific coast, and thus seems to form part of a regional NE-SW pattern oblique to the proposed westward or northwestward motion of the Jalisco block. The orientation of this regional structural pattern at the northern end of the Tepic-Zacoalco <span class="hlt">rift</span> seems consistent with proposed dominant SW-directed extension along the <span class="hlt">rift</span> during the Pliocene and Quaternary, rather than with NW-SE lateral strike-slip faulting. The orthogonal pattern that characterizes the northernmost boundary of the Tepic-Zacoalco <span class="hlt">rift</span> is oblique to the pattern observed in the Grande de Santiago river (which conforms the northern limit of the <span class="hlt">rift</span>) and for the <span class="hlt">central</span>-eastern sectors of the Ameca graben (south of the <span class="hlt">rift</span>). This spatial arrangement of major lineaments and structural elements points to a complex tectonic history for the region that includes the <span class="hlt">rifting</span> of the Gulf of California and margin deformation due to plate convergence and kinematic re-organization events, and which may have resulted in</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=112837','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=112837"><span id="translatedtitle">Genetic Reassortment of <span class="hlt">Rift</span> Valley Fever Virus in Nature</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Sall, A. A.; Zanotto, P. M. de A.; Sene, O. K.; Zeller, H. G.; Digoutte, J. P.; Thiongane, Y.; Bouloy, M.</p> <p>1999-01-01</p> <p><span class="hlt">Rift</span> Valley fever virus (RVFV), a phlebovirus of the Bunyaviridae family, is an arthropod-borne virus which emerges periodically throughout Africa, emphasizing that it poses a major threat for animal and human populations. To assess the genetic variability of RVFV, several isolates from diverse localities of Africa were investigated by means of reverse transcription-PCR followed by direct sequencing of a region of the small (S), medium (M), and large (L) genomic segments. Phylogenetic analysis showed the existence of three major lineages corresponding to geographic variants from West Africa, Egypt, and <span class="hlt">Central</span>-East Africa. However, incongruences detected between the L, M, and S phylogenies suggested that genetic exchange via reassortment occurred between strains from different lineages. This hypothesis, depicted by parallel phylogenies, was further confirmed by statistical tests. Our findings, which strongly suggest exchanges between strains from areas of endemicity in West and East Africa, strengthen the potential existence of a sylvatic cycle in the tropical rain forest. This also emphasizes the risk of generating uncontrolled chimeric viruses by using live attenuated vaccines in areas of endemicity. PMID:10482570</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2008QuRes..70..283M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2008QuRes..70..283M"><span id="translatedtitle">Middle Stone Age starch acquisition in the Niassa <span class="hlt">Rift</span>, Mozambique</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Mercader, Julio; Bennett, Tim; Raja, Mussa</p> <p>2008-09-01</p> <p>The quest for direct lines of evidence for Paleolithic plant consumption during the African Middle Stone Age has led scientists to study residues and use-wear on flaked stone tools. Past work has established lithic function through multiple lines of evidence and the spatial breakdown of use-wear and microscopic traces on tool surfaces. This paper focuses on the quantitative analysis of starch assemblages and the botanical identification of grains from flake and core tools to learn about human ecology of carbohydrate use around the Niassa woodlands, in the Mozambican <span class="hlt">Rift</span>. The processing of starchy plant parts is deduced from the occurrence of starch assemblages that presumably got attached to stone tool surfaces by actions associated with extractive or culinary activities. Specifically, we investigate starch grains from stone tools recently excavated in northern Mozambique at the site of Mikuyu; which presumably spans the middle to late Pleistocene and represents similar sites found along the Malawi/Niassa corridor that links East, Southern, and <span class="hlt">Central</span> Africa. Starch was extracted and processed with a diverse tool kit consisting of scrapers, cores, points, flakes, and other kinds of tools. The microbotanical data suggests consumption of seeds, legumes, caryopses, piths, underground storage organs, nuts, and mesocarps from more than a dozen families. Our data suggest a great antiquity for starch use in Africa as well as an expanded diet and intensification.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19820005722','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19820005722"><span id="translatedtitle">Papers presented to the Conference on the Processes of Planetary <span class="hlt">Rifting</span></span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p></p> <p>1981-01-01</p> <p>The basic problems of processes of planetary <span class="hlt">rifting</span> are addressed from the following viewpoints: (1) speculation as to the origin and development of <span class="hlt">rifts</span>; (2) <span class="hlt">rifts</span> on other planets; (3) tectonics; (4) geology; (5) chemistry of the lithosphere; (6) physics of the lithosphere; and (7) resources associated with <span class="hlt">rifting</span>. The state of ignorance on the subject and its remedy is debated.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://www.dtic.mil/docs/citations/ADA218352','DTIC-ST'); return false;" href="http://www.dtic.mil/docs/citations/ADA218352"><span id="translatedtitle">Transstadial and Horizontal Transmission of <span class="hlt">Rift</span> Valley Fever Virus in Hyalomma truncatum</span></a></p> <p><a target="_blank" href="https://publicaccess.dtic.mil/psm/api/service/search/search">DTIC Science & Technology</a></p> <p></p> <p>1989-01-01</p> <p>pendiculatus were established from the progeny <span class="hlt">Central</span> African Republic. Attempts to isolate of females collected from cattle in <span class="hlt">Rift</span> Valley RVF virus from...in this study.18 on a guinea pig. On day 15 post-inoculation, 20 For virus assays, ticks were triturated in 1 ml female and 15 male ticks were allowed...ticks (5 female , 5 male for Exp. 1 and 2) were of RVF virus 24 hr prior to estimated tick drop- sampled at predetermined intervals post-inocu- off. After</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70017645','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70017645"><span id="translatedtitle">Composition of the crust beneath the Kenya <span class="hlt">rift</span></span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Mooney, W.D.; Christensen, N.I.</p> <p>1994-01-01</p> <p>We infer the composition of the crust beneath and on the flanks of the Kenya <span class="hlt">rift</span> based on a comparison of the KRISP-90 crustal velocity structure with laboratory measurements of compressional-wave velocities of rock samples from Kenya. The rock samples studied, which are representative of the major lithologies exposed in Kenya, include volcanic tuffs and flows (primarily basalts and phonolites), and felsic to intermediate composition gneisses. This comparison indicates that the upper crust (5-12 km depth) consists primarily of quartzo-feldspathic gneisses and schists similar to rocks exposed on the flanks of the <span class="hlt">rift</span>, whereas the middle crust (12-22 km depth) consists of more mafic, hornblende-rich metamorphic rocks, probably intruded by mafic rocks beneath the <span class="hlt">rift</span> axis. The lower crust on the flanks of the <span class="hlt">rift</span> may consist of mafic granulite facies rocks. Along the <span class="hlt">rift</span> axis, the lower crust varies in thickness from 9 km in the southern <span class="hlt">rift</span> to only 2-3 km in the north, and has a seismic velocity substantially higher than the samples investigated in this study. The lower crust of the <span class="hlt">rift</span> probably consists of a crust/mantle mix of high-grade metamorphic rocks, mafic intrusives, and an igneous mafic residuum accreted to the base of the crust during differentiation of a melt derived from the upper mantle. ?? 1994.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/19212408','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/19212408"><span id="translatedtitle">Magma-compensated crustal thinning in continental <span class="hlt">rift</span> zones.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Thybo, H; Nielsen, C A</p> <p>2009-02-12</p> <p>Continental <span class="hlt">rift</span> zones are long, narrow tectonic depressions in the Earth's surface where the entire lithosphere has been modified in extension. <span class="hlt">Rifting</span> 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 <span class="hlt">rift</span> zones. Conventional models of <span class="hlt">rift</span> 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 <span class="hlt">rift</span> 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 <span class="hlt">rifting</span> processes with significant implications for estimation of stretching factors and modelling of sedimentary basins around failed <span class="hlt">rift</span> structures.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014Tecto..33..485P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014Tecto..33..485P"><span id="translatedtitle">Evolution, distribution, and characteristics of <span class="hlt">rifting</span> in southern Ethiopia</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Philippon, Melody; Corti, Giacomo; Sani, Federico; Bonini, Marco; Balestrieri, Maria-Laura; Molin, Paola; Willingshofer, Ernst; Sokoutis, Dimitrios; Cloetingh, Sierd</p> <p>2014-04-01</p> <p>Southern Ethiopia is a key region to understand the evolution of the East African <span class="hlt">rift</span> system, since it is the area of interaction between the main Ethiopian <span class="hlt">rift</span> (MER) and the Kenyan <span class="hlt">rift</span>. However, geological data constraining <span class="hlt">rift</span> evolution in this remote area are still relatively sparse. In this study the timing, distribution, and style of <span class="hlt">rifting</span> in southern Ethiopia are constrained by new structural, geochronological, and geomorphological data. The border faults in the area are roughly parallel to preexisting basement fabrics and are progressively more oblique with respect to the regional Nubia-Somalia motion proceeding southward. Kinematic indicators along these faults are mainly dip slip, pointing to a progressive rotation of the computed direction of extension toward the south. Radiocarbon data indicate post 30 ka faulting at both western and eastern margins of the MER with limited axial deformation. Similarly, geomorphological data suggest recent fault activity along the western margins of the basins composing the Gofa Province and in the Chew Bahir basin. This supports that interaction between the MER and the Kenyan <span class="hlt">rift</span> in southern Ethiopia occurs in a 200 km wide zone of ongoing deformation. Fault-related exhumation at ~10-12 Ma in the Gofa Province, as constrained by new apatite fission track data, occurred later than the ~20 Ma basement exhumation of the Chew Bahir basin, thus pointing to a northward propagation of the Kenyan <span class="hlt">rift</span>-related extension in the area.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2001AGUSM...S22A03T','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2001AGUSM...S22A03T"><span id="translatedtitle">Tectonic Framework of the Kachchh <span class="hlt">Rift</span> Basin</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Talwani, P.; Gangopadhyay, A. K.</p> <p>2001-05-01</p> <p>Evaluation of available geological data has allowed us to determine the tectonic framework of the Kachchh <span class="hlt">rift</span> 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 <span class="hlt">rift</span> 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 <span class="hlt">rift</span>) 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</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2009AGUFM.T31B1817D','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2009AGUFM.T31B1817D"><span id="translatedtitle">Dynamics of <span class="hlt">Rifting</span> in two Active <span class="hlt">Rift</span> Segments in Afar - Geodetic and Structural Studies - DoRA Project</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Doubre, C.; Socquet, A.; Masson, F.; Jacques, E.; Grandin, R.; Nercessian, A.; Kassim, M.; Vergne, J.; Diament, M.; Hinderer, J.; Ayele, A.; Lewi, E.; Calais, E.; Peltzer, G.; Toussaint, R.; de Chaballier, J.; Ballu, V. S.; Luck, B.; King, G. C.; Vigny, C.; Cattin, R.; Tiberi, C.; Kidane, T.; Jalludin, M.; Maggi, A.; Dorbath, C.; Manatschal, G.; Schmittbuhl, J.; Le Moigne, N.; Deroussi, S.</p> <p>2009-12-01</p> <p>The DoRA project aims to conduct complementary studies in two volcano-tectonic <span class="hlt">rifts</span> in the Afar Depression. In Northern Afar, the Wal’is Dabbahu <span class="hlt">Rift</span> (WD, Ethiopia) is currently undergoing a major <span class="hlt">rifting</span> episode. This event started in September 2005 with a significant seismic activity. InSAR data revealed the injection of a 65 km-long mega-dyke that opened by up to 8 m, the slip of numerous normal faults and opening of fissures, and a rhyolitic eruption. Similarly, the Asal-Ghoubbet <span class="hlt">Rift</span> (AG, Djibouti) was affected in 1978 by a smaller episode of <span class="hlt">rifting</span> associated with the intrusion of a 2 m wide dyke into the crust. Since then, a large catalog of geodetic data that includes recent InSAR time series reveals the importance of non-steady deformation controlling the <span class="hlt">rift</span> dynamics. Our goal is to gain an understanding of such volcano-tectonic segments on several time scales, including the dyking period itself and the post-event period. The study of the behavior of the AG <span class="hlt">Rift</span> during its whole post-<span class="hlt">rifting</span> period offers an image at t+30 years of the WD segment, while keeping in mind important structural and scale differences. First, we propose to build a complete and accurate set of geodetic data (InSAR, cGPS, GPS), covering the period under study. With a narrow temporal sample window, we will precisely describe the aseismic slip affecting the normal faults of these <span class="hlt">rifts</span>, the periods of sudden slip and/or slip acceleration but also measure the deformation associated with probable future dyke intrusion. Second, we aim to constrain the origin of these displacements and their relation with mass transfers within the crust. Series of gravity measurements will be pursue or initiated in both <span class="hlt">rifts</span>. Third, the recording of seismic activity is essential to constrain the relative importance of seismic and aseismic deformation. This will also help to evaluate the thickness of the seismogenic layer. Together with structural data collected during a seismic survey in the AG</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.8259G','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.8259G"><span id="translatedtitle">Ambient noise tomography of the western Corinth <span class="hlt">Rift</span>, Greece</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Giannopoulos, Dimitrios; Rivet, Diane; Sokos, Efthimios; Deschamps, Anne; Paraskevopoulos, Paraskevas; Lyon-Caen, Hélène; Pascal, Bernard; Tselentis, G.-Akis</p> <p>2016-04-01</p> <p>The Corinth <span class="hlt">Rift</span> separates Peloponnesus to the south from main-land Greece to the north. It is one of the most active extensional intra-continental <span class="hlt">rifts</span> 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 <span class="hlt">Rift</span> using ambient noise recordings. We used 3 yrs (01/2012-12/2014) of continuous waveform data recorded at 24 stations from the Corinth <span class="hlt">Rift</span> 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 <span class="hlt">Rift</span> 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-<span class="hlt">rift</span> sediments such as off-shore regions of the <span class="hlt">rift</span>, the Mornos delta and a large part of the southern coast. Higher velocities are observed in pre-<span class="hlt">rift</span> 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 <span class="hlt">Rift</span> structure using long time-series of ambient noise data.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/6197865','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/6197865"><span id="translatedtitle">Early opening of initially closed Gulf of Mexico and <span class="hlt">central</span> North Atlantic ocean</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Van Siclen, D.C.</p> <p>1984-09-01</p> <p>This paper presents ideas on the early opening and evolution of the Gulf of Mexico and the <span class="hlt">central</span> North Atlantic ocean. It discusses <span class="hlt">rifting</span> activity, plate tectonics, magnetic anomalies, and the geologic time elements involved.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_15");'>15</a></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li class="active"><span>17</span></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_17 --> <div id="page_18" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li class="active"><span>18</span></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="341"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70126185','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70126185"><span id="translatedtitle">Stratigraphic and structural evolution of the Selenga Delta Accommodation Zone, Lake Baikal <span class="hlt">Rift</span>, Siberia</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Scholz, C.A.; Hutchinson, D.R.</p> <p>2000-01-01</p> <p>Seismic reflection profiles from the Lake Baikal <span class="hlt">Rift</span> reveal extensive details about the sediment thickness, structural geometry and history of extensional deformation and syn-<span class="hlt">rift</span> sedimentation in this classic continental <span class="hlt">rift</span>. The Selenga River is the largest single source of terrigenous input into Lake Baikal, and its large delta sits astride the major accommodation zone between the <span class="hlt">Central</span> and South basins of the lake. Incorporating one of the world's largest lacustrine deltas, this depositional system is a classic example of the influence of <span class="hlt">rift</span> basin structural segmentation on a major continental drainage. More than 3700 km of deep basin-scale multi-channel seismic reflection (MCS) data were acquired during the 1989 Russian and the 1992 Russian–American field programs. The seismic data image most of the sedimentary section, including pre-<span class="hlt">rift</span> basement in several localities. The MCS data reveal that the broad bathymetric saddle between these two major half-graben basins is underlain by a complex of severely deformed basement blocks, and is not simply a consequence of long-term deltaic deposition. Maximum sediment thickness is estimated to be more than 9 km in some areas around the Selenga Delta. Detailed stratigraphic analyses of the Selenga area MCS data suggest that modes of deposition have shifted markedly during the history of the delta. The present mode of gravity- and mass-flow sedimentation that dominates the northern and southern parts of the modern delta, as well as the pronounced bathymetric relief in the area, are relatively recent developments in the history of the Lake Baikal <span class="hlt">Rift</span>. Several episodes of major delta progradation, each extending far across the modern <span class="hlt">rift</span>, can be documented in the MCS data. The stratigraphic framework defined by these prograding deltaic sequences can be used to constrain the structural as well as depositional evolution of this part of the Baikal <span class="hlt">Rift</span>. An age model has been established for this stratigraphy, by</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.7643G','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.7643G"><span id="translatedtitle">Is the Gop <span class="hlt">rift</span> oceanic? A reevaluation of the Seychelles-India conjugate margins</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Guan, Huixin; Werner, Philippe; Geoffroy, Laurent</p> <p>2016-04-01</p> <p> Gop <span class="hlt">Rift</span> axis. We propose that the conspicuous buoyant <span class="hlt">central</span> part of the Gop <span class="hlt">Rift</span> is likely associated with a continental C-Block as described in a recent paper on conjugated VPMs8, at least in the southern part of the Gop <span class="hlt">Rift</span>. The crust below the Laxmi basin is probably transitional continental i.e. strongly intruded. West of India and west of the Laxmi Ridge, the transition to the Carlsberg Basin occurs along a clearly-expressed transform fault, not through an extended and thinned continental margin. We reinterpret the whole system based on those observations and propositions, giving some explanations on controversial magnetic anomalies based on similar observations from the southern Atlantic Ocean. 1: Collier et al., 2008. Age of the Seychelles-India break-up. Earth and Planetary Science Letters. 2: Minshull et al., 2008. The relationship between riftingand magmatism in the northeastern Arabian Sea. Nature Geoscience. 3 : Armitage et al., 2010. The importance of <span class="hlt">rift</span> history for volcanic margin. Nature. 4 : Krishna et al., 2006. Nature of the crust in the Laxmi Basin (14 degrees-20 degrees N), western continental margin of India. Tectonics. 5 : Misra et al., 2015. Repeat ridge jumps and microcontinent separation: insights from NE Arabian Sea. Marine and Petroleum Geology. 6 : Biswas, 1982. <span class="hlt">Rift</span> basins in the western margin of India and their hydrocarbon prospects. Bull. Am. Assoc. Pet. Geol. 7 : Chatterjee et al., 2013. The longest voyage: Tectonic, magmatic, and paleoclimatic evolution of the Indian plate during its northward flight from Gondwana to Asia. Gondwana Research. 8 : Geoffroy et al., 2015. Volcanic passive margins: anotherway to break up continents. Scientific Reports.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19850029005&hterms=DALI&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D10%26Ntt%3DDALI','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19850029005&hterms=DALI&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D10%26Ntt%3DDALI"><span id="translatedtitle">Venus - Volcanism and <span class="hlt">rift</span> formation in Beta Regio</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Campbell, D. B.; Harmon, J. K.; Hine, A. A.; Head, J. W.</p> <p>1984-01-01</p> <p>A new high-resolution radar image of Beta Regio, a Venus highland area, confirms the presence of a major tectonic <span class="hlt">rift</span> system and associated volcanic activity. The lack of identifiable impact craters, together with the apparent superposition of the Theia Mons volcanic structure on the <span class="hlt">rift</span> system, suggest that at least some of the volcanic activity occurred in relatively recent geologic time. The presence of topographically similar highland areas elsewhere on Venus (Aphrodite Terra, Dali Chasma, and Diana Chasma) suggests that <span class="hlt">rifting</span> and volcanism are significant processes on Venus.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/17812797','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/17812797"><span id="translatedtitle">Precambrian <span class="hlt">rift</span>: genesis of strata-bound ore deposits.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Kanasewich, E R</p> <p>1968-09-06</p> <p>Study of deep seismic reflections has detected a Precambrian <span class="hlt">rift</span> valley below flat-lying sediments in southern Alberta. The anomalous magnetic and gravity trends show that the <span class="hlt">rift</span> is continuous across Alberta and British Columbia (through the Kimberley lead-zinc field) and possibly the Coeur d'Alene mining district of Idaho. There is evidence that these ore bodies were deposited in a Precambrian <span class="hlt">rift</span> under conditions similar to those prevailing in the hot-brine areas of the modern Red Sea.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/21240849','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/21240849"><span id="translatedtitle"><span class="hlt">Rift</span> Valley fever: the Nigerian story.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Adeyeye, Adewale A; Ekong, Pius S; Pilau, Nicholas N</p> <p>2011-01-01</p> <p><span class="hlt">Rift</span> Valley fever (RVF) is an arthropod-borne zoonotic disease of livestock. It is characterised by fever, salivation, abdominal pain, diarrhoea, mucopurulent to bloody nasal discharge, abortion, rapid decrease in milk production and death in animals. Infected humans experience an influenza-like illness that is characterised by fever, malaise, headaches, nausea and epigastric pain followed by recovery, although mortality can occur. RVF was thought to be a disease of sub-Saharan Africa but with the outbreaks in Egypt and the Arabian Peninsula, it may be extending its range further afield. Virological and serological evidence indicates that the virus exists in Nigeria and, with the warning signal sent by international organisations to countries in Africa about an impending outbreak, co-ordinated research between veterinarians and physicians in Nigeria is advocated.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/23347790','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/23347790"><span id="translatedtitle"><span class="hlt">Rift</span> Valley fever, Sudan, 2007 and 2010.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Aradaib, Imadeldin E; Erickson, Bobbie R; Elageb, Rehab M; Khristova, Marina L; Carroll, Serena A; Elkhidir, Isam M; Karsany, Mubarak E; Karrar, Abdelrahim E; Elbashir, Mustafa I; Nichol, Stuart T</p> <p>2013-02-01</p> <p>To elucidate whether <span class="hlt">Rift</span> Valley fever virus (RVFV) diversity in Sudan resulted from multiple introductions or from acquired changes over time from 1 introduction event, we generated complete genome sequences from RVFV strains detected during the 2007 and 2010 outbreaks. Phylogenetic analyses of small, medium, and large RNA segment sequences indicated several genetic RVFV variants were circulating in Sudan, which all grouped into Kenya-1 or Kenya-2 sublineages from the 2006-2008 eastern Africa epizootic. Bayesian analysis of sequence differences estimated that diversity among the 2007 and 2010 Sudan RVFV variants shared a most recent common ancestor circa 1996. The data suggest multiple introductions of RVFV into Sudan as part of sweeping epizootics from eastern Africa. The sequences indicate recent movement of RVFV and support the need for surveillance to recognize when and where RVFV circulates between epidemics, which can make data from prediction tools easier to interpret and preventive measures easier to direct toward high-risk areas.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19850023287&hterms=Continental+Drift&qs=N%3D0%26Ntk%3DAll%26Ntx%3Dmode%2Bmatchall%26Ntt%3DContinental%2BDrift','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19850023287&hterms=Continental+Drift&qs=N%3D0%26Ntk%3DAll%26Ntx%3Dmode%2Bmatchall%26Ntt%3DContinental%2BDrift"><span id="translatedtitle">Regional magnetic anomaly constraints on continental <span class="hlt">rifting</span></span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Vonfrese, R. R. B.; Hinze, W. J.; Olivier, R.; Bentley, C. R.</p> <p>1985-01-01</p> <p>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 <span class="hlt">rifted</span> 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.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012AGUFM.T53C2732T','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012AGUFM.T53C2732T"><span id="translatedtitle">Control of initial heterogeneities and boundary conditions on the deformation partitioning of continental <span class="hlt">rift</span>: a comparison between Rio Grande <span class="hlt">Rift</span> and Main Ethiopian <span class="hlt">Rift</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Thieulot, C.; Philippon, M.; Sokoutis, D.; Van Wijk, J. W.; Willingshofer, E.; Cloetingh, S.</p> <p>2012-12-01</p> <p>Understanding how initial heterogeneities and boundary conditions control the development of fault network and the deformation partitioning mechanisms in <span class="hlt">rifts</span> that affect continental crust still remains a challenge. Among the active continental <span class="hlt">rifts</span> affecting the Earth lithosphere, the Main Ethiopian <span class="hlt">Rift</span> (MER) and the Rio Grande <span class="hlt">Rift</span> (RGR) appear to be ideal natural laboratories to investigate this issues as they share structural characteristics but develop under different geological settings. From a structural point of view both <span class="hlt">rifts</span> show the same length (~1000km), width (50 to 70 km) and asymmetry. The MER is the NE-SW trending plate boundary between the Nubian and Somalian plates that has been developing for the past 11 Ma above a paleo-proterozoic lithospheric-scale weak zone re-heated by the Afar hotspot, whereas the RGR is the eastern "boundary" of the basin & range system that has been developing for the past 30 Ma in the frame of a westward retreating Farallon subduction zone. However, the RGR shows evidence of low angle normal faulting whereas the MER shows steeply dipping normal faults and conversely the MER shows a larger volume of erupted lavas than the RGR. Combined with a structural analysis of both <span class="hlt">rifts</span>, we present here a series of preliminary numerical models that allows for a better understanding of the influence of initial heterogeneities such as 1) the state of the crust (craton versus crust affected by "wide <span class="hlt">rift</span>" type extension); 2) the presence of a crustal-scale to lithospheric-scale weak zone, 3) the effect of the orientation of these weak zones & 4) the effects of the presence of magma.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015EGUGA..17.6393M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015EGUGA..17.6393M"><span id="translatedtitle">The role of inheritance in structuring hyperextended <span class="hlt">rift</span> systems</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Manatschal, Gianreto; Lavier, Luc; Chenin, Pauline</p> <p>2015-04-01</p> <p>A long-standing question in Earth Sciences is related to the importance of inheritance in controlling tectonic processes. In contrast to physical processes that are generally applicable, assessing the role of inheritance suffers from two major problems: firstly, it is difficult to appraise without having insights into the history of a geological system; and secondly all inherited features are not reactivated during subsequent deformation phases. Therefore, the aim of our presentation is to give some conceptual framework about how inheritance may control the architecture and evolution of hyperextended <span class="hlt">rift</span> systems. We use the term inheritance to refer to the difference between an "ideal" layer-cake type lithosphere and a "real" lithosphere containing heterogeneities and we define 3 types of inheritance, namely structural, compositional and thermal inheritance. Moreover, we assume that the evolution of hyperextended <span class="hlt">rift</span> systems reflects the interplay between their inheritance (innate/"genetic code") and the physical processes at play (acquired/external factors). Thus, by observing the architecture and evolution of hyperextended <span class="hlt">rift</span> systems and integrating the physical processes, one my get hints on what may have been the original inheritance of a system. Using this approach, we focus on 3 well-studied <span class="hlt">rift</span> systems that are the Alpine Tethys, Pyrenean-Bay of Biscay and Iberia-Newfoundland <span class="hlt">rift</span> systems. For the studied examples we can show that: 1) strain localization on a local scale and during early stages of <span class="hlt">rifting</span> is controlled by inherited structures and weaknesses 2) the architecture of the necking zone seems to be influenced by the distribution and importance of ductile layers during decoupled deformation and is consequently controlled by the thermal structure and/or the inherited composition of the curst 3) the location of breakup in the 3 examples is not significantly controlled by the inherited structures 4) inherited mantle composition and <span class="hlt">rift</span></p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013GGG....14.3032M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013GGG....14.3032M"><span id="translatedtitle">Early-stage <span class="hlt">rifting</span> of the northern Tyrrhenian Sea Basin: Results from a combined wide-angle and multichannel seismic study</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Moeller, S.; Grevemeyer, I.; Ranero, C. R.; Berndt, C.; Klaeschen, D.; Sallares, V.; Zitellini, N.; Franco, R.</p> <p>2013-08-01</p> <p>Extension of the continental lithosphere leads to the formation of <span class="hlt">rift</span> basins and ultimately may create passive continental margins. The mechanisms that operate during the early stage of crustal extension are still intensely debated. We present the results from coincident multichannel seismic and wide-angle seismic profiles that transect across the northern Tyrrhenian Sea Basin. The profiles cross the Corsica Basin (France) to the Latium Margin (Italy) where the early-<span class="hlt">rift</span> stage of the basin is well preserved. We found two domains, each with a distinct tectonic style, heat flow and crustal thickness. One domain is the Corsica Basin in the west that formed before the main <span class="hlt">rift</span> phase of the northern Tyrrhenian Sea opening (˜8-4 Ma). The second domain is <span class="hlt">rifted</span> continental crust characterized by tilted blocks and half-graben structures in the <span class="hlt">central</span> region and at the Latium Margin. These two domains are separated by a deep (˜10 km) sedimentary complex of the eastern portion of the Corsica Basin. Travel-time tomography of wide-angle seismic data reveals the crustal architecture and a subhorizontal 15-17 ± 1 km deep Moho discontinuity under the basin. To estimate the amount of horizontal extension we have identified the pre-, syn-, and post-tectonic sedimentary units and calculated the relative displacement of faults. We found that major faults initiated at angles of 45°-50° and that the <span class="hlt">rifted</span> domain is horizontally stretched by a factor of β ˜ 1.3 (˜8-10 mm/a). The crust has been thinned from ˜24 to ˜17 km indicating a similar amount of extension (˜30%). The transect represents one of the best imaged early <span class="hlt">rifts</span> and implies that the formation of crustal-scale detachments, or long-lived low-angle normal faults, is not a general feature that controls the <span class="hlt">rift</span> initiation of continental crust. Other young <span class="hlt">rift</span> basins, like the Gulf of Corinth, the Suez <span class="hlt">Rift</span> or Lake Baikal, display features resembling the northern Tyrrhenian Basin, suggesting that half</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFM.T53C..07I','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFM.T53C..07I"><span id="translatedtitle"><span class="hlt">Rift</span> Structure along the Eastern Continental Margin of India - new constraints on style of breakup of the Indian landmass from the eastern Gondwanaland</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Ismaiel, M.; Krishna, K. S.; Karlapati, S.; Mishra, J.; D, S.</p> <p>2015-12-01</p> <p>The Eastern Continental Margin of India (ECMI), a classical passive margin has evolved after breakup of the Indian landmass from the East Antarctica during the Early Cretaceous. Anomalous thick sediments and lack of cohesive magnetic signatures in the Bay of Bengal hampered delineation of <span class="hlt">rift</span>-structure and age assignment for the continental breakup between India and East Antarctica. Further, absence of lithological and geochronological information and a few seismic profiles from the margin led to put forward several competing models for the <span class="hlt">rift</span> initiation and evolution of the ECMI. Here, we analyze long streamer seismic reflection data and deep-water drill well information from the western Bay of Bengal to infer the buried <span class="hlt">rift</span> structure, crustal architecture and stratigraphy along the ECMI. Following the structural pattern of the margin, the region is divided into four domains as decoupled, coupled, exhumed and oceanic, which in turn helped us to demarcate the variations in <span class="hlt">rift</span> structure from south to north along the margin. The southern segment in the vicinity of Cauvery Basin consists of steep continental shelf associated with few major normal faults, which indicates that the segment was evolved as mix shear-<span class="hlt">rifted</span> margin. The <span class="hlt">central</span> segment off southern part of the Krishna-Godavari Basin is controlled by a series of fault-bounded half-graben structures and presence of thinned continental crust over the exhumed mantle body, revealing that the segment was formed under hyper-<span class="hlt">rifting</span> process. While the northern segment extends up to Mahanadi Basin shows relatively less gradient continental slope with a few major faults, suggesting that the segment was evolved by hypo-extended process. Variable crustal architecture lying along the ECMI supports each segment of the margin formed in a specific <span class="hlt">rift</span> process. A breakup unconformity considered as important geological constraint for completion of <span class="hlt">rift</span> process between India and East Antarctica is clearly mapped on</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70034427','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70034427"><span id="translatedtitle">Low lower crustal velocity across Ethiopia: Is the Main Ethiopian <span class="hlt">Rift</span> a narrow <span class="hlt">rift</span> in a hot craton?</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Keranen, K.M.; Klemperer, S.L.; Julia, J.; Lawrence, J. F.; Nyblade, A.A.</p> <p>2009-01-01</p> <p>[1] The Main Ethiopian <span class="hlt">Rift</span> (MER) is a classic narrow <span class="hlt">rift</span> that developed in hot, weak lithosphere, not in the initially cold, thick, and strong lithosphere that would be predicted by common models of <span class="hlt">rift</span> mode formation. Our new 1-D seismic velocity profiles from Rayleigh wave/receiver function joint inversion across the MER and the Ethiopian Plateau indicate that hot lower crust and upper mantle are present throughout the broad region affected by Oligocene flood basalt volcanism, including both the present <span class="hlt">rift</span> and the adjacent Ethiopian Plateau hundreds of kilometers from the <span class="hlt">rift</span> valley. The region of hot lithosphere closely corresponds to the region of flood basalt volcanism, and we interpret that the volcanism and thermal perturbation were jointly caused by impingement of the Afar plume head. Across the affected region, Vs is 3.6-3.8 km/s in the lowermost crust and ???4.3 km/s in the uppermost mantle, both ??0.3 km/s lower than in the eastern and western branches of the East African <span class="hlt">Rift</span> System to the south. We interpret the low Vs in the lower crust and upper mantle as indicative of hot lithosphere with partial melt. Our results lead to a hybrid <span class="hlt">rift</span> mode, in which the brittle upper crust has developed as a narrow <span class="hlt">rift</span> along the Neoproterozoic suture between East and West Gondwana, while at depth lithospheric deformation is distributed over the broad region (??400 km wide) thermally perturbed by the broad thermal upwelling associated with the Afar plume head. Development of both the East African <span class="hlt">Rift</span> System to the south (in cold, strong lithosphere) and the MER to the north (in hot, weak lithosphere) as narrow <span class="hlt">rifts</span>, despite their vastly different initial thermal states and depth-integrated lithospheric strength, indicates that common models of <span class="hlt">rift</span> mode formation that focus only on temperature, thickness, and vertical strength profiles do not apply to these classic continental <span class="hlt">rifts</span>. Instead, inherited structure and associated lithospheric weaknesses are</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014EGUGA..16.4870P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014EGUGA..16.4870P"><span id="translatedtitle">Link between extension, dyking and subsidence as the reconstruction tool of intraplate <span class="hlt">rifting</span> mechanism (backstripping data, modelling and geochronology)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Polyansky, Oleg P.; Reverdatto, Vladimir V.; Babichev, Alexey V.</p> <p>2014-05-01</p> <p> predicted over a range of γ = 0.01-0.12 (average value 0.06, i.e., ~6%). This value seems acceptable if we consider cumulative sills and dykes thicknesses in the outcrops and borehole sections in the Upper Devonian-Lower Carboniferous sediments. For better <span class="hlt">rifting</span> age resolution we sampled dolerites from the western, eastern and <span class="hlt">central</span> dyke swarms, which were then analyzed by stepwise 40Ar/39Ar heating. The determined ages of the monomineral plagioclase and pyroxene fractions from the dyke samples (345-378 Ma) coincide well with fast subsidence and sedimentation phase in the Devonian defined by backstripping data. Dating of dyke intrusions shows following succession of <span class="hlt">rifting</span> episodes: extension by dyking starts near the <span class="hlt">central</span> axes (364-378 Ma), extending to the periphery areas on the opposite sides of the <span class="hlt">rift</span> through (345-362 Ma). The established sequence of dyke intrusions is alternative to the oceanic crust spreading, at which the age of intrusions is increased from the spreading center in <span class="hlt">rift</span>-perpendicular direction. Results of thermo-mechanical modelling of formation of the Vilyui <span class="hlt">rift</span> basin are presented. The goal of 2D finite elements modelling is to demonstrate that the deep troughs can form in the continental crust under its limited extension. The 2D numerical simulation shows the possibility of considerable localized subsidence near the <span class="hlt">rift</span> axis and explains why mafic dike swarms are concentrated on the basin periphery. Thus, the mechanism of the Vilyui basin formation by means of magmatic <span class="hlt">rifting</span> is corroborated [Polyansky at al., 2013]. RFBR grant no. 14-05-00188.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2005AGUFM.G42A..04D','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2005AGUFM.G42A..04D"><span id="translatedtitle">Eight Years of Surface Deformation in the Asal-Ghoubbet <span class="hlt">Rift</span> (Afar Depression) Observed With SAR Data</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Doubre, C.; Peltzer, G.; Manighetti, I.; Jacques, E.</p> <p>2005-12-01</p> <p>The volcano-tectonic Asal-Ghoubbet <span class="hlt">rift</span> (Djibouti) is the youngest spreading segment of the Aden oceanic ridge propagating inland into the Afar Depression. The deformation in the <span class="hlt">rift</span> is characterized by magmatic inflation and dilatation (dyking), distributed extension, fissure opening, and normal faulting, contributing to a far field opening velocity of ~1.5 cm/yr. We use radar interferometry data acquired by the Canadian satellite Radarsat on 24-day repeat, descending passes to measure the surface deformation in a 100 km wide region centered on the <span class="hlt">rift</span>. The data set defines 87 epochs of acquisitions distributed between 1997 and 2005. We combined the SAR data into 354 full-resolution interferograms and solved for incremental displacements between epochs using a least-square approach [Berardino et al., 2002]. The resulting line of sight displacement map time series shows the following features: - A 40 km-wide zone centered on the <span class="hlt">rift</span> is uplifted as a dome at a steady rate. - The <span class="hlt">central</span> <span class="hlt">rift</span> is subsiding with respect to the north and south shoulders. The velocity field shows a marked asymmetry with faster rates occurring along the northern edge of the <span class="hlt">rift</span>. The mean velocity of the relative movement of the subsiding inner floor with respect to the northern up-lifting shoulder reaches 7 mm/yr. - Subsidence is faster in the north half of the inner floor of the <span class="hlt">rift</span> and is associated with episodic creep events on normal faults. These includes a slip of 16 mm on the north-dipping δ fault in 2003 and an episode of accelerated creep of 10 mm occurring in 2000 on the γ fault, which is creeping at a steady rate of 3.5 mm/yr. A northern-dipping normal fault is slipping with a mean rate of 1.4 mm/yr and accommodates also the subsidence of the northern part of the inner floor. Unlike other active faults, this one does not coincide with a topographic scarp but shows evidence of surface creep in the velocity field. - The southeastern part of F fault system is the only fault</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013AGUFM.T14A..05F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013AGUFM.T14A..05F"><span id="translatedtitle">Deep electrical conductivity structure of the Rio Grande <span class="hlt">Rift</span> in Colorado and New Mexico: Early results from a two-year magnetotelluric study</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Feucht, D. W.; Bedrosian, P.; Sheehan, A. F.</p> <p>2013-12-01</p> <p>A wideband and long-period magnetotelluric experiment is underway across the Rio Grande <span class="hlt">Rift</span> in Colorado and New Mexico in order to provide constraints on the thermal and rheological state of the lithosphere beneath this region of intra-continental extension. Magnetotellurics is a passive source electromagnetic technique that at long periods has depth penetration into the deep crust and upper mantle. Important questions about continental <span class="hlt">rifting</span> remain unresolved, including the role of magmatism, volatiles and inherited lithospheric structure in the initiation and development of <span class="hlt">rifting</span>. Recent seismic imaging studies show thinned crust and low seismic wavespeeds in the crust and upper mantle beneath the Rio Grande <span class="hlt">Rift</span>. New and ongoing geodetic work confirms the low strain-rate environment of the region yet shows surprisingly uniform deformation over an area far wider than the <span class="hlt">rift</span>'s physiographic expression. Electrical conductivity models from this experiment will provide information complementary to these studies and can be used to determine the relative contributions of thermal and compositional heterogeneity in the crust and upper mantle to processes of continental extension. Over the past two years, magnetotelluric data has been collected at ~100 site locations along three 450 km long east-west transects of the <span class="hlt">rift</span> axis. These three profiles extend across the northern, <span class="hlt">central</span>, and southern portions of the <span class="hlt">rift</span> and include sites in the High Plains, Colorado Front Range, southern Rocky Mountains, San Juan Basin, Sangre de Cristo Mountains, and southern Basin and Range along the New Mexico/Mexico border. A comparison of results from these segments will be used to examine along-strike variation in the spatial extent of <span class="hlt">rifting</span> and associated modification of the lithosphere. Data assessment shows high-quality signal to periods in excess of 10 000 s, which corresponds to upper-mantle depths in this region of high upper-crustal conductivity and low crustal</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012EGUGA..1413065R','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012EGUGA..1413065R"><span id="translatedtitle"><span class="hlt">Rifting</span> of the Tyrrhenian Basin, a complex interaction among faulting , magmatism and mantle exhumation.</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Ranero, C. R.; Sallarés, V.; Grevemeyer, I.; Zitellini, N.; Guzman, M.; Prada, M.; Moeller, S.; de Franco, R.; Medoc Cruise Party</p> <p>2012-04-01</p> <p>The Tyrrhenian basin has been created during the extension of continental lithosphere driven by the retreat of a Ionian slab across the mantle. The basin does not seem to be actively extending, but its preserved crustal structure provides information of the time evolution of the processes involved in <span class="hlt">rifting</span>. The basin <span class="hlt">rifted</span> from north to south, with <span class="hlt">rifting</span> stopping after progressively larger stretching factor towards the south. The northern region stopped opening after a relatively low extension factor. Towards the south extension increased up to full crustal separation that produced mantle exhumation. The final structure displays two conjugate margins with asymmetric structures. Thus, the basin provides a natural laboratory to investigate a full <span class="hlt">rift</span> system, that displays variable amounts of extension. We present observations from a two-ship seismic experiment that took place in spring 2010. The cruise took place on two legs. In the first leg, the Spanish R/V Sarmiento de Gamboa and the Italian R/V Urania collected five E-W trending wide-angle seismic (WAS) profiles across the entire basin using 17 Ocean Bottom Seismometers and 25 Ocean Bottom Hydrophones and a 4800 c.i. G-II gun array. The profiles were extended with land stations that recorded the marine shots. During a second leg the R/V Sarmiento de Gamboa collected 16 Multichannel Seismic Reflection (MCS) profiles using a 3.75 km-long streamer and a 3000 c.i. G-II gun array. MCS profiles were acquired coincident with the WAS profiles, and a number of additional lines concentrated in the <span class="hlt">central</span> region of the basin where mantle exhumation took place. The seismic profiles were located to cover regions of the basin that displays different amount of extension, and the coincident wide-angle and MCS transects cross the entire basin to image the two conjugate margins. In this presentation we compare observations from different transects mapping the structures produced at different extension factors. A comparison</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/5062407','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/5062407"><span id="translatedtitle">Geology and petroleum resources of <span class="hlt">central</span> and east-<span class="hlt">central</span> Africa</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Peterson, J.A.</p> <p>1986-05-01</p> <p>The petroleum provinces of <span class="hlt">central</span> and east-<span class="hlt">central</span> Africa include the Somali basin, the Ethiopian plateau and <span class="hlt">rift</span> belt, and the <span class="hlt">central</span> African <span class="hlt">rift</span> basins. The western shelf on the Somali basin in the horn of Africa contains a sedimentary cover 1000-10,000 m (3000-3500 ft) thick of Mesozoic and Tertiary marine and continental clastic, carbonate, and evaporite deposits with good reservoir and potentially adequate source rock properties. To date, no commercial oil or gas discoveries have been made. The Ethiopian plateau and <span class="hlt">rift</span> belt is a high-risk area with minimum potential for commercial petroleum because the Neogene <span class="hlt">rifting</span> origin of the province has resulted in high geothermal gradients, extensive volcanism, and inadequate marine deposits and petroleum source rocks. The <span class="hlt">central</span> Africa interior basins are continental craton-<span class="hlt">rifted</span> depressions of Late Cretaceous and Tertiary age containing as much as 4000 m (13,000 ft) or more of fluvial and lacustrine clastic beds, which inter-tongue with nearshore marine clastic and carbonate beds in the western basins. Since the mid-1970s, approximately 13 oil discoveries have been made in the Upper Nile, Doba-Doseo, and Chad basins. These basins produce from fluvial and lacustrine sandstone reservoirs, sourced by lacustrine organic shale beds of Cretaceous age. The Benue trough is a <span class="hlt">rifted</span> depression of middle Cretaceous age adjoined on the southwest by the prolific Niger Delta petroleum province. The trough is filled with 6000 m (20,000 ft) or more of clastic and carbonate marine and continental rocks of late Early Cretaceous and early Cenozoic age. Very few exploratory wells have been drilled, and no commercial discoveries have been made although good reservoir and source rocks are present.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/25688166','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/25688166"><span id="translatedtitle">Sociocultural and economic dimensions of <span class="hlt">Rift</span> Valley fever.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Muga, Geoffrey Otieno; Onyango-Ouma, Washington; Sang, Rosemary; Affognon, Hippolyte</p> <p>2015-04-01</p> <p>Health researchers have advocated for a cross-disciplinary approach to the study and prevention of infectious zoonotic diseases, such as <span class="hlt">Rift</span> Valley Fever. It is believed that this approach can help bring out the social determinants and effects of the zoonotic diseases for the design of appropriate interventions and public health policy. A comprehensive literature review using a systematic search strategy was undertaken to explore the sociocultural and economic factors that influence the transmission and spread of <span class="hlt">Rift</span> Valley Fever. Although the findings reveal a paucity of social research on <span class="hlt">Rift</span> Valley Fever, they suggest that livestock sacrificial rituals, food preparation and consumption practices, gender roles, and inadequate resource base for public institutions are the key factors that influence the transmission. It is concluded that there is need for cross-disciplinary studies to increase the understanding of <span class="hlt">Rift</span> Valley Fever and facilitate appropriate and timely response and mitigation measures.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70170231','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70170231"><span id="translatedtitle">The seismic history of the Rio Grande <span class="hlt">Rift</span></span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Hoffman, J.P.</p> <p>1975-01-01</p> <p>The Rio Grande <span class="hlt">Rift</span>, one of the major geologic structures of the Southwest, cuts through the center of New Mexico from north to south. The <span class="hlt">rift</span> is also referred to as the Rio Grande Trench and as the Rio Grande Trough. It extends from the northern end of the San Luis Valley in Colorado southward 725 kilometres along the course of the Rio Grande River through New Mexico to near El Paso, Tex. The Rio Grande <span class="hlt">Rift</span> is not a single trough but a series of north-trending basins arranged en echelon and separated by narrow constrictions or channels. The <span class="hlt">rift</span> follows the western flank of the southern Rocky Mountains and apparently was formed at the same time as the moutains. </p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/21979933','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/21979933"><span id="translatedtitle">Lithospheric thinning beneath <span class="hlt">rifted</span> regions of Southern California.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Lekic, Vedran; French, Scott W; Fischer, Karen M</p> <p>2011-11-11</p> <p>The stretching and break-up of tectonic plates by <span class="hlt">rifting</span> control the evolution of continents and oceans, but the processes by which lithosphere deforms and accommodates strain during <span class="hlt">rifting</span> remain enigmatic. Using scattering of teleseismic shear waves beneath <span class="hlt">rifted</span> zones and adjacent areas in Southern California, we resolve the lithosphere-asthenosphere boundary and lithospheric thickness variations to directly constrain this deformation. Substantial and laterally abrupt lithospheric thinning beneath <span class="hlt">rifted</span> regions suggests efficient strain localization. In the Salton Trough, either the mantle lithosphere has experienced more thinning than the crust, or large volumes of new lithosphere have been created. Lack of a systematic offset between surface and deep lithospheric deformation rules out simple shear along throughgoing unidirectional shallow-dipping shear zones, but is consistent with symmetric extension of the lithosphere.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li class="active"><span>18</span></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_18 --> <div id="page_19" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li class="active"><span>19</span></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="361"> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19850031795&hterms=continents+oceans&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D30%26Ntt%3Dcontinents%2Boceans','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19850031795&hterms=continents+oceans&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D30%26Ntt%3Dcontinents%2Boceans"><span id="translatedtitle">Preferential <span class="hlt">rifting</span> of continents - A source of displaced terranes</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Vink, G. E.; Morgan, W. J.; Zhao, W.-L.</p> <p>1984-01-01</p> <p>Lithospheric <span class="hlt">rifting</span>, while prevalent in the continents, rarely occurs in oceanic regions. To explain this preferential <span class="hlt">rifting</span> of continents, the total strength of different lithospheres is compared by integrating the limits of lithospheric stress with depth. Comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of three. Also, a thickened crust can halve the total strength of normal continental lithosphere. Because the weakest area acts as a stress guide, any <span class="hlt">rifting</span> close to an ocean-continent boundary would prefer a continental pathway. This results in the formation of small continental fragments or microplates that, once accreted back to a continent during subduction, are seen as displaced terranes. In addition, the large crustal thicknesses associated with suture zones would make such areas likely locations for future <span class="hlt">rifting</span> episodes. This results in the tendency of new oceans to open along the suture where a former ocean had closed.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70017108','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70017108"><span id="translatedtitle">Metallogeny of the midcontinent <span class="hlt">rift</span> system of North America</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Nicholson, S.W.; Cannon, W.F.; Schulz, K.J.</p> <p>1992-01-01</p> <p>The 1.1 Ga Midcontinent <span class="hlt">rift</span> system of North America is one of the world's major continental <span class="hlt">rifts</span> and hosts a variety of mineral deposits. The rocks and mineral deposits of this 2000 km long <span class="hlt">rift</span> are exposed only in the Lake Superior region. In the Lake Superior region, the <span class="hlt">rift</span> cuts across Precambrian basement terranes ranging in age from ??? 1850 Ma to more than 3500 Ma. Where exposed, the <span class="hlt">rift</span> consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks. Beneath the center of Lake Superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection profiles. This region hosts two major classes of mineral deposits, magmatic and hydrothermal. All important mineral production in this region has come from hydrothermal deposits. <span class="hlt">Rift</span>-related hydrothermal deposits include four main types: (1) native copper deposits in basalts and interflow sediments; (2) sediment-hosted copper sulfide and native copper; (3) copper sulfide veins and lodes hosted by <span class="hlt">rift</span>-related volcanic and sedimentary rocks; and (4) polymetallic (five-element) veins in the surrounding Archean country rocks. The scarcity of sulfur within the <span class="hlt">rift</span> rocks resulted in the formation of very large deposits of native metals. Where hydrothermal sulfides occur (i.e., shale-hosted copper sulfides), the source of sulfur was local sedimentary rocks. Magmatic deposits have locally supported exploration and minor production, but most are subeconomic presently. These deposits occur in intrusions exposed near the margins of the <span class="hlt">rift</span> and include CuNiPGE and TiFe (V) in the Duluth Complex, U-REE-Nb in small carbonatites, and breccia pipes resulting from local hydrothermal activity around small felsic intrusions. Mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization. Most of the sulfide deposits in intrusions, however, contain sulfur derived from</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19810021160','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19810021160"><span id="translatedtitle">The geology and geophysics of the Oslo <span class="hlt">rift</span></span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Ruder, M. E.</p> <p>1981-01-01</p> <p>The regional geology and geophysical characteristics of the Oslo graben are reviewed. The graben is part of a Permian age failed continental <span class="hlt">rift</span>. Alkali olivine, tholefitic, and monzonitic intrusives as well as basaltic lavas outline the extent of the graben. Geophysical evidence indicates that <span class="hlt">rifting</span> 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 <span class="hlt">rift</span> 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 <span class="hlt">rift</span> 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.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=4385765','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=4385765"><span id="translatedtitle">Sociocultural and Economic Dimensions of <span class="hlt">Rift</span> Valley Fever</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Muga, Geoffrey Otieno; Onyango-Ouma, Washington; Sang, Rosemary; Affognon, Hippolyte</p> <p>2015-01-01</p> <p>Health researchers have advocated for a cross-disciplinary approach to the study and prevention of infectious zoonotic diseases, such as <span class="hlt">Rift</span> Valley Fever. It is believed that this approach can help bring out the social determinants and effects of the zoonotic diseases for the design of appropriate interventions and public health policy. A comprehensive literature review using a systematic search strategy was undertaken to explore the sociocultural and economic factors that influence the transmission and spread of <span class="hlt">Rift</span> Valley Fever. Although the findings reveal a paucity of social research on <span class="hlt">Rift</span> Valley Fever, they suggest that livestock sacrificial rituals, food preparation and consumption practices, gender roles, and inadequate resource base for public institutions are the key factors that influence the transmission. It is concluded that there is need for cross-disciplinary studies to increase the understanding of <span class="hlt">Rift</span> Valley Fever and facilitate appropriate and timely response and mitigation measures. PMID:25688166</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/1144929','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/1144929"><span id="translatedtitle">Experimental <span class="hlt">Rift</span> Valley fever in West African Dwarf sheep.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Fagbami, A H; Tomori, O; Fabiyi, A; Isoun, T T</p> <p>1975-05-01</p> <p>West African Dwarf sheep were challenged with a low mouse brain-passaged <span class="hlt">Rift</span> Valley fever virus (Ib-AR 55172) isolated from Nigeria. Viraemia, mild febrile reaction and neutralising antibodies were demonstrated in inoculated animals.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/1988JGR....93.4258D','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/1988JGR....93.4258D"><span id="translatedtitle">Growth and persistence of Hawaiian volcanic <span class="hlt">rift</span> zones</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Dieterich, James H.</p> <p>1988-05-01</p> <p>Hawaiian volcanic <span class="hlt">rift</span> zones are modeled by representing the <span class="hlt">rifts</span> and adjacent volcano flanks as long ridges with the geometry of flattened triangular prisms. The intrusion of dikes along the axis of a <span class="hlt">rift</span> requires a mechanism to generate the appropriate dike-trapping stress field within the prism. Possible factors that affect the state of stress in the prism include multiple dike intrusion along the ridge axis, faulting, and gravitational sagging of the topography. In extreme models with very steep slopes and high Poisson's ratio, corresponding to the gelatin models of <span class="hlt">rift</span> zones by Fiske and Jackson (1972), results of finite element calculations indicate that gravity-induced stresses are sufficient to trap a dike into propagating within the prism and parallel to the <span class="hlt">rift</span> zone as proposed by Fiske and Jackson. However, the mechanism does not work for gently sloping flanks or a more acceptable Poisson's ratio of about 0.25. Additionally, trapping stresses in the gravity-loading and density stratification models will not persist after a few dike injection episodes. Therefore in mature Hawaiian <span class="hlt">rift</span> zones with possibly thousands of dikes, additional processes must act to control the stresses that permit continued dike intrusion and <span class="hlt">rift</span> persistence. It is proposed that accommodation to dike emplacement occurs by slip on deep faults, possibly of the type proposed for the 1975 Kalapana, Hawaii, earthquake. As suggested by others for this earthquake, the faults could coincide with the contact of the volcano with the seafloor within the weak seafloor sediments. Such faulting not only provides a means for the flanks to adjust continuously to intrusions but also generates the stress patterns needed to constrain future dikes to propagate along the <span class="hlt">rift</span> axis. Other possible faulting mechanisms, such as shallow gravity slides and normal faulting of the flanks, do not appear to favor <span class="hlt">rift</span> zone persistence. In this model the horizontal stress generated by a standing column of</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2005GeoJI.161..707S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2005GeoJI.161..707S"><span id="translatedtitle">An updated global earthquake catalogue for stable continental regions: reassessing the correlation with ancient <span class="hlt">rifts</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Schulte, Saskia M.; Mooney, Walter D.</p> <p>2005-06-01</p> <p>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 <span class="hlt">rifts</span>, we assess the correlation of intraplate seismicity with ancient <span class="hlt">rifts</span> on a global scale. Each tectonic event is put into one of five categories based on location: (i) interior <span class="hlt">rifts</span>/taphrogens, (ii) <span class="hlt">rifted</span> continental margins, (iii) non-<span class="hlt">rifted</span> crust, (iv) possible interior <span class="hlt">rifts</span> and (v) possible <span class="hlt">rifted</span> margins. We find that approximately 27 per cent of all events are classified as interior <span class="hlt">rifts</span> (i), 25 per cent are <span class="hlt">rifted</span> continental margins (ii), 36 per cent are within non-<span class="hlt">rifted</span> crust (iii) and 12 per cent (iv and v) remain uncertain. Thus, over half (52 per cent) of all events are associated with <span class="hlt">rifted</span> crust, although within the continental interiors (i.e. away from continental margins), non-<span class="hlt">rifted</span> crust has experienced more earthquakes than interior <span class="hlt">rifts</span>. 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 <span class="hlt">rifts</span> (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 <span class="hlt">rifts</span>/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 <span class="hlt">rifts</span>/taphrogens that have experienced the largest earthquakes, seismicity within the Kutch <span class="hlt">rift</span>, India, and the East China <span class="hlt">rift</span> system, may be controlled by diffuse plate boundary deformation more than by the presence</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70029536','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70029536"><span id="translatedtitle">An updated global earthquake catalogue for stable continental regions: Reassessing the correlation with ancient <span class="hlt">rifts</span></span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Schulte, S.M.; Mooney, W.D.</p> <p>2005-01-01</p> <p>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 <span class="hlt">rifts</span>, we assess the correlation of intraplate seismicity with ancient <span class="hlt">rifts</span> on a global scale. Each tectonic event is put into one of five categories based on location: (i) interior <span class="hlt">rifts</span>/taphrogens, (ii) <span class="hlt">rifted</span> continental margins, (iii) non-<span class="hlt">rifted</span> crust, (iv) possible interior <span class="hlt">rifts</span> and (v) possible <span class="hlt">rifted</span> margins. We find that approximately 27 per cent of all events are classified as interior <span class="hlt">rifts</span> (i), 25 per cent are <span class="hlt">rifted</span> continental margins (ii), 36 per cent are within non-<span class="hlt">rifted</span> crust (iii) and 12 per cent (iv and v) remain uncertain. Thus, over half (52 per cent) of all events are associated with <span class="hlt">rifted</span> crust, although within the continental interiors (i.e. away from continental margins), non-<span class="hlt">rifted</span> crust has experienced more earthquakes than interior <span class="hlt">rifts</span>. 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 <span class="hlt">rifts</span> (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 <span class="hlt">rifts</span>/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 <span class="hlt">rifts</span>/taphrogens that have experienced the largest earthquakes, seismicity within the Kutch <span class="hlt">rift</span>, India, and the East China <span class="hlt">rift</span> system, may be controlled by diffuse plate boundary deformation more than by the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2003AGUFM.S32A0831G','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2003AGUFM.S32A0831G"><span id="translatedtitle">Lithospheric Velocity Structure of the Upper Rhine Graben; A new Model for the Development of Europe's Largest Continental <span class="hlt">Rift</span>.</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Granet, M.; Lopes Cardozo, G.; Herquel, G.</p> <p>2003-12-01</p> <p>The lithospheric structure of the Rhine Graben, part of the European Cenozoic <span class="hlt">Rift</span> system (ECRIS), is studied with the help of a regional 3D teleseismic travel time tomography. Joint interpretation of the tomography, the SKS anisotropy, and a local earthquake tomographic study leads to a new model for the <span class="hlt">rifting</span> and development of the Graben. Two seismic campaigns provided 2696 P and PKP travel time residuals of teleseismic events. The two data sets are combined and inverted together. This inversion yields detailed images of the P velocity structure of the lower crust and the lithospheric mantle down to a depth of 125 kilometers. An absence of strong low velocity anomalies in the region confirms that there is no shallow mantle diapir, this demonstrates the passive development of the <span class="hlt">rift</span>. The observed velocity structures have an orientation 10° -20° oblique to the graben axis and are interpreted as inherited Variscan structures. These structures are thought to have played a dominant role during the Oligocene <span class="hlt">rifting</span> of the Rhine Graben. Similar structures are known to have influenced the development of the Limagne graben (Massif <span class="hlt">Central</span>), also part of the ECRIS. An absence of structures that are orientated parallel to the graben axis shows that the Upper Rhine Graben did not form following a traditional McKenzie or Wernicke model. A new model is presented in which strike slip movements on the Variscan structures in the lithospheric mantle are responsible for the extension in the crust. This conceptual model is supported by SKS anisotropy and helps to explain some of the features in the development of the continental <span class="hlt">rift</span> in the overall compressive setting of the Alpine foreland.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.8403S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.8403S"><span id="translatedtitle">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 <span class="hlt">Rift</span> System</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Smets, Benoît; d'Oreye, Nicolas; Kervyn, Matthieu; Kervyn, François</p> <p>2016-04-01</p> <p>The East African <span class="hlt">Rift</span> System (EARS) is often mentioned as the modern archetype for <span class="hlt">rifting</span> and continental break-up (Calais et al., 2006, GSL Special Publication 259), showing the complex interaction between <span class="hlt">rift</span> 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 <span class="hlt">rifting</span>, magmatism and volcanism. This is the case of the Virunga Volcanic Province (VVP) located in the <span class="hlt">central</span> 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 <span class="hlt">rift</span> 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 <span class="hlt">rift</span>. The presence of transfer zones north and south of the Lake Kivu <span class="hlt">rift</span> basin favoured the development of volcanic provinces at these locations. <span class="hlt">Rift</span> faults, including reactivated Precambrian structures influenced the location of volcanism within the volcanic provinces and the <span class="hlt">rift</span> 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.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015DokES.465.1191S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015DokES.465.1191S"><span id="translatedtitle">Modern fault formation in the Earth's crust of the Baikal <span class="hlt">rift</span> system according to the data on the mechanisms of earthquake sources</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>San'kov, V. A.; Dobrynina, A. A.</p> <p>2015-11-01</p> <p>The spatial characteristics of seismotectonic deformations and the most likely fracture planes in the earthquake sources of the Baikal <span class="hlt">rift</span> system (BRS) are determined using the method of cataclastic analysis of fractures [1]. It is shown that extension conditions with a strike of modern fractures parallel to the <span class="hlt">rift</span>-controlling faults are dominant in the <span class="hlt">central</span> zone and in most of the NE flank of the BRS. The flat average dip of fractures in the earthquake sources of the main fault zones for some <span class="hlt">rift</span> depressions allow a suggestion about the flattening of faults in the middle crust. The antithetic faults are steeper. The BRS flanks are characterized by dominant shear deformations and more diverse morphogenetic faults in the earthquake sources (strike-slip faults, reversed faults, and normal faults). The modern faults at the BRS flanks weakly inherit the neotectonic structure.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/127662','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/127662"><span id="translatedtitle">Buried Mesozoic <span class="hlt">rift</span> basins of Moroccan Atlantic continental margin</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Mohamed, N.; Jabour, H.; El Mostaine, M.</p> <p>1995-08-01</p> <p>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 <span class="hlt">Rift</span> basins buried beneath the post-<span class="hlt">rift</span> 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 <span class="hlt">rift</span> basins beneath the Atlantic coastal plain were determined by analysis of drilled-hole data in combination with gravity anomaly and aeromagnetic maps. These <span class="hlt">rift</span> basins are characterized by several half graben filled by synrift sediments of Triassic age probably deposited in lacustrine environment. Coeval <span class="hlt">rift</span> basins are known to be present in the U.S. Atlantic continental margin. Basin modeling suggested that many of the less deeply bored <span class="hlt">rift</span> basins beneath the coastal plain are still within the oil window and present the most attractive exploration targets in the area.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/5862861','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/5862861"><span id="translatedtitle">Sedimentary deposits in response to <span class="hlt">rift</span> geometry in Malawi, Africa</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Bishop, M.G. )</p> <p>1991-03-01</p> <p>Sedimentary deposits of the Malawi continental <span class="hlt">rift</span> basin are a direct result of topography and tectonics unique to <span class="hlt">rift</span> structure. Recent models describe <span class="hlt">rifts</span> as asymmetric half-graben connected in series by transfer of accommodation zones. Half-graben consist of roughly parallel, tilted fault blocks stepping up from the bounding fault zone where maximum subsidence occurs. The <span class="hlt">rift</span> becomes a local baselevel and depocenter as regional drainage is shed away by the <span class="hlt">rift</span> shoulders. Most of the sediments are derived locally due to internal drainage of connected basins, individual basins, and individual fault blocks. The patterns of sedimentation and facies associations depend on structural position at both fault block and half-graben scales. Drainage is directed and dammed by tilted fault blocks. Forward-tilted fault blocks form basinward-thickening sediment wedges filled with facies of axial fluvial systems, alluvial fault-scarp fans, and ponded swamp and lake deposits. These deposits are asymmetrically shifted toward the controlling fault and onlap the upthrown side of the block, ordinarily the site of erosion or nondeposition. Rivers entering the lake on back tilted fault blocks form large deltas resulting in basinward fining and thinning sediment wedges. Lacustrine, nearshore, shoreline, and lake shore plain deposits over multiple fault blocks record lake levels, water chemistry, and tectonic episodes. Tectonic movement periodically changes the basin depth, configuration, and baselevel. This movement results in widespread unconformities deposition and reworking of sediments within the <span class="hlt">rift</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2001AGUFM.T51B0871B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2001AGUFM.T51B0871B"><span id="translatedtitle">Regional Hydrothermal Cooling During the Initiation of Continental <span class="hlt">Rifting</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Buck, W. R.</p> <p>2001-12-01</p> <p>Thermochronologic data from the uplifted flanks of the Red Sea clearly indicate at least two distinct phases of regional cooling: the first at about 32 Ma and the second at about 21 Ma according to Omar and Steckler (1995). The latter phase correlates well with the rapid subsidence in the <span class="hlt">rift</span> and uplift of the flanks. Thus this cooling data is generally interpreted to result from tectonic denudation. The earlier cooling is more problematic, since sediments of the same age are not clearly identified in <span class="hlt">rift</span> basins. A different explanation for the early cooling episode is suggested by numerical models of lithospheric stretching. Those models show that <span class="hlt">rift</span> initiation can result in a very broad region of lithospheric downbending. This bending produces small magnitude (several percent) surface extension of the sides of the <span class="hlt">rift</span>. Eventually, these <span class="hlt">rift</span> flanks are bent in the opposite sense and uplifted. The earlier phase of extensional strain may create a system of cracks and faults of sufficient permeability to allow significant water circulation. Hydrothermal flow could produce cooling of rocks as deep as 5-10 km. The cooling would be recorded in low-temperature thermochrologic systems such as apatite fission tracks. Model thermal histories for different <span class="hlt">rift</span> and hydrothermal flow histories are compared to fission track data from the Red Sea region.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2001AGUFM.S11D..12B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2001AGUFM.S11D..12B"><span id="translatedtitle">P Wave Velocity Structure Beneath the Baikal <span class="hlt">Rift</span> Axis</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Brazier, R. A.; Nyblade, A. A.; Boman, E. C.</p> <p>2001-12-01</p> <p>Over 100 p wave travel times from the 1500 km en echelon Baikal <span class="hlt">Rift</span> system are used in this study.The events range 3 to 13 degrees from Talaya, Russia (TLY) along the axis of southwest northeast trending <span class="hlt">rift</span> 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 <span class="hlt">rift</span> at about 400km from TLY. The Baikal <span class="hlt">Rift</span> hosts the deepest lake and is the most seismically active <span class="hlt">rift</span> in the world. It is one of the few continental <span class="hlt">rifts</span>, 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.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013EGUGA..1513183B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013EGUGA..1513183B"><span id="translatedtitle">Ground deformation across the Corinth <span class="hlt">rift</span> from 22 years of GPS observations</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Briole, Pierre</p> <p>2013-04-01</p> <p>Between 1990 and 2012 sixteen GPS campaigns have been carried out in the western and <span class="hlt">central</span> <span class="hlt">rift</span> of Corinth area and surroundings in order to map and monitor the extension and the vertical motions of the <span class="hlt">rift</span>. Since 2001 five permanent GPS stations are also operated in the western <span class="hlt">rift</span> plus a EUREF station located at the University of Patras. Fieldwork involved in total more than eighty people. The campaign network was designed as the sum of a "first order" network with approximately 70 points measured in several campaigns during sessions of several days and a "second order" with more than 100 points measured once during a few hours and available for future measurements. Spacing between points in the centre of the network (Corinth <span class="hlt">Rift</span> Laboratory, CRL, http://crlab.eu) is approximately one point every five kilometres. This density was defined so as sample each main known fault of the southern coast of the <span class="hlt">rift</span> and to provide in principle ten to twenty points with significant motion in case of a M=6 earthquake like the June 15, 1995 Aigion earthquake, and possibly one point with significant motion in case of M=5 shallow earthquake like those of January 18 and 22, 2010 near Nafpaktos. The density of points is less outside the <span class="hlt">central</span> area but enough to provide constraints on the boundary conditions of the <span class="hlt">rift</span>, in particular at its western termination near the city of Patras. The percentage of destroyed points is approximately 10% in 10 years and if maintained the network could be usable during several more decades. Kinematic GPS data were also acquired along several hundred of kilometres of roads on both sides of the <span class="hlt">rift</span>. The ten years time series at the five permanent stations provides an accurate determination of the overall extension and a local reference frame for the network. The permanent station located at Trizonia island exhibits a 2mm/yr subsidence rate, the four others (Kounina, Psaromita, Lidoriki, Efpalio) show no vertical velocity. However Efpalio</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015SedG..325..177Z','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015SedG..325..177Z"><span id="translatedtitle">Turbidite systems of lacustrine <span class="hlt">rift</span> basins: Examples from the Lake Kivu and Lake Albert <span class="hlt">rifts</span>, East Africa</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Zhang, Xuewei; Scholz, Christopher A.</p> <p>2015-07-01</p> <p>The Holocene turbidite systems of Lake Kivu and the Pliocene turbidite systems of Lake Albert in the East African <span class="hlt">Rift</span> were examined using high-resolution 2-D and 3-D seismic reflection data and sediment core information. Based on investigations of seismic facies and lithofacies, several key turbidity-flow depositional elements were observed, including channels, overbank levees with sediment waves, and depositional lobes. Analyses of the sources of the recent and ancient turbidite systems in these two extensional basins suggest that flood-induced hyperpycnal flows are important triggers of turbidity currents in lacustrine <span class="hlt">rift</span> basins. From source to sink, sediment dispersal, facies distribution, and depositional thickness of the turbidite systems are strongly influenced by <span class="hlt">rift</span> topography. The Lake Kivu and Lake Albert <span class="hlt">rifts</span> serve as excellent analogues for understanding the sedimentary patterns of lacustrine turbidites in extensional basins.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2006AGUFM.T52C..02R','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2006AGUFM.T52C..02R"><span id="translatedtitle">Along-strike supply of volcanic <span class="hlt">rifted</span> margins: Implications for plume-influenced <span class="hlt">rifting</span> and sudden along-strike transitions between volcanic and non-volcanic <span class="hlt">rifted</span> margins</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Ranero, C. R.; Phipps Morgan, J.</p> <p>2006-12-01</p> <p>The existence of sudden along-strike transitions between volcanic and non-volcanic <span class="hlt">rifted</span> margins is an important constraint for conceptual models of <span class="hlt">rifting</span> and continental breakup. We think there is a promising indirect approach to infer the maximum width of the region of upwelling that exists beneath a <span class="hlt">rifted</span> margin during the transition from <span class="hlt">rifting</span> to seafloor-spreading. We infer this width of ~30km from the minimum length of the ridge-offsets that mark the limits of the `region of influence' of on-ridge plumes on the axial relief, axial morphology, and crustal thickness along the ridge and at the terminations of fossil volcanic <span class="hlt">rifted</span> margins. We adopt Vogt's [1972] hypothesis for along-ridge asthenospheric flow in a narrow vertical slot beneath the axis of plume-influenced `macro-segments' and volcanic <span class="hlt">rifted</span> margins. We find that: (1) There is a threshold distance to the lateral offsets that bound plume-influenced macrosegments; all such `barrier offsets' are greater than ~30km, while smaller offsets do not appear to be a barrier to along-axis flow. This pattern is seen in the often abrupt transitions between volcanic and non-volcanic <span class="hlt">rifted</span> margins; these transitions coincide with >30km ridge offsets that mark the boundary between the smooth seafloor morphology and thick crust of a plume- influenced volcanic margin and a neighboring non-volcanic margin, as recorded in 180Ma <span class="hlt">rifting</span> of the early N. Atlantic, the 42Ma <span class="hlt">rifting</span> of the Kerguelen-Broken Ridge, and the 66Ma Seychelles-Indian <span class="hlt">rifting</span> in the Indian Ocean. (2) A similar pattern is seen in the often abrupt transitions between `normal' and plume-influenced mid- ocean ridge segments, which is discussed in a companion presentation by Phipps Morgan and Ranero (this meeting). (3) The coexistance of adjacent volcanic and non-volcanic <span class="hlt">rifted</span> margin segments is readily explained in this conceptual framework. If the volcanic margin macrosegment is plume-fed by hot asthenosphere along an axial ridge slot</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFM.T31B2875S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFM.T31B2875S"><span id="translatedtitle">Concentration of strain in a marginal <span class="hlt">rift</span> zone of the Japan backarc during post-<span class="hlt">rift</span> compression</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Sato, H.; Ishiyama, T.; Kato, N.; Abe, S.; Shiraishi, K.; Inaba, M.; Kurashimo, E.; Iwasaki, T.; Van Horne, A.; No, T.; Sato, T.; Kodaira, S.; Matsubara, M.; Takeda, T.; Abe, S.; Kodaira, C.</p> <p>2015-12-01</p> <p>Late Cenozoic deformation zones in Japan may be divided into two types: (1) arc-arc collision zones like those of Izu and the Hokkaido axial zone, and (2) reactivated back-arc marginal <span class="hlt">rift</span> (BMR) systems. A BMR develops during a secondary <span class="hlt">rifting</span> event that follows the opening of a back-arc basin. It forms close to the volcanic front and distant from the spreading center of the basin. In Japan, a BMR system developed along the Sea of Japan coast following the opening of the Japan Sea. The BMR appears to be the weakest, most deformable part of the arc back-arc system. When active <span class="hlt">rifting</span> in the marginal basins ended, thermal subsidence, and then mechanical subsidence related to the onset of a compressional stress regime, allowed deposition of up to 5 km of post-<span class="hlt">rift</span>, deep-marine to fluvial sedimentation. Continued compression produced fault-related folds in the post-<span class="hlt">rift</span> sediments, in thin-skin style deformation. Shortening reached a maximum in the BMR system compared to other parts of the back-arc, suggesting that it is the weakest part of the entire system. We examined the structure of the BMR system using active source seismic investigation and earthquake tomography. The velocity structure beneath the marginal <span class="hlt">rift</span> basin shows higher P-wave velocity in the upper mantle/lower crust which suggests significant mafic intrusion and thinning of the upper continental crust. The syn-<span class="hlt">rift</span> mafic intrusive forms a convex shape, and the boundary between the pre-<span class="hlt">rift</span> crust and the mafic intrusive dips outward. In the post-<span class="hlt">rift</span> compressional stress regime, the boundary of the mafic body reactivated as a reverse fault, forming a large-scale wedge thrust and causing further subsidence of the <span class="hlt">rift</span> basin. The driver of the intense shortening event along the Sea of Japan coast in SW Japan was the arrival of a buoyant young (15 Ma) Shikoku basin at the Nankai Trough. Subduction stalled and the backarc was compressed. As the buoyant basin cooled, subduction resumed, and the rate of</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012EGUGA..14.4355F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012EGUGA..14.4355F"><span id="translatedtitle"><span class="hlt">Rifting</span> and volcanism: Examples from volcanic <span class="hlt">rifted</span> and magma-poor margins based on multichannel seismic data</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Franke, D.</p> <p>2012-04-01</p> <p>Great efforts in the research of passive <span class="hlt">rifted</span> margins in the last decades highlighted also that lots of open questions remain. A considerable controversy exists about the role of the mantle during <span class="hlt">rifting</span> and the subsequent formation of oceanic crust and about the interaction of mantle and surface processes, i.e. the precise nature of volcanism in the <span class="hlt">rifting</span> process. There are two end-member extremes of passive <span class="hlt">rifted</span> margins. Volcanic <span class="hlt">rifted</span> margins evolve by a combination of extension, and extensive extrusive flood volcanism over short time periods during breakup, manifested in reflection seismic data as seaward dipping reflectors. These margins are commonly related to mantle plumes; however, in the past years this has been questioned. Magma-poor <span class="hlt">rifted</span> margins in contrast show wide extensional features as rotated faults blocks and detachment surfaces near the base of the continental crust, but limited magmatism that in addition seems to be delayed to post-breakup. In this presentation examples from three locations that are less frequently cited in the discussion about (either magma-poor or volcanic) <span class="hlt">rifted</span> margins will be shown: The Laptev Sea margin in the Arctic Ocean, where the active Arctic mid-oceanic ridge meets continental lithosphere at a high angle, the southernmost South Atlantic with well expressed conjugate volcanic <span class="hlt">rifted</span> margins in a comparably "simple" configuration, potentially influenced by a mantle plume, the Tristan hot spot, and the South China Sea that may represent an intermediary form of continental extension between the end member extremes. The role of (hot-spot related) volcanism during break-up will be discussed for the three example margins that evolved in the Early Cretaceous, the Paleocene and the Oligocene, respectively.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li class="active"><span>19</span></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_19 --> <div id="page_20" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li class="active"><span>20</span></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="381"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2005E%26PSL.239..352C','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2005E%26PSL.239..352C"><span id="translatedtitle">Numerical modelling of quaternary deformation and post-<span class="hlt">rifting</span> displacement in the Asal-Ghoubbet <span class="hlt">rift</span> (Djibouti, Africa) [rapid communication</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Cattin, Rodolphe; Doubre, Cécile; de Chabalier, Jean-Bernard; King, Geoffrey; Vigny, Christophe; Avouac, Jean-Philippe; Ruegg, Jean-Claude</p> <p>2005-11-01</p> <p>Over the last three decades a host of information on <span class="hlt">rifting</span> process relating to the geological and thermal structure, long-time scale deformation (Quaternary and Holocene) and <span class="hlt">rifting</span> cycle displacement across the Asal-Ghoubbet <span class="hlt">rift</span> has been made available. These data are interpreted with a two-dimensional thermo-mechanical model that incorporates rheological layering of the lithosphere, dyke inflation and faulting. Active fault locations and geometry are mainly controlled by both thermal structure and magma intrusion into the crust. The distributed slip throughout the inner <span class="hlt">rift</span> is related to the closeness of magma chamber, leading to additional stress into the upper thinned crust. Assuming a constant Arabia-Somalia motion of 11 mm/year, the variation of subsidence rate between the last 100 and 9 ka is associated with a decrease of the average injection rate from 10 to 5 mm/year. These values, about equal to the regional opening rate, suggest that both volcanism and tectonic play an equivalent role in the <span class="hlt">rifting</span> process. Our modelled sequence of events gives one possible explanation for both vertical and horizontal displacements observed since the 1978 seismovolcanic crisis. Although part of the post-<span class="hlt">rifting</span> deformation could be due to viscous relaxation, the high opening rate in the first years after the event and the abrupt velocity change in 1984-1986 argue for a large dyke inflation of 12 cm/year ending in 1985. The asymmetric and constant pattern of the GPS velocity since 1991 suggests that present post-<span class="hlt">rifting</span> deformation is mainly controlled by fault creep and regional stretching. This study demonstrates the internal consistency of the data set, highlights the role of magmatism in the mechanics of crustal stretching and reveals a complex post-<span class="hlt">rifting</span> process including magma injection, fault creep and regional stretching.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013AGUFM.T12A..01M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013AGUFM.T12A..01M"><span id="translatedtitle"><span class="hlt">Rifting</span>, Volcanism, and the Geochemical Character of the Mantle Beneath the West Antarctic <span class="hlt">Rift</span> System (Invited)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Mukasa, S. B.; Aviado, K. B.; Rilling-Hall, S.; Bryce, J. G.; Cabato, J.</p> <p>2013-12-01</p> <p>The West Antarctic <span class="hlt">Rift</span> System (WARS) is one of the largest extensional alkali volcanic provinces on Earth, but the mechanisms responsible for generating the massive amounts of its associated magmatism remain controversial. The failure of both passive and active decompression melting models to adequately explain the observed lava volumes has prompted debate about the relative roles of thermal plume-related melting and ancient subduction-related flux melting. 40Ar/39Ar dating and geochemical analyses of the lavas, as well as volatile and trace-element determinations of olivine-hosted melt inclusions shed light on the relationship between <span class="hlt">rifting</span> and volcanism, and also improve our understanding of the geochemical character of the mantle beneath the WARS. Results show that the magmatism post-dates the main phase of extension along the Terror <span class="hlt">Rift</span> within the WARS, which supports a decompression-melting model without the benefit of a significant thermal anomaly. However, the observed large magma volumes seem to require a volatile-fluxed mantle, a notion supported by a long history of subduction (>500 Myr) along the paleo-Pacific margin of Gondwana. In fact, the legacy of that subduction may manifest itself in the high H2O concentrations of olivine-hosted melt inclusions (up to 3 wt% in preliminary results from ion probe measurements). The major oxide compositions of lavas in the WARS are best matched to experimental melts of garnet pyroxenite and carbonated peridotite sources. The Pb and Nd isotopic systems are decoupled from each other, suggesting removal of fluid-mobile elements from the mantle source possibly during the long history of subduction along this Gondwana margin. Extremely unradiogenic 187Os/188Os ranging to as low as 0.1081 × 0.0001 hints at the involvement of lithospheric components in generation of magmas in the WARS.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2008AGUFM.T43A1980H','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2008AGUFM.T43A1980H"><span id="translatedtitle">Seismic Observations From the Afar <span class="hlt">Rift</span> Dynamics Project: Preliminary Results</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Hammond, J. O.; Guidarelli, M.; Belachew, M.; Keir, D.; Ayele, A.; Ebinger, C.; Stuart, G.; Kendall, J.</p> <p>2008-12-01</p> <p>Following the 2005 Dabbahu <span class="hlt">rifting</span> event in Afar, 9 broadband seismometers were installed around the active <span class="hlt">rift</span> segment to study the microseismicity associated with this and subsequent dyking events. These recorded more than one year of continuous data. In March 2007, 41 stations were deployed throughout Afar and the adjacent <span class="hlt">rift</span> flanks as part of a large multi-national, collaboration involving universities and organisations from the UK, US and Ethiopia. This abstract describes the crustal and upper mantle structure results of the first 19 months of data. Bulk crustal structure has been determined using the H-k stacking of receiver functions and thickness varies from ~45 km on the <span class="hlt">rift</span> margins to ~16 km beneath the northeastern Afar stations. Estimates of Vp/Vs show normal continental crust values (1.7-1.8) on the <span class="hlt">rift</span> margins, and very high values (2.0-2.2) in Afar. A study of seismic noise interferometry is in early stages, but inversions using 20 s Green's function estimates, with some control from regional surface waves, show evidence for thin crustal regions around the recently <span class="hlt">rifted</span> Dabbahu segment. To improve our understanding of the physical and compositional properties of the crust and locate regions of high attenuation (an indicator of melt), we determine attenuation (Q) using t* values measured from spectra of P wave arrivals. We present whole path attenuation from source to receiver, which will provide a starting point for a future tomographic inversion. SKS-wave splitting results show sharp changes over small lateral distances (40° over <30 km), with fast directions overlying the Dabbahu segment aligning parallel with the recent diking. This supports ideas of melt dominated anisotropy beneath the Ethiopian <span class="hlt">rift</span>. Seismic tomography inversions show that in the top 150 km low velocities mimic the trend of the seismicity in Afar. The low velocity anomalies extend from the main Ethiopian <span class="hlt">rift</span> NE, towards Djibouti, and from Djibouti NW towards the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2009EGUGA..11.3548I','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2009EGUGA..11.3548I"><span id="translatedtitle">Recent geodynamics and evolution of the Moma <span class="hlt">rift</span>, Northeast Asia.</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Imaev, V. S.; Imaeva, L. P.; Kozmin, B. M.; Fujita, K. S.; Mackey, K. G.</p> <p>2009-04-01</p> <p>The Cenozoic Moma <span class="hlt">rift</span> system is a major tectonic feature in northeast Russia. It is composed of a series of basins (Selennyakh, Kyrin,Lower Moma,Upper Moma,etc.) filled with up to one km thick and bounded by the Chersky Range (up to 3100 m high) on the southwest and the Moma Range (up to 2400 m high) on the northeast. Northeast of the Moma Range is the Indigirka-Zyryanka foreland basin, composed of thick, up to 2.5 km, Eocene, Oligocene, and Miocene coal-bearing sequences, while on the southwestern side of the Chersky Range there are a number of piedmont basins (Tuostakh, Upper Adycha, Derbeke, etc.) containing up to several hundred meters of Miocene and Oligocene coal-bearing deposits. Despite considerable study over the past half-century, there is considerable debate over the origin, present-day tectonics, and evolution of the Moma <span class="hlt">rift</span> system. The Cenozoic deposits of the basins generally become younger from northwest to southeast with the exception of the Seimchan-Buyunda basin. In the northeast, fan-shaped coal-bearing basins (e.g., Nenneli, Olzhoi, Selennyakh, Uyandina, Tommot, and others) are filled with Miocene to Pliocene deposits, while basins in the southeast (e.g., Taskan) are filled with Neogene sediments. The Seimchan-Buyunda basin, however, has sediments of Oligocene age. The Moma <span class="hlt">rift</span> system is reflected a major step in the gravity field, presumably separating denser rocks of the Kolyma-Omolon superterrain from somewhat less dense rocks of the Verkhoyansk fold belt (margin of the North Asian Craton). Analysis of travel-times of Pn and Pg waves from local earthquakes indicates an area of thinned crust (30-35 km) southwest of the Moma <span class="hlt">rift</span> system, extending as a "tongue" from the Lena River delta and the Laptev Sea to the upper part of the Kolyma River, as compared to 40-45 km in the surrounding areas. This region of thinned crust also coincides with a region of high heat flow values measured in boreholes of the Chersky Range (up to 88 mW/m2). Hot</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/1987Tectp.133..257W','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/1987Tectp.133..257W"><span id="translatedtitle">The Fenwei <span class="hlt">rift</span> and its recent periodic activity</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Wang, Jing-Ming</p> <p>1987-02-01</p> <p>The Fenwei <span class="hlt">rift</span> on the southern sector of the Jin—Shaan <span class="hlt">rift</span> system of China is marked by a crescent-shaped valley 600 km in length and 30-90 km in width depressed up to 10 km and filled with about 7000 m of Cenozoic deposits, bounded on both northern and southern sides by majestic mountain ranges. The geometry of the <span class="hlt">rift</span> valley is characterized by six branch depressions and five intervening swells extending east-northeastward in a dextral en-echelon pattern and bounded on both sides by abrupt topographic slopes reflecting the underlying faults. These are typically a system of growth faults having downthrows ranging from 800 m to 10 km and dipping toward the centre of the valley forming an asymmetric graben structure. The geometry, kinematics and evolution of these faults have had controlling influences on the neotectonic movement of the <span class="hlt">rift</span> and its recent periodic activity as the present overall form of the <span class="hlt">rift</span> valley. Estimates of the amount of extension across the <span class="hlt">rift</span> for various recent geological periods were obtained from calculations made on the fault separation of corresponding stratigraphie horizons. The total amount of extension in response to tensile stresses, acting in a direction varying from 25° NW on the west to 70° NW on the northeast is estimated to be 9065 m, since the beginning of the <span class="hlt">rift</span> formation in the Eocene whereas the rate of extension in the Recent is 4.5 mm/yr and in modern times it is 8-24 mm/yr. The amount of left-lateral displacement across the <span class="hlt">rift</span> during various stages of its development was also calculated from the observed effects of strike-slip movement on the drainage system. The left-lateral offset since the mid-Pleistocene is approximately 7170 m and the offset rate in modern times is 6 mm/yr. These estimates suggest that the Fenwei <span class="hlt">rift</span> has been a place of intense neotectonic activity. Details of more recent activity of the <span class="hlt">rift</span> were investigated in terms of the various <span class="hlt">rift</span>-related phenomena such as seismic events</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/25732680','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/25732680"><span id="translatedtitle">A review of mosquitoes associated with <span class="hlt">Rift</span> Valley fever virus in Madagascar.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Tantely, Luciano M; Boyer, Sébastien; Fontenille, Didier</p> <p>2015-04-01</p> <p><span class="hlt">Rift</span> Valley fever (RVF) is a viral zoonotic disease occurring throughout Africa, the Arabian Peninsula, and Madagascar. The disease is caused by a Phlebovirus (RVF virus [RVFV]) transmitted to vertebrate hosts through the bite of infected mosquitoes. In Madagascar, the first RVFV circulation was reported in 1979 based on detection in mosquitoes but without epidemic episode. Subsequently, two outbreaks occurred: the first along the east coast and in the <span class="hlt">central</span> highlands in 1990 and 1991 and the most recent along the northern and eastern coasts and in the <span class="hlt">central</span> highlands in 2008 and 2009. Despite the presence of 24 mosquitoes species potentially associated with RVFV transmission in Madagascar, little associated entomological information is available. In this review, we list the RVFV vector, Culex antennatus, as well as other taxa as candidate vector species. We discuss risk factors from an entomological perspective for the re-emergence of RVF in Madagascar.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2009AGUFM.T53D1623P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2009AGUFM.T53D1623P"><span id="translatedtitle">Seismic anisotropy of the Rio Grande <span class="hlt">Rift</span> and surrounding regions</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Pulliam, J.; Rockett, C. V.; Grand, S. P.</p> <p>2009-12-01</p> <p>The Rio Grande <span class="hlt">Rift</span>, located between the Colorado Plateau and the Great Plains, has a complex tectonic history comprised of two distinct phases in the Cenozoic era. An early stage of <span class="hlt">rifting</span> began in the mid-Oligocene (~30 Ma) and lasted until the early Miocene (~18 Ma), followed by a lull and then an apparent reactivation along previous zones of weakness during a separate extension event in the late Miocene (~10 Ma), which continues today. The <span class="hlt">rift</span> now extends more than 1000 km in length, trending north-south from Southern Colorado through New Mexico and Western Texas and into Chihuahua, Mexico. Structure of the <span class="hlt">rift</span> is complex due to its multiple events of extension. The LA RISTRA seismic study (1999-2006) deployed broadband seismographs on a transect from Texas to Utah to investigate the structure and processes that control the <span class="hlt">Rift</span>. Among other results, they found distinct differences in orientation of the fast polarization direction, as measured from SKS splitting, in the three main regions: the Colorado Plateau, the Rio Grande <span class="hlt">Rift</span>, and the Great Plains. In 2008 71 EarthScope FlexArray stations were installed between Transportable Array stations to form a broad 2D deployment on the eastern flank of the RGR in southeastern New Mexico and western Texas as part of the SIEDCAR (Seismic Investigation of Edge Driven Convection Associated with the Rio Grande <span class="hlt">Rift</span>) study. SKS splitting measurements from these, as well as from TA stations in the vicinity, show a more complex 2D pattern, but one which conforms with variations in crustal thickness and velocity anomalies in the uppermost mantle. We will report on these measurements and their implications for the style of convection associated with the RGR.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014JAfES.100..203A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014JAfES.100..203A"><span id="translatedtitle"><span class="hlt">Rift</span> architecture and evolution: The Sirt Basin, Libya: The influence of basement fabrics and oblique tectonics</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Abdunaser, K. M.; McCaffrey, K. J. W.</p> <p>2014-12-01</p> <p> zones and adjoining highs. Late Eocene rocks exposed in the western part of the basin exhibit a complex network of branching segmented normal and strike-slip faults, generally with a NNW-SSE structural orientations. Many surface structural features have been interpreted from satellite images which confirm sinistral strike-slip kinematics. Relay ramp structures, numerous elongate asymmetric synclines associated with shallow west limbs and steeper dipping east limbs are developed in the hangingwalls adjacent to west downthrowing normal faults. These structural patterns reflect Cretaceous/Tertiary extensional tectonics with additional control by underlying pre-existing Pan-African basement fabrics and ENE-WSW trending Hercynian structures. We relate the Sirt Basin <span class="hlt">rift</span> development as exemplified in our study area to the break-up of Gondwana represented by the structural evolution of the West-<span class="hlt">Central</span> African <span class="hlt">rift</span> system, and the South and <span class="hlt">Central</span> Atlantic, the Tethys and the Indian Oceans.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2005AGUFM.V33D..04R','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2005AGUFM.V33D..04R"><span id="translatedtitle">Istopically Defined Source Reservoirs of Primitive Magmas in the East African <span class="hlt">Rift</span>.</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Rooney, T. O.; Furman, T.; Hanan, B.</p> <p>2005-12-01</p> <p>Extension within the East African <span class="hlt">Rift</span> is a function of the interaction between plume-driven uplift and far-field stresses associated with plate tectonic processes. Geochemical and isotopic investigation of primitive basalts from the Main Ethiopian <span class="hlt">Rift</span> (MER) reveals systematic spatial variations in the contributions from distinct and identifiable source reservoirs that, in turn help identify the mechanisms by which along-axis <span class="hlt">rifting</span> has progressed. The Sr-Nd-Pb isotopic characteristics of MER basalts can be described by a three-component mixing model involving the long-lived Afar plume, a depleted mantle component similar to the source region for Gulf of Aden MORB from east of 48° E and a reservoir that is likely lithospheric (sub-continental mantle lithosphere, magmatic underplate or lower crust). Quaternary basalts in the <span class="hlt">central</span> MER exhibit a systematic decrease in plume influence southward from 9.5° N to 8° N, i.e., away from the modern surface expression of the Afar plume in Djibouti and Erta 'Ale. The composition of the Afar plume component is comparable to the "C" mantle reservoir. This southward decrease in plume influence is coupled with an increase in the influence of the lithospheric and depleted mantle components. Linear arrays observed within Pb-Pb isotopic space at each eruptive center require distinctive ratio of lithospheric + depleted mantle components mixing with variable amounts of the "C"-like plume component. This isotopic evidence suggests the depleted mantle and lithosphere mixed prior to the generation of the recent magmas. To the south, the Sr-Nd-Pb isotopic compositions of Turkana (Kenya) <span class="hlt">rift</span> basalts record a mix of a similar "C"-like plume component and a fourth HIMU-like source component. Low 3He/4He values observed in the HIMU-dominated lavas from Turkana contrast with the higher ratios found in basalts associated with the "C"-like Afar plume. Further analysis of "C"-HIMU lavas at Turkana is required to fully constrain the He</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016Tectp.688...65J','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016Tectp.688...65J"><span id="translatedtitle">Jurassic failed <span class="hlt">rift</span> system below the Filchner-Ronne-Shelf, Antarctica: New evidence from geophysical data</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Jokat, Wilfried; Herter, Ulrich</p> <p>2016-10-01</p> <p>During the austral summer of 1994/95, reasonable ice conditions in the Weddell Sea allowed the acquisition of new high quality seismic refraction data parallel to the Filchner-Ronne Ice Shelf (FRS), Antarctica. Although pack ice conditions resulted in some data gaps, the final velocity-depth/2D-density models cover the entire FRS in E-W direction using all available deep seismic data/picks from this remote area. The velocity-depth model shows a sedimentary basin with a thickness up to 12 km and a large velocity inversion in the lowermost sedimentary unit. The crustal thickness reaches a maximum of 40 km along the basin's margins in the Antarctic Peninsula and East Antarctica. In the <span class="hlt">central</span> shelf area, numerous interfering seismic phases occur from the crust-mantle boundary at decreasing distances indicating a thinning of the crust. Here, the modelled velocities and densities reveal a thickness of 20 km for the igneous crust. This corridor of overthickened oceanic or close to oceanic crust is 160 km wide. The corridor is characterized by weak, but in general continuous magnetic anomalies, which we interpret as isochrons developed during the <span class="hlt">rifting</span> or the initial formation of oceanic crust. If the crustal composition represents an old stripe of oceanic crust, a minimum estimate for the early formation of the oceanic crust is 145/148 Ma (Late Jurassic). However, based on the velocity of <span class="hlt">rift</span> propagation during the initial opening of the adjacent Weddell Sea the oceanic crust is likely to have formed around 160 Ma. The onset of <span class="hlt">rifting</span> and development of a thick igneous crust can be related to stresses developed between the interior and the southwestern paleo-Pacific subduction margin of the fragmenting Gondwana supercontinent in combination with additional melt supply from a deeper mantle source that arrived and spread in the period 183-155 Ma.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.8908S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.8908S"><span id="translatedtitle">Orogenic structural inheritance and <span class="hlt">rifted</span> passive margin formation</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Salazar Mora, Claudio A.; Huismans, Ritske S.</p> <p>2016-04-01</p> <p>Structural inheritance is related to mechanical weaknesses in the lithosphere due to previous tectonic events, e.g. <span class="hlt">rifting</span>, subduction and collision. The North and South Atlantic <span class="hlt">rifted</span> passive margins that formed during the breakup of Western Gondwana, are parallel to the older Caledonide and the Brasiliano-Pan-African orogenic belts. In the South Atlantic, 'old' mantle lithospheric fabric resulting from crystallographic preferred orientation of olivine is suggested to play a role during <span class="hlt">rifted</span> margin formation (Tommasi and Vauchez, 2001). Magnetometric and gravimetric mapping of onshore structures in the Camamu and Almada basins suggest that extensional faults are controlled by two different directions of inherited older Brasiliano structures in the upper lithosphere (Ferreira et al., 2009). In the South Atlantic Campos Basin, 3D seismic data indicate that inherited basement structures provide a first order control on basin structure (Fetter, 2009). Here we investigate the role of structural inheritance on the formation of <span class="hlt">rifted</span> passive margins with high-resolution 2D thermo-mechanical numerical experiments. The numerical domain is 1200 km long and 600 km deep and represents the lithosphere and the sublithospheric mantle. Model experiments were carried out by creating self-consistent orogenic inheritance where a first phase of orogen formation is followed by extension. We focus in particular on the role of varying amount of orogenic shortening, crustal rheology, contrasting styles of orogen formation on <span class="hlt">rifted</span> margin style, and the time delay between orogeny and subsequent <span class="hlt">rifted</span> passive formation. Model results are compared to contrasting structural styles of <span class="hlt">rifted</span> passive margin formation as observed in the South Atlantic. Ferreira, T.S., Caixeta, J.M., Lima, F.D., 2009. Basement control in Camamu and Almada <span class="hlt">rift</span> basins. Boletim de Geociências da Petrobrás 17, 69-88. Fetter, M., 2009. The role of basement tectonic reactivation on the structural evolution</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFM.T51F2965S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFM.T51F2965S"><span id="translatedtitle">Neogene Development of the Terror <span class="hlt">Rift</span>, western Ross Sea, Antarctica</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Sauli, C.; Sorlien, C. C.; Busetti, M.; De Santis, L.; Wardell, N.; Henrys, S. A.; Geletti, R.; Wilson, T. J.; Luyendyk, B. P.</p> <p>2015-12-01</p> <p>Terror <span class="hlt">Rift</span> is a >300 km-long, 50-70 km-wide, 14 km-deep sedimentary basin at the edge of the West Antarctic <span class="hlt">Rift</span> System, adjacent to the Transantarctic Mountains. It is cut into the broader Victoria Land Basin (VLB). The VLB experienced 100 km of mid-Cenozoic extension associated with larger sea floor spreading farther north. The post-spreading (Neogene) development of Terror <span class="hlt">Rift</span> is not well understood, in part because of past use of different stratigraphic age models. We use the new Rossmap seismic stratigraphy correlated to Cape Roberts and Andrill cores in the west and to DSDP cores in the distant East. This stratigraphy, and new fault interpretations, was developed using different resolutions of seismic reflection data included those available from the Seismic Data Library System. Depth conversion used a new 3D velocity model. A 29 Ma horizon is as deep as 8 km in the south, and a 19 Ma horizon is >5 km deep there and 4 km-deep 100 km farther north. There is a shallower northern part of Terror <span class="hlt">Rift</span> misaligned with the southern basin across a 50 km right double bend. It is bounded by steep N-S faults down-dropping towards the basin axis. Between Cape Roberts and Ross Island, the Oligocene section is also progressively-tilted. This Oligocene section is not imaged within northern Terror <span class="hlt">Rift</span>, but the simplest hypothesis is that some of the Terror <span class="hlt">Rift</span>-bounding faults were active at least during Oligocene through Quaternary time. Many faults are normal separation, but some are locally vertical or even reverse-separation in the upper couple of km. However, much of the vertical relief of the strata is due to progressive tilting (horizontal axis rotation) and not by shallow faulting. Along the trend of the basin, the relief alternates between tilting and faulting, with a tilting margin facing a faulted margin across the <span class="hlt">Rift</span>, forming asymmetric basins. Connecting faults across the basin form an accommodation zone similar to other oblique <span class="hlt">rifts</span>. The Neogene basin is</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://www.osti.gov/scitech/servlets/purl/799225','SCIGOV-STC'); return false;" href="http://www.osti.gov/scitech/servlets/purl/799225"><span id="translatedtitle">Field Studies of Geothermal Reservoirs Rio Grande <span class="hlt">Rift</span>, New Mexico</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>James C Witcher</p> <p>2002-07-30</p> <p>The Rio Grande <span class="hlt">rift</span> provides an excellent field laboratory to study the nature of geothermal systems in an extensional environment. Much of the geologic complexity that is found in the Basin and Range is absent because the <span class="hlt">rift</span> is located on cratonic crust with a thin and well-characterized Phanerozoic stratigraphy and tectonic history. On the other hand, the Neogene thermo-tectonic history of the <span class="hlt">rift</span> has many parallels with the Basin and Range to the west. The geology of the southern Rio Grande <span class="hlt">rift</span> is among the best characterized of any <span class="hlt">rift</span> system in the world. Also, most geologic maps for the region are rather unique in that detailed analyses of Quaternary stratigraphic and surficial unit are added in concert with the details of bedrock geology. Pleistocene to Holocene entrenchment of the Rio Grande and tributaries unroofs the alteration signatures and permeability attributes of paleo outflow plumes and upflow zones, associated with present-day, but hidden or ''blind,'' hydrothermal systems at Rincon and San Diego Mountain.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/22094700','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/22094700"><span id="translatedtitle">East Antarctic <span class="hlt">rifting</span> triggers uplift of the Gamburtsev Mountains.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Ferraccioli, Fausto; Finn, Carol A; Jordan, Tom A; Bell, Robin E; Anderson, Lester M; Damaske, Detlef</p> <p>2011-11-16</p> <p>The Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 1958. The preservation of Alpine topography in the Gamburtsevs may reflect extremely low long-term erosion rates beneath the ice sheet, but the mountains' origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long <span class="hlt">rift</span> system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1 Gyr ago), was preserved as in some old orogens and was rejuvenated during much later Permian (roughly 250 Myr ago) and Cretaceous (roughly 100 Myr ago) <span class="hlt">rifting</span>. Much like East Africa, the interior of East Antarctica is a mosaic of Precambrian provinces affected by <span class="hlt">rifting</span> processes. Our models show that the combination of <span class="hlt">rift</span>-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that <span class="hlt">rifting</span> and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/6887650','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/6887650"><span id="translatedtitle">Episodic <span class="hlt">rifting</span> and subsidence in the South China sea</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Ru, K.; Pigott, J.D.</p> <p>1986-09-01</p> <p>The South China Sea experienced at least three stages of <span class="hlt">rifting</span> and two intervening stages of sea-floor spreading since the Early Cretaceous. Its evolution can be described by an episodic model of tectonism, one of thermal cooling and subsidence, pulsed by temporally and spatially confined heating events. Analysis of regional geologic and geophysical data suggests episodes of <span class="hlt">rifting</span> and associated thermal activities initiated during the Late Cretaceous, the late Eocene, and the late early Miocene. The <span class="hlt">rift</span> system corresponding to the first episode trends northeast-southwest, whereas those of the second and third trend east-west. These two trends coincide with the orientations of the major tectonic lineations within the basin. Age estimates from heat-flow and bathymetric data suggest the oceanic crust in the Southwest subbasin is considerably older (55 Ma) than that in the Northwest (35-36 Ma) or East (32 Ma) subbasins. In terms of hydrocarbon potential, the episodes of <span class="hlt">rifting</span> and drifting would be conducive to the development of overprinted structures and the deposition of several discrete transgressive packages of source rocks and reservoirs, separated by widespread unconformities. The thermal maturity of sedimentary organic matter affected by episodic <span class="hlt">rifting</span> and subsidence may be greater than expected on a purely passive margin of equivalent age that had not experienced repeated heating. 21 figures.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2006JHyd..316..290A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2006JHyd..316..290A"><span id="translatedtitle">Hydrogeochemical and lake level changes in the Ethiopian <span class="hlt">Rift</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Alemayehu, Tamiru; Ayenew, Tenalem; Kebede, Seifu</p> <p>2006-01-01</p> <p>The Ethiopian <span class="hlt">Rift</span> is characterized by a chain of lakes varying in size, hydrological and hydrogeological settings. The <span class="hlt">rift</span> lakes and feeder rivers are used for irrigation, soda extraction, commercial fish farming and recreation, and support a wide variety of endemic birds and wild animals. The level of some lakes shows dramatic changes in the last few decades. Lakes Abiyata and Beseka, both heavily impacted by human activities, show contrasting lake level trends: the level of Abiyata has dropped by about 5 m over three decades while Beseka has expanded from an area of 2.5-40 km 2 over the same span of time. Changes in lake levels are accompanied by dilution in ionic concentration of lake Beseka and increase in salinity of lake Abiyata. Although the principal hydrogeochemical process in the <span class="hlt">rift</span> lakes is controlled by the input and output conditions and carbonate precipitation, anthropogenic factors such as water diversion for irrigation and soda ash extraction played important role. The recent changes appear to have grave environmental consequences on the fragile <span class="hlt">rift</span> ecosystem, which demands an integrated basin-wide water management practice. This paper demonstrates the drastic changes of lake levels and associated changes in lake chemistry of the two studied lakes. It also gives the regional hydrogeochemical picture of the other <span class="hlt">rift</span> lakes that do not show significant response due to climate change and human impact.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70025197','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70025197"><span id="translatedtitle">Kilauea east <span class="hlt">rift</span> zone magmatism: An episode 54 perspective</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Thornber, C.R.; Heliker, C.; Sherrod, D.R.; Kauahikaua, J.P.; Miklius, Asta; Okubo, P.G.; Trusdell, F.A.; Budahn, J.R.; Ridley, W.I.; Meeker, G.P.</p> <p>2003-01-01</p> <p>On January 29 30, 1997, prolonged steady-state effusion of lava from Pu'u'O'o was briefly disrupted by shallow extension beneath Napau Crater, 1 4 km uprift of the active Kilauea vent. A 23-h-long eruption (episode 54) ensued from fissures that were overlapping or en echelon with eruptive fissures formed during episode 1 in 1983 and those of earlier <span class="hlt">rift</span> zone eruptions in 1963 and 1968. Combined geophysical and petrologic data for the 1994 1999 eruptive interval, including episode 54, reveal a variety of shallow magmatic conditions that persist in association with prolonged <span class="hlt">rift</span> zone eruption. Near-vent lava samples document a significant range in composition, temperature and crystallinity of pre-eruptive magma. As supported by phenocryst liquid relations and Kilauea mineral thermometers established herein, the <span class="hlt">rift</span> zone extension that led to episode 54 resulted in mixture of near-cotectic magma with discrete magma bodies cooled to ???1100??C. Mixing models indicate that magmas isolated beneath Napau Crater since 1963 and 1968 constituted 32 65% of the hybrid mixtures erupted during episode 54. Geophysical measurements support passive displacement of open-system magma along the active east <span class="hlt">rift</span> conduit into closed-system <span class="hlt">rift</span>-reservoirs along a shallow zone of extension. Geophysical and petrologic data for early episode 55 document the gradual flushing of episode 54 related magma during magmatic recharge of the edifice.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70016969','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70016969"><span id="translatedtitle">Speculations on the origin of the North American Midcontinent <span class="hlt">rift</span></span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Cannon, W.F.; Hinze, W. J.</p> <p>1992-01-01</p> <p>The Midcontinent <span class="hlt">rift</span> is an example of lithospheric extension and flood basalt volcanism induced when a new mantle plume arrived near the base of the lithosphere. Very large volumes of basaltic magma were generated and partly erupted before substantial lithospheric extension began. Volcanism continued, along with extension and deep <span class="hlt">rift</span> subsidence, for the ensuing 15 m.y. Much of the basaltic magma, including some of the earliest flows, was formed by partial melting of isotopically primitive asthenosphere contained in the plume head. The intense but relatively short duration of <span class="hlt">rifting</span> and magmatism is a result of the dissipation of thermal and mechanical energy in the plume head. As the plume head spread beneath the lithosphere, it stretched the overlying lithosphere radially away from the Lake Superior region, the triple junction of the <span class="hlt">rift</span> system, and partially melted to form the great volume of basalt and related intrusive rocks of the region. The plume arrived beneath a continent that was under compression as a result of the ongoing Grenville orogeny that affected a large region east of the <span class="hlt">rift</span>. That compression prevented full continental separation and eventually returned the region to compressional tectonics as the energy of the plume head waned. ?? 1992.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015EGUGA..17.8800B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015EGUGA..17.8800B"><span id="translatedtitle">Basement depth and sedimentary infill from deep seismic reflection data at the western tip of the offshore Corinth <span class="hlt">Rift</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Beckers, Arnaud; Tripsanas, Efthymios; Hubert-Ferrari, Aurélia; Beck, Christian; Sakellariou, Dimitris</p> <p>2015-04-01</p> <p>The Corinth <span class="hlt">rift</span> is a young continental <span class="hlt">rift</span> located in <span class="hlt">central</span> Greece. The active part of the <span class="hlt">rift</span> forms an E-W striking depression - the Gulf of Corinth - that is the deepest in its <span class="hlt">central</span> part. Extensive seismic surveys have imaged the basin's basement and allowed to estimate the total extension across most of the Gulf except its western tip. Extension is high in the <span class="hlt">central</span> part and decreases westward and eastward, as reflected in the present-day bathymetry. Two decades of GPS measurements have shown that the extension rate increases westwards from ~5 to 10-15 mm yr-1, but this is not consistent with the long term pattern. However, no data allowed so far to estimate the basement depth at the western tip of the Gulf, where the geodetic extension rate is the largest. Such data would allow to check the apparent inconsistency between the present rate and the long-term estimates of crustal extension. We present here an unpublished multichannel seismic line dating from 1979 and crossing the western tip of the Gulf of Corinth. The line is 22 km long and strikes WNW-ESE, from the Mornos delta to the West-Channel fault. A Maxipulse source has been used, allowing to image the basement below the synrift sedimentary infill. To the east, a ~1.6 km deep basin is imaged between the southern margin of the Gulf and an inactive south-dipping fault located between the Aigion and the Trizonia faults. The sedimentary infill consists in an alternation between basin-focused bodies made of incoherent reflections and more extensive high-amplitude reflectors. Attributing this alternation to eustatic variations give an age of 300-350 ka to the oldest well imaged deposits. Northwest of the Trizonia fault, the basement is imaged at shallower depth, i.e. ~450 m. The western tip of the seismic line reaches the Mornos delta, close to the northern shoreline. There, the depth to the basement is larger, reaching ~1.2 km. The infill is made of 3 units : on the basement lies a thin unit of</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/7012987','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/7012987"><span id="translatedtitle">Caribbean basin framework, 2: Northern <span class="hlt">Central</span> America</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Tyburski, S.A.; Gordon, M.B.; Mann, P. )</p> <p>1991-03-01</p> <p>There are four Jurassic to Recent basin-forming periods in northern <span class="hlt">Central</span> America (honduras, Honduran Borderlands, Belize, Guatemala, northern Nicaragua): (1) Middle Jurassic-Early Cretaceous <span class="hlt">rifting</span> and subsidence along normal faults in Honduras and Guatemala; <span class="hlt">rifts</span> are suggested but are not well defined in Honduras by the distribution of clastic sediments and associated volcanic rocks. <span class="hlt">Rifting</span> is attributed to the separation of <span class="hlt">Central</span> America from the southern margin of the North American plate; (2) Cretaceous subsidence recorded by the development of a Cretaceous carbonate platform in Honduras, Guatemala, and Belize; subsidence is attributed to thermal subsidence of the <span class="hlt">rifted</span> margins of the various blocks; (3) Late Cretaceous-Recent development of a volcanic arc along the western margin of Middle America and the northern margin of Honduras; (4) Late Cretaceous large-scale folding in Honduras, ophiolite obduction, and formation of a foredeep basin in Guatemala (Sepur trough); deformation is attributed to the collision between a north-facing arc in northern Honduras and the Nicaraguan Rise and the passive margin of Guatemala and Belize; and (5) Eocene to Recent strike-slip faulting along the present-day North American-Caribbean plate boundary in Guatemala, northern Honduras, and Belize. Strike-slip faults and basins form a California-type borderlands characterized by elongate basins that appear as half-grabens in profile. Counterclockwise rotation of the <span class="hlt">central</span> honduras plateau, a thicker and topographically higher-than-average block within the plate boundary zone, is accommodated by <span class="hlt">rifting</span> or strike-slip faults at its edges.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li class="active"><span>20</span></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_20 --> <div id="page_21" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li class="active"><span>21</span></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_23");'>23</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="401"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015EGUGA..1712068L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015EGUGA..1712068L"><span id="translatedtitle">Frontier of the underthrusting Indian lithosphere beneath the <span class="hlt">central</span> Tibet from finite frequency tomography</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Liang, Xiaofeng; Chen, Yun; Tian, Xiaobo; Wang, Minling; Xu, Tao; Sun, Changqing; Si, Shaokun; Lan, Haiqiang; Teng, Jiwen</p> <p>2015-04-01</p> <p>Combining the new collected teleseismic body waves recorded by TIBET-31N passive seismic array with waveforms from several previous temporary seismic arrays, we carried out finite-frequency tomographic inversions to image three-dimensional velocity structures beneath southern-<span class="hlt">central</span> Tibet to examine the roles of the upper mantle in the formation of the Tibetan plateau. Strong low P- and S-wave velocity anomalies that extend from the lower crust to about 200 km depth beneath the Comei <span class="hlt">rift</span>, Yadong-Gulu <span class="hlt">rift</span>, Tangra Yum Co <span class="hlt">rift</span>, suggesting that <span class="hlt">rifting</span> in southern Tibet is probably a process that involves the entire lithosphere. At the same time there is only the low velocity close to Yadong-Gulu <span class="hlt">rift</span> extending further north and connecting with the massive upper mantle low velocity beneath <span class="hlt">central</span> Tibet, and moreover, the other two are limited in southern Tibet. This observation implies that the previous proposed fragmentation of underthrusting Indian lithosphere might not happen underneath all the north-south trending <span class="hlt">rifts</span>. Instead, it only happens close to Yadong-Gulu <span class="hlt">rift</span>, then hot temperature upwelling materials fill up this lithospheric crack and might stuff the other weak zones in shallow depths beneath southern Tibet. Continuous high velocities are observed beneath Himalayas and Lhasa Terrance with a moderate northward inclination angle. We interpret this anomaly as the subducting/underthrusting Indian continental lithosphere.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..1811901B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..1811901B"><span id="translatedtitle">Tectonics and sedimentology of post-<span class="hlt">rift</span> anomalous vertical movements: the <span class="hlt">rifted</span> margin of Morocco</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Bertotti, Giovanni; Charton, Remi; Luber, Tim; Arantegui, Angel; Redfern, Jonathan</p> <p>2016-04-01</p> <p>Roughly 15 years ago it was discovered that substantial parts of the Morocco passive continental margin experienced km-scale, post-<span class="hlt">rift</span> exhumation. It was predicted that the sands resulting from the associated erosion would be present in the offshore and potentially form hydrocarbon reservoirs. At the same time, anomalous post-<span class="hlt">rift</span> vertical movements have been documented in various localities of the world and <span class="hlt">rifted</span> continental margins are at present exciting objects of research. Following intense research efforts the knowledge of the kinematics of vertical movements and their implications for sedimentary systems is increasing. The low-T geochronology initially limited to the classical Meseta-Massif Ancien de Marrakech transect has been expanded reaching the Reguibate Massif to the S and covering, possibly more importantly, one transect in E-W direction along the Anti Atlas. Exhumation occurred along two dominant trends. In N-S direction a several hundred-kilometers long exhuming domain developed roughly parallel to the Atlantic margin. Changes in magnitude and timing of exhumation are observed along this elevated domain associated with E-W trending undulations. The timing of main stage of upward movement of E-W trending highs seems to be Late Jurassic-Early Cretaceous in the Meseta and High Atlas and somewhat older, Early to Middle Jurassic, in the Anti-Atlas and Reguibate. The discovery of E-W trending highs and lows has major implication for sediment distribution and dispersal. At the large scale, it means that the drainage basins were smaller than initially predicted. This seems to be compatible with the paucity of sands encountered by recent exploration wells drilled offshore Morocco. At the scale of several kilometers, W-E trending anticlines and synclines developed in a generally subsiding coastal environment. These folds often had an expression at the sea floor documented by ravinement surfaces and (Jurassic) reef build-ups on top of the anticlines</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19830030613&hterms=geothermal+energy&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D50%26Ntt%3Dgeothermal%2Benergy','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19830030613&hterms=geothermal+energy&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D50%26Ntt%3Dgeothermal%2Benergy"><span id="translatedtitle">A groundwater convection model for Rio Grande <span class="hlt">rift</span> geothermal resources</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Morgan, P.; Harder, V.; Daggett, P. H.; Swanberg, C. A.</p> <p>1981-01-01</p> <p>It has been proposed that forced convection, driven by normal groundwater flow through the interconnected basins of the Rio Grande <span class="hlt">rift</span> is the primary source mechanism for the numerous geothermal anomalies along the <span class="hlt">rift</span>. A test of this concept using an analytical model indicates that significant forced convection must occur in the basins even if permeabilities are as low as 50-200 millidarcies at a depth of 2 km. Where groundwater flow is constricted at the discharge areas of the basins forced convection can locally increase the gradient to a level where free convection also occurs, generating surface heat flow anomalies 5-15 times background. A compilation of groundwater data for the <span class="hlt">rift</span> basins shows a strong correlation between constrictions in groundwater flow and hot springs and geothermal anomalies, giving strong circumstantial support to the convection model.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/20090027899','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/20090027899"><span id="translatedtitle">The First Prediction of a <span class="hlt">Rift</span> Valley Fever Outbreak</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Anyamba, Assaf; Chretien, Jean-Paul; Small, Jennifer; Tucker, Compton J.; Formenty, Pierre; Richardson, Jason H.; Britch, Seth C.; Schnabel, David C.; Erickson, Ralph L.; Linthicum, Kenneth J.</p> <p>2009-01-01</p> <p>El Nino/Southern Oscillation (ENSO) related anomalies were analyzed using a combination of satellite measurements of elevated sea surface temperatures, and subsequent elevated rainfall and satellite derived normalized difference vegetation index data. A <span class="hlt">Rift</span> Valley fever risk mapping model using these climate data predicted areas where outbreaks of <span class="hlt">Rift</span> Valley fever in humans and animals were expected and occurred in the Horn of Africa from December 2006 to May 2007. The predictions were subsequently confirmed by entomological and epidemiological field investigations of virus activity in the areas identified as at risk. Accurate spatial and temporal predictions of disease activity, as it occurred first in southern Somalia and then through much of Kenya before affecting northern Tanzania, provided a 2 to 6 week period of warning for the Horn of Africa that facilitated disease outbreak response and mitigation activities. This is the first prospective prediction of a <span class="hlt">Rift</span> Valley fever outbreak.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFM.T51G2999G','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFM.T51G2999G"><span id="translatedtitle">Middle Miocene <span class="hlt">rifting</span> and volcanic history of the Berufjordor- Breiddalur region, eastern Iceland revealed by 40Ar/39Ar geochronology</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Gans, P. B.; Askew, R. A.; Thordarson, T.</p> <p>2015-12-01</p> <p>Eighteen new 40Ar/39r incremental heating analyses of groundmass concentrates from fresh holocrystalline interiors of basalt lavas and dikes collected in an E-W, 35-km-long transect across the Berufjordor- Breiddalur region, eastern Iceland shed important new light on the Miocene spreading history and age of the Breiddalur <span class="hlt">central</span> volcano. Despite fine-grain sizes and low K contents, most samples yield high quality ages (either simple plateaus or spectra indicative of modest recoil ± low T argon loss) with estimated uncertainties of ±0.1 to 0.2 Ma. Ages decrease monotonically westward in the eastern half of transect from 12.1 to 10.0 Ma over an 18 km distance, in excellent agreement with the estimated half spreading rate of 0.9 cm/yr. In contrast, the western 15 km of the transect (and ~ 1 km of "apparent" vertical succession), including lavas below, above and within the Breiddalur volcano shows no systematic westward younging - all lavas in this region range from 9.1 to 9.8 Ma. Ages from diabase dikes similarly range from 9.1 to 9.8 Ma, except one distinctly younger at 7.8 Ma. The simplest interpretation of the new age determinations is that up until ~9 Ma, a ≥15 km-wide <span class="hlt">rift</span> zone/plate boundary was situated in the eastern half of the transect. The entire <span class="hlt">rift</span> zone (including Breiddalur volcano) was then accreted to the Eurasian plate during a westward <span class="hlt">rift</span> jump of ≥ 15 km. These types of minor jumps in the plate boundary and accretion of entire <span class="hlt">rift</span> zones to one plate or the other may help explain the "excess width" of Iceland. Also, the data raise questions about the strict applicability of the Palmeson (1973) model, as in this case, a paleo <span class="hlt">rift</span> zone is not reflected by a dip reversal, and "proximal" or within <span class="hlt">rift</span> volcanic and plutonic rocks are well exposed at the present surface and are not buried by younger flows as the spreading center moves away.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2003AGUFM.T11E..06L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2003AGUFM.T11E..06L"><span id="translatedtitle">On the formation of ultra-thick sedimentary basins on <span class="hlt">rifted</span> margins: a comparison of the Scotian and Labrador margins</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Louden, K.; Funck, T.</p> <p>2003-12-01</p> <p>Sedimentary basins that form on <span class="hlt">rifted</span> continental margins exhibit a great variety of shapes and sizes. In particular, the total sediment thickness can vary significantly and in certain sub-basins can approach 15-20 km. The deeper structure of these ultra-thick basins is typically not well resolved by seismic reflection profiles due to poor penetration within the thickest parts of the basin. Wide-angle seismic reflection/refraction profiles can help resolve these deeper features. We compare two such sub-basins that occur on the eastern Canadian margins, where reflection and refraction profiles are able to define the complete sedimentary and crustal structures: the Sable sub-basin on the northeast Nova Scotian margin and the Hopedale sub-basin on the <span class="hlt">central</span> Labrador margin. We compare the development of these basins by converting the sediment refraction velocities to density and back-stripping assuming local isostasy. Although these basins formed during completely different episodes of <span class="hlt">rifting</span> on different types of continental crust, we find a surprising similarity in the characteristics of crustal thinning across each margin, especially for the lower crust. Initial thinning of the crust by 50-60% occurs within 50 km followed by more gradual thinning over the subsequent 100 km. This leaves a tongue of lower continental crust extending 150 km seaward of the unstretched continental crust. This outer region becomes the location of the thickest initial sediment deposition, followed by up-building and out-building of the shelf. The local form of this deposition differs between the two margins: with much larger syn- and immediately post-<span class="hlt">rift</span> sediments on the Scotian margin and thicker recent deposition on the Labrador margin, probably controlled by the local availability of sediment fill. Comparison with previous models of <span class="hlt">rifting</span> based on borehole observations for the Scotia margin compare well with the overall width of the <span class="hlt">rifting</span> (150 km), but our results suggest more</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015EGUGA..17.5164P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015EGUGA..17.5164P"><span id="translatedtitle">The Angola-Gabon <span class="hlt">rifted</span> margin: reappraisal of the upper- and lower-plate concept</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Peron-Pinvidic, Gwenn; Manatschal, Gianreto; Masini, Emmanuel; Sutra, Emilie; Flament, Jean Marie; Haupert, Isabelle; Unternehr, Patrick</p> <p>2015-04-01</p> <p>In this contribution we summarize observations from the South Atlantic Angola-Gabon <span class="hlt">rifted</span> margin. Our study is based on interpretation of a selection of deep penetration depth migrated seismic reflection profiles. We describe the dip architecture of the margin under five structural domains (proximal, necking, distal, outer and oceanic), listing their characteristics. We further explain the necking domain and discuss the architecture of the distal domain as a combination of hyper-extended crust and exhumed mantle. The mapping and characterization of these domains permit to illustrate the along strike structural and stratigraphic variability of the margin. We interpret this variability as the result of a shift from an upper-plate setting (<span class="hlt">central</span> segment, South Congo to North Angola) to lower-plate settings (southward with the inner Kwanza Basin, and northward with the Gabon Basin). The transfer from one setting to the other is either sharp, typified by a major regional normal fault on the northern flank of a (residual) H-block, identified offshore Cabinda-Zaire, or more diffuse southward. First order screening of conjugate profiles confirmed the segmentation and the structural characteristics of the transfer zones. The studied dataset also permitted identifying key sections that can be considered as type-examples of upper-plate and lower-plate settings, what permits us reviewing the characteristics of upper- and lower-plate <span class="hlt">rifted</span> margins.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013EGUGA..15..730M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013EGUGA..15..730M"><span id="translatedtitle">Non-extensivity and complexity in the earthquake activity at the West Corinth <span class="hlt">rift</span> (Greece)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Michas, Georgios; Vallianatos, Filippos; Sammonds, Peter</p> <p>2013-04-01</p> <p>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 <span class="hlt">rift</span> (<span class="hlt">central</span> 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 <span class="hlt">rift</span>. 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).</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=5221768','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=5221768"><span id="translatedtitle">Phylogeography of <span class="hlt">Rift</span> Valley Fever Virus in Africa and the Arabian Peninsula</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Peterson, A. Townsend; Hall, Matthew</p> <p>2017-01-01</p> <p><span class="hlt">Rift</span> Valley Fever is an acute zoonotic viral disease caused by <span class="hlt">Rift</span> Valley Fever virus (RVFV) that affects ruminants and humans in Sub-Saharan Africa and the Arabian Peninsula. We used phylogenetic analyses to understand the demographic history of RVFV populations, using sequence data from the three minigenomic segments of the virus. We used phylogeographic approaches to infer RVFV historical movement patterns across its geographic range, and to reconstruct transitions among host species. Results revealed broad circulation of the virus in East Africa, with many lineages originating in Kenya. Arrival of RVFV in Madagascar resulted from three major waves of virus introduction: the first from Zimbabwe, and the second and third from Kenya. The two major outbreaks in Egypt since 1977 possibly resulted from a long-distance introduction from Zimbabwe during the 1970s, and a single introduction took RVFV from Kenya to Saudi Arabia. Movement of the virus between Kenya and Sudan, and CAR and Zimbabwe, was in both directions. Viral populations in West Africa appear to have resulted from a single introduction from <span class="hlt">Central</span> African Republic. The overall picture of RVFV history is thus one of considerable mobility, and dynamic evolution and biogeography, emphasizing its invasive potential, potentially more broadly than its current distributional limits. PMID:28068340</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/425497','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/425497"><span id="translatedtitle">Petroleum system of the Shelf <span class="hlt">Rift</span> Basin, East China Sea</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Cunningham, A.C.; Armentrout, J.M.; Prebish, M.</p> <p>1996-12-31</p> <p>The Tertiary section of the Oujioang and Quiontang Depressions of the East China Sea Basin consists of at least eight <span class="hlt">rift</span>-related depositional sequences identified seismically by regionally significant onlap and truncation surfaces. These sequences are calibrated by several wells including the Wenzhou 6-1-1 permitting extrapolation of petroleum system elements using seismic facies analysis. Gas and condensate correlated to non-marine source rocks and reservoired in sandstone at the Pinghu field to the north of the study area provides an known petroleum system analogue. In the Shelf <span class="hlt">Rift</span> Basin, synrift high-amplitude parallel reflections within the graben axes correlate with coaly siltstone strata and are interpreted as coastal plain and possibly lacustrine facies with source rock potential. Synrift clinoform seismic facies prograding from the northwest footwall correlate with non-marine to marginal marine conglomerate, sandstone and siltstone, and are interpreted as possible delta or fan-delta facies with reservoir potential although porosity and permeability is low within the Wenzhou 6-1-1 well. Post-<span class="hlt">rift</span> thermal sag sequences are characterized by parallel and relatively continuous seismic reflections and locally developed clinoform packages. These facies correlate with porous and permeable marine sandstone and siltstone. Shales of potential sealing capacity occur within marine flooding intervals of both the synrift and post-<span class="hlt">rift</span> sequences. Traps consist of differentially rotated synrift fill, and post-<span class="hlt">rift</span> inversion anticlines. Major exploration risk factors include migration from the synrift coaly source rocks to the post-<span class="hlt">rift</span> porous and permeable sandstones, and seismic imaging and drilling problems associated with extensive Tertiary igneous intrusions.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/6591249','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/6591249"><span id="translatedtitle">Petroleum system of the Shelf <span class="hlt">Rift</span> Basin, East China Sea</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Cunningham, A.C.; Armentrout, J.M.; Prebish, M. )</p> <p>1996-01-01</p> <p>The Tertiary section of the Oujioang and Quiontang Depressions of the East China Sea Basin consists of at least eight <span class="hlt">rift</span>-related depositional sequences identified seismically by regionally significant onlap and truncation surfaces. These sequences are calibrated by several wells including the Wenzhou 6-1-1 permitting extrapolation of petroleum system elements using seismic facies analysis. Gas and condensate correlated to non-marine source rocks and reservoired in sandstone at the Pinghu field to the north of the study area provides an known petroleum system analogue. In the Shelf <span class="hlt">Rift</span> Basin, synrift high-amplitude parallel reflections within the graben axes correlate with coaly siltstone strata and are interpreted as coastal plain and possibly lacustrine facies with source rock potential. Synrift clinoform seismic facies prograding from the northwest footwall correlate with non-marine to marginal marine conglomerate, sandstone and siltstone, and are interpreted as possible delta or fan-delta facies with reservoir potential although porosity and permeability is low within the Wenzhou 6-1-1 well. Post-<span class="hlt">rift</span> thermal sag sequences are characterized by parallel and relatively continuous seismic reflections and locally developed clinoform packages. These facies correlate with porous and permeable marine sandstone and siltstone. Shales of potential sealing capacity occur within marine flooding intervals of both the synrift and post-<span class="hlt">rift</span> sequences. Traps consist of differentially rotated synrift fill, and post-<span class="hlt">rift</span> inversion anticlines. Major exploration risk factors include migration from the synrift coaly source rocks to the post-<span class="hlt">rift</span> porous and permeable sandstones, and seismic imaging and drilling problems associated with extensive Tertiary igneous intrusions.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19930005126','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19930005126"><span id="translatedtitle">Flexural analysis of uplifted <span class="hlt">rift</span> flanks on Venus</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Evans, Susan A.; Simons, Mark; Solomon, Sean C.</p> <p>1992-01-01</p> <p>Knowledge of the thermal structure of a planet is vital to a thorough understanding of its general scheme of tectonics. Since no direct measurements of heat flow or thermal gradient are available for Venus, most estimates have been derived from theoretical considerations or by analog with the Earth. The flexural response of the lithosphere to applied loads is sensitive to regional thermal structure. Under the assumption that the yield strength as a function of depth can be specified, the temperature gradient can be inferred from the effective elastic plate thickness. Previous estimates of the effective elastic plate thickness of Venus range from 11-18 km for the foredeep north of Uorsar Rupes to 30-60 km for the annular troughs around several coronae. Thermal gradients inferred for these regions are 14-23 K km(exp -1) and 4-9 K km(exp -1) respectively. In this study, we apply the same techniques to investigate the uplifted flanks of an extensional <span class="hlt">rift</span>. Hypotheses for the origin of uplifted <span class="hlt">rift</span> flanks on Earth include lateral transport of heat from the center of the <span class="hlt">rift</span>, vertical transport of heat by small-scale convection, differential thinning of the lithosphere, dynamical uplift, and isostatic response to mechanical uploading of the lithosphere. The 1st hypothesis is considered the dominant contributor to terrestrial <span class="hlt">rift</span> flanks lacking evidence for volcanic activity, particularly for <span class="hlt">rift</span> structures that are no longer active. In this study, we model the uplifted flanks of a venusian <span class="hlt">rift</span> as the flexural response to a vertical end load.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015JGRB..120.1210L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015JGRB..120.1210L"><span id="translatedtitle">Thermal perturbations beneath the incipient Okavango <span class="hlt">Rift</span> Zone, northwest Botswana</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Leseane, Khumo; Atekwana, Estella A.; Mickus, Kevin L.; Abdelsalam, Mohamed G.; Shemang, Elisha M.; Atekwana, Eliot A.</p> <p>2015-02-01</p> <p>We used aeromagnetic and gravity data to investigate the thermal structure beneath the incipient Okavango <span class="hlt">Rift</span> Zone (ORZ) in northwestern Botswana in order to understand its role in strain localization during <span class="hlt">rift</span> initiation. We used three-dimensional (3-D) inversion of aeromagnetic data to estimate the Curie Point Depth (CPD) and heat flow under the <span class="hlt">rift</span> and surrounding basement. We also used two-dimensional (2-D) power-density spectrum analysis of gravity data to estimate the Moho depth. Our results reveal shallow CPD values (8-15 km) and high heat flow (60-90 mW m-2) beneath a ~60 km wide NE-trending zone coincident with major <span class="hlt">rift</span>-related border faults and the boundary between Proterozoic orogenic belts. This is accompanied by thin crust (<30 km) in the northeastern and southwestern parts of the ORZ. Within the Precambrian basement areas, the CPD values are deeper (16-30 km) and the heat flow estimates are lower (30-50 mW m-2), corresponding to thicker crust (~40-50 km). We interpret the thermal structure under the ORZ as due to upward migration of hot mantle fluids through the lithospheric column that utilized the presence of Precambrian lithospheric shear zones as conduits. These fluids weaken the crust, enhancing <span class="hlt">rift</span> nucleation. Our interpretation is supported by 2-D forward modeling of gravity data suggesting the presence of a wedge of altered lithospheric mantle centered beneath the ORZ. If our interpretation is correct, it may result in a potential paradigm shift in which strain localization at continental <span class="hlt">rift</span> initiation could be achieved through fluid-assisted lithospheric weakening without asthenospheric involvement.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012AGUFM.T43C2689F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012AGUFM.T43C2689F"><span id="translatedtitle">Magnetotelluric pilot study in the Rio Grande <span class="hlt">Rift</span>, southwest USA</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Feucht, D. W.; Bedrosian, P. A.; Sheehan, A. F.</p> <p>2012-12-01</p> <p>A magnetotelluric (MT) pilot study consisting of approximately 25 stations distributed in and around the Rio Grande <span class="hlt">Rift</span> of the southwest United States was carried out in the summer of 2012. Both broadband (100 Hz to 1000 s) and long-period (up to 10 000 s) MT data were collected across two profiles that run perpendicular to the <span class="hlt">rift</span> axis near Denver, Colorado and Taos, New Mexico, respectively. Time-domain EM data was also collected at each site to account for galvanic distortion in the near-surface. The tectonic forces and rheologic properties behind the initiation and propagation of the <span class="hlt">rift</span> are poorly understood. Surface mapping of volcanism, normal faulting and sedimentary basins reveals a narrow band of crustal deformation confined to a region in close proximity to the <span class="hlt">rift</span> axis while geophysical results suggest that deformation is distributed across a much broader and deeper region of the lithosphere. In particular, seismic tomography shows low seismic wave speeds into the lower crust and upper mantle. The magnetotelluric technique is a well-proven passive electromagnetic method that allows for the detection of apparent resistivity at a wide range of depth scales. Complimenting the seismic results with MT data will provide important new information on the geologic and geophysical properties that control the <span class="hlt">rifting</span> process in this low-strain rate environment. Properties to which the MT method is particular sensitive include temperature, fluid content, and mineral alteration. Preliminary results from this most recent survey are encouraging, showing good data quality up to 10 000 s. In an important precursor to full 2D modeling, the magnetotelluric phase tensor has been used to assess the dimensionality of the electrical resistivity structure at depth. This pilot study provides proof of concept for a much larger magnetotelluric experiment planned to take place in the Rio Grande <span class="hlt">Rift</span> in 2013.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015Litho.224..256A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015Litho.224..256A"><span id="translatedtitle">Geochemical characteristics and petrogenesis of phonolites and trachytic rocks from the České Středohoří Volcanic Complex, the Ohře <span class="hlt">Rift</span>, Bohemian Massif</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Ackerman, Lukáš; Ulrych, Jaromír; Řanda, Zdeněk; Erban, Vojtěch; Hegner, Ernst; Magna, Tomáš; Balogh, Kadosa; Frána, Jaroslav; Lang, Miloš; Novák, Jiří K.</p> <p>2015-05-01</p> <p>Trachyandesites, trachytes and phonolites represent the most evolved rock types within the České Středohoří Volcanic Complex (CSVC) in the Ohře/<span class="hlt">Eger</span> <span class="hlt">Rift</span>. The K-Ar ages of the suite range from ~ 33.8 to ~ 25.8 Ma. Major and trace element variation in the basanite - trachybasalt - trachyandesite series can be explained by several stages of modification of parental magmas by assimilation-fractional crystallization (AFC) involving fractionation of olivine, clinopyroxene, apatite, amphibole and Ti-oxide and bulk continental crust (BCC) as an assimilate. Relative to plausible basanitic starting compositions, the trachytes are moderately depleted in Sr, exhibit more pronounced depletions in P and Ti and some of them also show mild MREE depletion. Such composition requires variable amphibole, clinopyroxene, plagioclase ± apatite, titanite and/or Ti-magnetite fractionation and BCC assimilation. Two types of phonolites (type A and B phonolite) can be distinguished on the basis of overall REE patterns, Gd/Gd* ratios and Ba and Sr contents. Type B phonolites are depleted in Ba, Sr and MREE as a result of extensive alkali feldspar, plagioclase and amphibole fractionation. Modelling of trace element distributions implies basanitic magmas as the most likely parental composition of the basanite - trachybasalt - trachyandesite - trachyte - phonolite suite formed through magmatic differentiation. The Sr-Nd isotopic compositions in the samples can be explained with the assimilation of continental crust by such parental magmas. The highly radiogenic 87Sr/86Sr found in some phonolites are contrasted by uniform Nd isotopic signature; this feature may be explained by contamination and/or overprint of source magmas by Na-Rb-rich material with radiogenic Sr signature formed due to high-Rb (> 200 ppm) character of these melts/fluids. The nature of such contaminant is further evidenced by elevated Li (and Cs in some cases) abundances in type B phonolites although at least two</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..1814331C','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..1814331C"><span id="translatedtitle">Orthorhombic faults system at the onset of the Late Mesozoic-Cenozoic Barents Sea <span class="hlt">rifting</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Collanega, Luca; Breda, Anna; Massironi, Matteo</p> <p>2016-04-01</p> <p>The structures of the Late Mesozoic/Cenozoic Barents Sea <span class="hlt">rifting</span> have been investigated with multichannel 3D seismics, covering an area of 7700 sqKm in the Hoop Fault Complex, a transitional area between the platform and the marginal basins. The main structural lineaments have been mapped in a time domain 3D surface and their activity ranges have been constrained through the sin-sedimentary thickness variations detected in time-thickness maps. Two main fault systems have been identified: an orthorhombic fault system consisting of two fault sets trending almost perpendicularly one to the other (WNW-ESE and NNE-SSW) and a graben/half-graben system, elongated approximately N-S in the <span class="hlt">central</span> part of the study area. While the graben/half-graben system can be explained through the theory of Anderson, this landmark theory fails to explain the simultaneous activity of the two fault sets of the orthorhombic system. So far, the models that can better explain orthorhombic fault arrangements are the slip model by Reches (Reches, 1978; Reches, 1983; Reches and Dieterich, 1983) and the odd-axis model by Krantz (Krantz, 1988). However, these models are not definitive and a strong quest to better understand polymodal faulting is actual (Healy et al., 2015). In the study area, the presence of both a classical Andersonian and an orthorhombic system indicates that these models are not alternative but are both effective and necessary to explain faulting in different circumstances. Indeed, the Andersonian plain strain and the orthorhombic deformation have affected different part of the succession during different phases of the <span class="hlt">rifting</span>. In particular, the orthorhombic system has affected only the Late Mesozoic-Cenozoic interval of the succession and it was the main active system during the initial phase of the <span class="hlt">rifting</span>. On the other hand, the graben/half-graben system has affected the whole sedimentary succession, with an increasing activity during the development of the <span class="hlt">rifting</span>. It has</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/5558823','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/5558823"><span id="translatedtitle">Intermittent upwelling of asthenosphere beneath the Gregory <span class="hlt">Rift</span>, Kenya</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Tatsumi, Yoshiyuki Kyoto Univ. ); Kimura, Nobukazu ); Itaya, Tetsumaru ); Koyaguchi, Takehiro ); Suwa, Kanenori )</p> <p>1991-06-01</p> <p>K-Ar dates and chemical compositions of basalts in the Gregory <span class="hlt">Rift</span>, Kenya, demonstrate marked secular variation of lava chemistry. Two magmatic cycles characterized by incompatible element relative depletion are recognized; both occurring immediately after the peak of basaltic volcanism and coeval with both trachyte/phonolite volcanism and domal uplift of the region. These cycles may be attributed to increasing degree of partial melting of mantle source material in association with thinning of the lithosphere by thermal erosion through contact with hot upwelling asthenospheric mantle. Cyclic variation in asthenosphere upwelling may be considered an important controlling process in the evolution of the Gregory <span class="hlt">Rift</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19810056997&hterms=Continental+Drift&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D20%26Ntt%3DContinental%2BDrift','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19810056997&hterms=Continental+Drift&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D20%26Ntt%3DContinental%2BDrift"><span id="translatedtitle">Continental <span class="hlt">rifting</span> and the origin of Beta Regio, Venus</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Mcgill, G. E.; Steenstrup, S. J.; Barton, C.; Ford, P. G.</p> <p>1981-01-01</p> <p>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 <span class="hlt">rift</span> systems on earth. This interpretation is consistent with the commonly quoted analogy between the East African <span class="hlt">rift</span> 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.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70017946','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70017946"><span id="translatedtitle">Thermal budget of the lower east <span class="hlt">rift</span> zone, Kilauea Volcano</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Delaney, Paul T.; Duffield, Wendell A.; Sass, John H.; Kauahikaua, James P.; ,</p> <p>1993-01-01</p> <p>The lower east <span class="hlt">rift</span> zone of Kilauea has been the site of repeated fissure eruptions fed by dikes that traverse the depths of interest to geothermal explorations. We find that a hot-rock-and-magma system of low permeability extending along the <span class="hlt">rift</span> zone at depths below about 4 km and replenished with magma at a rate that is small in comparison to the modern eruption rate Kilauea can supply heat to an overlying hydrothermal aquifer sufficient to maintain temperatures of about 250??C if the characteristic permeability to 4-km depth is about 10-15m2.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2640145','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2640145"><span id="translatedtitle">New vectors of <span class="hlt">Rift</span> Valley fever in West Africa.</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Fontenille, D.; Traore-Lamizana, M.; Diallo, M.; Thonnon, J.; Digoutte, J. P.; Zeller, H. G.</p> <p>1998-01-01</p> <p>After an outbreak of <span class="hlt">Rift</span> Valley fever in Southern Mauritania in 1987, entomologic studies were conducted in a bordering region in Sénégal from 1991 to 1996 to identify the sylvatic vectors of <span class="hlt">Rift</span> Valley fever virus. The virus was isolated from the floodwater mosquitoes Aedes vexans and Ae. ochraceus. In 1974 and 1983, the virus had been isolated from Ae. dalzieli. Although these vectors differ from the main vectors in East and South Africa, they use the same type of breeding sites and also feed on cattle and sheep. Although enzootic vectors have now been identified in West Africa, the factors causing outbreaks remain unclear. PMID:9621201</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li class="active"><span>21</span></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_23");'>23</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_21 --> <div id="page_22" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li class="active"><span>22</span></li> <li><a href="#" onclick='return showDiv("page_23");'>23</a></li> <li><a href="#" onclick='return showDiv("page_24");'>24</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="421"> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/9621201','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/9621201"><span id="translatedtitle">New vectors of <span class="hlt">Rift</span> Valley fever in West Africa.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Fontenille, D; Traore-Lamizana, M; Diallo, M; Thonnon, J; Digoutte, J P; Zeller, H G</p> <p>1998-01-01</p> <p>After an outbreak of <span class="hlt">Rift</span> Valley fever in Southern Mauritania in 1987, entomologic studies were conducted in a bordering region in Sénégal from 1991 to 1996 to identify the sylvatic vectors of <span class="hlt">Rift</span> Valley fever virus. The virus was isolated from the floodwater mosquitoes Aedes vexans and Ae. ochraceus. In 1974 and 1983, the virus had been isolated from Ae. dalzieli. Although these vectors differ from the main vectors in East and South Africa, they use the same type of breeding sites and also feed on cattle and sheep. Although enzootic vectors have now been identified in West Africa, the factors causing outbreaks remain unclear.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/1991Tecto..10.1257B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/1991Tecto..10.1257B"><span id="translatedtitle">Geophysical studies of the West Antarctic <span class="hlt">Rift</span> System</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Behrendt, J. C.; Lemasurier, W. E.; Cooper, A. K.; Tessensohn, F.; TréHu, A.; Damaske, D.</p> <p>1991-12-01</p> <p>The West Antarctic <span class="hlt">rift</span> system extends over a 3000 × 750 km, largely ice covered area from the Ross Sea to the base of the Antarctic Peninsula, comparable in area to the Basin and Range and the East African <span class="hlt">rift</span> system. A spectacular <span class="hlt">rift</span> shoulder scarp along which peaks reach 4-5 km maximum elevation marks one flank and extends from northern Victoria Land-Queen Maud Mountains to the Ellsworth-Whitmore-Horlick Mountains. The <span class="hlt">rift</span> shoulder has maximum present physiographic relief of 5 km in the Ross Embayment and 7 km in the Ellsworth Mountains-Byrd Subglacial Basin area. The Transantarctic Mountains part of the <span class="hlt">rift</span> shoulder (and probably the entire shoulder) has been interpreted as rising since about 60 Ma, at episodic rates of ˜1 km/m.y., most recently since mid-Pliocene time, rather than continuously at the mean rate of 100 m/m.y. The <span class="hlt">rift</span> system is characterized by bimodal alkaline volcanic rocks ranging from at least Oligocene to the present. These are exposed asymmetrically along the <span class="hlt">rift</span> flanks and at the south end of the Antarctic Peninsula. The trend of the Jurassic tholeiites (Ferrar dolerites, Kirkpatric basalts) marking the Jurassic Transantarctic <span class="hlt">rift</span> is coincident with exposures of the late Cenozoic volcanic rocks along the section of the Transantarctic Mountains from northern Victoria Land to the Horlick Mountains. The Cenozoic <span class="hlt">rift</span> shoulder diverges here from the Jurassic tholeiite trend, and the tholeiites are exposed continuously (including the Dufek intrusion) along the lower- elevation (1-2 km) section of Transantarctic Mountains to the Weddell Sea. Widely spaced aeromagnetic profiles in West Antarctica indicate the absence of Cenozoic volcanic rocks in the ice covered part of the Whitmore-Ellsworth-Mountain block and suggest their widespread occurrence beneath the western part of the ice sheet overlying the Byrd Subglacial Basin. A German Federal Institute for Geosciences and Natural Resources (BGR)-U.S. Geological Survey (USGS) aeromagnetic</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012AGUFMED23B0759G','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012AGUFMED23B0759G"><span id="translatedtitle">Using Earthquake Intensity Data to Determine Earthquake Locations and Magniitudes of PRE-1966 Events in the Rio Grande <span class="hlt">Rift</span> Region</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Galvan, P.; Castro, J.; Doser, D. I.</p> <p>2012-12-01</p> <p>Prior to the use of modern seismographs beginning in the mid-1960's, not much information exists on the location and magnitudes of earthquakes in the Rio Grande <span class="hlt">rift</span> and surrounding regions of Colorado, New Mexico, west Texas, and Chihuahua, Mexico. However, a fair amount of intensity information is available for these earthquakes. Using procedures originally developed by Bakun and Wentworth (1997) we can use intensity information for well located, recent earthquakes as calibration events to develop intensity-distance attenuation models. The intensity attenuation models can then be used to determine the epicenters and magnitudes for the older events. Preliminary analysis of intensity data for recent events in the Rio Grande <span class="hlt">rift</span>-southeastern Rocky Mountains area suggest this region has a similar intensity attenuation relationship to that determined by Bakun (2006) for the Basin and Range province. Intensity data for recent events in the eastern Colorado Plateau and westernmost Great Plains appears consistent with intensity attenuation models developed by Bakun and Hopper (2004) for eastern North America. We will use these attenuation models to determine the magnitudes and locations of pre-1966 events, with special emphasis on events occurring in the <span class="hlt">central</span> Rio Grande <span class="hlt">rift</span> between 1905 and 1950.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016JGRB..121.8068R','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016JGRB..121.8068R"><span id="translatedtitle">Passive <span class="hlt">rifting</span> of thick lithosphere in the southern East African <span class="hlt">Rift</span>: Evidence from mantle transition zone discontinuity topography</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Reed, Cory A.; Liu, Kelly H.; Chindandali, Patrick R. N.; Massingue, Belarmino; Mdala, Hassan; Mutamina, Daniel; Yu, Youqiang; Gao, Stephen S.</p> <p>2016-11-01</p> <p>To investigate the mechanisms for the initiation and early-stage evolution of the nonvolcanic southernmost segments of the East African <span class="hlt">Rift</span> System (EARS), we installed and operated 35 broadband seismic stations across the Malawi and Luangwa <span class="hlt">rift</span> zones over a 2 year period from mid-2012 to mid-2014. Stacking of over 1900 high-quality receiver functions provides the first regional-scale image of the 410 and 660 km seismic discontinuities bounding the mantle transition zone (MTZ) within the vicinity of the <span class="hlt">rift</span> zones. When a 1-D standard Earth model is used for time-depth conversion, a normal MTZ thickness of 250 km is found beneath most of the study area. In addition, the apparent depths of both discontinuities are shallower than normal with a maximum apparent uplift of 20 km, suggesting widespread upper mantle high-velocity anomalies. These findings suggest that it is unlikely for a low-velocity province to reside within the upper mantle or MTZ beneath the nonvolcanic southern EARS. They also support the existence of relatively thick and strong lithosphere corresponding to the widest section of the Malawi <span class="hlt">rift</span> zone, an observation that is consistent with strain localization models and fault polarity and geometry observations. We postulate that the Malawi <span class="hlt">rift</span> is driven primarily by passive extension within the lithosphere attributed to the divergent rotation of the Rovuma microplate relative to the Nubian plate, and that contributions of thermal upwelling from the lower mantle are insignificant in the initiation and early-stage development of <span class="hlt">rift</span> zones in southern Africa.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.1381P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.1381P"><span id="translatedtitle">Discussion on final <span class="hlt">rifting</span> evolution and breakup : insights from the Mid Norwegian - North East Greenland <span class="hlt">rifted</span> system</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Peron-Pinvidic, Gwenn; Terje Osmundsen, Per</p> <p>2016-04-01</p> <p>In terms of <span class="hlt">rifted</span> margin studies, the characteristics of the distal and outer domains are among the today's most debated questions. The architecture and composition of deep margins are rarely well constrained and hence little understood. Except from in a handful number of cases (eg. Iberia-Newfoundland, Southern Australia, Red Sea), basement samples are not available to decipher between the various interpretations allowed by geophysical models. No consensus has been reached on the basement composition, tectonic structures, sedimentary geometries or magmatic content. The result is that non-unique end-member interpretations and models are still proposed in the literature. So, although these domains mark the connection between continents and oceans, and thus correspond to unique stages in the Earth's lithospheric life cycle, their spatial and temporal evolution are still unresolved. The Norwegian-Greenland Sea <span class="hlt">rift</span> system represents an exceptional laboratory to work on questions related to <span class="hlt">rifting</span>, <span class="hlt">rifted</span> margin formation and sedimentary basin evolution. It has been extensively studied for decades by both the academic and the industry communities. The proven and expected oil and gas potentials led to the methodical acquisition of world-class geophysical datasets, which permit the detailed research and thorough testing of concepts at local and regional scales. This contribution is issued from a three years project funded by ExxonMobil aiming at better understanding the crustal-scale nature and evolution of the Norwegian-Greenland Sea. The idea was to take advantage of the data availability on this specific <span class="hlt">rift</span> system to investigate further the full crustal conjugate scale history of <span class="hlt">rifting</span>, confronting the various available datasets. In this contribution, we will review the possible structural and sedimentary geometries of the distal margin, and their connection to the oceanic domain. We will discuss the definition of 'breakup' and introduce a first order conceptual</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013GeoJI.194..961O','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013GeoJI.194..961O"><span id="translatedtitle">The uppermost mantle shear wave velocity structure of eastern Africa from Rayleigh wave tomography: constraints on <span class="hlt">rift</span> evolution</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>O'Donnell, J. P.; Adams, A.; Nyblade, A. A.; Mulibo, G. D.; Tugume, F.</p> <p>2013-08-01</p> <p>An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath eastern Africa has been developed using earthquakes recorded by the AfricaArray East African Seismic Experiment in conjunction with data from permanent stations and previously deployed temporary stations. The combined data set comprises 331 earthquakes recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this study, data from 149 earthquakes were used to determine fundamental-mode Rayleigh wave phase velocities at periods ranging from 20 to 182 s using the two-plane wave method, and then combined with the similarly processed published measurements and inverted for a 3-D shear wave velocity model of the uppermost mantle. New features in the model include (1) a low-velocity region in western Zambia, (2) a high-velocity region in eastern Zambia, (3) a low-velocity region in eastern Tanzania and (4) low-velocity regions beneath the Lake Malawi <span class="hlt">rift</span>. When considered in conjunction with mapped seismicity, these results support a secondary western <span class="hlt">rift</span> branch striking southwestwards from Lake Tanganyika, likely exploiting the relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block and the Congo Craton. We estimate a lithospheric thickness of ˜150-200 km for the substantial fast shear wave anomaly imaged in eastern Zambia, which may be a southward subsurface extension of the Bangweulu Block. The low-velocity region in eastern Tanzania suggests that the eastern <span class="hlt">rift</span> branch trends southeastwards offshore eastern Tanzania coincident with the purported location of the northern margin of the proposed Ruvuma microplate. Pronounced velocity lows along the Lake Malawi <span class="hlt">rift</span> are found beneath the northern and southern ends of the lake, but not beneath the <span class="hlt">central</span> portion of the lake.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70023659','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70023659"><span id="translatedtitle">Anomalously high b-values in the South Flank of Kilauea volcano, Hawaii: Evidence for the distribution of magma below Kilauea's East <span class="hlt">rift</span> zone</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Wyss, M.; Klein, F.; Nagamine, K.; Wiemer, S.</p> <p>2001-01-01</p> <p>The pattern of b-value of the frequency-magnitude relation, or mean magnitude, varies little in the Kaoiki-Hilea area of Hawaii, and the b-values are normal, with b = 0.8 in the top 10 km and somewhat lower values below that depth. We interpret the Kaoiki-Hilea area as relatively stable, normal Hawaiian crust. In contrast, the b-values beneath Kilauea's South Flank are anomalously high (b = 1.3-1.7) at depths between 4 and 8 km, with the highest values near the East <span class="hlt">Rift</span> zone, but extending 5-8 km away from the <span class="hlt">rift</span>. Also, the anomalously high b-values vary along strike, parallel to the <span class="hlt">rift</span> zone. The highest b-values are observed near Hiiaka and Pauahi craters at the bend in the <span class="hlt">rift</span>, the next highest are near Makaopuhi and also near Puu Kaliu. The mildest anomalies occur adjacent to the <span class="hlt">central</span> section of the <span class="hlt">rift</span>. The locations of the three major and two minor b-value anomalies correspond to places where shallow magma reservoirs have been proposed based on analyses of seismicity, geodetic data and differentiated lava chemistry. The existence of the magma reservoirs is also supported by magnetic anomalies, which may be areas of dike concentration, and self-potential anomalies, which are areas of thermal upwelling above a hot source. The simplest explanation of these anomalously high b-values is that they are due to the presence of active magma bodies beneath the East <span class="hlt">Rift</span> zone at depths down to 8 km. In other volcanoes, anomalously high b-values correlate with volumes adjacent to active magma chambers. This supports a model of a magma body beneath the East <span class="hlt">Rift</span> zone, which may widen and thin along strike, and which may reach 8 km depth and extend from Kilauea's summit to a distance of at least 40 km down <span class="hlt">rift</span>. The anomalously high b-values at the center of the South Flank, several kilometers away from the <span class="hlt">rift</span>, may be explained by unusually high pore pressure throughout the South Flank, or by anomalously strong heterogeneity due to extensive cracking, or by both</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.4604N','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.4604N"><span id="translatedtitle">Permian and Triassic Meliata-related <span class="hlt">rift</span> and drift processes in Eastern Alps: middle and lower crust and its potential correlation with sedimentary units</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Neubauer, Franz</p> <p>2016-04-01</p> <p>The poorly studied middle and lower crust (MLC) of passive continental margins, a key plate tectonic element, is often preserved in thick-skinned tectonic wedges of mountain belts. We studied the Permian to early Norian Meliata Ocean-related <span class="hlt">rift</span>-characteristics of MLC and detached upper crust (UC) of the Austroalpine nappes of Eastern Alps (and Western Carpathians) with the aim to assess <span class="hlt">rift</span> models, composition and temporal and spatial distribution of magmatism. We also compare the development of UC sedimentary units with similar successions of the eastern Southalpine unit. There, <span class="hlt">rifting</span> started already during Early Permian, and a major extensional event occurred during late Early Permian separating Lower and Upper Permian carbonate platforms. In Austroalpine units, <span class="hlt">rifting</span> occurred later, and the response of LMC to <span class="hlt">rifting</span> includes acidic and subordinate, mostly earlier mafic magmatism, high-temperature/low-pressure metamorphism (ca. 0.46 GPa, 540 °C), and pure and simple shear deformation in shallow parts of MLC. In UC, the poorly dated <span class="hlt">rift</span>-onset unconformity formed in Early or Middle Permian and resulted in N-S to NE-SW striking halfgrabens filled with up to 1.5 km thick terrestrial clastics. First marine ingressions occurred during latest Permian, since Anisian carbonate deposition dominated (loss of the clastic hinterland). We recently detected a break-up angular unconformity in <span class="hlt">central</span> Northern Calcareous Alps (NCA) on top of tilted Lower Anisian Gutenstein Limestone and wedge-shaped Middle Triassic carbonates covered by Norian Dachstein Reef Limestone indicating the break-off and spreading in the Meliata oceanic tract. In Permian evaporites, polyhalite veins and grains crystallized between 235 and 225 Ma and at ca. 210 Ma testifying intense fluid flow along normal faults similar as Anisian/Ladinian strata-bound iron and Pb-Zn-Ba-F mineralizations do. In the underlying basement, we detected similar Permian to Triassic ductile shear zones with Ar</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2010AGUFM.T31C2195V','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2010AGUFM.T31C2195V"><span id="translatedtitle">Geometry of the Arabia-Somalia Plate Boundary into Afar: Preliminary Results from the Seismic Profile Across the Asal <span class="hlt">Rift</span> (Djibouti)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Vergne, J.; Doubre, C.; Mohamed, K.; Tiberi, C.; Leroy, S.; Maggi, A.</p> <p>2010-12-01</p> <p>In the Afar Depression, the Asal-Ghoubbet <span class="hlt">Rift</span> in Djibouti is a young segment on land at the propagating tip of the Aden Ridge. This segment represents an ideal laboratory to observe the mechanisms of extension and the structural evolutions involved, from the continental break-up to the first stage of oceanic spreading. However, we lack first order information about the crustal and upper mantle structure in this region, which for example prevent detailed numerical modeling of the deformations observed at the surface from GPS or InSAR. Moreover the current permanent network is not well suited to precisely constrain the ratio of seismic/aseismic deformation and to characterize the active deformation and the <span class="hlt">rifting</span> dynamics. Since November 2009 we have maintained a temporary network of 25 seismic stations deployed along a 150 km-long profile. Because we expect rapid variations of the lithospheric structure across the 10 km-wide <span class="hlt">central</span> part of the <span class="hlt">rift</span>, we gradually decreased the inter-stations spacing to less than 1 km in the middle section of the profile. In order to obtain a continuous image of the plate boundary, from the topographic surface to the upper mantle, several techniques and methods will be applied: P and S wave receiver functions, tomographies based on body waves, surface waves and seismic noise correlation, anisotropy, and finally a gravity-seismic joint inversion. We present some preliminary results deduced from the receiver functions applied to the data acquired during the first months of the experiment. We migrate several sets of receiver functions computed in various frequency bands to resolve both mantle interfaces and fine scale structures within the thin crust in the center of the <span class="hlt">rift</span>. These first images confirm a rapid variation of the Moho depth on both sides of the <span class="hlt">rift</span> and a very complex lithospheric structure in the <span class="hlt">central</span> section with several low velocity zones within the top 50km that might correspond to magma lenses.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014EGUGA..16.9676M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014EGUGA..16.9676M"><span id="translatedtitle">Thermochronological response to <span class="hlt">rifting</span> and subduction in the Corsica-Sardinia block</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Malusà, Marco Giovanni; Danišík, Martin; Kuhlemann, Joachim</p> <p>2014-05-01</p> <p>The linkage between deep-seated tectonic processes and surface processes provides a key to investigate the geological evolution of complex plate boundaries starting from the analysis of low-temperature geochronological systems. Here, we integrate published thermochronological data from Corsica (Danišík et al., 2007) with a new multi-thermochronological dataset (i.e., zircon and apatite fission track (ZFT and AFT), and apatite (U-Th)/He (AHe) data) from Sardinia, in order to tackle the Western Mediterranean tectonic issue and constrain the problematic transition in space and time between the opposite-dipping Alpine (European) and Apenninic (Adriatic) subductions. Mesozoic AFT ages (169-201 Ma) and AHe ages (133-204 Ma), found on mountain ridges of <span class="hlt">central</span> Sardinia and on the eastern coast of the island, indicate that rocks now exposed at the surface have resided since Jurassic times at very shallow depth, i.e., above the partial annealing zone of the AFT system (~60-110°C) or even above the partial retention zone of the AHe system (~40-80°C). The observed age pattern and track length distributions are consistent with those predicted after rising of isothermal surfaces during <span class="hlt">rifting</span> and subsequent thermal relaxation after continental break-up. We demonstrate that the crustal sections now exposed in <span class="hlt">central</span> and eastern Sardinia were originally located closer to the Tethyan <span class="hlt">rift</span> axis than crustal sections exposed in NW Sardinia and Corsica, pointing to a NNE trend for the continental crust isopachs of the northern Tethyan margin (ENE before Corsica-Sardinia rotation), with burial depth progressively increasing from SE to NW. In Alpine Corsica, the low-T geochronological evidence of Jurassic <span class="hlt">rifting</span> was largely obliterated by Cenozoic metamorphism, but it is still recognized in high-T systems. AFT and AHe ages set after Tethyan <span class="hlt">rifting</span> but not thermally affected by Neogene backarc extension, define a SE-NW trend of decreasing ages from southern Sardinia to northern</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2011AGUFM.T43D2360S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2011AGUFM.T43D2360S"><span id="translatedtitle">Broad, Undulated <span class="hlt">Rift</span> Structure Hidden Under Thick Sediment in the Niigata region, Japan</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Sekiguchi, S.; Enescu, B.; Takeda, T.; Asano, Y.; Obara, K.</p> <p>2011-12-01</p> <p>Niigata area is part of a broader region, located in the <span class="hlt">central</span> and north-eastern part of Japan, known for its high strain rates (Sagiya et al., 2000). The reverse fault system in the Niigata area may indicate present reactivation of an ancient <span class="hlt">rift</span> structure formed in the Miocene, at the opening of the Sea of Japan (Sato, 1994). To have a detailed understanding of seismotectonics in the Niigata area, we have installed a dense temporary network of 300 seismic stations. Enescu et al. (2011) presented preliminary tomography results, using earthquake data recorded during intense observations. The present study uses an improved dataset and refined data processing to reveal the detailed velocity structure and accurate earthquake locations. The data consists of 1805 events that have 151,780 P-picks and 169,696 S-picks, recorded at 434 temporary and permanent seismic stations. We first use a JHD algorithm (Kissling et al., 1994) to determine an optimum 1D velocity structure and more accurate hypocenters (both used later as input for the tomography inversion). As a result of relocation, the hypocenters in the on-shore Niigata basin region are shifted upwards by ~3 km, while the off-shore events become shallower by as much as 10 km, in agreement with results obtained in previous studies (e.g., Kato et al., 2009). We next use the "tomoDD" software (Zhang and Thurber, 2003) to invert for the 3D velocity structure and relocate simultaneously the hypocenters. The horizontal and vertical grid spacing were of 5 ~ 10 km and 2 ~ 4 km, respectively. The obtained velocity model shows a wide and relatively low velocity (< 5.5 km/s for the P-wave velocity and < 3 km/s for the S-wave velocity) band extending in a NE-SW direction, which widens and narrows along its extent. The thickness of the low-velocity region varies from place to place and exceeds 8 km in some parts. We have constructed iso-velocity maps to better visualize the shape and depth extent of the low-velocity region. Such</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70019799','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70019799"><span id="translatedtitle">Sedimentation and subsidence patterns in the <span class="hlt">central</span> and north basins of Lake Baikal from seismic stratigraphy</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Moore, T.C.; Klitgord, Kim D.; Golmshtok, A.J.; Weber, E.</p> <p>1997-01-01</p> <p>Comparison of sedimentation patterns, basement subsidence, and faulting histories in the north and <span class="hlt">central</span> basins of Lake Baikal aids in developing an interbasinal seismic stratigraphy that reveals the early synrift evolution of the <span class="hlt">central</span> portion of the Baikal <span class="hlt">rift</span>, a major continental <span class="hlt">rift</span> system. Although there is evidence that the <span class="hlt">central</span> and northern <span class="hlt">rift</span> basins evolved at approximately the same time, their sedimentation histories are markedly different. Primary sediment sources for the initial <span class="hlt">rift</span> phase were from the east flank of the <span class="hlt">rift</span>; two major deltas developed adjacent to the <span class="hlt">central</span> basin: the Selenga delta at the south end and the Barguzin delta at the north end. The Barguzin River system, located at the accommodation zone between the <span class="hlt">central</span> and north basins, also fed into the southern part of the north basin and facilitated the stratigraphic linkage of the two basins. A shift in the regional tectonic environment in the mid Pliocene(?) created a second <span class="hlt">rift</span> phase distinguished by more rapid subsidence and sediment accumulation in the north basin and by increased subsidence and extensive faulting in the <span class="hlt">central</span> basin. The Barguzin delta ceased formation and parts of the old delta system were isolated within the north basin and on Academic Ridge. These isolated deltaic deposits provide a model for the development of hydrocarbon plays within ancient <span class="hlt">rift</span> systems. In this second tectonic phase, the dominant sediment fill in the deeper and more rapidly subsiding north basin shifted from the flexural (eastern) margin to axial transport from the Upper Angara River at the north end of the basin.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/86606','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/86606"><span id="translatedtitle">Masirah Graben, Oman: A hidden Cretaceous <span class="hlt">rift</span> basin</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Beauchamp, W.H.; Ries, A.C.; Coward, M.P.</p> <p>1995-06-01</p> <p>Reflection seismic data, well data, geochemical data, and surface geology suggest that a Cretaceous <span class="hlt">rift</span> basin exists beneath the thrusted allochthonous sedimentary sequence of the Masirah graben, Oman. The Masirah graben is located east of the Huqf uplift, parallel to the southern coast of Oman. The eastern side of the northeast-trending Huqf anticlinorium is bounded by an extensional fault system that is downthrown to the southeast, forming the western edge of the Masirah graben. This graben is limited to the east by a large wed